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Article

Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs

by
Chuan-Hsiung Chung
,
Chen-Feng You
* and
Yi-Ling Yeh
Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan
*
Author to whom correspondence should be addressed.
Water 2025, 17(1), 71; https://doi.org/10.3390/w17010071
Submission received: 27 November 2024 / Revised: 23 December 2024 / Accepted: 27 December 2024 / Published: 31 December 2024
(This article belongs to the Section Hydrogeology)
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Abstract

:
This study investigates water–rock interactions of Taiwan hot springs by analyzing rare earth elements (REEs) concentrations and strontium (Sr) isotopes. REEs were separated from samples using RE resin, and their concentrations were measured by HR-ICPMS. Strontium was isolated using SrSPEC resin, and the strontium isotopic ratio was determined by MC-ICPMS. The ΣREE in the hot springs ranges from 3.17 ng/L to 29.7 µg/L, with the highest levels found in the Tatun Volcano Group, followed by springs from sedimentary and metamorphic regions. The primary factors controlling REE compositions are lithology and pH. REE patterns of hot springs can be categorized into five types, indicating that the hot springs were affected by various mechanisms. The most distinct hot spring samples are from Tatun Volcano, Ginshan, and Kuantzuling. The 87Sr/86Sr ratios range from 0.70468 to 0.71730, with the most radiogenic samples originating from metamorphic regions, reflecting the nature of the parent rock interacting with the hot spring water. Seawater intrusion and preferential weathering of carbonate also have minor effects on Sr isotope composition. The findings indicate that the types of surrounding rocks and the pH values of the hot springs significantly influence REE patterns and Sr isotope compositions in Taiwan’s hot springs.

1. Introduction

Located at the convergent boundary between the Eurasian and Philippine plates, Taiwan is a geothermally active country. There are >150 hot springs that can be found in this region and they are considered potential resources for recreation and geothermal energy. Therefore, the understanding of hydrogeology and chemical properties of hot springs becomes critical. Although many studies have focused on Taiwan hot springs [1,2,3,4,5,6,7,8], the source and path of the geothermal water are not clarified due to its complex geology. Based on origins, three major categories could be derived from igneous, sedimentary, and metamorphic rocks [9], whereas classification based on ionic constituents was also used [10]. Apart from classification purposes, the ionic compositions of hot springs can also provide insights into circulation patterns and geochemical processes. Several hydrochemical parameters are capable of being used to identify the formation processes of hot springs. Earlier studies showed that chemical compositions, geological settings, and host-rock interaction provide critical information on the spring dynamic and release mechanism of dissolved matters [11,12]. For instance, the silica geothermometer is effective in predicting aquifer temperatures under ideal conditions, as demonstrated in Yellowstone National Park [13]. The chemical composition of hot springs, including ionic facies such as HCO3:Cl:SO4 and Na + K:Mg:Ca, helps trace processes like solute acquisition and CO2 degassing, providing insights into the geochemical environment and potential sources of water [14]. The origins of waters from igneous, sedimentary, and metamorphic rocks significantly influence their hydrochemical profiles due to the distinct mineral compositions and weathering processes associated with each rock type. In summary, the hydrochemical profiles of waters originating from igneous, sedimentary, and metamorphic rocks are shaped by the specific mineral compositions and weathering processes of each rock type. These differences influence the concentrations of various ions in the water, ultimately determining its overall chemical characteristics. While characterizing hot springs based on ionic constituents is beneficial, there are several limitations. The ionic composition of hot spring waters can change due to seasonal variations, anthropogenic influences, or natural geological processes. This variability may lead to inconsistent interpretations if not monitored regularly [15]. Relying solely on ionic constituents may oversimplify the complex interactions within hot spring systems. Other factors such as temperature, pressure, and biological activity also play significant roles in defining the characteristics of hot springs but are not captured through ionic analysis alone.
Rare earth elements (REEs) are a group of trace elements whose chemical properties gradually change with their decreasing ionic radii across the lanthanide series (lanthanide contraction), from lanthanum to lutetium, causing slightly different behaviors for light REEs (LREE, La–Sm) and heavy REEs (HREE, Dy–Lu) during dissolution, precipitation and adsorption. REEs have been widely utilized in the fields of geochemistry and marine science [16,17,18,19,20]. REEs are excellent indicators, not only aiding in understanding their sources [21,22], but also being applied as tracers for groundwater [23,24], in the assessment of anthropogenic pollution [25,26], and in the interactions between water as well as its surrounding environment (e.g., redox reactions, weathering, and complexation). The REE distribution patterns and fractionation behavior may be changed during water–rock reaction [27,28,29,30] and transportation in natural systems [31,32,33]. The REE patterns in natural water are routinely used as tracers for weathering source identification [29,34,35,36] or to study the mixing processes of water masses in oceanic or continental environments [17,31,37,38,39,40,41]. The concentration of REEs in natural waters is quite low, generally ranging from ng/mL to pg/mL (ppb to ppt) in unpolluted freshwater, and even lower in seawater [42,43]. The concentration of rare earth elements in hot spring water also mostly falls within the ppb ppt range. Due to the influence of surrounding rocks, geological conditions, and processes, the distribution of rare earth elements in hot springs can vary significantly. Generally, the distribution pattern of rare earth elements in aqueous solutions, especially in geothermal fluids, does not necessarily match that of the surrounding rocks [44,45].
Physicochemical properties are crucial in the behavior and distribution of REEs in various aqueous environments. The unique chemical characteristic of REEs, including their capacity to exist in multiple oxidation states, wide span of ionic radii, and varying electronegativities, render them highly susceptible to interaction with diverse dissolved constituents, pH levels, redox potentials, and other water parameters [46]. Previous research has shown that the REEs could have affinities with the temperature of waters and the concentration and distribution pattern of REEs may be discriminated by the variable temperatures in the aqueous environment [47,48]. Zhang et al. [49] show that water salinity plays an important role in modulating the content and speciation of aqueous REEs of thermal water [49]. Redox conditions in waters are the primary way to modulate Ce and Eu anomalies due to their attributes as variable valence elements [50,51]. Michard [44] found that the concentration of rare earth elements in fluids from different geothermal areas is not related to rock type or temperature but increases with decreasing pH. High-temperature solutions (>230 °C) from different rock types may exhibit similar rare earth element distribution patterns. Lewis et al. [52] also found that the concentration of rare earth elements in the hot waters of Yellowstone Park is inversely related to pH, indicating that pH is one of the main factors affecting the transport of REEs. The presence of colloidal substances in hot springs may be quite important. The normalized distribution pattern of rocks in hot springs further indicates that the positive Eu anomaly in the solution may result from the leaching of REEs from rocks by hot spring water, particularly Eu, which may preferentially dissolve from feldspar minerals. Bau et al. [53] studied the Nishinuma hot spring in Hokkaido, Japan, and found that low-temperature acidic springs often result in a negative Eu anomaly, while the surrounding rocks do not exhibit an Eu anomaly. This is because Eu in rocks, compared to other REEs, is more easily enriched in feldspar, which is relatively stable in slightly acidic conditions, thus reducing the Eu concentration in water. However, in high-temperature environments (temperature > 200 °C), such as black smokers, Eu3+ is reduced to Eu2+, causing a positive Eu anomaly. Mőller et al. [54] used REEs and Pb isotopes to investigate the hydrothermal sources in Turkey, classifying different hot springs based on their REE patterns. The results showed that REEs can be an important tool for identifying the source rocks’ hot springs and the anomalies of Eu and Y can be used to determine whether water–rock interactions are in a stable state.
Strontium (Sr) isotopes have been used in environmental and geological studies to distinguish between different sources of material, particularly as a diagnostic of fluid sources and migration pathways in nature. 87Sr is the stable daughter isotope decay product produced by the decay of 87Rb (halflife = 48 Ga) and 86Sr is a non-radiogenic stable isotope. Therefore, the 87Sr/86Sr ratios depend on mainly the ages and Rb/Sr ratios of the source rocks. Therefore, different geological materials exhibit unique Sr isotope ratios. This makes Sr isotopes effective tracers for identifying fluid sources and migration pathways in natural systems due to their distinct isotopic signatures, which can reflect the geological history and interactions of fluids with surrounding materials. There is a general consensus that Sr isotopes will not be significantly fractionated by biological and low-temperature abiotic chemical reactions. Hence, the 87Sr/86Sr in natural waters will be a function primarily of chemical weathering of different materials, and thus to first order reflect the 87Sr/86Sr in source areas. Sr isotopes have been utilized in watershed studies to trace fluid pathways and weathering reactions [55,56,57,58,59,60,61,62]. Most studies use a classic two-end-member mixing model to quantify the contributions of Sr in spring water from atmospheric inputs versus bedrock weathering. This mixing model, which assumes that Sr in water originates from two primary sources: atmospheric deposition and the weathering of bedrock, is a simplified approach used to quantify the contributions of Sr in spring water. The implications of using this model are significant in understanding the geochemical processes and provenance of water sources and can be beneficial in providing a clear framework for initial assessments of Sr contributions in various environments [63]. The accuracy of the two-end-member mixing model is highly dependent on the specific environmental conditions and the presence of additional Sr sources. In systems where Sr primarily originates from bedrock weathering and atmospheric inputs, the model can perform well. However, the model’s accuracy diminishes in more complex systems where multiple sources contribute to the Sr pool [64]. Incorporating additional sources and processes, such as the three-source mixing model, can significantly improve the accuracy of Sr source quantification [65].
In this study, hot springs were collected from various geological settings in Taiwan, and their major elements, REE contents, and Sr isotopes were measured. We combine REE patterns, Sr isotopes, and other evidence to provide a better understanding of water–rock interactions in Taiwan’s hydrothermal system.

