Pressure and temperature calculations for magmas of the Öræfajökull volcano, Iceland

Abstract

The Öræfajökull volcano is an ice-covered stratovolcano located by the southeast coast of Iceland, ≥50 km from the divergent plate boundary. Öræfajökull is the highest volcano in Iceland with its highest peak, Hvannadalshnjúkur, rising ~2110 m above sea level. Two eruptions have occurred in Öræfajökull in historical time (i.e. the past 1130 years); the silicic Plinian eruption of 1362, (Ö1362) with a Volcanic Explosivity Index (VEI) of 6 and the hybrid (explosive to effusive) flank eruption in 1727, (Ö1727) with VEI of ~4. Both eruptions had a significant impact and the eruption in 1362 is thought to have been the largest explosive eruption in Europe since Mt. Vesuvius in Italy erupted in 79 AD. Although Öræfajökull and its eruptions have been the subject of a range of volcanological and petrological studies, many aspects of its activity are poorly understood, in particular, the nature of the plumbing system. The principal aim of this study is to estimate pressure (i.e. depth) of magma residence beneath the volcano along with the pre-eruption magma temperature using thermobarometric equations. This is achieved by analysing clinopyroxene in lava samples from five different eruptions at Öræfajökull volcano to obtain at what depth and temperature the crystals were growing from the magma as it resided within the crust. This information will improve our understanding of the plumbing system beneath the volcano. The samples were obtained from Ingólfshöfði, a basaltic, Surtseyan and peripheral eruption; Salthöfði, a basaltic lava flow from a parasitic vent on the volcano´s south flank; the recently discovered andesitic cone row from 1727; Hrafnarkambsskriða, an andesitic subglacial hyaloclastite and pillow lava formation and Svarthamrar, a Pleistocene basaltic lava flow. These units are all mildly alkalic and range in composition from alkali basalt to trachytic (SiO2). The 1727 cone row and Hrafnarkambsskriða showed that magma mixing had occurred before the eruptions and additionally contained rhyolitic glass fragments suggesting that older rhyolitic domes are present below the locations as remnants from an older caldera.