A study of strontium diffusion in plagioclase using Rutherford backscattering spectroscopy

Abstract

Strontium chemical diffusion has been measured in plagioclase of three intermediate compositions (between An 23 and An 67) under anhydrous, 1-atmosphere conditions. A strontium oxide-aluminosilicate powder mixture was used as the diffusant source material, with Rutherford backscattering spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725–1075°C, the following Arrhenius relations were obtained for diffusion normal to (001) ( D in m 2 s −1): Oligoclase (An 23): log D = (−6.07 ± 0.58) + (−273 ± 13 kJ mol −1)/RT . Andesine (An 43): log D = (−6.75 ± 0.33) + (−265 ± 8 kJ mol −1)/RT . Labradorite (An 67): log D = (−7.03 ± 0.37) + (−268 ± 8 kJ mol −1)/RT . In the labradorite, diffusion of Sr normal to (010) was somewhat slower (by ~0.7 log units) than diffusion perpendicular to (001). The activation energy for diffusion normal to (010) appears to follow a trend similar to that observed for diffusion perpendicular to (001). In oligoclase, diffusion normal to (001) also appears to be somewhat faster than diffusion normal to (010), but the differences are less pronounced than in labradorite. No significant crystallographic anisotropy was noted in the andesine. The results show a clear trend of increasing Sr diffusivity with decreasing An content of the plagioclase. Profiles of Ca and Na, measured with RBS and Nuclear Reaction Analysis (NRA) respectively, indicate that Sr is exchanging with both of these species in the plagioclase. In each feldspar composition, decreases in Ca and Na in the near-surface region are nearly proportional to their bulk concentrations, indicating that there is no significant substitutional preference of Sr for either cation in those feldspars investigated. NRA measurements of Al, showing a corresponding uptake of Al and Sr, suggest that a coupled exchange, possibly Sr +2 + Al +3 → Na +1 + Si +4 , is taking place when Sr exchanges with Na. Chemical diffusion of Sr in calcic plagioclase is dominated by Sr +2 → Ca +2 exchange.