Origins and pathways of deeply derived carbon and fluids observed in hot spring waters from non-active volcanic fields, western Kumamoto, Japan

Abstract

Natural springs containing volcanic and magmatic components occur in association with these activities. However, features of deeply originated fluids and solutes were less documented from fields, where active volcanic and magmatic activities are not distributed. To characterize the presence of deep components and identify their major pathways 28 groundwater samples (~ 1230 m deep) were collected from hot spring sites located at western coast of Kumamoto, southwestern Japan, where the typical subduction related magmatisms are absent. The samples were measured for dissolved ion concentrations and stable isotope ratios (δ²H_H2O, δ¹⁸O_H2O, δ¹³C_DIC and δ³⁴S_SO4) that were compared with data of 33 water samples from vicinity surface systems. The groundwaters were classified into three types based on major hydrochemistry: high Cl⁻ fluid, low concentration fluid, and high HCO₃⁻ fluid. Our data set suggests that the high Cl⁻ fluid was formed by saline water mixing with aquifer waters of meteoric origin and subsequently evolved by reverse cation exchange. The low concentration fluid is identical to regional aquifer water of meteoric origin that was subjected to cation exchange. The high HCO₃⁻ fluid showed the highest HCO₃⁻ concentrations (~ 3,888 mg/l) with the highest δ¹³C_DIC (up to − 1.9‰). Based on our carbon mixing model and observed δ²H_H2O and δ¹⁸O_H2O shifts, it is suggested that dissolved carbon of mantle origin and small fraction of fluids generated in deep crust were transported towards surface through structural weakness under open tectonic setting. These deeply derived components were then mixed with waters in the surface systems and diluted. Their impacts on surface hydrological systems were limited in space except few locations, where deeply connected pathways are anticipated along active structural deformations.

Introduction

Artesian springs with high temperature and/or elevated concentrations occur in major active volcanic and seismotectonic regions globally. These highly concentrated dissolved solutes are facilitated with incorporation of volcanic and magmatic gases and fluids (Aiuppa et al. 2009, 2017), and simultaneous high temperature weathering or alterations that can occur in crust deeper than active hydrological systems in surface (Hedenquist and Lowenstern 1994). Discharge of these fluids is commonly observed in vicinity of active volcano-hydrothermal fields. For example, relevant number of studies tried to depict origins and pathways of deeply sourced components to quantify the impact of volcanic and magmatic activities on lateral material transportations from land to ocean (Chiodini et al. 2000; Dessert et al. 2009; Gaillardet et al. 2011; Rive et al. 2013; Hosono et al. 2018; Perez-Fodich and Derry 2019). Similar high concentration hot spring waters are found in regions, where active volcanic and magmatic activities are not existing (Tomaru et al. 2007; Kusuda et al. 2014; Togo et al. 2014; Morikawa et al. 2016; Kusuhara et al. 2020 and references therein). However, origin, distribution, pathway, and thus their impact of hydrochemical features on surface systems are less well known for these waters.

The major sources invoked to explain for the origin of deeply originated fluids include magmatic fluids, slab fluids and deep crustal fluids (Hedenquist and Lowenstern 1994; Ague 2014; Morikawa et al. 2016). The magmatic fluids present in a crust beneath active volcanoes that are located typically along the volcanic front with some possible exceptions on back arc. Dehydration of subducting oceanic plates releases waters and fluids from slab sediments and rocks (here we call slab fluids) into mantle wedges. Some many hot springs in southwest Japan are thought to be originated directly of these fluids on fore arc system (Tomaru et al. 2007; Kusuda et al. 2014; Togo et al. 2014; Yoshida et al. 2015; Morikawa et al. 2016; Kusuhara et al. 2020). Alternatively, fluids generated in lower crust (here we call deep crustal fluids) can also contribute to surface systems along major pathways especially at regions, where substantial heat source exists beneath lower crust, such as upwelling asthenospheric mantle (e.g., Zhao et al. 2018); however, this phenomenon has been less advocated from nature of lived hot spring discharges due to rare opportunity to test this hypothesis.

