Planetary atmospheres can have three different origins: nebular gas accreted from the protoplanetary disk (primordial); early, syn-accretionary gas release from planetesimal accretion or outgassing during the cooling of a magma ocean (primary); or long-term release of volatiles from the planetary interior for example, through volcanism (secondary). The atmospheres of rocky planets may evolve from primordial/primary to secondary as they undergo significant modification by geological and atmospheric processes, altering the atmosphere's mass fraction and chemical composition. Modification processes include hydrodynamic escape, erosion and volatile addition from impacts, and volcanic outgassing of volatiles from the interior (see Table 1 for references). A more extensive list of processes which can modify planetary atmospheres, and associated literature which has investigated these effects, can be found in Table 1.In this paper series, we focus on secondary atmospheres which form as a result of volcanic outgassing. The chemistry of volcanic gases is dependent on the oxygen fugacity (fO 2 ) of the magma (and likewise the mantle from which the magma was formed, e.g., Burgisser et al., 2015;Gaillard et al., 2015;Ortenzi et al., 2020), surface pressure through volatile solubility in magmas (Gaillard & Scaillet, 2014), and the relative abundance of volatile elements (i.e., H, C, O, S and N) within the magma. The secondary atmospheres of volcanically active rocky planets are therefore inextricably linked to their geological state. This is particularly useful when considering
The behaviour of sulfur in magmas is complex because it dissolves as both sulfide (S 2- ) and sulfate (S 6+ ) in silicate melt. An interesting aspect in the behaviour of sulfur is the solubility minima (SS min ) and maxima (SS max ) with varying oxygen fugacity ( f O 2 ). We use a simple ternary model (silicate–S 2 –O 2 ) to explore the varying f O 2 paths where these phenomena occur. Both SS min and SS max occur when S 2- and S 6+ are present in the silicate melt in similar quantities due to the differing solubility mechanisms of these species. At constant T , a minimum in dissolved total S content in vapour-saturated silicate melt ( w S T m ) occurs along paths of increasing f O 2 and either constant f S 2 or P . For paths on which w S T m is held constant with increasing f O 2 , the SS min is expressed as a maximum in P . The SS min occurs when the fraction of S 6+ in the melt ([S 6+ /S T ] m ) is 0.25 for constant f S 2 and [S 6+ /S T ] m ≅ 0.75 for constant w S T m and P . A minimum in w S T m is not encountered during closed- or open-system depressurisation in the simple system we modelled. However, the SS min marks a change from reduction to oxidation during degassing. Various SS max occur when the silicate melt is multiply-saturated with at least two phases: vapour, sulfide melt, and/or anhydrite. The SS min and SS max are important features of magmatic process involving S, such as mantle melting, magma mixing, and degassing. These concepts influence calculations of the pressures of vapour-saturation, f O 2 , and SO 2 emissions using melt inclusions. Supplementary material: Additional information and data used to create the figures are available at https://doi.org/10.6084/m9.figshare.c.6274527 . The code used to generate the data is available at https://github.com/eryhughes/SSminmax . Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/topic/collections/sulfur-in-the-earth-system
Two of the most widely observed co‐eruptive volcanic phenomena—Ground deformation and volcanic outgassing—Are fundamentally linked via the mechanism of magma degassing and the development of compressibility, which controls how the volume of magma changes in response to a change in pressure. Here we use thermodynamic models—Constrained by petrological data—To reconstruct volatile exsolution and the consequent changes in magma properties. We use the fraction of SO2 exsolved during decompression to predict co‐eruptive SO2 flux and magma compressibility to predict co‐eruptive surface deformation (both normalized by erupted volume). We conduct sensitivity tests using properties of typical basalts to assess how varying magma volatile content, crustal properties, and chamber geometry affect co‐eruptive deformation and degassing. We find that magmatic H2O content has the most impact on both SO2 flux and volume change. Our findings have general implications for typical basaltic systems in arc and ocean island settings. The higher water content of arc magmas makes them more compressible than ocean island magmas and leads to muted or non‐existent deformation being observed during arc eruptions. Our models are consistent with observation: Deformation has been detected during 48% of basaltic eruptions in ocean island settings (16/33) during the satellite era (2005–2020), but only 11% of basaltic eruptions in arc settings (7/61).
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