英文背景知识：(地幔介绍)

　　The Earth's mantle is a 2,900 km (1,800 mi) thick shell of compressed and heated rock, beginning below the Earth's crust (lithosphere), which extends 5 km (3.1 mi) below the ocean floor and 30 to 50 km (19 - 31 mi) below the continents. The Earth's mantle makes up 70% of Earth's volume, in comparison to the Earth's crust which makes up less than 1% of the total. In fact, the crust is just a thin layer of frozen rock shielding the mantle from outer space. The crust and mantle are separated by a transition area called the Mohorovičić discontinuity (the "Moho") where a certain type of seismic wave quickly speeds up when transiting through.

　　Like the crust, the mantle is largely composed of oxide compounds such as olivine, pyroxenes, spinel, garnet, peridotite, and eclogite. The mantle differs in its chemical ratios from the crust, however. It is composed of roughly 45% oxygen, 23% magnesium, 22% silicon, 6% iron, 2% aluminum, 2% calcium, with trace amounts of sodium, potassium, and other elements. Like the crust, the mantle can broadly be regarded as silicate. Below the mantle are the outer core and inner core of the Earth, making up about 29% of the Earth's volume, and composed primarily of molten (outer core) or solid (inner core) iron and nickel.

　　英文背景知识：(如何探测地幔成分及构造)

　　The Earth has a radius of 6371 km, yet the deepest boreholes reach to a depth of only 12 km. As we cannot probe the inner Earth directly, our knowledge of its

　　interior stems mainly from indirect evidence. Studies of the chemistry of the Solar System and the composition of meteorites can give us broad bounds on the chemistry of the Earth as a whole. Geochemical evidence from magma and rock fragments brought tectonically to the surface from depths of 200 km together with geophysical evidence such as density and magnetic studies and, in particular, seismic studies allow us to constrain further the Earth’s composition and structure.

　　英文背景知识：(关于第三段的实验)

　　A team of scientists, made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite. McNamara, a geophysicist, modeled the stresses the mineral would typically undergo as convection(对流)"This the first time the deformation properties of this mineral have been studied at lower mantle temperatures and pressures," says McNamara. "The goal was to observe where the weak planes are in its crystal structure and how they are oriented." The results of the combined laboratory tests and computer models, he says, show that post-perovskite doesn't fit what is known about conditions in the lowermost mantle.

　　Earth's mantle is a layer that extends from the bottom of the crust, about 25 miles down, to the planet's core, 1,800 miles deep. Scientists divide the mantle into two layers separated by a wide transition zone centered around a depth of about 300 miles. The lower mantle(下地幔)lies below that zone. Most of Earth's lower mantle is made of a magnesium silicate mineral called perovskite(钙钛矿). In 2004, earth scientists discovered that under the conditions of the lower mantle, perovskite can change into a high-pressure form, which they dubbed post-perovskite. Since its discovery, post-perovskite has been geophysicists' favorite candidate to explain the composition of a mysterious layer that forms the bottom of Earth's lower mantle.

　　**中文背景资料：关于第三段科学家实验结论，和旧理论比较。(不确定文章中比较的是哪一组)

　　一个日本研究小组在最新一期英国《自然》杂志上报告说，他们通过模拟实验发现，地幔的上层与下层物质构成不同，更深处的下地幔是由富含硅的矿物形成。

　　地幔位于地壳以下数十公里到约2900 公里深处，包裹着地球内部的地核。根据地球内部地震波传播速度的不同，地幔被认为在地下660 公里处分为上下两层，上地幔由橄榄石等镁含量很高的矿物形成，但是下地幔的构成并不清楚。以前研究人员认为，由于经历地球诞生后约40 亿年的内部对流运动，地幔已经均一化，上地幔与下地幔的构成应是相同的。

　　东北大学副教授村上元彦领导的研究小组注意到，不同矿物中地震波的传播速度不一样，于是他们开发出能够在与下地幔环境相当的高温高压条件下测定地震波速度的实验装置，检测各种矿物传播地震波的速度。实验发现，下地幔主要构成与上地幔不同，而是含有更多硅的钙钛矿。研究小组认为，地幔自从地球诞生以来就在持续对流运动，不过上层与下层没有混合，而是分别独立对流。

　　一种观点认为，地球的成分受到40 多亿年前陨石冲击的影响。研究小组分析下地幔的化学组成后发现，它与太阳系其它行星的平均构成是一致的，陨石冲击并没有带来显著变化。研究小组认为，这一发现改变了对于地球内部基本结构以及地球形成的传统看法。