Argon enters the retentive zone: reassessment of diffusion parameters for K-feldspar in the South Cyclades Shear Zone, Ios, Greece
Published:January 01, 2010
M. A. Forster, G. S. Lister, 2010. "Argon enters the retentive zone: reassessment of diffusion parameters for K-feldspar in the South Cyclades Shear Zone, Ios, Greece", Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling, M. I. Spalla, A. M. Marotta, G. Gosso
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40Ar/39Ar apparent age spectra have been measured for unusually retentive potassium feldspars (K-feldspar) from the South Cyclades Shear Zone, Ios, Greece. Our results imply that the Argon Partial Retention Zone (Ar PRZ) for the most retentive domains in potassium K-feldspar can expand into the ductile regime, even when temperatures in excess of about 400–450 °C apply. In such cases K-feldspar could be used as a geochronometer to estimate the timing and duration of deformation and metamorphism events. Therefore, we have reassessed traditional methods used to analyse Arrhenius plots by simulating the effect of step-heating experiments on argon loss. Fractal multidomain diffusion models were used, with theoretical distributions of diffusion domain size and volume. We discovered a Fundamental Asymmetry Principle that offers objective constraints on slope fitting to allow an analysis to be consistent with the multidomain diffusion hypothesis, and which consistently leads to the estimation of higher activation energies. Reanalysis of existing datasets is encouraged to allow reassessment of the significance of the average values reported. Retentive diffusion parameters for K-feldspar might prove to be commonplace.
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Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling
Iterative comparison of analytical results and natural observations with predictions of numerical models improves interpretation of geological processes. Further refinements derive from wide-angle comparison of results from various scales of study. In this volume, advances from field, laboratory and modelling approaches to tectonic evolution – from the lithosphere to the rock scale – are compared. Constructive use is made of apparently discrepant or non-consistent results from analytical or methodological approaches in processing field or laboratory data, P–T estimates, absolute or relative age determinations of tectonic events, tectonic unit size in crustal scale deformation, grain-scale deformation processes, various modelling approaches, and numerical techniques. Advances in geodynamic modelling critically depend on new insights into grain- and subgrain-scale deformation processes. Conversely, quantitative models help to identify which rheological laws and parameters exert the strongest control on multi-scale deformation up to lithosphere and upper mantle scale.