Effects of hydrothermal processes on the chemical and isotopic composition of mantle-derived gases in SE China

Chemical and isotopic (He, Ar and C) compositions have been measured for bubbling gases discharging from two closely-spaced earthquake prediction wells in Guangdong, SE China. Observed 3He/aHe ratios (R/RA ranging from 1.1 to 4.9) indicate the occurrence of a mantle component that has been diluted by a crustal radiogenic 4He component. 613C values of CO2 suggest a different origin in the two wells: magmatic for ZK#1 (-6.8 to -8.4%0) and biogenic for ZK#7 (- 14.5 to - 18.2%o). Observed 4°Ar/36Ar and air-corrected Nz/Ar ratios suggest that nitrogen and argon of both wells are mostly meteoric. While the CO2/~He ratios (-1 × 109) in ZK#1 are close to that of MORB (Middle Oceanic Ridge Basalts), the ZK#7 is characterized by much lower CO2/3H e ratios (~1×107) with lower CO2 concentration. The latter cannot be ascribed in terms of a simple mixing of the two components but has to be explained by removal of CO2, probably caused by calcite precipitation. Alternatively, CO2 removal from and 4He addition to the magmatic component may be responsible for the coupled variations of 3He/4He and CO2/He ratios in the two wells, which is most likely controlled by the degree of gas-water-rock interaction. © 1997 CNR. Published by Elsevier Science Ltd. All rights reserved