Developing luminescence analysis of Icelandic volcanic glass: a case study using the Þórsmörk Ignimbrite

Large volcanic eruptions from Iceland can produce significant volumes of glass-rich rhyolitic tephra, which are then deposited across NW Europe and the North Atlantic-Arctic region, forming time-parallel marker horizons useful to palaeoenvironmental studies. Here we investigate new ways of improving the tephrochronological record of Iceland using (thermo)luminescence analysis of rhyolitic volcanic glass shards that dominate airfall ash deposits of the Þórsmörk Ignimbrite (ÞIG), tephra from the Askja 1875 AD, Öræfi 1362 AD eruptions, and the Óþoli tephra from NW Iceland. Following screening experiments, which showed that pure volcanic glass samples retained age-related TL signals, we undertook glass-phase TL dating of the ÞIG and Óþoli tephra. Our TL age estimate of c. 40 ± 10 ka for the ÞIG supports the phenocryst-based radiometric age of c. 50 ka rather than older age estimates of c. 200 ka. Results from the Óþoli tephra were consistent with the fission track age established at c. 2 Ma age, but further investigations of high dose sensitivity changes and longer-term stability factors such as athermal fading are required for quantitative dating of volcanic glass deposits >100 ka. However, as thermoluminescence signals from purified glass fractions of Icelandic tephra can be obtained over 100–1,000,000-year time scales, luminescence characterisation of glass shards can be used alongside geochemical and morphological analysis to distinguish between distal tephras with similar geochemical signatures, and assist with tephrochronological investigations beyond the limits of radiocarbon dating.