Numerical simulations of crystallization processes of plagioclase in complex melts: the origin of major and oscillatory zoning in plagioclase

Abstract

Numerical simulations of the growth of a large crystal face of plagioclase in response to an instantaneous undercooling below the equilibrium temperature are presented for model granodiorite and basalt melts with varying water contents. The simulations suggest that the anorthite content of plagioclase decreases uniformly from the composition in equilibrium with the bulk melt as undercooling is increased, and that the water content in the melt has little influence on this result. Comparison of the simulations with sharp compositional changes in natural profiles suggests that undercoolings of tens of degrees C can be rapidly imposed on plutonic phenocrysts. Large changes of undercooling most likely result from chilling of the magma and local convection around growing crystals. The observation in experiments that growth rate does not increase rapidly with increasing water content in the starting melting composition can be attributed to the concentration of water at the crystal face during growth; the action of water to reduce liquidus temperature and undercooling has a greater effect on growth rate than its action to increase transport rates. Even at large undercooling, there is no significant increase in temperature at the interface caused by the release of heat of crystallization.

Simulations are presented to illustrate how disequilibrium growth processes due to undercooling can modify the normal zoning profiles expected from fractionation. Assuming that an undercooling is necessary to cause nucleation, normal zoning can result if crystal growth takes place at constant or increasing undercooling, but reverse zoning can occur at decreasing undercooling. Undercooling during growth is controlled by the relative rate of cooling and the rate at which the liquidus temperature is decreased by the accumulation of residual components and volatiles in the melt. Consequently, normal zoning should be promoted by rapid cooling, contemporaneous crystallization of other phases, and absence of volatiles, while reverse zoning should be expected in phenocrysts grown in slowly-cooled melts or in melts where volatiles are concentrated. The zoning patterns found in many plutonic plagioclase crystals suggest that their compositions are in significant disequilibrium with the melt; consequently, they are unsuitable for use in geothermometers.

Approximate calculations suggest that the amount of water concentrated at the surface of growing phenocrysts in plutons can promote local convection. Comparison of simulated and observed oscillatory zoning profiles suggests that oscillatory zoning is not explained by a re-nucleationdiffusion model (Harloff 1927), but is readily explained by periodic local convection.