2. Geological Background and Sampling

Taiwan is located at the boundary between the Philippine Sea plate and the Eurasian Continental plate and constitutes a well-known arc-continent collision belt in the Western Pacific (Figure 1). The island of Taiwan consists of tectonic units separated by N-S oriented major thrust faults [66]. The western coastal plain is composed mainly of alluvial deposits and some terrace deposits. The Western Foothills (sedimentary rock zone) and the Hsueshan Range are composed of Cenozoic shallow-marine siliciclastic overlaid by Quaternary alluvial deposits [67]. The Central Range (metamorphic rock zone) is composed of Miocene deep-marine turbidites and late Paleozoic to Mesozoic metamorphic rocks, while the Coastal Range in eastern Taiwan is composed of Miocene–Pliocene sedimentary rocks overlaid by Plio–Pleistocene turbidite deposits [68]. In northern Taiwan, agglomerate masses of igneous rock fragments are the potential volcanic geothermal area [69]. Due to different geologic settling, hot spring samples were collected from three geothermal fields located under different geological conditions (Figure 1). These geothermal fields include (1) Tatun Volcanic Group (TVG), (2) Central Range and Hsueshan Range, (3) Coastal Range, and (4) Western Foothill. The TVG geothermal field is mainly composed of volcanoclastic rock (andesite), whereas the Central Range and Hsueshan Range fields are mainly of metamorphic rocks. Hot springs collected from the Coastal Range and Western Foothill field are exposed in sedimentary rock. In this study, sixteen hot spring samples were collected from these four geothermal fields. Sampling information is shown in Table 1 and localities are shown in Figure 1. Hot spring waters were collected in acid-cleaned polypropylene bottles. Then, the samples were filtered through 0.45 μm cellulose nitrate filters (Micro Filtration Systems, 47 mm diameter) using a vacuum filtration unit in the laboratory on the same day of collection. Filtered samples were stored in pre-cleaned bottles then acidified with double-distilled HNO3 and stored at a temperature of 4 °C.

