Determining melt productivity of mantle sources from 238U-230Th and 235U-231Pa disequilibria; an example from Pico Island, Azores

We use coupled U-238-Th-230 and U-235-Pa-231 disequilibria measurements from Pico Island, Azores to examine the melting behavior of the underlying mantle. U-series disequilibria in young, mafic lavas are dependent on the melting rate of their source, which in most cases is primarily controlled by its melt productivity. Mafic lithologies such as eclogite and pyroxenite have much higher melt productivities than peridotite and so U-series measurements may provide constraints on the mineralogy of the melting mantle. Recent Pico Mountain lavas show limited geochemical variations and a restricted range of U-series disequilibria with (Th-230/U-238) = 1.22-1.25 and (Pa-231/U-235) = 1.46-1.50. Using a simple, dynamic melting model of a homogeneous source, these results can be reproduced with melting rates of