Previous studies (Nakajima and Hasegawa 2007; Hirose et al. 2008; Zhao et al. 2021) showed the depth contours of subducting slabs under Japanese archipelago demonstrating that western Kyushu is located on unique setting, where possible subduction plate is in mantle deeper than 200 km and typical subduction related magmatisms are absent. Some hot springs occur along the coast of western Kumamoto in western Kyushu without linkage of active volcanisms in surface (Fig. 1a). Notably, these historically recognized hot springs and other recently developed hot spring facilities are located along and on extension of the major faults (Fig. 1) that possibly allow deeply originated gases and fluids in the crust passing toward surface. There have been no reports comprehensively investigating the origin and formation of these hot springs; however, we thought that precise geochemical characterization on these hot springs may be able to put more constraint on a presence, properties, and discharging processes of these components.

Fig. 1

Map showing the study area and its geological background. a Tectonic structure of the Kyushu Islands with location of active volcanoes (red triangle) and some important localities explained in the main text. The Beppu–Shimabara and Kagoshima grabens are after Matsumoto (1979) and Hayasaka (1987). Yellow hatch indicates an area, where subsurface high conductivity blocks distribute in and beneath lower crust which were depicted from recent electrical resistivity survey (Hata et al. 2017). b Sampling locations shown on topographical map with locality name and major fault systems. c A–A’ cross section in panel a showing the locations of low frequency micro earthquakes during 2002–2016 (purple circles) and low seismic velocity zones (red color) in the mantle wedge and crust (Zhao et al. 2018). d Simplified geological map modified after Moreno et al. (2016). Geographical distribution of three types of fluids, High-Cl⁻, Low-concentration and High-HCO₃⁻ fluids, are also shown

Stable isotopes are useful for identifying the origin of waters and dissolved ions, and for interpreting the biogeochemical processes in aquifers (Kendall 1998; Cook and Herczeg 2000). Stable isotope ratios of oxygen and hydrogen in water molecule (δ²H and δ¹⁸O) have been used to examine the contribution of fluids that had experienced high temperature water–rock interaction resulting in δ¹⁸O isotopic shift towards enriched compositions leaving δ²H unchanged (Giggenbach 1992). Stable carbon isotope ratio of dissolved inorganic carbon (δ¹³C) is a fundamental tracer for identifying a contribution of magmatic or mantle derived carbon in fluids (e.g., Chiodini et al. 2000; Yamada et al. 2011; Rive et al. 2013). Sulfur isotope ratio of dissolved sulfate (δ³⁴S) is also applicable for evaluating volcanic influence on spring waters and for interpreting redox processes in aquifer environments (e.g., Kusakabe et al. 2000; Hosono et al. 2014a, 2018). This study integrates the results from these isotopic analyses and characterized geochemical features of hot spring waters with comparison of those from other surface waters for trying to reveal the upwelling signatures of deeply originated components in crust and to assess their impact on surface hydrological systems.

Outline of the study area

The study area is located on the western coast of the Kumamoto Prefecture in Kyushu Islands and consists of four local domains, Uto, Amakusa, Yatsushiro and Ashikita districts (Fig. 1b). Uto and Amakusa are located on western peninsula and small islands, while Yatsushiro and Ashikita are on western coast of the main island Kyushu, respectively. Coastal flat plain in Yatsushiro is the reclaimed land on tidal flat. The Kyushu has humid subtropical climate and average annual temperature and precipitation of Amakusa and Yatsushiro are 16.4 and 16.8 °C and 2,076 and 1,979 mm during 1981–2010, respectively (http://www.jma.go.jp/jma/menu/report.html). Around 40% of total precipitation occurs during summer rainy season in June and July.

