Field sampling and measurements
Field works at Heinitang were carried out in August 2013 and March 2018. Two water samples (S2* and S4*) were collected in 2013 and another six water samples (S1, S2, S3, S4, S5 and S6) were collected in 2018. S7 could not be collected since it was surrounded by mud. The water samples were collected as close to the vents as possible. Water samples for chemical and isotopic analyses were collected in 550 ml glass bottles which were water-rinsed at least 3 times prior to field sampling. The water samples were sent to the laboratory as soon as possible.
In situ measurements of some physicochemical characteristics of hot springs were completed at all the sampling points. Water samples were measured for free carbon dioxide (CO2), temperature, pH and Eh in the field (Table 1). The temperatures of water samples were determined with a MIK-TP101 thermometer. The values of pH and Eh were determined in situ using a CT-6821 pH meter. The concentrations of CO2 were measured by NaOH-HCl titration in situ using digital titrator manufactured by the HACH company immediately after samples collection (Table 1).