Revisited: The Ni isotopic compositions of ureilite metal and olivines.

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This study is tailored to revisit and expand on the hypothesis of exogenous, impact-delivered ureilite vein material and follows up on the results of my recent work with ureilites. The working hypothesis is that the ureilite parent body (UPB) was a regular differentiated body consisting of a metal core, ultramafic mantle and a thin basaltic crust. The impactor was also large, incompletely solidified and possibly primarily metallic. Upon collision, a liquid metal phase from the impactor mixed with the silicate debris of the UPB and was injected into fractures and pore space (similar to the suggested formation of the pallasites). The exogenous carbon-metal component then sparked the reduction of olivine and, to a lesser extent, pyroxene. Depending on fragment size and pore space geometry, the reduction reaction varied from inside out and ceased quickly within a matter of days (see predecessor study). The Ni isotopic investigation proposed here is the last phase of the ureilite project I first presented in 2017 and started in August 2020. It is to follow up on my recent findings when exploring the reduction of ureilite olivines that took place contemporaneously with the catastrophic disruption of the UPB. The Ni isotopic fingerprints of ureilite olivines and vein metal will lead to crucial genetic constraints on their origins. If both constituents are confirmed to be unrelated, this would lend strong support to my earlier findings that ureilite olivines and vein metal are not co-genetic, providing the third and final line of evidence to my working hypothesis.