Isotopic evidence for a terminal lunar cataclysm

Abstract

Most highland total rock samples define a single U-Pb isochron which corresponds to a metamorphism age of ∼ 3.9AE. This age is also obtained for internal U-Pb isochrons for some of these samples. The data on 18 rock samples range from concordant samples with ²³⁸U/²⁰⁶Pb∼ 1.2 to discordant ones with ²³⁸U/²⁰⁶Pb∼ 0.02. This feature coupled with a correlated pattern of ²³⁸U/²⁰⁴Pb ratios, indicates that Pb was extensively mobilized at ∼ 3.9AE. The observed Pb-U fractionation is essentially due to Pb volatilization during the metamorphic events. Volatile Pb transport is not accompanied by similar effects in Rb and must therefore be attributed to a specific process. Rb-Sr internal isochrons for the same rocks determine distinct metamorphic events in the interval 3.85–4.00 AE. We conclude that highland samples from widely separated areas bear the imprint of an event or series of events in a narrow time interval which can be identified with a cataclysmic impacting rate of the moon at ∼ 3.9AE, although differentiation by internal magma generation cannot be excluded. This cataclysm is associated with the Imbrium impact and very possibly the formation of Crisium and Orientale and possibly several other major basins in a narrow time interval (∼2 × 10⁸yr or less). The U-Pb data indicate formation of the lunar crust at ∼ 4.4AE, which is distinctly younger than 4.6 AE, the time generally associated with planetary formation. If the lunar crust started being formed at ∼ 4.6AE, then this process must have continued until times significantly younger than 4.4 AE. Rb-Sr data also indicate formation of the lunar crust around 4.5 AE with only minor additions of high Rb/Sr materials to the crust at times younger than 4.3 AE. Using the U-Pb systematics, K/U and the average U concentration of the moon as obtained from heat-flow measurements, we estimate the lunar concentrations: primordial Pb= 35ppb;Rb= 0.5ppm with Rb/Sr= 0.006.