Nature - USA (2020-10-15)

400 | Nature | Vol 586 | 15 October 2020

Article

of different cratons^35 ,^36. Ages of these deep-seated events usually (but
not always) correspond to the ages of thermal/tectonic events in the
overlying crust, suggesting linkages between crust and mantle of cra-
tons^36. In the case of subduction-induced viscous underplating, the
overlying older lithospheric mantle could be linked with the crust,
whereas the protokeel should be somewhat younger than the crust.
Indeed some tectono-magmatic crustal reworking of pre-existing
continental crust could be expected at cratonic margins during the
arc-continent collision that terminates both oceanic subduction and
viscous underplating (Fig. 1d).

Mantle keels are of complex origin

This study presents a new geotectonic mechanism: viscous underplat-
ing and the thickening of cratonic lithosphere, related to early plate
tectonics. This mechanism requires greatly elevated mantle potential
temperatures (≥1,500 °C, Fig. 4a) and has a very different physical origin
to modern plate tectonics. The mechanism of viscous underplating of

depleted oceanic mantle beneath arrested protocratonic uppermost

mantle implies that many cratonic mantle keels would be stratified by

degree of depletion, and this feature could be a direct consequence of

the onset of Archaean subduction and plate tectonics at about 3.5–3.0 Ga

(ref.^37 ) (Fig.  4 ). Major parts of these keels were formed in proximity to

palaeosubduction zones and thus they are of rather foreign and distant

origin relative to the continental crust of the cratons. However, this

mechanism does not obliterate the record preserved in pre-existing

subcontinental lithospheric mantle, which also contributes to the for-

mation of the keel (Fig. 1c). Furthermore, the laterally emplaced ‘new’

depleted material would contribute to the stability and preservation

of the overlying continental crust (a ‘life-raft’ model^13 ). Mantle keels

thus may be of complex origin, built up at different times and through

different processes (for example, see refs.^21 ,^38 ,^39 ). It seems that the two

last stages of mantle keel evolution—viscous underplating during sub-

duction, followed by conductive cooling (Fig. 5b, c)—are becoming

clearer, whereas the earlier stages (Fig. 5a)—related to the formation

of continental crust and its uppermost mantle—remain unknown and

controversial.

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availability are available at https://doi.org/10.1038/s41586-020-2806-7.

1. Arndt, N. T. et al. Origin of Archean subcontinental lithospheric mantle: some
   petrological constraints. Lithos 109 , 61–71 (2009).


2. Peslier, A. H. et al. Olivine water contents in the continental lithosphere and the longevity
   of cratons. Nature 467 , 78–81 (2010).


3. Lee, C. T. A., Luffi, P. & Chin, E. J. Building and destroying continental mantle. Annu. Rev.
   Earth Planet. Sci. 39 , 59–90 (2011).


4. Eaton, D. W. & Perry, H. K. C. Ephemeral isopycnicity of cratonic mantle keels. Nat. Geosci.
   6 , 967–970 (2013).


5. Griffin, W. L. et al. The evolution of lithospheric mantle beneath the Kalahari craton and its
   margins. Lithos 71 , 215–241 (2003a).


6. Griffin, W. L. et al. The origin and evolution of Archaean lithospheric mantle. Precambr.
   Res. 127 , 19–41 (2003b).


7. Yuan, H. & Romanowicz, B. Lithospheric layering in the North American craton. Nature
   466 , 1063–1068 (2010).


8. Herzberg, C. et al. Temperatures in ambient mantle and plumes: constraints from basalts,
   picrites, and komatiites. Geochem. Geophys. Geosyst. 8 , Q02006 (2007).


9. Condie, K. C. & Kroner, A. When did plate tectonics begin? Evidence from the geologic
   record. In When Did Plate Tectonics Begin On Planet Earth? (eds Condie, K. C. & Pease, V.)
   440, 281–294 (Geological Society of America, 2008).


10. Korenaga, J. Initiation and evolution of plate tectonics on Earth: theories and
    observations. Annu. Rev. Earth Planet. Sci. 41 , 117–151 (2013).


11. Gerya, T. V. Precambrian geodynamics: concepts and models. Gondwana Res. 25 ,
    442–463 (2014).


12. Dhuime, B., Wuestefeld, A. & Hawkesworth, C. J. Emergence of modern continental crust
    about 3 billion years ago. Nat. Geosci. 8 , 552–555 (2015).


13. Griffin, W. L. et al. The composition and evolution of lithospheric mantle: a re-evaluation
    and its tectonic implications. J. Petrol. 50 , 1185–1204 (2009).


14. Griffin, W. L., O’Reilly, S. Y. & Ryan, C. G. The composition and origin of subcontinental
    lithospheric mantle. In Mantle Petrology: Field Observations and High-Pressure
    Experimentation: A Tribute to Francis (eds Fei, Y. et al.) 6, 13–45 (The Geochemical Society,
    1999).


15. Artemieva, I. M. & Mooney, W. D. Thermal thickness and evolution of Precambrian
    lithosphere: a global study. J. Geophys. Res. 106 , 16387–16414 (2001).


16. Boyd, F. R. Compositional distinction between oceanic and cratonic lithosphere. Earth
    Planet. Sci. Lett. 96 , 15–26 (1989).


17. Stein, M. & Hofmann, A. W. Mantle plumes and episodic crustal growth. Nature 372 ,
    63–68 (1994).


18. Davies, G. F. Punctuated tectonic evolution of the Earth. Earth Planet. Sci. Lett. 136 ,
    363–379 (1995).


19. Griffin, W. L. & O’Reilly, S. Y. Cratonic lithospheric mantle: is anything subducted?
    Episodes 30 , 43–53 (2007).


20. Helmstaedt, H. H. & Schulze, D. J. Southern African kimberlites and their mantle sample:
    implications for Archean tectonics and lithosphere evolution. Geol. Soc. Aust. Spec. Publ
    14 , 358–368 (1989).


21. Beall, A. P., Moresi, L. & Cooper, C. M. Formation of cratonic lithosphere during the
    initiation of plate tectonics. Geology 46 , 487–490 (2018).


22. Perchuk, A. L. et al. Hotter mantle but colder subduction in the Precambrian: what are the
    implications? Precambr. Res. 330 , 20–34 (2019).

Graphite

Craton

Graphite

Asthenosphere

protokeel stage

Lithosphere and

depleted asthenosphere

Continent

Asthenosphere

Plumes

Mantle transition

zone Stagnant slabs

Asthenosphere

a Continent Oceanic crust

b

c

Lithosphere

Mantle

keel

Lithosphere

Mantle

protokeel

Viscous underplate source mantle

Diamondiferous area

Lithosphere and

depleted asthenosphere

Viscous underplate

source mantle

Eclogitized

crust

Diamond

Diamond

Fig. 5 | A dynamic model of cratonic keel formation. a, Beginning of plate
tectonics. Early stage of subduction of the oceanic plate with highly depleted
mantle (both lithospheric and asthenospheric). b, Protokeel formation caused
by viscous f low under the continent of depleted mantle originally related to the
oceanic plate. The lower part of the protokeel is formed by diapirs originating
from hydrated stagnant slabs in the mantle transition zone. c, Culmination of
the compositionally stratified mantle keel formation by conductive cooling of
the protokeel structure during secular cooling of the Earth.