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Article

Geochemical Characterization of Laminated Crystalline Crust Travertines Formed by Ca2+-Deficient Hot Springs at Sobcha (China)

by
Zhipeng Lu
1,2,
Huaguo Wen
1,2,3,4,*,
Lianchao Luo
5,
Liang Li
6 and
Ying Nie
7
1
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
2
Institute of Sedimentary Geology, Chengdu University of Technology, Chengdu 610059, China
3
CNPC Key Laboratory of Carbonate Reservoir, Chengdu University of Technology, Chengdu 610059, China
4
Key Laboratory of Deep-Time Geographical Environment Reconstruction and Application, Ministry of Natural Resources, Chengdu 610059, China
5
Department of Earth Sciences, University of Florence, 50121 Firenze, Italy
6
College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
7
School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(2), 220; https://doi.org/10.3390/min13020220
Submission received: 18 November 2022 / Revised: 29 January 2023 / Accepted: 30 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Geochemistry of Travertines and Calcareous Tufas)
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Abstract

:
Travertines formed of crystalline crust have been widely reported, but there has not been focus on their geochemical characteristics. We therefore carefully conducted a series of geochemical investigations and U-Th dating on a travertine mound mainly composed of crystalline crust from Sobcha (southwest China) to determine their geochemical features and geological implications. The Sobcha travertines dominantly consist of granular crystals and fan crystals and show δ13C from 3.4‰ to 4.9‰ V-PDB, δ18O from −26.7‰ to −23.7‰ V-PDB, and 87Sr/86Sr from 0.712458 to 0.712951. When normalized to PASS, the Sobcha travertines exhibit MREE enrichment relative to HREE and LREE, HREE enrichment relative to LREE, and positive Eu anomalies. The δ13C signatures and mother CO2 evaluation of the Sobcha travertines show that the Sobcha travertines were thermogene travertines largely receiving mother CO2 from (upper) mantle (i.e., magmatic CO2) or a mixture of soil-derived CO2 and CO2 related to carbonate decarbonation. The 87Sr/86Sr of the Sobcha travertines is out of the 87Sr/86Sr ranges of local deposits exposed at Sobcha and surrounding areas but is well matched with the mean 87Sr/86Sr of Nadi Kangri volcanic rocks which cropped out to the northeast of the studied travertines (over 20 km away). This might indicate the important role of the Nadi Kangri volcanic rocks in suppling Sr to the studied travertines, but more studies are required. The LREE depletion compared to MREE and HREE in the Sobcha travertines was interpreted to be caused by the difference in geochemical mobility between LREEs and HREEs during water–rock interaction at depth, while the MREE enrichment compared to HREE was considered to be most likely inherited from reservoir/aquifer rocks. The positive Eu anomalies of the Sobcha travertines may result from very high reservoir temperatures and/or preferential dissolution of Eu-rich minerals/rocks (especially plagioclase). The Sobcha travertine mounds displays no or very slight vertical variations in δ13C, 87Sr/86Sr, and REE patterns, indicating the compositional stability of mother CO2 and paleo-fluids. However, a significant vertical increase in δ18O was observed and was explained as the result of gradual water temperature decrease related to climate cooling, self-closure of the vents, or mound vertical growth. The findings in this study might help us better understand the deposition of crystalline crust in Ca2+-deficient hot spring systems.

1. Introduction

Travertines (i.e., thermogene/hypogean travertines) are typical terrestrial carbonate rocks/deposits associated with warm-hot springs on the earth’s surface [1]. Travertine systems are often characterized by diverse morphologies, rapid changes of lithofacies and environments, rapid precipitation rates, and the deficiency of macrophytes and faunas [1,2,3,4]. More attention has recently been paid to travertines largely owing to their great potentials in enhancing our understanding of tectonic activity [5,6,7,8,9,10,11,12,13,14,15], paleo-climate [16,17,18,19], paleo-environment [16,17,20,21], thermal fluid circulation in geothermal systems [8,22], CaCO3 polymorphism and crystallization [23,24,25], microbe-mediated carbonate precipitation [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40], simulation of engineered geological storage and leakage of CO2 [41,42], and pre-salt carbonate hydrocarbon reservoirs [43,44,45].
The lithofacies composition of travertines is quite complex [46,47], but one attractive lithofacies in many travertine systems is abiotic crystalline crust, which is formed of abundant well-packed bright calcite and/or aragonite crystals and often has a low porosity. Similar lithofacies in carbonate deposits from other environments (e.g., marine, lake) is commonly treated as diagenetic products. In contrast, abiotic crystalline crust in travertine systems is commonly considered to be primary deposits mainly formed by rapid passive CO2 degassing [47,48,49,50,51,52,53]. However, calcite/aragonite crystals constituting abiotic crystalline crust shows various morphologies and sizes, such as dendritic crystals over 1 cm high, and raft-like crystals more than 5 cm long [49,50,51,52,54]. Therefore, abiotic crystalline crust is often further subdivided according to the morphology and size of the main fabrics. Gandin and Capezzuoli [47], for example, split abiotic crystalline crust in tufa and travertine systems up into five subtypes: feather-like/dendritic crystals, fan/ray crystals, banded palisade crystals, and foam rock and calcite rafts, which can be formed under numerous deposition conditions.
Many works on the sedimentological, petrological, and mineralogical characteristics of abiotic crystalline crust have been carried out over the past few decades [47,48,49,50,51,52,55,56,57]. Geochemical characteristics of travertines have also been investigated in some studies [58,59,60,61]. However, most of them only focused on the stable carbon and oxygen isotope compositions of travertines [52,61,62,63,64,65,66,67]. The 87Sr/86Sr and rare-earth elements signatures of travertines were poorly examined and interpreted in past studies. This study thus carefully characterized the geochemical compositions (δ13C, δ18O, 87Sr/86Sr, and rare-earth elements) of laminated crystalline crust travertines at Sobcha, Tibet, southwestern China. Based on the geochemical results, this study attempted to determine the genesis, (paleo-)fluid source(s), and fluid evolution of the Sobcha travertine system. Some geochronological data were also obtained in this study. Considering that the Sobcha travertine system was likely deposited by hot springs deficient in Ca2+ (Ca2+ concentrations of active hot springs at Sobcha are 0.17 mM) [68], the findings obtained in this study might aid in the geochemical interpretation of crystalline crust travertines, especially those formed in Ca2+-deficient hot spring systems.

