Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts

Abstract

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. The results show that garnet retains U preferentially over Th ( D gt melt U = 0.0059 , D gt melt Th = 0.0014 ), while clinopyroxene shows the opposite sense of partitioning ( D cpx melt U = 0.0127 , D cpx melt Th = 0.014 ). The experimental Th, U and Pb partition coefficients for garnet-melt and cpx-melt are consistent with garnet-cpx pairs from garnet-bearing ultramafic rocks which exhibit U-Pb isochrons, thus demonstrating equilibrium ( D gt cpx U = 0.30 , D gt cpx Th = 0.072 , D gt cpx Pb = 0.016 ). The partition coefficients for Th and U between clinopyroxene and basaltic melt vary systematically as a function of the tetrahedral Al content of clinopyroxene. Garnet/melt values for Th, U and Pb agree with previous determinations, indicating that mid-ocean ridge basalt (MORB) generation begins in the stability field of garnet lherzolite. However, high 226 Ra 230 Th 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 segregation is not in the spinel lherzolite field. This requires either rapid transport of MORB magmas from ϵ 70 km, or some degree of disequilibrium during melt generation and/or transport.