INTERPRETATION OF H2O/CO2/NaCl PHASE EQUILIBRIA IN EXPERIMENTS AT MAGMATIC TEMPERATURES AND PRESSURES

Cold-seal hydrothermal experimentation relevant to geological systems such as magmatic-hydrothermal Cu-Au ore environments requires the variation and control of many intensive variables, including initial H₂O/CO₂/NaCl_(eq.) ratios, run fO_2, and the estimation of fluid inclusion (FI) pressure, final salinity and H₂O/CO₂ ratios. CO₂ production via oxalic acid (OA) is common in experiments, but H₂ gas is generated during OA decomposition and must diffuse across the Pt capsule wall. The initial internal fH₂ⁱ (e.g., fH₂ⁱ ~9.7 MPa; log fO₂ ~ -16.5) can be calculated upon OA decomposition from mass balance and phase relations (Duan et. al. 1995, GCA). By using the diffusion equation of Chou (1987: Hydrothermal Techniques), an instantaneous diffusion rate (dH₂/dt) is obtained. Iterative calculation of dH₂/dt and internal fH₂ yields the time required for equilibration with the external fH₂ (~0.32 MPa, or log fO₂ ~ -14). Experiments performed at 800^oC, 100 MPa, and H₂O/CO₂ ~ 10 in Pt capsules (0.125 mm thick) require ~30 minutes for H₂ equilibration. Raman spectroscopy on the trapped vapor inclusions confirms the absence of CO and CH₄, which would be expected with prolonged elevation of fH₂ⁱ at 800^oC.

The determination of pressure and salinity is also vital to characterizing FI. Inclusion pressure for vapor inclusions (P) can be estimated using an isochoric quench model. Given an initial estimate of inclusion salinity, the volume of condensed aqueous liquid (L) and CO₂ (V) are calculated. The CO₂ density is converted to P given a suitable equation of state. Clathrate melting temperature Tm^clath is a function of salinity and pressure (Diamond 2003, MAC, SC 32) for inclusions containing (V). Given a Tm^clath, P and salinity are then iterated until they converge at the clathrate stability boundary. Run product vapor inclusions containing L and V, and which nucleate clathrate (Tm^clath~ 4^oC), have calculated P ~ 25-26 bars @ 1-3 wt% salinity, consistent with observed phase relations. In summary, H₂ generated from OA decomposition has minimal effect on fO₂, at the above experimental conditions. Given published phase equilibria, mass balance calculations, an equation of state for CO₂, and empirical confirmation, an isochoric quench model for FI may provide a useful tool in evaluating salinity in CO₂/clathrate-bearing vapor inclusions.