Fig. 5: Results of model simulations of magmatic degassing.
From: Escalation of caldera unrest indicated by increasing emission of isotopically light sulfur
All data from Supplementary Data 1. The plots illustrate the pressure dependency of equilibrium melt (a) and gas (b–f) inferred from model simulations27 of decompression-driven, closed-system degassing of trachybasaltic CF melts (Methods). Results of nine model runs, initialized at different CO2 contents and redox conditions (Supplementary Data 1), are shown. a, Dissolved sulfur in the melt, demonstrating relatively deep sulfur degassing for CF melts. b, Pressure-dependent evolution of the model magmatic gas CO2/ST ratio (ST, total sulfur). c, Sulfur speciation in the gas phase, indicating that, at pressures higher than 100 MPa, SO2 makes up from <40% at NNO to ~80% at NNO + 0.75, to >95% at NNO + 1.5. This implies that, in the magmatic scenario hypothesis, fumarolic H2S can serve as a proxy of total magmatic sulfur only at condition that the magma source is reduced (~NNO), deep (>100 MPa, or > 4 km) and that any SO2 is converted into H2S during gas expansion and cooling, from magmatic temperature to surface temperature (150–165 °C). d–f, Model-predicted evolution of H2O, CO2 and ST molar fractions in the equilibrium magmatic gas phase. The gas becomes increasingly CO2-rich (and ST-rich) with increasing model run pressure.
