An investigation of disequilibrium growth processes of plagioclase in the system anorthite-albite-water by methods of numerical simulation

Abstract

Compositional zoning of plagioclase is useful as a recorder of dynamic geological conditions if the mechanisms of crystal growth are known. Although the present lack of quantitative information on specific kinetic processes limits their accuracy, numerical simulations of phenoycryst plagioclase growth are useful both for identifying the most influential kinetic processes (for example, diffusion) that should receive priority in experimental measurements and for designing informative growth experiments. The interaction of kinetic processes at a crystal face is so complex that the overall result cannot be assessed intuitively. A primary purpose of these papers is to explore this interaction in the plagioclase system as quantitatively as data permit.

The growth of a single face of a plagioclase crystal in an infinite melt was simulated in computer models for: (1) anhydrous and hydrous plagioclase melts; (2) for different undercoolings; and (3) for both interface-controlled and melt-transport controlled growth. Major uncertainties include the velocity and nonequilibrium partitioning laws in the interface-controlled model, and transport coefficients for melt components. Comparison of computed models with published growth velocity data for anhydrous melts was used to estimate a transport coefficient (with the form for diffusion), and that coefficient was extrapolated to hydrous melts on the basis of the Stokes-Einstein relationship.

The results of simulations suggest that undercoolings reasonable for plutonic systems could result in deviations of crystal composition from that in equilibrium with the melt of several mole % An; geothermometers based on the assumption of equilibrium partitioning will be in error significantly. Similarly, the volatile content and composition of melt trapped during growth would deviate significantly from bulk melt properties. The velocity of crystal growth and deviation of crystal composition from equilibrium show low sensitivity to water content because larger water contents result in greater accumulation of water at the interface and a consequent depression of effective undercooling.

The large magnitude of the derived transport parameter suggests that local convection as well as diffusion may occur during growth in the anhydrous system. The addition of water to the system reduces viscosity and increases the density gradients near the crystal, making local convection even more probable. Our meagre knowledge of transport by diffusion and convection in the melt is probably the most important factor limiting the accuracy of growth simulations.