Meteoritics amp Planetary Science 40 Nr 3 343ndash360 (2005)Abstract available online at httpmeteoriticsorg

343 copy The Meteoritical Society 2005 Printed in USA

Northwest Africa 011 A ldquoeucriticrdquo basalt from a non-eucrite parent body

Christine FLOSS1 Larry A TAYLOR2 Prinya PROMPRATED2 and Douglas RUMBLE III3

1Laboratory for Space Sciences Washington University One Brookings Drive Saint Louis Missouri 63130 USA2Planetary Geosciences Institute University of Tennessee Knoxville Tennessee 37996 USA

3Geophysical Laboratory Carnegie Institution Washington D C 20015 USACorresponding author E-mail flosswustledu

(Received 02 July 2004 revision accepted 17 January 2005)

AbstractndashWe have carried out a detailed petrographic mineralogical and trace element study ofNorthwest Africa (NWA) 011 This meteorite bears many similarities to the eucrites it was initiallyidentified with although oxygen isotopic compositions rule out a genetic relationship Like manyeucrites NWA 011 crystallized from a source with approximately chondritic proportions of REEalthough a slightly LREE-enriched bulk composition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances (Korotchantseva et al 2003) suggestthat the NWA 011 source experienced some pyroxene andor olivine fractionation Thermalmetamorphism resulted in homogenization of REE abundances within grains but NWA 011 did notexperience the intergrain REE redistribution seen in some highly metamorphosed eucrites Despite asimilarity in oxygen isotopic compositions NWA 011 does not represent a basaltic partial melt fromthe acapulcoitelodranite parent body The material from which NWA 011 originated may have beenlike some CH or CB chondrites members of the CR chondrite clan which are all related throughoxygen isotopic compositions The NWA 011 parent body is probably of asteroidal origin possiblythe basaltic asteroid 1459 Magnya

INTRODUCTION

Melting and differentiation of asteroids was a commonprocess in the early solar system as attested to by the varietyof magmatic iron meteorite types (eg Scott 1979) Howeverfew of the corresponding silicate differentiates that must haveformed on these planetesimals are represented in ourmeteorite collection Among the basaltic achondrites only theeucrites are reasonably abundant and have with some degreeof confidence been associated with a specific parent body theasteroid 4 Vesta (McCord et al 1970 Binzel and Xu 1993)Northwest Africa (NWA) 011 was originally classified as anon-cumulate eucrite (Afanasiev et al 2000) However theoxygen isotopic composition of NWA 011 was determined tobe very different from that of the eucrites and other knownbasaltic achondrites (Yamaguchi et al 2002) In addition theFeMn values in NWA 011 pyroxene are significantly higherthan the values typically associated with eucrites (Afanasievet al 2000) Thus NWA 011 appears to have originated on adistinct parent body and as such has the potential to provide uswith fresh insights into asteroidal differentiation processes inthe early solar system

We have carried out a detailed petrographic

mineralogical and trace element investigation of NWA 011 inorder to provide additional constraints on the nature of thisparent body Our data show that while NWA 011 is verysimilar to eucrites there are also differences includingevidence for formation under more oxidizing conditions thanthose experienced by eucrites Preliminary results of our workhave been reported by Promprated et al (2003) and Flosset al (2004)

EXPERIMENTAL

We studied the petrography and mineralogy of a thinsection of NWA 011 with a total area of 17 cm2 Majorelement compositions were determined using the fullyautomated Cameca SX-50 electron microprobe at theUniversity of Tennessee operated at an accelerating voltageof 15kV with a 20 nA beam current and counting times of20 sec Beam sizes varied depending on the phase beinganalyzed a 10 microm beam was used for plagioclase analyses toavoid volatilization of Na and 5 microm and 1 microm beam sizeswere used for pyroxene and olivine and for opaque mineralsrespectively Data were corrected for matrix effects usingstandard PAP procedures Prior to trace element analysis the

344 C Floss et al

section was further characterized using the JEOL 840ascanning electron microscope (SEM) at WashingtonUniversity X-ray mapping for ten elements (Al Ca Cr FeMg Na P S Si Ti) was done of the entire section and customsoftware written in LabVIEW was used to identify phasesand calculate the modal composition of the section (mode 1 inTable 1) The resolution of the X-ray maps (256 times 256 pixelsat a magnification of 75times 1 pixel = sim5 microm) did not allowpigeonite and augite to be distinguished We also did notspecifically search for silica this phase may in fact be presentin our section but is not included in the mode The majorsource of uncertainty in the modal determination is likely tobe due to the difficulty of determining exact phaseboundaries given the limited resolution of the X-ray mapsHowever it is difficult to assign a quantitative error to themode based on this uncertainty

Concentrations of the REE and other trace elements weredetermined using the modified Cameca IMS 3f ionmicroprobe at Washington University according to techniquesdescribed by Zinner and Crozaz (1986a) All analyses weremade using an Ominus primary beam and energy filtering at lowmass resolution to remove complex molecular interferencesThe resulting mass spectrum is deconvolved in the massranges K-Ca-Sc-Ti Rb-Sr-Y-Zr and Ba-REE to removesimple molecular interferences that are not eliminated withenergy filtering (Alexander 1994 Hsu 1995) Sensitivityfactors for the REE in pyroxene and Ca-phosphate are fromZinner and Crozaz (1986b) and for plagioclase are from Flossand Jolliff (1998) Sensitivity factors for other elements inplagioclase and pyroxene are from Hsu (1995) and are listedin Table 1 of Floss et al (1998) Absolute concentrations aredetermined using sensitivity factors relative to Si for thesilicates and Ca for the phosphates The CaO concentrationsused for the phosphates (merrillite) are the average of lunarmerrillites from Jolliff et al (1993) SiO2 concentrations forplagioclase are the average of the values determined byelectron microprobe Pyroxene in NWA 011 is pigeonitecontaining abundant augite exsolution lamellae We obtainedelectron microprobe analyses of the augite lamellae as well asthe host pigeonite Because the ion probe beam was largerthan the width of the lamellae we used an average of the

augite and pigeonite concentrations as the reference SiO2concentration for the pyroxene analyses REE abundances inthe figures are normalized to the CI abundances of Andersand Grevesse (1989)

Oxygen isotopic analyses were carried out at theGeophysical Laboratory Carnegie Institution on two separateportions of NWA 011 using a high-precision laser-fluorination technique (Wiechert et al 2001) Samplesweighing 1ndash2 mg were loaded into a Sharp-type reactionchamber (Sharp 1990) evacuated pre-fluorinated and heatedwith a CO2 laser in a 25 torr atmosphere of purified BrF5 Theprimary δ18O standard was UWG-2 garnet (Valley et al1995) Two secondary standards were also analyzed tobracket the δ18O of NWA 011 The ∆17O offset of NWA 011from the terrestrial fractionation line was determinedaccording to the procedures of Miller (2002)

RESULTS

NWA 011 is a 40 g stone (Grossman 2000) that appears tobe a monolithic unbrecciated achondrite consisting mainly ofpigeonite and plagioclase with minor amounts of olivinesilica phosphate and the opaque minerals ulvˆspinel andilmenite (Fig 1) Pigeonite is commonly subhedral and oftenhas curved boundaries at the contacts with plagioclase Grainsrange in size from 02 to 08 mm and contain exsolutionlamellae of augite that are typically a few microns wide butcan reach up to sim10 microm in width Plagioclase grains areanhedral up to 01 mm in size and usually form crystalaggregates (up to 2 mm across) or straddle along grainboundaries of pigeonite grains (Fig 1a) This textureprobably indicates recrystallization possibly as a result ofthermal metamorphism However we did not observe relictplagioclase or augite grains such as those noted byYamaguchi et al (2002) Opaque minerals occur as smallclusters usually together with olivine and include ulvˆspinelilmenite and small FeNi metal-sulfide intergrowths (Fig 1b)The Ca-phosphates merrillite and apatite are common butare not evenly distributed

Modal abundances are given in Table 1 where they arecompared with others from the literature Mode 2 was

Table 1 Mineralogical modes for NWA 011 (vol)1a 2b 3c Average

Pyroxene (pigeonite + augite) 546 585 553 561 plusmn 21Plagioclase 438 396 422 419 plusmn 21Olivine 02 tr 01 02 plusmn 01Silica nd 07 15 11 plusmn 06Phosphate 11 05 04 07 plusmn 04Opaque minerals 02 07 04 04 plusmn 03Total area 17 cm2 29 cm2 098 cm2

aThis workbPromprated et al (2003)cYamaguchi et al (personal communication)tr = trace nd = not determined

Northwest Africa 011 345

reported by Promprated et al (2003) and was determined byKorochantseva et al (2003) on a different thin section ofNWA 011 Mode 3 was determined by A Yamaguchi on athird thin section of NWA 011 An average mode based on allthree determinations is also given in Table 1 There aremoderate variations in the relative abundances of plagioclaseand pyroxene between the three sections which probably to alarge degree reflect the uncertainties associated with themodal determinations The accessory phases also appear to beheterogeneously distributed as has been noted by others (egYamaguchi et al 2002)

Mineral compositions are summarized in Table 2Compositions of pigeonite (Wo57ndash65Fs632ndash651) and the augite

lamellae (Wo361ndash392Fs352ndash383) are similar to those of NuevoLaredo trend eucrites (Fig 2 Warren and Jerde 1987)However as has been noted (eg Afanasiev et al 2000) theFeMn ratios are significantly higher than those of typicaleucrites (Fig 3) The plagioclase in NWA 011 is bytownite incomposition with low K2O contents (An802ndash877Or07ndash02) Theolivine is Fe-rich and has little variation in composition(Fa795ndash814 Fig 2) The compositions of ulvˆspinel andilmenite are given in Table 2 Phosphate compositions werenot analyzed by electron microprobe but EDS spectra takenof the grains analyzed by ion microprobe show that all aremerrillite rather than apatite

Results of the oxygen isotopic analyses are shown in

Fig 1 Backscattered electron (BSE) images of NWA 011 a) subhedral pigeonite with augite exsolution lamellae (px) and anhedral plagioclase(pl) crystal aggregates b) opaque mineral cluster with ulvˆspinel (usp) ilmenite (ilm) and olivine (ol)

Fig 2 Major element compositions of pyroxene and olivine from NWA 011 The solid lines in the pyroxene quadrilateral show thecompositions of pyroxenes from the Nuevo Laredo trend (NL) Stannnern trend (St) and main group (Juvinas Jv) eucrites Data from BVSP(1981) and Warren and Jerde (1987)

346 C Floss et al

Table 3 and Fig 4 The average isotopic composition of thefour aliquots of NWA 011 that we measured is δ18O = 292 plusmn022 and δ17O = minus006 plusmn 012 consistent with the results ofYamaguchi et al (2002) These data confirm that NWA 011has an unusual oxygen isotopic composition that is farremoved from the known eucrite field (∆17O = minus158 forNWA 011 versus minus025 for the eucrites) The oxygen isotopiccomposition falls within the range of values observed for CRchondrites (Fig 4) but is not like that of any other knownachondrites (Clayton 1993 Clayton and Mayeda 1996)

Minor and trace element compositions of plagioclase

pyroxene and merrillite are given in Table 4 and the REEpatterns are shown in Fig 5 Two plagioclase grains haveparallel LREE-enriched patterns with large positive Euanomalies (Fig 5a) REE abundances differ by about 50between the two grains Four merrillite grains have virtuallyidentical REE abundances The patterns are LREE-enrichedwith negative Eu anomalies all four grains also show smallpositive Ce anomalies (Fig 5b) The REE abundances inpyroxene are uniform indicating that we adequately sampledboth the augite lamellae and host pigeonite compositionsMoreover we did not observe any core to rim zoning in three

Table 2 Major element concentrations (in wt) in NWA 011 mineralsPlagioclase Pigeonite Augite lamellae Olivine Ilmenite Ulvˆspinel

SiO2 4645 plusmn 063 4779 plusmn 037 4855 plusmn 040 3086 plusmn 031 016 plusmn 018 006 plusmn 002TiO2 nm 052 plusmn 007 084 plusmn 011 034 plusmn 032 5272 plusmn 072 2419 plusmn 031Cr2O3 nm 018 plusmn 003 039 plusmn 007 nm 010 plusmn 004 1373 plusmn 041Al2O3 3342 plusmn 043 050 plusmn 004 127 plusmn 010 003 plusmn 003 011 plusmn 014 441 plusmn 020V2O3 nm nm nm nm 026 plusmn 004 043 plusmn 005FeO 076 plusmn 070 3611 plusmn 034 2041 plusmn 054 5960 plusmn 034 4512 plusmn 058 5327 plusmn 076MnO nm 055 plusmn 003 033 plusmn 003 055 plusmn 003 037 plusmn 001 030 plusmn 003NiO nm nm nm 001 plusmn 004 nm nmMgO 005 plusmn 009 968 plusmn 023 851 plusmn 026 809 plusmn 031 066 plusmn 007 051 plusmn 002CaO 1721 plusmn 042 279 plusmn 010 1755 plusmn 048 020 plusmn 005 018 plusmn 010 007 plusmn 003Na2O 162 plusmn 022 002 plusmn 001 009 plusmn 001 nm nm nmK2O 007 plusmn 002 nm nm nm nm nmP2O5 nm nm nm nm nm nmFe2O3

a bd 124 111 bd bd 229Total 9958 9937 9905 9967 9966 9926 analyses 16 16 12 9 3 5

An822ndash877Or07ndash02 Wo57ndash65Fs632ndash651 Wo361ndash392Fs352ndash383 Fa795ndash814FeMn 67 65 108

aCalculated after the method of Spear (1995)nm = not measured bd = below detection Errors represent the standard deviation from the mean

Fig 3 Molar Fe versus Mn in pyroxenes from NWA 011 The FeMn ratios are significantly higher than those of typical eucrites Correlationlines are based on data from Papike (1998)

Northwest Africa 011 347

pyroxene grains (Figs 5cndashe) consistent with the largelyuniform major element compositions of the pigeonite(Table 2 Yamaguchi et al 2002)

DISCUSSION

NWA 011 was originally classified as a non-cumulateeucrite and indeed this meteorite seems to share manysimilarities with the eucrites including texture mineralogyand major element compositions except for higher FeMnratios (eg Yamaguchi et al 2002) In portions of thefollowing discussion therefore we will compare the traceelement data of NWA 011 with that of certain non-cumulateeucrites in order to elucidate its crystallization (and post-crystallization) history

Ce Anomalies in Merrillite Terrestrial Weathering

Because NWA 011 is a meteorite recovered from a hotdesert terrestrial alteration and its possible effect on traceelement compositions must be evaluated Recent studies have

shown that hot desert meteorites are quite susceptible toterrestrial contamination (Bland et al 1996 Dreibus et al2001 Crozaz and Wadhwa 2001) and may experiencesignificant chemical changes that can include enrichments ofRb Sr Ba LREE U and Th decreased abundances of C Nand H the presence of occasional Ce anomalies and theaddition of terrestrial noble gases (Scherer et al 1994 Ashand Pillinger 1995 Barrat et al 1999 Crozaz et al 2003)

