Paper No. 7
Presentation Time: 3:15 PM
COMBINED MICROANALYTICAL METHODS DETAIL HISTORY OF MAGMATISM AT MOUNT ST. HELENS
Combining microanalytical techniques in extracting the geochemical and isotopic composition of zircon and associated glasses provides an unprecedented record of the magmatic history of the Mount St. Helens (MSH) system, spanning 500,000 years and providing time-correlated temperature (Ti), compositional (Hf, U, Th, REEs), and potentially oxygen fugacity (Ce) information on magmas from intrusion into the crust through eruption, illuminating the dynamics of the plumbing system through time. Traditional SHRIMP (in-situ) trace element and U-Series isotopic analyses of zircon from samples spanning the eruptive history of MSH combine to reveal the history of the magmatic system. Wide ranges of ages and compositions of zircon within each sample indicate an active plutonic body beneath MSH, in which magmas and their zircon cargo are stored for up to hundreds of thousands of years, cooling and crystallizing, and are then rejuvenated, mixed, and entrained by hot, young magmas on the way to eruption. Supplementing traditional SHRIMP analyses of zircon interiors with analyses of zircon surfaces (outermost rims represent the most recent intact growth) allows us to understand the events leading up to eruption of any given sample, despite the antecrystic nature of the zircon cargo. Absence of eruption age zircon surfaces in samples known to have glass-hosted zircons suggests undersaturation of the magmas in zircon, and given timescales of zircon dissolution, suggests rapid (~100 years) transport of zircon from entrainment to eruption. Zircon surfaces exhibiting ages within error of the sample eruption age, though relatively rare, allow us to further characterize events preceding eruption. Comparing glass major and trace element compositions (SEM and LA-ICPMS) with zircon surface compositions further illuminates the pre-eruptive events. Glass analyses from samples of a given age compared with zircon of that age allow us to place limits on the homogeneity of the MSH system at a given time and begin to construct a model of the connectivity and activity of the magmatic plumbing system through time. In cases where zircon and glasses are definitively coeval, this comparison allows us to test the tools that employ zircon trace element compositions by providing direct record of zircon-trace element partitioning.