EVOLUTION OF MOUNT ST. HELENS SUB-VOLCANIC MAGMATIC SYSTEM REVEALED BY ZIRCON: EVIDENCE FROM SHRIMP-RG ANALYSIS OF TRACE ELEMENTS AND U-TH DISEQUILIBRIA

Zircons from four samples that span the 300,000 year history of Mount St. Helens volcanic center reveal clues to the time-temperature-compositional history of the sub-volcanic plumbing system. Application of Ti-in-zircon thermometry (Watson et al. 2006) suggests that the zircons grew at temperatures ranging from ~640 to ~840 C, with 90% of analyzed spots recording temperatures between ~670 and ~770 C (T's carry uncertainties of tens of degrees, mostly from uncertainty in a(TiO2)). These temperatures are significantly lower than the eruption temperatures of the material from which they were extracted (~800-950 C). This discrepancy between T of zircon crystallization and that of erupting magma is consistent with the rounded, resorbed surface morphology of many of the grains. SHRIMP-RG U-Pb ages in the oldest sample, a dacite erupted 295 ka, reveal that zircons grew between ~320 and 520 ka. 238U-230Th age spectra in the three youngest samples indicate multiple ages of growth in each sample. The oldest of these three samples (constrained to ~35-50 ka) contains zircons ranging from ~50 ka to 300. Zircons from a 35 ka dacite range from ~65 to ~230 ka in age, with a dominant episode of growth ~130 ka. Dacite from the current eruption, sampled from the dome in 2005, contains zircons ranging from ~40 to ~170, with distinct populations at ~130 and ~170 ka. As a group, these data suggest that somewhere beneath the volcano lies a relatively cool, crystal-rich reservoir - either a slightly sub-solidus rock or a crystal mush - that has experienced repeated episodes of zircon saturation since 500 ka. The geometry, location and physical state of this reservoir remain uncertain. Did these zircons grow on the boundaries of a widened conduit, similar to models of the modern reservoir, or in an accumulating silicic intrusion at slightly greater depth? Were they extracted during rejuvenation of recharged crystal mush or by disaggregation of a recently solidified portion of the system? Calculated dissolution rates suggest that zircons could have survived only tens to hundreds of years in magma at temperatures recorded at eruption, so regardless of its shape, position or physical state, upward migrating magma must move through this reservoir, incorporate these zircons, and continue relatively rapidly toward eruption.