2. Geological Setting

Sobcha is located in central Tibet, southwestern China (32°31’36.7” N, 89°56’39” E; altitude: ca. 4735 m) (Figure 1) and lies to the north of Qixiang Co Lake (ca. 2 km). The sedimentary sequences cropping out at Sobcha and surrounding areas mainly include (1) Upper Triassic–Lower Jurassic Sobcha Formation (also known as Xiaochaka Formation and Suobucha Formation) marine deposits mainly consisting of limestone, (2) Lower Jurassic Quse Formation and Middle Jurassic Sewa Formation marine deposits mainly composed of clastic deposits (e.g., shale, sandstone, and claystone), and (3) Neogene and Quaternary terrestrial clastic deposits [69,70,71]. A series of E–W faults developed at Sobcha and surrounding areas. The studied travertines were formed on the Upper Triassic–Lower Jurassic Sobcha Formation marine deposits and are situated very close to two faults (Figure 1).
The studied Sobcha travertines are fossil travertine deposits, but modern hot springs and associated deposits, though very limited, are still present near the studied travertines. Liao [68] called this place Quse or Sobcha Hot Fountain. Specifically, this place is a hot spring area composed of 12 vents [68]. The location information of these hot spring vents given by Liao [68] is very close to that of the Sobcha travertines (89°56′25″ E, 32°31′35″ N; altitude: 4770 m). A pioneering study by Liao [68] showed that most of the hot springs at Sobcha have vent temperatures from 45 °C to 50 °C and only two hot springs at Sobcha show vent temperatures near 60 °C. The water composition of one hot spring was also reported by Liao [68]: pH = 8.62, Ca2+ concentration = 0.17 mM, Mg2+ concentration = 0.69 mM, K+ concentration = 0.87 mM, Na+ concentration = 22.17 mM, HCO3− concentration = 14.95 mM, SO42− concentration = 1.58 mM, Cl− concentration = 3.55 mM, and CO32− concentration = 1.68 mM. Na+ is absolutely dominant in the cation composition. Thus, the studied fossil travertines were anticipated to be formed by Ca2+-deficient hot springs, similar to those in Tengchong (China) [72], Lake Bogoria (Kenya) [73], and Waikite (New Zealand) [74].

