A CONCEPTUAL MODEL FOR NEAR-SURFACE KINETIC CONTROLS ON THE TRACE- ELEMENT AND STABLE ISOTOPE COMPOSITION OF ABIOGENIC CALCITE CRYSTALS

Abstract

The surface of a crystal in equilibrium with solute-bearing fluid generally has a composition that differs from that of the bulk crystal. If the crystal is growing, the surface composition may be “captured” by the newly formed lattice to a degree that depends upon the growth rate and the mobility of atoms in the near-surface region: rapid growth promotes this growth “entrapment,” high near-surface mobility works against it. Natural calcites may be particularly susceptible to this kind of kinetic disequilibrium, because their precipitation rates from aqueous solution can be relatively high even at near-ambient temperatures, where ion mobility in the critical near-surface region may be limited.