Hirschmann The mechanism and timing of partial melting and melt extraction in the mantle beneath arcs constitutes one of the key unresolved questions regarding arc magmatism. This collaboration between researchers at the Universities of Minnesota and Iowa will use analyses of U-series nuclides and particularly of 231Pa in young volcanic rocks to better understand the melting process beneath the Central American arc and to determine how this process is affected by fluxes of components from the subducting slab. Preliminary studies show that 231Pa is strongly fractionated from U in the sub-arc mantle prior to eruption on a time scale similar to or less than the half life of 231Pa (33 kyr), strongly suggesting that 231Pa is a sensitive indicator of the timing and mechanism of melting and melt extraction. Measurements of (231Pa)/(235U) and (230Th)/(238U) activity ratios using thermal ionization mass spectrometry and (226Ra)/(230Th) ratios using alpha counting techniques will be made for an along-arc suite of young volcanic rocks from the southern Central American arc in Costa Rica and Nicaragua. The southern Central American arc is an ideal location for studying melting and mass transfer processes using U-series isotopes because many of the parameters affecting arc volcanism, such as the composition of the mantle and the mass flux of components from the subducting slab, vary across the region in a systematic and well-characterized fashion. The resulting data set will be to our knowledge the most complete study of U-series isotopes associated with an arc province, and combined with the trace element and long-lived isotopic data will yield a unique body of data against which hypotheses for melting and mass transfer in arcs can be tested.