3. Methods

Fifty-eight fossil travertines samples were collected in the field. Specific sampling sites for geochemical analysis are shown in Figure 2D. To evaluate their mineralogical composition, thirty powder samples were made using an agate motor and a pestle and were then analyzed with a DX-2700 X-ray diffractometer (XRD) (Cu-Kα radiation; 2θ from 5° to 60°). Fifty-eight thin sections were prepared and were observed with a Nikon LV100POL polarizing microscope to check the petrographic and mineralogical features of the Sobcha travertines. Both the XRD and thin section analyses were conducted at Chengdu University of Technology, China.
δ13C, δ18O, 87Sr/86Sr, and concentrations of some trace elements (including rare-earth elements, manganese, strontium, zirconium, copper) of the Sobcha travertines were examined to determine the genesis of the travertines and the characteristics of paleo-fluids. Travertine samples used for geochemical analyses were ground into powder using an agate motor and a pestle and the powder samples were then sifted using a 200 mesh sieve. δ13C and δ18O signatures were determined for thirty samples using a Thermo Fisher Scientific DELTA V Advantage isotope ratio mass spectrometer and their values were reported relative to the Vienna Pee Dee Belemnite standard (V-PDB). The analytical precision is commonly better than 0.2‰ for δ13C and 0.3‰ for δ18O. 87Sr/86Sr values were determined for thirty travertine samples using a Thermo Fisher Scientific Triton Plus mass spectrometer. The final 87Sr/86Sr results were corrected by assuming a non-radiogenic 86Sr/88Sr isotopic ratio of 0.1194. Rare-earth elements (i.e., REE), Mn, Sr, Zr, and Cu concentrations were determined for twenty travertine samples using a Jena Plasma Quant MS ICP-MS. U-Th dating of three travertine samples collected from the bottom (Sample 1), middle (Sample 21), and top (Sample 30) of the sampling profile at Sobcha, respectively, was also conducted using a Neptune Plus MC-ICP-MS. The δ13C-δ18O analyses, 87Sr/86Sr analyses, trace element analyses, and U-Th dating were completed at Yangtze University (China), Chengdu University of Technology (China), Beijing Createch Testing Technology Co., Ltd. (Beijing, China), and the Institute of Geology and Geophysics (Chinese Academy of Sciences), respectively.
In this study, rare-earth elements were divided into three groups: light REE (i.e., LREE: La, Ce, Pr, and Nd), middle REE (i.e., MREE: Sm, Eu, Gd, Tb, Dy, and Ho), and heavy REE (i.e., HREE: Er, Tm, Yb, and Lu) [75]. To better interpret REE characteristics of the Sobcha travertines, REE of Post-Archean Australian Shale (PAAS) [76] was utilized to normalize the REE values of the studied travertines. Additionally, anomalies of Eu were presented as (Eu/Eu*)N, and were calculated using the following equations: (Eu/Eu*)N = EuN/(SmN2 × TbN)1/3 [77]. The relative enrichments of LREE, MREE, and HREE were presented as (Pr/Yb)N (LREE compared to HREE), (Pr/Tb)N (LREE compared to MREE), and (Tb/Yb)N (MREE compared to HREE) [75].

4. Results

4.1. Description of the Sobcha Travertines

The studied travertine system is a domical mound system developed on a slightly inclined slope (Figure 2), morphologically similar to the travertine mounds found in Chusang (southwestern China) [78]. It is characterized by an asymmetric outline with a nearly flat surface at the upslope side and a relatively steep surface at the downslope side. An orifice which had been nearly fully sealed was found on the top surface of the mound (Figure 2C). Determining the exact scale of the mound system was handicapped by the fact that some of the travertine mound was covered by clastic deposits and/or soils and that fluvial incision eroded parts of the travertine mound (Figure 2A). However, the residual travertines were observed to be at least over twenty meters wide. The fluvial incision was largely caused by a north-flowing river (i.e., Sobcha River). Due to the fluvial incision, a four-meter-high travertine profile was exposed, providing a good place for our field observation and sampling.

4.2. Petrology and Mineralogy

The outcropping travertines at Sobcha show great laminated structures, which are characterized by the alternation of wavy dark laminae and wavy light laminae (Figure 2B). Both dark laminae and wavy light laminae have highly variable thicknesses, but in general, light laminae are thicker and more dominant. Furthermore, the Sobcha travertines are consolidated and show visible large calcite crystals ranging from 1 to 10 mm in hand specimens. Large cavities (up to 2 cm in diameter) are also visible in the outcrop, but their development is very limited in fresh samples. Therefore, these cavities are largely secondary (probably generated by the dissolution of meteoric water or temporary flood water).
According to the lithofacies classification of Gandin and Capezzuoli [47], most of the Sobcha travertines are identified as (abiotic) crystalline crust. In this study, the Sobcha travertines were termed ‘laminated crystalline crust’ to clearly indicate their textural and petrological characteristics. Apart from the laminated crystalline crust, a small amount of clotted peloidal boundstone was also observed in the studied travertine system. XRD and thin-section analyses shows the predominance of calcite in both the laminated crystalline crust and the clotted peloidal boundstone.
The clotted peloidal boundstone is formed of dark micrite peloids and sparite (Figure 3A), while components constituting the laminated crystalline crust include fan crystals and granular crystals (Figure 3B,C). The fan crystals range from 0.2 to 2 mm long and usually develop as thin layers ca. 30 mm thick (Figure 3B,C). The granular crystals are the most widely developed crystals in the Sobcha travertines and have highly variable sizes between 0.1 and 0.5 mm in diameter (Figure 3B). The granular crystals have a more irregular morphology and are subhedral to anhedral in shape (Figure 3B).

