Search

We report the results of the geochemical monitoring of the fumarolic discharges at the Pico do Fogo volcano in Cape Verde from 2007 to 2016. During this period Pico do Fogo experienced a volcanic eruption (November 23, 2014) that lasted 77 days, from a new vent ∼2.5 km from the fumaroles. Two fumaroles were sampled, a low (F1∼100°C) and a medium (F2∼300°C) temperature. The variations observed in the δ18O and δ2H in F1 and F2 suggest different fluid source contributions and/or fractionation processes. Although no significant changes were observed in the outlet fumarole temperatures, two clear increases were observed in the vapor fraction of fumarolic discharges during the periods November 2008–2010 and 2013–2014. Also, two sharp peaks were observed in CO2/CH4 ratios at both fumaroles, in November 2008 and November 2013. This confirms that gases with a strong magmatic component rose towards the surface within the Pico do Fogo system during 2008 and 2013. Further, F2 showed two CO2/Stotal peaks, the first in late 2010 and the second after eruption onset, suggesting the occurrence of magmatic pulses into the volcanic system. Time series of He/CO2, H2/CO2 and CO/CO2 ratios are low in 2008–2009, and high in 2013–2014 period, supporting the hypothesis of fluid input from a deeper magmatic source. Regarding to the isotopic composition, increases in air-corrected 3He/4He ratios are observed in both fumaroles; F1 showed a peak in 2010 from a minimum in 2009 during the first magmatic reactivation onset and another in late 2013, while F2 displayed a slower rise to its maximum in late 2013. The suite of geochemical species analyzed have considerably different reactivities, hence these integrated geochemical time-series can be used to detect the timing of magmatic arrivals to the base of the system, and importantly, indicate the typical time lags between gas release periods at depth and their arrival at the surface. The high 3He/4He ratios in both fumaroles in the range observed for mid-ocean ridge basalts, indicating that He is predominantly of upper mantle origin. This work supports that monitoring of the chemical and isotopic composition of the fumaroles of the Pico do Fogo volcano is a very important tool to understand the processes that take place in the magmatic-hydrothermal system and to be able to predict future episodes of volcanic unrest and to mitigate volcanic risk.

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. 3He/4He ratio measured 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. 3He and mantle 4He 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 3He (1.7 × 10−2 and 1.5 × 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 , 4He and 3He emissions is observed due to the rapid degassing of the magma responsible of the 2014–2015 eruptive process. • We report the temporal evolution of 3He/4He ratio and thermal energy from the crater of Pico do Fogo between 2007 and 2018. • An early mantle He was observed in 2010, but the most important was measured several months before eruption onset. • A link between 3He and thermal energy has provided a discriminator for intrusions with enough energy to produce an eruption. [ABSTRACT FROM AUTHOR]