⁴⁰Ar* diffusion in Fe-rich biotite

Hydrothermal bulk-loss experiments employing radiogenic Ar(⁴⁰Ar*) were performed to determine whether ⁴⁰Ar* 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 $(Xannite=0.71) D=0.40−0.28+0.96exp[−(50500±2.2)/RT]$ and Cooma biotite $(Xannite=0.54) D=0.075−0.021+0.049exp[−(47100±1.5)/RT]$⁠. 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 ⁴⁰Ar* 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 ⁴⁰Ar* loss by intercrystalline diffusion. The measured rates of ⁴⁰Ar* 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 ⁴⁰Ar* 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 ⁴⁰Ar* diffusion experiments and discuss how diffusion parameters determined in the laboratory may be applied to thermochronology provided suitable constraints are available.