scholarly journals The evolution and distribution of recycled oceanic crust in the Earth’s mantle: Insight from geodynamic models

2020 ◽  
Author(s):  
Jun Yan ◽  
Maxim D. Ballmer ◽  
Paul J. Tackley
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Bulk Composition ◽  
Global Scale ◽  
Slab Thickness ◽  
Mantle Viscosity ◽  
The Earth ◽  
Viscosity Profile ◽  
Tectonic Style ◽  
Geodynamic Models

<p>A better understanding of the Earth’s compositional structure is needed to place the geochemical record of surface rocks into the context of Earth accretion and evolution. Cosmochemical constraints imply that lower-mantle rocks may be enriched in silica relative to upper-mantle pyrolite, whereas geophysical observations support whole-mantle convection and mixing. To resolve this discrepancy, it has been suggested that subducted mid-ocean ridge basalt (MORB) segregates from subducted harzburgite to accumulate in the mantle transition zone (MTZ) and/or the lower mantle. However, the key parameters that control basalt segregation and accumulation remain poorly constrained. Here, we use global-scale 2D thermochemical convection models to investigate the influence of mantle-viscosity profile, planetary-tectonic style and bulk composition on the evolution and distribution of mantle heterogeneity. Our models robustly predict that, for all cases with Earth-like tectonics, a basalt-enriched reservoir is formed in the MTZ, and a harzburgite-enriched reservoir is sustained at 660~800 km depth, despite ongoing whole-mantle circulation. The enhancement of basalt and harzburgite in and beneath the MTZ, respectively, are laterally variable, ranging from ~30% to 50% basalt fraction, and from ~40% to 80% harzburgite enrichment relative to pyrolite. Models also predict an accumulation of basalt near the core mantle boundary (CMB) as thermochemical piles, as well as moderate enhancement of most of the lower mantle by basalt. While the accumulation of basalt in the MTZ does not strongly depend on the mantle-viscosity profile (explained by a balance between basalt delivery by plumes and removal by slabs at the given MTZ capacity), that of the lowermost mantle does: lower-mantle viscosity directly controls the efficiency of basalt segregation (and entrainment) near the CMB; upper-mantle viscosity has an indirect effect through controlling slab thickness. Finally, the composition of the bulk-silicate Earth may be shifted relative to that of upper-mantle pyrolite, if indeed significant reservoirs of basalt exist in the MTZ and lower mantle.</p>

scholarly journals Compositional mantle layering revealed by slab stagnation at ~1000-km depth

2015 ◽  
Vol 1 (11) ◽  
pp. e1500815 ◽  
Author(s):  
Maxim D. Ballmer ◽  
Nicholas C. Schmerr ◽  
Takashi Nakagawa ◽  
Jeroen Ritsema
Keyword(s):  
Numerical Modeling ◽  
Seismic Tomography ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Global Scale ◽  
Compositional Gradient ◽  
Mantle Enrichment ◽  
Geodynamic Models ◽  
Neutrally Buoyant

Improved constraints on lower-mantle composition are fundamental to understand the accretion, differentiation, and thermochemical evolution of our planet. Cosmochemical arguments indicate that lower-mantle rocks may be enriched in Si relative to upper-mantle pyrolite, whereas seismic tomography images suggest whole-mantle convection and hence appear to imply efficient mantle mixing. This study reconciles cosmochemical and geophysical constraints using the stagnation of some slab segments at ~1000-km depth as the key observation. Through numerical modeling of subduction, we show that lower-mantle enrichment in intrinsically dense basaltic lithologies can render slabs neutrally buoyant in the uppermost lower mantle. Slab stagnation (at depths of ~660 and ~1000 km) and unimpeded slab sinking to great depths can coexist if the basalt fraction is ~8% higher in the lower mantle than in the upper mantle, equivalent to a lower-mantle Mg/Si of ~1.18. Global-scale geodynamic models demonstrate that such a moderate compositional gradient across the mantle can persist can in the presence of whole-mantle convection.

scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

scholarly journals On the Nature of the Nonhydrostatic Quadrupole Excess Moment of the Earth

1980 ◽  
Vol 78 ◽  
pp. 153-156
Author(s):  
T. V. Ruzmaikina
Keyword(s):  
Lower Mantle ◽  
Earth’S Rotation ◽  
Earth's Rotation ◽  
Mantle Viscosity ◽  
The Earth ◽  
Mass Flows

I wish to discuss an effect that is caused by the secular decrease in the Earth's rotation. I shall show that this deceleration induces mass flows across level surfaces and that these flows redistribute temperature and density in the Earth and produce an excess equatorial bulge. This mechanism does not require large lower mantle viscosity, unlike mechanisms discussed by Munk and MacDonald (1960) and McKenzie (1966). Therefore it does not suffer from the difficulties pointed out by Goldreich and Toomre (1969).

