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Building cratonic keels in Precambrian plate tectonics

Abstract

The ancient cores of continents (cratons) are underlain by mantle keels—volumes of melt-depleted, mechanically resistant, buoyant and diamondiferous mantle up to 350 kilometres thick, which have remained isolated from the hotter and denser convecting mantle for more than two billion years. Mantle keels formed only in the Early Earth (approximately 1.5 to 3.5 billion years ago in the Precambrian eon); they have no modern analogues1,2,3,4. Many keels show layering in terms of degree of melt depletion5,6,7. The origin of such layered lithosphere remains unknown and may be indicative of a global tectonics mode (plate rather than plume tectonics) operating in the Early Earth. Here we investigate the possible origin of mantle keels using models of oceanic subduction followed by arc-continent collision at increased mantle temperatures (150–250 degrees Celsius higher than the present-day values). We demonstrate that after Archaean plate tectonics began, the hot, ductile, positively buoyant, melt-depleted sublithospheric mantle layer located under subducting oceanic plates was unable to subduct together with the slab. The moving slab left behind craton-scale emplacements of viscous protokeel beneath adjacent continental domains. Estimates of the thickness of this sublithospheric depleted mantle show that this mechanism was efficient at the time of the major statistical maxima of cratonic lithosphere ages. Subsequent conductive cooling of these protokeels would produce mantle keels with their low modern temperatures, which are suitable for diamond formation. Precambrian subduction of oceanic plates with highly depleted mantle is thus a prerequisite for the formation of thick layered lithosphere under the continents, which permitted their longevity and survival in subsequent plate tectonic processes.

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Fig. 1: Development of subduction/collision-induced mantle keel at elevated mantle temperature ΔT = 200 °C (Tp = 1,500 °C).
Fig. 2: Effect of mantle temperature on the development of mantle keels in subduction/collision zones.
Fig. 3: Compositional stratification of lithospheric keels beneath well known cratons compared to the predictions of the model.
Fig. 4: Comparison of estimated mantle potential temperature, thickness of depleted oceanic sublithospheric ages and cratonic ages.
Fig. 5: A dynamic model of cratonic keel formation.

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Data availability

All input and output files used in the petrologic thermal-mechanical modelling are available on request.

Code availability

The numerical code I2VIS and MatLab code used for the calculations are available at https://doi.org/10.17605/OSF.IO/SYJF7.

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Acknowledgements

This work was supported by SNF project IZSEZO-189211 (to A.L.P.), by SNF projects 200021_182069 and 200021_192296 (to T.V.G.) and by RFBR project 20-05-00329 (to A.L.P.). The simulations were performed on the ETH-Zurich Euler and Leonhard cluster and on the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University. This is contribution 1531 from the ARC Centre of Excellence for Core to Crust Fluid Systems (www.ccfs.mq.edu.au; W.L.G.) and 1404 from the GEMOC Key Centre (www.gemoc.mq.edu.au; W.L.G.), and is related to IGCP-662 (W.L.G.).

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A.L.P. conceived and designed the study and conducted some of the numerical experiments; T.V.G. programed the numerical code and designed boundary conditions for the models; V.S.Z. programmed an automated input of varied model geometry and parameters and conducted some of the numerical experiments; and W.L.G. compiled and annotated Extended Data Figs. 4–7 and provided related text. All authors discussed the results, problems and methods, and contributed to interpretation of the data and writing the paper.

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Correspondence to A. L. Perchuk.

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Extended data figures and tables

Extended Data Fig. 1 Design and boundary conditions of the numerical model.

White lines are isotherms shown for increments of 200 °C starting from 100 °C. Colours indicate materials (for example, rock type or melt). Mantle with a degree of melt-depletion of more than 20% is shown in dark blue. Viscous underplate source mantle (T > 1,300 °C, melt depletion >20%) is outlined in magenta for better visibility. Model parameters are for elevated mantle potential temperature (Tp) of 1,500 °C (ΔT = 200 °C). The zoomed-in area shows the prescribed incipient subduction zones. The colour key for different materials is shown at the bottom.

Extended Data Fig. 2 The development of the protokeel during subduction at elevated mantle potential temperature.

The evolution of the effective viscosity (left column, panels ad) and density (right column, panels eh) fields computed for the reference model (Fig. 1) at elevated mantle potential temperature of Tp = 1,500 °C (ΔT = 200 °C) is shown. Arrows in the left column show evolution of the velocity field. ‘lg’ is used for the decimal logarithm.

Extended Data Fig. 3 Termination of the protokeel detachment from the slab after the beginning of arc-continent collision.

The evolution of the experiment shown in Fig. 2a, b is shown (40-Myr-old lithosphere, subducting plate velocity of 5 cm yr−1, elevated mantle potential temperature (Tp = 1,550 °C, ΔT = 250 °C)) for longer experiment run times. a, Growth of basaltic arc on the former oceanic crust at 18.2 Myr ago. b, Growth of the arc composed of basaltic and felsic volcanic rocks derived by fluid-fluxed melting of the mantle wedge and melting of the hydrated slab, respectively, at 19.8 Myr ago. We note the preservation of the protokeel thickness and its underplating by hydrated diapirs derived from slab fragments. The reduced degree of decompression melting (narrow red zones) is due to the strong upper mantle depletion. The colour key is shown in Fig. 1. The protokeel source mantle (T > 1,300 °C, melt depletion >20%) under the subducting plate is outlined in magenta for better visibility.

