Study on investigation method of fracture distribution based on data obtained at the Mizunami Underground Research Laboratory, central Japan

Sasao, Eiji

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 29(2), p.112 - 118, 2022/12

https://doi.org/10.3327/jnuce.29.2_112

Since fractures in granite act as pathways for groundwater flow and mass transport, understanding of fracture distribution is an important subject for the disposal of high-level nuclear waste. Fracture data obtained through the borehole investigation from ground surface contains errors due to the crossing angle between boreholes and fractures. I studied method of borehole investigation that can effectively characterize the fracture distribution based on fracture information obtained from geological investigation of shaft wall at the Mizunami Underground Research Laboratory. As a result, it was found that inclined boreholes captured larger number of fractures than vertical boreholes. Therefore, inclined boreholes are preferable for efficient characterization of the fractures. Fracture orientation at surface exposure is well concordant with that at the shaft, which implies that the drilling direction should be determined based on the result of surface fracture mapping.

Micropores and mass transfer in the formation of myrmekites

Yuguchi, Takashi*; Yuasa, Haruka*; Izumino, Yuya*; Nakashima, Kazuo*; Sasao, Eiji; Nishiyama, Tadao*

American Mineralogist, 107(3), p.476 - 488, 2022/03

https://doi.org/10.2138/am-2021-7956

The methodology and interpretations in this study provide new insights into the mechanism of myrmekite formation in a granitic system. The presence of micropores in the myrmekites and this study clarified that the genesis of micropores is related to the formation of myrmekites in the Toki granite. The results led to an increased understanding of (1) an estimate of mass transfer between the reactant and product minerals, and the inflow and outflow of components with consideration of the volume change due to micropore formation, (2) the factor controlling the formation of micropores during myrmekitization, and (3) the sequential variations in the hydrothermal fluid chemistry during sub-solidus conditions. The micropores act as a migration pathway for hydrothermal fluid and thus enhance the mass transfer within a granitic body starting with the myrmekite formation and continuing through the present day and into the future.

Mass transfer associated with chloritization in the hydrothermal alteration process of granitic pluton

Yuguchi, Takashi*; Matsuki, Takanobu*; Izumino, Yuya*; Sasao, Eiji; Nishiyama, Tadao*

American Mineralogist, 106(7), p.1128 - 1142, 2021/07

https://doi.org/10.2138/am-2020-7353

This study reveals the hydrothermal alteration processes in a pluton, with a focus on the mass transfer between minerals and hydrothermal fluid. Hydrothermal alteration of the Toki granite in Tono area, central Japan, progressed through the successive processes of chloritization, plagioclase alteration, and precipitation of a carbonate mineral. This paper describes the alteration process of hornblende chloritization, K-feldspar chloritization, and the formation of fracture-filling chlorite through petrography and mineral chemistry. Several types of chloritization reactions (including biotite chloritization) can be characterized by their reaction with the inflow of Al, Fe, Mn, and Mg and the outflow of HSiO, Ca, K+, and F. The reactions of chloritization and plagioclase alteration represent the sequential variations in fluid chemistry at temporal conditions from 68 Ma to 51 Ma as the temperature decreased from 350C to 180C.

Fluvial sediments and porcelain clay of the Miocene to Pleistocene Seto Group, central Japan

Hatano, Nozomi*; Yoshida, Koki*; Sasao, Eiji

Chishitsugaku Zasshi, 127(6), p.345 - 362, 2021/06

https://doi.org/10.5575/geosoc.2020.0028

This paper is guidebook of field excursion at the 127th annual meeting of the Geological Society of Japan, originally planned to be held on September, 2020 (postponed for one year). Miocene to Pleistocene Seto Group yields porcelain clay and are studied in the point of view of clay mineralogy and geology. In this field excursion, genesis of porcelain clay will be discussed through the observation of paleosols, paleoweathering profiles of the Seto Group and the basement strongly weathered granite.

Genesis and development processes of fractures in granite; Petrographic indicators of hydrothermal alteration

Yuguchi, Takashi*; Izumino, Yuya*; Sasao, Eiji

PLOS ONE (Internet), 16(5), p.e0251198_1 - e0251198_17, 2021/05

https://doi.org/10.1371/journal.pone.0251198

This study analyzes the relationships among alteration indicators, areal microvoid fractions in chloritized biotite, and macroscopic fracture frequencies in the Toki granite, central Japan, to establish the genesis and development processes of fractures in granite. Petrographic alteration indicators using biotite chloritization as innovative methods are proposed to evaluate the extent of hydrothermal alteration and fracture frequency within granites. Samples with high macroscopic fracture frequencies correspond to a high number of areal microvoid fractions and large alteration indicators. The alteration indicators contribute to the characterization of present and future distributions of macroscopic fracture frequencies.

