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KREEP

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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[1] (those that prefer a liquid state during magma crystallization) and the heat-producing elements potassium, uranium and thorium.[2]

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.[3]

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.[4] This impact put a large amount of material into circum-terrestrial orbit that ultimately re-accreted to form the Moon.[5] 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.[6]

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.[7]

See also

References

  1. ^ G. Jeffrey Taylor (August 31, 2000). "A New Moon for the Twenty-First Century". Planetary Science Research Discoveries. Retrieved August 11, 2009.
  2. ^ Charles Shearer and 15 coauthors, C. K. (2006). "Thermal and magmatic evolution of the Moon". Reviews in Mineralogy and Geochemistry. 60 (1). Mineralogical Society of America: 365–518. doi:10.2138/rmg.2006.60.4. Retrieved August 11, 2009. {{cite journal}}: More than one of |work= and |journal= specified (help)CS1 maint: numeric names: authors list (link)
  3. ^ C. R. Neal, and L. A. Taylor, "K-Frac + REEP-Frac": A New Understanding of KREEP in Terms of Granite and Phosphate Petrogenesis, Abstracts of the Lunar and Planetary Science Conference, volume 19, page 831 (1988)
  4. ^ Belbruno, E.; Gott III, J. Richard (2005). "Where Did The Moon Come From?". The Astronomical Journal. 129 (3): 1724–1745. arXiv:astro-ph/0405372. Bibcode:2005AJ....129.1724B. doi:10.1086/427539.
  5. ^ G. Jeffrey Taylor (November 22, 2005). "Gamma Rays, Meteorites, Lunar Samples, and the Composition of the Moon". Planetary Science Research Discoveries. Retrieved August 11, 2009.
  6. ^ Mark Wieczorek and 15 coauthors, M. A. (2006). "The constitution and structure of the lunar interior". Reviews in Mineralogy and Geochemistry. 60 (1). Mineralogical Society of America: 221–364. doi:10.2138/rmg.2006.60.3. Retrieved August 11, 2009. {{cite journal}}: More than one of |work= and |journal= specified (help)CS1 maint: numeric names: authors list (link)
  7. ^ Jolliff, Bradley; Gillis, Jeffrey; Haskin, Larry; Korotev, Randy; Wieczorek, Mark (2000). "Major lunar crustal terranes: Surface expressions and crust-mantle origins". Journal of Geophysical Research: 4197–4216. doi:10.1029/1999JE001103. Retrieved August 11, 2009.

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