3. Analytical Methods

The chemical compositions and strontium isotopic ratios of the hot spring samples were determined in the Earth Dynamic System Research Center at National Cheng Kung University, Tainan, Taiwan. All preparation procedures for elemental and isotopic compositions were operated in a Class 1000–10,000 clean room. A high-resolution sector field ICP-MS (Thermo Fisher Scientific, Waltham, MA USA, Element2) was used for the determination of the major elements and Sr, as well as REEs. Two introduction systems were employed in this study: (1) a low flow self-aspiration micro-concentric nebulizer coupled to a Cyclone-Scott tandem type spray chamber, and (2) a CETAC (Omaha, NE USA) Aridus desolvation system incorporating a PFA micro-concentric nebulizer and heated PTFE spray chamber. The former setting, which delivers stable signals, is for Sr and major element analysis, and the latter one is responsible for reducing the polyatomic interferences during the determination of REEs. In order to avoid interference in REE analysis, a column packed with RE resin (Eichrom, Lisle, IL, USA) was used to separate matrix elements. The extraction procedures were modified from Huff and Huff [70]. Recovery, reproducibility, and blank were evaluated by passing SLRS-4 and deionized water through the column. Recovery was found to be 97 ± 2% and the contribution of blank was less than 1%. No elemental fractionation was observed after extraction. Indium was used as an internal standard in order to compensate for matrix and time-dependent signal variations, and its content was adjusted accurately so that it was present in equal concentrations in all solutions. The international standard SLRS-4 was also analyzed to check the validity of the measurement; since there is no formally certified REE data, the concentrations of REEs in SLRS-4 were adopted from Yeghicheyan et al. [71]. Matrix-matched standards were also used to achieve high-precision measurements. The analytical errors in major cations and Sr concentrations were estimated to be better than 3% and data reproducibility for REEs was 3–10% due to the extremely low concentration in hot spring water.
The isotopic compositions of strontium were determined by a Multi-Collectors Inductively Coupled Plasma Mass Spectrometry (Thermo Fisher Scientific, Neptune). Sr separation was performed by SrSpec resin (Eichrom, Lisle, IL, USA). The mobile phases used were 3N HNO3 and ultra-pure water (18.3 MΩ cm, MilliQ, Molsheim, France). Acids were prepared by sub-boiling distillation using quartz stills. This separation procedure shows negligible total blank. All measured 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194. Instrumental performance was monitored by duplicated analysis of the SRM 987 standard (0.710254 ± 30, n = 34).

4. Results

4.1. Major Element Compositions

The results of major components, Sr concentrations, as well as Sr isotope measurements of collected hot spring samples are presented in Table 2. The concentrations of major cations exhibited significant variations, varied from 2.7 to 8440 mg/L for Na+, 0.77–267 mg/L for K+, 0.16–249 mg/L for Mg2+, and 1.17–293 mg/L for Ca2+ (Table 2). Except for DP, the major element content of hot springs in the TVG area is lower than that of hot springs in other areas. The results were in good agreement with previously published results [1,8]. DP, CS, and KTL hot springs featured high sodium and chloride content.

4.2. Sr Concentration and Sr Isotope

The elemental and isotopic compositions of Sr in the hot springs are given in Table 2. The Sr contents vary from 0.03 to 5.34 mg/L. The 87Sr/86Sr falls in a range of 0.70468 to 0.71730. Summarized Sr isotopic compositions for studied samples are as follows: (1) 0.71480 to 0.71730 for samples collected in the metamorphic region, (2) 0.70931 to 0.71066 for hot spring exposed in sedimentary strata, and (3) 0.70468 to 0.70168 for hot spring exposed in volcanic strata.

4.3. REEs Compositions

The REE concentrations and Ce, Eu in Taiwan hot spring waters are presented in Table 3 while the chondrite-normalized REE patterns are shown in Figure 2. The Ce and Eu anomalies are determined by Ce/Ce* = CeN/(LaN × PrN)0.5 and Eu/Eu* = EuN/(SmN × GdN)0.5, respectively, where N refers to normalization by C1 chondrite [73]. The results of these calculations are listed in Table 2. In general, the total REE concentrations show a large variation. The TVG samples contain significantly higher REE abundances than sedimentary and metamorphic ones. The REE patterns are diverse. TVG hot spring samples generally show flat patterns except sample GS which has a V-shape pattern. Metamorphic region samples also have relatively flat patterns with very low REE abundances. Most samples have negative Eu anomalies. The hot springs exposed in sedimentary rock have a LREE enrich pattern with negative Eu anomalies. The total concentration of REEs (ΣREE) in hot springs ranges from approximately 3.17 ng/L to 29.7 µg/L, with significant variations among different hot springs. When categorizing the REE concentrations based on the exposed rock types into three groups (Figure 2), it is observed that the hot springs in the TVG have higher REE concentrations, generally exceeding those in sedimentary and metamorphic rock areas. The hot springs in sedimentary rock areas have the next highest concentrations, while most hot springs in metamorphic rock areas have lower concentrations. This variation is likely related to differences in the surrounding rocks and the pH of the hot spring water (Figure 3).

5. Discussion

5.1. Division of Water-Type by Major Component

High sulfate contents were typical characteristics of TVG hot springs. Low SO42− was observed in a specimen collected in the sedimentary and metamorphic region, except in the hot spring AT (422 mg/L, located in the sedimentary region) and WS (790 mg/L, located in the metamorphic region) which also featured high Ca2+ concentration. The possible sources of SO42− in these two hot spring specimens could be the dissolution of sulfur-containing minerals such as chalcopyrite, pyrite, and gypsum [74]. However, the lack of gypsum occurrence suggests that sedimentary and metamorphic pyrite were responsible sources of dissolved SO42− [74,75]. High solute Ca2+ and SO42− are attributed to coupled sulfide oxidation and carbonate weathering [76]. The major ion contents showed that hot springs from central and NE Taiwan were Na-HCO3 type, whereas in TVG were Ca-SO4 and Na-Cl-SO4 type. In comparison, the water in hot springs collected from mud volcano area in southern Taiwan were Na-Cl type, suggesting marine origin [6].

