He, Ar, C and N isotopes in thermal springs of the Chukotka Peninsula: Geochemical evidence of the recent rifting in the north-eastern Asia

Abstract Thermal springs with water temperatures up to 97 °C were sampled in the Chukotka Peninsula of the north-eastern Asia on the area of ~ 60,000 km 2 . He and Ar isotope ratios in bubbling gases were measured in 30 samples of from 23 spring groups. 3 Не/ 4 Не = R values vary in the range of (20.2 ÷ 172) × 10 − 8  = (0.14 ÷ 1.23)R atm . The over-atmospheric 40 Ar/ 36 Ar ratio values were found out in 18 groups. The compositions of gas phase were determined in 26 groups. In 16 of them N 2 content exceeds 90 vol.%. at δ 15 N values = (0.0 ÷ + 4.5) ‰. CO 2 contents vary from 13 C CO 2 values determined in 19 samples vary from − 20.4 to − 4.4‰. In several samples δ 13 C values were also measured in methane and ethane. Geochemical specificity of gases was revealed in Kolyuchin–Mechigmen Zone (KMZ) of the Peninsula. The zone coincides with the trough of NW–SE strike that dissected the Precambrian basement of this part of the Chukotka–Seward microcontinent during the Triassic phase of tectonic activation. The latest re-activation of KMZ resulted in its subsidence during Quaternary, seismic activity and basic volcanism. The KMZ gases contain maximal contribution of light (mantle-derived) 3 He and carbon dioxide whereas this CO 2 , and associated non-atmospheric N 2 and Ar are all enriched by heavy isotopes ( 13 C, 40 Ar, and 15 N, respectively). The waters of these springs are characterized by the highest reservoir temperatures estimated by solute geothermometers. These features are related to upwelling of mantle-derived melt supplying into the crust 3 He-enriched helium and thermal energy stimulating rock metamorphism.