Intergranular diffusion rates from the analysis of garnet surfaces: implications for metamorphic equilibration

Dempster, T. J. , Symon, S. and Chung, P. (2017) Intergranular diffusion rates from the analysis of garnet surfaces: implications for metamorphic equilibration. Journal of Metamorphic Geology, 35(6), pp. 585-600. (doi:10.1111/jmg.12247)

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Abstract

Novel approaches to garnet analysis have been used to assess rates of intergranular diffusion between different matrix phases and garnet porphyroblasts in a regionally metamorphosed staurolite-mica-schist from the Barrovian-type area in Scotland. X-ray maps and chemical traverses of planar porphyroblast surfaces reveal chemical heterogeneity of the garnet grain boundary linked to the nature of the adjacent matrix phase. The garnet preserves evidence of low temperature retrograde exchange with matrix minerals and diffusion profiles documenting cation movement along the garnet boundaries. Garnet–quartz and garnet–plagioclase boundaries preserve evidence of sluggish Mg, Mn and Fe diffusion at comparable rates to volume diffusion in garnet, whereas diffusion along garnet–biotite interfaces is much more effective. Evidence of particularly slow Al transport, probably coupled to Fe3+ exchange, is locally preserved on garnet surfaces adjacent to Fe-oxide phases. The Ca distribution on the garnet surface shows the most complex behaviour, with long-wavelength heterogeneities apparently unrelated to the matrix grain boundaries. This implies that the Ca content of garnet is controlled by local availability and is thought likely to reflect disequilibrium established during garnet growth. Geochemical anomalies on the garnet surfaces are also linked to the location of triple junctions between the porphyroblasts and the matrix phases, and imply enhanced transport along these channels. The slow rates of intergranular diffusion and the characteristics of different boundary types may explain many features associated with the prograde growth of garnet porphyroblasts. Thus, minerals such as quartz, Fe-oxides and plagioclase whose boundaries with garnet are characterized by slow intergranular diffusion rates appear to be preferentially trapped as inclusions within porphyroblasts. As such grain boundary diffusion rates may be a significant kinetic impediment to metamorphic equilibrium and garnet may struggle to maintain chemical and textural equilibrium during growth in pelites.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Symon, Ms Shona and Chung, Mr Peter and Dempster, Dr Tim
Authors: Dempster, T. J., Symon, S., and Chung, P.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Journal of Metamorphic Geology
Publisher:Wiley
ISSN:0263-4929
ISSN (Online):1525-1314
Published Online:03 March 2017
Copyright Holders:Copyright © 2017 John Wiley & Sons Ltd
First Published:First published in Journal of Metamorphic Geology 35(6): 585-600
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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