Rb-Sr ages and initial Sr-87/Sr-86 for Apollo 17 basalts and KREEP basalt 15386

The Rb-Sr data reported for Apollo 17 mare basalts and for KREEP basalt 15386 is used to determine mineral isochrons. The weighted average age of four Apollo 17 basalts is 3.76 + or - 0.06 AE, while the age determined for 15386 is 3.94 + or - 0.04 AE. The isotopic data for the Apollo 17 basalts are discussed in the context of Sm and Eu data for the same samples. The Sr-isotopic data are fit best by a three-stage model evolution involving evolution of Sr-87/Sr-86 in an environment with Rb/Sr greater than in the basalts, production of mare basalt source regions of lower but variable Rb/Sr sometime in the interval 4.6 to 3.75 AE ago, and extraction of lavas from these sources 3.75 AE ago. Other possibilities are considered.