HR: 1330h
AN: V42C-0372    [PDF]
TI: Highly Siderophile Element Partitioning in Partial Melting Residues : a Case Study in Some Pyrenean Harzburgites (France).
AU: * Luguet, A A
EM: luguet@dtm.ciw.edu
AF: Carnegie Institution of Washington-Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015 United States
AU: Horan, M F
EM: horan@dtm.ciw.edu
AF: Carnegie Institution of Washington-Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015 United States
AU: Shirey, S B
EM: shirey@dtm.ciw.edu
AF: Carnegie Institution of Washington-Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015 United States
AU: Lorand, J
EM: lorand@mnhn.fr
AF: Museum National d Histoire Naturelle, Laboratoire de Mineralogie, CNRS FRE 2456, 61 rue Buffon, Paris, 75005 France
AU: Carlson, R W
EM: carlson@dtm.ciw.edu
AF: Carnegie Institution of Washington-Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015 United States
AB: Highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd and Re) in mantle peridotites are mainly ($>$90%) concentrated into base-metal sulfide microphases (BMS). During partial melting, BMS are extracted along with the partial melt and are completely consumed by 15-20% partial melting. At the same time, Pd and Re progressively enter the partial melt while Os, Ir, Ru and sometimes Pt remain in the peridotitic residue even when BMS have been completely extracted. Identifying the mineral(s) concentrating the refractory HSE (Os, Ir, Ru) and Pt in refractory peridotites will provide a better understanding of the behavior of these elements and their radiogenic isotopes during partial melting and melt extraction processes. Such a study has been done on three spinel harzburgites from the Lherz massif (French Pyrenees) using electron microprobe (EMP) and scanning electron microscope (SEM) coupled with isotope dilution (ID)-ICP-MS analyses of whole-rock and mineral separates. These samples (0.3-1.22 wt. % Al$_{2}$O$_{3}$) show S contents ranging between 100 and $<$5 ppm (Lorand et al., 1999 and unpublished data). Whole-rock HSE concentrations are 2-5 ppb for Ir, Ru and Pt, 0.2-2 ppb for Pd and 15-70 ppt for Re, in good agreement with the previous analyses of Lorand et al. (1999). The CI-normalized patterns show a progressive depletion from Ir and Ru to Pt, Pd and Re. Pt$_{N}$/Ir$_{N}$, Pd$_{N}$/Ir$_{N}$ and Re$_{N}$/Ir$_{N}$ are subchondritic (0.7-0.77; 0.52-0.08; 0.23-0.05, respectively; N: CI-normalization after Horan et al., 2003). Pd and Re concentrations as well as Pd$_{N}$/Ir$_{N}$ and Re$_{N}$/Ir$_{N}$ decrease with the extent of partial melting. Orthopyroxene contains $<$10 ppt Ir, $<$40-255 ppt Ru,$ <$10-420 ppt Pt, 4-76 ppt Pd and $<$5-335 ppt Re; these concentrations vary widely between different aliquots separated from the same sample. Clinopyroxene broadly shows the same range of HSE concentrations (50-210 ppt Ir, $<$30-320 ppt Ru, $<$215 ppt Pt, $<$30 ppt Pd and $<$5 ppt Re) while Cr-spinel is HSE-richer (1200-4300 ppt Ir, $<$30-2500 ppt Ru, 1700-11300 ppt Pt, 350-7100 ppt Pd and 40-340 ppt Re). Because of their low HSE contents and/or their low modal abundances, neither orthopyroxene, clinopyroxene nor spinel can account for the whole-rock HSE abundances of Pyrenean harzburgites. The large variations of concentrations in pyroxenes or spinel suggest that HSE do not reside in the lattices of these minerals but more likely occur as included microphases. The other potential HSE-rich minerals are then olivine and/or intergranular HSE-rich microphases such as HSE alloys or sulfides. High-resolution electron microbeam techniques have been unsuccessful in detecting these microphases, suggesting that, if they exist, they are smaller than 1 $\mu$m. Lorand et al., 1999. J. Pet. 40, 957-981. Horan et al., 2003. Chem. Geol. 196, 5-20.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 1065 Trace elements (3670)
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting