Rare earth elements (REE) - Core

Chemical Geology 417 (2015) 58–67

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Rare earth elements (REE) in deep groundwater from granite and fracture-filling calcite in the Tono area, central Japan: Prediction of REE fractionation in paleo- to present-day groundwater Takashi Munemoto ⁎, Kazuaki Ohmori, Teruki Iwatsuki Mizunami Underground Research Laboratory, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 1-64 Yamanouchi, Akiyo, Mizunami 509-6132, Gifu, Japan

a r t i c l e

i n f o

Article history: Received 28 May 2015 Received in revised form 28 August 2015 Accepted 29 September 2015 Available online 24 October 2015 Keywords: Groundwater Rare earth elements Fracture-filling calcite Water–rock interaction

a b s t r a c t Rare earth elements (REEs) combined with yttrium (YREE) in deep groundwater from granite and fracture-filling calcite are being studied at the Mizunami Underground Research Laboratory (MIU, Tono area, central Japan). The groundwater was sampled from cored boreholes drilled from the MIU galleries at 200, 300, 400 and 500 m depths. The fracture-filling calcite was collected from the same boreholes and dissolved in 1 M NH4 acetate buffer solution (pH 4.8). Major chemical composition of groundwater changed with depth; and the concentrations of some major elements (Na+, K+, Ca2+ Mg2+, Cl−, Br− and B) increased with depth, while alkalinity, SO2− 4 , and F− decreased. Isotopic compositions of δ18O and δD indicated that the groundwater originated from meteoric water. YREE patterns of groundwater also changed with depth; with heavy REE (HREE) enrichment in “shallow” groundwater (−200 to −400 m) and light REE (LREE) enrichment in “deep” groundwater (−500 m). The pre− + − + dominant YREE species are YREECO+ 3 and YREE(CO3)2 (LaCO3 : 65–70%, La(CO3)2 : 8–27%, LuCO3 : 20–38% and − Lu(CO3)2 : 53–77%). The LREE enrichment in the “shallow” groundwater resulted in preferential scavenging of positively charged YREECO+ 3 by negatively charged bedrock surfaces in slightly alkaline groundwater pH (7.4– 8.6). Because of the construction and operation of the underground facility, the “shallow” groundwater has been influenced by infiltration of surface water undersaturated in calcite. In contrast, the “deep” groundwater was barely affected by the mixing of surface water, indicating that the YREE originated from YREE fractionation by bedrock granite. Irrespective of depth, a negative Ce anomaly preserved in the groundwater, is probably indicative of paleo-oxic conditions during paleo-recharge. The negative Ce anomaly in the groundwater was possibly fractionated from sedimentary calcite with positive Ce anomalies during the paleo-recharge. The YREE patterns of the fracture-filling calcite were enriched in LREE. The isotopic variations of δ18O and δ13C in the fracture-filling calcite indicated that it originated from a mixture of sea water and fresh water at a temperature b 100°C. The YREE patterns of paleo-groundwater from which the fracture calcite precipitated were calculated using YREE abundances in the fracture calcite and distribution coefficient reported in previous laboratory experiments. The paleo-groundwater was found to be enriched in LREE and HREE depending on the aqueous YREE species. Our results suggest that changes in the paleo- to present-day YREE fractionation range roughly from that of present-day shallow to deep groundwater. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction The chemical properties of rare earth elements (REEs) and combined yttrium (YREE), which act as geochemical tracers in aqueous environments, have been studied widely (Goldstein and Jacobsen, 1988; Gosselin et al., 1992; Takahashi et al., 2002; Willis and Johannesson, 2011). REEs exhibit a gradual decrease in their ionic radii with increasing atomic number, which leads to systematic changes in chemical properties across the YREE series. The YREE abundance in natural ⁎ Corresponding author. E-mail address: [email protected] (T. Munemoto).

materials is mainly regulated by mineral–water interactions such as dissolution/precipitation of minerals, and oxidation/reduction of ions during adsorption/desorption onto mineral surfaces (Bau, 1999; Sholkovitz, 1992; Tweed et al., 2006). The YREE abundance in groundwater is ascribed to the interaction with YREE-bearing minerals along flow paths, and it varies with groundwater conditions including pH, redox conditions, and dissolved anions (Johannesson et al., 1999; Smedley, 1991). For instance in seawater, YREE abundance is characterized by input sources, transport of aqueous YREE in rivers to the ocean, stability of colloidal materials, hydrothermal seafloor vents, and salinity and redox state of seawater (Bolhar et al., 2004; Haley et al., 2004). However, global YREE variability in natural groundwater is very complex and

http://dx.doi.org/10.1016/j.chemgeo.2015.09.024 0009-2541/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

T. Munemoto et al. / Chemical Geology 417 (2015) 58–67

dependent on geologies, water flow paths, mixing and chemical condition of groundwater (Noack et al., 2014). Therefore, the variability of YREE behavior in groundwater at the regional scale provides useful information for enhancing our understanding of YREE behavior in natural water systems. Calcium carbonates are some of the most common secondary minerals in nature including sedimentary rocks and igneous rocks because they precipitate readily from fluids. Many studies have focused on the incorporation of metals in calcium carbonates, to reflect the precipitation conditions in the source solution (Munemoto et al., 2014a). In particular, YREE abundance in calcite has been used as a record for assessing paleo-environments (Tanaka et al., 2004; Zhou et al., 2012). For example, the REE patterns of carbonate precipitates are similar to those of the original CO2-rich groundwater in Kangwon, Korea (Choi

