A STUDY OF STRONTIUM DIFFUSION IN K-FELDSPAR, NA-K FELDSPAR AND ANORTHITE USING RUTHERFORD BACKSCATTERING SPECTROSCOPY

Sr chemical diffusion has been measured in orthoclase, anorthoclase and anorthite under dry, 1-atm conditions. A strontium oxide-aluminosilicate powder mixture was used as the source material, with Rutherford Backscattering Spectroscopy (RBS) used to measure diffusion profiles. Over the temperature range 725-1075-degrees-C the following Arrhenius relations were obtained. Orthoclase (normal to (001)): D = 5.97 x 10(-0.0060/+0.0030) exp(- 67900+/-1600/RT)cm2 s-1 Measurements of diffusion perpendicular to (010) and (100) indicate little diffusional anisotropy in orthoclase. Anorthoclase: normal to (010): D = 4.51 x 10(+1)(+358/-5.6) exp(- 89100+/-4800/RT) cm2 s-1 normal to (001): D = 2.25 x 10(+2)(+1615/31) exp(- 89100+/-4600/RT) cm2 s-1 Anorthite (normal to (010)): D = 3.85 x 10(-2)(+0.380/-0.0039) exp(- 78800+/-5400/RT) cm2 s-1 The correlation of Sr uptake with a reduction of K and Ca in orthoclase and anorthite, respectively, indicates that Sr is exchanging with these species. Nuclear reaction analysis (NRA) measurements of Al in orthoclase suggests that the coupled exchange Sr2+ + Al3+ --> K1+ + Si4+ is taking place, as expected. Sr closure temperatures for feldspars, calculated with the above parameters, are relatively high compared to those of some other minerals (e.g., biotite, muscovite, apatite). However, closure temperatures of feldspars may be significantly depressed if they are exsolved or twinned, leading to small effective diffusion lengths. Feldspar crystals of millimeter size should retain Sr-87/Sr-86 ratios except when subjected to thermal events of extreme temperature or duration.