The importance of phase morphology for rheology of ferropericlase-bridgmanite mixtures

2021 ◽  
Author(s):  
Marcel Thielmann ◽  
Gregor Golabek ◽  
Hauke Marquardt
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Effective Viscosity ◽  
Large Scale ◽  
Numerical Models ◽  
Phase Morphology ◽  
Strong Impact ◽  
Analytical Approximations ◽  
The Impact ◽  
Strong Control

<p>The rheology of the Earth’s lower mantle is poorly constrained due to a lack of knowledge of the rheological behaviour of its constituent minerals. In addition, the lower mantle does not consist of only a single, but of multiple mineral phases with differing deformation behaviour. The rheology of Earth’s lower mantle is thus not only controlled by the rheology of its individual constituents (bridgmanite and ferropericlase), but also by their interplay during deformation. This is particularly important when the viscosity contrast between the different minerals is large. Experimental studies have shown that ferropericlase may be significantly weaker than bridgmanite and may thus exert a strong control on lower mantle rheology.</p><p>Here, we thus explore the impact of phase morphology on the rheology of a ferropericlase-bridgmanite mixture using numerical models. We find that elongated ferropericlase structures within the bridgmanite matrix significantly lower the effective viscosity, even in cases where no interconnected network of weak ferropericlase layers has been formed. In addition to the weakening, elongated ferropericlase layers result in a strong viscous anisotropy. Both of these effects may have a strong impact on lower mantle dynamics, which makes is necessary to develop upscaling methods to include them in large-scale mantle convection models. We develop a numerical-statistial approach to link the statistical properties of a ferropericlase-bridgmanite mixture to its effective viscosity tensor. With this approach, both effects are captured by analytical approximations that have been derived to describe the evolution of the effective viscosity (and its anisotropy) of a two-phase medium with aligned elliptical inclusions, thus allowing to include these microscale processes in large-scale mantle convection models.</p>

The interplay between recycled and primordial heterogeneities: predicting Earth's mantle dynamics via numerical modeling

2021 ◽  
Author(s):  
Matteo Desiderio ◽  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Large Scale ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Mantle Dynamics ◽  
Density Anomaly ◽  
Mantle Mineral ◽  
Intrinsic Strength

<p>According to geochemical and geophysical observations, Earth's lower mantle appears to be strikingly heterogeneous in composition. An accurate interpretation of these findings is critical to constrain Earth's bulk composition and long-term evolution. To this end, two main models have gained traction, each reflecting a different style of chemical heterogeneity preservation: the 'marble cake' and 'plum pudding' mantle. In the former, heterogeneity is preserved in the form of narrow streaks of recycled oceanic lithosphere, stretched and stirred throughout the mantle by convection. In the latter, domains of intrinsically strong, primordial material (enriched in the lower-mantle mineral bridgmanite) may resist convective entrainment and survive as coherent blobs in the mid mantle. Microscopic scale processes certainly affect macroscopic properties of mantle materials and thus reverberate on large-scale mantle dynamics. A cross-disciplinary effort is therefore needed to constrain present-day Earth structure, yet countless variables remain to be explored. Among previous geodynamic studies, for instance, only few have attempted to address how the viscosity and density of recycled and primordial materials affect their mutual mixing and interaction in the mantle.</p><p>Here, we apply the finite-volume code <strong>STAGYY</strong> to model thermochemical convection of the mantle in a 2D spherical-annulus geometry. All models are initialized with a lower, primordial layer and an upper, pyrolitic layer (i.e., a mechanical mixture of basalt and harzburgite), as is motivated by magma-ocean solidification studies. We explore the effects of material properties on the style of mantle convection and heterogeneity preservation. These parameters include (i) the intrinsic strength of basalt (viscosity), (ii) the intrinsic density of basalt, and (iii) the intrinsic strength of the primordial material.</p><p>Our preliminary models predict a range of different mantle mixing styles. A 'marble cake'-like regime is observed for low-viscosity primordial material (~30 times weaker than the ambient mantle), with recycled oceanic lithosphere preserved as streaks and thermochemical piles accumulating near the core-mantle boundary. Conversely, 'plum pudding' primordial blobs are also preserved when the primordial material is relatively strong, in addition to the 'marble cake' heterogeneities mentioned above. Most notably, however, the rheology and the density anomaly of basalt affect the appearance of both recycled and primordial heterogeneities. In particular, they control the stability, size and geometry of thermochemical piles, the enhancement of basaltic streaks in the mantle transition zone, and they influence the style of primordial material preservation. These results indicate the important control that the physical properties of mantle constituents exert on the style of mantle convection and mixing over geologic time. Our numerical models offer fresh insights into these processes and may advance our understanding of the composition and structure of Earth's lower mantle.</p>

scholarly journals Neo-Tethys geodynamics and mantle convection: from extension to compression in Africa and a conceptual model for obduction

