Changes in the thermal energy and the diffuse 3He and 4He degassing prior to the 2014-2015 eruption of Pico do Fogo volcano, Cape Verde

We report the temporal evolution of helium isotopes emission and thermal energy release in the period 2007-2018 at the summit crater of Pico do Fogo volcano, Cape Verde. He-3/He-4 ratiomeasured in a fumarolic discharge, ranged from 6.69 to 8.63 R-A, being the maximum mantle-derived composition measured in November 2013, one year before the eruption onset in 23 November 2014, and confirm the beginning of a possible magmatic intrusion four years and eight months before the 2014-2015 eruption. He-3 and mantle He-4 emission rates showed similar trends in temporal evolution. Thermal energy release calculated for the study period ranged from 1 up to 72 MW, being the maximum values linked to maximum He-3 (1.7 x 10(-2) and 1.5 x 10(-3) mol d(-1) using the first and the second approach respectively), confirming a close relationship between both parameters. After the eruptive event, a simultaneous decrease in R-A, He-4 and He-3 emissions is observed due to the rapid degassing of the magma responsible of the 2014-2015 eruptive process. (C) 2021 Elsevier B.V. All rights reserved.