GEOGRAPHY

Scientists reveal global regional differences in the onset of plate subduction


Plate tectonics are the main tectonic feature that distinguishes Earth from other terrestrial planets in the Solar System. The earth is the common home of human beings and all known living species, and its livability is established, improved and maintained for a long time, including the cyclic replenishment of nutrients needed for life activities, suitable atmospheric temperature, humidity, oxygen content, etc., all depend on the continuous material-energy exchange in the various spheres of the earth driven by plate tectonics. Plate tectonics have been proposed for more than half a century, and although they have been widely verified and supported, there are still many unsolved mysteries. Among them, the start time of plate tectonics has been the most controversial hot issue.

According to the geogeochemical research and geophysical data of today’s plate tectonics, scientists have summarized some recognized identification marks of plate tectonics, including geochemical characteristics, high-pressure metamorphism, oceanic crust fragments/ophiolite, paleomagnetism and other aspects. These markers and indicators are used to determine the plate tectonics of the Phanerozoic are successful, but there are many uncertainties in the Archean epoch that extends to the early Earth, and there are large deviations in the initiation era used to identify and confirm the initiation of plate tectonics, and the initiation epochs of plate tectonics given by different methods can range from less than 1 billion years later to more than 4 billion years earlier (Figure 1). For example, by studying the N content and C-N isotopes of diamonds, scientists (Smart et al., 2016) believe that the high N content and positive δ15N isotope in diamond from the Kaapvaal craton, South Africa, indicate the recycling of surface rocks, and plate tectonics have begun in 3.5 billion years. By studying the components of inclusions in diamonds, some scholars have found that diamonds 3.2 billion years ago were single mantle rock inclusions, and duriolite inclusions began to appear in diamonds 3 billion years ago, suggesting that plate tectonics may have occurred 3 billion years later (Shirey et al., 2011). By studying the S isotopes of sulfide inclusions in diamonds, scientists (Smit et al., 2019) found that sulfide inclusions over 3 billion years have 33S non-mass fractionation of the Archean surface environment, and plate tectonics should occur after 3 billion years. Using the continental crustal growth curve to level off at 3 billion years, scholars believe that the recirculation of the continental crust at 3 billion years may indicate the initiation of plate tectonics (Dhuime et al., 2012). By studying the evolution of continental crustal composition, some scholars have established the connection between continental crust composition and Ni/Co and Cr/Zn in shale and moraine rocks, and believe that a large number of felsic crust began to appear in 3 billion years, and global plate tectonics should occur in 3 billion years (Tang et al., 2016). By summarizing the geothermal gradients of global metamorphism, some scholars believe that around 2.7 billion years, the early Earth’s double metamorphic zone began to appear, indicating that plate tectonics began at this time (Brown, 2006). Also from the perspective of metamorphism, some scholars (Stern et al., 2005) believe that the first appearance of blueschists (cold-subducted metamorphic rocks) indicates that the initiation of plate tectonics occurred 1 billion years later. Different from the earliest shell structure of the earth, vertical structure is predominant, and plate tectonic is dominated by horizontal motion. Ancient horizontal structures have been recorded in some ancient cratons. For example, ancient hypertrophic structures of about 3.7 billion to 3.8 billion years have been found in Greenland (Komiya et al., 1998), horizontal structures of 3.2 billion years in Pilbara cratons in Western Australia (van Kranendonk et al., 2007), and 2.7 billion years of horizontal structures in the Superior craton, Canada (Lin et al., 2005) are interpreted as evidence of plate tectonic initiation. Geophysical data also contributed to the initiation of plate tectonics. Scientists have found a residual 3.5 billion year oblique interface in Slave Kraton, Canada, with an inclination angle of less than 20° at depths of 96-124 km, which is interpreted as traces of plate subduction (Chen et al., 2009). East of the Pilbara craton in Western Australia, scientists have recorded paleolatitude deflections using paleomagnetism to discover 3.2 billion-year-old basalt, possibly reflecting horizontal movements driven by plate tectonics (Brenner et al., 2020). Based on the short-period intensive observation profile data of the North China craton, combined with other global craton geophysical data, some scholars (Wan et al., 2020) pointed out that the globally linked plate tectonics occurred in 2 billion years.

The above study gives a plate tectonic initiation era from 1 billion years later to 4 billion years earlier. This difference stems from differences between various markers and indicators, as well as from the essentially regional nature of specific records resulting from incomplete geological records on the early Earth. Therefore, choosing a generally developed geological record and observing the change law of plate tectonic related indicators may be able to obtain a clearer understanding.