The evidence for terrestrial alteration of NWA 011 ismixed Yamaguchi (2001) notes that the rock is quiteweathered but although Korochantseva et al (2003) observedsignificant terrestrial Ar in their analysis of NWA 011 theyalso found that Ba and Sr abundances were low indicatinglittle to no contamination of these elements from the desertenvironment Our data also do not show elevated abundancesof Sr and Ba However we do observe small positive Ceanomalies in the four grains of merrillite measured here(Table 4 Fig 4) although plagioclase and pyroxene grains donot exhibit any Ce anomalies Cerium anomalies in mineralsfrom hot desert meteorites are generally associated withLREE enrichments and are most often found in phases withlow abundances of the LREE such as olivines and pyroxenes(Crozaz et al 2003) An exception is the brachinite EaglesNest which shows an LREE enrichment and large Cedepletion in apatite (Swindle et al 1998 Crozaz et al 2003)However both olivine and whole rock analyses of thismeteorite show the same features indicating that thismeteorite is strongly contaminated Excesses in Ce could beproduced through the oxidation of Ce (III) to Ce (IV) (eg

Fig 4 Three isotope plots showing the oxygen isotopic composition of NWA 011 (black circle) Errors (022 permil for δ18O and 012 permil for δ17O1σ) are smaller than the size of the symbol Also shown are the terrestrial fractionation (TF) line the carbonaceous chondrite anhydrousmineral (CCAM) line and the fields for CR chondrites CM chondrites Martian meteorites (SNC) the HED (howardite-eucrite-diogenite)meteorites and the acapulcoiteslodranites (Aca-Lod) Fields for meteorites other than NWA 011 are from Clayton (2003) and referencestherein

Table 3 Oxygen isotopic data for NWA 011Aliquot δ18O δ17O

A1 323 010A2 284 minus010B1 288 minus008B2 271 minus017Average 292 plusmn 022 minus006 plusmn 012

348 C Floss et al

Mittlefehldt and Lindstrom 1991) which is more insolublethan trivalent Ce Dissolution of the Ca phosphates duringaqueous alteration would result in the preferential retention ofCe (IV) relative to the trivalent REE leading to positive Ceanomalies such as those observed in NWA 011 Although wefind it unlikely that REE-rich Ca-phosphate would be affectedby this process while phases with lower REE concentrationssuch as plagioclase and pyroxene are not we cannot rule outthat terrestrial weathering is responsible for the Ce anomaliesobserved in NWA 011 merrillite Moreover Schreiber et al(1980) have shown experimentally that Ce (IV) in magmaticsystems will be reduced by Fe (II) to Ce (III) These authorsnote that ldquoit is difficult to visualize a magma under realisticconditions where the Ce (IV) species would be stabilizedrdquoThus we conclude that the Ce anomalies in merrillite are notprimary but could be a secondary feature due to terrestrialalteration However other trace element compositions inNWA 011 do not appear to have been compromised byterrestrial contamination

Bulk Compositions of NWA 011

Whole rock REE compositions of NWA 011 werecalculated using the averages of our mineral compositions(Table 4) and the modes reported in Table 1 For thepurposes of this calculation the REE abundances in olivinesilica and opaque minerals were assumed to be negligibleFigure 6 shows the three calculated bulk REE patternstogether with the average mineral compositions of NWA011 All three patterns are approximately flat to slightlyLREE-enriched with abundances of sim10 times CI and havesmall positive Eu anomalies The patterns also have smallpositive Ce anomalies that reflect the influence of phosphateon the bulk composition of the meteorite The REE patternfor bulk 1 has LREE abundances that are about a factor oftwo higher than those of bulk 2 and bulk 3 largely due to thehigher modal abundance of Ca-phosphate in this thin section(Table 1)

Published whole rock REE patterns for NWA 011however are heterogeneous and differ from those calculatedhere Both Yamaguchi et al (2002) and Korotchantseva et al(2003) analyzed chips of NWA 011 the REE patterns ofwhich are HREE-enriched with large positive Eu anomaliesThese REE patterns can be reproduced by assuming modalproportions of plagioclase and pyroxene with little to no Ca-phosphate (Fig 7a) Another chip analyzed byKorotchantseva et al (2003) has a LREE-rich pattern with apositive Ce anomaly and is clearly dominated by excess Ca-phosphate (Fig 7b) Moreover the negative Eu anomaly inthis pattern requires a much larger fraction of pyroxene (andcorrespondingly less plagioclase) than is present in oursections (sim88 versus 55ndash58)

Table 5 shows two whole rock major elementcompositions for NWA 011 from the literature Whole rock Y

was determined by Yamaguchi et al (2002) using wetchemical analysis whereas whole rock K was determined byKorochantseva et al (2003) using mineral chemistry and themode published by Promprated et al (2003) (ie mode 2 inTable 1) We also calculated bulk compositions from modes 12 and 3 in Table 1 and the mineral compositions from Table 2in order to compare them with the literature whole rockcompositions For the purposes of these calculations weassumed that the proportions of augite to pigeonite andilmenite to ulvˆspinel were 12 The results are shown inTable 5 Our calculated bulk compositions generally showdifferences that are similar in magnitude to those between thetwo literature whole rock compositions and that are consistentwith the differences in the three modes of Table 1 Thus forexample bulk 1 has the highest concentrations of Al2O3CaO and Na2O consistent with the fact that this mode has thehighest proportion of plagioclase Similarly the bulkcomposition calculated from mode 2 which has the highestproportion of pyroxene has the highest concentrations ofMgO and FeO Concentrations of SiO2 are intermediatebetween the whole rock values of Korochantseva et al (2002)and Yamaguchi et al (2002) and not surprisingly are highestin the two that contain silica in the mode Moreover all threeof our calculated bulk compositions have somewhat higherAl2O3 CaO and Na2O and lower FeO and MgO than thewhole rock determinations Y and K Since the modalabundances for whole rock K and bulk 2 are the same someof these differences must reflect the average mineralcompositions used to calculate the bulk compositionsHowever modes 1 and 3 also have more plagioclase thanmode 2 Whole rock composition Y from Yamaguchi et al(2002) has the highest MgO and FeO abundances and thelowest SiO2 which seems to indicate a higher proportion ofolivine in this analysis The compositions calculated frommode 2 determined over the largest area of NWA 011(Table 1) are most similar to the whole rock determination ofYamaguchi et al (2002) this mode may be the mostrepresentative of NWA 011

Equilibrium Melt Compositions and CrystallizationHistory

If plagioclase and pyroxene in NWA 011 have preservedtheir original igneous signatures it should be possible tocalculate the REE compositions of the melts from which theycrystallized Although textural evidence and the uniformity ofmajor element compositions in both plagioclase and pyroxenesuggest that NWA 011 has undergone some re-equilibrationsuch a calculation can still provide some useful constraints onthe nature of the NWA 011 parent melt as well as possiblesecondary processes that may have affected the sample

The distribution coefficients listed in Table 6 were usedto calculate the compositions of liquids in equilibrium withcore plagioclase and pyroxene grains from NWA 011 The D

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

eral

s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

xene

1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

12

6 plusmn

01

53

plusmn 0

26

0 plusmn

02

41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

plusmn 0

24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

plusmn 1

Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

plusmn 0

060

42 plusmn

00

30

39 plusmn

00

40

43 plusmn

00

30

41 plusmn

00

30

45 plusmn

00

3Pr

006

9 plusmn

000

70

10 plusmn

00

10

11 plusmn

00

10

10 plusmn

00

10

074

plusmn 0

007

008

1 plusmn

000

70

11 plusmn

00

1N

d0

22 plusmn

00

10

34 plusmn

00

20

50 plusmn

00

20

42 plusmn

00

30

39 plusmn

00

20

45 plusmn

00

30

51 plusmn

00

2Sm

005

2 plusmn

001

00

074

plusmn 0

014

026

plusmn 0

02

028

plusmn 0

04

025

plusmn 0

02

022

plusmn 0

02

026

plusmn 0

02

Eu1

5 plusmn

01

16

plusmn 0

10

029

plusmn 0

006

003

9 plusmn

002

001

7 plusmn

001

30

025

plusmn 0

010

004

1 plusmn

001

0G

d0

037

plusmn 0

010

008

0 plusmn

001

40

40 plusmn

00

40

26 plusmn

00

60

34 plusmn

00

30

45 plusmn

00

40

35 plusmn

00

4Tb

000

76 plusmn

00

023

000

84 plusmn

00

038

008

3 plusmn

000

70

092

plusmn 0

014

008

3 plusmn

000

90

089

plusmn 0

007

010

plusmn 0

01

Dy

004

0 plusmn

000

60

09 plusmn

00

10

79 plusmn

00

30

70 plusmn

00

40

62 plusmn

00

30

82 plusmn

00

30

76 plusmn

00

3H

ob

db

d0

19 plusmn

00

10

15 plusmn

00

20

15 plusmn

00

10

19 plusmn

00

10

17 plusmn

00

1Er

bd

bd

064

plusmn 0

02

056

plusmn 0

04

055

plusmn 0

03

067

plusmn 0

02

061

plusmn 0

02

Tmb

db

d0

11 plusmn

00

10

098

plusmn 0

010

009

6 plusmn

000

90

099

plusmn 0

007

011

plusmn 0

01

Yb

001

7 plusmn

000

60

023

plusmn 0

009

068

plusmn 0

04

077

plusmn 0

07

067

plusmn 0

05

091

plusmn 0

04

080

plusmn 0

04

Lub

db

d0

13 plusmn

00

10

13 plusmn

00

20

14 plusmn

00

10

15 plusmn

00

10

10 plusmn

00

1Eu

Eua

9762

027

043

017

024

041

Ce

Cea

a Err

ors a

re 1

σ fr

om c

ount

ing

stat

istic

s onl

yn

d =

not

det

erm

ined

bd

= b

elow

det

ectio

n

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 345

reported by Promprated et al (2003) and was determined byKorochantseva et al (2003) on a different thin section ofNWA 011 Mode 3 was determined by A Yamaguchi on athird thin section of NWA 011 An average mode based on allthree determinations is also given in Table 1 There aremoderate variations in the relative abundances of plagioclaseand pyroxene between the three sections which probably to alarge degree reflect the uncertainties associated with themodal determinations The accessory phases also appear to beheterogeneously distributed as has been noted by others (egYamaguchi et al 2002)

Mineral compositions are summarized in Table 2Compositions of pigeonite (Wo57ndash65Fs632ndash651) and the augite

lamellae (Wo361ndash392Fs352ndash383) are similar to those of NuevoLaredo trend eucrites (Fig 2 Warren and Jerde 1987)However as has been noted (eg Afanasiev et al 2000) theFeMn ratios are significantly higher than those of typicaleucrites (Fig 3) The plagioclase in NWA 011 is bytownite incomposition with low K2O contents (An802ndash877Or07ndash02) Theolivine is Fe-rich and has little variation in composition(Fa795ndash814 Fig 2) The compositions of ulvˆspinel andilmenite are given in Table 2 Phosphate compositions werenot analyzed by electron microprobe but EDS spectra takenof the grains analyzed by ion microprobe show that all aremerrillite rather than apatite

Results of the oxygen isotopic analyses are shown in

Fig 1 Backscattered electron (BSE) images of NWA 011 a) subhedral pigeonite with augite exsolution lamellae (px) and anhedral plagioclase(pl) crystal aggregates b) opaque mineral cluster with ulvˆspinel (usp) ilmenite (ilm) and olivine (ol)

Fig 2 Major element compositions of pyroxene and olivine from NWA 011 The solid lines in the pyroxene quadrilateral show thecompositions of pyroxenes from the Nuevo Laredo trend (NL) Stannnern trend (St) and main group (Juvinas Jv) eucrites Data from BVSP(1981) and Warren and Jerde (1987)

346 C Floss et al

Table 3 and Fig 4 The average isotopic composition of thefour aliquots of NWA 011 that we measured is δ18O = 292 plusmn022 and δ17O = minus006 plusmn 012 consistent with the results ofYamaguchi et al (2002) These data confirm that NWA 011has an unusual oxygen isotopic composition that is farremoved from the known eucrite field (∆17O = minus158 forNWA 011 versus minus025 for the eucrites) The oxygen isotopiccomposition falls within the range of values observed for CRchondrites (Fig 4) but is not like that of any other knownachondrites (Clayton 1993 Clayton and Mayeda 1996)

Minor and trace element compositions of plagioclase

pyroxene and merrillite are given in Table 4 and the REEpatterns are shown in Fig 5 Two plagioclase grains haveparallel LREE-enriched patterns with large positive Euanomalies (Fig 5a) REE abundances differ by about 50between the two grains Four merrillite grains have virtuallyidentical REE abundances The patterns are LREE-enrichedwith negative Eu anomalies all four grains also show smallpositive Ce anomalies (Fig 5b) The REE abundances inpyroxene are uniform indicating that we adequately sampledboth the augite lamellae and host pigeonite compositionsMoreover we did not observe any core to rim zoning in three

Table 2 Major element concentrations (in wt) in NWA 011 mineralsPlagioclase Pigeonite Augite lamellae Olivine Ilmenite Ulvˆspinel

SiO2 4645 plusmn 063 4779 plusmn 037 4855 plusmn 040 3086 plusmn 031 016 plusmn 018 006 plusmn 002TiO2 nm 052 plusmn 007 084 plusmn 011 034 plusmn 032 5272 plusmn 072 2419 plusmn 031Cr2O3 nm 018 plusmn 003 039 plusmn 007 nm 010 plusmn 004 1373 plusmn 041Al2O3 3342 plusmn 043 050 plusmn 004 127 plusmn 010 003 plusmn 003 011 plusmn 014 441 plusmn 020V2O3 nm nm nm nm 026 plusmn 004 043 plusmn 005FeO 076 plusmn 070 3611 plusmn 034 2041 plusmn 054 5960 plusmn 034 4512 plusmn 058 5327 plusmn 076MnO nm 055 plusmn 003 033 plusmn 003 055 plusmn 003 037 plusmn 001 030 plusmn 003NiO nm nm nm 001 plusmn 004 nm nmMgO 005 plusmn 009 968 plusmn 023 851 plusmn 026 809 plusmn 031 066 plusmn 007 051 plusmn 002CaO 1721 plusmn 042 279 plusmn 010 1755 plusmn 048 020 plusmn 005 018 plusmn 010 007 plusmn 003Na2O 162 plusmn 022 002 plusmn 001 009 plusmn 001 nm nm nmK2O 007 plusmn 002 nm nm nm nm nmP2O5 nm nm nm nm nm nmFe2O3

a bd 124 111 bd bd 229Total 9958 9937 9905 9967 9966 9926 analyses 16 16 12 9 3 5

An822ndash877Or07ndash02 Wo57ndash65Fs632ndash651 Wo361ndash392Fs352ndash383 Fa795ndash814FeMn 67 65 108

aCalculated after the method of Spear (1995)nm = not measured bd = below detection Errors represent the standard deviation from the mean

Fig 3 Molar Fe versus Mn in pyroxenes from NWA 011 The FeMn ratios are significantly higher than those of typical eucrites Correlationlines are based on data from Papike (1998)

Northwest Africa 011 347

pyroxene grains (Figs 5cndashe) consistent with the largelyuniform major element compositions of the pigeonite(Table 2 Yamaguchi et al 2002)