4.3. δ13C, δ18O, and 87Sr/86Sr

The δ13C, δ18O, and 87Sr/86Sr signatures of the Sobcha travertines are listed in Table S1. The Sobcha travertines show δ13C from 3.4‰ to 4.9‰ V-PDB (average = 4.0‰ V-PDB). δ18O of the Sobcha travertines has a range larger than δ13C (from −26.7‰ to −23.7‰ V-PDB, average = −25.6‰ V-PDB). A positive δ13C-δ18O correlation was found in the Sobcha travertines, but the correlation is not very strong (R2 = 0.50, n = 30) (Figure 4). Additionally, it is notable that there is a gradual positive δ18O shift from the bottom to the top of the studied profile, although a few anomalous values are also present (Figure 5A). Such increase was also found in δ13C of the travertines, but the δ13C excursion is very slight (Figure 5B). Unlike visible δ13C and δ18O variations, 87Sr/86Sr of the Sobcha travertines are nearly unchanged and show a very narrow range from 0.712458 to 0.712951 (average = 0.712737, n = 30) (Figure 5C).

4.4. U-Th Dating

The U-Th dating results of three travertine samples (i.e., Samples 1, 21, and 30) are listed in Table S2. Samples 1, 21, and 30 were collected from the bottom, middle, and top of the sampling profile at Sobcha, respectively. Sample 1 has a measured depositional age of 8981 ± 420 yr BP, while Sample 21 exhibits a depositional age of 6502 ± 1542 yr BP. Sample 30 is the youngest of the three samples and its corrected age is 1785 ± 642 yr BP. The dating results show that travertine deposition of the studied travertine mound system began, at least, in the early Holocene and ceased in the late Holocene.

4.5. Rare-Earth Elements, Mn, Sr, Zr, and Cu

Rare-earth elements, Mn, Sr, Zr, and Cu concentrations and some main calculated geochemical parameters of the Sobcha travertines are listed in Tables S3 and S4. ΣREE of the Sobcha travertines is highly variable (ranging from 0.61 to 23.05 µg g−1, average = 8.04 µg g−1), but most of the samples show ΣREE below 13.00 µg g−1. The Sobcha travertines show significant MREE and HREE enrichment relative to LREE, as shown by their very low (Pr/Tb)N (from 0.10 to 0.55, mostly between 0.10 and 0.25, average = 0.16) and (Pr/Yb)N (from 0.12 to 0.60, commonly between 0.12 and 0.24, average = 0.19). (Tb/Yb)N ratios of the Sobcha travertines vary from 1.09 to 1.21 (average = 1.14), indicating the Sobcha travertines are weakly MREE-enriched compared to HREE.
Elemental anomaly calculations show that the Sobcha travertines have (Eu/Eu*)N ratios range from 1.05 to 1.68 (average = 1.36) and most of the (Eu/Eu*)N data are between 1.28 and 1.43. This reflects unneglectable positive Eu anomalies in the Sobcha travertines.
The Sobcha travertines have very low Zr and Mn concentrations and Mn/Sr ratios. Their Zr concentrations are from 0.38 to 3.24 µg g−1 (average = 1.73 µg g−1), but most of the values are lower than 2 µg g−1. Mn concentrations of the Sobcha travertines are between 279 and 1280 µg g−1 and have an average value of 825.75 µg g−1. The calculated Mn/Sr ratios are within a small range from 0.53 to 2.48 (average = 1.63). Cu concentrations of the Sobcha travertines are between 0.19 and 1.17 µg g−1 and have an average value of 0.52 µg g−1.

5. Interpretation and Discussion

5.1. Interpretation of δ13C: Source of Mother CO2

δ13C analyses of travertines are good tools in the determination of mother CO2 sources [52,62,79]. δ13C of mother CO2 (i.e., δ13Cmother-CO2) of spring-related carbonates have been used to divide them into thermogene travertine and meteogene travertine [79,80,81,82,83]. For example, according to Pentecost and Viles [83], δ13C of thermogene travertines ranges from −4‰ to 8‰ V-PDB, whereas δ13C of meteogene travertines ranges from −11‰ to 0 V-PDB. δ13C values of the Sobcha travertines are between 3.4‰ and 4.9‰ V-PDB, which is obviously in the δ13C range of thermogene travertines. In addition, Figure 6 also shows that the Sobcha travertines belongs to hypogean CATT (calcitic or aragonitic travertine and tufa) [60]. However, spring-related carbonates with δ13C in the travertine range may show highly different CO2 sources. Thus, δ13Cmother-CO2 of the Sobcha travertines were calculated and compared with δ13C of potential CO2 sources to determine their genesis and mother CO2 source(s).
The empirical equation developed by Panichi and Tongiorgi [84] was adapted to calculate δ13Cmother-CO2 of the Sobcha travertines. The calculated δ13Cmother-CO2 varies from −6.5‰ and −4.6‰ V-PDB (Table S1). A simple comparison in Figure 7 shows that the calculated δ13Cmother-CO2 overlaps the δ13C range of mantle-derived CO2, and is significantly different from δ13C of soil-related CO2 and δ13C of marine carbonate rocks in the study area and surrounding areas [69,85,86,87,88]. This might be indicative of the close relationship between the Sobcha travertines and mantle-derived CO2. Such magmatic CO2-containing volatiles has been found in hot springs from Naqu (close to the study area) and was interpreted as the product of magma bodies in the shallow crust [89,90]. Unfortunately, detailed analyses of hot spring gases from the study area have not been performed. Thus, such a mantle-derived CO2 origin is possible but is not the only interpretation.
An alternative explanation of the similarity between the calculated δ13Cmother-CO2 range and δ13C of mantle-derived CO2 is underground CO2 mixing. Apart from magmatic CO2, there are at least two main CO2 sources for travertine systems: soil CO2 (C3 or C4 plants) and carbonate-related CO2 (either by the dissolution or decarbonation of carbonate rocks) [82]. In our study area, the distribution of C4 plants is very limited, probably due to its high altitude [91]. Therefore, the plants in the study area are mainly C3 plants. For the carbonate-related CO2, given the low Ca2+ concentration (0.17 mM) and high HCO3− concentrations (14.95 mM) of the modern spring in the study area [68], the dissolution of carbonate rocks is considered to only supply minor CO2 to the spring waters and travertines at Sobcha. However, this cannot exclude the CO2 contribution of carbonate decarbonization because this process can provide CO2 to the spring water (in other words, elevating HCO3− concentration) without increasing the concentrations of Ca2+ and Mg2+. If there was an underground mixing between soil CO2 and carbonate-related CO2, their mixture may yield similar δ13C to mantle-derived CO2. Thus, mixing, if present, might happen between soil CO2 (C3 plants) and CO2 related to carbonate decarbonation.