In the Kyushu Islands the active volcanoes are generally situated in two major graben structures, Beppu–Shimabara and Kagoshima grabens (Fig. 1a) (Nakada et al. 2016) that are facilitated by a unique open tectonic setting partly associated with wet upwelling flow in the mantle wedge (Zhao et al. 2021). The study area is located outside of these graben structures, lying between two major active fault systems, Hinagu and Futagawa strike-slip faults (Fig. 1b), for example, which activities generated the 2016 M_w 7.0 Kumamoto destructive earthquakes (e.g., Hosono et al. 2019, 2020). As stated earlier, neither active volcanic activities nor typical subduction related magmatisms are recognized within and beneath the study area. For instance, low frequency micro earthquakes generally occur beneath active volcanoes such as Aso, Kuju and Tsurumi somehow accompanied with low seismic velocity anomalies in the crust, while there is no significant evidence of major magmas or melts anticipated in the crust beneath the study area (Fig. 1c). In contrast, some recent tomography images (Zhao et al. 2018, 2021) depicted the presence of upwelling asthenospheric mantle in the corner of mantle wedge beneath crust under the study area.

Paleozoic to Mesozoic low-temperature and high-pressure metamorphic rocks and metasedimentary rocks of accretionary prism are widely distributed in the southern part of the Usuki–Yatsushiro Tectonic Line (Fig. 1d). In the northern part of this tectonic line Cretaceous Himenoura Group, alternative sedimentary layers of sandstone, mudstone, shale and conglomerate, comprises of the basement rocks of the study area. Cretaceous metamorphic rocks are also outcropped in Amakusa but their distributions are quite limited. These geological basements are overline by Paleogene sedimentary formations which are further intruded by Miocene dikes and overline by Miocene to Pleistocene volcanic rocks mainly of andesitic to dacitic compositions. Most of these young volcanic rocks are found at Uto peninsula (1.7 to 4.2 Ma; Yokose et al. 2009) and Ashikita district (Fig. 1d). Some limestone formations are interlayered but their distribution is quite limited that are not seen near majority hot spring sites.

Samples and methods

In total 28 groundwater samples were collected from borehole of the hot spring facilities except one sample that was collected from artesian spring during June 2019 and May 2020 (Fig. 1b, Additional file 1: Table S1). The depth of borehole is ranging from 70 to 1,230 m and are generally deeper than 800 m and temperature of the waters were ranging from 19.8 to 45.2 °C (Additional file 1: Table S1). Shallow boreholes (70–100 m) are of Yunoko and Shimoda hot springs, which hot springs with Oyano hot spring were first discovered as artesian flowing from the ground (Fig. 1a) and then later explored subsurface by drilling. Other facilities explored groundwater with elevated temperature though originally there were no hot spring discharge symptoms on a surface land. In addition, 19 stream waters, 10 mountain spring waters, and 4 shallow well (~ 45 m in depth) waters were collected within the same area during July 2019 and April 2020 for isotopic and geochemical comparisons (Fig. 1b), which samples are representative for waters in meteologically driven surface hydrological systems (we termed ‘surface water’ for all these waters). No precipitation occurred during 3 days before the sampling.

Groundwaters from hot spring sites were collected directly from the pipes of the pumping or artesian flows. Stream and artesian mountain spring waters were collected on sites, while shallow well waters were obtained using electric pumping or Baylor tube sampler. Field parameters such as temperature, electrical conductivity, pH, oxidation reduction potential, and dissolved oxygen were monitored using field meters (HORIBA D-54) and water samples were collected after these values became constant. The HCO₃⁻ concentration was determined by in situ alkalinity measurement following titrating with sulfuric acid. Water samples were stored in 100, 500 and 2,000 ml polyethylene bottles for major ion concentrations (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, and NO₃⁻), δ¹³C and δ³⁴S analyses, while 20 ml glass vials were used for collecting water samples for δ²H and δ¹⁸O analyses, respectively. The samples were filtered through ADVANTEC 0.2 µm cellulose–acetate filters onsite before storing for 100 ml bottles and immediately after taking back to laboratory for 500 and 2,000 ml bottles, respectively. The filtered water samples in latter two bottles were adjusted to pH 12 to 12.5 with NaOH and to pH 2.5 to 3 with 6 M HCl, respectively, then dissolved inorganic carbon and SO₄²⁻ were collected as BaCO₃ and BaSO₄ by adding 10% BaCl₂ for δ³⁴S and δ¹³C analyses.

