09:00 〜 09:15
[BCG06-01] Secular variations of Hf isotopes of detrital and autochthonous granitic zircons: Secular change of formation processes of granitic rocks and plate tectonics
キーワード：地球進化、プレートテクトニクスの開始、冥王代・原太古代、大陸成長
When did the plate tectonics begin? This is the most important and fundamental issue in the Earth history because the onset of plate tectonics creates the Earth's unique geology, namely granitic continental crusts. The formation of continental crusts resulted in not only solid earth evolution, such as topographical diversity and formation of a depleted mantle reservoir, but also a strong influence on the evolution of life, such as the formation of a wide landmass and continental shelves for providing habitats for terrestrial and shallow-sea organisms and sufficient nutrient supplies to the oceans. However, the period of the beginning of plate tectonics is highly controversial, and various estimates ranging from the Hadean through the Eoarchean to Neoarchean have been proposed. Because the main product of plate tectonics is the formation of granitic continental crusts, the most important constraint is their occurrence of granitic continental crusts.
Bauer et al. (2020) showed that the Hadean and Eoarchean detrital zircons have statistically lower initial Hf isotope values, εHf(t), than the Paleoarchean zircons and proposed that the difference is due to the onset of plate tectonics after the Paleoarchean. The continental crusts were considered to have been formed by partial melting of old basaltic crusts of stagnant lids in the Hadean and Eoarchean, whereas continental crusts were formed by partial melting of subducted basaltic crusts after the Paleoarchean.
We compiled detrital zircons in sedimentary rocks and magmatic zircons within granitic rocks of different ages. The detrital zircon compilation covers zircons from 4358 Ma to the present in Pb-Pb ages and includes ca. 13,700 spots. On the other hand, the compilation of zircon in granitic rocks covers the Pb-Pb age from 4202 Ma to 4 Ma and includes ca. 10400 spots. Approximately 2500 zircons in mafic and ultramafic rocks from 3800 Ma to the present are also compiled.
The compilation of detrital zircons shows that the maximum εHf(t) values in the Hadean and Eoarchean are significantly lower than the Hf isotope evolution of depleted mantle reservoir (DMM-εHf(t)), whereas those in the Paleoarchean and Neoarchean are relatively high, close to the DMM-εHf(t) values. This is consistent with the previous studies. The maximum εHf(t) values are slightly lower than the DMM-εHf(t) values after the Neoarchean, and the difference between the maximum εHf(t) and DMM-εHf(t) values is equivalent to those in the Hadean to Eoarchean. However, the maximum εHf(t) values were close to or even higher than the DMM-εHf(t) values in some periods. The minimum εHf(t) values began to be very low after 2 Ga and extremely low at 500 Ma. In contrast, the minimum εHf(t) values were relatively high at 14, 7.5, and 400 Ma.
On the other hand, the compilation of zircons within granitic rocks shows that the maximum εHf(t) values in the Eoarchean are relatively high, and there is no difference between the Eoarchean and Paleoarchean. The maximum εHf(t) values in the Mesoarchean were low. The maximum εHf(t) values are low in most periods after 2.0 Ga, but high values occur around 2200, 1500, and 400 Ma. The compilation of zircons within granitic rocks is inconsistent with the compilation of detrital zircons, possibly because the detrital zircons underwent Pb-loss owing to later metamorphism. Zircons contain large amounts of Hf relative to Lu; thus, U-Pb ages easily change, but Hf isotope ratios do not change due to later thermal events. As a result, the initial values of the Hf isotopes decreased because of later thermal events.
The low maximum εHf(t) values in the Mesoarchean were mainly obtained from the granitic bodies of the Barberton Greenstone Belt. Previous studies have shown that greenstones in this area have undergone relatively high-pressure metamorphism (e.g., Kato et al., 2018). Therefore, it is considered that the low maximum εHf(t) values were due to subduction of a relatively old oceanic plate. On the other hand, the Isua supracrustal rocks underwent relatively high-temperature, intermediate-pressure-type metamorphism, consistent with the subduction of a young plate, namely, high maximum εHf(t) values. In addition, the extremely low Hf isotope values since 2 Ga are considered to be due to the enhanced recycling of crustal materials due to the formation of supercontinents.
