美国加州理工学院Bednarick, Amanda L.小组在最新研究中，报道了20亿年来岛弧Sr同位素与海水化学的时间协变。这一研究成果于2024年10月15日发表在国际顶尖学术期刊《美国科学院院刊》上。

据悉，岛弧玄武岩（IAB）的化学成分反映了地幔以及俯冲板块的流体和熔体的贡献。放射性海水Sr通过热液蚀变和随后的俯冲作用被添加到洋壳中，经常被用来解释现代IAB中⁸⁷Sr/⁸⁶Sr特征的升高。然而，岛弧岩浆岩⁸⁷Sr/⁸⁶Sr随时间的变化尚未得到充分研究，限制了人们对整个地球历史上影响岛弧Sr收支因素的理解。

为了解决这个问题，研究人员收集了从现代到古元古代的岛弧岩浆岩的⁸⁷Sr/⁸⁶Sr数据（只包括最能保存原始岩浆⁸⁷Sr/⁸⁶Sr的岛弧位置的数据）。初始⁸⁷Sr/⁸⁶Sr值中位数与亏损地幔8⁸⁷Sr/⁸⁶Sr相比持续升高，证明岛弧中的放射性Sr持续富集。岛弧⁸⁷Sr/⁸⁶Sr相对于亏损地幔的高度是可变的。新元古代晚期岛弧⁸⁷Sr/⁸⁶Sr的显著升高，与海水⁸⁷Sr/⁸⁶Sr和Sr浓度的急剧升高相吻合。

为了研究这种潜在的连通性，研究人员输入亏损地幔⁸⁷Sr/⁸⁶Sr、海水⁸⁷Sr/⁸⁶Sr和海水Sr浓度，建立了0-830Ma岛弧岩浆的⁸⁷Sr/⁸⁶Sr模型。该模型再现了汇编数据的总体轨迹。研究人员认为，岛弧⁸⁷Sr/⁸⁶Sr值的时间变化及其与海水化学波动的密切联系表明，海洋地球化学的变化强烈影响了岛弧岩浆的Sr同位素记录。

附：英文原文

Title: Temporal covariation of island arc Sr isotopes and seawater chemistry over the past 2 billion years

Author: Bednarick, Amanda L., Bucholz, Claire E., DePaolo, Donald J., Stolper, Daniel A.

Issue&Volume: 2024-10-15

Abstract: The chemical compositions of island arc basalts (IAB) reflect contributions from the mantle as well as fluids and melts from the subducting slab. Addition of radiogenic seawater Sr to oceanic crust through hydrothermal alteration and subsequent subduction is often invoked to explain elevated ⁸⁷Sr/⁸⁶Sr signatures in modern IAB. However, changes in the ⁸⁷Sr/⁸⁶Sr of island arc magmatic rocks through time has not been investigated, limiting our understanding of the factors influencing the Sr budgets of arcs throughout Earth’s history. To address this, we compiled ⁸⁷Sr/⁸⁶Sr values from island arc magmatic rocks ranging in age from modern to Paleoproterozoic, only including data from island arc localities that best preserve initial magmatic ⁸⁷Sr/⁸⁶Sr. Median initial ⁸⁷Sr/⁸⁶Sr values are consistently elevated compared to depleted mantle ⁸⁷Sr/⁸⁶Sr over this period, indicating persistent enrichment in radiogenic Sr in island arcs. Moreover, the elevation in island arc ⁸⁷Sr/⁸⁶Sr relative to the depleted mantle is variable. A notable rise in island arc ⁸⁷Sr/⁸⁶Sr during the late Neoproterozoic coincides with a steep increase in seawater ⁸⁷Sr/⁸⁶Sr and Sr concentration. To investigate this potential connectivity, we modeled the ⁸⁷Sr/⁸⁶Sr of island arc magmas between 0 and 830 Ma with inputs of depleted mantle 87Sr/86Sr, seawater ⁸⁷Sr/⁸⁶Sr, and seawater Sr concentration. The model reproduces the overall trajectory of the compiled data. We interpret the observed temporal variation in island arc ⁸⁷Sr/⁸⁶Sr values and its close association with fluctuations in seawater chemistry as evidence that changes in marine geochemistry have strongly influenced the Sr isotopic record of island arc magmas over time.

DOI: 10.1073/pnas.2401832121

Source: https://www.pnas.org/doi/abs/10.1073/pnas.2401832121