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. 2021 Nov 3;6(45):30674–30685. doi: 10.1021/acsomega.1c04533

Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan–Yunnan Block, China

Bingkun Meng †,‡,§, Shixin Zhou †,‡,*, Jing Li †,‡,*, Zexiang Sun †,‡,§
PMCID: PMC8600645  PMID: 34805695

Abstract

graphic file with name ao1c04533_0012.jpg

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan–Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan–Yunnan block. Chemical and isotopic compositions were analyzed in N2-dominant hot spring gases. The 3He/4He ratio (0.068–0.541 Ra) indicates the occurrence of mantle-derived helium throughout the Sichuan–Yunnan block, which has been diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between 4He, 20Ne, and N2 concentrations implies a common trapping mechanism for 4He, 20Ne, and N2 in hot spring gases. The 40Ar/36Ar ratios and N2/Ar ratios indicate that N2 and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ13C(CO2)o values calculated by Rayleigh fractionation and CO2 concentration suggest that CO2 has inorganic and organic origins. The plot of Rc/Ra versus δ13C(CO2) indicates that the spring gas CO2 origin in the Sichuan–Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO2. The δ13C(CH4) versus CH4/3He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan–Yunnan block.

1. Introduction

In recent years, extensive reports and studies have been conducted on the geochemical characteristics of geothermal fluids in the southeastern margin of the Qinghai–Tibet Plateau.1,2 The Sichuan–Yunnan block is close to Tengchong, Ning’er, and Pingbian volcanic fields in the southeastern margin of the Qinghai–Tibet Plateau. However, there are relatively few reports of noble gas and carbon isotopes of the geothermal fluids in the Sichuan–Yunnan block. The 3He/4He value between crustal helium (∼10–8) and mantle-derived helium (∼10–5) has a remarkable difference.3 Unlike rocks, natural fluids can integrate helium isotope ratios of mantle-derived and crustal-derived helium to varying degrees. Therefore, the 3He/4He ratio in fluids provides a possibility to indicate local to regional geological characteristics.4 Mantle volatiles are released through fractures and volcanic pipes connecting the mantle and the surface of the earth.5 When mantle fluids are ejected from the subsurface through hot spring, they can be blended with crustal fluids in faults or volcanic channels.1 In structurally stable regions, helium is formed by α decay of uranium and thorium series elements, while active extension or young volcanism areas are characterized by mantle-derived helium.4 Therefore, the overflow of mantle-derived fluids is mainly controlled by tectonism. The 3He/4He ratio has a significant positive correlation with the deformation rate of active crust.6 Therefore, the 3He/4He ratio is usually used to track mantle-derived materials.7 In addition, the correlation between the carbon isotopes of CO2 and CH4 and the abundance of 3He can indicate geochemical information related to their sources and carbon cycle.

To discuss the mantle degassing mechanism in an intraplate tectonic environment, this paper studies the noble gas and carbon isotopic distributions of the geothermal fluids in the Sichuan–Yunnan block. By defining the relationship between the 3He/4He ratio and the abundance of main gases and carbon isotope data in the subsurface geothermal fluids, the correlation between the mantle degassing mechanism, tectonics, and magmatism is summarized.

2. Geological Settings

The study area is located at the intersection of Yunnan and Sichuan Provinces, PR China and is geologically situated in the Sichuan–Yunnan block in China (Figure 1a). It is a part of the Yanyuan–Lijiang platform fold belt in the western margin of the upper Yangtze block.8 It is adjacent to Sanjiang structural belt in the west and Kangdian ancient land in the east.8 Ninglang-Yanyuan Basin is distributed in the study area (Figure 1b). The tectonic location of the Ninglang-Yanyuan Basin is a part of the southwest margin of the upper Yangtze block, which is mainly surrounded by the Jinhe-Qinghe fault, Xiaojinhe fault, Maijiaping fault, Guobaoshan-Yanfeng fault, Woluo fault, and Baizidi-Yongsheng fault.8 The basement is the Proterozoic stratum, on which the Sinian, Lower Cambrian, Lower Ordovician–Lower Jurassic, and Paleogene sedimentary strata are deposited, missing the Middle-Upper Cambrian and Jurassic–Middle Cretaceous and Neogene, and the sedimentary stratum is more than 5000–10 000 m thick.9 Carbonate rocks (limestone, marble, and dolomite) are developed in the Upper Sinian, Middle-Upper Ordovician, Middle-Upper Silurian, Middle-Upper Devonian, Carboniferous, Permian Maokou formation, the upper of Middle Triassic, and the lower of Upper Triassic.10 The upper wall of the Jinhe-Qinghe thrust fault is the Sinian stratum, while the footwall is the Upper Triassic–Lower Jurassic strata in the Kangdian ancient land, and the upper wall of other faults is the Permian stratum and the footwall is the Triassic stratum in the other study areas.8 The study area mainly developed the Jinning, Caledonian Early Hercynian, late Hercynian, Indosinian, and Yanshanian magmatism.10 Except that the Late Permian Emeishan basalt of the Hercynian period erupted in a large area and had a huge thickness, other magmatic activities were mainly intrusive in the study area.10 The magmatic rocks in the Kangdian uplift and surrounding areas have high U and Th contents.11,12

Figure 1.

