Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling
Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling
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DOI：
10.1073/pnas.2009663117
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发表时间：
2020-11
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通讯作者：
M. Jackson;J. Blichert‐Toft;S. Halldórsson;A. Mundl‐Petermeier;M. Bizimis;M. Kurz;A. Price;Sunna Harðardóttir;L. Willhite;K. Breddam;T. Becker;R. Fischer
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作者：
M. Jackson;J. Blichert‐Toft;S. Halldórsson;A. Mundl‐Petermeier;M. Bizimis;M. Kurz;A. Price;Sunna Harðardóttir;L. Willhite;K. Breddam;T. Becker;R. Fischer
意义 最近在现代羽流热点熔岩中发现的异常 182W 特征为自早期冥古宙以来地球内部区域的生存提供了证据。只有具有古老、高 3He/4He 特征的熔岩才能保留异常的 182W 特征。然而，目前尚不清楚为什么只有高3He/4He熔岩具有异常182W，而其他热点熔岩却没有。我们表明，只有缺乏强再生地壳特征的热点熔岩才会表现出高 3He/4He 和异常 182W。这一观察结果可以通过富含 W 和 4He 的地壳俯冲来解释，该地壳具有低 3He/4He 和正常 182W：俯冲后，地壳掩盖了古代地幔域特有的高 3He/4He 和异常 182W 特征。我们的模型将冥古宙地球化学特征的破坏与板块俯冲和再循环的地球动力学过程联系起来。火山热点喷发的海岛玄武岩 (OIB) 中罕见的高 3He/4He 特征源自地球内部保存的深层区域。只有高 3He/4He OIB 表现出异常 182W（地球最早历史期间继承的同位素特征），支持高 3He/4He 的古老起源。然而，不明白为什么有些 OIB 主机异常 182W，而其他 OIB 主机却没有。我们提供来自冰岛的具有异常 182W 的最高 3He/4He 熔岩（高达大气压的 42.9 倍）的地球化学数据，并研究了 Sr-Nd-Hf-Pb 同位素变化（可用于追踪俯冲、再循环地壳）与高 3He/4He 和异常 182W 的关系。这些数据与全球 OIB 数据一起表明，最高的 3He/4He 和最大量级 182W 异常仅在地球化学贫化的地幔域中发现（具有高 143Nd/144Nd 和低 206Pb/204Pb），缺乏再生材料的强烈特征。相比之下，与回收材料相关的具有最强特征的 OIB 具有较低的 3He/4He 且缺乏异常 182W。这些观测结果为了解古代 He 和 W 特征在地幔中的生存提供了重要线索。我们表明，与回收材料相比，具有异常 182W 的高 3He/4He 地幔域具有较低的 W 和 4He 浓度，因此非常容易被低 3He/4He 和俯冲地壳的正常（非异常）182W 特征叠印。因此，高 3He/4He 和异常 182W 仅保存在受再循环地壳改造最少的地幔域中。该模型将古代高 3He/4He 和异常 182W 的长期保存置于地壳俯冲和再循环的地球动力学背景中，并为地球内部其他早期形成的异质性的生存提供信息。
Significance The recent discovery of anomalous 182W signatures in modern, plume-derived hotspot lavas provides evidence for survival of domains in Earth’s interior since the early Hadean. Only lavas with ancient, high-3He/4He signatures preserve anomalous 182W signatures. However, it is not known why only high-3He/4He lavas have anomalous 182W, while other hotspot lavas do not. We show that only hotspot lavas lacking strong recycled crust signatures exhibit high 3He/4He and anomalous 182W. This observation is explained by subduction of W- and 4He-rich crust that has low 3He/4He and normal 182W: Following subduction, the crust masks high-3He/4He and anomalous 182W signatures characteristic of ancient mantle domains. Our model links destruction of Hadean geochemical signatures with the geodynamic process of plate subduction and recycling. Rare high-3He/4He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-3He/4He OIB exhibit anomalous 182W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high 3He/4He. However, it is not understood why some OIB host anomalous 182W while others do not. We provide geochemical data for the highest-3He/4He lavas from Iceland (up to 42.9 times atmospheric) with anomalous 182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high 3He/4He and anomalous 182W. These data, together with data on global OIB, show that the highest-3He/4He and the largest-magnitude 182W anomalies are found only in geochemically depleted mantle domains—with high 143Nd/144Nd and low 206Pb/204Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low 3He/4He and lack anomalous 182W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-3He/4He mantle domains with anomalous 182W have low W and 4He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low 3He/4He and normal (not anomalous) 182W characteristic of subducted crust. Thus, high 3He/4He and anomalous 182W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high 3He/4He and anomalous 182W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.