Extended Data Fig. 4 Structure of the lithospheric keel beneath the Daldyn Kimberlite Field, Siberian craton, Russia60,62.

A pronounced highly depleted layer extends about 140–190 km depth, and is progressively melt-metasomatized towards its base. This is overlain by a less depleted layer that still has very magnesian olivine but shows a strong trend towards decreasing XMgOliv (MgO/(MgO+FeO) in olivine) with depth and a marked kink near 140 km depth. Chromite is most abundant and most Cr-rich around 170–180 km. The highly depleted root may have extended to about 220–230 km depth. The nature of the stratification, whether primary or metasomatically overprinted, was evaluated using subsidiary data including the distribution of chromites, and profiles of whole-rock Al2O3 (estimated from Cr and/or Y contents of garnets) and XMgOliv calculated from garnet data89. The colour key shows rock types based on major- and trace-element patterns in garnet xenocrysts (see Methods). Harzburgites are defined as having mineral assemblages of olivine+opx+garnet ± chromite. Depleted lherzolites have minor clinopyroxene and depleted trace-element patterns. Depleted/metasomatized lherzolites contain minor clinopyroxene but have metasomatically enriched trace-element signatures. Fertile lherzolites contain abundant clinopyroxene and have been highly enriched in trace elements by (usually carbonatitic) metasomatism. Melt-metasomatism results in a rapid decrease in XMgOliv, and increases in Zr and Ti, ascribed to percolation of mafic melts and related fluids. LAB, lithosphere–asthenosphere boundary. %Cr2O3 and %TiO2 indicate weight per cent of these oxides in chromite.

Extended Data Fig. 5 Structure of the lithospheric keel beneath the Lac de Gras area, Slave craton, northern Canada72.

This is one of the most striking examples of a layered sub-cratonic lithospheric mantle. An ultradepleted layer extends <100–150 km, where there is a sharp boundary to a more fertile layer, with a high degree of melt-related metasomatism14. The Al2O3-enriched boundary zone corresponds to a high concentration of eclogites, which are notably diamond-rich. Re-Os data indicate that the upper layer experienced depletion around 3.4 Ga, and the lower layer at about 3.27 Ga (refs. 78,79). Chromite is most abundant, and most Cr-rich, but also most Ti-rich, just below the boundary between layers. Lower-mantle diamonds59 are regarded as evidence that the deeper layer represents a plume head, but other interpretations are possible14,81,82.

Extended Data Fig. 6 Structure of the lithospheric keels beneath South Africa.

a, The Limpopo Belt area5. The section shows an ultradepleted layer from 140–180 km depth, overlain by a more fertile layer with high XMgOliv (refs. 5,29,30). The deeper part is moderately melt-metasomatized with the introduction of Al and Fe, corresponding to sheared lherzolites, but chromite is most abundant at 170–190 km depth, and the depleted root may originally have extended to depth of about 210 km. Van der Meer et al.83 have verified the structure with xenolith studies and suggest that the two layers have distinct provenances. b, The northern Lesotho area. Harzburgites are mostly confined to the more fertile layer (high Al2O3) above 120 km, but the section is dominated by depleted lherzolites from 140–180 km depth, and the base is marked by a dominance of lherzolites produced by intense melt-related metasomatism. c, The northern Botswana area. A relatively depleted (but mainly lherzolitic) section from 120–190 km depth is overlain by more fertile (higher-Al2O3) rocks, but with similarly high XMgOliv more characteristic of depleted rocks. This suggests that the upper layer in this case represents the refertilization of a depleted section, rather than a separate unit.

Extended Data Fig. 7 Structure of lithospheric keel beneath the eastern Gawler Craton, South Australia81.

The upper part of the section is relatively fertile, with high whole-rock Al2O3 but magnesian olivine. It is sharply underlain at about 140 km by a more harzburgite-rich section with lower Al2O3 but also lower XMgOliv. Chromite is more abundant and more Cr-rich in the lower layer, but also has higher mean TiO2, reflecting the increasing melt-related metasomatism in this layer towards greater depth. The presence of super-deep diamonds in the kimberlites suggests that the deeper layer may be plume-related85.

Extended Data Fig. 8 Protokeel formation in the reference model after switching off the prescribed plate convergence.

The reference model is described in Fig. 1: Tp = 1,500 °C, ΔT = 200 °C. ad, Slab-pull controlled retreating subduction and slab break-off of the oceanic plate with highly depleted mantle. Prescribed convergence is switched off at 4.8 Myr after the beginning of the experiment; eh, Building of the mantle protokeel during slab-pull controlled retreating subduction and slab break-off of the oceanic plate with highly depleted mantle after switching of the prescribed convergence at 8.4 Myr from the beginning of the experiment. Dotted lines indicate boundaries of the mantle transition zone. We note the reduced volume of the protokeel in h (see Fig. 1d). The colour-code is shown in Fig. 1. Protokeel source mantle (>1,300 °C, melt depletion >20%) under subducting plate is outlined by magenta colour for better visibility.

Extended Data Table 1 Conditions and results for the two-dimensional numerical experiments
Extended Data Table 2 Physical properties of materials used in the numerical experiments

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Perchuk, A.L., Gerya, T.V., Zakharov, V.S. et al. Building cratonic keels in Precambrian plate tectonics. Nature 586, 395–401 (2020). https://doi.org/10.1038/s41586-020-2806-7

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