Motohiko Murakami

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Murakami

First Name

Motohiko

ORCID

0000-0002-2165-2994

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09495 - Murakami, Motohiko / Murakami, Motohiko

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Publications 1 - 10 of 23

Nitrogen Speciation in Silicate Melts at Mantle Conditions From Ab Initio Simulations
Huang, Dongyang; Brodholt, John; Sossi, Paolo A.; et al. (2022)
Geophysical Research Letters
Nitrogen (N) is a major ingredient of the atmosphere, but a trace component in the silicate Earth. Its initial inventory in these reservoirs during Earth's early differentiation requires knowledge of N speciation in magmas, for example, whether it outgasses as N-2 or is sequestered in silicate melts as N3-, which remains largely unconstrained over the entire mantle regime. Here we examine N species in anhydrous and hydrous pyrolitic melts at varying P-T-redox conditions by ab-initio calculations, and find N-N bonding under oxidizing conditions from ambient to lower mantle pressures. Under reducing conditions, N interacts with the silicate network or forms N-H bonds, depending on the availability of hydrogen. Redox control of N speciation is demonstrated valid over a P-T space encompassing probable magma ocean depths. Finally, if the Earth accreted from increasingly oxidized materials toward the end of its accretion, an N-enriched secondary atmosphere might be produced and persist until later impacts.
Structure of disordered materials under ambient to extreme conditions revealed by synchrotron x-ray diffraction techniques at SPring-8—recent instrumentation and synergic collaboration with modelling and topological analyses
Ohara, Koji; Onodera, Yohei; Murakami, Motohiko; et al. (2021)
Journal of Physics: Condensed Matter
The structure of disordered materials is still not well understood because of insufficient experimental data. Indeed, diffraction patterns from disordered materials are very broad and can be described only in pairwise correlations because of the absence of translational symmetry. Brilliant hard x-rays from third-generation synchrotron radiation sources enable us to obtain high-quality diffraction data for disordered materials from ambient to high temperature and high pressure, which has significantly improved our grasp of the nature of order in disordered materials. Here, we introduce the progress in the instrumentation for hard x-ray beamlines at SPring-8 over the last 20 years with associated results and advanced data analysis techniques to understand the topology in disordered materials.
Anomalous density, sound velocity, and structure of pressure-induced amorphous quartz
Petitgirard, Sylvain; Sahle, Christoph J.; Malfait, Wim J.; et al. (2022)
Physical Review B
The study of quartz and other silica systems under pressure is one of the most prolific domains of research over the past 50 years because of their applications in material science and fundamental relevance to planetary interiors. The characterization of the amorphous state is essential for the comprehension of pressure-induced amorphization of minerals, the metamorphism observed in shocked materials, and the study of melt structures under pressure. Here, we measured in situ, under static compression the density, sound velocities, and electronic structure of quartz as it passes through its pressure-induced amorphization transition. The transition pressure could be derived from the abrupt increase in density and sound velocity at 24 GPa, and from strong changes in the silicon L2,3 edge and oxygen K edge between 22 and 27 GPa observed in x-ray Raman scattering data, confirming previous results from x-ray diffraction. Above this pressure, our data show an anomalous behavior in density, sound velocity, and electronic fine structure compared to the cold compressed glass and other silica polymorphs. The pressure-induced amorphous quartz has a lower density relative to that of the compressed glass, consistent with the lower average coordination inferred from a different signature in the Si L2,3 and O K electronic absorption edges measured by x-ray Raman scattering spectroscopy. This behavior sheds light on the pressure limit of tetrahedral units in SiO2 components and the existence of polyamorphism in network-forming materials, and highlights the possibility to discriminate between different amorphous states with x-ray Raman scattering spectroscopy.
Thermoelastic Properties of Liquid Fe-Rich Alloys Under Martian Core Conditions
Huang, Dongyang; Li, Yunguo; Khan, Amir; et al. (2023)
Geophysical Research Letters