5.2. Water–Rock Interaction Revealed by Sr Isotope

The strontium isotope values, 87Sr/86Sr, of hot springs range from 0.70468 to 0.71730. There are three possible Sr sources in the TVG hydrothermal system: meteoric or rainwater, host rocks, and seawater. The Sr isotope ratios of local rain water and seawater are 0.709014~0.70993 [77] and 0.70917 [78], respectively. The 87Sr/86Sr of host rocks will be discussed in detail in the following sections. Strontium isotope ratios of Taiwan hot springs in the three major rock types differ significantly (Figure 4A). Hot springs in igneous rock areas have the lowest strontium isotope ratios, ranging from 0.70468 to 0.70618. Hot springs in sedimentary rock areas have intermediate values, ranging from 0.70931 to 0.71066. Hot springs in metamorphic rock areas have the highest values, ranging from 0.71480 to 0.71730. Comparing these values with the strontium isotope ratios of rocks, the andesite in the TVG has strontium isotope ratios ranging from 0.70428 to 0.70469 [79]. The strontium isotope ratios of hot springs in this area are close to those of andesite, indicating that the hot water in this area reacts with andesite after infiltration [80,81].
The strontium isotope ratios of sedimentary rocks are higher than those of igneous rocks (e.g., the sediments of the Erhjen River range from 0.71450 to 0.72020; Miocene sedimentary rocks range from 0.71515 to 0.71785; the strontium isotope ratios of the laterite gravel layer range from 0.71500 to 0.73000) [82,83,84]. However, the strontium isotope ratios of hot spring water in this area are lower than those of sedimentary rocks or sediments.
As for metamorphic rocks, the ratios are even higher, such as the gneiss in Kinmen, which ranges from 0.70749 to 0.72247, and the metamorphic mudstone, which ranges from 0.71017 to 0.71358 [85,86]. This indicates that the strontium isotope ratios of hot springs can mostly reflect the nature of the original rocks interacting with the hot spring water.
Several source components must be involved in explaining all data presented in Figure 4. The less radiogenic signature in TVG (range from 0.70468 to 0.70618) is explained in terms of andesite weathering which contributed Sr isotopic ratio of 0.7041–0.7049 [87]. Figure 4B shows the strontium isotope ratios of hot spring water plotted against rare earth element content. It shows that hot springs in igneous rock areas have low strontium isotope ratios and high rare earth element content; sedimentary rock areas are next; and hot springs in metamorphic rock areas have high strontium isotope ratios and low rare earth element content. Figure 4A shows three hot springs that fall outside the range of each rock type: DP, WS, and AT hot springs. DP and WS hot springs fall within the range of sedimentary rock hot springs, while AT hot spring falls within the range of igneous rock hot springs. Although DP hot spring is generally classified as an igneous-related hot spring due to its location in the Tatun volcanic area. The surrounding rock of the DP hot spring is Holocene alluvium. The much radiogenic strontium isotope ratio of DP hot spring (0.71240) compared with other hot springs in the same area (0.70468–0.70618) indicates that the strontium source of DP hot spring is not simply a mixture of andesite or meteoric water. Chen [81] believes that may originate from aquifers in the Wuzhishan Formation, forming acidic hot water after mixing with volcanic gasses according to the sulfur isotope ratios of DP spring. Taiwan sedimentary rock featured high 87Sr/86Sr values ranging from 0.71424 to 0.72053 [88]. Water–rock interactions dissolve these more radiogenic Sr. Therefore, the high strontium isotope ratio in the DP area may reflect the result of mixing sedimentary rocks or surface sedimentary rocks with andesite and meteoric water. Principle component analysis of trace elements compositions of TVG hot springs show notable sedimentary contributions [8]. Boron isotope and hydrogen–oxygen isotope studies in the TVG region also support this theory [8].
AT hot spring is located in the middle section of the Coastal Range and is generally classified as a sedimentary rock hot spring. However, its low strontium isotope ratio (0.70564) shows a significant difference from typical sedimentary rock hot springs, and its ratio is closer to that of igneous rock hot springs. AT hot spring emerges from mudstone or tuff breccia blocks in the Lichi Mélange at the surface. Since the rock compositions of the Lichi Mélange include serpentinite, sandstone, mudstone, and andesite fragments, the low strontium isotope ratio of AT hot spring indicates that part of its source should be related to igneous rocks, possibly andesite or tuff breccia blocks in the local rocks.
WS hot spring is in the Central Mountain Range area and is hosted by Tananao schist. Its strontium isotope ratio (0.71004) is significantly lower than other metamorphic rock hot springs. Yu and Lan [89] measured the strontium isotope ratio of calcium carbonate in marble in the Tananao area, showing a ratio of 0.70746–0.70775. Therefore, it is speculated that the preferential weathering of carbonate is one of the endmembers which provide the low strontium isotope component to WS hot spring.
The strontium isotope ratios of hot spring specimens in sedimentary rock areas are not as high as those of sedimentary rocks. The strontium isotope ratio of the DKS hot spring (0.70931) is close to that of seawater (0.70917). This indicates that its main source of Sr may reflect the influence of nearby limestone, and its high calcium concentration in the hot spring water also supports this argument. The strontium isotope ratio of KTL (0.71066) hot springs is lower than those in the surrounding sedimentary rocks. The China Petroleum Corporation once drilled oil and gas wells in the KTL area, revealing that the host rock is Changchikeng Formation which is a marine formation. Also, the hot spring contains higher salt and CO2. Therefore, the strontium isotope ratio of the KTL hot spring is significantly affected by connate seater in the marine sediments.

