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Chen B.,Peking University | Jahn B.-M.,National Taiwan University | Suzuki K.,Japan Agency for Marin Earth Science and Technology
Geology | Year: 2013

The Mesozoic high-Mg dioritic rocks in the North China Craton have been suggested to be part of adakitic rocks. The origin of the high-Mg diorites has been attributed to equilibration of partial melts from delaminated mafi c crust (eclogite) with mantle peridotite. Here we present petrological and Os isotopic data against the delamination model, and propose a process of magma mixing between siliceous crustal melts and basaltic magma from metasomatized mantle in a post-kinematic setting for their origin. The magma mixing process is supported by (1) euhedral overgrowths of high-Ca plagioclase and high-Mg pyroxene over low-Ca plagioclase and low-Mg pyroxene, respectively, and (2) highly radiogenic Os isotopic compositions, and negatively correlated Nd and Sr isotopic ratios. Our proposed model is probably applicable to the general mode of origin and tectonic settings of high-Mg adakitic magmas. © 2012 Geological Society of America.

Blaes O.,University of California at Santa Barbara | Krolik J.H.,Johns Hopkins University | Hirose S.,Japan Agency for Marin Earth Science and Technology | Shabaltas N.,University of California at Santa Barbara | Shabaltas N.,Cornell University
Astrophysical Journal | Year: 2011

Standard models of radiation-supported accretion disks generally assume that diffusive radiation flux is solely responsible for vertical heat transport. This requires that heat must be generated at a critical rate per unit volume if the disk is to be in hydrostatic and thermal equilibrium. This raises the question of how heat is generated and how energy is transported in MHD turbulence. By analysis of a number of radiation/MHD stratified shearing-box simulations, we show that the divergence of the diffusive radiation flux is indeed capped at the critical rate, but deep inside the disk, substantial vertical energy flux is also carried by advection of radiation. Work done by radiation pressure is a significant part of the energy budget, and much of this work is dissipated later through damping by radiative diffusion. We show how this damping can be measured in the simulations and identify its physical origins. Radiative damping accounts for as much as tens of percent of the total dissipation and is the only realistic physical mechanism for dissipation of turbulence that can actually be resolved in numerical simulations of accretion disks. Buoyancy associated with dynamo-driven, highly magnetized, nearly isobaric nonlinear slow magnetosonic fluctuations is responsible for the radiation advection flux and also explains the persistent periodic magnetic upwelling seen at all values of the radiation to gas pressure ratio. The intimate connection between radiation advection and magnetic buoyancy is the first example we know of in astrophysics in which a dynamo has direct impact on the global energetics of a system. © 2011. The American Astronomical Society. All rights reserved.

Okazaki K.,Hiroshima University | Katayama I.,Hiroshima University | Noda H.,Japan Agency for Marin Earth Science and Technology
Geophysical Research Letters | Year: 2013

We have determined the permeability of antigorite serpentinite gouge in three orthogonal directions during frictional experiments using a triaxial gas apparatus to better understand fluid flow in subduction fault zones. The experiments were conducted at room temperature, a confining pressure of 150 MPa, a pore pressure of 100 MPa, and a constant slip rate of 0.58 μm/s. Although the initial permeabilities are similar in all directions, anisotropy develops during deformation, with the permeability normal to the shear plane becoming one order of magnitude lower than in the other directions when the shear stress reaches steady state. Formation of such permeability anisotropies may enhance fluid flow along the subduction plate interface and serpentinite-bearing fault zones, important in evaluating potentially heterogeneous distribution of fluid pressure along the fault and in understanding the spatio-temporal variation in the seismic activity. Key Points Shear deformation of serpentine gouge results a strong permeability anisotropy Fluid flow is enhanced along the interface of subducting plates and fault zones Subduction seismicity is controlled by heterogeneous fluid flow at the boundary ©2013. American Geophysical Union. All Rights Reserved.

