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A complex corona between olivine and plagioclase from the Jotun Nappe, Norway, and the diffusion modelling of multimineralic layers

Published online by Cambridge University Press:  05 July 2018

J. R. Ashworth ,
J. J. Birdi  and
T. F. Emmett
J. R. Ashworth
Affiliation:
School of Earth Sciences, Univcrsity of Birmingham, Edgbaston, Birmingham BI5 2TT, U.K.
J. J. Birdi
Affiliation:
School of Earth Sciences, Univcrsity of Birmingham, Edgbaston, Birmingham BI5 2TT, U.K.
T. F. Emmett
Affiliation:
Geology Division, Anglia Polytechnic, East Road, Cambridge CB1 IPT, U.K.
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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 waterundersaturated 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

coronaskyanitestaurolitetschermakitic amphibolemetasomatic phase rule
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 385 , December 1992 , pp. 511 - 525
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

Present address: 17 Stonedown Close, Bilston, Wolverhampton WV7 9YN, U.K.

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