The interpretation of 40Ar/39Ar apparent age spectra produced by mixing: application of the method of asymptotes and limits

A method is presented for the analysis and interpretation of apparent age spectra produced during 40Ar/39Ar step-heating experiments. Application of this method is particularly relevant to complex apparent age spectra produced from minerals taken from exhumed metamorphic tectonites, particularly when these rocks once resided in an ancient Argon Partial Retention Zone. Such rocks may have been subject to localised deformation and/or recrystallisation during orogenesis. Yet microstructures relict of earlier episodes of deformation and/or metamorphism may retain relatively old apparent ages. As a result complex apparent age spectra can be produced during a conventional step-heating experiment, and these age spectra often cannot be simply interpreted. The recognition (and the interpretation) of a plateau in such spectra is sometimes uncertain, and in any case, the definition of a ‘plateau’ is then a concept of limited theoretical validity. To avoid these difficulties an alternative strategy is presented, based on a theory of mixing gas from different microstructural/microchemical reservoirs. This method relies on the definition of asymptotes and limits in sequences of apparent ages in apparent age spectra obtained from step-heating experiments. Frequently measured ages (FMAs) in individual datasets can then be recognized using statistical analysis. The significance of FMAs must be independently assessed.