INVESTIGATION OF VOLCANIC PROCESSES DURING APE CANYON STAGE OF MOUNT ST. HELENS

Most previous work and research done concerning the eruptive stages of Mount St. Helens (MSH), Washington focuses on the more recent eruptive period, the Spirit Lake Stage. There is a lack of information on the oldest eruptive stage the Ape Canyon eruptive stage (300 to 35 ka), which is problematic given the complexity of MSH. MSH lies behind the main volcanic arc of the Cascades; additionally some research suggests the presence of adakites that may represent slab melts. The main objective of this study was to obtain a better understanding of the magmatic processes that took place during the early history of MSH. Samples were collected from outcrops that had previously identified as being from the Ape Canyon stage. During the processing of the samples, some melt inclusion assemblages were found that defined crystal growth zones in plagioclase crystals. Melt inclusions within the growth zones were typically small (5-40 μm) while isolated inclusions between growth zones were typically larger (25-75 μm). Many of the melt inclusions were initially partially devitrified so homogenization experiments were run on the samples. A majority of the melt inclusions within the assemblages homogenized at 1050⁰C; large isolated melt inclusions were not always completely glassy at the end of the experiments Electron microprobe and laser ablation tests were run on the samples to determine the chemistry of the minerals and the melt inclusions. The results of the analyses were used to determine the magmatic processes that took place during the Ape Canyon eruptive period.