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

International audienceThe influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ionmicroprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relativevolumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eightdifferent islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from OceanDrilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7Ma. B/Be is variable (5.8 –122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from ;3 to4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18–607).These values are too high to be related simply to a sediment influence on petrogenesis. Together with veryhigh d11B values (211.6 to 137.5) for the same shards and lavas these data indicate that most of the B isderived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. Highd11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11Bvalues noted in the Kermadec Arc (17 to 24.4) are related to the influence of sediment eroded from NewZealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4Ma 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 fieldstrength element depletion. This suggests maximum degrees of partial melting at this time. Although thinningof the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry,variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other traceelement systematics in the Tonga-Kermadec Arc with no indication of slab melting