HR: 1330h
AN: V42C-0376    [PDF]
TI: Towards a Precise and Accurate Estimate of the Platinum Group Element Composition of the Primitive Upper Mantle
AU: * Becker, H
EM: hbecker@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Horan, M F
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington., 5241 Broad Branch Road, NW., Washington, DC 20015 United States
AU: Walker, R J
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Lorand, J
AF: Laboratoire de Mineralogie, Museum National D'Naturelle, Unite CNRS FRE 2456., 61 Rue Buffon, Paris, 75005 France
AU: Gao, S
AF: Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an, 710069 China
AU: Rudnick, R L
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AB: Determining precise and accurate platinum group element (PGE) abundances in peridotites is key for the detection of primordial and secondary (i.e., magmatic) heterogeneities of the PGE in the mantle, and may be used to establish model PGE compositions of the primitive upper mantle (PUM). In conjunction with $^{187}$Os/$^{188}$Os (reflecting the Re/Os of PUM), the PGE model composition of PUM, in particular its Pd/Ir, may help in tracing the source of the late siderophile element influx at 1 AU by comparison with data for chondrites (Horan et al. 2003). We have determined Os, Ir, Ru, Pt, Pd and Re abundances in samples from continental peridotite massifs and peridotite xenoliths. The new approach uses isotope dilution and high-pressure, high-temperature digestion in Carius tubes at $345\deg$ C, as most other techniques are either not precise enough, or do not access the complete PGE budget during the digestion of peridotites (Meisel et al. 2003, and in press). Reproducibility and accuracy are controlled by multiple analysis of the peridotite standard UB-N, for which the PGE content is now well constrained through high-T-P digestion and Na2O2-NaOH fusion. New data for peridotites from the Pyrenees and Lanzo indicate that, in some cases, Ir and Ru contents may have been underestimated by up to 50-80% by older techniques using NiS-fire assay, resulting in overestimates of Pd/Ir. The new data also show a good correlation of Pd/Ir with Al, suggesting a Pd/Ir of PUM between 1.5 and 2.0, comparable to or higher than EH chondrites. Data for peridotite xenoliths from Hannuoba (China), while also correlating with Al, show larger scatter in Pd/Ir than the European samples. This could be an analytical artifact, because most of the Chinese samples were digested in standard Carius tubes at $220\deg$ C. High-T-P digestions will be performed to test this hypothesis and data on other peridotites are being obtained to constrain the composition of PUM.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1025 Composition of the mantle
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting