How is the Density of Fluid Phase Computed?

[zihanwei]

In the output of rhyolite-MELTS, the density of the fluid phase is given. I'm wondering how the density of H2O and CO2 under magmatic temperature and pressure is computed in the MELTS algorithm. Thanks!

[Paula]

Taken from Understanding rhyolite-MELTS versions on the MELTS website:

rhyolite-MELTS + H2O-CO2 fluid (v 1.2.x)
This version is a replacement and upgrade from rhyolite-MELTS 1.0.x. It is the rhyolite-MELTS model with water properties completely replaced with the H2O-CO2 fluid saturation model of Ghiorso and Gualda (2015). Thermodynamic properties of the fluid phase are calculated from the model of Duan and Zhang (2006). Use this version to model crystallization in bulk compositions other than those that saturate in quartz and crystallize to the two-feldspar-, quartz-, fluid-saturated ternary minimum.
Cite as Gualda et al., 2012, and as Ghiorso and Gualda, 2015.

rhyolite-MELTS + H2O-CO2 fluid (v 1.1.x)
Use this version ONLY to model crystallization in bulk compositions that saturate in quartz and crystallize to the two-feldspar-, quartz-, fluid-saturated ternary minimum. This version is the rhyolite-MELTS model with CO2 melt properties and mixed-fluid energetic terms from Ghiorso and Gualda (2015), utilizing H2O melt properties and H2O-anhydrous component energetic mixing terms from MELTS (which are the same as in rhyolite-MELTS v 1.0.x). Thermodynamic properties of the fluid phase are calculated from the model of Duan and Zhang (2006).
Cite as Gualda et al., 2012, and as Ghiorso and Gualda, 2015 (water model from Ghiorso and Sack, 1995).

rhyolite-MELTS (original, v 1.0.x)
This version is the MELTS model (Ghiorso and Sack, 1995) with corrections to the free energies of quartz and sanidine so that the eutectic-like crystallization at the ternary minimum in the quartz-two-feldspar+fluid system is recovered. (This version replaces MELTS, which should no longer be used to calculate phase relations in magmatic systems.) Use this version to model crystallization in natural composition liquids at pressures below 2 GPa. The fluid phase is modeled as pure H2O. PhasePlot implements the rhyolite-MELTS model.
Cite as Gualda et al., 2012.

• Gualda G.A.R., Ghiorso M.S., Lemons R.V., Carley T.L. (2012) Rhyolite-MELTS: A modified calibration of MELTS optimized for silica-rich, fluid-bearing magmatic systems. Journal of Petrology, 53, 875-890.
• Ghiorso M.S. and Gualda, G.A.R. (2015) An H2O-CO2 mixed fluid saturation model compatible with rhyolite-MELTS. Contributions to Mineralogy and Petrology 2015, in press.
• Duan Z. and Zhang Z. (2006) Equation of state of the H2O, CO2, and H2Oââ,¬â€œCO2 systems up to 10 GPa and 2573.15 K: Molecular dynamics simulations with ab initio potential surface. Geochimica et Cosmochimica Acta, 70, 2311ââ,¬â€œ2324.

[zihanwei]

Thanks a lot!