In this study, changes in the Hf isotope evolution of zircons are considered to be due to changes in the age of the subducting plate and the influence of recycled materials, rather than due to the onset of plate tectonics.
Bauer et al. (2020) showed that the Hadean and Eoarchean detrital zircons have statistically lower initial Hf isotope values, εHf(t), than the Paleoarchean zircons and proposed that the difference is due to the onset of plate tectonics after the Paleoarchean. The continental crusts were considered to have been formed by partial melting of old basaltic crusts of stagnant lids in the Hadean and Eoarchean, whereas continental crusts were formed by partial melting of subducted basaltic crusts after the Paleoarchean.
We compiled detrital zircons in sedimentary rocks and magmatic zircons within granitic rocks of different ages. The detrital zircon compilation covers zircons from 4358 Ma to the present in Pb-Pb ages and includes ca. 13,700 spots. On the other hand, the compilation of zircon in granitic rocks covers the Pb-Pb age from 4202 Ma to 4 Ma and includes ca. 10400 spots. Approximately 2500 zircons in mafic and ultramafic rocks from 3800 Ma to the present are also compiled.
The compilation of detrital zircons shows that the maximum εHf(t) values in the Hadean and Eoarchean are significantly lower than the Hf isotope evolution of depleted mantle reservoir (DMM-εHf(t)), whereas those in the Paleoarchean and Neoarchean are relatively high, close to the DMM-εHf(t) values. This is consistent with the previous studies. The maximum εHf(t) values are slightly lower than the DMM-εHf(t) values after the Neoarchean, and the difference between the maximum εHf(t) and DMM-εHf(t) values is equivalent to those in the Hadean to Eoarchean. However, the maximum εHf(t) values were close to or even higher than the DMM-εHf(t) values in some periods. The minimum εHf(t) values began to be very low after 2 Ga and extremely low at 500 Ma. In contrast, the minimum εHf(t) values were relatively high at 14, 7.5, and 400 Ma.
On the other hand, the compilation of zircons within granitic rocks shows that the maximum εHf(t) values in the Eoarchean are relatively high, and there is no difference between the Eoarchean and Paleoarchean. The maximum εHf(t) values in the Mesoarchean were low. The maximum εHf(t) values are low in most periods after 2.0 Ga, but high values occur around 2200, 1500, and 400 Ma. The compilation of zircons within granitic rocks is inconsistent with the compilation of detrital zircons, possibly because the detrital zircons underwent Pb-loss owing to later metamorphism. Zircons contain large amounts of Hf relative to Lu; thus, U-Pb ages easily change, but Hf isotope ratios do not change due to later thermal events. As a result, the initial values of the Hf isotopes decreased because of later thermal events.
The low maximum εHf(t) values in the Mesoarchean were mainly obtained from the granitic bodies of the Barberton Greenstone Belt. Previous studies have shown that greenstones in this area have undergone relatively high-pressure metamorphism (e.g., Kato et al., 2018). Therefore, it is considered that the low maximum εHf(t) values were due to subduction of a relatively old oceanic plate. On the other hand, the Isua supracrustal rocks underwent relatively high-temperature, intermediate-pressure-type metamorphism, consistent with the subduction of a young plate, namely, high maximum εHf(t) values. In addition, the extremely low Hf isotope values since 2 Ga are considered to be due to the enhanced recycling of crustal materials due to the formation of supercontinents.
In this study, changes in the Hf isotope evolution of zircons are considered to be due to changes in the age of the subducting plate and the influence of recycled materials, rather than due to the onset of plate tectonics.