Geofluids
Volume 2018 (2018), Article ID 1985784, 12 pages
https://doi.org/10.1155/2018/1985784

Research Article

Origin of Boron and Brine Evolution in Saline Springs in the Nangqen Basin, Southern Tibetan Plateau

Ji-long Han,1,2,3 Feng-qing Han,1,3 Syed-Asim Hussain,2,3 Wen-yu Liu,² Xiu-qing Nian,² and Qing-fei Mao²

¹Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China
²University of Chinese Academy of Sciences, Beijing 100049, China
³Key Laboratory of Salt Lake Geology and Environment of Qinghai Province, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, Qinghai 810008, China

Correspondence should be addressed to Feng-qing Han; hanfq@isl.ac.cn

Received 11 April 2017; Revised 8 August 2017; Accepted 8 November 2017; Published 21 January 2018

Academic Editor: Ferenc Molnar

Copyright © 2018 Ji-long Han et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The Nangqen Basin is a typical shearing-extensional basin situated in the hinterland of the Tibetan Plateau. It contains abundant saline spring resources and abnormal trace element enrichments. The hydrochemical molar ratios (Na/Cl, B/Cl, and Br/Cl), H-O isotopes, and B isotopes of the saline spring were systematically measured to describe the evolution of brines and the origin of the boron. The sodium chloride coefficient of the water samples in this area is around 1.0 or slightly greater, which is characteristic of leached brines; the highest B/Cl value is 4.25 (greater than that of seawater). The Na/Cl, B/Cl, and Br/Cl values of the springs are clear indicators of a crustal origin. The ¹⁸O values of the spring waters range from −12.88 to −16.05, and the D values range from −100.91 to −132.98. Meanwhile the B content and B isotopes in the saline springs are in the ranges of 1.00 to 575.56 ppm and +3.55 to +29.59, respectively. It has been proven that the saline springs in the Nangqen Basin are a type of leached brine, suggesting that the saline springs have a terrestrial origin. The ¹¹B-B characteristics of the springs are similar to those observed in the Tibetan geothermal area, indicating that these two places have the same B source. Moreover, they have a crustal origin (marine carbonate rocks and volcanic rocks) instead of a deep mantle source.

1. Introduction

The Nangqen Basin is located within the area bounded by 96° 00′-97° 00′ E and 32° 00′-32° 40′ N and is a typical Tertiary pull-apart basin on the northeastern Qinghai-Tibetan Plateau. It was formed between the Late Eocene and Early Oligocene and the Eogene sandstone-conglomerate red beds are widely distributed within the basin. The evolutionary history in the area includes an early stage extrusion thrust foreland basin, a middle stage strike-slip pull-apart basin, and a late stage extrusion strike-slip foreland basin [1, 2]. Previous research on the Nangqen Basin focused on the sedimentary characteristics of the basin fill [3–5], the basin’s structural evolution [6], and the potassium-rich volcanic rocks [7]. However, little work has been done on the saline springs in this area.

According to our preliminary work, the terrain of the Nangqen Basin is very steep with a series of northwest-southeast fault zones. This terrain results in exposing of many saline springs. Chemical analysis of the springs indicates that they are rich in Na⁺, Ca²⁺, K⁺, Mg²⁺, Cl⁻, and with trace amounts of Br⁻ Li⁺ and boron (B). The saline springs in the study area are a source of table salt, but they also produce a small amount of Glauber salt. However, the formation and material sources of these salts have not been studied. Investigation of the tracer element B and its isotopes (¹¹B) in the saline springs can help determine the formation environment and chemical source of these saline springs.

B is representative of the crustal elements [17]. It is a relatively lighter element with an atomic number of five, and it has two stable isotopes; the abundances of ¹⁰B and ¹¹B are 19.82% and 80.18%, respectively [18]. B and its isotopes are good tracers of the mass transfer processes in terrestrial systems [19–23].

B is highly soluble in aqueous environments and is highly reactive [24–26]. The large relative mass difference between the two isotopes (¹¹B and ¹⁰B) and the different properties of the two dominant aqueous species of boron (i.e., B(OH)₃ and ) lead to a significant isotopic fractionation of B [10, 27]. Due to their soluble and incompatible character, the unique geochemical characteristics of B isotopes are widely used to assess numerous key geological processes, such as dehydration and metamorphism during slab subduction, ancient oceanic pH levels, the evolution of the formation of continental crust [28–33], magmatism and the formation of hydrothermal ore deposits [25, 26, 34, 35], the causes of anthropogenic contamination, wastewater recharge [36–38], the origin of salt lakes and groundwater [27, 39, 40], sedimentary environment, and water-rock interactions [41, 42].

