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  • 1
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    Torsion of a cylinder of partially molten rock with a spherical inclusion: Theory and simulation (2015)
    Wiley
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    Publication Date: 2015-12-16
    Description: The processes that are involved in migration and extraction of melt from the mantle are not yet fully understood. Gaining a better understanding of material properties of partially molten rock could help shed light on the behavior of melt on larger scales in the mantle. In this study, we simulate three-dimensional torsional deformation of a partially molten rock that contains a rigid, spherical inclusion. We compare the computed porosity patterns to those found in recent laboratory experiments. The laboratory experiments show emergence of melt-rich bands throughout the rock sample, and pressure shadows around the inclusion. The numerical model displays similar melt-rich bands only for a small bulk-to-shear-viscosity ratio (five or less). The results are consistent with earlier two-dimensional numerical simulations; however, we show that it is easier to form melt-rich bands in three dimensions compared to two. The addition of strain-rate dependence of the viscosity causes a distinct change in the shape of pressure shadows around the inclusion. This change in shape presents an opportunity for experimentalists to identify the strain-rate dependence and therefore the dominant deformation mechanism in torsion experiments with inclusions. This article is protected by copyright. All rights reserved.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    The energetics of melting fertile heterogeneities within the depleted mantle (2011)
    Wiley
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    Publication Date: 2011-10-21
    Description: To explore the consequences of mantle heterogeneity for primary melt production, we develop a mathematical model of energy conservation for an upwelling, melting body of recycled oceanic crust embedded in the depleted upper mantle. We consider the end-member geometric cases of spherical blobs and tabular veins. The model predicts that thermal diffusion into the heterogeneity can cause a factor-of-two increase in the degree of melting for bodies with minimum dimension smaller than ∼1 km, yielding melt fractions between 50 and 80%. The role of diffusion is quantified by an appropriately defined Peclet number, which represents the balance of diffusion-driven and adiabatic melting. At intermediate Peclet number, we show that melting a heterogeneity can cool the ambient mantle by up to ∼20 K (spherical) or ∼60 K (tabular) within a distance of two times the characteristic size of the body. At small Peclet number, where heterogeneities are expected to be in thermal equilibrium with the ambient mantle, we calculate the energetic effect of pyroxenite melting on the surrounding peridotite; we find that each 5% of recycled oceanic crust diminishes the peridotite degree of melting by 1–2%. Injection of the magma from highly molten bodies of recycled oceanic crust into a melting region of depleted upper mantle may nucleate reactive-dissolution channels that remain chemically isolated from the surrounding peridotite.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Experimental study of dislocation damping using a rock analogue (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract In order to explore the effects of dislocations on seismic velocity and attenuation, we conducted a series of forced oscillation and ultrasonic tests on rock analogue samples (polycrystalline borneol) that were pre‐deformed under various differential stress ∆σ. Additionally, creep experiments were conducted to determine the steady‐state flow law for borneol. The dominant deformation mechanism of polycrystalline borneol changes from diffusion to dislocation creep at about ∆σ =2 MPa. At high stresses, power law creep with a stress exponent of ~4 was measured. Microstructure of the deformed samples showed wavy grain boundaries due to dislocation‐induced migration, and the occasional existence of microcracks. A borneol sample deformed in the dislocation creep regime showed a significant reduction in Young's modulus E and a slight increase in attenuation Q‐1 at frequencies lower than 100 Hz, whereas E at ultrasonic frequency (106 Hz) did not reduce. Therefore, a major part of the dislocation creep‐induced anelastic relaxation is a peak with a characteristic frequency between 100 and 106 Hz, which is much higher than the range of grain boundary‐induced anelasticity of this material. Further experiments under higher confining pressure are needed to assess the relative contribution from dislocations and microcracks to this peak.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Rate of Melt Ascent Beneath Iceland From the Magmatic Response to Deglaciation (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Observations of the time lag between the last deglaciation and a surge in volcanic activity in Iceland constrain the average melt ascent velocity to be ≥50 m/year. Although existing theoretical work has explained why the surge in eruption rates increased 5–30‐fold from the steady‐state rates during the last deglaciation, they cannot account for large variations of Rare Earth Element (REE) concentrations in the Icelandic lavas. Lavas erupted during the last deglaciation are depleted in REEs by up to 70%; whereas existing models, which assume instantaneous melt transport, can only produce at most 20% depletion. Here we develop a numerical model with finite melt ascent velocity and show that the variations of REEs are strongly dependent on the melt ascent velocity. When the average melt ascent velocity is 100 m/year, the variation of La calculated by our model is comparable to that of the observations. In contrast, when the melt ascent velocity is 1,000 m/year or above, the model variation of La becomes significantly lower than observed, which explains why previous models with instantaneous melt transport did not reproduce the large variations. We provide the first model that takes account of the diachronous response of volcanism to deglaciation. We show by comparing our model calculations of the relative volumes of different eruption types (subglacial, finiglacial, and postglacial) and the timing of the bursts in volcanic eruptions with the observations across different volcanic zones that the Icelandic average melt ascent velocity during the last deglaciation is likely to be ∼100 m/year.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Uplift histories of Africa and Australia from linear inverse modeling of drainage inventories (2015)
    Wiley
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    Publication Date: 2015-02-24
