Physics and Chemistry of the Deep Earth -  - ebook

Physics and Chemistry of the Deep Earth ebook

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Opis

Though the deep interior of the Earth (and other terrestrial planets) is inaccessible to humans, we are able to combine observational, experimental and computational (theoretical) studies to begin to understand the role of the deep Earth in the dynamics and evolution of the planet. This book brings together a series of reviews of key areas in this important and vibrant field of studies. A range of material properties, including phase transformations and rheological properties, influences the way in which material is circulated within the planet. This circulation re-distributes key materials such as volatiles that affect the pattern of materials circulation. The understanding of deep Earth structure and dynamics is a key to the understanding of evolution and dynamics of terrestrial planets, including planets orbiting other stars. This book contains chapters on deep Earth materials, compositional models, and geophysical studies of material circulation which together provide an invaluable synthesis of deep Earth research. Readership: advanced undergraduates, graduates and researchers in geophysics, mineral physics and geochemistry.

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Table of Contents

Title Page

Copyright

Contributors

Preface

Part 1: Materials' Properties

Chapter 1: Volatiles under High Pressure

Summary

1.1 Introduction: What Are Volatiles and Why Are They Important?

1.2 Earth's Volatile Budget

1.3 Water

1.4 Carbon

1.5 Other Volatiles

1.6 Outlook: Mantle Degassing and the Evolution of Atmosphere and Oceans

Acknowledgements

References

Chapter 2: Earth's Mantle Melting in the Presence of C-O-H-Bearing Fluid

Summary

2.1 Introduction

2.2 High-Pressure Experimental Techiques for Fluid-Bearing Systems

2.3 Temperature Profiles and Oxidation State in the Mantle

2.4 An Outline of Experimental Studies at Pressures below 6–7 GPa

2.5 Diamond Formation in Fluid-Bearing Systems

2.6 Melting Phase Relations in Peridotite and Eclogite Systems at Pressures to 20–30 GPa

2.7 Melting Behavior in Different Mantle Systems with Volatiles

2.8 Redox Melting, Redox Freezing, and Diamond Formation

2.9 The Big Mantle Wedge Model and Carbonates

2.10 Concluding Remarks

Acknowledgements

References

Chapter 3: Elasticity, Anelasticity, and Viscosity of a Partially Molten Rock

Summary

3.1 Introduction

3.2 Melt Geometry

3.3 Phenomenological Representation

3.4 Contiguity Model

3.5 Properties of a Texturally Equilibrated System

3.6 Anelasticity

3.7 Applications

3.8 Concluding Remarks

Acknowledgments

References

Chapter 4: Rheological Properties of Minerals and Rocks

Summary

4.1 Introduction

4.2 Mechanisms of Plastic Deformation and Flow Laws

4.3 Experimental Methods in Deformation Studies

4.4 Basic Experimental Observations

4.5 Theoretical Studies

4.6 Some Applications

4.7 Some Speculations on the Rheological Properties of Other Planets

4.8 Summary and Perspectives

Acknowledgements

References

Chapter 5: Electrical Conductivity of Minerals and Rocks

Summary

5.1 Introduction

5.2 Microscopic Physics of Electrical Conductivity

5.3 Issues on the Experimental Studies

5.4 Experimental Results

5.5 Some Applications

5.6 Summary and Perspectives

Acknowledgment

References

Part 2: Compositional Models

Chapter 6: Chemical Composition of the Earth's Lower Mantle: Constraints from Elasticity

