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Origin of reverse zoning in branching orthopyroxene and acicular plagioclase in orbicular diorite, Fisher Lake, California

Published online by Cambridge University Press:  05 July 2018

D. G. Durant  and
A. D. Fowler
D. G. Durant*
Affiliation:
University of Ottawa, Department of Earth Sciences, 140 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada
A. D. Fowler
Affiliation:
University of Ottawa, Department of Earth Sciences, 140 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada
*
*E-mail: durant@geologist.com
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Abstract

An orbicular diorite from Fisher Lake, California, USA, contains multi-shelled, magmatic orbicules with branching and budding orthopyroxene crystals as well as feather and acicular plagioclase crystals that are oriented perpendicular to the growth horizon. Plagioclase and orthopyroxene show gradual, reverse compositional zoning along the long axes and normal zoning along the short axes. The reverse zoning varies from An87 to An93 and Mg68 to Mg74 over distances of 4 mm and 8 mm respectively. The close proximity of these two minerals makes it likely that only one mechanism is responsible for the reverse zoning. This zoning can be explained by using relevant temperature-composition diagrams and Gibbs free energy-composition plots. Under sudden and moderate undercoolings, which produce high growth but low nucleation rates, the difference in Gibbs free energy (ΔG) between the crystals and liquid is not initially maximized, i.e. initial compositions are not near-to-equilibrium. This results in crystal compositions that are closer to that of the bulk liquid than expected for crystallization under near-to-equilibrium conditions (i.e. very small ΔT). Over time, and under isothermal crystallization conditions, ΔG gradually increases to a maximum producing crystal compositions that also gradually attain near-to-equilibrium compositions. Subsequent to attaining these conditions, normal zoning occurs perpendicular to the crystal growth axes.

Keywords

reverse zoningorthopyroxeneplagioclaseorbicular dioritecomb textureunidirectional growthorbiculesbranchingfeatherFisher LakeCalifornia
Type
Research Article
Information
Mineralogical Magazine , Volume 66 , Issue 6 , December 2002 , pp. 1003 - 1019
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

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