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Solution-redeposition and the orthoclase-microcline transformation: evidence from granulites and relevance to 18O exchange

Published online by Cambridge University Press:  05 July 2018

Kim Waldron ,
Ian Parsons  and
William L. Brown
Kim Waldron
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland
Ian Parsons
Affiliation:
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland
William L. Brown
Affiliation:
CNRS-CRPG, BP 20, F-54501 Vandoeuvre-lés-Nancy Cedex, France
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Abstract

The Or-rich part of optically blebby to lamellar mesoperthite crystals from an Adirondack granulite has been shown by TEM to be a lamellar cryptoperthite, composed dominantly of tweed orthoclase. A fluid-absent, two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution below ∼350°C probably during uplift. The mechanism was most probably homogeneous coherent nucleation rather than spinodal decomposition. However, cutting the orthoclase cryptoperthite are thin (<1 μm) seams of tartan microcline with sharp boundaries, often defined locally by {110} planes, and micropores. The microcline has replaced orthoclase by solution-redeposition along narrow planes infiltrated by fluid during minor retrogression at T < 350°C Solution-redeposition is a common process in feldspars at T < 500°C potentially accompanied by 18O exchange, because release of elastic strain energy in coherent perthite lamellar boundaries and twin-domain walls, followed by Si, Al ordering, provide driving forces for dissolution and reprecipitation of unstrained, more ordered phases.

Keywords

orthoclasc-microcline transformationsolution-redepositiongranulitesup18/supO exchangeperthiteAdirondackslow-temperature reactivity
Type
Mineralogy
Information
Mineralogical Magazine , Volume 57 , Issue 389 , December 1993 , pp. 687 - 695
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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Footnotes

*

Present address: Department of Geology, Colgate University, Hamilton, NY 13346 USA.

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