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Understanding the mass transfer characteristics of matrix diffusion and sorption is important in the safety assessment of geological disposal of high-level radioactive waste in crystalline rock (granite) by contributing to radionuclide retardation through mass transfer within the rock body. We present a comparative discussion of the effective diffusion coefficient (De), porosity, and petrological data for rock samples collected from the Toki Granite in central Japan, to evaluate the role of micropores within minerals in retardation by matrix diffusion and sorption in granitic rocks. De was derived from the through-diffusion experiments using uranine, barium, strontium, and chloride ions as tracers. Petrological data consist of the fracture frequency, the extent of hydrothermal alteration in the minerals, the micropore volume in the minerals, and the three-dimensional modal mineralogy (mineral assemblage and ratio) for the target rock samples. The relationship between the De, porosity, and petrological data has the following implications: 1) Micropores in minerals related to the alteration act as ‘storage pores’ that contribute to retardation due to matrix diffusion and sorption; 2) Once the uranine, cations (Ba2+ and Rb+), and anion (Cl−) penetrate the micropores in the minerals through matrix diffusion, the cations are sorbed on the micropore surfaces, whereas the uranine and conservative chloride anion is trapped at the end of the micropore network, resulting in retardation; 3) Regions with a high fracture frequency are associated with not only active advection–dispersion through fractures, but also retardation due to matrix diffusion and sorption; 4) The grain-boundary pores between colorless minerals act as ‘transport pores’ owing to matrix diffusion, and the retardation within grain-boundary pores is less than that within micropores in minerals.

This study presents the K-Ar geochronology for hydrothermal K-feldspar in plagioclase alteration, including methodology and application to the Toki granite, in central Japan. Borehole samples from the Toki granite were collected and mechanically and chemically processed to separate plagioclase from the rock and remove bulk impurities. The sample fraction of cleaned plagioclase powder was further processed to a smaller size fraction, allowing separation of the altered K-feldspar from the plagioclase host. The resulting K-feldspar represented the hydrothermal alteration product and was characterized crystallographically as microcline, and its K-Ar ages were measured. The results of the K-Ar dating and petrographic characterization indicated that in this setting, plagioclase alteration occurred through a combination of solid-state replacement and dissolution–precipitation processes. The K-feldspathization age enables constraint of the temporal conditions of the solid-state replacement process to 62.2 ± 1.4 Ma. The time-temperature (t-T) path of the sampling site is an effective tool for determining both the timing and thermal conditions of the hydrothermal microcline formation in plagioclase alteration. The combination of the t-T path and the microcline K-Ar age provides formation temperatures of about 307–325 °C. The timing and thermal conditions of solid-state replacement (62.2 ± 1.4 Ma and 325–307 °C) indicate an older age and a higher temperature than those of dissolution–precipitation (59.2 ± 1.4 Ma and 305–290 °C: Yuguchi et al., 2019A). The plagioclase alteration consists of serial processes from solid-state replacement to dissolution–precipitation. Addition of the thermal conditions and timing into petrography have implications for the sequential phenomenal variation in granite.