OXYGEN ISOTOPES IN ZIRCON AND RHYOLITE CHARACTERIZATION FROM ESJUFJÖLL VOLCANO, ICELAND

The Öræfi volcanic belt (ÖVB) comprises three off-rift volcanoes (Öræfajökull, Esjufjöll, and Snæfell) in eastern Iceland. The nature of ÖVB volcanism is enigmatic, complicated by coverage by Vatnajökull icecap. We present the first zircon-based geochemical investigation into rhyolite petrogenesis at the subglacial volcano Esjufjöll, the middle volcano in the ÖVB, based on limited subaerial exposures. Five nunatak samples were analyzed for whole rock major and trace element compositions via XRF. Esjufjöll rhyolites have SiO₂ ~65 wt% to 77 wt% and are tholeiitic, as is common for Icelandic rocks. Elemental compositions are in line with those observed at other ÖVB volcanoes. Zircons (n=205) from four of the samples were CL imaged and analyzed for oxygen isotope ratios at WiscSIMS. Grains are commonly sector zoned and occasionally display oscillatory zoning and evidence of antecrystic cores. Median δ¹⁸O values for the four samples are +0.1, 1.4, 4.2, and 4.2‰, with a total range of -1 to +4.5‰. All grains have δ¹⁸O less than that of zircon in equilibrium with the mantle (δ¹⁸O =5.3 +/- 0.6 (2s); Valley, 2003), which indicates that these zircons crystallized from magmas that contained a variable volume of a low-¹⁸O component—likely hydrothermally altered crust. Two Esjufjöll samples have zircon with a restricted δ¹⁸O range, consistent with neighboring Oræfajökull and Snæfell (zircon δ¹⁸O~3 to 5‰; Carley et al. 2011; Banik et al. 2021), which likely formed via a combination of fractional crystallization and crust (or silicic melts thereof) assimilation. The two samples with significantly lower δ¹⁸O (<2‰) suggest a mechanism for rhyolite generation at Esjufjöll that has not been previously identified in the ÖVB—namely partial melting of the pre-existing, hydrothermally altered crust. Volcanoes in the ÖVB are especially important to understand because the rhyolite they generate may erupt explosively; their subglacial position has associated hazards of phreatomagmatic eruptions and jökulhlaups. As glaciers melt in response to climate change, the eruption rate may increase in response to decompression (e.g., Pagli & Sigmundsson, 2008). Appropriate mitigation for these potentially dangerous volcanoes relies on better understanding of their magmatic systems.