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

Abstract:

Geologic, geochemical, and geophysical data from both volcanic and deep plutonic materials point to magmatic mushes as a long-term physical state for these systems. Samples from both settings indicate magma mixing commonly occurs, despite apparently high crystallinity and high viscosity melts. Previously proposed mechanisms for mush unlocking cannot recover the diversity of expressions of open system behavior observed at the crystal or meso-scale. Using discrete element simulations containing up to millions of crystals, we observed visco-elastic behavior within the mush. This indicates magma mushes could fail initially as a Mohr-Coulomb material during open system events. We call the failure planes soft faults, delimiting a volume that is subsequently unlocked. In addition we recover the diversity of mixing styles occurring simultaneously: porous media flow, crystal entrainment, new melt mixing with crystal free reservoir melt, etc.

Specifically, the open-system models show melt injection rate strongly determines the dynamic response of the mush. We compare injection rates with the minimum fluidization velocity (Umf), the lowest magma injection rate calculated to fluidize a bed of packed crystals. When injection rates are below or near Umf, melt passes through the bed as porous flow and the mush remains locked. Rates higher than Umf create soft faults within the crystal mush, which then bound a fluidized region of crystals we call the mixing bowl. A nearly crystal-free chimney of new melt within the mixing bowl continually entrains crystals, bringing them from the base to the top of the crystal pile. This leads to recurring overturn of the crystals, even at low Reynolds number conditions. We use the Lacey statistical mixing index to quantify the amount of mixing that occurs between particles. Results show the extent of crystal mixing can be predicted by the injection rate of the melt entering into the mush.