Figure 1

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Location and geologic map of the Sichuan–Yunnan block, PR China. (a) Location of the study area in China; (b) sketch of geological tectonics and distribution of thermal springs in the Sichuan–Yunnan block in China; (c) stratigraphic and fault distribution of sections A–B. An∈: Precambrian; ∈-O: Cambrian-Ordovician; S: Silurian; D: Devonian; C: Carboniferous; C-P: Carboniferous-Permian; D-P1: Devonian-Lower Permian; P2β: Permian Emeishan basalt; T2y-P2l: Middle Triassic Yantang formation-Middle Permian Leping formation; T2b-T3xb: Middle Triassic Baishan formation-Upper Triassic Xiaboda formation; T3-J1: Upper Triassic–Lower Jurassic; γ4+P2β: Diabase + Permian Emeishan basalt; AnJ + γ: Pre-Jurassic + Granite.

3. Samples and Analytical Methods

Two free gases (YNWQ, YBWQ) and three dissolved gases (JHWQ, SSYQ, and HGWQ) were collected from the Sichuan–Yunnan block, China, in August 2020. The free gas is collected by a gas drainage method using a 500 mL glass bottle with a funnel on the mouth and then sealed with a rubber cap under water when the gas occupies about one-half of the volume of the glass bottle. The dissolved gas adopts the same glass bottle and rubber cap to start sampling when the hot spring water flows out of the spring mouth within 10 min to avoid air pollution and ensure temperature balance. When the glass bottle is filled with hot spring water, it is sealed under water with a rubber cap. YNWQ is close to the Xiaojinhe fault; YBWQ, JHWQ, and SSYQ are located in the Jinhe-Qinghe fault; and HGWQ is sited in the Kangdian uplift. Chemical compositions and carbon and noble gas isotopes were analyzed in the Key Laboratory of Petroleum Resources of Gansu Province, China.

Chemical compositions were analyzed by a GC-9560-PDD gas chromatography (GC) instrument with a relative standard deviation of <5%, installed with a Porapak Q (2 m × 1.60 mm) column using He as the carrier gas. The oven temperature was programmed as follows: the initial temperature was set at 100 °C and maintained for 5 min and then the temperature was increased to 240 °C at 10 °C min–1 and maintained for 12 min. The detector temperature was 250 °C.13

The carbon isotopes of spring gas were tested by a 6890A GC instrument linked to a Finnigan MAT Delta Plus XP mass spectrometer.14 The hydrocarbon compounds and CO2 can be separated by an HP-PLOT column using He as the carrier gas. The GC oven temperature was programmed as follows: the initial temperature was set at 35 °C and maintained for 3 min, increased to 80 °C at 8 °C min–1, and then to 260 °C with 5 °C min–1, maintained for 10 min. The individual compounds were oxidized to CO2 in a high-temperature (940 °C) oxidation furnace (an oxidation ceramic microreactor loaded with twisted wires) and detected by a Delta Plus XP isotope mass spectrometer with uncertainties of ±0.5‰.14 The values of δ13C are reported relative to V-PDB (Vienna Pee Dee Belemnite) in per mill.15

Noble gas contents and isotope compositions were measured by a Noblesse SFT noble gas mass spectrometer.16 The analysis system is divided into four parts: sample introduction, sample purification, noble gas separation, and noble gas testing.17 The detailed processes of sample introduction are described in the refs (14, 16). The gas was purified first using a spongy titanium furnace at 800 °C to remove active gases (H2O, hydrocarbons, CO2, N2, O2, etc.). H2 in the gas can be eliminated by Zr–Al getters running at room temperature. Purified noble gases were separated by a cryogenic trap (10–475 K) filled with activated charcoal. He, Ne, Ar, Kr, and Xe were released for analysis at the cryogenic trap temperatures of 15, 50, 100, 150, and 230 K, respectively. The details of analytical procedures were described in ref (16). 4He, 20Ne, 22Ne 40Ar, and 36Ar were examined with a Faraday collector, and 3He, 21Ne, 38Ar, Kr, and Xe isotopes were analyzed with an electron multiplier. Experimental uncertainties for the noble gas concentrations were <10%.16 The details of data correction are described in refs (18, 19).

4. Results

4.1. Chemical Compositions and Carbon Isotopes

The basic information and chemical compositions of the hot spring gases collected from the Sichuan–Yunnan block are presented in Table 1. These hot spring gases are dominated by N2, which is present at a concentration ranging from 67.05 to 97.13%. This N2-dominant hot spring gas differs from the CO2-rich hot spring gas discovered in Tengchong20 and Wudalianchi volcanic areas.21 The O2 content in HGWQ hot spring gas (9.80%) is higher than those in the other hot spring gases (0.27–4.62%). However, CO2 contents in YNWQ and YBWQ hot spring gases (12.77 and 30.92%, respectively) are higher than those in the other hot spring gases (0.09–1.45%). In addition, O2 and CO2 contents in N2-rich hot spring gases are higher than those in the other major components (Table 1). The N2/O2 ratios (8.97–359.74) in the N2-dominant gas are greater than N2/O2 ratios (3.71) in the atmosphere (Table 1). The contents of H2 and CH4 are very low (2–32 ppm and 0.10–1.24%, respectively) in these N2-dominant hot spring gases.

