Rb-Sr and Sm-Nd chronology of an Apollo 17 KREEP basalt

Abstract

Sm-Nd and Rb-Sr mineral isochrons were determined for an Apollo 17 KREEP (pigeonite) basalt clast (,543) from breccia 72275 collected from Boulder 1, Station 2 in the Valley of Taurus-Littrow. Sm-Nd analyses of the basalt yield a precise mineral isochron age of 4.08 ± 0.07 Ga for λ( 147Sm= 0.00654 Ga −1. The Rb-Sr mineral isochron age of 4.13 ± 0.08 Ga for λ( 87Rb) = 0.0139 Ga −1 is concordant with the Sm-Nd age. However, the Rb-Sr isotopic system has been slightly disturbed, probably by the Serenitatis impact event ∼ 3.95 Ga ago. The Sm-Nd age is interpreted as the crystallization age of this Apollo 17 KREEP basalt. Ages and initial 87Sr/ 86Sr and ε Nd values for this basalt and Apollo 14 and 15 KREEP basalts suggest that these two types of KREEP basalts could be produced from sources having similar Rb/Sr and Sm/Nd ratios. These new age data suggest that KREEP basaltic volcanism started ∼ 200 Ma earlier than the ∼ 3.85 Ga age generally accepted from studies of Apollo 15 pristine KREEP basalts. Apollo 17 KREEP basalts probably pre-date the Serenitatis impact, strongly indicating that not all KREEP basaltic volcanism was necessarily related to basin-forming impacts. The younger Apollo 15 and 14 KREEP basalts probably are related to the Imbrium impact, however. The ages and ε Nd values for KREEP basalts, granites, troctolites and norites from a variety of lunar sites form a linear array corresponding to source(s) with Sm/Nd lower by ∼ 15% than the chondritic value (slightly enriched in LREE), suggesting that these rocks could be genetically related. Formation of a common Sm-Nd isotopic growth curve by these data is most easily explained by an early moon-wide differentiation accompanying the formation and subsequent solidification of a global magma ocean.