1. Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
2. Department of Earth and Environmental Sciences, CEPSAR, Walton Hall, The Open University, Milton Keynes, MK7 6AA, UK
3. Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK
4. Institut für Planetologie, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany
5. Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano-ku, Tokyo 164-8639, Japan

Correspondence to: Kentaro Terada¹Mahesh Anand^2,3 Correspondence and requests for materials should be addressed to K.T. (Email: terada@sci.hiroshima-u.ac.jp) or M.A. (Email: M.Anand@open.ac.uk).

Abstract

The origin and evolution of the Moon remain controversial^{1, 2}, with one of the most important questions for lunar evolution being the timing and duration of basaltic (mare) magmatism^{1, 3, 4, 5, 6, 7, 8}. Here we report the result of ion microprobe U–Pb dating of phosphates in a lunar meteorite, Kalahari 009, which is classified as a very-low-Ti mare-basalt breccia. In situ analyses of five phosphate grains, associated with basaltic clasts, give an age of 4.35  0.15 billion years. These ancient phosphate ages are thought to represent the crystallization ages of parental basalt magma, making Kalahari 009 one of the oldest known mare basalts. We suggest that mare basalt volcanism on the Moon started as early as 4.35 Gyr ago, relatively soon after its formation and differentiation, and preceding the bulk of lunar volcanism which ensued after the late heavy bombardment around 3.8-3.9 Gyr (refs 7 and 8). Considering the extremely low abundances of incompatible elements such as thorium and the rare earth elements in Kalahari 009 (ref. 9) and recent remote-sensing observations illustrating that the cryptomaria tend to be of very-low-Ti basalt type^{10, 11, 12}, we conclude that Kalahari 009 is our first sample of a very-low-Ti cryptomare from the Moon.

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