U-SERIES ISOTOPE EVIDENCE FOR THE ORIGIN OF CONTINENTAL BASALTS

U-series isotopic data may uniquely constrain the depth of melting of basalts and present-day compositional variability of the mantle under continents, because the (Th-230/U-238) activity ratio of basalts depends strongly on the residual mineralogy. Rift basalts from the Pinacate Volcanic Field (PVF) in Mexico have (Th-230/U-238) Of 1.22-1.35. In contrast, basalts from the San Francisco volcanic field (SFVF), Colorado Plateau, USA, part of a stable continental craton, have equilibrium (Th-230/ U-238) values. The U-series isotopic data, consistent with recent mineral/melt partitioning results [1-5], require that some continental basalts from areas of thick crust be derived from the shallower lithospheric mantle source, and rift basalts from a deeper OIB-type asthenospheric source.