Seawater-oceanic crust interaction constrained by triple oxygen and hydrogen isotopes in rocks from the Saglek-Hebron complex, NE Canada: Implications for moderately low-δ18O Eoarchean Ocean

Estimations of Earth's earliest surface conditions assume a strong connection between the temperature and oxygen isotopic composition of oceans, balanced by surface weathering and submarine hydrothermal alteration. The oldest preserved supracrustal rocks provide rare opportunities to study and constrain the earliest surface conditions prevailing on the Earth. Here, we present a study of triple oxygen and hydrogen isotopes of hydrothermally altered Eoarchean metamorphosed basalts, ultramafic rocks, and detrital and chemical sediments, from the Saglek-Hebron Complex in northern Labrador, Canada. For the metavolcanic rocks, δ’¹⁸O values range from 4.83 ‰ to 8.56 ‰, while Δ’¹⁷O values vary from −0.076 ‰ to −0.023 ‰, both higher and lower than the mantle. Accounting for the effects of metamorphism on oxygen and hydrogen isotopic compositions, we demonstrate that triple oxygen isotopic values are preserved from the hydrothermal suboceanic stage, while none of the hydrogen isotope compositions (δD from −77.9 ‰ to −10.7 ‰) are interpreted as primary. Several metabasalt samples from the Saglek-Hebron Complex yielded Δ’¹⁷Ο values lower than modern mantle values, which cannot be explained by direct interaction with modern seawater and indicate complex upstream interactions. Our numerical models and Monte Carlo simulation considers one- and two-stage mechanisms of water-rock interaction, including the δ’¹⁸Ο and Δ’¹⁷Ο isotopic shift effects due to interaction between basalts and chemical sediment-derived fluids. The modelling favors Eoarchean seawater characterized by low δ’¹⁸Ο < −8 ‰ at Δ’¹⁷Ο up to 0.01 ‰. This model also works for higher Δ’¹⁷Ο at lower δ’¹⁸Ο. Our results also suggest that without proper modelling of multi-stage water-rock interaction, involving isotopic shifts and input of sediment-derived fluids, exposed sections of altered oceanic crust present only remote evidence of the original seawater. Due to the modeled isotopic shifts and fluid mixing, we favor “weak” coupling of seawater-oceanic crust interaction globally. This potentially reduces the relative importance of submarine hydrothermal alteration in explaining the oxygen isotopic record in submarine basalts across the geologic history.