The effect of pre-eruptive fluid exsolution on the volatile budgets of large explosive eruptions in Central America: constraints from fluid inclusions and thermobarometry

Abstract The amounts of volatiles emitted from large Plinian eruptions are typically estimated using the difference between their concentration in silicate melt inclusions formed at depth, and their concentration in the partially degassed glassy groundmass of tephras (the so-called petrologic method). However, a pre-eruptive fluid phase coexisting with the magma prior to eruption may add significantly to the emission budgets. We have combined previously published chlorine emission data obtained by the petrologic method from seven Plinian eruptions along the Central American Volcanic Arc (CAVA) with new data obtained from magmatic fluid inclusions in the same samples. The presence of the magmatic fluid inclusions demonstrates the pre-eruptive criticality of these volcanic systems. The pre-eruptive magmatic fluid phase of silicic CAVA eruptions is water dominated, and contains on average 5 ± 3.5 mass% NaCl equivalents and 5 ± 4 mass% CO 2 with no systematic along-arc variations. The pressures obtained from the typical magmatic fluid inclusions range between 140 ± 30 and 170 ± 30 MPa for the various eruptions, which corresponds to minimum pre-eruptive water contents in the melts between 4.8 and 5.2 mass%. We consider a scenario where each magma coexists with between 1 and 5 volume % pre-eruptive fluid, which is erupted together with the magma, and thus adds on average 6 to 30 mass % “excess” Cl to the degassing budgets determined by the petrologic method, and by inference also represents minimum values for “excess” Br degassing. The high efficiency of Br for ozone destruction in the stratosphere is enhanced through interactions with sulfur aerosols present in an eruption column, causing average increases in stratospheric halogen loading, referred to as equivalent effective stratospheric chlorine (EESC), between 6 and 97% per eruption. Depending on the amount and composition of a pre-eruptive fluid phase, the estimated stratospheric loading of a Plinian eruption may thus be doubled compared to data from the petrologic method. Fluid inclusion data from other large eruptions may therefore be used to significantly revise the global emission budgets and the effects of stratospheric ozone destruction related to Cl and Br from large explosive eruptions.