KREEP
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KREEP stands for potassium (atomic symbol K), rare earth elements (REE), and phosphorus (P). KREEP-rich basalts are found on the Earth's Moon. KREEPs are also enriched in uranium and thorium.
More than 4.5 billion years ago the surface of the Moon was a liquid magma ocean. It is believed that KREEPs represent the last chemical remnants of that magma ocean after the lunar crust formed. KREEPs floated to the surface because their component elements are "incompatible", that is, they did not incorporate into compact crystal structures. KREEPs are used by scientists to trace the volcanic history of the Moon and to record the impacts by meteorites.
Results from Lunar Prospector's gamma ray spectrometer shows that KREEP-containing rocks are concentrated in the Mare Imbrium rim, the nearside maria and highlands near Imbrium and the Mare Ingenii South Pole-Aitken basin and are distributed at a lower level in the highlands. The distribution seen by Lunar Prospector supports the idea that the impact which formed Mare Imbrium excavated KREEP-rich rocks and ejected them over the Moon and the South Pole-Aitkin basin impact also exposed KREEP-rich material.