Burying Earth's Primitive Mantle in the Slab Graveyard
Burying Earth's Primitive Mantle in the Slab Graveyard
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DOI：
10.1029/2020gc009396
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发表时间：
2021-03-01
影响因子：
3.5
通讯作者：
van Keken, P. E.
中科院分区：
文献类型：
--
作者：
Jones, T. D.;Sime, N.;van Keken, P. E.
地幔成分的演化可以看作是一个破坏的过程，在这个过程中，初始的化学状态随着时间的推移而叠加和重新加工。对海洋岛屿玄武岩的分析表明，地幔的某些部分在这一过程中幸存下来，保留了化学上的“原始”特征。一个仍然存在的问题是，这个原始的特征是如何在45亿年的剧烈对流中幸存下来的。我们假设地球的一些原始地幔被埋在地核-地幔边界的一块厚板墓地中。我们利用地幔对流的高分辨率有限元模型探讨了这种可能性，其中海洋岩石圈在板块扩张带产生，在板块收敛带俯冲。在俯冲过程中，致密的海洋地壳下沉到地幔的底部，并逐渐积聚形成宽广而坚固的热化学堆。下沉的海洋地壳夹带着周围的地幔，其成分在模式演化的早期主要是原始的。因此，热化学堆最初提供了相对高浓度的原始材料——总计约占其总质量的30%。密集的海洋地壳控制着岩石，阻止了有效的混合，并保留了混合的原始物质。这一过程的重要性与随时间推移的地幔加工速率和在地幔压力和温度下海洋地壳的过剩密度成正比。与其他关于地球原始地幔存在的理论不同，这个理论不要求早期地球具有异常高密度和/或粘度的大范围域。
The evolution of mantle composition can be viewed as a process of destruction whereby the initial chemical state is overprinted and reworked with time. Analyses of ocean island basalts reveals that some portion of the mantle has survived this process, retaining a chemically "primitive" signature. A question that remains is how this primitive signature has survived four and a half billion years of vigorous convection. We hypothesize that some of Earth's primitive mantle is buried within a slab graveyard at the core-mantle boundary. We explore this possibility using high-resolution finite element models of mantle convection, in which oceanic lithosphere is produced at zones of plate spreading and subducted at zones of plate convergence. Upon subduction, dense oceanic crust sinks to the base of the mantle and gradually accumulates to form broad, robust thermochemical piles. Sinking oceanic crust entrains the surrounding mantle whose composition is predominantly primitive early in the model's evolution. As a result, thermochemical piles are initially supplied with relatively high concentrations of primitive material-summing up to similar to 30% their total mass. The dense oceanic crust dominating the piles resists efficient mixing and preserves the primitive material that it is intermingled with. The significance of this process is shown to be proportional the rate of mantle processing through time and the excess density of oceanic crust at mantle pressures and temperatures. Unlike other theories for the survival of Earth's primitive mantle, this one does not require the early Earth to have large-scale domains of anomalously high density and/or viscosity.