THE NATURE AND ORIGINS OF INTERNAL ZONATION WITHIN GRANITIC PEGMATITES (Invited Presentation)

The development of zones within granitic pegmatites stems from magmatic processes that have been elucidated by experimental and numerical simulations. Understanding the origins of zonation in pegmatites aids in the exploration of gem minerals within them.

Liquidus Undercooling. The crystallization of pegmatites commences at ~ 450°-500°C, ~ 200°C below the liquidus temperature of granitic melts. In response to undercooling, the first-formed zones exhibit anisotropy as unidirectional solidification inward, graphic intergrowths, and sequential mineralogical zones. Liquidus undercooling of granitic liquids by ~ 200°C is the single most important step in the formation of pegmatites.

Thermodynamic Relations. At 500°C, Gibbs Free Energies of crystallization (Ḡ_i,liquid → Ḡ_i,crystal, where i is a component of the melt with crystalline stoichiometry) strongly favor feldspars over quartz. As a result, the first-formed outer zones of pegmatites (and experiments) are feldspathic, containing only 23-25% modal quartz, and the ensuing internal zones are commensurately quartz-rich relative to the bulk composition (33-35% quartz).

Boundary Layer Pile-Up. At 450°-550°C, the concentrations of components that are excluded from the first crystalline assemblage build up in a boundary layer of melt adjacent to the crystallization front. Boundary layer pile-up has two consequences for zonation: (1) the crystallization front is alternately saturated in multiple mineral assemblages, or (2) constitutional zone refining leads to an accumulation of excluded components in the boundary layer liquid until the bulk melt has been exhausted, whereupon the mineralogy changes from ordinary to exotic. The formation of coarse-grained pegmatitic texture, and eventually gems, results more from process (2) than from (1).

Far-Field Chemical Diffusion. When crystallization begins with a plagioclase or a K-feldspar assemblage along one margin of a melt body, the complementary assemblage nucleates on the opposite side of the melt body. The chemical potential gradients caused by the initial assemblage appear at the far end of the melt column because that is the finite boundary condition for the diffusive supply of ions to the crystallization front. Far-field chemical diffusion leads to spatial segregation of plagioclase and K-feldspar.