DISCUSSION

NWA 011 was originally classified as a non-cumulateeucrite and indeed this meteorite seems to share manysimilarities with the eucrites including texture mineralogyand major element compositions except for higher FeMnratios (eg Yamaguchi et al 2002) In portions of thefollowing discussion therefore we will compare the traceelement data of NWA 011 with that of certain non-cumulateeucrites in order to elucidate its crystallization (and post-crystallization) history

Ce Anomalies in Merrillite Terrestrial Weathering

Because NWA 011 is a meteorite recovered from a hotdesert terrestrial alteration and its possible effect on traceelement compositions must be evaluated Recent studies have

shown that hot desert meteorites are quite susceptible toterrestrial contamination (Bland et al 1996 Dreibus et al2001 Crozaz and Wadhwa 2001) and may experiencesignificant chemical changes that can include enrichments ofRb Sr Ba LREE U and Th decreased abundances of C Nand H the presence of occasional Ce anomalies and theaddition of terrestrial noble gases (Scherer et al 1994 Ashand Pillinger 1995 Barrat et al 1999 Crozaz et al 2003)

The evidence for terrestrial alteration of NWA 011 ismixed Yamaguchi (2001) notes that the rock is quiteweathered but although Korochantseva et al (2003) observedsignificant terrestrial Ar in their analysis of NWA 011 theyalso found that Ba and Sr abundances were low indicatinglittle to no contamination of these elements from the desertenvironment Our data also do not show elevated abundancesof Sr and Ba However we do observe small positive Ceanomalies in the four grains of merrillite measured here(Table 4 Fig 4) although plagioclase and pyroxene grains donot exhibit any Ce anomalies Cerium anomalies in mineralsfrom hot desert meteorites are generally associated withLREE enrichments and are most often found in phases withlow abundances of the LREE such as olivines and pyroxenes(Crozaz et al 2003) An exception is the brachinite EaglesNest which shows an LREE enrichment and large Cedepletion in apatite (Swindle et al 1998 Crozaz et al 2003)However both olivine and whole rock analyses of thismeteorite show the same features indicating that thismeteorite is strongly contaminated Excesses in Ce could beproduced through the oxidation of Ce (III) to Ce (IV) (eg

Fig 4 Three isotope plots showing the oxygen isotopic composition of NWA 011 (black circle) Errors (022 permil for δ18O and 012 permil for δ17O1σ) are smaller than the size of the symbol Also shown are the terrestrial fractionation (TF) line the carbonaceous chondrite anhydrousmineral (CCAM) line and the fields for CR chondrites CM chondrites Martian meteorites (SNC) the HED (howardite-eucrite-diogenite)meteorites and the acapulcoiteslodranites (Aca-Lod) Fields for meteorites other than NWA 011 are from Clayton (2003) and referencestherein

Table 3 Oxygen isotopic data for NWA 011Aliquot δ18O δ17O

A1 323 010A2 284 minus010B1 288 minus008B2 271 minus017Average 292 plusmn 022 minus006 plusmn 012

348 C Floss et al

Mittlefehldt and Lindstrom 1991) which is more insolublethan trivalent Ce Dissolution of the Ca phosphates duringaqueous alteration would result in the preferential retention ofCe (IV) relative to the trivalent REE leading to positive Ceanomalies such as those observed in NWA 011 Although wefind it unlikely that REE-rich Ca-phosphate would be affectedby this process while phases with lower REE concentrationssuch as plagioclase and pyroxene are not we cannot rule outthat terrestrial weathering is responsible for the Ce anomaliesobserved in NWA 011 merrillite Moreover Schreiber et al(1980) have shown experimentally that Ce (IV) in magmaticsystems will be reduced by Fe (II) to Ce (III) These authorsnote that ldquoit is difficult to visualize a magma under realisticconditions where the Ce (IV) species would be stabilizedrdquoThus we conclude that the Ce anomalies in merrillite are notprimary but could be a secondary feature due to terrestrialalteration However other trace element compositions inNWA 011 do not appear to have been compromised byterrestrial contamination

Bulk Compositions of NWA 011

Whole rock REE compositions of NWA 011 werecalculated using the averages of our mineral compositions(Table 4) and the modes reported in Table 1 For thepurposes of this calculation the REE abundances in olivinesilica and opaque minerals were assumed to be negligibleFigure 6 shows the three calculated bulk REE patternstogether with the average mineral compositions of NWA011 All three patterns are approximately flat to slightlyLREE-enriched with abundances of sim10 times CI and havesmall positive Eu anomalies The patterns also have smallpositive Ce anomalies that reflect the influence of phosphateon the bulk composition of the meteorite The REE patternfor bulk 1 has LREE abundances that are about a factor oftwo higher than those of bulk 2 and bulk 3 largely due to thehigher modal abundance of Ca-phosphate in this thin section(Table 1)

Published whole rock REE patterns for NWA 011however are heterogeneous and differ from those calculatedhere Both Yamaguchi et al (2002) and Korotchantseva et al(2003) analyzed chips of NWA 011 the REE patterns ofwhich are HREE-enriched with large positive Eu anomaliesThese REE patterns can be reproduced by assuming modalproportions of plagioclase and pyroxene with little to no Ca-phosphate (Fig 7a) Another chip analyzed byKorotchantseva et al (2003) has a LREE-rich pattern with apositive Ce anomaly and is clearly dominated by excess Ca-phosphate (Fig 7b) Moreover the negative Eu anomaly inthis pattern requires a much larger fraction of pyroxene (andcorrespondingly less plagioclase) than is present in oursections (sim88 versus 55ndash58)

Table 5 shows two whole rock major elementcompositions for NWA 011 from the literature Whole rock Y

was determined by Yamaguchi et al (2002) using wetchemical analysis whereas whole rock K was determined byKorochantseva et al (2003) using mineral chemistry and themode published by Promprated et al (2003) (ie mode 2 inTable 1) We also calculated bulk compositions from modes 12 and 3 in Table 1 and the mineral compositions from Table 2in order to compare them with the literature whole rockcompositions For the purposes of these calculations weassumed that the proportions of augite to pigeonite andilmenite to ulvˆspinel were 12 The results are shown inTable 5 Our calculated bulk compositions generally showdifferences that are similar in magnitude to those between thetwo literature whole rock compositions and that are consistentwith the differences in the three modes of Table 1 Thus forexample bulk 1 has the highest concentrations of Al2O3CaO and Na2O consistent with the fact that this mode has thehighest proportion of plagioclase Similarly the bulkcomposition calculated from mode 2 which has the highestproportion of pyroxene has the highest concentrations ofMgO and FeO Concentrations of SiO2 are intermediatebetween the whole rock values of Korochantseva et al (2002)and Yamaguchi et al (2002) and not surprisingly are highestin the two that contain silica in the mode Moreover all threeof our calculated bulk compositions have somewhat higherAl2O3 CaO and Na2O and lower FeO and MgO than thewhole rock determinations Y and K Since the modalabundances for whole rock K and bulk 2 are the same someof these differences must reflect the average mineralcompositions used to calculate the bulk compositionsHowever modes 1 and 3 also have more plagioclase thanmode 2 Whole rock composition Y from Yamaguchi et al(2002) has the highest MgO and FeO abundances and thelowest SiO2 which seems to indicate a higher proportion ofolivine in this analysis The compositions calculated frommode 2 determined over the largest area of NWA 011(Table 1) are most similar to the whole rock determination ofYamaguchi et al (2002) this mode may be the mostrepresentative of NWA 011

Equilibrium Melt Compositions and CrystallizationHistory

If plagioclase and pyroxene in NWA 011 have preservedtheir original igneous signatures it should be possible tocalculate the REE compositions of the melts from which theycrystallized Although textural evidence and the uniformity ofmajor element compositions in both plagioclase and pyroxenesuggest that NWA 011 has undergone some re-equilibrationsuch a calculation can still provide some useful constraints onthe nature of the NWA 011 parent melt as well as possiblesecondary processes that may have affected the sample

The distribution coefficients listed in Table 6 were usedto calculate the compositions of liquids in equilibrium withcore plagioclase and pyroxene grains from NWA 011 The D

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

eral

s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

xene

1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

12

6 plusmn

01

53

plusmn 0

26

0 plusmn

02

41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

plusmn 0

24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

plusmn 1

Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

plusmn 0

060

42 plusmn

00

30

39 plusmn

00

40

43 plusmn

00

30

41 plusmn

00

30

45 plusmn

00

3Pr

006

9 plusmn

000

70

10 plusmn

00

10

11 plusmn

00

10

10 plusmn

00

10

074

plusmn 0

007

008

1 plusmn

000

70

11 plusmn

00

1N

d0

22 plusmn

00

10

34 plusmn

00

20

50 plusmn

00

20

42 plusmn

00

30

39 plusmn

00

20

45 plusmn

00

30

51 plusmn

00

2Sm

005

2 plusmn

001

00

074

plusmn 0

014

026

plusmn 0

02

028

plusmn 0

04

025

plusmn 0

02

022

plusmn 0

02

026

plusmn 0

02

Eu1

5 plusmn

01

16

plusmn 0

10

029

plusmn 0

006

003

9 plusmn

002

001

7 plusmn

001

30

025

plusmn 0

010

004

1 plusmn

001

0G

d0

037

plusmn 0

010

008

0 plusmn

001

40

40 plusmn

00

40

26 plusmn

00

60

34 plusmn

00

30

45 plusmn

00

40

35 plusmn

00

4Tb

000

76 plusmn

00

023

000

84 plusmn

00

038

008

3 plusmn

000

70

092

plusmn 0

014

008

3 plusmn

000

90

089

plusmn 0

007

010

plusmn 0

01

Dy

004

0 plusmn

000

60

09 plusmn

00

10

79 plusmn

00

30

70 plusmn

00

40

62 plusmn

00

30

82 plusmn

00

30

76 plusmn

00

3H

ob

db

d0

19 plusmn

00

10

15 plusmn

00

20

15 plusmn

00

10

19 plusmn

00

10

17 plusmn

00

1Er

bd

bd

064

plusmn 0

02

056

plusmn 0

04

055

plusmn 0

03

067

plusmn 0

02

061

plusmn 0

02

Tmb

db

d0

11 plusmn

00

10

098

plusmn 0

010

009

6 plusmn

000

90

099

plusmn 0

007

011

plusmn 0

01

Yb

001

7 plusmn

000

60

023

plusmn 0

009

068

plusmn 0

04

077

plusmn 0

07

067

plusmn 0

05

091

plusmn 0

04

080

plusmn 0

04

Lub

db

d0

13 plusmn

00

10

13 plusmn

00

20

14 plusmn

00

10

15 plusmn

00

10

10 plusmn

00

1Eu

Eua

9762

027

043

017

024

041

Ce

Cea

a Err

ors a

re 1

σ fr

om c

ount

ing

stat

istic

s onl

yn

d =

not

det

erm

ined

bd

= b

elow

det

ectio

n

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

346 C Floss et al

Table 3 and Fig 4 The average isotopic composition of thefour aliquots of NWA 011 that we measured is δ18O = 292 plusmn022 and δ17O = minus006 plusmn 012 consistent with the results ofYamaguchi et al (2002) These data confirm that NWA 011has an unusual oxygen isotopic composition that is farremoved from the known eucrite field (∆17O = minus158 forNWA 011 versus minus025 for the eucrites) The oxygen isotopiccomposition falls within the range of values observed for CRchondrites (Fig 4) but is not like that of any other knownachondrites (Clayton 1993 Clayton and Mayeda 1996)

Minor and trace element compositions of plagioclase

pyroxene and merrillite are given in Table 4 and the REEpatterns are shown in Fig 5 Two plagioclase grains haveparallel LREE-enriched patterns with large positive Euanomalies (Fig 5a) REE abundances differ by about 50between the two grains Four merrillite grains have virtuallyidentical REE abundances The patterns are LREE-enrichedwith negative Eu anomalies all four grains also show smallpositive Ce anomalies (Fig 5b) The REE abundances inpyroxene are uniform indicating that we adequately sampledboth the augite lamellae and host pigeonite compositionsMoreover we did not observe any core to rim zoning in three

Table 2 Major element concentrations (in wt) in NWA 011 mineralsPlagioclase Pigeonite Augite lamellae Olivine Ilmenite Ulvˆspinel

SiO2 4645 plusmn 063 4779 plusmn 037 4855 plusmn 040 3086 plusmn 031 016 plusmn 018 006 plusmn 002TiO2 nm 052 plusmn 007 084 plusmn 011 034 plusmn 032 5272 plusmn 072 2419 plusmn 031Cr2O3 nm 018 plusmn 003 039 plusmn 007 nm 010 plusmn 004 1373 plusmn 041Al2O3 3342 plusmn 043 050 plusmn 004 127 plusmn 010 003 plusmn 003 011 plusmn 014 441 plusmn 020V2O3 nm nm nm nm 026 plusmn 004 043 plusmn 005FeO 076 plusmn 070 3611 plusmn 034 2041 plusmn 054 5960 plusmn 034 4512 plusmn 058 5327 plusmn 076MnO nm 055 plusmn 003 033 plusmn 003 055 plusmn 003 037 plusmn 001 030 plusmn 003NiO nm nm nm 001 plusmn 004 nm nmMgO 005 plusmn 009 968 plusmn 023 851 plusmn 026 809 plusmn 031 066 plusmn 007 051 plusmn 002CaO 1721 plusmn 042 279 plusmn 010 1755 plusmn 048 020 plusmn 005 018 plusmn 010 007 plusmn 003Na2O 162 plusmn 022 002 plusmn 001 009 plusmn 001 nm nm nmK2O 007 plusmn 002 nm nm nm nm nmP2O5 nm nm nm nm nm nmFe2O3

a bd 124 111 bd bd 229Total 9958 9937 9905 9967 9966 9926 analyses 16 16 12 9 3 5

An822ndash877Or07ndash02 Wo57ndash65Fs632ndash651 Wo361ndash392Fs352ndash383 Fa795ndash814FeMn 67 65 108

aCalculated after the method of Spear (1995)nm = not measured bd = below detection Errors represent the standard deviation from the mean

Fig 3 Molar Fe versus Mn in pyroxenes from NWA 011 The FeMn ratios are significantly higher than those of typical eucrites Correlationlines are based on data from Papike (1998)

Northwest Africa 011 347

pyroxene grains (Figs 5cndashe) consistent with the largelyuniform major element compositions of the pigeonite(Table 2 Yamaguchi et al 2002)

DISCUSSION

NWA 011 was originally classified as a non-cumulateeucrite and indeed this meteorite seems to share manysimilarities with the eucrites including texture mineralogyand major element compositions except for higher FeMnratios (eg Yamaguchi et al 2002) In portions of thefollowing discussion therefore we will compare the traceelement data of NWA 011 with that of certain non-cumulateeucrites in order to elucidate its crystallization (and post-crystallization) history

Ce Anomalies in Merrillite Terrestrial Weathering

Because NWA 011 is a meteorite recovered from a hotdesert terrestrial alteration and its possible effect on traceelement compositions must be evaluated Recent studies have

shown that hot desert meteorites are quite susceptible toterrestrial contamination (Bland et al 1996 Dreibus et al2001 Crozaz and Wadhwa 2001) and may experiencesignificant chemical changes that can include enrichments ofRb Sr Ba LREE U and Th decreased abundances of C Nand H the presence of occasional Ce anomalies and theaddition of terrestrial noble gases (Scherer et al 1994 Ashand Pillinger 1995 Barrat et al 1999 Crozaz et al 2003)

