Argon Transport in Chain and Sheet Silicates: A Combined Laser Microprobe and TEM

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Chain and sheet silicates are important minerals in metamorphic rocks because of their common occurence and their usefulness in isotopic dating. Most radiometric mineral dates record the times which the minerals cooled through their blocking temperatures for isotopic diffusion and these differ for the various isotopes and the various chain and sheet silicates. Geologists exploit this variability to delineate the "temperature versus time" histories of metamorphic terranes. The calculated blocking temperatures for many minerals, however, such as for argon in hornblende and micas, is poorly constrained unless one assumes a diffusion geometry reflecting the anisotropy of these silicate lattices, and a diffusion dimension (or radius) reflecting their microtextural homogeneity. By measuring the argon spatial distribution in both natural and hydrothermally treated samples of hornblende, biotite, and muscovite with the laser microprobe, it is planned to estimate these properties. These estimates will be further tested by observing microtextures of the minerals with the transmission electron microscope (TEM). Results from the controlled heating experiments will also facilitate interpretation of the spatial information being derived from 40Ar/39Ar laser microprobes.