What's under the hood : petrologic mapping of the magmatic plumbing system prior to the most recent mount hood eruptions

Abstract

This study examines pre-eruptive magmatic processes and timescales for the Timberline and Old Maid eruptive periods at Mount Hood, OR, using plagioclase compositions, petrologic mapping, and Mg diffusion modeling. Mount Hood is an active Cascade volcano where eruptions are linked to mafic recharge into a long-lived, crystal-rich reservoir, though the timing of final mixing remains poorly constrained. We characterize this event using electron probe microanalysis and anorthite (An) mapping in individual plagioclase crystals. Both eruptive units contain normal, reverse, and single-zoned crystals with overlapping core and rim compositions, indicating similar crystal populations within a shared plumbing system. Systems analysis of An zoning reveals distinct exchange patterns between low and high-An populations. While overall mixing processes are similar, differences in An reservoirs suggest variability in recharge magma or mafic input proportions. Mg diffusion profiles yield bimodal mixing timescales of 1–3 and 17–20 days, indicating rapid, multiple recharge events that trigger eruption and reflect consistent behavior over the past ~1.5 ka.