Stable chlorine isotopes in saline springs from the Nangqen basin, Qinghai–Tibet Plateau: Brine genesis and evolution

Abstract

Chlorine isotopes can be used to study the evolution of different fluids, sources and the causes of various related deposits. In this study, Cl concentrations and chlorine isotope (d 37 Cl, IAEA ISL-354 NaCl standard) values were determined for brine samples from Nangqen basin, located on the southern boundary of the Qinghai-Tibet plateau to study the source and the processes of these saline springs. The results demonstrated that the saline springs are distributed around a fault or fault zone, with a high average salinity of 228.30 g/l and flow rates ranging from 1.7 to 0.01 l/s. Na + and Cl À are the predominant cations and anions, respectively, accounting for more than 90% of the total. The d 37 Cl values range from À1.55% to +0.97%, and the Cl/Br ratios are from 1739 to 175,260. Coupled with the previous H, O and B isotope compositions (dD, VSMOW2 standard, ranges from À100.91% to À132.98%, d 18 O, VSMOW2 standard, from À12.88% to À16.05% and d 11 B, NIST 951 standard, from +3.55% to +29.59%), it can be interpreted that the saline springs are mainly the result of the dissolution of halite hosted in mudstone and volcanic country rocks.

Origin and Evolution of Eocene Rock Salts in Pakistan and Implications for Paleoclimate Studies: Insights From Chemistry and Cl Stable Isotopes

“…Figure 6 shows that the Bahadurkhel and Karrak samples are mostly characterized by rather constant δ 37 Cl ∼−0.2‰ but varying Br/Cl ratios, whereas the Jatta samples display somewhat positive δ 37 Cl values with considerably constant Br/Cl ratios. This may be explained by frequent and multiple dissolution and precipitation processes as Br contents decline with successive recycling processes (Eggenkamp et al, 2019b;Han et al, 2019). It also indicated that the basin was disconnected from the main water reservoir (i.e.…”

“…Brennan et al (2013) reported Br − concentrations of 11 ppm for halites from the Catalan sub-basin, whereas Moretto (1986) reported Br − concentrations of 10 ppm for the Bresse Basin. The Eocene salts from Bhadur Khel have lower Br − concentrations compared to the other Paleogene and Neogene basins (1.3-2.37 ppm) that indicate an entry of non-marine water (Shouakar-Stash, 2008) or the dissolution or recycling of ancient marine halite (Brennan et al, 2013;Han et al, 2019;Eggenkamp et al, 2019b). The study of the Cl/Br ratio has proved to be one of the most effective and sensitive indexes when defining the geological environment, brine concentrations and the depositional stages associated to the brine evaporation process (e.g.…”

“…The study of the Cl/Br ratio has proved to be one of the most effective and sensitive indexes when defining the geological environment, brine concentrations and the depositional stages associated to the brine evaporation process (e.g. Han et al, 2019;Hussain et al, 2019). Experiments have demonstrated that this Cl/Br ratio is related to the brine and salt evolution (Braitsch, 1962): the more advanced brine solutions produce lower Cl/Br ratios in the precipitates (Eggenkamp et al, 2019a;Eggenkamp et al, 2019b).…”

“…Guo et al, 2017;Hussain et al, 2020a). Classical chemistry and isotope systematics have already demonstrated their added value to better constrain the sources and processes leading to the formation of these evaporites (Han et al, 2019;Hussain et al, 2020a;Hussain et al, 2021). For example, the Br/Cl ratio and Na-Cl-Br concentrations measured in evaporites can be used to characterize the formation and evolution, including recrystallizsation, dissolution and redeposition processes, of basin fluids (e.g.…”