Degassing of volatiles (H2O, CO2, S, Cl) during ascent, crystallization, and eruption at mafic monogenetic volcanoes in central Mexico

Mafic monogenetic volcanoes (cinder cones, maars) have eruption styles that include highly explosive, mildly explosive, and effusive regimes. Here we investigate the degassing and vapor-melt partitioning of volatiles (H2O, CO2, S, Cl) in monogenetic volcanoes from the subduction-related Michoacán–Guanajuato Volcanic Field (MGVF) in central Mexico. Olivine-hosted melt inclusions from these volcanoes contain variably degassed melts that were trapped over a wide range of pressures from < 50 MPa to ∼ 300 MPa. Variations in melt compositions and volatile contents provide evidence that crystallization and differentiation were driven by degassing of H2O. Melt CO2 and H2O concentrations are highly variable, and much of the variation does not conform to equilibrium open- or closed-system degassing paths. Instead, we suggest that gas-fluxing – partial re-equilibration of magmas with CO2-rich gases rising from depth – can explain the variable CO2 and H2O concentrations in the melts. Such fluxing may be common in basaltic systems, and it increases the extent of crystallization during magma ascent by removing dissolved H2O from vapor-saturated (but H2O-undersaturated) melts. Strong degassing of S and Cl during magma ascent and crystallization begins at pressures of approximately 50 MPa. Using the relationship between degassing and crystallization, we calculate apparent vapor-melt partition coefficients for S and Cl. Our results show that, overall, S partitions more strongly into the vapor phase than Cl, consistent with published experimental data and thermodynamic models, and that vapor-melt partitioning of S increases more strongly with decreasing pressure than Cl. The S and Cl partitioning behavior inferred from the melt inclusion data are consistent with the gas fluxing model suggested by the H2O and CO2 data.