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首页> 外文期刊>Nature >Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle
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Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle

机译：铁在热熔岩包裹体中的氧化态表明有热的古生地幔

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Komatiites are volcanic rocks mainly of Archaean age that formed by unusually high degrees of melting of mantle peridotite. Their origin is controversial and has been attributed to either anhydrous melting of anomalously hot mantle or hydrous melting at temperatures only modestly greater than those found today. Here we determine the original Fe~(3+)/ΣFe ratio of 2.7-Gyr-old komatiitic magma from Belingwe, Zimbabwe, preserved as melt inclusions in olivine, to be 0.10 ± 0.02, using iron K-edge X-ray absorption near-edge structure spectroscopy. This value is consistent with near-anhydrous melting of a source with a similar oxidation state to the source of present-day mid-ocean-ridge basalt. Furthermore, this low Fe~(3+)/ΣFe value, together with a water content of only 0.2-0.3 wt% (ref. 7), excludes the possibility that the trapped melt contained significantly more water that was subsequently lost from the inclusions by reduction to H_2 and diffusion. Loss of only 1.5 wt% water by this mechanism would have resulted in complete oxidation of iron (that is, the Fe~(3+)/ΣFe ratio would be ～1). There is also no petrographic evidence for the loss of molecular water. Our results support the identification of the Belingwe komatiite as a product of high mantle temperatures (～1,700 ℃), rather than melting under hydrous conditions (3-5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.
机译：科马蒂岩是主要为古生代时代的火山岩，是由地幔橄榄岩异常高度融化形成的。它们的起源是有争议的，并且被归因于异常热地幔的无水融化或仅比今天发现的温度略高的水合融化。在这里，我们利用近距离的铁K边缘X射线吸收，确定了津巴布韦贝林格威的2.7吉尔的日生岩浆岩的Fe〜（3 +）/ΣFe比值，其作为熔融夹杂物保存在橄榄石中，为0.10±0.02。边缘结构光谱学。该值与具有与当今中海脊玄武岩来源相似的氧化态的来源近乎无水融化相一致。此外，这种低的Fe〜（3 +）/ΣFe值，以及仅0.2-0.3 wt％的水含量（参考7），排除了被困熔体包含大量水的可能性，这些水随后从夹杂物中流失通过还原为H_2并扩散。通过这种机理仅损失1.5 wt％的水就可以使铁完全氧化（即，Fe〜（3 +）/ΣFe的比值约为1）。也没有岩石学证据证明分子水的损失。我们的结果支持将Belingwe科马铁矿鉴定为地幔温度高（〜1,700℃）的产物，而不是在含水条件下（3-5％（重量）水）融化，从而证实了古生时代异常热的地幔的存在。

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