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 bell- shaped Ar loss profiles. Recognizing the importance of the respective rates in field situations leads to re-evaluating the ages and the interpretive paradigms in classic examples such as the Central Alpine "Lepontine" amphibolite facies event and the Western Alpine eclogitic event.
Villa, I. (2015). 39Ar-40Ar geochronology of mono- and polymetamorphic basement rocks. PERIODICO DI MINERALOGIA, 84(3B), 615-632 [10.2451/2015PM0454].
39Ar-40Ar geochronology of mono- and polymetamorphic basement rocks
Villa, IM
2015
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 bell- shaped Ar loss profiles. Recognizing the importance of the respective rates in field situations leads to re-evaluating the ages and the interpretive paradigms in classic examples such as the Central Alpine "Lepontine" amphibolite facies event and the Western Alpine eclogitic event.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.