Elsevier

Earth and Planetary Science Letters

Volume 484, 15 February 2018, Pages 1-14
Earth and Planetary Science Letters

Geodynamics of kimberlites on a cooling Earth: Clues to plate tectonic evolution and deep volatile cycles

https://doi.org/10.1016/j.epsl.2017.12.013Get rights and content

Highlights

  • Secular mantle cooling controlled late appearance of global kimberlite magmatism.

  • Preservation had only a minor influence on the global kimberlite age distribution.

  • Frequent kimberlite-forming incipient melting occurred beneath cratons after 2 Ga.

  • Abundant kimberlite magmatism after 1.2 Ga linked to global weakening of cratons.

  • The onset of plate tectonics cannot be inferred from kimberlite age distributions.

Abstract

Kimberlite magmatism has occurred in cratonic regions on every continent. The global age distribution suggests that this form of mantle melting has been more prominent after 1.2 Ga, and notably between 250–50 Ma, than during early Earth history before 2 Ga (i.e., the Paleoproterozoic and Archean). Although preservation bias has been discussed as a possible reason for the skewed kimberlite age distribution, new treatment of an updated global database suggests that the apparent secular evolution of kimberlite and related CO2-rich ultramafic magmatism is genuine and probably coupled to lowering temperatures of Earth's upper mantle through time.

Incipient melting near the CO2- and H2O-bearing peridotite solidus at >200 km depth (1100–1400 °C) is the petrologically most feasible process that can produce high-MgO carbonated silicate melts with enriched trace element concentrations akin to kimberlites. These conditions occur within the convecting asthenospheric mantle directly beneath thick continental lithosphere. In this transient upper mantle source region, variable CHO volatile mixtures control melting of peridotite in the absence of heat anomalies so that low-degree carbonated silicate melts may be permanently present at ambient mantle temperatures below 1400 °C. However, extraction of low-volume melts to Earth's surface requires tectonic triggers. Abrupt changes in the speed and direction of plate motions, such as typified by the dynamics of supercontinent cycles, can be effective in the creation of lithospheric pathways aiding kimberlite magma ascent.

Provided that CO2- and H2O-fluxed deep cratonic keels, which formed parts of larger drifting tectonic plates, existed by 3 Ga or even before, kimberlite volcanism could have been frequent during the Archean. However, we argue that frequent kimberlite magmatism had to await establishment of an incipient melting regime beneath the maturing continents, which only became significant after secular mantle cooling to below 1400 °C during post-Archean times, probably sometime shortly after 2 Ga. At around this time kimberlites replace komatiites as the hallmark mantle-derived magmatic feature of continental shields worldwide.

The remarkable Mesozoic–Cenozoic ‘kimberlite bloom’ between 250–50 Ma may represent the ideal circumstance under which the relatively cool and volatile-fluxed cratonic roots of the Pangea supercontinent underwent significant tectonic disturbance. This created more than 60% of world's known kimberlites in a combination of redox- and decompression-related low-degree partial melting. Less than 2% of world's known kimberlites formed after 50 Ma, and the tectonic settings of rare ‘young’ kimberlites from eastern Africa and western North America demonstrate that far-field stresses on cratonic lithosphere enforced by either continental rifting or cold subduction play a crucial role in enabling kimberlite magma transfer to Earth's surface.

Section snippets

Rationale

Plate tectonics and magmatism are consequences of heat loss from a planet's interior. Earth was significantly hotter in the distant past and has been cooling for most of its history (Korenaga, 2008, Davies, 2009, Michaut et al., 2009, Ganne and Feng, 2017). The effects of an early hotter Earth on the intensity of mantle convection, volcanism, and volatile element cycling continue to be debated, because they had critical influence on the formation of a life-supporting atmosphere (Lyons et al.,

Constraints from an updated global kimberlite age database

We have compiled a high-quality geochronology database for bona fide kimberlites1 to better understand their global magma emplacement patterns (Supplementary file A). The database contains published age information for 1,133 kimberlite localities, which represents approximately 20% of the known kimberlite occurrences worldwide (Fig. 1). Although quality age information is not available for every

Where, when, and how do kimberlites form?

Kimberlites are arguably the deepest and least understood melting products of Earth's mantle. Kimberlite magmatism occurred on every craton (Jelsma et al., 2009, Yaxley et al., 2013), and the pulsed nature over the past 1.2 billion years (Fig. 2, Fig. 7) has provided room for speculation about melt origins and tectonic trigger mechanisms (England and Houseman, 1984, Griffin et al., 2014). Some models prefer kimberlite melt origins from mantle plume sources (Le Roex, 1986, Haggerty, 1994,

Summary and conclusion

Integration of a wide variety of datasets (geochronology, isotope geology, experimental petrology, volcanology, paleogeography; Supplementary files A to E) enables us to refine existing models for the origin and temporal evolution of global kimberlite magmatism. The new model fuses two concepts: (1) secular mantle cooling to below 1400 °C during post-Archean times established a ‘kimberlite-friendly’ incipient melting regime beneath thick continental lithosphere, and (2) supercontinent cyclicity

Acknowledgments

ST acknowledges support from the National Research Foundation (NRF) of South Africa through the IPRR grant programme. The DEEP Research Group at the University of Johannesburg is financially supported by the CIMERA DST-NRF Centre of Excellence. We gratefully acknowledge support of our research on kimberlites by the Geological Society of South Africa, Petra Diamonds, Gem Diamonds, Ekapa Diamonds, Tsodilo Resources, and De Beers. THT acknowledges the Research Council of Norway, through its

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