The Changing Character of Carbon in Fluids with Pressure

Abstract

Decades of research have now firmly established that aqueous fluids in Earth's crust from groundwater to deep basinal brines to shallow mid-ocean ridge hydrothermal systems can contain metastable equilibria involving C-species in which methane does not participate. It now appears, however, that this situation does not extend to the upper mantle. Instead, field evidence from theoretical models, experimental diamond-anvil cell and fluid inclusion studies, and high-pressure metamorphic rocks all indicate a wide variety of aqueous C-species at high pressures. At pressures above about 2.0-3.0 GPa, methane and all other aqueous C-species with different oxidation states between -4.0 and +4.0 can coexist in variable proportions. Furthermore, the aqueous fluids can coexist with immiscible hydrocarbon fluids. As a consequence, C-species with a mixture of oxidation states in aqueous fluids and hydrocarbon fluids at high pressures in the upper mantle can influence the oxidation state of their upper mantle environment.

Methane-hydrogen-rich fluid migration triggers seismic failure in subduction at forearc depths

“…Figure 8 also suggests that, if the CH4-rich fluids remained trapped below the talc seal during decompression, fluid volume expansion would promote hydrofracturing for even smaller amounts of trapped fluid. H2O, H2, CO2, and CH4 are immiscible over a wide range of P-T conditions in subduction zones [22][23][24][25]62 . Evidence of immiscibility has been described in the carbonated serpentinites representing the source rock 25,63 .…”

“…In subduction zones, CO2-rich fluid can accumulate under low-permeability rocks leading to high pore pressure and seismicity at crustal depth [16][17][18][19][20] . Additionally, the larger wetting angles of CO2 compared to an aqueous fluid together with fluid immiscibility can segregate CO2 from the aqueous fluid and generate preferential CO2 migration via hydrofracturing [21][22][23][24][25][26] . Finally, CO2-rich fluids expand more compared to aqueous fluids during exhumation and decompression of subducted rocks leading to failure in low-permeability rock types 27 .…”

Deciphering the Origin of Abiotic Organic Compounds on Earth: Review and Future Prospects

“…However, all or some of the CH 4 present in these inclusions might form through the decomposition or degradation of more complex organicbearing fluids. Indeed, the presence of daughter minerals, such as graphite, calcite, etc., clearly indicates that some carbon redistribution occurs within the fluid inclusions (Sverjensky, 2020).…”

“…Going further, Sverjensky et al (2014) developed the Deep Earth Water Model (DEW) for thermodynamic calculations, up to 6 GPa, 1200°C, involving solid minerals, mixed solvent species, neutral solute molecules, and charged ions to predict changes in fluid species. Sverjensky et al (2014) and Sverjensky (2020) predicted that abundant and diverse aqueous abiotic organic and inorganic C species will emerge at high pressures: at crustal pressures and at temperatures greater than about 500°C, the expected thermodynamic equilibrium speciation of C is CO 2 (aq), CH 4 (aq), HCO 3 − , CO 3 2− , whereas at high P-T (5 GPa, 600°C ) CH 3 COO − and CH 3 COOH(aq) are predicted. Aqueous speciation calculations along a representative mantle geotherm show that at a depth of about 100 km, the fluid is predicted to transition from CO 2 -dominated to CH 4dominated (Fig.…”

Water–Gas Shift Reaction Produces Formate at Extreme Pressures and Temperatures in Deep Earth Fluids

“…C–H–O fluids in the deep crust and upper mantle are fundamentally important carbon carriers in the deep carbon cycle and were traditionally modeled as simple mixtures of small volatile molecules, e.g., H 2 O, CO 2 , CO, CH 4 , H 2 . Recently, experimental and theoretical studies have started to consider aqueous ions, complexes, and chemical speciation in C–H–O fluids, ,− and many of them focus on fully oxidized carbon forms in supercritical water. − With increasing depth, the Earth’s interior becomes more reducing, so reduced carbon is of great importance in deep Earth, but due to the lack of reliable data at extreme P-T conditions, the water–gas shift reaction was simply assumed to adjust fluid composition in reducing environments. , …”