Ar40 diffusion in homogenous orthoclase and an interpretation of Ar diffusion in K-feldspars

Ar⁴⁰ diffusion in a natural, non-perthitic orthoclase has been studied isothermal heating experiments between 500° and 800°C under both vacuum and hydrothermal (2 kbar) conditions. The sample is a one-phase K-feldspar without detectable chemical of structural inhomogeneities as verified by heating experiments, chemical and microprobe analyses, and cell refinements. The orthoclase does not disorder detectably and is stable for the duration of the heating interval. Diffusion coefficients were calculated using an isotropic model for spherical grains. Agreement of diffusion coefficients obtained on grain-sizes which differed by a factor of four indicate that the effective dimension for Ar⁴⁰ diffusion is the actual particle size. A series of experiments at 700°C show that Ar⁴⁰ loss may be described by the ideal spherical model and that the diffusion coefficient does not change with time. The Arrhenius relation is obeyed with a single activation energy and the diffusion coefficients are described by: D = (0·00982) exp — (43800/RT). Agreement of experiments conducted under vacuum and hydrothermally (up to 2 kbar) indicate that pressure and H₂O do not significantly affect Ar⁴⁰ loss. Relatively small amounts of alkali exchange between the feldspar and hydrothermal salt solutions do not affect the loss behavior.The simple behavior obtained for this orthoclase is attributed of the use of a simple technique within the region of sample stability and to the homogeneous nature of the feldspar. Effects due to sample instability and to the use of perthites are discussed. The new data are compared to those for homogeneous feldspars showing that the orthoclase gives diffusion coefficients which are as low as those sanidine. It is suggested that perthitization of feldspars in nature may reduce the effective grain size for diffusion and thereby allow diffusional loss of Ar⁴⁰ at relatively low temperatures.