Earth's Deep Mantle: Structure, Composition, and Evolution

336 Pages·2005·35.034 MB·English

Geophysical Monograph Series
Including
IUGG Volumes
Maurice Ewing Volumes
Mineral Physics Volumes
Geophysical Monograph Series
122 Dynamics of Fluids in Fractured Rock Boris 143 Mt. Etna: Volcano Laboratory Alessandro Bonaccorso,
Faybishenko, Paul A. Witherspoon, and Sally M. Benson Sonia Calvari, Mauro Coltelli, Ciro Del Negro, and
(Eds.) Susanna Falsaperla (Eds.)
123 Atmospheric Science Across the Stratopause David E. 144 The Subseafloor Biosphere at Mid-Ocean Ridges
Siskind, Stephen D. Eckerman, and Michael E. Summers William S. D. Wilcock, Edward F. DeLong, Deborah S.
(Eds.) Kelley, John A. Baross, and S. Craig Cary (Eds.)
124 Natural Gas Hydrates: Occurrence, Distribution, and 145 Timescales of the Paleomagnetic Fieldyarrces E. T.
Detection Charles K. Paull and Willam P. Dillon (Eds.) Channell, Dennis V. Kent, William Lowrie, and Joseph
125 Space Weather Paul Song, Howard J. Singer, and G. Meert (Eds.)
George L. Siscoe (Eds.) 146 The Extreme Proterozoic: Geology, Geochemistry,
126 The Oceans and Rapid Climate Change: Past, Present, and Climate Gregory S. Jenkins, Mark A. S.
and Future Dan Seidov, Bernd J. Haupt, and Mark McMenamin, Christopher P. McKay, and Linda Sohl
Maslin (Eds.) (Eds.)
127 Gas Transfer at Water Surfaces M. A. Donelan, W. M. 147 Earth's Climate: The Ocean-Atmosphere Interaction
Drennan, E. S. Saltzman, and R. Wanninkhof (Eds.) Chuzai Wang, Shang-Ping Xie, and James A. Carton
(Eds.)
128 Hawaiian Volcanoes: Deep Underwater Perspectives
Eiichi Takahashi, Peter W. Lipman, Michael O. Garcia, 148 Mid-Ocean Ridges: Hydrothermal Interactions
Jiro Naka, and Shigeo Aramaki (Eds.) Between the Lithosphere and Oceans Christopher R.
German, Jian Lin, and Lindsay M. Parson (Eds.)
129 Environmental Mechanics: Water, Mass and Energy
Transfer in the Biosphere Peter A.C. Raats, David 149 Continent-Ocean Interactions Within East Asian
Smiles, and Arthur W. Warrick (Eds.) Marginal Seas Peter Clift, Wolfgang Kuhnt, Pinxian
Wang, and Dennis Hayes (Eds.)
130 Atmospheres in the Solar System: Comparative
Aeronomy Michael Mendillo, Andrew Nagy, and J. H. 150 The State of the Planet: Frontiers and Challenges in
Waite (Eds.) Geophysics Robert Stephen John Sparks
and Christopher John Hawkesworth (Eds.)
131 The Ostracoda: Applications in Quaternary Research
Jonathan A. Holmes and Allan R. Chivas (Eds.) 151 The Cenzoic Southern Ocean: Tectonics,
Sedimentation, and Climate Change Between
132 Mountain Building in the Uralides Pangea to the
Australia and Antarctica Neville Exon, James P.
Present Dennis Brown, Christopher Juhlin, and Victor
Kennett, and Mitchell Malone (Eds.)
Puchkov (Eds.)
152 Sea Salt Aerosol Production: Mechanisms, Methods,
133 Earth's Low-Latitude Boundary Layer Patrick T. Newell
Measurements, and Models Ernie R. Lewis and
and Terry On sage (Eds.)
Stephen E. Schwartz
134 The North Atlantic Oscillation: Climatic Significance
153 Ecosystems and Land Use Change Ruth S. DeFries,
and Environmental Impact James W. Hurrell, Yochanan
Gregory P. Anser, and Richard A. Houghton (Eds.)
