Isotopic signals in fracture-filling calcite showing anaerobic oxidation of methane in a granitic basement

Abstract

Understanding the long-term redox conditions and the related carbon cycle in groundwater is essential for long-term safety assessment because they affect the performance of barrier systems and radionuclide transport in geological disposal. However, it is difficult to identify those long-term changes directly. To help understand this, we conducted a paleohydrogeological study on calcite mineralization associated with fracture-controlled groundwater flow-paths in the Toki Granite in central Japan, focusing on its carbon and oxygen stable isotope characteristics. Previous studies revealed four generations of fracture-filling calcite in the Toki Granite. Therefore, we conducted isotopic analysis on both bulk samples of calcite and spatially-resolved microsamples of discrete generations of calcite within zoned crystals. The δ18OVPDB of calcite ranging between −32.7‰ to −0.59‰ revealed that the groundwater that precipitated the calcite was derived from various origins over the geological history of the area, including early hydrothermal fluids associated with the late-stage cooling of the granite (less than −17.2‰); freshwater invasion from the surface following regional uplift (−18.5‰∼ −8.3‰), and; seawater that penetrated during periods of marine transgression (−8.7‰ ∼ −0.3‰). The range in δ13CVPDB values (−56.5‰ ∼ +6.0‰) was wider than the isotopic range of dissolved inorganic carbon (DIC) that originated from hydrothermal, meteoric, and seawater sources (−25‰ ∼ +2‰). Calcite with low δ13CVPDB values less than −25‰ is believed to have precipitated from groundwater with DIC that was provided by anaerobic oxidation of methane (AOM), whereas calcite with δ13CVPDB higher than +2‰ is believed to have precipitated from groundwater containing 13C-enriched DIC as a carbon source derived during methanogenesis. These processes influencing the formation of calcite mineralization in the Toki Granite are comparable to those at other crystalline rock sites in European countries. The AOM calcite and calcite associated with methanogenesis in the Toki Granite precipitated during the transition of the groundwater origin from meteoric to seawater. Understanding these redox processes and the related carbon cycle in granitic groundwater can provide important insights into processes relevant to assessing the long-term evolution of geoenvironmental systems.