CO2 Solubility in Natural Rhyolitic Melts at High Pressures - Implications for Carbon Flux in Subduction Zones by Sediment Partial Melts

Partial melts of subducting sediments is thought to be a critical agent in carrying trace elements and water to arc basalt source regions. For subduction zones that contain significant amount of carbonates in ocean-floor sediments, sediment melts likely also act as a carrier of CO₂. However, the CO₂ carrying capacity of natural rhyolitic melts at sub-arc depths remains unconstrained. We conducted experiments on a synthetic composition, similar to average, low-degree experimental partial melt of pelitic sediments. The composition was constructed with reagent grade oxides and carbonates, the source of excess CO₂. Experiments were conducted between 1 and 3 GPa at 1200 °C in Au₈₀Pd₂₀ capsules using a piston cylinder apparatus with a half-inch BaCO₃ assembly at Rice University. Quench products showed glasses with bubbles, the latter suggesting saturation of the melt with a CO₂-rich vapor phase. Oxygen fugacity during the experiments was not strictly controlled but the presence of CO₂ bubbles and absence of graphite indicates fO₂ above the CCO buffer. Major element concentrations of glasses were measured using EPMA. The CO₂ and H₂O contents of experimental doubly polished (50-110 μm), bubble-free portions of the glass chips were determined using a Thermo Nicolet Fourier Transform Infrared Spectrometer. Spectra were recorded with a resolution of 4 cm^-1, 512 scans, from 650 to 4000 cm^-1, under a nitrogen purge to eliminate atmospheric gases. Dissolved volatile concentrations were quantified using the Beer-Lambert law and linear molar absorption coefficients from previous studies [1, 2]. Total dissolved carbon dioxide of experimental glasses was determined from the intensity of the ν₃ antisymmetric stretch bands of CO₃^2- at 1430 cm^-1 and CO₂^mol at 2348 cm^-1. Dissolved water content of experimental glasses was determined from the intensity of O-H stretching at 3520 cm^-1. Estimated total CO₂ concentrations at 3 GPa are in the range of 1-2 wt%, for melts with H₂O contents between 1.5 and 2.5 wt%. Compared to previous work on CO₂ solubility in complex rhyolitic melts at lower pressures [3-5], there is a general trend of increasing CO₂ solubility with pressure. Dissolved CO₂ is present both as molecular CO₂ and as CO₃^2-, consistent with previous, simple system studies at high pressures [e.g. 2, 6]. The CO₂^mol/CO₂^Tot values are within the range of previous high pressure studies [e.g. 7] and range from 0.35 to 0.55. Experiments at variable P, T, and melt water content are underway. [1] Fine and Stolper (1985), CMP, 91, 105-121; [2] Stolper et al. (1987), AM, 72, 1071-1085; [3] Blank et al. (1993), EPSL, 119, 27-36; [4] Fogel and Rutherford (1990), AM, 75, 1331-1326; [5] Tamic et al. (2001), CG, 174, 333-347; [6] Mysen and Virgo (1980), AM, 65, 855-899; [7] Mysen (1976), AJS, 276, 969-996.