Gas Leakage From Shallow Ponding Magma and Trapdoor Faulting at Sierra Negra Volcano (Isabela Island, Galápagos)

We report on new volcanic gas composition results acquired in October 2017 at Minas de Azufre, a persistent fumarolic field topping the resurgent Sierra Negra caldera, in the Galápagos archipelago. Our results indicate that the Minas de Azufre fumaroles are moderately hydrous (52–64 mol.% H2O) and rich in CO2 (35–46 mol.%), with total sulfur (ST) being 21–35 times less abundant than CO2. SO2, the most abundant S species, is released at an average rate of 19 ± 9 tons/day. Using a volatile saturation model that provides the composition of magmatic gases at equilibrium with western Galápagos basaltic melt (48 wt. % SiO2) in the 400–0.1 MPa pressure range, we infer that Minas de Azufre fumarolic emissions consist of a mixture of (a) magma-derived gases coexisting with a melt at ∼50–60 MPa and (b) shallow meteoric water. We thus propose that the fumaroles are supplied by outgassing of magma stored in a ∼2 km deep sill-like reservoir underneath the caldera floor, and that the trapdoor fault system at the western margin of the resurgent caldera block acts as a preferential pathway for magmatic gas ascent and surface discharge. Our results thus suggest that, in contrast to the majority of the volcano-hosted hydrothermal systems worldwide, Minas de Azufre releases a relatively pristine magmatic gas.