Science2014-06-10 3:08 PM

地球深部的地震秘密 Melting of subducted basalt at the core-mantle boundary

论文摘要 

两项应用精密工具分析地震现象及地球内部化学组成的新的实验揭示了地震异常会出现在多深的地方。地球的内部像一块分层的蛋糕,其每一层的化学性质都不同。位于地球炽热的核心与地壳之间的地幔层有着最丰富的层次。距离地核最近的地幔部分是地核热量的一个直接的传输管道,它能将地热向上转移--这一过程促成了火山活动。确实,许多迄今为止最大的火山事件看来与地幔深部产生的地震异常相关。如今,在2项独立的研究中,Denis Andrault等与Li Zhang等帮助解释了其原因。

Andrault的小组研究了在地幔及其液态地核之间区域——即核幔边界(CMB)——中洋底壳的熔化性质。(随着板块的潜没地壳会被自然地带到CMB。)由于无法直接进入CMB,对其的观察是通过地震波速度的变化而间接进行的,地震波的传播速度在地核区域是缓慢的。以往的研究曾经发现了接近CMB的异常慢的地震波传播速度。为了了解其来源,Andrault及其同事用CMB区域特征性的压力和温度研究了地壳的熔化行为。他们发现,即使当CMB仍然还是固态时,地壳会在这些极端情况下熔化,而超慢速度区会在该过程中产生。

与此同时,Zhang等人用金刚石压腔技术来压缩被认为组成了地球低层地幔的矿物质,他们报告了一个惊人的发现;尽管过去的研究提出低层地幔在组成上均质并由钙钛矿组成的,但Zhang及其同事发现了一种可能会在那里发现的新的、富含铁的硅酸盐材料。由于地幔物质会控制地震波的行为,这种新材料或能帮助解释过去的研究在非常低的深部所发现的超慢速度。文章的作者说,该新的相位可能在过去被漏掉了,因为跨越其的速度改变太小而令其无法被检测到。由Quentin Williams撰写的一则《观点栏目》提出了更多的见解。

Abstract 

The geological materials in Earth's lowermost mantle control the characteristics and interpretation of seismic ultra–low velocity zones at the base of the core-mantle boundary. Partial melting of the bulk lower mantle is often advocated as the cause, but this does not explain the nonubiquitous character of these regional seismic features. We explored the melting properties of mid-oceanic ridge basalt (MORB), which can reach the lowermost mantle after subduction of oceanic crust. At a pressure representative of the core-mantle boundary (135 gigapascals), the onset of melting occurs at ~3800 kelvin, which is ~350 kelvin below the mantle solidus. The SiO2-rich liquid generated either remains trapped in the MORB material or solidifies after reacting with the surrounding MgO-rich mantle, remixing subducted MORB with the lowermost mantle.

Editor's Summary

Delving deeper into the lower mantle

Earth's lower mantle is an enigmatic region, a transition zone between slowly churning solids and a liquid outer core. Large seismic structures and discontinuities in this region are probably due to sharp gradients in temperature, composition, or mineralogy. Teasing apart the precise effects of these factors requires experiments at lower mantle temperatures and pressures (see the Perspective by Williams). Zhang et al. found that the major mineral phase of the lower mantle decomposes into two minerals. Andrault et al. show how the melting of subducted basalt from the oceanic crust will form pile-like structures on top of the core/mantle boundary.

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