Mineral/melt partitioning of U and Th during partial melting of garnet pyroxenite L.J. ELKINS, G.A. GAETANI, K.W.W. SIMS Woods Hole Oceanographic Institution, Woods Hole, MA, USA (lelkins@whoi.edu; ggaetani@whoi.edu; ksims@whoi.edu) The range of isotopic variability found in ocean island basalts (OIB) points to the presence of several distinct components in their source regions. At least one of these components is thought to comprise mafic material such as recycled oceanic crust. Many ocean islands are characterized by silica-poor, alkalic lavas that Hirschmann et al. (Hirschmann et al., 2003) showed can plausibly be produced by partial melting of silica-deficient garnet pyroxe- nite. Here we place additional constraints on this hypothesis through an experimental determination of the mineral/melt parti- tioning of U and Th during partial melting of garnet pyroxenite. Our results demonstrate that the U–Th systematics of most OIB can be explained through mixing of partial melts of peridotite and pyroxenite. Experiments were carried out in a piston-cylinder device at 2.5 GPa and 1450 °C on a synthetic basalt with the composition of a 21% partial melt of the silica deficient garnet pyroxenite MIX1G of Hirschmann et al. (Hirschmann et al., 2003). The major element compositions of coexisting clinopyroxene, garnet and silicate melt were determined by electron microprobe and the concentrations of U and Th by ion microprobe. The garnet/melt and clinopyroxene/melt partition coefficients determined in this study are at the high end of the range of previ- ously published partitioning data. They exceed partition coeffi- cients predicted by the model of Salters et al. (Salters et al., 2002) by up to three orders of magnitude. Dynamic melting calcu- lations for peridotite and MIX1G pyroxenite at depths of 100 km, using these partition coefficients and a moderate upwelling rate of 5 cm/yr, reveal that mixing trajectories between peridotite and pyroxenite melts can account for the U–Th isotope systematics of most OIB rocks. These same calculations can explain the Sm/Nd systematics of many OIB as well. Our results support the conclusion of Hirschmann et al. (Hirschmann et al., 2003) that partial melting of silica-poor garnet pyroxenite may play a role in the generation of nepheline-normative OIB. References Hirschmann, M.M., Kogiso, T., Baker, M.B., Stolper, E.M., 2003. Geology 31, 481–484. Salters, V.J.M., Longhi, J.E., Bizimis, M., 2002. Geochem. Geophys. Geosys. 3, 2001GC000148. doi:10.1016/j.gca.2006.06.1384 Li isotopic evidence for subduction induced mantle heterogeneity T. ELLIOTT 1 , A. JEFFCOATE 1 , S. KASEMANN 2 1 Department of Earth Sciences, University of Bristol, BS8 1RJ, UK (tim.elliott@bris.ac.uk) 2 School of Geosciences, University of Edinburgh, EH9 3JW, UK (simone.kasemann@ed.ac.uk) Cycling of Li through the hydrosphere causes significant (30&) fractionations of 7 Li/ 6 Li. As a result, a flux of isotopical- ly heavy Li is ultimately transferred to the mantle via subduction. Although little of this heavy Li appears to be lost to subduction zone lavas, the slab itself becomes isotopically light during dehy- dration. The Li isotopic system thus provides two valuable tracers of the ancient subduction, a heavy signature in past mantle wedge (and possibly subducted serpentinised mantle lithopsphere) and a light signature in the slab itself. Deviations of the Li isotopic compositions of mantle derived basalts from a terrestrial baseline can provide evidence of these recylced components within the convecting mantle. Such a terres- trial baseline, however, has not previously been well defined, but recent high precision analyses (±0.3&) have put quite tight con- straints on the composition of primitive mantle. The composition fertile peridotites determined by analysis of constituent minerals yield values of d 7 Li 3–4&, overlapping with measurements of depleted mid-ocean ridge basalts (MORB). These values are also compatible with Li isotopic measurements of eucrites and type 3 chondritic meteorites from a range of groups (CO, CV, LL, H). Compared to these reference values, the variability of Li isoto- pic ratios in ocean island basalts (OIB) is dramatic and provides clear evidence for the role recycling. d 7 Li ranges from values close to the mantle baseline (3&) up to very heavy values of 11&. In general, OIB from locations that display with extreme radiogenic isotopic compositions (EMI, EMII and HIMU) also show isoto- pically heavy Li,. This implies that the most marked forms of mantle heterogeneity result from mixing of subduction modified mantle wedge, or serpentinised subducted lithopsphere, back into the convecting mantle. These signatures are widespread and also evident in enriched MORB. In contrast, the much invoked role of the recycled slab in generating mantle heterogeneity is not evi- dent in the Li isotopic system. OIB all tend towards higher d 7 Li than the mantle baseline and no lighter Li isotopic compositions have been measured in whole rock samples. Whilst some very light mineral phases from peridotite nodules have been reported, it appears that these are likely generated by late diffusive processes on a grain length-scale and are not representative of bulk compositions. doi:10.1016/j.gca.2006.06.1385 Goldschmidt Conference Abstracts 2006 A159