Archean-Proterozoic border

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The Archean-Proterozoic boundary marked in the Earth's history the beginning of a profound change process in Geodynamics and environment, around 2500 million years BP started and its effects should be felt about another 500 million years.

term

The Archean - Proterozoic border (abbreviated to AP border), engl. Archean-Proterozoic boundary , can also be called Neoarchean-Paleoproterozoic boundary or Neoarchean-Siderium boundary . In the course of the planned reorganization of the Precambrian , the Archean with the Siderium is expected to end at 2,420 million years. Since the oxygenium follows the siderium in this scheme , the AP limit would have to be referred to as the siderium-oxygenium limit in the future .

introduction

Clearly recognizable is the strong decline in new crust formation at the Archean-Proterozoic boundary (after Condie, 2006)

It is now recognized that new continental crust was formed worldwide in the late Neo-Archean . This is underlined by the frequency distribution of zircon ages , which for all cratons show a clear maximum at 2700 million years BP, which is usually followed by a secondary maximum around 2500 million years BP. It was likely that the Kraton had grown very rapidly and intensively and possibly even the formation of supercontinent ( Superia or Kenorland ). Taylor and McLennan (1985) estimate this rapid crustal growth to be around 40% of today's crustal stock (from 30% around 2,700 million years BP to 70% around 2,500 million years BP).

However, the geodynamic principle that caused this rapid crustal growth remains controversial. Today's crust formation takes place according to the paradigm of plate tectonics at subduction zones , but it is uncertain whether the same processes took place in the late Neo-Archean. In today's plate tectonic regime, crust production and crust recycling (ie subduction) are balanced at 2.5 cubic kilometers / year. A crust-growing process as important as that at the Archean-Proterozoic boundary therefore had to take place relatively quickly in order to protect the newly formed crust from being recycled into the Earth's mantle .

Changes at the Archean-Proterozoic boundary

The geodynamic changes at the Archean-Proterozoic boundary can generally be traced back to two themes:

  • Changes caused by falling temperatures of the earth's mantle
  • Changes as an aftereffect of the crust growth event or the supercontinent formation

Falling jacket temperatures

The following are to be classified under the subject of falling jacket temperatures:

  • Komatiite the greenstone belt :
    The komatiites show a clear decline at the Archean-Proterozoic boundary; their frequency decreases with the beginning of the Proterozoic from more than 15% (around 2700 million years BP) to only around 5% of all volcanic rocks . At the same time, the MgO content of these ultramafic rocks was reduced from originally over 30% to around 22%. The MgO content can, however, be directly correlated with the outlet temperature of the komatiitic lavas and thus indirectly with the temperature of the rising mantle source ( mantle diapir ), with high MgO contents representing high formation temperatures . As a result, the upper mantle had experienced a strong cooling at the AP border.
  • Ribbon Ores :
    The band ores manifest a sudden decrease in their nickel / iron ratio at the AP boundary . Their Ni / Fe values ​​are reduced from 0.0005 to 0.0002. Konhauser et al. (2009) interpret this decline as a reduced nickel input into the world's oceans, caused by falling jacket temperatures, which in turn depressed komatiite production.
  • Incompatible elements :
    Several incompatible elements show an increase in concentration at the AP limit, recognizable by the increased element ratios Nb / Yb, La / Yb, Zr / Y, La / Sm and Gd / Yb in greenstone belt basalts (including basalts of the oceanic ridges, oceanic plateau basalts and oceanic island basalts). This is likely to be due to a reduced melting rate and thus also to lower jacket temperatures.

Aftermath of the supercontinent formation

The after-effects of the supercontinent formation (or the rapid continental growth) include:

