The accretionary history of ureilites: A systematic re-evaluation.

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Ureilites are petrographically complex meteorites that - as mantle restites - show evidence for high-temperature alteration in addition to a few features known from primitive, unaltered meteorites. They constitute a significant portion of Solar System material, being the second most abundant type of non-primitive extraterrestrial rocks. Yet, several open questions regarding their mode of origin and steps of evolution remain. This project will address two key topics concerning ureilite components, namely the assumed exogenous origin of vein material (constituting up to 10 vol%) and the generation of reduction rims ubiquitously exhibited by major silicate minerals (olivine and pyroxene). Unraveling constraints on the evolution of those ureilite components will concurrently reveal important geological processes shaping the early Solar System.The working hypothesis for the evolution of ureilites is a modified impact scenario: It has been established that the ureilite parent body (UPB) suffered a disruptive impact very early in its history, i.e. while partial melting was still underway (Goodrich et al. 2015 and references therein). The impactor was large and - so assumed here - still incompletely solidified like the UPB. The UPB was also already structurally weakened from previous encounters (“pre-shattered”: Michel et al. 2004). Upon impact, a carbon-rich liquid metal phase got injected into the pore space of the UPB [in analogue to the formation of pallasites: Yang et al. (2010) and Scheinberg et al. (2015)]. The exogenous carbon-metal component then sparked reduction of olivine and pyroxene. Depending on fragment size and pore space geometry, the reduction reaction varied from inside out and probably ceased quickly (i.e., within a matter of days as based on preliminary results [Patzer et al. 2017]).