Table 1. Chemical Compositions and Carbon Isotopes of Hot Spring Gases in the Sichuan–Yunnan Blocka.

          chemical composition (%)
 carbon isotopes (‰)
    
no. sampling site longitude (E°) latitude (N°) T (°C) N2 CO2 O2 CH4 H2 δ13C(CO2) δ13C(CO2)o δ13C(CH4) CO2/3He CH4/3He
1 YNWQ 100.704255 27.812170 52 84.84 12.77 1.37 0.12 0.0002 –9.0 –8.3 –19.4 1.49 × 109 1.40 × 107
2 YBWQ 101.265715 27.009167 42 67.05 30.92 1.02 0.10 0.0002 –5.0 –4.1 15.8 1.30 × 109 4.20 × 106
3 JHWQ 101.955357 27.712511 38 93.85 0.33 4.62 0.11 0.0005 –18.9 –13.5 –6.2 1.65 × 108 5.52 × 107
4 SSYQ 101.351221 26.971885 16 97.13 1.45 0.27 0.11 0.0032 –21.6 –18.6   5.05 × 108 3.83 × 107
5 HGWQ 101.951365 26.529469 57 87.88 0.09 9.80 1.24 0.0003 –26.3 –12.4 –40.9 3.48 × 106 4.79 × 107
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a

Note: The value of δ13C(CO2) is measured using the samples. δ13C(CO2)o is the original value of δ13C(CO2).

The δ13C(CO2) values analyzed using YNWQ and YBWQ hot spring gases (−9.0 and −5.0‰, respectively) are higher than other hot spring gases (−26.3 to −18.3‰) (Table 1). The δ13C(CO2)o values calculated by the Rayleigh fractionation in YNWQ and YBWQ hot spring gases (−8.3 and −4.1‰, respectively) are higher than other hot spring gases (−18.6 to −12.4‰) (Table 1). CO2 contents and the δ13C values of CO2 (Table 1) exhibit a positive correlation, indicating enriched 13C and depleted 12C with an increase of CO2 contents. The δ13C(CO2) values and CO2 concentrations in YNWQ and YBWQ hot spring gases are more than −10‰ and 10%, respectively, indicating an inorganic origin.22 However, the δ13C(CO2) values and CO2 concentrations in other hot spring gases are less than −10‰ and 10%, respectively, indicating a predominantly organic origin.22 The δ13C values of CH4 in YNWQ, YBWQ, JHWQ, and HGWQ are −19.4, 15.8, −6.2, and −40.9‰, respectively.

4.2. Noble Gases

4.2.1. Helium

4He contents in YNWQ, YBWQ, and HGWQ hot spring gases (850, 528, and 1649 ppm, respectively) are higher than those in JHWQ and SSYQ hot spring gases (33 and 54 ppm, respectively) (Table 2). 4He concentrations in all samples are higher than that in the atmosphere (5.24 ppm),23 indicating the contribution of helium mainly from nonatmospheric sources. The measured 3He/4He ratio of all samples are 0.072–0.616 Ra (Ra = 1.4 × 10–6).24 To eliminate the contamination of atmospheric helium, 3He/4He ratios are air-corrected assuming atmospheric origin for 20Ne. The Rc values range from 0.068 to 0.541 Ra, which is greater than crustal-derived (0.02 Ra) but much lower than mantle-derived (8 Ra) values.3 Based on a ternary mixture model,25 three end-members of atmospheric, crustal, and mantle-derived were used to estimate the contribution percentages of atmospheric helium (HeA), crustal helium (HeC), and mantle-derived helium (HeM) to the helium in hot spring gases. The detailed method is listed in Table 2 footnote. The calculated results show that the contribution percentages of HeA, HeC, and HeMrange 0.33–16.14, 78.2–98.9, and 30.74–5.67%, respectively. These results suggest the significant contribution of crustal helium to the hot spring gases in the study area.

Table 2. Noble Gas Contents and Their Isotope Ratios in Hot Spring Gases in the Sichuan–Yunnan Blocka.
no. sampling site 4He (ppm) 20Ne (ppm) 40Ar (ppm) 3He/4He err (1σ) 20Ne/22Ne err (1σ) 21Ne/22Ne err (1σ) 40Ar/36Ar err (1σ) R/Ra Rc/Ra 4He/20Ne HeA (%) HeM (%) HeC (%)
1 YNWQ 850 11 1054 1.01 × 10–7 1.1 × 10–8 10.32 0.41 0.0328 0.0061 498.0 2.2 0.072 0.068 80.7 0.36 0.74 98.9
2 YBWQ 528 9 1170 4.51 × 10–7 3.4 × 10–8 10.45 0.19 0.0294 0.0006 317.8 2.0 0.322 0.318 57.3 0.52 3.90 95.6
3 JHWQ 54 17 1094 3.72 × 10–7 1.1 × 10–8 9.90 0.01 0.0278 0.0004 314.9 0.6 0.265 0.186 3.25 9.76 2.02 88.2
4 SSYQ 33 17 1095 8.62 × 10–7 2.0 × 10–8 9.78 0.01 0.0281 0.0002 310.8 0.5 0.616 0.541 1.97 16.14 5.67 78.2
5 HGWQ 1649 19 908 1.57 × 10–7 5.8 × 10–9 10.63 0.05 0.0281 0.0011 610.2 4.9 0.112 0.109 88.2 0.33 1.25 98.4
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a

