Dating lunar rocks
Studies like this one are filling in the picture of how the initial crust of the Moon formed, which in turn sheds light on the formation of the terrestrial planets. However, asteroids are relatively small bodies and the existence of now-extinct radioactive isotopes such as article Hafnium, Tungsten, and the Differentiation of the Moon and Mars) in some igneous meteorites show that their parent bodies must have cooled rapidly and experienced little geological activity since they formed.Although igneous meteorites provide important information about what was happening on small bodies in the early Solar System, they provide only a general guide to the nature of events that built the larger planets.One possibility is that the young ages reflect impact events, not the original time of igneous crystallization.My colleagues Lars Borg (University of New Mexico) and Larry Nyquist and Don Bogard (Johnson Space Center) and I studied an anorthosite (rock 67215) relatively rich in pyroxene, allowing us to determine a precise crystallization age of 4.40 billion years.It consists of fragments of plagioclase (white) and glass (dark gray).Rock fragments in breccias like these tell us a great deal about the early history of the Moon.While photographs and remote sensing data provide useful information about distant bodies, having real samples from the Moon available for detailed laboratory studies has revealed aspects of the geological evolution of the planets which otherwise could only be imagined.For example, the first studies of Moon rocks inspired John Wood (Smithsonian Astrophysical Observatory) to boldly imagine the idea that terrestrial planets must have been extensively molten soon after they formed.
The ages and chemical compositions of lunar anorthosites therefore provide ground truth tests for theoretical models of planetary accretion and differentiation.
Fortunately, nature has provided a keystone that links the record of early nebular events preserved in meteorites with the subsequent geological evolution of the terrestrial planets, and that keystone is the Moon.
For example, volcanism on the Earth and Moon overlapped in time for about a billion years, yet the Moon's crust is sufficiently old that it preserves direct evidence for planetary-scale events that occurred before the Earth's surface stabilized.
The long history of meteorite impacts into the lunar crust has disturbed or reset their K-Ar and U-Pb isotopic compositions.
The fact that most lunar anorthosites are, by definition, composed almost totally of plagioclase makes it difficult to obtain enough sample for mineral isochrons using more robust systems such as 147Sm-143Nd.