As we all know, the most important rock unit of the Archean continental crust is the tonalite-trondhjemite-granodiorite assemblage (TTG rock set), which can account for more than 50% of the rock composition of the continental crust, and has a high proportion distribution in various Archean continents/cratons around the world. At the same time, TTG rock is a rock crystallized by silicon-saturated magma, and a large number of dateable mineral zircon can be accurately dated, which is a very ideal research object for studying early geodynamic processes. In the 50-year history of TTG genesis research, academics tend to think that TTG is formed by partial melting of aqueous basalts in the metamorphism of amphibolite facies (mid-low subfacies) to eclogite facies (high-pressure facies). Among them, the medium-low pressure phase system may represent the suboceanic plateau crustal environment, while the high pressure phase system is related to subduction. Due to the different pressures formed, the trace elements of TTG also show different characteristics. However, the formation of TTG does not only rely on the partial melting process, and the magma undergoes separation crystallization and assimilation and mixing during the process from formation to invasion, which will modify the trace element content of the magma, so that the pressure of relying on trace element discrimination loses its effectiveness. For example, plagioclase stacks have been found in TTGs developed in South Africa and Canada. Due to the high Sr content in plagioclase feldspar and the lack of Y and heavy rare earth content, the magma formed under medium and low pressure conditions can form “high-pressure TTG” through plagioclase feldspar crystals. In comparison, the partition coefficient of Ba between plagioclase feldspar and feldsine melt is close to 1, that is, the plagioclase feldspar does not cause changes in magmatic Ba content. Of the major rock-forming minerals, Ba is only compatible with potassium feldspar and biotite. However, the addition of these minerals also causes an increase in magma K and Ni content, which is not consistent with the composition of natural TTG. The separation and crystallization of incompatible minerals will cause a decrease in magma Mg#, which is also inconsistent with natural TTG components. Therefore, the Ba content in TTG is more likely to represent the composition of magma formation. If the Ba content of TTG magma formed by subduction and the melting of the earth’s crust under the ocean plateau is different, then the long-term change of Ba content in global TTG over time can determine the onset of subduction.

Based on the above understanding, Huang Guangyu, associate researcher of the Institute of Geology and Geophysics, Chinese Academy of Sciences, researcher Ross Mitchell and researcher Guo Jinghui jointly carried out phase equilibrium simulation using Archean mean basalt as the original rock. The results show that TTG magma with high Ba content can only be formed in a low temperature and high pressure environment (Figure 2), that is, TTG magma with high Ba can only be formed in a subduction environment. By analyzing the existing TTG databases of different global cratons, it is found that the Ba content in TTG in the world has evolved over time, and there have been three positive changes, which occurred around 3.7 billion years, 3.1 billion years and 2.8 billion years. Further tests showed that the positive change time of Ba content in different cratons TTG was not the same. Among them, the subduction of the slave craton may have begun in 4 billion years, followed by the North Atlantic craton and the Kaapvaal craton, which began to subduct in 3.7 billion and 3.5 billion years, respectively. Around 3 billion years , subduction has involved most cratons. Younger cratons, such as the Superior craton, began to subduct at 2.7 billion (Figure 3). These are consistent with the regional geological record previously reported (Figure 1).

Figure 1 The era of plate tectonics initiated by different authors

Fig. 2 Comparison of natural TTG with phase equilibrium simulation and experimental petrology results (background color yellow for low temperature, orange for high temperature)

Fig. 3 Long-term variation of Ba content in different craton TTGs globally over time

The regional differences in the onset of plate subduction are controlled by many factors, including the thickness of the lithosphere, the temperature of the mantle, the density difference between the lithospheric mantle and asthenospheric mantle, and the content of radioactive thermogenic elements. In these respects, the Earth is not uniform. Therefore, the occurrence of subduction depends on the coupling of various factors, and the subduction effect will occur first when all conditions are suitable. The horizontal movement of any one place does not exist in isolation, but is balanced by corresponding movements in other areas. Therefore, subduction will occur successively in various locations around the world, eventually forming a globally linked plate structure, which should appear 2.7 billion years later. This is consistent with Earth’s long-term history of thermal evolution (Figure 4).

Figure 4 Historical model of Earth’s thermal evolution

The research was published in Nature Communications. (Source: Institute of Geology and Geophysics, Chinese Academy of Sciences)

Related paper information:https://doi.org/10.1038/s41467-022-34343-0

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