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Nature:地球铀同位素的循环

摘要 : Morten Andersen等人发现,下潜到地幔中的铀在同位素组成上应当是截然不同的,238U/235U比例较高,这是由含氧海洋底部的蚀变(alteration)过程造成的,而且“洋中脊”玄武岩也的确比整个地球拥有更高的238U/235U比例,从而证实上层地幔中铀的再循环普遍存在。

 

Morten Andersen等人发现,下潜到地幔中的铀在同位素组成上应当是截然不同的,238U/235U比例较高,这是由含氧海洋底部的蚀变(alteration)过程造成的,而且“洋中脊”玄武岩也的确比整个地球拥有更高的238U/235U比例,从而证实上层地幔中铀的再循环普遍存在。

虽然曾有人认为很多海岛玄武岩也含有一个循环过的成分,但本文作者发现,它们的铀同位素组成与整个地球的铀同位素组成相比并未受到扰动。

这表明,海岛玄武岩中下潜的铀在距今6亿年前左右发生的完全海洋氧化之前从同位素组成上来讲并未被分馏,从而反映了“海岛玄武岩”这个来源更为古老。

相比之下,“洋中脊”玄武岩的铀同位素组成则要求循环过的铀过去6亿年内在整个上层地幔中进行对流搅动。

原文链接:

The terrestrial uranium isotope cycle

Changing conditions on the Earth’s surface can have a remarkable influence on the composition of its overwhelmingly more massive interior. The global distribution of uranium is a notable example. In early Earth history, the continental crust was enriched in uranium. Yet after the initial rise in atmospheric oxygen, about 2.4 billion years ago, the aqueous mobility of oxidized uranium resulted in its significant transport to the oceans and, ultimately, by means of subduction, back to the mantle. Here we explore the isotopic characteristics of this global uranium cycle. We show that the subducted flux of uranium is isotopically distinct, with high 238U/235U ratios, as a result of alteration processes at the bottom of an oxic ocean. We also find that mid-ocean-ridge basalts (MORBs) have 238U/235U ratios higher than does the bulk Earth, confirming the widespread pollution of the upper mantle with this recycled uranium. Although many ocean island basalts (OIBs) are argued to contain a recycled component, their uranium isotopic compositions do not differ from those of the bulk Earth. Because subducted uranium was probably isotopically unfractionated before full oceanic oxidation, about 600 million years ago, this observation reflects the greater antiquity of OIB sources. Elemental and isotope systematics of uranium in OIBs are strikingly consistent with previous OIB lead model ages, indicating that these mantle reservoirs formed between 2.4 and 1.8 billion years ago. In contrast, the uranium isotopic composition of MORB requires the convective stirring of recycled uranium throughout the upper mantle within the past 600 million years.(doi:10.1038/nature14062)

Earth science: Mixing it up in the mantle

Analysis reveals that the uranium isotopic composition of oceanic crust that is being subducted into Earth's interior is distinctive, allowing the development of chemical heterogeneity in the mantle to be tracked.(doi:10.1038/517275a)

对应Nature杂志: 2015年01月15日Nature杂志精选

来源: Nature 浏览次数:77

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