A carbon-rich region in Miller Range 091004 and implications for ureilite petrogenesis

TitleA carbon-rich region in Miller Range 091004 and implications for ureilite petrogenesis
Publication TypeJournal Article
Year of Publication2017
AuthorsDay JMD, Corder C.A, Cartigny P., Steele A., Assayag N., Rumble D., Taylor L.A
JournalGeochimica Et Cosmochimica Acta
Date Published2017/02
Type of ArticleArticle
ISBN Number0016-7037
Accession NumberWOS:000390987900023
Keywordsachondrites; chondrites; feo-rich; Graphite; Highly siderophile elements; Late-stage carbon; Melt-depletion; meteorites; MIL 091004; origin; oxygen-isotope; parent body; polymict; reduction; trace-element; Ureilite

Ureilite meteorites are partially melted asteroidal-peridotite residues, or more rarely, cumulates that can contain greater than three weight percent carbon. Here we describe an exceptional C-rich lithology, composed of 34 modal % large (up to 0.8 mm long) crystalline graphite grains, in the Antarctic ureilite meteorite Miller Range (MIL) 091004. This C-rich lithology is embedded within a silicate region composed dominantly of granular olivine with lesser quantities of low-Ca pyroxene, and minor FeNi metal, high-Ca pyroxene, spinel, schreibersite and troilite. Petrological evidence indicates that the graphite was added after formation of the silicate region and melt depletion. Associated with graphite is localized reduction of host olivine (Fo(88-89)) to nearly pure forsterite (Fo(99)), which is associated with FeNi metal grains containing up to 11 wt.% Si. The main silicate region is typical of ureilite composition, with highly siderophile element (HSE) abundances similar to 0.3 x chondrite, Os-187/Os-188 of 0.1260-0.1262 and Delta O-17 of -0.81 +/- 0.16 parts per thousand. Mineral trace-element analyses reveal that the rare earth elements (REE) and the HSE are controlled by pyroxene and FeNi metal phases in the meteorite, respectively. Modeling of bulk-rock REE and HSE abundances indicates that the main silicate region experienced similar to 6% silicate and >50% sulfide melt extraction, which is at the lower end of partial melt removal estimated for ureilites. Miller Range 091004 demonstrates heterogeneous distribution of carbon at centimeter scales and a limited range in Mg/(Mg + Fe) compositions of silicate grain cores, despite significant quantities of carbon. These observations demonstrate that silicate rim reduction was a rapid disequilibrium process, and came after silicate and sulfide melt removal in MIL 091004. The petrography and mineral chemistry of MIL 091004 is permissive of the graphite representing late-stage C-rich melt that pervaded silicates, or carbon that acted as a lubricant during anatexis and impact disruption in the parent body. Positive correlation of Pt/Os ratios with olivine core compositions, but a wide range of oxygen isotope compositions, indicates that ureilites formed from a compositionally heterogeneous parent body that experienced variable sulfide and metal melt-loss that is most pronounced in relatively oxidized ureilites with Delta O-17 between similar to 1.5 and similar to 0 parts per thousand. (C) 2016 Elsevier Ltd. All rights reserved.

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