Gauthier, Pierre-J.

Las Sierras-Masaya volcanic system is a persistently active basaltic caldera complex in Nicaragua. While there has been almost no juvenile material erupted since 1772, Masaya volcano has been persistently degassing for >150 years. An additional unusual behaviour for the Las Sierras-Masaya volcanic complex is its ability to produce large caldera-forming basaltic Plinian eruptions with the most recent occurring about 1800 years ago. Here we present melt inclusion analyses that provide constraints on the magmatic system over time. Melt inclusions hosted in plagioclase and olivine crystals were analyzed for major, trace and volatile elements (S, Cl, F, H2O). The data supports a consistent parental magmatic source with restricted compositional variability explained by simple fractional crystallization of plagioclase, olivine, clinopyroxene and magnetite at a nearly constant temperature. This broadly agrees with previous whole rock geochemical studies showing that the overall chemical signature of volcanic products at Masaya has remained largely unchanged for ~60,000 years and that both shallow fractionation and degassing processes dominate the whole evolution of the magmatic series. Based on volatile element in melt inclusions and gas composition and flux measurements, we determine the magmatic flux to be ~0.19 km3 yr−1 implying that up to 47 km3 of magma may have degassed since the last effusive eruption. As at other persistently active basaltic volcanoes (e.g., Mt. Etna, Italy; Kilauea, Hawaii, USA), this magmatic flux must involve significant endogenous storage which is likely accommodated by extensional tectonics. However, Masaya volcano differs in its apparent simplicity with respect to its stable chemistry and its fully interconnected magmatic system.