Abstract: A comprehensive understanding of the complex internal zonation of pegmatites has eluded petrologists for over a century. Explaining the origins of the zonation represents a challenge to our understanding of igneous and hydrothermal processes, and our ability to discern them.
Liquidus Undercooling. Liquidus undercooling of hydrous and flux-bearing (but not exotic) granitic liquids by ~ 200oC is the single most important step in the formation of zoned pegmatites. The crystallization of pegmatites commences at ~ 450o-500oC, ~ 200oC 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 assemblages.
Feldspar and Quartz. The most common and prominent manifestation of zonation entails feldspathic outer zones and quartz-rich cores. This zonation arises from differences in the Gibbs Free Energies of crystallization (Ḡi,liquid → Ḡi,crystal, where i is a component of the melt with crystalline stoichiometry) in the highly undercooled state of pegmatite-forming melts. At 500oC and 200 MPa, the energy released by the crystallization of (typical) plagioclase (Ab85An15) is - 26839 J/m, that of alkali feldspar (Or70Ab30) is -26760 J/m, and of quartz (8 oxygen basis) is - 18586 J/m. Consequently, the greater energy release from feldspars favors their crystallization over quartz at the start, and the commensurate quartz component missing in the outer zones is deposited sequentially in the interior portions of pegmatite bodies.
Zonation of Alkali Feldspar and Plagioclase. 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.
Oscillating Zonation and Chemical Fractionation. At 450o-550oC, 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).
Bio: David London obtained his B.A. in geology (1975) at Wesleyan University, Connecticut, after which he worked as a field geologist for the U.S. Geological Survey from 1975-1976. He received his M.S. (1979) and Ph.D. (1981) in geology from Arizona State University. Following a postdoctoral research fellowship at the Geophysical Laboratory of the Carnegie Institution of Washington (1981-1982), London joined the faculty of the School of Geology and Geophysics at the University of Oklahoma, where he is the Stubbeman-Drace Presidential Professor, the Norman R. Gelphman Professor of Geology, and director of the University’s electron microprobe lab. London also is the chair and managing editor of the “Pegmatite Interest Group” of the Mineralogical Society of America. He is the author of the book Pegmatites, which was published in 2008 as Canadian Mineralogist Special Publication 10, and he is the namesake of the mineral londonite, isometric CsAl4Be4[B11Be]O28 (Can. Mineral. 39: 747-755).
