U-Th
isotope studies of recent volcanic rocks may be used to evaluate the depths and
rates of melt generation beneath midocean ridges and ocean islands. Island arc
rocks exhibit a similar range in Th isotopes to that seen in both MORB and om,
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.