TRACE-ELEMENT TRANSPORT RATES IN SUBDUCTION ZONES - EVIDENCE FROM TH,SR AND PB ISOTOPE DATA FOR TONGA-KERMADEC ARC LAVAS

Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec are in the southwest Pacific suggest that geochemical v ariations in the lavas along the are are linked to differences in the material being subducted beneath the are. Lavas from the southern (Ker madec) segment of the are have relatively radiogenic Pb isotope compos itions, which reflects a contribution from subducted sediment. In cont rast, much of the Pb in Tonga lavas is derived from the altered oceani c crust in the subducting Pacific Plate, and lavas from the northernmo st Tonga islands of Tafahi and Niuatoputapu contain Pb and Sr derived from the subducted part of the Louisville Seamount Chain. The origin o f the Pb in the lavas from these two islands can thus be traced to a p oint on the subducting slab, and this observation is used to estimate the rate at which truce elements are transported beneath the are. Our calculations suggest that fluid-soluble elements such as U, Sr and Pb are transported from the subducted slab, across the mantle wedge and b ack to the surface in lavas over a period of approximately 2-3 Ma, and that magmas are erupted at the surface less than 350 ka after the mel ts are generated in the mantle wedge. (C) 1997 Elsevier Science B.V.