MULTI-STAGE DEGASSING AT MASAYA VOLCANO, NICARAGUA

The Masaya caldera in Nicaragua has erupted numerous times in recorded history, producing lava lakes and fluid lava flows. Moreover Masaya is an atypical basaltic arc volcano known to produce large explosive eruptions. Despite its unusual products and eruptive style only a few studies have examined Masaya's geochemistry. Moreover pre-eruptive volatiles from this setting have not been studied although considerable work has been conducted on its gas emissions. Here we present results from melt inclusion at Masaya caldera. These inclusions are basaltic, with low H₂O (< 0.5 wt. %), low S (<300 ppm) and high CO₂ concentrations (up to ~ 6000 ppm). Relationships between water, sulfur, Cl and F in combination with Masaya's high CO₂ and Ba/Zr and Ba/Nb ratios suggest that Masaya has undergone a multi stage degassing process involving 1) shallow degassing, 2) recycling of magma into a deeper reservoir, and 3) fluxing of previously degassed magma with a nearly pure CO₂ vapor. Trace element signatures of melt inclusions are consistent with contributions of melts derived from mantle that has variably metasomatized by fluids generated by dehydration of subducted sediments and/or altered oceanic crust.