Evolution of hydrothermal explosions at Rehai geothermal field, Tengchong volcanic region, China

During the period 1993-2003 over 20 hydrothermal eruptions occurred at Rehai geothermal field, in Tengchong volcanic region, Yunnan Province, southwestern China. Initially, the explosions occurred on the bed of the Zaotang River, and later along a NW-SE striking fault cross-cutting the river. The explosion activity intensified with time. The free gas samples collected from springs created by the explosions show 3He/4He ratios of 1.17, 2.22 and 4.05 Ra (Ra = 1.4 × 10−6) for the three distinguishable stages of ever-increasing magnitude of the explosions, respectively. Data on helium isotopes and gas chemistry indicate that the source of the gases feeding the explosions has progressively shifted from shallower to greater depth. Following this trend, larger hydrothermal explosions will probably occur in the future, so that this hazard must be taken into serious consideration.