40Ar/39Ar geochronology and the diffusion of 39Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo
Published:January 01, 2014
Marnie A. Forster, Gordon S. Lister, 2014. "40Ar/39Ar geochronology and the diffusion of 39Ar in phengite–muscovite intergrowths during step-heating experiments in vacuo", Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences, F. Jourdan, D. F. Mark, C. Verati
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Step-heating experiments in vacuo are routine when conducting 40Ar/39Ar geochronology, including for white mica. White mica can break down, due to dehydroxylation and delamination, so experiments involving mica are often conducted in relative haste, and not with the care and precision necessary when intending to apply multi-diffusion-domain theory to model the results. Here we show, however, that carefully managed step-heating experiments appear to allow release of argon through solid-state diffusion processes alone. We analysed phengite-muscovite intergrowths in high-pressure metamorphic rocks exhumed in and beneath extensional ductile shear zones during continental extension. Such materials often yield Arrhenius plots in which there is a distinct steepening of slope mid-way through the step-heating sequence. This steepening appears to correspond with steps in which release of argon from phengite components dominate. We analysed the data using a computer program (eArgon) and numerically simulated mixing of gas released from multiple diffusion domains. The results suggest that diffusion of 39Ar in phengitic white mica involves radically different diffusion parameters in comparison with muscovite. If these results extrapolate to nature then 40Ar/39Ar geochronology may allow direct dating of white mica mineral growth during metamorphism.
Data files A, B and C are available at http://www.geolsoc.org.uk/SUP18619. Data file A C++ computer code used to infer data for an Arrhenius plot, assuming different diffusion geometries. These methods are excerpted from the eArgon computer program used to analyse these data.
Data file B Analytical methods and procedures used in the laboratory for 40Ar/39Ar geochronology performed on the samples reported.
Data file C XML formatted data tables for the step-heating experiments reported in this study, in a form that can be read by the eArgon computer program.
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Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences
Decoding the complete history of Earth and our solar system requires the placing of the scattered pages of Earth history in a precise chronological order, and the 40Ar/39Ar dating technique is one of the most trusted dating techniques to do that. The 40Ar/39Ar method has been in use for more than 40 years, and has constantly evolved since then. The steady improvement of the technique is largely due to a better understanding of the K/Ar system, an appreciation of the subtleties of geological material and a continuous refinement of the analytical tools used for isotope extraction and counting. The 40Ar/39Ar method is also one of the most versatile techniques with countless applications in archaeology, tectonics, structural geology, orogenic processes and provenance studies, ore and petroleum genesis, volcanology, weathering processes and climate, and planetary geology. This volume is the first of its kind and covers methodological developments, modelling, data handling, and direct applications of the 40Ar/39Ar technique.