A DETRITAL ZIRCON INVESTIGATION INTO THE MAGMATIC HISTORY OF THE ÖRÆFAJÖKULL VOLCANO IN SOUTHEASTERN ICELAND

The Vatnajökull ice sheet in SE Iceland obscures the intersection of the Icelandic hotspot and Mid-Atlantic Ridge and several central volcanoes (Barðarbunga, Grímsvötn, Kverkfjöll, Öræfajökull, Esjufjöll) with fissure swarms. Vatnajökull has lost 10% of its volume over the past 100 years and will likely disappear in the next 200 years (Aðalgeirsdóttir et al 2006; Hannesdóttir et al. 2013). Past deglaciation in Iceland is linked to decompression melting, resulting in increased magma generation and eruption (Jull 1996; Maclennan et al 2002; Schmidt et al 2013). Understanding the magmatic history of subglacial systems is vital to future hazard mitigation. We use zircon from tephra and subglacially derived sediments to study these active and obscured systems. We focus on Iceland’s tallest volcano, Öræfajökull, on Vatnajökull’s SE margin, whose 1362 AD eruption (VEI 5, ~10 km³) is the most voluminous silicic eruption in historical Iceland. Trace elements (SHRIMP-RG) are presented for 52 zircons from 6 detrital samples, and compared to 40 zircons from 1362 AD pumice. Detrital zircons consistently follow geochemical trends characteristic of the tephra zircon record on bivariate plots (U/Yb vs. Nb/Yb, Gd/Yb vs. Sc/Yb, Hf vs. Ti). Yb ranges from ~300 ppm to ~3,900 ppm in both but the detrital record extends compositional fields towards more fractionated compositions, with Hf exceeding the tephra record’s upper limit of ~9,000 ppm to reach ~12,500 ppm. These trends support the hypothesis that the detrital record is more comprehensive then tephra from a single eruption. New partition coefficients (Kds) based on Ti in zircon (temp. proxy - Claiborne et al 2016; zircon/melt) and an empirically derived power law formula (Chapman et al 2016; zircon/whole rock) were used to constrain zircon host-melt compositions. Results of Ti-based Kds follow the same general pattern as measured glass compositions from 1362 AD pumice, and are enriched in HREEs relative to other Icelandic magmas. This is consistent with measured REE concentrations in Öræfajökull zircons, which are also HREE-enriched relative to other Icelandic systems. Power law Kds yield consistently low REE concentrations, possibly because Icelandic melts are unusually hot. Soon to be obtained U-Pb and U-Th zircon ages will further clarify Öræfajökull’s magmatic evolution.