Origin of a rhyolite that intruded a geothermal well while drilling at the Krafla Volcano, Iceland

• Published in: Geology (Boulder) 2011-03, Vol.39 (3), p.231-234
• Main Authors: Elders, Wilfred A, Fridleifsson, Gudmundur, Zierenberg, Robert A, Pope, Emily C, Mortensen, Anette K, Gudmundsson, Asgrimur, Lowenstern, Jacob B, Marks, Naomi E, Owens, Lara, Bird, Dennis K, Reed, Mark, Olsen, Nellie J, Schiffman, Peter
• Format: Article
• Language: English
• Subjects: assimilation; Basalt; basalts; Crystals; deep drilling; degassing; Dissolution; drilling; Economic geology; emplacement; energy sources; Europe; geologic thermometry; Geothermal; geothermal energy; geothermal wells; Geothermometry; Glass; glasses; Hydrogen; hydrothermal alteration; Iceland; igneous and metamorphic rocks; igneous rocks; in situ; intrusions; Isotopes; Krafla; Magma; magma chambers; magma transport; magmas; Melts; metasomatism; observations; partial melting; Petrology; Pressure; quenching; Rhyolite; rhyolites; textures; transport; vesicular texture; volcanic glass; volcanic rocks; Volcanoes; Western Europe
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G31393.1

Magma flowed into an exploratory geothermal well at 2.1 km depth being drilled in the Krafla central volcano in Iceland, creating a unique opportunity to study rhyolite magma in situ in a basaltic environment. The quenched magma is a partly vesicular, sparsely phyric, glass containing ∼1.8% of dissolved volatiles. Based on calculated H2O-CO2 saturation pressures, it degassed at a pressure intermediate between hydrostatic and lithostatic, and geothermometry indicates that the crystals in the melt formed at ∼900°C. The glass shows no signs of hydrothermal alteration, but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle-derived magmas, indicating that this rhyolite originated by anhydrous mantle-derived magma assimilating partially melted hydrothermally altered basalts.