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

Content Provider	Springer Nature Link
Author	Flude, Stephanie Lee, Martin R. Sherlock, Sarah C. Kelley, Simon P.
Copyright Year	2011
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.
Starting Page	983
Ending Page	994
Page Count	12
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	163
Issue Number	6
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	2011-12-03
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Feldspars SEM Charge contrast imaging Microtexture Mineralogy Mineral Resources Geology
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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