ACTIVE INTRUSIVE COMPLEXES BENEATH ARC VOLCANOES: THE ZIRCON RECORD OF MOUNT ST. HELENS

U-Series disequilibria and U-Pb SHRIMP ages of zircons from 24 samples spanning the nearly 300 kyr eruptive history of Mount St. Helens (MSH) reveal that an active intrusive complex underlies the volcano. Here magmas repeatedly stall or are left behind following eruptions and are stored, experiencing cooling and crystallization and periodic rejuvenation of portions that are mixed into young, hot magmas traversing the intrusion, and erupted. All samples have multiple zircon age populations, including ages at least 150 kyr and up to 300 kyr older than their eruption age. Age groups, which range from approximately 6 to 600 ka, are present in most but not all, samples. Ti-in-zircon thermometry and zircon saturation calculation suggests that much of the zircon growth was from highly fractionated melts and at low temperatures, requiring that the common MSH dacitic compositions would have been immobile crystal mushes by the time zircon saturation was reached. The geochemistry of the various magmas indicates that they are lower crustal melts and the relatively young ages of the major phases in erupted magmas indicate that they move rapidly from generation to eruption, and thus are not the products of segregation from the stalled magmas that are growing the zircons. However, this active plutonic body must contribute some material to the erupting magmas, as the presence of zircon in many erupted units requires that mushes or perhaps even rocks have repeatedly melted, been rejuvenated, and some portion of these melts and the zircons they contain incorporated into erupting magmas. The portions of the MSH intrusive body, then, that have not experienced any rejuvenation or zircon+melt extraction may be the unerupted equivalents of the volcanic rocks, while the portions that have undergone repeated rejuvenation and have contributed material to erupting magmas would likely be more like crystal cumulate residues. This intrusive component of the system is mostly cryptic in the volcanic record, with the exception of the zircons that survive the reheating events and mix into the lower crustal melts before eruption.