Helium emission at Cumbre Vieja volcano, La Palma, Canary Islands

We report herein the results of three soil helium surveys undertaken at Cumbre Vieja volcano, La Palma Island, Canary Islands. Helium content analyses at the surface environment of the volcano were carried out in the summers of 2002, 2003 and 2004. To estimate the diffusion contribution of helium emission, soil porosity was estimated indirectly from the granulometry data measured on a previous soil survey carried out in 1997. Helium enrichments in the soil layer with respect to the air concentration measured on Cumbre Vieja indicate a strong structural control in the degassing processes of the volcano and the excess helium seems to be emitted mainly along both N-S and N-W rifts of the volcano. He-4 flux values, calculated following a pure diffusive model, ranged between non-detectable values and 1.0 mg m(-2) d(-1) in 2002, 2003 and 2004 surveys. Total He-4 emission rate at Cumbre Vieja was estimated in the range of 18-38 kg d(-1). Isotopic composition of helium measured in soil gases along the N-S volcanic-rift of Cumbre Vieja showed small contributions of MORB-type helium increasing towards the southern part of the volcano. A He-3/He-4 ratio of 9.95 +/- 0.12 R-A was measured at a cold spring (geothermal) sample, showing a 7.2% contribution of plume-type helium. He-3 emission results in the range of 0.6-0.7 mol/yr support the hypothesis of contemporary degassing at Cumbre Vieja coming from magmas stored at different depths under La Palma island. (C) 2012 Elsevier B.V. All rights reserved.