[S-IT20] 地球深部科学

[SIT20-10] Influence of sulfur on metal-silicate partitioning of highly siderophile elements at high pressures

*近藤 望¹、浅沼 尚²、芳野 極¹ (1.岡山大学惑星物質研究所、2.京都大学人間・環境学研究科)

キーワード：硫黄、強親鉄元素、火星、高温高圧実験

The highly siderophile elements (HSE), which comprise platinum-group elements along with Re and Au, are known for their strong affinities to Fe-metal rather than coexisting silicates. Geochemical and experimental investigation on the HSE composition of the bulk silicate Earth (BSE) have provided a long-standing issue known as the “excess HSE problem”. One important feature of the “excess HSE problem” is that the HSE concentrations in the primitive upper mantle (PUM) are higher than the HSE concentrations in the silicate portion after core-mantle differentiation, which were predicted from experimentally determined partition coefficients. In addition, the relative HSE abundances of the PUM are broadly chondritic, despite that the experimental data showed largely different affinities of the HSE to Fe-metal. To reconcile the “excess HSE problem”, the Late Veneer hypothesis, which propose addition of a small amount of chondritic material after core-mantle differentiation, has been argued. On the other hand, previously determined metal-silicate partition coefficients of the HSEs themselves contain uncertainties. HSE partitioning can be affected by HSE contents in the starting material, whereas most previous studies have conducted partitioning experiments with starting materials of excessively high HSE contents (wt.% order), and also can be affected by sulfur in the Fe-metal. In our previous work, we revised metal-silicate partition coefficients of HSEs with the starting Fe-HSE alloy of low HSE content (in this alloy, each HSE content is around 1000 ppm) and found that the HSEs showed similar affinities to Fe-metal and have lower metal/silicate partition coefficients. In this study, we investigate influence of sulfur on the HSE partitioning.
We conducted high-pressure and high-temperature partitioning experiments with starting Fe-S-HSE alloys of different sulfur content (sulfur content in the alloys are 9, 16, and 36 wt.%). In the starting alloys, each HSE content is around 1000 ppm. The partitioning experiments were carried out from 6 to 18 GPa and 2423 to 2673 K with Kawai-type multi-anvil apparatus at Institute for Planetary Materials, Okayama University. The HSE contents in the recovered samples were measured by LA-ICP-MS at Kyoto University. In this presentation, we report the influence of sulfur on the HSE partitioning and discuss the HSE composition of the Earth and Martian mantles.