Modeling past and present activity of a subarctic hydrothermal system using O, H, C, U and Th isotopes

The hot springs of the Pymvashor subarctic hydrothermal system are of considerable interest because the area is devoid of recent volcanism and is located in the permafrost region. We attempted to evaluate the activity of thermal waters with respect to host rocks to quantify the water residence time in this system and date the associated travertine. Therefore, we used the chemical composition of the thermal waters, thermodynamic modeling, δ18O and δ2H labels and isotopes, such as 14C–δ13C, 234U–238U, and 230Th–232Th. The δ18O and δ2H values indicated the infiltration of atmospheric water in the recharge area of the hydrothermal system and suggested a stable paleoclimate in the area over the last 5–7.9 thousand years. The fresh water flows through deep parts of the aquifer system where it mixes with brine followed by discharge. The hot springs geothermal water total dissolved solid (TDS) ranged from 1.8 to 2 g/L, and in the deep wells, the TDS ranged from 7.1 to 198 g/L. The ratios of Na/Cl (mol), Br/10−3Cl (ppm), and Ca/Cl (ppm) in the thermal springs ranged from 0.89 to 0.90, 1.8 to 1.9, and close to 0.12, respectively, reflecting participation of deep brines in their formation. The composition of the thermal water can be formed via a mixture of one part of the brines with 130 parts of the cold water end member with a TDS of 291 mg/L. The results of thermodynamic modeling and mixing diagram analysis indicate that during water–rock interaction in the aquifer, the precipitation of calcite and the dissolution of gypsum and magnesite were accompanied by hydrolysis of the sodium aluminosilicates with precipitating clay secondary minerals. The low uranium concentration in the Pymvashor groundwater (0.24–0.34 ppb) and the sufficiently long water residence time combined with the relatively high 234U/238U activity ratios (3–5) suggest a high α recoil loss and low dissolution rates of the host rocks or a high precipitation rate and adsorption of uranium. The elevated values of the α recoil loss may be due to radioactive decay of the precipitated and adsorbed 238U because in this case, the probability of 234Th release and 234U appearance in water increases ∼4-fold compared to the probability of emission directly from the rock. The 14C age of the water was estimated to be between 4960 and 7870 years, and the 230Th/U age of the travertine ranged from 1970 to 7650 years. Overall, these results allow for a better understanding of the nature and evolution of the thermal waters in this unique subarctic hydrothermal system.