Mineralogic Constraints on Magma Assembly beneath Augustine

Abstract:

Outstanding questions at Augustine volcano, as with most active arc volcanoes, include both the indications of an imminent eruption and the evolution of magma supplying the volcanic eruptions. The suggested complexity of the Augustine magmatic system makes it a promising candidate to test competing hypotheses for magma assembly: 1) “cold- storage, 2) “warm-storage”, or 3) vertically extensive models. The objectives of this work are to evaluate the pacing of magma accumulation through the elemental diffusion within volcanic minerals, compare the ²³⁸U-²³⁰Th zircon ages from previous work (Coombs and Vazquez 2014) on Augustine eruptive products to diffusion timescales, and determine if the magma system is dominated by cold- or warm- storage conditions. To achieve these objectives, we have constructed timescales from records of elemental distribution within plagioclase and pyroxene from the 2006 Augustine eruptions. Residence times constructed from diffusive profiles of Sr and Mg with plagioclase crystals at higher temperatures (950-995°C) are on the order of 5 to 10 years, and also align with remotely sensed records of volcanic activity. Although, these geologically short residence times are promising, they do not align with previously ²³⁸U-²³⁰Th dated zircons. With an average age of 9 ka ± 6 ka, the ²³⁸U-²³⁰Th ages are reflected in the residence times of Cr and Ti diffusion profiles in orthopyroxene above 900°C as well as profiles in plagioclase near 750°C. Our findings of seemingly disparate ²³⁸U-²³⁰Th zircon ages, distinct storage temperatures and timelines of plagioclase and pyroxene can be reconciled with a vertically integrated magma system of cold-storage mini-plutons in the upper crust and a plexus of dikes facilitating the migration of new warm-storage magma from depth. The crystal-rich pods of magma are defrosted from basaltic intrusions, resulting in magma mixing, remobilization, and eventual eruption. Combining temperatures and timescale information from early crystallizing dense minerals expected to settle to the edges of magma bodies with those gleaned from later crystallizing and less dense minerals may ultimately improve the ability to forecast eruptions and provide a means to synthesize timescales of crustal magmatic processes.