Noda H.,California Institute of Technology | Noda H.,Japan Agency for Marin Earth Science and Technology | Shimamoto T.,China Earthquake Administration
Journal of Structural Geology | Year: 2012

Generation of large earthquakes involves with behaviors of whole plate boundaries or faults from brittle to ductile regimes. This paper reports stability analyses of halite shear zones using a recently developed rate-and-state friction to flow law with an emphasis on the behaviors across the brittle-ductile transition. The law smoothly connects the friction law with pressure-insensitive flow law without any additional constitutive parameter. Behavior upon a velocity step is characterized by an instantaneous change in shear resistance followed by transient behavior toward a steady-state. These transient behaviors are in opposite directions between friction and flow regimes, resulting in variable transient behaviors across the brittle-ductile transition. Linear stability analyses of a spring-slider system around steady-state solutions predict pressure and temperature conditions for unstable fault motion that are consistent with experimental results. The condition for potential instability is not equal to, but includes that for rate-weakening. A nonlinear analysis at the stable-unstable boundary has revealed that a sub-critical Hopf bifurcation takes place and thus a permanently sustained oscillation around a destabilized steady-state solution does not exist although experimental results suggest it. This issue deserves further study including the investigation of the friction law and construction of a physical model for brittle-ductile transition. © 2011 Elsevier Ltd.

Shi J.,Johns Hopkins University | Krolik J.H.,Johns Hopkins University | Hirose S.,Japan Agency for Marin Earth Science and Technology
Astrophysical Journal | Year: 2010

We study the properties of the turbulence driven by the magnetorotational instability in a stratified shearing box with outflow boundary conditions and an equation of state determined by self-consistent dissipation and radiation losses. A series of simulations with increasing resolution are performed within a fixed computational box. We achieve numerical convergence with respect to radial and azimuthal resolution. As vertical resolution is improved, the ratio of stress to pressure increases slowly, but the absolute levels of both the stress and the pressure increase noticeably. These results are in contrast with those of previous work on unstratified shearing boxes, in which improved resolution caused a diminution in the magnetic field strength. We argue that the persistence of strong magnetic field at higher resolution found in the stratified case is due to buoyancy. In addition, we find that the time-averaged vertical correlation length of the magnetic field near the disk midplane is ≃3 times larger than that found in previous unstratified simulations, decreasing slowly with improved vertical resolution. We further show that the undulatory Parker instability drives the magnetic field upwelling at several scale heights from the midplane that is characteristic of stratified magnetohydrodynamics-turbulent disks. © 2010. The American Astronomical Society. All rights reserved.

Almeev R.R.,Leibniz University of Hanover | Kimura J.-I.,Japan Agency for Marin Earth Science and Technology | Ariskin A.A.,Moscow State Textile University | Ozerov A.Y.,Institute of Volcanology and Seismology
Journal of Volcanology and Geothermal Research | Year: 2013

We present a new dataset for whole-rock major, trace, isotopic, and phenocryst compositions indicating a genetic link between andesites of the Holocene eruptions of the Bezymianny stratovolcano (the Bezymianny stage), the andesitic to dacitic Late Pleistocene lava dome complex (the pre-Bezymianny stage), and the magnesian to high-alumina basalts of the adjacent Kliuchevskoi Volcano. We demonstrate that volcanic products from the Bezymianny stage of volcano evolution are most likely the products of magma mixing between silicic products of the earliest stages of magma fractionation and the less evolved basaltic andesite parental melts periodically injected into the magma reservoir. In contrast, the intermediate and silicic magmas of the pre-Bezymianny stage together with basalts from Kliuchevskoi much more closely resemble the liquid line of descent and may represent a unique prolonged and continuous calc-alkaline trend of magma evolution from high-magnesian basalt to dacite. As a result of the geothermobarometry, we recognize variable conditions of magma fractionation and magma storage beneath Bezymianny for different magma types during its evolution since the Late Pleistocene: (1) 1100-1150°C, 500-640MPa, 1-2.5wt.% H2O for parental basaltic andesite; (2) 1130-1050°C, 700-600MPa, 2.5-5wt.% H2O for two-pyroxene andesites; (3) 1040-990°C, 560-470MPa, 5-6.5wt.% H2O for orthopyroxene-bearing andesites; (4) 950-1000°C, 450-150MPa, 3.5-5.5wt.% H2O for hornblende-bearing andesites; and (5) 950-900°C, 410-250MPa, 6-7wt.% H2O for dacites. Repeated basalt injections and magma fractionation combined with internal mixing in the magma chamber are the main processes responsible for both the complex petrography and the geochemical trends observed in the lavas of Bezymianny Volcano. © 2013 Elsevier B.V.