Some studies have focused on the B isotopic composition of brines from different salt lakes in Inner Qinghai, Xinjiang, and the Tibetan Provinces in western China. These studies reported that B values vary in different regions [27, 43] and even vary among the salt lakes in the Qaidam Basin in Qinghai Province [44–48]. However, no studies have been conducted on saline springs of Nangqen Basin.

The purpose of this study is to delineate the geochemical origins of the saline springs and to use B isotopes of the saline springs as a tracer to determine their solute source and the geochemical processes that formed them.

2. Geology and Tectonic Setting

A continental collision between the Indian and Eurasian plates took place about 45–50 Ma ago (Middle Eocene), resulting in deformation within Tibet, and as a result, lithospheric shortening and thickening occurred [1]. This was one of the greatest geological events in the Asian continent. Subsequent subduction toward the northern Indian plate resulted in lateral resistance from the Yangtze plate, including main body of the Qinghai-Tibetan Plateau [49].

The Nangqen Basin is a typical shearing-extensional basin [1] situated in the hinterland of the Tibetan Plateau [50] and is very similar to the adjacent Dengqen and Baxoi Basins [49]. It is a lacustrine-dominated Cenozoic sedimentary basin on the eastern-central Tibetan Plateau near the headwaters of the Zha Qu/Lancangjiang River (also called the Mekong River in Thailand) with an average elevation of over 4000 m [50]. This NW-SE elongated basin is approximately 55 km long and 18 km wide [51] and is situated in the central part of the Qiangtang terrane [50]. To the east of the basin a thrust fault is in contact with Late Triassic limestone, while to the west Carboniferous and Permian organic limestone is unconformably overlain by Tertiary units [49]. It is an asymmetric continental sedimentary basin whose basement dips to the east, and its strata are thicker on the eastern side [1, 49].

This basin contains volcanic and subvolcanic rocks. The volcanic rocks of the Nangqen Basin include trachytes, latites, shoshonites, tephrites, tephriphonolites, and phonotephrites that erupted 38.7 Ma (⁴⁰Ar-³⁹Ar plagioclase age) [52]. These volcanic rocks belong to the shoshonitic series and were concluded to be formed by partial melting of an enriched mantle source [52, 53]. All of the rocks are characterized by high K₂O, as is typical of the shoshonite series [54].

Early Tertiary sandstone-conglomerate red beds are widely distributed within the basin. The sandstone-conglomerate red beds are overthrust by Late Triassic limestones in the eastern part of the basin, while the limestones are unconformably overlain by these red beds in the western part of the basin [1]. These red beds exhibit an intrusive contact, although a few of them have large-scale bedding contacts. Pull-apart basins are well developed along the Lancangjiang fault due to the intracontinental deformation conditions present in this area. Only when the basement fault reaches the upper mantle, can partial melting of deep mantle material occur, which leads to volcanism. The following stages are a series of intimately related events: the closing of the Tethys, intracontinental deformation, strike-slip movement of the Lancangjiang fault, development of the terrestrial pull-apart basin, and eruption of the potassic magmas. These lithospheric evolutionary phases provide tectonic evidence for the intraplate postcollisional environments of the volcanism. Volcanism in this area is characteristic of a typical postcollisional setting and is similar to the potassic volcanism in the Alps and the Roman province. The trace elements characteristics of these volcanics are analogous to the potassic volcanic rocks in northern Tibet, which can be ascribed to the same geotectonic environment [49].

The hydrology of the area is controlled by five rivers, that is, the Zhaqu, Ziqu, Jiqu, Baqu, and Requ Rivers (Figure 1). Until now, 7 saline springs had been located and utilized. In the course of this study, we identified several more saline springs in an untraversed region that had not previously been found or utilized because of the inconvenience of reaching the area. The exposed saline springs are predominantly located in sandstone composed of volcanic rocks of the Eocene sequences. The other lithology of the area around the saline springs (from bottom to top) is interbedded mud stone, silt stone, with an intermixed layer of gypsum, and mudstone. In the study area, a series of fractures, which are very important to the formation and development of the basin, can be found to the northwest and southeast, and saline springs are exposed in the fault zones. The unbalanced development and multistage activity of these fractures are concluded to be conducive to the formation of a salt basin tectonic environment [4, 5].

