KREEP
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KREEP is an acronym used in geochemistry to represent a mixture of K-potassium, REE-rare earth elements, and P-phosphorus. It is not only the main source of these elements on the moon, but also many other trace elements such as uranium, thorium, fluorine, chlorine, and zirconium. Many scientists consider this component of lunar rocks to be the remnant of a magma ocean which is widely accepted to have covered the surface of the moon approximately 4.5 billion years ago after the moon formed from debris cast out in a cataclysmic collision between earth and another celestial body. It is considered a convenient tracer for researchers seeking to chronicle the volcanic history of the lunar surface, as well as impacts of celestial bodies.
KREEP is a composite of what scientists term "incompatible elements": elements which cannot fit into the crystal structures formed by cooling magma. This resulted in a KREEP rich magma sandwiched between the floating anorthositic plagioclase of the lunar crust and the precipitating minerals that formed the mantle. although is was common belief that this process would form an evenly distributed global layer of KREEP magma, the Gamma Ray Spectrometer aboard Lunar Prospector shows that KREEP is mainly concentrated in the area of Oceanus Procellarum and Mare Imbrium. This unusual geological region is now referred to as the Procellarum KREEP Terrane. Deep penetrating impact basins away from this region show only modest or no KREEP concentrations in their ejecta and rims.
KREEP is kicked up in the ejecta or "splash out" produced in impacts taking place on or in close proximity to basaltic lava flows, as well as in major impacts which penetrate the crust. Some of this splash out was brought back by the Apollo 15 mission in fragments generally smaller than 1cm. These samples are believed to have originated after the impact which formed the Imbrium basin[1].