The evidence for terrestrial alteration of NWA 011 ismixed Yamaguchi (2001) notes that the rock is quiteweathered but although Korochantseva et al (2003) observedsignificant terrestrial Ar in their analysis of NWA 011 theyalso found that Ba and Sr abundances were low indicatinglittle to no contamination of these elements from the desertenvironment Our data also do not show elevated abundancesof Sr and Ba However we do observe small positive Ceanomalies in the four grains of merrillite measured here(Table 4 Fig 4) although plagioclase and pyroxene grains donot exhibit any Ce anomalies Cerium anomalies in mineralsfrom hot desert meteorites are generally associated withLREE enrichments and are most often found in phases withlow abundances of the LREE such as olivines and pyroxenes(Crozaz et al 2003) An exception is the brachinite EaglesNest which shows an LREE enrichment and large Cedepletion in apatite (Swindle et al 1998 Crozaz et al 2003)However both olivine and whole rock analyses of thismeteorite show the same features indicating that thismeteorite is strongly contaminated Excesses in Ce could beproduced through the oxidation of Ce (III) to Ce (IV) (eg

Fig 4 Three isotope plots showing the oxygen isotopic composition of NWA 011 (black circle) Errors (022 permil for δ18O and 012 permil for δ17O1σ) are smaller than the size of the symbol Also shown are the terrestrial fractionation (TF) line the carbonaceous chondrite anhydrousmineral (CCAM) line and the fields for CR chondrites CM chondrites Martian meteorites (SNC) the HED (howardite-eucrite-diogenite)meteorites and the acapulcoiteslodranites (Aca-Lod) Fields for meteorites other than NWA 011 are from Clayton (2003) and referencestherein

Table 3 Oxygen isotopic data for NWA 011Aliquot δ18O δ17O

A1 323 010A2 284 minus010B1 288 minus008B2 271 minus017Average 292 plusmn 022 minus006 plusmn 012

348 C Floss et al

Mittlefehldt and Lindstrom 1991) which is more insolublethan trivalent Ce Dissolution of the Ca phosphates duringaqueous alteration would result in the preferential retention ofCe (IV) relative to the trivalent REE leading to positive Ceanomalies such as those observed in NWA 011 Although wefind it unlikely that REE-rich Ca-phosphate would be affectedby this process while phases with lower REE concentrationssuch as plagioclase and pyroxene are not we cannot rule outthat terrestrial weathering is responsible for the Ce anomaliesobserved in NWA 011 merrillite Moreover Schreiber et al(1980) have shown experimentally that Ce (IV) in magmaticsystems will be reduced by Fe (II) to Ce (III) These authorsnote that ldquoit is difficult to visualize a magma under realisticconditions where the Ce (IV) species would be stabilizedrdquoThus we conclude that the Ce anomalies in merrillite are notprimary but could be a secondary feature due to terrestrialalteration However other trace element compositions inNWA 011 do not appear to have been compromised byterrestrial contamination

Bulk Compositions of NWA 011

Whole rock REE compositions of NWA 011 werecalculated using the averages of our mineral compositions(Table 4) and the modes reported in Table 1 For thepurposes of this calculation the REE abundances in olivinesilica and opaque minerals were assumed to be negligibleFigure 6 shows the three calculated bulk REE patternstogether with the average mineral compositions of NWA011 All three patterns are approximately flat to slightlyLREE-enriched with abundances of sim10 times CI and havesmall positive Eu anomalies The patterns also have smallpositive Ce anomalies that reflect the influence of phosphateon the bulk composition of the meteorite The REE patternfor bulk 1 has LREE abundances that are about a factor oftwo higher than those of bulk 2 and bulk 3 largely due to thehigher modal abundance of Ca-phosphate in this thin section(Table 1)

Published whole rock REE patterns for NWA 011however are heterogeneous and differ from those calculatedhere Both Yamaguchi et al (2002) and Korotchantseva et al(2003) analyzed chips of NWA 011 the REE patterns ofwhich are HREE-enriched with large positive Eu anomaliesThese REE patterns can be reproduced by assuming modalproportions of plagioclase and pyroxene with little to no Ca-phosphate (Fig 7a) Another chip analyzed byKorotchantseva et al (2003) has a LREE-rich pattern with apositive Ce anomaly and is clearly dominated by excess Ca-phosphate (Fig 7b) Moreover the negative Eu anomaly inthis pattern requires a much larger fraction of pyroxene (andcorrespondingly less plagioclase) than is present in oursections (sim88 versus 55ndash58)

Table 5 shows two whole rock major elementcompositions for NWA 011 from the literature Whole rock Y

was determined by Yamaguchi et al (2002) using wetchemical analysis whereas whole rock K was determined byKorochantseva et al (2003) using mineral chemistry and themode published by Promprated et al (2003) (ie mode 2 inTable 1) We also calculated bulk compositions from modes 12 and 3 in Table 1 and the mineral compositions from Table 2in order to compare them with the literature whole rockcompositions For the purposes of these calculations weassumed that the proportions of augite to pigeonite andilmenite to ulvˆspinel were 12 The results are shown inTable 5 Our calculated bulk compositions generally showdifferences that are similar in magnitude to those between thetwo literature whole rock compositions and that are consistentwith the differences in the three modes of Table 1 Thus forexample bulk 1 has the highest concentrations of Al2O3CaO and Na2O consistent with the fact that this mode has thehighest proportion of plagioclase Similarly the bulkcomposition calculated from mode 2 which has the highestproportion of pyroxene has the highest concentrations ofMgO and FeO Concentrations of SiO2 are intermediatebetween the whole rock values of Korochantseva et al (2002)and Yamaguchi et al (2002) and not surprisingly are highestin the two that contain silica in the mode Moreover all threeof our calculated bulk compositions have somewhat higherAl2O3 CaO and Na2O and lower FeO and MgO than thewhole rock determinations Y and K Since the modalabundances for whole rock K and bulk 2 are the same someof these differences must reflect the average mineralcompositions used to calculate the bulk compositionsHowever modes 1 and 3 also have more plagioclase thanmode 2 Whole rock composition Y from Yamaguchi et al(2002) has the highest MgO and FeO abundances and thelowest SiO2 which seems to indicate a higher proportion ofolivine in this analysis The compositions calculated frommode 2 determined over the largest area of NWA 011(Table 1) are most similar to the whole rock determination ofYamaguchi et al (2002) this mode may be the mostrepresentative of NWA 011

Equilibrium Melt Compositions and CrystallizationHistory

If plagioclase and pyroxene in NWA 011 have preservedtheir original igneous signatures it should be possible tocalculate the REE compositions of the melts from which theycrystallized Although textural evidence and the uniformity ofmajor element compositions in both plagioclase and pyroxenesuggest that NWA 011 has undergone some re-equilibrationsuch a calculation can still provide some useful constraints onthe nature of the NWA 011 parent melt as well as possiblesecondary processes that may have affected the sample

The distribution coefficients listed in Table 6 were usedto calculate the compositions of liquids in equilibrium withcore plagioclase and pyroxene grains from NWA 011 The D

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

eral

s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

xene

1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

12

6 plusmn

01

53

plusmn 0

26

0 plusmn

02

41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

plusmn 0

24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

plusmn 1

Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

plusmn 0

060

42 plusmn

00

30

39 plusmn

00

40

43 plusmn

00

30

41 plusmn

00

30

45 plusmn

00

3Pr

006

9 plusmn

000

70

10 plusmn

00

10

11 plusmn

00

10

10 plusmn

00

10

074

plusmn 0

007

008

1 plusmn

000

70

11 plusmn

00

1N

d0

22 plusmn

00

10

34 plusmn

00

20

50 plusmn

00

20

42 plusmn

00

30

39 plusmn

00

20

45 plusmn

00

30

51 plusmn

00

2Sm

005

2 plusmn

001

00

074

plusmn 0

014

026

plusmn 0

02

028

plusmn 0

04

025

plusmn 0

02

022

plusmn 0

02

026

plusmn 0

02

Eu1

5 plusmn

01

16

plusmn 0

10

029

plusmn 0

006

003

9 plusmn

002

001

7 plusmn

001

30

025

plusmn 0

010

004

1 plusmn

001

0G

d0

037

plusmn 0

010

008

0 plusmn

001

40

40 plusmn

00

40

26 plusmn

00

60

34 plusmn

00

30

45 plusmn

00

40

35 plusmn

00

4Tb

000

76 plusmn

00

023

000

84 plusmn

00

038

008

3 plusmn

000

70

092

plusmn 0

014

008

3 plusmn

000

90

089

plusmn 0

007

010

plusmn 0

01

Dy

004

0 plusmn

000

60

09 plusmn

00

10

79 plusmn

00

30

70 plusmn

00

40

62 plusmn

00

30

82 plusmn

00

30

76 plusmn

00

3H

ob

db

d0

19 plusmn

00

10

15 plusmn

00

20

15 plusmn

00

10

19 plusmn

00

10

17 plusmn

00

1Er

bd

bd

064

plusmn 0

02

056

plusmn 0

04

055

plusmn 0

03

067

plusmn 0

02

061

plusmn 0

02

Tmb

db

d0

11 plusmn

00

10

098

plusmn 0

010

009

6 plusmn

000

90

099

plusmn 0

007

011

plusmn 0

01

Yb

001

7 plusmn

000

60

023

plusmn 0

009

068

plusmn 0

04

077

plusmn 0

07

067

plusmn 0

05

091

plusmn 0

04

080

plusmn 0

04

Lub

db

d0

13 plusmn

00

10

13 plusmn

00

20

14 plusmn

00

10

15 plusmn

00

10

10 plusmn

00

1Eu

Eua

9762

027

043

017

024

041

Ce

Cea

a Err

ors a

re 1

σ fr

om c

ount

ing

stat

istic

s onl

yn

d =

not

det

erm

ined

bd

= b

elow

det

ectio

n

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 347

pyroxene grains (Figs 5cndashe) consistent with the largelyuniform major element compositions of the pigeonite(Table 2 Yamaguchi et al 2002)

DISCUSSION

NWA 011 was originally classified as a non-cumulateeucrite and indeed this meteorite seems to share manysimilarities with the eucrites including texture mineralogyand major element compositions except for higher FeMnratios (eg Yamaguchi et al 2002) In portions of thefollowing discussion therefore we will compare the traceelement data of NWA 011 with that of certain non-cumulateeucrites in order to elucidate its crystallization (and post-crystallization) history

Ce Anomalies in Merrillite Terrestrial Weathering

Because NWA 011 is a meteorite recovered from a hotdesert terrestrial alteration and its possible effect on traceelement compositions must be evaluated Recent studies have

shown that hot desert meteorites are quite susceptible toterrestrial contamination (Bland et al 1996 Dreibus et al2001 Crozaz and Wadhwa 2001) and may experiencesignificant chemical changes that can include enrichments ofRb Sr Ba LREE U and Th decreased abundances of C Nand H the presence of occasional Ce anomalies and theaddition of terrestrial noble gases (Scherer et al 1994 Ashand Pillinger 1995 Barrat et al 1999 Crozaz et al 2003)

The evidence for terrestrial alteration of NWA 011 ismixed Yamaguchi (2001) notes that the rock is quiteweathered but although Korochantseva et al (2003) observedsignificant terrestrial Ar in their analysis of NWA 011 theyalso found that Ba and Sr abundances were low indicatinglittle to no contamination of these elements from the desertenvironment Our data also do not show elevated abundancesof Sr and Ba However we do observe small positive Ceanomalies in the four grains of merrillite measured here(Table 4 Fig 4) although plagioclase and pyroxene grains donot exhibit any Ce anomalies Cerium anomalies in mineralsfrom hot desert meteorites are generally associated withLREE enrichments and are most often found in phases withlow abundances of the LREE such as olivines and pyroxenes(Crozaz et al 2003) An exception is the brachinite EaglesNest which shows an LREE enrichment and large Cedepletion in apatite (Swindle et al 1998 Crozaz et al 2003)However both olivine and whole rock analyses of thismeteorite show the same features indicating that thismeteorite is strongly contaminated Excesses in Ce could beproduced through the oxidation of Ce (III) to Ce (IV) (eg

Fig 4 Three isotope plots showing the oxygen isotopic composition of NWA 011 (black circle) Errors (022 permil for δ18O and 012 permil for δ17O1σ) are smaller than the size of the symbol Also shown are the terrestrial fractionation (TF) line the carbonaceous chondrite anhydrousmineral (CCAM) line and the fields for CR chondrites CM chondrites Martian meteorites (SNC) the HED (howardite-eucrite-diogenite)meteorites and the acapulcoiteslodranites (Aca-Lod) Fields for meteorites other than NWA 011 are from Clayton (2003) and referencestherein

Table 3 Oxygen isotopic data for NWA 011Aliquot δ18O δ17O

A1 323 010A2 284 minus010B1 288 minus008B2 271 minus017Average 292 plusmn 022 minus006 plusmn 012

348 C Floss et al

Mittlefehldt and Lindstrom 1991) which is more insolublethan trivalent Ce Dissolution of the Ca phosphates duringaqueous alteration would result in the preferential retention ofCe (IV) relative to the trivalent REE leading to positive Ceanomalies such as those observed in NWA 011 Although wefind it unlikely that REE-rich Ca-phosphate would be affectedby this process while phases with lower REE concentrationssuch as plagioclase and pyroxene are not we cannot rule outthat terrestrial weathering is responsible for the Ce anomaliesobserved in NWA 011 merrillite Moreover Schreiber et al(1980) have shown experimentally that Ce (IV) in magmaticsystems will be reduced by Fe (II) to Ce (III) These authorsnote that ldquoit is difficult to visualize a magma under realisticconditions where the Ce (IV) species would be stabilizedrdquoThus we conclude that the Ce anomalies in merrillite are notprimary but could be a secondary feature due to terrestrialalteration However other trace element compositions inNWA 011 do not appear to have been compromised byterrestrial contamination

Bulk Compositions of NWA 011

Whole rock REE compositions of NWA 011 werecalculated using the averages of our mineral compositions(Table 4) and the modes reported in Table 1 For thepurposes of this calculation the REE abundances in olivinesilica and opaque minerals were assumed to be negligibleFigure 6 shows the three calculated bulk REE patternstogether with the average mineral compositions of NWA011 All three patterns are approximately flat to slightlyLREE-enriched with abundances of sim10 times CI and havesmall positive Eu anomalies The patterns also have smallpositive Ce anomalies that reflect the influence of phosphateon the bulk composition of the meteorite The REE patternfor bulk 1 has LREE abundances that are about a factor oftwo higher than those of bulk 2 and bulk 3 largely due to thehigher modal abundance of Ca-phosphate in this thin section(Table 1)

Published whole rock REE patterns for NWA 011however are heterogeneous and differ from those calculatedhere Both Yamaguchi et al (2002) and Korotchantseva et al(2003) analyzed chips of NWA 011 the REE patterns ofwhich are HREE-enriched with large positive Eu anomaliesThese REE patterns can be reproduced by assuming modalproportions of plagioclase and pyroxene with little to no Ca-phosphate (Fig 7a) Another chip analyzed byKorotchantseva et al (2003) has a LREE-rich pattern with apositive Ce anomaly and is clearly dominated by excess Ca-phosphate (Fig 7b) Moreover the negative Eu anomaly inthis pattern requires a much larger fraction of pyroxene (andcorrespondingly less plagioclase) than is present in oursections (sim88 versus 55ndash58)

