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 (230Th/238U) activity ratio of basalts depends strongly on the residual mineralogy. Rift basalts from the Pinacate Volcanic Field (PVF) in Mexico have (230Th/238U) 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 (230Th/238U) 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.