Abstract number 1451: 32nd Lunar and Planetary Science Conference, March 2001, Houston, Texas.

TRACE-ELEMENT CONCENTRATIONS IN NORTHWEST AFRICA 032. 1R. L. Korotev, 1B. L. Jolliff, 1A. Wang, 1J. J. Gillis, 1L. A. Haskin, 2T. J. Fagan, 2G. J. Taylor, and 2K. Keil; 1Department of Earth and Planetary Sciences, Campus Box 1169, Washington University, Saint Louis, MO, 63130; 2Hawai’i Institute of Geophysics and Planet-ology, School of Ocean and Earth Science and Technology, University of Hawai’i at Manoa, Honolulu, HI, 96822 (rlk@levee.wustl.edu)

Northwest Africa 032 (NWA 032) is the third lunar meteorite which is a crystalline mare basalt [1]. We have obtained data for trace element concentrations on four samples of NWA 032 by instrumental neutron ac-tivation analysis (INAA) and we have examined weathering products by Raman spectroscopy.

Table 1. Results of INAA for four samples of NWA 032. sample A B C D mean ± s.s.d. N 4 4 2 2 12 1 4 Na2O % 0.348 0.362 0.330 0.344 0.347 0.004 0.013 Sc µg/g 56.0 55.0 52.4 58.2 55.7 0.6 2.4 Cr µg/g 2690 2570 3190 2640 2740 30 285 FeO % 22.22 22.10 22.32 21.90 22.12 0.22 0.18 Co µg/g 42.5 41.0 44.4 40.0 41.8 0.5 1.9 Ni µg/g 45 45 65 50 50 40 10 Sr µg/g 126 139 176 140 142 24 22 Zr µg/g 170 175 165 180 175 30 6 Ba µg/g 224 152 397 232 242 12 104 La µg/g 11.00 11.41 11.06 11.49 11.24 0.11 0.25 Ce µg/g 29.4 30.7 29.2 29.6 29.7 1.0 0.7 Nd µg/g 20. 19. 20. 23. 21. 3 2 Sm µg/g 6.52 6.72 6.39 6.77 6.61 0.07 0.18 Eu µg/g 1.08 1.13 1.07 1.11 1.10 0.02 0.03 Tb µg/g 1.53 1.60 1.52 1.60 1.56 0.03 0.04 Yb µg/g 5.71 5.89 5.58 5.94 5.79 0.06 0.16 Lu µg/g 0.789 0.819 0.781 0.815 0.802 0.010 0.019 Hf µg/g 4.93 5.05 4.85 5.13 5.00 0.07 0.13 Ta µg/g 0.61 0.64 0.60 0.62 0.62 0.04 0.02 Au ng/g 5 4 5 0 4 2 2 Th µg/g 1.86 1.92 1.79 1.99 1.90 0.04 0.09 U µg/g 0.41 0.44 0.54 0.45 0.45 0.09 0.06 mass mg 58.67 41.12 33.78 50.38 184.

Columns A, B, C, and D each contain mass-weighted mean concen-trations of N subsamples; “mean” is the mass weighted mean of all 12 subsamples. “anal. unc.” = estimated analytical uncertainty (1 std. dev.) of a single analysis. “s.s.d.” = sample standard deviation of columns A–D. Other elements: As (<2 µg/g), Br (<1.5 µg/g), Rb (<15 µg/g), Sb (<0.15 µg/g), Cs (<0.4 ppm), W (<4 µg/g). Samples were not dried for INAA, which might account for lower FeO com-pared to [1] (23.0%).

Samples. Samples A and B were broken from op-posite ends of a 1-g slab. Samples C and D were each obtained from different sources. For INAA, samples A and B were each subdivided into 4 subsamples and samples B and C were each subdivided into 2 subsam-ples. In total, 12 subsamples with a combined mass of 184 mg were analyzed (Table 1).

Weathering. We investigated several small chips of NWA 032 that contain exterior surfaces. These sur-faces are coated extensively with a thin crust of white granular material identified by laser Raman spectros-copy to be calcite. Other surface coatings are reddish to orange in color and appear to be some ferric oxide or oxyhydroxide alteration, but these are extremely fine-grained and did not yield distinctive Raman spectra, only the spectra of underlying mafic silicates, espe-

cially olivine. In some areas, a pinkish color zone sur-rounds calcite, but this feature also did not yield dis-tinctive Raman spectra. Sawn interior surfaces are completely free of alteration.