Table 5 shows two whole rock major elementcompositions for NWA 011 from the literature Whole rock Y

was determined by Yamaguchi et al (2002) using wetchemical analysis whereas whole rock K was determined byKorochantseva et al (2003) using mineral chemistry and themode published by Promprated et al (2003) (ie mode 2 inTable 1) We also calculated bulk compositions from modes 12 and 3 in Table 1 and the mineral compositions from Table 2in order to compare them with the literature whole rockcompositions For the purposes of these calculations weassumed that the proportions of augite to pigeonite andilmenite to ulvˆspinel were 12 The results are shown inTable 5 Our calculated bulk compositions generally showdifferences that are similar in magnitude to those between thetwo literature whole rock compositions and that are consistentwith the differences in the three modes of Table 1 Thus forexample bulk 1 has the highest concentrations of Al2O3CaO and Na2O consistent with the fact that this mode has thehighest proportion of plagioclase Similarly the bulkcomposition calculated from mode 2 which has the highestproportion of pyroxene has the highest concentrations ofMgO and FeO Concentrations of SiO2 are intermediatebetween the whole rock values of Korochantseva et al (2002)and Yamaguchi et al (2002) and not surprisingly are highestin the two that contain silica in the mode Moreover all threeof our calculated bulk compositions have somewhat higherAl2O3 CaO and Na2O and lower FeO and MgO than thewhole rock determinations Y and K Since the modalabundances for whole rock K and bulk 2 are the same someof these differences must reflect the average mineralcompositions used to calculate the bulk compositionsHowever modes 1 and 3 also have more plagioclase thanmode 2 Whole rock composition Y from Yamaguchi et al(2002) has the highest MgO and FeO abundances and thelowest SiO2 which seems to indicate a higher proportion ofolivine in this analysis The compositions calculated frommode 2 determined over the largest area of NWA 011(Table 1) are most similar to the whole rock determination ofYamaguchi et al (2002) this mode may be the mostrepresentative of NWA 011

Equilibrium Melt Compositions and CrystallizationHistory

If plagioclase and pyroxene in NWA 011 have preservedtheir original igneous signatures it should be possible tocalculate the REE compositions of the melts from which theycrystallized Although textural evidence and the uniformity ofmajor element compositions in both plagioclase and pyroxenesuggest that NWA 011 has undergone some re-equilibrationsuch a calculation can still provide some useful constraints onthe nature of the NWA 011 parent melt as well as possiblesecondary processes that may have affected the sample

The distribution coefficients listed in Table 6 were usedto calculate the compositions of liquids in equilibrium withcore plagioclase and pyroxene grains from NWA 011 The D

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

eral

s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

xene

1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

12

6 plusmn

01

53

plusmn 0

26

0 plusmn

02

41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

plusmn 0

24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

plusmn 1

Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

plusmn 0

060

42 plusmn

00

30

39 plusmn

00

40

43 plusmn

00

30

41 plusmn

00

30

45 plusmn

00

3Pr

006

9 plusmn

000

70

10 plusmn

00

10

11 plusmn

00

10

10 plusmn

00

10

074

plusmn 0

007

008

1 plusmn

000

70

11 plusmn

00

1N

d0

22 plusmn

00

10

34 plusmn

00

20

50 plusmn

00

20

42 plusmn

00

30

39 plusmn

00

20

45 plusmn

00

30

51 plusmn

00

2Sm

005

2 plusmn

001

00

074

plusmn 0

014

026

plusmn 0

02

028

plusmn 0

04

025

plusmn 0

02

022

plusmn 0

02

026

plusmn 0

02

Eu1

5 plusmn

01

16

plusmn 0

10

029

plusmn 0

006

003

9 plusmn

002

001

7 plusmn

001

30

025

plusmn 0

010

004

1 plusmn

001

0G

d0

037

plusmn 0

010

008

0 plusmn

001

40

40 plusmn

00

40

26 plusmn

00

60

34 plusmn

00

30

45 plusmn

00

40

35 plusmn

00

4Tb

000

76 plusmn

00

023

000

84 plusmn

00

038

008

3 plusmn

000

70

092

plusmn 0

014

008

3 plusmn

000

90

089

plusmn 0

007

010

plusmn 0

01

Dy

004

0 plusmn

000

60

09 plusmn

00

10

79 plusmn

00

30

70 plusmn

00

40

62 plusmn

00

30

82 plusmn

00

30

76 plusmn

00

3H

ob

db

d0

19 plusmn

00

10

15 plusmn

00

20

15 plusmn

00

10

19 plusmn

00

10

17 plusmn

00

1Er

bd

bd

064

plusmn 0

02

056

plusmn 0

04

055

plusmn 0

03

067

plusmn 0

02

061

plusmn 0

02

Tmb

db

d0

11 plusmn

00

10

098

plusmn 0

010

009

6 plusmn

000

90

099

plusmn 0

007

011

plusmn 0

01

Yb

001

7 plusmn

000

60

023

plusmn 0

009

068

plusmn 0

04

077

plusmn 0

07

067

plusmn 0

05

091

plusmn 0

04

080

plusmn 0

04

Lub

db

d0

13 plusmn

00

10

13 plusmn

00

20

14 plusmn

00

10

15 plusmn

00

10

10 plusmn

00

1Eu

Eua

9762

027

043

017

024

041

Ce

Cea

a Err

ors a

re 1

σ fr

om c

ount

ing

stat

istic

s onl

yn

d =

not

det

erm

ined

bd

= b

elow

det

ectio

n

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

348 C Floss et al

Mittlefehldt and Lindstrom 1991) which is more insolublethan trivalent Ce Dissolution of the Ca phosphates duringaqueous alteration would result in the preferential retention ofCe (IV) relative to the trivalent REE leading to positive Ceanomalies such as those observed in NWA 011 Although wefind it unlikely that REE-rich Ca-phosphate would be affectedby this process while phases with lower REE concentrationssuch as plagioclase and pyroxene are not we cannot rule outthat terrestrial weathering is responsible for the Ce anomaliesobserved in NWA 011 merrillite Moreover Schreiber et al(1980) have shown experimentally that Ce (IV) in magmaticsystems will be reduced by Fe (II) to Ce (III) These authorsnote that ldquoit is difficult to visualize a magma under realisticconditions where the Ce (IV) species would be stabilizedrdquoThus we conclude that the Ce anomalies in merrillite are notprimary but could be a secondary feature due to terrestrialalteration However other trace element compositions inNWA 011 do not appear to have been compromised byterrestrial contamination

Bulk Compositions of NWA 011

Whole rock REE compositions of NWA 011 werecalculated using the averages of our mineral compositions(Table 4) and the modes reported in Table 1 For thepurposes of this calculation the REE abundances in olivinesilica and opaque minerals were assumed to be negligibleFigure 6 shows the three calculated bulk REE patternstogether with the average mineral compositions of NWA011 All three patterns are approximately flat to slightlyLREE-enriched with abundances of sim10 times CI and havesmall positive Eu anomalies The patterns also have smallpositive Ce anomalies that reflect the influence of phosphateon the bulk composition of the meteorite The REE patternfor bulk 1 has LREE abundances that are about a factor oftwo higher than those of bulk 2 and bulk 3 largely due to thehigher modal abundance of Ca-phosphate in this thin section(Table 1)

Published whole rock REE patterns for NWA 011however are heterogeneous and differ from those calculatedhere Both Yamaguchi et al (2002) and Korotchantseva et al(2003) analyzed chips of NWA 011 the REE patterns ofwhich are HREE-enriched with large positive Eu anomaliesThese REE patterns can be reproduced by assuming modalproportions of plagioclase and pyroxene with little to no Ca-phosphate (Fig 7a) Another chip analyzed byKorotchantseva et al (2003) has a LREE-rich pattern with apositive Ce anomaly and is clearly dominated by excess Ca-phosphate (Fig 7b) Moreover the negative Eu anomaly inthis pattern requires a much larger fraction of pyroxene (andcorrespondingly less plagioclase) than is present in oursections (sim88 versus 55ndash58)

Table 5 shows two whole rock major elementcompositions for NWA 011 from the literature Whole rock Y

was determined by Yamaguchi et al (2002) using wetchemical analysis whereas whole rock K was determined byKorochantseva et al (2003) using mineral chemistry and themode published by Promprated et al (2003) (ie mode 2 inTable 1) We also calculated bulk compositions from modes 12 and 3 in Table 1 and the mineral compositions from Table 2in order to compare them with the literature whole rockcompositions For the purposes of these calculations weassumed that the proportions of augite to pigeonite andilmenite to ulvˆspinel were 12 The results are shown inTable 5 Our calculated bulk compositions generally showdifferences that are similar in magnitude to those between thetwo literature whole rock compositions and that are consistentwith the differences in the three modes of Table 1 Thus forexample bulk 1 has the highest concentrations of Al2O3CaO and Na2O consistent with the fact that this mode has thehighest proportion of plagioclase Similarly the bulkcomposition calculated from mode 2 which has the highestproportion of pyroxene has the highest concentrations ofMgO and FeO Concentrations of SiO2 are intermediatebetween the whole rock values of Korochantseva et al (2002)and Yamaguchi et al (2002) and not surprisingly are highestin the two that contain silica in the mode Moreover all threeof our calculated bulk compositions have somewhat higherAl2O3 CaO and Na2O and lower FeO and MgO than thewhole rock determinations Y and K Since the modalabundances for whole rock K and bulk 2 are the same someof these differences must reflect the average mineralcompositions used to calculate the bulk compositionsHowever modes 1 and 3 also have more plagioclase thanmode 2 Whole rock composition Y from Yamaguchi et al(2002) has the highest MgO and FeO abundances and thelowest SiO2 which seems to indicate a higher proportion ofolivine in this analysis The compositions calculated frommode 2 determined over the largest area of NWA 011(Table 1) are most similar to the whole rock determination ofYamaguchi et al (2002) this mode may be the mostrepresentative of NWA 011

Equilibrium Melt Compositions and CrystallizationHistory

If plagioclase and pyroxene in NWA 011 have preservedtheir original igneous signatures it should be possible tocalculate the REE compositions of the melts from which theycrystallized Although textural evidence and the uniformity ofmajor element compositions in both plagioclase and pyroxenesuggest that NWA 011 has undergone some re-equilibrationsuch a calculation can still provide some useful constraints onthe nature of the NWA 011 parent melt as well as possiblesecondary processes that may have affected the sample

The distribution coefficients listed in Table 6 were usedto calculate the compositions of liquids in equilibrium withcore plagioclase and pyroxene grains from NWA 011 The D

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

eral

s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

xene

1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

12

6 plusmn

01

53

plusmn 0

26

0 plusmn

02

41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

plusmn 0

24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

plusmn 1

Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

plusmn 0

060

42 plusmn

00

30

39 plusmn

00

40

43 plusmn

00

30

41 plusmn

00

30

45 plusmn

00

3Pr

006

9 plusmn

000

70

10 plusmn

00

10

11 plusmn

00

10

10 plusmn

00

10

074

plusmn 0

007

008

1 plusmn

000

70

11 plusmn

00

1N

d0

22 plusmn

00

10

34 plusmn

00

20

50 plusmn

00

20

42 plusmn

00

30

39 plusmn

00

20

45 plusmn

00

30

51 plusmn

00

2Sm

005

2 plusmn

001

00

074

plusmn 0

014

026

plusmn 0

02

028

plusmn 0

04

025

plusmn 0

02

022

plusmn 0

02

026

plusmn 0

02

Eu1

5 plusmn

01

16

plusmn 0

10

029

plusmn 0

006

003

9 plusmn

002

001

7 plusmn

001

30

025

plusmn 0

010

004

1 plusmn

001

0G

d0

037

plusmn 0

010

008

0 plusmn

001

40

40 plusmn

00

40

26 plusmn

00

60

34 plusmn

00

30

45 plusmn

00

40

35 plusmn

00

4Tb

000

76 plusmn

00

023

000

84 plusmn

00

038

008

3 plusmn

000

70

092

plusmn 0

014

008

3 plusmn

000

90

089

plusmn 0

007

010

plusmn 0

01

Dy

004

0 plusmn

000

60

09 plusmn

00

10

79 plusmn

00

30

70 plusmn

00

40

62 plusmn

00

30

82 plusmn

00

30

76 plusmn

00

3H

ob

db

d0

19 plusmn

00

10

15 plusmn

00

20

15 plusmn

00

10

19 plusmn

00

10

17 plusmn

00

1Er

bd

bd

064

plusmn 0

02

056

plusmn 0

04

055

plusmn 0

03

067

plusmn 0

02

061

plusmn 0

02

Tmb

db

d0

11 plusmn

00

10

098

plusmn 0

010

009

6 plusmn

000

90

099

plusmn 0

007

011

plusmn 0

01

Yb

001

7 plusmn

000

60

023

plusmn 0

009

068

plusmn 0

04

077

plusmn 0

07

067

plusmn 0

05

091

plusmn 0

04

080

plusmn 0

04

Lub

db

d0

13 plusmn

00

10

13 plusmn

00

20

14 plusmn

00

10

15 plusmn

00

10

10 plusmn

00

1Eu

Eua

9762

027

043

017

024

041

Ce

Cea

a Err

ors a

re 1

σ fr

om c

ount

ing

stat

istic

s onl

yn

d =

not

det

erm

ined

bd

= b

elow

det

ectio

n

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 349Ta

ble

4 M

inor

and

trac

e el

emen

t con

cent

ratio

ns (p

pm) i

n N

WA

011

min

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s

Plag

iocl

ase

1Pl

agio

clas

e 2

Pyro

xene

1ndash2

(c

ore)

Pyro

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1ndash2

(r

im)

Pyro

xene

3ndash4

(cor

e)Py

roxe

ne 3

ndash4

(rim

)Py

roxe

ne 5

ndash6

(cor

e)

Na

nd

nd

320

plusmn 1

365

plusmn 2

380

plusmn 1

310

plusmn 1

340

plusmn 1

Mg

225

plusmn 2

485

plusmn 3

nd

nd

nd

nd

nd

P34

0 plusmn

379

5 plusmn

410

40 plusmn

511

40 plusmn

967

0 plusmn

452

5 plusmn

464

5 plusmn

4K

460

plusmn 1

585

plusmn 2

24 plusmn

139

plusmn 1

46 plusmn

126

plusmn 1

27 plusmn

1C

an

dn

dn

dn

dn

dn

dn

dSc

31

plusmn 0

24

6 plusmn

02

28 plusmn

125

plusmn 1

23 plusmn

123

plusmn 1

23 plusmn

1Ti

100

plusmn 1

100

plusmn 1

2150

plusmn 4

2200

plusmn 7

2670

plusmn 5

2320

plusmn 4

2190

plusmn 4

Mn

84 plusmn

198

plusmn 1

5320

plusmn 6

5150

plusmn 1

049

00 plusmn

650

20 plusmn

648

60 plusmn

6Fe

4560

plusmn 3

517

190

plusmn 75

nd

nd

nd

nd

nd

Sr22

0 plusmn

122

5 plusmn

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6 plusmn

01

53

plusmn 0

26

0 plusmn

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41

plusmn 0

15

6 plusmn

01

Y0

31 plusmn

00

20

97 plusmn

00

55

6 plusmn

01

50

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24

8 plusmn

01

52

plusmn 0

15

3 plusmn

01

Zr0

30 plusmn

00

30

48 plusmn

00

411

plusmn 1

24 plusmn

128

plusmn 1

21 plusmn

123

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Ba

62 plusmn

185

plusmn 1

26

plusmn 0

17

3 plusmn

04

99

plusmn 0

36

5 plusmn

03

78

plusmn 0

3La

028

plusmn 0

02

054

plusmn 0

04

008

6 plusmn

001

0b

d0

10 plusmn

00

10

074

plusmn 0

010

010

plusmn 0

01

Ce

082

plusmn 0

05

10

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060

42 plusmn

00

30

39 plusmn

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43 plusmn

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30

41 plusmn

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9 plusmn

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5 plusmn

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19 plusmn

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068

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077

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Ce

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350 C Floss et alTa

ble

4 C

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ued

Min

or a

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ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

