Plagioclase-rich inclusions in carbonaceous chondrite meteorites: Liquid condensates?

Abstract

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 named “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. Type C CAIs have Group I or Group II trace element patterns, low initial 26Al 27Al ratios of ~5 × 10 −6, typical ak 35–55 melilite, Tpx with 2–12% TiO 2, and opaque grains (OsIrRuMoPt-alloy nuggets and fremdlinge) smaller than 10 μm. 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. The few CAIs with the coarse laths cooled slowly (2°–5°/h?) from below the liquidus, but the majority cooled faster from completely molten material. As the elemental abundances in Type C's are dominated by volatility, the melts were not formed by planetary (magmatic) processes. Evaporative residues and remelted solid condensates are also ruled out because the compositions of naturally occurring, experimentally produced, and thermodynamically calculated residues and solid condensates differ from those of Type C CAIs. Type C CAIs have compositions similar to those calculated for liquid condensates under high solar gas pressure (~ 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.