REFINING SUBGLACIAL EMPLACEMENT OF HÁGÖNGUR ON THE PERIMETER OF VATNAKÖKULL ICE SHEET, ICELAND

Subglacial volcanoes can produce explosive, phreatomagmatic eruptions due to the interaction of magmatic heat and ice that produce ash clouds, pyroclastic flows, and jökulhlaups. When ice loss occurs, the associated drop in pressure may cause shallow magma bodies to destabilize and erupt, and may promote decompression melting at shallow depths in rifting environments like Iceland (Jull et. al., 1996). We focus our attention on Hágöngur, a subglacially erupted mafic mountain (now exposed subaerially) on the southern perimeter of Vatnakökull ice cap (Iceland), near Þórðarhyrna, in the Grímsvötn-Laki volcanic system. We investigate the relationship between magma composition, magmatic water content, magma emplacement behavior, and environmental conditions during subglacial eruptions. We use portable X-ray fluorescence (pXRF) to constrain bulk-rock chemistry of a basaltic sample, thin sections to characterize the mineral assemblage, x-ray powder diffraction (XRD) to confirm and quantify crystal content, and secondary electron (SE) and backscatter electron (BSE) imaging to investigate bubble morphology and vesiculation gradients. We also use rhyolite-MELTS (Ghiorso et al., 2012) to model pre-eruptive melt evolution pathways and magmatic storage conditions. Secondary electron and BSE images reveal significant vesiculation gradients from our sample margin to the sample interior, indicating potential variations in gas behavior during eruption and rapid cooling at the lava-ice interface. Continued investigation of water concentration and interpretation of bubble number density and bubble size distribution along our vesiculation gradient will help to refine our understanding of the magmatic history and lava emplacement at Hágöngur.