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Ma X, Tkalčić H. Seismic low-velocity equatorial torus in the Earth's outer core: Evidence from the late-coda correlation wavefield. SCIENCE ADVANCES 2024; 10:eadn5562. [PMID: 39213349 PMCID: PMC11364092 DOI: 10.1126/sciadv.adn5562] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/15/2023] [Accepted: 07/26/2024] [Indexed: 09/04/2024]
Abstract
Thermochemical inhomogeneities in the Earth's outer core that enhance our understanding of the geodynamo have been elusive. Seismic constraints on such inhomogeneities would provide clues on the amount and distribution of light elements in the core apart from iron and nickel. Here, we present evidence for a low-velocity volume within the outer core via the global coda correlation wavefield. Several key correlogram features with a unique sensitivity to the liquid core show variations with wave paths remarkably slower in the equatorial than polar planes. We constrain a torus structure at low latitudes with ~2% lower velocity than the surrounding liquid outer core via waveform modeling. We propose a thermochemical origin for such a low-velocity torus, providing important constraints on the dynamical processes of the Earth's outer core.
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Affiliation(s)
- Xiaolong Ma
- Research School of Earth Sciences, The Australian National University, Canberra 2601, ACT, Australia
- Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
| | - Hrvoje Tkalčić
- Research School of Earth Sciences, The Australian National University, Canberra 2601, ACT, Australia
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Zhang Y, Wang Y, Huang Y, Wang J, Liang Z, Hao L, Gao Z, Li J, Wu Q, Zhang H, Liu Y, Sun J, Lin JF. Collective motion in hcp-Fe at Earth's inner core conditions. Proc Natl Acad Sci U S A 2023; 120:e2309952120. [PMID: 37782810 PMCID: PMC10576103 DOI: 10.1073/pnas.2309952120] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2023] [Accepted: 08/15/2023] [Indexed: 10/04/2023] Open
Abstract
Earth's inner core is predominantly composed of solid iron (Fe) and displays intriguing properties such as strong shear softening and an ultrahigh Poisson's ratio. Insofar, physical mechanisms to explain these features coherently remain highly debated. Here, we have studied longitudinal and shear wave velocities of hcp-Fe (hexagonal close-packed iron) at relevant pressure-temperature conditions of the inner core using in situ shock experiments and machine learning molecular dynamics (MLMD) simulations. Our results demonstrate that the shear wave velocity of hcp-Fe along the Hugoniot in the premelting condition, defined as T/Tm (Tm: melting temperature of iron) above 0.96, is significantly reduced by ~30%, while Poisson's ratio jumps to approximately 0.44. MLMD simulations at 230 to 330 GPa indicate that collective motion with fast diffusive atomic migration occurs in premelting hcp-Fe primarily along [100] or [010] crystallographic direction, contributing to its elastic softening and enhanced Poisson's ratio. Our study reveals that hcp-Fe atoms can diffusively migrate to neighboring positions, forming open-loop and close-loop clusters in the inner core conditions. Hcp-Fe with collective motion at the inner core conditions is thus not an ideal solid previously believed. The premelting hcp-Fe with collective motion behaves like an extremely soft solid with an ultralow shear modulus and an ultrahigh Poisson's ratio that are consistent with seismic observations of the region. Our findings indicate that premelting hcp-Fe with fast diffusive motion represents the underlying physical mechanism to help explain the unique seismic and geodynamic features of the inner core.
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Affiliation(s)
- Youjun Zhang
- Institute of Atomic and Molecular Physics, Sichuan University, Chengdu610065, China
- International Center for Planetary Science, College of Earth Sciences, Chengdu University of Technology, Chengdu610059, China
| | - Yong Wang
- National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China
| | - Yuqian Huang
- Institute of Atomic and Molecular Physics, Sichuan University, Chengdu610065, China
| | - Junjie Wang
- National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China
| | - Zhixin Liang
- National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China
| | - Long Hao
- National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang621900, China
| | - Zhipeng Gao
- National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang621900, China
| | - Jun Li
- National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang621900, China
| | - Qiang Wu
- National Key Laboratory for Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang621900, China
| | - Hong Zhang
- College of Physics, Sichuan University, Chengdu610065, China
| | - Yun Liu
- International Center for Planetary Science, College of Earth Sciences, Chengdu University of Technology, Chengdu610059, China
| | - Jian Sun
- National Laboratory of Solid State Microstructures, School of Physics and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing210093, China
| | - Jung-Fu Lin
- Department of Earth and Planetary Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX78712
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Costa de Lima T, Phạm TS, Ma X, Tkalčić H. An estimate of absolute shear-wave speed in the Earth's inner core. Nat Commun 2023; 14:4577. [PMID: 37516735 PMCID: PMC10387060 DOI: 10.1038/s41467-023-40307-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2023] [Accepted: 07/19/2023] [Indexed: 07/31/2023] Open
Abstract
Observations of seismic body waves that traverse the Earth's inner core (IC) as shear (J) waves are critical for understanding the IC shear properties, advancing our knowledge of the Earth's internal structure and evolution. Here, we present several seismological observations of J phases detected in the earthquake late-coda correlation wavefield at periods of 15-50 s, notably via the correlation feature I-J, found to be independent of the Earth reference velocity model. Because I-J is unaffected by compressional wave speeds of the Earth's inner core, outer core, and mantle, it represents an autonomous class of seismological measurements to benchmark the inner core properties. We estimate the absolute shear-wave speed in the IC to be 3.39 ± 0.02 km/s near the top and 3.54 ± 0.02 km/s in the center, lower than recently reported values. This is a 3.4 ± 0.5% reduction from the Preliminary Reference Earth Model (PREM), suggesting a less rigid IC than previously estimated from the normal mode data. Such a low shear-wave speed requires re-evaluating IC composition, including the abundance of light elements, the atomic properties and stable crystallographic phase of iron, and the IC solidification process.
