Magma degassing and fragmentation during the 1918 Katla eruption

Owen, Jacqueline and Tuffen, Hugh and Coats, Becky (2015) Magma degassing and fragmentation during the 1918 Katla eruption. In: IUGG, 2015-06-22 - 2015-07-02.

Abstract

The last eruption of the restless, ice-covered Katla volcanic system in south Iceland was a 1 km3 (DRE) explosive subglacial basaltic event in 1918, which released a 8 km3 meltwater flood. We have sampled both the 1918 jökulhlaup deposit and airfall tephra preserved on Sólheimajökull. Sampling of multiple layers allows examination of discrete phases of eruption/emplacement. Tephra was sieved and clasts thin sectioned to analyse bubble textures, and for compositional analysis using electron probe microanalysis (EPMA) and laser ablation inductively coupled mass spectrometry (LA-ICP-MS). Part of the clasts were analysed for volatile contents using fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). Jökulhlaup samples show evidence of interaction with water; the glass is microlite-poor with ~0.2-0.3 wt.% H2O consistent with quenching under elevated pressure. Some air-fall clasts are microlite rich, completely degassed (~0.1 wt.% H2O) and have interior bubbles that are significantly larger than those at the clast margin. We interpret this as post-fragmentation degassing. These clasts should have taken seconds to cool had they quenched in water. Hot-stage experiments show average bubble growth rates of ~1 µm s-1, suggesting that such clasts have had very little/no interaction with water. Many clasts show repeated episodes of fragmentation and/or degassing, suggesting recycling of material in the upper conduit and/or vent. Preliminary LA-ICP-MS data hints that the magma chamber may have been compositionally stratified. Further work will quantify internal textures and external clast morphologies to determine the relative roles of magmatic and phreatomagmatic fragmentation for the various phases of the eruption.