REFRACTORY INCLUSIONS IN THE PROTOTYPICAL CM CHONDRITE, MIGHEI

Refractory inclusions in the Mighei CM2 meteorite are very abundant, 35 being found in three thin sections of the meteorite. The most common are pyroxene-rimmed spinel-rich inclusions that occur in a range of structural and textural types. Some inclusions are simple aggregates of crystals or rimmed crystals, ranging from loose clusters of spinel grains to distended chains of pyroxene-rimmed spinel knots. Others are more compact and may have experienced recrystallization or melting; these include ''nodular'' spinel inclusions with polygonal-granular-textured spinel interiors and pyroxene rims, ribbons of spinel (in some cases with hibonite and/or perovskite) that are again rimmed by pyroxene, and refractory spherules. Four inclusions are complex aggregates of several texturally distinct objects each. Collectively, Mighei inclusions are similar to those in Murchison but appear to be somewhat more altered. Volatility fractionated and unfractionated trace element patterns similar to those found in refractory inclusions in other carbonaceous chondrites are represented in Mighei inclusions and, in addition, some Mighei inclusions have volatility fractionated pattern types not previously found in refractory inclusions. In three of the complex aggregates, fundamentally different trace element patterns are present within different portions of each inclusion, meaning that these objects accreted from genetically different components that never equilibrated with one another subsequent to accretion. Bulk inclusion magnesium isotopic compositions show minor mass fractionation effects: F-Mg values range from -2.3 to +7.5 parts per thousand, but the average F-Mg value is indistinguishable from normal, 1.8 +/- 1.2 parts per thousand. There are no correlations between magnesium isotopic composition and textural type or trace element pattern type. The diverse inclusion types in Mighei represent a variety of origins and histories. Textures suggest that the nodular and banded inclusions may have experienced solid-state recrystallization, whereas the porous and chain-like varieties and complex aggregates have not. Magnesium isotopic compositions and trace element abundances suggest that the Mighei inclusions are unlikely to be evaporation residues and are more likely to be the products of condensation or recondensation processes.