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The occurrence of sector zoning in minerals of regional metamorphic or low-temperature intrusive origin implies that rapid growth is not required for the development of this particular form of homogeneous disequilibrium. It is shown here that sector zoning can be a natural consequence simply of slow lattice diffusion. Given anisotropic surface en­richment coupled with the low dilfusivities typical of highly charged elements in refractory accessory minerals such as zircon and titanite, sector zoning can arise even in cases of growth rjates as low as a few micrometers per million years. According to the proposed model, the development of sector zoning depends upon the competition between growth rate (V) and lattice diffusion (D i ) within the near-surface layer (l), such that above a critical value of Vl/D i  (~0.5-3), sector zoning is unavoidable in crystals that exhibit selective enrichment on some growth surfaces. Known diffusivities of rare earth elements in zircon and titanite lead to the expectation of sector zoning in these minerals with respect to REEs for reasonable geologic growth rates. Even in the case of clinopyroxene, diffusion of REEs and high field-strength elements may be slow enough to contribute to the development of sector zoning in laboratory-grown crystals.

american mineralogist

A simple model for sector zoning in slowly grown crystals: Implications for growth rate and lattice diffusion, with emphasis on accessory minerals in crustal rocks