2016 ◽  
Vol 53 (11) ◽  
pp. 1190-1204 ◽  
Author(s):  
Laurent Jolivet ◽  
Claudio Faccenna ◽  
Philippe Agard ◽  
Dominique Frizon de Lamotte ◽  
Armel Menant ◽  
...  
Keyword(s):  
Transition Zone ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Large Scale ◽  
Time Window ◽  
Numerical Models ◽  
Conveyor Belt ◽  
Arabian Plate ◽  
African Plate ◽  
Northern Margin

Since the Mesozoic, Africa has been under extension with shorter periods of compression associated with obduction of ophiolites on its northern margin. Less frequent than “normal” subduction, obduction is a first order process that remains enigmatic. The closure of the Neo-Tethys Ocean, by the Upper Cretaceous, is characterized by a major obduction event, from the Mediterranean region to the Himalayas, best represented around the Arabian Plate, from Cyprus to Oman. These ophiolites were all emplaced in a short time window in the Late Cretaceous, from ∼100 to 75 Ma, on the northern margin of Africa, in a context of compression over large parts of Africa and Europe, across the convergence zone. The scale of this process requires an explanation at the scale of several thousands of kilometres along strike, thus probably involving a large part of the convecting mantle. We suggest that alternating extension and compression in Africa could be explained by switching convection regimes. The extensional situation would correspond to steady-state whole-mantle convection, Africa being carried northward by a large-scale conveyor belt, while compression and obduction would occur when the African slab penetrates the upper–lower mantle transition zone and the African plate accelerates due to increasing plume activity, until full penetration of the Tethys slab in the lower mantle across the 660 km transition zone during a 25 Myr long period. The long-term geological archives on which such scenarios are founded can provide independent time constraints for testing numerical models of mantle convection and slab–plume interactions.

The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection

2020 ◽  
Vol 77 (9) ◽  
pp. 3119-3137
Author(s):  
Marcin J. Kurowski ◽  
Wojciech W. Grabowski ◽  
Kay Suselj ◽  
João Teixeira
Keyword(s):  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Small Scale ◽  
Strong Impact ◽  
Shallow Convection ◽  
Liquid Water Path ◽  
Strong Response ◽  
The Impact ◽  
Benchmark Solutions

Abstract Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.

scholarly journals FINAL DESIGN OF THE NAGS HEAD BEACH NOURISHMENT PROJECT USING A LONGSHORE NUMERICAL MODEL

2012 ◽  
Vol 1 (33) ◽  
pp. 64 ◽  
Author(s):  
Haiqing Liu Kaczkowski ◽  
Timothy W Kana
Keyword(s):  
Numerical Model ◽  
Large Scale ◽  
Numerical Models ◽  
Normal Wave ◽  
Beach Nourishment ◽  
Shoreline Evolution ◽  
Longshore Transport ◽  
Final Design ◽  
Model Setup ◽  
The Impact

Nags Head, located at the northeastern part of North Carolina in the U.S., has sustained chronic erosion over the past 50 years. In 2005, Coastal Science & Engineering (CSE) was retained by the town of Nags Head to develop an interim beach restoration plan. Profile volume change was used in the planning and preliminary design of the project, and longshore and cross-shore numerical models were used in the final design to refine the preliminary nourishment plan and increase potential longevity of the project. This paper focuses on the key factors of the longshore numerical model setup for the project. These include model selection, input data and parameters, model calibration, and applications under different design alternatives. The Generalized Model for Simulating Shoreline Changes (GENESIS) was used in this study to evaluate shoreline evolution under normal wave conditions during various stages of the design life following the beach nourishment project. The model was used to identify the potential occurrence of erosional hotspots and to optimize the nourishment design so that the effects of such hotspots could be avoided or minimized where possible. Model results were also used to evaluate the impact of borrow area dredging on longshore transport in the project area and the impact of nourishment on shoaling in the adjacent inlet. The project encompasses 10.11 miles (mi) (16.28 kilometers-km) of ocean shoreline, and the design nourishment volume is based on the total permitted volume of 4 million cubic yards (cy) (3 million cubic meters-m³). [Note: As-built length was 10.0 mi and volume was 4.615 million cubic yards.] The final design has fill densities varying from north to south in relation to historical erosion rates and model projections. The average fill density is 75 cubic yards per foot (cy/ft) (188 m³/m) and ranges from 38 cy/ft to 150 cy/ft (95 m³/m to 375 m³/m). In conclusion, it is shown that the numerical model selected in this study was capable of predicting the overall performance of the large scale beach nourishment project in Nags Head as well as the performance at a particular location within or adjacent to the project, and its design methods can offer guidance to future projects.

