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Ab Initio Two-Phase Molecular Dynamics on the Melting Curve of SiO_2

Journal of Earth Science 2010年第5期21卷 801-810页

作者： Yusuke Usui taku tsuchiya Geodynamics Research Center Ehime University

Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20-160 GPa and temperatures of 2 500-6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature （Tin） of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary （CMB） of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD （molecular dynamics） simulations. For liquid silica, the O-O coordination number is found to be 12 along the Tm and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10^-9-3.3×10^-9 m2/s, and the viscosity is 0.02-0.03 Pa＇s along the Tin. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.

关键词： Ab Initio Two-Phase Molecular Dynamics on the Melting Curve of SiO2 SiO CMB MgO than

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Expanding-contracting Earth

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Geoscience Frontiers 2013年第3期4卷 341-347页

作者： taku tsuchiya Kenji Kawai Shigenori Maruyama Geodynamic Research Center Ehime UniversityMatsuyamaEhime 790-8577Japan Department of Earth and Planetary Sciences Tokyo Institute of TechnologyMeguroTokyo 152-8551Japan Department of Earth Science University of CaliforniaSanta Barbara 93106CAUSA

The Earth was born from a giant impact at 4.56 Ga. It is generally thought that the Earth subsequently cooled, and hence shrunk, over geologic time. However, if the Earth＇s convection was double-layered, there must have been a peak of expansion during uni-directional cooling. We computed the expansion- contraction effect using first principles mineral physics data. The result shows a radius about 120 km larger than that of the present Earth immediately after the consolidation of the magma-ocean on the surface, and subsequent shrinkage of about 110 km in radius within about 10 m.y., followed by gradual expansion of 11 km in radius due to radiogenic heating in the lower mantle in spite of cooling in the upper mantle in the Archean. This was due to double-layered convection in the Archean with final collapse of overturn with contraction of about g km in radius, presumably by the end of the Archean. Since then, the Earth has gradually cooled down to reduce its radius by around 12 km. Geologic evidence supports the late Archean mantle overturn ca. 2.6 Ga, such as the global distribution of super-liqnidus flood basalts on nearly all cratonic fragments （〉35 examples）. If our inference is correct, the surface environment of the Earth must have undergone extensive volcanism and emergence of local landmasses, because of the thin ocean cover （3-5 km thickness）. Global unconformity appeared in cratonic fragments with stromatolite back to 2.9 Ga with a peak at 2.6 Ga. The global magmatism brought extensive crustal melting to yield explosive felsic volcanism to transport volcanic ash into the stratosphere during the catastrophic mantle overturn. This event seems to be recorded by sulfur mass-independent fractionation （SMIF） at 2.6 Ga. During the mantle overturn, a number of mantle plumes penetrated into the upper mantle and caused local upward doming of by ca. 2-3 km which raised local landmasses above sea-level. The consequent increase of atmospheric oxygen enabled life evolutio

关键词： Mantle overturn Expanding Earth Global unconformity Nutrients Eukaryotes

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The second continent:Existence of granitic continental materials around the bottom of the mantle transition zone

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Geoscience Frontiers 2013年第1期4卷 1-6页

作者： Kenji Kawai Shinji Yamamoto taku tsuchiya Shigenori Maruyama Department of Earth Science University of CaliforniaSanta BarbaraCA 93106USA Department of Earth and Planetary Sciences Tokyo Institute of TechnologyOokayama 2-12-1MeguroTokyo 152-8551Japan Department of Earth and Planetary Sciences Tokyo Institute of TechnologyOokayama 2-12-1MeguroTokyo 152-8551.Japan Department of Earth Science and Astronomy Graduate School of Arts and SciencesThe University of TokyoKomaba 3-8-1MeguroTokyo 153-8902Japan Geodynamics Research Center Ehime University.Bunkyo-cho 2-5MatsuyamaEhime 790-8577Japan

It has been thought that granitic crust, having been formed on the surface, must have survived through the Earth＇s evolution because of its buoyancy. At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction, subduction erosion, and continental subduction. Granitic rocks, the major constituent of the continental crust, are lighter than the mantle at depths shallower than 270 km, but we show here, based on first principles calcu- lations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone, and thus can be subducted in the depth range of 270-660 km. This suggests that there can be two reservoirs of granitic material in the Earth, one on the surface and the other at the base of the mantle transition zone （MTZ）. The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust. Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle. This could explain the anomalous seismic-wave velocities observed around 660 km depth. The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation, chemical discontinuities between granitic material and the surrounding mantle, or a combination thereof.

关键词： Continental growthGraniteFirst principlesMantle transition zoneTectonicsGeodynamics

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Evolving roles of magnifying endoscopy and endoscopic resection for neoplasia in inflammatory bowel diseases

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World Journal of Gastrointestinal Oncology 2022年第3期14卷 646-653页

作者： Shintaro Akiyama taku Sakamoto Joshua M Steinberg Yutaka Saito Kiichiro tsuchiya Department of Gastroenterology University of TsukubaIbaraki 305-8575Japan Endoscopy Division National Cancer Center HospitalTokyo 104-0045Japan Department of Inflammatory Bowel Disease Gastroenterology of the RockiesDenverCO 80218United States

Magnifying endoscopy is a useful technique to differentiate neoplasia from non-neoplastic lesions. Data regarding the clinical utility of magnifying endoscopy for neoplasia in patients with inflammatory bowel disease(IBD) has been *** Kudo’s pit pattern types Ⅲ-Ⅴ are findings suggestive of neoplasia in non-IBD patients, these pit patterns are predictive of IBD-associated neoplasia as ***, active chronic inflammatory processes, particularly regenerative changes, can mimic neoplastic pit patterns and may affect a meticulous evaluation of pit pattern diagnosis in patients with IBD. The clinical evidence regarding the utility of magnifying endoscopy with narrow band imaging or endocytoscopy has also been evolving in regard to the diagnosis of IBD-associated neoplasia. These advanced endoscopic techniques are promising for multiple reasons;not only for making an accurate diagnosis of neoplasia, but also in determining if endoscopic resection is appropriate for such lesions in patients with IBD. In this review, we discuss the diagnostic accuracy and limitations of magnifying endoscopy in assessing IBD-associated neoplasia and examine the feasibility and outcomes of endoscopic resection for these lesions.

关键词： Magnifying endoscopy Neoplasia Ulcerative colitis Inflammatory bowel disease Endoscopic resection

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