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Understanding Transport Processes in Rocks |
1 Institute of Mineralogy and Geochemistry, University of Lausanne, BFSH2, 1015 Lausanne, Switzerland nicolas.kramar{at}imag.unil.ch
2 Centre Interdisciplinaire de Microscopie Electronique, Ecole Polytechnique Fédérale de Lausanne, MX-C, 1015 Lausanne, Switzerland philippe.buffat{at}epfl.ch
Recent advances in microscale 40Ar/39Ar geochronology have revealed argon concentration gradients in naturally deformed muscovite that are incompatible with volume diffusion uniquely, and have been interpreted to result from intragranular defect-enhanced diffusion. Defects and heterogeneously spaced stacking faults observed by transmission electron microscopy in such muscovites are evaluated as potential fast pathways for argon diffusion.
Two-dimensional defects, such as stacking faults, are of particular interest for noble gas diffusion because of the net dilatation effect that a stacking fault is able to generate in minerals. In micas, partial dislocations (and the area between them known as stacking faults) within the interlayer displace the potassium atoms from a stable hexagonally centred position between opposing tetrahedral layers to an unstable position relative to one of the tetrahedral layers such that repulsive forces lead to a localized net dilatation effect within the interlayer. Such a dilatation effect may have direct consequences for argon retention in micas. Numerical modelling of the effects stacking faults have on argon diffusion was performed on the basis of the calculated interlayer spacing, measured isotope data, and observed linear stacking fault density. These calculations result in effective diffusivity ratios defined by volume diffusion to defect-enhanced diffusion of 106 to 107, which are comparable with diffusivity ratios in other materials (ceramics or metals).
In the absence of defects causing physical grain size reduction (e.g. kink bands or subgrain boundaries), stacking faults are potentially the main defect in sheet silicates exerting a measurable influence on intragranular argon diffusion. Stacking-fault-enhanced argon diffusion differs from pipe diffusion, whose significance on bulk diffusion depends on high dislocation densities, by the small volume fraction of dislocations required to affect bulk diffusivities. In contrast to pipe diffusion, the linked occurrence of dislocations and stacking faults within mica interlayers represents a potentially significant volume fraction, even in samples that do not have high apparent dislocation densities.
