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Petrologic Mapping of the Magmatic Plumbing System Prior to the Most Recent Mount Hood Eruptions

Bethany Janssen , Rebecca deGraffenried

Abstract

Understanding how volcanic systems change in response to magma recharge is critical, and changes in crystal composition can be used to track this response.  Mount Hood is generally thought to erupt though cycles of mafic recharge, as has been noted for the Old Maid (OM) and Timberline (T) eruptions. However, it is currently unclear whether the subsurface magmatic recharge system responds the same was every time to recharge events. Here we use changes in plagioclase, a long-lived mineral in magmatic systems, composition from core to rim to investigate how the magmatic system of Mount Hood responds to recharge events. By combining anorthite composition and systems analysis, we can track magma migration in the subsurface in response to recharge for both OM and T eruptions.   Our preliminary results indicate that all zoning types, normal, reverse, and complex zoning patterns, are present in both eruptions. However, when comparing core and rim compositions present in both eruptions, we note that each eruption has different peak core and rim compositions for the different zoning patterns, which indicates that different magmatic bodies were tapped in each eruption. This result is unexpected as past behavior of Mount Hood would suggest that eruptions tap into similar magmatic reservoirs. Further analysis of the connections between the OM and T eruptions. The possibility of different magmatic reservoirs being involved in different eruptions for Mount Hood has implications for hazard assessment for future eruptions.