Cj. Hawkesworth et al., U-TH ISOTOPES IN ARC MAGMAS - IMPLICATIONS FOR ELEMENT TRANSFER FROM THE SUBDUCTED CRUST, Science, 276(5312), 1997, pp. 551-555
Uranium-thorium isotope results from island are volcanic rocks were us
ed to investigate the rates of transfer of fluids and sediments from t
he downgoing slab. Uranium, but not thorium, is readily mobilized in t
he fluid. A negative array between thorium/cerium and neodymium-143/ne
odymium-144 indicates that significant amounts of the thorium in are r
ocks are derived from subducted sediments, although perhaps only about
30 percent of the thorium in subducted sediments is returned to the c
rust in this way. The transfer times for fluid through the mantle wedg
e are about 30,000 to 120,000 years, whereas those for sediment melts
may be several million years. The law average uranium/thorium ratios o
f bulk crust primarily reflect different crustal generation processes
in the Archaean.