Highly Reduced Metals and Sulphides in Ureilites: Remnants of the Upb Core?

Introduction: Ureilites are ultramafic, carbon-rich achondrite meteorites, which are believed to represent mantle residues from the ureilite parent body (UPB). Approximately 1015% are polymict (brecciated) ureilites, which contain many exotic lithic and mineral clasts [e.g. 1,2]. Whether metal and sulphide phases in ureilites are indigenous to the UPB or foreign (i.e. impact-derived) is highly controversial. We present EPMA major element data (analysed at the NHM) for metal and sulphide phases in five paired Frontier Mountains ureilites: 90036, 90054, 90168, 90233, 90228 [3]. Results: The samples contained silicide, phosphide, sulphide and Fe,Ni metal phases. Metals and sulphides occur in three different contexts: as veins (FRO 90054); closely associated with carbon (FRO 90233); or interstitial to silicates (FRO 90228). Some samples contain metals in more than one of these. We observed significant interand intra-sample variations in composition, abundance and size. Individual metal grains, however, appeared internally homogeneous on a micron scale. Si and P are present in metal and sulphides (tabulated below). Cr is abundant in some sulphides (up to 7 wt%); these also have minor wt% Ti, Mn and Cu. Compositions are consistent with those in other work [e.g. 4,5]. There are also rare grains that appear to be a mixtures of phases, such as high-Si and high-P kamacites and Fe,Ni metal with higher Ni than typical kamacite (up to 7.6wt% Ni) [4]. Co is also present (up to 0.5 wt%). Ranges for each phase are in wt%: Phase Nominal Formula Fe Si P S Ni