Variations of geothermometry and chemical-isotopic compositions of hot spring fluids in the Rehai geothermal field, southwestern China

Abstract

Geothermal variations, origins of carbon-bearing components and reservoir temperatures in the Rehai geothermal field (RGF) of Tengchong volcanic area, Yunnan Province, southwestern China, are discussed on the basis of carbon isotope compositions, combined with helium isotope ratios and geothermal data from 1973 to 2000. δ 13C values of CO 2, CH 4, HCO 3 –, CO 3 = and travertine in the hot springs range from −7.6‰ to −1.18‰, −56.9‰ to −19.48‰, −6.7‰ to −4.2‰, −6.4‰ to −4.2‰ and −27.1‰ to +0.6‰, respectively. The carbon dioxide probably has a mantle/magma origin, but CH 4 and He have multiple origins. HCO 3 – and CO 3 = in RGF thermal fluids are predominantly derived from igneous carbon dioxide, but other ions originate from rocks through which the fluids circulate. The 13C values of CO 2, HCO 3 – (aq) and CO 3 = (aq) illustrate that isotopic equilibriums between CO 2 and HCO 3 – (aq), and CO 3 = (aq) and between DIC and travertine were not achieved, and no carbon isotope fractionation between HCO 3 – (aq) and CO 3 = (aq) of the hot springs in RGF was found. Using various geothermometers, temperatures of the geothermal reservoirs are estimated in a wide range from 69 °C to 450 °C that fluctuated from time to time. The best estimate of subsurface reservoir temperature may be 250–300 °C. Contributions of mantle fluids and shallow crust fluids in Rehai geothermal field varied with time, which resulted in variations of chemical and isotopic compositions and reservoir temperatures.