A model for the saturation of C-O-H-S fluids in silicate melts

Abstract

The behaviour of volatile components in magmas is crucial for magmatic and volcanic processes, from the deep regions of magma generation and storage to the shallow regions of magma eruption and emplacement. Water, carbon dioxide, and sulphur compounds are the main volatile components in natural magmas, generally comprising more than 99% of the volcanic gases released before, during, and after eruption. We have set up a method to calculate the chemical equilibrium between a fluid phase in the C-O-H-S system and a silicate liquid with a composition defined by ten major oxides. The method is based on previous models for the saturation of H20 CO2 fluids (Papale 1997(Papale , 1999 and sulphur solubility (Moretti 2002) in silicate liquids, and for the fugacities of components in fluids with complex composition (Belonoshko et al. 1992). The model calculations provide estimates of the partitioning of H20, CO2 and S between the silicate liquid and the coexisting fluid, and the composition of the fluid phase in terms of H20, CO2, SO2, and HzS, as a function of pressure, temperature, volatile-free liquid composition, oxygen fugacity, and total amount of each volatile component in the system. Model calculations are presented for silicate liquids with tholeiitic and rhyolitic composition, oxygen fugacities in the NNO _+ 2 range, and pressures from a few hundred MPa to atmospheric, with the simplifying assumption that no reduced or oxidized sulphur-bearing solid or liquid phases nucleate or separate from the liquid-gas system. The results are in good agreement with the bulk of experimental data from the literature, and show the well-known minima in sulphur saturation contents as a function of oxygen fugacity, the mutual effects of volatiles on their saturation contents, and the complex relationships between the saturation surface of multi-component fluids and the liquid composition, volatile abundance, and pressure-temperature oxygen fugacity conditions. The new model therefore represents a powerful tool for the prediction of multicomponent gas-liquid equilibria in natural maglnatic systems, for the simulation of magmatic and volcanic processes, and for the interpretation of data on the degassing of magma bodies and composition of volcanic plumes, provided that the assumptions on which the model rests, first of all that no separation of additional S-bearing phases occurs, are satisfied.

The H2O solubility of alkali basaltic melts: an experimental study

“…Pichavant 2003, 2005;Moretti et al 2003;Burgisser and Scaillet 2007;Burgisser et al 2008). The empirical Eqs.…”

Vapor segregation and loss in basaltic melts

“…The fraction of H 2 S in the vapor increases with pressure, owing to the volume difference in the reaction SO 2 + 3H 2 ↔ H 2 S + 2H 2 O. The abundance of H 2 S will be greater than SO 2 in the gas phase at lithostatic pressures greater than a few MPa, or a few hundred meters (for the redox buffer of NNO-0.5 at 1400 K) over a range of basic magma compositions and initial sulfur content (Moretti et al, 2003).…”

Simulating the behavior of volatiles belonging to the C–O–H–S system in silicate melts under magmatic conditions with the software D-Compress

“…These efforts include largely empirical models (e.g., Moore et al, 1998;Liu et al 2005), semi-empirical models (e.g., Iacono-Marziano et al, 2012;Ariskin et al, 2013;Duan, 2014), and thermodynamical models using various formalisms (e.g., Papale, 1999;Moretti et al, 2003). These models have found a wide range of applications, which includes the interpretation of melt inclusion data (Papale, 2005;Moore, 2008), the interpretation of gas measurements on active volcanoes (Aiuppa et al, 2007;Oppenheimer et al, 2011), the feedback between chemistry and physics in conduit flow models (Papale and Polacci, 1999;Burgisser et al, 2008), and the assessment of the impact of volcanic gases on the atmosphere of terrestrial planets Scaillet, 2009, 2014;Gaillard et al, 2011).…”

“…A good example of these complexities is the modeling of degassing in the Erebus magmatic system in Antarctica (Oppenheimer et al, 2011). In this comprehensive attempt to bring melt inclusion data, petrologic observations, and gas chemistry measurements of the emission of an active lava lake, the equilibrium saturation model of Moretti et al (2003) was combined with a regular mixture approach for H 2 O and CO 2 (Papale et al, 2006), a polymeric treatment of silicate melts for S-related computations (Moretti and Ottonello, 2005;Moretti and Papale, 2004), a thermodynamical model for iron (Ottonello et al, 2001) and its interaction with S (Moretti and Baker, 2008), and non-ideal equations of state for gas species (Belonoshko and Saxena, 1992). Owing to such complexity, not all of these models have been released to the volcanological community in a userfriendly format.…”