Portal de Periódicos da CAPES

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

1992; Elsevier BV; Volume: 108; Issue: 4 Linguagem: Inglês

10.1016/0012-821x(92)90023-o

1385-013X

C.‐Y. Shih, L. E. Nyquist, B. M. Bansal, H. Wiesmann,

Geological and Geochemical Analysis

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 εNd 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 εNd 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.