Simulations of convection with crystallization in the system KAlSi2O6-CaMgSi2O6: implications for compositionally zoned magma bodies

Abstract

A model has been developed and applied to study the origin of compositional and phase heterogeneity in magma bodies undergoing simultaneous convection and phase change. The simulator is applied to binary-component solidification of an initially superheated and homogeneous batch of magma. The model accounts for solidified, mushy (two- or three-phase), and all-liquid regions self-consistently, including latent heat effects, perco- lative flow of melt through mush, and the variation of system enthalpy with composition, temperature, and solid fraction. Phase Equilibria and thermochemical and transport data for the system KAlSi2O6-CaMgSi2O6 were utilized to address the origin of compositional zonation in model peralkaline magmatic systems. Momentum transport is accomplished by Darcy percolation in solid-dominated regions and by internal viscous stress diffusion in melt-dominated regions within which relative motion between solid and melt is not allowed. Otherwise, the mixture advects as a pseudofluid with a viscosity that depends on the local crystallinity. Energy conservation is written as a mixture-enthalpy equation with subsidiary expressions that are based on thermochemical data and phase relations that relate the mixture enthalpy to temperature, composition, and phase abundances at each location. Species conservation is written as the low-density component (KAlSi2O6) and allows for advection and diffusion as well as the relative motion between solid and melt.

Systematic simulations were performed to assess the role of thermal boundary conditions, solidification rates, and magma-body shape on the crystallization history. Examination of animations showing the spatial development of the bulk (mixture) composition (C) melt composition (C1), temperature (T), solid fraction (fs), mixture enthalpy (h), and velocity (V), reveals the unsteady and complex nature of convective solidification owing to nonlinear coupling among the momentum, energy, and species conservation equations. A consequence of the coupling includes the spontaneous development of compositional heterogeneity in terms of the modal abundances of diopside and leucite in the all-solid parts of the domain (i.e., modal mineralógical heterogeneity) as well as spatial variations in melt composition particularly within mushy regions where phase relations strongly couple compositional and thermal fields. Temporal changes in the rate of heat extraction because of bursts of crystallization and concommitant buoyancy generation are also found. Crystallization of diopside, the liquidus phase in all cases, enriches residual melt in low- density K-rich liquid. The upward flow of this material near the mush-liquid interface leads to the development of a strong vertical compositional gradient. The main effect of magma-body shape and different thermal boundary conditions is in changing the rate of solidification; in all cases compositional heterogeneities develop. The rate of formation of the compositional stratification is highest for the sill-like body because of its high cooling rate. Compositional zonation in a fully solidified body is found to be both radial and vertical. The most salient feature of this simple model is the spontaneous development of large-scale magma heterogeneity from homogeneous and slightly superheated initial states, assuming local equilibrium prevails during the course of phase change.