Diverse Metals and Sulfides in Polymict Ureilites Eet 83309 and Eet 87720

Introduction: Ureilites are a group of carbonbearing ultramafic achondrites. The majority of samples are monomict with major and trace element compositions consistent with a restitic origin after extensive loss of basaltic melts and significant loss of their metallic component during anatexis. Monomict ureilites are thought to represent largely intact samples of the ureilite parent body (UPB) mantle. Polymict ureilites, by contrast, are fragmental breccias consisting of welded lithic clasts and isolated mineral fragments thought to be regolith that assembled after major disruption fragmented large portions of the UPB mantle. In most polymict ureilites, the majority of clasts consist of material similar to monomict ureilites gardened from the UPB mantle but other materials, both endogenic and xenogenic to the UPB are also found in polymict ureilites, including clasts texturally and compositionally similar to known chondrite types as well as feldspathic melt rocks and clasts of Ca–Al–Ti-rich assemblages [1-4]. In this study, we demonstrate that polymict ureilites also contain a variety of metal and sulfide compositions of diverse origins. They offer insight into the final equilibrium conditions of disrupted portions of the UPB mantle and the diversity of materials locally available for regolith formation, and provide evidence for only limited post-regolith formation thermal metamorphism. Methods: We analyzed major and trace elements in metal and sulfide phases in three polished thick sections each of EET 83309 and EET 87720 (pairing relationship uncertain) from the U.S. Antarctic Meteorite Collection first by EPMA at NASA Johnson Space Center and then by LA-ICP-MS at the National High Magnetic Field Laboratory/Florida State University using a New Wave UP213 (Nd:YAG 213 nm) laser ablation system coupled to a Finnigan Element ICP-MS. A working range of spot sizes of 40-100 μm were used. The IVB iron meteorite Hoba, the IIA iron meteorite Filomena, NIST SRM 1263a, and an in-house Peruvian pyrite standard (#1419) were used as external calibration standards. Corrected intensities were converted to elemental abundances by normalization wherein the sum of all analytes totals 100 wt% [5]. Analytical precision for most elements was <10% based on repeat measurements of calibration standards. In addition, we analyzed metals in DAG 999, DAG 1000, DAG 1023, and monomict ureilite NWA 1241 by EPMA. Results and Discussion: Five distinct metal and sulfide phases were identified in samples EET 83309 and EET 87720. With the possible exception of metallic veins, this disequilibrium assemblage was inherited from breccia assembly. Temperatures of subsequent metamorphism were insufficient to have equilibrated metal and sulfide within these samples. Troilite containing 0.1-0.9 wt% Ni and <0.1 wt% Co typically occurs in association with Fe,Ni or Fe,Si metals. In at least one instance, it appears to form a cross-cutting vein together with kamacite. Pentlandite containing 6.6-10.9 wt% Ni and 0.2-0.3 wt% Co occurs within chondrite-like fragments resembling R-chondrites. These clasts contain no discernable Fe,Ni-metal phase. These clasts and the sulfides they contain are unrelated to any known portion of the UPB and never equilibrated with ureilitic material even after incorporation into the regolith.