229-19 Equations-of-State for Fluids in the C-O-H System: A Comparison between Predicted and Measured Values

Session: Crustal Petrology (Posters)

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Abstract:

Fluids in the Earth’s interior influence rocks properties, including melting temperatures, mineralogies, and rheologies.  H₂O is the most abundant species in the crust and upper mantle, CO₂ is also common and, in the C-O-H system, CH₄ is a significant component under reducing conditions.  Models that describe the thermodynamic properties of fluids provide insight into fluid related processes, and this study compares equations-of-state (EOS) with experimentally determined properties to determine which EOS, if any, best reproduces these data.  This comparison is restricted to EOS that, at a minimum, include H₂O, CO₂, and CH₄, at temperatures (T) and pressures (P) from 400 to 1600˚C and 0.2 to 6 GPa.  Four equations-of-state are included:  Belonoshko & Saxena (1992) Geochim Cosmochim Acta, v. 56, p 361, (BeSa(92)); Holland & Powell (1991) Contrib Min Pet, v. 109, p. 265, (HP(91)); Churakov & Gottschalk, M. (2003a) Geochim Cosmochim Acta, v. 67, p. 2397 (CG(03)); and Zhang & Duan, (2009) Geochim Cosmochim Acta, v.73, p. 2089 (ZD(09)).

H₂O:  All EOS yield values of VH₂O (V - molar volume) consistent with measured values to 900˚C and 0.6 GPa.  At P > 1.5 GPa CG(03) and ZD(09) yield values of VH₂O that, on average, provide the best agreement at T from 1000 to 1600˚C.  CO₂:  At P < 1 GPa and T from 400˚C to 1400˚C no EOS is clearly most consistent with measured values of VCO₂.  At T > 1000˚C and P between 0.8 and 2.0 GPa, BeSa(92) yields the most accurate values of ƒCO₂ (ƒ - fugacity), while at higher pressures ZD(09) yield better agreement at T from 925˚C to 1650˚C.  CH₄:  Measured values of CH₄ are available to 1 GPa at T < 400˚C and, therefore, choice of EOS is difficult at elevated T and P > 1 GPa.

No single EOS best describes the measured values of V or ƒ over the entire range of P-T and, therefore, choice of EOS depends on the P-T conditions.  However, at P > 1.5 GPa CG(03) and ZD(09) yield values of H₂O most consistent with the measurements.  ZD(09) yields the most accurate values of ƒCO₂ at P > 1.5 GPa, although CG(03) also agrees with the measured values within uncertainty.  Extrapolation of any EOS beyond the P and T range included in experimental studies may involve significant uncertainties.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10133

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