5.2. Interpretation of 87Sr/86Sr: Sr Source Rocks

The 87Sr/86Sr analysis of travertines can be used to determine their Sr sources [72]. With respect to the studied travertines, all the rocks exposing in the study area are their potential Sr source rocks. A simple comparison between 87Sr/86Sr of the studied travertines and 87Sr/86Sr of potential Sr source rocks [92,93,94] was made and is given in Figure 8. The comparison shows that the 87Sr/86Sr range of the Sobcha travertines is significantly higher than 87Sr/86Sr ranges of carbonate rocks and clastic rocks exposed in/near the study area (mainly the Quse Formation) and Cambrian to Cenozoic marine carbonates (Figure 8). This suggests that the studied travertines must acquire more radiogenic Sr from other Sr source rocks, instead of from the local rocks cropping out in the study area. Interestingly, the studied travertines display 87Sr/86Sr very close to the average 87Sr/86Sr value of Nadi Kangri volcanic rocks. However, the nearest exposed Nadi Kangri volcanic rocks are located in the Bilong Co area and are ca. 20 km away from the study area [95]. Thus, simply treating the Nadi Kangri volcanic rocks as the main Sr source rocks of the studied travertines is not very appropriate. Conclusively, further studies are required to uncover the main Sr source rocks of the Sobcha travertines.

5.3. Interpretation of REE Patterns and Eu Anomalies

REE of carbonate rocks might not be pristine, because they can be easily contaminated by other materials, such as clastic detritus [96,97,98,99,100,101,102,103] and Fe-Mn (oxyhydr) oxides [104] during or after deposition. Thus, prior to the analysis of REE of carbonates, a contaminant evaluation is often required. The Sobcha travertines shows Zr concentrations lower than 4 μg g−1 (Table S3). This indicates that there is no significant contamination from clastic detritus [98]. In addition, the Sobcha travertines display no Cu–ΣREE correlations and have very low Mn concentrations (279 to 1660 μg g−1) (Table S3, Figure 9), reflecting that the contamination of Fe-Mn (oxyhydr)oxides is not important [105]. The REE modification of post-depositional processes can be also neglected because the Mn/Sr ratios of the Sobcha travertines are all less than 3 (Table S4) [106,107,108]. These show that our REE data are not significantly contaminated and can provide original geological information.
There are two striking REE characteristics of the Sobcha travertines: (1) MREE-enrichment relative to both HREE and LREE and HREE-enrichment relative to LREE, and (2) positive Eu anomalies (1.28 and 1.43; Figure 10 and Table S4). There are two main factors which might influence REE patterns of the studied travertine: solution complexation and REE of aquifer rocks. Paleo-fluids depositing the Sobcha travertines might be compositionally similar to active hot springs at Sobcha (i.e., bicarbonate- and carbonate-rich) [68]. Such fluids could form strong HREE-complexes (i.e., preferential HREE incorporation into fluids) and the resulting fluids and associated deposits would be HREE-enriched compared to REE of their aquifer rocks [109]. However, it is strange that although the Sobcha travertines are HREE-enriched compared to LREE, their HREE are depleted compared to MREE. This indicates that their MREE-enrichment was largely inherited from aquifer rocks, instead of caused by solution complexation. However, this does not mean that the influence of solution complexation on the REE pattern of the Sobcha travertines is very weak. It is here believed that solution complexation might indeed cause the HREE-enrichment in the Sobcha travertines, but such HREE-enrichment did not fully mask the aquifer rock REE information recorded in the travertines (i.e., MREE-enrichment). In Figure 11 the REE patterns of the Sobcha travertines are compared with those of rocks cropping out in the study area, such as shale, calci-mudstone, and marl of the Quse Formation and Nadi Kangri volcanic rocks (including basalt, rhyolite, dacite, and tuff) [70,110,111,112,113,114,115]. It is clear that none of the surface rocks display MREE-enrichment, excluding the possibility that these rocks are aquifer rocks. Such inference is consistent with the results of the 87Sr/86Sr analyses in Section 5.2 because aquifer rocks were often considered as predominant Sr source rocks [116,117,118].
Positive Eu anomalies in marine carbonate rocks are often indicative of a hydrothermal origin of their parent fluids [75,119]. At high temperatures (often >250 °C), Eu might be fractionated from other REE because of the reduction of Eu into Eu2+ [120], which would finally cause Eu enrichment in high-temperature fluids. As a result, carbonate rocks deposited from these fluids, or their cooled products might show positive Eu anomalies. At Sobcha, the temperature of its reservoir(s) has not been investigated. However, geothermal systems with reservoir temperatures over 250 °C have been found in Central Tibet (e.g., Yangbajing geothermal field) [121]. Therefore, it is possible that positive Eu anomalies of the Sobcha travertines might be related to undiscovered high-temperature reservoirs beneath Sobcha. An alternative explanation of the positive Eu anomalies in the Sobcha travertines might be the preferential dissolution of Eu-rich minerals/rocks (especially plagioclase). Indeed, such a process is not uncommon in groundwater systems [122]. Therefore, positive Eu anomalies of the studied travertines might be caused by high-temperature geothermal reservoir(s), preferential dissolution of Eu-rich minerals/rocks, or both, and more studies are still necessary in this region.

