An Experimental Study on Kinetics-Controlled Ca-Carbonate Aqueous Reduction into CH4 (1 and 2 GPa, 550oC): Implications for C Mobility in Subduction Zones

Abiotic methane (CH4) generation under subduction zone conditions has been experimentally investigated through aqueous reduction of pure C-bearing materials (e.g. carbonate minerals and organic matter). However, quantitative assessments of CH4 production in these experiments, as well as the potential effects of other components such as SiO2 on the reduction processes, have not yet been well established. Here, we performed experiments to quantitatively evaluate the time-resolved Ca-carbonate aqueous reduction into CH4 at P = 1 and 2 GPa and T = 550oC in the CaO + COH, CaO + SiO2 + COH, and CaO + SiO2 + MgO + COH systems, employing calcite + water ± quartz ± serpentine (synthetic chlorine (Cl)-bearing chrysotile and natural Fe-Al-bearing antigorite) as starting materials. Redox conditions of the experiments were buffered by iron-wüstite (IW) using a double capsule setting, corresponding to oxygen fugacity (fO2) values (expressed as log units relative to the fayalite-magnetite-quartz buffer, deltaFMQ) in the inner capsule of deltaFMQ approximately equal to -5.5 at 1 GPa and deltaFMQ approximately equal to-6.0 at 2 GPa. The solid products are mainly composed of portlandite ± larnite ± wollastonite ± brucite, while Ca-carbonate and/or silicate reactants commonly occur as relicts. Quadrupole mass spectrometric analysis shows that CH4 and H2O are the major COH molecular species in the fluid products, with molar ratios between CH4 and starting calcite representing the reaction progress ranging from ~0.13 to ~1.00. Comparisons of experimental run products with thermodynamically predicted phase assemblages, together with time-series experiments, indicate that the reduction processes are primarily controlled by reaction kinetics. At 1 GPa and 550oC, rate constants of 4.0 × 10-6 s-1, 7.4 × 10-6 s-1, and 2.6 × 10-6 s-1 were retrieved for reactions starting with calcite + quartz + water, calcite + synthetic Cl-bearing chrysotile + water, and calcite + natural Fe-Al-bearing antigorite + water, respectively, significantly higher than the constant of 0.8 × 10-6 s-1 for the silicate-absent reaction. Besides, an increase in pressures can also enhance the reduction efficiency of Ca-carbonates until reaching equilibrium with the fluids. Our data provide experimental evidence for kinetics-controlled Ca-carbonate aqueous reduction into CH4 in subduction zones, indicating that silicate involvement and/or pressure increase can accelerate the reaction rates through short-lived f luid-rock interactions, which may have important implications for deep C mobility.