PLUTONIC BEHAVIOR OF A CASCADE ARC VOLCANO: THE ZIRCON RECORD OF THE MOUNT ST. HELENS MAGMATIC SYSTEM

Studies of timescales of magmatic processes at many of the world’s arc volcanoes, including the Cascades, suggest that often only a few thousand years pass between petrogenesis of a magma at its deep source and eruption of that magma. However, zircon U-series and U-Pb geochronology and trace element geochemistry from 24 samples spanning the approximately 300,000 year eruptive history of Mount St. Helens (MSH) indicate that young, erupting magmas invariably incorporate zircons from older material. Most of the zircons are cognate to the MSH magmatic system but not to the magma in which they erupt. The zircons generally range from 10,000 to 200,000 years older than the eruption age of their host rock, with dominant age populations discernible in the full MSH data set. Eruption-age zircon is rare, potentially due to limitations of traditional SIMS analyses that employ cross-sectional analyses. Analyses of the surfaces of grains scheduled for August 2009 should allow us to determine whether this paucity of eruption ages indicates erupting magmas are characteristically unsaturated in zircon or not. Relatively low Ti and high Hf concentrations in the zircons, as well as mineral saturation calculations, indicate these zircons grew at temperatures discernibly lower than proposed magma reservoir and eruption temperatures for MSH magmas. This, combined with the complex zircon age populations exhibited by all units, indicates that MSH magmas regularly stall in the crust, cool and crystallize, and are then rejuvenated by hotter, young magmas on their way to eruption. The zircon data reflect increasing influence of mafic components in the magmatic system through time (higher T, lower Hf, and wider ranging compositions), supporting existing chemical and petrologic data while providing a new, time-correlated record of these changes independent of the eruption history. These data as a whole appear to describe an active plutonic body beneath this active arc volcano, with melt volume, composition, and location that vary through time. The location of melt-rich zones within this body at any given time could potentially dictate the path erupting magmas take through the crust beneath the current edifice and must certainly affect the distribution of zircon ages in any one sample.