Solubility of CO2 and H2O in graphite-saturated haplogranitic melt

In the models of the geological carbon cycle, the contribution from crustal magmas is generally overlooked. However, organic matter that has been transformed to graphite through metamorphism could represent an important source of carbon in the lower crust, which can be re-mobilized during partial melting (Cesare et al., 2005). A COH fluid produced solely by dehydration in the presence of graphite will retain the H/O ratio of H₂O (H/O = 2) and thus maximize the H₂O activity and form of a ternary H₂O-CO₂-CH₄ mixture (Connolly and Cesare, 1993). Previous solubility studies have mainly considered oxidizing conditions (H/O < 2) and cannot be used to interpret graphitic systems, in which conditions are more reducing (Carvalho et al., 2023). Therefore, it is essential to obtain new solubility data for graphite-saturated silicate melt coexisting with a ternary H₂O-CO₂-CH₄ fluid.

In this study, solubility experiments were carried out in a single-stage piston cylinder apparatus at 5 to 10 kbar and in a temperature range of 800-1000°C. To simulate an anatectic melt formed in the mid to lower metasedimentary crust, a haplogranitic glass was synthesized, and the experimental charge was loaded with graphite and H₂O as the source for the COH fluid. To buffer the fluid composition at the condition H/O = 2 during the run, the double-capsule technique was utilized, and graphite and H₂O was added in the outer capsule. The speciation of the experimental fluid was analyzed ex-situ by a capsule-piercing quadrupole mass spectrometer (Tiraboschi et al., 2016). In all experiments H₂O was the major fluid component, in accordance with thermodynamic predictions, followed by variable amounts of CO₂ and CH₄. The relative amount of H₂O to carbonic species in the fluid changes with pressure and temperature, and the experiments cover a range of XH₂O_fluid = 0.67-0.99.

The experimental glasses contain bubbles which have been analyzed by micro-Raman spectroscopy, revealing that they mainly consist of pure CH₄ or binary CH₄-CO₂ mixtures and graphite. The H₂O content of the glasses have been determined by micro attenuated total reflectance Fourier transform infrared spectroscopy. Dissolved H₂O generally increases with pressure, and there is no visible temperature dependence. To quantify the total carbon (CO₂) content of the glasses, secondary ion mass spectrometry will be used. The new solubility data for carbon will complement the existing experimental dataset to allow better interpretation of complex systems where graphite, melts and fluids are present.

References:

Carvalho et al. (2023) Chem Geol 631

Cesare et al. (2005) Contrib Mineral Petrol 149, 129-240

Connolly, J.A.D. & Cesare, B. (1993) J Metamorph Geol 11, 379-388

Tiraboschi et al. (2016) Geofluids 16, 841-855