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.
A part of one of the rock samples collected on Apollo 12, lunar sample 12013, has a composition which is remarkably similar to some tektites. It is especially similar to high-magnesium javenites (part of the Australasian strewn field). Sample 12013 is inhomogenous in that it is composed of two types of materials, light and dark. The light, acidic portion is composed of up to 71 percent silicon dioxide. The dark portion resembles KREEP rocks. The abundances of 20 of 23 elements tested from the acidic portion of the sample showed a striking similarity to high-magnesium tektites. The major elements matched well, the minor and trace elements did not. However, other lunar samples matched some microtektites very well.
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.