Figure 1: Simplified geologic map of the Nangqen Basin (modified after Han et al. [8]).

3. Samples and Analytical Methods

3.1. Sample Collection

During our field investigations 7 saline spring resources were identified in the Nangqen Basin. Samples were collected from the 7 saline springs in June 2014 and November 2014. The sampling locations are listed in Table 1. Due to the negligible effects of seasonal variations on the runoff of the saline springs in our research area, we selected only one season for δ¹¹B, δD, and δ¹⁸O analysis. All water samples were filtered in situ and then stored in separate 2 L polyethylene bottles, which were thoroughly rinsed with deionized water before use. Unstable parameters, such as water temperature (), pH, and electrical conductivity (EC), were determined on site using a thermometer, a pH meter (SevenGo SG2, Mettler Toledo, Swiss), and a conductivity meter (SevenGo SG3, Mettler Toledo, Swiss), respectively.

Table 1: Sampling sites.

Saline spring
Location
Rock type
Age

Dagai
N32° 12′ 15.1′′ E96°
Sandstone + mudstone
Paleogene

Riazhong
N32° 15′ 58.8′′ E96°
Sandstone, gypsum + mudstone
Paleogene

Niuriwa
N32° 08′ 38.7′′ E96°
Sandstone, gypsum + mudstone
Paleogene

Duolunduo
N32° 04′ 48.5′′ E96°
Sandstone, volcanic rock + mudstone
Paleogene

Baizha
N31° 56′ 02.6′′ E96°
Sandstone + mudstone
Paleogene

Ranmu
N32° 26′ 43.1′′ E95°
Clay
Paleogene

Gayang
N32° 03′ 47.9′′ E95°
Sandstone + mudstone
Paleogene

3.2. Analytical Methods

All sample analyses were performed at the Salt Lake Analytical and Test Department of the Qinghai Institute of Salt Lakes, Chinese Academy of Sciences.

The concentrations of Ca²⁺, Mg²⁺, and Cl⁻ were determined using chemical titration for the concentrations of K⁺ and were obtained using gravimetric analysis. The concentration of Na⁺ was estimated via subtraction, and the concentrations of B₂O₃ and Li were analyzed by Plasma spectrometry (ICAP6500DUO, USA). Br was analyzed using a Uv-vis spectrophotometer (TU-1810, China). Analytical uncertainties are less than ±5% for B₂O₃, Li, and Br, less than ±2% for Na⁺, and less than ±0.5% for all of the other elements analyzed.

The analytical procedure for B analysis is described in detail below.

A 2-column procedure was used for the B separation. The first column is a mixed cation-anion resin column, and the second column is an Amberlite IRA743 B-specific resin column. The mixed resin consists of a 200–400 mesh strongly acidic cation resin (Dowex 50 W × 8 H⁺, USA) and a 60–100 mesh weak alkaline anion resin (Ion Exchanger II, , Germany). The Amberlite IRA 743 B-specific resin was first conditioned with HCl solution. Then, the anion resin was leached with a saturated NaHCO₃ solution. Finally, the column was washed with DI water to achieve a neutral pH prior to the next use (see also [55]).

B purification and separation were accomplished in three steps.

The samples were passed through a special B adsorption resin column, with a velocity of 2.5 mL min⁻¹. Then the leachate was collected and processed with a B HCl elution (500 L, 75°C, and 0.1 molL⁻¹).

Next, 0.5 mL of solution was concentrated and purified in a furnace at 60°C. Then it was loaded into the mixed anion and cation exchange resin column, after which the leachate was collected and processed with an elution of 15 mL of low B water.

Appropriate amounts of Cs₂CO₃ and mannitol were added in order to keep the mole ratio of B/Cs at 2 : 1 and that of B/mannitol at 1 : 1. Then, they were concentrated to 1 mg mL⁻¹ by heating at <60°C.

