Pyroxene geothermometry applied to a three-pyroxene achondrite from Allan Hills, Antarctica and ordinary chondrites.

A unique achondrite (ALHA77005) found by the joint U. S.-Japan team in Allan Hills, Antarctica is composed of olivine, pyroxenes, maskelynite, chromite, Ca-phosphate. The first occurrence of a three-pyroxene assemblage (orthopyroxene+pigeonite+augite) in meteorites is reported. The detailed chemistries and distribution of each phase indicate that the chemistries of the initial equilibrated assemblage are mostly preserved, but are partly modified by some processes related to shock events. A minor chemical zoning is observed from low-Ca pyroxene towards pigeonite. The orthopyroxene-like phase gives the X-ray diffraction pattern of pigeonite. Three kinds of pyroxene geothermometers developed by Ishii (Ishii et al., 1976) have been applied to this meteorite and the equilibrated ordinary chondrites. The estimated temperatures indicate that the initial equilibrated temperature of ALHA77005 (∼1160°C) is considerably higher than those of the ordinary chondrites (925°C–1010°C).