Hydrous mantle transition zone indicated by ringwoodite included within diamond
Hydrous mantle transition zone indicated by ringwoodite included within diamond
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DOI：
10.1038/nature13080
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发表时间：
2014-03-13
期刊：
影响因子：
64.8
通讯作者：
Vincze, L.
中科院分区：
文献类型：
--
作者：
Pearson, D. G.;Brenker, F. E.;Vincze, L.
地球水圈中水的最终起源在地球深处——地幔。理论(1)和实验（2-4）表明，尽管以橄榄石为主的浅层地幔的储水能力有限，但位于410至660公里深处的地球过渡带可能是一个主要的储水库，因为橄榄石的高压多晶岩——瓦德斯莱岩和环纹岩——能够容纳足够的水，占其重量的2.5%左右。含水过渡带可能在陆地岩浆活动和板块构造中起着关键作用（5-7），然而，尽管实验证明了这些阶段的含水能力，但地球物理探测（如电导率）提供了相互矛盾的结果（8-10），过渡带是否含有丰富的水的问题仍然存在高度争议（11）。在这里，我们报告了x射线衍射、拉曼和红外光谱数据，据我们所知，这些数据提供了陆地上存在高压橄榄石多晶型的第一个证据：我们在巴西Juina的一颗钻石中发现了环伍德石。红外吸收表明，这个包裹体的富水性质，以及环伍德岩的保存，是直接的证据，至少在局部，过渡带是含水的，约占1%的重量。这一发现还表明，一些金伯利岩的主要来源一定是在这个深地幔区域。
The ultimate origin of water in the Earth's hydrosphere is in the deep Earth-the mantle. Theory(1) and experiments(2-4) 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(5-7), yet despite experimental demonstration of the water-bearing capacity of these phases, geophysical probes such as electrical conductivity have provided conflicting results(8-10), and the issue of whether the transition zone contains abundant water remains highly controversial(11). 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 Juina, 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.