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Pressure dependence of metal-silicate partitioning explains the mantle phosphorus abundance.

ABSTRACT: Previous experiments performed below 20 GPa suggested that the metal/silicate partition coefficient of phosphorus (P), D_P, extrapolated to typical high-pressure and -temperature conditions of the Earth's core formation gives too high P concentration in the core unless a large amount of silicon was included in metals. Here we examined D_P between liquid metal and coexisting molten silicate at 27-61 GPa and 3820-4760 K, corresponding to conditions of core-forming metal segregation from silicate, by measuring recovered samples using a high-resolution imaging technique coupled with secondary ion mass spectrometry. The results demonstrate that the pressure dependence of D_P changes from positive to negative above 15 GPa, likely because of an increase in the coordination number of P⁵⁺ in silicate melt. With the present new partitioning data, the observed mantle P abundance may indicate ~ 0.2 wt% P in the core, consistent with the cosmo-/geochemical estimates, based on both single-stage and multi-stage core formation models without involving high amounts of silicon in metals.

SUBMITTER: Ikuta N

PROVIDER: S-EPMC10786851 | biostudies-literature | 2024 Jan

REPOSITORIES: biostudies-literature

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Pressure dependence of metal-silicate partitioning explains the mantle phosphorus abundance.

Ikuta Nagi N Sakamoto Naoya N Tagawa Shoh S Hirose Kei K Tsutsumi Yutaro Y Yokoo Shunpei S Yurimoto Hisayoshi H

Scientific reports 20240112 1

Previous experiments performed below 20 GPa suggested that the metal/silicate partition coefficient of phosphorus (P), D<sub>P</sub>, extrapolated to typical high-pressure and -temperature conditions of the Earth's core formation gives too high P concentration in the core unless a large amount of silicon was included in metals. Here we examined D<sub>P</sub> between liquid metal and coexisting molten silicate at 27-61 GPa and 3820-4760 K, corresponding to conditions of core-forming metal segrega ...[more]

PMID: 38216648

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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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Pressure dependence of metal-silicate partitioning explains the mantle phosphorus abundance.

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Pressure dependence of metal-silicate partitioning explains the mantle phosphorus abundance.

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