Helium and carbon isotopic compositions of thermal springs in the earthquake zone of Sichuan, Southwestern China

The origins of gases from hot springs and earthquake activity in the western part of Sichuan Province, Southwestern China, are discussed in terms of helium and carbon isotopes and energy released by earthquakes. δ13C values of CO2 in free and dissolved gases range from −3.34 to −17.09‰, and 3He/4He ratios vary from 1.5×10−8 to 3.63×10−6. The isotopic compositions indicate that carbon dioxide from the hot springs is a mixture of crustal and mantle CO2, and helium is derived from the crust, mantle and atmosphere. The thermal springs in different earthquake and/or fault zones, as well as in different parts of the same zone, have different values of δ13C and 3He/4He. The more tectonically active the district is, the larger the δ13C and 3He/4He values and the greater the frequency of earthquakes. This means that more heat energy is derived from the deep earth with upward migration of anatectic fluids in the active-tectonic district, such as in the Kangding district. Therefore, hot spring gases (He, CO2 etc.) may be important geochemical markers for determination of seismological and tectonic activity.