CO2 favors the accumulation of excess magmatic fluids

Abstract:

Deformation and gas emissions at active volcanoes provide insights into volcanic plumbing systems. Large discrepancies are observed between the volumes calculated from deformation data and the volumes of erupted magmas and are ascribed to the amount of excess fluids in magma reservoirs, which hinders our capacity to predict the magnitude of an imminent eruption. High-pressure (1240 bar) and high-temperature (1200 °C) hot isostatic press experiments show that the amount of trapped excess fluids in haplogranitic magmas with variable crystal contents (30, 50, 60, and 70 vol.%) depends strongly on fluid composition. Magmas with CO₂ excess fluids become permeable at much larger porosities (44% higher) with respect to the H₂O-rich counterparts at equivalent crystallinity. Available excess gas geochemistry data calculated from volatile-saturated melt inclusion record, syn-eruptive SO₂ emission, and erupted juvenile porosity data collected for silica-rich volcanic systems with known eruption magnitude and intensity (Mt St Helens 1980, Pinatubo 1991, Soufriére Hills 1996, and Merapi 2010) reveal that the discrepancy between erupted magma volume and SO₂ released during the eruption increases with CO₂ excess in magmas. In agreement with our experiments, these data highlight that CO₂-rich fluids enhance magma’s capacity to store excess volatiles and shed light on the largest discrepancies between pre-eruptive deformation and eruption intensity and magnitude.