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5-6

(rim

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(cor

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roxe

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(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

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Mg

nd

nd

nd

nd

nd

nd

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P45

5 plusmn

320

00 plusmn

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895

plusmn 6

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K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

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Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

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Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

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006

8 plusmn

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90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

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Ce

044

plusmn 0

03

044

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05

051

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1310

plusmn 7

1270

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1390

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100

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Pr0

071

plusmn 0

006

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1 plusmn

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10

11 plusmn

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113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

350 C Floss et alTa

ble

4 C

ontin

ued

Min

or a

nd tr

ace

elem

ent c

once

ntra

tions

(ppm

) in

NW

A 0

11 m

iner

als

Pyro

xene

5-6

(rim

)Py

roxe

ne 7

(cor

e)Py

roxe

ne 8

(cor

e)M

erril

lite

1M

erril

lite

2M

erril

lite

3M

erril

lite

4

Na

275

plusmn 1

605

plusmn 2

340

plusmn 1

nd

nd

nd

nd

Mg

nd

nd

nd

nd

nd

nd

nd

P45

5 plusmn

320

00 plusmn

10

895

plusmn 6

nd

nd

nd

nd

K13

plusmn 1

93 plusmn

127

plusmn 1

nd

nd

nd

nd

Ca

nd

nd

nd

nd

nd

nd

nd

Sc22

plusmn 1

32 plusmn

126

plusmn 1

nd

nd

nd

nd

Ti28

10 plusmn

426

20 plusmn

718

10 plusmn

5n

dn

dn

dn

dM

n49

50 plusmn

556

60 plusmn

950

20 plusmn

8n

dn

dn

dn

dFe

nd

nd

nd

nd

nd

nd

nd

Sr3

7 plusmn

01

68

plusmn 0

24

6 plusmn

02

nd

nd

nd

nd

Y4

9 plusmn

01

46

plusmn 0

25

7 plusmn

02

nd

nd

nd

nd

Zr32

plusmn 1

28 plusmn

112

plusmn 1

nd

nd

nd

nd

Ba

44

plusmn 0

210

plusmn 1

94

plusmn 0

4n

dn

dn

dn

dLa

006

8 plusmn

000

90

13 plusmn

00

20

069

plusmn 0

015

290

plusmn 3

290

plusmn 3

320

plusmn 4

260

plusmn 5

Ce

044

plusmn 0

03

044

plusmn 0

05

051

plusmn 0

04

1310

plusmn 7

1270

plusmn 8

1390

plusmn 8

100

plusmn 11

Pr0

071

plusmn 0

006

009

1 plusmn

001

10

11 plusmn

00

113

0 plusmn

214

5 plusmn

215

0 plusmn

311

5 plusmn

3N

d0

42 plusmn

00

20

42 plusmn

00

30

52 plusmn

00

349

0 plusmn

551

0 plusmn

554

0 plusmn

642

5 plusmn

8Sm

022

plusmn 0

01

019

plusmn 0

02

027

plusmn 0

03

120

plusmn 4

125

plusmn 4

140

plusmn 5

100

plusmn 5

Eu0

038

plusmn 0

007

004

6 plusmn

001

80

056

plusmn 0

018

14 plusmn

113

plusmn 1

16 plusmn

112

plusmn 1

Gd

032

plusmn 0

02

035

plusmn 0

04

055

plusmn 0

05

110

plusmn 4

115

plusmn 4

115

plusmn 5

92 plusmn

6Tb

008

0 plusmn

000

50

082

plusmn 0

010

011

plusmn 0

01

20 plusmn

123

plusmn 1

23 plusmn

116

plusmn 2

Dy

070

plusmn 0

02

064

plusmn 0

04

089

plusmn 0

04

120

plusmn 3

120

plusmn 3

130

plusmn 3

115

plusmn 3

Ho

016

plusmn 0

01

013

plusmn 0

02

019

plusmn 0

02

24 plusmn

124

plusmn 1

26 plusmn

123

plusmn 1

Er0

58 plusmn

00

20

58 plusmn

00

30

70 plusmn

00

459

plusmn 2

65 plusmn

264

plusmn 2

50 plusmn

2Tm

010

plusmn 0

01

010

plusmn 0

01

010

plusmn 0

01

81

plusmn 0

58

4 plusmn

05

81

plusmn 0

68

2 plusmn

06

Yb

075

plusmn 0

03

074

plusmn 0

05

082

plusmn 0

05

37 plusmn

240

plusmn 2

41 plusmn

235

plusmn 2

Lu0

13 plusmn

00

10

12 plusmn

00

20

15 plusmn

00

24

8 plusmn

05

35

plusmn 0

54

8 plusmn

06

40

plusmn 0

5Eu

Eua

043

054

043

03

60

330

380

38C

eC

ea1

601

491

511

53a E

rror

s are

1σ

from

cou

ntin

g st

atis

tics o

nly

nd

= n

ot d

eter

min

ed b

d =

bel

ow d

etec

tion

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 351

Fig 5 CI chondrite-normalized REE patterns of minerals in NWA 011 a) core analyses of two plagioclase grains b) analyses of four differentmerrillite grains cndashe) core and rim analyses of three pyroxene grains f) core analyses of two pyroxene grains

Table 5 Whole rock major element concentrations (in wt) for NWA 011Whole rock Ya Whole rock Kb Bulk 1 Bulk 2 Bulk 3 Average bulk

SiO2 4563 4822 4669 4720 4775 4766TiO2 092 085 043 067 052 054Cr2O3 024 019 015 018 016 016Al2O3 1312 1286 1507 1369 1454 1455FeO 2113 1995 1802 1934 1826 1845MnO 040 029 028 029 027 028MgO 666 605 516 548 519 526CaO 1111 1076 1231 1158 1174 1193Na2O 045 054 073 067 071 071K2O 003 003 003 003 003 003P2O5 lt002 022 050 023 018 030Total 9971 9996 9935 9933 9935 9987

aY Yamaguchi et al (2002) wet chemical analysisbK Korochantseva et al (2003) calculated composition (see text)Bulks 1 2 and 3 calculated from modes 1 2 and 3 of Table 1 and mineral compositions from Table 2

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

352 C Floss et al

values used for pyroxene were calculated from the parametersgiven by McKay et al (1986) for pigeonite of Wo20composition which approximates the average CaO content ofNWA 011 pyroxene reconstructed from the pigeonite andaugite lamellae Figure 8 shows the REE patterns of theliquids in equilibrium with plagioclase 1 and the core ofpyroxene 3ndash4 (Table 4) together with the average bulkcomposition of NWA 011 calculated from the mineral dataBoth parent melt compositions have approximately flat toslightly LREE-enriched REE patterns at sim20 times CI indicatingthat the two minerals crystallized from similar melts TheREE pattern for the melt in equilibrium with plagioclase has asmall positive Eu anomaly similar to that seen in the bulkREE pattern suggesting that this feature is real and is notsimply an artifact reflecting an overabundance of plagioclasein the mode determined from our thin sections The melt REEpatterns for both pyroxene and plagioclase are approximatelyparallel to the bulk REE pattern but have somewhat elevatedabundances particularly for the HREE Moreover the meltcalculated to be in equilibrium with pyroxene is somewhatLREE-enriched relative to the bulk pattern This enrichmentprobably reflects homogenization of the REE withinoriginally zoned grains In plagioclase equilibration of theREE will tend to increase REE abundances relative to theoriginal core compositions without significantly affecting theshape of the pattern equilibration of the REE in pyroxenewill in addition tend to increase the LREE abundances (fordetailed discussions of the effects of equilibration on REEabundances and patterns in minerals see Hsu and Crozaz[1996] and Floss et al [1998]) Thus the REE composition ofthe original parent melt for NWA 011 was probably quitesimilar to the average bulk REE pattern shown in Fig 8 Theeffects of thermal metamorphism on the trace elementcompositions of NWA 011 minerals will be discussed in moredetail below

The whole rock major element compositions ofNWA 011 (K and Y Table 5) were used with the MELTSprogram to model the crystallization of NWA 011 at an fO2 ofIW Figure 9 shows that equilibrium crystallization from thebulk composition of Korotchantseva et al (2003) results in acrystallization sequence of pigeonite + plagioclase rarr spinelrarr olivine rarr augite rarr merrillite rarr ilmenite rarr silica Thecrystallization order from the bulk composition of Yamaguchiet al (2002) is slightly different (spinel rarr plagioclase rarrolivine rarr pigeonite rarr augite rarr merrillite) and lacks primaryilmenite and silica These differences are probably due tosample heterogeneity for the determination of bulkcompositions as discussed above for the bulk REEcompositions For example the earlier crystallization ofspinel from whole rock Y probably reflects the higherconcentrations of Fe and Cr in this bulk composition becausespinel stability is very sensitive to Cr contents Bothcompositions indicate extensive co-crystallization ofplagioclase and pigeonite consistent with the REE parentmelt compositions calculated above However both alsopredict significant augite crystallization after the onset ofpigeonite crystallization although no augite is present in oursection Yamaguchi et al (2002) did note the presence ofrelict augite grains in their section of NWA 011 but did notgive an estimate of their abundance In addition pigeonitecompositions show a large range in FeMg that is not seen inour mineral compositions (Table 2) post-crystallizationprocesses are probably responsible for the equilibration ofpyroxene compositions in NWA 011

Metamorphism on the NWA 011 Parent Body

Other evidence also suggests that thermal metamorphismhas played a role in the evolution of NWA 011 Yamaguchiet al (2002) noted that NWA 011 exhibits a number of

Fig 6 CI chondrite-normalized bulk REE patterns for NWA 011 along with the average mineral REE patterns used to calculate the whole rockcompositions Bulks 1 2 and 3 correspond to the respective modes listed in Table 1

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 353

mineralogical features seen in highly metamorphosedeucrites such as Ibitira EET 90020 and Y-86763 (Floss et al2000 Yamaguchi et al 2001) The low-Ca pyroxene inNWA 011 consists of pigeonite with fine exsolution lamellaeof augite similar to equilibrated pyroxenes in type 5 eucrites(Takeda and Graham 1991) Application of the two-pyroxenethermometer of Kretz (1982) indicates an equilibrationtemperature of sim860 degC whereas the compositions of relictaugite and pigeonite observed by Yamaguchi et al (2002)suggest a peak metamorphic temperature of sim1090ndash1100 degCNWA 011 also contains oxide assemblages (ilmenite and Ti-rich chromite) associated with Fe-rich olivine and pyroxene(eg Fig 1b) In the highly equilibrated eucrites suchfeatures which are not in equilibrium with the rest of themeteorite have been attributed to a rapid reheating andcooling event following thermal metamorphism on the eucriteparent body (Floss et al 2000 Yamaguchi et al 2001)

In a study of trace element distributions in eucrites Hsu

and Crozaz (1996) showed that pyroxene and plagioclasefrom many non-cumulate eucrites have LREEHREE ratiosthat fall along a single correlation line For plagioclase thisline also represents the abundances expected forcrystallization from melts with chondritic proportions of theREE However these authors as well as Floss et al (2000)noted that some highly metamorphosed eucrites haveplagioclase and pyroxene compositions that are LREE-enriched and thus fall to the right of these lines Figure 10plots the LREEHREE ratios of plagioclase and pyroxenefrom NWA 011 relative to the correlation lines seen in non-cumulate eucrites NWA 011 plagioclase falls on the non-cumulate eucrite line suggesting that it crystallized from achondritic melt Pyroxene from NWA 011 however fallssomewhat to the right of the line In the highlymetamorphosed eucrites studied by Hsu and Crozaz (1996)and Floss et al (2000) the LREE enrichments observed inplagioclase and pyroxene were attributed to extensiveredistribution of the REE between Ca-phosphates and thesilicate minerals possibly due to partial melting of mesostasismaterial which contains the phosphates and interaction ofthis melt with plagioclase and pigeonite to produce the LREE-enriched signature It is unlikely that a similar processaccounts for the LREE-rich nature of NWA 011 pyroxenebecause such a mechanism would be expected to enrich bothsilicate minerals A more likely explanation is thathomogenization of the REE within originally zoned pyroxenegrains during thermal metamorphism accounts for theobserved LREE enrichment This is consistent with theobservation above that equilibrium melt compositionscalculated for pyroxene are LREE-enriched relative to thosedetermined for plagioclase Moreover the fact thatplagioclase LREEHREE ratios fall on the non-cumulatecorrelation line while pyroxene ratios do not suggests thatredistribution of the REE was limited to intra-grain re-equilibration and did not involve redistribution betweenmineral grains

The distributions of other trace elements in NWA 011plagioclase and pyroxene relative to eucrites are shown in

Fig 7 CI chondrite-normalized bulk REE patterns for NWA 011The white symbols are data from Yamaguchi et al (2002) the greysymbols are data from Korotchantseva et al (2003) and the solidlines represent patterns calculated from the mineral compositionsmeasured in this study a) HREE-rich patterns can be reproducedusing 585 pyroxene 396 plagioclase (Table 1) and = 005Ca-phosphate b) the LREE-enriched pattern can be reproducedusing 88 pyroxene 10 plagioclase and 16 Ca-phosphate

Table 6 Partition coefficients used to calculate NWA 011 equilibrium melts

Plagioclasea Pigeonite (Wo20)b

La 0051 plusmn 0010 0007 plusmn 0003Ce 0044 plusmn 0009 0014 plusmn 0005Nd 0038 plusmn 0008 0055 plusmn 0015Sm 0031 plusmn 0006 0106 plusmn 0018Eu 115 plusmn 023Gd 0021 plusmn 0004 0129 plusmn 0036Yb 00038 plusmn 00008 0238 plusmn 0029Lu 0242 plusmn 0029

aData from Jones (1995) bData from McKay et al (1986)Errors are estimated at 20 relative for plagioclase and are the average

relative errors given by McKay et al (1986) for pigeonite

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

354 C Floss et al

Figs 11 and 12 Sodium Ba Sr and K abundances inNWA 011 plagioclase generally fall within or near the fieldspreviously defined for non-cumulate eucrites (Hsu andCrozaz 1996) This is also true for the highly metamorphosedeucrites EET 90020 and Y-86763 an observation that ledFloss et al (2000) to suggest that abundances of these traceelements in plagioclase had not been affected by inter-mineralredistribution despite the fact that the REE had been affectedFor NWA 011 the similarities provide another link of thismeteorite to the eucrites The abundances of Ti Y and Zr inpyroxene from NWA 011 are intermediate between those innon-cumulate eucrites and those in highly metamorphosedeucrites (Fig 12) This implies some redistribution of theseelements in NWA 011 pyroxene although it seems to havebeen more limited in NWA 011 than in Ibitira EET 90020and Y-86763 Alternatively it could simply reflect primarysource differences In summary if NWA 011 can be comparedwith the eucrites it appears that although thermalmetamorphism resulted in some REE redistribution inNWA 011 pyroxene overall the effects on trace elementcompositions are less than in some of the highlymetamorphosed eucrites In particular there is no clearevidence for any partial melting and resultant redistribution ofthe REE from phosphates to silicate minerals

