Element budget of the Tonga subduction zone

Over the past three years we have been studying the element budget of the Tonga arc, where old oceanic crust is being subducted beneath an oceanic island arc for the past 30 million years. The Tonga arc is ideal for studying the mobility of elements during subduction because old crust that could interact and contaminate magmas on their ascent from the zone of melt generation in the mantle wedge is absent in this region. The arc crust is thin (12 km) and young (30 Myr), thereby limiting ingrowth of radiogenic isotopes and associated contamination of mantle melts.

As part of this NSF-funded project (NSF-OCE-0117936) we have analyzed the sedimentary and igneous portion of the subducting slab (DSDP Sites 204, 595 and 596), the Tonga arc (Late to Tofua, Tafahi and Niuatoputapu) as well as the back arc lavas (Valu Fa Ridge) for Sr, Nd, Li, B, O and Os isotopes, major and trace element composition, including platinum group elements. A preliminary interpretation of the large geochemical dataset indicates that a combination of crustal contamination, transport of slab-derived Os and additional fractionation processes is best suited to explain the complex geochemical observations.