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

Abstract

Wereport 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 alongbothN–SandN–Wriftsofthevolcano.4Hefluxvalues,calculatedfollowingapurediffusivemodel,ranged betweennon-detectablevaluesand1.0 mg m−2 d−1in2002,2003and2004surveys.Total 4Heemissionrateat CumbreViejawasestimatedintherangeof18–38 kg d−1.Isotopiccompositionofheliummeasuredinsoilgases along the N–S volcanic-rift of Cumbre Vieja showed small contributions of MORB-type helium increasing towardsthesouthernpartofthevolcano.A 3He/4Heratioof9.95±0.12RAwasmeasuredatacoldspring(geothermal) sample, showing a 7.2% contribution of plume-type helium. 3He emission results in the range of 0.6–0.7 mol/yr support the hypothesis of contemporarydegassing at CumbreVieja coming frommagmasstored at different depths under La Palma island