Unexpected Rhyolites: A Microanalytical Isotopic Investigation of Rhyolitic Domes at Krafla, Iceland based on

The Krafla volcano in NW Iceland gained attention in 2009 when the IDDP-1 geothermal drill intercepted a low- 𝛅¹⁸O rhyolitic magma body. To understand rhyolite petrogenesis at Krafla, this study presents major phenocryst 𝛅¹⁸O by laser fluorination and zircon 𝛅¹⁸O and U-Pb measured by SIMS from rhyolite domes exposed along the caldera margin. We also present numerical modeling of the thermal conditions necessary to create these domes at Krafla. The U-Pb dating of rhyolites at Krafla revealed three phases of rhyolitic magmatism: Phase 1 caldera forming tuff dated at ca. 105 ka, Phase 2 eruptions of three coeval rhyolite domes at ca. 67 ka, and Phase 3 eruptions of obsidian which produced no zircon for dating. Zircon extracted out of the domes show a 𝛅¹⁸O range of +0.6 - +2.0‰ that is out of equilibrium with other major phenocryst phases and whole rock 𝛅¹⁸O values. Major phases analyzed individually and in bulk also show consistently low and heterogenous values: plagioclase (-1.6‰ to +2.7‰), clinopyroxene (+1.6‰ to +4.8‰), high-silica glass groundmass (+2.1 to +3.6‰). Samples of IDDP-1 drill core felsite and glass were also analyzed. The process of creating these low 𝛅¹⁸O rhyolites began as continuous injections of basalt at the ridge thermally primed the crust for millions of years. Zircon ages show two crystallization events (ca.105 ka - ca. 67 ka) separated by ca. 38 ka years, during which recharge partially melted the basaltic wall rock hydrothermally altered by meteoric water, incorporating low 𝛅¹⁸O silicic partial melt into the magma. This magma was combined with partially melted rhyolite (𝛅¹⁸O = 2.1‰) that remained unerupted after the caldera forming event to create a high silica rhyolite with a heterogeneous crystal population and very low 𝛅¹⁸O values. 𝛅¹⁸O of felsite from IDDP-1 provides evidence for continuation of the process of recharge induced partial melting into the present. The generation of partial melt is modeled using a modified version of Heat2D; modeling results confirm that dike intrusions along the ridge can create the necessary volumes of partial melt to induce the observed isotopic signature. Together 𝛅¹⁸O measurements in phenocrysts, including zircons, combined with numerical modeling of thermal conditions offers a new look at the petrogenesis of low 𝛅¹⁸O rhyolites that still lurk underneath Krafla.