A pyroxene-oldhamite clast in Bustee: Igneous aubritic oldhamite and a mechanism for the Ti enrichment in aubritic troilite

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A pyroxene-oldhamite clast in the Bustee aubrite contains major enstatite and minor diopside and forsterite as the silicate host, with abundant (∿30vol%), included 1-5mm spherules of oldhamite. Within the oldhamite (∿5vol% of the oldhamite) are a variety of phases, including osbornite (TiN), titanoan troilite (17.2-25.2wt% Ti), heideite (28.7-29.4wt% Ti), niningerite, daubreelite, forsterite, and Fe metal. This clast represents only the second occurrence of an oldhamite-rich lithology in aubrites. An igneous origin for oldhamite within the Bustee pyroxene-oldhamite clast is suggested by its large size, rounded shapes, presence of included phases formed from multiple, immiscible sulfides, and inclusion of phases unknown from enstatite chondrites. The Bustee clast contains a variety of Ti-rich phases. This work suggests Ti-rich troilite observed in aubrites formed by co-crystallization with or exsolution from Ti-poor sulfides (e.g., oldhamite). In the Bustee case, the undetectably low Ti concentrations in oldhamite and experimental Ti partition coefficient between FeS and CaS approaching infinity support such a suggestion. This mechanism solves the long-standing problem of enriching aubritic troilite in Ti and weakens arguments against a direct derivation of aubrites from known enstatite chondrites.
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