A STUDY OF STRONTIUM DIFFUSION IN PLAGIOCLASE USING RUTHERFORD BACKSCATTERING SPECTROSCOPY

Strontium chemical diffusion has been measured in plagioclase of three intermediate compositions (between An23 and An67) under anhydrous, 1- atmosphere conditions. A strontium oxide-aluminosilicate powder mixtur e was used as the diffusant source material, with Rutherford backscatt ering spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725-1075-degrees-C, the following Arrhenius relation s were obtained for diffusion normal to (001) (D in m2 S-1): Oligoclas e (An23): log D = (-6.07 +/- 0.58) + (-273 +/- 13 kJ mol-1)/RT. Andesi ne (An43): log D = (-6.75 +/- 0.33) + (-265 +/- 8 kJ mol-1)/RT. Labrad orite (An67): 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 approximately 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 (010) . In oligoclase, diffusion normal to (001) also appears to be somewhat faster than diffusion normal to (010), but the differences are less p ronounced than in labradorite. No significant crystallographic anisotr opy was noted in the andesine. The results show a clear trend of incre asing Sr diffusivity with decreasing An content of the plagioclase. Pr ofiles of Ca and Na, measured with RBS and Nuclear Reaction Analysis ( NRA) respectively, indicate that Sr is exchanging with both of these s pecies 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 substitut ional preference of Sr for either cation in those feldspars investigat ed. NRA measurements of Al, showing a corresponding uptake of Al and S r, 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 S r in calcic plagioclase is dominated by Sr+2 --> Ca+2 exchange.