Enrichment of CO 2 in fluid inclusions in quartz by removal of H 2O during crystal-plastic deformation

Abstract

Ductile strain-induced leakage of H 2O from mixed CO 2 + H 2O fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure CO 2 fluid inclusions in metamorphic rocks. The H 2O needed for hydrolytic weakening of quartz under stress may be provided by the H 2O in fluid inclusions. With dislocation creep, as dislocations nucleate on the walls of fluid inclusions or intersect them, H 2O 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 H 2O. The H 2O taken to the grain boundaries would, due to its wetting properties, be wicked out along the grain boundaries. If the H 2O 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 CO 2 + H 2O inclusion would, as a consequence of the process, become a pure CO 2 fluid inclusion. During dynamic recrystallization, the CO 2 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 CO 2 inclusions could result in CO 2 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 CO 2 fluid in these inclusions.