Interior-surface-atmosphere interactions of rocky planets: simulation of volcanic outgassing and volatile chemical speciation in the C-O-H system

Abstract

The characterization of the volcanic volatile outgassing is central for investigating the composition and the development of rocky planet atmospheres. I have analysed the volcanic gas release via numerical simulations of the volatile pathway from the mantle to the atmosphere of a planet. The thesis has been carried out as a subproject within the Transregional Collaborative Research Center TRR 170 ”Late accretion onto terrestrial planets” which gave the opportunity to collaborate with other subprojects and research institutions. The aim of the thesis was to develop a numerical model for characterising the volcanic outgassed composition during the early Earth evolution. The core of this research is the volatile chemical speciation model which simulates the volcanic outgassing considering the C-O-H system. I designed the model following the ”Mass balance and equilibrium” method (French, 1966; Holloway, 1981; Holland, 1984; Huizenga, 2005; Fegley, 2013; Gaillard and Scaillet, 2014; Schaefer and Fegley, 2017) taking into consideration the principal factors that define the outgassed composition of a silicate melt namely: pressure, temperature and redox state. The volatile chemical speciation is calculated considering that the volatile species are in chemical equilibrium with the silicate melt and an ideal gas behaviour. The redox state of the system was reproduced by using some of the most common petrological mineral buffers (Holloway et al., 1992), for both reducing or oxidising conditions. The collected results demonstrate how the magma redox state is the driving parameter that affects the final outgassed composition. In reducing conditions (QIF and IW buffers) the principal outgassed species are H2 and CO whereas, in oxidising states (NiNiO and QFM buffers) the dominant volatile species are H2O and CO2. In order to investigate the volatile outgassing at a global scale, the volatile chemical speciation model was coupled with different models that reproduce the mantle convection regime (Noack and Breuer, 2013; Noack et al., 2014, 2017) and the corresponding outgassed atmospheres (Dorn et al., 2018). The coupling of the models is employed to investigate the volatile outgassing at different conditions. In Guimond et al. (2021), we investigated the early Earth evolution outgassing during the magma ocean stage. Still considering a global Earth magma ocean, in Katyal et al. (2020) we simulated the degassed atmospheric composition, H2 escape and the infrared emission/transmission. Ortenzi et al. (2020) analyses the degassing for rocky planets considering a stagnant lid regime and calculating the outgassed atmospheric compositions and radial extents. The volatile chemical speciation model was extended to include also the sulphur species (H2S, S2 and SO2) and for simulating the real gas behaviour. At the moment the simulations of the C-O-H-S system are only for a limited range of temperature and pressure, and the model needs further improvements for being employed in global outgassing simulations. In conclusion, the thesis includes a detailed description of the developed volatile chemical speciation models showing both their points of weakness and their versatility for investigating the volatile composition of outgassed atmospheres at different planet evolutionary stages.