Volatile outgassing and chemical speciation of the C-O-H system during the Earth Magma Ocean evolution

Abstract

During the early phase of its evolution the Earth was likely characterized by a magma ocean stage. The planet was completely or partly molten and there was a strong interaction between: the interior differentiation process, the silicate melt phase and the atmosphere. Interpretations of the Earth differentiation process suggest a shift of the magma ocean redox state from reduced condition to a much more oxidizing ambient, with values of oxygen fugacity similar to the present day mantle. The aim of the research is to better characterize the volcanic degassing of the magma ocean and the related development of the early Earth's atmosphere. The magma ocean outgassing and the volatile chemical speciation of the C-O-H system are investigated with the equilibrium and mass balance method. The most representative reactions for volcanic degassing of the C-O-H system are simulated at different thermodynamic conditions and redox states. The composition of the gas phases are analysed for a wide range of pressures and temperatures which are characteristic for the magma ocean context. The gas chemical speciation is influenced not only by the thermodynamic properties but, one of the most important aspects is the redox state of the melt during the outgassing process. The different level of oxygen fugacity is investigated using the most commons petrological mineral buffers and some meteoritic compositions representative for different accretion scenarios. The results collected through this method show the strong change in the volatile composition due to the redox condition and to the temperature. For low temperatures and low level of oxygen fugacity (QIF and IW buffers) the principal gas phases are reduced chemical species such as CO, CH4 and H2. On the other hand in oxidizing condition (NiNiO and QFM buffers) the volatile composition is dominated by H2O and CO2. Considering the variations of magma ocean composition in the early Earth evolution, the results from our simulations show the direct link between the interior and the atmosphere development.