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Hydrothermal bulk-loss experiments employing radiogenic Ar ( 40 Ar ∗ ) were performed to determine whether  40 Ar ∗  diffusivity in biotite increases with Fe content. Diffusion laws determined for intermediate and Fe-rich biotite assuming single-domain diffusion (infi­nite-cylinder geometry) are remarkably similar: Fe-mica biotite (X anitee  = 0.71) D = 0.40 +096  -0.28 exp[-(50500 ± 2.2)/RT] and Cooma biotite (X anite  = 0.54) D = 0.075 +0.049  -0.021 exp [-(47100 ± 1.5)/RT]. The nearly identical results for Fe-mica biotite and Cooma biotite and their similarity to those from previous studies indicate that most biotite grains of intermediate composition possess comparable  40 Ar ∗  diffusion properties. Because limited grain breakage and volumetrically minor recrystallization is unavoidable during hydrothermal heating in bulk diffusion experiments, these diffusion laws necessarily provide upper limits to  40 Ar ∗  loss by intercrystalline diffusion. The measured rates of  40 Ar ∗  loss from biotite agree reasonably well with expectations based on single-domain volume dif­fusion using infinite-cylinder geometry when experimental uncertainties are taken into account. However, lack of information regarding  40 Ar ∗  gradients within the hydrothermally treated mica prevents us from precluding more complex diffusion mechanisms in­volving high diffusivity pathways. In this paper we consider the significance of bulk-loss  40 Ar ∗  diffusion experiments and discuss how diffusion parameters determined in the lab­oratory may be applied to thermochronology provided suitable constraints are available.

40 Ar* diffusion in Fe-rich biotite