Melting of the Earth's lithospheric mantle inferred from protactinum-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 lithosphere(1). Here we present a method of placing constraints on melting in the lithospheric mantle using Pa-231-U-235 data obtained from continental basalts in the southwestern United States and Mexico. Combined with Th-230-U-238 data(2,3), the Pa-231-U-235 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 compaction(4) owing to melting-induced deformation(5). The large observed Pa-231 excesses require that the timescale of melt generation and transport within the lithosphere is small compared to the half-life of Pa-231 (similar to 32.7 kyr). The Pa-231-Th-230 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 Th-230 excesses are not expected during melting at depths shallower than 85 km.