by Acad. Mikhail KUZMIN, Vinogradov Institute of Geochemistry (Irkutsk)
When was the first continental crust formed on our planet? What was the mechanism of this process? There are a lot of puzzles left, and now only more or less reasoned assumptions are pronounced on the processes at the giant natural laboratory, which took place more than 4 bln years ago, and on the "ingredients" used in these processes. The up-to-date methods of geochemical science of last years enable scientists to better imagine the happenings of so remote past.
Fig. 1. Model of the core formation by "using" the deep magmatic ocean (Wood, Walter, 2006; Wood, 2011).
Fig. 2. Change of the isotope ratio of tungsten isotopes determined in different systems (metal core, carbonaceous chondrites, silicate mantle).
THE NEW COGNITION PHASE
Until recently, in the first half of the 20th century, when the geosincline concept was the basic paradigm of geology or more simply of mobile belts of the Earth, it was considered that the main geological history of the planet covered Phanerozoic (it included Paleozoic, Mesozoic and Cenozoic eras of a total duration of 540 mln years up to the present time). With the advent of the plate tectonics concept* (the 1960s), there started studies of lateral ties of coeval structures of magmatic and ore formations. In this connection it was necessary to determine the age of formation of the asthenosphere**, which is a source of the oceanic magmatic rocks. Of great significance was search for the most ancient ophiolitic complexes***, which had once formed spaces under oceans. Discovery of such complexes comparable with modern rocks under oceans has showed that the plate tectonics similar to the modern one began "to work" on the planet about 2 bln years ago.
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