5.4. Evolution of Paleo-Fluids

Travertine deposition may be affected by various factors, such as tectonic activity, climate, hydrodynamics, and hydrochemistry [10,12,17,123] and is thus not always steady. For example, some of these complex controlling factors (either internal or external), such as climate change, may lead to the changes in the flow path and/or discharge. However, changes in (paleo-)fluids (e.g., the source and physicochemical properties of the mother water) might be reflected by the geochemical features of travertine deposits, such as δ13C, 87Sr/86Sr, and REE. The δ13C of the Sobcha travertines shows a great stability (Table S1, Figure 5B). The small range of δ13C indicates that the CO2 source of the Sobcha travertines did not change. The 87Sr/86Sr and REE signatures of the Sobcha travertines also display little changes in the sampling profile (Table S1 and Figure 5C), indicating the source and physicochemical stability of the mother water of the Sobcha travertines
The δ18O of the Sobcha travertines has a larger variation than δ13C and shows a striking gradual increase from bottom to top of the sampling profile (Table S1 and Figure 5A). The δ18O signature of travertines is important to the recovery of paleo-temperature of the mother water in travertine deposition systems [52,62,79]. We were unable to recover the exact temperature changes of the mother water depositing the Sobcha travertines. However, according to common traditional oxygen isotope thermometers, there is an inverse relationship between δ18O of carbonate deposits and water temperature [124]. Thus, the positive δ18O excursion in the studied travertine profile of Sobcha, at least, indicates that paleo-fluid temperature forming the Sobcha travertines decreased gradually. However, the factors that cause such a δ18O increase or temperature decrease remain unknown.
Climate (mainly atmospheric temperature and rainfall) may influence the temperature of mother water of travertines [63,64,125]. The Sobcha travertine deposition began, at least, in the early Holocene and ceased in the late Holocene. From the early Holocene to the middle Holocene and to the late Holocene, the southern Qinghai–Tibet Plateau was generally in a climate background from drought to humid and to drought, as recorded in the δ18O composition of cave deposits [126,127,128]. In the Holocene, the temperature of the Qinghai–Tibet Plateau also experienced a gradual increase from the early to the middle period and a gradual decrease from the middle to the late period [126]. In general, the trend of δ18O during the deposition period of the Sobcha travertines are similar to that of δ18O recorded in cave deposits of the southern Qinghai–Tibet Plateau at the same time. This seems to indicate that the δ18O of the Sobcha travertines, or rather the temperature of the mother water, is affected by climate.
In addition to climate, a gradual decrease in the discharge of the mother water may also lead to a faster decrease in the temperature of the mother water (assuming the air temperature is stable). This would in turn lead to a gradual increase in the δ18O of the deposits. The gradual decrease in the discharge of the mother water of the Sobcha travertines may also be due to the gradual drought of the climate during the deposition period. However, the self-closure of the vents and mound vertical growth might also lead to a gradual decrease in water discharge of mound springs. For the Sobcha travertines, we found that the paleo-vent was completely closed due to the growth of deposit (Figure 2C). Therefore, it is possible that water discharge of paleo-fluids depositing the Sobcha travertines gradually decreased due to the gradual closure of the vent(s) and/or mound vertical growth.