5.3. Hot Spring REE Patterns

Hot springs in igneous rock areas have similar REE concentrations, which are the highest among the three rock types, corresponding to the previously mentioned high concentration type. Their distribution mainly shows a nearly flat pattern (CSL, DP, and KTP), or slight LREE depletion (MT, PY, and HYK), with no or slight negative Eu anomalies (Figure 2A). All these hot springs are located in the TVG, and the absence or slight presence of Eu anomalies may be influenced by their parent rock (andesite). GS hot spring is an exception, showing a pattern of middle REE depletion, which is not similar to the distribution pattern of andesite in the TVG [79]. Compared to other hot springs, those in igneous rock areas have distinct REE distribution patterns, indicating that the REE distribution of hot springs in this area can effectively distinguish them from hot springs in other regions.
Hot springs in metamorphic rock areas (Figure 2B) have the lowest REE concentrations, enriched corresponding to the previously mentioned low concentration type, with more complex distribution patterns. HY, CS, and WS hot springs show light REE- patterns with slight negative Eu anomalies, similar to AT and other sedimentary rock hot springs. SA and JS hot springs show slight light REE depletion patterns, along with slight negative Eu anomalies.
Hot springs in sedimentary rock areas have REE concentrations between those of igneous and metamorphic rock areas. AT and DKS hot springs show a light REE-enriched inclined pattern with slight negative Eu anomalies, similar to the REE distribution patterns of typical sedimentary rocks [45,54]. The KTL hot spring, however, differs from the other two, showing a heavy REE-enriched pattern with a positive Ce anomaly (Figure 2C).
REE patterns in hot springs provide valuable information about geochemical processes and water–rock interaction in geothermal systems. Many hot springs exhibit light REE (LREE) enrichment with chondrite-normalized patterns. This pattern is observed in geothermal waters from regions such as Oregon, Nevada, and California, as well as the Idaho Batholith [90,91]. In contrast, some springs, like those in the Manza area, show heavy REE (HREE) enrichment, reflecting the underlying hydrothermal reservoir [92]. These patterns are influenced by factors such as water acidity [91,93], particulate matter [90], and underlying rock compositions [94]. Understanding these patterns not only aids in geothermal exploration but also enhances our knowledge of geochemical processes in these dynamic systems.
The anomalies of Ce and Eu are valuable indicators in interpreting hydrogeochemical processes. These anomalies help in understanding the redox conditions, mineral interactions, and fluid migration patterns in various environments. Ce anomalies are often used to infer redox conditions. Negative Ce anomalies typically indicate oxidizing conditions, as Ce is preferentially removed from solution through oxidative scavenging [95,96]. Conversely, positive Ce anomalies can suggest reducing conditions. Ce anomalies can also reflect interactions between water and rock, where Ce is either adsorbed or desorbed depending on the redox state and mineral composition [96,97,98,99].
Eu anomalies in hot springs are crucial for understanding the stages of mineral decomposition and water–rock interactions. Eu anomalies can arise from the dissolution of Eu-rich minerals like feldspars, which is common in volcanic and granitic settings [95,96,97,98]. Experimental studies suggest that at the earliest stage of rock alteration in acidic environments, the interstitial materials with relatively low Eu contents were decomposed, which resulted in the positive Eu anomaly. In the following stage, feldspar that had a relatively high Eu content was decomposed, which led to the negative Eu anomaly [100]. The presence of negative Eu anomalies exhibited in most of Taiwan’s hot springs suggests significant water–rock interactions, particularly the dissolution of feldspar, which is a key process in the geochemical evolution of hot springs. The Ce and Eu anomalies in Taiwan’s hot springs will be discussed in the following sections.

