No plate tectonic movement before Neoproterozoic?

The initiation of plate tectonics has been a great debate among geologists from late 1990s. Some scientists insist that the plate tectonics began before Archean (> 4.2Ga), while others suggest it started at 0.85 Ga although the majority suggest the initiation of modern plate tectonics during Archean based on various evidence. Stern et al. (2005) have suggested 1 Ga Neoproterozoic era as the beginning of modern plate tectonics, based on the first appearance of ophiolitic complexes, blueschist facies metamorphic rocks, and ultrahigh-pressure metamorphic terranes. Eleven years later, he confirmed the same number using different evidence, kimberlites.

Kimberlites are potassic, volatile-rich ultramafic igneous rocks, containing abundant compatible and incompatible elements, known for host rocks of diamonds. Kimberlite magma is the deepest-sourced terrestrial magma and they often host xenocrysts and xenoliths from base of mantle lithosphere. Previous scientists have attributed the ascent of kimberlite magma to CO2-rich fluid in deep mantle produced by exsolution process related to orthopyroxene (Brett et al., 2015; Kamenetsky et al., 2015). The high content of volatiles in kimberlite magma suggest high CO2 and H2O contents in the deep mantle, which may have resulted from subduction process. Stern et al., therefore, argue that age distribution of kimberlites will answer the question of ‘when does the plate tectonics begin?’

The first assumption of Stern is that present age data represent kimberlite production through time. Another basic assumption of Stern is equivalence of plate tectonics and deep subduction (depth about 670 km). This author highlights the part that 95 % of kimberlites are younger than 750 Ma. Stern strongly insists that volatile content increase in the mantle around Neoproteozoic is cause of young kimberlites. Other processes such as cooling of Earth, activity of plumes are not sufficient to explain such increase around early Neoproterozoic. Volatile increase in mantle is consequence of large amount of injection from deep penetration of slabs into mantle.

One of their supporting evidence is the Farallon plate that subducted to 660 km depth in mantle and resulting kimberlite chain in Northwest Canada. Also the authors take negative S-wave velocity anomaly in imaged in subduction zones (van der Lee, 2008) as signature of returning subducted volatiles to surface. Also other indicators of plate tectonics such as ophiolites and high pressure metamorphic rocks (Stern 2005) match well with age distribution of kimberlites (mostly younger than 1.0 Ga) supporting the theory of Stern. There explanation regarding kimberlites older than Neoproterozoic is delamination of carbonate rich lithosphere, which was not an effective kimberlite factory.

However, there are some critical questions that should be confirmed; How much time lag is required between kimberlite magma eruption and subduction of volatiles to deep mantle? Did plates subduct always deep to 670 km depth? Actually, definition of plate tectonics can range from drift of plates, convective mantle movement or subduction of cold materials to deep mantle. The latter is what these authors (Stern) took, but other scientists chose other definitions. Existing Arhcean ophiolites on surface might have involved in shallow subduction (Ernst, 2017), which do not satisfy the definition of Stern. Also, the production of cold oceanic crust that can subduct into deep mantle requires too much time for cooling if there was not subduction at all (Korenaga 2013). At least subduction of small plates should have existed before subduction of large scale oceanic crusts as now. What Stern insist as plate tectonics is limited to modern style. However, there were subductions, drift of plates which different from the modern type.

In conclusion, there are many remaining controversy in the initiation of plate tectonics. Combination of different use of sample from nature and different definition of plate tectonics lead to various age of plate tectonics initiation. Start of plate tectonics from 1.0 Ga is one possibility as Stern insists. However, there is no critical evidence that rejects Archean initiation of plate tectonics. Unified definition and insight of plate tectonics should be made before severe discussions.

References

Brett., R. C., Russell, J. K., Andreqs, G. D. M., Jones, T. J., 2015, The ascent of kimberlite: Insights from olivine, Earth and Planetary Science Letters, vol 424., p. 119-131.

Ernst, W. G., 2017, Kimberlites and the start of plate tectonics, GEOLOGY FORUM, e405, doi:10.1130/G38681C.1.

Kamenetsky, V. S., and Yaxley, G. M., 2015, Carbonate-silicate liquid immiscibility in the mantle propels kimberlite magma ascent, Geochimica et Cosmochimica Acta, Vol 158, p. 48-56.

Korenaga, J., 2013, Initiation and Evolution of Plate Tectonics on Earth: Theories and Observations, Annu. Rev. Earth Planet. Sci., Vol 41, p. 117-151.

Stern, R. J., 2005, Evidence from ophiolites, blueschists, and ultrahigh-pressure metamorphic terrances that the modern episode of subduction tectonics began in Neoproterozoic time, GEOLOGY, vol 33, no. 7, p. 557-560.

Stern, R. J., Leybourne, M. I., Tsujimori, T., 2016, Kimberlites and the start of plate tectonics, GEOLOGY, vol 44, no. 10, p. 799-802.