KREEP
Encyclopedia
KREEP, an acronym built from the letters K (the atomic symbol
for potassium
), REE (Rare Earth Elements
) and P (for phosphorus
), is a geochemical
component of some lunar impact melt breccia
and basalt
rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements (those that prefer a liquid state during magma crystallization
) and the heat-producing element
s potassium, uranium
and thorium
.
Canonical composition of KREEP includes 1% (wt) of potasium and phosphorus oxides, 20-25 ppm of Rb, and concentrations of Lanthanum that are 300-350 times the concentrations found in chondrites.
Indirectly, the origin of KREEP is a result of the Moon's origin, which is now commonly believed to be the result of a Mars-sized object that impacted the Earth
4.5 billion years ago. This impact put a large amount of material into circum-terrestrial orbit that ultimately re-accreted to form the Moon. Given the large amount of energy that was liberated in this event, it is predicted that a large portion of the Moon would have initially been molten, forming a near-global magma ocean
. As crystallization of this magma ocean proceeded, minerals such as olivine
and pyroxene
would have precipitated and sunk to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase
would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a "KREEP"-rich magma that initially should have been sandwiched between the crust and mantle. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.
Before the Lunar Prospector
mission, it was commonly thought that KREEP-rich materials would have formed a near global layer beneath the crust. However, results from the gamma ray
spectrometer
on this mission show that KREEP-containing rocks are primarily concentrated within the region of Oceanus Procellarum
and Mare Imbrium
, a unique geological province that is now known as the Procellarum KREEP Terrane. Basins far from this province that excavated deep into the crust (and possibly mantle) such as the Crisium
, Orientale
, and South Pole-Aitken
show only modest, or no, enhancements of KREEP within their rims or ejecta. The enhancement of heat producing elements within the crust (and/or mantle) of the Procellarum KREEP Terrane is almost certainly responsible for the longevity and intensity of mare volcanism
on the nearside of the Moon.
Chemical symbol
A chemical symbol is a 1- or 2-letter internationally agreed code for a chemical element, usually derived from the name of the element, often in Latin. Only the first letter is capitalised...
for potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
), REE (Rare Earth Elements
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
) and P (for phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
), is a geochemical
Geochemistry
The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
component of some lunar impact melt breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
and basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
rocks. Its most significant feature is somewhat enhanced concentration of a majority of so-called "incompatible" elements (those that prefer a liquid state during magma crystallization
Igneous differentiation
In geology, igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement or eruption.-Primary melts:...
) and the heat-producing element
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
s potassium, uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
and thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
.
Canonical composition of KREEP includes 1% (wt) of potasium and phosphorus oxides, 20-25 ppm of Rb, and concentrations of Lanthanum that are 300-350 times the concentrations found in chondrites.
Indirectly, the origin of KREEP is a result of the Moon's origin, which is now commonly believed to be the result of a Mars-sized object that impacted the Earth
Giant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
4.5 billion years ago. This impact put a large amount of material into circum-terrestrial orbit that ultimately re-accreted to form the Moon. Given the large amount of energy that was liberated in this event, it is predicted that a large portion of the Moon would have initially been molten, forming a near-global magma ocean
Lunar magma ocean
According to the giant impact hypothesis a large amount of energy was liberated in the formation of the Moon and it is predicted that as a result a large portion of the Moon was once completely molten, forming a lunar magma ocean...
. As crystallization of this magma ocean proceeded, minerals such as olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
would have precipitated and sunk to form the lunar mantle. After crystallization was about three-quarters complete, anorthositic plagioclase
Anorthite
Anorthite is the calcium endmember of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8.-Mineralogy :...
would have begun to crystallize, and because of its low density, float, forming an anorthositic crust. Importantly, elements that are incompatible (i.e., those that partition preferentially into the liquid phase) would have been progressively concentrated into the magma as crystallization progressed, forming a "KREEP"-rich magma that initially should have been sandwiched between the crust and mantle. Evidence for this scenario comes from the highly anorthositic composition of the lunar highland crust, as well as the existence of KREEP-rich materials.
Before the Lunar Prospector
Lunar Prospector
The Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
mission, it was commonly thought that KREEP-rich materials would have formed a near global layer beneath the crust. However, results from the gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
on this mission show that KREEP-containing rocks are primarily concentrated within the region of Oceanus Procellarum
Oceanus Procellarum
Oceanus Procellarum is a vast lunar mare on the western edge of the near side of the Earth's Moon. Its name derives from the old superstition that its appearance during the second quarter heralded bad weather...
and Mare Imbrium
Mare Imbrium
Mare Imbrium, Latin for "Sea of Showers" or "Sea of Rains", is a vast lunar mare filling a basin on Earth's Moon and one of the larger craters in the Solar System. Mare Imbrium was created when lava flooded the giant crater formed when a very large object hit the Moon long ago...
, a unique geological province that is now known as the Procellarum KREEP Terrane. Basins far from this province that excavated deep into the crust (and possibly mantle) such as the Crisium
Mare Crisium
Mare Crisium is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. This basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. This mare is in diameter, and 176,000 km2 in area. It has a very flat floor, with a ring of wrinkled ridges...
, Orientale
Mare Orientale
Mare Orientale is one of the most striking large scale lunar features, resembling a target ring bull's-eye. Located on the extreme western edge of the lunar nearside, this impact basin is difficult to see from an Earthbound perspective.Material from this basin was not sampled by the Apollo program...
, and South Pole-Aitken
South Pole-Aitken basin
The South Pole-Aitken basin is an impact crater on Earth's Moon. Roughly in diameter and deep, it is one of the largest known impact craters in the Solar System. It is the largest, oldest and deepest basin recognized on the Moon. This moon basin was named for two features on opposing sides; the...
show only modest, or no, enhancements of KREEP within their rims or ejecta. The enhancement of heat producing elements within the crust (and/or mantle) of the Procellarum KREEP Terrane is almost certainly responsible for the longevity and intensity of mare volcanism
Lunar mare
The lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
on the nearside of the Moon.
See also
- Geology of the MoonGeology of the MoonThe geology of the Moon is quite different from that of the Earth...
- Lunar mareLunar mareThe lunar maria are large, dark, basaltic plains on Earth's Moon, formed by ancient volcanic eruptions. They were dubbed maria, Latin for "seas", by early astronomers who mistook them for actual seas. They are less reflective than the "highlands" as a result of their iron-rich compositions, and...
- MoonMoonThe Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
- Lunar ProspectorLunar ProspectorThe Lunar Prospector mission was the third selected by NASA for full development and construction as part of the Discovery Program. At a cost of $62.8 million, the 19-month mission was designed for a low polar orbit investigation of the Moon, including mapping of surface composition and possible...
External links
- Moon articles in Planetary Science Research Discoveries, including articles about KREEP