Kawano Katsutoshi, Nishi Masayuki, Kuwahara Hideharu, Kakizawa Sho, Inoue Toru, Kondo Tadashi
Department of Earth and Space Science, Osaka University, Toyonaka, 560-0043, Japan.
Geodynamics Research Center, Ehime University, Matsuyama, Ehime, 790-8577, Japan.
Nat Commun. 2024 Oct 15;15(1):8701. doi: 10.1038/s41467-024-52677-9.
Seismological observations indicate the presence of chemical heterogeneities at the lowermost mantle, just above the core-mantle boundary (CMB), sparking debate over their origins. A plausible explanation for the enigmatic seismic wave velocities observed in ultra-low-velocity zones (ULVZs) is the process of iron enrichment from the core to the silicate mantle. However, traditional models based on diffusion of atoms and penetration of molten iron fail to account for the significant iron enrichment observed in ULVZs. Here, we show that the chemical reaction between silicate bridgmanite and iron under hydrous conditions leads to profound iron enrichment within silicate, a process not seen in anhydrous conditions. Our findings suggest that the interaction between the core and mantle facilitates deep iron enrichment over a few kilometres at the bottom of the mantle when water is present. We propose that the seismic signatures observed in ULVZs indicate whole mantle convection, accompanied by deep water cycles from the crust to the core through Earth's history.
地震学观测表明，在核幔边界（CMB）上方的下地幔底部存在化学不均一性，这引发了关于其起源的争论。对于在超低速带（ULVZs）观测到的神秘地震波速度，一个合理的解释是铁从地核向硅酸盐地幔富集的过程。然而，基于原子扩散和铁水渗透的传统模型无法解释在超低速带中观测到的显著铁富集现象。在这里，我们表明，在含水条件下，硅酸盐布里奇曼石与铁之间的化学反应会导致硅酸盐内部大量铁富集，这一过程在无水条件下未见。我们的研究结果表明，当有水存在时，地核与地幔之间的相互作用有助于在地幔底部几公里范围内实现深部铁富集。我们提出，在超低速带观测到的地震特征表明整个地幔对流，伴随着地球历史上从地壳到地核的深部水循环。