The ¹¹B/¹⁰B ratio was measured on a thermal ionization mass spectrometer (Thermo Fisher Finnigan, Germany). The B isotopic composition can be expressed as follows:

NIST 951 = 4.05262 ± 0.00077 (%, ).

For D and ¹⁸O isotopes, we used MAT253 (Thermo Fisher Scientific, USA) and a Flash HT-IRMS analysis system [56].

VSMOW2 was used as the H₂O standard with / = 0 ± 0.3 and ¹⁸/ = 0 ± 0.02.

Standards GBW04401-GBW04404 were also used

with GBW04401 having / = −0.4 ± 1.0 and ¹⁸/ = 0.32 ± 0.19,

GBW04402 having / = −64.8 ± 1.1 and ¹⁸/ = −8.79 ± 0.14,

GBW04403 having / = −189.1 ± 1.1 and ¹⁸/ = −24.52 ± 0.20,

and GBW04401 having / = −428.3 ± 1.2 and ¹⁸/ = −55.16 ± 0.24.

4. Results

see Tables 2 and 3.

Table 2: Geochemical analysis of the saline springs, Nangqen Basin.

Samples
(°C)
pH
TDS (g/L)
Concentrations of major ions

Na⁺ (g/L)
Ca²⁺ (g/L)
K⁺ (g/L)
Mg²⁺ (g/L)
Li⁺ (10⁻³ g/L)
B (10⁻³ g/L)
Cl⁻ (g/L)
(g/L)
Br⁻ (10⁻³ g/L)

Dagai
10.7
7.75
175.36
66.28
1.34
0.08
0.30
2.66
3.01
100.49
6.62
<0.01

-
7.52
165.75
62.07
1.13
0.08
0.21
1.84
3.56
96.12
5.38
3.09

Riazhong
-
6.84
320.44
120.50
0.66
0.36
2.11
27.17
13.55
183.97
12.63
42.55

-
7.02
316.58
117.29
1.01
0.31
1.99
20.93
12.19
180.96
13.32
6.26

Niuriwa
11.3
6.96
148.24
56.50
0.85
0.23
0.26
2.44
1.86
87.34
2.97
3.47

-
7.18
150.89
57.11
1.13
0.19
0.29
3.91
2.17
87.98
2.98
3.98

Duolunduo
11.9
7.54
332.39
126.73
0.34
0.09
0.25
9.38
174.77
174.41
29.41
22.83

-
7.75
292.75
107.02
0.37
0.11
0.21
9.68
200.56
154.97
29.10
19.13

Baizha
8.2
7.09
273.98
105.53
1.21
0.08
0.15
5.04
20.10
162.40
4.60
6.08

-
7.04
269.48
103.77
1.23
0.07
0.14
4.88
21.38
159.73
4.45
4.70

Ranmu
6.1
6.98
220.43
84.30
1.26
0.20
0.33
1.03
1.00
130.53
3.70
15.10

-
7.14
229.26
86.03
1.19
0.29
0.41
0.75
1.74
135.77
4.49
20.30

Gayang
12
7.05
150.34
56.28
1.86
0.08
0.13
2.20
1.91
86.55
5.05
5.18

-
7.22
150.28
55.93
1.70
0.09
0.35
2.24
2.07
86.29
5.52
5.25

Italic indicates samples collected during the summer season, and bold indicates samples collected during the winter season.

Table 3: B, H, and O isotopes for the saline springs, Nangqen Basin, June 2014.

Samples
¹¹B/¹⁰B
¹¹B ()
D ()
¹⁸O ()

Dagai
4.07
4.85
−127.39
−14.52

Riazhong
4.10
12.7
−128.28
−14.68

Niuriwa
4.13
18.01
−124.14
−14.38

Duolunduo
4.07
3.55
−100.91
−12.88

Baizha
4.11
12.96
−132.98
−16.00

Ranmu
4.17
29.59
−128.36
−16.05

Gayang
4.13
18.27
−129.57
−15.93

5. Discussion

5.1. Geochemical Analysis

The geochemical analysis results for the samples in this study are reported in Table 2. The salinity of the saline springs in this area is 150 g/L or greater. Na⁺ and Cl⁻ were found to be the main cation and anion, respectively, in our water samples. Very small concentration (changes of less than an order of magnitude) was obtained for all of the other ions analyzed, such as Ca²⁺, Mg²⁺, K⁺, and . Trace amounts of some ions (e.g., B³⁺, Li⁺, and Br⁻) were also present in most of the saline springs.

