Metallic iron limits silicate hydration in Earth’s transition zone
Metallic iron limits silicate hydration in Earth’s transition zone
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DOI：
10.1073/pnas.1908716116
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发表时间：
2019-10
期刊：
影响因子：
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通讯作者：
F. Zhu;Jie Li;Jiachao Liu;Junjie Dong;Zhenxian Liu
中科院分区：
文献类型：
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作者：
F. Zhu;Jie Li;Jiachao Liu;Junjie Dong;Zhenxian Liu
Significance Some studies suggested that the Earth’s mantle transition zone (MTZ) consists of hydrous silicates while others proposed that it contains iron metal. Here we show that the metallic iron dehydrates hydrous silicates at MTZ conditions, implying that global hydration of silicates and metal saturation are incompatible. Comparing iron production in and water injection to the MTZ, we found that the hydration of MTZ silicates is likely limited to <0.1 wt %, while large amount of the hydrogen can be stored as iron hydride and hydrogen fluid instead. Water-rich domains may still exist near the modern active subducted slabs. Our finding connects the water content to the oxidation state of the MTZ, thus providing a different perspective on volatile cycles in the mantle. The Earth’s mantle transition zone (MTZ) is often considered an internal reservoir for water because its major minerals wadsleyite and ringwoodite can store several oceans of structural water. Whether it is a hydrous layer or an empty reservoir is still under debate. Previous studies suggested the MTZ may be saturated with iron metal. Here we show that metallic iron reacts with hydrous wadsleyite under the pressure and temperature conditions of the MTZ to form iron hydride or molecular hydrogen and silicate with less than tens of parts per million (ppm) water, implying that water enrichment is incompatible with iron saturation in the MTZ. With the current estimate of water flux to the MTZ, the iron metal preserved from early Earth could transform a significant fraction of subducted water into reduced hydrogen species, thus limiting the hydration of silicates in the bulk MTZ. Meanwhile, the MTZ would become gradually oxidized and metal depleted. As a result, water-rich region can still exist near modern active slabs where iron metal was consumed by reaction with subducted water. Heterogeneous water distribution resolves the apparent contradiction between the extreme water enrichment indicated by the occurrence of hydrous ringwoodite and ice VII in superdeep diamonds and the relatively low water content in bulk MTZ silicates inferred from electrical conductivity studies.