Major element and REE compositions of Pliocene sediments in southwest Japan; Implications for paleoweathering and paleoclimate

Hatano, Nozomi*; Yoshida, Koki*; Mori, Saori*; Sasao, Eiji

Sedimentary Geology, 408, p.105751_1 - 105751_13, 2020/10

https://doi.org/10.1016/j.sedgeo.2020.105751

The history of the East Asian monsoon and the relationship between the development of the monsoon climate, tectonics and global climate are complicated and controversial. The present study clarifies the chemical weathering conditions in southwest Japan based on the concentrations of major elements and rare earth elements (REEs) in lacustrine muddy sediments. Between 3.8 and 3.4 Ma, chemical weathering significantly intensified, as indicated by the high values of the chemical index of alteration as well as the high concentrations of REEs and light REEs against heavy REEs and kaolinite-rich clay mineral compositions. The intense chemical weathering on land in southwest Japan from 3.8 to 3.4 Ma may have been regulated by alternating periods of warm and humid climate brought by the invasion of the Kuroshio Current and the intensification of the East Asian summer monsoon.

Mizunami Underground Research Laboratory Project, Annual report for fiscal year 2018

Takeuchi, Ryuji; Iwatsuki, Teruki; Matsui, Hiroya; Nohara, Tsuyoshi; Onoe, Hironori; Ikeda, Koki; Mikake, Shinichiro; Hama, Katsuhiro; Iyatomi, Yosuke; Sasao, Eiji

JAEA-Review 2020-001, 66 Pages, 2020/03

JAEA-Review-2020-001.pdf:7.6MB

The Mizunami Underground Research Laboratory (MIU) Project is being pursued by the Japan Atomic Energy Agency (JAEA) to enhance the reliability of geological disposal technologies through investigations of the deep geological environment in the crystalline rock (granite) at Mizunami City, Gifu Prefecture, central Japan. On the occasion of JAEA reformation in 2014, JAEA identified three remaining important issues on the geoscientific research program based on the synthesized latest results of research and development (R&D): "Development of countermeasure technologies for reducing groundwater inflow", "Development of modeling technologies for mass transport" and "Development of drift backfilling technologies". The R&D on three remaining important issues have been carrying out in the MIU Project. In this report, the current status of R&D and construction activities of the MIU Project in fiscal year 2018 is summarized.

Mizunami Underground Research Laboratory Project, Plan for fiscal year 2019

Takeuchi, Ryuji; Iwatsuki, Teruki; Matsui, Hiroya; Ikeda, Koki; Mikake, Shinichiro; Hama, Katsuhiro; Iyatomi, Yosuke; Matsuoka, Toshiyuki; Sasao, Eiji

JAEA-Review 2019-014, 30 Pages, 2019/10

JAEA-Review-2019-014.pdf:4.72MB

The Mizunami Underground Research Laboratory (MIU) Project is being pursued by the Japan Atomic Energy Agency(JAEA) to enhance the reliability of geological disposal technologies through investigations of the deep geological environment in the crystalline host rock(granite) at Mizunami City, Gifu Prefecture, central Japan. On the occasion of the reform of the entire JAEA organization in 2014, JAEA identified three important remaining issues on the geoscientific research program based on the synthesized latest results of research and development (R&D): "Development of countermeasure technologies for reducing groundwater inflow", "Development of modeling technologies for mass transport" and "Development of drift backfilling technology". The R&D on three remaining important issues have been carrying out in the MIU Project. This report summarizes the R&D activities planned for fiscal year 2019 on the basis of the MIU Master Plan updated in 2015 and Investigation Plan for the Third Medium to Long-term Research Phase.

Characterizing the permeability of drillhole core samples of Toki granite, central Japan to identify factors influencing rock-matrix permeability

Kubo, Taiki*; Matsuda, Norihiro*; Kashiwaya, Koki*; Koike, Katsuaki*; Ishibashi, Masayuki; Tsuruta, Tadahiko; Matsuoka, Toshiyuki; Sasao, Eiji; Lanyon, G. W.*

Engineering Geology, 259, p.105163_1 - 105163_15, 2019/09

https://doi.org/10.1016/j.enggeo.2019.105163

Rock matrix permeability is mainly controlled by microcracks. This study aims to identify the factors influencing the permeability of the Toki granite, central Japan. Permeability of core samples, measured by a gas permeameter, largely increases in the fault and fracture zones. Although a significant correlation is identified between permeability and P-wave velocity, this correlation is enhanced by classifying the samples into two groups by the Mn/Fe concentration ratio. Thus, lithofacies is another control factor for permeability due to the difference in mineral composition. Moreover, permeability shows significant negative and positive correlations with Si and Ca concentrations, respectively. These concentrations are probably affected by dissolution of silicate minerals and calcite generation in the hydrothermal alteration process. Therefore, a combination of hydrothermal alteration and strong faulting are the predominant processes for controlling permeability.

Effects of grain size on the chemical weathering index; A Case study of Neogene fluvial sediments in southwest Japan

Hatano, Nozomi*; Yoshida, Koki*; Sasao, Eiji

Sedimentary Geology, 386, p.1 - 8, 2019/05

https://doi.org/10.1016/j.sedgeo.2019.03.017

Grain size variation of sediments generally complicates the assessment of the degree of chemical weathering. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan to evaluate the effects of grain size on the degree of chemical weathering. We indicated that the AlO/SiO ratio may be suitable as a grain size index. Most of the fine-grained sediments have higher CIA values. In contrast, coarse-grained sediments, display a wide range of CIA values. The CIA values for intermediate grain sized samples reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. In the intermediate grain sized samples, the Miocene sediments have higher CIA values than Pliocene sediments. To compare the degree of chemical weathering, it is desirable to evaluate with the AlO/SiO ratios limited to the intermediate grain size range.