Evolution of low-Al orthopyroxene in the Horoman Peridotite, Japan: An unusual indicator of metasomatizing fluids

Unusually alumina-poor orthopyroxene is found in a spinel peridolite 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 × 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 × 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 therzolite via a CO₂ + H₂O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.