Early lunar differentiation: 4.42-AE-old plagioclase clasts in Apollo 16 breccia 67435

We report on a⁴⁰Ar-³⁹Ar study of the Apollo 16 breccia 67435 and present ages of five samples representing matrix, lithic clasts and plagioclase clasts. While the matrix age spectrum does not have a well-defined plateau, the two lithic clasts gave plateau ages of 3.96 and 4.04 AE. Since all samples had apparent ages of ～1 AE in the fractions ≤600°C extraction temperature, the breccia might have been assembled in a rather mild process at about that time or even more recently out of material with different metamorphic ages. The two plagioclase samples, of which one was a single 9-mg mineral clast and the other a 15-mg composite of several clasts, also have ages of ～1 AE in the low-temperature release fractions, but are apparently undisturbed by any ～4-AE events since they both have well-defined plateaux at 4.42 AE. The age of these strongly calcic plagioclase clasts, believed to be remnants of the anorthositic lunar crust, establishes a lower age limit to the end of the early lunar differentiation and thus places a strong constraint to the lunar evolution.