Long-term core-mantle interaction explains W-He isotope heterogeneities.
Long-term core-mantle interaction explains W-He isotope heterogeneities.
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DOI：
10.1073/pnas.2215903120
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发表时间：
2023-01-24
影响因子：
11.1
通讯作者：
Korenaga, Jun
中科院分区：
文献类型：
--
作者：
Ferrick, Amy L.;Korenaga, Jun
洋岛玄武岩被认为来自深部地幔物质，表现出钨和氦同位素特征之间的相关性。这种相关性的来源仍然难以捉摸：虽然地幔氦同位素不均匀通常被归因于原始的、未脱气的下地幔储集层，但必须调用额外的过程来进一步解释与钨同位素特征的相关性。结果表明，地核与深部地幔之间的直接相互作用可以自然地解释洋岛玄武岩的钨、氦同位素组成。这种可能性破坏了长期以来的观点，即对地幔的处理肯定是低效的，无法保存原始信号。长期以来，洋岛玄武岩的同位素特征一直被用来推断其来源的性质和地球地幔的长期演化。钨和氦同位素特征之间的反相关性是这些玄武岩中一个特别令人费解的特征，似乎没有单一的过程可以解释这一点。传统上，高的3He/4He特征归因于深部地幔中未脱气的储集层。为获得较低的182W/184W所需的额外工艺往往会带来未观察到的辅助地球化学效应。然而，有人提出，地核为下地幔提供原始氦，从而消除了对未脱气的地幔储藏库的需要。独立地说，富钨核被认为赋予了烟柱来源异常的钨同位素特征。我们提出了同位素扩散可以同时将钨和氦跨越核-地幔边界的观点，这意味着扩散可以自然地解释所观察到的同位素趋势。通过模拟地幔区域的长期同位素演化，我们证明了这种机制可以解释足够多的热柱源物质的同位素比值，在动力输送到地球表面后，与今天的地幔W-He同位素不均一性是一致的。不需要未脱气的地幔储集层，这对早期地球条件，如岩浆海洋的范围具有重要意义。
Ocean island basalts, thought to originate from deep mantle material, exhibit a correlation between tungsten and helium isotopic signatures. The source of this correlation remains elusive: While mantle helium isotope heterogeneities are often attributed to a primitive, undegassed lower mantle reservoir, additional processes must be invoked to further explain the correlation with tungsten isotope signatures. We show that direct interaction between the core and the deep mantle can naturally explain the tungsten and helium isotopic composition of ocean island basalts. This possibility undermines the long-standing view that the processing of the Earth’s mantle must be inefficient to preserve primordial signals. The isotopic characteristics of ocean island basalts have long been used to infer the nature of their source and the long-term evolution of the Earth’s mantle. Anticorrelation between tungsten and helium isotopic signatures is a particularly puzzling feature in those basalts, which no single process appears to explain. Traditionally, the high 3He/4He signature has been attributed to an undegassed reservoir in the deep mantle. Additional processes needed to obtain low 182W/184W often entail unobserved ancillary geochemical effects. It has been suggested, however, that the core feeds the lower mantle with primordial helium, obviating the need for an undegassed mantle reservoir. Independently, the tungsten-rich core has been suggested to impart the plume source with anomalous tungsten isotope signatures. We advance the idea that isotopic diffusion may simultaneously transport both tungsten and helium across the core–mantle boundary, with the striking implication that diffusion can naturally account for the observed isotopic trend. By modeling the long-term isotopic evolution of mantle domains, we demonstrate that this mechanism can account for more than sufficient isotopic ratios in plume-source material, which, after dynamical transport to the Earth’s surface, are consistent with the present-day mantle W-He isotopic heterogeneities. No undegassed mantle reservoir is required, bearing significance on early Earth conditions such as the extent of magma oceans.
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