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

Öræfajökull is an ice-covered central volcano in southeast Iceland, situated ≥50 km from the divergent plate boundary. It is the highest volcano in Iceland rising ~2110 m a.s.l. Öræfajökull has had two historical eruptions (i.e. in the last 1140 years); a VEI 6 silicic Plinian eruption in 1362 CE and a VEI 4 hybrid flank eruption in 1727 CE. Both eruptions had a significant impact on the local population. The 1362 CE event is the largest explosive eruption in Europe since Mt. Vesuvius in 79 AD. Although Öræfajökull has been subjected to range of volcanological and petrological studies, the nature of its plumbing system is poorly understood. The aim of this study is to provide estimates of pressure (i.e. depth) of magma residence beneath the volcano and the pre-eruptive temperature of the magma. This is achieved by analyzing clinopyroxene in lava samples from five different Late Glacial, Holocene and historic eruptions at Öræfajökull. The samples are from Ingólfshöfði, a basaltic, Surtseyan eruption at the periphery of the volcano; Salthöfði, a basaltic lava flow from a parasitic vent on the volcano's south flank; the recently discovered andesitic cone row from the historic 1727 event; Hrafnarkambsskriða, an andesitic subglacial hyaloclastite and pillow lava formation and Svarthamrar, an early Holocene basaltic lava flow. These units are all mildly alkalic and range in composition from alkali basalt to trachyte. The samples from the 1727 CE cone row and Hrafnarkambsskriða show evidence indicating that mixing of two magmas took place sometime before erupting. The magmas contain lithics of silicic glass that predate the current edifice, therefore we suggest they are associated with the formation of an older caldera. The results indicate that at least two main magma storage zones currently residing under the volcano occurring at similar depths for each sample location; ~2-4 km and ~8-10 km for Salthöfði, ~1-2 km and ~7-8 km for the 1727 cone row, ~4-6 km and ~9-11 km for Hrafnarkambsskriða and at ~2-4 km and ~9-12 km for Svarthamrar. Thus, magmas that feed eruptions on the flank and summit of Öræfajökull pass through these magma storage zones. However, the magma that fed the peripheral vent at Ingólfshöfði appears to be derived from even deeper source at ~19-20 km and did not pass through the shallow magma storage zones underneath the volcano.