HF-ZR INTERDIFFUSION IN SINGLE-CRYSTAL ZIRCON

Interdiffusivity of Hf and Zr in the zircon (ZrSiO4)-hafnon (HfSiO4) solid solution system was measured by (1) coupled annealing of synthetic hafnon and natural zircon and (2) heating of zircon crystal with a layer of hafnon epitaxially grown over the surface of zircon crystal. Because of the unexpectedly low diffusivity of Hf-Zr pair, practically usable value of diffusion coefficient was not obtained by the conventional coupled annealing technique. By use of the sample prepared by an epitaxial over-growth technique the upper limit of diffusion coefficient for Hf-Zr interdiffusion in the zircon-hafnon system was found to be less than or similar 1 x 10(-15) cm2 . sec-1 at 1600-degrees-C. The observed low diffusivity of Hf-Zr pair in the zircon-hafnon system implies that diffusion of U, Th and radiogenic daughters does not affect lo age of zircon significantly during its residence in the parental magma, assuming that the diffusivities of those elements do not differ significantly from that observed for Hf-Zr.