Explosive subglacial rhyolitic eruptions in Iceland are fuelled by high magmatic H2O and closed-system degassing

Abstract

Rhyolitic eruptions beneath Icelandic glaciers can be highly explosive, as demonstrated by Quaternary tephra layers dispersed throughout northern Europe. However, they can also be small and effusive. A subglacial rhyolitic eruption has never been observed, so behavioral controls remain poorly understood and the influence of preeruptive volatile contents is unknown. We have therefore used secondary ion mass spectrometry to characterize pre-eruptive volatile contents and degassing paths for five subglacial rhyolitic edifices within the Torfajökull central volcano, formed in contrasting styles of eruption under ice ~400 m thick. This includes the products of the largest known eruption of Icelandic subglacial rhyolite of ~16 km 3 at ca. 70 ka. We find pre-eruptive water contents in melt inclusions (H 2 O MI ) of up to 4.8 wt%, which indicates that Icelandic rhyolite can be significantly more volatile rich than previously thought. Our results indicate that explosive subglacial rhyolite eruptions correspond with high H 2 O MI , closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H 2 O MI and show open-system degassing and more sluggish ascent rates.

Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

“…12), closedsystem degassing of obsidian cannot explain observed low water contents in vesicular samples. This would indicate that either 1) the magma experienced non-equilibrium closed-system degassing, related to its high viscosity (Sparks, 1978) or 2) a proportion of the gas was also being removed from the vesiculating magma (open-system degassing, Owen et al, 2013). High bubble connectivity in vesicular samples supports this theory (Fig.…”

“…Lower H 2 O t vesicular samples have on average 0.13 wt.% dissolved water, compared to approximately 0.19 wt.% in the obsidian samples. Using a simple model based on the ideal gas law (similar to that used in Owen et al, 2013) we calculate that a 0.06 wt.% decrease in dissolved water content can be explained by exsolution of bubbles corresponding to the observed porosity increase of 35% ('vesiculation' arrow in Fig. 20).…”

An updated calibration of the plagioclase-liquid hygrometer-thermometer applicable to basalts through rhyolites

“…The Torfajökull rhyolites are saturated with ≥4 mineral phases: plagioclase + alkali feldspar + clinopyroxene + titanomagnetite ± olivine ± apatite ± zircon ± biotite ± amphibole; neither ilmenite nor orthpyroxene is found (MacDonald et al 1990). Although pre-eruptive temperatures are not known for these rhyolites, Owen et al (2013) analyzed water concentrations in feldspar-hosted melt inclusions in several samples; the highest value of 4.8 wt% was found in an explosively erupted unit. This maximum value for the melt water concentration in the Torfajökull rhyolites provides an opportunity to use the plagioclase-liquid thermometer from this study to constrain preeruptive temperatures.…”

“…fIgurE 14. A plot showing estimates of minimum pre-eruptive temperatures (°C) that are calculated by applying the plagioclaseliquid thermometer to 19 rhyolite lavas from the Torfajökull Volcanic Complex, for which water contents (≤4.8 wt%) have been measured in melt inclusions (Owen et al 2013). Plagioclase compositions (An 15-27 ) are from MacDonald et al (1990).…”

Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

“…Ice deformation models suggest that declining magma discharge rates encourage cavity filling and the transition to effusive activity. Owen et al (2013) have recently determined pre-eruptive volatile contents and degassing paths for Dalakvísl and four other Torfajökull edifices. Dalakvísl displays two distinct H 2 O-Cl trends, with explosivelygenerated samples having high pre-eruptive H 2 O contents and undergoing closed system degassing with fast ascent rates, whilst effusivelygenerated samples show lower pre-eruptive H 2 O content, open system degassing and slower ascent rates.…”