Abstract
An experimental study, implicated in the revealing of the conditions for the origin for Fe3+-bearing magnesiowüstite in the lithospheric mantle, was performed using Mössbauer spectroscopy of pre-synthesized samples. Experiments were carried out using a multi-anvil high-pressure split-sphere apparatus at 6.3–7.5 GPa, in the range of 1100–1650 °C in carbonate-metal, carbonate–oxide-metal, carbonate-oxide, carbide-oxide and carbonate–metal- sulphur systems. In three experimental series, oxygen fugacity gradient in the samples was created, which enabled the study of the processes of magnesiowüstite formation under oxidizing and reducing conditions (ΔlogfO2 (FMQ) values from −1 to −5). It was established that Fe3+-bearing magnesiowüstite can form both in assemblage with oxidized phases, such as carbonate or with reduced ones—metal, carbides, sulphides, graphite and diamond. According to the Mössbauer spectroscopy, the composition of synthesized magnesiowüstite varied within a range of Fe3+/ΣFe values from 0 to 0.3, with IV and VI coordination of Fe3+ depending on P, T, fO2, x-parameters. It was established that Fe3+-bearing magnesiowüstite formation processes under upper mantle P,T-conditions include redox reactions, with magnesiowüstite being (1) reductant or (2) product of interaction, (3) crystallization processes of magnesiowüstite from an oxidized melt, where magnesiowüstite acts as a sink for ferric iron and (4) iron disproportionation.
Highlights
Experimental studies devoted to the revealing of the formation processes of Fe3+ -bearing magnesiowüstite and wüstite at P, T, f O2 and environment composition parameters typical of the lithospheric mantle were carried out using a multi-anvil high-pressure split-sphere apparatus (BARS) [69]
We found that under redox gradient in the carbonate-iron system, ferric iron concentrations in magnesiowüstite consistently change from 0–5 at.% in the most reduced sample areas
FMQ-5 log units)), where magnesiowüstite is in an assemblage with cohenite and graphite/diamond, up to 34 at.% in the most oxidized, where magnesiowüstite co-exists with carbonate melt, graphite and diamond
Summary
Distribution of Mg, Fe-Oxides in the Earth’s Interior. According to experimental and theoretical data, ferropericlase and magnesiowüstite are considered the most abundant oxide minerals in the Earth’s interior; they are thought to comprise approximately. 16–20 wt.% of the lower mantle [1,2,3,4,5,6,7,8,9,10]. The presence of ferropericlase in the lower mantle is caused by the decomposition of transition zone minerals to Mg–Si perovskite and MgO at the 650-km discontinuity [11,12]. A distribution area of Mg,Fe-oxides in the Earth’s interior is not limited to the lower mantle. Minor amounts of ferropericlase and magnesiowüstite occur in the Earth’s crust, in the rocks altered in high-temperature contact metamorphic processes. The upper mantle occurrences of ferropericlase were proposed to exist after comprehensive study and criticism of “lower mantle”
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