Summary

6.1 Introduction

6.2 Brillouin Scattering Spectroscopy

6.3 Representative Recent Experimentnal Results

6.4 Applications to the Lower Mantle Mineralogy

6.5 Concluding Remarks and Outlook

Acknowledgements

References

Chapter 7: Ab Initio Mineralogical Model of the Earth's Lower Mantle

Summary

7.1 Introduction

7.2 Ab Initio Methods for Mineral Physics

7.3 Major Lower Mantle Phases

7.4 Elastic Properties of Materials with Crustal Compositions under Lower Mantle Conditions

7.5 Dense Hydrous Phases

7.6 Insight into the Lower Mantle Heterogeneity

7.7 Concluding Remarks

References

Chapter 8: Chemical and Physical Properties and Thermal State of the Core

Abstract

8.1 Introduction

8.2 Core–Mantle Boundary

8.3 Outer Core: Melting and Melt Properties of the Core Materials

8.4 Inner Core: Structure, Composition, and Physical Properties

Acknowledgments

References

Chapter 9: Composition and Internal Dynamics of Super-Earths

Summary

9.1 Introduction

9.2 A Primer on the Data

9.3 Composition of Planets

9.4 Interior Dynamics

9.5 Challenges for the Future

9.6 Final Remarks

References

Part 3: Geophysical Observations and Models of Material Circulation

Chapter 10: Seismic Observations of Mantle Discontinuities and Their Mineralogical and Dynamical Interpretation

Summary

10.1 Introduction

10.2 Seismological Data and Methods

10.3 The Transition Zone

10.4 Upper Mantle

10.5 Lower Mantle

10.6 Geodynamical Interpretation

10.7 Concluding Remarks

Acknowledgements

References

Chapter 11: Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

Summary

11.1 Introduction

11.2 An Introduction to Linearised Inverse Theory

11.3 Isotropic Velocity Tomography

11.4 Anisotropic Velocity Tomography

11.5 Density Tomography

11.6 Attenuation Tomography

11.7 Promising Future Directions

References

Chapter 12: Mantle Mixing: Processes and Modeling

Summary

12.1 Introduction

12.2 Evidence for Mantle Heterogeneity

12.3 Adding Heterogeneity

12.4 Mixing Processes and Modeling

12.5 Sampling Heterogeneity

12.6 Modeling Perspectives

Acknowledgments

References

Chapter 13: Fluid Processes in Subduction Zones and Water Transport to the Deep Mantle

Summary

13.1 Introduction

13.2 Subduction Zone Processes

13.3 Water Transport to Deep Mantle

13.4 Geochemical Constraints and an Integrated Model

13.5 Concluding Remarks

Acknowledgements

References

Index

This edition first published 2013 © 2013 by John Wiley & Sons, Ltd.

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Library of Congress Cataloging-in-Publication Data

Physics and chemistry of the deep Earth / Shun-ichiro Karato.

pages cm

Includes bibliographical references and index.

ISBN 978-0-470-65914-4 (cloth)

1. Geophysics. 2. Geochemistry. 3. Earth— Core. I. Karato, Shun-ichiro, 1949-

QE501.K325 2013

551.1′2— dc23

2012045123

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Cover image: © iStockphoto.com/Thomas Vogel

Cover design by Design Deluxe

Contributors

Jennifer AndrewsBullard Laboratory, Cambridge University, Cambridge, UK
Elizabeth DayBullard Laboratory, Cambridge University, Cambridge, UK
Arwen DeussBullard Laboratory, Cambridge University, Cambridge, UK
Andreas FichtnerDepartment of Earth Sciences, Utrecht University, Utrecht, The Netherland
Hikaru IwamoriDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
Shun-ichiro KaratoDepartment of Geology and Geophysics, Yale University, New Haven, CT, USA
Kenji KawaiDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
Hans KepplerByerisched Geoinstitut, Universität Bayreuth, Bayreuth, Germany
Konstantin LitasovDepartment of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
Motohiko MurakamiDepartment of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
Tomoeki NakakukiDepartment of Earth and Planetary Systems Science, Hiroshima University, Hiroshima, Japan
Eiji OhtaniDepartment of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
Anton ShatskiyDepartment of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Sendai, Japan
Yasuko TakeiEarthquake Research Institute, University of Tokyo, Tokyo, Japan
Jeannot TrampertDepartment of Earth Sciences, Utrecht University, Utrecht, The Netherland
Taku TsuchiyaGeodynamic Research Center, Ehime University, Matsuyama, Ehime, Japan
Diana ValenciaDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
Peter van KekenDepartment of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA
Duojun WangGraduate University of Chinese Academy of Sciences, College of Earth Sciences, Beijing, China

Preface

Earth's deep interior is largely inaccessible. The deepest hole that human beings have drilled is only to ∼11 km (Kola peninsula in Russia) which is less than 0.2 % of the radius of Earth. Some volcanoes carry rock samples from the deep interior, but a majority of these rocks come from less than ∼200 km depth. Although some fragments of deep rocks (deeper than 300 km) are discovered, the total amount of these rocks is much less than the lunar samples collected during the Apollo mission. Most of geological activities that we daily face occur in the shallow portions of Earth. Devastating earthquakes occur in the crust or in the shallow upper mantle (less than ∼50 km depth), and the surface lithosphere (“plates”) whose relative motion controls most of near surface geological activities has less than ∼100 km thickness. So why do we worry about “deep Earth”?

