The partitioning behavior 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. Secondary (i.e., pedogenic) components were distinguished from primary (igneous) components on the basis of Nd and Sr isotope ratios. Most of the primary components have Nd, Pb, and Sr isotope ratios within the ranges observed in oceanic basalts.

The 238U/204Pb (mu) ratios of residual (i.e., type 1A) clinopyroxene separates analyzed in this study are generally high (60--120) and show an excellent positive correlation with 147Sm/144Nd ratios. Because the latter are consistently greater than the chondritic ratio and because clinopyroxene contains most of the U in the nodules, we suggest this mineral is a ''high mu'' component in the upper mantle. Calculated ''whole rock'' U contents, based on analyses of purified separates of the major silicate minerals, are consistent with recent estimates for mid-ocean ridge basalt sources. Clinopyroxene/melt partition coefficients, based on the assumption that the lavas bearing the nodules are the melts with which the nodule minerals last equilibrated, decrease in the order Ds/lU>Ds/lTh >Ds/lpb, consistent with available experimental data. Sulfide is inferred to be a major Pb-bearing phase in the nodules. Bulk partition coefficinets for Pb, Th, and U during melting are inferred to depend primarily on the abundance 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. ¿ American Geophysical Union 1990