Volatile solubility in fluid-saturated haplogranitic melts under graphite-buffered redox conditions

Graphite is a common accessory mineral in metasedimentary rocks and is a potential source of carbon during anatexis. Its presence constrains the composition of C-O-H fluids, and thus acts as a fluid buffer. To date, experiments on H 2 O-CO 2 mixed volatile solubility have focused primarily on binary H 2 O-CO 2 fluids which are not representative of graphite-buffered systems where ternary H 2 O-CO 2 -CH 4 mixtures occur. In this study, we have conducted solubility experiments in graphite- and fluid-saturated haplogranitic melts at 0.5 and 1 GPa, between 800 and 1000 °C. The double-capsule technique was employed to impose graphite-buffered redox conditions. Equilibration of graphite and H 2 O produces a ternary C-O-H fluid, modelling the behavior of an internally-buffered graphitic system during dehydration melting. The resultant composition of the fluid phase (determined ex-situ) corresponds to a ternary H 2 O-CO 2 -CH 4 mixture with H 2 O contents ranging from 68 to 96 mol%. The maximum CO 2 contents dissolved in the glasses range from 0.12 to 0.2 wt%, and the H 2 O contents vary from 8 to 11 wt%. Although present in the C-O-H fluid, dissolved CH 4 was not detected in the glass, indicating negligible solubility. Comparison with previous experimental studies reveals that the carbon contents of the glasses are consistently lower than those previously documented in experiments equilibrated with binary H 2 O-CO 2 fluids. We further observe that solubility models overestimate the CO 2 contents of the melts studied here. Since melting of metasediments can occur in various geological settings, our results emphasize the need to consider ternary C-O-H fluids in solubility models in order to address graphite-buffered anatectic systems. • Graphite and H 2 O was used to buffer the fluid composition in the C-O-H system. • The fluid phase was measured ex-situ and is close to the thermodynamic prediction. • Graphite-saturated melts have lower carbon contents than carbonate-saturated melts. • Solubility models overestimate the CO 2 solubility of graphite-saturated melts.