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Petrogenesis and chronology of lunar meteorite Northwest Africa 4472: A KREEPy regolith breccia from the Moon |
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| Authors: | KH Joy R Burgess VA Fernandes IA Crawford AT Kearsley EIMFg |
| Institution: | a Department of Earth and Planetary Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK b The Centre for Planetary Sciences at UCL/Birkbeck, Department of Earth Sciences, Gower Street, London WC1E 6BT, UK c IARC, Department of Mineralogy, The Natural History Museum, Cromwell Road, London, UK d Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, TX 77058, USA e NASA Lunar Science Institute, CA, USA f School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK g Edinburgh Ion Microprobe Facility, Grant Institute, School of Geosciences, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, UK h Berkeley Geochronological Center, 2455 Ridge Road, Berkeley, CA 94709, USA i Department of Earth and Planetary Sciences, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720-4767, USA j Physics Institute, Space Research & Planetary Sciences, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland k University of Washington, 4000 15th Avenue NE, Seattle, WA 98195-1310, USA |
| Abstract: | Northwest Africa (NWA) 4472 is a polymict lunar regolith meteorite. The sample is KREEP-rich (high concentrations of potassium, rare earth elements and phosphorus) and comprises a heterogeneous array of lithic and mineral fragments. These clasts and mineral fragments were sourced from a range of lunar rock types including the lunar High Magnesian Suite, the High Alkali Suite, KREEP basalts, mare basalts and a variety of impact crater environments. The KREEP-rich nature of NWA 4472 indicates that the sample was ejected from regolith on the nearside of the Moon in the Procellarum KREEP Terrane and we have used Lunar Prospector gamma-ray remote sensing data to show that the meteorite is most similar to (and most likely sourced from) regoliths adjacent to the Imbrium impact basin.U-Pb and Pb-Pb age dates of NWA 4472 phosphate phases reveal that the breccia has sampled Pre-Nectarian (4.35 Ga) rocks related to early episodes of KREEP driven magmatism. Some younger phosphate U-Pb and Pb-Pb age dates are likely indicative of impact resetting events at 3.9-4 Ga, consistent with the suggested timing of basin formation on the Moon. Our study also shows that NWA 4472 has sampled impact melts and glass with an alkali-depleted, incompatible trace element-rich (high Sc, low Rb/Th ratios, low K) compositional signature not related to typical Apollo high-K KREEP, or that sampled by KREEPy lunar meteorite Sayh al Uhaymir (SaU) 169. This provides evidence that there are numerous sources of KREEP-rich protoliths on the Moon. |
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