J. Maarten de Moor
Alessandro Aiuppa
Geoffroy Avard
Henriette Bakkar
Marcello Bitetto
M. M. Mora Fernandez
Peter J. Kelly
Gaetano Giudice
Dario Delle Donne
Hairo Villalobos
Angelo Battaglia
2019
<p><span>Understanding the trigger mechanisms of phreatic eruptions is key to mitigating the effects of these hazardous but poorly forecastable volcanic events. It has recently been established that high-rate volcanic gas observations are potentially very suitable to identifying the source processes driving phreatic eruptions, and to eventually detecting precursory changes prior to individual phreatic blasts. In February-May 2017, we deployed a Multi-GAS instrument to continuously monitor gas concentrations in the crater lake plume of Rincón de la Vieja, a remote and poorly monitored active volcano in Costa Rica, site of frequent phreatic/phreatomagmatic eruptions. Forty-two phreatic/phreatomagmatic eruptions were seismically recorded during our investigated period, 9 of which were also recorded for gas by the Multi-GAS. To the best of our knowledge, these represent the first instrumentally measured gas compositions during individual phreatic/phreatomagmatic explosions at an active volcano. Our results show that during background quiescent degassing the Rincón de la Vieja crater lake plume was characterized by high CO</span><sub>2</sub><span>/SO</span><sub>2</sub><span> ratios of 64 ± 59 and H</span><sub>2</sub><span>S/SO</span><sub>2</sub><span> ratios of 0.57 ± 0.20. This composition is interpreted as reflecting hydrothermal (re)processing of magma-sourced gas in the sub-limnic environment. Phreatic blasts were recorded by the Multi-GAS as brief (1–2 min long) pulses of elevated gas mixing ratios (up to ~52 ppmv SO</span><sub>2</sub><span> and >3,000 ppmv CO</span><sub>2</sub><span>), or more than an order of magnitude higher than during background degassing (~1 ppmv SO</span><sub>2</sub><span> and ~450 ppmv CO</span><sub>2</sub><span>). During the phreatic eruption(s), the H</span><sub>2</sub><span>S/SO</span><sub>2</sub><span> ratio was systematically lower (<0.18) than during background degassing, but the CO</span><sub>2</sub><span>/SO</span><sub>2</sub><span> ratio remained high (and variable), ranging from 37 to 390. These S-poor compositions for the eruptive gas imply extensive processing of the source magmatic gas during pre-eruptive hydrothermal storage, likely by deposition of native S and/or sulfate. Our gas results are thus overall consistent with a mechanism of phreatic eruptions triggered by accumulation of magmatic-hydrothermal gases beneath a hydrothermal seal. We claim that real-time Multi-GAS monitoring is urgently needed at other crater lake-hosting volcanoes (e.g., Ruapehu, Aso), where phreatic eruptions may similarly be preceded by phases of reduced S degassing at the surface.</span></p>
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10.3389/feart.2018.00247
en
Frontiers Media
Insights into the mechanisms of phreatic eruptions from continuous high frequency volcanic gas monitoring: Rincón de la Vieja volcano, Costa Rica
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