EXPLORING SUBGLACIAL MAGMATISM IN ICELAND USING DETRITAL ZIRCON: A CASE STUDY FROM NORTHEASTERN VATNAJöKULL

Iceland is a unique geologic setting on modern Earth—an oceanic island with high volumes of magma production, of which ~10-15% are silicic. Iceland is the product of a hotspot and the Mid-Atlantic Ridge, but Vatnajökull (Iceland’s largest ice cap) obscures the intersection of these two magmato-tectonic features. Due to climate change, Vatnajökull has lost 10% of its volume in the last century. Prior deglaciation in this region directly correlates to increased magma production and a corresponding increase in eruption rate. Therefore, understanding the nature of magmatism under Vatnajökull is critical to accurate future hazard assessment. While magmatic systems under western Vatnajökull (e.g. Grímsvötn, Barðarbunga) are some of Iceland’s most active and well-studied, magmatism under eastern Vatnajökull is more enigmatic. To investigate this obscured magmatic region, we separated zircons from glacially-derived sediment collected near Vatnajökull’s NE margin. We present SHRIMP-RG U-Pb ages (n=16) and trace element compositions (n=40) for these detrital zircons. We also modeled potential magma compositions using new zircon partition coefficients. Zircon U-Pb ages range from ~0.7 to 2.3 Ma (avg. 1.4 Ma) and do not correlate with ages of any known magmatic centers in the region (e.g., Snæfell, Askja, etc.), suggesting these zircons are not derived from tephra contamination from regional explosive volcanoes. Trace element concentrations (Hf~6,500-12,000 ppm; Ti~5-35 ppm; U/Yb~0.1-0.4; Yb/Nd~10-100; Th/U=0.32-0.83) indicate a moderately evolved source and are consistent with compositions of other Icelandic zircons. These compositions are similar to—but more restricted than—detrital zircon analyses (e.g. Sc/Yb vs. Gd/Yb; U/Yb vs. Nb/Yb) from Öræfajökull, a large silicic volcano at the southeastern margin of Vatnajökull and the Öræfi volcanic belt. All known silicic magmas produced under eastern Vatnajökull are associated with the Öræfi volcanic belt, a potential embryonic rift. The presence of zircons in our glacially derived sands, plus their ages and compositions, suggest previously undocumented silicic magmatic centers beneath northeastern Vatnajökull.