To investigate the Li fluxes and its isotopic fractionation during low-temperature seafloor alteration, we present the Li isotope compositions of fifty fresh and altered basalts drilled from the Integrated Ocean Drilling Program (IODP) 329 Expedition Sites U1365 and U1368, which show different low-temperature (<150 °C) alteration styles and intensities (i.e., total volume of secondary minerals and degrees of elemental enrichment) with meager sedimentation rates (0.71 and 1.1 m/Myr, respectively). The fresh basalts (δLi vary from +2.7 to +6.4) show Li isotope compositions consistent with the published N-MORB (+3.5 ± 1.0‰), whereas the altered basalts exhibit considerably higher δLi values (+1.7‰ to +11.6‰). Furthermore, their δLi values show positive correlations with alkali elements and loss on ignition and follow a mixing trend between a MORB-like endmember and a Li-rich seawater-derived endmember (+9.2 ± 0.2‰), which indicates the effects of low-temperature seafloor alteration on Li isotopic compositions. Based on previously reconstructed paleo-seawater Li isotope compositions, the Li isotopic fractionation scale between seawater and altered basalts is estimated (+12.6‰ and +18.0‰ for Sites U1365 and U1368, respectively), which further supports previous conclusions that the scale of Li isotope fractionation changes over time. A mass calculation suggests the upper limits of the Li-uptake fluxes during low-temperature seafloor alteration are 14.9 ± 2.2 × 10 mol/year and 3.6 ± 1.3 × 10 mol/year for Sites U1365 and U1368, respectively. In addition, our estimation shows the Li-uptake flux during low-temperature seafloor alteration in the Cretaceous is comparable to the total Li-input flux, whereas, in the Neogene, the Li-uptake flux during low-temperature seafloor alteration is much lower than that of the Li-input flux, which suggests a large diagenetic Li sink may be necessary to maintain the seawater Li budget balance if we assume the seawater Li concentration is roughly constant.

中文翻译：

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低温蚀变洋壳的锂同位素组成及其对锂循环的影响

为了研究低温海底蚀变期间的锂通量及其同位素分馏，我们提出了从综合海洋钻探计划（IODP）329探险站点U1365和U1368钻探的50个新鲜和蚀变玄武岩的锂同位素组成，它们显示出不同的低温温度（<150 °C）蚀变类型和强度（即次生矿物的总体积和元素富集程度）与微薄的沉积速率（分别为 0.71 和 1.1 m/Myr）。新鲜玄武岩（δLi 范围为 +2.7 至 +6.4）显示的 Li 同位素组成与已发表的 N-MORB（+3.5 ± 1.0‰）一致，而蚀变玄武岩则表现出相当高的 δLi 值（+1.7‰ 至 +11.6‰） 。此外，它们的δLi值与碱元素和烧失量呈正相关，并遵循类MORB端元和富含锂的海水衍生端元之间的混合趋势（+9.2±0.2‰），这表明低-海底温度变化对锂同位素组成的影响。根据先前重建的古海水锂同位素组成，估算了海水与蚀变玄武岩之间的锂同位素分馏规模（U1365和U1368站点分别为+12.6‰和+18.0‰），这进一步支持了先前的结论，即锂的规模同位素分馏随时间而变化。质量计算表明，U1365 和 U1368 站点低温海底蚀变期间的锂吸收通量上限分别为 14.9 ± 2.2 × 10 mol/年和 3.6 ± 1.3 × 10 mol/年。此外，我们的估计表明，白垩纪低温海底蚀变期间的锂吸收通量与总锂输入通量相当，而在新近纪，低温海底蚀变期间的锂吸收通量要低得多大于锂输入通量，这表明如果我们假设海水锂浓度大致恒定，则可能需要大的成岩锂汇来维持海水锂预算平衡。