Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p11a1591b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P11A-1591
Other
[1015] Geochemistry / Composition Of The Core, [1025] Geochemistry / Composition Of The Mantle, [1060] Geochemistry / Planetary Geochemistry
Scientific paper
Reliable determination of highly siderophile element (HSE) distribution between metal and silicate melt is key to understanding the relative roles of equilibrium core-formation and ongoing accretion in establishing the observed level of these elements in the primitive upper mantle (PUM). Platinum is of particular interest among the HSE due to its parental role in the long-lived 190Pt-186Os isotopic system, with measured 186Os/188Os providing a time-integrated constraint on a chondritic Pt/Os ratio for the PUM. Two issues currently plague the results of metal solubility experiments designed to estimate partitioning between Fe-metal and silicate melt. Firstly, experiments to measure Pt solubility at the low fO2 relevant to core metal segregation show evidence for a highly dispersed, discrete metal phase. It is debated whether this phase is formed during quench, or is stable during the experiment; implied melt solubilities differ considerably depending on the interpretation. Secondly, past experiments have utilised graphite capsules, but the association of quenched CO vesicles with metal particles in quenched run products from the highest temperature experiments (T≥2300C) suggest that some HSEs (e.g. Au and Pt) may dissolve into molten silicate, in part, as a carbonyl species. The question is whether dissolved carbon can therefore enhance HSE solubility. Our work aims to resolve these issues. We have conducted piston-cylinder experiments at 2GPa and 1600-2300C, over an fO2 range of several log units, spanning iron-wustite. Capsules were made from either graphite or Pt-Ir-Fe alloy. The melt used in the experiments is an Fe-bearing basalt that forms a glass on quench. Experiments are analysed by LA-ICP-MS, allowing glass homogeneity to be assessed on a micron-scale. Our results show that, unlike previous studies, we have been able to suppress the formation of metal particles through the addition of small quantities of silicon metal to the experiment. Measured glass concentrations can therefore be unambiguously assigned to a dissolved component in the melt phase. It is likely that oxidants present in the samples initially enhance HSE solubility before slow reduction towards equilibrium fO2 causes precipitation of a dispersed metal phase. In our experiments, the addition of a strong reducing agent is effective in rapidly reducing the experimental fO2, thus avoiding transient high solubilities and later metal oversaturation. The effect of carbon will be appraised by comparing graphite versus Pt-Ir-Fe capsule experiments run at fixed P-T conditions but variable fO2. If carbon enhances Pt solubility we expect higher concentrations of dissolved Pt in experiments run in graphite capsules but otherwise identical P-T-fO2 conditions. We may also expect to see a difference in the relationship between platinum concentration and fO2 if the formation of carbonyl species affects Pt valence, an observation which would have significant impact on the extrapolation of results to fO2 conditions beyond the experimentally studied range. Our results to date support the idea that ongoing accretion and not equilibrium core-formation is responsible for the chondritic nature of HSEs in the PUM.
Bennett Norman R.
Brenan J.
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