Major ion concentrations were measured by ion chromatography (Compact IC 761, Metrohm, Switzerland). Both δ¹³C of BaCO₃ and δ³⁴S of BaSO₄ were determined using a continuous-flow gas ratio mass spectrometer (Delta V Advantage, Thermo Fisher Scientific, USA) interfaced with an elemental analyzer (Thermo Fisher Flash 2000, USA) (Hosono et al. 2014a, 2015). International standards (NBS123 and OGS-1) and several carbonate and sulfate materials were used for quality check of the measurement data. The precision of both δ¹³C and δ³⁴S analyses were generally better than ± 0.2‰. The δ²H and δ¹⁸O of water molecular were determined by a gas-ratio mass spectrometer (Delta V Advantage, Thermo Fisher Scientific, USA) coupled with an automatic water–gas equilibrization devise (Nakano Denshi Co., Ltd., Japan). Based on replicate measurements of standards and samples, the analytical precisions for δ²H and δ¹⁸O were better than ± 0.5‰ and ± 0.05‰, respectively.

Results and discussion

Hydrochemical classification

According to trilinear diagram hydrochemical features of groundwater samples from hot spring sites clearly differ from those for surface waters (Fig. 2a). The surface water samples are broadly plotted on compositional field between Ca–HCO₃⁻ type with pristine signature and modern sea water, suggesting presence of sea water mixing into fresh surface waters with lower dissolved solutes (electric conductivity of around 50 μS/cm increased near 1,000 μS/cm, Additional file 1: Table S1). On the other hand, the groundwater samples from hot spring sites are exclusively plotted along two different trends with increased concentrations (electric conductivity of ~ 42,500 μS/cm), high Cl⁻ with variable Ca²⁺ ratio and high Na⁺ with variable HCO₃⁻ ratio (Fig. 2b). Thus, these groundwaters changed their hydrochemical features owing to several factors including mixing of waters with other origins and geochemical evolutions, such as cation exchange and redox processes.

Fig. 2

Hydrochemical classification based on trilinear diagram. a Comparison between surface waters and groundwaters from hot spring sites. b Classification of three different types of groundwaters from hot spring sites. c Difference in Cl⁻ concentrations among three types of fluids. d Diversity in HCO₃⁻ concentrations among three types of fluids

The groundwater samples categorized by the former trend are relatively more enriched in Cl⁻ and Na⁺ and thus we classified this type of waters as High-Cl⁻ fluid (Fig. 2b, c, Additional file 1: Table S1). The groundwater samples categorized by the latter trend are subdivided into two groups, one characterized with greater HCO₃⁻ concentrations (~ 3,888 mg/l) and the other with least dissolved concentrations but with relatively higher SO₄²⁻ ratio (Fig. 2d). We classified these two types of groundwaters and named High-HCO₃⁻ fluid and Low-concentration fluid, respectively (Fig. 2b, d, Additional file 1: Table S1). Geographical distribution of three types of fluids, High-Cl⁻, Low-concentration and High-HCO₃⁻ fluids, are shown in Fig. 1d. This hydrochemical classification and terminology are used in the following discussion.

Origin of water

Figure 3 shows δ²H versus δ¹⁸O diagrams plotting all collected samples and data of some hypothesized source waters for comparison. This comparison allows us to identify the possible origins of water samples. All surface water samples were closely plotted on local meteoric water line except one stream water sample that is slightly deviated towards composition of modern sea water (Fig. 3a). This suggests that all these waters are mainly of meteoric origin except one sample that was clearly affected by a mixing of sea water. We admit that some of these waters are affected by sea water component in terms of major ion chemistry. However, this effect was small and not obviously reflected on δ²H and δ¹⁸O shifts. Thus their compositions were regard as representative feature of waters in surface hydrological systems over the study area.