Almeev R.R.,Leibniz University of Hanover | Ariskin A.A.,Moscow State Textile University | Kimura J.-I.,Japan Agency for Marin Earth Science and Technology | Barmina G.S.,Moscow State Textile University
Journal of Volcanology and Geothermal Research | Year: 2013

Using the updated COMAGMAT model, the crystallization sequences of a Bezymianny Volcano basaltic andesite (Kamchatka, Russia) are simulated in a wide range of thermodynamic conditions (P-T-fO2) as a function of H2O concentration. Comparison of the modeled liquid lines of descent with petrochemical trends of the volcanic suite indicates the parental melts contain 1.5-2wt.% H2O stored under 490-520MPa pressure in the magma plumbing system beneath Bezymianny Volcano. The initial magma originates as a result of the polybaric evolution of mantle-derived high-Mg basaltic magmas of the adjacent Kliuchevskoi Volcano. The subsequent evolution of derivative hydrous and alumina-rich basaltic andesite magmas may proceed under polybaric conditions with an average decompression of ~12MPa per 1% of crystallization. In the course of polybaric crystallization, compositions of pyroxene-bearing andesites can be numerically reproduced and the modeled liquid compositions follow the natural liquid line of descent. However, hornblende-bearing magmas cannot be produced as a result of continued crystallization from parental basaltic andesite through the stage of pyroxene-bearing andesite formation. They require high water contents and high pressures of crystallization. In this case, liquid composition should deviate from the chemical trend defined by the whole rock compositions. © 2013 Elsevier B.V.

Yoshikawa S.,University of Tokyo | Okino K.,University of Tokyo | Asada M.,Japan Agency for Marin Earth Science and Technology
Marine Geology | Year: 2012

This study presents the first detailed geomorphological characterization of field-scale geological features associated with hydrothermal systems in the southern Mariana Trough, using near-bottom swath mapping data collected by the autonomous underwater vehicle (AUV) Urashima during cruise YK09-08 and dive observation data acquired by the submersible Shinkai6500 during cruise YK10-11. The motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity, and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area. Two of the hydrothermal sites in the study area (near 12°57'N, 143°37'E) are located on the active backarc spreading axis (the Snail and Yamanaka sites), one is located at the eastern foot of the axial high (the Archean site), and two are located on an off-axis knoll about 5. km from the spreading axis (the Pika and Urashima sites). The on-axis area is divided into tectonically dominant and volcanically dominant zones; volcanically dominant zones are characterized by mounds (height, 5-30 m; diameter, 250-320 m) cut by fissures. The Snail and Yamanaka sites are located adjacent to these fissures, and are possibly represented local activity associated with a 4th order segment-scale diking event (on the basis of comparisons with previously studied cases on the East Pacific Rise with similar on-axis geological characteristics). In contrast to the on-axis sites, the off-axis sites show no evidence of faulting. The Archean site at the foot of the axial high is characterized by a single mound (height, 50-100; diameter, 250-300 m), pronounced off-axis lava flows, and the presence of high-amplitude rugged seafloor features; the site is located at the top of the mound. Numerous ridge lines (height, mainly 2-6 m) extend radially from the top of the mound, and several chimney-like structures (up to approximately 6 m high) occur on the top and slopes of the mound. The Pika site is located on the western peak of an off-axis knoll, and the newly discovered Urashima site is located at the northern foot of the western peak of the same knoll. The western peak is characterized by bumpy seabed textures formed by numerous smaller-scale mounds and ridge lines; however, the eastern peak has a very smooth top and slope, and shows no signs of hydrothermal activity. Numerous mounds (heights, 5-75 m; diameters, 50-350 m) are developed on the comparatively gentle slope of the knoll, in contrast to the numerous ridge lines (height, mainly 1-6 m) developed on the relatively steep slopes of the knoll. On the basis of the associated geomorphological features, the three off-axis sites (Archean, Pika, and Urashima) were identified as localities created by relatively long-term large-scale hydrothermal activity, as compared with sites in the on-axis area. The sustained activity at off-axis sites appears closely related to an off-axis upwelling magma system. The three off-axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting. These areas are characterized by numerous ridge lines, conical mounds, and bumpy seabed texture, whereas the on-axis sites are characterized by the absence of ridge lines, and the presence of white smoker and shimmering observed on dome-shaped pillow mounds. Hence, the distribution of ridge lines, mound morphology, and bumpy seabed texture is likely to correlate with hydrothermal activity. © 2012 Elsevier B.V.

