Multiphase Simulations Constraining the Characteristic Volumes and Efficiency of Mixing within Magmatic Mushes

Abstract:

Mixing in crystal-rich magmas (mushes) during open-system events is governed by both granular and fluid dynamics. To clarify the granular (crystal-crystal-melt) controls on the volumes and timescales of crystal-rich mixing, we use discrete element-computational fluid dynamics (DEM-CFD) simulations that resolve mixing and multiphase flow at the crystal scale. We will report on three central findings: 1) that the crystal-crystal mixing time is well-recovered by an exponential relationship, 2) that the mixing of melts and crystals have different time and spatial scales and are not simply indexed to strain-rate, 3) a comparison of DEM-CFD with mixture theory and suspension rheology produces very different outcomes and illustrates the limitations of mixture theory when applied crystal-rich magmas.

To quantify the progress of crystal mixing, we introduce the Initial Neighbor Distance (IND) metric, which varies between zero (unmixed) and unity (mixed), indicating the goodness-of-mixing of the crystals throughout the domain with a single value at each time step. The IND is calculated by comparing the distance between every particle and its initial nearest neighbor, to the distance between every particle and a randomly selected particle. We find an exponential relationship between the IND and time during an open-system event. Although the coefficients of the exponential function depend on the crystal and melt properties, mush size and geometry, and intrusive velocity of incoming magma, the exponential behavior is very robust, and allows for predictions of characteristic crystal mixing times. For example consider a meter-scale olivine-rich basaltic mush intruded by a sub-meter scale dike at a flow-rate roughly ten times that needed to unlock and fluidize the crystals of the mush. The crystals achieve nearly complete mixing with an IND value of 0.9 after approximately four minutes of open-system intrusion.