In a sense, the importance of deep processes to understand the surface processes controlled by plate tectonics is obvious. Although plate motion appears to be nearly two-dimensional, the geometry of plate motion is in fact three-dimensional: Plates are created at mid-ocean ridges and they sink into the deep mantle at ocean trenches, sometimes to the bottom of the mantle. Plate motion that we see on the surface is part of the three-dimensional material circulation in the deep mantle. High-resolution seismological studies show evidence of intense interaction between sinking plates and the deep mantle, particularly the mid-mantle (transition zone) where minerals undergo a series of phase transformations. Circulating materials of the mantle sometimes go to the bottom (the core–mantle boundary) where chemical interaction between these two distinct materials occurs. Deep material circulation is associated with a range of chemical processes including partial melting and dehydration and/or rehydration. These processes define the chemical compositions of various regions, and the material circulation modifies the materials' properties, which in turn control the processes of materials circulation.

In order to understand deep Earth, a multi-disciplinary approach is essential. First, we need to know the behavior of materials under the extreme conditions of deep Earth (and of deep interior of other planets). Drastic changes in properties of materials occur under the deep planetary conditions including phase transformations (changes in crystal structures and melting). Resistance to plastic flow also changes with pressure and temperature as well as with water content. Secondly, we must develop methods to infer deep Earth structures from the surface observations. Thirdly, given some observations, we need to develop a model (or models) to interpret them in the framework of physical/chemical models.

In this book, a collection of papers covering these three areas is presented. The book is divided into three parts. The first part (Keppler, Litasov et al., Takei, Karato, Karato and Wang) includes papers on materials properties that form the basis for developing models and interpreting geophysical/geochemical observations. The second part (Murakami, Tsuchiya and Kawai, Ohtani, Valencia) contains papers on the composition of deep Earth and planets including the models of the mantle and core of Earth as well as models of super-Earths (Earth-like planets orbiting stars other than the Sun). And finally the third part (Deuss et al., Trampert and Fichtner, van Keken, Iwamori) provides several papers that summarize seismological and geochemical observations pertinent to deep mantle materials circulation and geodynamic models of materials circulation where geophysical/geochemical observations and mineral physics data are integrated. All of these papers contain reviews of the related area to help readers understand the current status of these areas.

I thank all the authors and reviewers and editors of Wiley-Blackwell who made it possible to prepare this volume. I hope that this volume will help readers to develop their own understanding of this exciting area of research and to play a role in the future of deep Earth and planet studies.

Shun-ichiro KaratoNew Haven, Connecticut

Part 1

Materials' Properties

Chapter 1

Volatiles under High Pressure

Hans Keppler

Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany

Summary

Hydrogen and carbon are the two most important volatile elements in the Earth's interior, yet their behavior is very different. Hydrogen is soluble in mantle minerals as OH point defects and these minerals constitute a water reservoir comparable in size to the oceans. The distribution of water in the Earth's interior is primarily controlled by the partitioning between minerals, melts and fluids. Most of the water is probably concentrated in the minerals wadsleyite and ringwoodite in the transition zone of the mantle. Carbon, on the other hand, is nearly completely insoluble in the silicates of the mantle and therefore forms a separate phase. Stable carbon-bearing phases are likely carbonates in the upper mantle and diamond or carbides in the deeper mantle. Already minute amounts of water and carbon in its oxidized form (as carbonate or ) greatly reduce the solidus of mantle peridotite. Melting in subduction zones is triggered by water and both water and contribute to the melting below mid-ocean ridges and in the seismic low-velocity zone. Redox melting may occur when oxygen fugacity increases upon upwelling of reduced deep mantle, converting reduced carbon species to carbonate or that strongly depress solidus temperatures. The large contrast of water storage capacity between transition zone minerals and the mineral assemblages of the upper and lower mantle implies that melt may form near the 440 and 660 km seismic discontinuities. Water and carbon have been exchanged during the Earth's history between the surface and the mantle with typical mantle residence times in the order of billions of years. However, the initial distribution of volatiles between these reservoirs at the beginning of the Earth's history is not well known. Nitrogen, noble gases, sulfur and halogens are also continuously exchanged between mantle, oceans and atmosphere, but the details of these element fluxes are not well constrained.