Fig. 3

δ²H versus δ¹⁸O diagrams. a–d Isotopic signatures for the surface waters, High-Cl⁻, Low-concentration, and High-HCO₃⁻ fluids, respectively. Isotopic compositional field for hypothesized magmatic water is after Giggenbach (1992). Local meteoric water line in Kumamoto (ca. 20 km northeast from Uto district) is after Okumura et al. (2018)

In Fig. 3b, samples of High-Cl⁻ fluid are broadly plotted along an array between compositional field of surface waters and modern sea water. In combination with Cl⁻ concentration (Additional file 2: Fig. S1), our simple mixing model suggests that this fluid was originated of the surface fresh water admixed with modern sea water with variable mixing proportions ranging from few to nearly 100% due to sea water intrusion and/or contribution of saline pore waters in sediments. This interpretation agrees well with the fact that all High-Cl⁻ fluids are located just on the coasts or on the reclaimed land in Yatsushiro (Fig. 1d). In turn, for Low-concentration fluid all sample plots were fallen on the compositional field of the surface water (Fig. 3c), indicating their intrinsic meteoric origin analogous to surface circulated waters.

In contrast to High-Cl⁻ and Low-concentration fluids, some samples of High-HCO₃⁻ fluid are plotted on right to the local meteoric water line towards hypothesized magmatic water proposed by Giggenbach (1992) (Fig. 3d), although this isotopic shift seems not to be much drastic compared to ones reported from some volcanic fields and other hot spring waters of subduction slab origin (e.g., Kita et al. 2009; Kusuda et al. 2014; Morikawa et al. 2016). Nevertheless, observed isotopic shift possibly indicates incorporation of deep fluids that have experienced high temperature water–rock interaction in deep crust as explained more later.

Geochemical evolution

Cation exchange

High-Cl⁻ fluid showing variable Ca²⁺ ratio (Fig. 2b) possibly indicates the occurrence of other processes than source inputs, i.e., geochemical evolution. The chloro-alkaline indices (CAI) (Schoeller 1977) expressed by the following equations in milliequivalent concentrations can be used to test the occurrence of cation exchange reactions that control cation concentrations in aquifers.

$${\text{CAI}} - 1 = \frac{{{\text{Cl}}^{ - } - \left( {{\text{Na}}^{ + } + {\text{K}}^{ + } } \right)}}{{{\text{Cl}}^{ - } }}$$

(1)

$${\text{CAI}} - 2 = \frac{{{\text{Cl}}^{ - } - \left( {{\text{Na}}^{ + } + {\text{K}}^{ + } } \right)}}{{{\text{SO}}_{4}^{2 - } + {\text{HCO}}_{3}^{ - } + {\text{CO}}_{3}^{2 - } + {\text{NO}}_{3}^{ - } }}$$

(2)

Here, the cation exchange and reverse cation exchange are expected to occur when both CAI-1 and CAI-2 show negative and positive values, respectively. The parameter CO₃²⁻ can be neglected when pH of solutions is lower than 10.3. The pH measured for our samples were all below 10.3 (Additional file 1: Table S1). Figure 4 displays both CAI-1 and CAI-2 yielded positive values and the values are tended to increase with increase in Ca²⁺ ratio for the High-Cl⁻ fluid. We can thus conclude that hydrochemical features of High-Cl⁻ fluid was formed by the sea component mixing and subsequent progressive reverse cation exchange reaction in aquifer. Exchange of calcium ion in exchangeable site in aquifer’s solid phase was released by Na⁺ in water, assuming to easily be enhanced by additives of saline water with high Na⁺ concentration. Moderately, but similar variation in Ca²⁺ ratio was observed for Low-concentration and High-HCO₃⁻ fluids (Fig. 2). However, both fluids had negative CAI-1 and CAI-2 values which tended to decrease with decrease in Ca²⁺ ratios (Fig. 4). Thus, these fluids had evolved along normal cation exchange reaction in contrast to High-Cl⁻ fluid.

Fig. 4

Biplot diagram between Ca²⁺ ratio and chloro-alkaline indices. a, b Plot diagrams compare for CAI-1 and CAI-2, respectively, three types of groundwaters from hot spring sites, High-Cl⁻, Low-concentration, and High-HCO₃⁻ fluids. Ca²⁺ ratio denotes proportion of calcium ion among four major cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺) in milliequivalent concentrations

Redox process (sulfur cycle)

