Growth zoning of garnet porphyroblasts: grain boundary and microtopographic controls

Dempster, T. J. , Coleman, S., Kennedy, R., Chung, P. and Brown, R. W. (2020) Growth zoning of garnet porphyroblasts: grain boundary and microtopographic controls. Journal of Metamorphic Geology, 38(9), pp. 1011-1027. (doi: 10.1111/jmg.12558)

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Abstract

Chemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite‐facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X‐ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from <7 mol% grs to >16 mol% grs, across distances of less then 50 mm. This small‐scale patchy compositional zoning is as much variation as the core‐rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P‐T‐t paths for crustal rocks. The evidence of extremely localized (10‐50 mm scale) equilibrium during growth further undermines these approaches.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Brown, Professor Roderick and Chung, Mr Peter and Dempster, Dr Tim and Coleman, Ms Sarah
Authors: Dempster, T. J., Coleman, S., Kennedy, R., Chung, P., and Brown, R. W.
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:14 July 2020
Copyright Holders:Copyright © 2020 The Authors
First Published:First published in Journal of Metamorphic Geology 38(9): 1011-1027
Publisher Policy:Reproduced under a Creative Commons License

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