Ureilite meteorites provide a new model of early planetesimal formation and destructionOPEN ACCESS 

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N. Rai, H. Downes, C. Smith

Geochemical Perspectives Letters 14, pp. 20–25

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“Ureilite meteorites are ultramafic rocks derived from parts of the depleted silicate mantle of their parent planetesimal. We 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. Our 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.”

Figure 4: Model of stages in history of ureilite planetesimal. (a) Accretion of MgO-rich and FeO-rich chondrules; MgO-rich chondrules accrete slightly earlier, leading to a compositional gradient within the growing planetesimal. (b) Heating and formation of eutectic metal-sulfide melt, migration of melt and coalescence to form a core. (c) Silicate partial melting and removal of magma. (d) Disruption of planetesimal by a glancing blow from an impactor; more of the shallow FeO-rich material was removed from the planetesimal than the deeper MgO-rich material, and the S-rich core was not disrupted. (e) Fractured breakaway portion. (f) Re-accreted present day ureilite asteroid composed of jumbled fragments of the breakaway portion. Darker shading = FeO-rich ureilites; lighter shading = MgO-rich ureilites; red = silicate melt-enriched material.

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