The redox process is another important geochemical evolutional process that must be considered. This is true, because some groundwaters from hot spring sites showed low dissolved oxygen (< 2 mg/l) and negative oxidation reduction potential (mV) although surface waters generally showed aerobic regime (Additional file 1: Table S1), implying the occurrence of some anaerobic microbial reductions facilitated in some aquifers. In general, Cl⁻ concentrations are not affected by redox changes and NO₃⁻ was not highly concentrated (mostly < 2.0 mg/l except three samples with > 10 mg/l due to local contamination) in the water samples investigated (Additional file 1: Table S1). Thus Cl⁻ and NO₃⁻ are not of major interests for the subject to discuss about the issue. Here, additional source characteristics and redox processes will be discussed for SO₄²⁻ with conjunction of δ³⁴S (Fig. 5).

Fig. 5

SO₄²⁻ concentration versus δ³⁴S diagrams. a, b Sample plots for three types of groundwaters from hot spring sites, High-Cl⁻, Low-concentration, and High-HCO₃⁻ fluids, with comparison of those for surface waters. Compositions of modern sea water are after Rees et al. (1978)

The δ³⁴S and SO₄²⁻ variations for surface waters were considered as juvenile indices that are defined as − 7‰ < δ³⁴S < 15‰ and 2 mg/l < SO₄²⁻ < 30 mg/l, respectively (Fig. 5a), taking their overall aerobic (Additional file 1: Table S1) and the least contaminated hydrochemical features. In Fig. 5b, some High-Cl⁻ fluids showed elevated SO₄²⁻ (~ 1,700 mg/l) towards compositions of modern sea water (δ³⁴S: ca. 20.3‰, SO₄²⁻: ca. 2649 mg/l; Rees et al. 1978), suggesting significant saline water contribution as explained previously. However, other samples showed lower concentrations with elevated δ³⁴S up to 28‰, indicating an occurrence of microbial reduction of concentrated sulfate of marine origin (Strebel et al. 1990; Tuttle et al. 2009). Some samples were not measured for δ³⁴S because of their low SO₄²⁻ concentrations probably due to progressive sulfate reduction reaction. Such phenomenon is commonly seen for coastal aquifers either at near study fields in Kumamoto (Hosono et al. 2014a) or some other countries (Hosono et al. 2010, 2011, 2014b).

The Low-concentration fluid displayed compositions almost identical to those for surface water except two samples (HS19007 and HS20004) with moderately elevated SO₄²⁻ (~ 280 mg/l) and δ³⁴S (~ 17.4‰) and one sample (HS19002) with slightly elevated SO₄²⁻ (57 mg/l) and quite depleted δ³⁴S (− 13.7‰) (Fig. 5b). The observed signature for former two samples can be explained by mixing of sulfate of marine origin similar to High-Cl⁻ fluid but that intensity was too small to clearly be detected on stable isotope ratios of water molecular (Fig. 3c). The signature of later sample can be explained by the oxidation of sulfide with lower δ³⁴S (Taylor et al. 1984; van Everdingen and Krouse 1985). In summary, overall hydrochemical formation process of Low-concentration fluid resembles to that for surface waters except cation exchange reaction.

The High-HCO₃⁻ fluid has the least SO₄²⁻ concentrations, which are lower than 16 mg/l, and for majority samples, ranging between detection limit and 5 mg/l (Additional file 1: Table S1). Two samples had similar δ³⁴S compositions to those for surface waters but other four samples showed most enriched δ³⁴S signature (~ 45‰) among all samples with depleted SO₄²⁻ concentrations (Fig. 5b). No striking signatures of contribution from marine sulfate are observed, corresponding to results of δ²H and δ¹⁸O analyses. Overall, hydrochemistry of the High-HCO₃⁻ fluid was changed their features with similar manner as found for Low-concentration fluid, except least contribution from marine sulfate and tremendous HCO₃⁻ enrichment (Fig. 2d).

Upwelling signatures of deeply derived carbon and fluids

Significant contribution of sulfur from some heat sources was not detected from any water samples collected from our study area (Fig. 5), which δ³⁴S values resemble a feature of arc volcanic magmas typically in a range between 0 and 10‰ (Ueda and Sakai 1984). On the other hand, elevated HCO₃⁻ in High-HCO₃⁻ fluid may be originated of CO₂ gas released from some heat sources that was dissolved in waters and preferentially transported towards surface leaving SO₂ and other gasses (e.g., Ohsawa et al. 2002; Gultekin et al. 2011). The use of δ¹³C tracer in dissolved inorganic carbon can provide clue to test this hypothesis (Chiodini et al. 2000; Yamada et al. 2011; Rive et al. 2013).

