A MICROANALYTICAL OXYGEN ISOTOPIC AND U-TH GEOCHRONOLOGIC INVESTIGATION OF RHYOLITE PETROGENESIS AT THE KRAFLA CENTRAL VOLCANO, ICELAND

Understanding the petrogenesis of silicic magmas is critical for understanding the volcanic hazards they pose through explosive activity, their geothermal energy potential, and the creation of continental crust. In this study we explore the origin of rhyolitic magmas in basaltic crust at the Krafla Central Volcano in Iceland. We present laser fluorination oxygen isotope analyses of plagioclase, pyroxene and groundmass from eight rhyolites and six selected basalts, as well as in situ oxygen isotope analyses and U-Th geochronology of zircons from three rhyolitic domes erupted around the caldera margins. Zircon U-Th geochronology yields ages of 67.6 ± 9.4 ka for Jörundur, 65.9 ± 8.6 ka for Hlídarfjall, and 67.2 ± 9.3 ka for Gæsafjallarani domes, some 20-30 ka after the eruption of the Halarauður ignimbrite and formation of Krafla caldera. Oxygen isotope analyses identify some instances of isotopic disequilibrium between groundmass (~3.5‰) in some units around the caldera and in the major phases in the post caldera rhyolite domes, implying assimilation of diverse low δ¹⁸O crustal material. However, zircon is largely in equilibrium with glass, suggesting it crystallized from homogenized low-δ¹⁸O magma. Furthermore, trace element patterns (Hf, Yb, Th, U) are indicative of cooling and fractional crystallization of this low-δ¹⁸O magma prior to eruption of the three domes. Pairing these observations with two dimensional numerical thermal modeling (Heat2D) and mass balance chemical modeling using the Magma Chamber Simulator, we suggest that petrogenesis of rhyolitic magma at Krafla requires at least two-steps: first the δ¹⁸O of basaltic parents are first lowered through assimilation of hydrothermally altered material (generated in the high temperature region in the crust surrounding the magma chamber) to produce low δ¹⁸O mafic to intermediate magmas, and second its separation from magma generation zones into the colder crust, leading to further fractional crystallization at shallower depths. We additionally demonstrate that prior hydrothermal alteration of the mafic crust greatly increases the volume of partial melt that can be produced and assimilated, and we thus suggest that long-lived hydrothermal systems may play an important role in further encouraging the production of larger volumes of rhyolitic magmas in basalt-dominated environments. This may explain the greater abundance of rhyolites in Iceland and may offer insight into the formation of the continents on the early Earth.