Hidekazu Yoshida

W- and M-type tetrad effects in REE patterns for water-rock systems in the Tono uranium deposit, central Japan

Abstract Abundances of rare earth elements plus Y (REE) were determined for granitic rocks, Tertiary sedimentary rocks, and related groundwater in the Tono area, central Japan. Tetrad effects, concave curves in REE patterns, were found for these rocks and groundwater samples. Conjugate M- and W-types of tetrad effects were simultaneously observed in these specimens which constitute the water–rock system; the granitic rocks show the M-type tetrad effects while the groundwater and the sedimentary rocks exhibit the W-type tetrad effects. Non-chondritic Y/Ho ratios were found especially in the groundwater; Y/Ho ratios showed larger values than the chondritic value for the samples showing W-type tetrad effects, and vice versa. These results suggest that: (1) preference of the groundwater for a W-type tetrad effect and larger Y/Ho ratio than chondrite produces an M-type tetrad effects and lower Y/Ho ratios in the granitic rocks during weathering processes; (2) the W-type tetrad effect and larger Y/Ho ratio for the sedimentary rocks is brought about by the fixation of groundwater REE onto the sedimentary rocks. The variation of the tetrad effect and Y/Ho ratio was confirmed by laboratory experiments on the leaching of REE from the granitic rocks. The W-type tetrad effect and large Y/Ho ratio in the U-mineralization zone in the sedimentary rocks were also found by SIMS analysis, which is consistent with the W-type tetrad effect and larger Y/Ho ratio in the whole rock abundances of the sedimentary rocks. Disequilibrium of the U series ( 234 U/ 238 U), analyzed by SIMS in the U-mineralization zone, suggests that the U-bearing minerals were formed through precipitation from groundwater as it became reducing. These results suggest that the REE in the sedimentary rocks were transported by the groundwater from the granitic rocks in the Tono area. Based on this study, it is implied that the degree of the tetrad effects and the variation of Y/Ho ratio can be valuable tools to investigate the migration of REE during water–rock interactions in natural systems.

Isotopic and morphological features of fracture calcite from granitic rocks of the Tono area, Japan: a promising palaeohydrogeological tool

Abstract This study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system, using fracture-filling and fracture-lining calcite. Fracture calcite in deep (to ca. 1000 m) granitic rocks of the Tono area, central Japan, was investigated by optical and electron microscopy, and chemical and isotopic analysis. Coupled with geological evidence, these new data imply 3 main origins for the waters that precipitated calcite: (1) relatively high-temperature hydrothermal solutions, precipitating calcite distinguished by δ18OSMOW from −3 to ca. 10‰, and with δ13CPDB from ca. −18 to −7‰; (2) seawater, probably partly of Miocene age, which precipitated calcite distinguished by δ13CPDB of ca. 0‰ and δ18OSMOW > ca. 20‰; (3) fresh water, with a variable δ13CPDB composition, but which precipitated calcite distinguished by δ13CPDB that was significantly ca. 17‰. Data for 14C suggest that at least some of the fresh-water calcite formed within the last 50 ka. The present day hydrogeological regime in the Tono area is also dominated by fresh groundwater. However, the marine calcite of probable Miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved. Studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters. When this correlation is applied to the Tono observations, the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data. Thus, the present study suggests that even in presently low-salinity groundwater systems, calcite morphological variations may record the changing salinity of coexisting groundwaters. It is suggested that calcite morphological data, coupled with isotopic data, could provide a powerful palaeohydrogeological tool in such circumstances.

A new method for the determination of CeIII/CeIV ratios in geological materials; application for weathering, sedimentary and diagenetic processes

Abstract Relative abundances of rare earth elements (REE) in geological materials are used widely to investigate diverse geochemical issues such as the origins of igneous rocks or the degree of stratification of water columns in oceans. One of the REE, cerium (Ce), can exist in either trivalent or tetravalent forms depending on the redox condition. Thus, knowledge of the oxidation state of Ce in rocks and minerals could potentially be used to constrain the redox states of past and present geological environments. However, the use of Ce for this purpose has been hampered by an inability to measure its oxidation state directly. Here, we present a new method, employing the X-ray absorption near-edge structure, for making such determinations in samples with Ce concentrations as low as 15 ppm. By analyzing a range of diverse geological materials (granites, manganese nodules and cherts), we showed that the method could identify significant differences in Ce speciation between materials formed under widely differing redox conditions. Used together with the degrees of Ce anomalies measured in REE patterns, this information has promise for refining our knowledge of various geochemical processes such as weathering, sedimentary and diagenetic processes that control the REE behavior. Such a refinement should in turn improve geological interpretations based upon REE data.

Redox front formation in fractured crystalline rock: an analogue of matrix diffusion in an oxidizing front along water-conducting fractures

Redox fronts may be formed in the geological environment surrounding a shallow geological repository for radioactive waste due to oxidized pore water infiltration. Such oxidation fronts can exert an influence on contaminant retardation during matrix diffusion via redox changes and direct fixation by accumulated oxides. However, the process of matrix diffusion in oxidizing fractured crystalline rocks has rarely been described. This study focuses on a redox front in Cretaceous granitic rocks from central Japan. Detailed observations reveal that ferric iron has accumulated at grain boundaries and formed micro-spherical Fe-oxyhydroxide aggregations. Geochemical analyses with X-ray fluorescence, inductively coupled plasma–mass spectrometry and electron probe microanalysis examination show that rare earth elements, Cs and U have migrated to the front. Uranium, in particular, has been enriched at the front edge and probably retained by Fe-oxyhydroxides and this relationship persists in the oxidizing zone, presumably due to the strong adsorption capacity of the oxyhydroxides. These findings suggest that matrix diffusion in the oxidizing zone also can be an effective contaminant retardation.

