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

Abstract

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 δ18O = 8-12‰, Δ′17O=-0.02 to −0.05‰, and δD of −30-40‰ 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°C) as evidenced by a positive correlation of δ18O with silica, elevated Δ′17O 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 δ18O of the early oceans could have been comparable to that of the modern to perhaps −5‰ but silica-saturated. Models with an ancient ocean with δ18O of +3‰ or <−5‰ 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 δ18O values to near modern values.