Insights from Fumarole Gas Geochemistry on the Recent Volcanic Unrest of Pico do Fogo, Cape Verde

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.

Insights from fumarole gas geochemistry on the recent volcanic unrest of Pico do Fogo, Cape Verde

<p>The Cape Verde islands are located about 800 km west of Senegal, at 14°-17° latitude and 21°-25° longitude. The archipelago consists of a volcanic chain of 10 major islands and eight minor islands The only currently active volcano in the Cape Verde archipelago is Pico do Fogo, which is located on the island of Fogo. Rising to 2829 m a.s.l., it is the most active volcano of the Cabo Verde Island. 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. Two fumaroles were sampled, a low (F1~100ºC) and a medium (F2~300ºC) temperature. The variations observed in the δ<sup>18</sup>O and δ<sup>2</sup>H 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 2008-2009 and 2013-2014. Also, two sharp peaks were observed in CO<sub>2</sub>/CH<sub>4</sub> ratios at both fumaroles, in November 2008 and November 2013, coinciding with significant increases in the emission rate of diffuse CO<sub>2</sub> and He, and heat flow measured in the crater of Pico do Fogo volcano. 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 CO<sub>2</sub>/St 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/CO<sub>2</sub>, H<sub>2</sub>/CO<sub>2</sub> and CO/CO<sub>2</sub> 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 <sup>3</sup>He/<sup>4</sup>He (R/R<sub>A</sub>)<sub>cor</sub> are observed in both fumaroles; F1 showed a peak in 2010 from a minima 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 high <sup>3</sup>He/<sup>4</sup>He ratios in both fumaroles are close to the magmatic end-member, 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.</p>

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

<p>Cape Verde archipelago is a cluster of several volcanic islands arranged in a westward opening horseshoe shape located in the Atlantic Ocean, between 550 and 800 km-west of the coast of Senegal (Africa). Fogo Island is located in the southwest of the archipelago, and as main feature is a 9-km-north to south wide collapse caldera opened toward the east, within Pico do Fogo volcano rises 2,829 m.a.s.l. Pico do Fogo crater has an area of 0.142 km<sup>2</sup> and its characterized by a fumarolic field composed by low and moderate temperature fumaroles, with temperatures around 95ºC and reaching 400ºC respectively. The last eruption of Fogo volcanic system took place between November 2014 and February 2015, when four new eruptive vents were formed, and destroyed partially the villages of Portela and Bangaeira (Silva et. al., 2015) forcing the evacuation of 1,300 inhabitants. In this work we present the temporal evolution of <sup>3</sup>He/<sup>4</sup>He isotopic ratio, <sup>3</sup>He and <sup>4</sup>He emission and thermal energy released data measured from March 2007 to November 2018 in the crater of Pico do Fogo. In all the studied temporal evolutions, we can observe two main increases in the above parameters, the first in early 2010, suggesting a magmatic intrusion, and the second several months before the eruption onset. We have also observed that changes in the <sup>3</sup>He emission might be accompanied by a significant increase in thermal output if the system is in an eruptive cycle. Our results confirm <sup>3</sup>He emission studies are highly reliable indicator of imminent volcanic eruption and constitute a powerful tool to monitor the activity of volcanic areas around the world.</p><p>Silva et al., (2015), Geophysical Research Abstracts Vol. 17, EGU2015-13378, EGU General Assembly.</p>

Three-Dimensional interpretation of broadband magnetotelluric data at Fogo Volcano, Azores Islands

