SR, Y, AND REE DIFFUSION IN FLUORITE

Abstract

The diffusion of Sr, Y and three rare earth elements (Nd, Dy and Yb) has been measured in natural fluorite under dry 1-atm conditions. Sources of diffusant consisted of mixtures of Sr, Y, or REE fluorides and CaF2 powders, pre-reacted in vacuo in silica glass tubes. Experiments were performed by sealing the prepared source material with pieces of cleaved fluorite in silica glass tubes under vacuum, and annealing in vertical tube furnaces for times ranging from 20 min to a few months. Following diffusion anneals, concentration profiles were measured with Rutherford Backscattering Spectroscopy (RBS). Over the temperature range 700-1050°C, the following Arrhenius relations are obtained:DSr=2.3x101 exp(-450 kJ mol-1/RT) m2/sDY=1.2x102 exp(-454 kJ mol-1/RT) m2/sDNd=4.8x10-2 exp(-385 kJ mol-1/RT) m2/sDDy=3.2x100 exp(-419 kJ mol-1/RT) m2/sDYb=3.1x10-1 exp(-395 kJ mol-1/RT) m2/sDiffusion rates for the REE (and Y) are all quite similar, and faster than diffusion of Sr. This is in contrast to observations in other mineral systems (e.g., feldspars, zircon) where diffusion of more highly charged cations proceeds more slowly than that of cations with lower charge. However, the ionic radius of Sr2+ (1.26 Α) is larger than that of Y3+ (1.019 Α) or the investigated REE (0.985-1.109 Α), suggesting that differences in cationic size may exert greater influence on diffusion rates in fluorite than do differences in cation charge. This size dependence is weak, however, perhaps due to similarities in size between these cations and Ca, for which they likely substitute in the fluorite lattice, or as a consequence of the relatively flexible fluorite lattice.The diffusion findings also have important implications for Rb-Sr and Sm-Nd isotopic dating and can be useful in interpreting zoning patterns observed in fluorite. These data indicate that REE zoning on the 100 μm scale will be retained for times on order of the age of the earth at temperatures below typical crystallization temperatures for fluorite in igneous systems (i.e., ~500°C), provided that the zoning is modified solely by volume diffusion.