3. Minerals and the interior of the Earth

2014 ◽  
Author(s):  
David Vaughan
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Plate Tectonics ◽  
Inner Core ◽  
Indirect Evidence ◽  
Outer Core ◽  
The Earth ◽  
Granite Formation ◽  
Temperature And Pressure

‘Minerals and the interior of the Earth’ looks at the role of minerals in plate tectonics during the processes of crystallization and melting. The size and range of minerals formed are dependent on the temperature and pressure of the magma during its movement through the crust. The evolution of the continental crust also involves granite formation and processes of metamorphism. Our understanding of the interior of the Earth is based on indirect evidence, mainly the study of earthquake waves. The Earth consists of concentric shells: a solid inner core; liquid outer core; a solid mantle divided into a lower mantle, a transition zone, and an upper mantle; and then the outer rigid lithosphere.

The formation, preservation and seismic signatures of chemical heterogeneities in the lower mantle

2020 ◽  
Author(s):  
Anna J. P. Gülcher ◽  
Maxim D. Ballmer ◽  
Paul J. Tackley ◽  
Paula Koelemeijer
Keyword(s):  
Physical Properties ◽  
Lower Mantle ◽  
Numerical Models ◽  
Bulk Composition ◽  
Oceanic Lithosphere ◽  
Global Scale ◽  
Seismic Velocities ◽  
Mantle Dynamics ◽  
Wide Range ◽  
Scientific Challenge

<p>Despite stirring by vigorous convection over billions of years, the Earth’s lower mantle appears to be chemically heterogeneous on various length scales. Constraining this heterogeneity is key for assessing Earth’s bulk composition and thermochemical evolution, but remains a scientific challenge that requires cross-disciplinary efforts. On scales below ~1 km, the concept of a “marble cake” mantle has gained wide acceptance, emphasising that recycled oceanic lithosphere, deformed into streaks of depleted and enriched compositions, makes up much of the 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 intrinsically-strong materials. Intrinsically dense materials may accumulate as piles at the core-mantle boundary, while intrinsically viscous domains (e.g., enhanced in the strong mineral bridgmanite) may survive as “blobs” in the mid-mantle for large timescales, such as plums in the mantle “plum pudding”<sup>1,2</sup>. While many studies have explored the formation and preservation of either intrinsically-dense (recycled) or intrinsically-strong (primordial) heterogeneity, only few if any have quantified mantle dynamics in the presence of different types of heterogeneity with distinct physical properties.<span> </span></p><p>To address this objective, we use state-of-the-art 2D numerical models of global-scale mantle convection in a spherical-annulus geometry. We explore the effects of the <em>(i)</em> physical properties of primordial material (density, viscosity), <em>(ii)</em> temperature/pressure dependency of viscosity, <em>(iii)</em> lithospheric yielding strength, and <em>(iv)</em> Rayleigh number on mantle dynamics and mixing. Models predict that primordial heterogeneity is preserved in the lower mantle over >4.5 Gyr as discrete blobs for high intrinsic viscosity contrast (>30x) and otherwise for a wide range of parameters. In turn, recycled oceanic crust is preserved in the lower mantle as “marble cake” streaks or piles, particularly in models with a relatively cold and stiff mantle. Importantly, these recycled crustal heterogeneities can co-exist with primordial blobs, with piles often tending to accumulate beneath the primordial domains. This suggests that the modern mantle may be in a hybrid state between the “marble cake” and “plum pudding” styles.<span> </span></p><p>Finally, we put our model predictions in context with recent discoveries from seismology. We calculate synthetic seismic velocities from predicted temperatures and compositions, and compare these synthetics to tomography models, taking into account the limited resolution of seismic tomography. Convection models including preserved bridgmanite-enriched domains along with recycled piles have the potential of reconciling recent seismic observations of lower-mantle heterogeneity<sup>3</sup> with the geochemical record from ocean-island basalts<sup>4,5</sup>, and are therefore relevant for assessing Earth’s bulk composition and long-term evolution.<span> </span></p><p><sup>1</sup> Ballmer et al. (2017), <em>Nat. Geosci</em>., 10.1038/ngeo2898<br><sup>2</sup> Gülcher et al. (in review), <em>EPSL</em>: Variable dynamic styles of primordial heterogeneity preservation in Earth’s lower mantle <br><sup>3</sup> Waszek et al. (2018), <em>Nat. Comm., </em>10.1038/s41467-017-02709-4 <br><sup>4</sup> Hofmann (1997), <em>Nature, </em>10.1038/385219a0; <br><sup>5</sup> Mundl et al. (2017), <em>Science, </em>10.1126/science.aal4179</p>

