northwest africa 5131: another … · northwest africa 5131: another tafassasset-like metachondrite...
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NORTHWEST AFRICA 5131: ANOTHER TAFASSASSET-LIKE
METACHONDRITE RELATED TO THE CR CHONDRITE PARENT BODY
James Wittke1, Ted Bunch1,5, Anthony Irving2,5, Douglas Rumble, III3 and Paul Sipiera4,5
1Geology Program, SESES, Northern Arizona University, Flagstaff, AZ 86011, USA, [email protected]; 2Dept. of Earth & Space Sciences, University of Washington, Seattle, WA, USA; 3Geophysical Laboratory,
Carnegie Institution, Washington, DC, USA; 4Field Museum of Natural History, Chicago, IL, USA; 5Planetary Studies Foundation, Galena, IL, USA.
NWA 5131 is a 533 gram achondrite (Figure 1)
previously thought to have affinities with LL
chondrites (based largely on its olivine composition);
however, new oxygen isotopic data clearly indicate
that it is instead related to CR chondrites, and in fact
may be a piece of Tafassasset (Figures 2, 3). The
specimen is an inequigranular metamorphic
aggregate of olivine (60 vol%; Fa29.3, FeO/MnO = 68),
orthopyroxene (26 vol%; Fs24.7Wo2.), poikilitic
plagioclase, An35.5Or4, and interstitial plagioclase,
An46.6Or3 (6 vol%), kamacite and taenite (Ni = 6.2
wt% and 10.8 wt%, respectively; 8 vol%) with
accessory clinopyroxene, chromite (molar Cr/[Cr+Al]
= 0.82), troilite (Ni = 4-10.3 wt%) and merrillite (Mg =
3.6 wt%, Na2O = 2.6 wt%. Finer grained portions
consist of polygonal grains meeting at 120º junctions,
but there also are regions with poikiloblastic texture
(Figures 4, 5, 6). Orthopyroxene poikiloblasts up to
4mm across enclose smaller chadacrysts of olivine
and metal. Chromite and plagioclase poikiloblasts are
also present and enclose small, subrounded grains of
olivine. The highly serrated metal grain boundaries
against orthopyroxene and olivine, imply replace-
ment.. No chondrules were found, but the
poikiloblastic texture may represent former chondrule
regions that have been highly recrystallized.
Petrography
Oxygen Isotopes
Results obtained by laser fluorination of acid-washed
NWA 5131 material are essentially identical to results
for Tafassasset measured in the same laboratory, and
plot along the trend for CR chondrites (Figure 6).
[1] Bunch T. et al. (2008) Lunar Planet. Sci. XXXIX, #1991; [2]
Bunch T. et al. (2005) Lunar Planet. Sci. XXXVI, #2308; [3]
Yamaguchi A. et al. (2002) Science 296, 334-336; Floss C. et al.
(2005) MAPS 40, 343-360; Spivak-Birndorf L. et al. (2009) 72nd
Met. Soc Mtg., #5390; Bouvier A. and Wadhwa M. (2010) Lunar
Planet. Sci. XLI, #489.
References
Figure 6. Above: False colored backscattered electron
image of Tafassasset. Green = olivine; yellow =
orthopyroxene; pale blue = metal and troilite; red =
plagioclase. Below: False colored backscattered
electron image of NWA 5131. Colors as for Tafassasset
with dark blue = chromite..
envisioned. Like the CV-CK chondrite parent body, it
may well have a metallic core surrounded by a
metamorphic mantle (perhaps containing plutonic
igneous bodies) and a chondrule-rich regolithic crust.
Previously, we have shown [1] that Tafassasset, NWA
2994 and NWA 3100 exhibit fractionated, non-
chondritic compositional characteristics. Hypotheses
about the size and/or thermal history of the CR
chondrite parent body may need to be reconsidered
in light of discoveries of highly equilibrated specimens
like NWA 5131 and Tafassasset.
Figure 2. Plane light photomicrograph of NWA 5131 showing
the strongly recrystallized texture that includes orthopyroxene
poikiloblasts. Base width = 9 mm.
Figure 3. Plane light photomicrograph of Tafassasset, showing
the same strongly recrystallized texture, although with a
slightly smaller mean grain size. Base width = 9 mm.
Figure 5. Plane light photomicrograph of NWA 5131 showing
chadacrysts of olivine (pinks and blues) within plagioclase
oikocyrsts (white-gray). Image width = 4 mm.
Figure 1. Sawn surface of NWA 5131 that shows
heterogeneous distribution of metal similar to that of some
mesosiderites. Modal analysis of this surface gives a metal +
sulfide content of 16 volume% in contrast to the less than 10
volume% from a thin section, illustrating the heterogeneous
distribution of metal. Photo courtesy of Fabien Kuntz.
Figure 4. SEM-BSE image of NWA 5131 that shows chromite
(Chr) and plagioclase (Pl) poikiloblasts with olivine (Ol)
inclusions set within a large olivine grain, an orthopyroxene
(Opx) poikiloblast is present in the bottom right of the image.
Discussion
After consideration of all of the macroscopic,
microscopic and mineralogical evidence, we conclude
that NWA 5131 is very likely a stone from the massive
(110 kg) Tafassasset fall. This meteorite provides
evidence for the existence of at least some
extensively thermally metamorphosed regions within
the CR chondrite parent body. Although the vast
majority of CR chondrites are hydrated (petrologic
type 2), there are two known highly equilibrated CR6
chondrites (NWA 2994, NWA 3100) [1].
Both Tafassasset and NWA 5131 are best termed CR
metachondrites, since they are completely lacking in
chondrules. As we have already argued [1, 2], such
thermally metamorphosed specimens, as well as the
paired, very ancient igneous-textured achondrites
NWA 011/2400/2976/4901/4587 with very similar
oxygen isotopic compositions [3], imply much more
complexity within the CR parent body than originally
-8
-4
0
4
-5 -2 1 4 7 10
d18
O, per mil
d17 O
, p
er
mil .
CR chondrites NWA 2994
NWA 3100 LEW 88763
Tafassasset NWA 5131
NWA 468 Sombrerete
TFL
CR
CR Related Meteorites
CV
CO