Spatially heterogeneous argon-isotope systematics and apparent 40Ar/39Ar ages in perlitised obsidian

These complex interactions essentially represent the mixing of three distinct Ar reservoirs: initial trapped Ar that may or may not be fractionated, an isotopically atmospheric Ar component introduced during hydration, and radiogenic 40Ar*. Such reservoir mixing is the underlying reason for poor correlations on isotope correlation diagrams and the difficulties in validating the composition of the non-radiogenic Ar component. We thus suggest that high 36Ar yields are a combination of the incomplete degassing of initial (possibly magmatic) Ar and the gain of Ar during interaction between the obsidian and meteoric/atmospheric fluids. Our analyses emphasise the challenging nature of 40Ar/39Ar dating obsidian samples, but also point to possible solutions by careful sample characterisation and selection of highly degassed samples.