Abstract
An isochemical kelyphite (orthopyroxene+spinel+plagioclase) that has nearly the same bulk chemical composition as the precursor garnet was found within a matrix of ordinary kelyphites (orthopyroxene+clinopyroxene+spinel±amphibole) in garnet peridotites from the Czech part of the Moldanubian Zone. It was shown that the kelyphitization of garnet took place in three stages: (1) the garnet-olivine reaction, accompanied by a long-range material transfer across the reaction zone, and (2) the isochemical breakdown of garnet, essentially in a chemically-closed system, and finally, (3) an open-system hydration reaction producing a thin hydrous zone (amphibole+spinel+plagioclase), which is located between the isochemical kelyphite and relict garnet. The presence of relict garnet suggests that this breakdown reaction of the second stage did not proceed to a completion probably being hindered by the formation of the hydrous zone at the reaction front. It was found by electron back-scattered diffraction method that orthopyroxene and spinel do not show any topotaxic relationship in the first type of kelyphite; whereas they show locally topotaxic relationship in the isochemical kelyphite. The transition from the first type to the second type of kelyphite is discussed on the basis of the detailed observations in the transition zone between the two kelyphites. More widespread occurrence of isochemical kelyphite is expected to occur in orogenic peridotites as well as from xenoliths brought by volcanics.
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Acknowledgments
We are grateful to Prof. T. Hirajima (Kyoto Univ.) and Dr. M. Svojtka (Academy of Science, Czech Republic) for their cooperations in the field and for discussions. We are also grateful to Prof. H. Nagahara for her permission of the use of FE-SEM and EBSD facilities at Univ. Tokyo, and to Dr. H. Yoshida for his technical assistance and to Professor Emeritus I. Kushiro for his discussions on the garnet breakdown reactions. Thanks are extended to Dr. T. Kawakami and T. Ueda for their assistance in the microprobe work, to Dr. Y. Seto (Kobe Univ.) for his assistance with the TEM analytical work and to Mr. Tutumi for his thin section preparations. The paper was benefitted by constructive criticisms of Petr Jerabek and an anonymous reviewer and their efforts are acknowledged. Editorial handling by R. Abart is greatly acknowledged. We thank Oxford University Press for letting us reuse Fig. 1 in Naemura et al (2009) published in Journal of Petrology.
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Editorial handling: R. Abart
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Obata, M., Ozawa, K., Naemura, K. et al. Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics. Miner Petrol 107, 881–895 (2013). https://doi.org/10.1007/s00710-012-0260-4
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DOI: https://doi.org/10.1007/s00710-012-0260-4