EXPERIMENTAL-DETERMINATION OF U-PARTITIONING AND TH-PARTITIONING BETWEEN CLINOPYROXENE AND NATURAL AND SYNTHETIC BASALTIC LIQUID

Clinopyroxene-silicate liquid partition coefficients for U and Th have been determined by particle track radiography from 1 atm crystallization experiments at controlled fO2. Two natural basaltic and one synthetic composition were studied at fO2 values from the Ni-NiO oxygen buffer to 1 log unit more oxidizing than Fe-FeO (IW + 1). Over the range Of fO2 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 fO2. 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 fO2. This demonstrates that crystal-liquid U-Th fractionation is fO2 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 fO2, possibly as a result of changes in composition with fO2. 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 U-238-Th-230 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 U-238-Th-230 disequilibrium in MORB may not be caused by partial melting at all.