Note: R/Ra is the measured 3He/4He ratio divided by that of the air (Ra = 1.4 × 10–6).24Rc/Ra is the air-corrected 3He/4He ratio. Rc/Ra = (R/Ra – X)/(1 – X), r = (4He/20Ne)air/(4He/20Ne)sample, where (4He/20Ne)air = 0.318.25 HeA, HeM, HeC represent the proportion of Helium from air, mantle, and crust, respectively. The calculation formula is as follows: (Rc/Ra)sample = A × (R/Ra)air + M × (R/Ra)mantle + C × (R/Ra)crust, 1/(4He/20Ne)sample = A/(4He/20Ne)air + M/(4He/20Ne)mantle + C/(4He/20Ne)crust, 1 = A + M + C,25 where (R/Ra)air = 1, (R/Ra)mantle = 8 Ra,34 (R/Ra)crust = 0.02 Ra,35 (4He/20Ne)air = 0.318, (4He/20Ne)mantle = (4He/20Ne)crust = 1000.25

4.2.2. Neon

The contents of 20Ne in hot spring gases range from 9 to 19 ppm (Table 2). In Figure 2, the 20Ne/22Ne ratios ranging from 10.3 to 10.6 in YNWQ, YBWQ, and HGWQ hot spring gases show slight deviations from the atmospheric 20Ne/22Ne ratios (9.8)26 and are lower than mantle 20Ne/22Ne ratios (12.5).27 However, 20Ne/22Ne ratios in JHWQ and SSYQ hot spring gases (9.8 and 9.9, respectively) are similar to the atmospheric 20Ne/22Ne ratios (9.8).26 The 21Ne/22Ne values (0.0281–0.0328) of the hot spring gases vary from the atmospheric and crust values of 21Ne/22Ne = 0.029 and 0.03–0.70, respectively.28 The 21Ne/22Ne values of the hot spring gases are less than 0.029 except for YNWQ spring gas, indicating that 21Ne in these hot spring gases is mainly derived from the atmosphere without evident crustal addition of 21Ne. Previously reported data also show the same isotopic pattern.30 All of the Ne isotopic ratios can be accounted for by two processes: one is variable crustal radiogenic 21Ne addition to the air Ne, and the other is the mass fractionation process (Figure 2).

Figure 2.

Figure 2

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Plot of 20Ne/22Ne ratios vs 21Ne/22Ne ratios. The Ne isotopic pattern can be accounted for by the crustal 21Ne addition to the air ratio and a mass fraction-related process. The 20Ne/22Ne and 21Ne/22Ne values for atmospheric and crustal were from refs (26, 29), respectively. Although the error of the 21Ne/22Ne ratio in YNWQ is 0.0061, it can still be explained by the mixing of mass fractionation and the crustal 21Ne addition to the air ratio. MFL, mass fractionation line. Samples that do not show the error bars indicate 1σ uncertainties are smaller than symbols.

4.2.3. Argon

40Ar contents in the hot spring gases are 908–1170 ppm, and 40Ar/36Ar ratios vary between 310.8 and 610.2 (Table 2). In Figure 3, 40Ar/36Ar ratios of the hot spring gases are higher than the atmospheric 40Ar/36Ar ratios (298).31 Because of the minor mantle contribution proved by the 3He/4He ratio, therefore, excess 40Ar generally was derived from radioactive decay of 40K in the crust. Based on excess 40Ar* = 40Ar – 296.0 × 36Ar,32 the proportion of excess 40Ar* to total 40Ar is 40.56 and 51.49% in YNWQ and HGWQ hot spring gases, respectively, and 6.86, 5.99, and 4.78% in YBWQ, JHWQ, and SSYQ hot spring gases, respectively.

Figure 3.

Figure 3

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Plot of 1/36Ar ratios vs 40Ar/36Ar ratios. Atmospheric values of 1/36Ar = 3.18 and 40Ar/36Ar = 298.31 The proportion of excess 40Ar* to total 40Ar is negatively correlated with atmospheric 36Ar. 1σ uncertainties are smaller than symbols.

