NORTHWEST AFRICA 5131: ANOTHER TAFASSASSET-LIKE METACHONDRITE RELATED TO THE CR CHONDRITE PARENT BODY James Wittke 1 , Ted Bunch 1,5 , Anthony Irving 2,5 , Douglas Rumble, III 3 and Paul Sipiera 4,5 1 Geology Program, SESES, Northern Arizona University, Flagstaff, AZ 86011, USA, [email protected]; 2 Dept. of Earth & Space Sciences, University of Washington, Seattle, WA, USA; 3 Geophysical Laboratory, Carnegie Institution, Washington, DC, USA; 4 Field Museum of Natural History, Chicago, IL, USA; 5 Planetary 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%; Fa 29.3 , FeO/MnO = 68), orthopyroxene (26 vol%; Fs 24.7 Wo 2. ), poikilitic plagioclase, An 35.5 Or 4 , and interstitial plagioclase, An 46.6 Or 3 (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%, Na 2 O = 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) 72 nd 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 d 18 O, per mil d 17 O, per mil CR chondrites NWA 2994 NWA 3100 LEW 88763 Tafassasset NWA 5131 NWA 468 Sombrerete TFL CR CR Related Meteorites CV CO