Experimental determination of U and Th partitioning between clinopyroxene and natural and synthetic basaltic liquid

Abstract

Clinopyroxene-silicate liquid partition coefficients for U and Th have been determined by particle track radiography from 1 atm crystallization experiments at controlled fO_2. Two natural basaltic and one synthetic composition were studied at fO_2 values from the Ni—NiO oxygen buffer to 1 log unit more oxidizing than Fe—FeO (IW+ 1). Over the range of fO_2 values and compositions studied, D_U^(cpx/liq) = 0.0034–0.015,D_(Th)^(cpx/liq) = 0.008–0.036, and D_(Th)/D_U= 3.4–1.1. With decreasing fO_2, D_(Th)/D_U can decrease by up to a factor of 3 for a given composition, primarily from an increase in D_U^(cpx/liq), which we interpret as resulting from an increase in the proportion of tetravalent U in the system with decreasing fO_2. This demonstrates that crystal-liquid U—Th fractionation is fO_2 dependent and that U in terrestrial magmas is not entirely tetravalent. D_(Th)^(cpx/liq) appears to decrease in the two basalts at the lowest fO_2, possibly as a result of changes in composition with fO_2. Our data show the sense of U—Th fractionation by clinopyroxene-liquid partitioning is consistent with previous experimental determinations, in that D_(Th)^(cpx)/D_U^(cpx)> 1 in all cases. This indicates that, during partial melting, the liquid will have a Th/U ratio less than the clinopyroxene in the source. The observed ^(238)—U^(230)Th disequilibrium in MORB requires that the partial melt should have a Th/U ratio greater than the bulk source, and, therefore, cannot result from clinopyroxene-liquid partitioning. Further, the magnitudes of the measured partition coefficients are too small to generate significant U—Th fractionation in either direction. Assuming that clinopyroxene contains the bulk of the U and Th in MORB source, our results indicate that ^(238)U—^(230)Th disequilibrium in MORB may not be caused by partial melting at all.