Crustal‐scale degassing due to magma system destabilization and magma‐gas decoupling at Soufrière Hills Volcano, Montserrat

Abstract

Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well correlated with seismicity and ground deformation cycles. Large variations in SO 2 flux do not correlate with these alternations, but high and low HCl/SO 2 characterize lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO 2 emissions have continued at an average rate of 374 tonnes/day (6 140 t/d), and incandescent fumaroles (temperatures up to 610 o C) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher ( 10x) SO 2 flux have been accompanied by swarms of volcano-tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO 2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km 3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur-rich gases from the magma. These observations are consistent with a vertically extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (10 3 to 10 4 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilization of the mush system leads to eruption, characterized by magma mixing and release of volatiles with different ages, compositions and sources.

Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

“…Recent observations of degassing and deformation at volcanoes demonstrate that transport can be rapid in igneous mush systems. For example, episodes of magmatic fluid transport from depths of at least 10 km are illustrated by volumetric strain data and SO 2 gas fluxes associated with eruptive activity at the andesitic Soufrière Hills volcano on Montserrat (Gottsmann et al, 2011;Hautmann et al, 2014;Christopher et al, 2015). Transport speeds of several meters per second are indicated within the Soufrière Hills volcano mush region and are consistent with transport through transient fracture systems.…”

“…This is consistent with having predominantly compressible components involved in the material transfer and matches scenario S2. In addition, there is also the possibility that there is downward transfer of crystal-rich mush to compensate for the upward (potentially decoupled) movement of melt and magmatic volatiles beneath the region of overall inflation (Christopher et al, 2015;Sparks and Cashman, 2017). The magmatic column and bulge can thus be interpreted to represent a vertical protrusion of the upper surface of the APMB.…”

Magma evolution beneath Bequia, Lesser Antilles, deduced from petrology of lavas and plutonic xenoliths

“…Colours indicate the SiO 2 content of the melts. Arrows show the ascent and differentiation of melts from different regions within the crust connected lenses of fractionated melt segregated from dense mafic cumulates, which migrate upwards finally combining to form magma reservoirs at physically favourable depths (Christopher et al 2015). Beneath Bequia, these lenses may never have attained significant thicknesses.…”

The 2011 eruption of Nabro volcano, Eritrea: perspectives on magmatic processes from melt inclusions

“…This would explain the occurrence of different textural batches with similar bulk chemistry. It is plausible that gravitational instability in the mush is important in triggering eruption as suggested for eruptions of andesite magmas at Soufrière Hills volcano, Montserrat (Christopher et al 2015). Mush destabilisation can also lead to copious release of SO 2 gas previously stored at different levels, accounting for the large emission of SO 2 during the 2011 eruption of Nabro relative to the sulphur content that could have been contained within the erupted magma (Bourassa et al 2012).…”