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

• Published in: Geology
• Main Authors: Owen, Jacqueline, Tuffen, Hugh, McGarvie, Dave
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2013
• Subjects: Torfajökull; Iceland
• Online Access: https://oro.open.ac.uk/35269/; https://oro.open.ac.uk/35269/1/G33647-Owen-edited.pdf; https://doi.org/10.1130/G33647.1

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 pre-eruptive 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 km3 at ca. 70 ka. We find pre-eruptive water contents in melt inclusions (H2OMI) 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 H2OMI, closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H2OMI and show open-system degassing and more sluggish ascent rates. Volatile controls on eruption style thus appear similar to those for subaerial eruptions, suggesting that ice plays a subsidiary role in controlling the behavior of subglacial rhyolitic eruptions.