Decoupling between the carbon isotopic signature of CO2-bearing aqueous fluids and of their graphite+carbonate source at subarc depth

Subducted limestones and carbonate-bearing sediments can be very rich in organic matter. Once they are subducted, portions of organic matter are transported down to great depths, as suggested by the ¹²C-enriched isotopic signature of diamonds.

We studied the carbon isotopic exchange among graphite (99% ¹²C), aragonite (99% ¹³C) and aqueous fluids at subarc conditions (P = 3 GPa, T = 700°C), with and without the presence of coesite. The redox state of the experiments was constrained close to FMQ conditions. The isotope ratios and the quantitative analysis of the volatile phase in runs quenched to rooms pressure and temperature has been accomplished by quadrupole mass spectrometry. The isotope ratios of solids (graphite, aragonite) has been performed by means of laser-ablation ICP-MS and secondary ion mass spectrometry (nano-SIMS).

Results show that, while the initial isotopic ratio of graphite remains unchanged in quenched products, the isotopic ratio of aragonite becomes lighter, gaining ~ 20% of ¹²C even after only a few hours, and it is almost independent on run duration. This isotopic lightening of aragonite is enhanced when SiO₂ is present in the system, where aragonite contains 40-50% ¹²C. The isotopic ratio of the fluid phase is always comparable to that of aragonite. Results suggest that ¹³C aragonite reacts at high-pressure/temperature conditions with CO_2(aq) produced mainly by oxidation of graphite, which is enhanced by the presence of silica. CO₂ evolves rapidly towards ¹³C-rich ratios, due to the isotopic exchange with the dissolution products of aragonite (mainly CaHCO₃⁺ at the investigated P-T-pH conditions). Aragonite continuously undergoes dissolution/reprecipitation, the higher the amount of dissolved graphite, the higher the ¹²C/¹³C final ratio.

The isotopic signature of volatiles released at subarc conditions is therefore controlled by carbonates, even if the major process underlying the CO₂ formation is the oxidation of graphite. Our results challenge the conventional assumption that the isotopic signature of a devolatizing carbon source and the isotopic signature of the corresponding COH fluid are coupled in a straightforward manner.