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et al., 2009). The similarity in REE patterns between the precipitates and original water may be ascribed to co-precipitation with carbonate minerals without ligand exchange of the carbonate aqueous complex between the precipitates and original water (Négrel et al., 2000). Calcite precipitated in granite fractures broadly represents the YREE abundance of paleo-groundwater when it precipitated, and back-calculation of paleo-groundwater chemistry based on distribution coefficients of calcite suggests that present-day groundwater in Laxemar, Sweden has been stable for millions of years (Maskenskaya et al., 2015; Mathurin et al., 2014). The chondrite-normalized REE pattern of calcite fracturefilling with a positive Eu anomaly is indicative of reducing conditions in the paleo-groundwater at the Samkwang Mine Site, Korea (Lee et al., 2003). Hence the YREE abundance in calcite is a useful indicator of the paleo-groundwater environment. Because the groundwater moves

Fig. 1. Geologic map and cross section of the Tono area and sampling locations in the Mizunami Underground Research Laboratory (MIU). The groundwater was sampled from boreholes 07MI07, 09MI20 and 10MI26 drilled at depths of 200, 300 400 m, respectively, and 12MI33, 13MI38, 13MI39 and 13MI40 at a depth of −500 m.

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through granite fractures along water flow paths (Illman et al., 2009), the YREE abundance in facture filling calcite precipitated in the fracture represents paleo-groundwater environment in granite. Takahashi et al. (2002) studied YREE behavior during water–rock interactions at the Tono uranium deposit in the Tono area (Fig. 1), central Japan, and suggested that groundwater YREE leached from bedrock granite was transported to the overlying sedimentary rocks near the unconformity. However, YREE behavior at depth in granite is poorly understood due to difficulty in sampling of the deep groundwater. The Mizunami Underground Research Laboratory (MIU) has been constructed in the Tono area. The facility has galleries at several levels and two shafts excavated to depth of 500 m in sedimentary rock and bedrock granite. The origin of this groundwater considered meteoric (Iwatsuki et al., 2005). However, hydrochemical conditions of the groundwater have been disturbed because of the construction and operation of the underground facility, and fresh surface water has mixed with deep groundwater (Iwatsuki et al., 2015). The groundwater at the MIU site may reflect YREE fractionation during solute transport, and mixing and infiltration of fresh meteoric water with deep groundwater. The identification and study of various geochemical processes and remarkable YREE behaviors appear to indicate that the mobility of natural YREEs in a deep hydrogeochemical setting is analogous to the behavior of radioactive nuclear waste because the chemical properties of REEs are comparable with those of actinides and are thus relevant to the safe storage and disposal of nuclear waste (e.g. Wood 1993; Krauskopf 1986). In the present study, the YREE distribution between groundwater and the fracture-filling calcite system deep in a granite medium, and the fractionation of YREE in shallow to deep (− 200 to 500 m) groundwater composition were investigated at the MIU in Tono, central Japan. We assessed YREE fractionation during solute transport, mixing and infiltration of fresh meteoric water with deep groundwater.

2. Site description and hydrochemical settings MIU (Japan Atomic Energy Agency), the study area, is located in Tono, central Japan (Fig. 1). Around the MIU site, tertiary sedimentary rocks of the Seto Group (12–1.5 Ma) and the Mizunami Group (20–15 Ma) unconformably overlie the Late Cretaceous (70 Ma) Toki granite (Sasao, 2013). The Mizunami Group mainly comprises tuffaceous sandstone and mudstone with intercalation of tuff and lignite. The group is divided into four formations in ascending stratigraphic order; the Toki Lignite-bearing, the Hongo, the Akeyo and Oidawara Formations. Constituent minerals of the sedimentary rocks include quartz, plagioclase, smectite, carbonate, and sulfide. Some carbonate-rich domains have been observed in the Toki Lignite-bearing, and the Akeyo Formations (Dobashi and Shikazono 2008). The Toki granite divided into three structural domains based on the intensity of fractures and alteration: an uppermost weathering domain, upper highly fractured domain (approximately − 150 to 450 m depth) and a lower sparsely fractured domain (approximately below − 450 m depth). Various secondary minerals precipitated in the upper fractured domain, mainly calcite, chlorite, and pyrite (Iwatsuki et al. 2015). Because of the sparse fracture in the deeper domain, secondary minerals were hardly found on the fractures. Isotopic analysis of calcite precipitated on the fractures indicates that the waters that precipitated calcite originated mainly from the following three sources: (1) high temperature hydrothermal solutions; (2) seawater; and (3) fresh water (Iwatsuki et al. 2002). The groundwater is of the Na-Ca-HCO3 type in the upper sedimentary rock and of the Na-(Ca)-Cl type in the lower horizons and in the basement granite (Iwatsuki et al., 2005). The groundwater pH and HCO− 3 concentrations were controlled by equilibrium with calcite (Arthur et al. 2006).