scholarly journals The Subduction Dichotomy of Strong Plates and Weak Slabs

2016 ◽  
Author(s):  
Robert I. Petersen ◽  
Dave R. Stegman ◽  
Paul J. Tackley
Keyword(s):  
Lower Mantle ◽  
Plate Tectonics ◽  
Material Strength ◽  
Control Material ◽  
The Earth ◽  
Dynamic Forces ◽  
Lithospheric Plates ◽  
Geodynamic Models

Abstract. A key element of plate tectonics on Earth is that the lithosphere is subducting into the mantle. Subduction results from forces that bend and pull the lithosphere into the interior of the Earth. Once subducted, lithospheric slabs are further modified by dynamic forces in the mantle and their sinking is inhibited by the increase in viscosity of the lower mantle. These forces are resisted by the material strength of the lithosphere. Using geodynamic models we investigate several subduction models wherein we control material strength by setting a maximum viscosity for the surface plates and the subducted slabs independently. We find that the models which produce results most analogous to observations of subduction on Earth are characterized by a dichotomy of lithosphere strengths. These models have strong lithospheric plates at the surface which promotes Earth-like single-sided subduction. At the same time these models have weakened lithospheric subducted slabs which pile, bend or lie flat at the top of the lower mantle reproducing the spectrum of slab morphologies observed on Earth.

scholarly journals An investigation into the sensitivity of postglacial decay times to uncertainty in the adopted ice history

2019 ◽  
Author(s):  
Joseph Kuchar ◽  
Glenn Milne ◽  
Alexander Hill ◽  
Lev Tarasov ◽  
Maaria Nordman
Keyword(s):  
Sea Level ◽  
Decay Time ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Solution Space ◽  
Hudson Bay ◽  
James Bay ◽  
Mantle Viscosity ◽  
Viscosity Models ◽  
Decay Times

Abstract At the centers of previously glaciated regions such as Hudson Bay in Canada and the Gulf of Bothnia in Fennoscandia, it has been observed that the sea level history follows an exponential form and that the associated decay time is relatively insensitive to uncertainty in the ice loading history. We revisit the issue of decay time sensitivity by computing relative sea level histories for Richmond Gulf and James Bay in Hudson Bay and Ångerman River in Sweden for a suite of reconstructions of the North American and Fennoscandian Ice Sheets and Earth viscosity profiles. We find that while some Earth viscosity models do indeed show insensitivity in computed decay times to the ice history, this is not true in all cases. Moreover, we find that the location of the study site relative to the geometry of the ice sheet is an important factor in determining ice sensitivity, and based on our set of ice sheet reconstructions, conclude that the location of James Bay is not well-suited to a decay time analysis. We describe novel corrections to the RSL data to remove the effects associated with the spatial distribution of sea level indicators as well as for other signals unrelated to regional ice loading (ocean loading, rotation and global mean sea-level changes) and demonstrate that they can significantly affect the inference of viscosity structure. We performed a forward modelling analysis based on a commonly adopted 2-layer, sub-lithosphere viscosity structure to determine how the solution space of viscosity models changes with the input ice history at the three study sites. While the solution spaces depend on ice history, for both Richmond Gulf and Ångerman River there are regions of parameter space where solutions are common across all or most ice histories, indicating low ice load sensitivity for these mantle viscosity parameters. For example, in Richmond Gulf, upper mantle viscosity values of (0.3–0.5)x1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s tend to satisfy the data constraint consistently for most ice histories considered in this study. Similarly, the Ångerman River solution spaces contain a solution with an upper mantle viscosity of 0.3 × 1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s common to 9 of the 10 ice histories considered there. However, the dependence of the viscosity solution space on ice history suggests that joint estimation of ice and Earth parameters is the optimal approach.

scholarly journals GRACE constraints on Earth rheology of the Barents Sea and Fennoscandia

2019 ◽  
Author(s):  
Marc Rovira-Navarro ◽  
Wouter van der Wal ◽  
Valentina R. Barletta ◽  
Bart C. Root ◽  
Louise Sandberg Sørensen
Keyword(s):  
Solid Earth ◽  
Upper Mantle ◽  
Barents Sea ◽  
Ice Sheet ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Mantle Viscosity ◽  
The Barents Sea ◽  
Grace Data ◽  
Geodynamic Models

