ORCID Profile
0000-0001-5036-4572
Current Organisation
Massachusetts Institute of Technology
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Seismology and Seismic Exploration | Mineralogy and Crystallography | Geology | Igneous and Metamorphic Petrology | Extraterrestrial Geology
Publisher: American Geophysical Union (AGU)
Date: 06-2004
DOI: 10.1029/2003JB002407
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-4258
Abstract: & & Although the seismic properties of polycrystalline olivine have been the subject of systematic and comprehensive study at seismic frequencies, the role of orthopyroxene as the major secondary phase in the shallow parts of the Earth& #8217 s upper mantle has so far received little attention. Accordingly, we have newly prepared synthetic melt-free polycrystalline specimens containing different proportions of olivine (Ol, Fo& sub& & /sub& ) and orthopyroxene (Opx, En& sub& & /sub& ) by the solution-gelation method. The resulting specimens, ranging in composition between Ol& sub& & /sub& Opx& sub& & /sub& and Ol& sub& & /sub& Opx& sub& & /sub& & composition, were mechanically tested by torsional forced oscillation at temperatures of 1200 & #186 C to 400 & #186 C accessed during staged cooling under a confining pressure of 200 MPa. The microstructures of tested specimens were evaluated by BSE, EBSD and TEM. The forced-oscillation data, i.e. shear modulus and associated strain-energy dissipation at 1-1000 s period, were closely fitted by a model based on an extended Burgers-type creep function. This model was also required to fit data from previous ultrasonic and Brillouin spectroscopic measurements at ns-& #181 s periods. Within the observational window (1-1000 s), the shear modulus and dissipation vary monotonically with period and temperature for each of the tested specimens, which is broadly comparable with that previously reported for olivine-only s les. There is no evidence of the superimposed dissipation peak reported by Sundberg and Cooper (2010) for an Ol& sub& & /sub& Opx& sub& & /sub& & specimen prepared from natural precursor materials and containing a melt fraction of 1.5%. The higher orthopyroxene concentrations are associated with systematically somewhat lower levels of dissipation and corresponding weaker modulus dispersion. The new findings suggest that the olivine-based model for high-temperature viscoelasticity in upper-mantle olivine requires only modest modification to accommodate the role of orthopyroxene, including appropriate compositional dependence of the unrelaxed modulus and its temperature derivative.& &
Publisher: Elsevier BV
Date: 10-2016
Publisher: Annual Reviews
Date: 30-05-2015
DOI: 10.1146/ANNUREV-EARTH-060313-054732
Abstract: Energy dissipation due to intrinsic attenuation occurs at elevated temperatures in rocks as a result of a range of processes. Ex les where small-strain, transient deformation occurs are seismic waves, tidal deformation, and at longer timescales post-glacial rebound and far-field post-seismic deformation. Experiments at mantle temperatures and seismic frequencies show that grain boundary sliding is a key process that results in a broad absorption band, as indicated by seismic observations. Models of grain boundary sliding predict a smooth transition from elastic behavior through an anelastic regime toward viscous (Maxwell) behavior, consistent with experimental observations. Other mechanisms that may contribute to dissipation in Earth, at least locally, are dislocations and melt. Extrapolation of the laboratory data shows that first-order observations of planetary behavior and structure can be explained by the effects of temperature and pressure on transient creep properties, but that locally, additional mechanisms are required.
Publisher: American Geophysical Union (AGU)
Date: 06-2004
DOI: 10.1029/2003JB002406
Publisher: Wiley
Date: 07-04-2021
Publisher: American Association for the Advancement of Science (AAAS)
Date: 20-04-2012
Abstract: The driving forces behind plate tectonics act on a relatively weak upper mantle, such that the stress accumulated from colliding plates in the crust dissipates with depth. The physical properties of common mantle minerals, such as olivine, may be important in controlling mantle rheology, but they are difficult to measure directly. Farla et al. (p. 332 ) monitored the deformation of randomly oriented olivine crystals at the pressures and temperatures of the upper mantle. Linear defects, known as dislocations, dissipated energy in s les that should normally be stable in several regions of the mantle, including below oceanic crust and around actively subducting slabs. Contrary to previous models, dislocations may d en low-frequency seismic waves traveling through Earth's interior.
