Diffuse He and H2 degassing survey for geothermal exploration at La Palma, Canary Islands

During geothermal exploration, the geochemical methods are extensively used and play a major role in both exploration and exploitation phases. Discovery of new geothermal systems at those areas where the resources are either hidden or lie at great depths, the geochemical methods for geothermal exploration must include soil gas surveys, based on the detection of anomalously high concentrations of some hydrothermal gases in the soil atmosphere. We report herein the results of an intensive soil gas study, focused on non-reactive and/or highly mobile gases such as helium (He) and hydrogen (H₂), in Cumbre Vieja volcano, the volcanically ac tive part of La Palma island (Canary Islands, Spain). He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water. H2 is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas.

A detailed geochemical survey was carried out in an area of 25 km² at the western side of La Palma. A total of 766 sites were sampled at 40 cm depth using a metallic probe with 60 cc hypodermic syringes and stored in 10 cc glass vials for later laboratory analyses. Spatial distribution maps of diffuse He and H₂ emission were constructed to study the presence of enhanced vertical permeability areas related to high temperature hydrothermal activity at depth. As a result, the main He emission anomalies show different size well-defined concentric-shape structures, distributed along a west to east main direction. On the other hand, H₂ highest fluxes are slightly more dispersed in different areas, although some of the main ones are coincident with the distribution of the well-known anomalous volcanic CO₂ active diffuse degassing in Puerto Naos and La Bombilla villages. Soil He and H₂ surveys have demonstrated to provide meaningful insights of areas that could be acting as preferential zones of vertical permeability that allow deep source gases migration to surface and, therefore, of potential geothermal system structures.