In a previous study of YREE geochemistry at the Tono Mine two kilometers north-east of the present study area (Fig. 1), chondrite-normalized YREE patterns of groundwater in the sedimentary rock and granite, and those of bedrock granite were observed to be enriched in LREE (Takahashi et al., 2002). The Y/Ho ratio of the granite was chondritic (28), whereas that of the groundwater at the depth of − 150 m was higher than chondrite (41.8–79.7). The higher Y/Ho ratio in the groundwater than in the bedrock granite can be ascribed to the degree of rock– water interaction. Dobashi and Shikazono (2008) studied REE incorporation into carbonate minerals precipitated in the sedimentary rock, which were found to be enriched in LREE. Our previous study showed the REE patterns of the groundwater obtained from boreholes at a depth of − 300 and − 400 m depth and excavation wall in − 300 m were enriched in HREE. The groundwater contained REE-bearing colloids with N0.2 μm diameter, as determined by batch ultrafiltration. 3. Materials and methods 3.1. Groundwater sampling and analyses Groundwater sampling was carried out with hydrochemical monitoring systems installed in approximately 100 m long boreholes 07MI07, 09MI20, 10MI26, 12MI33, 13MI38, 13MI39, and 13MI40 (Fig. 1). Boreholes 07MI07, 09MI20, and 10MI26 were drilled horizontally from galleries at depths of −200 m, −300 m, and −400 m, respectively. Boreholes 12MI33, 13MI38, 13MI39, and 13MI40 were drilled from the −500 m gallery. Based on distinguishable YREE patterns, we refer to the groundwater sampled from −200 to −400 m as “shallow” groundwater and that sampled at − 500 m as “deep” groundwater (see Section 4.1). The boreholes have been divided into isolated sampling intervals using hydraulic packers, and groundwater was collected from the packer intervals at the defined depths. The detailed sampling procedure is described in Munemoto et al. (2014b). Before groundwater sampling, a sufficient volume of groundwater was purged by flushing the borehole to eliminate initial fluid in the borehole. The pH and temperature of the groundwater were measured using a portable multi-parameter meter (HORIBA, D-55), which was calibrated with pH 4.01, 6.86, and 9.18 buffers. The groundwater was filtered through a 0.22 μm membrane, and the filtrates were immediately acidified by high-purity 60 wt.% HNO3 so that they contained 1.2 wt.% HNO3 to lower the pH and preserve for YREE analysis. Chemical analyses to determine major elements were performed by ion chromatography (Dionex, ICS-1000) and inductively coupled plasma optical emission spectrometry (ICP-OES, Rigaku, CIROS-Mark II). Total organic carbon was measured using an infrared gas analyzer (Analytik Jena, multi N/C 2100S). The concentrations of REEs in groundwater are generally very low. The analytical method used in this study for determining REEs involved pre-concentration with a 3M EmporeTM Chelating Resin Disk (Ohima et al., 2000; Munemoto et al. 2014b). After the chelating disk was conditioned by sequential injection of 5 mL of 2 M HNO3, ultrapure water, and 0.1 M ammonium acetate solution, 40 mL of the groundwater sample was injected onto the chelating disk. The REEs were eluted by 2 mL of 2 M HNO3 solution. The concentrated REE were measured by inductively coupled plasma mass spectrometry (ICP-MS, Perkin Elmer, ELAN DRCII). The recovery of REEs with this method was 102 ± 4% for all REEs in preliminary analysis tests, which were within analytical error of ICP-MS (±10%), and preferential sorption and fractionation of YREE was not observed in the procedure. In order to estimate the origin of groundwater and its chemical evolution, the isotopic compositions (δD and δ18O) of the groundwater samples were analyzed by the CO2 equilibration method (Isoprime, MultiFlow Bio) for δ18O and the Zn reduction method (Elementar, Vario EL cube) for δD, respectively, coupled with isotope ratio mass spectrometer (IR-MS, Isoprime). The isotopic ratios are expressed in the international δ-notation as the per mil (‰) deviation from Standard Mean Ocean Water (SMOW).


3.2. Fracture-filling calcite Six secondary mineral samples from granite fractures were obtained from the drill cores from the 07MI07, 09MI20 and 10MI26 boreholes that they were drilled into the upper highly fractured domain. Secondary minerals in the fractures were hardly found from the drill cores at the depth of − 500 m (12MI33, 13MI38, 13MI39, and 13MI40 boreholes), because that depth corresponds to the sparsely fractured domain. The secondary minerals on − 500 m drill cores cannot be subject to the present study. X-ray diffraction analysis (XRD, Rigaku, RINT-2100, CuKα, 40 mA, 30 kV) showed that the secondary minerals comprise mainly calcite. Powdered 100 or 50 mg samples were dissolved for 6 h in 50 mL of 1 M NH4-acetate buffer solution (pH = 4.8) (Tessier et al., 1979). The extracted amounts of Ca ranged from 0.19 to 0.33 g/L, indicating 0.19–0.83 g of CaCO3 was extracted from 1 g of powdered sample (Table S3). After the reactions, the aged suspensions were filtered through a 0.22 μm membrane. The filtrates were immediately acidified by high-purity 60 wt.% HNO3. The concentrations of REEs in the leached solutions were measured by ICP-MS without pre-concentration and Ca was measured by ICP-OES. The quantity of YREEs in unit mass of the fracture-filling calcite was calculated from YREE concentrations and Ca concentration in the leachate solutions assuming Ca was eluted from calcite. In order to estimate the origin of the fracture-filling calcite, the isotopic composition (δ13C and δ18O) of the calcite was measured using mass spectrometer (Isoprime, IsoPrime100) with phosphate acid treatment. The observed δ13C and δ18O were expressed relative to PeeDee Belenite (PDB) and SMOW, respectively. 3.3. Speciation calculation Thermodynamic calculations were performed using the Geochemist's Workbench (GWB) and the database “thermos. com.v8.r6+.dat” (Bethke, 1992). Speciation and saturation calculations were conducted using REACT in GWB. The equilibrium constants for aqueous YREE species in the groundwater system used for the speciation and saturation calculations are listed in Table 1. The measured concentrations of major elements and YREEs were used as input parameters for the calculations. 4. Results 4.1. Groundwater Major chemical composition of groundwater is listed in Table S1. The groundwater had a slightly alkaline pH (7.4–8.6). The concentrations of

