[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-01] Isotopic Modification of ∂¹³C via CO₂-Mineral Equilibria: Implications on the origin of CO₂ degassing at Earth’s surface.

*Antonio Caracausi¹, Dario Buttitta¹, Giorgio Capasso¹, Emilyne Beaudet², Lili loth^2,3 (1.Istituto Nazionale di Geofisica e Vulcanologia (INGV), Palermo, Italy, 2.University of Lorraine, France, 3.University of Genève, Switzerland )

Keywords:deep CO2, CO2 vs rock interaction, carbon isotopes, equilibrium processes, geochemical modeling

The isotopic composition of carbon dioxide (CO₂) emitted from the deep and/or shallow portions of the Earth interiors is widely used to trace its origin in natural fluids emitted in atmosphere, distinguishing between mantle, biogenic, and crustal sources (e.g., Sano & Marty, 1987; Hoefs, 2009). Classical models in continental regions often assume CO₂ maintains its pristine isotopic signature during migration and storage within the crust, with a substantial degree of dependence on binary mixing between sources (e.g., Chiodini et al., 2011). However, recent studies show that gas-water-rock interactions are critical in controlling the isotopic value of CO₂ degassing in active tectonic regions (e.g., Buttitta et al., 2023). These interactions lead to chemical and isotopic fractionation as CO₂ equilibrates with the local lithologies and aquifers due to processes such as carbonate dissolution, mineral precipitation, and degassing processes. This study highlights the significant influence of CO₂-rock interactions on modifying the isotopic composition of CO₂ within the crust, a factor often overlooked in conventional models (e.g., Barry et al., 2021). By evaluating equilibrium processes between CO₂ and different C-bearing minerals under 1) different temperature-pressure conditions and 2) CO₂/mineral volume ratios, we show that CO₂-mineral reactions can induce significant isotopic shifts (e.g., Deines et al., 1974). These interactions alter the original ∂¹³C values of CO₂ before its emergence at the surface, complicating source identification when applying traditional mixing models. Experimental and theoretical considerations indicate that equilibrium with carbonate minerals, such as calcite and dolomite, can lead to both positive and negative deviations in ∂¹³C depending on crustal conditions (e.g., Bottinga, 1969). This study considers several mineralogical phases to evaluate CO₂-mineral equilibrium and isotopic fractionation processes. Among the primary carbonate minerals examined, calcite, dolomite, and aragonite, siderite and graphite are also analyzed, given its relevance in crustal environments where iron-bearing carbonates interact with CO₂. By investigating these mineralogical phases, the study aims to better understand how CO₂ equilibrates with C-bearing lithologies, thereby altering its isotopic composition during its migration and storage within the crust. Our initial findings indicate that the isotopic signature of CO₂ undergoes significant variations in ∂¹³C values depending on environmental conditions with a shift of the carbon isotopic composition between -6‰ to +9‰ for CO₂ interacting with calcite. Our findings underscore the necessity of incorporating CO₂-mineral equilibria into geochemical models based on mixing processes to avoid misinterpretations of CO₂ sources. This study demonstrates the potential of CO₂-rock interaction processes to modify the isotopic signature of original CO₂ across a range of geological conditions. This finding has significant implications for the interpretation of signals at the surface and for the reconstruction of gas origin and processes, leading to more precise interpretations.

References: Barry, P. H., et al. (2021). Nature Communications, 12, 6324. Bottinga, Y. (1969). Geochim. Cosmochim. Acta, 33, 49–64. Buttitta, D., et al. (2023). Science of the Total Environment, 897, 165367. Chiodini, G., et al. (2011). Earth Planet. Sci. Lett., 304, 85–95. Deines, P., et al. (1974). Geochim. Cosmochim. Acta, 38, 1147–1164. Hoefs, J. (2009). Stable Isotope Geochemistry. Springer. Randazzo, L., et al. (2021). Geochim. Cosmochim. Acta, 303, 221–241. Sano, Y., Marty, B. (1987). Geochim. Cosmochim. Acta, 51, 2753–2763. Scheele, N., Hoefs, J. (1992). Contrib. Mineral. Petrol., 112, 35–45. Zhang, Y., et al. (2022). Earth Planet. Sci. Lett., 579, 117335.