5.2. Genetic Analysis of Brine

5.2.1. H-O Isotopes

In order to ascertain the source of the saline springs, hydrogen and oxygen isotopes were measured for all samples. In the same area five saline springs were identified and H and O isotopic analyses were performed by the Qinghai geophysical prospecting team in 1987 (Figure 2). The ¹⁸O values in the spring waters ranged from −12.88 to −16.05, and the D values ranged from −100.91 to −132.98 (Table 3). The H-O isotopic values of the five saline springs analyzed in 1987 were very similar to the seven saline springs analyzed in 2014. Similar values of H-O isotopes were reported for saline springs in Canada [57]. In a thermal spring study in India, the D-¹⁸O plot also illustrated that all of the thermal waters fall near the LMWL [12].

Figure 2: The H-O isotopic composition of the saline springs.

The hydrogen and oxygen isotopic compositions of all seven samples were near the global water line value. According to Figure 2, the source of the saline spring waters is atmospheric precipitation and infiltration.

5.2.2. Na/Cl and B/Cl Ratios

Na/Cl and B/Cl molar ratios can be used to distinguish between the various sources of dissolved solids in salt lakes [27, 58]. Water with low   B/Cl ratios suggests a sea water source [13]. In our study, we used the relationship between these two parameters to investigate the sources of salts in the saline springs (Figure 3).

Figure 3: Relationship between the B/Cl and Na/Cl molar ratios.

The Na/Cl ratio reflects the degree of enrichment of sodium in the salt brine, which is bounded at 0.86. The coefficient of sodium chloride in a sedimentary brine is considered to be less than 0.86, and the coefficient of sodium chloride in a leached brine is generally high, close to 1 [59]. The coefficient of sodium chloride for the water samples in the study area is around 1.0 or slightly greater than 1.0, indicating that they are leached brines. The B/Cl and Na/Cl ratios display little variability in the salt springs of the Nangqen Basin. The Na/Cl ratios of the salt springs (Na/Cl = 1.0) are greater than that of seawater (Na/Cl = 0.86) and are consistent with a halite influence. These values could also suggest the input of halite (Na/Cl = 1, B/Cl = 0) with little addition of B, whereas high Na/Cl values indicate the influence of Na₂SO₄ salts, which are abundant in some saline springs (e.g., Duolunduo, Riazhong). The majority of our samples have B/Cl values away from the marine line (Na/Cl = 0.86; B/Cl = 7 × 10⁻⁴) [27].

5.2.3. Br/Cl Ratio

The main water reservoir (the ocean) exhibits fairly uniform Cl and Br concentrations and its Cl/Br molar ratio is about 655 ± 4 [60, 61] (Br × 10³/Cl (g/g) is about 3.44 ± 0.02). Thus, this ratio has been used as a tracer to determine the source and evolution of groundwater and surface water from many years [62, 63] and has become an important tool in studies of surface and groundwater with low-to-moderate salinity [64–67] as well as studies of the input of salinity in lakes and rivers [61, 68–70] (Figure 4).

Figure 4: Relationship between Br × 10³/Cl and Cl content. Br × 10³/Cl of the black points on the concentration curve of water from the Yellow Sea are from Chen [9].

To further distinguish the genetic type of the saline spring water, we calculated the Br content and the coefficient of the Br/Cl ratio of all of the samples, as they are the most effective and sensitive index for determining the degree of concentration of the brine. All of the samples collected in our study area have low levels of bromide. Their maximum value is 42.2 ppm (Riazhong).

Br is generally nonmetallogenic by itself, so it can only be accessed based on the presence of isomorphism in the mineral crystal lattice, staying in the mother liquor [71]. Br in the brine is often depleted by dilution, while migration of the brine also results in adsorption, so the coefficient of Br × 10³/Cl should be low in leached brines and high in sedimentary brines. If the coefficient of sodium chloride is 0.87–0.99 or higher and the coefficient of Br × 10³/Cl is 0.87–0.08 or smaller, the saline spring is generally considered to be a leached brine [72]. The Br content and the coefficient of Br × 10³/Cl indicate that these springs are leached brines.

