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

Previous experiments performed below 20 GPa suggested that the metal/silicate partition coefficient of phosphorus (P), DP, 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 DP 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 DP 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.

Melting phase relations in Fe-Si-H at high pressure and implications for Earth's inner core crystallization.

Hydrogen could be an important light element in planetary cores, but its effect on phase diagrams of iron alloys is not well known because the solubility of H in Fe is minimal at ambient pressure and high-pressure experiments on H-bearing systems have been challenging. Considering that silicon can be another major light element in planetary cores, here we performed melting experiments on the Fe-Si-H system at ~ 50 GPa and obtained the ternary liquidus phase relations and the solid/liquid partition coefficient, D of Si and H based on in-situ high-pressure X-ray diffraction measurements and ex-situ chemical and textural characterizations on recovered samples. Liquid crystallized hexagonal close-packed (hcp) (Fe0.93Si0.07)H0.25, which explains the observed density and velocities of the Earth's solid inner core. The relatively high DSi = 0.94(4) and DH = 0.70(12) suggest that in addition to Si and H, the liquid outer core includes other light elements such as O, which is least partitioned into solid Fe and can thus explain the density difference between the outer and inner core. H and O, as well as Si, are likely to be major core light elements, supporting the sequestration of a large amount of water in the Earth's core.

Stratification in planetary cores by liquid immiscibility in Fe-S-H.

Liquid-liquid immiscibility has been widely observed in iron alloy systems at ambient pressure and is important for the structure and dynamics in iron cores of rocky planets. While such previously known liquid immiscibility has been demonstrated to disappear at relatively low pressures, here we report immiscible S(±Si,O)-rich liquid and H(±C)-rich liquid above ~20 GPa, corresponding to conditions of the Martian core. Mars' cosmochemically estimated core composition is likely in the miscibility gap, and the separation of two immiscible liquids could have driven core convection and stable stratification, which explains the formation and termination of the Martian planetary magnetic field. In addition, we observed liquid immiscibility in Fe-S-H(±Si,O,C) at least to 118 GPa, suggesting that it can occur in the Earth's topmost outer core and form a low-velocity layer below the core-mantle boundary.

Experimental evidence for hydrogen incorporation into Earth's core.

Hydrogen is one of the possible alloying elements in the Earth's core, but its siderophile (iron-loving) nature is debated. Here we experimentally examined the partitioning of hydrogen between molten iron and silicate melt at 30-60 gigapascals and 3100-4600 kelvin. We find that hydrogen has a metal/silicate partition coefficient DH ≥ 29 and is therefore strongly siderophile at conditions of core formation. Unless water was delivered only in the final stage of accretion, core formation scenarios suggest that 0.3-0.6 wt% H was incorporated into the core, leaving a relatively small residual H2O concentration in silicates. This amount of H explains 30-60% of the density deficit and sound velocity excess of the outer core relative to pure iron. Our results also suggest that hydrogen may be an important constituent in the metallic cores of any terrestrial planet or moon having a mass in excess of ~10% of the Earth.