1.1 Introduction: What Are Volatiles and Why Are They Important?

Volatiles are chemical elements and compounds that tend to enter the gas phase in high-temperature magmatic and metamorphic processes. Accordingly, one can get some idea about the types of volatiles occurring in the Earth's interior by looking at compositions of volcanic gases. Table 1.1 compiles some typical volcanic gas analyses. As is obvious from this table, water and carbon dioxide are the two most abundant volatiles and they are also most important for the dynamics of the Earth's interior (e.g. Bercovici & Karato, 2003; Mierdel et al., 2007; Dasgupta & Hirschmann, 2010). Other, less abundant volatiles are sulfur and halogen compounds, particularly HCl, and HF. Noble gases are only trace constituents of volcanic gases, but they carry important information on the origins and history of the reservoirs they are coming from (Graham, 2002; Hilton et al., 2002). Nitrogen is a particular case. Volcanic gas analyses sometimes include nitrogen, but it is often very difficult to distinguish primary nitrogen from contamination by air during the sampling process. The most conclusive evidence for the importance of nitrogen as a volatile component in the Earth's interior is the occurrence of -filled fluid inclusions in eclogites and granulites (Andersen et al., 1993). Ammonium appears to be a common constituent in metamorphic micas, which may therefore recycle nitrogen into the mantle in subduction zones (Sadofsky & Bebout, 2000).

Table 1.1 Composition of volcanic gases (in mol%).

Generally, the composition of fluids trapped as fluid inclusions in magmatic and metamorphic rocks of the Earth's crust is similar to volcanic gases. Water and carbon dioxide prevail; hydrous fluid inclusions often contain abundant dissolved salts. Methane containing inclusions are also sometimes found, particularly in low-grade metamorphic rocks of sedimentary origin and in sediments containing organic matter (Roedder, 1984). Fluid inclusions in diamonds are an important window to fluid compositions in the mantle. Observed types include -rich inclusions, carbonatitic compositions, water-rich inclusions with often very high silicate content, and highly saline brines (Navon et al., 1988; Schrauder & Navon, 1994; Izraeli et al., 2001). Methane and hydrocarbon-bearing inclusions have also been reported from xenoliths in kimberlites (Tomilenko et al., 2009).

Although volatiles are only minor or trace constituents of the Earth's interior, they control many aspects of the evolution of our planet. This is for several reasons: (1) Volatiles, particularly water and carbon dioxide, strongly reduce melting temperatures; melting in subduction zones, in the seismic low velocity zone and in deeper parts of the mantle cannot be understood without considering the effect of water and carbon dioxide (e.g. Tuttle & Bowen, 1958; Kushiro, 1969; Kushiro, 1972; Tatsumi, 1989; Mierdel et al., 2007; Hirschmann, 2010). (2) Even trace amount of water dissolved in major mantle minerals such as olivine can reduce their mechanical strength and therefore the viscosity of the mantle by orders of magnitude (Mackwell et al., 1985; Karato & Jung, 1998; Kohlstedt, 2006). Mantle convection and all associated phenomena, such as plate movements on the Earth's surface, are therefore intimately linked to the storage of water in the mantle. (3) Hydrous fluids and carbonatite melts only occur in trace amounts in the Earth's interior. Nevertheless they are responsible for chemical transport processes on local and on global scales (e.g. Tatsumi, 1989; Iwamori ., 2010). (4) The formation and evolution of the oceans and of the atmosphere is directly linked to the outgassing of the mantle and to the recycling (“ingassing”) of volatiles into the mantle (e.g. McGovern & Schubert, 1989; Rüpke ., 2006; Karato, 2011).

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