  • Increasing Nb / Yb ratio in basalts (except basalts of the island arches ):
    At the AP limit, there was a sudden increase in the peak values ​​of the niobium / ytterbium ratio from 3 to 10 (tripling). For comparison: primitive mantle rocks have Nb / Yb values ​​around 1.4. The Nb / Yb ratio serves as a proxy for continental crust growth, since intensive enamel secretion leaves a depleted trace element signature in the remaining ( restitic ) mantle. This relationship is also expressed in the ratios Nb / Ta and Nb / La.
  • Increase in the ratio Nb / Th and ε Nd (T) in basalts (except basalts of the island arcs) and in associated TTG complexes :
    The peak values ​​in the ratio Nb / Th rise from 13 at the end of the Archean to the HIMU value of 16 by 2000 million years BP (today's peak value is around 20 and corresponds to the value of the depleted earth's mantle ).
    At ε Nd (T), the peak values ​​at the AP limit increase steadily from + 4 to today's value of + 10. Bennett (2006) explains this with a rapid increase in the continental crust towards the end of the Archean, which had severely impaired the Sm / Nd ratio of the mantle springs.
  • Incompatible elements in TTG complexes and in the continental crust:
    Towards the end of the Archean, the concentration of lithophilic elements with large ionic radii ( LILE ) and those with high valences ( HFSE ) in continental crustal rocks increased, while the strontium concentration decreased at the same time . The reason for this is a change from TTG magmas to calcareous magmas . This magma exchange was primarily expressed in an increased K 2 O / Na 2 O ratio (from 0.7 to 0.9) in the paleoproterozoic upper crust. The La / Yb values ​​fell, however, as the mafic parent rocks were now depleted in garnet . The Sr / Y values ​​had also dropped sharply, which could be explained by restitic plagioclase and / or fractional crystallization. The Eu / Eu * ratio due to plagioclase fractionation (defined as: Eu / Eu * = Eu / (Sm × Gd) 1/2 , where the values ​​normalized for chondrites are used) also shows a decrease from 1.05 to 0.65 .
  • Increase in the δ 18 O value in granitic zirconia:
    At the AP limit, the maximum δ 18 O values rise steadily, starting from 7.5 ‰ VSMOW, to reach 11.0 ‰ VSMOW at the end of the Proterozoic. Since δ 18 O values react very sensitively to weathering and to rocks that have come into contact with the hydrosphere, such as shale , conclusions can be drawn about the incorporation of a clear, sedimentary component in the granitic magma formation from the Proterozoic onwards. It is possible that in the Archean a much more effective recycling of the newly formed, Si-rich crustal rocks restricted longer contact with the hydrosphere.
  • Gold :
    The orogenic (orogen type) and also those bound to massive sulphides (VHMS type) gold deposits pass through an absolute maximum between 2700 and 2600 million years, and then go back to zero by 2300 million years BP. Orogenic and VHMS gold deposits (VHMS - volcanic-hosted massive sulfides ) are directly linked to subduction processes and the creation of juvenile crust. The IOCG type (IOCG - iron oxide copper-gold ), which is linked to the rift association , does not appear until later around 2,600 million years BP in the Carajas deposit in Brazil .
  • Crust thickening:
    Between 2700 and 2650, a thickening of the earth's crust can be determined for the first time, datable using the Re-Os depletion age T RD , which reach a clear maximum.
  • Island arch basalts:
    The island arch basalts increase in frequency from 2700 million years onwards. If at the beginning of the Neo-Archaic period they had only placed about 30% under all basalts, their share at the AP limit had already increased to a little more than 60%.

Individual evidence

  1. ^ Felix M. Gradstein et al .: On the Geologic Time Scale . In: Newsletters on Stratigraphy . tape 45/2 , 2012, p. 171-188 .
  2. ^ KC Condie: Episodic continental growth and supercontinents: a mantle avalanche connection? In: Earth and Planetary Science Letters . tape 163 , 1998, pp. 97-108 .
  3. ^ A b S. R. Taylor, SM McLennan: The Continental Crust: Composition and Evolution . Blackwell Scientific Publications, 1985, ISBN 0-632-01148-3 .
  4. ↑ DW Scholl, R. von Huene: Crustal recycling at modern subduction zones applied to the past-Issues of growth and preservation of continental basement crust, mantle geochemistry, and supercontinent reconstruction . In: Geological Society of America, Memoir . tape 200 , 2007, pp. 9-32 .
  5. ^ MJ De Wit, LD Ashwal: Greenstone Belts . Oxford University Press, Oxford 1997, pp. 809 .
  6. ^ NT Arndt, SJ Barnes, CM Lesher: Komatiite . Cambridge University Press, Cambridge, United Kingdom 2008, pp. 488 .
  7. ↑ KO Konhause, E. Pecoits, SV Lalonde, D. Papineau, EG Nisbet, ME Barley, NT Arndt, K. Zahnle, BS Kamber: Oceanic nickel depletion and a methanogen famine before the great oxidation event . In: Nature . tape 458 , 2009, p. 750-754 .
  8. ↑ JA Pearce: Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust . In: Lithos . tape 100 , 2008, p. 14-48 .
  9. ^ PJ Sylvester, IH Campbell, DA Bowyer: Niobium / uranium evidence for early formation of the continental crust . In: Science . tape 275 , 1997, pp. 521-523 .
  10. ^ KC Condie: Incompatible element ratios in oceanic basalts and komatiites: tracking deep mantle sources and continental growth rates with time . In: Geochemistry Geophysics Geosystems . tape 4 (1) , 2003, p. 1005 .
  11. ^ VC Bennett: Compositional evolution of the mantle . In: Treatise on Geochemistry . v. 4, chapter 13, 2004, pp. 493-519 .
  12. ^ KC Condie: Did the character of subduction change at the end of the Archean? Constraints from convergent-margin granitoids . In: Geology . tape 36 (8) , 2008, pp. 611-614 .
  13. ↑ JW Valley, JS Lackey, AJ Cavosie, CC Clechenko, MJ Spicuzza, MAS Basei, IN Bindeman, VP Ferreira, AN Sial, EM King, WH Peck, AK Sinha, CS Wei: 4.4 years billion of crustal maturation: oxygen isotope ratios of magmatic zircon . In: Contributions to Mineralogy and Petrology . tape 150 , 2005, pp. 561-580 .
  14. ↑ DI Groves, RM Much rich, RJ Goldfarb, KC Condie: Controls on the heterogeneous distribution of mineral deposits through time . In: Geological Society, London, Special Publication . tape 248 , 2005, pp. 71-101 .
  15. ^ DG Pearson, RW Carlson, SB Shirey, FR Boyd, PH Nixon: Stabilization of Archean lithospheric mantle: A Re-Os isotope study of peridotite xenoliths from the Kaapvaal craton . In: Earth and Planetary Science Letters . tape 134 , 1995, pp. 341-357 .
  16. ^ KC Condie: High field strength element ratios in Archean basalts: a window to evolving sources of mantle plumes . In: Lithos . tape 79 , 2005, pp. 491-504 .