Trude V. V. King

Accretionary dark rims in unequilibrated chondrites

Textural and qualitative EDX investigations of dark-rimmed particles in six low petrologic type chondrites indicate that the rims accreted on host particles over a wide range of temperatures prior to initial accumulation and lithification of the meteorites in which the rimmed particles are now contained. Many dark rims are enriched in moderately volatile trace elements such as Na, Cl, P, and K, relative to the host particles and matrix. The range of physical/chemical environments associated with hypervelocity impacts may have offered the setting for the formation of dark-rimmed particles early in solar system history.

A model for the internal structures of asteroids

Abstract Interpretation of reflectance spectra indicates that most belt asteroids are composed of materials similar to carbonaceous chondrites. Also, there is considerable evidence to support the origin of many, if not most, lunar and meteoritic chondrules by impact processes. The accretional histories of the carbonaceous asteroids must have influenced greatly their internal structures and textures. A model for this accretional history can be divided conveniently into three temporal stages that produce distinctly different lithologies: (1) low-velocity accretion of fine silicate and carbonaceous grains producing chondrule-free petrologic type 1 lithology; (2) continued accretion of low-velocity fine silicate and carbonaceous grains, but with a few larger, higher-velocity bodies also impacting the surface thereby producing both fluid drop and lithic chondrules (the resultant lithology would be that of petrologic type 2 and 3 carbonaceous chondrites); and (3) dominance of high-velocity low-mass meteoroid impacts, producing a sparse, thin, erosive lunar-like regolith. The lithologic product of stage 3 is not ideally represented among the presently described carbonaceous chondrites, but texturally analogous samples are known from the achondrites. The greater proportion of chondrules in the C V group meteorites, in contrast to the C M2 and C O3 groups, may be due to the origin of the C V chondrites on larger asteroid parent bodies that could withstand more numerous and higher-energy chondrule-producing impacts prior to fragmentation.