scholarly journals Impact of upper mantle convection on lithosphere hyperextension and subsequent horizontally forced subduction initiation

Solid Earth ◽  
2020 ◽  
Vol 11 (6) ◽  
pp. 2327-2357
Author(s):  
Lorenzo G. Candioti ◽  
Stefan M. Schmalholz ◽  
Thibault Duretz
Keyword(s):  
Upper Mantle ◽  
Mantle Convection ◽  
Driving Forces ◽  
Numerical Models ◽  
Passive Margin ◽  
Continuous Simulation ◽  
Subduction Initiation ◽  
Lithosphere Dynamics ◽  
The Impact

Abstract. Many plate tectonic processes, such as subduction initiation, are embedded in long-term (>100 Myr) geodynamic cycles often involving subsequent phases of extension, cooling without plate deformation and convergence. However, the impact of upper mantle convection on lithosphere dynamics during such long-term cycles is still poorly understood. We have designed two-dimensional upper-mantle-scale (down to a depth of 660 km) thermo-mechanical numerical models of coupled lithosphere–mantle deformation. We consider visco–elasto–plastic deformation including a combination of diffusion, dislocation and Peierls creep law mechanisms. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. Our models exhibit realistic Rayleigh numbers between 106 and 107, and the model temperature, density and viscosity structures agree with geological and geophysical data and observations. We tested the impact of the viscosity structure in the asthenosphere on upper mantle convection and lithosphere dynamics. We also compare models in which mantle convection is explicitly modelled with models in which convection is parameterized by Nusselt number scaling of the mantle thermal conductivity. Further, we quantified the plate driving forces necessary for subduction initiation in 2D thermo-mechanical models of coupled lithosphere–mantle deformation. Our model generates a 120 Myr long geodynamic cycle of subsequent extension (30 Myr), cooling (70 Myr) and convergence (20 Myr) coupled to upper mantle convection in a single and continuous simulation. Fundamental features such as the formation of hyperextended margins, upper mantle convective flow and subduction initiation are captured by the simulations presented here. Compared to a strong asthenosphere, a weak asthenosphere leads to the following differences: smaller value of plate driving forces necessary for subduction initiation (15 TN m−1 instead of 22 TN m−1) and locally larger suction forces. The latter assists in establishing single-slab subduction rather than double-slab subduction. Subduction initiation is horizontally forced, occurs at the transition from the exhumed mantle to the hyperextended passive margin and is caused by thermal softening. Spontaneous subduction initiation due to negative buoyancy of the 400 km wide, cooled, exhumed mantle is not observed after 100 Myr in model history. Our models indicate that long-term lithosphere dynamics can be strongly impacted by sub-lithosphere dynamics. The first-order processes in the simulated geodynamic cycle are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyperextended rifted margins, which might have been the case for the Alpine orogeny.

Impact of Abrupt Land Cover Changes by Tropical Deforestation on Southeast Asian Climate and Agriculture

2017 ◽  
Vol 30 (7) ◽  
pp. 2587-2600 ◽  
Author(s):  
Merja H. Tölle ◽  
Steven Engler ◽  
Hans-Jürgen Panitz
Keyword(s):  
Land Cover ◽  
Large Scale ◽  
Layer Structure ◽  
Surface Fluxes ◽  
Maximum Temperature ◽  
Natural Mode ◽  
Land Cover Changes ◽  
Strong Impact ◽  
Se Asia ◽  
The Impact

