Читать книгу Mantle Convection and Surface Expressions - Группа авторов - Страница 14

The viscosity of the mantle governs large‐scale material flow within our planet and ultimately dictates the interaction between Earth's deep interior and surface geology. The first chapter of this book, contributed by Maxwell Rudolph et al., focuses on the viscosity profile of the mid‐ and lower mantle and examines its connection to mantle dynamics. Based on analysis of the geoid and seismic tomographic models, the authors favor the presence of a low‐viscosity channel in the mid‐mantle.

Mantle dynamic processes and relevant properties, including mantle viscosity, are governed by the physical behavior of mantle minerals and rocks. Lowell Miyagi reviews the progress made in the experimental study of the plastic behavior of lower mantle materials at high pressures and temperatures. While the overwhelming amount of past experiments on lower‐mantle deformation has been performed on mono‐mineralic samples, he emphasizes the importance of understanding the effective rheology of rocks, which may behave dramatically different than the sum of their parts as a result of strain‐partitioning between the minerals.

The interpretation of seismic observations always critically relies on our knowledge about the elastic properties of Earth's mantle materials at the conditions expected at depth. This information is largely derived from high‐pressure/‐temperature experiments as well as computations. Johannes Buchen reviews the current state of high‐pressure/‐temperature elasticity measurements and outlines future directions. Exemplarily, he discusses the possible effect of the iron spin transition in ferropericlase, and evaluates the potential to resolve compositional variations in the mantle from seismic observations.

Geophysical observations, and their interpretation, can only provide a snapshot of the current state of Earth’s mantle, but do not directly constrain the causative history of mantle‐dynamic processes. Bernhard Schuberth and Tobias Bigalke review approaches to quantitatively link predictions from dynamic mantle evolution models to present‐day seismological observations. After providing a general overview of past works, they focus on the importance of better constraining anelastic effects and their uncertainties to interpret seismic observations, as may be constrained by future mineral physics work.

Complementary constraints on compositional heterogeneity within the Earth’s mantle, and thus on mantle convection and material cycles between the surface and the mantle, are provided through the analysis of trace elements and isotope ratios in various basaltic rocks exposed on Earth’s surface. Takeshi Hanyu and Li‐Hui Chen review our current understanding of chemical diversity in the mantle, as derived from the composition of surface basalts. They highlight some recent alternative models, and discuss implications for the deep cycling of volatile elements.

Part of the geochemical variation described in the chapter on isotopic data could result from selective sampling of the mantle through melting of different source lithologies as a function of pressure and temperature. Ananya Mallik et al. examine the role of melting a mantle assemblage of different lithologies, melt‐rock reaction, and magma differentiation for the genesis of mid‐ocean ridge basalts (MORB), ocean island basalts (OIB), and volcanic arcs.

Diamonds provide complementary information on the composition and mineralogy of the deep mantle. Tiny mineral and glass inclusions in diamonds directly sample the petrology of the deep Earth, providing constraints on its chemistry and mineralogy beyond those derived through the interpretation of geophysical observations and geochemical analyses of surface volcanic rocks. Evan Smith and Fabrizio Nestola summarize the progress in this field during the last decade, focusing on findings of major relevance to our understanding of deep Earth material and volatile cycles.

The D" layer in the lowermost mantle is characterized by several unique geophysical features that are distinct from the bulk of the lower mantle, including ultralow velocity zones, ULVZ. Jennifer Jackson and Christine Thomas contribute a review of seismic observables and their possible interpretations based on mineral‐physics data, and present a case study focusing on the deep mantle below the Bering Sea and Alaska.

Paula Koelemeijer discusses the challenges involved in determining the topography of the core–mantle boundary (CMB) from geodynamic and seismic constraints. She highlights the close correlation with the density structure in the lowermost mantle and its association with the observed large low shear velocity provinces (LLSVP) as are observed under Africa and the Pacific. She concludes that more work is needed to better constrain CMB topography, which would be a critical step toward understanding the nature of LLSVPs and their relationship to mantle dynamics.