<p>The architecture of volcanic systems is essential to know as (1) it yields knowledge on evolution of the volcanic system, thus improving our capability to project future behaviour; (2) as it provides insights regarding geohazards (such as seismic activity, landslides, increased outgassing), crucial for mitigating risk to human population; (3) as it contributes to the assessment of potential for renewable energy resources. High electrical conductivity values are typically associated with volcanic-hydrothermal systems and the magnetotelluric (MT) method has proven to be successful in mapping such conductivity contrasts and constraining volcanic processses.</p><p>The Azores archipelago (Portugal) is formed by nine volcanic islands located in the North Atlantic Ocean where the American, Eurasian, and African plates meet at a triple junction. São Miguel Island is the largest of the archipelago and<span> hosts</span> three trachytic polygenetic volcanoes: Sete Cidades, Fogo (Água de Pau) and Furnas. Following our earlier MT studies at Furnas, 44 high-quality (to ~1000s) broadband MT sites were collected during 2018 across Fogo Volcano and the adjacent Congro region that is prone to seismic swarm activity.</p><p>Our MT studies comprised two avenues: generating geoelectrical models that provided new insights into this unique setting, and investigating and assessing new tools for the MT community.</p><p>(1) Fogo has a resistive core and we do not see a magma chamber beneath.</p><p>(2) Shallow conductive channels are observed beneath Congro and their presence have been tested and validated through forward modelling and additional sensitivity tests.</p><p>(3) The MT results can be used to map clay alteration, with the highly conductive zone on the northern flank of Fogo corresponding to the smectite zone. The alteration temperature distribution is consistent with the formation temperature recorded within the area.</p><p>(4) A potential new geothermal resource has been identified. An area north of Ribeira Cha, on the southern flank of Fogo has very similar characteristics of the Ribeira Grande geothermal system that is located on the northern flank. This area may be key in increasing the energy self-sufficiency of the island. <br><br>Depth slices through the final 3-D MT inversion volume will be presented.</p><p>The new MT processing code of University of Frankfurt was compared against two long-standing codes commonly used in the MT community and it proved to yield superior responses for every site and examples will be presented. See presentation in this session “FFproc - an improved multivariate robust statistical data processing code for the estimation of MT transfer functions” (Castro et al).</p><p>A novel approach exploiting the Jacobian matrix elements for the 3-D MT inversion strategy will be presented. The Jacobian matrix elements map the relationship between the model and model responses, thus portions of the model with low sensitivities infer that the sensitivity structure is algorithmically influenced more strongly by the regularisation term than by the data-fitting term. Our results will show that the computation of the Jacobian matrix (albeit computationally expensive) is a powerful tool in aiding interpretation.</p>

Assessing the impact of explosive eruptions of Fogo volcano (São Miguel, Azores) on the tourism economy

The Azores are an active volcanic region that offers exceptional conditions for nature-based tourism, one of the main axes of economic growth in the archipelago. A future volcanic eruption may have long-term consequences to this economic sector. Therefore, it is fundamental to assess its vulnerability to volcanic hazards in order to try to mitigate the associated risk. This study proposes a new approach to assessing the economic impact of explosive eruptions on the tourism sector. We considered two eruptive scenarios for Fogo volcano (São Miguel Island), the most probable (Volcanic Explosivity Index, VEI, 4 sub-Plinian eruption) and the worst-case (VEI 5 Plinian eruption), both producing tephra fallout and pyroclastic density currents. The results of numerical simulations were overlaid with tourism-related buildings and infrastructure of Vila Franca do Campo municipality to identify the elements at risk. The loss present value method was used to estimate the benefits generated by the accommodation units over 30 years for different economic scenarios. The assessment of the economic impact using 2018 indicators reveals that in a near-total-destruction scenario, the economic loss is approximately EUR 145 million (considering a 2 % discount rate). This approach can also be applied to other volcanic regions, geologic hazards and economic sectors.

Assessing the impact of explosive eruptions of Fogo volcano (São Miguel, Azores) on the tourism economy

The Azores is an active volcanic region that offers exceptional conditions for nature-based tourism, one of the main axes of economic growth in this region. A future volcanic eruption in the archipelago may have long-term consequences to this economic sector. Therefore, it is fundamental to assess its vulnerability to volcanic hazards in order to try to mitigate the associated risk. This study proposes a new approach to assess the economic impact of explosive eruptions on the tourism sector. We considered two eruptive scenarios for Fogo volcano (São Miguel Island), the most probable (VEI 4 sub-Plinian eruption) and the worst-case (VEI 5 Plinian eruption), both producing tephra fallout and PDCs. The results of numerical simulations were overlaid with tourism-related buildings and infrastructure of Vila Franca do Campo municipality to identify the elements at risk. The Loss Present Value method was used to estimate the benefits generated by the accommodation units over 30 years for different economic scenarios. The assessment of the economic impact using 2018 indicators reveals that in a near total destruction scenario the economic loss is approximately 145 million euros. Such approach can also be adopted to other volcanic regions, other geologic hazards and other economic sectors.