The Origin of Rhyolitic Magmas at Krafla Central Volcano (Iceland)

Abstract We present a detailed petrologic study of rhyolites from seven eruptions spanning the full (?190 ky) history rhyolitic volcanism at Krafla volcano, northeast Iceland. The vary widely in size and style, but all are crystal-poor (<6 modal%: plagioclase + augite ± pigeonite orthopyroxene titanomagnetite fayalite) have similar evolved compositions (73.7–75.8 wt% normalized whole-rock SiO2) trace element patterns. Macrocryst rim each eruption cluster within narrow range appropriate for equilibrium with their carrier melt. Crystal cores interiors display complex growth patterns commonly host resorption surfaces, compositional variations slight (e.g. typically <10 mol% An plagioclase, Mg# pyroxene), consistent an overall trend cooling differentiation by crystal fractionation. Although most core interior broadly melts to whole-rock, variable histories, juxtaposition grains distinct compositions, scatter melt inclusion indicate mixing antecrysts compositionally and/or assimilated felsic mush or intrusions before final growth. Evidence mafic recharge coupled increases Fe plagioclase) is absent crystals; rhyolite storage fractionation thus occurred largely isolation underlying system. Comparison observed matrix glass published experimental work on melting altered (meta)basalts casts doubt previous models favouring generation partial hydrothermally basalts, instead supporting recent isotopic modelling arguments crystallization-driven process [Hampton, R. L. et al. (2021). Journal Volcanology Geothermal Research 414, 107229]. MELTS fractional crystallization assimilation-fractional (AFC) 1 kbar predict liquid major after ?60–70 vol% quartz tholeiite representative basalts erupted caldera. suggest that stalling these shallow depth forms mushes which (broadly dacitic rhyolitic) extracted. These ascend mix other bodies assimilate intrusive material uppermost crust. Daly gap between ?57 71 SiO2 preferential extraction ?50–70% crystallinity window. Residual solid (cumulate) predicted exclusively mafic, hence efficient silicic may also explain apparent bimodality some Icelandic plutonic suites.