Solution-Redeposition and the Orthoclase-Microcline Transformation: Evidence from Granulites and Relevance to 18O Exchange

Kim Waldron*, Ian Parsons and William L. Brown
Department of Geology and Geophysics, The University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, Scotland
CNRS-CRPG, BP 20, F-54501 Vandoeuvre-lés-Nancy Cedex, France
*Present address: Department of Geology, Colgate University, Hamilton, NY 13346 USA.

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, A1 ordering, provide driving forces for dissolution and reprecipitation of unstrained, more ordered phases.

Keywords: orthoclase-microcline transformation • solution-redeposition • granulite • 18O exchange • perthite • Adirondacks • low-temperature reactivity

Mineralogical Magazine; December 1993 v. 57; no. 389; p. 687-695; DOI: 10.1180/minmag.1993.057.389.13
© 1993, The Mineralogical Society
Mineralogical Society (