The timing and mechanism of crust formation has always been a key scientific issue of great concern in the geological community, and is of great significance for understanding the history of plate tectonics, the evolution of atmospheric and ocean chemistry, and the evolution of life. TTG represents the main material composition of the Archean crust, but there is still debate in the academic community as to whether TTG is formed by subductioning the oceanic crust or by thickening the lower crust by melting the earth's crust extensively separated crystallization through basalt magma in the lower middle-lower crust. Recent studies have proposed through heavy silicon isotope features that the TTG source region needs to have the addition of shell material, and tends to the horizontal tectonic pattern of the Archean Age. However, the existing problem is that the trace element ratio commonly used to infer the genesis of TTG does not lead to very precise conclusions. Therefore, it is questionable whether the shell material (e.g. flint) entered the TTG source region through subduction or other processes.
In view of the above problems, Dr. Michael A. Antonelli from the University of Paris and his collaborators proposed to restrict the surface geothermal gradient of TTG magma formation by stabilizing calcium isotopes, and then judge the tectonic environment of ancient land crust formation. The results were published in Nature Communications (Antonelli et al., 2021).
The authors performed calcium isotope analysis on modern Edak rocks, TTG of the Archean Age, and la-plaque granite samples that already had other isotopic data (Si, Hf, and Nd). First, in order to better illustrate the opposite isotope fractionation effects brought about by increased temperature and pressure, as well as the changes in the proportion and composition of minerals in the TTG source rock during the gradual melting process, they combined equilibrium calcium isotope fractionation and phase equilibrium simulation studies. The results of the study showed that the overall trend of δ44Ca change was constrained by the confrontation between temperature and pressure, i.e., the temperature increased and the isotopic fractionation decreased; the pressure increased, and the isotopic fractionation increased due to the increase in residual garnet (Figure 1a). In addition, they found that most of the TTG and modern Edak rocks can be explained by geothermal gradient trajectories of 500-750°C/GPa (Figure 1b). This geothermal gradient range is similar to the geothermal gradient of modern thermal subduction, but higher than the temperature required to melt the mafic crust directly.
Subsequently, the authors used the wet basalt solid phase line as the lower temperature limit (Figure 1c), and the results were consistent with previous predictions of the genesis of TTG (Palin et al., 2016), as well as the estimated P-T results of the modern Edak rocks, which typically form in the subduction environment of the hot/young oceanic crust. In contrast, melting at the bottom of the crust under thickening occurs at a higher geothermal gradient (>700°C/GPa). Therefore, using calcium isotope data combined with recent geophysical models, the authors suggest that the Archean TTG is likely to have formed in a thermal subduction environment (Figure 2).
Figure 1 Results of phase equilibrium simulations of loss-making Taikooera porphyry basalt (DAT) and comparison with δ44Ca measurements and modern subduction zone P-T estimates (Antonelli et al., 2021). (A) Effects of geothermal gradients on δ44Ca; (B) δ44Ca vs. Dy/Yb simulation results and data (color numbers represent pressure, GPa) ;(C) combinations of modern thermo subduction zones (garnet and blue schist (Penniston-Dorland et al., 2015)), modern Eddack rocks (Hastie et al., 2016), and P-T estimation of TTG samples of the Archean
Figure 2 Dynamic model supported by data from this paper (Antonelli et al., 2021)
In addition, the authors found that two granite samples with unusually low δ44Ca predicted by their 500°C/GPa model could not be explained by the equilibrium magma process and were most likely caused by carbonate deposits (Figure 2). Therefore, the data in this paper provide independent evidence of the presence of carbonate deposits on the ocean floor in the early Archean archeans, bringing forward the time of the oldest carbonate units preserved. This suggests that the silicate-carbonate cycle existed before 3.8Ga and provides a reservoir for a large amount of volcanic CO2 degassing. The above results have important implications for the emergence/evolution of continental weathering over time.
Finally, the authors note that while the data in this paper do not necessarily attest to the existence of global lattices with plate boundaries connected to each other (some plate tectonics are so defined), they demonstrate the conclusion that subduction events are repeated in the Archean Ageus, with a growing body of evidence indicating that plate tectonics were initiated before 3.5Ga (Arndt, 2013; Keller et al., 2020; Ptácek et al., 2020) is consistent.
Acknowledgements: Thank you to the colleagues of the former Cambrian Research Group in the Lithosphe Chamber for their valuable revision suggestions.
Key References (Swipe up and down to view)
Antonelli M A, Kendrick J, Yakymchuk C, et al. Calcium isotope evidence for early Archaean carbonates and subduction of oceanic crust[J]. Nature Communications, 2021, 12: 2534.（原文链接）
Arndt N T. Formation and evolution of the continental crust, Geochem[J]. Perspectives, 2013, 2: 405-533.
Hastie A R, Fitton J G, Bromiley G D, et al. The origin of Earth’s first continents and the onset of plate tectonics[J]. Geology, 2016, 44(10): 855-858.
Keller C B, Harrison T M. Constraining crustal silica on ancient Earth[J]. Proceedings of the National Academy of Sciences, 2020, 117(35): 21101-21107.
Palin R M, White R W, Green E C R. Partial melting of metabasic rocks and the generation of tonalitic–trondhjemitic–granodioritic (TTG) crust in the Archaean: Constraints from phase equilibrium modelling[J]. Precambrian Research, 2016, 287: 73-90.
Penniston-Dorland S C, Kohn M J, Manning C E. The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models[J]. Earth and Planetary Science Letters, 2015, 428: 243-254.
Ptácek M P, Dauphas N, Greber N D. Chemical evolution of the continental crust from a data-driven inversion of terrigenous sediment compositions[J]. Earth and Planetary Science Letters, 2020, 539: 116090.
Written by: Shan Houxiang /Institute of Geology, China Earthquake Administration, Zhou Yanyan/Lithosphere Chamber
Editor: Chen Feifei
Proofreader: Zhang Tengfei Jiang Xuejiao