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cations (Na+, K+, Ca2 +, and Mg2 +) and anions (Cl−, Br−, and B) in− creased with depth, whereas alkalinity, SO2− 4 , F decreased. Using the concentrations of major cations and anions, the groundwater are grouped into Na-Cl type. The δ18O and δD values of the groundwater ranged from − 8.3 to − 9.0‰ and from − 56 to − 60‰, respectively, and they were almost constant with depth (Fig. 2a). A plot of these data shows trends close to that of the global meteoric water line, indicating that the tested groundwater samples, irrespective of sampling depth, have meteoric origin (Fig. 2b). The concentrations of YREEs in groundwater are presented in Table S2. Total YREEs (ΣYREEs) ranged widely from 23 to 533 ng/L. Fig. 3 shows chondrite-normalized YREE patterns of the groundwater samples. According to geochemical similarities including ionic radii, chondrite-normalized abundance of Y was plotted between Dy and Ho. With the exception of one sample (1.0), the groundwater samples from depths of −200 to − 400 m exhibited LaCN/YbCN ratios N 1 (0.14 to 0.86), where subscript CN denotes chondrite normalization, indicating that the YREE patterns of the groundwater were enriched in HREE relative to LREE (Fig. 3a). In contrast, the LaCN/YbCN ratios of the groundwater sampled at −500 m were higher than unity (1.2 to 6.0), which indicated LREE enrichment in the groundwater (Fig. 3b). The YREE patterns of the groundwater from the sedimentary rock overlying granite were reported in Takahashi et al. (2002). These patterns show LREE enrichment relative to HREE (LaCN/YbCN ≈ 2.0) (Fig. 3c). Therefore, YREE fractionation varied with depth, and we classified the groundwater from − 200 to − 400 m as “shallow” groundwater and that from −500 m as “deep” groundwater. The results of speciation calculations are shown in Fig. 4. The predominant YREE aqueous species in the groundwater samples were − REECO+ 3 and REE(CO3)2 . The percentages of the lightest REE, in partic+ ular, LaCO3 and La(CO3)− 2 were 66–70% and 8–27%, respectively, and − those of the heaviest REE, in particular, LuCO+ 3 and Lu(CO3)2 were 20–38% and 53–77%, respectively. The other minor REE species of La were La3 + (3–9%), LaF2 + (3–8%) and LaF+ 2 (2–3%), and those of Lu were Lu3+ (0–2%), LuF2+ (1–2%) and LuF+ 2 (2–8%). 4.2. Fracture-filling calcite The isotopic variations in the fracture-filling calcite are shown in Fig. 5. The δ13C ranged from − 14.4 to − 6.2‰ and δ18O from 8.9 to 17.4‰. A systematic depth trend of δ18O was not observed. In contrast, δ13C values were lower (−12.5 and −14.4‰) in the fracture-filling calcite from −200 m, while higher (− 6.2 to −7.9‰) in the calcite from −300 m and −400 m. YREE contents in the fracture-filling calcite are listed in Table S3. ΣYREE varied from 28.3 to 1175 ppm, irrespective of depth.

Table 1 Formation constants (log unit) for aqueous REE species.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ref.

Ln(CO3)2 −

LnCO+ 3

LnCl2+

LnCl+ 2

LnCl03

LnCl− 4

LuF2+

LnF+ 2

LnF03

LnF− 4

LnHCO2+ 3

LnOH2+

LnSO+ 4

9.36 8.9 8.58 8.49 8.13 8.03 8.18 7.88 7.75 7.66 7.54 7.39 7.36 7.29 [a]

3.6 3.27 3.1 3.05 2.87 2.85 2.94 2.87 2.77 2.78 2.72 2.65 2.53 2.58 [a]

−0.74 −0.68 −0.71 −0.71 −0.67 −0.69 −0.71 −0.67 −0.7 −0.71 −0.7 −0.71 −0.69 −0.67 [b]

−0.51 −0.46 −0.45 −0.47 −0.42 −0.44 −0.46 −0.43 −0.43 −0.45 −0.45 −0.46 −0.44 −0.43 [b]

