Rb-Sr ages and initial strontium in basalts from Apollo 15

Abstract

Internal isochrons have been determined for six basalts returned by the Apollo 15 mission. The rocks represent each of the major sampling stops during Apollo 15. The ranges in age and I are 3.28 ± 0.04 to 3.44 ± 0.07AE and 0.69923 ± 6 to 0.69937 ± 4. The range in I is very narrow and appears to represent a rather uniform magma source. A detailed study of the initial ⁸⁷Sr/⁸⁶Sr was made on geometrically distinct pieces of 15555. The I value was demonstrated to be well defined to within 3 × 10⁻⁵ and therefore reports of variations in I for this rock are in error. The close similarity in the ages of basalts from both the Apollo 12 and 15 sites indicates that widespread extrusion of basaltic lavas took place at 3.2–3.3 AE. From all of the evidence currently available, the last major episode of mare flows took place at 3.16 ± 0.09AE. No clear demonstration of major extrusions subsequent to this time has yet been found. The onset of mare flooding must have begun prior to 3.7 AE. All the results to date show that subsequent to the formation of the Moon the lunar basalts evolved in a magma reservoir with Rb/Sr ∼ 10⁻² and cannot be derived from a mantle which has a chondritic Rb/Sr.