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Example output from VolatileCalc for rhyolite.

Isobars (gray) represent locus of values for dissolved H2O andCO2 in rhyolitic melt in equilibrium with H2O–CO2 vapor at 800°C and selected pressures. Isopleths represent locus of rhyolitic melt compositions in equilibrium with the given vapor compositions (20, 50 and80 mol% H2O shown here) at 800°C. From Newman and Lowenstern (2002) (Public domain.)

VolatileCalc provides solution models for the rhyolite–H2O–CO2 and basalt–H2O–CO2 systems at magmatic temperatures and pressures below 5000 bar. The models are coded as macros written in Visual Basic for Applications, for use within Microsofts Excel . The series of macros, entitled VolatileCalc, can calculate the following: (1) Saturation pressures for silicate melt of known dissolved H2O andCO2 concentrations and the corresponding equilibrium vapor composition; (2) open- and closed-system degassing paths (melt and vapor composition) for depressurizing rhyolitic and basaltic melts; (3) isobaric solubility curves for rhyolitic and basaltic melts; (4) isoplethic solubility curves (constant vapor composition) for rhyolitic and basaltic melts; (5) polybaric solubility curves for the two endmembers and (6) endmember fugacities of H2O andCO2 vapors at magmatic temperatures. The output agrees well with the published solution models and experimental data for silicate melt–vapor systems for pressures below 5000 bar.

Now in use for almost 20 years, the software is easy to use and provides accurate results for rhyolitic and standard basalt compositions. Its functionality for calculating degassing paths and integration into Excel makes it particularly practical for research and educational purposes.