GEOTHERMOBAROMETRY OF GLACIAL OUTWASH SEDIMENT FROM ÖRÆFAJÖKULL, ICELAND

Iceland is geologically unique, forming from the combination of the Mid-Atlantic Ridge and a hotspot, but also generating rhyolitic volcanism (~10%). The Vatnajökull ice sheet overlies the junction of the hotspot and the ridge, as well as many active volcanoes (e.g. Öræfajökull, Bárðarbunga, and Grímsvötn) capable of hazardous eruptions (e.g., Öræfajökull 1362 AD: VEI 5). Vatnajökull is receding due to global climate change, potentially destabilizing existing magma bodies or initiating decompression melting. Understanding the past magmatic conditions at these subglacial volcanoes will establish a baseline for their behavior, but direct study of them is challenging due to ice cover. We posit that detrital sediment from carefully selected localized glacial outwash deposits provides insight into the eroding systems below the ice.

In this study, we aim to establish the temperature and pressure conditions of magma storage in a subglacial magmatic system using olivine-augite (Loucks 1996) and Fe-Ti oxide (Sauerzapf et al. 2008, Ghiorso and Evans 2008, Gualda and Ghiorso 2012) geothermobarometry. Phenocryst-bearing mafic rock chips were hand-picked from detrital sediments associated with the Virkisa outlet glacier on the western flank of Öræfajökull, on the south margin of Vatnajökull. Using a backscattered electron detector attached to an SEM, we determined that the rock chips contain a mineral assemblage of olivine, pyroxenes, Ca-rich plagioclase, and Fe-Ti oxides. Evidence of equilibrium crystallization of relevant mineral phases (e.g., inclusions, rims in contact, rapid cooling textures) indicate these samples are viable for use in geothermobarometry.

The next stage of this study is to use an electron microprobe to gather compositional data for geothermobarometry calculations. Calculated temperatures and pressures combined with measured mineral compositions will reveal conditions in this system and provide a basis for rhyolite-MELTS modeling to explore the impact on magmatic stability and composition in response to environmental changes (e.g. reduction in glacial overpressure, fractional crystallization). The validity of using glacial outwash for this purpose can be tested against published findings for Öræfajökull (a well-studied system), then applied to other subglacial systems.