Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe) |
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| Authors: | Lucie Taj?manová Ji?í Konopásek James A D Connolly |
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| Institution: | (1) Institute of Petrology and Structural Geology, Charles University, Albertov 6, 128 43 Prague, Czech Republic;(2) Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic;(3) Institut für Mineralogie und Petrographie, ETH-Zentrum, 8092 Zürich, Switzerland |
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| Abstract: | Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the
high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona
is a manifestation of diffusion-driven transfer of CaO and Na2O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This
process establishes Si-undersaturated conditions along the plagioclase–kyanite interface, which allow crystallization of spinel
during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermodynamically assuming local
equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length
for Na and Ca that extends ∼400–450 and ∼450–550 μm, respectively, around each kyanite crystal. Low estimated bulk diffusion
coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim. |
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| Keywords: | Equilibration volume Coronal structures Diffusion Low-pressure granulites Perple_X Bohemian Massif |
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