The generation of uranium series disequilibria by partial melting of spinel peridotite: constraints from partitioning studies

Abstract

Alkaline earth, Pb and actinide partition coefficients have been measured for olivine-, orthopyroxene- and clinopyroxene-melt pairs in experimental charges using secondary ion mass spectrometry. The bulk partition coefficients for spinel peridotites in equilibrium with basaltic melts are 1.1 × 10 −4 and 1.7 × 10 −4 for U and Th, respectively; both are dominated by the presence of clinopyroxene. These new data demonstrate that the bulk partition coefficient for uranium is lower than that for thorium in spinel peridotites, even if hydrous or accessory phases are stable at the solidus. U is similarly incompatible compared to Th for spinel peridotite in equilibrium with carbonate melts. These results demonstrate that the partial melting of a spinel peridotite at chemical equilibrium cannot account for the ( 230Th/ 238U) ratios in excess of unity observed in MORB. The generation of the observed ( 230Th) excesses by disequilibrium partial melting of spinel peridotite requires that the diffusivity of Th is greater than that of U, which is improbable, as the ionic radius of Th is greater than that of U. Similarly, ( 230Th) excesses can be produced on melt percolation only if the bulk partition coefficient of Th is smaller than that of U, and cannot therefore be generated in the spinel stability field. ( 230Th/ 238U) ratios greater than unity might be produced by slow, near fractional partial melting in the garnet stability field, if D U * gt/L is large compared to D Th * gt/L and D Th * cpx/L.