A STUDY OF STRONTIUM DIFFUSION IN APATITE USING RUTHERFORD BACKSCATTERING SPECTROSCOPY AND ION-IMPLANTATION

Strontium diffusion in Durango fluorapatite has been measured under anhydrous conditions using a combination of techniques. Diffusants were introduced into the apatite by two methods: (1) ion implantation of Sr, and (2) immersion in a strontium oxide reservoir. Resulting diffusion profiles were measured by Rutherford Backscattering Spectrometry (RBS) and fit to the appropriate solutions of the diffusion equation to obtain diffusion coefficients. Previous determinations of Sr diffusion in apatite obtained under hydrous conditions (FARVER and GILETTI, 1989) indicate that a decrease in activation energy occurs at approximately 1000-degrees-C. In contrast, diffusivities obtained in the present work may be described by a simple Arrhenius relationship: D = 2.7 X 10(-3) exp (-65000 +/- 2200 cal/mol/RT) cm2 sec-1 for diffusion perpendicular to c over the temperature range 700-1050-degrees-C. Similar results are obtained for transport parallel to c. As in the case of Pb diffusion in apatite (CHERNIAK et al., 199 1 ), the results of the present study lie on the down-temperature extrapolation of diffusion coefficients determined under dry conditions at higher temperatures (WATSON et al., 1985). Radiation damage induced by ion implantation may, in some cases, enhance or otherwise affect diffusion parameters. To assess the significance of such effects in apatite, the Sr results obtained by ion implantation were compared with those from a set of experiments in which Sr was introduced by immersion of the crystals in strontium oxide powder. Excellent agreement of diffusion coefficients from the two data sets indicates that radiation damage does not adversely affect these measurements of Sr diffusion in apatite made under anhydrous conditions. The measured diffusivities suggest that strontium isotope ratios in the cores of apatite crystals entrained in felsic magmas or residual to crustal anatexis may be unaffected by the Strontium isotopic compositions of their surroundings for the temperatures and durations typical of these events.