Southeast Asia (SE Asia) undergoes major and rapid land cover changes as a result of agricultural expansion. Landscape conversion results in alterations to surface fluxes of moisture, heat, and momentum and sequentially impact the boundary layer structure, cloud-cover regime, and all other aspects of local and regional weather and climate occurring also in regimes remote from the original landscape disturbance. The extent and magnitude of the anthropogenic modification effect is still uncertain. This study investigates the biogeophysical effects of large-scale deforestation on monsoon regions using an idealized deforestation simulation. The simulations are performed using the regional climate model COSMO-CLM forced with ERA-Interim data during the period 1984–2004. In the deforestation experiment, grasses in SE Asia, between 20°S and 20°N, replace areas covered by trees. Using principal component analysis, it is found that abrupt conversion from forest to grassland cover leads to major climate variability in the year of disturbance, which is 1990, over SE Asia. The persistent land modification leads to a decline in evapotranspiration and precipitation and a significant warming due to reduced latent heat flux during 1990–2004. The strongest effects are seen in the lowlands of SE Asia. Daily precipitation extremes increase during the monsoon period and ENSO, differing from the result of mean precipitation changes. Maximum temperature also increases by 2°C. The impacts of land cover change are more intense than the effects of El Niño and La Niña. In addition, results show that these land clearings can amplify the impact of the natural mode ENSO, which has a strong impact on climate conditions in SE Asia. This will likely have consequences for the agricultural output.

scholarly journals What controls the large-scale magnetic fields of M dwarfs?

2013 ◽  
Vol 9 (S302) ◽  
pp. 166-169
Author(s):  
T. Gastine ◽  
J. Morin ◽  
L. Duarte ◽  
A. Reiners ◽  
U. Christensen ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Numerical Models ◽  
Strong Impact ◽  
M Dwarfs ◽  
Relative Contribution ◽  
Broad Variety ◽  
Field Geometry ◽  
M Stars

AbstractObservations of active M dwarfs show a broad variety of large-scale magnetic fields encompassing dipole-dominated and multipolar geometries. We detail the analogy between some anelastic dynamo simulations and spectropolarimetric observations of 23 M stars. In numerical models, the relative contribution of inertia and Coriolis force –estimated by the so-called local Rossby number– is known to have a strong impact on the magnetic field geometry. We discuss the relevance of this parameter in setting the large-scale magnetic field of M dwarfs.

scholarly journals Evaluation of Large-Scale Public-Sector Reforms

2016 ◽  
Vol 38 (2) ◽  
pp. 226-245 ◽  
Author(s):  
Karen N. Breidahl ◽  
Gunnar Gjelstrup ◽  
Hanne Foss Hansen ◽  
Morten Balle Hansen
Keyword(s):  
Comparative Analysis ◽  
Public Sector ◽  
Large Scale ◽  
Evaluation Process ◽  
Strong Impact ◽  
Cultural Perspective ◽  
Vital Importance ◽  
Public Sector Reforms ◽  
The Impact

Research on the evaluation of large-scale public-sector reforms is rare. This article sets out to fill that gap in the evaluation literature and argues that it is of vital importance since the impact of such reforms is considerable and they change the context in which evaluations of other and more delimited policy areas take place. In our analysis, we apply four governance perspectives (rational-instrumental perspective, rational interest–based perspective, institutional-cultural perspective, and chaos perspective) in a comparative analysis of the evaluations of two large-scale public-sector reforms in Denmark and Norway. We compare the evaluation process (focus and purpose), the evaluators, and the organization of the evaluation, as well as the utilization of the evaluation results. The analysis uncovers several significant findings including how the initial organization of the evaluation shows strong impact on the utilization of the evaluation and how evaluators can approach the challenges of evaluating large-scale reforms.

Coupled dynamics of primordial and recycled heterogeneity in Earth's lower mantle, and their present-day seismic signatures

2021 ◽  
Author(s):  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer ◽  
Paul J. Tackley
Keyword(s):  
Oceanic Crust ◽  
Seismic Tomography ◽  
Lower Mantle ◽  
Large Scale ◽  
Numerical Models ◽  
Global Scale ◽  
Physical Parameters ◽  
Compositional Heterogeneity ◽  
Seismic Velocities ◽  
S Wave

