The role of pre-eruptive gas segregation on co-eruptive deformation and SO2 emissions

The presence of exsolved gas bubbles influences measurements of both volcanic surface deformation and SO2 emissions. In a closed-system, exsolved volatiles remain within the melt but in an open-system, the decoupled gas phase can either outgas or accumulate, leading to large variations magmatic gas fraction. Here we investigate the role of gas volume fraction and gas segregation processes on magma properties and co-eruptive monitoring data. First we use thermodynamic models of gas exsolution to model gas volume fraction and magma compressibility, and use these to calculate SO2 emissions and co-eruptive volume change. We find that volume change is equally sensitive to magma compressibility and chamber compressibility over realistic parameters ranges, and both must be considered when interpreting surface deformation data. Reservoir depth and magma composition are the dominant controls on gas volume fraction, but the initial content of H2O and S have strong influences on volume change and SO2 emissions, respectively. Pre-eruptive gas accumulation produces increased SO2 emissions and muted co-eruptive deformation, while degassing has the opposite effect. We then compare our models to a compilation of data from 20 recent eruptions where measurements of volume change, SO2 emissions and erupted volume are available. To the first order, shallow reservoirs produce smaller volume changes per volume erupted and silica-poor magmas yield greater co-eruptive volume changes than silica-rich systems, consistent with closed system degassing. Co-eruptive degassing causes high SO2 emissions during effusive eruptions. Comparison between model predictions and observations suggests that all magmatic systems experience a certain degree of outgassing prior to an eruption. Our findings are consistent with current conceptual models of transcrustal magmatic systems consisting of heterogeneous mixtures of gas and melt and have important implications for the interpretation of surface deformation and SO2 emission signals at all stages of the eruption cycle.