5. Discussion

5.1. Correlation between 4He, 20Ne, and N2 Contents in Hot Spring Gases

The contents of 20Ne and 4He show a positive correlation in hot spring gases except for SSYQ and JHWQ samples in the Sichuan–Yunnan block (Figure 4a). 20Ne is almost completely derived from the atmosphere and enters the subsurface by dissolving in groundwater.334He is formed by α decay of uranium and thorium series elements.33 Generally, there is no correlation between 4He and 20Ne from different sources, and they are also subject to variable dilution of main gas components in the later stage. The positive correlation between 4He and 20Ne in the sample indicates that 4He and 20Ne have mixed before the main gas component is charged. In addition, this correlation provides direct evidence of the important role of groundwater in the enrichment process of crustal-derived gas. Similar relationships exist between 4He/N2 and 20Ne/N2 (Figure 4b) and N2 and 4He concentrations (Figure 4c), implying a common trapping mechanism for 4He, 20Ne, and N2 in the geothermal fluids. Notably, 4He vs 20Ne, 4He vs N2, and 4He/N2 and 20Ne/N2 show two trend distributions in the hot spring gases; this is may be caused by the difference in 4He contents in the spring gases. The difference of 4He contents in the samples is mainly determined by the difference of U and Th concentrations in rocks in different regions. This also shows that the U and Th contents of rocks in SSYQ and JHWQ areas are lower than those in other sample distribution areas. However, 20Ne vs N2 shows only one trend (Figure 4d), indicating that 20Ne and N2 have the same source and geological migration pathway.

Figure 4.

Figure 4

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Plots of (a) 4He vs 20Ne content, (b) 4He/N2 vs 20Ne/N2, (c) 4He vs N2 content, and (d) 20Ne vs N2 concentration indicating the strong correlation between 4He, 20Ne, and N2 contents.

5.2. Origins of Gases Discharging from the Sichuan–Yunnan Block

5.2.1. N2–Ar–He System

The magma fluids, crustal fluids, and atmospheric precipitation fluids have different distribution positions in N2–Ar–He triangle diagram; therefore, the N2–Ar–He triangle diagram is usually used to determine the source of the subsurface fluids.36,37 The N2, He, and Ar contents of the three major sources of gas usually have the following distribution characteristics (Figure 5). The air and air-saturated water (ASW) usually have lower He contents, and N2/Ar ratios are 83 and 38, respectively. However, mantle-sourced gases usually have higher He contents and lower N2 contents, with N2/He less than 200.38 Arc-type gases usually have high N2 contents, N2/He greater than 1000, and N2/Ar greater than 200. The hot spring gases from the Sichuan–Yunnan block are distributed along between mantle–crustal-derived and air or ASW in Figure 5, and the N2/Ar values of samples are greater than that of air (83), except for the YBWQ hot spring gas, the N2/Ar ratio is 77 between air (83) and ASW (38). The excessively high N2/Ar ratio may be caused by excessive N2 addition or N2/Ar fractionation during the fluid migration process.39 Compared with the hot spring gases from the Sichuan–Yunnan block, Tengchong and Wudalianchi gases show mantle–crustal-derived and air or ASW distribution, but Tengchong and Wudalianchi gases have higher mantle He and Ar concentrations. In addition, some Tengchong gases fall along the arc-type gas region.

Figure 5.

Figure 5

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N2–Ar–He ternary diagram. Modified from ref (40). Wudalianchi data from ref (41); Tengchong data from ref (42). The hot spring gases from the Sichuan–Yunnan block are distributed along between mantle–crustal-derived and air or ASW.

5.2.2. Helium in Hot Spring Gases

Due to the different isotopic compositions of noble gases in the different geospheres, 4He/20Ne and 3He/4He can further reveal deep material information and the source of gas components.43 As shown in Figure 6, a simple ternary mixture model including atmospheric (Ra), mantle (∼8.0 Ra), and crustal (0.02 Ra) can be used to indicate the source of geothermal fluids in the Sichuan–Yunnan block.25 The 4He/20Ne values range from 57.32 to 88.18 except for JHWQ and SSYQ hot spring gases with values of 3.25 and 1.97, respectively, and Rc/Ra ratios (0.068–0.541 Ra) all are less than 0.6 Ra. It can be clearly seen that all hot spring gases in the Sichuan–Yunnan block are scattered between air and crustal mixing lines. Therefore, all the hot spring gases are typical of a crustal origin. Usually, 3He/4He ratios of natural gases are greater than 0.1 Ra, suggesting the presence of mantle He components.44 The proportion of mantle He in YBWQ, JHWQ, and SSYQ hot spring gases are 3.90, 2.02, and 5.67%, respectively, and this result is consistent with the Jinhe-Qinghe fault cut through the lower crust.45 However, the proportion of mantle He in the YNWQ and HGWQ hot spring gases (0.74 and 1.25%, respectively) are relatively lower than that in the hot spring gases located in the Jinhe-Qinghe fault.

Figure 6.

Figure 6

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Plot of Rc/Ra vs 4He/20Ne ratios. Atmospheric: 3He/4He = Ra, 4He/20Ne = 0.318; crustal-derived: 3He/4He = 0.02 Ra, 4He/20Ne = 1000; mantle-derived: 3He/4He = 8 Ra, 4He/20Ne = 1000.25 All samples from the Sichuan–Yunnan block are scattered between air and crustal mixing lines, indicating helium of all of the hot spring gases is typical of a crustal origin. Wudalianchi data are from refs (41, 47); Changbai Mountains data are from ref (47) Tengchong data are from refs (46, 47).

