EFFECTS OF VOLCANISM, CRUSTAL THICKNESS, AND LARGE SCALE FAULTING ON THE HE ISOTOPE SIGNATURES OF GEOTHERMAL SYSTEMS IN CHILE

摘要

The Chilean cordillera provides a unique geologic setting to evaluate the influence of volcanism, crustal thickness, and large scale faulting on fluid geochemistry in geothermal systems. In the Central Volcanic Zone (CVZ) of the Andes in the northern part of Chile, the continental crust is quite thick (50-70 km) and old (Mesozoic to Paleozoic), whereas the Southern Volcanic Zone (SVZ) in central Chile has thinner (～40 km) and younger (Cenozoic to Mesozoic) crust. In the SVZ, the Liquine-Ofqui Fault System, a major intra-arc transpressional dextral strike-slip fault system which controls the magmatic activity from 38°S to 47°S, provides the opportunity to evaluate the effects of regional faulting on geothermal fluid chemistry. Measurements of ~3He/~4He and ~4He/~(36)Ar ratios in Chilean geothermal water and gas samples have been used to guide exploration efforts. He gas samples were collected from the El Tatio geothermal system in the CVZ and the Tinguirgirica, Chillan, and Tolhuaca geothermal systems in the SVZ. The Rc/Ra values for gas samples collected from hot springs and fumaroles at El Tatio range from 1.39 to 2.44. This range of values supports the model that magmatic fluids in the CVZ mix with crustally derived ~4He, which would be more abundant in the thicker, older silicic crust that is present in northern Chile, resulting in lower ~3He/~4He values for this region. Higher ~3He/~4He values were observed for most of the thermal features sampled in the SVZ. Gas sampled from a distal chloride hot spring at Tinguiririca yielded an Rc/Ra value of 1.4, and a sample collected from a fumarole at Chillan had a value of 3.21. Three fumaroles and two steam-heated bicarbonate hot springs were sampled at the Tolhuaca geothermal system, which is located near the Liquine-Ofqui Fault System. The Tolhuaca samples had He Rc/Ra values ranging from 6.16 to 6.71, suggesting that the geothermal fluids from this system have a significant mantle noble gas component, similar to that observed in fluids from active volcanic arc complexes. These new values agree with previously published results for the Chilean Andes.