Variations in grain size and viscosity based on vacancy diffusion in minerals, seismic tomography, and geodynamically inferred mantle rheology

Glišović, P.; Forte, A. M.; Ammann, M. W.

doi:10.1002/2015gl065142

Cited by 26 publications

(26 citation statements)

References 40 publications

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“…In particular, we demonstrate that viscosity variations in the mantle are stronger than expected from models assuming a constant grain size. This result is in contrast to previous studies, which predicted that an evolving grain size would reduce-instead of increase-lateral viscosity variations [Gli sović et al, 2015]. Because they infer grain size only from present-day temperatures, they find that regions with high (low) temperatures always feature Geochemistry, Geophysics, Geosystems 10.1002/2017GC006944 large (small) grain sizes, which is not necessarily the case in dynamically evolving models (see for example, supporting information Figure S8).…”

Section: Geodynamicscontrasting

confidence: 99%

The importance of grain size to mantle dynamics and seismological observations

Dannberg

Eilon

Faul

et al. 2017

Geochem Geophys Geosyst

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Grain size plays a key role in controlling the mechanical properties of the Earth's mantle, affecting both long‐time‐scale flow patterns and anelasticity on the time scales of seismic wave propagation. However, dynamic models of Earth's convecting mantle usually implement flow laws with constant grain size, stress‐independent viscosity, and a limited treatment of changes in mineral assemblage. We study grain size evolution, its interplay with stress and strain rate in the convecting mantle, and its influence on seismic velocities and attenuation. Our geodynamic models include the simultaneous and competing effects of dynamic recrystallization resulting from dislocation creep, grain growth in multiphase assemblages, and recrystallization at phase transitions. They show that grain size evolution drastically affects the dynamics of mantle convection and the rheology of the mantle, leading to lateral viscosity variations of 6 orders of magnitude due to grain size alone, and controlling the shape of upwellings and downwellings. Using laboratory‐derived scaling relationships, we convert model output to seismologically observable parameters (velocity and attenuation) facilitating comparison to Earth structure. Reproducing the fundamental features of the Earth's attenuation profile requires reduced activation volume and relaxed shear moduli in the lower mantle compared to the upper mantle, in agreement with geodynamic constraints. Faster lower mantle grain growth yields best fit to seismic observations, consistent with our reexamination of high‐pressure grain growth parameters. We also show that ignoring grain size in interpretations of seismic anomalies may underestimate the Earth's true temperature variations.

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Section: Geodynamicscontrasting

confidence: 99%

The importance of grain size to mantle dynamics and seismological observations

Dannberg

Eilon

Faul

et al. 2017

Geochem Geophys Geosyst

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“…Recent efforts by Yang and Gurnis (2016) to reconcile both gravity anomalies and topography in mantle flow calculations suggest quite modest LVV in the sublithospheric mantle indicating that several opposing microphysical controls on rheology may be important, as found by Glišović et al (2015). The Yang and Gurnis (2016) study extends earlier work by Ghosh et al (2010) and Forte, Moucha, et al (2010) showing similarly modest impacts of LVV.…”

Section: Mantle Heterogeneity Modelsmentioning

confidence: 89%

“…Indeed, as shown in Moucha et al (2007), the impact of LVV is overshadowed by much stronger variability in the geodynamic predictions associated with uncertainties in the tomography models themselves. To underline these points, Figure S13 shows the impact of LVV throughout the mantle (as calculated by Glišović et al, 2015) on dynamic surface topography calculated by Kajan et al (2018). The effect of these LVV is modest and much smaller than the differences between topography predictions obtained using two distinct tomography models, where each model was optimized to yield a best fit to the geodynamic surface observables.…”

Section: Mantle Heterogeneity Modelsmentioning

confidence: 92%

The Sensitivity of Joint Inversions of Seismic and Geodynamic Data to Mantle Viscosity

