U-TH-PB PARTITIONING BEHAVIOR DURING PARTIAL MELTING IN THE UPPER MANTLE - IMPLICATIONS FOR THE ORIGIN OF HIGH MU-COMPONENTS AND THE PB PARADOX

The partitioning behaviour of U, Th, and Pb in the upper mantle has been investigated through the analysis of ultramafic nodules found in lava flows at two localities in the southwestern United States. Bulk partition coefficients for Pb, Th, and U during melting are inferred to depend primarily on the abundance of clinopyroxene and sulfide in the residual mantle. On the basis of the model presented, the "Pb paradox' is explained as a direct consequence of melt depletion from the upper mantle. -from Authors