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雅思听力材料：天上掉钻石

2018-04-09 15:15:23来源：网络 柯林斯词典

　　This theory holds that the organic material, when exposed to the extreme heat and pressure within the Earth for millions of years, produced the carbon in diamonds. But the fossil record, and the dating of diamonds, indicate that this carbon is at least three billion years older than animal and plant life.

　　Haggerty suspects that some of the carbon in these diamonds was in fact produced in supernovae: the explosions of dying stars. The carbon was incorporated into our solar system, where it is the fourth most abundant element.

　　This carbon, plus some that was brought to Earth on meteorites, may well be the source of diamonds, Haggerty says. The study of diamond has seen a recent burst of activity as new research methods have become available. The new theory is based on an evaluation of this scientific literature, he notes.

　　Evidence lending weight to the "stardust" theory includes the antiquity of the diamonds, and the similarity of carbon isotopic ratios to those found in meteorites, Haggerty says.

　　Also, the bulk composition of the Earth is chondritic; that is, similar to a class of meteorites called chondrites. Chondrites contain several forms of carbon, including diamonds older than our sun.

　　"Because the early Earth was bombarded by meteorites," he says, "it is reasonable to conclude that the carbon in diamonds on the Earth is primordial."

　　Scientists have shown that most diamonds are brought from the Earth's interior to its surface by volcanoes. But the volcanoes that bring forth these precious stones are much younger than the diamonds themselves, according to Haggerty.

　　"This combination of old diamonds and young volcanoes indicates that the diamonds were already formed when magma brought them to the surface," said Haggerty.

　　Additionally, there were two geologically short time periods during which hundreds of diamond-producing volcanoes erupted all over the Earth. One group erupted about one billion years ago, and the other 100 million years ago.

　　Haggerty suggests that the eruptions were the result of the "blooming" of molten plumes from the Earth's core. The volcanoes occurred randomly around the planet, rather than along continental plate boundaries, lending support to the model of deep primary carbon.



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