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

Abstract

AbstractThe 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.