39Ar-40Ar geochronology of mono- and polymetamorphic basements

Igor Maria Villa

Abstract


The 39Ar-40Ar  technique is often used to date the metamorphic evolution of basement rocks. The present review article examines systematic aspects of the K-Ar decay system in different mineral chronometers frequently found in mono- and polymetamorphic basements (amphibole, biotite, muscovite/phengite, K-feldspar). A key observation is that the measured dissolution rate of silicates in aqueous fluids is many orders of magnitude faster, and has a much lower activation energy, than the rate of Fickian diffusion of Ar. The effects of this inequality are patchy age zonations, very much like those observed in many U-Pb chronometers, unaccompanied by intra-crystalline bell­shaped Ar loss profiles. Recognizing the importance of the respective rate constants in field situations leads to re-evaluating the ages and the interpretive paradigms in classic examples such as the Central Alpine "Lepontine" amphibolite event and the Western Alpine eclogitic event. 


Keywords


39Ar-40Ar dating; diffusion; geochronology; eclogite dating; Lepontine event

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DOI: http://dx.doi.org/10.2451/2015PM0454



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