U-Th-Pb systematics on lunar rocks and inferences about lunar evolution and the age of the moon

Abstract

All of the available data on terra rocks define a striking linear array when plotted on the U-Pb evolution diagram with a generally limited scatter around a reference isochron which intersects the concordia curve at 4.42 and 3.95 AE and defines an initial radiogenic ²⁰⁷Pb/²⁰⁶Pb of I_(Pb) = 1.45. In addition to the observed wide range of 201Pbf06Pb (0.4-1.45) the samples exhibit a vast range for μ = ²³⁸U/²⁰⁴Pb from 0.9 to 10⁴ which is generally correlated with the degree of discordance. On the basis of similar observations, on a limited number of terra rocks, we had previously inferred that a major impact had caused global metamorphism in the lunar crust. The more extensive results presented here demonstrate that the terra rocks in general fall on the linear array although it is not clear as to how much of the scatter reflects lunar processes and how much is due to insufficient precision in some of the existing data. The data array is taken as an evidence for the validity of the terminal lunar cataclysm hypothesis which ascribes large-scale metamorphism at --3.9 AE to a period of intense bombardment of an ancient lunar crust, effecting element redistribution and extensive mobilization of Pb. A time interval of up to 200 m.y. (3.8-4.0 AE) is assigned to the duration of the terminal cataclysmic bombardment during which several of the major ringed basins may have been formed. Some of the impacted materials lost almost all their Pb and consequently fall on the concordia curve at the lower intersection. The mobilized Pb was deposited and/or incorporated in other rocks which became discordant. Some rocks are dominated by this labile Pb and lie close to the intercept (e.g. 15415). While the target rocks may be of variable age and the mobilized Pb not have a unique isotopic composition the gross isotopic composition of the radiogenic comp.Jnent of the mobilized lead (I_(Pb) ~ 1.45) presumably represents the average value of ²⁰⁷Pb/²⁰⁶Pb in the lunar crust at ~3.8-3.9 AE. This I_(Pb) is consistent with its production in a single stage in the time span 4.4-3.9 AE. The upper intersection of the linear array with the concordia curve appears well defined by the fact that two terra rocks and one mare basalt are concordant by the U-Th-Pb method at 4.42 AE indicating a very strong possibility that this is close to the actual time of differentiation of the lunar crust. A firm interpretation of the upper intersection is not yet possible. It must be recognized, however, that from the point of view of the U-Th-Pb systematics that it is not possible to distinguish between differentiates produced by magmatic activity at ~ 3.9 AE and those rocks produced in cataclysmic impact metamorphism. Thus, a sample might fall accidentally on the array without having a genetic relationship to terra rocks on the array. This might apply to the orange soil and basalt 75055 which plot on the reference isochron. New data on two lunar basalts show that the ²⁰⁴Pb concentration levels are much lower than reported previously.