Seismic measurements made on Mars indicate that the liquid iron-nickel core is rich in light elements; however, the effects of these light components on the elasticity of Mars' core remain poorly constrained. Here, we calculate elastic properties of various liquid Fe-X (X = Ni, S, C, O and H) mixtures using ab initio molecular dynamics simulations. We find that, at martian core conditions, the addition of S and O most effectively decreases the density of liquid iron, followed by C and H, while Ni has a minimal effect. As for compressional sound velocity (Vp), C increases Vp of liquid Fe throughout Mars' core, while both S and O reduce Vp, the intensity of which diminishes with increasing pressure. Assuming a martian core made of a binary mixture, the seismically-inferred density would require the presence of at least 30 wt% S.
Ultrahigh‐Pressure Acoustic Velocities of Aluminous Silicate Glass up to 155 GPa With Implications for the Structure and Dynamics of the Deep Terrestrial Magma Ocean
Saha, Pinku; Murakami, Motohiko; McCammon, Catherine; et al. (2023)
Geophysical Research Letters
We have carried out in situ high-pressure acoustic velocity measurements of (Fe2+, Al)-bearing MgSiO3 glass up to pressures of 155 GPa, which confirmed a distinct pressure-induced trend change in the transverse acoustic velocity (VS) profile around 98 GPa, likely caused by the Si-O coordination number (CN) change from 6 to 6+. Although it has been reported that the substitution of Fe2+ in MgSiO3 glass induces almost linear velocity reduction up to ∼160 GPa, we revealed that the VS profile of (Fe2+, Al)-bearing MgSiO3 becomes anomalously steeper above ∼100 GPa and eventually came to be equivalent to MgSiO3 glass above ∼125 GPa. This implies the incorporation of Al into Fe-bearing MgSiO3 glass significantly facilitates making it far elastically stiffer and thus the densification under pressures well within the Earth's lower mantle. Our results indicate the possible presence of stiff and highly dense silicate melts in deep MOs in the rocky terrestrial planets.
Atomic and Electronic Structure in MgO–SiO₂
Shuseki, Yuta; Kohara, Shinji; Kaneko, Tomoaki; et al. (2024)
The Journal of Physical Chemistry A
Understanding disordered structure is difficult due to insufficient information in experimental data. Here, we overcome this issue by using a combination of diffraction and simulation to investigate oxygen packing and network topology in glassy (g-) and liquid (l-) MgO–SiO₂ based on a comparison with the crystalline topology. We find that packing of oxygen atoms in Mg₂SiO₄ is larger than that in MgSiO₃, and that of the glasses is larger than that of the liquids. Moreover, topological analysis suggests that topological similarity between crystalline (c)- and g-(l-) Mg₂SiO₄ is the signature of low glass-forming ability (GFA), and high GFA g-(l-) MgSiO₃ shows a unique glass topology, which is different from c-MgSiO₃. We also find that the lowest unoccupied molecular orbital (LUMO) is a free electron-like state at a void site of magnesium atom arising from decreased oxygen coordination, which is far away from crystalline oxides in which LUMO is occupied by oxygen’s 3s orbital state in g- and l-MgO–SiO₂, suggesting that electronic structure does not play an important role to determine GFA. We finally concluded the GFA of MgO–SiO₂ binary is dominated by the atomic structure in terms of network topology.
The Composition of Earth's Lower Mantle
Murakami, Motohiko; Khan, Amir; Sossi, Paolo A.; et al. (2024)
Annual Review of Earth and Planetary Sciences
Determining the composition of Earth's lower mantle, which constitutes almost half of its total volume, has been a central goal in the Earth sciences for more than a century given the constraints it places on Earth's origin and evolution. However, whether the major element chemistry of the lower mantle, in the form of, e.g., Mg/Si ratio, is similar to or different from the upper mantle remains debated. Here we use a multidisciplinary approach to address the question of the composition of Earth's lower mantle and, in turn, that of bulk silicate Earth (crust and mantle) by considering the evidence provided by geochemistry, geophysics, mineral physics, and geodynamics. Geochemical and geodynamical evidence largely agrees, indicating a lower-mantle molar Mg/Si of ≥1.12 (≥1.15 for bulk silicate Earth), consistent with the rock record and accumulating evidence for whole-mantle stirring. However, mineral physics–informed profiles of seismic properties, based on a lower mantle made of bridgmanite and ferropericlase, point to Mg/Si ∼ 0.9–1.0 when compared with radial seismic reference models. This highlights the importance of considering the presence of additional minerals (e.g., calcium-perovskite