A. Boujibar

Cosmochemical fractionation by collisional erosion during the Earth's accretion.

Early in the Solar Systems history, energetic collisions of differentiated bodies affected the final composition of the terrestrial planets through partial destruction. Enstatite chondrites (EC) are the best candidates to represent the primordial terrestrial precursors as they present the most similar isotopic compositions to Earth. Here we report that collisional erosion of >15% of the early Earths mass can reconcile the remaining compositional differences between EC and the Earth. We base our demonstration on experimental melting of an EC composition at pressures between 1 bar and 25 GPa. At low pressures, the first silicate melts are highly enriched in incompatible elements Si, Al and Na, and depleted in Mg. Loss of proto-crusts through impacts raises the Earths Mg/Si ratio to its present value. To match all major element compositions, our model implies preferential loss of volatile lithophile elements and re-condensation of refractory lithophile elements after the impacts.

Incorporation of Fe2+ and Fe3+ in bridgmanite during magma ocean crystallization

Abstract Using large volume press, samples of bridgmanites (Bg) in equilibrium with both silicate melt and liquid Fe-alloy were synthesized to replicate the early period of core-mantle segregation and magma ocean crystallization. We observe that the Fe partition coefficient between Bg and silicate melt (DFeBg/melt) varies strongly with the degree of partial melting (F). It is close to 1 at very low F and adopts a constant value of ~0.3 for F values above 10 wt%. In the context of a partially molten mantle, a larger F (closer to liquidus) should yield Fe-depleted Bg grains floating in the liquid mantle. In contrast, a low F (closer to solidus) should yield buoyant pockets of silicate melt in the dominantly solid mantle. We also determined the valence state of Fe in these Bg phases using X-ray absorption near-edge spectroscopy (XANES). Combining our results with all available data sets, we show a redox state of Fe in Bg more complex than generally accepted. Under the reducing oxygen fugacities (fO2) of this study ranging from IW-1.5 and IW-2, the measured Fe3+ content of Bg is found moderate (Fe3+/ΣFe = 21 ± 4%) and weakly correlated with Al content. When fO2 is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When fO2 remains between IW and Re/ReO2 buffers, Fe3+/ΣFe ratio becomes independent of fO2 and exclusively correlated with Al content. Due to the incompatibility of Fe in Bg and the variability of its partition coefficient with the degree of melting, fractional crystallization of the magma ocean can lead to important chemical heterogeneities that will be attenuated ultimately with mantle stirring. In addition, the relatively low-Fe3+ contents found in Bg (21%) at the reducing conditions (IW-2) prevailing during core segregation seem contradictory with the 50% previously suggested for the actual Earth’s lower mantle. This suggests the presence of 1.7 wt% Fe3+ in the lower mantle, which reduces the difference with the value observed in the upper mantle (0.3 wt%). Reaching higher concentrations of trivalent Fe requires additional oxidation processes such as the late arrival of relatively oxidized material during the Earth accretion or interaction with oxidized subducting slabs.