Pre-eruptive storage constraints of an active crystal mush using mineral-scale techniques
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Authors
Winslow, Heather
Issue Date
2023
Type
Dissertation
Language
Keywords
Andes , crystal mush , diffusion , igneous petrology , olivine , volcanology
Alternative Title
Abstract
Crystal mushes are common conceptual models used to interpret the structure of a magmatic reservoirs in many volcanic environments and are drivers of differentiation that can generate explosive silicic eruptions. They are composed of a crystal-rich (45-65 vol%) framework, that may represent a plexus of intrusions, and their high crystallinity produces differentiated interstitial melt that is extracted to form an eruptible crystal-poor silicic melt lens cap. Here, I present a comprehensive study to determine petrologic constraints of an active crystal mush system that is hosted within a continental arc. I focus on crystal-rich mafic enclaves that are interpreted to represent fragments of a crystal mush and were hosted in a crystal-poor rhyolite flow from the 2011-12 eruption of Cordón Caulle, Chile. Mafic enclaves are commonly associated with mafic injection origins, but this study utilizes mineral textures, whole-rock geochemistry, and mineral chemistry and chemical zonation to argue for a more nuanced investigation of highly crystalline enclaves that points toward crystal mush origins. The Cordón Caulle mafic enclaves are basaltic endmembers in comparison to enclaves globally and display interlocking grain textures with simple zonation patterns indicative of slow continual growth within a crystal mush. Melt chemistry from the interstitial glass of the mafic enclaves determined a genetic relationship between the enclaves and their host lavas. This further corroborated the mush conceptual model as opposed to mafic injection magmas that intrude chemically distinct and pre-existing reservoirs. I determine quantitative storage constraints of the crystal mush using a variety of thermobarometers (Mg in plagioclase thermometer, Al in olivine-spinel thermometer, clinopyroxene-liquid barometer) that utilize chemical zonation in individual crystals. These methods reveal the basaltic mush resides at shallow crustal levels and relatively cooler temperatures compared to typical basalts (~100-350 MPa, ~920-970°C). Diffusion chronometry, which takes advantage of the chemical gradients in crystals, is used to determine timescales of magma residence (~104 yr), cooling paths (~103 yr), and the final stages of differentiation prior to eruption (~months to years) at Cordón Caulle. This study reports first-ever parameters on the underlying crystal mush that generated explosive silicic eruptions but can also be used as a model to interpret crystal mush systems globally and better understand the dynamics of rhyolite formation in continental arcs.
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License
Creative Commons Attribution 4.0 United States