U-Th isotope studies of recent volcanic rocks may be used to evaluate the depths and rates of melt generation beneath mid- ocean ridges and ocean islands. Island arc rocks exhibit a similar range in Th isotopes to that seen in both MORB and OIB,
but while those with high (230Th/232Th) tend to have excess 238U, many of those with low (230Th/232Th) are in isotope
equilibrium. The excess 238U is attributed to the preferential mobilisation of U in hydrous fluids from the subducted slab
within the last 100,000 years, whereas the low Th isotope ratios in the higher-K arcs are primarily due to the introduction
of subducted sediment. Preliminary results from mineral separates from lavas on Soufriere on the island of St. Vincent
indicate that some of the minerals crystallised up to 50,000 years prior to eruption.