The Case for Irreversible Chemical Stratification of the Mantle
The Case for Irreversible Chemical Stratification of the Mantle
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地幔不可逆化学分层的案例
DOI：
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发表时间：
2002
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通讯作者：
D. L. Anderson
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作者：
D. L. Anderson
地球的化学分化成一个浮力，橄榄石丰富的上地幔，沿着原地壳材料，钙钛矿丰富的深层，和一个富铁的核心不断发生在吸积。致密的科马提质液体和榴辉岩固体下沉到中地幔深处。大离子亲石元素和原始气体在原上地幔中富集。在随后的演化过程中，大部分地壳元素被从上地幔中蒸出;地幔底部的层收集了来自核的轻浮渣和来自地幔的致密残渣，并与核反应。这种原始物质根据密度、溶解度、硅酸盐相容性和熔点的分离和重力分类，随着行星的生长而变得不可逆转，因为压力对热膨胀的影响。化学边界很难用地震技术探测到，但有证据表明在1000公里附近有一个这样的边界。在此之下，地幔中的挥发物和产热元素可能已经耗尽，代表着除浮力和可熔化合物以及伴随的微量元素之外的增生物质。观测结果还显示，在地幔底部有一个厚的、化学性质不同的层，在某些地方，它可能从核幔边界延伸超过1000公里。该层表现出大规模的迟滞行为，适合于高普朗特数，低瑞利数对流。这种化学和重力分层解决了各种地球动力学和地球化学悖论，并且比单层和双层模型以及可逆分层更符合岩石学和矿物物理学。
Chemical differentiation of the Earth into a buoyant, olivine-rich upper mantle, along with protocrustal materials, a perovskite-rich deeper layer, and an iron-rich core occurred continuously during accretion. Dense komatiitic liquids and eclogitic solids sank to mid-mantle depths. The large-ion lithophile elements and primordial gases accumulated in the proto-upper mantle. During subsequent evolution, most of the crustal elements were sweated out of the upper mantle; the layer at the base of the mantle collected light dross from the core and dense dregs from the mantle and reacted with the core. This fractionation and gravitational sorting of primordial materials according to density, solubility, silicate compatibility, and melting point became irreversible as the planet grew because of the effect of pressure on thermal expansion. Chemical boundaries are hard to detect by seismic techniques, but evidence favors one such boundary near 1000 km. Below this, the mantle is probably depleted in volatiles and the heat-producing elements, and represents the accreted material minus the buoyant and fusable compounds and the accompanying trace elements. Observations also favor a thick, chemically distinct layer at the base of the mantle that may extend, in places, more than 1000 km from the core-mantle boundary. This layer exhibits large-scale sluggish behavior as appropriate for high Prandtl number, low Rayleigh number convection. This kind of chemical and gravitational stratification resolves various geodynamic and geochemical paradoxes, and is more consistent with petrology and mineral physics than one- and two-layer models, and reversible stratification.