Forte

Simmons

et al. 2020

Geochem Geophys Geosyst

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Seismic tomography has revealed the existence of large-scale velocity heterogeneities in the mantle. The interpretation of seismic velocity anomalies in terms of temperature and chemical composition is nonunique. We use geodynamic observations including gravity, plate motions, dynamic topography, and excess ellipticity of the core-mantle boundary combined with seismic observations to investigate the thermo-chemical structure of the mantle through joint inversions. An outstanding issue, however, is the physical connection between mantle density anomalies and the surface geodynamic observations, which requires knowledge of the mantle viscosity structure. Here we perform joint inversions assuming different viscosity profiles and examine the dependence of the results on the viscosity. We first assume that mantle heterogeneity is due to thermal variations, which places a constraint on the relation between seismic velocity and density, and we subsequently relax the constraint to allow for potential nonthermal effects. In all of our joint inversions, a nonthermal origin of density anomalies is required to explain the geodynamic data, though the amount varies with the assumed viscosity structure. A common observation is a high-density chemical signal in the center of the large low-shear-velocity provinces at the base of the mantle resulting in a near neutral or slightly dense overall buoyancy there. Using the derived density models and their corresponding viscosity profiles, we also calculate instantaneous mantle flow fields. The predicted flow fields derived from joint inversions are generally similar but are quite different from flow fields using density models derived from a posteriori scaling of pure seismic tomography models.Plain Language Summary The origin and evolution of Earth's mantle have been long-standing fundamental questions in geosciences. We use both global seismological and geodynamical (gravity, topography, plate motions, and excess ellipticity of the core-mantle boundary) data sets to investigate whether lateral changes in mantle structure can be explained solely by temperature variations or whether the mantle must also have significant chemical variations. Our results indicate the presence of chemically distinct mantle anomalies. In particular, we find two large regions at the base of the mantle that appear to be chemically distinct with hot mantle upwellings surrounding them. We also derived several models of 3D density variations in the mantle assuming different viscosity profiles. These models were then used to predict the present-day mantle convective flow. We show that the mantle viscosity structure does not have a strong influence on the pattern of large-scale mantle flow. We find, however, that 3D density models derived by simple (1D) a posteriori scaling of tomography models obtained only from seismic data yield predictions of significantly different mantle flow.

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“…In addition, the viscosity of the mantle has also been inferred to decrease with the presence of water based on laboratory measurements of olivine [ Karato and Jung , ], the most common mineral of the upper mantle, if it is composed primarily of peridotite. The weak mantle inferred by our model could also be explained by a decrease in grain size [ Glišović et al ., ] or partial melting [ Ringwood , ; Kohlstedt , ]. In order to understand how many of these factors may be significant, we compare our results to known geologic and geophysical studies from the Basin and Range as discussed below.…”

Section: Discussionmentioning

confidence: 99%

Inference of the viscosity structure and mantle conditions beneath the Central Nevada Seismic Belt from combined postseismic and lake unloading studies

Dickinson

Freed

Andronicos

2016

Geochem. Geophys. Geosyst.

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We test whether a single depth‐dependent Newtonian viscosity structure can be found to explain measured surface deformation in Western Nevada from two separate loading events: tectonic loading from a series of seven historic earthquakes in the Central Nevada Seismic Belt and nontectonic loading from the formation and evaporation of co‐located Pleistocene‐aged Lake Lahontan. Rheologic studies are generally plagued with nonuniqueness issues due to the limitations of observational constraints. Here, we reduce nonuniqueness by solving for a single rheologic structure that can simultaneously satisfy all observational constraints associated with all events. Model results suggest that Western Nevada is underlain by a strong lower crust (order 1020 Pa s), a relatively weak mantle (order 5 × 1018 Pa s) from 40 to 80 km, and a much weaker mantle (order 1018 Pa s) below 80 km. We would thus place the mechanical lithosphere/asthenosphere boundary (LAB) at 40 km depth. Thermal modeling of conductive geothermal gradients, combined with melting curves calculated for enriched and depleted mantle compositions suggest that the viscosity decrease at 40 km depth (the LAB) is associated with the onset of wet melting of mantle lithosphere hydrated by past subduction and is about 10 km shallower than the inferred transition from conduction to convection.

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Variations in grain size and viscosity based on vacancy diffusion in minerals, seismic tomography, and geodynamically inferred mantle rheology

Cited by 26 publications

References 40 publications

The importance of grain size to mantle dynamics and seismological observations

The importance of grain size to mantle dynamics and seismological observations

The Sensitivity of Joint Inversions of Seismic and Geodynamic Data to Mantle Viscosity

Inference of the viscosity structure and mantle conditions beneath the Central Nevada Seismic Belt from combined postseismic and lake unloading studies

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