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Open-system dynamics and mixing in magma mushes


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.

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Figure 1: Three time steps from the simulation of an open-system event in basaltic mush.
Figure 2: Crystal trajectories and zoning.
Figure 3: Crystal–crystal mixing efficiency.


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Financial support was provided by National Science Foundation grants EAR-1049884 and EAR-1447266 to G.W.B. and DGE-1256082 to J.M.S. Access to computational facilities was provided by grant TG-EAR140013 to G.W.B. from the NSF-funded XSEDE consortium.

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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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Correspondence to G. W. Bergantz.

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The authors declare no competing financial interests.

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Bergantz, G., Schleicher, J. & Burgisser, A. Open-system dynamics and mixing in magma mushes. Nature Geosci 8, 793–796 (2015).

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