Grain boundary melt films in an experimentally deformed olivine-orthopyroxene rock: Implications for melt distribution in upper mantle rocks

1996 ◽  
Vol 23 (7) ◽  
pp. 701-704 ◽  
Author(s):  
Martyn R. Drury ◽  
John D. Fitz Gerald
Keyword(s):  
Grain Boundary ◽  
Upper Mantle ◽  
Melt Distribution

scholarly journals Upper Mantle Melt Distribution From Petrologically Constrained Magnetotellurics

2019 ◽  
Vol 20 (7) ◽  
pp. 3328-3346 ◽  
Author(s):  
K. Selway ◽  
J. P. O'Donnell ◽  
Sinan Özaydin
Keyword(s):  
Upper Mantle ◽  
Melt Distribution

Origin and significance of seismic discontinuities in the continental upper mantle: An interdisciplinary study

2021 ◽  
Author(s):  
Shun-ichiro Karato ◽  
Lidong Dai ◽  
Gary Egbert ◽  
Jennifer Girard ◽  
Benjamin Murphy ◽  
...  
Keyword(s):  
Electrical Conductivity ◽  
Grain Boundary ◽  
Pressure Dependence ◽  
Upper Mantle ◽  
Grain Boundary Sliding ◽  
Seismic Attenuation ◽  
Depth Range ◽  
Seismic Discontinuities ◽  
Boundary Sliding ◽  
Interdisciplinary Study

<p>              The mid-lithosphere discontinuity (MLD) and the lithosphere-asthenosphere-boundary (LAB) are two well-known seismic discontinuities in the continental upper mantle. Both MLD and LAB are present in most of the continents but at different depths and with different magnitude of velocity change and sharpness. Understanding the causes for these discontinuities including their regional variations is critical in inferring the evolution of the continents from geophysical observations on these discontinuities.</p><p>              Among various models, we focus on the elastically-accommodated grain-boundary sliding (EAGBS) model that provides plausible and unified explanations for the MLD and the LAB (Karato and Park, 2019). This model has a few testable predictions, and the main purpose of this talk is to review the current status of these tests.</p><ul><li>(i) One assumption of the EAGBS model is that EAGBS is enhanced by water. A recent paper by Cline et al. (2018) challenges this hypothesis by showing that water has no effects on attenuation in Ti-doped hydrated olivine. However, the relevance of the results on highly Ti-doped olivine to Ti-poor real upper mantle is unclear.</li> <li>(ii) A clear and unique prediction of the EAGBS is the presence of a peak in seismic attenuation at/near the MLD. However, inferring an attenuation peak in a narrow depth range is challenging and this hypothesis has not been tested.</li> <li>(iii) Another prediction of the “dry” version of the EAGBS model for the MLD is that although seismic wave velocity drops and there is a peak in attenuation, electrical conductivity does not change.</li> <li>(iv) If the MLD is caused by EAGBS, then materials below are in the “relaxed” state. This would explain the lack of large velocity drop at the LAB. However, the validity of this explanation depends on the pressure dependence of grain-boundary sliding. If pressure dependence of EAGBS is large, then the un-relaxed state will re-establish itself at a relatively shallow depth within the lithosphere. In this case, a deeper thermal transition to the relaxed state should produce stronger LAB than reported.  </li> </ul><p>We have conducted an interdisciplinary study to address these issues including mineral physics and seismology. We found that the addition of Ti modifies the defect-related properties of olivine and complicates the application of Cline et al. (2018) to actual upper-mantle conditions. We determined the pressure dependence of olivine grain-growth, from which we infer that the pressure dependence of grain-boundary sliding is small. Regarding the seismological test of attenuation peak, we forward-modeled surface-wave dispersion in a dispersive medium. Calculations show that the over-tones of Love waves are a key to detecting an attenuation peak near the GBS transition. Combined with a comparison of seismological studies (on velocity and attenuation) and MT estimates of electrical conductivity, we will have better constraints on the validity of the EAGBS model for the origin of the MLD.</p>

Partial melting under spreading ridges

1978 ◽  
Vol 288 (1355) ◽  
pp. 501-525 ◽  
Keyword(s):  
Partial Melting ◽  
Upper Mantle ◽  
Thermal Evolution ◽  
Depth Interval ◽  
Ridge Axis ◽  
Eruption Rate ◽  
Ridge Crest ◽  
Spreading Ridges ◽  
Melt Distribution ◽  
Basalt Composition

