The Gravitational Stability of Lenses in Magma Mushes: Confined Rayleigh‐Taylor Instabilities

G Seropian,R S J Sparks,A C Rust

doi:10.1029/2018jb015523

G Seropian, R S J Sparks + Show 1 more

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https://doi.org/10.1029/2018jb015523

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Abstract

AbstractIn the current paradigm, magma primarily exists in the crust as a crystalline mush containing distributed melt lenses. If a melt‐rich (or fluid) lens is less dense than the overlying mush, then Rayleigh‐Taylor (RT) instabilities will develop and could evolve into spheroids of ascending melt. Due to contrasting melt‐mush rheologies, the theoretical RT instability wavelength can be orders of magnitude larger than the magmatic system. We explored how this confinement affects the gravitational stability of melt lenses through laboratory experiments with pairs of liquids with one layer much thinner and up to 2.2·105 times less viscous than the other; we extended the viscosity ratio to 106 with linear stability analysis. We found the growth rate of a bounded RT instability is approximately , where Δρ is the difference in density between the fluids, g is gravity, D is the container diameter, and μ2 is the viscosity of the thicker viscous layer. This differs from the unbounded case, where the growth rate also depends on the thickness and viscosity of the thin, low‐viscosity layer. Applying the results to melt lenses in magmatic mushes, we find that for the ranges of expected rheologies, the timescales for development of the instability, and the volumes of packets of rising melt generated span very wide ranges. They are comparable with the frequencies and sizes of volcanic eruptions and episodes of unrest and so suggest that RT instabilities in mush systems can cause episodic volcanism.

Highlights

A major challenge of modern volcanology concerns subsurface magma transport and accumulation
Due to contrasting melt-mush rheologies, the theoretical RT instability wavelength can be orders of magnitude larger than the magmatic system. We explored how this confinement affects the gravitational stability of melt lenses through laboratory experiments with pairs of liquids with one layer much thinner and up to 2.2 ⋅ times less viscous than the other; we extended the viscosity ratio to with linear stability analysis
For a wide range of expected viscosities, the large viscosity contrast between the mush and melt lens means that the theoretical fastest-growing wavelength is unfeasibly large and so the wavelength of the instability is the largest available: the diameter of the lens. This lateral confinement means that the growth rate of the instability is reduced compared to the theoretical unconfined scenario

Summary

Introduction

A major challenge of modern volcanology concerns subsurface magma transport and accumulation. Conceptual models are emerging that depict subsurface systems as large uneruptible crystalline networks (mushes) containing heterogeneously distributed pockets of eruptible magma and exsolved volatiles that can extend deep in the crust and down to the mantle (e.g., Bachmann & Huber, 2016; Cashman et al, 2017). The dynamics of igneous mush systems has become a dominant theme in contemporary magma physics (e.g., Bergantz et al, 2017; Dufek & Bachmann, 2010; Parmigiani et al, 2014) and a key feature of interpretations of geophysical, geochemical, and petrological data (e.g., Jaxybulatov et al, 2014; Putirka, 2017). Melts, and fluids are commonly less dense than the overlying mush; Rayleigh-Taylor (RT) instabilities develop naturally wherever buoyant layers form. For some conditions (e.g., sufficiently high mush viscosity) the growth rate may be sufficiently slow that other processes (e.g., solidification due to cooling) dominate

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