Þingmúli revisited: New zircon-based perspectives on rhyolite petrogenesis in Iceland

Þingmúli (Thingmuli), an extinct volcano in eastern Iceland, has long been an archetype for fractional crystallization of tholeiitic basalt to produce rhyolite. However, a recent re-examination of Þingmúli suggests that some silicic magmas were produced under more oxidizing conditions from a combination of magma mixing and assimilation, thereby indicating two petrogenetic pathways. We present the first zircon-based analyses from Þingmúli to evaluate the extent to which the zircon record corroborates these recent findings. Zircon U-Pb age dates of ~ 8.5–10 Ma agree with published whole-rock and regional values. Zircon εHf ranges from ~ + 13.6 to 16.3; zircon δ18O is largely restricted to ~ 3.1–3.9‰, with crystals from one sample averaging ~ 2‰. Trace elements reveal two distinct compositional groups: Group 1—common at Þingmúli but rare in the Iceland zircon record; and Group 2, which is representative of typical Icelandic zircon, but under-represented at Þingmúli. The rhyolite from which Group 1 zircon crystallize results from extreme fractional crystallization of incompatible-element enriched melt with variable, but probably small, amounts of unusually low-18O material. Group 2 zircon, with compositions distinct from Group 1 (notably, Eu/Eu*>0.1), reveal a different origin story—one counter to the traditionally held view of petrogenesis at Þingmúli. A combination of variable mantle input composition and differentiation dominated by AFC processes involving crustal input are required to generate the evolved melts from which Group 2 zircon crystallized. This dynamic petrogenesis, common in active Icelandic central volcanic systems, dominates rhyolite petrogenesis at Þingmúli.