Kushnir, Geir Ottersen, and Martin Visbeck (Eds.)
154 The Rocky Mountain Region—An Evolving
135 Prediction in Geomorphology Peter R. Wilcock and
Lithosphere: Tectonics, Geochemistry, and
Richard M. Iverson (Eds.)
Geophysics Karl E. Karlstrom and G. Randy Keller (Eds.)
136 The Central Atlantic Magmatic Province: Insights
155 The Inner Magnetosphere: Physics and Modeling
from Fragments of Pangea W. Hames, J. G. McHone,
Tuija I. Pulkkinen, Nikolai A. Tsyganenko,
P. Renne, and C. Ruppel (Eds.)
and Reiner H. W. Friedel (Eds.)
137 Earth's Climate and Orbital Eccentricity: The Marine
156 Particle Acceleration in Astrophysical Plasmas:
Isotope Stage 11 Question Andre W. Droxler, Richard
Geospace and Beyond Dennis Gallagher, James
Z. Poore, and Lloyd H. Burckle (Eds.)
Horwitz, Joseph Perez, Robert Preece, and John
138 Inside the Subduction Factory John Eiler (Ed.)
Quenby (Eds.)
139 Volcanism and the Earth's Atmosphere Alan Robock
157 Seismic Earth: Array Analysis of Broadband
and Clive Oppenheimer (Eds.)
Seismograms Alan Levander and Guust Nolet (Eds.)
140 Explosive Subaqueous Volcanism James D. L. White,
158 The Nordic Seas: An Integrated Perspective Helge
John L. Smellie, and David A. Clague (Eds.)
Drange, Trond Dokken, Tore Furevik, Rudiger Gerdes,
141 Solar Variability and Its Effects on Climate Judit M. and Wolfgang Berger (Eds.)
Pap and Peter Fox (Eds.)
159 Inner Magnetosphere Interactions: New Perspectives
142 Disturbances in Geospace: The Storm-Substorm From Imaging James Burch, Michael Schulz, and Harlan
Relationship A. Surjalal Sharma, Yohsuke Kamide, and Spence (Eds.)
Gurbax S. Lakhima (Eds.)
Geophysical Monograph 160
Earth's Deep Mantle: Structure,
Composition, and Evolution
Robert D. van der Hilst
Jay D. Bass
Jan Matas
Jeannot Trampert
Editors
§8 American Geophysical Union
Washington, DC
Published under the aegis of the AGU Books Board
Jean-Louis Bougeret, Chair, Gray E. Bebout, Cari T. Friedrichs, James L. Horwitz, Lisa A. Levin, W. Berry Lyons,
Kenneth R. Minschwaner, Andy Nyblade, Darrell Strobel, and William R. Young, members.
Library of Congress Cataloging-in-Publication Data
Earth's deep mantle : structure, composition, and evolution / Robert D. van der
Hilst... [et al.], editors.
p. cm. - (Geophysical monograph, ISSN 0065-8448 ; 160)
Includes bibliographical references.
ISBN-13: 978-0-87590-425-2
ISBN-10: 0-87590-425-4
1. Earth—Mantle—Research. 2. Thermochemistry—Research. 3.
Seismology—Research. 4. Heat—Convection, Natural—Research. I. Hilst, Robert
Dirk van der, 1961- II. Series.
QE509.4.E27 2005
551.1'16-dc22 2005028262
ISBN-10:0-87590-425-4 (hardcover)
ISBN-13: 978-0-87590-425-2 (hardcover)
ISSN 0065-8448
Back Cover: Images modified after a cartoon published in F. Albarede and R. D. van der Hilst,
EOS, Transactions, American Geophysical Union, 45, 535-539, 1999.
Copyright 2005 by the American Geophysical Union
2000 Florida Avenue, N.W.
Washington, DC 20009
Figures, tables, and short excerpts may be reprinted in scientific books and journals if the source is properly cited.
