COMBINED INSIGHT FROM TEPHRA AND GLACIALLY-DERIVED SEDIMENTS: A ZIRCON-BASED STUDY ORAEFAJOKULL, SE ICELAND

Climate change makes investigating the history and behavior of subglacial volcanic systems a critical endeavor, as deglaciation may prompt decompression melting and magmatic destabilization. Öræfajökull, whose 1362 AD eruption was historical Iceland’s most voluminous explosive eruption, is an off-rift volcano that underlies Vatnajökull ice cap and has been suggested to overlie ancient continental crust. We present elemental (SiO₂ = 72.4 wt.%) and isotopic (ε_Hf = 12.4; ε_Nd = 5.8; ²⁰⁶Pb/²⁰⁴Pb = 18.542; ²⁰⁷Pb/²⁰⁴Pb = 15.523) compositions for 1362 AD pumice. Pumice zircon ages typically predate the eruption by 1-10’s ka (median ~12 ka; rarely > 100 ka; n = 15). Their median ε_Hf (12.1, n=13) is lower than that of analyzed Icelandic zircons (13.5, n = 345), particularly those from older systems like Breiðavík (13 Ma; 14.7, n = 73) and Hrafnfjörður (14 Ma; 15.5, n = 14), suggesting influence of a less-depleted mantle source or possible ancient crust at this system. Compared to other active Icelandic volcanoes, Öræfajökull ε_Hf is lower than Krafla (14.7, n = 11), comparable to Kerlingarfjöll (12.5, n = 9) and higher than Torfajökull (10.9, n = 26). Oxygen isotopes for 1362 AD zircons are heavy, with median δ¹⁸O of 4.0 ‰ (n = 16) compared to 3.1‰ for analyzed Icelandic zircons (n=572); 1.2 ‰ at Torfajökull (n=65), 1.3 ‰ at Krafla (n=25) and 2.5 ‰ at Kerlingarfjöll (n = 17). Altered crust is less significant in petrogenesis at Öræfajökull than at on- or propagating rift systems. Trace elements form coherent trends within typical Icelandic compositional fields but have restricted and low Hf (5500-9000 ppm) and Ti (8-16 ppm), suggesting limited fractionation at moderate and restricted temperatures.

Zircons from glacial sediments expand the magmatic history captured by the Öræfajökull tephra record. While overlapping in age, 1362 AD tephra zircons (U-Th, n=13; U-Pb, n=2) tend to be younger than detrital zircons (U-Th, n = 18; U-Pb, n=10). Tephra zircons comprise ~60% of ages <50 ka, but only 15% of ages > 50 ka. Ages in the detrital record extend continuously from ~0 to ~150 ka, reaching ~475 ka. Detrital zircons consistently follow and expand geochemical trends of the tephra zircon record, revealing more diverse crystallizing conditions than recorded in tephra (e.g., Hf to ~12500 ppm, Ti to ~30 ppm; n=57).