An extensive and short-lived venting of fayalite-bearing high-silica rhyolite at Mono Craters during the mid-Holocene

With ~30 high-silica rhyolite domes and flows and a total eruptive volume of ~5-8.5 km3(DRE), the Mono Craters volcanic chain is one of the youngest and most active silicic systems in North America. Effusive products range in age from ~42 ka to 0.7 ka and exhibit changes in mineralogy (biotite-, orthopyroxene-, or fayalite-bearing) and crystallinity through time. Ion microprobe 238U-230Th dating of allanite and zircon surfaces was previously used to obtain final crystallization ages for the four older biotite-bearing domes (~40-20 ka) and two orthopyroxene-bearing domes (~12-9 ka). Whole-rock trace-element compositions and zircon-saturation temperatures from these domes reveal compositional trends and provide insights on changing magmatic conditions through time. We report new ion microprobe 238U-230Th isochron ages for all nine fayalite-bearing Mono domes from paired allanite and zircon surfaces, which are assumed to be eruption ages. Whole-rock and titanomagnetite compositions reveal that the fayalite-bearing domes are compositionally indistinguishable despite having the largest spatial distribution of all the Mono dome types, stretching over ~12 km from the northernmost to the southernmost dome. Overall, our results reveal an unprecedented change in eruptive frequency over the history of the volcanic chain, with all fayalite-bearing domes erupting 6 ± 0.4 ka and evacuating >0.3 km3 of high-silica rhyolite lava.

Speaker: Genna Chiaro, USGS California Volcano Observatory