In addition, Wudalianchi, Changbai Mountain, and Tengchong hot spring gases are also plotted in Figure 6 for comparison. All Wudalianchi and Changbai Mountain hot spring gas 3He/4He values are greater than 1.0 Ra, indicating that these spring gases are mainly derived from the mantle origin. The hot spring gases from Tengchong, Yunnan Province can be divided into CO2- and N2-rich gases.4 Generally, N2-rich gases have a higher crust source gas input, and CO2-rich gases have a higher mantle source gas input.46

5.2.3. CO2 and CH4 in Hot Spring Gases

The δ13C values of carbonaceous compounds in the hot spring gases contain abundant geochemical information. Generally, the different sources of CO2 have different carbon isotope distribution characteristics. The δ13C value of CO2 with the range of −10 to 0‰ and the CO2 contents greater than 15% can be regarded as an inorganic origin.22 However, CO2 contents and the δ13C values of CO2 less than 10 and −10‰, respectively, can be regarded as an organic origin.48 The δ13C value of CO2 is less than −14‰, which generally indicates the source of organic matter,49 while the δ13C value of marine limestone is usually 0 ± 3‰.50

If the mantle-derived fluids are effectively mixed through the lithosphere without significant fractionation, the CO2/3He ratio in the surface fluids is between the mantle (2 × 109) and crustal (>1010) values.51 For example, the samples from the Sichuan Basin and YNWQ and YBWQ spring gases have a CO2/3He ratio between the mantle and crustal values (Figure 7). However, compared with the mantle and crustal end-member,52 the JHWQ, SSYQ, and HGWQ have significantly low CO2/3He ratios (1.65 × 108, 5.05 × 108, and 3.48 × 106, respectively). 3He production within the crust is dominated by thermal neutron capture by 6Li in reaction 6Li(n,α)3H(β–)3He.53 Even if the Li content in surrounding rocks reaches 100 ppm,54 the local 3He/4He ratio of radiogenic helium cannot exceed 0.1 Ra.55 Thus, 3He is mainly derived from the mantle, and it is impossible for 3He to be added during the upward migration of gas. 3He belongs to noble gas, and there is no correlation between the geothermal system and volcanic activity in the study area. Thus, except for dissolution, almost no process could alter 3He concentrations in the geothermal fluids. When the gas is released from the geothermal fluids, due to the solubility difference in the aqueous solution, CO2 and He may be fractionated.56 Helium prefers to partition into exsolved vapor phase relative CO2, rendering the residual phase CO2/3He values elevated compared to the original values. The CO2/3He ratio in the dissolved gases (JHWQ, SSYQ, and HGWQ) is lower than the bubbling gas (YNWQ and YBWQ), which is similar to the mantle CO2/3He ratio (1–10 × 109).52 Therefore, hydrothermal degassing is not the cause of CO2/3He ratio change.

Figure 7.

Figure 7

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Plot of δ13C(CO2) vs CO2/3He. Except for YNWQ and YBWQ spring gas, other sample data plot below standard mixing fields. Ranges of different sources are from refs (52, 60). The δ13C(CO2) value of the sample with the frame is the initial value, and the δ13C(CO2) value of the sample without the frame is the correction value. Sichuan Basin data are from ref (61).

In the ascending channel, calcite precipitates when the CO2 partial pressure decreases, which leads to the decrease of CO2/3He ratio and the δ13C(CO2) changes.57 Based on the Rayleigh fractionation (δ13C(CO2) = δ13C(CO2)o + ε ln f,58 the value of δ13C(CO2) is measured by the samples, the δ13C(CO2)o is the original value of δ13C(CO2), f is the fraction of CO2 remaining in the geothermal fluids, and ε is the carbon isotope fractionations for precipitation. The specific calculation method is shown in ref (59); the original value of δ13C(CO2) values ranges from −18.6 to −4.1‰. The gas plots below the crustal and mantle range in the CO2/3He-δ13C(CO2) space (Figure 7) imply the loss of CO2 relative to He, as previously observed in numerous natural CO2 accumulations.59 About 83.5, 49.5, and 99.5% of CO2 has been lost in JHWQ, SSYQ, and HGWQ, respectively, assuming mantle-derived CO2 with a typical magmatic range of 1–10 × 109.52 In addition, the loss contents of CO2 reflected by the CO2/3He values are positively correlated with the carbon isotope fractionation degree of CO2 (Figure 7).

