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 f erroan 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 epsi lon(Nd) = 0.1 +/- 0.2 (2sigma optimized error) relative to the Murchin son 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 betw een 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 for mation of the Moon but implies a relatively early event (pre 4.50 Ga) and, therefore, rapid accretion and differentiation of the terrestrial planets.