Open-system dynamics and mixing in magma mushes

Journal name:
Nature Geoscience
Volume:
8,
Pages:
793–796
Year published:
DOI:
doi:10.1038/ngeo2534
Received
Accepted
Published online

Magma dominantly exists in a slowly cooling crystal-rich or mushy state1, 2, 3. Yet, observations of complexly zoned crystals4, some formed in just one to ten years5, 6, 7, 8, 9, as well as time-transgressive crystal fabrics10 imply that magmas mix and transition rapidly from a locked crystal mush to a mobile and eruptable fluid5, 6. Here we use a discrete-element numerical model that resolves crystal-scale granular interactions and fluid flow, to simulate the open-system dynamics of a magma mush. We find that when new magma is injected into a reservoir from below, the existing magma responds as a viscoplastic material: fault-like surfaces form around the edges of the new injection creating a central mixing bowl of magma that can be unlocked and become fluidized, allowing for complex mixing. We identify three distinct dynamic regimes that depend on the rate of magma injection. If the magma injection rate is slow, the intruded magma penetrates and spreads by porous media flow through the crystal mush. With increasing velocity, the intruded magma creates a stable cavity of fluidized magma that is isolated from the rest of the reservoir. At higher velocities still, the entire mixing bowl becomes fluidized. Circulation within the mixing bowl entrains crystals from the walls, bringing together crystals from different parts of the reservoir that may have experienced different physiochemical environments and leaving little melt unmixed. We conclude that both granular and fluid dynamics, when considered simultaneously, can explain observations of complex crystal fabrics and zoning observed in many magmatic systems.

At a glance

Figures

  1. Three time steps from the simulation of an open-system event in basaltic mush.
    Figure 1: Three time steps from the simulation of an open-system event in basaltic mush.

    Computer simulation is archived as Supplementary file V1. The settled crystals are identical, and are coloured black and white for visualization. The basaltic liquid is blue. The new magma is red and the dimensionless velocity U is 9.3. Pie charts indicate the percentage of crystals residing in different melt compositions. The resident (blue), and new (red), magma are represented by 0.0 and 1.0, respectively. a, Formation of the mixing bowl by viscoplastic failure of the mush along two conjugate granular faults. b, Continued input unlocks the mixing bowl and entrains a crystal cargo from the bottom and core of the mush. c, Continued input induces circulation.

  2. Crystal trajectories and zoning.
    Figure 2: Crystal trajectories and zoning.

    a, Trajectories of two crystals that originate from a common location, illustrating complex crystal dispersal and gathering. The grey strip in the bottom centre is the location of new magma input and provides scale. b, Concentration of the melt that the two crystals in a encounter during transport. This can be considered a synthetic crystal zoning record.

  3. Crystal-crystal mixing efficiency.
    Figure 3: Crystal–crystal mixing efficiency.

    LI or goodness of crystal–crystal mixing as a function of scaled velocity U at t equal to 0.238. These results demonstrate that a self-similar regime is established for a range of U once the mixing bowl is fully fluidized.

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Affiliations

  1. Department of Earth and Space Sciences, Box 351310, University of Washington, Seattle, Washington 98195, USA

    • G. W. Bergantz &
    • J. M. Schleicher
  2. Institut des Sciences de la Terre, CNRS—IRD—Université de Savoie, Campus Scientifique, 73376 Le Bourget du Lac, France

    • A. Burgisser

Contributions

G.W.B. wrote the manuscript and directed the numerical experiments. J.M.S. performed the simulations, created the figures and contributed to the Supplementary Information. A.B. contributed to the performance of the simulations. All authors participated in the workflow and revisions.

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The authors declare no competing financial interests.

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