Comparison of Lunar Rocks and Meteorites: Implications to Histories of the Moon and Parent Meteorite Bodies

There are many similarities between lunar samples and stone meteorites. Lunar samples, especially from the highlands, indicate that they have been affected by complex and repeated impact processes that can result in breccias with a wide variety of features produced by crushing; grinding; comminution; cataclasis; mixing (with other rock types and meteorites); partial or complete melting and mobilization; production of glass as matrix, agglutinates, spherules, fragments, and chondrules; solid-state recrystallization; impact melt as matrix or as essentially wholly new igneous rocks with ophitic, intersertal, poikilitic, or other textures; etc. Similar complex and repeated impact processes have also been operative on the achondritic and chondritic meteorites, but there has been much less detailed study of the effects of these processes and discussion of their implications. In this study, we draw attention to a number of similarities between lunar and meteoritic rocks and suggest that this comparison is essential for a clear- understanding of meteorites as probes of the early history of the solar systems. (1) Monomict and polymict breccias occur in lunar rocks, as well as in achondritic and chondritic meteorites, having resulted from complex and repeated impact processes. (2) Chondrules are present in lunar meteorites, as well as in a few achondritic and most chondritic meteorites. They apparently crystallized spontaneously from molten highly supercooled droplets which may have formed from impact melts or, perhaps, volcanic processes (as well as from the solar nebula, in the case of meteoritic chondrites). It is pointed out that because chondrules may form in several different ways and in different environments, a distinction between the different modes of origin and an estimate of their relative abundance is important if their significance as sources of information on the early history of the solar system is to be clearly understood. (3) Lithic fragments are very useful in attempts to understand the preimpact and postimpact history of lunar and meteoritic breccias. They vary from little modified (relative to the apparent original texture) to partly or completely melted and recrystallized lithic fragments. Their detailed study allows conclusions to be drawn about their parent rock types and their origin, thereby gaining insight into preimpact histories of lunar and meteoritic breccias. There is considerable evidence that cumulate rocks were involved in the early history of both the Moon and parent meteorite bodies. In the light of knowledge derived from the lunar program and by continued study of meteoritic samples, we should learn more about the preimpact history of both rock groups and thereby be better prepared to make comparative studies of planetary bodies, particularly regarding their early histories.