Rb-Sr ages of lunar rocks from the sea of tranquillity

Abstract

⁸⁷Rb-⁸⁷Sr internal isochrons have been determined for six crystalline rocks from the Sea of Tranquillity, and all yield ages within the narrow range 3.65 ± 0.06 × 10⁹yr. Differences in initial (⁸⁷Sr/⁸⁶Sr)_I define at least two main rock groups which coincide with the two groups indicated by alkali content of the rocks. This demonstrates that at least two magnetic reservoirs were involved in the 3.65 × 10⁹ yr event on the moon. The soil yields a model Rb-Sr age of 4.6 × 10⁹ yr which suggests that it consists of an average over a variety of rock systems of different ages which represent a closed total system 4.6 × 10⁹ yr old or the widespread existence of rocks of 4.6 × 10⁹ yr age. A single peculiar rock fragment from the soil gives a model age of 4.4 × 10⁹ yr. Initial Sr compositions for the lunar rocks and for soil fragments define a narrow range (0.6990–0.6994) whose lower limit is equal to the basaltic achondrite initial Sr. The moon, earth and basaltic achondrites represent planets with Rb/Sr much lower than in the sun. The moon and the basaltic achondrites must have separated from the solar nebula within a time interval of less than 4 × 10⁶ yr. From consideration of Sr-Rb relations, if the moon formed by fission from the earth, it must have occurred prior to 4.4 × 10⁹ yr ago.

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