Factors of importance in partial melting calculations are discussed. The thermal evolution of a geochemical and petrological model of the upwelling asthenosphere beneath a ridge crest is studied numerically. Partial melting, basalt eruption and differentiation of the upwelling asthenosphere is modelled. Melt distribution and density distribution in the top 100 km of the upper mantle are calculated. Partial melting takes place in a depth interval of 25—60 km below the ridge crest. The degree of partial melting is somewhat less than 20 %. About 2.5 times more liquid is produced by partial melting in the upwelling asthenosphere than is erupted at the ridge centre. This excess liquid solidifies in the lithosphere, off-ridge axis below the Moho. The calculated results are in agreement with the observations on the oceanic ridge basalt composition, its average eruption rate, and geochemical estimates of the degree of partial melting in the sub-ridge upper mantle.

scholarly journals Dislocation-accommodated grain boundary sliding as the major deformation mechanism of olivine in the Earth’s upper mantle

2015 ◽  
Vol 1 (9) ◽  
pp. e1500360 ◽  
Author(s):  
Tomohiro Ohuchi ◽  
Takaaki Kawazoe ◽  
Yuji Higo ◽  
Ken-ichi Funakoshi ◽  
Akio Suzuki ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Upper Mantle ◽  
Laboratory Data ◽  
Grain Boundary Sliding ◽  
Dislocation Creep ◽  
Softening Effect ◽  
Water Fugacity ◽  
Boundary Sliding

Understanding the deformation mechanisms of olivine is important for addressing the dynamic processes in Earth’s upper mantle. It has been thought that dislocation creep is the dominant mechanism because of extrapolated laboratory data on the plasticity of olivine at pressures below 0.5 GPa. However, we found that dislocation-accommodated grain boundary sliding (DisGBS), rather than dislocation creep, dominates the deformation of olivine under middle and deep upper mantle conditions. We used a deformation-DIA apparatus combined with synchrotron in situ x-ray observations to study the plasticity of olivine aggregates at pressures up to 6.7 GPa (that is, ~200-km depth) and at temperatures between 1273 and 1473 K, which is equivalent to the conditions in the middle region of the upper mantle. The creep strength of olivine deforming by DisGBS is apparently less sensitive to pressure because of the competing pressure-hardening effect of the activation volume and pressure-softening effect of water fugacity. The estimated viscosity of olivine controlled by DisGBS is independent of depth and ranges from 1019.6to 1020.7Pa·s throughout the asthenospheric upper mantle with a representative water content (50 to 1000 parts per million H/Si), which is consistent with geophysical viscosity profiles. Because DisGBS is a grain size–sensitive creep mechanism, the evolution of the grain size of olivine is an important process controlling the dynamics of the upper mantle.

Impedance Spectroscopy of Upper Mantle Earth Materials

1995 ◽  
Vol 411 ◽  
Author(s):  
J. A. Tyburczy ◽  
J. J. Roberts
Keyword(s):  
Grain Boundary ◽  
Pressure Dependence ◽  
Upper Mantle ◽  
Boundary Behavior ◽  
Complex Impedance ◽  
Major Constituent ◽  
Boundary Conductivity ◽  
Earth’S Interior ◽  
Earth's Interior ◽  
Grain Boundary Transport

ABSTRACTEarth scientists study electrical properties of Earth materials to understand mechanisms of transport and deformation under conditions existing in the Earth's interior, and for comparison with direct determinations of the conductivity – depth structure of the Earth. We have performed impedance spectroscopic studies of natural and artificial (hot-pressed and sintered) rocks composed of (Mg9Fe1)2SiO4 olivine, a major constituent of the Earth's upper mantle between 40 and 400 km depth. The studies were performed over the frequency range 105 to 10−4 Hz at 1 bar total pressure and temperatures of 800 – 1400 °C under controlled oxygen atmospheres. Complex impedance plane analysis of the results shows depressed impedance arcs corresponding to grain interior and grain boundary transport in series, analogous to the behavior of zirconia and other materials. Distinct grain boundary phases that might cause the resistive grain boundary behavior are not observed. The exponent of the oxygen pressure dependence of the grain boundary conductivity ranges from 0.02 to −0.08, which is very different than the 1/5.5 to 1/7 dependence of the polaronic grain interior mechanism. However, lack of constraints on the composition of the intergranular material limit interpretation of these exponents in terms of mechanism. Key issues for application to the Earth's interior are determination of the mechanism and pressure dependence of the grain boundary transport.

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