Authorization to photocopy items for internal or personal use, or for the internal or personal use of specific clients, is grant­
ed by the American Geophyscial Union for libraries and other users registered with the Copyright Clearance Center (CCC)
Transactional Reporting Service, provided that the base fee of $1.50 per copy plus $0.35 per page is paid directly to CCC, 222
Rosewood Dr., Danvers, MA 01923. 0065-8448/05/$01.50+0.35.
This consent does not extend to other kinds of copying, such as copying for creating new collective works or for resale. The
reproduction of multiple copies and the use of full articles or the use of extracts, including figures and tables, for commercial
purposes requires permission from the American Geophysical Union.
Printed in the United States of America.
CONTENTS
Preface
Robert D. van der Hilst, Jay D. Bass, Jan Matas, and Jean not Trampert vii
Earth's Deep Mantle: Structure, Composition, and Evolution—An Introduction
Robert D. van der Hilst, Jay D. Bass, Jan Matas, and Jeannot Trampert 1
Noble Gas Models of Mantle Structure and Reservoir Mass Transfer
Darrell Harrison and Chris J. Ballentine 9
The Survival of Mantle Geochemical Heterogeneities
Francis Albarede 27
Towards a Quantitative Interpretation of Global Seismic Tomography
Jeannot Trampert and Robert D. van der Hilst 47
Seismic Modeling Constraints on the South African Super Plume
Don V. Helmberger and Sidao Ni 63
Numerical and Laboratory Studies of Mantle Convection: Philosophy, Accomplishments, and
Thermochemical Structure and Evolution
Paul J. Tackley, Shunxing Xie, Takashi Nakagawa, and John W. Hern I und 83
Heterogeneous Lowermost Mantle: Compositional Constraints and Seismological Observables
H. Samuel, C.G. Farnetani, and D, Andrault 101
Numerical Study of the Origin and Stability of Chemically Distinct Reservoirs Deep in Earth's Mantle
P. van Thienen, J. van Summeren, R. D. van der Hilst, A. P. van den Berg, and N. ). Vlaar 117
Self-Gravity, Self-Consistency, and Self-Organization in Geodynamics and Geochemistry
Don L Anderson 137
The Role of Theoretical Mineral Physics in Modeling the Earth's Interior
Mark S. T. Bukowinski and Sofia Akber-Knutson 165
The Uncertain Major Element Bulk Composition of Earth's Mantle
Q. Williams and E. Knittle 187
Highly Siderophile Elements: Constraints on Earth Accretion and Early Differentiation
Kevin Righter 201
Mantle Oxidation State and Oxygen Fugacity: Constraints on Mantle Chemistry, Structure, and Dynamics
Catherine A. McCammon 219
Thermochemical State of the Lower Mantle: New Insights From Mineral Physics
James Badro, Guillaume Fiquet, and Frangois Guyot 241
Stability of MgSiO Perovskite in the Lower Mantle
s
Sang-Heon Shim 261
Synthetic Tomographic Images of Slabs From Mineral Physics
Y. Ricard, E. Mattern, and J. Matas 283
Compositional Dependence of the Elastic Wave Velocities of Mantle Minerals: Implications for Seismic
Properties of Mantle Rocks
Sergio Speziale, Fuming Jiang, and Thomas S. Duffy 301
Recent Progress in Experimental Mineral Physics: Phase Relations of Hydrous Systems and the Role of Water
in Slab Dynamics
Fiji Ohtani 321
PREFACE
Understanding the inner workings of our planet and its re­ In the last decade geoscientists have begun to respond to
lationship to processes closer to the surface remains a fron­ such questions with more quantitative integrations of ob­
tier in the geosciences. Manmade probes barely reach -10 km servations and constraints from different lines of inquiry—
depth and volcanism rarely brings up samples from deeper an approach that is changing our views on the structure,
than -150 km. These distances are dwarfed by Earth's di­ composition, and evolution of Earth's deep mantle. Studies
mensions, and our knowledge of the deeper realms is pieced within and across traditional disciplinary boundaries have
together from a range of surface observables, meteorite and inspired many special sessions at annual meetings of the
solar atmosphere analyses, experimental and theoretical main professional societies as well as many topical work­
mineral physics and rock mechanics, and computer simula­ shops. The Union Session on Geophysical and Geochemi-
