Diffusion domains determined by 39Ar released during step heating

Abstract

The Arrhenius plot for 39Ar released during step heating of a sample containing a distribution of diffusion domains will depart from a straight line in ways that depend on the domain size distribution and the heating schedule used to extract the 39Ar. In a new series of experiments we make use of the heating schedule effect to confirm the multidomain nature of several alkali feldspar samples. We find that temperature cycling is particularly useful for determining the diffusion parameters (activation energy and frequency factor) of the different domains. We also use the log (r/r0) versus cumulative percent 39Ar released plot presented in an earlier paper to display better the sizes of the different domains and the proportion of argon that they contain. We conclude by showing that a particular choice of the geometry and number of domains is not critical to the determination of the thermal history provided the 39Ar data are satisfactorily fitted by the particular choice mode. Experiments on single crystals show that multiple diffusion domains are an intrinsic property of the feldspars and thus are not separable by a careful selection of the grains to be analyzed.