T264 Petrology and Volcanology of Earth and Other Planets (Posters)

Wednesday, 25 September 2024
8:00 AM - 5:30 PM
Hall D

A YOUNG, MULTI-STAGE, RHYOLITIC HISTORY AT ESJUFJÖLL VOLCANO, ICELAND, REVEALED BY ZIRCON AGES AND TRACE ELEMENTS

Paper number: T264-8
Program: Technical Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

The nature of volcanism in the Öræfi volcanic belt (ÖVB) in eastern Iceland is difficult to ascertain due to coverage by Vatnajökull icecap. Subglacial volcanoes like those in the ÖVB can be uniquely dangerous because of incomplete understanding of eruptive histories, increased eruption rates in response to deglaciation and decompression (e.g., Pagli & Sigmundsson, 2008), and associated hazards of phreatomagmatic eruptions and jökulhlaups. We use zircon-based evidence to investigate rhyolite petrogenesis at Esjufjöll, the middle—and most ice-covered—volcano in the ÖVB. Zircon U-Pb and U-Th ages and trace element compositions (n~100) were obtained from four rhyolitic nunatak samples via SHRIMP-RG at Stanford University. The zircon crystals lie in two distinct groups (Group 1 and Group 2, with each group consisting of zircon from two unique rock samples) with differences in ages, elemental composition, and d18O values. Group 1 zircons are significantly younger than Group 2 zircons, with Group 1 ranging from ~zero age to ~300 ka and Group 2 ranging from 1.4 to 2.0 Ma. Group 1 has higher Ti (median 8.3 vs 6.3 ppm) than Group 2, suggesting that Group 1 zircons crystallized at temperatures ~30°C higher than those in Group 2. Group 1 has higher Hf (median 10,110 vs. 7,850 ppm) and a slightly more pronounced Eu anomaly (median Eu/Eu* = 0.32 vs 0.26) than Group 2. The Th/U ratio in Group 1 is lower than Group 2 (median 0.5 vs 0.7). Group 1 has a more depleted and slightly steeper REE/chondrite profile (Yb/Gd=11.5 vs 8.8) relative to Group 2. Together, these data indicate that Group 1 and 2 zircons represent two different stages of magmatism at Esjufjöll with varying mechanisms of rhyolite production. Previously presented oxygen isotope data (Driggs et al., 2023, GSA) revealed that magma associated with Group 1 zircons was likely produced via the partial melting of existing, hydrothermally altered crust, while Group 2 zircons crystallized from melts with a more-pronounced history of fractional crystallization; these new age and trace element data help to refine that petrogenetic hypothesis. The recent crystallization ages of Group 1 zircons suggest Esjufjöll is still active and a potential hazard and worthy of continued study and monitoring.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-403679

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