Melting of the Earth's lithospheric mantle inferred from protactinium– thorium–uranium isotopic data

The processes responsible for the generation of partial melt in the Earth's lithospheric mantle and the movement of this melt to the Earth's surface remain enigmatic, owing to the perceived difficulties in generating large-degree partial melts at depth and in transporting small-degree melts through a static lithosphere1. Here we present a method of placing constraints on melting in the lithospheric mantle using 231Pa–235U data obtained from continental basalts in the southwestern United States and Mexico. Combined with 230Th–238U data2,3, the 231Pa–235U data allow us to constrain the source mineralogy and thus the depth of melting of these basalts. Our analysis indicates that it is possible to transport small melt fractions—of the order of 0.1%—through the lithosphere, as might result from the coalescence of melt by compaction4 owing to melting-induced deformation5. The large observed 231Pa excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of 231Pa (∼32.7 kyr). The 231Pa–230Th data also constrain the thorium and uranium distribution coefficients for clinopyroxene in the source regions of these basalts to be within 2% of one another, indicating that in this setting 230Th excesses are not expected during melting at depths shallower than 85 km.