Institute of Earth Sciences, Academia Sinica, Nankang, 11529 Taipei, Taiwan;
Institute of Earth Sciences, Academia Sinica, Nankang, 11529 Taipei, Taiwan.
Proc Natl Acad Sci U S A. 2018 Apr 17;115(16):4099-4104. doi: 10.1073/pnas.1718557115. Epub 2018 Apr 2.
Iron may critically influence the physical properties and thermochemical structures of Earth's lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8-10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.
铁可能会严重影响地球下地幔的物理性质和热化学结构。然而，其对热导率的影响，以及对热传递和地幔动力学的可能影响，在很大程度上仍然未知。我们使用超快光学泵浦探测技术在金刚石压腔中测量了下地幔尖晶石的晶格热导率，最高压力可达 120GPa。铁含量为 56%的尖晶石在约 53GPa 处的自旋转变时，热导率显著下降了 1.8 倍，而铁含量为 8-10%的尖晶石则随压力单调增加，导致低自旋态下的铁替代效应增强。结合布里奇曼石的数据，我们的结果建模提供了下地幔热导率的自洽径向分布，它主要受压力、温度和铁效应的控制，并显示出从下地幔顶部到底部的两倍增加。这种热导率的增加可能会延迟地核的冷却，而其随铁含量的减少可能会增强大低速剪切波速度区域的动力学。我们的研究结果进一步表明，如果地震超低速区富含铁且温度较高，其导电性会异常低，从而延迟冷却。
