Abstract The record of the Li-isotopic composition of ancient-seawater has the potential to provide important insight into the Earth system. However, we still have an incomplete understanding of the controls on the magnitude and isotopic compositions of the fluxes of Li into and out of the ocean. An important sink for Li from seawater is through low-temperature hydrothermal alteration of the upper oceanic crust. Here we present a detailed study of three adjacent drill cores in 120 Myr old Atlantic crust (Holes 417A, 417D and 418A) that had very different sedimentation histories. Samples from Hole 417A, drilled on a topographic high and exposed to seawater for >20 Myr before sediment started to accumulate, have much higher Li contents (up to ∼70 ppm) than samples from the other sites where sediment began accumulating immediately after crustal accretion (up to ∼20 ppm). Despite this difference in Li content, the δ7Li of the altered basalts in all cores increases with depth over the upper ∼40 m from around -1‰ at the lava-sediment interface to 6±2‰ at ∼40 m depth, remaining roughly constant with depth after this. If the uppermost lavas equilibrated with seawater, with a bulk fractionation factor of ∼0.980±0.003, then the isotopic composition of seawater 120 Myr ago was ∼19±3‰, within the range of other estimates for Cretaceous seawater. Fluid-rock reaction models, with the hydrological regime varying as a function of depth and sediment accumulation history, can explain the observed rock compositions. However, modelling shows that complexities related to the time-varying hydrological regime, along with the possibility of time-varying δ7Li of seawater, mean that the Li-isotopic composition of altered oceanic lavas cannot be uniquely interpreted. The bulk Li content and isotopic composition of the well-sampled 120 Myr crust in Holes 417A, 417D and 418A suggest that Cretaceous upper oceanic crust was a sink for ∼4x109 mol yr-1 of Li. Surprisingly, the bulk Li-isotopic composition of the lavas sampled at these sites (∼6‰) is not much higher than that of fresh MORB, and within uncertainty of that of high-temperature hydrothermal vent fluids; i.e., high- and low-temperature alteration processes approximately balanced one another for Li-isotopes in the late Cretaceous. This could be fortuitous; alternatively, if the ocean was at steady state the main source of Li to the ocean at this time may have had a δ7Li similar to that of high-temperature hydrothermal fluids.

中文翻译：

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DSDP 417和418站点上大洋地壳低温蚀变过程中的锂同位素交换

摘要 古代海水锂同位素组成的记录有可能为了解地球系统提供重要的信息。然而，我们对锂进出海洋通量的大小和同位素组成的控制仍然不完全了解。海水中锂的一个重要汇是通过上大洋地壳的低温热液蚀变。在这里，我们详细研究了 120 Myr 古老的大西洋地壳（417A、417D 和 418A 孔）中三个相邻的钻孔岩心，这些岩心具有非常不同的沉积历史。来自 417A 孔的样品，在地形高处钻孔并在沉积物开始积聚之前暴露在海水中 >20 Myr，与来自地壳吸积后沉积物立即开始积累的其他地点的样品（高达 20 ppm）相比，锂含量高得多（高达 70 ppm）。尽管 Li 含量存在这种差异，但所有岩心中蚀变玄武岩的 δ7Li 都随着上部 40 m 的深度从熔岩-沉积物界面处的约 -1‰ 增加到约 40 m 处的 6±2‰，保持大致恒定在这之后有深度。如果最上面的熔岩与海水平衡，体积分馏系数为~0.980±0.003，那么120 Myr 前海水的同位素组成为~19±3‰，在白垩纪海水的其他估计范围内。流体-岩石反应模型，水文状况随深度和沉积物堆积历史而变化，可以解释观察到的岩石成分。然而，建模表明，与随时间变化的水文状况相关的复杂性，以及随海水 δ7Li 发生变化的可能性，意味着改变的海洋熔岩的锂同位素组成无法得到唯一解释。在 417A、417D 和 418A 孔中采样良好的 120 Myr 地壳的整体锂含量和同位素组成表明，白垩纪上大洋地壳是约 4x109 mol yr-1 锂的汇。令人惊讶的是，在这些地点采样的熔岩的整体锂同位素组成（~6‰）并不比新鲜 MORB 高多少，并且在高温热液喷口流体的不确定性范围内；即，白垩纪晚期锂同位素的高温和低温蚀变过程大致相互平衡。这可能是偶然的；或者，