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Journal of Petrology | Volume 44 | Number 7 | Pages 1237-1246 | 2003
© Oxford University Press 2003
Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids
1 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
2 GRADUATE SCHOOL OF NATURAL SCIENCE AND TECHNOLOGY, KANAZAWA UNIVERSITY, KAKUMA, KANAZAWA 920-1192, JAPAN
3 DEPARTMENT OF EARTH SCIENCES, KANAZAWA UNIVERSITY, KAKUMA, KANAZAWA 920-1192, JAPAN
* Corresponding author. Telephone: +81-76-264-5723. Fax: +81-76-264-5746. E-mail: moripta{at}kenroku.kanazawa-u.ac.jp
Unusually alumina-poor orthopyroxene is found in a spinel peridotite from the Horoman Peridotite Complex, Japan. Al2O3, Cr2O3 and CaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter) are <0·25 wt %, <0·04 wt % and <0·3 wt %, respectively, and are distinctively lower than those in orthopyroxene porphyroclasts. The Low-Al OPX occurs in two modes, both at the margin of olivine. The first mode of occurrence is as the rim of a large orthopyroxene porphyroclast in contact with olivine. This type of Low-Al OPX occurs only locally (15 µm x 45 µm), and the orthopyroxene rim in contact with olivine more commonly has normal Al2O3 contents (>2 wt %). In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5 µm x 50 µm in dimension, at a grain boundary between olivine and clinopyroxene. Trace element compositions of porphyroclast clinopyroxene in the sample indicate that the sample having the Low-Al OPX underwent metasomatism although there are no hydrous minerals around the Low-Al OPX. Petrographic observations and trace element compositions of clinopyroxene combined with an inferred P–T history of the Horoman peridotite suggest that the Low-Al OPX was formed through a very local reaction between peridotite and invasive fluids, probably formed by dehydration of a subducted slab, in a late stage of the history of the Horoman peridotite. Crystallization of orthopyroxene, representing addition of silica to mantle lherzolite via a CO2 + H2O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.
KEY WORDS: Horoman Peridotite Complex; low-Al orthopyroxene; metasomatism; mantle wedge
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