EXPERIMENTAL AND NATURAL PARTITIONING OF TH, U, PB AND OTHER TRACE-ELEMENTS BETWEEN GARNET, CLINOPYROXENE AND BASALTIC MELTS

Partition coefficients for Th, U, Pb, rare-earth elements (REE), high field strength elements (HFSE), alkaline-earth elements, Sc, Cr, V and K were measured by ion microprobe techniques in two experiments on a natural high-alumina basalt composition from Medicine Lake, California . All elements were measured at natural abundance levels except Th, U and Pb, which were each present in the starting mix at 1-wt% levels. T he results show that garnet retains U preferentially over Th (D-U(gt/m elt)=0.0059, D-Th(gt/melt)=0.0014), while clinopyroxene shows the oppo site sense of partitioning (D-U(cpx/melt)=0.0127, D-Th(cpx/melt)=0.014 ). The experimental Th, U and Pb partition coefficients for garnet-mel t and cpx-melt are consistent with garnet-cpx pairs from garnet-bearin g ultramafic rocks which exhibit U-Pb isochrons, thus demonstrating eq uilibrium (D-U(gt/cpx)=0.30, D-Th(gt/cpx)=0.072, D-Pb(gt/cpx)=0.016). The partition coefficients for Th and U between clinopyroxene and basa ltic melt vary systematically as a function of the tetrahedral Al cont ent of clinopyroxene. Garnet/melt values for Th, U and Pb agree with p revious determinations, indicating that mid-ocean ridge basalt (MORB) generation begins in the stability field of garnet lherzolite. However , high Ra-226/Th-230 ratios in MORB require very small porosities near the region where the melts lose chemical equilibrium with the mantle. Partitioning data for HFSE and REE suggest that this region of melt s egregation is not in the spinal lherzolite field. This requires either rapid transport of MORB magmas from greater than or equal to 70 km, o r some degree of disequilibrium during melt generation and/or transpor t.