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Affiliation(s)
- Thuany Costa de Lima
- Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.
| | - Thanh-Son Phạm
- Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
| | - Xiaolong Ma
- Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
| | - Hrvoje Tkalčić
- Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia
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Phạm TS, Tkalčić H. Up-to-fivefold reverberating waves through the Earth's center and distinctly anisotropic innermost inner core. Nat Commun 2023; 14:754. [PMID: 36810283 PMCID: PMC9944935 DOI: 10.1038/s41467-023-36074-2] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/02/2022] [Accepted: 01/11/2023] [Indexed: 02/24/2023] Open
Abstract
Probing the Earth's center is critical for understanding planetary formation and evolution. However, geophysical inferences have been challenging due to the lack of seismological probes sensitive to the Earth's center. Here, by stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth's diameter. Differential travel times of these exotic arrival pairs, hitherto unreported in seismological literature, complement and improve currently available information. The inferred transversely isotropic inner-core model contains a ~650-km thick innermost ball with P-wave speeds ~4% slower at ~50° from the Earth's rotation axis. In contrast, the inner core's outer shell displays much weaker anisotropy with the slowest direction in the equatorial plane. Our findings strengthen the evidence for an anisotropically-distinctive innermost inner core and its transition to a weakly anisotropic outer shell, which could be a fossilized record of a significant global event from the past.
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Affiliation(s)
- Thanh-Son Phạm
- Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.
| | - Hrvoje Tkalčić
- grid.1001.00000 0001 2180 7477Research School of Earth Sciences, The Australian National University, Canberra, ACT Australia
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He Y, Sun S, Kim DY, Jang BG, Li H, Mao HK. Superionic iron alloys and their seismic velocities in Earth's inner core. Nature 2022; 602:258-262. [PMID: 35140389 DOI: 10.1038/s41586-021-04361-x] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2020] [Accepted: 10/21/2021] [Indexed: 11/09/2022]
Abstract
Earth's inner core (IC) is less dense than pure iron, indicating the existence of light elements within it1. Silicon, sulfur, carbon, oxygen and hydrogen have been suggested to be the candidates2,3, and the properties of iron-light-element alloys have been studied to constrain the IC composition4-19. Light elements have a substantial influence on the seismic velocities4-13, the melting temperatures14-17 and the thermal conductivities18,19 of iron alloys. However, the state of the light elements in the IC is rarely considered. Here, using ab initio molecular dynamics simulations, we find that hydrogen, oxygen and carbon in hexagonal close-packed iron transform to a superionic state under the IC conditions, showing high diffusion coefficients like a liquid. This suggests that the IC can be in a superionic state rather than a normal solid state. The liquid-like light elements lead to a substantial reduction in the seismic velocities, which approach the seismological observations of the IC20,21. The substantial decrease in shear-wave velocity provides an explanation for the soft IC21. In addition, the light-element convection has a potential influence on the IC seismological structure and magnetic field.
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Affiliation(s)
- Yu He
- Key Laboratory of High-Temperature and High-Pressure Study of the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China. .,Center for High Pressure Science and Technology Advanced Research, Shanghai, China.
