REE DIFFUSION IN CALCITE

Abstract

Chemical diffusion of four rare-earth elements (La, Nd, Dy and Yb) has been measured in natural calcite under anhydrous conditions, using rare-earth carbonate powders as the source of diffusants. Experiments were run in sealed silica capsules along with finely ground calcite to ensure stability of the single-crystal samples during diffusion anneals. Rutherford backscattering spectroscopy (RBS) was used to measure diffusion profiles. The following Arrhenius relations were obtained over the temperature range 600-850°C: DLa =2.6x10-14 exp(-147+/-14 kJ mol-1/RT) m2 s-1, DNd =2.4x10-14 exp(-150+/-13 kJ mol-1/RT) m2 s-1, DDy =2.9x10-14 exp(-145+/-25 kJ mol-1/RT) m2 s-1, DYb =3.9x10-12 exp(-186+/-23 kJ mol-1/RT) m2 s-1. In contrast to previous findings for refractory silicates (e.g. zircon), differences in transport rates among the REE are not pronounced over the range of temperature conditions investigated in this study. Diffusion of the REE is significantly slower than diffusion of the divalent cations Sr and Pb and slower than transport of Ca and C at temperatures above ~650°C. Fine-scale zoning and isotopic and REE chemical signatures may be retained in calcites under many conditions if diffusion is the dominant process affecting alteration.