In Figure 8, the correlation of Rc/Ra versus δ13C(CO2) indicates that all spring gases from the Sichuan–Yunnan block are plotted within the two mixing lines (between mantle and limestone and between the mantle and organic sediments). The YNWQ and YBWQ spring gases contain 12.77 and 30.92% CO2, respectively, with δ13C(CO2) values of −9.0 and −5.0‰, respectively, which are overlapping that for typical magmatic CO2. However, considering the outcrops of organic-rich shale and carbonate rocks in the Sichuan–Yunnan area8 and the relatively low 3He/4He ratio, CO2 is mainly contributed by limestone mixed with minor mantle and organic CO2. The JHWQ, SSYQ, and HGWQ spring gases from the Sichuan–Yunnan block with low CO2 contents and δ13C(CO2) values are −18.9, −21.6, and −26.3‰, respectively, which are significantly less than those for typical inorganic carbon. Therefore, the spring gas CO2 origins in the Sichuan–Yunnan block are mainly derived from mixing of limestone and organic sediments with minor mantle CO2. The organic-rich shale and carbonate decomposition provides organic sediments and limestone-type carbon as two end-members of the crust that pollute the rising mantle volatiles. However, the δ13C(CO2) value and the 3He/4He ratio of the samples in the Sichuan Basin are less than −10‰ and 0.02 Ra, respectively, indicating that CO2 is mainly derived from the contribution of organic sediments CO2 (Figure 8). In addition, the δ13C(CO2) value and the 3He/4He ratio of the samples in the Quaternary volcanos (Wudalianchi, Tengchong, and Changbai Mountains) are greater than −10‰ and 1.0 Ra, respectively, suggesting the CO2 is mainly derived from the contribution of mantle CO2 (Figure 8).

Figure 8.

Figure 8

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Plot of δ13C(CO2) vs Rc/Ra. Limestone: δ13C(CO2) = 0‰, 3He/4He = 0.02 Ra, organic sediments: δ13C(CO2) = −30‰, 3He/4He = 0.02 Ra; mantle: δ13C(CO2) = −5‰, 3He/4He = 8.0 Ra.25 The δ13C(CO2) value of the sample with the frame is the initial value, and the δ13C(CO2) value of the sample without the frame is the correction value. All samples are distributed between the two end-members of limestone and organic sediments. Wudalianchi data are from refs (21, 41, 47); Changbai Mountains and Tengchong data are from ref (47); Sichuan Basin data are from ref (61).

Generally, the formation of natural CH4 is reported in four ways: (a) biogenic methane formed by bacteria at a temperature less than 100 °C,62 (b) thermal decomposition of organic matter at a temperature greater than 100 °C,63 (c) degassing of the mantle,64 and (d) formation by chemical reactions, such as the Fischer–Tropsch synthesis reaction.65 In Figure 9, the plot of the δ13C(CH4) versus CH4/3He values can effectively identify the four origins of CH4.66 The HGWQ spring gas is located in the mixing between thermogenic methane and biogenic methane. The other spring gas methane is thermogenic methane, which tends to be a heavy carbon isotope. The hot spring gases have a tendency to approach the end components of the EPR (abiogenic) (Figure 9), which suggests that microbial oxidation processes are likely to exist in geothermal systems.66 The microorganisms seem to preferentially consume 12C in methane, resulting in the enrichment of 13C in methane. Therefore, methane has a heavier carbon isotope and a decrease of CH4/3He values in the hot spring gases from the Sichuan–Yunnan block. Compared with the other hot spring gases, the CH4 in YBWQ spring gas may undergo the greatest impact on the degree of microbial oxidation.

Figure 9.

Figure 9

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Plot of δ13C(CH4) vs CH4/3He. The hot spring gas has a tendency to approach the end components of the EPR (abiogenic), which suggests that microbial oxidation processes are likely to exist in geothermal systems.

5.3. Mantle Helium Distribution in an Intracontinental Crust and Its Tectonic Implications

Assuming that the upward transmission rate of helium along the stratum in the study area is equal to that in Southern California (147 mm a–1),67 it takes 4.4 Ma for helium to pass through the 40 km crust. At the same time, helium is in the whole granite (DHe = 5 × 10–7 m2 a–1)53 migration distance is only 1 mm, indicating the importance of faults for the upward transport of helium. Deep and large faults are one of the important channels through which mantle fluids pass through the crust, and the ratio of 3He/4He can reflect the rate of mantle fluids passing through fault zones.68 Usually, 3He/4He ratio in the volcanic areas is higher than that in the areas without magmatic activity.44 The high 3He/4He ratios are usually related to subsurface melting or magmatism.69 The 3He/4He ratio has a significant positive correlation with the deformation rate of the active crust.6 Therefore, the migration of mantle helium in the crust may be influenced by the scale of fault, fault activity rate, and magmatic activity in the Sichuan–Yunnan block.

The hot spring gas is mainly distributed along the fault in the Sichuan–Yunnan block. YNWQ is close to the Xiaojinhe fault, YBWQ, JHWQ, and SSYQ are close to the Jinhe-Qinghe fault, and HGWQ is located in the Kangdian uplift. The Moho depth of the Yunnan block is 50–60 km.70,71 In addition, there is a low-velocity zone within ∼30 km in central Yunnan.72 According to the previous structural geological profile,70−73 the stratigraphic contact relationship on both sides of the Jinhe-Qinghe fault is that the Sinian Dengying formation (Zbd) in the north is thrust napped onto the Triassic Jurassic formation (T3-J1) in the south (Figure 1c). Based on the characteristics of the surface geological structure and deep electrical anomaly, it is considered that the Jinhe-Qinghe fault is a lithospheric fault passing through the Moho surface (Figure 10).45 Therefore, the contribution ratio of mantle-derived helium of hot spring gases (the proportions of mantle-derived helium in YBWQ, SSYQ, and JHWQ samples are 3.90, 5.67, and 2.02%, respectively) near the Jinhe-Qinghe fault is higher than that of hot spring gases distributed in other fault zones (the proportions of mantle-derived helium in YNWQ and HGWQ are 0.74 and 1.25%, respectively). In addition, the lower proportion of mantle-derived helium in YNWQ and HGWQ than other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault.

