[SIT31-08] Light elements in iron-hydrous silicate system: Searching for core formation process using in-situ high-pressure and high-temperature neutron and X-ray observations
Keywords:light elements, Core-mantle formation, in-situ high pressure and high temperature observation, hydrogen
A starting material simulates an ideal composition of the primitive Earth, iron-enstatite (MgSiO3) system with water. An Fe powder was pelletized and placed in the center of the capsule. Powder mixture of quartz (SiO2), brucite Mg(OD)2 (or MgO for comparison of water existence) surrounds the Fe pellet to avoid the contamination of carbon from the capsule into Fe. High-PT neutron diffraction experiments up to 6.5−11.5 GPa, 750 °C were performed using our original multi-anvil 6-6 type assembly  and a six-axis multi-anvil press “Atsuhime” installed at PLANET (BL11), J-PARC. The diffraction data obtained were refined using the Rietveld method to determine structural and atomic parameters of Fe polymorphs. In-situ X-ray imaging and diffraction measurements were also conducted at NE7A, PF-AR, KEK in order to identify the products and reaction process. A camera system with a high spatial resolution was newly developed to clearly observe the formation and motion of iron droplets in the silicate melts within the area of ~1 mm square. All the recovered samples were analyzed using an X-ray diffractometer and a SEM-EDS to more precisely identify the products, their compositions, and the elements partition.
A series of reactions (phase transitions of Fe, dehydration of Mg(OD)2, and formation of FeS-V and silicates) were observed with temperature increase. X-ray radiography successfully visualized that Fe droplets coalesce together, migrate by thermal convection, and then sink to the core. The solubility of D in fcc-Fe increased with temperature (i.e., endothermic reaction) and also had positive correlation with pressure. The maximum D content x in fcc-FeDx was about 0.25 at 11.5 GPa, 750 °C for the sample without sulfur, accompanied with a slow hydrogenation process through the redox reaction. On the other hand, the cell volume of FeS-V did not increase regardless of time, although sulfur seemed to inhibit the hydrogen dissolution into iron. Recovered samples exhibited that S and Fe were not mobile and could not react with each other without water, suggesting that the formation of FeS was promoted in the presence of water. Sulfur seemed to be a siderophile element and selectively concentrated at the Fe surface to reduce the interfacial energy. Both H and S can be preferentially incorporated into solid Fe and lower the melting point. It suggests that the other light elements (C, O, Si) could have dissolved into molten iron hydride and/or FeS in the later process of Earth’s core-mantle differentiation at much higher-PT condition.
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