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RESEARCH PRODUCT

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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OVSICORI 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 CO2/SO2 ratios of 64±59 and H2S/SO2 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 (1–2min long) pulses of elevated gas mixing ratios (up to ∼52 ppmv SO2 and >3,000 ppmv CO2), or more than an order of magnitude higher than during background degassing (∼1 ppmv SO2 and ∼450 ppmv CO2). During the phreatic eruption(s), the H2S/SO2 ratio was systematically lower (3000 ppmv de CO2), o más de un orden de magnitud superior que durante la desgasificación de fondo ( ∼1 ppmv SO2 y ∼450 ppmv CO2). Durante la(s) erupción(es) freática(s), la relación H2S/SO2 fue sistemáticamente más baja (<0,18) que durante la desgasificación de fondo, pero la relación CO2/SO2 se mantuvo alta (y variable), con un rango de 37 a 390. Estas composiciones pobres en S para el gas eruptivo implica un procesamiento extenso de la fuente de gas magmático durante el almacenamiento hidrotermal previo a la erupción, probablemente por la deposición de S nativo y/o sulfato. Nuestros resultados de gas son, por lo tanto, en general consistentes con un mecanismo de erupciones freáticas provocadas por la acumulación de gases magmático-hidrotermales debajo de un sello hidrotermal. Afirmamos que se necesita urgentemente el monitoreo Multi-GAS en tiempo real en otros volcanes que albergan lagos de cráter (por ejemplo, Ruapehu, Aso), donde las erupciones freáticas pueden estar precedidas de manera similar por fases de desgasificación reducida de S en la superficie. Università di Palermo, Italy Universidad Nacional, Costa Rica Instituto Costarricense de Electricidad, Costa Rica Universidad de Costa Rica Red Sismológica Nacional, Costa Rica United States Geological Survey Istituto Nazionale di Geofisica e Vulcanologia, Italy Observatorio Vulcanológico y Sismológico de Costa Rica