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Lunar and Planetary Science XXXVI (2005) 1639.pdf

PETROLOGY AND OF MIL 03346: A NEW FROM M. Anand ([email protected]), C.T. Williams, S.S. , G. , S. James, and M.M. Grady Department of Mineralogy, The Natural History Museum, Cromwell Road, , SW7 5BD, UK

MIL 03346 is a newly discovered meteorite origin. In textural appearance, this rocks appears most from Miller Range in Antarctica, which belongs to the similar to the nakhlite group nakhlite, of Martian NWA 817 [3]. [1]. It is an The unbrecciated, modal medium- mineralogy grained of MIL - 03346 is bearing dominated clinopy- by cumulus roxenite with pyroxenes a cumulate (~70%) fol- texture, lowed by similar to the glassy 6 other pre- mesostasis viously (~25%). known Olivine is . only present in minor amounts and no crystalline pla- This is only gioclase was observed in our sample. However, the the 2nd composition of the glassy mesostasis is most closely nakhlite in Antarctic collections, the other being Ya- mato 000593 and its pairs. The Meteorite Working Group (MWG) allocated us two polished sections (MIL 03346,102 & MIL 03346,116), and 1 g rock chip (MIL 03346,37) for carrying out petrological and geochemi- cal investigations. Petrography and Mineral Chemistry: The rock displays a cumulate texture (Fig. 1) consisting predominantly of zoned matched by a Na-K-rich-feldspar. The majority of the euhedral pyroxene grains show extensive zoning from core to clinopyroxene the rim. The outer 10-20 µm zones of pyroxene grains (0.5-1 mm display several generations of growth (sometimes up to long) and 4), clearly evident in back scatter electron images. The glassy cores of pyroxene crystals are remarkably homogene- mesostasis, ous in composition at Wo41En38Fs21 (Fig. 4). Different with rare generations of rim growth can be demarcated based on olivine grains Wo-En-Fs contents. In general, there is a gradual de- (up to 1 mm, Fig. 2). Accessory minerals mostly occur crease in Wo and En contents from core to rim except in the mesostasis, and include Ti-magnetite, fayalite, a in the outermost growth zone (overgrowth), which is sulphide phase, and free silica. Shock effects are evi- marked by an increase in Wo content (Fig. 4). Olivine dent in the form of extensive cracks and shock induced grains are also zoned in terms of Fe-Mg and vary from twinning in some pyroxenes. The mineralogy, Fe to Fo44 in the core to Fo13 in the rim regions. This is the Mn ratios in pyroxene and olivine (Fig. 3, after [2]) and highest forsterite content measured in any nakhlite oli- bulk-rock composition (Table 1) confirm vine. A secondary alteration phase has also been iden- tified in the cracks of the olivine grains, chemical com- Lunar and Planetary Science XXXVI (2005) 1639.pdf

position of which closely matches those of ‘iddingsite- sample by ICP-AES after digestion by hydrofluoric and smectite’ mineral assemblage reported from Nakhla, perchloric acids and re-dissolution in dilute nitric acid. Lafayette and other nakhlites. Fayalitic grains (Fo5-12) The instrument was calibrated with synthetic standards, are abundant in the mesostasis areas but because of and a number their skeletal growth and spongy texture, reliable com- of CRMs were positions were difficult to measure. The CaO and MnO also prepared contents in vary from 0.1 to 0.6 wt% and 0.8 to check data to 1.9 wt%, respectively. Feldspathic-glass composi- quality and to tions in the mesostasis are Ab71-62Or7-14. MIL 03346 correct for any also matrix effects. contains The same pure silica instrumental pods (~5 µm analytical in size) conditions distributed were used as throughout for the major the mesosta- elements sis. Opaque analysis. In minerals are terms of major abundant and trace and Ti- elements (9.3 magnetite is wt% MgO, 3.6 most com- wt% Al2O3, 46 mon (as ppm Sc), MIL euhedral and 03346 is a skeletal nakhlite, most similar to NWA 817. grains; Fig. Summary: MIL 03346 is a new member of the 5), with sig- nakhlite group of Martian meteorites. Texturally, this nificant pro- rock appears to be slightly different from other portions of nakhlites because it lacks plagioclase crystals in the Fe3+. Close mesostasis. This is probably a result of rapid cooling of inspection the MIL 03346 parental upon extrusion, pre- reveals venting crystallization of plagioclase crystals. Remark- submicron able skeletal Ti-magnetite and fayalite grains in the level mesostasis areas of this rock also support this interpre- exsolution, probably of ilmenite, in the Ti-magnetite tation. However, the occurrence and preservation of up grains but the composition of the exsolved phase could to 4 distinct growth zones on cumulus pyroxenes indi- not be determined. cates that the parental magma was subjected to com- Bulk-rock Major- and Trace-element chemistry: plex magma chamber processes before erupting on the The major- and trace-element composition of MIL , similar to those inferred for other 03346 was determined using an Axial Varian Vista Pro nakhlites. The similarity in chemical composition but ICP-AES (Table 1). Major elements were determined slightly more fractionated nature and faster cooling following a lithium metaborate fusion of 20 mg pow- history than other nakhlites suggest that MIL 03346 dered rock sample in a Pt crucible and re-dissolution in might have been derived from the upper levels of the dilute nitric acid. Primary calibrations were against lava flow sequence on the Martian surface and thus the international Certified Reference Materials (CRM). geochemical signature of this meteorite may provide Other CRMs were used as a check, where samples fell complementary information about petrogenesis of the outside the primary calibration range. A sample of nakhlite group of martian meteorites. Nakhla (BM 1913, 25) was also analyzed in the same References: [1] Ant. Met. News Lett. (2004), batch for comparison purposes. Trace-element concen- 27(2); [2] Papike et al. (2003) Am. Min., 88, 469-472; trations were determined on a 100 mg powdered rock [3] Sautter et al. (2002) EPSL, 195, 223-238.