MINERAL LAYERS AROUND COARSE-GRAINED, CA-AL-RICH INCLUSIONS IN CV3 CARBONACEOUS CHONDRITES - FORMATION BY HIGH-TEMPERATURE METASOMATISM

Coarse-grained (Type A, B) Ca-Al-rich inclusions (CAIs) in carbonaceou s chondrites typically are surrounded by thin mineral layers (''rims'' ) that have puzzled researchers for two decades. Quantitative reaction -diffusion models can account for the overall mineral zoning structure s of rims and the major-element zoning of the ubiquitous clinopyroxene layer, suggesting that the layers formed by metasomatism. Melilite-be aring CAIs appear to have reacted with an external medium that primari ly contained Mg-Si-rich vapor (with atomic Mg/[Mg+Si] less than or equ al to 0.66) and forsteritic olivine. Different reactant compositions i n the external medium appear to have been largely responsible for prod ucing different rim types. Various rims formed either in different loc al environments or at different times in an evolving system. It is sug gested that layer formation occurred in a nebular setting, while silic ates were being vaporized and olivine was condensing around CAIs. Stea dy state layer growth models do not adequately explain the presence of melilite layers or patches in some rims and consistently underestimat e the spinel/clinopyroxene ratios of runs, probably because of a failu re to attain complete steady state conditions as a result of changing pressure, temperature, or reactant compositions during layer growth. R oughly 3-50% of the spinel in rims can be attributed to metasomatic gr owth, but the remaining spinel formed by another process, possibly as a residue of partial melting during a brief vaporization event, or by preferential nucleation on the surfaces of molten CAIs. The thermal ev ents accompanying CAI metasomatism can be constrained by modeling Mg i sotope exchange that occurred between some CAIs and the external mediu m. Based on one well-studied CAI, it is inferred that isotopic exchang e and layer formation was initiated either in a high-temperature (>145 0 degrees C) heating event <10 hours in duration, or at lower temperat ures (less than or equal to 1450 degrees C) during cooling at a rate o f less than or equal to 0.1-2 degrees C/hr.