Degassing regime of Hekla volcano 2012–2013

Abstract

Hekla is a frequently active volcano with an infamously short pre-eruptive warning period. Our project contributes to the ongoing work on improving Hekla's monitoring and early warning systems. In 2012 we began monitoring gas release at Hekla. The dataset comprises semi-permanent near-real time measurements with a MultiGAS system, quantification of diffuse gas flux, and direct samples analysed for composition and isotopes (d 13 C, dD and d 18 O). In addition, we used reaction path modelling to derive information on the origin and reaction pathways of the gas emissions.Hekla's quiescent gas composition was CO 2 -dominated (0.8 mol fraction) and the d À1 are sourced from a small area at the volcano's summit. There was no detectable gas flux at other craters, even though some of them had higher ground temperatures and had erupted more recently.Our measurements are consistent with a magma reservoir at depth coupled with a shallow dike beneath the summit. In the current quiescent state, the composition of the exsolved gas is substantially modified along its pathway to the surface through cooling and interaction with wall-rock and groundwater. The modification involves both significant H 2 O condensation and scrubbing of S-bearing species, leading to a CO 2 -dominated gas emitted at the summit. We conclude that a compositional shift towards more S-and H 2 O-rich gas compositions if measured in the future by the permanent MultiGAS station should be viewed as sign of imminent volcanic unrest on Hekla.

Tunable diode laser measurements of hydrothermal/volcanic CO<sub>2</sub> and implications for the global CO<sub>2</sub> budget

“…The large error in our flux estimate (±46 %) reflects the poor quality of our plume transport speed measurement, the determination of which was complicated by the strong winds blowing across the top of Hekla at the time of our measurements. We still observe, however, that our 15 ± 7 t d −1 estimate matches closely the recently reported CO 2 flux for Hekla summit (13.7 ± 3.7 t d −1 ), obtained using conventional (accumulation chamber) soil survey techniques (Ilyinskaya et al, 2014).…”

“…In this area, the background CO 2 mixing ratio was evaluated at around 400 ppm. The peak CO 2 mixing ratio (∼ 1050 ppm) was detected in the central portion of the investigated area, in the same sector where the highest soil CO 2 fluxes have been observed (Ilyinskaya et al, 2014).…”

“…1b). Given the positioning of GasFinder and retro-reflectors, the Matlab-derived contour map is here relative to an hypothetical horizontal cross section, taken at about 1 m height above the warm degassing ground identified by Ilyinskaya et al (2014) on the rim of the 1980-1981 summit crater of Hekla (Figs. 1b and 4).…”

“…Gas information has long remained missing, because Hekla appears to be only degassing during eruptions. Very recently, Ilyinskaya et al (2014) the Hekla 1980-1981 crater (Fig. 1b), and studied the composition of this gas using data from a permanent Multi-GAS instrument and field campaigns using an accumulation chamber installed by INGV-PA (Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo) and IMO (Icelandic Meteorological Office) in 2012.…”

Reaction path models of magmatic gas scrubbing

“…The resulting gas-water-rock interactions cause partitioning of water-soluble species (S, halogens) into aqueous solutions, and irreversibly modify the composition of the primary magmatic gas phase. Quantitative assessment of scrubbing is, therefore, essential for interpreting mechanisms and evolution of volcanichydrothermal unrests (Doukas and Gerlach, 1995;Gerlach et al, 2008;Ilyinskaya et al, 2015;Symonds et al, 2001Symonds et al, , 2003Werner et al, 2008Werner et al, , 2012Shinohara et al, 2015). The mechanism of magmatic gas scrubbing by hydrothermal systems was introduced in the fifties (White, 1957), but it was only in the 1990s that scrubbing was invoked as a most important process to explain the anomalous low fluxes of magmatic SO 2 and HCl observed at many volcanoes worldwide, both before and after eruptions (Doukas and Gerlach, 1995;Reed, 1997).…”