The solubility of H2O and CO2 in rhyolitic melts in equilibrium with a mixed CO2-H2O fluid phase

H2O and CO2 solubilities in a natural rhyolite melt (Erevan Dry Fountain (E DF), Armenia) in equilibrium with H2O-CO2 fluids were determined at 200 and 500 MPa and at 800 degreesC and 1100 degreesC. The composition of the flui d phase after experiment was determined by gravimetry, except for extreme C O2-rich fluids for which mass balance was used. Water and CO2 contents of t he glasses were measured using IR spectroscopy. At 200 MPa, the water solub ility changes from a square root dependence on mole fraction of H2O in the fluid phase (X-H2O(f)) at low X-H2O(f) to a linear dependence above X-H2O(f ) = 0.25. Up to about 5 wt.% dissolved water in the melt(corresponding to X -H2O(f) approximate to 0.5), a similar trend is observed at 500 MPa. At hig her X-H2O(f) however, the dependence of water solubility on X-H2O(f) is mor e pronounced than at 200 MPa. A negative temperature dependence of water so lubility is observed in the whole experimental range at 200 MPa (e.g. the w ater solubility at X-H2O(f)= 1 decreases from 5.97 to 5.58 wt.% when temper ature rises from 800 degreesC to 1100 degreesC). In contrast, at 500 MPa th e temperature dependence of water solubility changes from positive at high X-H2O(f) (e.g., increase from 9.84 to 11.04 wt.% for a temperature increase from 800 to 1100 degreesC) to negative at low X-H2O(f). An empirical model to predict water solubility in rhyolitic melts in the P-T range 75-500 MPa and 800-1100 degreesC was derived from our new solubility data and data fr om Blank et al. (1993). The model reproduces the data at 75 and 200 MPa wit hin +/-2.5% relative and the data at 500 MPa within +/-5% relative. The CO2 solubility shows a non-linear dependence on X-H2O(f) with deviation from linearity, which increases with pressure. The maximum CO2 solubilitie s (equilibrium with pure CO2) predicted from the data trends are 0.11 +/- 0 .01 and 0.28 +/- 0.03 wt.% at 1100 degreesC and 200 Mpa and 500 MPa, respec tively. The temperature dependence of CO2 solubility is found to be almost negligible at 200 MPa and slightly positive at 500 MPa. Our solubility data at 200 MPa are reproduced slightly better by the model of Papale (1999) th an by that of Holloway and Blank (1994). (C) 2001 Elsevier Science B.V. All rights reserved.