From mantle to atmosphere: Magma degassing, explosive eruptions, and volcanic volatile budgets

Over the past 25 years there has been a growing body of evidence based on petrologic, remote sensing, and volcanic gas data that andesitic, dacitic, and rhyolitic magmas in crustal reservoirs are saturated with a multicomponent C-O-H-S vapor phase before eruption. A key piece of evidence for magmatic vapor saturation is the "excess" S problem: the common observation that much more S is released by explosive eruptions than was originally dissolved in the erupted volume of silicate melt. If all of the "excess" S is derived from a magmatic vapor phase, then intermediate to silicic magmas must commonly contain several wt% exsolved vapor prior to eruption. The large amounts of volatiles implied by these estimates suggest that exsolved vapor accumulates in the apical regions of magma bodies during repose periods between eruptions. A major uncertainty in understanding volcanic SO2 emissions has been lack of information on S partitioning between silicate melt and coexisting hydrous vapor. Recently published experimental data on melt-vapor partitioning show that S partitions strongly into the vapor phase under conditions relevant for most dacitic and rhyolitic magmas. Thermodynamic modeling based on these results suggest that the pre-eruptive magmatic vapor phase for dacitic to rhyolitic magmas typically contains 0.5 to 6 mol% total S. Andesitic, dacitic, and rhyolitic magmas in crustal reservoirs are probably vapor saturated due to recharge and underplating by vapor-saturated basaltic magma. CO2 is particularly important because it is abundant in mantle-derived basaltic magma but has low solubility in silicate melts at crustal pressures. Understanding budgets of the major volatiles requires integrating remote sensing and volcanic gas data for volatile fluxes from volcanoes with petrologic data for both differentiated magma stored in crustal reservoirs and mafic magma recharging these systems. Comparison of repose times, eruptive volumes, and basaltic magma supply rates for a spectrum of volcanic systems suggests that the flux of S, CO2 and H2O from explosive eruptions is approximately balanced by the mantle-derived supply rate of these volatiles provided by mafic recharge into the crust.