Prolonged KREEP magmatism on the Moon indicated by the youngest dated lunar igneous rock

Abstract

Primordial solidification of the Moon (or its uppermost layer) resulted in the formation of a variety of rock types that subsequently melted and mixed to produce the compositional diversity observed in the lunar sample suite1,2. The initial rocks to crystallize from this Moon-wide molten layer (the magma ocean) contained olivine and pyroxene and were compositionally less evolved than the plagioclase-rich rocks that followed. The last stage of crystallization, representing the last few per cent of the magma ocean, produced materials that are strongly enriched in incompatible elements including potassium (K), the rare earth elements (REE) and phosphorus (P)—termed KREEP3,4,5. The decay of radioactive elements in KREEP, such as uranium and thorium, is generally thought to provide the thermal energy necessary for more recent lunar magmatism4,6,7. The ages of KREEP-rich samples are, however, confined to the earliest periods of lunar magmatism between 3.8 and 4.6 billion years (Gyr) ago8,9, providing no physical evidence that KREEP is directly involved in more recent lunar magmatism. But here we present evidence that KREEP magmatism extended for an additional 1 Gyr, based on analyses of the youngest dated lunar sample.

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Figure 1
Figure 2: Age versus initial ɛNd plot for KREEP-rich lunar rocks (Mg and alkali-suites, and KREEP basalts) demonstrating that NWA 773 is derived from the most evolved (LREE-enriched) lunar source region yet known.
Figure 3: Plot of age versus Rb/Sr ratios of sample source regions calculated from their initial Sr isotopic compositions using a single-stage growth model.

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Acknowledgements

We thank L. Nyquist for the use of his heavy liquid separations facility at the Johnson Space Center, M. Grady and S. Russell of the Natural History Museum, London for providing us with the sample, and two anonymous reviewers for their comments. This work was supported by NASA Cosmochemistry Program.

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Correspondence to Lars E. Borg.

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