# Hydrogeochemistry of Fault-Related Hot Springs in the Qaidam Basin, China
## 4 Discussion
### Chemical Characteristics of the Hot Spring Gas
#### 4.2.2 Isotopic Characteristics of Helium and Neon  
Helium gas in the hot springs has multiple origins and is the most sensitive geochemical tracer used to identify mantle-sourced gases. Since the \({}^{3}\)He/\({}^{4}\)He values in the mantle source, crust source, and air have their respective characteristics, the corresponding sources may be further analyzed based on the \({}^{3}\)He/\({}^{4}\)He values of the samples [70]. In order to estimate the proportions of mantle-derived helium (Hem), it was proposed that the Rm values should be corrected according to the atmospheric value of the \({}^{3}\)He/\({}^{4}\)He ratio to exclude atmospheric contamination. This is expressed as R\({}_{\rm c}\)/R\({}_{\rm a}\), where R\({}_{\rm c}\) denotes the \({}^{3}\)He/\({}^{4}\)He value of the sample after the removal of atmospheric interference. The calibration method uses the following equation:  
\[\rm R_{c}=\frac{R\times(\frac{4 He}{\beta_{Ne}})_{V}-R_{a}\times(\frac{4 He}{ \beta_{Ne}})_{a}}{(\frac{4 He}{\beta_{Ne}})_{V}-(\frac{4 He}{\beta_{Ne}})_{a}} \tag{7}\]  
In Equation (7), R denotes the \({}^{3}\)He/\({}^{4}\)He value of the sample, R\({}_{\rm a}\) denotes the atmospheric \({}^{3}\)He/\({}^{4}\)He, which is equal to 1.39 \(\times\) 10\({}^{-6}\), \((\rm{{}^{4}He}/^{20}Ne)_{V}\) denotes the \({}^{4}\)He/\({}^{20}\)Ne value of the sample, and \((\rm{{}^{4}He}/^{20}Ne)\)a denotes the atmospheric \({}^{4}\)He/\({}^{20}\)Ne, which is equal to 0.318. The R\({}_{\rm C}\) values obtained for the gas samples from the YTBLK and AK hot springs were 0.1496 R\({}_{\rm a}\) and 0.4389 R\({}_{\rm a}\). Both values were less than 1 R\({}_{\rm a}\), indicating the characteristics of upper crustal-sourced helium. The \({}^{3}\)He/\({}^{4}\)He gas ratios in the Valles Caldera region, New Mexico indicate a deep magmatic source (R/R\({}_{\rm a}\) up to 6) [71]. The \({}^{3}\)He/\({}^{4}\)He ratios of the hot springs in this study area are 2.085 and 6.116, and the values of the AK springs are close to those of Valles Caldera region. After excluding atmospheric helium interference, it was observed that R\({}_{\rm c}\)/R\({}_{\rm a}\) > 0.1, which suggested that mantle-sourced helium additions occur in the hot springs of the present study region [72; 73]. Since the \({}^{4}\)He/\({}^{20}\)Ne-R/Ra relationships of the mantle, crust, and atmosphere are relatively fixed, a \({}^{4}\)He/\({}^{20}\)Ne-R/Ra diagram may be employed to visually and effectively discern the genesis and the source regions of the fluids and rare gas. \({}^{3}\)He/\({}^{4}\)He = 1.39 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 0.318 in the atmosphere; \({}^{3}\)He/\({}^{4}\)He = 12 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 100,000 in the mantle; \({}^{3}\)He/\({}^{4}\)He = 0. 02 \(\times\) 10\({}^{-6}\), \({}^{4}\)He/\({}^{20}\)Ne = 100,000 in the continental crust. The area between the two black lines represents the mixing of atmospheric, crustal, and mantle helium [74]. The percentage of mantle-sourced helium could then be calculated by casting the hot spring gas data into the \({}^{4}\)He/\({}^{20}\)Ne-R/Ra relationship diagram. The release of the mantle-sourced helium from theYTBLK spring was 3.06%, and that from the AK spring was 7.38% (Figure 7). These results suggested an intrusion of mantle-sourced material in the study region. Large amounts of mantle-derived helium are evident in volcanic regions [75]. However, no active Cenozoic volcanoes have been located in the vicinity of the study region, which suggests that the mantle-derived helium in the study region did not have a volcanic release. The Altun Tagh fault and the East Kunlun fault belt serve as good channels for the upward transport of mantle-derived helium, with dilution of the mantle-source helium via upper crustal-source helium during the upward transport, and this phenomenon is also observed in the Xianshuihe fault [38].