Precipitation sequence of fracture-filling calcite in fractured granite and changes in the fractionation process of rare earth elements and yttrium

Abstract

This study has focused on the paragenetic sequence, and variation in rare earth elements with yttrium (REY) composition, of fracture-filling calcite in the Toki Granite in the Mizunami area, central Japan. The morphological, chemical, and isotopic characteristics of the calcite and chemistry of fluid inclusions reveal that the calcite in the Toki Granite can be differentiated into four discrete generations: Calcite I (oldest) to Calcite IV (most recent). The precipitation history of calcite reflects the changes in the hydrogeochemical regime of paleo-groundwaters, controlled by the evolution of groundwater by seawater infiltration associated with marine transgression and surface water infiltration associated with marine regression and uplift. The post-Archean average shale-normalized REY patterns in each generation of calcite show no significant Ce anomaly, negative Eu anomaly, and a light REY (LREY)-depleted pattern dominates. These features are also common to the Toki Granite. The consistency of the features in each generation of calcite indicates that REY was supplied from the Toki Granite by water-rock interaction. The lack of a Ce anomaly in the calcite demonstrates that groundwaters have maintained reducing conditions during the calcite precipitation. However, the fractionation of LREY and heavy REY in each generation of the calcite is more pronounced than in the granite. The fractionation process in the paleo-groundwaters from which each generation of calcite precipitated closely relates to the systematic variation of carbonate complex in the REY series and/or pH in palaeo-groundwater. The findings of this study will be important for assessing the long-term safety of geological disposal of high-level radioactive waste.