Helium, Argon and Carbon Isotopic Compositions of Spring Gases in the Hainan Island, China

Chemical and isotopic compositions have been measured for N2-He-rich bubbling gases discharging from hot springs in the Hainan Island, Southern China. Observed 3He/4He ratios (0.11.3 RA) indicate the occurrence of a mantle component throughout the island, which has been highly diluted by a crustal radiogenic 4He component. The occurrence of mantle-derived helium is high in the northern island (12%16% of total He) and gradually decreases towards southern coast (l%3% of total He). Such a distribution pattern is most likely controlled by the Pleocene-Quaternary volcanic activities in the northern island and groundwater circulation along the deep major faults. The 40Ar/36Ar and N2/Ar ratios suggest that N2 and Ar of the hot spring gases are mostly meteoric. Although d13C values of CO2 (20 parts per thousand to -27 parts per thousand) with low concentrations are consistent with the biogenic origin, the combination of 3He/4He and d13CCO2 suggests a two end-member mixing of mantle and crustal components with CO2/3He ratios of 2x109 and 8x1011, respectively. However, the low CO2/3He ratios (122x106) can not be ascribed in terms of the simple mixing but has to be explained by the addition of radiogenic 4He and loss of CO2 by calcite precipitation in the hydrothermal system, which is most likely controlled by the degree of gas-water-rock interaction.