STRONTIUM DIFFUSION KINETICS IN PLAGIOCLASE FELDSPARS

A pronounced, systematic variation, by a factor of 10(4), in strontium tracer diffusion kinetics has been measured in four feldspar specimens that span the range of plagioclase compositions. The experiments were carried out in air, using isotopically enriched Sr-86, and analyzed with an ion microprobe. The Arrhenius relations for strontium diffusion are: [GRAPHICS] No difference was found, within the measurement uncertainty, between these results and those for albite under hydrothermal conditions at one kbar( 100 MPa). Strontium diffusion rates parallel to, and normal to the albite twin lamellae are the same. There is no effect on the strontium diffusion rate in albite between an oxygen partial pressure of 0.2 bar (air) and similar to 10(-19) bar (graphite buffer). A relation is developed that permits calculation of the strontium diffusion Arrhenius parameters for any normal plagioclase composition. The large variation in diffusion rate with composition has a major effect on the ease of strontium isotope exchange at high temperatures between plagioclases and coexisting phases in rocks, which may prove useful in determining thermal histories.