tions. A major unresolved issue concerns the nature of man­ cal Models for the Structure and Composition of Earth's
tle convection, the slow (1-5 cm/year) solid-state stirring that Mantle at the joint EGS-EUG-AGU meeting (Nice, France,
helps cool the planet by transporting radiogenic and primor­ April 7-11, 2003) and a workshop on Mantle Composition,
dial heat from Earth's interior to its surface. Structure, and Phase Transitions (Frejus, France, April
Expanding our knowledge here requires input from a 2-6, 2003) brought together seismologists, geodynami­
range of geoscience disciplines, including seismology, cists, mineral physicists, and geochemists to discuss new
geodynamics, mineral physics, and mantle petrology and observations and changing views on Earth's mantle. The
chemistry. At the same time, with better data sets and faster current volume derives from these meetings, along with
computers, seismologists are producing more detailed mod­ other invited contributions. Most contributors have com­
els of 3-D variations in the propagation speed of different bined a review component with a presentation of new de­
types of seismic waves; new instrumentation and access to velopments in the respective disciplinary fields. We hope,
state-of-the-art community facilities such as synchrotrons therefore, that readers will consider this volume both as an
have enabled mineral physicists to measure rock and min­ overview of achievements made during the recent past and
eral properties at ever larger pressures and temperatures; as a source of inspiration for new investigations.
new generations of mass spectrometers are allowing geo- We thank the coorganizers of the two workshops that in­
chemists to quantify minute concentrations of diagnostic spired the production of this volume and Centre Nationale
isotopes; and with supercomputers geodynamicists are de la Recherche Scientific (CNRS), the Institut Nationale
making increasingly realistic simulations of dynamic pro­ des Sciences de TUnivers (INSU), the French Ministry of
cesses at conditions not attainable in analogue experiments. Research and Technology, the U.S. National Science Foun­
But many questions persist. What causes the lateral varia­ dation (NSF), the Consortium for Materials Properties Re­
tions in seismic wavespeed that we can image with mount­ search in Earth Sciences (COMPRES), and the Ecole Nor-
ing accuracy? How reliable are extrapolations of laboratory male Superieure de Lyon, France, for their financial support
measurements on simple materials over many orders of of the workshop in Frejus, which enabled the broad par­
magnitude of pressure and temperature? What are the ef­ ticipation of students and post-docs and of scientists from
fects of volatiles and minor elements on rock and mineral Japan and the USA. We thank the authors for their contribu­
properties under extreme physical conditions? Can ab initio tions and also the more than 40 reviewers for their invalu­
calculations help us understand material behavior in condi­ able help with evaluating and improving the manuscripts.
tions that are still out of reach of laboratory measurement?
What was the early evolution of our planet and to what ex­ Robert D. van der Hilst
tent does it still influence present-day dynamics? And how Jay D. Bass
well do we know such first-order issues as the average bulk Jan Matas
composition of Earth? Jeannot Trampert
Earth's Deep Mantle: Structure, Composition, and Evolution
Geophysical Monograph Series
Copyright 2005 by the American Geophysical Union
10.1029/160GM01
vii
Earth's Deep Mantle: Structure, Composition,
and Evolution—An Introduction
Robert D. van der Hilst1, Jay D. Bass2, Jan Matas3, and Jeannot Trampert4
Here we present the general scope of the monograph and introduce the different
chapters. The chapters are organized by theme instead of scientific discipline, and
most combine a review of past accomplishments with discussions of results from
current research. Collectively, they document the tremendous progress in under­
standing achieved over the past decade or so, but they also demonstrate that many
controversies and challenges remain for future collaborative studies of Earth's
deep interior.
