Dynamic mixing in magma bodies: theory, simulations, and implications

Considerable geochemical and petrographic evidence suggests that magma mixing phenomena are important in producing the chemical heterogeneity commonly observed in plutonic and volcanic rocks on a variety of scales in both space and time. Simulations of time-dependent, variable viscosity, double-diffusive convection have been carried out to quantitatively investigate the mixing dynamics of magma in melt-dominated magma bodies. Mixing times are at a minimum for equant bodies, while for sill-like bodies, mixing is inhibited by the formation of multiple cells of different composition in the horizontal. Assimilation and fractional crystallization geochemical models that assume "well-mixed' magma bodies may be grossly misleading. A viscous (ie crystal laden), large (d~5km) magma body heated weakly from below and initially strongly chemically stratified will remain unmixed for several Ma. A large-volume, thermally well-connected basaltic body will mix rapidly (10³-10⁴yr). Because flow reversals may occur in dynamic mixing (Rr>0), crystal distributions within convecting magma bodies will be different from those predicted assuming steady state velocity fields. -from Authors