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’O values range from 4.83 ‰ to 8.56 ‰, while Δ’O 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’Ο 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’Ο and Δ’Ο isotopic shift effects due to interaction between basalts and chemical sediment-derived fluids. The modelling favors Eoarchean seawater characterized by low δ’Ο < −8 ‰ at Δ’Ο up to 0.01 ‰. This model also works for higher Δ’Ο at lower δ’Ο. 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.

中文翻译：

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加拿大东北部萨格莱克-希伯伦杂岩岩石中的氧氢三重同位素限制了海水-洋壳相互作用：对中等低 δ18O 太古代海洋的影响


对地球最早表面状况的估计假设海洋的温度和氧同位素组成之间存在密切联系，并通过表面风化和海底热液蚀变来平衡。保存最古老的表层岩石为研究和限制地球上最早的地表条件提供了难得的机会。在这里，我们对来自加拿大拉布拉多北部萨格莱克-希伯伦杂岩的热液蚀变的太太古代变质玄武岩、超镁铁岩以及碎屑和化学沉积物进行了三氧和氢同位素研究。对于变火山岩，δ'O值范围为4.83‰至8.56‰，而Δ'O值范围为-0.076‰至-0.023‰，均高于或低于地幔。考虑到变质作用对氧和氢同位素组成的影响，我们证明三重氧同位素值从洋底热液阶段就被保留下来，而没有任何氢同位素组成（δD从-77.9 ‰到-10.7 ‰）被解释为主要的。 。来自萨格莱克-希伯伦杂岩体的几个变玄武岩样本产生的 Δ'O 值低于现代地幔值，这不能通过与现代海水的直接相互作用来解释，并表明复杂的上游相互作用。我们的数值模型和蒙特卡罗模拟考虑了水-岩石相互作用的一阶段和两阶段机制，包括由于玄武岩和化学沉积物衍生流体之间的相互作用而产生的 δ'O 和 Δ'O 同位素位移效应。该模型有利于太太古代海水，其特征是 δ'O < -8 ‰ 较低，Δ'O 高达 0.01 ‰。该模型也适用于较低 δ'O 时较高的 Δ'O。 我们的结果还表明，如果没有对多阶段水-岩石相互作用进行适当的建模，包括同位素变化和沉积物衍生流体的输入，改变的洋壳的裸露部分只能提供原始海水的遥远证据。由于模拟的同位素变化和流体混合，我们支持全球海水-洋壳相互作用的“弱”耦合。这可能降低了海底热液蚀变在解释整个地质历史中海底玄武岩氧同位素记录方面的相对重要性。