5.4. Water–Rock Interaction Revealed by REE Compositions

REEs in water can originate from various sources, one of which is the weathering of rocks or the release of minerals. The andesite in the TVG contains approximately 80–100 µg/g of REEs [79], which may be released into the water through water–rock interactions. Figure 3 clearly shows that the REE concentrations in hot springs are related to the distribution of rock types, with higher concentrations in igneous rock areas compared to the other two areas. Additionally, many studies have shown a significant correlation between pH and REE concentrations. Generally, in geothermal systems, REE concentrations decrease with increasing pH and are not necessarily related to water temperature or rock type [44,45,52,101,102]. When the solution pH is below 7, REEs may be released into the water through dissolution or desorption from mineral surfaces [103]. The pH of Taiwan hot springs in sedimentary and metamorphic regions mostly ranges from 6 to 8, with REE concentrations ranging from 0.03 to 0.28 µg/L. In igneous areas, the pH of hot springs mostly ranges from 2 to 4, with REE concentrations ranging from 1.17 to 29 µg/L. It is evident that the hot springs in the TVG, which have lower pH values, exhibit higher REE concentrations (Figure 3). Lewis et al. [104] found that in hot springs with low pH and high sulfate content, the dominant REE complexes in water are Ln3+ and (Ln-SO4)+, consistent with the observations of Haas et al. [105]. Due to the high sulfate concentration in the hot springs of the Tatun Volcano Group, Ln3+ and sulfate complexes (Ln-SO4)+ may play a more significant role. Many studies have shown that sulfate complexes are primarily complex with light REEs, and under specific conditions (such as low pH and high sulfate water), the ratio of light to heavy REEs can be quite close. Ln3+ preferentially complexes with heavy REEs [21,104,106]. The combined effects of these interactions may result in the REE distribution patterns observed in the Tatun Volcano Group.
To compare the distribution of REEs (named REEs) in hot springs from different regions and facilitate comparison with rock models, the REE concentrations were normalized to C1-chondrite. The plot clearly shows significant differences in REE content and distribution patterns among the hot springs (Figure 2), with variations up to 104 times. Based on REE content and distribution patterns, three main types can be identified: (1) high concentration with a nearly flat pattern, (2) medium concentration, and (3) low concentration. The latter two types can be further divided into light REE-enriched and heavy REE-enriched patterns. In the first type, REE concentrations range from 10−2 to 10−3 times that of C1-chondrite, with most hot springs showing a nearly flat pattern or slightly lower light REE levels, and no or slight Eu anomalies. This distribution pattern is not observed in other types of hot springs, except for the DKS hot spring, which falls into the first type but shows a light REE-enriched pattern similar to the medium and low concentration types. The second type has REE concentrations of about 10−4 to 10−5 times that of C1-chondrite, characterized by light REE enrichment and Eu anomalies. CS and KTL hot springs are unique within this type, with CS showing a middle REE depletion and heavy REE enrichment pattern, and KTL showing a significant heavy REE enrichment pattern with a positive Ce anomaly. The third type has REE concentrations ranging from 10−5 to 10−7 times that of C1-chondrite, with very low REE content and slight Eu anomalies. The REE content and distribution patterns may be influenced by sources and different chemical processes, making concentration a basic classification criterion. The classification based on REE distribution patterns highlights the uniqueness of the first type, CS, and KTL hot springs, indicating that these hot springs likely have significantly different sources or processes compared to other medium- and low-concentration hot springs. Despite concentration differences, the other two types show similar distribution patterns, suggesting other correlations. The low concentration of the third type may result from surface water dilution, stronger rock resistance to weathering, leading to low REE dissolution in water, or specific chemical processes removing REEs from the water [52,53,107].
The (La/Yb)N of hot springs generally increases with the strontium isotope ratio, while Eu/Eu* decreases with the strontium isotope ratio. This trend may be inherited from the characteristics of the parent rock, i.e., as the lithology changes from igneous to metamorphic rock, (La/Yb)N increases, and Eu/Eu* decreases. The distribution of REEs in hot springs can be used to identify hot springs that may be influenced by similar sources and chemical processes.
Due to similar sources and chemical processes causing similar rare earth element (REE) distribution patterns, REEs can serve as tracers for understanding sources and impacts [45,54]. However, REE content in different rocks can vary greatly [54,85,87,101], so the differences in REE content among hot springs may be related to the surrounding lithology or reflect differences in hydrothermal pathways or duration. Additionally, the lower concentration group of hot springs in metamorphic rock areas may be due to the input of meteoric water which has very low REEs. These may lead to hot springs from different regions and lithologies having similar REE contents. Since the factors influencing REE concentrations are quite complex, using both concentration and REE distribution patterns as criteria can better elucidate the relationships between hot springs and clarify their influencing factors and sources [52,54,101]. Based on their distribution patterns, they can be divided into five types:
The first type (Figure 5) exhibits a LREE-enriched inclined pattern with Eu negative anomalies. Two distinct concentration groups can be identified: higher concentrations are mostly from hot springs in sedimentary rock areas, while lower concentrations are from metamorphic rock areas, with a concentration difference of about 1000 times. Moreover, this distribution is not entirely controlled by the commonly assumed lithology or spring quality. The REE distribution pattern of hot spring water does not necessarily fully reflect the pattern of the surrounding rocks [44,45]. For example, the lithology and spring quality of the hot spring shown in Figure 5 are similar to those of the hot spring in Figure 5 (both are hot springs in metamorphic rock areas), but their distribution patterns are completely different. This suggests that lithology is not the only controlling factor on hot springs REE patterns.
The second type (Figure 5) of rare earth element distribution pattern is nearly flat or slightly depleted in HREE, with lower concentrations similar to those of CS Hot Spring in Figure 5. The lower concentrations of REEs in JS and SA Hot Springs may be due to dilution by meteoric or surface water, as both hot springs are located near rivers. The differences in the REE distribution patterns between the two hot springs may be influenced by the presence of calcium carbonate or CO2, especially since SA Cold Spring releases carbon dioxide when it flows to the surface. REEs may form carbonate and hydroxide complexes in water, which are more significant in near-neutral and high pH environments compared to other types of complexes [52,104]. The carbonate complexes of HREE are more stable than those of HREE [106], resulting in a more pronounced depletion of HREE in SA Cold Spring compared to JS Hot Spring.
The third type (Figure 5) and the fourth type (Figure 5) have similar patterns, both being nearly flat but differing in concentration. The third type of hot spring has the highest concentration among all samples, with a nearly flat pattern and a slight negative Eu anomaly, including CSL, KTP, and DP Hot Springs. The fourth type of hot spring (MT, PY, and SYK) has concentrations about 10 times lower than the third type, with a slight depletion in HREE and no Eu anomaly. The presence or absence of Eu anomalies in hot springs may be inherited from the Eu anomaly characteristics of the rocks [79,87]. Both types of hot springs are located in the northern TVG area. The slight differences in REE patterns and concentrations may be attributed to different sources of REEs. The andesite of the TVG can be further subdivided into several rock types, each with slightly different mineral compositions [108]. Hot spring waters may flow through different andesite, therefore resulting in different REE distribution patterns. The dilution and mixing effects caused by meteoric or surface water input may also affect the concentrations of REEs.
The last type includes GS and KTL hot springs (Figure 5), which have the most distinct patterns, completely different from the other four types. The REE pattern shows middle rare earth depletion or HREE enrichment, and the KTL hot spring also has a significant positive Ce anomaly. These two hot springs differ from others in that they both emerge in unconsolidated loose sediment areas, especially the KTL hot spring, which is a mud spring with a large amount of clay minerals suspended in the water. Both hot springs may be affected by seawater intrusion or mixing. GS hot spring is located by the coast. Based on hydrogen and oxygen isotope results, Liu et al. (1984) suggested GS hot spring is mixed with intruded seawater. This can explain the high contents of sulfate, bicarbonate, and chloride in GS hot spring, and may have a significant impact on the composition of REEs. In low pH conditions, carbonate complexes mainly combine with HREE, while sulfate and chloride complexes easily combine with HREE [21,52,104,105,106]. The results suggest that the unique REE pattern of GS hot spring is caused by seawater intrusion and the mixture of different proportions of REEs. Previous studies based on major ion analysis [109] and the B/Cl ratio [8] also indicated the mixing of thermal fluid with seawater in GS hot spring. The KTL hot spring is a mud spring located in a mud volcano area, where seawater may be preserved as sedimentary pore water. The enrichment of HREE in water may be related to the mixing of seawater and ground water, or the adsorption of clay minerals which have an obvious fraction of the HREE [106,110]. Generally, seawater mainly shows an HREE enrichment and HREE depletion distribution pattern, often with Ce anomalies [26,111]. Comparing the distribution patterns of seawater and the KTL hot spring, it is found that except for the special positive Ce anomaly, the distribution pattern of the KTL hot spring is similar to that of seawater. Therefore, the low concentration of LREE in the KTL hot spring may be due to the influence of seawater mixing. The positive Ce anomaly in the KTL hot spring may be due to redox reactions and complexation effects in the water [111,112,113]. Among REEs, Ce is particularly prone to redox-driven behavior [16]. De Baar et al. [111] observed the presence of a positive Ce anomaly in surface waters of the North Atlantic, with increasing negative Ce anomalies with depth. Positive Ce anomalies have also been observed in the surface pore water of some marine sediments [114,115]. Additionally, Mőller and Bau [112] found a positive Ce anomaly in the alkaline Lake Van, attributing it to the oxidation of Ce (III) to Ce (IV) in the solution, forming stable (penta)carbonato–CeIV-complexes, leading to positive Ce anomaly in the water. In alkaline, carbonate-rich water bodies, the formation of (penta)carbonato–CeIV-complexes can prevent the removal of Ce from the solution [112,113]. The KTL hot spring is rich in carbonates and is in a reducing environment underground, so the presence of a positive Ce anomaly may be related to these two mechanisms.
Using REE contents and distribution patterns as indicators in hot springs offers crucial information. REE patterns can serve as tracers for geochemical processes, such as water–rock interactions and redox conditions, providing insights into the geothermal system’s history and dynamics. REE signatures in hot-spring deposits can help identify geothermal reservoir rock types, offering valuable information for geothermal exploration. REE analyses, combined with other methods, can help track geothermal anomalies and understand the spatial extent of hydrothermal activity. However, REE patterns can be influenced by detrital contamination, which may obscure the true geochemical signals and complicate interpretations [116]. Accurate REE analysis requires precise analytical techniques due to the low content in hot springs, and factors like oxide formation during measurement can affect results. The REE distribution can be affected by multiple factors, including pH, redox conditions, and mineral dissolution, making it challenging to isolate specific processes. The substantial variability in REE content among different rock types can have a significant influence on the interpretation of REE distributions observed in hot springs, as the composition of the host rock is a key factor in determining the REE signature of the spring water [117]. These factors can hinder the interpretation of hot spring geochemical processes derived from REE results. Integrating REE compositions of hot springs with isotopic analysis, such as Sr isotope, could be a valuable approach for characterizing geothermal systems.

