Plagioclase, Amphibole, and Magnetite in the 1991 Pinatubo Reservoir: Timescales of Crystallization and Storage

U-series crystal ages can provide information regarding crystal storage timescales and the chemical evolution of magma within a reservoir. We present 238U-230Th disequilibria for plagioclase (pl.), amphibole (am.), and magnetite (mag.) separated from the crystal-rich white dacite of the 1991 Pinatubo eruption. We evaluate the compatibility of each phase with the host liquid and constrain the crystallization and thermal history of the 1991 eruptive products from Pinatubo, using eruptive units from the Lassen Volcanic Center (LVC) and Mount Hood (MH) for comparison. All three phases are in 238U-230Th disequilibria. Am. is Th-enriched (1.081). Mag. (0.307) and glass (.970) are both Th-depleted. Am. and mag. each plot within error of a zero-age two-point model age when compared with the glass composition. Maximum ages calculated from two-point mineral-glass isochrons indicate average crystallization ages no more than 64 years and 15,729 years for mag. and amp., respectively, assuming crystallization from the host liquid. However, the three phases do not define an isochron, suggesting a more complex crystallization history. The activity ratios of amp., mag., and glass support different possible crystallization histories: crystallization during distinct events separated in time; crystallization from different liquid compositions; or both. Trace element diffusion data for pl. phenocrysts from the same sample suggests that pl. could have only been stored at 750°C or higher (i.e., above rheological lock-up) for less than a century and was likely stored at lower temperatures for longer periods of time. If the Pinatubo reservoir shares a long-lived crystallization and storage history like that at MH, the U-series data for the pl. bulk separate should produce an older age compared to the amp. and mag. bulk separates. A younger age would suggest that crystallization and storage within this system may occur on much shorter timescales compared to MH and the LVC.