Dynamics of diffuse helium degassing from Cumbre Vieja volcano, La Palma, Canary Islands

La Palma island (730 km²) is the northwestern most island of the Canarian archipelago. Cumbre Vieja volcano (220 km²) is the result of the volcanic activity in the southern part of the island during the last 1 Ma. Six historical eruptions had occurred at Cumbre Vieja, and the most recent one took place at the extreme south of this volcano in 1971. Three major volcanic rift-zones trending N-S, NW-SE and NE-SW constitute Cumbre Vieja\'{ }s major structural features. The aim of this study is evaluate the use of diffuse helium degassing for monitoring Cumbre Vieja volcano since (1) diffuse degassing studies seems to become a powerful geochemical tool for volcano surveillance (Hernández et al., 2001), and (2) helium is an ideal geochemical gas tracer because it is chemically inert, physically stable, sparingly soluble in water under ambient conditions. Since 1997 diffuse degassing surveys are regularly performed at Cumbre Vieja. During the last 2 years these surveys have investigated helium in the soil atmosphere. Soil gas samples were collected at 40 cm deep using a metallic probe and stored in vacutainers by means of water displacement technique. Soil gas samples were analyzed for ⁴He and CO₂ contents by means of a QMS within 24 hours. CO₂ efflux measurements were also performed by means of a portable NDIR sensor according to the accumulation chamber method. Δ He contour maps (Δ He = He_{soil atmosphere}-He_air) were constructed using kriging as interpolation method. Both surveys showed a good spatial agreement for Δ He, and their peak values (> 1,800 ppbV) were mainly observed at the summit area of Cumbre Vieja along the N-S rift-zone, suggesting a deep origin for the degassing through this major structure. The total output for diffuse ⁴He emission rate at Cumbre Vieja was estimated by multiplying CO2 efflux times Δ He /Δ CO₂ ratio at each sampling site. The results showed an increase on the diffuse ⁴He emission rate from 67 to 167 kg d^-1. Monitoring these results could be useful for the volcanic surveillance.