AN ANCIENT SM-ND AGE FOR A FERROAN NORITIC ANORTHOSITE CLAST FROM LUNAR BRECCIA-67016

Strontium and neodymium systematics have been examined in a clast of ferroan noritic anorthosite from Apollo 16 breccia 67016. Two splits (,328 and 326) of the same clast give different Sm-Nd results. Split 328 gives a well defined internal isochron age of 4.562 +/- 0.068 Ga and an initial Nd-143/Nd-144 ratio of 0.50673 +/- 10 corresponding to epsilon(Nd) = 0.1 +/- 0.2 (2sigma optimized error) relative to the Murchinson carbonaceous chondrite. The pyroxene separate from split 326 lies on the same isochron. In contrast, the plagioclase and whole-rock from split 326 fall below this line, indicating a small-scale disturbance of the Sm-Nd system. This may reflect either an isotopic exchange between the plagioclase and a low Sm/Nd mineral or a loss of radiogenic Nd-143 from the plagioclase, possibly during the period of major impacts at approximately 3.9 Ga. The preservation of an extremely old age for the noritic ferroan anorthosite 67016,328 suggests a rapid cooling of this rock at an early stage in the evolution of the lunar magma ocean. This old age is also consistent with giant impact models for the formation of the Moon but implies a relatively early event (pre 4.50 Ga) and, therefore, rapid accretion and differentiation of the terrestrial planets.