Volcanic gas emissions from Soufrière Hills Volcano, Montserrat 1995–2009, with implications for mafic magma supply and degassing

Abstract

Volcanic gas emissions illustrate a complex volatile budget for Soufrière Hills Volcano, Montserrat. Fluxes of sulphur dioxide, carbon dioxide, hydrogen sulphide and hydrogen chloride (and probably water) are substantial from this arc volcano. Additional sources of volatiles in addition to the erupting andesite are required to satisfy the mass balance defined by gas emissions and petrological constraints. Mafic magma is intruded at depths of >10 km, supplying volatiles by quenching, crystallising and vesiculating at the andesite‐mafic magma interface. Sulphur, carbon dioxide and other volatiles supplied to the system migrate to the surface and their fluxes remain high while mafic recharge at depth is sustained. Decompression degassing of porphyritic andesite supplies chlorine and water to the gas plume. Chlorine partitions into vapour from the melt during periods of active extrusion, making the HCl/SO2 ratio of volcanic gases a reliable indicator of the onset of eruptive activity. Studies of this volatile budget highlight the role of deep degassing and vapour production at arc volcanoes, which are important mechanisms for driving the eruption due to its effects on magma buoyancy and the advection of heat through the system.

Chapter 16 Pre-eruptive vapour and its role in controlling eruption style and longevity at Soufrière Hills Volcano

“…We have observed, throughout the eruption so far, that 95 far more sulphur gases are degassed than can be accounted for by degassing of erupting 96 magma alone (Edmonds et al, 2001;Christopher et al, 2010), and this has been ascribed to a 97 pre-eruptive fluid phase containing the bulk of the sulphur (e.g. Wallace and Edmonds, 2011), 98 and fluid replenishment by mafic magma intrusion and degassing (Edmonds et al, 2010). 99…”

“…As the proportion of exsolved fluid is increased, 92 the increase in compressibility leads to a larger mass of magma having to be erupted in order 93 to relieve a given overpressure, and hence the duration of the resulting effusive eruption will 94 be larger (Huppert and Woods, 2002). We have observed, throughout the eruption so far, that 95 far more sulphur gases are degassed than can be accounted for by degassing of erupting 96 magma alone (Edmonds et al, 2001;Christopher et al, 2010), and this has been ascribed to a 97 pre-eruptive fluid phase containing the bulk of the sulphur (e.g. Wallace and Edmonds, 2011), 98 and fluid replenishment by mafic magma intrusion and degassing (Edmonds et al, 2010).…”

Insights into processes and deposits of hazardous vulcanian explosions at Soufrière Hills Volcano during 2008 and 2009 (Montserrat, West Indies)

“…This represents the continuous exsolution of sulfur-rich gases from mafic magma at depth and their migration to the shallow andesitic volcanic system [Edmonds et al, 2003a;Christopher et al, 2010]. By advecting heat to the andesitic magma, increasing its bulk volatile concentration, and affecting it's buoyancy, continued injection of mafic magma at depth drives the eruption of andesite magma at the surface [Christopher et al, 2010]. In a marked departure from established long-term trends, the SO 2 emission rate remained well below the LTM with a mean of 302 ± 102 tons/day between 30 May and 15 July (see Figure S6) Robertson et al, 2009].…”

“…[16] The long-term mean SO 2 emission rate (LTM) for the entire eruption (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)) is 570 tons per day [Christopher et al, 2010]. This represents the continuous exsolution of sulfur-rich gases from mafic magma at depth and their migration to the shallow andesitic volcanic system [Edmonds et al, 2003a;Christopher et al, 2010].…”

Chapter 21 Controls on variations in cristobalite abundance in ash generated by the Soufrière Hills Volcano, Montserrat in the period 1997 to 2010

“…Such fluctuations may result from, for example, pauses in magma extrusion, and in response to changes in volatile exsolution from magma at depth. It is known that some major dome collapses are immediately followed by a marked increase in SO 2 content (Christopher et al 2010), suggesting that magmatic gas is trapped beneath the dome prior to collapse, then subsequently released. This may result from permeability occlusion at depth, which may, in turn, lead to pressurization beneath the dome, contributing to its failure (Horwell et al 2013).…”