Using calculated chemical potential relationships to account for coronas around kyanite: an example from the Bohemian Massif P. S ˇ TI ´ PSKA ´ , 1 R. POWELL, 2 R. W. WHITE 3 AND J. A. BALDWIN 4 1 Ecole et Observatoire des Sciences de la Terre, IPGS (CNRS UMR7516), Universite ´ de Strasbourg, 1 rue Blessig, 67084 Strasbourg, France (stipska@gmail.com) 2 School of Earth Sciences, The University of Melbourne, Melbourne, Vic. 3010, Australia 3 Institute for Geosciences, University of Mainz, D-55099 Mainz, Germany 4 Department of Geosciences, University of Montana, Missoula, MT 59812, USA ABSTRACT Corona textures around kyanite, involving for example zoned plagioclase separating kyanite from the matrix, reflect the instability of kyanite with the matrix on changing P–T conditions, commonly related to decompression. The chemical potential gradients set up between the kyanite and the matrix as a consequence of slow Al diffusion drive corona development, with the zoning of the plagioclase reflecting the gradients. Calculated mineral equilibria are used to account for corona textures involving plagioclase ± garnet around kyanite, and replacement of kyanite by plagioclase + spinel symplectite, in quartz + plagioclase + K-feldspar + garnet + kyanite granulite facies gneiss from the Blansky´ les massif in the Bohemian massif, Czech Republic. In the garnet-bearing coronas, a commonly discontinuous garnet layer lies between the kyanite and the continuous plagioclase layer in the corona, with both the garnet and the plagioclase appearing mainly to replace matrix rather than kyanite. The garnet layer commonly extends around kyanite from original matrix garnet adjacent to the kyanite. Where garnet is missing in the corona, the kyanite itself may be replaced by a spinel–plagioclase corona. In a local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in l(Na 2 O)–l(CaO)–l(K 2 O)–l(FeO)–l(MgO)–l(SiO 2 ) in the model chemical system, Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 (NCKFMAS). The discontinuous nature of the garnet layer in coronas is accounted for by the effect of the adjacent original garnet on the chemical potential relationships. The replacement of kyanite by spinel + plagioclase appears to be metastable with respect to replacement by corundum + plagioclase, possibly reflecting the difficulty of nucleating corundum. Key words: Bohemian massif; chemical potential; ky–K-feldspar granulite; plagioclase ± garnet around kyanite; plagioclase–spinel symplectite. INTRODUCTION In an equilibrium model of metamorphism, a mineral assemblage and the mineral compositions are at equilibrium on some length scale at the prevailing pressure–temperature (P–T) conditions (e.g. Powell et al., 2005), building on the idea of local or mosaic equilibrium (Korzhinskii, 1959; Thompson, 1959). If a new mineral does not nucleate with changing condi- tions, the equilibrium will be a metastable one, rather than the stable one. If the scale of equilibration is vanishingly small, no visible change occurs. Equili- bration itself is dependent on rates of intergranular (i.e. grain boundary, or intercrystalline) and intracrystal- line diffusion, which in turn depend on temperature, rate of change of temperature with time, fluid ⁄ melt presence ⁄ absence, grain size, strain in the minerals, etc. Assuming equalized P–T between the different parts of a mineral assemblage, equilibration involves equal- izing of chemical potentials. If, with changing P–T, minerals in an assemblage are no longer in equilibrium with each other, chemical potential gradients are established between them. Reaction will then occur in an attempt to flatten out the gradients. As discussed in White et al. (2008), such reaction may proceed to completion, with chemical potential gradients removed, while still leaving spatial and compositional consequences of the reaction path, particularly for reaction in the prograde history. Reaction in the retrograde history commonly does not proceed to completion, with effective reaction ceasing during cooling because the rate of equilibration becomes too slow even though chemical potential gradients are still present. This, combined with the coarser-grained nat- ure of higher temperature mineral assemblages, results in the characteristic feature of retrograde reaction in higher grade rocks: the development of coronas. A situation in which corona development may occur is in response to chemical potential gradients estab- lished between a mineral and other parts of a mineral J. metamorphic Geol., 2010, 28, 97–116 doi:10.1111/j.1525-1314.2009.00857.x Ó 2009 Blackwell Publishing Ltd 97