How continental keels and cratons may have formed

There is Byzantine ring to the word craton: hardly surprising as it stems from the Greek kratos meaning ‘might’ or ‘strength’. Yes, the ancient cores of the continents were well named, for they are mighty. Some continents, such as Africa, have several of them: probably relics of very ancient supercontinents that have split and spread again and again. Cratons overlie what are almost literally the ‘keels’ of continents. Unlike other mantle lithosphere beneath continental crust (150 km on average) cratonic lithosphere extends down to 350 km and is rigid. Upper mantle rocks at that depth elsewhere are mechanically weaker and constitute the asthenosphere. Geologists only have evidence from the near-surface on which to base ideas of how cratons formed. Their exposed rocks are always Precambrian in age, from 1.5 to 3.5 billion years old, though in some cases they are covered by a thin veneer of later sedimentary rocks that show little sign of deformation. No cratons formed after the Palaeoproterozoic and they are the main repositories of Archaean rock. Their crust is thicker than elsewhere and dominated at the surface by crystalline rocks of roughly granitic composition. Cratons have the lowest amount of heat flowing out from the Earth’s interior; i.e. heat produced by the decay of long-lived radioactive isotopes of uranium, thorium and potassium. This relative coolness provides an explanation for the rigidity of cratons relative to younger continental lithosphere. Because granitic rocks are well-endowed with heat-producing isotopes, the implication of low heat flow is that the deeper parts of the crust are strongly depleted in them. As a result the deep mantle in cratonic keels is at higher pressure and lower temperature than elsewhere beneath the continental surface. Ideal conditions for the formation of diamonds in mantle rock, so that cratonic keels are their main source – they get to the surface in magma pipes when small amounts of partial melting take place in the lithospheric mantle.

The low heat flow through cratons beckons the idea that the heat-producing elements U, Th and K were at some stage driven from depth. An attractive hypothesis is that they were carried in low-density granitic magmas formed by partial melting of mantle lithosphere during the Precambrian that rose to form continental crust. Yet there is an abundance of younger granite plutons that are associated with thinner continental lithosphere. This seeming paradox suggests different kinds of magmagenesis and tectonics during the early Precambrian. Russian and Australian geoscientists have proposed an ingenious explanation (Perchuk, A.L. et al. 2020. Building cratonic keels in Precambrian plate tectonics. Nature, v. 586, p. 395-401; DOI: 10.1038/s41586-020-2806-7). The key to their hypothesis lies in the 2-layered nature of mantle keels beneath cratons, as revealed by seismic studies. Modelling of the data suggests that the layering resulted from different degrees of partial melting in the upper mantle during Precambrian subduction.

Development of a cratonic keel from melt-depleted lithospheric mantle during early Precambrian subduction. Mantle temperature is 250°C higher than it is today. The oceanic lithosphere being subducted in (a) has become a series of stagnant slabs in (b) (credit: Perchuk et al.; Fig. 2)

Perchuk et al. suggest that high degrees of partial melting of mantle associated with subduction zones produced the bulk of magma that formed the Archaean and Palaeoproterozoic crust. This helps explain large differences between the bulk compositions of ancient and more recent continental crust, which involves less melting. The residue left by high degrees of melting of mantle rock in the early Precambrian would have had a lower density than the rest of the mantle. While older oceanic crust at ancient subduction zones would be transformed to a state denser than the mantle as a whole and thus able to sink, this depleted lithospheric mantle would not. In its hot ductile state following partial melting, this mantle would be ‘peeled’ from the associated oceanic crust to be emplaced below. The figure shows one of several outcomes of a complex magmatic-thermomechanical model ‘driven’ by assumed Archaean conditions in the upper mantle and lithosphere An excellent summary of modern ideas on the start of plate tectonics and evolution of the continents is given by:Hawkesworth, C.J., Cawood, P.A. & Dhuime, B. 2020. The evolution of the continental crust and the onset of plate tectonics. In Topic: The early Earth crust and its formation, Frontiers in Earth Sciences; DOI: 10.3389/feart.2020.00326

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