Characterizing the chemical containment properties of the deep geosphere: water-rock interactions in relation to fracture systems within deep crystalline rock in the Tono area, Japan

Abstract A water/rock interaction study has been integrated with interpretations of geological structures, in order to understand the geochemical conditions in deep granitic rock at the Tono study site, central Japan. Geological investigations show that fracture systems in the granitic rock can be classified into: intact zones; moderately fractured zones; and intensely fractured zones. This classification is based upon the frequency and width of fractures and fractured zones. Isotopic analyses of the groundwaters indicate that the groundwater at SL 80m (SL: sea level) has recharged within the last 30 years. Petrological and mineralogical observations were undertaken, and coupled with theoretical calculations using the PHREEQE geochemical computer code. The results of this approach suggest that the chemical evolution of the groundwater is generally controlled by water-rock interactions involving plagioclase, clay minerals and ferric hydroxide in the intact zones and moderately fractured zones. The geochemical condition of the groundwater is correlated closely with the nature of the fracture systems, mineral compositions and water-rock interaction processes. Different fractures contain chemically distinct groundwaters, which have differing potential capacities to mobilize radionuclides. The investigation method, based on a comparison between geological structures and groundwater chemistry, can be applied to develop a realistic hydrogeochemical model for deep, fractured granitic rocks. Such an approach is necessary if the chemical containment potential of such lithologies is to be evaluated adequately. The work demonstrates that structural controls on chemical heterogeneities in groundwaters should be understood, before the disposal of redox-sensitive wastes can be undertaken safely in fractured crystalline rocks.

Relation between U-series nuclide migration and microstructural properties of sedimentary rocks

Abstract Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments. Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 105 a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides. Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.

Mineralogical and petrological evidence for the hydrogeological characteristics of the Tsukiyoshi Fault, Japan

Generic approaches for determining the spatial and temporal variability of a faults hydrogeological properties are being developed by studying the reversed Tsukiyoshi Fault at Tono, central Japan. This fault is associated with zones of deformation/damage that are sometimes wider than the faults displacement. The hydraulic characteristics probably vary laterally over metres to tens of metres and the hanging wall and foot wall have different hydraulic properties. It is also possible that some of these properties may have changed over time, due to physical processes and/or to water/rock interactions, even at relatively low-temperatures and pressures (to a few tens of degrees centigrade and hundreds of bars). It is suggested that mineralogical and structural data can be used to estimate the most transmissive conditions within a fault in the past. These estimates can then be used to select conservative (‘worst-case’) fault parameters for assessing the future safety of underground waste repositories. Such information can also be used to design a repository to avoid faults with permeable crush zones. The study demonstrates the value of integrating detailed mineralogical and petrological studies into a borehole drilling programme for characterizing the hydraulic properties of faults.

A biogeochemical assessment of the Tono site, Japan

Abstract When designing investigations of microbial populations in the subsurface, it is extremely valuable to undertake scoping calculations to estimate the likely microbial abundances and evaluate the effects of contamination during sampling. A biogeochemical assessment of the groundwater and lithologies of the Tono mine, Japan, has been made using the BGS/NAGRA computer code BGSE (Bacterial Growth in Subsurface Environments). This code enables an assessment to be made of the maximum microbial growth rates that may be achieved in ideal circumstances, based on availability of nutrients and energy calculated from mineralogical and groundwater analyses. The effect of drilling fluid/groundwater mixing on biomass was assessed using a hypothetical drilling fluid composition. The results of modelling the mixing between groundwater and drilling fluid shows that the addition of only small concentrations of drilling fluid (

Early post-mortem formation of carbonate concretions around tusk-shells over week-month timescales.

Carbonate concretions occur in sedimentary rocks of widely varying geological ages throughout the world. Many of these concretions are isolated spheres, centered on fossils. The formation of such concretions has been variously explained by diffusion of inorganic carbon and organic matter in buried marine sediments. However, details of the syn-depositional chemical processes by which the isolated spherical shape developed and the associated carbon sources are little known. Here we present evidence that spherical carbonate concretions (diameters φ : 14 ~ 37 mm) around tusk-shells (Fissidentalium spp.) were formed within weeks or months following death of the organism by the seepage of fatty acid from decaying soft body tissues. Characteristic concentrations of carbonate around the mouth of a tusk-shell reveal very rapid formation during the decay of organic matter from the tusk-shell. Available observations and geochemical evidence have enabled us to construct a ‘Diffusion-growth rate cross-plot’ that can be used to estimate the growth rate of all kinds of isolated spherical carbonate concretions identified in marine formations. Results shown here suggest that isolated spherical concretions that are not associated with fossils might also be formed from carbon sourced in the decaying soft body tissues of non-skeletal organisms with otherwise low preservation potential.

Estimation of the Migration Parameters for the Boom Clay Formation by Percolation Experiments on Undisturbed Clay Cores

The safety assessment of the repository for high level radioactive waste in the Boom clay formation requires reliable data for the migration parameters. The experimental set-up and the interpretation method is briefly described for percolation experiments on undisturbed clay cores drilled from the formation and results are reported. The undisturbed clay cores are drilled perpendicular and parallel to the stratification of the formation to study the isotropy of the formation. To represent the real situation as close as possible, in-situ interstitial clay pore water is used as percolating liquid. Parameter values determined in the percolation experiments are presented for bromine, iodine and tritiated water. Darcy velocity and effective stress are used as variables. Anisotropy of the formation is demonstrated for the hydraulic conductivity, but is found to be trivial for the dispersion parameters. A general relation between the apparent dispersion constant, the diffusion accessible porosity and the Darcy velocity is also given.