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Each terrestrial planet displays a redox state different from all known chondritic meteorites. This suggests either an incomplete sampling of the nebular material through the meteoritic collection, or an important modification of the oxygen fugacity of terrestrial planets during their growth. Experimental studies allow to investigate chemical reactions occurring during planetary differentiation and to track any major change of planetary oxidation state.
Evolution of the total FeO/Fe ratio of terrestrial planets through core segregation
I used experimental data on metal-silicate equilibria to retrieve an empirical model that can estimate the final FeO content of any sample having liquid silicate and Fe-rich metal, subjected to high pressure and temperature. Using this empirical model predicting the FeO-content of planetary mantles, I demonstrated that Earth’s core segregation lead to an increase of the planet’s oxidation state. Earth likely accreted a majority of material with an oxidation state similar to enstatite chondrites and additional ~20% of oxidized material such as carbonaceous chondrites. These results are in agreement with a more efficient partitioning of Si than O into the Earth’s core, inducing an increase of FeO and decrease of SiO2 in the Earth’s mantle. Applying this same empirical model to other planetary objects, I established that the progressive increase with distance from the Sun of FeO-content in mantles of terrestrial planets and asteroids (Mercury, Venus, Earth, Mars then asteroid Vesta) must be related to the accretion of material formed at increasing oxygen fugacity. These results suggest that dynamical mixing during planetary growth did not erase zoning of the oxygen fugacity in the inner proto-planetary disk (Boujibar et al. 2015, Goldschmidt).
Can bridgmanite (Mg-perovskite) crystallization during magma ocean solidification change the oxidation state of the Earth’s mantle?
During the Earth’s accretion, the energy accumulated by radioactive decay and meteoritic bombardments allowed significant melting of its mantle and the formation of a large magma ocean. It was previously proposed that the crystallization of this magma ocean subsequently changed the redox state of the Earth. This process was suggested by the incorporation of Fe3+ in bridgmanite (Mg-silicate perovskite), the most abundant terrestrial mineral. However, previous studies that resulted in this hypothesis were conducted in relatively oxidized conditions. Using the high pressure equipment of the Laboratoire Magmas et Volcans (Clermont-Ferrand, France), I conducted experiments at very high pressure (25 GPa, 250000 times the atmospheric pressure) at the redox state of the Earth’s mantle solidification, when Fe metal was still fusing and progressively separating from the mantle. After collecting samples and analyzing their composition, we performed XANES (X-ray Absorption Near Edge Spectroscopy) measurements at the synchrotron Soleil (Saint-Aubin, France) to measure the valence state of Fe. We showed that the Fe3+-insertion in bridgmanite significantly dependent on the oxygen fugacity at the conditions of the Earth’s differentiation. Even with high Al-content, the crystallization of bridgmanite at the redox conditions of the Earth’s formation cannot yield high Fe3+ content. Hence, the crystallization of the magma ocean could not lead to substantial increase of the oxidation state of the Earth’s mantle (Boujibar et al. 2016, American Mineralogist). Change of iron’s valence in a crystallizing magma ocean has a minor role in modifying the Earth’s oxidation state in comparison to other processes such as oxygen exchange during core segregation.
Urey-Craig diagram showing the redox state of major chondrite groups compared to planetary compositions (top). a is a zoom of b
Illustration of reactions between core and mantle changing the FeO content of the Earth's mantle (bottom). If Si partitions into the core, released O can oxidize Fe metal: mantle's FeO increases (top). If O partitions into the core, FeO in the starting silicate is consumed, mantle's FeO decreases (Boujibar et al. 2015, Goldschmidt).
Example of sample from an experiment at 25 GPa containing a bridgmanite (=Mg-perovskite) crystal equilibrated with a silicate melt (top). XANES spectrum of bridgmanite collected at the synchrotron Soleil (Saint-Aubin, France) used to determine Fe3+ content (Boujibar et al. 2016, American Mineralogist).
Evolution of the Fe3+ content in Bridgmanite as a function of the oxygen fugacity of equilibrating samples. At low fO2, typical of core formation and magma ocean crystallization, the abundance of Fe3+ remains moderate (20%) in bridgmanite.
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