Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging |
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Authors: | Email author" target="_blank">Stephanie?FludeEmail author Martin?R?Lee Sarah?C?Sherlock Simon?P?Kelley |
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Institution: | (1) Department of Earth and Environmental Sciences, Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK;(2) School of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow, G12 8QQ, UK |
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Abstract: | Charge contrast imaging in the scanning electron microscope can provide new insights into the scale and composition of alkali
feldspar microtextures, and such information helps considerably with the interpretation of their geological histories and
results of argon isotope thermochronological analyses. The effectiveness of this technique has been illustrated using potassium-rich
alkali feldspars from the Dartmoor granite (UK). These feldspars contain strain-controlled lamellar crypto- and microperthites
that are cross-cut by strain-free deuteric microperthites. The constituent albite- and orthoclase-rich phases of both microperthite
generations can be readily distinguished by atomic number contrast imaging. The charge contrast results additionally show
that sub-micrometre-sized albite ‘platelets’ are commonplace between coarser exsolution lamellae and occur together to make
cryptoperthites. Furthermore, charge contrast imaging reveals that the orthoclase-rich feldspar is an intergrowth of two phases,
one that is featureless with uniform contrast and another that occurs as cross-cutting veins and grains with the {110} adularia
habit. Transmission electron microscopy shows that the featureless feldspar is tweed orthoclase, whereas the veins and euhedral
grains are composed of irregular microcline that has formed from orthoclase by ‘unzipping’ during deuteric or hydrothermal
alteration. The charge contrast imaging results are especially important in demonstrating that deuteric perthites are far
more abundant in alkali feldspars than would be concluded from investigations using conventional microscopy techniques. The
unexpected presence of such a high volume of replacement products has significant implications for understanding the origins
and geological histories of crustal rocks and the use of alkali feldspars in geo- and thermochronology. Whilst the precise
properties of feldspars that generate contrast remain unclear, the similarity between charge contrast images and corresponding
cathodoluminescence images of deuteric microperthites indicates that trace element chemistry and possibly also elastic strain
within the crystal play a major role. |
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