HR: 0800h
AN: V41D-1502    [Abstracts]
TI: Fractionation of Highly Siderophile Elements (HSEs) by Sulfide-Silicate Partitioning: A New Spin
AU: * Brenan, J M
EM: brenan@geology.utoronto.ca
AF: University of Toronto, Dept of Geology 22 Russell St, Toronto, ON M5S 3B1 Canada
AU: McDonough, W F
EM: mcdonoug@geol.umd.edu
AF: University of Maryland, Dept of Geology, College Park, MD 20742-4211 United States
AB: Existing sulfide-silicate partitioning data show that the HSEs are strongly sequestered by molten sulfide during mantle melting. However, partition coefficients for specific elements are either highly variable, or non-existent, so the specific role of sulfide-silicate partitioning in fractionating the HSEs is poorly known. Previous studies have relied on bulk analytical techniques for HSE analysis, so precise measurement of individual partition coefficients has been hampered by extreme sulfide-silicate partitioning and the likely presence of trace sulfide in the silicate separate. We report on a new technique to measure sulfide-silicate partitioning that involves subjecting sulfide-silicate mixtures to high acceleration at superliquidus conditions to promote phase separation. Time-resolved laser ablation ICP-MS analysis is used to document phase homogeneity and establish elemental concentrations. HSE-doped (approx. 500 ppm each) samples of Fe sulfide + synthetic basalt are contained in olivine crucibles. The crucible and sulfide buffer (either Pt-PtS or Ru-RuS2) are vacuum-sealed in a fused quartz ampoule and equilibated under static conditions at 1200 C for about 48 hours. Following the static soak, the sample is quenched, sealed into a fresh ampoule, and subject to about 400g, at 1200 C, using a modified large capacity centrifuge. By buffering the sulfur fugacity, the oxygen fugacity can be estimated through the heterogeneous equilibrium: FeO(silicate) + S2 = FeS(sulfide) + O2. Sulfide-silicate partition coefficients at the Pt-PtS/FMQ buffers are greater than 10,000 for the PGEs and 230 for Re. At Pt-PtS/FMQ-1, the measured values of Dsulfide/silicate are 1000-3000 for the PGEs and 700 for Re. Our data supports the model of Bockrath et al (2004), requiring residual mss (mono-sulfide solid solution) to produce the observed PGE fractionation in mantle-derived magmas. The observed Re-Os fractionation during crust-mantle differentation is adequately modeled with our new partitioning data, obviating the need for control by residual alloy, silicate or oxide phases.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005