    Description: We describe and apply a linear inverse model which calculates spatial and temporal patterns of uplift rate by minimizing the misfit between inventories of observed and predicted longitudinal river profiles. Our approach builds upon a more general, non-linear, optimization model, which suggests that shapes of river profiles are dominantly controlled by upstream advection of kinematic waves of incision produced by spatial and temporal changes in regional uplift rate. Here, we use themethod of characteristics to solve a version of this problem. A damped, non-negative, least squares approach is developed that permits river profiles to be inverted as a function of uplift rate. An important benefit of a linearized treatment is low computational cost. We have tested our algorithm by inverting 957 river profiles from both Africa and Australia. For each continent, the drainage network was constructed from a digital elevation model. The fidelity of river profiles extracted from this network was carefully checked using satellite imagery. River profiles were inverted many times to systematically investigate the trade-off between model misfit and smoothness. Spatial and temporal patterns of both uplift rate and cumulative uplift were calibrated using independent geologic and geophysical observations. Uplift patterns suggest that the topography of Africa and Australia grew in Cenozoic times. Inverse modeling of large inventories of river profiles demonstrates that drainage networks contain coherent signals that record the regional growth of elevation.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    The Viscosities of Partially Molten Materials Undergoing Diffusion Creep (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Partially molten materials resist shearing and compaction. This resistance is described by a fourth‐rank effective viscosity tensor. When the tensor is isotropic, two scalars determine the resistance: an effective shear and an effective bulk viscosity. Here calculations are presented of the effective viscosity tensor during diffusion creep for a 2‐D tiling of hexagonal unit cells and a 3‐D tessellation of tetrakaidecahedrons (truncated octahedrons). The geometry of the melt is determined by assuming textural equilibrium. The viscosity tensor for the 2‐D tiling is isotropic but that for the 3‐D tessellation is anisotropic. Two parameters control the effect of melt on the viscosity tensor: the porosity and the dihedral angle. Calculations for both Nabarro‐Herring (volume diffusion) and Coble (surface diffusion) creep are presented. For Nabarro‐Herring creep the bulk viscosity becomes singular as the porosity vanishes. This singularity is logarithmic, a weaker singularity than typically assumed in geodynamic models. The presence of a small amount of melt (0.1% porosity) causes the effective shear viscosity to approximately halve. For Coble creep, previous modeling work has argued that a very small amount of melt may lead to a substantial, factor of 5, drop in the shear viscosity. Here a much smaller, factor of 1.4, drop is obtained for tetrakaidecahedrons. Owing to a Cauchy relation symmetry, the Coble creep bulk viscosity is a constant multiple of the shear viscosity when melt is present.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Combined 147,146Sm-143,142Nd constraints on the longevity and residence time of early terrestrial crust (2014)
    Wiley
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    Publication Date: 2014-05-08
    Description: Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142 Nd anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here, we report new whole-rock 147,146 Sm- 143,142 Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147 Sm- 143 Nd data combined with literature data define an age of 3371±141 Ma (2 SD) and yield an initial ε 143 Nd of -5.6±2.1. These results are at odds with the Acasta zircon U-Pb record, which comprises emplacement ages of 3920-3960 Ma. Ten of our thirteen samples show 142 Nd deficits of -9.6±4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142 Nd anomalies and a mid-Archean 147 Sm- 143 Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early-enrichment of the Acasta source. Two compositional end-members can be identified: a felsic component with 142 Nd/ 144 Nd identical to the modern Earth, and a mafic component with 142 Nd/ 144 Nd as low as -14.1 ppm. The ca. 4310 Ma AGC source is ~200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd-Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142 Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Compaction around a rigid, circular inclusion in partially molten rock (2014)
    Wiley
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    Publication Date: 2014-06-14
    Description: Conservation laws that describe the behavior of partially molten mantle rock have been established for several decades, but the associated rheology remains poorly understood. Constraints on the rheology may be obtained from recently published experiments involving deformation of partially molten rock around a rigid, spherical inclusion. These experiments give rise to patterns of melt segregation that exhibit the competing effects of pressure shadows and melt-rich bands. Such patterns provide an opportunity to infer rheological parameters through comparison with models based on the conservation laws and constitutive relations that hypothetically govern the system. To this end, we have developed software tools to simulate finite strain, two-phase flow around a circular inclusion in a configuration that mirrors the experiments. Simulations indicate that the evolution of porosity is predominantly controlled by the porosity-weakening exponent of the shear viscosity and the poorly known bulk viscosity. In two-dimensional simulations presented here, we find that the balance of pressure shadows and melt-rich bands observed in experiments only occurs for bulk-to-shear-viscosity ratio of less than about five. However, the evolution of porosity in simulations with such low bulk viscosity exceeds physical bounds at unrealistically small strain due to the unchecked, exponential growth of the porosity variations. Processes that limit or balance porosity localization should be incorporated in the formulation of the model to produce results that are consistent with the porosity evolution in experiments.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production (2020)
    Wiley
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    Publication Date: 2020-08-13
    Print ISSN: 1354-1013
    Electronic ISSN: 1365-2486
    Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography
    Published by Wiley
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  • 10
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    Analysis of physical factors of the windstormXaverin Poland: ­post-hazard review (2017)
    Wiley
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    Publication Date: 2017-09-15
    Print ISSN: 0043-1656
    Electronic ISSN: 1477-8696
    Topics: Geosciences
    Published by Wiley
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