NEAR-PRIMARY MANTLE MELTS AND THEIR IMPLICATIONS FOR THE MECHANISM OF ISLAND ARC BASALT OXIDATION

Near-primary melt compositions (i.e., in equilibrium with >Fo₈₈ 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. This study presents 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. We selected melt inclusions from each type and determined major elements and sulfur 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 ≥Fo₈₈ olivine, consistent with host olivine compositions, and thus are near-primary melt compositions. Both have high Mg# (>65), and are basalt to basaltic andesite (49-55 wt% SiO₂). Samples from Type 1 show relatively flat REE patterns, classic high Ba/Th ratios, and positive anomalies in Pb and Sr. In contrast, samples from Type 2 exhibit steeply sloped REE patterns with strong depletions in the HREE that suggest garnet in the source lithology for these magmas. The slab surface temperature (SST) was calculated from H₂O/Ce data; Type 1 SST shows temperatures comparable to global arcs (~781°C), while Type 2 SST is the hottest yet constrained by this method (~1041°C). Volatile analysis reveals that both lava types have had some degassing of H₂O with CO₂, and give minimum H₂O contents of each magma: ~3 wt.% for Type 1, ~2.5 wt.% for Type 2. XANES analysis shows that Type 1 samples have Fe³⁺/∑Fe ratios similar to global arc basalts (~0.23), while Type 2 samples have Fe³⁺/∑Fe ratios that are among the highest measured in natural terrestrial glasses (~0.34), and have much higher concentrations of S. A global correlation between H₂O and Fe³⁺/∑Fe ratio suggests an oxidized, H₂O-rich component is common to most arcs. The Type 1 magma conforms to this global trend, but Type 2 does not. Despite its highly oxidized condition and high sulfur content, Type 2 is too dry to be the end-member component that appears to be delivering oxidation to most global arc magmas.