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Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle.

ABSTRACT: Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite-type FeO₂ and FeO₂H_x phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high-pressure transformation products of Fe³⁺OOH goethite by in situ X-ray absorption spectroscopy in laser-heated diamond-anvil cell. At pressures and temperatures of ~91 GPa and 1,500-2,350 K, respectively, that is, in the previously reported stability field of FeO₂H_x, a measured shift of -3.3 ± 0.1 eV of the Fe K-edge demonstrates that iron has turned from Fe³⁺ to Fe²⁺. We interpret this reductive valence change of iron by a concomitant oxidation of oxygen atoms from O^2- to O^-, in agreement with previous suggestions based on the structures of pyrite-type FeO₂ and FeO₂H_x phases. Such peculiar chemistry could drastically change our view of crystal chemistry in deep planetary interiors.

SUBMITTER: Boulard E

PROVIDER: S-EPMC6472328 | biostudies-literature | 2019 Feb

REPOSITORIES: biostudies-literature

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Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle.

Boulard E E Harmand M M Guyot F F Lelong G G Morard G G Cabaret D D Boccato S S Rosa A D AD Briggs R R Pascarelli S S Fiquet G G

Geophysical research letters 20190214 3

Recent experiments have demonstrated the existence of previously unknown iron oxides at high pressure and temperature including newly discovered pyrite-type FeO2 and FeO2Hx phases stable at deep terrestrial lower mantle pressures and temperatures. In the present study, we probed the iron oxidation state in high-pressure transformation products of Fe3+OOH goethite by in situ X-ray absorption spectroscopy in laser-heated diamond-anvil cell. At pressures ...[more]

PMID: 31007309

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Project description:Water (H2O) as one of the most abundant fluids present in Earth plays crucial role in the generation and transport of magmas in the interior. Though hydrous silicate melts have been studied extensively, the experimental data are confined to relatively low pressures and the computational results are still rare. Moreover, these studies imply large differences in the way water influences the physical properties of silicate magmas, such as density and electrical conductivity. Here, we investigate the equation of state, speciation, and transport properties of water dissolved in Mg1-xFexSiO3 and Mg2(1-x)Fe2xSiO4 melts (for x = 0 and 0.25) as well as in its bulk (pure) fluid state over the entire mantle pressure regime at 2000-4000 K using first-principles molecular dynamics. The simulation results allow us to constrain the partial molar volume of the water component in melts along with the molar volume of pure water. The predicted volume of silicate melt + water solution is negative at low pressures and becomes almost zero above 15 GPa. Consequently, the hydrous component tends to lower the melt density to similar extent over much of the mantle pressure regime irrespective of composition. Our results also show that hydrogen diffuses fast in silicate melts and enhances the melt electrical conductivity in a way that differs from electrical conduction in the bulk water. The speciation of the water component varies considerably from the bulk water structure as well. Water is dissolved in melts mostly as hydroxyls at low pressure and as -O-H-O-, -O-H-O-H- and other extended species with increasing pressure. On the other hand, the pure water behaves as a molecular fluid below 15 GPa, gradually becoming a dissociated fluid with further compression. On the basis of modeled density and conductivity results, we suggest that partial melts containing a few percent of water may be gravitationally trapped both above and below the upper mantle-transition region. Moreover, such hydrous melts can give rise to detectable electrical conductance by means of electromagnetic sounding observations.

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Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle.

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Ferrous Iron Under Oxygen-Rich Conditions in the Deep Mantle.

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