Tracing the evolving flux from the subducting plate in the Tonga-Kermadec arc system using boron in volcanic glass

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was i nvestigated using ion microprobe measurements of B/Be and boron isotope rat ios (B-11/B-10) to document the source and relative volumes of the fluids r eleased from the subducting oceanic plate. We analyzed young lavas from eig ht different islands along the Tonga-Kermadec Are, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which rec ord the variations in the chemistry of Tonga magmatism. since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from similar to3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for are lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Toge ther with very high delta B-11 values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid esca ped from the subducting altered Pacific oceanic crust, rather than from sed iment. High delta B-11 values also reflect large degrees of isotopic fracti onation in this cold fast subduction zone. Lower delta B-11 values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment ero ded from New Zealand and slower convergence. High fluid flux (B/Be) is sync hronous in Tonga and the Marianas at similar to3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma. correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maxi mum degrees of partial melting at this time. Although thinning of the arc l ithosphere during rifting to form the Lau Basin is expected to influence th e arc geochemistry, variable aqueous fluid flux from the subducting plate a lone appears capable of explaining boron and other trace element systematic s in the Tonga-Kermadec Arc with no indication of slab melting. Copyright ( C) 2001 Elsevier Science Ltd.