Akira Miyake

Bio: Akira Miyake is an academic researcher from Aichi University of Education. The author has contributed to research in topics: Monte Carlo method & Metamorphic rock. The author has an hindex of 5, co-authored 9 publications receiving 80 citations. Previous affiliations of Akira Miyake include Nagoya University.

Monte Carlo simulation of normal grain growth in 2- and 3-dimensions: the lattice-model-independent grain size distribution

Abstract: The phenomenon of normal grain growth in pure single phase systems is modeled with the Monte Carlo technique and a series of simulations are performed in 2- and 3-dimensions. The results are compared with natural and experimental monomineralic rock samples. In these simulations various lattice models with different anisotropic features in grain boundary energy are examined in order to check the universality of the simulation results. The obtained microstructure varies with the artificial parameter T ′ in each lattice model, where T ′ means scaled temperature and controls thermal fluctuation on grain boundary motion. As T ′ (thermal fluctuation) increases, the boundary energy anisotropy characterizing each lattice model becomes less important for the evolution of the microstructure. As a result the difference in the grain size distribution among the lattice models, which is significantly large for low T ′, is reduced with increasing T ′. The distribution independent of both the lattice model and the dimension is obtained at sufficiently high T ′ and is very close to the normal distribution when carrying out the weighting procedure with a weight of the square of each grain radius. A comparison of the planar grain size distribution of the natural and experimental rock samples with the 3-D simulation results reveals that the simulations reproduce very well the distributions observed in the real rock samples. Although various factors such as the presence of secondary minerals and a fluid phase, which are not included in the simulation modeling, are generally considered to influence the real grain growth behavior, the good agreement of the distribution indicates that the overall grain growth behavior in real rocks may still be described by the simplified model used in the present simulations. Thus the grain size distribution obtained from the present simulations is possessed of the universal form characterizing real normal grain growth of which the driving force is essentially grain boundary energy reduction through grain boundary migration.

Phase equilibria in the hornblende-bearing basic gneisses of the Uvete area, central Kenya

Abstract: The hornblende-bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio-clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A′(Al + Fe3+− (13/7)Na)-F(Fe2+)-M′(Mg)-C′(Ca-(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other. The phase relations in the Uvete area are in contrast with those in the staurolite-kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low-variance mineral assemblages formed under different values of μH2O and μCO2. These assemblages define overlapping composition volumes in the A′-F′-M′-C’tetrahedron. The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH2O value. Such a fluid composition was externally controlled by the supply of H2O-rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.

A new thermodynamic model for clino- and orthoamphiboles in the system Na2O?CaO?FeO?MgO?Al2O3?SiO2?H2O?O

Abstract: A recent thermodynamic model for the Na–Ca clinoamphiboles in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–O (NCFMASHO), is improved, and extended to include cummingtonite–grunerite and the orthoamphiboles, anthophyllite and gedrite. The clinoamphibole model in NCMASH is adopted, but the extension into the FeO- and Fe2O3-bearing systems is revised to provide thermodynamic consistency and better agreement with natural assemblage data. The new model involves order–disorder of Fe–Mg between the M2, M13 and M4 sites in the amphibole structure, calibrated using the experimental data on site distributions in cummingtonite–grunerite. In the independent set of end-members used to represent the thermodynamics, grunerite (rather than ferroactinolite) is used for FeO, with two ordered Fe–Mg end-members, and magnesioriebeckite (rather than ferritschermakite) is used for Fe2O3. Natural assemblage data for coexisting clinoamphiboles are used to constrain the interaction energies between the various amphibole end-members. For orthamphibole, the assumption is made that the site distributions and the non-ideal formulation is the same as for clinoamphibole. The data set end-members anthophyllite, ferroanthophyllite and gedrite, are used; for the others, they are based on the clinoamphibole end-members, with the necessary adjustments to their enthalpies constrained by natural assemblage data for coexisting clino- and orthoamphiboles. The efficacy of the models is illustrated with P–T grids and various pseudosections, with a particular emphasis on the prediction of mineral assemblages in ferric-bearing systems.

A Practical Guide to Rock Microstructure

Abstract: Preface 1. Background 2. Microstructures of sedimentary rocks 3. Microstructures of igneous rocks 4. Microstructures of metamorphic rocks 5. Microstructures of deformed rocks Mineral symbols used in this book Glossary of microstructural terms.

Denudation history of the high T/P Ryoke metamorphic belt, southwest Japan: constraints from CHIME monazite ages of gneisses and granitoids

Abstract: CHIME (chemical Th–U-total Pb isochron method) monazite ages were determined for gneisses and granitoids from the eastern and western parts of the Ryoke belt separated by about 500 km. The monazite ages for the gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c. 95 Ma in both the eastern and western parts. In the eastern part plutonism continued from c. 95 Ma to c. 68 Ma at intervals of 2–10 Ma, whereas in the western part it ceased at c. 85 Ma. The CHIME monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts. These lines of evidence are incompatible with a current view that the plutonometamorphism in the Ryoke belt becomes younger towards the east. The CHIME monazite ages, coupled with available data on the depth at which the Ryoke metamorphism took place and the emplacement of individual plutons, show that the western part was eroded more rapidly (about 1.5 mm year−1) than the eastern part (about 0.8 mm year−1) over the time span from 91 to 85 Ma. The denudation rates agree well with those in active orogenic belts like the Alps and Himalayas.

Evolution of permafrost on the Qinghai-Xizang (Tibet) Plateau since the end of the late Pleistocene

Abstract: [1] The present distribution of permafrost on the Qinghai-Xizang (Tibet) Plateau (QTP) is largely a relict of the permafrost formed during the late Pleistocene. It has been degrading and shrinking in areal extent under the fluctuating climates, with a general trend of warming, during the Holocene. The major criteria for the occurrence of relict permafrost include the remnants of ancient buried permafrost, relict permafrost tables, thawed sandwiches (taliks), thick-layered ground ice, and periglacial phenomena such as pingo scars, cryoturbations, primary sand and clayey silt wedges, ice wedge casts, aeolian sand dunes and loesses, thick layers of peat, and humic soils. On the basis of 14C dating of soils, comprehensive analyses, and comparisons of the spatiotemporal distribution of relict and modern permafrost and periglacial phenomena, the evolution of permafrost and periglacial environments since the late Pleistocene was divided into seven stages: (1) the cold period at the end of the late Pleistocene (35,000 to 10,800 years B.P.); (2) the period of significant climatic change during the early Holocene (10,800 to ∼8500–7000 years B.P.), (3) the Megathermal period in the middle Holocene (∼8500–7000 to ∼4000–3000 years B.P.), (4) the cold period in the late Holocene (∼4000–3000 to 1000 years B.P.), (5) the warm period in the later Holocene (1000 to 500 years B.P.), (6) the Little Ice Age (500 to 100 years B.P.), and (7) the recent warming period (100 years B.P. to present). The conditions for permafrost development, distribution, and the paleoclimates and paleoenvironments are discussed for each stage.