Did NWA 011 Originate on the Eucrite Parent Body

We have repeatedly noted the many similarities thatNWA 011 bears to the eucrites The mineralogy and texturesseen in NWA 011 are like those of some highlymetamorphosed eucrites (eg Yamaguchi et al 2002) andmajor element mineral compositions resemble those of non-cumulate eucrites (eg Fig 2) As we have seen thesimilarities also extend to trace element compositions The

whole rock REE composition of NWA 011 is around 10 times CI(Fig 6) similar to those typically seen in eucrites(Consolmagno and Drake 1977) and silicate mineral REEabundances fall within the ranges seen in non-cumulateeucrites (Hsu and Crozaz 1996) Abundances of other traceelements are also like those of the non-cumulate eucrites(Figs 11 and 12) Moreover NWA 011 has a cosmic rayexposure age of 30 Ma (Yamaguchi et al 2002) within therange of cosmic ray exposure ages for eucrites (7ndash70 MaEugster and Michel 1995) and its Sm-Nd age of 446 plusmn004 Ga is identical within errors to the age of 451 plusmn 004determined for EET 90020 (Nyquist et al 2003) 39Ar-40Ardata have been more difficult to interpret due to the effects ofsignificant weathering of this meteorite Both Korotchantsevaet al (2003) and Bogard and Garrison (2004) report ldquoagesrdquo ofsim32 Ga for the last major degassing event of NWA 011which is outside of the time period in which the 39Ar-40Ar agesof most eucrites have been reset (sim34ndash41 Ga Bogard andGarrison 2003) However Bogard and Garrison (2004) alsonoted that two whole rock analyses give higher maximumages (within the eucrite range) at intermediate temperaturesand pointed out that the space exposure age of NWA 011 isclose to a peak in cosmic ray exposure ages observed in manyother HED meteorites (Wakefield et al 2004)

Despite these similarities the oxygen isotopiccomposition of NWA 011 is clearly distinct from that of theHED meteorites and FeMn ratios are higher than those oftypical eucrites Our trace element data suggest additionaldifferences Although most trace elements in plagioclase haveabundances within the eucrite range Sr abundances areslightly elevated relative to eucrites Moreover REEabundances in merrillite are lower by a factor of 10 than intypical eucrites and both pyroxene and phosphate havesmaller Eu anomalies than most eucrites

Fig 8 CI chondrite-normalized REE patterns for liquids calculated to be in equilibrium with plagioclase (grey symbols) and pyroxene (whitesymbols) from NWA 011 Also shown (solid line) is the REE pattern calculated from the average modal composition (Table 1) the shadedarea shows 1 errors

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 355

In principle infiltration of an Fe-rich metasomatizingliquid could account for the elevated FeMn ratios inNWA 011 and some trace element features such as theelevated Sr abundances in plagioclase However comparisonof the whole rock compositions of NWA 011 (Table 2) withthose from a variety of eucrites (Kitts and Lodders 1998)suggests that NWA 011 is in fact depleted in Mn relative toother eucrites and only shows a slight enrichment in FeMoreover metasomatism would be expected to enrich allminerals in Fe but plagioclase compositions show FeOconcentrations within the ranges typically seen for eucrites Inaddition it is not clear how such a process might account formerrillite REE abundances that are 10times lower than in typicaleucrites

The most difficult characteristic of NWA 011 to reconcilewith the eucrites however is its oxygen isotopic composition(Fig 4) which falls far from the known eucrite fieldWiechert et al (2002) have noted that eucrites exhibit oxygenisotopic heterogeneity and have suggested that the eucritesmust come from at least four different parent bodies or fromisotopically different reservoirs of the same planetary bodyHowever the total ∆17O variation in oxygen isotopiccomposition measured by Wiechert et al (2002) spans a rangeof minus007 permil to minus025 permil far less than the ∆17O of minus158 permilmeasured for NWA 011 This large difference makes itunlikely that NWA 011 comes from a previously unsampledisotopically heterogeneous reservoir on the eucrite parentbody Moreover there are no known secondary processes thatcan produce such a change in oxygen isotopes Thus we

conclude that NWA 011 did not originate on the HED parentbody despite the many similarities this meteorite exhibitswith eucrites

Is NWA 011 Related to the AcapulcoitesndashLodranites

The acapulcoites and lodranites are a group of relatedprimitive achondrites that probably originated from the sameparent body (Mittlefehldt et al 1996 McCoy et al 19961997a) These meteorites experienced variable degrees ofheating resulting in complex partial melting and meltmigration processes (McCoy et al 1997b Floss 2000)Lodranites are typically depleted in troilite and plagioclaseand have lost sim15 or more silicate partial melts relative tothe acapulcoites which have generally retained chondriticmineralogies However geochemical data show that the twogroups are not always easily distinguished on petrographiccriteria alone and that the acapulcoitelodranite clan forms acontinuum exhibiting a range of degrees of heating andpartial melting that may or may not be accompanied by meltmigration (Floss 2000 Patzer et al 2004) The oxygenisotopic composition of NWA 011 falls near that of theacapulcoites and lodranites (Fig 4) raising the question ofwhether NWA 011 could be related to these meteorites Asnoted above the lodranites are residues that have lost silicatepartial melts However with the possible exception of acoarse-grained lithology in the anomalous acapulcoiteLEW 86220 (McCoy et al 1997b) these basaltic partial meltshave not been identified in the meteorite population We

Fig 9 Equilibrium crystallization sequences for NWA 011 whole rock compositions K and Y (Table 5) pig = pigeonite plag = plagioclasesp = spinel ol = olivine aug = augite mer = merrillite ilm = ilmenite sil = silica

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

356 C Floss et al

investigate here whether NWA 011 could represent a sampleof the basaltic partial melt complementary to the lodraniteresidues

The first silicate melt to form through heating of achondritic parent body should be enriched in incompatibleelements and the trace elements associated with a feldspathiccomponent Such a melt is likely to be LREE-enriched andone would expect the minerals crystallizing from such a meltto be LREE-enriched relative to those crystallizing from amelt with chondritic abundances of the REE In fact as wenoted earlier the parent melt composition calculated forNWA 011 is slightly LREE-enriched with a small positive Euanomaly (Fig 8) In addition Fig 13a shows that NWA 011merrillite is distinctly LREE-rich compared to merrillite fromthe acapulcoites However plagioclase from NWA 011 has aflatter rather than steeper LREE pattern than plagioclase fromthe acapulcoites (Fig 13b) and as we noted earlier NWA 011plagioclase has LREEHREE ratios that are consistent withcrystallization from a chondritic source Pyroxene REE

compositions do not provide any information in this situationLREE abundances in pyroxene vary with major elementcomposition (eg Wo contents) and the pyroxenes present inthe acapulcoites are orthopyroxene and augite whereas thosein NWA 011 are pigeonite Thus a direct comparison of REEabundances is not possible

Fig 10 Plots of a) La versus Y in plagioclase and b) Sm versus Ybin pigeonite from NWA 011 (grey symbols) The solid lines representcorrelations of these elements observed in non-cumulate eucrites(white symbols) Highly metamorphosed eucrites (black symbols)experienced redistribution of the REE and fall to the right of the lineEucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 11 Scatter plots of a) Na versus Sr b) Na versus Ba and c) Kversus Ba in NWA 011 plagioclase (black symbols) Also shown arethe fields for non-cumulate eucrites (Stannern Nuevo Laredo SiouxCounty Pasamonte Lakangaon and Chervony Kut) and for thehighly metamorphosed eucrites Ibitira EET 90020 and Y-86763Eucrite data are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 357

Despite the similarities noted here between the NWA 011parent melt composition and the REE compositions expectedfor a basaltic partial melt from the acapulcoitelodraniteparent body it is unlikely that NWA 011 is in fact a basalticsample from the acapulcoitelodranite parent body NWA 011has a very fractionated bulk composition with a molar FeMgratio of about 18 Goodrich and Delaney (2000) have shownthat low degrees of partial melting (2) of a chondriticprecursor will result in liquids that have higher FeMg ratiosthan the bulk composition and slightly lower FeMn As thedegree of melting increases these ratios move back towardthe original chondritic compositions Fractionalcrystallization of these melts produces strong increases in theFeMg ratio without much change in FeMn ratios while theresulting cumulates have lower FeMg ratios (cf Fig 5 inGoodrich and Delaney 2000) Very high FeMg ratios such asthose observed in NWA 011 cannot be produced throughpartial melting and thus are probably the result of a fractional

crystallization process on its parent body Such a scenariowould also be consistent with the slightly fractionated parentmelt composition that we calculated crystallization andaccumulation of mafic olivine andor pyroxene would enrichthe remaining melt in the LREE and could produce a smallpositive Eu anomaly as we observe for NWA 011 (Fig 8) Inthis context Korotchantseva et al (2003) noted thatNWA 011 is depleted in Sc suggesting pyroxenefractionation

It is not feasible that extensive fractional crystallizationlike that required to produce the Fe-rich composition ofNWA 011 could have occurred on the acapulcoitelodraniteparent body which has retained significant chondriticchemical and mineralogical characteristics Mafic cumulatesthat would represent the counterpart to the fractionated basaltrepresented by NWA 011 have also not been sampled from theacapulcoitelodranite parent body In addition siderophileelement abundances in NWA 011 are highly fractionated(Yamaguchi et al 2002 Korotchantseva et al 2003)suggesting core formation which has clearly not occurred onthe acapulcoitelodranite parent body Thus we conclude thatNWA 011 is not genetically related to the acapulcoites andlodranites despite the similarity in oxygen isotopes

Fig 12 Scatter plots of a) Ti versus Y and b) Ti versus Zr inNWA 011 pyroxene (black symbols) Also shown are the fields fornon-cumulate eucrites (Stannern Nuevo Laredo Sioux CountyPasamonte Lakangaon and Chervony Kut) and for the highlymetamorphosed eucrites Ibitira EET 90020 and Y-86763 Eucritedata are from Hsu and Crozaz (1996) Floss et al (2000) andYamaguchi et al (2001)

Fig 13 CI chondrite-normalized REE patterns for a) merrillite and b)plagioclase from NWA 011 Shaded areas show the range ofabundances observed in the acapulcoites (data from Floss 2000)

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

358 C Floss et al

Relationship to CR Chondrites

The oxygen isotopic composition of NWA 011 falls withinthe range noted for the CR chondrites The CR chondrites area group of fourteen meteorites named after the type memberRenazzo (Weisberg et al 1993 Krot et al 2002) They aregenerally of petrologic type 2 and contain abundant hydroussilicates Despite the aqueous alteration they haveexperienced they are considered to be highly primitivebecause the ratios of refractory lithophile elements to Mgabundances are similar to those of CI chondrites Boesenberg(2003) modeled possible parent body compositions that uponmelting would produce magmas consistent with the NWA 011bulk composition oxygen isotopic compositions were alsoconstrained to match that of NWA 011 Several modelsincluding H-Allende and a LL-Allende mixtures producedcompositions that were very similar to that of the bulk majorelement compositions of NWA 011 Surprisingly however themodel based on a pure CR chondrite composition produced thepoorest fit to the NWA 011 composition with CaOAl2O3ratios that were far too high The model was based on thecomposition of CR chondrite Y-790112 which is the closestmatch to NWA 011 in terms of oxygen isotopic compositionThe CR chondrites are part of a large CR chondrite clan thatalso includes the CH (high metal) chondrites the CB(Bencubbin) chondrites and ungrouped LEW 85332(Weisberg et al 1995 Krot et al 2002) Although thesemeteorite groups are related by a number of characteristicsincluding oxygen isotopic compositions there are chemicaland mineralogical differences between them Thus a differentmember of this clan may provide a better fit as the sourcematerial for NWA 011

Trace element compositions provide few constraints onthe NWA 011 parent as all the chondrites have unfractionatedREE patterns However as we noted earlier NWA 011 isdepleted in Mn relative to eucrites leading to its distinctiveFeMn ratio (Fig 3) The CR chondrite clan meteorites (CRCB and CH) are all highly depleted in volatile lithophileelements including Mn relative to other chondrites(Weisberg et al 1995) suggesting a possible link to theNWA 011 parent material Moreover some of the CRchondrite clan meteorites are highly enriched in refractoryand normal siderophile elements including the CH and CBchondrite groups (Krot et al 2002) NWA 011 has highsiderophile element abundances with highly fractionated NiIr and NiCo ratios Korotchantseva et al (2003) suggestedthat oxidizing conditions in the NWA 011 source may haveprevented complete equilibration with Fe metal (accountingfor the elevated siderophile element abundances) and notedthat the high FeMn and high P contents in NWA 011 supportan oxidized source region Our major element data alsoprovide some evidence for oxidizing conditions from thepresence of Fe2O3 in ulvˆspinel and pyroxene (Table 2)Ulvˆspinel from NWA 011 contains 23 wt of Fe2O3significantly higher than the Fe2O3 contents of HED

chromites which are typically less than 1 wt (Mittlefehldt1994 Mittlefehldt et al 1998)

Origin of NWA 011 on Mercury

Commenting on the discovery by Yamaguchi et al(2002) that NWA 011 has an oxygen isotopic compositiondifferent from that of the eucrites and therefore must comefrom a distinct source Palme (2002) suggested that perhapsNWA 011 is of Mercurian origin The discovery andidentification of a meteorite from Mercury would be of greatscientific value and the probability of a Mercurian samplereaching Earth while low is not negligible (estimated to beabout 1 of that from Mars Love and Keil [1995])However NWA 011 does not appear to be that sample Loveand Keil (1995) noted that Mercurian rocks should probablyhave low volatile contents and be moderately enriched inrefractory lithophile elements Most Mercurian surface rocksare probably volcanic and should contain lt5 FeO NWA011 with its very Fe-enriched composition clearly does notmatch these expectations Moreover the Sm-Nd age of NWA011 of 446 Ga (Nyquist et al 2003) is older than theestimated solidification ages of sim37 to sim44 Ga for rocksfrom Mercury (Love and Keil 1995) and suggests anasteroidal rather than Mercurian origin The exposure age ofNWA 011 is similar to that of eucrites (Yamaguchi et al 2002Bogard and Garrison 2004) and thus also suggests anasteroidal origin

The only known basaltic asteroid other than 4 Vesta inthe asteroid belt is 1459 Magnya (Lazarro et al 2000) Thissmall (sim30 km) rock has no known dynamical links to 4 Vestaor any other nearby large asteroids but orbital resonances inthe region of the asteroid belt around 1459 Magnya indicatethat it could be a rare surviving portion of a largerdifferentiated body that was disrupted long ago (Lazarro et al2000) NWA 011 could be a fragment of this parent body asalso suggested by Nyquist et al (2003)

