Earth?s earliest hydrosphere recorded by the oldest hydrothermally-altered oceanic crust: Triple oxygen and hydrogen isotopes in the 4.3-3.8 Ga Nuvvuagittuq belt, Canada

The origin, evolution, and the state of Earth's hydrosphere are relevant for the timing, style, and intensity of plate tectonics, continental and submarine weathering, and the emergence of the continents. We here present a study of triple oxygen isotopes of hydrothermally altered oceanic crust from the Nuvvuagittuq greenstone belt (NGB) in Canada (4.3-3.8 Ga), which provides a rare snapshot of the Earth's earliest hydrosphere on a planet without modern-style plate tectonics. High delta O-18 = 8-12%o, triangle O-iota 17=-0.02 to -0.05%o, and 8D of -30-40%o values measured in the NGB metavolcanic rocks are interpreted to indicate that they represent the upper section of ancient oceanic crust (pillows, sediments) altered at low temperature by seawater similar to modern submarine oceanic sections. Hydrothermal alteration was accompanied by silicification at low temperature (50-150 degrees C) as evidenced by a positive correlation of delta O-18 with silica, elevated triangle O-iota 17 values, and triple oxygen isotope thermometry. We discuss the effects of 2.7 Ga metamorphism on the preservation of triple oxygen isotope compositions and show that the hydrothermal alteration is protolithic and predated metamorphism. Triple oxygen isotopes allow reconstruction of the isotopic compositions of seawater and suggest that the 818O of the early oceans could have been comparable to that of the modern to perhaps -5%o but silica-saturated. Models with an ancient ocean with delta O-18 of +3%o or < -5%o seawater result in worse fits to the data. Submarine weathering and hydrothermal silicification could have played a more important role in Eoarchean world with subdued role of subaerial weathering due to insignificant continental exposure that helped balancing seawater delta O-18 values to near modern values. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the hydrothermally altered oceanic crust from the Nuvvuagittuq greenstone belt in Canada, providing valuable insights into the early hydrosphere of the Earth. The findings suggest that the ancient oceanic crust underwent hydrothermal alteration at low temperatures similar to modern submarine oceanic sections. The results also reveal the isotopic compositions of the early oceans and the potential role of submarine weathering and hydrothermal silicification in the Eoarchean world.