Study on the primordial state of the terrestrial planets from the abundance of transition metal elements

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The partitioning behavior of Mn, Co and Ni between metallic iron and olivine, pyroxene and magnesiowustite were experimentally investigated at 1400ﾟC and 3-11 GPa.The effect of dissolution of hydrogen in metallic iron on the partitioning behavior was also examined. At normal pressure, it is known that Co and Ni concentrate in metallic phase and Mn concentrates in oxide phases. These siderophile and lithophile nature of these elements reduce under high pressures : all the observed metal/silicate partition coefficients, are approaching to 1 with an increase in pressure. It is also found that the effect of dissolution of hydrogen in metallic iron is very small. Since the exchange reaction with Fe can not be defined on the trivalent elements, the pressure effect on the partition behavior of these elements were not determined precisely. However, it was suggested that partition coefficient of Cr, V,In, La, Tm and Sc may be also approaching to 1 with increase in pressure.It has been pointed out that the abundance of elements in the Earth's upper mantle can not be explained by metal/silicate equilibrium consideration estimated from the partition coefficients at normal pressure. From the present experimental results, however, it is found that partition coefficients approach to 1 with increase in pressure, and the observed upper mantle elemental abundance may be explained by the equilibrium between metal/silicates under extremely high pressure. When we compare the geochemical characters of Earth, Moon, Shergottite parent body and Eucrite parent body, it is found that the apparent partition coefficients of these terrestrial planes systematically changed with change in size of the planet. These observations suggest that pressure effect on partition coefficients playd an important role for the chemical evolution of the terrestrial planets.