Based on Br/Cl, Na/Cl, and B/Cl ratios and the H-O isotopes, it has been speculated that different kinds of saline springs were formed as a result of salt leaching when the groundwater and/or rainwater passed through salt rich stratum.

5.3. Origin of Boron

The above evidence indicates that the saline springs in the Nangqian Basin are the result of atmospheric precipitation dissolving a halite layer and that the B/Cl ratios in the halite are as low as 0 [27]. However, the B/Cl ratio in the saline springs in the study area is abnormal (0.03–3.29) with a maximum of 3.29 (Duolunduo). In order to understand the formation and evolution process of the saline springs and the material source of their boron, we have measured the boron isotope values. Furthermore, we will discuss the boron enriched rocks distributed in the region, which are likely to be the sources of the B in the saline springs. According to the geologic background of this area, there are carbonates and some volcanic rocks in the Nangqen Basin, with boron contents of 0.29–75.1 ppm and 378–688 ppm and boron isotopic values of −5.5–20 and −16.3 to −10.3, respectively [11].

5.3.1. The Influence of pH on B Isotopes

Several studies have shown that the boron isotopic composition is controlled by pH [10, 58, 73, 74]. According to Palmer et al. [75], pH can restrict the boron isotopic characteristics of a water body to a certain extent. Especially in the case of low concentrations of B, there is a linear relationship between pH and δ¹¹B. When the pH of a water sample increases, there is a linear decrease in the B isotopic value. Some experiments (a hydrothermal system in Taiwan) have shown that B isotopes are dependent on pH. The pH values, B concentrations, and B isotopic compositions show significant correlations with each other, suggesting that the δ¹¹B/B and pH/B ratios of hydrothermal systems have stable values [14]. In our study, the effect of pH on the change in the δ¹¹B value is investigated, as shown in Figure 5.

Figure 5: Relationship between ¹¹B and pH.

δ¹¹B is stable at around +15 when the salt spring’s pH is approximately neutral (pH of 6.8–7.2). However, ¹¹B in the saline springs decreased significantly (≥+5) with increasing pH, whereas the B isotopic value (+29.59) in the Ranmu sample indicates that it had a different source, so the influence of the pH value can be ignored. The linear relationship between δ¹¹B and the pH of the water samples is shown in Figure 5. The linear regression equation is = −19.01 + 150.60 with a low standard deviation value of 0.42. The correlation is very low. Overall, there is no obvious linear relationship between the B isotope and the pH value. Musashi et al., 1988 (Japan), also concluded that the B isotopic variation was independent of pH and showed a systematic regional variation [15], which can be seen in our study also.

Based on the relationship between δ¹¹B and pH (Figure 5), we suggest that the sample’s pH is not the main factor influencing the variation in the δ¹¹B values in the study area. Instead, the δ¹¹B characteristics of the salt springs mainly depend on their source area. Thus, we can determine the source of the material based on the δ¹¹B characteristics.

5.3.2. The Relationship between δ¹¹B and the B Content

The average δ¹¹B value of atmospheric precipitation on the Qinghai-Tibetan Plateau has not been reported. Lü et al. [11] speculated that the δ¹¹B value of atmospheric precipitation is between −6.0 and 6.8 on the Qinghai-Tibetan Plateau based on the location of δD-Cl in relation to the atmospheric precipitation line of B . Our δ¹¹B results are similar to those of a rainwater sample (5.4 ± 1.4) in Nepal collected by Rose et al. (2000) [76]. Therefore, we assume that the Nepal rainwater and rainwater in the Nangqen Basin share the same B isotopic characteristics. As shown in Figure 6, marine carbonates (B = 3.72–24.0 ppm) and Tertiary volcanic rocks (B = 378–688 ppm) are widely distributed in this area. Therefore, we speculate that they are most likely to be the B source of the saline springs.

Figure 6: Relationship between ¹¹B and B concentration in the saline springs of the Nangqen Basin. The green and pink shaded areas are the regions with boron geochemical characteristics of crustal marine carbonate rocks and magmatic rocks, respectively [10]. Data for Tibetan geothermal hot water is from [11].

The δ¹¹B-B relationships (Figure 6) indicate that the saline springs of the Nangqen Basin have the largest variations in δ¹¹B contents and boron concentrations of any saline springs, which could be related to differences in the geological and geochemical setting of the Nangqen Basin.

