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- Title
Isochemical breakdown of garnet in orogenic garnet peridotite and its implication to reaction kinetics.
- Authors
Obata, Masaaki; Ozawa, Kazuhito; Naemura, Kosuke; Miyake, Akira
- 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.
- Subjects
GARNET; PERIDOTITE; CHEMICAL kinetics; ORTHOPYROXENE; PLAGIOCLASE; HYDRATION; VOLCANOLOGY
- Publication
Mineralogy & Petrology, 2013, Vol 107, Issue 6, p881
- ISSN
0930-0708
- Publication type
Academic Journal
- DOI
10.1007/s00710-012-0260-4