4:30 PM - 4:45 PM
[SIT14-21] Partitioning of highly siderophiel elements between metal and silicate liquids up to lower mantle pressures
★Invited Papers
Keywords:Core, Highly siderophile elements , Late Venner, Element partitioning , High pressure and high temperature experiment
The highly siderophile elements (HSE), which comprise platinum-group elements (Os, Ir, Ru, Rh, Pt, and Pd) along with Re and Au, are known for their strong affinities to Fe-metal rather than coexisting silicates and regarded as the key tracer of the core-mantle differentiation and subsequent mantle evolution. Geochemical investigation on the HSE composition of chondrites (e.g., Fischer-Godde et al., 2010) and Earth’s primitive upper mantle (PUM) (e.g., Becker et al., 2006) and experimental studies on metal-silicate partitioning behavior of the HSE (e.g., Mann et al., 2012) 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 PUM are higher than the HSE concentrations in the bulk silicate Earth (BSE, silicate portion after core-mantle differentiation) 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 discrepancy between HSE composition of the PUM and the BSE, the late veneer hypothesis, which propose rate accretion 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 HSE themselves contain uncertainties. The HSE partition coefficients can have pressure dependence (Mann et al., 2012), but previous experiments mainly conducted pressures lower than 20 GPa. Besides, recent low-pressure experiments (Zhang & Li, 2021) reported that the HSE partition coefficients do not obey the Henry’s low and are affected by HSE contents in the starting material, whereas most previous studies have conducted partitioning experiments with starting materials of high HSE contents (wt.% order). In order to assess the effect of pressure and HSE contents on the HSE partitioning and to obtain HSE partition coefficients more appropriate to estimate HSE composition of the BSE, we conducted partitioning experiments of HSE (Re, Os, Ir, Ru, Pt, Pd) between metal and silicate liquids up to lower mantle pressures (> 24 GPa) with starting materials of varying HSE contents.
For the starting material, we prepared Fe-HSE alloys in which each HSE content varies from 1000 ppm to 10 wt. %. A small tip of the Fe-HSE alloy and powder mixture of chondritic mantle composition (composition from Andrault et al., 2018) are packed in a single-crystal MgO capsule and totally melted. We conducted high pressure and high temperature experiments by using Kawai-type multi-anvil apparatus at Institute for Planetary Materials, Okayama University. The HSE concentrations in the metal and silicate portions in recovered samples are measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at Graduate School of Human and Environmental Studies, Kyoto University, and Field Emission Electron Probe Microanalyzer (FE-EPMA) at Institute for Planetary Materials, Okayama University.
As a result, from experiments at 18 and 24 GPa with the starting Fe-HSE alloy of low HSE content (each HSE content is around 1000 ppm), we obtained lower values of metal/silicate partition coefficients than the previous data in Mann et al. (2012), and the HSE showed similar affinities to Fe-metal. In the case of experiments at 18 GPa with the starting Fe-HSE alloy of high-HSE content (each HSE content is around 10 wt.%), the HSE partition coefficients are consistent with previous data in Mann et al. (2012). By using newly obtained HSE partition coefficients, the relative HSE abundances in the BSE are predicted to be broadly chondritic, and the estimated HSE concentrations in the BSE get closer to the HSE concentration of the PUM. From these results, we discuss the necessity of the Late veneer during the early Earth’s evolution.
On the other hand, previously determined metal-silicate partition coefficients of the HSE themselves contain uncertainties. The HSE partition coefficients can have pressure dependence (Mann et al., 2012), but previous experiments mainly conducted pressures lower than 20 GPa. Besides, recent low-pressure experiments (Zhang & Li, 2021) reported that the HSE partition coefficients do not obey the Henry’s low and are affected by HSE contents in the starting material, whereas most previous studies have conducted partitioning experiments with starting materials of high HSE contents (wt.% order). In order to assess the effect of pressure and HSE contents on the HSE partitioning and to obtain HSE partition coefficients more appropriate to estimate HSE composition of the BSE, we conducted partitioning experiments of HSE (Re, Os, Ir, Ru, Pt, Pd) between metal and silicate liquids up to lower mantle pressures (> 24 GPa) with starting materials of varying HSE contents.
For the starting material, we prepared Fe-HSE alloys in which each HSE content varies from 1000 ppm to 10 wt. %. A small tip of the Fe-HSE alloy and powder mixture of chondritic mantle composition (composition from Andrault et al., 2018) are packed in a single-crystal MgO capsule and totally melted. We conducted high pressure and high temperature experiments by using Kawai-type multi-anvil apparatus at Institute for Planetary Materials, Okayama University. The HSE concentrations in the metal and silicate portions in recovered samples are measured by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) at Graduate School of Human and Environmental Studies, Kyoto University, and Field Emission Electron Probe Microanalyzer (FE-EPMA) at Institute for Planetary Materials, Okayama University.
As a result, from experiments at 18 and 24 GPa with the starting Fe-HSE alloy of low HSE content (each HSE content is around 1000 ppm), we obtained lower values of metal/silicate partition coefficients than the previous data in Mann et al. (2012), and the HSE showed similar affinities to Fe-metal. In the case of experiments at 18 GPa with the starting Fe-HSE alloy of high-HSE content (each HSE content is around 10 wt.%), the HSE partition coefficients are consistent with previous data in Mann et al. (2012). By using newly obtained HSE partition coefficients, the relative HSE abundances in the BSE are predicted to be broadly chondritic, and the estimated HSE concentrations in the BSE get closer to the HSE concentration of the PUM. From these results, we discuss the necessity of the Late veneer during the early Earth’s evolution.