Rb-Sr age of troctolite 76535

Abstract

We have studied the Rb-Sr systematics for the troctolite 76535 in phases covering a wide range of Rb and Sr concentrations. The Rb-Sr data define a precise linear array for most of the samples. Excluding two samples which are distinctly anomalous we obtain a line corresponding to an age of 4.61 ± 0.07 AE and an initial ⁸⁷Sr/⁸⁶Sr of 0.69900 + 0.00003. The Rb-Sr age of 4.55 ± 0.10 AE for the dunite (72417) and the Rb-Sr age of the troctolite are in approximate agreement and permit an interpretation that these rocks formed during a single major lunar differentiation which presumably produced an anorthositic-gabbroic lunar crust and layered upper mantle. Other dating methods (K-Ar, Sm-Nd) yield distinctly younger ages for the troctolite of 4.25 AE which are not in agreement with the Rb-Sr age. The causes of this discrepancy are not understood. The U-Pb system shows a young disturbance in the neighborhood of ~4.0 AE. Whereas the K-Ar ages could previously be considered as due to partial degassing of 76535 during the terminal lunar cataclysm, the Sm-Nd data cannot easily be interpreted in this fashion. Assuming that the ages of 4.61 and 4.25 AE are meaningful, our preferred interpretation is that the troctolite was formed at 4.61 AE and remained at elevated temperatures until 4.25 AE at which time it was excavated by a major impact. Throughout the interval 4.61-4.25 AE, ⁴⁰Ar diffused out of the rock and Sm and Nd exchanged chemically and isotopically; however, Rb-Sr systems protected inside olivine crystals remained closed. Except for some inconclusive U-Pb results and peculiarities in the ⁴⁰Ar-³⁹Ar data there are no results which clearly show that the troctolite was affected by the terminal lunar cataclysm.