Rb-Sr and Sm-Nd chronology and genealogy of mare basalts from the Sea of Tranquility

Abstract

Rb-Sr and Sm-Nd ages of two Apollo 11 mare basalts are 3.64 ± 0.05 and 3.57 ± 0.03 AE for high-K basalt 10072, and 4.01 ± 0.11 and 3.88 ± 0.06 AE for low-K basalt 10062. Rb-Sr, Sm-Nd, and ⁴⁰Ar-³⁹Ar ages are in good agreement and show an extensive time interval for filling of the Sea of Tranquility, presumably by thin lava flows, in agreement with similar observations for the Ocean of Storms. The older ages indicate that mare filling may have started prior to the Imbrium event. Initial Sr and Nd isotopic compositions on Apollo 11 basalts identify at least two parent sources producing basalts. The low-K basalts may have been produced from a single reservoir over an extended time interval. The Sm-Nd isotopic data demonstrate that low-K and high-Ti basalts from Apollo 11 and 17 have been derived from totally distinct reservoirs while low-Ti Apollo 15 mare basalt sources have Sm/Nd similar to the sources of Apollo 11 basalts. It appears that groupings of mare basalt based on Ti content and on isotopic data do not coincide. Apollo 11 high-K basalts show no significant Sm/Nd fractionation near the time of crystallization in contrast to characteristically large Rb/Sr fractionation for these basalts.

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