On the δ¹³C versus HCO₃⁻ concentration diagram (Fig. 6a), all surface water samples fall on δ¹³C compositions between − 31.3 and − 11.8‰ with the lowest HCO₃⁻ concentrations generally less than ca. 100 mg/l (Additional file 1: Table S1), except one sample (GW20002) with relatively elevated δ¹³C (− 8.9‰) and HCO₃⁻ concentration (295 mg/l). These geochemical features are of the typical ecosystem-mediated carbon globally (Clark and Fritz 1997) and aquifer systems near the study areas (Ohsawa et al. 2002; Hosono et al. 2014a) and thus are reasonably assigned as a representative composition of surface water systems before alteration by additional source inputs. On the same diagram, almost all groundwater samples from hot spring sites are plotted along the exponential curved trends based on two end-component mixing models between hypothesized soil CO₂ and deep originated CO₂, suggesting progressive contribution of CO₂ from deep heat source into shallower hydrological systems.

Fig. 6

Identification for deeply originated components. a HCO₃⁻ concentration versus δ¹³C diagram for all collected water samples. Simple two components mixing curved lines with relative contribution rates from hypothesized mantle sourced carbon in 10% interval are shown in the graph. Mixing of two sets of end-components, soil carbon (HCO₃⁻ and δ¹³C: 20 mg/l and − 30‰, 160 mg/l and − 30‰, respectively) and mantle sourced carbon (HCO₃⁻ and δ¹³C: 4,000 mg/l and − 4.5‰, 4,000 mg/l and − 1.5‰, respectively) were hypothesized taking compositional variations observed from this study, with a same method applied elsewhere (e.g., Chiodini et al. 2000; Yamada et al. 2011; Rive et al. 2013). b Geographical distribution of relative contribution from mantle sourced carbon in groundwater samples deduced from mixing model shown in panel a. c δ²H versus δ¹⁸O plot diagram for High-HCO₃⁻ fluid shown in different colors with variable δ¹⁸O. d Geographical distribution of δ¹⁸O values. The symbol color is same as in panel c

A contribution of deep originated CO₂ dominates in High-HCO₃⁻ fluid compared to other fluids (High-Cl⁻ and Low-concentration fluids) which contain only ~ 5% of carbon from that source (Fig. 6a). Therefore, significant contribution of CO₂ from some heat source is responsible for the formation of major hydrochemical features of High-HCO₃⁻ fluid. Figure 6b displays geographical distribution of relative contribution of carbon from the heat source. It is notable from this map that groundwaters with the greater contributions are sporadically distributed along the fault systems, and interestingly, these anomalies are characteristically located near the historical hot spring sites, i.e., Yunoko, Shimoda and Oyano, regardless of their host geological formations (Fig. 1). It seems plausible, therefore, that the surface geology is not necessarily associated with the occasion of observed deeply sourced carbon, instead, the presence of deeper heat source must be more critically important. The groundwaters with greater δ²H and δ¹⁸O compositional shifts tend to be seen on locations, where deeply originated CO₂ occurs (Fig. 6c, d), supporting above scenario.

If there are still lived magmas or melts beneath surface, the observed isotopic features of High-HCO₃⁻ fluid could be explained by involvement of these magmatic gases and fluids; however, the presence of these heat sources has not been reported for particular fields based on scientific evidences. Alternatively, based on well-known geotectonic setting for the study area, upwelling asthenospheric mantle (Zhao et al. 2018, 2021) is the most plausible origin of deeply sourced CO₂ that was released into metasomatized lower crust triggered by crustal assimilation with this huge heat source. Similar phenomenon was reported from the Hitoyoshi Basin in non-volcanic field at western Kumamoto (Fig. 1a) based on the same isotopic systematics (Sakai et al. 2011), with some additional noble gas isotopes, e.g., ³He/⁴He (Ohsawa et al. 2011).

