The implications of muscovite sub-spectra in phengitic white mica on the theory and practice of argon geochronology

Forster, Marnie1, Lister, Prof. Gordon1

1Research School of Earth Sciences, Australian National University, Canberra, Australia

This study illustrates a new method for the quantitative determination of the timing of movement in ductile shear bands formed in mylonites, or in strongly stretched metamorphic tectonites. The method is of particular use where phengitic white mica is involved, since interlaying in this mineral is usually so fine as to preclude the application of laser methods. In any case, laser methods as they are currently applied, do not have the capability of extracting exact and detailed age-temperature spectra. Laser methods also fail to achieve the multitudinous steps of the age spectrum evident from our high-definition UHV diffusion experiments. Laser methods also lose all information in terms of the low-volume early release of argon that is essential for the recognition of sub-spectra. Computer modelling and simulation shows that such detail in the age spectrum is essential in terms of being able to accurately infer the timing and duration of metamorphic events,

Here we show that high-definition ultra-high-vacuum (UHV) 39Ar diffusion experiments using phengitic white mica are routinely able to extract muscovite sub-spectra in the first 10-30% of 39Ar gas release during 40Ar/39Ar geochronology. A critical factor is that the recognition of muscovite sub-spectra requires Arrhenius data in order to recognise the steps dominated by release of 39Ar from muscovite. In turn this requires precise measurement of temperature during each heating step. The muscovite sub-spectrum is distinct and separate to the main spectrum, which is itself dominated by mixing of gas released from phengite as well as muscovite. The muscovite sub-spectra allow consistent estimates of the timing of the formation of microstructural shear bands in various mylonites, as well as allowing quantitative estimates of temperature variation with time during the tectonic history of shear zones.

Our new data reveals hitherto unsuspected variation in the timing of exhumation of individual slices of the eclogite-blueschist belt, caused by Eocene and Miocene detachment-related shear zones. With excellent outcrop, the eclogite-blueschist belt exposed in the Cycladic archipelago in the Aegean Sea, Greece, offers a spectacular natural laboratory in which to decipher the structural geology of a highly extended orogenic belt and to ascertain the history of the different fabrics and microstructures that can be observed. Using phengitic white mica we demonstrate a robust correlation of age with microstructure, once again dispelling the myth that 40Ar/39Ar geochronology using this mineral, produces cooling ages alone. Previous work in the Cycladic eclogite-blueschist belt has incorrectly assumed that the diffusion parameters for phengitic white mica were the same as for muscovite. Arrhenius data suggest this is not the case, and that phengitic white mica is considerably more retentive of argon than muscovite. Previous workers have also erred in dismissing microstructural variation in age as an artefact, supposedly as the result of the incorporation of excess argon. This has led to inconsistencies in interpretation, because phengite is able to retain argon at temperatures that exceed those estimated using metamorphic mineral parageneses.

The argon system has been treated as a thermochronometer. However, we demonstrate a robust correlation between microstructure and age, down to the detail present in complex tectonic sequence diagrams produced during fabric and microstructural analysis of individual thin-sections. This points to new strategies being required in terms of the theory and practice of argon geochronology.


Dr Marnie Forster is following in the footsteps of Prof Ian McDougall running the ANU Argon Laboratory  with the assistance of Davood Vasegh and Agnes the argon robot.

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