Water content, δD and δ11B tracking in the Vanuatu arc magmas (Aoba Island): Insights from olivine-hosted melt inclusions

Ion microprobe measurements of H and B isotopic ratios and H2O, B and trace element contents are reported here for a series of melt inclusions typical of alkaline basalts of Aoba Island in the central part of Vanuatu arc (Southwestern Pacific). The melt inclusions, hosted in olivine Fo(86-90), display large ranges in trace element concentrations and hydrogen (delta D from -48.2 to +61.7 parts per thousand) and boron (delta B-11 from -11.9 to + 6.4 parts per thousand.) isotopic compositions. The high deuterium enrichment (delta D >= 0 parts per thousand) observed in a small subset of melt inclusions requires a proton diffusion loss through the olivine network, in addition to late-stage magma interactions with aqueous saline fluids. These melt inclusions are therefore not considered as representative of the magma from which the olivine grew. In most melt inclusions, positive correlations between H2O, K2O, Ba and Sr lead us to determine the K2O/H2O (1.5 +/- 02), H2O/Ba (46 +/- 3 x 10(-4)) and H2O/Sr (29 +/- 2 x 10(-4)) ratios of Aoba basalts. Overall correlations between delta B-11, B/Nb, and B/Nd testify to the mixing between slab-derived fluids, preferentially enriched in delta B-11 and fluid mobile elements and a relatively depleted MORB-type mantle wedge beneath Aoba Island. Heavy delta B-11 (on average 5.4 +/- 0.7%.) indicate slab-derived fluids, possibly involving serpentine, which would have a mean delta D value of -28.4 +/- 7 parts per thousand.The chemical and isotopic variability recorded by Aoba magmas (melt inclusions) is consistent with the geodynamic context of ridge-arc collision in the central segment of Vanuatu arc. (C) 2014 Elsevier B.V. All rights reserved.