Synthesis and Microthermometric Analysis of H2S-H2O Fluid Inclusions
Abstract
Supercritical H2S-H2O mixtures are significant in geologic environments, but the lack of volumetric data inhibits calculation of mineral-fluid phase equilibria. The purpose of this study is to determine densities (ρ ) of supercritical H2S-H2O fluids as the basis for a thermodynamic model. The method consists of trapping H2S-H2O fluid inclusions in fractured quartz and calculating bulk ρ via an equation of state (EOS) from temperatures (T) of phase transitions measured on a heating-freezing stage. Preliminary work indicates that: 1) it is possible to trap fluids of a range of compositions at T as great as 600° C; 2) phase relations within the fluid inclusions are consistent with those reported in the literature; and 3) a new EOS is required to predict accurately phase equilibria and ρ of H2S-H2O mixtures. Each mixture is produced by reaction of Al2S3 and H2O in a gold capsule, which is pressurized in a cold-seal vessel with an Ar-H2 mixture. T has ranged from 600-800° C and pressure (P) from 0.50-1.83 kbar. Bulk mole fraction H2S (XH2S) has ranged up to 0.30. At 800° C, rapid dissociation of H2S and loss of hydrogen from capsules has rendered experiments impractical. At 600° C, experiments of 7-9 days duration yield dozens of measurable inclusions in the healed quartz. Observed phase relations are consistent with those reported in the literature for the H2S-H2O system. In inclusions of moderate XH2S, H2S hydrate, H2S-rich liquid (LS), aqueous liquid (LA), and vapor (G) coexist only at 29.4° C, indicating lack of significant contamination. At T between 29.4 and as high as ~106° C, LS, LA, and G coexist, as is consistent with a reported upper critical end-point T of 106.2° C. With increasing T, LS exits the 3-fluid-phase assemblage first, at TLS=LA+G. At higher T, homogenization occurs at Th either to liquid or vapor. If an EOS is available at subcritical P and T, measurements of both TLS=LA+G and Th allow calculation of XH2S and bulk ρ of an isochoric fluid inclusion. In contrast to cubic EOS, the 12-parameter equation of Span and Wagner (2003) represents vapor saturation P and liquid ρ of H2S with acceptable accuracy. Future work will include 1) extension of this EOS to H2S-H2O mixtures, 2) experiments over wider ranges of XH2S and T, and 3) spectrometric analyses in order to provide independent determination of XH2S and concentrations of impurities such as H2 or S2. Reference: Span, R. and W. Wagner, Equations of state for technical applications, I. Simultaneously optimized functional forms for nonpolar and polar fluids: Int. J. Thermophys., 24, 1, 2003.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSM.V21C..06J
- Keywords:
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- 3630 Experimental mineralogy and petrology