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Incorporation of Fe (super 2+) and Fe (super 3+) in bridgmanite during magma ocean crystallization

Asmaa Boujibar, Nathalie Bolfan-Casanova, Denis Andrault, M. Ali Bouhifd and Nicolas Trcera
Incorporation of Fe (super 2+) and Fe (super 3+) in bridgmanite during magma ocean crystallization
American Mineralogist (July 2016) 101 (7): 1560-1570

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 (D (sub Fe) (super Bg/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 approximately 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 (f (sub 02) ) of this study ranging from IW-1.5 and IW-2, the measured Fe (super 3+) content of Bg is found moderate (Fe (super 3+) /Sigma Fe = 21 + or - 4%) and weakly correlated with Al content. When f (sub O2) is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When f (sub O2) remains between IW and Re/ReO (sub 2) buffers, Fe (super 3+) /Sigma Fe ratio becomes independent of f (sub O2) 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-Fe (super 3+) 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% Fe (super 3+) 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.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 101
Serial Issue: 7
Title: Incorporation of Fe (super 2+) and Fe (super 3+) in bridgmanite during magma ocean crystallization
Affiliation: Universite Blaise Pascal, Laboratoire Magmas et Volcans, Clermont-Ferrand, France
Pages: 1560-1570
Published: 201607
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 58
Accession Number: 2016-079217
Categories: General mineralogySolid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Secondary Affiliation: Synchrotron SOLEIL, FRA, France
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201638
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