NORTHWEST AFRICA 5131: ANOTHER … · NORTHWEST AFRICA 5131: ANOTHER TAFASSASSET-LIKE METACHONDRITE...

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, james.wittke@nau.edu; 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

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0

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d18

O, per mil

d17 O

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mil .

CR chondrites NWA 2994

NWA 3100 LEW 88763

Tafassasset NWA 5131

NWA 468 Sombrerete

TFL

CR

CR Related Meteorites

CV

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