Earth and Planetary Science Letters, 119 (1993) 27-36 27 Elsevier Science Publishers B.V., Amsterdam [CL] Solubilities of carbon dioxide and water in rhyolitic melt at 850°C and 750 bars J.G. Blank a E.M. Stolper a and M.R. Carroll b a California Institute of Technology, Division of Geological and Planetary Sciences, 170-25, Pasadena, CA 91125, USA b University of Bristol, Department of Geology, Wills Memorial Building, Queens Road, Bristol BS8 1RI, UK Received October 26, 1992; revision accepted June 11, 1993 ABSTRACT Concentrations of carbon dioxide and water dissolved in glasses quenched from rhyolitic melts equilibrated with H20-CO 2 fluids at 850°C and 750 bar were measured using infrared spectroscopy; concentrations of H20 and CO 2 in the quenched fluids were measured manometrically. The mole fraction of CO 2 in the quenched fluid ranged from 0.06 to 0.91. Concentrations of CO 2 in the coexisting rhyolitic melt increased from 23( + 6) ppm for the sample equilibrated with the most CO2-poor fluid to 515(+ 16) ppm for that equilibrated with the most CO2-rich fluid. The water content of the melt varied from 0.51(+ 0.06) to 3.34(+0.08) wt%. Our results show that concentrations of molecular CO 2 and H20 in the glasses obey Henry's Law; i.e., the mole fractions of molecular CO 2 and molecular H20 in the quenched melts are proportional to their fugacities in the coexisting vapor. CO 2 contents of vapor-saturated melts are not enhanced by addition of water to CO2-rich vapor, contrary to previous reports for silicate melts at higher pressures. The Henrian behavior of CO 2 and H2 ° at low pressure considerably simplifies modeling of the degassing of silicic magmas. I. Introduction Water and carbon dioxide are the most abun- dant volatile components in terrestrial magmas [1-5]. They exsolve from molten silicate into bub- bles as magmas rise toward the earth's surface and depressurize. The vapor exsolving from mag- mas can be almost pure CO 2 at high pressure but becomes progressively enriched in H20 upon de- compression because carbon dioxide is much less soluble than water in most silicate liquids [e.g., 6,7]. On eruption at the surface all but a few ppm CO 2 and 0.1-0.2 wt% H20 have typically parti- tioned into bubbles or escaped into the atmo- sphere [8-10]. Escape of these and other gases from magmas in near-surface environments is a significant factor in powering explosive volcanic eruptions [11,12] and is the ultimate source of the earth's atmosphere and oceans [13]. Key to understanding degassing phenomena of magmas are the solubilities of carbon dioxide and water in silicate liquids similar in composition to magmas, and there has been much work over many decades directed toward this goal [14-23]. It is frequently assumed in efforts to model the volatile contents of magmas that Henry's Law is obeyed for gaseous species dissolved in silicate melts [10,24-28]; i.e., the fugacity (or partial pres- sure at sufficiently low total pressure) of a gaseous species is assumed to be proportional to the amount dissolved in the melt. If valid, this as- sumption considerably simplifies modeling of de- gassing of systems with both water and carbon dioxide since it means that one need only deter- mine the behavior of the end-member systems (i.e., silicate-CO 2 and silicate-H20) and the equation of state of mixed H20-CO 2 vapor. A potential complication, however, is that water may influence the solubility of carbon dioxide in silicate melts and vice versa. Thus it is essential that experimentation on the partitioning of water and carbon dioxide between vapor and silicate 0012-821X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved