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

Abstract

Unusually alumina-poor orthopyroxene is found in a spinel peridotite from the Horoman Peridotite Complex, Japan. Al2O3 ,C r 2 O 3and CaO contents in the low-Al orthopyroxene (named Low-Al OPX hereafter) are 5025 wt%, 5004 wt% and 503 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 (15mm 45mm), and the orthopyroxene rim in contact with olivine more commonly has normal Al2O3 contents (42 wt%). In the second mode of occurrence, the Low-Al OPX occurs as a thin film, 5mm 50mm 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 CO2a H2O-bearing fluid phase, is a mechanism for metasomatic alteration of mantle wedge peridotite.