ENRICHMENT OF CO2 IN FLUID INCLUSIONS IN QUARTZ BY REMOVAL OF H2O DURING CRYSTAL-PLASTIC DEFORMATION

Ductile strain-induced leakage of H2O from mixed CO2 + H2O fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure CO2 fluid inclusions in metamorphic rocks. The H2O needed for hydrolytic weakening of quartz under stress may be provided by the H2O in fluid inclusions. With dislocation creep, as dislocations nucleate on the walls of fluid inclusions or intersect them, H2O can be transported with the dislocations from the inclusion to the grain boundaries. The process should continue so long as there is stress on the host quartz and the inclusion contains H2O. The H2O taken to the grain boundaries would, due to its wetting properties, be wicked out along the grain boundaries. If the H2O is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal-plastic flow, then a residual inclusion bearing components other than water should remain. A mixed CO2 + H2O inclusion would, as a consequence of the process, become a pure CO2 fluid inclusion. During dynamic recrystallization, the CO2 would collect at grain boundary triple junctions and, following grain boundary migration, would become incorporated into the recrystallized quartz as fluid inclusions. This mechanism of generating pure CO2 inclusions could result in CO2 fluid inclusions having densities appropriate for the pressure-temperature conditions during deformation. Using estimates of temperature of deformation, the total confining pressure during deformation could be determined from the density of the CO2 fluid in these inclusions. © 1990.