Origin of plagioclase-olivine inclusions in carbonaceous chondrites

Abstract

Plagioclase-Olivine Inclusions (POIs) are an abundant group of chondrule-like objects with igneous textures found in carbonaceous chondrites. POIs consist of plagioclase, olivine, pyroxene, and spinel, and cover a wide range of compositions between Type C Ca-Al-rich Inclusions (CAIs) and ferromagnesian chondrules. POIs are distinguished from CAIs by the absence of melilite, lack of refractory siderophile-rich opaque assemblages, more sodic plagioclase, and abundance of olivine and aluminousenstatite. Rare accessory minerals including armalcolite, zirconolite, rutile, and sapphirine are found in several POIs. The petrographie and chemical properties of POIs indicate that they are not condensates or evaporative residues but formed by melting or partial melting of pre-existing solids. Seven of fourteen POIs contain isotopically fractionated Mg, and despite their textures these POIs are not isotopically homogeneous. Spinel is the major carrier of fractionated Mg in six POIs. The magnitude of Mg isotopic fractionation ( F Mg ) of spinel ranges from −8 to +11%./amu among the POIs but F Mg is always either positive or negative within an individual POL Within a single inclusion, F Mg of coexisting spinel is not constant but varies by up to 7%./amu. Isotopic fractionation of Mg in silicates is less common and always positive ( F Mg > 0). Pyroxenes in two POIs and plagioclase in a third contain isotopically heavy Mg. Radiogenic 26Mg∗ is rare in POIs as only two inclusions show evidence of excess 26Mg with 26Mg∗/ 27Al ~ 4 × 10 −6. The preservation of isotopic heterogeneity in objects whose igneous textures suggest crystallization from a homogeneous melt implies that melting was incomplete, allowing survival of relict phases. A comparison of the essential characteristics of POIs and CAIs suggests that the major processes leading to formation of POIs—including condensation, dust/gas fractionation, aggregation of chemically and isotopically disparate materials, and partial melting—are common to most CAIs and chondrules. We present a scenario for the formation of these objects and conclude that the homogeneity of the final assemblage—CAI, POI, or chondrule—is primarily a reflection of the thermal history rather than the nature of precursor materials.