Near-Primary Oxidized Basalts from the Submarine Vanuatu Arc

Abstract:

Near-primary melt compositions (i.e., in equilibrium with >Fo₈₉ olivine) are rare in arc systems. Yet, such melts provide essential views of mantle-derived melts, without further modification by fractional crystallization or other crustal processes, and reveal the diversity of melt compositions that exist in the arc mantle wedge. Here, we present new measurements of naturally glassy, near-primary olivine-hosted melt inclusions from one dredge of Evita seamount (SS07/2008 NLD-02) in the southern Vanuatu arc system. Two distinct basalt types were identified in hand sample upon collection, based on contrasting phenocryst assemblage (Type 1: 1% phenocrysts; Type 2: 15% phenocrysts). We selected melt inclusions from each type and determined major elements, S, and Cl by EMP, H₂O and CO₂ by FTIR, trace elements by LA-ICP-MS, and Fe³⁺/∑Fe ratios by XANES. Melt inclusions from both lava types show equilibrium with ≥Fo₉₀ olivine, consistent with host olivine compositions, and thus are near-primary melt compositions that have escaped major modification since departing the mantle wedge. Both have similar maximum dissolved H₂O (~2.3 wt.%), high Mg# (48-75), and are basalt to basaltic andesite (SiO2 49-55 wt.%). However, the two lava types have very different major and trace element compositions. Inclusions from Type 1 show relatively flat REE patterns and classic negative anomalies in Nb and Ta, and positive anomalies in Pb and Sr typical of normal arc basalts, and have Fe3+/∑Fe ratios similar to global arc basalts (~0.24). In contrast, melt inclusions from Type 2 exhibit steeply sloped REE patterns with strong depletions in the HREE that suggest garnet in the source lithology for these magmas, either in the subducting slab or the mantle wedge. Moreover, the Type 2 inclusions have high La/Yb (29.5-43) and Sr/Y (50-58), which are classically attributed to partial melting of the basaltic slab, although these inclusions are basaltic, not andesitic. Type 2 inclusions also have Fe3+/∑Fe ratios that are among the highest measured in natural terrestrial glasses (~0.32). The compositions of these near-primary basalts indicate that melts of the mantle wedge, and perhaps melts of the slab, form under oxidizing conditions.