High δ¹¹B values indicate a deviation in B values from that of seawater [17, 76], that is, δ¹¹B = 36.36–45.74 (¹¹B/¹⁰B ratio = 4.200–4.238) [77, 78], whereas lower δ¹¹B values support a continental origin of B [17, 77]. δ¹¹B of the saline springs is 3.55 to 29.59, which is significantly lower than that of seawater. Part of the Qinghai-Tibetan Plateau has been uplifted since the Cretaceous-Tertiary (especially in the Miocene) as discussed by Yin et al. [79, 80]. This evidence confirms that the late transgression did not penetrate into the internal part of the plateau. In summary, we conclude that B in the saline springs in the study area has a terrestrial source.

Previous studies have shown that the B content of the mantle is only 0.18–1.35 ppm [81, 82]. Consequently, according to the B content of the saline springs in the Nangqen Basin (1.00–200.56 ppm), we speculate that mantle processes cannot provide abundant B and that B-enriched water is always associated with B-enriched crustal rocks.

In some modern (shallow) marine carbonates boron isotope is recorded approximately as 30 [10, 83], and the B content of marine carbonate (0.29–75.1 ppm) in the Tibetan area is consistent with that of other marine carbonates (3.72–24.0 ppm) [11] and having similar chemical composition as reflected in our study. B in hot springs in the Tibet area (Xiqinqucan, δ¹¹B = 12.3 to 12.5; Kangbuqucan, δ¹¹B = 13.1) comes mainly from marine carbonates [11]. The boron sources, enriched in the saline springs of Dagai, Riazhong, Niuriwa, Baizha, and Gayang (B = 5.98–64.76 ppm, δ¹¹B = 4.85–18.27, green area in Figure 6), appear to be mainly from a marine carbonate source based on the fact that they share similar B characteristics with marine carbonate. B of the rain data is very close to that of the saline springs. The range of B isotopic values for precipitation on the southern Qinghai-Tibet Plateau is about −6.0–6.8 and the B content is 0.014 ppm [11]. Thus, precipitation has little influence on the B isotope values of the saline springs.

The Duolunduo saline spring has a low δ¹¹B value and a high B content. Zheng et al. [84] studied the B contents of various types of rocks on the Qinghai-Tibet Plateau and established that the Precambrian metamorphic rocks, Cretaceous volcanic rocks, Cenozoic rhyolites, granites, and so on are all rich in B, especially the Cretaceous-Paleogene volcanic rocks (378–688 ppm). Based on the large amount of volcanic rock in the Nangqen Basin and their similar isotopic values (3.55) and high B contents (174.77 ppm), we hypothesize that these B isotopic characteristics are the result of the combined influence of the volcanic rocks and the marine carbonates and that both rock types are the B source of the saline springs.

As compared with hot springs in Tibet, δ¹¹B of the saline springs in the Nangqen Basin is consistently higher, even for the Ranmu saline spring (high δ¹¹B value versus low B content). In contrast to Ranmu, the other springs (Riazhong, Niuriwa, Baizha, and Dagai) lie solely in the area of the marine carbonates (green area in Figure 6). This may be due to the fact that B isotopes can be adsorbed by a clay layer. In the water-rock reaction process, clay minerals can adsorb boron, and ¹⁰B preferentially goes into the solid phase [45], which results in high boron isotopic values in the saline springs. For all of the samples collected in the study area, plots of the δ¹¹B values versus the boron concentrations indicate that the saline springs in the Nangqen region have different boron sources. Moreover, the B characteristics of the two component end members were similar to those present in the Tibetan geothermal area. One end member has relatively low boron concentrations and low δ¹¹B, and the other end member exhibits relatively high boron concentrations and high δ¹¹B. There is a strong correlation between the strata and the fractures around the saline springs. Further investigation into the origin and evolution of boron in the saline springs in the Nangqen area is required to gain a deeper understanding of the geochemical processes that affect boron and its isotopes.

