Nature and Origins of Meteoritic Breccias

Abstract

Meteorite breccias provide information about impact processes on planetary bodies, their collisional evolution, and their structure. Fragmental and regolith breccias are abundant in both differentiated and chondritic meteorite groups and together with rarer impact-melt rocks provide constraints on cratering events and catastrophic impacts on asteroids. These breccias also constrain the stratigraphy of differentiated and chondritic asteroids and the relative abundance of different rock types among projectiles. Accretional chondritic breccias formed at low impact speeds (typically tens or hundreds of meters per second), while other breccias reflect hypervelocity impacts at higher speeds (~5 km/s) after asteroidal orbits were dynamically excited. Iron and stony-iron meteorite breccias only formed when their parent bodies were partly molten. Polymict fragmental breccias and regolith breccias in some meteorite groups contain unique types of clasts that do not occur as individual meteorites in our collections. For example, ureilite breccias contain feldspathic clasts from the ureilite parent body as well as carbonaceous chondritic projectile material. Such clasts provide new rock types from both unsampled parent bodies and unsampled parts of known parent bodies. We review breccias in all types of asteroidal meteorites and focus on the formation of regolith breccias and the role of catastrophic impacts on asteroids.