Age of an Apollo 15 mare basalt; Lunar crust and mantle evolution

An internal Rb-Sr isochron for the large basalt boulder 15555 returned from the edge of Hadley rille by the Apollo 15 mission yields an age3.32 ± 0.06AE and an initial88Sr/86Sr,I = 0.69934 ± 5. This age andI value fall well within the range obtained for the Apollo 12 basalts from the Ocean of Storms and may indicate that extensive lava flows occurred at ∼ 3.3 AE over widespread areas of the moon. The Sr composition of the anorthosite 15415 is as low as that of plagioclase extracted from the Apollo 11 low K rocks. The initial Sr composition of 15415 for anassumed age of 3.3 to 4.6 AE is extremely primitive and provides further evidence for an extremely short formation interval (3 to 1 × 106 yr) of a nonchondritic moon with respect to an origin in time defined by BABI. The initial87Sr/86Sr for crystalline rocks returned from all lunar missions is correlated with the concentration of Rb and correspondingly K, U and Th. This correlation places distinctive constraints on the evolution of lunar magmas and the internal structure of the moon.