<p>The nature of compositional heterogeneity in Earth’s lower mantle is a long-standing puzzle that can inform about the thermochemical evolution and dynamics of our planet. On relatively small scales (<1km), streaks of recycled oceanic crust (ROC) and lithosphere are distributed and stirred throughout the mantle, creating a “marble cake” mantle. On larger scales (10s-100s of km), compositional heterogeneity may be preserved by delayed mixing of this marble cake with either intrinsically-dense or -strong materials of e.g. primordial origin. Intrinsically-dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous (e.g., enhanced in the strong mineral MgSiO<sub>3 </sub>bridgmanite) may survive as blobs in the mid-mantle for large timescales (i.e., as plums in the mantle “plum pudding”). So far, only few, if any, studies have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.<br><br>Here, we use 2D numerical models of global-scale mantle convection to investigate the coupled evolution and mixing of (intrinsically-dense) recycled and (intrinsically-strong) primordial material. We explore the effects of ancient compositional layering of the mantle, as motivated by magma-ocean solidification studies, and the physical parameters of the primordial material. Over a wide parameter range, primordial and recycled heterogeneity is predicted to coexist with each other. Primordial material usually survives as mid-to-large scale blobs in the mid-mantle, and this preservation is largely independent on the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as piles at the base of the mantle and as small streaks everywhere else. The robust coexistence between recycled and primordial materials in the models indicate that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles.<br><br>Finally, we put our model predictions in context with geochemical studies on early Earth dynamics as well as seismic discoveries of present-day lower-mantle heterogeneity. For the latter, we calculate synthetic seismic velocities from output model fields, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Because of the competing effects of compositional and thermal anomalies on S-wave velocities, it is difficult to identify mid-mantle bridgmanitic domains in seismic tomography images. This result suggests that, if present, bridgmanitic domains in the mid-mantle may be “hidden” from seismic tomographic studies, and other approaches are needed to establish the presence/absence of these domains in the present-day deep Earth.</p>

Strain-weakening rheology in Earth’s lower mantle: a multi-scale numerical endeavour

2020 ◽  
Author(s):  
Gregor J. Golabek ◽  
Anna J. P. Gülcher ◽  
Marcel Thielmann ◽  
Paul J. Tackley ◽  
Maxim D. Ballmer
Keyword(s):  
Lower Mantle ◽  
Mantle Convection ◽  
Numerical Models ◽  
Order Effect ◽  
Small Scale ◽  
Localized Deformation ◽  
Earth Planet ◽  
Multi Scale ◽  
Macro Scale ◽  
Heterogeneous Rocks

<p>Rocks in the Earth’s interior are not homogeneous but consist of different mineralogical phases with different rheological properties. Deformation of heterogeneous rocks is thus also heterogeneous, and strongly depends on the rheological contrasts and spatial distribution of the mineral phases. In Earth’s lower mantle, the main rock constituents are bridgmanite (Br) and smaller amounts of ferropericlase (Fp). Bridgmanite is substantially stronger than ferropericlase [1]. Recent studies propose that lower mantle rheology is highly dependent on the relative mineral abundances and distribution of these two phases [1,2]. It has been suggested that for bridgmanite-depleted compositions, the viscosity decreases with accumulating strain due to the interconnection of the weaker ferropericlase. This implies that deformation may localize in the lower mantle, potentially aiding the formation and preservation of compositionally distinct and “hidden” reservoirs away from these regions of localized deformation [3]. Therefore, understanding the rheological nature of Br-Fp aggregates on a small-scale is crucial for assessing the dynamics of global mantle convection. Here, we address this objective with multi-scale numerical approaches.  </p><p>Using a numerical-statistical approach, a connection between ferropericlase morphology and effective rheology of Earth’s lower mantle has recently been established [4]. Results show that bulk-rock weakening depends on the topology of the weak phase as well on its rheology, but also that significant rheological weakening can already be achieved when ferropericlase does not (yet) form an interconnected weak layer.</p><p>In a second suite of models, we implement a macro-scale description of strain-weakening based on the micro-scale solutions found in [4] in a global mantle convection model to test the first-order effect of strain weakening on convection dynamics in the lower mantle. We present 2D numerical models of thermochemical convection in a spherical annulus geometry [5] that include a new implementation of tracking the strain ellipse at each tracer through time. We further allow lower mantle materials to rheologically weaken once a certain strain threshold has been reached. Preliminary results indicate that strain localizes along both up- and downwellings in the lower mantle and that rheological weakening has a stabilizing effect on these conduits. </p><p>This multi-scale approach is essential for addressing lower-mantle rheological behavior and our results form an important step towards addressing the feasibility of isolated, long-lived geochemical reservoirs in Earth’s lower mantle.</p><p>[1] Yamazaki and Karato (2001), Am. Mineral. 86, 385-391. [2] Girard et al. (2016), Science 351, 144-147. [3] Ballmer et al. (2017), Nat. Geosci. 10, 236-240. [4] Thielmann et al. (2020), Geochem. Geophys. Geosyst., doi:10.1029/2019GC008688. [5] Hernlund and Tackley (2008), Phys. Earth Planet. Int. 171, 48–54.</p>

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