441 research outputs found
Rheological control of oceanic crust separation in the transition zone
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95124/1/grl9376.pd
Adsorption and desorption of deuterium on partially oxidized Si(100) surfaces
Adsorption and desorption of deuterium are studied on the partially oxidized Si(100) surfaces. The partial oxygen coverage causes a decrease in the initial adsorption probability of D atoms. The observed D2 temperature-programmed-desorption (TPD) spectra comprise of multiple components depending on the oxygen coverage (θO). For θO=0.1ML the D2 TPD spectrum is deconvoluted into four components, each of which has a peak in the temperature region higher than the D2 TPD peaking at 780 K on the oxygen free surface. The highest TPD component with a peak around 1040 K is attributed to D adatoms on Si dimers backbonded by an oxygen atom. The other components are attributed to D adatoms on the nearest or second nearest sites of the O-backbonded Si dimers. D adatoms on the partially oxidized Si surfaces are abstracted by gaseous H atoms along two different abstraction pathways: one is the pathway along direct abstraction (ABS) to form HD molecules and the other is the pathway along indirect abstraction via collision-induced-desorption (CID) of D adatoms to form D2 molecules. The ABS pathway is less seriously affected by oxygen adatoms. On the other hand, the CID pathway receives a strong influence of oxygen adatoms since the range of surface temperature effective for CID is found to considerably shift to higher surface temperatures with increasing θO. Gradual substitution of D adatoms with H atoms during H exposure results in HD desorption along the CID pathway in addition to the ABS one. By employing a modulated beam technique the CID-related HD desorption is directly distinguished from the ABS-related one
Sharp thermal transition in the forearc mantle wedge as a consequence of nonlinear mantle wedge flow
Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 38 (2011): L13308, doi:10.1029/2011GL047705.In the forearc mantle wedge, the thermal field depends strongly on slab-driven mantle wedge flow. The flow is in turn affected by the thermal field via the temperature dependence of mantle rheology. Using thermal modeling, we show that the nonlinear feedback between the thermal and flow fields always leads to complete stagnation of the mantle wedge over a shallow, weakened part of the slab-mantle interface and an abrupt onset of mantle flow further down-dip. The abrupt increase in flow velocity leads to a sharp thermal transition from a cold stagnant to a hot flowing part of the wedge. This sharp thermal transition is inherent to all subduction zones, explaining a commonly observed sharp arc-ward increase in seismic attenuation.This research was partially supported by
National Science Foundation through a MARGINS postdoctoral fellowship
(NSF OCE‐0840800) and by Natural Environment Research Council
Mid-mantle deformation inferred from seismic anisotropy
With time, convective processes in the Earth's mantle will tend to align crystals, grains and inclusions. This mantle fabric is detectable seismologically, as it produces an anisotropy in material properties—in particular, a directional dependence in seismic-wave velocity. This alignment is enhanced at the boundaries of the mantle where there are rapid changes in the direction and magnitude of mantle flow, and therefore most observations of anisotropy are confined to the uppermost mantle or lithosphere and the lowermost-mantle analogue of the lithosphere, the D" region. Here we present evidence from shear-wave splitting measurements for mid-mantle anisotropy in the vicinity of the 660-km discontinuity, the boundary between the upper and lower mantle. Deep-focus earthquakes in the Tonga–Kermadec and New Hebrides subduction zones recorded at Australian seismograph stations record some of the largest values of shear-wave splitting hitherto reported. The results suggest that, at least locally, there may exist a mid-mantle boundary layer, which could indicate the impediment of flow between the upper and lower mantle in this region
Effects of pressure on diffusion and vacancy formation in MgO from non-empirical free-energy integrations
The free energies of vacancy pair formation and migration in MgO were
computed via molecular dynamics using free-energy integrations and a
non-empirical ionic model with no adjustable parameters. The intrinsic
diffusion constant for MgO was obtained at pressures from 0 to 140 GPa and
temperatures from 1000 to 5000 K. Excellent agreement was found with the zero
pressure diffusion data within experimental error. The homologous temperature
model which relates diffusion to the melting curve describes well our high
pressure results within our theoretical framework.Comment: 4 pages, latex, 1 figure, revtex, submitted to PR
Precession of a Freely Rotating Rigid Body. Inelastic Relaxation in the Vicinity of Poles
When a solid body is freely rotating at an angular velocity ,
the ellipsoid of constant angular momentum, in the space , has poles corresponding to spinning about the minimal-inertia and
maximal-inertia axes. The first pole may be considered stable if we neglect the
inner dissipation, but becomes unstable if the dissipation is taken into
account. This happens because the bodies dissipate energy when they rotate
about any axis different from principal. In the case of an oblate symmetrical
body, the angular velocity describes a circular cone about the vector of
(conserved) angular momentum. In the course of relaxation, the angle of this
cone decreases, so that both the angular velocity and the maximal-inertia axis
of the body align along the angular momentum. The generic case of an asymmetric
body is far more involved. Even the symmetrical prolate body exhibits a
sophisticated behaviour, because an infinitesimally small deviation of the
body's shape from a rotational symmetry (i.e., a small difference between the
largest and second largest moments of inertia) yields libration: the precession
trajectory is not a circle but an ellipse. In this article we show that often
the most effective internal dissipation takes place at twice the frequency of
the body's precession. Applications to precessing asteroids, cosmic-dust
alignment, and rotating satellites are discussed.Comment: 47 pages, 1 figur
Tidal torques. A critical review of some techniques
We point out that the MacDonald formula for body-tide torques is valid only
in the zeroth order of e/Q, while its time-average is valid in the first order.
So the formula cannot be used for analysis in higher orders of e/Q. This
necessitates corrections in the theory of tidal despinning and libration
damping.
We prove that when the inclination is low and phase lags are linear in
frequency, the Kaula series is equivalent to a corrected version of the
MacDonald method. The correction to MacDonald's approach would be to set the
phase lag of the integral bulge proportional to the instantaneous frequency.
The equivalence of descriptions gets violated by a nonlinear
frequency-dependence of the lag.
We explain that both the MacDonald- and Darwin-torque-based derivations of
the popular formula for the tidal despinning rate are limited to low
inclinations and to the phase lags being linear in frequency. The
Darwin-torque-based derivation, though, is general enough to accommodate both a
finite inclination and the actual rheology.
Although rheologies with Q scaling as the frequency to a positive power make
the torque diverge at a zero frequency, this reveals not the impossible nature
of the rheology, but a flaw in mathematics, i.e., a common misassumption that
damping merely provides lags to the terms of the Fourier series for the tidal
potential. A hydrodynamical treatment (Darwin 1879) had demonstrated that the
magnitudes of the terms, too, get changed. Reinstating of this detail tames the
infinities and rehabilitates the "impossible" scaling law (which happens to be
the actual law the terrestrial planets obey at low frequencies).Comment: arXiv admin note: sections 4 and 9 of this paper contain substantial
text overlap with arXiv:0712.105
Joint inversion of satellite-detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone
We present a new global electrical conductivity model of Earths mantle. The model was derived by using a novel methodology, which is based on inverting satellite magnetic field measurements from different sources simultaneously. Specifically, we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent Swarm and CHAMP data. The challenging task of properly accounting for the ocean effect in the data was addressed through full three-dimensional solution of Maxwell's equations. We show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution. Comparison with laboratory-based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0.01 wt and 0.1 wt in the upper mantle and transition zone, respectively
Stress, strain, and B‐type olivine fabric in the fore‐arc mantle: Sensitivity tests using high‐resolution steady‐state subduction zone models
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94925/1/jgrb15022.pd
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