Extremely low-Al orthopyroxene in the Horoman peridotite, Japan

Abstract

We report an occurrence of extremely low-alumina orthopyroxenes in a spinel lherzolite from the Horoman Peridotite Complex, Japan. The low-Al orthopyroxenes occur in two modes; the first occurs at the margin of a large orthopyroxene porphyroclast in contact with olivine and the second occurs at grain boundaries between clinopyroxene and olivine. Al₂O₃, Cr₂O₃ and CaO contents in the low-Al orthopyroxenes are less than 0.2 wt.% and are distinctively lower than those in orthopyroxene porphyroclasts. Petrographic observations combined with a inferred P-T history of the Horoman peridotite reveal that the low-Al orthopyroxenes were formed through a very local reaction between peridotite with aqueous fluids, which were not pervasively altering the precursor mineralogy.