Feldspathic Clasts in Polymict Ureilites

Introduction: Ureilites are ultramafic meteorites composed mainly of magnesian (mg#=76-95) olivine and pyroxene, with minor carbon-bearing phases and metal. Ureilite characteristics suggest that monomict ureilites are residues of partial melting on a carbonaceous chondrite-like asteroidal parent body [1]. Basaltic meteorites complementary to the monomict ureilites are currently unknown, but would provide compositional, isotopic, and geochronologic information critical to constraining the composition and evolution of the ureilite parent body (UPB). Polymict ureilites [2-4], regolith breccias composed mainly of monomict ureilite material, contain about 2% (vol.) feldspathic clasts. Oxygen isotopes [5] suggest these clasts are ureilitic, so they may represent ureilitic basalts. Among these feldspathic clasts, we are interested in distinguishing between primary igneous rocks and secondary products such as impact melts. This distinction is not straightforward for the UPB. Explosive volcanism on the UPB [6-8] might produce glassy and quench-textured rocks similar in texture to impact-melted rocks. Because metal separation and core formation probably did not occur on the UPB, siderophile elements are not depleted in ureilites and contamination by chondritic impactors will not be evident. Both igneous and secondary products may be strongly reduced by ubiquitous carbon in ureilites. Petrologic and chemical modeling requires knowledge of all coexisting phases and an accurate bulk composition, which may not be obtainable from single, small clasts. A survey of feldspathic clasts can identify populations of materials, whose characteristics can then be better understood and modeled. Feldspathic Clast Populations: We used X-ray element maps to easily identify feldspathic clasts >80 mm in size within thin sections of five polymict ureilites (DaG 164, 165, 319, and 665, and EET83309).