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.
Bibliographical noteFunding Information:
We thank D. Lynch for help with fieldwork in Mexico, D. Pickett for sharing unpublished Pa data and discussion, and M. Reagan and two anonymous journal reviewers for very thoughtful, thorough and constructive reviews. We have also benefited from additional reviews by our colleagues at UNM, A. Brearley, R. Jones and C. Yapp. Y.A. expresses his deep appreciation to the folks of Lake Wobegan for their generosity, which made the work pleasant. This project was supported by funds from the McKnight Foundation to L.E. \[CL\]