6. Conclusions

The REE content in hot springs ranges from 3.17 ng/L to 29.7 µg/L, with the highest levels in the TVG hot springs, followed by sedimentary rock areas, and the lowest in metamorphic rock areas, possibly controlled by lithology and pH of the hot spring water.
Major cations show little effect on the distribution of REEs. The correlation between REE content and sulfate may be due to sulfate dissolving in water to form sulfuric acid, lowering the pH and causing a slight correlation with REEs.
REE patterns of Taiwan hot springs are not only controlled by lithology, but also the mixtures with seawater and meteor water.
The stable REE complexes may be related to pH and the concentration of anions. The significant positive Ce anomaly in the KTL hot spring may be due to the reduction in tetravalent Ce to trivalent Ce in water or the formation of stable (penta)carbonato–CeIV-complexes, leading to higher Ce content in water compared to other REEs.
Hot spring water in igneous rock areas featured in lowest strontium isotope ratios, followed by sedimentary rock areas, and the highest 88Sr/86Sr were found in metamorphic rock areas, reflecting the nature of the parent rock interacting with the hot spring water. However, the ratios in DP, WS, and AT hot springs do not fall within their respective lithological zones, indicating other sources of influence. The strontium isotope ratio of the DKS cold spring is closest to that of seawater, possibly reflecting the influence of nearby limestone.

Author Contributions

Conceptualization, C.-H.C.; methodology, C.-H.C. and C.-F.Y.; validation, C.-H.C. and C.-F.Y.; investigation, C.-H.C., C.-F.Y. and Y.-L.Y.; writing—original draft preparation, C.-H.C.; writing—review and editing, C.-H.C., and C.-F.Y.; visualization, Y.-L.Y.; supervision, C.-F.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Science and Technology Council, R.O.C (NSTC, Taiwan) grant number 112–2116–M–006–008–, to C.-F.Y.

Data Availability Statement

The data are contained within the article.

