Comment on " Multipath Ar transport in K - feldspar deduced from isothermal heating experiments " by Igor Villa

Villa [1] interpreted 39Ar loss experiments performed with a low-temperature, igneous K-feldspar (gal) as being inconsistent with multi-diffusion domain model (MDD; see [2]). The applicability of the MDD model for gal was dismissed in [1] exclusively on the basis of two features of the experiments: (1) initial steps at lower temperature produced more gas loss than initial steps at higher temperature; and (2) a sharp drop in log(D/r 2) occurred in successive isothermal steps. However, failure of these experiments to reproduce previously determined 39Ar loss systematics for the same material [19] and improper characterization of the domain distribution of gal indicate that the basis for the conclusion is flawed. Three aliquots of gal were step-heated, with each experiment beginning with isothermal steps at a different temperature. While more Ar is normally expected to be released at higher temperatures, the first 580°C step of galc liberated nearly twice as much 39Ar (~ 7%) as the first 670°C step of galb ( ~ 4%), and about the same amount as the first 750°C step of gala ( ~ 7%). All heating durations were 10 rain (see table 1 of [1]). The generality of the conclusions arising from these isothermal experiments are contradicted by every K-feldspar step-heating experiment in which log(D/r 2) monotonically rises with 1 /T through the temperature interval 5 0 0 ~ 750°C. This