and stishovite) and possibly suggests a lower mantle varying compositionally with depth. In closing, we discuss how we can improve our understanding of lower-mantle and bulk silicate Earth composition, including its impact on the light element budget of the core. ▪ The chemical composition of Earth's lower mantle is indispensable for understanding its origin and evolution. ▪ Earth's lower-mantle composition is reviewed from an integrated mineral physics, geophysical, geochemical, and geodynamical perspective. ▪ A lower-mantle molar Mg/Si of ≥1.12 is favored but not unique. ▪ New experiments investigating compositional effects of bridgmanite and ferropericlase elasticity are needed to further our insight.
Structural evolution in a pyrolitic magma ocean under mantle conditions
Huang, Dongyang; Murakami, Motohiko; Brodholt, John; et al. (2022)
Earth and Planetary Science Letters
Structure and properties of terrestrial magma oceans control the co-evolution of the core, mantle and atmosphere of the early Earth, but are poorly understood because discrepancies remain between experiments and theoretical calculations. Here we combine acoustic velocity measurements and ab initio simulations on pyrolite glass/melt with a silicate Earth-like composition. In the complex system, we find a gradual increase of sound velocity with increasing pressure. Through ab initio simulations, this is explicable by the transition from four- to six-fold coordinated Si occurring over the entire mantle regime. These results are at odds with recent X-ray diffraction measurements, which show an abrupt change in Si-O coordination at 35 GPa. It is however consistent with recent high-pressure data, where Ni partitioning between molten metal and silicate exhibits a similar gradual change with pressure. Unlike amorphous silica, smooth structural evolution in a multicomponent system implies progressive changes in magma ocean properties with depth, such as density, element partitioning and transport properties, which, when incorporated into magma ocean models, may improve our understanding of early history of the Earth and other rocky planets.
The texture of the post-perovskite phase controls the characteristics of the D” seismic discontinuity
Murakami, Motohiko; Kobayashi, Shin-ichiro; Hirao, Naohisa; et al. (2025)
Communications Earth & Environment
The D” seismic discontinuity is one of the most prominent and enigmatic seismic anomalies in Earth’s lower mantle, and its origin remains unclear. Since the discovery of the post-perovskite phase, seismic anisotropy observed in the D” layer has been attributed to the texturing of this phase. However, no substantial seismic velocity jump has been observed across the isotropic post-perovskite phase transition boundary, revealing a critical gap in experimental data linking the discontinuity to phase texture. Here we present in situ high-pressure acoustic velocity and synchrotron X-ray diffraction data on both textured and randomly-oriented MgGeO3 post-perovskite, an analogue for MgSiO3 component, up to 115 gigapascals. The results show that texturing with a (001) slip plane reproduces the shear wave velocity jump at the D” discontinuity, while randomly oriented samples do not. These findings indicate that the texture of the post-perovskite phase can explain most of the key features of the D” discontinuity.
Nitrogen sequestration in the core at megabar pressure and implications for terrestrial accretion
Huang, Dongyang; Siebert, Julien; Sossi, Paolo; et al. (2024)
Geochimica et Cosmochimica Acta
Nitrogen (N) is the most abundant element in Earth's atmosphere, but is extremely depleted in the silicate Earth. However, it is not clear whether core sequestration or early atmospheric loss was responsible for this depletion. Here we study the effect of core formation on the inventory of nitrogen using laser-heated diamond anvil cells. We find that, due to the simultaneous dissolution of oxygen in the metal, N becomes much less siderophile (iron-loving) at pressures and temperatures up to 104 GPa and 5000 K, a thermodynamic condition relevant to the bottom of the magma ocean in the aftermath of the moon-forming giant impact. Using a core–mantle–atmosphere coevolution model, we show that the impact-induced processes (core formation and/or atmospheric loss) are unlikely to account for the observed N anomaly, which is instead best explained by the accretion of mainly N-poor impactors. The terrestrial volatile pattern requires severe N depletion on precursor bodies, prior to their accretion to the proto-Earth.

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Motohiko Murakami

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