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 (δ98/95Mo) of arc lavas is highly variable. While source processes play an important role in controlling the δ98/95Mo 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 δ98/95Mo of lavas and enclaves with adakitic compositions from Solander volcano, located above the incipient Puysegur subduction zone. These rocks show extreme δ98/95Mo variation (−0.35‰ to +0.79‰). The most mafic samples have light δ98/95Mo (−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 δ98/95Mo in the rest of the sample set is unlikely to result from source processes. Covariations of δ98/95Mo and Mo contents with MgO concentrations indicate that the δ98/95Mo 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 δ98/95Mo 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 δ98/95Mo and Mo concentration variation in the Solander adakitic magmas. Thus, the role of magmatic fluids on the δ98/95Mo of intermediate‐silicic arc rocks can be significant, at least in adakitic systems, and needs to be considered prior to a source assessment.

Keywords

• adakites
• subduction zones
• fluid exsolution
• fluid unmixing
• brine assimilation