Dawsonite-aragonite association in the Cretaceous Izumi Group, SW Japan: Evidence of CO_2-rich fluid invasion in the area of classical study

摘要

Dawsonite has been recently re-discovered in the Cretaceous Izumi Group, Southern Osaka Prefecture, the area where this mineral was first described in Japan. Dawsonite commonly occurs as thin veinlets that are strongly associated with aragonite. Marginal fringes of aragonite and dawsonite are found along mudstone walls of thick calcite-dominated veins that cut across the dawsonite-aragonite veins. The dawsonite-aragonite association is characterized by a large number of cavities of varying sizes, suggesting their precipitation in an open space formed by the forcible invasion of a CO_2-rich fluid. Thin- section observations reveal that the CO_2-rich fluid hardly reacts with mudstone. The order of formation of carbonate veins (from the dawsonite-dominated regime to the calcite-dominated regime) is generally concordant with the changes in the fluid chemistry and precipitated phases predicted on the basis of a geochemical simulation to the quartzofeldspathic CO_2 reservoir in CO_2 geological storage.