UNDERSTANDING ASSEMBLY AND STORAGE OF MAGMAS AT MOUNT ST. HELENS USING CHEMISTRY, TEXTURES, AND AGES OF CRYSTAL-RICH INCLUSIONS (Invited Presentation)

We use a variety of techniques to assess the rock record of magmatic processes and events from Mount St. Helens volcano. Mount St. Helens’ nearly 300 kyr eruptive history spans 10 km of central vent locations (Clynne et al. 2008). Zircon records conditions of longer term storage beneath the volcano (Claiborne et al. 2010), while most glasses and feldspars record events in the years to thousands of years before eruption (Cooper and Donnelly 2005). The physical state of this storage is elusive (melt rich? crystal-rich mush? solid rock?), as crystal growth may drastically slow or stop during storage, pausing the mineral record. To better understand this, we present zircon and feldspar data from crystal rich inclusions erupted during Pine Creek eruptive stage (3.9-3.3ka). The inclusions (60-70% phenocrysts) are less chemically evolved than the Pine Creek age host dacites (62-66 wt% and and 57-59 wt% SiO₂, respectively) with remarkably similar trace elements (~360-460 ppm Sr, 252-307 ppm Zr in hosts, 350-480 ppm Sr, 205-284 ppm Zr in inclusions). Zircon SHRIMP-RG U-Th dating of one inclusion indicates it crystallized entirely during the Cougar stage (~23ka), as one cohesive melt package, unlike all other dated samples which show mixing of various-aged zircon before eruption. The high crystal fraction suggests that this inclusion was erupted as a crystal mush; the euhedral phenocrysts and lack of resorption textures on crystal edges indicate it was undergoing crystallization, not melting. Host feldspars show normal zoning (calcic to sodic), while inclusion feldspars more often exhibit interior resorption textures overgrown by rims and reverse zoning. Host and inclusion feldspars span a similar range of compositions, but inclusion feldspars are bimodal, with no correlation between cores and rims among grains. Taken together, this suggests the inclusions represent batches of magma stored as crystal rich mush and exposed to fluctuating thermal conditions for extended periods of time (~15kyr in this case). The euhedral rims of inclusion feldspars indicate crystallization, not melting, of the mush preceded eruption and that entrainment and eruption was rapid.