Grain boundary migration of water and carbon dioxide during uplift of garnet‐zone Alpine Schist, New Zealand

Abstract

Abstract Deformed quartz veins in garnet‐zone schist adjacent to the active Alpine Fault, New Zealand, have fluid inclusions trapped along quartz grain boundaries. Textures suggest that the inclusions formed in their present shapes during annealing of the deformed veins. Many of the inclusions are empty, but some contain carbon dioxide with densities that range from 0.16 to 0.80 g cm−3. No water, nitrogen or methane was detected. The inclusions are considerably more CO2‐rich than either the primary metamorphic fluid (<5% CO2) or fluids trapped in fracture‐related situations in the same, or related, rocks (<50% CO2). Enrichment of CO2 is inferred to have resulted from selective migration (wicking) of saline water from the inclusions along water‐wet grain boundaries after cooling‐induced immiscibility of a water‐CO2 mixture. Inclusion volumes changed after loss of water. Non‐wetting CO2 remained trapped in the inclusions until further percolation progressively removed CO2 in solution. This mechanism of fluid migration dominated in ductile quartz‐rich rocks near, but below, the brittle‐ductile transition. At deeper levels, hydraulic fracturing is also an important mechanism for fluid migration, whereas at shallower levels advection through open fractures dominates the fluid flow regime.