Abstract. The Barents Sea is situated on a continental margin and was home to a large ice sheet at the Last Glacial Maximum. Studying the solid Earth response to the removal of this ice sheet (Glacial Isostatic Adjustment, GIA) can give insight in the sub-surface structure in this region. However, because the region is currently covered by ocean, uplift measurements from the center of the former ice sheet are not available, but GRACE data has been shown to be able to constrain GIA. Here we analyze GRACE data for the period 2003–2015 in the Barents Sea and use it to constrain a GIA models for the region. We study the effect of uncertainty in non-tidal ocean mass models that are used to correct GRACE data and find that it is not negligible and should be taken into account when studying solid Earth signals in oceanic areas from GRACE. We compare the obtained gravity rates with GIA model predictions for different ice deglaciation chronologies and infer a lower bound for the Earth's upper mantle viscosity of 2·1020 Pa·s. Following a similar procedure for Fennoscandia we find that the preferred upper mantle viscosity there is a factor 2 larger than in the Barents Sea for a range of lithospheric thickness values. This factor is shown to be consistent with the ratio of viscosities derived for both regions from global seismic models. The viscosity difference can serve as constraint for geodynamic models of the area.

scholarly journals The subduction dichotomy of strong plates and weak slabs

Solid Earth ◽  
2017 ◽  
Vol 8 (2) ◽  
pp. 339-350 ◽  
Author(s):  
Robert I. Petersen ◽  
Dave R. Stegman ◽  
Paul J. Tackley
Keyword(s):  
Seismic Tomography ◽  
Lower Mantle ◽  
Plate Tectonics ◽  
Material Strength ◽  
Control Material ◽  
The Earth ◽  
Dynamic Forces ◽  
Lithospheric Plates ◽  
Geodynamic Models

Abstract. A key element of plate tectonics on Earth is that the lithosphere is subducting into the mantle. Subduction results from forces that bend and pull the lithosphere into the interior of the Earth. Once subducted, lithospheric slabs are further modified by dynamic forces in the mantle, and their sinking is inhibited by the increase in viscosity of the lower mantle. These forces are resisted by the material strength of the lithosphere. Using geodynamic models, we investigate several subduction models, wherein we control material strength by setting a maximum viscosity for the surface plates and the subducted slabs independently. We find that models characterized by a dichotomy of lithosphere strengths produce a spectrum of results that are comparable to interpretations of observations of subduction on Earth. These models have strong lithospheric plates at the surface, which promotes Earth-like single-sided subduction. At the same time, these models have weakened lithospheric subducted slabs which can more easily bend to either lie flat or fold into a slab pile atop the lower mantle, reproducing the spectrum of slab morphologies that have been interpreted from images of seismic tomography.

scholarly journals Stability of ferrous-iron-rich bridgmanite under reducing midmantle conditions

2017 ◽  
Vol 114 (25) ◽  
pp. 6468-6473 ◽  
Author(s):  
Sang-Heon Shim ◽  
Brent Grocholski ◽  
Yu Ye ◽  
E. Ercan Alp ◽  
Shenzhen Xu ◽  
...  
Keyword(s):  
Oxidation State ◽  
Upper Mantle ◽  
Lower Mantle ◽  
Sound Speed ◽  
Bulk Composition ◽  
Mantle Flow ◽  
Mantle Minerals ◽  
Fe Content ◽  
Mixed Oxidation ◽  
Seismic Measurements

Our current understanding of the electronic state of iron in lower-mantle minerals leads to a considerable disagreement in bulk sound speed with seismic measurements if the lower mantle has the same composition as the upper mantle (pyrolite). In the modeling studies, the content and oxidation state of Fe in the minerals have been assumed to be constant throughout the lower mantle. Here, we report high-pressure experimental results in which Fe becomes dominantly Fe2+ in bridgmanite synthesized at 40–70 GPa and 2,000 K, while it is in mixed oxidation state (Fe3+/∑Fe = 60%) in the samples synthesized below and above the pressure range. Little Fe3+ in bridgmanite combined with the strong partitioning of Fe2+ into ferropericlase will alter the Fe content for these minerals at 1,100- to 1,700-km depths. Our calculations show that the change in iron content harmonizes the bulk sound speed of pyrolite with the seismic values in this region. Our experiments support no significant changes in bulk composition for most of the mantle, but possible changes in physical properties and processes (such as viscosity and mantle flow patterns) in the midmantle.

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