Metasomatism involving fluids in CO2-H2O-NaCl.

Abstract

Phase relations in the system CO2-H2O-NaCl are used to model metamorphic fluids in carbonate rocks. It is possible that two fluid phases coexist along most T-depth paths in metamorphic rocks if the chloride content of the fluid system is adequate. The presence of two fluid phases is shown to have considerable influence on the evolution of fluid composition in reacting metasomatic systems. An aqueous brine interacting with dehydration, decarbonation, hydration-decarbonation and dehydration- decarbonation equilibria is considered. Dilution of a one-phase fluid describes the process involving dehydration equilibria. Dilution of the brine by CO2 as in the remaining three reaction types drives fluids into a two-phase region of coexisting liquid and vapour. Saturation in NaCl is shown to be possible for systems undergoing hydration-decarbonation reactions. Irreversible reaction is possible in systems undergoing dehydration-decarbonation. The rate of reaction progress increases in systems that reach saturation with respect to halite. Still greater rates of reaction progress are possible in systems undergoing irreversible reaction. (Authors' abstract).-A.W.H.