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 (Type 1: 1% phenocrysts; Type 2: 15% phenocrysts). 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. Moreover, the Type 2 samples have low Ba/Th ratios and high La/Yb (29.5-43) and Sr/Y (50-58), which are classically attributed to partial melting of the basaltic slab. The slab surface temperature (SST) was calculated from H2O/Ce data; Type 1 SST shows temperatures comparable to global arcs (∼767°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. Mixing calculations suggest that Type 2 is not a simple mixture of the Type 1 basalt with an end-member slab melt. Alternate explanations include the possibility that Type 1 and Type 2 are instead the results of a mantle melt component mixing with either slab fluid or slab melt (but not both), or that they are distinct melts from different parts of the wedge that have migrated into the same volcanic system. A global correlation between H 2O and Fe3+/∑Fe ratio suggests an oxidized, H 2O-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.
"Near-primary mantle melts and their implications for the mechanism of island arc basalt oxidation"
Dissertations and Master's Theses (Campus Access).