Hata et al. (2017, 2020) provided key information for putting more constraint on the realization of the deep crustal fluids. Their recent electrical resistivity image finely identified significant high conductivity blocks exist in western Kyushu under shallow depth in the upper mantle and lower crust (~ 50 km), and argued the presence of deep hydrothermal fluids particularly under Hishikari (hydrothermal gold–silver ore deposits) (Fig. 1a), Hitoyoshi, Ashikita and Amakusa, where deep crustal fluid upwelling was anticipated. Thus, we propose fluids generated in a lower crust possibly contributes for the formation of our hypothesized deep fluids, although a possibility of partial involvement of recycled subduction slab fluids can’t be neglected as their ultimate origin (Nakamura et al. 2019; Sano and Marty 1994). However, observed degree of isotopic shift on δ²H and δ¹⁸O compositions (Fig. 6c) was not much dramatic as seen for deeply originated CO₂ (Fig. 6a) probably due to both dispersion of CO₂ gas and mitigation by the mixing with water of meteoric origin, while the deep crustal fluids ascent through long distance in the crust.

Discharging slab originated fluids ubiquitous in southwest Japan is thought to be transported through major tectonic structures, such as Median Tectonic Line, forearc thrust faults in the accretionary prisms and branching faults, driven by geo- and heat pressures under compressional tectonic setting (Kusuda et al. 2014; Togo et al. 2014; Yoshida et al. 2015; Morikawa et al. 2016). Deeply originated gases and fluids proposed in this study could also ascend through structural breaks in the Hinagu and Futagawa strike-slip faulting systems (Fig. 1) and possibly in relation to pressure release under open tectonic setting, with a similar law demonstrated from previous works in Kyushu (Hosono and Nakano, 2004; Hosono et al. 2018) and from other open tectonic regions (e.g., Cerón et al. 1998). It’s interesting to note that stronger signature of upwelling of the deeply sourced components (Fig. 6b, d) characteristically occurs near the locations, where more than two different faults are crossing (Fig. 1b). This may be due to generation of lower pressure around cross point of the different stresses in the crust, where pressurized deep fluids can more easily release towards air pressure near surface (Cappa et al. 2009; Takashima and Kano 2005).

The results of this study showed δ¹³C was the most sensible tracers among all measured stable isotope ratios for fingerprinting deeply originated components in groundwaters. It is also useful for identifying the locations of major structural pathways and could be applicable for further investigating deeply connecting fault systems and seismic responses (Barbieri et al. 2020; Barberio et al. 2021; Luccio et al. 2018). Giving that the mantle sourced carbon mediated groundwaters (High-HCO₃⁻ fluid) shared nearly half of total analyzed groundwaters (46%), this type of waters must to be extended wide in space bellow few hundred meters deeper in crust. Despite their prosperity in deep aquifers their discharges to near surface systems look somehow restrictive at, only where major fluid pathways exist, and thus, their overall impacts on surface hydrological systems seem to be small on this particular study setting. In our study area, only minor contamination with mantle originated carbon, i.e., ca. 5%, was observed locally in shallow well water (GW20002, 20 m in depth) near Yunoko hot spring (Fig. 6a). These aspects can be taken when we consider carbon cycle on earth surface systems in other fields.

Conclusions

The hydrochemical features of surface and subsurface waters were characterized to understand the origin, distribution and pathway of deeply originated components in hot springs in western Kumamoto using multiple stable isotope ratios (δ²H, δ¹⁸O, δ¹³C and δ³⁴S) for an area, where subduction-related active volcanic and magmatic activities are not anticipated. Our results implied that dissolved carbon and waters assumed to be originated from upper mantle and lower crust, respectively, could be transported to shallower aquifers along the major pathways. This study illustrates a possible genesis of some carbon rich hot springs appearing on ‘back-arc-side’ non-volcanic fields. Additional analysis on noble gas isotopes may provide firmer evidence. The aspects from our results are useful for interpreting data from other fields and for more global generalization, and beneficial to users for hot spring resources.