International Workshop on the mechanism of Phreatic Eruption

Hydrothermal - magmatic interactions driving phreatic eruptions
John Stix1 and Maarten de Moor2
1 McGill University, 2 UNA - OVSICORI
E-mail: stix(at)eps.mcgill.ca, maartenjdemoor(at)gmail.com

Many phreatic eruptions are the result of a complex interplay and relationship between
magmatic and hydrothermal contributions. The eruptions tend to be sudden, sometimes withfew precursors. As a result, the eruptions are not usually forecast, which poses a serious
problem for people in the vicinity including nearby inhabitants, scientists, civil defense
personnel, and tourists. An improved understanding of how the deeper magmatic system isconnected to the shallower hydrothermal system (or systems) is needed. At some volcanoes
which exhibit summit degassing and also have flank hydrothermal systems, phreaticeruptions may be preceded by changes in gas chemistry at both summit and hydrothermal
fumaroles. Another key issue is to better understand the locations and timescales of sealing
processes which can pressurize the subsurface prior to a phreatic eruption. Such seals mayform around a deeper crystallizing and degassing magma (e.g., Ohba et al., 2019) and also
at shallower levels within or above the hydrothermal system (e.g., de Moor et al., 2019). The
timescales of sealing appear to vary widely, from longer timescales of months to years toshorter timescales of days to weeks (Fig. 1). We need to understand the detailed
mechanisms and processes which determine timescales. To address these issues, we arecurrently working on two approaches. First, we are building a new type of MultiGas
instrument which will be able to assess magmatic and hydrothermal inputs in real time. The
instrument will measure CO2, H2O, CH4, H2S, and H2, with the main goal of measuring
CO2/CH4 (Chiodini 2009) in real time (Fig. 2). Second, we will measure δ13C of both CO2 andCH4 on a near real-time basis using portable mass analyzers, in order to constrain thesources of the gases and their inter-relationships. Magmatic-hydrothermal gases can beplotted in ternary space, as shown in Figure 3. The S-CO2 join reflects the variable
composition of gases associated with active volcanic systems. The CO2-CH4 join indicatesquiescent systems with variable inputs of magmatic and hydrothermal gases.
References cited
Chiodini G (2009) CO2/CH4 ratio in fumaroles a powerful tool to detect magma degassing
episodes at quiescent volcanoes. Geophys Res Lett 36: L02302,
doi:10.1029/2008GL036347
de Moor JM, Stix J, Avard G, Muller C, Corrales E, Diaz JA, Alan A, Brenes G, Pacheco J,
Aiuppa A, Fischer TP (2019) Insights on hydrothermal‐magmatic interactions and eruptive
processes at Poás volcano (Costa Rica) from high-frequency gas monitoring and drone
measurements. Geophys Res Lett 46: 1293-1302, doi:10.1029/2018GL080301
Ohba T, Yaguchi M, Nishino K, Numanami N, Daita Y, Sukigara C, Ito M, and Tsunogai U(2019) Time variations in the chemical; and isotopic composition of fumarolic gases at
Hakone volcano, Honshu Island, Japan, over the earthquake swarm and eruption in 2015,
interpreted by magma sealing model. Earth Planet Space, 71: 48,
doi:10.1186/s40623-019-1027-5

International Workshop on the mechanism of Phreatic Eruption

Figure 1: H2S/SO2 at Poás, 2016-2018

Figure 2: Schematic view of CO2-CH4 MultiGas

Figure 3: Ternary depiction of S - CO2 - CH4 for gases at Costa Rican volcanoes