Zircon from Swift Creek stage eruptions records the assembly and evolution of an intrusive magmatic complex beneath Mount St. Helens

Abstract

This study employs combined U-Th radiometric age and trace element analyses of zircon from rocks of the Swift Creek stage (16 to 10 ka) of Mount St. Helens in order to obtain time-temperature-composition records of the melts from which these crystals grew. Results illustrate that the Mount St. Helens magmatic system has been constructed over hundreds of thousands of years by repeated intrusion of new magma batches that stall and crystallize within the crust to produce a crystal storage zone (or zones). Zircon trace element signatures primarily record the long-term magmatic conditions and interactions within this active intrusive body and do not record conditions immediately preceding eruption. Pulses of ascending magma rejuvenate and interact with existing intrusions, mixing zircon crystal populations and inducing new phases of zircon growth and melt fractionation, while magmas that do not stall in the crust entrain and mix zircon crystals from the crystal storage zone(s) and carry them to eruption. Geochemical trends with time suggest that the deep-seated magmatic system of Mount St. Helens has been strongly influenced by input of hot, relatively primitive melt since ~100 to 60 ka. This study documents the existence of an intrusive complex beneath an active arc volcano and records a clear connection between Mount St. Helens volcanism and an underlying plutonic magmatic system. It also suggests that, similar to large-scale batholiths, small-scale volcanic systems accumulate in small increments over prolonged periods of time.