Impact melt breccia
An impact melt breccia is a rock in which fragments are held in a matrix of rock melted by an impact.
An impact melt breccia contains angular fragments of rock and minerals enclosed in material that was melted by an impact and then solidified. The melt may have crystallised into fine-grained rock or quenched to glass.
These breccias form in and around craters where the impact energy was enough to melt part of the target. They are common among lunar samples and lunar meteorites, in some ordinary chondrites and in howardites. Some chondrites, such as parts of the Chelyabinsk meteorite, contain dark impact melt lithologies.
Impact melt breccias are useful clocks. When rock melts, its argon is driven out, so argon–argon dating of the melt can give the age of the impact itself. Dating many impact melts from the Moon and from chondrites has helped researchers track the history of collisions in the inner Solar System, including large impacts recorded in L chondrites around 470 million years ago.
Related terms
Level: Expert Advanced