Synthetic H2O-CO2-NaCl fluid inclusions with relative salinities (R.S. = mass NaCl/ [mass H2O + mass NaCl]) of up to 50 wt% and mole fractions of CO2 (XCO2) ranging from 0.1 to 0.5 formed in spontaneously nucleated forsterite and orthopyroxene hosts are used to define the limits of fluid immiscibility at temperatures of 890-960 °C and pressures of 6600-7500 bar. Observations of the phase ratios (liquid CO2 to H2O-NaCl liquid) in the inclusions at 25 °C and microthermometric data were used to determine whether the inclusions trapped miscible or immiscible fluids. The results of this study show that fluid immiscibility in the system H2O-CO2-NaCl is possible for fluids with moderate to low salinities (>23 wt% relative to H2O) and intermediate mole fractions of CO2 (XCO2 >0.3) for temperatures as high as 940 °C at 6800-7700 bar. This result indicates that the H2O-CO2-NaCl miscibility gap expands rapidly with pressure, making fluid immiscibility viable even at the highest grades of metamorphism.

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