0.39 0.39 0.32 0.32 0.39 0.47 0.47 0.467 0.47 0.47 0.47 0.467 0.76 1.2 [c]

0.82 0.74 0.74 0.74 0.82 0.89 0.89 0.89 0.89 0.89 0.89 0.89 1.18 1.77 [c]

−3.63 −3.85 −3.86 −3.82 −4.14 −4.26 −4.23 −4.36 −4.39 −4.28 −4.27 −4.28 −4.37 −4.24 [b]

−6.69 −7.27 −7.34 −7.56 −7.64 −7.71 −7.93 −8.15 −8.15 −8.3 −8.3 −8.37 −8.37 −8.44 [c]

−8.71 −9.51 −9.66 −9.88 −10.03 −10.17 −10.47 −10.69 −10.76 −10.91 −10.9 −10.98 −11.05 −11.1 [c]

−10.36 −11.39 −11.53 −11.83 −11.98 −12.12 −12.49 −12.8 −12.86 −13 −13.1 −13.2 −13.22 −13.3 [c]

−2.34 −2.31 −2.25 −2.28 −2.34 −2.47 −2.36 −2.46 −2.5 −2.46 −2.49 −2.52 −2.53 −2.49 [a]

8.64 8.42 8.27 8.13 7.98 7.91 7.91 7.83 7.83 7.76 7.76 7.69 7.61 7.61 [c]

−3.64 − 3.69 − 3.69 −3.64 −3.64 −3.64 −3.4 −3.64 −3.64 −3.57 −3.57 −3.57 −3.57 −3.57 [c]

a Luo and Byrne (2004). b Migdisov et al. (2009). c Haas et al. (1995).

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Fig. 2. (a) Depth profile of δ18O and δD in the groundwater samples and (b) relationship between δ18O and δD in the groundwaters. Solid line in (b) represents global meteoric water line.

Chondrite-normalized YREE patterns of the fracture-filling calcite are shown in Fig. 6. Most of the fracture-filling calcite exhibited LaCN/YbCN ratios N1 (1.5 to 4.4). Although only 1 sample showed a HREE enriched shape (0.95), most of the fracture-filling calcites were enriched in LREE relative to HREE. The LREE enrichment was similar to bedrock granite (Takahashi et al., 2002). The Y/Ho ratios in the fracture calcite ranged from 34.5 to 52.2, which is higher than that in chondrite and bedrock granite (28). 5. Discussion 5.1. YREE in groundwater Chondrite-normalized REE patterns of shallow groundwater from − 200 to − 400 m were more enriched in HREE than those distinguished from LREE-enriched bedrock granite, the −150 m groundwater in Tono Mine and the present-day −500 m deep groundwater (Fig. 3). The speciation calculation showed that REECO+ 3 is predominant in LREE, while REE(CO3)− 2 is predominant in HREE (Fig. 4). The REE pattern of the groundwater is affected by complexation of dissolved aqueous components (Willis and Johannesson, 2011). In neutral to slightly alkaline pH groundwater, the positively charged REECO+ 3 sorbs preferentially

onto the negatively charged aquifer mineral surfaces compared with the negatively charged REE(CO3)− 2 complex (Johannesson and Zhou, 1999; Négrel et al., 2000). Laboratory experiments of REE adsorption onto the Carrizo Sand aquifer in Texas showed that adsorption decreased from LREE to HREE across the REE series in the presence of high carbonate ions (Tang and Johannesson, 2010). Compared with the effects of carbonate ions, ionic strength has less of an effect on the adsorption of REE onto the sand; however, REE adsorption is inhibited at high ionic strength. Moreover, LREE adsorption is inhibited effectively in high ionic strength groundwater. Fukushi et al. (2013) demonstrated Eu(III) sorption onto granite from the 10MI26 borehole (− 400 m) in the laboratory; the sorption capacity significantly decreased with increasing ionic strength. Because there are fewer ion exchange sites of mineral(s) at high ionic strength; indeed, YREE abundance in groundwater increased at high ionic strength. The pH of the −200 to −500 m groundwater was slightly alkaline (pH = 7.4–8.6), suggesting positively charged aqueous species were potentially scavenged by negatively charged bedrock mineral assemblages at all depths. The pH of the groundwater from the Toki granite by KNA-6 borehole in the Tono Mine was also slightly alkaline (8.2 and 8.5). The concentration of carbonate ions was 85 mg/L (as CaCO3) in the groundwater from granite (KNA-6 borehole) in the Tono Mine (Arthur et al., 2006), higher than that in the present-day shallow groundwater, which is approximately 50 mg/L, and deep groundwater, which is approximately 15 mg/L (Table S1). The ionic strength of the groundwater in the Tono uranium mine was calculated to be 0.002, which is lower than that of the present-day groundwater samples, 0.005 for − 200 and − 300 m, 0.007 for − 400 m and 0.016 for − 500 m groundwater samples. Preferential sorption of the REECO+ 3 complex in slightly alkaline pH groundwater in the shallow groundwater induced HREE enrichment of the groundwater. An increase in the ionic strength led to competition for sorption sites and depletion of LREE because of competition for negatively charged sorption sites. Consequently, the HREE enrichment in the shallow groundwater was largely affected by the REECO+ 3 complex and to a lesser extent by the increase in ionic strength. The YREE patterns of the deep groundwater showed LREE-enrichment similar to that of the bedrock granite (Figs. 3 and 6). In contrast to the shallow groundwater, the depletion of carbonate ions supported LREE enrichment in the deep groundwater. Sorption of REECO+ 3 onto aquifer minerals was probably inhibited by an increase in ionic strength, which induced LREE enrichment in the deep groundwater. Another important transport process is the binding of YREE to organic materials. For the YREE in the groundwater of meteoric origin in Laxemar and Forsmark, Sweden, speciation calculations showed that aqueous YREE was bound to humic substances, and the YREE patterns of the meteoric groundwater were dominantly controlled by the sorption of organic YREE complexes onto the fracture minerals (Mathurin

