CREEP

from Wikipedia, the free encyclopedia
KREEP basalt from near the Montes Apenninus .

KREEP is a geochemical component of different rocks of the earth's moon , which was directly detected in corresponding samples brought to earth as part of the Apollo missions as well as lunar meteorites found on earth. KREEP stands for an increased content of so-called incompatible elements , i.e. H. chemical elements , the incorporation of which into the ion lattice of the minerals olivine , pyroxene and plagioclase , which are generally typical of lunar rocks, was strongly inhibited due to an unfavorable ion radius during crystallization from the liquid interior of the moon during the early geological history of the moon. Rock of earthly origin with the KREEP signature is not yet known.

etymology

SEE profiles for KREEP-rich moon rock samples. The negative europium anomaly, which also occurs in numerous terrestrial rocks, is clearly recognizable.

KREEP is an acronym for K alium, R are E arth E lements ( rare earth elements, SEE ) and P hosphorus. The name was coined in 1971 by NASA geologist Norman J. Hubbard and colleagues in a scientific essay on the chemical composition of regolith samples from the area around the Apollo 12 landing site.

Characteristic

The KREEP rocks, which are quite variable in terms of the mineral stock, but mostly basaltic, contain around 0.5-3  % by weight of potassium oxide (K 2 O) and phosphorus oxide (P 2 O 5 ), as well as cerium with sometimes more than 1000, neodymium with sometimes significantly more than 100, dysprosium , erbium , lanthanum , rubidium , samarium and ytterbium with less than 100 ppm . The concentration of lanthanum is up to 600 times higher than in chondrites (i.e. in meteorites made from undifferentiated “primordial matter” of the solar system). Also typical for KREEP are a negative europium anomaly (i.e. the rare earth element europium is present in a significantly lower concentration than other rare earth elements) as well as a significantly higher proportion of the radioactive element thorium compared to non-KREEP moon rocks (10th century) -20 ppm).

Classic hypothesis for the formation of KREEP

Differentiation of the outer layers of the moon and the resulting accumulation of KREEP below the crust.

After the discovery of the KREEP signature in the first lunar rock samples, it was assumed that the magmas from which the KREEP rocks on the lunar surface emerged came from a few kilometers thick zone at the base of the lunar crust that formed during the differentiation phase of the lunar interior.

According to current theories, the moon was formed when an object roughly the size of a Mars hit the early earth around 4.5 billion years ago (see also Origin of the Moon ). This impact sent a large amount of terrestrial material into orbit around the earth that eventually formed the moon.

Due to the large amount of energy released during this impact and the subsequent formation of the moon, it can be assumed that a large part of the young moon was liquid. Due to slow cooling in the early pre-Nectarian period (> 4.2 billion years before today), mafic minerals such as olivine and pyroxene initially crystallized out ( fractional crystallization or magmatic differentiation ). These had a higher density than the melt of the magma ocean, sank and began to form the moon's mantle . In the next step feldspars crystallized , especially anorthite , which rose due to their lower density and formed the lunar crust from anorthosite . During these processes, the magma was enriched with incompatible elements that did not enter either the mafic minerals or the feldspars. This ultimately left a layer with the chemical signature typical of KREEP between the anorthosite crust and the mafic lunar mantle (so-called original KREEP ).

Based on this hypothesis for the differentiation of the outer layers of the moon, it would have been expected that the anorthosite crust and the underlying primordial KREEP layer would appear relatively evenly distributed over the entire moon. For the development of today's geology of the moon, the heavy asteroid impacts of the pre-Nectarian, Nectarian and early Imbrian periods (4.1-3.8 billion years before today) as well as the subsequent late-Imbrian and post-Imbrian Mare volcanism (3rd , 8–1.2 billion years ago). The Mare volcanism was mainly seen as the result of the crust thinning as a result of the severe impacts.

