Rb-Sr study of a lunar dunite and evidence for early lunar differentiates

Abstract

A Rb-Sr isochron is defined by data on a set of rock chip samples and handpicked mineral separates of the dunite clast 72417. Although this rock has undergone a complex multistage history, the data yield a well-defined age of 4.55 ± 0.10 AE and an initial ⁸⁷Sr/⁸⁶Sr, I = 0.69900 ∓ 0.00004. These observations estabμsh the dunite as an early lunar differentiate. The I value must be representative of the early lunar upper mantle and offers no evidence of I values for the moon significantly lower than BABI. Trace-element correlations show that the dunite contains at least three distinct phases with different Rb/Sr and, therefore, the isochron is not simply a mixing line. Samples of the dunite have extremely low K/Rb ~100 (by weight) as compared to most lunar rocks and minerals and require that the dunite were derived from a distinctive magma. Samples of the host breccia of the dunite indicate that the breccia was formed or disturbed at times younger than 4.10 AE but show that isotopic homogenization was not achieved over distances as short as 1 mm. The lack of significant element migration during metamorphism of the breccia is consistent with the preservation of the ancient Rb-Sr age of the dunite through the last stage of metamorphism at the geometrical scale studied. Data are presented which show that mineral separation procedures, using organic liquids, cause the differential dissolution of Rb and Sr from phases in the dunite. In general, the use of organic liquids in mineral separation may seriously disturb the isotopic systems.

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