Surface enrichment and trace-element uptake during crystal growth

Adsorption or enrichment of trace elements in the near-surface or interfacial regions of minerals has been documented in systems ranging from low-temperature aqueous environments to magmas. Under static conditions, this surface enrichment results from chemical equilibrium between the host medium of the crystal and the surface layer, which may exhibit diversity or flexibility in the types of atomic sites present. If the crystal is growing, any trace elements bound in the surface layer may be buried and trapped in the newly-formed lattice, resulting in lattice concentrations that deviate substantially from those predicted by equilibrium partitioning between the crystal and its growth medium. The effectiveness of this growth entrapment process depends upon the interplay between the growth rate, V, of the crystal (which can be thought of as the burial rate of surface-enriched element i) and the diffusivity, D-i, in the near-surface region of the crystal (which determines how efficiently i can escape to the surface by diffusion). The competition can be quantified in terms of the dimensionless number Vl/D-i (termed the growth Peclet number, Pe), where I is the half-thickness of the surface-enriched layer. Depending upon the degree of surface enrichment, some growth entrapment is possible if Pe > 0.1, and the process is highly efficient if Pe > 10. An assessment of Pe for a wide variety of geological situations suggests that growth entrapment is common in diagenetic and metamorphic environments but limited to slow lattice diffusants in silicic igneous systems. It is probably rare to nonexistent in basaltic systems except at laboratory growth rates.