Acknowledgments

We thank the two reviewers and the Editor for providing us with constructive comments/suggestions in the earlier draft, which have improved this paper significantly.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 00071 g001
Figure 1. Location map showing various sampling sites of hot springs and associated geological units same formatting. Sampling locations in the TVG area (red box) shown in the upper right corner.
Figure 1. Location map showing various sampling sites of hot springs and associated geological units same formatting. Sampling locations in the TVG area (red box) shown in the upper right corner.
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Water 17 00071 g002aWater 17 00071 g002b
Figure 2. The normalized distribution patterns of REEs in hot springs collected from (A) the igneous region; (B) the metamorphic region; (C) the sedimentary region.
Figure 2. The normalized distribution patterns of REEs in hot springs collected from (A) the igneous region; (B) the metamorphic region; (C) the sedimentary region.
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Figure 3. Plot of ΣREE versus pH in Taiwan hot springs.
Figure 3. Plot of ΣREE versus pH in Taiwan hot springs.
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Water 17 00071 g004
Figure 4. Plots of (A) 87Sr/86Sr versus 1/Sr; (B) 87Sr/86Sr versus ΣREE in Taiwan hot springs.
Figure 4. Plots of (A) 87Sr/86Sr versus 1/Sr; (B) 87Sr/86Sr versus ΣREE in Taiwan hot springs.
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Figure 5. Different types of REE distribution patterns in Taiwan hot springs.
Figure 5. Different types of REE distribution patterns in Taiwan hot springs.
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Table 1. Sampling information on Taiwan hot spring water.
Table 1. Sampling information on Taiwan hot spring water.
Sample IDSample LocationGeothermal FieldType of
Exposed Rock		pHTemperature
(°C)		Water Type
CSLChungshanlo(1)andesite3.274Acid SO4
KTPKengtzuping(1)andesite2.582Acid SO4
DPDapu(1)alluvium2.299SO4-Cl
MTMa-Tsao(1)andesite2.460Acid SO4
SYKSiaoyoukeng(1)tuff2.472Acid SO4
PYPayen(1)andesite2.391Acid SO4
GSGinshan(1)alluvium2.557SO4-Cl
CCTChangchuntzu(2)marble---
CSChingshui(2)argillite6.764Na-HCO3
JSJiaosi(2)metasandstone7.358Na-HCO3
SASuao(2)slate and argillite5.521Na-HCO3
WSWenshan(2)slate and schist7.446Neutral SO4
HYHungyeh(3)slate and phyllite7.380Na-HCO3
ATAntung(3)sandstone and shale8.766SO4-Cl
KTLKuantzuling(4)silt and shale7.566Na-HCO3-Cl
DKSDakangshan(4)limestone7.421Na-HCO3-Cl
Notes: Geothermal fields: (1) Tatun Volcanic Group (TVG), (2) Central Range and Hsueshan Range, (3) Coastal Range, and (4) Western Foothill.; -: not analyzed.
Table 2. Major elements, Sr and Sr isotope compositions of Taiwan hot springs.
Table 2. Major elements, Sr and Sr isotope compositions of Taiwan hot springs.
Sample IDNaKMgAlSiCaFeSr*F−*Cl−*HCO3−*SO42−87Sr/86Sr
CSL23.410.518.356.341.450.778.10.15ND59.4ND2120.70552
KTP5.760.952.692.779.435.592.070.0313.430ND3490.70558
DP247026724996.988.229375.92.865.456320ND20700.71240
MT16.96.6513.74.963.345.73.390.11.6219.617.12340.70468
SYK14.15.1212.36.4178.349.220.60.052.69907ND11600.70601
PY15.72.98.638.1243.623.64.610.07ND55.4ND5590.70516
GS58796.610340.010394.118.10.126.1814302192540.70618
CCT2.670.777.580.012.3138.70.010.24----0.70817
CS76537.40.620.3874.51.170.210.521.633.974981.110.71495
JS19810.35.410.2119.011.40.140.063.0827.44891.110.71571
SA12.81.225.910.169.4815.60.100.93ND10.5117210.71480
WS12210.437.00.1119.31880.100.072.6116.52217900.71004
HY38813.51.890.2948.29.640.273.201.2310671471.50.71730
AT5875.180.160.6428.497.70.290.144.9772425.54220.70564
KTL844026623.90.29.321.40.253.427.443620486037.40.71066
DKS1006.1295.43.5324.42325.525.34ND492311ND0.70931
Notes: Unit: mg/L. ND: below detection limit.; -: not analyzed; * Data from Chen et al. [72].
Table 3. REEs compositions of Taiwan hot spring ( unit: ng/L).
Table 3. REEs compositions of Taiwan hot spring ( unit: ng/L).
Sample IDLaCePrNdSmEuGdTbDyHoErTmYbLuΣREE(La/Yb)NCe/Ce* Eu/Eu*
CSL300085501110585017405182160372243052315802341480144297002.151.120.81
KTP28365567.53238126.411620.213932.510215.410216.219801.811.140.83
DP11802970413174042711660011177616145058.636426.193804.661.020.7
MT60.615726.415762.827.312522.517741.813519.51402211700.280.940.93
SYK17048079.346016161.822336.926056.217123.616524.223700.730.990.99
PY4821300227134048820169012389920664692.167111174700.450.941.05
GS45.688.210.237.68.391.66.010.97.142.139.191.7116.84.372401.080.980.68
CCT0.521.140.140.610.180.040.160.030.170.040.10.020.090.023.263.441.000.76
CS1.082.060.261.110.360.080.360.050.280.060.130.020.080.015.947.540.940.7
JS0.921.640.31.220.410.120.480.10.860.20.640.090.680.17.760.920.750.79
SA0.140.240.040.20.120.030.270.050.430.130.420.060.420.092.640.170.780.58
WS0.731.280.140.520.120.020.090.020.10.020.060.010.050.013.177.370.970.65
HY5011213.757.615.33.4713.91.869.581.533.380.372.080.2728519.41.020.72
AT48.190.210.544.111.82.8411.61.7610.11.995.460.754.80.732456.850.960.74
KTL5.8141.32.6912.740.922.980.677.062.9517.64.3145.69.231580.072.500.81
DKS42190110546912933.115622.911820.549.36.0333.85.0724708.61.030.71
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Chung, C.-H.; You, C.-F.; Yeh, Y.-L. Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs. Water 2025, 17, 71. https://doi.org/10.3390/w17010071

AMA Style

Chung C-H, You C-F, Yeh Y-L. Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs. Water. 2025; 17(1):71. https://doi.org/10.3390/w17010071

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Chung, Chuan-Hsiung, Chen-Feng You, and Yi-Ling Yeh. 2025. "Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs" Water 17, no. 1: 71. https://doi.org/10.3390/w17010071

APA Style

Chung, C.-H., You, C.-F., & Yeh, Y.-L. (2025). Strontium Isotopes and Rare Earth Elements as Tracers of Water–Rock Interactions in Taiwan Hot Springs. Water, 17(1), 71. https://doi.org/10.3390/w17010071

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