Constraining the Early Eruptive History of the Mono Craters Rhyolites, California, based on 238U-230Th Isochron Dating of their Explosive and Effusive Products

The Mono Craters are an overlapping chain of at least 28 domes and coulees located south of Mono Lake, east central California, and represent the most recent eruptions of high-silica rhyolite magma in the Mono Lake-Long Valley volcanic region. Regionally widespread tephra fall deposits from the Mono Craters serve as important chronostratigraphic markers for correlations of late Quaternary terrestrial archives in the western United States. A well-resolved eruption chronology for the Mono Craters is thus not only critical for volcanic hazard considerations but also for paleoclimatic and paleomagnetic studies. Here we constrain the timing of early Mono Craters volcanism using ion microprobe U-238-Th-230 dating of unpolished crystal faces of autocrystic zircon and allanite microphenocrysts, which samples the final stage of crystallization prior to eruption. The stratigraphically oldest domes (biotite-bearing rhyolites) yield U-238-Th-230 isochron dates between ca. 42 and ca. 19ka and are unambiguously linked to dated tephras in the Wilson Creek formation of ancestral Mono Lake via coupled U-238-Th-230 geochronology and titanomagnetite geochemistry. The second oldest set of domes (orthopyroxene-bearing rhyolites) have overlapping U-238-Th-230 isochron dates that are within uncertainty of published K/Ar and Ar-40/Ar-39 dates for the fayalite-bearing rhyolite domes, suggesting a period of intense effusive activity for the Mono Craters near the Pleistocene-Holocene boundary. Our new and previously published U-238-Th-230 dates for tephras in the Wilson Creek formation provide robust geochronologic constraints for the controversial geomagnetic excursion recorded in the Wilson Creek formation.