GENERAL SCOPE OF MONOGRAPH cally inferred heterogeneity, reproducing essential aspects
of the geochemical record of selected chemical elements,
In the past decade, spectacular advances in geochemis­ and satisfying geophysical observables such as the geoid
try, theoretical and experimental mineral physics, seismic and long wavelength gravity, dynamic topography, and
imaging, and computational geodynamics have produced heat flow.
new views on the inner workings of our planet. In con­ A challenge central to all these studies has been to under­
trast to previous single-disciplinary approaches, in the mid- stand the nature, scale, and geological history of mantle
1990s many investigators started to make more rigorous convection—the slow (1-5 cm/yr) solid-state stirring that
efforts to include constraints from other disciplines into helps cool the planet by transporting radiogenic and primor­
their models or interpretations. Experimental and theoreti­ dial heat from Earth's interior to its surface. Evidence for a
cal mineral physics research has been yielding increasingly chemically heterogeneous mantle, with multiple long-lived
accurate constraints on elastic parameters at pressure and "reservoirs", has long competed with views that convection
temperature conditions typical for Earth's mantle. With this driven by pure thermal buoyancy has effectively homog­
growing—but still vastly incomplete—data base, seismolo­ enized the mantle. The ensuing debate revolved around
gists have begun to realize that not all inferred wavespeed end-member models of either convective layering at 660
variations can have a thermal origin, and evidence for lateral km depth (with layers above and below this depth having
variations in the composition of Earth's mantle has been different—but uniform—compositions), or iso-chemical
mounting. In a parallel effort, geodynamicists have been whole mantle overturn. But none of these canonical models
using the input from mineral physics research and exploiting satisfies more than what can now be considered a fairly nar­
increased computational power to simulate thermo-chemical row subset of available constraints. Over the past decade,
convection, with important measures of their success being however, several key discoveries and the creative interplay
the ability to match the structure and spectra of tomographi- between various disciplines have begun to paint a picture
of the lower mantle that is far more interesting—but not
^assachusetts Institute of Technology, Cambridge, Massachu­ less enigmatic—than the one or two relatively bland shells
setts. of near-constant physical, chemical, and thermo-dynami­
2University of Illinois, Urbana-Champaign, Urbana, Illinois.
cal properties considered in the classical models. Some
3Ecole Normale Superieure de Lyon, Lyon, France.
scientists have been trying to explain the richness of mul-
4Utrecht University, Utrecht, The Netherlands.
tidisciplinary constraints with hybrid models that combine
Earth's Deep Mantle: Structure, Composition, and Evolution the successful ingredients of the canonical models; others
Geophysical Monograph Series 160
have been questioning the fundamental assumptions and the
Copyright 2005 by the American Geophysical Union
10.1029/160GM02 perceived paradoxes to produce interesting, albeit as yet dif-
1
2 INTRODUCTION: STRUCTURE, COMPOSITION, AND EVOLUTION OF EARTH'S DEEP MANTLE
ficult to validate, conceptual models of the thermo-chemical to major element composition is reasonable, what—if any­
evolution of the Earth. thing—does it tell us about the spatial distribution of the
In the late 1990s, new types of crosscutting research began trace elements and noble gases used in geochemical inves­
to provide important new insight. Joint interpretations of tigations of mantle structure? For example, if the deepest
seismic images of shear- and bulk properties and then avail­ mantle contains compositionally distinct domains, do they
able elasticity parameters for relevant mantle compositions represent refractory repositories or are they depleted in
and P-T conditions produced tentative indicators for lateral heat-producing elements? Questions such as these have key
variations in (major element) composition in the deep mantle implications for our understanding of the formation origin
(e.g., Su and Dziewonski, 1997; Kennett et al, 1998; van der of the structures and their evolution over geological time, the
Hilst andKdrason, 1999). This emerging evidence inspired early evolution of the core-mantle system, and the present-
several groups (e.g., Tackley, 1998; Kellogg et al, 1999; day thermal state of our planet.
