U-Th Isotopes in Arc Magmas: Implications for Element Transfer from the Subducted Crust
Uranium-thorium isotope results from island arc volcanic rocks were used to investigate the rates of transfer of fluids and sediments from the downgoing slab. Uranium, but not thorium, is readily mobilized in the fluid. A negative array between thorium/cerium and neodymium-143/neodymium-144 indicates that significant amounts of the thorium in arc rocks are derived from subducted sediments, although perhaps only about 30 percent of the thorium in subducted sediments is returned to the crust in this way. The transfer times for fluid through the mantle wedge are about 30,000 to 120,000 years, whereas those for sediment melts may be several million years. The low average uranium/thorium ratios of bulk crust primarily reflect different crustal generation processes in the Archaean.
Hawkesworth
CJ
C. J. Hawkesworth, S. P. Turner, D. W. Peate, and P. van Calsteren are with the Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA, UK. F. McDermott is with the Department of Geology, University College, Dublin, Eire.
Turner
SP
McDermott
F
Peate
DW
van Calsteren P