Publisher: Springer Science and Business Media LLC
Date: 28-12-2017
Publisher: American Geophysical Union (AGU)
Date: 11-01-2011
DOI: 10.1029/2009JB007174
Publisher: Oxford University Press (OUP)
Date: 11-02-2202
Publisher: Springer Science and Business Media LLC
Date: 03-2018
DOI: 10.1038/NATURE25764
Abstract: Lateral variations of seismic wave speeds and attenuation (dissipation of strain energy) in the Earth's upper mantle have the potential to map key characteristics such as temperature, major-element composition, melt fraction and water content. The inversion of these data into meaningful representations of physical properties requires a robust understanding of the micromechanical processes that affect the propagation of seismic waves. Structurally bound water (hydroxyl) is believed to affect seismic properties but this has yet to be experimentally quantified. Here we present a comprehensive low-frequency forced-oscillation assessment of the seismic properties of olivine as a function of water content within the under-saturated regime that is relevant to the Earth's interior. Our results demonstrate that wave speeds and attenuation are in fact strikingly insensitive to water content. Rather, the redox conditions imposed by the choice of metal sleeving, and the associated defect chemistry, appear to have a substantial influence on the seismic properties. These findings suggest that elevated water contents are not responsible for low-velocity or high-attenuation structures in the upper mantle. Instead, the high attenuation observed in hydrous and oxidized regions of the upper mantle (such as above subduction zones) may reflect the prevailing oxygen fugacity. In addition, these data provide no support for the hypothesis whereby a sharp lithosphere-asthenosphere boundary is explained by enhanced grain boundary sliding in the presence of water.
Publisher: Elsevier BV
Date: 30-05-2005
Publisher: Elsevier BV
Date: 03-2014
Publisher: Elsevier BV
Date: 12-2006
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-4014
Abstract: It is well known that increasing pressure and temperature along upper-mantle geotherms combine to produce a zone of low seismic wave speeds. Beyond such behaviour arising from the anharmonicity of the crystal lattices of the constituent minerals, viscoelastic relaxation may result in further reduction of the wave speeds, along with appreciable attenuation of seismic waves. In order to better constrain such sub-solidus relaxation in olivine-dominated lithologies, we have recently prepared and tested in torsional forced oscillation several new specimens of synthetic polycrystalline olivine (Fo90 olivine buffered by ~5 wt% En90). These specimens were prepared by hot pressing sol-gel precursor powder encapsulated within metal foils (of Ni70Fe30 or Pt) at high temperature (1200-1350& #186 C) and pressure (300 MPa). Enclosure within Ni-Fe foil yields relatively reducing anhydrous conditions and average grains sizes d & #8804 5 & #956 m. The more oxidising and hydrous conditions associated with Pt encapsulation are conducive to grain growth to at least 20 & #956 m. Our forced-oscillation methods have been refined by replacement of the polycrystalline alumina control specimen with single-crystal sapphire, discontinuation of the use of Ni-Fe foils at the ends of the specimens in favour of direct contact with alumina torsion rods, and selective use of austenitic stainless steel as an alternative to the usual mild-steel material for the enclosing jacket. Such testing of fine-grained olivine polycrystals at periods of 1-1000 s and shear strain litudes 10-5 has consistently revealed an essentially monotonically period- and temperature-dependent high-temperature background. The onset of high-temperature anelastic relaxation involves a superimposed dissipation peak of only modest litude plausibly attributed to elastically accommodated grain-boundary sliding. Grain-size sensitivity is incorporated into a Burgers type creep-function model fitted to the (G,Q-1) data for multiple specimens through power-law grain size dependencies of the key characteristic times. The Maxwell time & #964 M, varying as d-mV, defines the transition from anelastic to viscous background behaviour, and & #964 P ~ d-mA, the centre of the distribution of relaxation times for the dissipation peak. The data for the newly prepared pure synthetic specimens of 4-22 mm grain size, tested with the refined experimental methodology, require mV ~ 3 and mA 1.5. These inferences are consistent with micromechanical models for grain-boundary sliding, but yield markedly stronger grain-size sensitivity than previously reported. However, mapping of the tested s les by