Distribution of KREEP on the moon and implications for crust formation

Moon map with the distribution of thorium in the surface rocks (violet tones = low, red tones = high), whereby high thorium concentrations are interpreted as an indicator for significant occurrences of KREEP rocks.

However, the mapping of the thorium concentration in the surface rocks of the moon using the gamma spectroscope of the Lunar Prospector probe showed that KREEP is very unevenly distributed over the moon's surface. An area that extends over Mare Frigoris , Oceanus Procellarum , Mare Imbrium , Mare Cognitum and Mare Nubium is also known as Procellarum-KREEP-Terran (PKT) because there seem to be more KREEP rocks than anywhere else on the moon ( 30–40% of the crustal thorium within about 10% of the lunar crust). This area is strongly influenced by Mare volcanism, but it only contains a part of the Maria and the Mare Crisium , the Mare Orientale or the South Pole Aitken Basin apparently have far less KREEP. This leads to the conclusion that the homogeneous differentiation of the magma ocean with the subsequent highland-mare division of the lunar surface into two represents a greatly simplifying model. Instead, the geological development of the PKT seems to have diverged from that of the surrounding feldspar highland terran (FHT) already during the differentiation phase and the crust of the PKT was more mafic and KREEP-rich than that of the FHT from the start. The reason for this is assumed to be a globally uneven distribution of the feldspars floating on the magma ocean with the formation of an anorthositic "craton" or "supercontinent" (corresponds to today's core area of ​​the FHT), through which mafic, KREEP-rich residual melts were also laterally displaced . The comparatively high content of radioactive elements in and directly below the crust of the PKT, in particular of thorium and uranium, ensured thermal effects, which resulted in both intense and very long-lasting magmatism, including Mare volcanism, and which caused the Occurrence of mare basalts in the PKT could be among the youngest on the moon (approx. 1.2 Ma).

KREEP as SEE ore?

Even if KREEP is repeatedly mentioned as a possible source of raw materials, it should be noted that the content of rare earth metals is far behind the ores that are considered economically degradable on earth. Since there was neither an atmosphere nor liquid water nor plate tectonics on the moon, a locally stronger enrichment of metals in rocks and thus the formation of rich ores was not possible.

