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

• We report data on δD, δ11B, H2O, Cl and trace elements in basaltic melt inclusions in ridge-arc collision domain.
• Hydrogen isotope kinetic fractionation accounts for abnormal D-enrichments.
• δ11B and δD values of Aoba melt inclusions are typical of arc magmas.
• Boron isotopes possibly track the contribution of serpentine-derived fluids.
• Magma hydrogen isotopic signature is dominated by 11B-enriched slab fluids.

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 Fo86–90, display large ranges in trace element concentrations and hydrogen (δD from − 48.2 to + 61.7‰) and boron (δ11B from − 11.9 to + 6.4‰) isotopic compositions. The high deuterium enrichment (δD ≥ 0‰) 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 ± 0.2), H2O/Ba (46 ± 3 × 10− 4) and H2O/Sr (29 ± 2 × 10− 4) ratios of Aoba basalts.Overall correlations between δ11B, B/Nb, and B/Nd testify to the mixing between slab-derived fluids, preferentially enriched in δ11B and fluid mobile elements and a relatively depleted MORB-type mantle wedge beneath Aoba Island. Heavy δ11B (on average 5.4 ± 0.7‰) indicate slab-derived fluids, possibly involving serpentine, which would have a mean δD value of − 28.4 ± 7‰. 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.