CONCLUSIONS

The NWA 011 basalt originated from a source withroughly chondritic proportions of the REE like the eucrites itwas initially identified with A slightly LREE-enriched bulkcomposition with a small positive Eu anomaly as well ashighly fractionated FeMg ratios and depleted Sc abundances(Korotchantseva et al 2003) suggest that the NWA 011source experienced some pyroxene andor olivinefractionation Metamorphism of NWA 011 resulted inhomogenization of REE abundances within grains but thismeteorite does not seem to have experienced the intergrainREE redistribution seen in some highly metamorphosedeucrites Despite a similarity in oxygen isotopic compositionsNWA 011 is probably not genetically related to theacapulcoites and lodranites The source material for NWA 011may have been like some CH or CB chondrites members of

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

Northwest Africa 011 359

the CR chondrite clan with similar oxygen isotopiccompositions The NWA 011 parent body is probably ofasteroidal origin possibly the basaltic asteroid 1459 Magnya

AcknowledgmentsndashThis work was supported by NASA grantsNAG-NNG04GG49G to C F NAG5-11558 to L A T andNAG5-12948 to D R We thank T Smolar and E Inazaki formaintenance of the 3f ion microprobe at WashingtonUniversity Careful reviews by A Ruzicka and K Righterprovided significant improvements to this paper and are muchappreciated

Editorial HandlingmdashDr Kevin Righter

REFERENCES

Afanasiev S V Ivanova M A Korotchantseva E V KononkovaN N and Nazarov M A 2000 Dhofar 007 and NorthwestAfrica 011 Two new eucrites of different types (abstract)Meteoritics amp Planetary Science 35A19

Alexander C M O 1994 Trace element distributions withinordinary chondrite chondrules Implications for chondruleformation conditions and precursors Geochimica etCosmochimica Acta 583451ndash3467

Anders E and Grevesse N 1989 Abundances of the elementsMeteoritic and solar Geochimica et Cosmochimica Acta 53197ndash214

Ash R D and Pillinger C T 1995 Carbon nitrogen and hydrogenin Saharan chondrites The importance of weatheringMeteoritics 3085ndash92

Barrat J A Gillet P Lesourd M Blichert-Toft J and Poupeau G R1999 The Tatahouine diogenite Mineralogical and chemicaleffects of sixty-three years of terrestrial residence Meteoritics ampPlanetary Science 3491ndash97

Basaltic Volcanism Study Project 1981 Basaltic volcanism on theterrestrial planets New York Pergamon Press 1286 pp

Binzel R P and Xu S 1993 Chips off of asteroid 4 Vesta Evidencefor the parent body of basaltic achondrite meteorites Science260186ndash191

Bland P A Berry F J Smith T B Skinner S J and Pillinger C T1996 The flux of meteorites to the Earth and weathering in hotdesert ordinary chondrite finds Geochimica et CosmochimicaActa 60 2053ndash2059

Boesenberg J 2003 An oxygen isotope mixing model for theNorthwest Africa 011 basaltic achondrite (abstract 1239) 34thLunar and Planetary Science Conference CD-ROM

Bogard D and Garrison D 2003 39Ar-40Ar ages of eucrites andthermal history of asteroid 4 Vesta Meteoritics amp PlanetaryScience 38669ndash710

Bogard D and Garrison D 2004 39Ar-40Ar dating of unusual eucriteNWA 011 Is it from Vesta (abstract 1094) 35th Lunar andPlanetary Science Conference CD-ROM

Clayton R N 1993 Oxygen isotopes in meteorites Annual Reviewsin Earth and Planetary Science 21115ndash149

Clayton R N 2003 Oxygen isotopes in meteorites In Meteoritesplanets and comets edited by Davis A M Oxford ElsevierScience pp 129ndash142

Clayton R N and Mayeda T K 1996 Oxygen isotope studies ofachondrites Geochimica et Cosmochimica Acta 601999ndash2017

Consolmagno G J and Drake M J 1977 Composition and evolutionof the eucrite parent body Evidence from rare earth elementsGeochimica et Cosmochimica Acta 411271ndash1282

Crozaz G and Wadhwa M 2001 The terrestrial alteration of Saharanshergottites Dar al Gani 476 and 489 A case study of weatheringin a hot desert environment Geochimica et Cosmochimica Acta65971ndash978

Crozaz G Floss C and Wadhwa M 2003 Chemical alteration andREE mobilization in meteorites from hot and cold desertsGeochimica et Cosmochimica Acta 674727ndash4741

Dreibus G Huisl W Haubold R and Jagoutz E 2001 Influence ofterrestrial desert weathering in martian meteorites (abstract)Meteoritics amp Planetary Science 36A50ndashA51

Eugster O and Michel T 1995 Common asteroid break-up events ofeucrites diogenites and howardites and cosmic-ray productionrates for noble gases in achondrites Geochimica etCosmochimica Acta 59177ndash199

Floss C 2000 Complexities on the acapulcoite-lodranite parentbody Evidence from trace element distributions in silicateminerals Meteoritics amp Planetary Science 351073ndash1085

Floss C and Jolliff B 1998 Rare earth element sensitivity factors incalcic plagioclase (anorthite) In Secondary ion massspectrometry SIMS XI edited by Gillen G Lareau R Bennett Jand Stevie F New York John Wiley amp Sons pp 785ndash788

Floss C James O B McGee J J and Crozaz G 1998 Lunar ferroananorthosite petrogenesis Clues from trace element distributionsin FAN subgroups Geochimica et Cosmochimica Acta 621255ndash1283

Floss C Crozaz G Yamaguchi A and Keil K 2000 Trace elementconstraints on the origins of highly metamorphosed Antarcticeucrites Antarctic Meteorite Research 13222ndash237

Floss C Taylor L A and Promprated P 2004 Trace elementsystematics of Northwest Africa 011 A ldquoeucriticrdquo basalt from anon-eucrite parent body (abstract 1153) 35th Lunar andPlanetary Science Conference CD-ROM

Goodrich C A and Delaney J S 2000 FeMg-FeMn relations ofmeteorites and primary heterogeneity of primitive achondriteparent bodies Geochimica et Cosmochimica Acta 64149ndash160

Grossman J N 2000 The Meteoritical Bulletin no 84 Meteoriticsamp Planetary Science 35A199ndashA225

Hsu W 1995 Ion microprobe studies of the petrogenesis of enstatitechondrites and eucrites PhD thesis Washington University StLouis Missouri USA

Hsu W and Crozaz G 1996 Mineral chemistry and the petrogenesisof eucrites I Noncumulate eucrites Geochimica etCosmochimica Acta 604571ndash4591

Kitts K and Lodders K 1998 Survey and evaluation of eucrite bulkcompositions Meteoritics amp Planetary Science 33A197ndashA213

Korotchantseva E V Ivanova M A Lorenz C A Bouikine A ITrieloff M Nazarov M A Promprated P Anand M and TaylorL A 2003 Major and trace element chemistry and Ar-Ar age ofthe NWA 011 achondrite (abstract 1575) 34th Lunar andPlanetary Science Conference CD-ROM

Kretz R 1982 Transfer and exchange equilibria in a portion of thepyroxene quadrilateral as deduced from natural and experimentaldata Geochimica et Cosmochimica Acta 46411ndash421

Krot A N Meibom A Weisberg M K and Keil K 2002 The CRchondrite clan Implications for the early solar system processesMeteoritics amp Planetary Science 371451ndash1490

Jolliff B L Haskin L A Colson R O and Wadhwa M 1993Partitioning in REE-saturating minerals Theory experimentand modelling of whitlockite apatite and evolution of lunarresidual magmas Geochimica et Cosmochimica Acta 574069ndash4094

Jones J H 1995 Experimental trace element partitioning In Rockphysics and phase relations a handbook of physical constantsedited by Ahrens T J Washington D C American GeophysicalUnion pp 73ndash104

360 C Floss et al

Lazzaro D Michtchenko T Carvano J M Binzel R P Bus S JBurbine T H Mothe-Diniz T Florczak M Angeli C A andHarris A W 2000 Discovery of a basaltic asteroid in the outermain belt Science 2882033ndash2035

Love S G and Keil K 1995 Recognizing mercurian meteoritesMeteoritics 30269ndash278

McCord T B Adams J B and Johnson T V 1970 Asteroid VestaSpectral reflectivity and compositional implications Science1681445ndash1447

McCoy T J Keil K Clayton R N Mayeda T K Bogard D DGarrison D H Huss G R Hutcheon I D and Wieler R 1996A petrologic chemical and isotopic study of Monument Drawand comparison with other acapulcoites Evidence for formationby incipient partial melting Geochimica et Cosmochimica Acta602681ndash2708

McCoy T J Keil K Clayton R N Mayeda T K Bogard D DGarrison D H and Wieler R 1997a A petrologic and isotopicstudy of lodranites Evidence for early formation as partial meltresidues from heterogeneous precursors Geochimica etCosmochimica Acta 61623ndash637

McCoy T J Keil K Muenow D W and Wilson L 1997b Partialmelting and melt migration in the acapulcoite-lodranite parentbody Geochimica et Cosmochimica Acta 61639ndash650

McKay G Wagstaff J and Yang S R 1986 Clinopyroxene REEdistribution coefficients for shergottites The REE content of theShergotty melt Geochimica et Cosmochimica Acta 50927ndash937

Miller M F 2002 Isotopic fractionation and the quantification of 17Oanomalies in the oxygen three-isotope system An appraisal andgeochemical significance Geochimica et Cosmochimica Acta661881ndash1889

Mittlefehldt D W 1994 ALH 84001 a cumulate orthopyroxenitemember of the martian meteorite clan Meteoritics 29214ndash221

Mittlefehldt D W and Lindstrom M M 1991 Generation ofabnormal trace element abundances in Antarctic eucrites byweathering processes Geochimica et Cosmochimica Acta 5577ndash87

Mittlefehldt D W Lindstrom M M Bogard D D Garrison D Hand Field S W 1996 Acapulco- and lodran-like achondritesPetrology geochemistry chronology and origin Geochimica etCosmochimica Acta 60867ndash882

Mittlefehldt D W McCoy T J Goodrich C A and Kracher A1998 Non-chondritic meteorites from asteroidal bodies InPlanetary materials edited by Papike J J Washington D CMineralogical Society of America 195 p

Nyquist L Shih C-Y Wiesman H Yamaguchi A and Misawa K2003 Early volcanism on the NWA 011 parent body (abstract)Meteoritics amp Planetary Science 38A59

Palme H 2002 A new solar system basalt Science 296271ndash273Papike J J 1998 Comparative planetary melt-derived mineralogy In

Planetary materials edited by Papike J J Washington D CMineralogical Society of America 11 p

Patzer A Hill D H and Boynton W V 2004 Evolution andclassification of acapulcoites and lodranites from a chemicalpoint of view Meteoritics amp Planetary Science 3961ndash85

Promprated P Taylor L A Anand M Rumble D KorotchantsevaE V Ivanova M A Lorenz C A and Nazarov M A 2003Petrology and oxygen isotopic compositions of anomalousachondrite NWA 011 (abstract 1757) 34th Lunar and PlanetaryScience Conference CD-ROM

Schreiber H D Lauer H V Jr and Thanyasiri T 1980 The redoxstate of cerium in basaltic magmas An experimental study ofiron-cerium interactions in silicate melts Geochimica etCosmochimica Acta 441599ndash1612

Scott E R D 1979 Origin of iron meteorites In Asteroids edited by

Gehrels T Tucson Arizona The University of Arizona Press pp892ndash925

Scherer P Schultz L and Loeken T 1994 Weathering andatmospheric noble gases in chondrites In Noble gasgeochemistry and cosmochemistry edited by Matsuda J TokyoTerra Scientific Publishing Co pp 43ndash53

Sharp Z D 1990 A laser-based microanalytical method for the insitu determination of oxygen isotope ratios of silicates andoxides Geochimica et Cosmochimica Acta 541353ndash1357

Spear F S 1995 Metamorphic phase equilibria and pressure-temperature-time paths Washington D C MineralogicalSociety of America 799 pp

Swindle T D Kring D A Burkland M K Hill D H and BoyntonW V 1998 Noble gases bulk chemistry and petrography ofolivine-rich achondrites Eagles Nest and Lewis Cliff 88763Comparison to brachinites Meteoritics amp Planetary Science 3331ndash48

Takeda H and Graham A L 1991 Degree of equilibration of eucriticpyroxenes and thermal metamorphism of the earliest planetarycrust Meteoritics 26129ndash134

Valley J W Kitchen N E Kohn M J Niendorf C R and SpicuzzaM J 1995 UWG-2 a garnet standard for oxygen isotope ratiosStrategies for high precision and accuracy with laser heatingGeochimica et Cosmochimica Acta 595223ndash5231

Wakefield K Bogard D and Garrison D 2004 Cosmic rayexposure ages Ar-Ar ages and the origin and history of eucrites(abstract 1020) 35th Lunar and Planetary Science ConferenceCD-ROM

Warren P H and Jerde E A 1987 Composition and origin of NuevoLaredo trend eucrites Geochimica et Cosmochimica Acta 51713ndash725

Weisberg M Prinz M Clayton R and Mayeda T K 1993 The CR(Renazzo-type) carbonaceous chondrite group and itsimplications Geochimica et Cosmochimica Acta 571567ndash1586

Weisberg M Prinz M Clayton R Mayeda T K Grady M M andPillinger C T 1995 The CR chondrite clan Proceedings of theNIPR Symposium on Antarctic Meteorites 811ndash32

Wiechert U Halliday A N Lee D-C Snyder G Taylor L A andRumble D 2001 Oxygen isotopes and the moon-forming giantimpact Science 294345ndash348

Wiechert U Halliday A N Palme H and Rumble D 2002 Oxygenisotopic heterogeneity among eucrites (abstract) Geochimica etCosmochimica Acta 66A834

Yamaguchi A 2001 Mineralogical study of a highly metamorphosedeucrite Northwest Africa 011 (abstract 1578) 32nd Lunar andPlanetary Science Conference CD-ROM

Yamaguchi A Taylor G J Keil K Floss C Crozaz G Nyquist LE Bogard D D Garrison D Reese Y Wiesman H and ShihC-Y 2001 Post-crystallization reheating and partial melting ofeucrite EET 90020 by impact into the hot crust of 4 Vesta sim450Ga ago Geochimica et Cosmochimica Acta 653577ndash3599

Yamaguchi A Clayton R N Mayeda T K Ebihara M Oura YMiura Y N Haramura H Misawa K Kojima H and Nagao K2002 A new source of basaltic meteorites inferred fromNorthwest Africa 011 Science 296334ndash336

Zinner E and Crozaz G 1986a A method for the quantitativemeasurement of rare earth elements by ion microprobeInternational Journal of Mass Spectrometry and Ion Processes6917ndash38

Zinner E and Crozaz G 1986b Ion probe determination of theabundances of all the rare earth elements in single mineral grainsIn Secondary ion mass spectrometry SIMS V edited byBenninghoven A Colton R J Simons D S and Werner H WNew York Springer-Verlag pp 444ndash446