5.3.3. Comparison with Other Springs Worldwide

Terrestrial sources have low boron isotopic values, while higher boron isotopic values are characteristics of a marine source. Present-day seawater has a constant δ¹¹B value of +39.5 and a boron concentration of approximately 4.5 ppm [45]. The continental crust has relatively low δ¹¹B values of −20–7 [85]. Many studies of springs worldwide show that B concentrations and δ¹¹B values can be used to determine the source of the material. We have compared our data to that of the hot springs and saline springs in Indonesia, Japan, India, Greece, and the yellow and white springs in Taiwan.

As shown in Figure 7, some springs around the world have similar δ¹¹B-B relationships to that seen in our study area, while some are very different. There are many factors affecting the δ¹¹B values and B contents of these springs, such as the geological setting, wall rock compositions, and tectonic structures. Indonesia, Japan, and Greece have values similar to those of our study area; however their sources are different. The δ¹¹B values of hot springs in Indonesia range from −2.4 to +28.7 and the B concentrations range from 2.08 to 93.23 ppm. In the case of geothermal brines, the δ¹¹B values range from −0.02 to 0.26 and the B concentrations range from 262.54 to 593.6 ppm, which indicates the leaching of andesitic rocks, which is a similar conclusion to that reached regarding our saline springs (Duolunduo) [13]. In a study from (central) Japan, it was revealed that the δ¹¹B values of the springs in that area range from −7.56 to 24.77, and the B contents range from 0.366 to 208 ppm. The Japanese B isotopic study is independent of the pH value and reported a systematic regional variation, which can also be seen in our study. However, in Japan the source from which the aqueous B is derived and that of some springs are strongly influenced by sea water [15]. Although the data is similar to that of the Nangqen Basin springs, the source of B is entirely different because of the geological background of the areas and the influence of the surrounding rocks. In a hydrothermal system in Greece, the δ¹¹B values range from 4.90 to 24.36 with low B content 0.25–6.02 ppm, and it is concluded that the B source is sediment or magmatic influence in the deep reaction zone of the Milos hydrothermal system, which differs from the B source of the salt springs in the Nangqen Basin [16].

Figure 7: Comparison of springs worldwide. Data from India [12], Indonesia [13], Taiwan [14], Japan [15], and Greece [16].

Hot springs in India and Taiwan have shown different B and δ¹¹B values than those in our study area. δ¹¹B compositions range from +34 to +41 in springs in India, but in our study area the B values are lower (Table 3). Thus, we can easily differentiate the two difference sources; that is, one is terrestrial and the other (Indian) is marine [13]. A study of the yellow spring and white spring hydrothermal fluids in Taiwan confirmed that B of the hydrothermal fluids and plumes is higher and is derived from seawater and that only a small amount of it comes from andesite. The B concentrations and B isotopic compositions show significant correlation, which is absent in our study area [14].

6. Conclusions

The saline springs in the Nangqen Basin are leached brines according to analyses of hydrogen and oxygen isotopes, Na/Cl and B/Cl molar ratios, and the coefficient of Br × 10³/Cl.

The B concentrations (3.23–147.77 ppm) and δ¹¹B (3.55–29.59) of the saline springs in the Nangqen area vary significantly. However, the variations in the B/Cl molar ratios (0.03–3.29) are relatively small. In the Tibetan geothermal area, the B concentration is 0.036–472.4 ppm, and δ¹¹B ranges from −16.0 to 13.1 [11]. Both values are similar to those of the Nangqen Basin. These characteristics are in sharp contrast to the geochemical characteristics of seawater and typical geothermal waters of marine origin. Therefore, it is concluded that the saline springs in the Nangqen Basin have a terrestrial source.

The δ¹¹B-B binary mixing relationship in the study area indicates that marine carbonate rocks and volcanic rocks are the main sources of the B in the saline springs. Therefore, we conclude that B in the saline springs has a primarily crustal source (marine carbonate rocks and volcanic rocks), instead of a deep mantle source.

Based on our study, the Nangqen area has the lowest δ¹¹B values and the largest variation in B concentrations. These characteristics are most likely related to the unique tectonic history of the Nangqen Basin.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors thank Ma Yunqi, associate research fellow, for providing valuable suggestions and help during lab experiments. They thank Zhang Xiying and Zeng Fangming for their help with revision and acknowledge LetPub (http://www.letpub.com) for the linguistic assistance during the preparation of this manuscript. This research was supported by Science and Technology Project of Qinghai Province (Grant no. 2014-ZJ-702).

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