Research press release


Nature Geoscience

Earth’s early pulse formed first stable crust



今回Daniel Wiemerたちは、東ピルバラ・クラトンの岩石の年代を測定し、熱力学的モデルを用いて、想定される重力的なひっくり返りを再構成した。彼らは、この岩石が36億~34億年前の約1億年間継続したひっくり返り事象の際に形成されたと提案している。また、既存のデータを用いて、同様な事象がインドやアフリカのクラトンにも記録されていると主張している。彼らは、それぞれのクラトンに安定した大陸地殻を形成するためには、それぞれが約1億年間継続する計3回の重力的ひっくり返り循環が必要であると示唆している。従って、このような循環は初期地球の「パルス」を表している可能性がある。

Earth’s oldest stable crust may have formed during multiple pulses of overturning, reports a study published online this week in Nature Geoscience.

Continental crust today is formed through plate tectonics, mostly via the volcanic activity that occurs where one plate subducts under another. In the Archaean eon (4 to 2.6 billion years ago), however, Earth was likely probably too hot to sustain modern-style plate tectonics. Instead, continental crust may have formed through gravitational instabilities. In this scenario, a mixture of dense and lighter minerals initially separated from the mantle, crystallizing to form a proto-crust. The denser components were gravitationally unstable, causing large chunks of crust to overturn and be recycled back into the mantle. This process repeated until lower-density components were concentrated at the surface, forming a stable crust. Some have proposed that crust formed in this way is preserved in a craton - an old and geologically stable region of continental crust - in Western Australia’s Pilbara region.

Daniel Wiemer and colleagues dated the rocks in the East Pilbara Craton and used a thermodynamic model to reconstruct the purported gravitational overturns. They propose that the rocks formed around 3.6 to 3.4 billion years ago during an overturn event that lasted about 100 million years. The researchers also use existing data to argue that similar events are recorded in cratons in India and Africa. They suggest that to create stable continental crust in each craton, three cycles of gravitational overturn, each lasting about 100 million years, would have been required. These cycles may therefore represent the ‘pulse’ of the early Earth.

doi: 10.1038/s41561-018-0105-9


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