Composition. Despite their small sizes (8–26 mg), the analyzed subsamples are all very similar in compo-sition. Most of the compositional variation involves variation in the ratio of olivine+chromite (which occur together in glomerocrysts, see image) to plagioclase+ clinopyroxene (which occur together in mesostasis) (Fig. 2). Specifically, the composition of highest-FeO subsample corresponds to a mixture of (1) 87% of the lowest-FeO subsample and (2) 13% of an hypothetical assemblage of Fo45 olivine (97%) and chromite (3%) with 31% Cr2O3. Remarkably, this range in olivine abundance among the small subsamples of NWA 032 is equivalent to the range observed among 25 different large (~5-g) samples of Apollo 15 olivine-normative basalts [2].

Sample C is enriched in Sr (30%), Ba (96%), and U (24%) and is depleted in most other elements by about 4% compared to the mean of the other samples, pre-sumably as a result of weathering (Fig. 3).

Figure 1. Photographs of NWA 032. Top image is a sawn interior surface, olivine phenocrysts and thin plagioclase laths are prominent. Lower left shows calcite (white) rimmed by pink alteration. Bottom center shows exterior surface coated by white (calcite) and reddish-brown (ferric oxide?) crust. Lower right shows olivine phenocryst with chromite inclusion.

TRACE ELEMENTS IN NORTHWEST AFRICA 032: R. L. Korotev et al.

The bulk composition [1] of NWA 032 does not match that of any known mare basalts, including Asuka 881757 and Yamato 793169, but it resembles the Apollo 12 ilmenite and the Apollo 15 olivine basalts. Absolute concentrations of trivalent REE are at the high end of the range of low-Ti basalts (Fig. 4). Both relative and absolute REE concentrations strongly re-semble the group-3 aluminous basalts of Apollo 14 [3], but the Eu/Sm ratio is at the low end (deep Eu anom-aly) of the range for mare basalts with similar total REE abundances. The Th concentration is greater than that of any low-Ti basalt except the group-1 aluminous basalts of Apollo 14 [3]. The most intriguing composi-tional feature of NWA 032 is that it has the highest Th/REE ratio of any known mare basalt (Fig. 5). This feature suggests that NWA 032 may derive from a re-

gion distant from the Apollo sites. Using composition-ally constrained, global remote sensing data (FeO, 18–22 wt%, LP-GRS; TiO2, 1.5–5 wt%, Clementine; Th, 1–3 ppm LP-GRS [4,5]) we find that the most likely source areas for NWA 032 are, in order of probability (on the basis of compositional match and areal extent); M. Humorum, M. Fecunditatis, western M. Serenitatis, Crisium, and far western Procellarum.

Acknowledgment: This work was funded by NASA grants NAG5-4172 and NAG5-8609.

References: [1] Fagan et al. (2000) Meteorit. Planet. Sci. 35, A51; [2] Ryder & Schuraytz (in press) J. Geophys. Res; [3] Dickin-son et al. (1985) PLPSC15, C365–C374; [4] Lucey et al. (2000), J. Geophys. Res. 105, 20,297–20,305; [5] Lawrence et al. (2000) J. Geophys. Res. 105, 20,307–20,331.

TiO2 (%)

0 2 4 6 8 10 12 14

Th

/Sm

0.0

0.1

0.2

0.3

0.4

0.5

NWA 032

Apollo 15, 16, & 17

mean

Apollo 12 & 14

range of KREEP-bearing

impact-melt breccias

mare basalt types(mean ± 95% C.L.)

A14 A12

A15

A17

Luna

R2 = 0.513

A11-A

A11

Sc

(µ

g/g

)

48

50

52

54

56

58

60

62

Na

2O

(%

)

0.32

0.33

0.34

0.35

0.36

0.37

0.38

FeO (%)21 22 23 24

Co

(µ

g/g

)

36

38

40

42

44

46

48

50

52

R2 = 0.666

R2 = 0.562

R2 = 0.884

A

B

C

D

La Ce Nd Sm Eu Tb Yb Lu

ch

on

dri

te n

orm

aliz

ed

co

nc

en

tra

tio

n

910

15

20

30

40

50

60

70

80

A15 Ol

A15 Qtz

A12 Ol

A12 Pig

A12 IlmNWA 032A14 Al-3

A14 Al-2Luna 16

some mare basalttypes with <5% TiO2

Asuka 88

Figure 4. Rare earth element concentrations in some low-Ti mare basalts (normalized to volatile-free CI chondrites).

Ba (µg/g)

100 200 300 400 500

Sr

(µ

g/g

)

100

120

140

160

180

200

ABCD

Figure 3. The two subsamples of sam-ple C are enriched in Ba and Sr.

Figure 5. Variation of Th/ Smwith TiO2 in mare basalts.The error bars represent 95%confidence limits and thediagonal line is a least-squares fit weighted by theconfidence limits. There is aweak tendency for Th/ Sm todecrease with increasingTiO2. NWA 032 has an anom-alously high Th/Sm ratio.

Figure 2. Variation of Sc (carried mainlyby pyroxene), Na2O (plagioclase), and Co(olivine) with FeO in subsamples ofNWA 032.