| | - Shichuan Sun
- Key Laboratory of High-Temperature and High-Pressure Study of the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China.,University of Chinese Academy of Sciences, Beijing, China
| | - Duck Young Kim
- Center for High Pressure Science and Technology Advanced Research, Shanghai, China
| | - Bo Gyu Jang
- Center for High Pressure Science and Technology Advanced Research, Shanghai, China
| | - Heping Li
- Key Laboratory of High-Temperature and High-Pressure Study of the Earth's Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China
| | - Ho-Kwang Mao
- Center for High Pressure Science and Technology Advanced Research, Shanghai, China
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Huang H, Fan L, Liu X, Xu F, Wu Y, Yang G, Leng C, Wang Q, Weng J, Wang X, Cai L, Fei Y. Inner core composition paradox revealed by sound velocities of Fe and Fe-Si alloy. Nat Commun 2022; 13:616. [PMID: 35105891 PMCID: PMC8807611 DOI: 10.1038/s41467-022-28255-2] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2021] [Accepted: 01/10/2022] [Indexed: 11/08/2022] Open
Abstract
Knowledge of the sound velocity of core materials is essential to explain the observed anomalously low shear wave velocity (VS) and high Poisson's ratio (σ) in the solid inner core. To date, neither VS nor σ of Fe and Fe-Si alloy have been measured under core conditions. Here, we present VS and σ derived from direct measurements of the compressional wave velocity, bulk sound velocity, and density of Fe and Fe-8.6 wt%Si up to ~230 GPa and ~5400 K. The new data show that neither the effect of temperature nor incorporation of Si would be sufficient to explain the observed low VS and high σ of the inner core. A possible solution would add carbon (C) into the solid inner core that could further decrease VS and increase σ. However, the physical property-based Fe-Si-C core models seemingly conflict with the partitioning behavior of Si and C between liquid and solid Fe.
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Affiliation(s)
- Haijun Huang
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Lili Fan
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Xun Liu
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Feng Xu
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Ye Wu
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Gang Yang
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Chunwei Leng
- School of Science, Wuhan University of Technology, Wuhan, Hubei, 430070, China
| | - Qingsong Wang
- National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China
| | - Jidong Weng
- National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China
| | - Xiang Wang
- National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China
| | - Lingcang Cai
- National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China
| | - Yingwei Fei
- Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, 20015, USA.
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Feng J, Yao H, Chen L, Wang W. Massive lithospheric delamination in southeastern Tibet facilitating continental extrusion. Natl Sci Rev 2021; 9:nwab174. [PMID: 35386921 PMCID: PMC8982193 DOI: 10.1093/nsr/nwab174] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2021] [Revised: 09/09/2021] [Accepted: 09/09/2021] [Indexed: 11/13/2022] Open
Abstract
Significant left-lateral movement along the Ailao Shan-Red River fault accommodated a substantial amount of the late Eocene to early Miocene India-Asia convergence. However, the activation of this critical strike-slip fault remains poorly understood. Here, we show key seismic evidence for the occurrence of massive lithospheric delamination in southeastern Tibet. Our novel observation of reflected body waves (e.g. P410P and P660P) retrieved from ambient noise interferometry sheds new light on the massive foundered lithosphere currently near the bottom of the mantle transition zone beneath southeastern Tibet. By integrating the novel seismic and pre-existing geochemical observations, we highlight a linkage between massive lithospheric delamination shortly after the onset of hard collision and activation of continental extrusion along the Ailao Shan-Red River fault. This information provides critical insight into the early-stage evolution of the India-Asia collision in southeastern Tibet, which has significant implications for continental collision and its intracontinental response.
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Affiliation(s)
- Jikun Feng
- Laboratory of Seismology and Physics of Earth's Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
| | - Huajian Yao
- Laboratory of Seismology and Physics of Earth's Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
- CAS Center for Excellence in Comparative Planetology, University of Science and Technology of China, Hefei 230026, China
- Mengcheng National Geophysical Observatory, University of Science and Technology of China, Mengcheng 253500, China
| | - Ling Chen
- State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
- CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
| | - Weitao Wang
- Institute of Geophysics, China Earthquake Administration, Beijing 100081, China
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Ritterbex S, Tsuchiya T. Viscosity of hcp iron at Earth's inner core conditions from density functional theory. Sci Rep 2020; 10:6311. [PMID: 32286388 PMCID: PMC7156496 DOI: 10.1038/s41598-020-63166-6] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/28/2019] [Accepted: 03/18/2020] [Indexed: 11/25/2022] Open
Abstract
The inner core, extending to 1,221 km above the Earth’s center at pressures between 329 and 364 GPa, is primarily composed of solid iron. Its rheological properties influence both the Earth’s rotation and deformation of the inner core which is a potential source of the observed seismic anisotropy. However, the rheology of the inner core is poorly understood. We propose a mineral physics approach based on the density functional theory to infer the viscosity of hexagonal close packed (hcp) iron at the inner core pressure (P) and temperature (T). As plastic deformation is rate-limited by atomic diffusion under the extreme conditions of the Earth’s center, we quantify self-diffusion in iron non-empirically. The results are applied to model steady-state creep of hcp iron. Here, we show that dislocation creep is a key mechanism driving deformation of hcp iron at inner core conditions. The associated viscosity agrees well with the estimates from geophysical observations supporting that the inner core is significantly less viscous than the Earth’s mantle. Such low viscosity rules out inner core translation, with melting on one side and solidification on the opposite, but allows for the occurrence of the seismically observed fluctuations in inner core differential rotation.
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Affiliation(s)
- Sebastian Ritterbex
- Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577, Japan.
| | - Taku Tsuchiya
- Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577, Japan
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