literature

Web links

References and comments

  1. ^ Y. Lin, W. Shen, Y. Liu, L. Xu, BA Hofmann, Q. Mao, GQ Tang, F. Wu, XH Li: Very high-K KREEP-rich clasts in the impact melt breccia of the lunar meteorite SaU 169: New constraints on the last residue of the Lunar Magma Ocean . In: Geochimica et Cosmochimica Acta . 85, 2012, pp. 19-40. doi : 10.1016 / j.gca.2012.02.011 .
  2. ↑ G. Jeffrey Taylor: A New Moon for the Twenty First Century . Planetary Science Research Discoveries. August 31, 2000. Retrieved August 11, 2009.
  3. Jump up ^ Charles K. Shearer, Paul C. Hess, Mark A. Wieczorek, Matt E. Pritchard, E. Mark Parmentier, Lars E. Borg, John Longhi, Linda T. Elkins-Tanton, Clive R. Neal, Irene Antonenko, Robin M. Canup, Alex N. Halliday, Tim L. Grove, Bradford H. Hager, DC. Lee, Uwe Wiechert: Thermal and magmatic evolution of the Moon . In: Reviews in Mineralogy and Geochemistry . 60, No. 1, 2006, pp. 365-518. doi : 10.2138 / rmg.2006.60.4 .
  4. ↑ Wilhelms: Geologic history of the Moon , 1987 (see literature ), p. 140
  5. ↑ SB Simon, J. Papike, DC Gosselin: Petrology of Apollo 12 Regolith Breccias . In: Lunar and Planetary Science Conference Abstracts . 16, 1985, pp. 783-784. bibcode : 1985LPI .... 16..783S .
  6. ↑ Norman J. Hubbard, Charles Meyer Jr., Paul W. Gast: The composition and derivation of Apollo 12 soils . In: Earth and Planetary Science Letters . 10, No. 3, 1971, pp. 341-350. doi : 10.1016 / 0012-821X (71) 90040-9 .
  7. ↑ The specification of the concentration of the elements in the form of oxides is common practice in petrology. In fact, the elements are present as silicates. The proportion of the rare earth elements mentioned below is also determined in the form of their oxides, e.g. B. lanthanum oxide (La 2 O 3 ) is determined.
  8. ^ Clive R. Neal, Lawrence A. Taylor: "K-Frac + REEP-Frac": A New Understanding of KREEP in Terms of Granite and Phosphate Petrogenesis . In: Lunar and Planetary Science Conference Abstracts . 19, 1988, pp. 831-832. bibcode : 1988LPI .... 19..831N .
  9. ↑ Clive R. Neal, G. Kramer: The Composition of KREEP: A Detailed Study of KREEP Basalt 15386 . In: 34th Annual Lunar and Planetary Science Conference . . bibcode : 2003LPI .... 34.2023N .
  10. ↑ Graham Ryder: Quenching and disruption of lunar KREEP lava flows by impacts . In: Nature . 336, 1988, pp. 751-754. doi : 10.1038 / 336751a0 .
  11. ^ E. Belbruno, J. Richard Gott III: Where Did The Moon Come From? . In: The Astronomical Journal . 129, No. 3, 2005, pp. 1724-1745. arxiv : astro-ph / 0405372 . bibcode : 2005AJ .... 129.1724B . doi : 10.1086 / 427539 .
  12. ↑ G. Jeffrey Taylor: Gamma Rays, Meteorites, Lunar Samples, and the Composition of the Moon . Planetary Science Research Discoveries. November 22, 2005. Retrieved August 11, 2009.
  13. ^ Mark A. Wieczorek, Bradley L. Jolliff, Amir Khan, Matthew E. Pritchard, Benjamin P. Weiss, James G. Williams, Lon L. Hood, Kevin Righter, Clive R. Neal, Charles K. Shearer, I. Stewart McCallum, Stephanie Tompkins, B. Ray Hawke, Chris Peterson, Jeffrey J. Gillis, Ben Bussey: The constitution and structure of the lunar interior . In: Reviews in Mineralogy and Geochemistry . 60, No. 1, 2006, pp. 221-364. doi : 10.2138 / rmg.2006.60.3 .
  14. ↑ a b H. Hiesinger, JW Head III, U. Wolf, R. Jaumann, G. Neukum: Ages and stratigraphy of mare basalts in Oceanus Procellarum, Mare Nubium, Mare Cognitum, and Mare Insularum . In: Journal of Geophysical Research: Planets . 108, No. E7, 5065, 2003. doi : 10.1029 / 2002JE001985 .
  15. ↑ Wilhelms: Geologic history of the Moon , 1987 (see literature ), u. a. P. 276 f.
  16. ^ TH Prettyman, JJ Hagerty, RC Elphic, WC Feldman, GW Lawrence, DT Vaniman: Elemental composition of the lunar surface: Analysis of gamma ray spectroscopy data from Lunar Prospector . In: Journal of Geophysical Research . 111, E12007, 2006. doi : 10.1029 / 2005JE002656 .
  17. ↑ a b Bradley Jolliff, Jeffrey Gillis, Larry Haskin, Randy Korotev, Mark Wieczorek: Major lunar crustal terranes: Surface expressions and crust-mantle origins . In: Journal of Geophysical Research . 105, No. E2, 2000, pp. 4197-4216. doi : 10.1029 / 1999JE001103 .
  18. ↑ Rare Earth Elements and The Moon: KREEP Basalts. Mining the Moon for Rare Earth Elements - Is It Really Possible? ( Memento of the original from January 23, 2017 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Private website of Robert Beaufort (PhD in Geology and Planetary Science, curator of the University of Arkansas Meteorite Collection).  @1@ 2Template: Webachiv / IABot / robertbeauford.net