Ar⁴⁰-Ar³⁹ systematics in rocks and separated minerals from Apollo 14

Abstract

The Ar⁴⁰-Ar³⁹ dating technique has been applied to separated minerals (plagioclase, pyroxene, quintessence and an "ilmenite" concentrate), and whole rock samples of Apollo 14 rocks 14310 and 14073. Plagioclase shows the best gas retention characteristics, with no evidence of anomalous behavior and only a small amount of gas loss in the initial release. Ages determined from the plagioclase of 14310 and 14073 are (3.87 ± 0.05) and (3.88 ± 0.05) AE respectively. Low apparent ages at low release temperatures, which are frequently observed in whole rock Ar⁴⁰-Ar³⁹ experiments on lunar basalts, are shown here to be principally due to gas loss in the high-K interstitial glass (quintessence) phase, confirming earlier suggestions. The decrease in apparent ages in the high temperature release previously observed in several total rock samples of Apollo 14 basalts has been identified with the pyroxene. Plagioclase is also found to be the most suitable mineral for the determination of cosmic ray exposure ages, and exposure ages of 280 and 113 million years are found for 14310 and 14073, respectively, indicating that these rocks, which are very similar in many respects, have different exposure histories. The relative production rates of Ar³⁸ from Fe and Ca have been determined from a comparison of pyroxene and plagioclase measurements.

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