A test of the interlayer ionic porosity model as a measure of argon diffusivity in trioctahedral micas

Abstract

Micas are among the most commonly-dated minerals by the 40Ar/39Ar technique to determine the crystallization or cooling ages of rocks. Interpretation of the geological significance of these ages is largely dependent on understanding how 40Ar* (radiogenic argon) can be retained or released from the mineral during the thermal history of a terrane. The interlayer ionic porosity (Zi) model, serving as a measure of a mineral’s atomic packing density, has been proposed in an attempt to understand and explain the diffusive behaviour of argon (Ar) and its retention in micas. We tested this model by dating micas from several outcrops using the 40Ar/39Ar and Rb/Sr techniques along a ∼5.2 km transect in the Frontenac terrane, Grenville Province, Ontario. This transect is ideal because: (a) the rocks in this small area experienced the same geologic history, and (b) there are a variety of lithologies containing trioctahedral and dioctahedral micas with different chemical compositions. Our results show that trioctahedral and dioctahedral micas yielded 40Ar/39Ar plateau ages that differ by ∼150 Ma. Significantly, although both Mg# and F# correlate with Zi, as predicted by the model, there is no correlation between Mg#, F#, and Zi and the age of the micas in similar lithologies. This suggests that the interlayer ionic porosity model, in itself, may not be suitable for explaining mechanisms of Ar mobility in micas alone. Instead, both internal (chemical composition, bulk rock properties) and external (fluid composition, open/closed-system behaviour) factors that may influence Ar retentivity in micas should be more broadly considered in order to elucidate the significance of their 40Ar/39Ar ages.