40AR/39AR RESULTS FOR ALKALI FELDSPARS CONTAINING DIFFUSION DOMAINS WITH DIFFERING ACTIVATION-ENERGY

Extension of the single diffusion domain/activation energy closure model of Dodson to apply to minerals with a discrete distribution of domain sizes appears to reconcile 40Ar/39Ar age spectra of alkali feldspars with their associated Arrhenius plots. However, small remaining discrepancies, particularly apparent in log (r/r0) plots, suggest that some, and perhaps most,alkali feldspars contain diffusion domains with activation energies that may vary by as much as 8 kcal/mol. An important consequence of even relatively small variations in activation energy between domains is that the shape of an age spectrum can change dramatically by varying the laboratory heating schedule. We find that Arrhenius and log (r/r0) plots have the potential to reveal even small differences in activation energy (approximately 2 kcal/mol) between domains, at least in cases where the domains are well separated in size. Variations in activation energy of approximately 5 kcal/mol can result in differences in calculated closure temperature of up to 30-degrees-C from that obtained assuming equal activation energies for all domains. Overestimates of apparent activation energy and other inconsistencies resulting from reversed heating experiments may reflect annealing of subgrain features which define the smallest diffusion domain size. Use of the diffusion compensation relationship may provide a way to assess the relative distribution of diffusion domain sizes in a sample containing domains with multiple activation energies.