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

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• 作者：Emily R. Johnson ; Paul J. Wallace ; Katharine V. Cashman ; Hugo Delgado Granados
• 关键词：degassing ; volatiles ; melt inclusions ; basalt ; crystallization
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2010
• 出版时间：30 November 2010
• 年：2010
• 卷：197
• 期：1-4
• 页码：225-238
• 全文大小：2117 K

文摘

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.