A combined Sm-Nd, Rb-Sr, and U-Pb isotopic study of Mg-suite norite 78238: Further evidence for early differentiation of the Moon

摘要

Lunar Mg-suite norite 78238 was dated using the Sm—Nd, Rb—Sr, and U—Pb isotopic systems in order to constrain the age of lunar magma ocean solidification and the beginning of Mg-suite magmatism, as well as to provide a direct comparison between the three isotopic systems. The Sm—Nd isotopic system yields a crystallization age for 78238 of 4334 ± 37 M_a and an initial ε_Nd~143 value of-—0.27 ± 0.74. The age-initial ε_Nd~143 (T-I) systematics of a variety of KREEP-rich samples, including 78238 and other Mg-suite rocks, KREEP basalts, and olivine cumulate NWA 773, suggest that lunar differentiation was completed by 4492 ± 61 M_a assuming a Chondritic Uniform Reservoir bulk composition for the Moon. The Rb—Sr isotopic systematics of 78238 were disturbed by post-crystallization processes. Nevertheless, selected data points yield two Rb—Sr isochrons. One is concordant with the Sm-Nd crystallization age, 4366 ± 53 M_a. The other is 4003 ± 95 Ma and is concordant with an Ar-Ar age for 78236. The ~207Pb-~206Pb age of 4333 ± 59 M_a is concordant with the Sm-Nd age. The U-Pb isotopic systematics of 78238 yield linear arrays equivalent to younger ages than the Pb—Pb system, and may reflect fractionation of U and Pb during sample handling. Despite the disturbed nature of the U—Pb systems, a time-averaged μ (~238U/~204Pb) value of the source can be estimated at 27 ± 30 from the Pb-Pb isotopic systematics. Because KREEP-rich samples are likely to be derived from source regions with the highest U/Pb ratios, the relatively low ц value calculated for the 78238 source suggests the bulk Moon does not have an exceedingly high ц value.