The Role of Exsolved Fluids on the Mo Isotopic Composition of Arc Lavas: Insights From the Adakitic Rocks of Solander Volcano

Abstract

Abstract The Molybdenum isotopic composition (δ 98/95 Mo) of arc lavas is highly variable. While source processes play an important role in controlling the δ 98/95 Mo of arc magmas, the impact of differentiation on this variability remains poorly understood. In this study, we assess and deconvolute the effects of source and crustal processes on the δ 98/95 Mo of lavas and enclaves with adakitic compositions from Solander volcano, located above the incipient Puysegur subduction zone. These rocks show extreme δ 98/95 Mo variation (−0.35‰ to +0.79‰). The most mafic samples have light δ 98/95 Mo (−0.35‰ to −0.29‰) and low Mo/Ce ratios (0.006–0.007) consistent with published models invoking the addition of altered oceanic crust melts to their mantle source. However, the range of δ 98/95 Mo in the rest of the sample set is unlikely to result from source processes. Covariations of δ 98/95 Mo and Mo contents with MgO concentrations indicate that the δ 98/95 Mo and Mo content variability was generated during differentiation. Similar Sr‐Nd isotope compositions of all Solander lavas exclude a significant role for crustal assimilation. Mineral fractionation alone cannot reproduce the observed systematics without invoking unreasonable δ 98/95 Mo for the fractionated phases. Instead, we suggest that Mo mobility and associated isotopic fractionation during supercritical fluid exsolution and unmixing, followed by brine assimilation and magma mixing, are the first order controls on δ 98/95 Mo and Mo concentration variation in the Solander adakitic magmas. Thus, the role of magmatic fluids on the δ 98/95 Mo of intermediate‐silicic arc rocks can be significant, at least in adakitic systems, and needs to be considered prior to a source assessment. Plain Language Summary The Molybdenum isotopic composition (δ 98/95 Mo) of volcanic arc lavas is highly variable. While mantle source processes play an important role in controlling the δ 98/95 Mo of arc lavas, the impact of magmatic differentiation (dominantly occurring at crustal levels) on this variability remains poorly understood. In this study, we assess and deconvolute the effects of source and crustal processes on the δ 98/95 Mo of lavas and enclaves with adakitic compositions (a type of volcanic arc rock with high Sr/Y, La/Yb, and low Y and HREE contents) from Solander volcano, located above the incipient Puysegur subduction zone. These rocks show extreme δ 98/95 Mo variation. We show that, apart from the δ 98/95 Mo of the most mafic samples that are consistent with previously inferred mantle source processes at Solander volcano, the range of δ 98/95 Mo in the rest of the sample set is best explained by magmatic differentiation effects. We suggest that most of the δ 98/95 Mo variability in Solander lavas reflects the effects of fluid loss from the magma as well as the unmixing of that fluid into brine (high‐density and high‐salinity fluid) and vapor (low‐density and low‐salinity fluid) and the assimilation of brine by magma located at the roof of the chamber. Key Points The range of Mo isotopic compositions of the Solander adakitic rocks is extreme Most of this variation was created during magmatic differentiation but does not result from mineral fractionation or crustal assimilation Mo isotope fractionation during supercritical fluid exsolution and unmixing followed by brine assimilation led to the isotopic variability