Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle
Oxidation state of iron in komatiitic melt inclusions indicates hot Archaean mantle
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DOI：
10.1038/nature07377
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发表时间：
2008-10-16
期刊：
影响因子：
64.8
通讯作者：
Sutton, Stephen R.
中科院分区：
文献类型：
--
作者：
Berry, Andrew J.;Danyushevsky, Leonid V.;Sutton, Stephen R.
科马提岩是由地幔橄榄岩异常高的熔融程度形成的以太古代为主的火山岩。它们的起源是有争议的，并被归因于异常热的地幔的无水熔融(1-3)或仅略高于今天发现的温度的水合熔融(4，5)。本文利用铁K边X射线吸收近边结构光谱测定了津巴布韦贝灵圭2.7Gyr古老的科马提质岩浆(6)的原始Fe(3+)/Sigma Fe比值为0.10+/-0.02。这个值与接近无水熔融的源岩相一致，该源岩的氧化状态与今天大洋中脊玄武岩的源岩相似。此外，这种低的Fe(3+)/Sigma Fe值，加上只有0.2-0.3wt%的水分(参考文献)。7)，排除了捕获的熔体含有明显更多的水的可能性，这些水随后通过还原为H(2)和扩散从包裹体中损失。通过这种机制，只损失1.5wt%的水将导致铁的完全氧化(即，Fe(3+)/Sigma Fe比率将类似于1)。也没有岩石学证据表明分子水的损失。我们的结果支持贝灵韦科马提岩是地幔高温(类似于1,700℃)的产物，而不是在含水条件下(3-5-wt%水)熔融的产物，证实了太古代异常热地幔的存在。
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(1-3) or hydrous melting at temperatures only modestly greater than those found today(4,5). Here we determine the original Fe(3+)/Sigma Fe ratio of 2.7- Gyr- old komatiitic magma from Belingwe, Zimbabwe(6), 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+)/Sigma Fe 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+)/Sigma Fe ratio would be similar to 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 (similar to 1,700 degrees C), rather than melting under hydrous conditions ( 3-5-wt% water), confirming the existence of anomalously hot mantle in the Archaean era.