Rai, and two co-authors, reported in 2020 that ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. The authors used Monte Carlo modelling to explain the observed array of oxygen isotopes and major element chemistry shown by bulk ureilites, after restoration of their missing core and silicate melt components. Despite using a wide range of primitive nebular material, our modelling shows that only a combination of proxy material resembling Allende-type FeO-rich and MgO-rich chondrules, can account for the ureilite oxygen isotope trend and the reconstructed ureilite major element chemistry.
Their model predicts formation of a radial gradient in major elements and oxygen isotopes within the planetesimal, with a more Mg-rich silicate interior and a more Fe-rich silicate exterior. Temperatures recorded by ureilites were not high enough to form a magma ocean but were sufficiently high to form a metallic core and silicate melts. The ureilite parent planetesimal was then disrupted by impact. Re-accretion of the outer layers of more Fe-rich silicate material, at the expense of the more MgO-rich material and the core, explains the observed distribution of bulk rock and mineral compositions.
The authors concluded that many early Solar System processes are exemplified by ureilite meteorites. We have used constraints from the geochemical trends displayed by ureilites (Fig. 1), and their reconstructed bulk major element compositional range (Fig. 3), to produce a two component model consisting of MgO-rich and FeO-rich chondrules, that have been used in our compositional model as proxy for the real primitive material that formed the original ureilite planetesimal. This successfully explains the correlation between oxygen isotopes and Fo contents of olivine cores in ureilites. The model also explains the trace element systematics of the siderophile and rare earth elements, by formation of an FeS core and silicate melts, respectively.
Original working URL: https://www.geochemicalperspectivesletters.org/article2018/?hilite=%27Rare%20Earth%20Elements%27
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