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Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions

机译:通过在极端条件下测量六方磷酸铁的声速可以推断出对地球内芯成分的限制

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摘要

Hexagonal close-packed iron (hcp-Fe) is a main component of Earth’s inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density–sound velocity data of Earth’s core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (VP) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch’s law for hcp-Fe, which gives us the VP of pure hcp-Fe up to core conditions. We find that Earth’s inner core has a 4 to 5% smaller density and a 4 to 10% smaller VP than hcp-Fe. Our results demonstrate that components other than Fe in Earth’s core are required to explain Earth’s core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth’s core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.
机译:六方密堆积铁(hcp-Fe)是地球内核的主要成分。初步参考地球模型(PREM)中hcp-Fe与内芯之间的密度差异显示出密度不足,这意味着芯中存在轻元素。这样,声速就对核心中的轻元素的数量和种类提供了重要的约束。尽管地震观测提供了地核的密度-声速数据,但在受控实验室条件下很少有测量值可用于比较。我们报告了使用来自金刚石砧座单元中激光加热样品的非弹性X射线散射,hcp-Fe的压缩声速(VP)高达163 GPa和3000K。我们针对hcp-Fe提出了新的高温桦木定律,这使我们能够将纯hcp-Fe的VP提升到核心条件。我们发现,地球的内核比hcp-Fe的密度小4%至5%,VP则小4%至10%。我们的结果表明,与hcp-Fe相比,地球核心中除Fe以外的其他成分都需要用来解释地球的核心密度和速度缺陷。假设温度对铁合金的影响与对hcp-Fe的影响相同,我们根据速度不足来缩小内核中的轻元素。氢是一个很好的选择。因此,地球的核心可能是一个隐藏的储氢库。如果考虑到内核中存在某些熔体,无弹性和/或预熔效应,则硅和硫也是可能的候选物,并可能与PREM达成良好协议。

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