GEOTHERMOBAROMETRIC INVESTIGATION OF NUNATAKS FROM ÞÓRÐARHYRNA (THORDARHYRNA), ICELAND USING TEXTURES AND COMPOSITIONS OF CLINOPYROXENE AND PLAGIOCLASE

Þórðarhyrna is a subglacial silicic feature associated with the Grímsvötn-Laki volcanic system, located near the junction of a mantle plume and the Eastern Volcanic Zone of Iceland. Þórðarhyrna is obscured by Vatnajökull ice cap, so we must rely on limited outcrop exposures to investigate a potentially dangerous system with associated hazards such as phreatomagmatic explosions, jökulhlaups, and eruptions of rhyolite. Deglaciation due to climate change threatens to increase eruptability as isostasy influences ease of melt generation and the stability of magmas stored in the crust. Six rhyolitic (SiO₂ ≥65 wt.%) and two basaltic (SiO₂ ≤56 wt.%) nunatak exposures were sampled at Þórðarhyrna: Eystri- and Vestri-Geirvörtur, Húsbóndi, Nibba, Pálsfjall, Hágöngur and Vöttur. These samples provide a unique opportunity to characterize magma plumbing structure, storage depths and crystallization temperatures at this understudied system. Such information is crucial for understanding magmatic history and forecasting potential eruptive events. We will use geothermobarometry to estimate the pressure and temperature conditions of the melts that formed these nunataks. Equilibrium conditions between crystals and their host melts is a prerequisite for a geothermobarometric approach. We have assessed the suitability of our samples for this application using crystal textures and euhedral morphology in petrographic and BSE observations. Clinopyroxene-only thermometer (Putirka, 2008) and barometer (Neave and Putirka, 2017) equations rely on the composition of components (Jd: jadeite, DiHd: diopside + hedenbergite, EnFs: enstatite + ferrosilite) which have been determined via EDS analysis of crystals in Nibba (n = 20) and E-Geirvörtur (n = 12). Magma storage depths and crystallization temperatures will be calculated from this growing dataset and compared with previous investigations into magmatic plumbing at Grímsvötn-Laki. Plagioclase zoning and crystal rim compositions will provide additional insight into magma storage, evolution, and disruption processes, to complement the clinopyroxene perspective.