Experimental Studies of Early Differentiation and Evolution of the Earth

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In order to simulate the core segregation in a magma ocean (MO) extended deeper than 1000 km depth, we aimed to examine experimentally reactions and elemental partitioning between molten iron (MI) and silicates at ultrahigh pressures. Melting experiment at pressures higher than 30 GPa can be carried out by utilizing sintered diamond as anvil material of the splitsphere multi-anvil apparatus. Innovation of analytical method using a secondary ion mass spectrometer has made it possible to determine concentration of elements less than 1ppm over an area of 5mum diameter. Based on these newly developed methods, melting relations of silicate and iron silicate systems have been examined and the partitioning of major and trace elements between MI and silicate liquid (SL) or coexisting solid phases has been determined as function of pressure. Important results obtained in the present research are summarized as follows : (1) Liquidus phase of the primary Earth's material is magnesiow ustite (Mw) … More at the bottom of MO with depths of 600-1000 km. However, a very narrow temperature interval of the SL+Mw field suggests that perovskite (Pv) would also have coexisted with MI prior to core formation. (2) At higher than 20 GPa, dissolution of Si and O in MI from SL proceeds noticeably with pressure. Thus Mg/Si ratio of the mantle could remarkably be increased by the core segregation in a deep MO.(3) Partition co efficients of Mn, Ni, and Co between MI and SL,Mw, and Pv to 26 GPa indicate that Ni and Co abundance of the mantle can be explained if the core material would have been equilibrated with SL and Mw at the bottom of MO deeper than 900 km. However, the coexistence of Pv demands a much deeper MO.(4) It has been found that appreciable amounts of rare earth elements (REE) dissolve in MI from SL at pressures higher than 20 GPa with a remarkable fractionation between light and heavy REE,resulting in depletion in light REE in the primitive mantle after the core formation. The results are of crucial importance for the mantle chemistry because the relative abundace of REE for the primitive mantle have intuitively assumed so far to be same as those of chondrites. Less