Nature2014-06-24 4:45 PM

尖晶橄榄石证明地幔过渡带含水 Hydrous mantle transition zone indicated by ringwoodite included within diamond

论文摘要 

目前尚不清楚固体地球内还残留有多少水以及这些残留的水到底在哪里,其中很多间接测量工作都产生了冲突的结果。在这篇论文中,Graham Pearson等人提供了来自巴西Juína一个金刚石包裹体、关于尖晶橄榄石在陆地上已知首次出现的证据。尖晶橄榄石是橄榄石的一种高压多晶型,首次是在陨石中发现的,被认为是地幔过渡带的一个主要组成成分。这种包裹体富含水的性质提供了直接证据,证明这个过渡带至少是局部含水的,含量大约为1% (重量比)。

Abstract 

The ultimate origin of water in the Earth’s hydrosphere is in the deep Earth—the mantle. Theory and experiments have shown that although the water storage capacity of olivine-dominated shallow mantle is limited, the Earth’s transition zone, at depths between 410 and 660 kilometres, could be a major repository for water, owing to the ability of the higher-pressure polymorphs of olivine—wadsleyite and ringwoodite—to host enough water to comprise up to around 2.5 per cent of their weight. A hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics, yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results, and the issue of whether the transition zone contains abundant water remains highly controversial. Here we report X-ray diffraction, Raman and infrared spectroscopic data that provide, to our knowledge, the first evidence for the terrestrial occurrence of any higher-pressure polymorph of olivine: we find ringwoodite included in a diamond from Juína, Brazil. The water-rich nature of this inclusion, indicated by infrared absorption, along with the preservation of the ringwoodite, is direct evidence that, at least locally, the transition zone is hydrous, to about 1 weight per cent. The finding also indicates that some kimberlites must have their primary sources in this deep mantle region.

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