Lunar chronology as determined from the radiometric ages of returned lunar sample
We have heard earlier in this Discussion Meeting that from the systematics of Sr and Pb isotopes in lunar samples, it is possible to ascertain that the Moon had a solid crust about 4.6 Ga ago, that is, very soon after the formation of the solar system. In addition, it would seem that the major ring-basins on the Earth side of the Moon were all formed before 3.8 Ga ago. After the formation of the basins by impact, there was extensive magmatic activity in the form of basalt flows, expecially in the major ring-basins. About 3.1 Ga ago, all major lava-flow activity on the Moon had ceased. This outline of lunar chronology is accepted in practically all proposed interpretations of the radiometric ages. The first 600 Ma of lunar chronology is not as clear. During this time the multi-ring basins were formed. It has been proposed by G. Turner and by G. J. Wasserburg at this meeting, that the multi-ring basins formed in a rather short interval of time, perhaps as short as 100 Ma. This would imply an intense cratering rate some 3.9 Ga ago and a rapid decline in cratering rate thereafter. Such an event would have probably greatly altered the other bodies in the solar system, especially the Earth, and as such is of no small significance. It has been pointed out previously by Tera et al. (1974) that an alternative interpretation of the radiometric data is that widespread, simultaneous metamorphism (which the systematics of the lead isotopes in the highland rocks seems to imply) could as well result from a single widespread event such as Imbrium as from a multiple basin-forming sequence in a short period of time. In that case, the chronology of the multi-ring basins is an open question.