Okuzumi S.,Nagoya University | Hirose S.,Japan Agency for Marin Earth Science and Technology
Astrophysical Journal | Year: 2011

Turbulence driven by magnetorotational instability (MRI) crucially affects the evolution of solid bodies in protoplanetary disks. On the other hand, small dust particles stabilize MRI by capturing ionized gas particles needed for the coupling of the gas and magnetic fields. To provide an empirical basis for modeling the coevolution of dust and MRI, we perform three-dimensional, ohmic-resistive MHD simulations of a vertically stratified shearing box with an MRI-inactive "dead zone" of various sizes and with a net vertical magnetic flux of various strengths. We find that the vertical structure of turbulence is well characterized by the vertical magnetic flux and three critical heights derived from the linear analysis of MRI in a stratified disk. In particular, the turbulent structure depends on the resistivity profile only through the critical heights and is insensitive to the details of the resistivity profile. We discover scaling relations between the amplitudes of various turbulent quantities (velocity dispersion, density fluctuation, vertical diffusion coefficient, and outflow mass flux) and vertically integrated accretion stresses. We also obtain empirical formulae for the integrated accretion stresses as a function of the vertical magnetic flux and the critical heights. These empirical relations allow us to predict the vertical turbulent structure of a protoplanetary disk for a given strength of the magnetic flux and a given resistivity profile. © 2011. The American Astronomical Society. All rights reserved.

Okuzumi S.,Nagoya University | Hirose S.,Japan Agency for Marin Earth Science and Technology
Astrophysical Journal Letters | Year: 2012

Turbulence driven by magnetorotational instability (MRI) affects planetesimal formation by inducing diffusion and collisional fragmentation of dust particles. We examine conditions preferred for planetesimal formation in MRI-inactive "dead zones" using an analytic dead-zone model based on our recent resistive MHD simulations. We argue that successful planetesimal formation requires not only a sufficiently large dead zone (which can be produced by tiny dust grains) but also a sufficiently small net vertical magnetic flux (NVF). Although often ignored, the latter condition is indeed important since the NVF strength determines the saturation level of turbulence in MRI-active layers. We show that direct collisional formation of icy planetesimal across the fragmentation barrier is possible when the NVF strength is lower than 10 mG (for the minimum-mass solar nebula model). Formation of rocky planetesimals via the secular gravitational instability is also possible within a similar range of the NVF strength. Our results indicate that the fate of planet formation largely depends on how the NVF is radially transported in the initial disk formation and subsequent disk accretion processes. © 2012. The American Astronomical Society. All rights reserved..

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