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 (F1~100ºC) and a medium (F2~300ºC) 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. 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.

中文翻译：

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关于佛得角 Pico do Fogo 最近火山动荡的喷气孔气体地球化学见解

我们报告了 2007 年至 2016 年佛得角 Pico do Fogo 火山喷气孔排放的地球化学监测结果。距离喷气孔约 2.5 公里。采样了两个喷气孔，低 (F1~100ºC) 和中 (F2~300ºC) 温度。在 F1 和 F2 中观察到的 δ18O 和 δ2H 变化表明不同的流体源贡献和/或分馏过程。尽管出口喷气孔温度没有观察到显着变化，但在 2008-2010 年和 2013-2014 年期间，观察到喷气孔排放的蒸汽分数有两次明显增加。此外，在 2008 年 11 月和 2013 年 11 月，在两个喷气孔的 CO2/CH4 比率中都观察到了两个尖峰。这证实了在 2008 年和 2013 年期间，具有强烈岩浆成分的气体在 Pico do Fogo 系统内向地表上升。此外，F2 显示出两个 CO2/Sttal 峰，第一个在 2010 年末，第二个在喷发开始后岩浆脉冲进入火山系统。He/CO2、H2/CO2 和 CO/CO2 比值的时间序列在 2008-2009 年较低，在 2013-2014 年较高，支持流体来自更深岩浆源的假设。关于同位素组成，在两个喷气孔中都观察到经空气校正的 3He/4He 比率增加。分析的地球化学物种套件具有相当不同的反应性，因此这些综合地球化学时间序列可用于检测岩浆到达系统底部的时间，重要的是，表示深部气体释放期与其到达地表之间的典型时间滞后。在大洋中脊玄武岩观察到的范围内，两个喷气孔中的 3He/4He 比率都很高，表明 He 主要来自上地幔。这项工作支持监测 Pico do Fogo 火山喷气孔的化学和同位素组成是了解岩浆-热液系统中发生的过程并能够预测未来火山动荡和以减轻火山风险。