I believe you can find a detailed answer to your question is in chapter 3 of our book: Mints et al. "East European Craton: Early Precambrian History and 3D Models of Deep Crustal Structure" GSA Special Paper 510, 2015. You may copy Chapter 3 from ResearchGate.

In page 111 you can read:

Nature of sanukitoid magmas. As was shown above, the model Nd ages of sanukitoids definitely depend on the age of country TTG gneisses, which reflects either the age of the crust and the lithospheric mantle. Sanukitoids in the young Kuhmo–Segozero microcontinent have Nd isotopic parameters of the depleted mantle, whereas sanukitoids from the older Vodlozero and Kianta microcontinents are characterized by lowered initial εNd values. In the opinion of Kovalenko et al. [2005], the isotopic heterogeneity of sanukitoids is a result of different duration of a gap between enrichment of the mantle source and the moment of partial melting of that source, which produced sanukitoid magma. The two-stage model of the formation of sanukitoid magmas is a natural consequence of this statement. The first stage corresponds to the mantle metasomatism, i.e., enrichment of the mantle under effect of fluids or melts generated in the process of subduction or underplating of the crust by mantle-derived magmas. The second stage, which started 2.76–2.70 Ga ago, corresponds to the tectonothermal event (vigorous influx of extracrustal heat), which gave rise to the partial melting of the previously metasomatized mantle and generation of sanukitoid magma.

It should be noted, however, that a gap between formation of the crust (with or without participation of subduction) and formation of sanukitoids is cardinally different for three microcontinents making up the Karelian Craton. For the Kuhmo–Segozero microcontinent, this gap does not exceed 60 Ma. For the Kianta microcontinent, the gap is estimated at 60–100 Ma. Finally, for the ancient Vodlozero microcontinent, a gap could have been more than 150 Ma. These estimates convincingly show the absence of genetically predetermined succession of events: subduction → formation of continental crust of granite–greenstone domain → sanukitoid magmatism that completes formation of this domain. Following Kovalenko et al. [2005], Lobach–Zhuchenko et al. [2005b], and Bibikova et al. [2005b], we assume that the two-stage model is consistent with available geological and isotopic data, which reflect tectonic and geodynamic independence of the consecutive stages in the formation of sanukitoid magma. The crucial factor that initiated appearance of sanukitoids irrespective of prehistory of the crust is a thermal pulse dated at 2.76–2.70 Ga.

I highly appreciate the detailed answer by Prof. Mints. The crux of the problem is where did the mantle get metasomatized. One possible way, as you mention, is the 'enrichment of the mantle under effect of fluids or melts generated in the process of subduction'. Could you please elaborate how mantle can get metasomatized by 'underplating of the crust by mantle-derived magmas'. Thanks in advance for your time. 

In fact, starting from Shirey and Hanson (1984), all researchers of the sanukitoid problem based solely on geochemical models. No one was trying to discover within territories and/or in geological maps those subduction zones, which were to play a decisive role in the models of sanukitoid origin. In our work, we on the other hand based on geological (tectonic, structural) aspects. In the book Mints et al, (2015), association of sanukitoids in the Karelian craton (Karelia - Finland) was studied from a geological point of view. It is shown that the appearance of sanukitoid magma in the region cannot be associated with any subduction event. Most likely, this magma was created due to mantle-plume event. Accordingly, I believe that the idea of metasomatized mantle (in the previous text by Kovalenko et al [2005]) has not sufficient geological basis. I believe that the geochemical models of sanukitoid magmas formation should be reviewed taking into account the geological data. All our materials, including maps (in Appendices), you can found in ResearchGate.

Thanks Prof. Mints for such insightful answer. 

You are welcom!

Dear Sukanta Dey,

I am a little late in this answer, but I really like this topic!

Many authors believe that Drip Tectonics during Archean times may also be an efficient process to metasomatize the upper mantle with crust-derived fluids.

Please take a look in our 2020 publication https://doi.org/10.1016/j.jsames.2020.102817 and references therein.

Regards,

Felipe

Thanks Felipe, for the comment. But can drip tectonics intruduce the copious amount of water that is required to generate the sanukitoids?

Best

Sukanta

Dear Sukanta,

You are welcome, and thank you for the reply!

Yes, I do believe that drip tectonics may introduce the copious amount of water that is required to generate the sanukitoids.

There is a growing idea that in the Early Earth we had a lot of surface water, with much larger oceans that we have today (e.g., Dong et al, 2021), and probably, with very few or absent land surfaces (e.g., Korenaga, 2021).

The Archean crustal drips are mostly composed of effusive mafic/ultramafic rocks (greenstone belts) that once interacted with seawater during its formation. Therefore, these huge packs of chlorite-schists and serpentinites (± trapped sediments) dripping into the upper mantle may introduce the required crust-derived fluids (including water) to latter formation of sanukitoids.

Dripping is a kind of subduction in a less stiff (hotter) lithosphere, but the processes of water transfer from surface to mantle depths may work similarly. Please see also Nebel et al. (2018) and Mole et al. (2019).

Best regards,

Felipe

References

Dong, J., Fischer, R. A., Stixrude, L. P., & Lithgow-Bertelloni, C. R. (2021). Constraining the volume of Earth's early oceans with a temperature-dependent mantle water storage capacity model. AGU Advances, 2, e2020AV00032.

Korenaga, J. Was There Land on the Early Earth? Life. 2021, 11, 1142.

Nebel, O., Capitanio, F., Moyen, J.F., Weinberg, R., Clos, F., Nebel–Jacobsen, Y.J., Cawood, P.A., 2018. When crust comes of age: on the chemical evolution of Archaean, felsic continental crust by crustal drip tectonics. Phil. Trans. R. Soc. A. 376, 20180103.

Mole, D.R., Kirkland, C.L., Fiorentini, M.L., Barnes, S.J., Cassidy, K.F., Isaac, C., Belousova, E.A., Hartnady, M., Thebaud, N., 2019. Time–space evolution of an archean craton: a Hf–isotope window into continent formation. Earth Sci. Rev. 196, 102831.

Thanks Felipe, this option is really thought-provoking.

Best wishes

Felipe,

Nothing is ever going to drip into the mantle unless it is denser than the mantle. So, we all know that eclogitised mafic crust can sink into the mantle, but what about "huge packs of chlorite-schists and serpentinites"?! What are the density contrasts here with ambient mantle, with TTGs, and with the unaltered mafic crust?

Food for thought...

Article Fluid processes in the early Earth and the growth of continents

More food for thought...