ALKALI FRACTIONATION AND FELDSPAR ZONATION IN GRANITIC PEGMATITES

Spatial zonation of alkali feldspars is a fundamental characteristic of granitic pegmatites. Zonation at the scale of the pegmatite body gives rise to Na-rich margins and K-rich centers, to K-rich upper and Na-rich lower portions, and to sequential alterations of albitic and potassic feldspars. Previous models hinged on flux-rich, low viscosity melts to enhance elemental diffusion throughout the entire melt volume, and buoyant ascent of aqueous vapor through melt to fractionate K from Na. The high viscosities of hydrous granitic melts at liquidus temperatures (~ 10⁵ Pa•s, the viscosity of window putty, at 680°C, 200 MPa H₂O) or below (e.g., ~ 10⁸ Pa•s, the viscosity of cold asphalt, at 400°C, 200 MPa H₂O) were thought to work against long-range diffusion, but also would impede the buoyant separation of an aqueous phase. Petrologic experiments have simulated the zonation of feldspars in pegmatites via substantial liquidus undercooling (by ~ 200°C) of high-viscosity granitic melts. Recent experiments have demonstrated that the diffusion and fractionation of alkalis occur rapidly throughout the melt, even in viscous, simple granitic liquids. When chemical potential gradients of the components H₂O, B, P, F, and Al exist in the melt, Na diffuses toward and associates with these components, whereas K diffuses (with or opposed to Na) so as to maintain charge balance on ^IVAl. Such gradients are created when fractionated melt boundary layers form at the margins of crystallization fronts, where the accumulation of excluded components such as H₂O, B, P, F draw Na for local charge balance, creating a distal melt enriched in K. Chemical potential gradients also are created by sequential feldspar growth: hypersolvus Na-rich plagioclase tends to be the first-formed feldspar in natural granitic compositions, and its crystallization at one location in an experiment can (re)move sufficient Na to induce K-feldspar crystallization at the opposite end of the (diffusive distance in the) melt volume. The experiments and natural pegmatites, spanning variations of ~ 10⁶ in diffusion length, are similar because they originate from the same process: when sidewall crystallization advances inward from the margins, diffusion and fractionation of alkalis is scaled to the size of the body, independent of distance, volume, and time.