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

Strontium diffusion in Durango fluorapatite has been measured under an hydrous 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 diffusio n profiles were measured by Rutherford Backscattering Spectrometry (RB S) and fit to the appropriate solutions of the diffusion equation to o btain 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 approximatel y 1000-degrees-C. In contrast, diffusivities obtained in the present w ork 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 perpendic ular to c over the temperature range 700-1050-degrees-C. Similar resul ts are obtained for transport parallel to c. As in the case of Pb diff usion in apatite (CHERNIAK et al., 199 1 ), the results of the present study lie on the down-temperature extrapolation of diffusion coeffici ents 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 th e 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 oxi de powder. Excellent agreement of diffusion coefficients from the two data sets indicates that radiation damage does not adversely affect th ese measurements of Sr diffusion in apatite made under anhydrous condi tions. The measured diffusivities suggest that strontium isotope ratio s in the cores of apatite crystals entrained in felsic magmas or resid ual to crustal anatexis may be unaffected by the Strontium isotopic co mpositions of their surroundings for the temperatures and durations ty pical of these events.