Abstract

New experimental data on fluid immiscibility in the H 2 O-NaCl-CO 2 system at 900 degrees C and 5-7 kbar have been obtained using the synthetic fluid-inclusion technique. The main result is a significant enlargement of the immiscibility field as pressure decreases from 7 to 5 kbar. Combined with previous data, our experiments show that immiscibility is probably a widespread phenomenon in low-pressure granulite-facies rocks. Because CO 2 -rich fluids and NaCl-rich aqueous fluids have very contrasting wetting behaviour, fluid unmixing could result in a selective entrapment of the CO 2 -rich component in granulites (Watson & Brenan, 1987). To check this hypothesis, we performed an experiment in which polycrystalline quartz was heat-treated in the presence of small volume percentages of the two immiscible fluids. The observed pore geometry is characterized by a combination of large, isolated CO 2 -rich bubbles, and an interconnected network of NaCl-H 2 O-filled channels along quartz edges. A model combining unmixing and the subsequent escape of the aqueous fluid by porous flow could therefore explain the CO 2 -rich fluid inclusions in low-pressure granulites.

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