Replenishment of volatile-rich mafic magma into a degassed chamber drives mixing and eruption of Tungurahua volcano

Abstract

In July and August of 2006 and May of 2010, Tungurahua volcano, Ecuador, produced pyroclastic flow-forming eruptions, representing increased explosivity compared to the Strombolian events that characterized its behavior since its renewal in 1999. Volatiles (H2O, CO2, S, Cl) and major elements were analyzed in 35 melt inclusions hosted in olivine and pyroxene phenocrysts in tephra from both events to reconstruct the pre-eruptive magmatic conditions and mechanisms that led to these more explosive episodes. Melt inclusion composition paired with host phenocryst zonation indicate mixing of two distinct magmas: a volatile-rich (∼4.0 wt% H2O and ∼1,800 ppm S) basaltic andesite containing olivine phenocrysts and a degassed (∼1.0 wt% H2O and 100–500 ppm S) andesite with plagioclase and pyroxene phenocrysts that contain andesitic to dacitic melt inclusions. We attribute the lower volatile concentrations in the evolved melt inclusions to degassing that occurred during residence in a shallow reservoir, where fractional crystallization led to the production of dacitic melt. Our melt inclusion data confirm the hypothesis made on the basis of phenocryst zoning profiles (J Volcanol Geotherm Res 199:69–84, 2011) that the intrusion of a volatile-rich basaltic andesite into a more evolved chamber and subsequent mixing led to explosive eruption in 2006. Melt inclusions from the 2006 and 2010 eruptive products have comparable volatile and major element compositions. High H2O concentrations in melt inclusions from 2010 olivine indicate little diffusive loss from the melt inclusions following mixing with the degassed andesitic reservoir, which requires that the 2010 eruption be the result of a new recharge event and not remobilization of the 2006 hybrid.