Department of Geological Sciences, Stanford University, Stanford, California 94305, USA.
Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China.
Nature. 2017 Nov 22;551(7681):494-497. doi: 10.1038/nature24461.
Ultralow-velocity zones (ULVZs) at Earth's core-mantle boundary region have important implications for the chemical composition and thermal structure of our planet, but their origin has long been debated. Hydrogen-bearing iron peroxide (FeOH) in the pyrite-type crystal structure was recently found to be stable under the conditions of the lowermost mantle. Using high-pressure experiments and theoretical calculations, we find that iron peroxide with a varying amount of hydrogen has a high density and high Poisson ratio as well as extremely low sound velocities consistent with ULVZs. Here we also report a reaction between iron and water at 86 gigapascals and 2,200 kelvin that produces FeOH. This would provide a mechanism for generating the observed volume occupied by ULVZs through the reaction of about one-tenth the mass of Earth's ocean water in subducted hydrous minerals with the effectively unlimited reservoir of iron in Earth's core. Unlike other candidates for the composition of ULVZs, FeOH synthesized from the superoxidation of iron by water would not require an extra transportation mechanism to migrate to the core-mantle boundary. These dense FeOH-rich domains would be expected to form directly in the core-mantle boundary region and their properties would provide an explanation for the many enigmatic seismic features that are observed in ULVZs.
地核-地幔边界地区的超低速度带 (ULVZ) 对地球的化学成分和热结构具有重要意义，但它们的起源长期以来一直存在争议。最近在最下地幔的条件下发现，具有黄铁矿型晶体结构的含氢氧化铁过氧化物 (FeOH) 是稳定的。我们使用高压实验和理论计算发现，具有不同氢含量的氧化铁过氧化物具有高密度、高泊松比以及极低的声速，与 ULVZ 一致。在这里，我们还报告了在 86 吉帕斯卡和 2200 开尔文条件下铁与水之间的反应，生成了 FeOH。这将提供一种机制，通过俯冲含水矿物中地球海洋水中约十分之一的质量与地球核心中无限的铁储量之间的反应，产生观测到的 ULVZ 所占据的体积。与 ULVZ 成分的其他候选物质不同，由水对铁的超氧化作用合成的 FeOH 不需要额外的输运机制迁移到核幔边界。这些富含高密度 FeOH 的区域预计将直接在地核-地幔边界区域形成，其性质将为在 ULVZ 中观察到的许多神秘地震特征提供解释。
