On the dynamics of magma mixing by reintrusion: implications for pluton assembly processes

Many plutons have formed by repeated intrusion, with complex internal contacts characterized by sharp and diffuse kinematic and compositional domains. We performed numerical experiments of mixing following magma chamber recharge that explicitly considers the fluid dynamics of multiphase mixtures. In the limit of zero diffusivity of intensive scalar quantities and low Stokes number, mingling by fluid instability is equivalent to deformation, and persistent fluid structures are kinematic ‘attractors’. Three distinct regimes are exemplified, and can be described by their multiphase Reynolds (or Grashof) number. For a Reynolds number greater than about 100, an internal intrusive contact will collapse by internal wave-breaking, and chaotic magma mingling and mixing yield a nearly chamber-wide stratification. This flow may scour mushy regions at the walls and widely distribute previously crystallized material. For Reynolds numbers from 10 to 100, the internal wave does not break, and the stratification occupies less of the chamber, leaving islands of unmixed material. For Reynolds numbers around 1, internal slumping and folding can occur, and is the most likely to be preserved by a mineral fabric. Internal, sub-vertical contacts require a high absolute viscosity in the resident magma, representing a time-break between episodes of reintrusion. The shape of perched mafic domains captures the progress of sinking from higher levels into regions of increasing strength and crystallinity.