PACROFI VI - Electronic Program

Volumetric Properties and Solvus Locations in the Ternary System H2O - NaCl - CO2 at Elevated Temperatures, Pressures, and Salinities up to 20 Wt% Sodium Chloride

C. Schmidt and R. J. Bodnar

Fluids Research Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0420, USA

Fluid inclusions having compositions of 6 or 20 wt% NaCl and 10, 20 and 30 mole% CO2, both relative to H2O, were synthesized in cold-seal pressure vessels at pressures from 2 to 5 kbar and temperatures between 400o and 700oC. The solvus locations and the P-T slopes of iso-Th lines were determined for bulk fluid densities above the critical density based on microthermometric analysis. For solvus pressures above about 2 kbar, the liquid + vapor => liquid boundary was delineated in P-T space using microthermometrically determined salinities. Molar volumes were calculated from microthermometric and Raman spectroscopic data for the density of the CO2 phase.

For H2O + 20 wt% NaCl + 10 mol% CO2, the high density portion of the bubble-point curve shows a pressure minimum at about 1300 bars and 440oC (see Figure). At lower temperatures, the solvus has a steep negative slope, whereas the solvus pressure increases only slightly at temperatures above 440oC. Addition of CO2 to an aqueous solution of 20 wt.% NaCl results in a significant shift of the bubble point curve towards higher pressures: the minimum pressure is between 2 and 3 kbar for a 20 mol% CO2 content. The immiscibility field extends into granulite-facies conditions at a CO2 concentration of 30 mol% relative to water. With the possible exception of the 700oC, 5 kbar run, no fluid inclusions formed in the one-phase field even at 5 kbar pressure. The slopes of the iso-Th lines for a constant composition of H2O + 20 wt.% NaCl + 10 mole% CO2 decrease nonlinearly from about 20 bars/oC for Th(L-V) of 360oC, to approximately 3 bars/oC for Th(L-V) = 600oC (see figure). The bulk molar volumes, isochores and solvus locations along the pseudobinary (H2O + 6 wt.% NaCl) - CO2 are consistent with the data of GEHRIG (1980).

Measured clathrate melting temperatures of H2O - NaCl - CO2 inclusions which have formed in the one-phase field are in excellent agreement with the equation of DIAMOND (1992).