HR: 1340h
AN: MR13A-0066    [Abstracts]
TI: Majoritic garnet-silicate melt partitioning of trace elements between 10 and 17 GPa in an anhydrous fertile mantle composition
AU: Armstrong, L
EM: l.armstrong@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: * Corgne, A
EM: a.corgne@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: Keshav, S
EM: s.keshav@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AU: Minarik, W
EM: minarik@eps.mcgill.ca
AF: McGill University, 3450 University Street, Montreal, H3A2A7 Canada
AU: McDonough, W F
EM: mcdonoug@geol.umd.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: Fei, Y
EM: fei@gl.ciw.edu
AF: Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, DC 20015 United States
AB: High-pressure crystal-melt partition coefficients (D) in fertile mantle composition are necessary in order to evaluate the possible effects of melting and crystallization in early history of the Earth. Unfortunately, current partitioning data are too sparse to allow an accurate analysis to be made. Here we report new majoritic garnet-melt partition coefficients for REE and other trace elements for experiments run between 10 and 17 GPa. The starting composition is a synthetic fertile mantle composition prepared following the pyrolite major element composition of McDonough and Sun (1995). Trace elements were added as atomic absorption standard solutions. Temperature gradients were minimized using small Re-capsules drilled by spark-erosion. Runs were 30-150 min long. Recovered samples were polished and analyzed by electron microprobe for major elements (GL) and by LA-ICPMS for trace elements (McGill, UMD). Preliminary results show that similar patterns of D are obtained at 15-17 GPa (this study) and 23.5 GPa (Walter et al., 2004). The garnet signature is preserved at high-pressure with DREE varying from <0.01 (e.g. DLa) to >1 (e.g. DLu). Isovalent cations entering the X-site exhibit the predicted near-parabolic dependence of D on ionic radius (e.g. Blundy and Wood, 1994). This underlines the important contribution made by the crystal structure to the control of element partitioning. One important observation is that the optimum partition coefficient D03+ (top of the 3+ parabola for the X-site) decreases with increasing pressure, i.e. with increasing Si content in garnet. In other words, partitioning of heavy REE are sensitive to pressure. In combination with published data at higher pressures (Walter et al., 2004), we derived a predictive model for REE partitioning between majoritic garnet and anhydrous silicate melt in the 10-24 GPa pressure range. These data are used to model the origin of Archean Al-depleted komatiites. We also use our data to discuss the possible origin of diamonds containing inclusions of majoritic garnet.
DE: 1025 Composition of the mantle
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005