A Complex Corona Between Olivine and Plagioclase from the Jotun Nappe, Norway, and the Diffusion Modelling of Multimineralic Layers

J. R. Ashworth, J. J. Birdi* and T. F. Emmett
School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K.
Geology Division, Anglia Polytechnic, East Road, Cambridge CB1 1PT, U.K.
*Present address: 17 Stonedown Close, Bilston, Wolverhampton WV7 9YN, U.K.

Abstract: Coronas containing Ca-amphibole with aluminous minerals have been characterised optically and by scanning electron microscopy, analytical transmission electron microscopy and electron-probe microanalysis. The layers nearest to plagioclase are amphibole + epidote + kyanite, followed by amphibole + epidote + staurolite + spinel. These assemblages are consistent with water-undersaturated conditions, possibly at lower metamorphic grade than the commoner assemblage amphibole + spinel. Observed mineral proportions and compositions were used in a seven-layer model of steady-state, diffusion-controlled growth with local equilibrium. This model is not fully realistic, because the observed amphibole is strongly zoned from tschermakitic to actinolitic away from plagioclase, suggesting disequilibrium. However, the four-mineral layer has been successfully modelled assuming local equilibrium, with diffusion coefficients Lii larger for i = FeO and MgO than for SiO2, AlO3/2, CaO and FeO3/2. Retarded grain-boundary diffusion of the latter components is explicable by crystal-chemical effects. The number of minerals per layer is constrained by a modified form of the metasomatic phase rule of Korzhinskii, with the role of ‘inert’ components played by relatively immobile ones (having relatively small fluxes and relatively small diffusion coefficients).

Keywords: coronas • kyanite • staurolite • tschermakitic amphibole • metasomatic phase rule

Mineralogical Magazine; December 1992 v. 56; no. 385; p. 511-525; DOI: 10.1180/minmag.1992.056.385.07
© 1992, The Mineralogical Society
Mineralogical Society (www.minersoc.org)