Figure 10.

Figure 10

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Distribution of structural faults in the Sichuan–Yunnan region (modified from ref (45)).

Although a large amount of mantle-derived helium exists in Tengchong, Pingbian, and Ning’er volcanic areas, there is low mantle-derived helium observed in the Sichuan–Yunnan block. Compared with the mantle-derived helium of typical intraplate volcanic areas in China, the maximum proportion of mantle-derived helium in Tengchong, Changbai Mountain, Hainan, and Wudalianchi volcanic fields can reach 65.6,69 80,74 15.6, and 38%,4,21 respectively. However, the proportion of mantle-derived helium (0.74–5.67%) in the Sichuan–Yunnan block is significantly lower. In addition, when the mantle volatiles was transported along the deep fault in the granitic crust, the α decay of uranium and thorium series elements produces 4He, which is added to mantle fluids. Therefore, the 3He/4He ratio in hot spring gas shows a certain functional relationship with uranium and thorium contents in rock, indicating the time of extracting 4He into the fluids and passing through the crust.75 Geochemical evidence shows that the Mesozoic granites in the Sichuan–Yunnan block have high U and Th contents.11,12 Therefore, mantle-derived helium is diluted by the radiogenic 4He produced in the crust. Besides, the slow transport of mantle-derived helium in deep faults (e.g., the average flow rate of the Karakoram fault is 19 mm a–1)76 will lead to more time mixing of crustal helium and mantle-derived helium.

Fault activity and deformation have a positive correlation with the migration of mantle-derived helium in the crust. The increase of fault slip rate can significantly improve and maintain the high permeability of fault.77 According to the statistical relationship of the helium isotope (Ra) and the strike-slip rate (mm a–1) of the S-wave low-speed anomaly occurring in the 70 km deep region (3He/4He (Ra) = strike-slip rate of fault × 0.127 + 0.551, R2 = 0.869),78 the results show that the strike-slip rates of the Xiaojinhe fault, Jinhe-Qinghe fault, and Kangdian uplift are 0.10, 1.03–3.82, and 0.42 mm a–1, respectively. The fault activity rate calculated by 3He/4He is consistent with the current activity rate characterization of main faults in the Sichuan–Yunnan region through GPS.79 Therefore, the crustal deformation rate is positively correlated with the migration rate of mantle fluids in the crust. According to the above analysis, the low fault activity rate, the small fault scale, and the not obvious magmatic activity result in the low proportion of mantle-derived helium of the geothermal fluids in the Sichuan–Yunnan block.

6. Conclusions

The N2-dominant component (67.05–97.13%) were observed in the hot spring gas samples. contents of the YBWQ sample (67.05%).The helium isotope of all hot spring gases shows 0.068–0.541 Ra, and the percentages of atmospheric, crustal, and mantle-derived helium range 0.33–16.14, 78.2–98.9, and 0.74–5.67%, respectively. The N2–Ar–He triangle diagram and 40Ar/36Ar ratios indicate that N2 and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The low CO2/3He ratio (e.g., 3.48 × 106 in HGWQ) is probably accused by the loss of CO2 by calcite precipitation. The δ13C(CO2)o values calculated by Rayleigh fractionation and the CO2 concentrations suggest that YNWQ and YBWQ are consistent with the inorganic origin and JHWQ, SSYQ, and HGWQ are consistent with the organic origin. The plot of Rc/Ra versus δ13C(CO2)o indicates that the spring gas CO2 origin in the Sichuan–Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO2. The δ13C(CH4) versus CH4/3He values indicate that the origin of methane is thermogenic and microbial oxidation. Mantle helium contents in YBWQ, SSYQ, and JHWQ samples near the Jinhe-Qinghe fault are 3.90, 5.67, and 2.02%, respectively, and higher than those in hot spring gases distributed in other fault zones (mantle helium contents in YNWQ and HGWQ are 0.74 and 1.25%, respectively). Such distribution patterns and low mantle-derived helium proportions are most likely controlled by the low fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan–Yunnan block.

Acknowledgments

The authors are very grateful to the Associate Editor Dr. Mohamed Mahmoud and three anonymous reviewers for their constructive comments and suggestions, which greatly improved the professionalism and standardization of the manuscript. This study was supported by the National Science and Technology Projects of Ministry of Science and Technology of China (No. 2016ZX05003002004), the National Natural Science Foundation of China (Nos. 41072105 and 41872147), and the Project of Exploration Branch of China Petrochemical Corporation “Research on Geological Conditions of Helium Accumulation and Block Optimization in Peripheral Areas of the South China” (No. 35450003-20-ZC0607-0014).

The authors declare no competing financial interest.

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