Re-examination of crystal ages in recent Mount St. Helens lavas: Implications for magma reservoir processes

U-series data for recent Mount St. Helens lavas suggest that crystallization preceded eruption by more than 0.5 ka but are complicated by possible evidence of crystal recycling and/or addition of radium to the liquid after crystallization. We report new ion and electron microprobe trace- and major-element data for plagioclase and pyroxene in these recent Mount St. Helens lavas and use these data to reassess ²²⁶Ra-²³⁰Th crystal ages by taking into account differences in the partitioning behavior of radium and barium and the effects of impurities in mineral separates. Revised ²²⁶Ra-²³⁰Th model crystallization ages are ∼2-4 ka for plagioclase (with the exception of the 1982 dacite) and ∼0.15-5.7 ka for pyroxene. In contrast to previous interpretations, no late-stage addition of Ra to the liquid after precipitation of the minerals is required. The variability of Ba concentrations measured in plagioclase is too large to be consistent with progressive crystallization from the same liquid or with diffusive re-equilibration of xenocrysts with a new host liquid. Ba heterogeneity limits the residence time of the crystals in a magma at high temperatures and also suggests that in most cases Ra-Th ages have not been significantly modified by Ra diffusion into or out of the crystals. High (²²⁶Ra)/Ba in plagioclase in the 1982 dacite relative to the host liquid likely reflects crystallization processes that precluded bulk crystal-liquid chemical equilibrium. One possibility is that of growth entrapment of surface enrichments during rapid crystallization, which could lead to less discrimination between Ra and Ba than predicted by calculated bulk partition coefficients. ²²⁶Ra-²³⁰Th crystal ages for the Castle Creek andesite and basalt that are younger than ²³⁰Th-²³⁸U ages of the same crystals could be explained by mixing of crystals into melts with different ²³⁰Th/²³²Th ratios, by combinations of older and younger crystal growth within the same magma, or, for the basalt, by diffusion of Ra into crystals at high magmatic temperatures. Average plagioclase ages in most of the samples of > 2 ka imply that some significant mass fraction of the crystals in each flow was present simultaneously beneath the volcano. This observation could be consistent either with simultaneous storage of physically distinct magma batches or with incorporation of a population of similarly aged crystals into each successive magma batch.