Systematic Analysis of K-feldspar 40Ar/39Ar Step Heating Results II: Relevance of Laboratory Argon Diffusion Properties to Nature

We examine a database containing the results of Ar/Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of Ar/Ar systematics reveals that about half of the K-feldspars examined are sufficiently well-behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8 while roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multi-diffusion domain (MDD) model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield Ar/Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar upon thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion (HD) model. We find that the MDD approach can successfully recover imposed thermal histories from HD-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar Ar/Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms – a requirement implicitly met by those samples exhibiting high degrees of correlation.