and their relationship to mantle dynamics.
Part II: Material Transport Across the Mantle: Geophysical Observations and Geodynamic Predictions
When combined with mineral physics constraints, observations of seismic anisotropy in Earth’s mantle can be used to infer mantle flow patterns and understand material transport through the mantle. Thorsten Becker and Sergei Lebedev summarize seismic anisotropy observations made in the upper mantle and discuss their relation to laboratory measurements. It is shown that regional convection patterns can be resolved using anisotropy observations, but uncertainties remain and systematic relationships need to be further refined and established.
An essential regional component of mantle convection is the subduction and deep cycling of oceanic lithosphere. Indeed, the sinking of subducted slabs through the mantle is a key driver of mantle circulation and plate tectonics. Zhouchuan Huang and Dapeng Zhao discuss the analysis of seismic anisotropy in subduction zone settings that is used to infer deformation and flow patterns around subducting slabs in the upper mantle.
Subducted oceanic crust often descends deep into the mantle, commonly reaching the core–mantle boundary. Understanding the fate of subducted material and its chemical interaction with the surrounding mantle is pivotal to our understanding of deep Earth material cycles, the interpretation of geophysical heterogeneity, and the diversity of geochemical signatures found in surface rocks. Mingming Li reviews our understanding of the distribution, physical behavior and chemical interaction of subducted oceanic crust in the deep mantle that emerges by integrating evidence from different disciplines.
In regions where subducting slabs reach the core‐mantle boundary, they may cause deformation strong enough to induce seismic anisotropy, in a similar way as discussed above for the upper mantle. Andy Nowacki and Sanne Cottaar explore the potential and limitations of multidisciplinary approaches to infer deep mantle flow from recent observations of lowermost mantle seismic anisotropy.
While the downwelling limbs of mantle convection are commonly observed in seismic images, at least in some cases from the surface to the core–mantle boundary (see above), the nature of upwellings remains more under‐resolved and uncertain. However, while their existence has been highly debated for a long time, researchers now converge towards a consensus in terms of the sheer existence and nature of plume‐like upwellings. Jeroen Ritsema et al. report on the challenges involved in imaging mantle plumes by seismic methods. They conclude that the deployment of seismic receivers on the ocean floors would lead to a significant advancement in our ability to image the interior of our planet in general, and mantle plumes in particular.
Part III: Surface Expressions: Mantle Controls on Planetary Evolution and Habitability
An important surface expression of mantle dynamics is dynamic topography. Dynamic topography arises directly from mantle flow and the related stresses at the base of the lithosphere. It thus contains information about the density structure of the mantle. The study of dynamic topography can potentially provide a picture of Earth’s mantle flow and structure through time, when coupled with geological constraints of subsidence and uplift. Mark Hoggard et al. review the progress made in quantifying dynamic topography, summarize the inherent limitations, and point out future research directions.
Trond Torsvik et al. bring together the main topics discussed in this book in a single chapter. They discuss the role of mantle upwellings (plumes) and downwellings (slabs) on atmospheric evolution and climate. Plume heads may rise from the margins of LLSVPs and sustain flood‐basalt volcanism, outgassing and silicate weathering at the surface. Slab subduction and related arc volcanism, in turn, control the ingassing of atmospheric species, including greenhouse gases. Together, these key ingredients of mantle convection stabilize Earth’s long‐term atmospheric conditions, climate, and habitability.
While Earth’s surface is currently reshaped by plate tectonic processes that are intimately linked to mantle convection, other terrestrial bodies in the solar system display a stagnant lid that does not participate in underlying mantle convection. In other words, the surfaces of these planets consist of just one single plate with very restricted surface deformation. Nicola Tosi and Sebastiano Padovan present key processes and observations to understand mantle convection in the stagnant‐lid regime, which is much more common in our solar system, and most probably throughout the universe, than the plate‐tectonic regime. The comparison of mantle convection styles in both these regimes leads to a more coherent picture of planetary interior dynamics and evolution.
Hauke Marquardt University of Oxford, UK Maxim Ballmer University College London, UK Sanne Cottaar University of Cambridge, UK Jasper Konter University of Hawaii at Mānoa, USA
1 Long‐Wavelength Mantle Structure: Geophysical Constraints and Dynamical Models
Maxwell L. Rudolph1, Diogo L. Lourenço1, Pritwiraj Moulik2, and Vedran Lekić2
1 Department of Earth and Planetary Sciences, University of California, Davis, CA, USA
2 Department of Geology, University of Maryland, College Park, MD, USA
ABSTRACT
The viscosity of the mantle affects every aspect of the thermal and compositional evolution of Earth’s interior. Radial variations in viscosity can affect the sinking of slabs, the morphology of plumes, and the rate of convective heat transport and thermal evolution. Below the mantle transition zone, we detect changes in the long‐wavelength pattern of lateral heterogeneity in global tomographic models, a peak in the the depth‐distribution of seismic scatterers, and changes in the dynamics plumes and slabs, which may be associated with a change in viscosity. We analyze the long‐wavelength structures, radial correlation functions, and spectra of four recent global tomographic models and a suite of geodynamic models. We find that the depth‐variations of the spectral slope in tomographic models are most consistent with a geodynamic model that contains both a dynamically significant phase transition and a reduced‐viscosity region at the top of the lower mantle. We present new inferences of the mantle radial viscosity profile that are consistent with the presence of such a feature.
1.1 INTRODUCTION
Heterogeneity in Earth’s mantle is dominated by its very long‐wavelength components in the upper mantle, transition zone, and the lowermost mantle. Such long‐wavelength variations reflect the distribution of the continents and the ocean basins in the uppermost mantle, subducted slabs in the transition zone and the degree‐2 dominant continent‐sized large low shear velocity provinces (LLSVPs) in the lowermost mantle. The long‐wavelength structure of the upper mantle is positively correlated with the lowermost mantle structure (Figure 1.1), supporting the well‐established idea that the lower mantle and lithospheric plate systems mutually interact through subduction and upwelling to produce related large‐scale structures. However, the transition zone and shallow lower mantle contain large‐scale heterogeneity that is weakly anticorrelated with the upper mantle and lowermost mantle. Observations of deflected upwellings, slab stagnation above and below the 650 km phase transition, the presence of seismic scatterers, and changes in the large‐scale pattern of mantle structure suggest the possibility of changes in mantle properties across this region that lack a single agreed‐upon explanation.
The