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Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging

Flude, Stephanie; Lee, Martin R.; Sherlock, Sarah C. and Kelley, Simon P. (2012). Cryptic microtextures and geological histories of K-rich alkali feldspars revealed by charge contrast imaging. Contributions to Mineralogy and Petrology, 163(6) pp. 983–994.

DOI (Digital Object Identifier) Link: http://dx.doi.org/10.1007/s00410-011-0710-8
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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.

Item Type: Journal Article
Copyright Holders: 2011 Springer-Verlag
ISSN: 1432-0967
Project Funding Details:
Funded Project NameProject IDFunding Body
A new approach to Ar/Ar thermochronology: investigating the effects of recrystallisation and deformation on alkali feldspar crystalsNE/E018629/1NERC (Natural Environment Research Council)
Keywords: feldspars; SEM; charge contrast imaging; microtexture
Academic Unit/Department: Science > Environment, Earth and Ecosystems
Interdisciplinary Research Centre: Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR)
Item ID: 31694
Depositing User: Sarah Sherlock
Date Deposited: 30 Jan 2012 14:59
Last Modified: 20 Mar 2014 14:24
URI: http://oro.open.ac.uk/id/eprint/31694
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