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 H2O-CO2 mixed volatile solubility have focused primarily on binary H2O-CO2 fluids which are not representative of graphite-buffered systems where ternary H2O-CO2-CH4 mixtures occur. In this study, we have conducted solubility experiments in graphite- and fluid-saturated haplogranitic melts at 0.5 and 1 GPa, between 800-1000 °C. The double-capsule technique was employed to impose graphite-buffered redox conditions. Equilibration of graphite and H2O 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 H2O-CO2-CH4 mixture with H2O contents ranging from 68-96 mol%. The maximum CO2 contents dissolved in the glasses range from 0.12 wt.% to 0.2 wt.%, and the H2O contents vary from 8 to 11 wt.%. Although present in the C-O-H fluid, dissolved CH4 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 H2O-CO2 fluids. We further observe that solubility models overestimate the CO2 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.