electron back-scattered diffraction indicates that the density of geometrically necessary dislocations, responsible for lattice curvature, decreases systematically with increasing grain size, raising the possibility that any contribution from dislocation d ing might enhance the apparent grain-size sensitivity. A preliminary extrapolation of the new model for grain-size sensitive viscoelastic relaxation in dry, melt-free dunite to upper-mantle conditions of grain size and pressure suggests shear modulus relaxation 2% and dissipation Q-1 0.01 & #8211 thus unable to account for seismological observations of the mantle beneath young oceanic lithosphere and in subduction zones. Uncertainties in such extrapolation will be discussed, along with other factors that might enhance sub-solidus viscoelastic relaxation including the segregation of trace-element impurities to olivine grain boundaries, and the influence of oxygen and water fugacities.&
Publisher: Geological Society of London
Date: 2011
DOI: 10.1144/SP360.13
Publisher: American Geophysical Union (AGU)
Date: 10-2021
DOI: 10.1029/2021JB022504
Abstract: Although the seismic properties of polycrystalline olivine have been the subject of systematic and comprehensive study at seismic frequencies, the role of orthopyroxene as the major secondary phase in the shallow parts of the Earth's upper mantle has so far received little attention. Accordingly, we have newly prepared three synthetic melt‐free polycrystalline specimens containing different proportions of olivine (Ol, Fo 90 ) and orthopyroxene (Px, En 90 ) by hot pressing precursor powders produced with the solution‐gelation method at 1,200°C and 300 MPa. The resulting specimens (of Ol 95 Px 5 , Ol 70 Px 30 , and Ol 5 Px 95 phase proportions) were mechanically tested by torsional forced‐oscillation from 1,300 or 1,200–400°C during staged cooling under a confining pressure of 200 MPa. Within the observational window (1–1,000 s), the shear modulus and dissipation vary monotonically with period and temperature for each of the tested specimens. There is no evidence of the superimposed high‐temperature dissipation peak reported by Sundberg and Cooper (2010, 0.1080/14786431003746656 ) for an Ol 60 Px 40 specimen derived from natural precursor material and containing ∼1.5% melt. The forced‐oscillation data for each specimen are well‐described by a model based on an extended Burgers‐type creep function. The findings suggest that an olivine‐based model for high‐temperature viscoelasticity in upper mantle olivine requires only modest adjustment of the unrelaxed shear modulus to accommodate the role of orthopyroxene.
Publisher: American Geophysical Union (AGU)
Date: 07-2018
DOI: 10.1029/2018GC007534
Abstract: Some seismic models derived from tomographic studies indicate elevated shear‐wave velocities (≥4.7 km/s) around 120–150 km depth in cratonic lithospheric mantle. These velocities are higher than those of cratonic peridotites, even assuming a cold cratonic geotherm (i.e., 35 mW/m 2 surface heat flux) and accounting for compositional heterogeneity in cratonic peridotite xenoliths and the effects of anelasticity. We reviewed various geophysical and petrologic constraints on the nature of cratonic roots (seismic velocities, lithology/mineralogy, electrical conductivity, and gravity) and explored a range of permissible rock and mineral assemblages that can explain the high seismic velocities. These constraints suggest that diamond and eclogite are the most likely high‐ V s candidates to explain the observed velocities, but matching the high shear‐wave velocities requires either a large proportion of eclogite ( vol.%) or the presence of up to 3 vol.% diamond, with the exact values depending on peridotite and eclogite compositions and the geotherm. Both of these estimates are higher than predicted by observations made on natural s les from kimberlites. However, a combination of ≤20 vol.% eclogite and ~2 vol.% diamond may account for high shear‐wave velocities, in proportions consistent with multiple geophysical observables, data from natural s les, and within mass balance constraints for global carbon. Our results further show that cratonic thermal structure need not be significantly cooler than determined from xenolith thermobarometry.
Publisher: American Geophysical Union (AGU)
Date: 04-2007
DOI: 10.1029/2006JB004586
Publisher: Springer Science and Business Media LLC
Date: 05-12-2010
Publisher: Elsevier BV
Date: 2010
Location: United States of America
Start Date: 2013
End Date: 12-2017
Amount: $370,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 05-2020
End Date: 12-2024
Amount: $252,000.00
Funder: Australian Research Council
View Funded Activity