Lunar Highland chronology

Abstract

The distribution of lunar highland rock ages is of twofold significance. At first, the highest ages provide a lower limit of ∼4.4 b.y. for the time of the initial differentiation of the lunar crust. Secondly, the ages reflect the collisional history of the moon in the impact-dominated period between 4.4 and 3.8 b.y. ago in which the large multiring-basins were formed. The way in which rock ages respond to impacts is rather complex and it is therefore not possible to date a basin or another lunar feature from a single rock age. Instead, rock age distributions must be considered. More than 150 highland rock ages have been dated, mainly by the KAr (Ar 39Ar 40) method. The generally broad distribution of ages for breccias from a common landing site as well as the coexistence of various ages within the same breccia favor a “crater-dominated chronology” in which partial age resetting is achieved by numerous local craters in a statistical manner. This is in accordance with petrological observations which indicate that the highland breccias contain large proportions of local material reworked by secondary impacts. A statistical analysis of all available KAr data reveals distribution maxima at 3.9 b.y. for A 14 Fra Mauro highland basalts which are considered to be Imbrium ejecta; at 4.0 b.y. for Apollo 16 rocks from the Descartes region in the Central Highlands and, indistinguishable therefrom, also at 4.0 b.y. for the A 17 Taurus Littrow (Serenitatis) rocks. However, the age distribution is bimodal and a secondary maximum occurs for A 17 rocks at ∼4.25 b.y. Evidently, the time between 3.8 and 4.0 b.y. has been a very active period in shaping the lunar crust and the major basin-forming collisions occurred within that interval. However, the frequent occurrence of rock ages between 4.0 and 4.3 b.y. raises serious objections against a purely basin-dominated chronology. Instead, a rapidly decreasing cratering rate seems to have enabled a statistically increasing number of rocks to survive the bombardment as time proceeded. The obviously limited resetting power of basin excavations prevents establishing a detailed absolute basin chronology for the sequence Serenitatis-Nectaris-Humorum-Crisium-Imbrium-Orientale as inferred from remote crater counts. However, all basins must be older than 3.8 b.y. since this is the age of Taurus Littrow mare basalt fillings which are not covered by Orientale ejecta. Most likely all circular basins formed within less than 150 m.y. approximately from 3.8 to 3.92 b.y.