Core origin of seismic velocity anomalies at Earth’s core–mantle boundary
Core origin of seismic velocity anomalies at Earth’s core–mantle boundary
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DOI：
10.1038/s41586-023-05713-5
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发表时间：
2023-02
期刊：
影响因子：
64.8
通讯作者：
S. Fu;S. Chariton;V. Prakapenka;S. Shim
中科院分区：
文献类型：
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作者：
S. Fu;S. Chariton;V. Prakapenka;S. Shim
地震研究发现，在核幔边界（CMB），如超低速区（ULVZ）和核心刚性区，精细尺度的异常。超低频带被认为是与地幔有关的过程，-，但对可能的地核起源知之甚少。外核中轻元素的沉淀被认为是解释核心刚性区的原因，但目前还不清楚是什么过程导致了这种沉淀。尽管它的重要性，外芯，熔化行为的Fe-Si-H在相关的压力-温度条件下没有得到很好的理解。在这里，我们报告的观察B2 FeSi从Fe-9wt%Si熔化在氢的存在下，高达125 GPa和3,700 K的使用激光加热的金刚石对顶砧细胞的结晶。氢使B2晶体中的Si浓度急剧增加到Si：Fe = 1，而它主要保留在共存的Fe液体中。B2相中的高Si含量使其在最外层核心温度下以固体形式稳定，并且密度低于周围的液体。因此，富硅微晶可以形成、漂浮和沉积到CMB界面的下侧，这很好地解释了芯侧刚度异常。如果少量的FeSi晶体能够被纳入地幔，它们将在CMB上方形成致密的低速结构，这可能是一些ULVZ的原因。B2 FeSi沉淀促进H在最外层的核心提供了一个单一的核心驱动的起源在CMB的两种类型的异常。这种情况也可以解释的核心一样的钨同位素的签名在洋岛玄武岩，后与沉淀物平衡的物质被夹带到地幔的最上部的地幔柱连接到ULVZ。
Seismic studies have found fine-scale anomalies at the core–mantle boundary (CMB), such as ultralow velocity zones (ULVZs),and the core rigidity zone,. ULVZs have been attributed to mantle-related processes, , , , –, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core, the melting behaviour of Fe–Si–H at relevant pressure–temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe–9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio of Si:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies,. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.