COMPOSITIONAL RE-EQUILIBRATION OF FLUID INCLUSIONS IN QUARTZ

Compositional modifications to salt-water fluid inclusions in quartz w ere observed following exposure to disequilibrium conditions in a seri es of laboratory experiments in which samples containing inclusions of known composition were annealed at 3 kbar and 600 less than or equal to T less than or equal to 825 degrees C in the presence of fluids hav ing different compositions for time intervals ranging from a few days to one month. Changes in inclusion compositions following re-equilibra tion were monitored using salt crystal dissolution temperatures and/or IR (infra red) spectroscopy. The behaviors of both synthetic and natu ral fluid inclusions were studied. The synthetic samples were re-equil ibrated under P-int = P-conf conditions to minimize stress in the crys tal surrounding the inclusions, and were subjected to both f(H2O)(int) > f(H2O)(conf) and f(H2O)(int) < f(H2O)(conf). After re-equilibration for four days at T greater than or equal to 600 degrees C, most inclu sions displayed significant compositional changes without decrepitatio n. Salinity variations as large as approximate to 25 wt% were inferred for brine inclusions exposed to f(H2O)(int) not equal f(H2O)(conf) fo r 16 days at 825 degrees C. The majority of our observations are consi stent with the net diffusion of water toward the reservior having the lowest mu(H2O); i.e., synthetic NaCl-H2O fluid inclusions exhibited in creased Tm(NaCl)s (implying lower relative H2O contents) when re-equil ibrated in the presence of fluids having lower mu H2O, whereas, simila r (and, in some cases, the same) inclusions exhibited decr eased Tm(Na CI)s (implying higher H2O contents) after exposure to fluids having hi gher mu(H2O) The behavior of natural salt-water fluid inclusions durin g re-equilibration was generally consistent with corresponding observa tions on synthetic samples verifying that compositional changes are no t restricted to synthetic inclusions. Our results clearly show that th ere was chemical communication between fluids trapped as inclusions in quartz and the external fluid reservoir. Additionally, it is evident that although applied stress can significantly enhance the re-equilibr ation rate, strain in the crystal host around the inclusions resulting from large pressure differentials between the internal and confining fluids is not a necessary prerequisite for compositional change. Final ly, because significant compositional changes can be induced in brine inclusions in quartz during shortterm exposure to non-equilibrium cond itions at 600 less than or equal to T less than or equal to 825 degree s C in the laboratory, it is likely that similar changes may result at much lower temperatures during exposure of natural rocks to non-equil ibrium conditions over geologic time.