Plagioclase-rich inclusions in carbonaceous chondrite meteorites: liquid condensates

Plagioclase-rich inclusions in carbonaceous chondrites are of two types: (1) mm/cm-sized Ca-Al-rich inclusions (CAIs) with 30-60 vol.% anorthite and up to 35 vol.% each of Ti-Al-pyroxene (Tpx), melilite and spinel, and (2) smaller (<3 mm) CA chondrules similar to those found in ordinary chondrites, containing more sodic plagioclase, pyroxene, olivine and spinel. The CAIs, which were formerly names 'Type I' and thought to be 'Intermediate' between Type A and Type B CAIs, are shown to be an independent type, renamed 'Type C', with distinctive chemical and mineralogical compositions. The textures, sequence of crystallization, and concentric zonation indicate that they crystallized from melt droplets. The texture of the anorthite ranges from coarse laths to a fine-grained groundmass, reflecting increasing retardation of its nucleation. As the elemental abundances in Type C's are dominated by volatility, the melts were not formed by planetary (magmatic) processes. Type C CAIs have compositions similar to those calculated for liquid condensates under high solar gas pressure (approx. 1 atm) or in dust-rich environments, and are probably liquid condensates. The few Type C CAIs that crystallized from incompletely molten materials may have been reheated liquid condensates or mixed liquid/solid condensates. (author)