New geochemical insights into volcanic degassing

Abstract

Magma degassing plays a fundamental role in controlling the style of volcanic eruptions. Whether a volcanic eruption is explosive, or effusive, is of crucial importance to approximately 500 million people living in the shadow of hazardous volcanoes worldwide. Studies of how gases exsolve and separate from magma prior to and during eruptions have been given new impetus by the emergence of more accurate and automated methods to measure volatile species both as volcanic gases and dissolved in the glasses of erupted products. The composition of volcanic gases is dependent on a number of factors, the most important being magma composition and the depth of gas-melt segregation prior to eruption; this latter parameter has proved difficult to constrain in the past, yet is arguably the most critical for controlling eruptive style. Spectroscopic techniques operating in the infrared have proved to be of great value in measuring the composition of gases at high temporal resolution. Such methods, when used in tandem with microanalytical geochemical investigations of erupted products, are leading to better constraints on the depth at which gases are generated and separated from magma. A number of recent studies have focused on transitions between explosive and effusive activity and have led to a better understanding of gas-melt segregation at basaltic volcanoes. Other studies have focused on degassing during intermediate and silicic eruptions. Important new results include the recognition of fluxing by deep-derived gases, which buffer the amount of dissolved volatiles in the melt at shallow depths, and the observation of gas flow up permeable conduit wall shear zones, which may be the primary mechanism for gas loss at the cusp of the most explosive and unpredictable volcanic eruptions. In this paper, I review current and future directions in the field of geochemical studies of volcanic degassing processes and illustrate how the new insights are beginning to change the way in which we understand and classify volcanic eruptions.

Inverting for volcanic SO<sub>2</sub> flux at high temporal resolution using spaceborne plume imagery and chemistry-transport modelling: the 2010 Eyjafjallajökull eruption case study

“…The behaviour of ash discharge is different, with weak emissions until the end of 5 May, and a more or less abrupt increase (depending on the meteorological field used to force the simulation) culminating on 6 May with the major ash event of the period characterised by a rate of ∼ 20 t s −1 . These different release behaviours of ash and SO 2 could be compatible with processes of gas/melt separation during magma ascent in the days preceding the major explosive phase on 6 May (Gonnermann and Manga, 2007;Edmonds, 2008).…”

“…Therefore, if peaks in the ash release rate are observed during a specific eruptive episode, it can be expected that peaks in the gas emission rate have occurred simultaneously. However, gas can be released from the volcano without being accompanied by ash-rich explosions, for instance if processes of gas-melt separation occur during magma ascent (Gonnermann and Manga, 2007;Edmonds, 2008). Figure 6 shows the temporal evolution of Eyjafjallajökull SO 2 degassing determined by this study and the ash release rate estimated by Stohl et al (2011).…”

A model of degassing for Stromboli volcano

“…The so-called "excess degassing" (Shinohara, 2008), the fact that basaltic volcanoes no doubt emit more gas than potentially contributed by erupted magma, implies an effective gas bubble-melt separation at some point during the ascent. However, while it is universally accepted that separate gas transfer exerts a key control on both quiescent (Burton et al, 2007a) and eruptive (Edmonds and Gerlach, 2007) degassing of basaltic volcanoes, the mechanisms (structural vs. fluid-dynamic control) and depths (shallow vs. deep) of such gas separation are still not entirely understood (Edmonds, 2008). …”

Volcanic Plume CO2 Flux Measurements at Mount Etna by Mobile Differential Absorption Lidar

“…These have included the first instrumental networks of scanning Differential Optical Absorption Spectrometers (DOAS) for volcanic SO 2 flux monitoring, the implementation of satellite-based volcanic gas observations, and the advent of sensor units for in situ gas monitoring [1][2][3][4][5][6]. Owing to this technical progress, volcanic gas plume composition and fluxes have increasingly been used to extract information on degassing mechanisms/processes [4], and to derive constraints on shallow volcano plumbing systems [5]. However, work still needs to be done to increase the number of volcanic gas species that can be detected in plumes, which remain few if compared to the countless number of chemicals quantified from fumarole direct sampling [1].…”