The Blue Angel: I. The mineralogy and petrogenesis of a hibonite inclusion from the Murchison meteorite

A detailed mineralogie, chemical, and petrologic study of the Blue Angel, a relatively large (~1.5 mm) hibonite-containing inclusion from the Murchison meteorite, was performed in an attempt to understand the mechanisms of formation and modification of hibonite-rich inclusions. The Blue Angel inclusion is composed of roughly equal amounts of hibonite and calcite, with minor amounts of spinel, perovskite, diopside, and an Fe-rich silicate. The inclusion can be divided into three roughly concentric zones—a hibonite-rich core, a calcite-rich mantle, and a spinel-rich layered rim. The mineral chemistry and petrography of the Blue Angel are consistent with a three-stage formation history: (1) an early stage of nebular condensation which produced the hibonite, perovskite, and spinel; (2) a moderate temperature stage of aqueous alteration and metamorphism occurring on a small planetary body containing CO_2 and H_2O during which calcite was formed in the inclusion; and (3) the final emplacement of the Blue Angel into its present position in Murchison. The study of the Blue Angel indicates that extensive alteration of Ca-Al-rich inclusions (CAI) may have occurred by aqueous alteration and thermal metamorphism followed by explosive mixing processes on a parent body. Such metamorphic reactions may involve formation and destruction of phases, such as melilite and diopside, which have been previously thought to be primary condensates. The mechanisms proposed for the formation and modification of the Blue Angel help to explain the secondary phases and oxygen isotope anomalies found in many CAI and eliminate the need for invoking kinetically-complicated back-reactions at very low pressures with a cool part of the solar nebula. The contribution of planetary metamorphism in the formation and alteration of CAI must be considered along with nebular processes in order to understand the formation of carbonaceous chondrites