Fig. 3. Chondrite-normalized YREE patterns of groundwater samples from (a) shallow groundwater (−200 m to −400 m), (b) deep groundwater (−500 m), and (c) sedimentary rock overlying granite (Takahashi et al. 2002).


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Fig. 4. Percentages of calculated REE speciation in groundwater; (a) −200 m, (b) −300 m, (c) −400 m and (d) −500 m.

et al. 2014). In the present study area, our previous study showed changes in TOC concentration correlated with U during groundwater flow-through in the borehole, while no correlation was observed for REE (Munemoto et al. 2014b). Aosai et al. (2014) investigated organic matter in the −300 m groundwater with the infra-red (IR) spectroscopy. They showed that the IR spectra of the organic matter were different from those of the humic acid standard. Furthermore, TOC concentrations were mostly below the detection limit (Table S1). Although the contribution of organic matter to YREE fractionation and transport is inconclusive in the present study, our results suggest that the changes in

Fig. 5. (a) Depth profile of δ13C and δ18O in fracture-filling calcite and (b) relationship between δ13C and δ18O.

the dominant REE carbonate complex affect YREE fractionation in the groundwater in granite. To assess the interaction between groundwater and the secondary fracture-filling calcite, we calculated saturation indexes for calcite as follows: . SI ¼ log IAP

K sp

ð1Þ

where IAP is the ion activity product and Ksp is the solubility product of the corresponding solid phase. Saturation indices for calcite (Ksp = 10–8.48 at 25°C (Plummer and Busenberg, 1982)) were calculated using GWB. Fig. 7 shows the depth profile of the saturation indexes for calcite in groundwater. The groundwater from − 200 and − 300 m were undersaturated or in equilibrium with respect to calcite. The SI approached equilibrium in the −400 and −500 m groundwater. The deep groundwater (− 500 m) was mostly at equilibrium with calcite except for one sample. The major element chemistry of the groundwater at various depths has been monitored since drilling of the boreholes, as reported in Iwatsuki et al. (2015). The concentrations of Ca and alkalinity dramatically changed for a few years. We calculated changes in the saturation for calcite after drilling (Fig. 8). At −200 m, the groundwater was undersaturated with respect to calcite for approximately three years after drilling (2008–2010) and approached equilibrium in the period 2010–2015. Similarly, the groundwater at a depth of − 300 m depth was undersaturated for one year after drilling (2010–2011), and then approached equilibrium with respect to calcite. The −400 m groundwater was undersaturated with respect to calcite, similar to groundwater from −200 and −300 m, but rapid changes in the saturation were not observed. In the Tono region, saturation of calcite had been investigated from surface boreholes before construction of the underground facility (Arthur et al., 2006). The regional groundwater was undersaturated with respect to calcite in the sedimentary rock overlying the Toki granite, but it was in equilibrium or supersaturated in the granite. The δ18O and δD values indicate that the groundwater originated from meteoric

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Fig. 6. Chondrite-normalized YREE patterns of fracture-filling calcite collected at −200 to −400 m (a) and that of bedrock granite (Takahashi et al. 2002) and calcite in sedimentary rock overlying granite (Dobashi and Shikazono 2008). YREE concentrations in the calcite are for normalized CaCO3 using Ca concentrations in the leachate 1 M NH4 acetate pH 4.8 buffer solution (Table S3).

water. Tritium and chlorofluorocarbons, an index of surface water infiltration, were observed in the shallow groundwater (−200 to −400 m), but not in the deep groundwater (−500 m) (Iwatsuki et al., 2015). Consequently, the deep groundwater (−500 m) YREE that originated from bedrock granite has been scarcely (re)fractionated by the fracture-filling calcite. Changes in the undersaturation to equilibrium possibly occurred because of the infiltration of surface water into the shallow

groundwater (−200–400 m), which induces the dissolution of the fracture-filling calcite. 5.2. Ce anomaly Cerium anomalies have been widely used as redox indicators because insoluble Ce4+ precipitates easily in oxic water, and it is preferentially incorporated into redox sensitive hydrous precipitates such as Mn-oxides and Fe-(oxy)hydroxide (Bolhar et al., 2004; Takahashi et al., 2007). The Ce anomaly (Ce/Ce*) has been defined as follows: Ce=Ce ¼ ð2CeCN =ðLaCN þ PrCN ÞÞ;

ð2Þ

where CeCN, LaCN, and PrCN denote the chondrite normalized concentration. Negative Ce anomalies (b1) occur in oxic water while positive Ce anomalies (N1) occur in counterpart Ce scavengers. The Ce anomalies are hindered by anomalously high abundances of La relative other LREE in water. According to Bau and Dulski (1996), “true” Ce anomalies are defined as a combination of Ce and Pr anomalies, where Pr/Pr* is given as follows: Pr= Pr ¼ ð2 PrCN =ðCeCN þ NdCN ÞÞ:

ð3Þ

The calculated Ce/Ce* and Pr/Pr* are shown in Fig. 9. Apparently, almost all the groundwater samples showed a negative Ce anomaly (Ce/Ce* b 1), and the magnitudes of the negative Ce anomalies increased

Fig. 7. Depth profiles of saturation indices for calcite in groundwater.

