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3.2 b. The influence of temperature, pressure and oxygen fugacity on the metal/oxide and metal/silicate partition coefficients of siderophile elements (C.K. Geßmann and D.C. Rubie)

The partitioning behaviour of the siderophile elements Ni, Co, Cr, V und Mn and the solubility of the light elements Si und O in liquid metal have been studied systematically in the system liquid metal/magnesiowüstite at 9 - 23 GPa and 1800 - 2400 °C. The siderophile elements were added as traces to a Fe-Ni-powder starting material which was contained in an MgO capsule. A wide range of different oxygen fugacities (from iron-wüstite (IW) to 3.5 log-units below IW) were imposed by varying the Si/O ratio in the starting material. In experiments with high contents of Si in the starting material a silicate liquid formed, thus providing metal-silicate partitioning data in addition to the metal/oxide partitioning results. At all experimental temperatures a metal liquid formed, which equilibrated with the MgO capsule to form magnesiowüstite. Magnesiowüstite is one of the mineral phases of the Earth´s lower mantle and therefore the metal-oxide partitioning data obtained here provide constraints on the accretion of the Earth´s core.

Experiments at constant pressure and temperature over a range of oxygen fugacities (3.5 log units) has enabled the valence states of the siderophile elements to be determined at high pressures and temperatures (e.g. 9 GPa, 2200 °C). All observed partition coefficients (metal/oxide and metal/sililcate) show a distinct dependence on oxygen fugacity (increasing partition coefficients with decreasing oxygen fugacity). The results indicate divalency for Fe, Ni and Co and a valence state of 3+ for Cr. Surprisingly the data for Mn and V give valence states between 2+ and 3+. No changes of valence state were observed as a function of pressure or temperature. In contradiction to the general assumption, these fO2-dependent partitioning experiments show some evidence for nonideal mixing behaviour between Fe and Ni at high pressures and temperatures.

In order to evaluate the effect of temperature on metal/oxide partition coefficents of the siderophile elements, experiments have been performed between 1800 and 2400 °C at constant pressure. Over this temperature range, the partition coefficients for Ni, Fe and Co show no detectable temperature dependence while partitioning of the moderately siderophile elements Mn, Cr and V is weakly temperature dependent.

The pressure dependence of the partitioning behaviour of siderophile elements was obtained from experiments at constant temperature and constant fO2. Partitioning of Ni and Co show a slight pressure dependence, while the partition coefficients of the moderately siderophile elements Mn, Cr and V are not pressure dependent. The partition coefficients of Mn are lower than those of Cr. The behaviour of Mn is less siderophile than that of Cr, thus supporting arguments that explain the low abundances of Mn in the mantle by its volatile character. Although the pressure dependence observed for Ni and Co could explain their abundances in the mantle by an equilibrium model, the results obtained for the moderate siderophile elements, point to more complicated accretion models.

The solubility of Si and O in the metal liquid in the pressure-temperature range of this study suggest that both elements might be at least partly responsible for the density deficit in the Earth's core. The main argument against Si + O in the core, established previously by Ringwood & Hibberson, is weakened because the solubilities of Ti in liquid metal are much lower than those of Si in all performed experiments.

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