• journal_title：Geology
• Contributor：Jacqueline Owen ; Hugh Tuffen ; David W. McGarvie
• Publisher：Geological Society of America
• Date：2013-02-01
• Format：text/html
• Language：en
• Identifier：10.1130/G33647.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：2
• firstpage：251
• section：Articles

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 (H₂O_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 H₂O_MI, closed-system degassing, and rapid magma ascent, whereas their effusive equivalents have lower H₂O_MI 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 influencing the behavior of subglacial rhyolitic eruptions.