Fig. 8. Changes in saturation indices for calcite in groundwater (−200–400 m) after drilling of boreholes.


with depth. The Pr/Pr* values were scattered from 0.74 to 1.27, suggesting that the groundwater samples have a positive La anomaly and a negative Ce anomaly. Although the groundwater is highly reducing (Munemoto et al., 2014b; Suzuki et al., 2014), negative Ce anomalies appeared in the groundwater. Negative Ce anomalies have been reported in reducing conditions and ascribed to episodic oxic conditions (Feng et al., 2010; Feng et al., 2013). Kim et al. (2012) suggested that the observed negative Ce anomalies might not always be an indicator of paleo-oxic conditions. For REE fractionation in marine pore water, the positive Ce anomaly appears in particulate organic material and counterpart deep marine water shows the negative Ce anomaly, resulting in Ce scavenging (Haley et al. 2004). In the present study, the isotopic compositions of δ18O and δD indicate that the present-day groundwater has meteoric origin. The origin of the sedimentary rock overlying granite has been considered to be marine and lacustrine (Iwatsuki et al., 2002). The negative Ce anomalies in the present-day groundwater probably preserved the paleo-oxic conditions during infiltration of meteoric water. In contrast to the groundwater from granite, positive Ce anomalies were observed in calcite collected from sedimentary rock (Dobashi and Shikazono, 2008). See Fig. 9 Another possibility for the negative Ce anomaly in reducing groundwater may be the preferential uptake of Ce by organic compounds and Fe-Mn oxides (Pourret et al., 2008; Takahashi et al., 2007). In the study area, the concentrations of total organic carbon, total Fe, and Mn are significantly higher in the sedimentary rock than in the bedrock granite, and the redox potential is controlled by the formation of Feoxide (Iwatsuki et al., 2005). Consequently, this result suggests that Ce fractionation during paleo-infiltration was preserved in the sedimentary calcite and the groundwater from granite, and the Ce anomaly could potentially be affected during paleo-infiltration. 5.3. YREE in fracture-filling calcite and paleo-groundwater YREE patterns of the fracture-filling calcite show LREE enrichment similar to that of the bedrock granite. The LREEs are preferentially scavenged during the precipitation of calcite (Terakado and Masuda, 1988; Zhong and Mucci, 1995). YREE fractionation may be caused by ligand exchange between water and calcite (Négrel et al. 2000), and the REE

65

patterns of modern calcite preserve the patterns of original groundwater, indicating that no fractionation of REE patterns occurred between calcite and the reacting groundwater without ligand exchange (Choi et al., 2009). The LREE-enriched groundwater was probably produced by water–rock interaction with bedrock granite in the present-day deep groundwater. The YREE fractionation between groundwater and the fracture-filling calcite should be considered as fractionation between the paleo-groundwater, from which the fracture calcite precipitated. The δ13C values were lower in the fracture calcite from −200 m, but higher in the calcite from −300 m and −400 m (Fig. 5). Assuming isotopic equilibrium between the calcite and the water, the precipitation temperature of the fracture calcite was estimated to be 37 to 88°C by using following Eq. (4): 1000lnα ¼ 2:78 106 T−2 –2:89;

ð4Þ

where α is the isotope fractionation factor between calcite and water and T is the water temperature (O'Neil et al. 1969). In the Tono region, the origin of calcite in the sedimentary rock overlying granite has been classified into the marine (δ13C = −11 to 3‰, δ18O = 20 to 28‰) and lacustrine (δ13C = −19 to −6‰, δ18O = 18 to 24‰) (Iwatsuki et al. 2002). The isotopic composition of the fracture-filling calcite suggests that the original waters from which the calcite precipitated were hydrothermal solution (δ13C = −18 to −7‰, δ18O − 3 to 10‰), sea water (δ13C = approximately 0‰, δ18C N approximately 20‰), fresh water (δ13C b 0‰, δ13O N 17‰), and a mixture of the above. 14C dating of the fracture-filling calcite and dissolved inorganic carbon in the groundwater indicated that most of the calcite precipitated before 50 ka and groundwater infiltration occurred within 14 ka (Iwatsuki et al., 2002). Furthermore, present-day groundwater has been altered by the mixing of deep and shallow groundwater due to the construction and operation of the underground facility (Iwatsuki et al. 2015). Accordingly, in the present study, the isotopic composition indicated that the fracture calcite originated from a mixture of sea water and fresh water at temperatures b 100°C. Maskenskaya et al. (2015) studied the REE geochemistry of paleo-groundwater in granitoids from which calcite was precipitated in Laxemar, Sweden, by back-calculating from the reported distribution coefficient considering preferential scavenging of LREE (Zhong and Mucci 1995). For the Sweden site, the back-calculated REE patterns indicate that the patterns of the paleo-groundwater are broadly similar to those of the present groundwater and that carbonate aqueous species play an important role in the fractionation of REE. Experimental distribution coefficients have been reported in REECO+ 3 dominant water (Tanaka et al. 2004) and REE(CO3)− 2 water (Zhong and Mucci 1995). In the present study, the predominant aqueous carbonate species and overall YREE patterns of the present groundwater varied with depth (Fig. 4). To estimate the YREE fractionation between the paleo-groundwater and the fracture-filling calcite, the YREE patterns of paleogroundwater were back-calculated from the distribution coefficients of REEs on calcite reported by Zhong and Mucci (1995) and Tanaka et al. (2004). The distribution coefficient, DREE, is expressed as follows: DREE ¼

