Laser probe 40Ar/39Ar dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 ± 1 to 761 ± 1 ka. Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 ± 0.03 to 2.3 ± 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts ∼1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

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