Solubility of C-O-H mixturesin natural melts: new experimental data and application range of recent models

R. Botcharnikov, M. Freise, F. Holtz, H. Behrens

Abstract


The effect of pressure, temperature, and melt composition on CO2 and H2O solubilities in aluminosilicate
melts, coexisting with CO2-H2O fluids, is discussed on the basis of previously published and new experimental
data. The datasets have been chosen so that CO2 and H2O are the main fluid components and the conclusions
are only valid for relatively oxidizing conditions. The most important parameters controlling the solubilities
of H2O and CO2 are pressure and composition of melt and fluid. On the other hand, the effect of temperature
on volatile solubilities is relatively small. At pressures up to 200 MPa, intermediate compositions
such as dacite, in which both molecular CO2 and carbonate species can be dissolved, show higher volatile
solubilities than rhyolite and basalt. At higher pressures (0.5 to 1 GPa), basaltic melts can incorporate higher
amounts of carbon dioxide (by a factor of 2 to 3) than rhyolitic and dacitic melts. Henrian behavior is observed
only for CO2 solubility in equilibrium with H2O-CO2 fluids at pressures <100 MPa, whereas at higher
pressures CO2 solubility varies nonlinearly with CO2 fugacity. The positive deviation from linearity with
almost constant CO2 solubility at low water activity indicates that dissolved water strongly enhances the solubility
of CO2. Water always shows non-Henrian solubility behavior because of its complex dissolution
mechanism (incorporation of OH-groups and H2O molecules in the melt). The model of Newman and Lowenstern
(2002), in which ideal mixing between volatiles in both fluid and melt phases is assumed, reproduces
adequately the experimental data for rhyolitic and basaltic compositions at pressures below 200 MPa but
shows noticeable disagreement at higher pressures, especially for basalt. The empirical model of Liu et al.
(2004) is applicable to rhyolitic melts in a wide range of pressure (0-500 MPa) and temperature (700-
1200°C) but cannot be used for other melt compositions. The thermodynamic approach of Papale (1999) allows
to calculate the effect of melt composition on volatile solubilities but needs an update to account for
more recent experimental data. A disadvantage of this model is that it is not available as a program code. The
review indicates a crucial need of new experimental data for scarcely investigated field of pressures and fluid
compositions and new models describing evident non-ideality of H-C-O fluid solubility in silicate melts
at high pressures.

Keywords


CO2;H2O;solubility;mixed fluid;silicate melt;experimental data

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References


DOI: https://doi.org/10.4401/ag-3224
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Published by INGV, Istituto Nazionale di Geofisica e Vulcanologia - ISSN: 2037-416X