X REE X Ca solid ½REE ½Ca aq

ð5Þ

where XREE and XCa are the mole fractions of REEs and Ca ions in the solid, respectively, and [REE] and [Ca] are the molar concentrations of Þ REE and Ca in the solution. The ðXXREE Ca

solid

Fig. 9. Relationship between Ce/Ce* and Pr/Pr* in groundwater (−200–500 m) and fracture-filling calcite. Sedimentary calcite in the Mizunami Group overlying bedrock granite is plotted as open circles (Dobashi and Shikazono, 2008).

values were obtained from se-

quential extraction (Table S3). The concentration of Ca in the paleogroundwater was assumed from the present-day groundwater values (Table S1). Although the Ca concentration affected the total concentration of REE in the back-calculated paleo-groundwater, the overall REE pattern was independent in Eq. (4).

66


Fig. 10. Chondrite-normalized YREE patterns of paleo-groundwater back-calculated using distribution coefficient from (a) Zhong and Mucci (1995) and (b) Tanaka et al. (2004) obtained + from REE(CO3)− 2 dominant water and REECO3 water, respectively. To compare paleo- to present-day groundwater, averaged patterns of present-day groundwater (−200 to −500 m) are also shown.

The back-calculated REE patterns of paleo-groundwater from the distribution coefficients of Zhong and Mucci (1995) are enriched in HREE relative to LREE, irrespective of depth (Fig. 10a). The REE patterns of the present-day shallow groundwater overlap with the range of backcalculated patterns. At greater depths (− 500 m), the back-calculated HREE-enriched patterns are different from the present-day LREEenriched groundwater. The back-calculated REE patterns of paleogroundwater from the distribution coefficient obtained in a partitioning experiment (1 ppb of initial REE) by Tanaka et al. (2004) were enriched in LREE (Fig. 10b). The back-calculated LREE-enriched patterns were different from those of the present-day shallow groundwater, but in agreement with the present-day deep groundwater. The predominant (Y)REE species changed in the present-day shal+ low and deep groundwater as REE(CO3)− 2 dominant and REECO3 , respectively; the shallow groundwater was enriched in LREE and the deep groundwater was enriched in HREE (Figs. 3 and 4). Although the true condition of the paleo-groundwater was unknown, the back-calculated YREE patterns indicated that the LREE-enrichment and HREE-enrichment varied with the predominant YREE species. The YREE patterns of the fracture-filling calcite were unknown because bedrock granite at − 500 m is characterized as a sparsely fractured domain. Moreover depending on the predominant aqueous REE species, changes in the YREE patterns of the paleo-groundwater possibly range roughly from that of the present-day shallow to deep groundwater.

in LREE and HREE depending on the aqueous YREE species. The YREE content in our back-calculated paleo-groundwater from fracture-filling calcite suggests that the changes in the paleo- to present-day YREE fractionation range roughly from that of the present-day shallow and deep groundwater. Even in a deep underground environment in a different location and geologic history, similar to the case of Sweden (Maskenskaya et al. 2015), the fractionation of YREE between calcite and groundwater suggests that changes in the geochemical conditions generate the changes in the distribution of YREE via water–rock interactions.

6. Conclusions

References

Our results show that the YREE behavior of groundwaters from granite and fracture-filling calcite at depths of −200 to −500 m in the MIU can be characterized by HREE enrichment in “shallow” groundwater (− 200 to − 400 m) and LREE enrichment in “deep” groundwater (− 500 m). The predominance of negatively charged YREE(CO3)− 2 in HREE in the “shallow” groundwater, less scavenged by negatively charged bedrock surfaces, provides HREE enrichment in the groundwater. The shallow groundwater was affected by the infiltration of surface water undersaturated with respect to calcite, while the deep groundwater was hardly altered and affected by surface water infiltration. Irrespective of the infiltration, the negative Ce anomalies preserved in the groundwater suggest paleo-oxic conditions and possibly fractionation from sedimentary calcite with a positive Ce anomaly during paleo-recharge. The isotopic variations of δ18O and δ13C in the fracture-filling calcite indicated that it originated from a mixture of sea water and fresh water at temperature b100°C. The YREE patterns of paleo-groundwater from which the fracture-filling calcite precipitated were enriched

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