6. Conclusions

Through the geochemical and geochronological study on the Sobcha fossil travertine mound, we mainly draw the following conclusions:
(1)
The Sobcha travertines are dominantly composed of laminated crystalline crust mainly composed of granular crystals and fan crystals. In addition, a small amount of clotted peloidal boundstone was also observed. Calcite has an absolute predominance in the Sobcha travertines.
(2)
δ13C and δ18O analyses show that the Sobcha travertines belong to thermogene travertines, and its parent CO2 may be derived from mantle-derived CO2 or the mixture of soil-derived CO2 and CO2 related to carbonate decarbonation.
(3)
The Sobcha travertines display 87Sr/86Sr very close to the average 87Sr/86Sr value of Nadi Kangri volcanic rocks. However, further studies are required to uncover the main Sr source rocks of the Sobcha travertines.
(4)
For the PAAS-normalized REE patterns of the Sobcha travertines, the LREE depletion relative to HREE of the Sobcha travertines may be due to the differences in geochemical mobility between LREEs and HREEs during water–rock interaction at depth, while the MREE enrichment of the Sobcha travertines may be inherited from aquifer rocks. Positive Eu anomalies were also observed in the Sobcha travertines and may result from the hydrothermal property of the mother water and/or preferential dissolution of Eu-rich minerals/rocks.
(5)
Travertine deposition in the studied travertine mound began, at least, in the early Holocene and ceased in the late Holocene. During the whole deposition period, the source of CO2 and the source of the mother water of Sobcha travertines remained stable. However, δ18O of the studied travertines gradually decreased, probably because of climate drying, self-closure of the vents, or mound vertical growth.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min13020220/s1. Table S1: δ13C, δ18O, and 87Sr/86Sr values of the Sobcha travertines and their calculated δ13Cmother-CO2 (i.e., δ13C of mother CO2). δ13Cmother-CO2 was evaluated using the equation from Panichi and Tongiorgi [84]. Table S2: U-Th dating results of the Sobcha travertines. Table S3: Concentrations (µg g−1) of trace elements in the travertine samples from Sobcha. Table S4: Mn/Sr, enrichment indexes, and elemental anomalies of the studied travertines from Sobcha.

Author Contributions

Conceptualization, Z.L., H.W. and L.L. (Lianchao Luo); methodology, Z.L., H.W. and L.L. (Lianchao Luo); software, Z.L. and L.L. (Lianchao Luo); validation, Z.L., H.W. and L.L. (Lianchao Luo); formal analysis, Z.L. and L.L. (Lianchao Luo); investigation, Z.L., L.L. (Liang Li), and Y.N.; resources, H.W.; data curation, H.W.; writing—original draft preparation, Z.L.; writing—review and editing, Z.L., H.W., L.L. (Lianchao Luo), L.L. (Liang Li), and Y.N.; visualization, Z.L. and L.L. (Lianchao Luo); supervision, H.W.; project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 41972116 and 41572097 to H. Wen).

Data Availability Statement

Not applicable.

