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3.2 a. The influence of pressure and temperature on metal-silicate partitioning of highly siderophile elements (A. Holzheid and D.C. Rubie, in collaboration with H. Palme/Köln and H.St.C. O'Neill/Canberra)

The mantle of the Earth has low contents of siderophile (metal-loving) elements when compared to chondritic meteorites. This is readily explained by partitioning of these elements into the Earth's metallic core during core formation. However, metal/silicate equilibration experiments at low pressure have shown that at a silicate FeO-content characteristic of the Earth's present upper mantle, the depletions of siderophile elements produced by metal extraction should be much higher than those found in the Earth's mantle. This phenomenon is known as the "siderophile element anomaly". One possible resolution of this problem is that metal/silicate partition coefficients of siderophile elements at very high temperatures and/or pressures are different from those at low pressure and temperature.

In the past several years new experimental data on the pressure and temperature dependence of metal/silicate partition coefficients of moderately siderophile elements, e.g. Ni and Co, have been obtained. The results show that concentrations of the moderately siderophile elements in the Earth's mantle cannot be explained by metal-silicate equilibrium within the pressure and temperature range of the present Earth's upper mantle. Recent preliminary studies on the partitioning of highly siderophile elements, especially of Re, at high pressure (up to 2 GPa) and high temperature (Righter and Drake, LPL, Univ. of Arizona) suggest that Re metal-silicate partition coefficients depend strongly on pressure.

In this study the pressure dependence of the partition coefficients of other highly siderophile elements (Pd, Pt, Ir) were experimentally determined. Experiments were performed from 1 to 20 GPa and 1300 to 1500°C in piston cylinder (PC) and multianvil (MA) devices. Fe-metal alloys, e.g. Pd90Fe10, were equilibrated with an FeO-containing silicate melt close to a composition of komatiitic basalt. Small pieces of silicate glass were inserted into the metal capsule, and to avoid contamination of the capsule material with the sample, the metal alloy was used as the capsule material. After reaching the desired pressure, samples were heated to run temperature. Solid metal and liquid silicate coexist during the experiment, and silicate liquids were quenched to glass by turning off the power. At 2 GPa (PC apparatus) a time series, e.g. experiments at a single pressure and temperature but different run durations, were performed to investigate if chemical equilibrium between solid metal and liquid silicate had been obtained. Run temperatures were varied at 4 GPa (MA device) to investigate the temperature dependence of the solubility at higher pressure.

All samples were mounted in epoxy, cut through the center of the metal capsule and polished as microprobe sections. Metal contents and major element silicate compositions were determined by electron microprobe. Highly siderophile element concentrations in silicates (expected concentration levels: ppb-ppm range) will be analysed by laser ablation ICP-MS at ANU, Canberra. The effective oxygen fugacities that prevailed during the experiments were calculated from the measured mole fractions of Fe and FeO in metal and silicate phases, respectively. The oxygen fugacities range from 0.6 to 1.4 log units below the Fe-FeO buffer. Preliminary metal-silicate partition coefficients using upper limits of Pd-contents in silicate melt (< 1000 ppm), determined by electron microprobe, are still too high to explain the highly siderophile element abundances in the Earth's present mantle by simple metal-silicate (core-mantle) equilibrium. This discrepancy will increase with lower Pd concentrations in the melt.

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