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Unusually alumina-poor orthopyroxene is found in a spinel peridotite from the Horoman Peridotite Complex, Japan. Al 2 O 3 , Cr 2 O 3  and CaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter) are &lt;0·25 wt %, &lt;0·04 wt % and &lt;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 × 45 µm), and the orthopyroxene rim in contact with olivine more commonly has normal Al 2 O 3  contents (&gt;2 wt %). In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5 µm × 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 CO 2  + H 2 O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.

Evolution of Low-Al Orthopyroxene in the Horoman Peridotite, Japan: an Unusual Indicator of Metasomatizing Fluids