Acknowledgments

We are grateful to Mingshi Fen (Chengdu University of Technology) for his help in SEM analyses, Di Yang (Chengdu University of Technology) for her help in Sr isotope analyses, and Miao Deng (Chengdu University of Technology) for his help in XRD analyses, Xiugen Fu (Southwest Petroleum University) for his help in field survey, and Xuefeng Wang (Uranium chronology laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences) for his help in U-Th dating analyses.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) The location of Sobcha in the Tibetan Plateau, southwestern China. (B) Simplified geological map of the study area (modified from Fu et al. [70]).
Figure 1. (A) The location of Sobcha in the Tibetan Plateau, southwestern China. (B) Simplified geological map of the study area (modified from Fu et al. [70]).
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Figure 2. (A) General view of the studied travertine mound at Sobcha. (B) Nearly horizontal laminated abiotic crystalline crust travertines; (C) Paleo-vent situated at the top of the studied travertine mound (its location can be found in panel (A)). (D,E) Schematic diagram of sampling sites of the Sobcha travertines for geochemical analysis. The red dots represent the sample site for geochemical analysis, the number represents the sample number, and the same number represents the same sample.
Figure 2. (A) General view of the studied travertine mound at Sobcha. (B) Nearly horizontal laminated abiotic crystalline crust travertines; (C) Paleo-vent situated at the top of the studied travertine mound (its location can be found in panel (A)). (D,E) Schematic diagram of sampling sites of the Sobcha travertines for geochemical analysis. The red dots represent the sample site for geochemical analysis, the number represents the sample number, and the same number represents the same sample.
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Figure 3. Thin-section microphotographs of the Sobcha travertines (A–C); plane-polarized light. (A) Clotted peloidal boundstone and fan crystals. (B) Fan crystals and granular crystals. (C) Fan crystals.
Figure 3. Thin-section microphotographs of the Sobcha travertines (A–C); plane-polarized light. (A) Clotted peloidal boundstone and fan crystals. (B) Fan crystals and granular crystals. (C) Fan crystals.
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Figure 4. Bivariant graph of δ18O and δ13C of travertines from Sobcha.
Figure 4. Bivariant graph of δ18O and δ13C of travertines from Sobcha.
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Figure 5. (A–C) Vertical variations of δ18O, δ13C, and 87Sr/86Sr of the Sobcha travertines in the studied profile.
Figure 5. (A–C) Vertical variations of δ18O, δ13C, and 87Sr/86Sr of the Sobcha travertines in the studied profile.
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Figure 6. δ13C and δ18O composition for the Sobcha travertines. δ13C ranges for thermogene and meteogene travertines were from Pentecost and Viles [64]. δ13C and δ18O ranges of different CATT (i.e., calcitic or aragonitic travertine and tufa) were from Teboul et al. [65].
Figure 6. δ13C and δ18O composition for the Sobcha travertines. δ13C ranges for thermogene and meteogene travertines were from Pentecost and Viles [64]. δ13C and δ18O ranges of different CATT (i.e., calcitic or aragonitic travertine and tufa) were from Teboul et al. [65].
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Figure 7. Calculated δ13Cmother-CO2 of the Sobcha travertines in comparison with δ13C of potential carbon sources. δ13C ranges of mantle-derived CO2 and CO2 related to C3 plants and C4 plants were from Vogel [69] and Prokoph et al. [68]. δ13C ranges of carbonate rocks of the Sewa Formation and Sobcha to Quse Formation were from Yi [70] and Yi [49]; Fu et al. [67]; Yi [70].
Figure 7. Calculated δ13Cmother-CO2 of the Sobcha travertines in comparison with δ13C of potential carbon sources. δ13C ranges of mantle-derived CO2 and CO2 related to C3 plants and C4 plants were from Vogel [69] and Prokoph et al. [68]. δ13C ranges of carbonate rocks of the Sewa Formation and Sobcha to Quse Formation were from Yi [70] and Yi [49]; Fu et al. [67]; Yi [70].
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Figure 8. 87Sr/86Sr in the studied travertines in comparison with 87Sr/86Sr ranges of potential Sr source rocks: oil shales and micritic limestones of the Quse Formation (data from Fu et al. [76]), average values of Nadi Kangri volcanic rocks (data from Fu et al. [75]), and Cambrian to Cenozoic marine carbonates (data from McArthur et al. [77]).
Figure 8. 87Sr/86Sr in the studied travertines in comparison with 87Sr/86Sr ranges of potential Sr source rocks: oil shales and micritic limestones of the Quse Formation (data from Fu et al. [76]), average values of Nadi Kangri volcanic rocks (data from Fu et al. [75]), and Cambrian to Cenozoic marine carbonates (data from McArthur et al. [77]).
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Figure 9. Bivariant graph of Cu and ΣREE of travertines from Sobcha.
Figure 9. Bivariant graph of Cu and ΣREE of travertines from Sobcha.
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Figure 10. PAAS-normalized REE patterns for the travertine samples of Sobcha.
Figure 10. PAAS-normalized REE patterns for the travertine samples of Sobcha.
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Figure 11. Comparison of PAAS-normalization patterns between Sobcha travertines and potential source rocks: basalt, rhyolite, and dacite of the Nadi Kangri volcanic rocks (data from Fu et al. [93]); tuff of the Nadi Kangri volcanic rocks (data from Wang et al. [96]; Fu et al. [97]; Wang et al. [98]); shale of the Quse Formation (data from Fu et al. [50]); mudstone of the Quse Formation (data from Nie et al. [95]); and marl of the Quse Formation (data from Fu et al. [94]).
Figure 11. Comparison of PAAS-normalization patterns between Sobcha travertines and potential source rocks: basalt, rhyolite, and dacite of the Nadi Kangri volcanic rocks (data from Fu et al. [93]); tuff of the Nadi Kangri volcanic rocks (data from Wang et al. [96]; Fu et al. [97]; Wang et al. [98]); shale of the Quse Formation (data from Fu et al. [50]); mudstone of the Quse Formation (data from Nie et al. [95]); and marl of the Quse Formation (data from Fu et al. [94]).
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Lu, Z.; Wen, H.; Luo, L.; Li, L.; Nie, Y. Geochemical Characterization of Laminated Crystalline Crust Travertines Formed by Ca2+-Deficient Hot Springs at Sobcha (China). Minerals 2023, 13, 220. https://doi.org/10.3390/min13020220

AMA Style

Lu Z, Wen H, Luo L, Li L, Nie Y. Geochemical Characterization of Laminated Crystalline Crust Travertines Formed by Ca2+-Deficient Hot Springs at Sobcha (China). Minerals. 2023; 13(2):220. https://doi.org/10.3390/min13020220

Chicago/Turabian Style

Lu, Zhipeng, Huaguo Wen, Lianchao Luo, Liang Li, and Ying Nie. 2023. "Geochemical Characterization of Laminated Crystalline Crust Travertines Formed by Ca2+-Deficient Hot Springs at Sobcha (China)" Minerals 13, no. 2: 220. https://doi.org/10.3390/min13020220

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