Becker et al, 1999; Davaille, 1999) to speculate on the
existence of compositionally distinct domains in the deep OVERVIEW OF MONOGRAPH
mantle as an explanation of some of the hitherto conflicting
observations. These exciting developments followed earlier For this monograph we have solicited contributions that
suggestions of large-scale compositional heterogeneity that relate to the general scope described above. Other important
were based on seismic wave speeds and the high-pressure developments in the quest to understand Earth's deep mantle
densities of different rock lithologies, but with much less are not covered in detail. For instance, the lowermost mantle,
extensive elasticity and phase equilibrium data for miner­ including the so-called D" region of enhanced structural
als (e.g., Bass and Anderson, 1984; Anderson, 1989). In and, probably, compositional heterogeneity, is not a primary
fact, the model described by Anderson (this volume) shares target of the chapters of this volume; the progress achieved
with the proposition of Kellogg et al the premise of a three- in the 1990s is reviewed elsewhere (e.g., Gurnis et al, 1998;
layer mantle and a compositionally distinct bottom third of Garnero, 2000; Karato et al, 2000). The recent discovery
the mantle. The early seismological evidence for composi­ of a post-perovskite phase transition is covered briefly, but
tional heterogeneity was far from uncontroversial, however, an overview of this rapidly evolving topic would perhaps be
but through detailed analysis by, for instance, Ishii and premature. The emphasis on the deep mantle also means that
Tromp (1999), Masters et al (2000), Saltzer et al (2001, the numerous recent studies of the upper mantle and transi­
2004), Karato and Karki (2001), Anatolik et al (2003), and tion zone structures, including the related pressure-induced
Trampert et al (2004), the notion has become inescapable discontinuities, and the investigation of mantle "plumes" are
that much of deep mantle heterogeneity does not have a not focused on here.
purely thermal origin. The first two chapters of this volume concern old and new
Despite these developments, there are many uncertain­ constraints from geochemistry. In the past decade, many
ties, and even first-order aspects of the nature, volume, new proposals have been made and fundamental assump­
and long-term evolution of deep mantle heterogeneity have tions questioned. For example, the difference in mid-ocean
remained enigmatic. The early thermo-chemical convec­ island basalt (MORB) and ocean island basalt (OIB) trace
tion models each have their problems (see overviews by, for element and noble gas distributions, which has inspired much
instance, Tackley, this volume), and the interpretation of the discussion about geochemical reservoirs, can perhaps be
tomographic images is neither straightforward nor unique. explained by differences in sampling from a (statistically)
But these are perhaps the least of all problems. Constraints similar source (e.g., Anderson, 2001; Kellogg et al, 2002; Ito
as fundamental as Earth's average bulk composition remain and Mahoney, 2005). This issue, however, is not addressed
controversial; the experimental and theoretical constraints in detail here. Harrison and Ballentine (this volume) review
on the temperature and pressure dependencies of elasticity noble gas geochemistry, the so-called helium/heat and argon
mostly involve simple compositions and ignore effects of "paradoxes", and the development of Earth models based on
important minor elements (such as aluminum and calcium), noble gas systems. They show that models based on selected
the oxidation state of Fe (e.g., the amount of Fe3+ in ferro- systems can be misleading and that a wide range of noble
magnesian-silicates), and mineralogy; and extrapolations gases must be considered when building robust mantle mod­
over large temperature and pressure ranges remain a major els. The changing views on the helium system are reviewed,
concern. Moreover, with few exceptions most scaling from with emphasis on the uncertainty in absolute concentrations
wavespeed to temperature, composition, and mass density and in the level of compatibility of parent and daughter ele­
assume predominance of a thermal dependence, which may ments. For the latest development on the latter topic we refer
be incorrect. Even if the transformation from wavespeed the reader to Parman et al (2005). Harrison and Ballentine
VAN DER HILST ET AL. 3
discuss the pitfalls of comparing systems with very different Mantle convection experiments, both analog and numerical,
characteristic time scales and suggest that a "zero paradox" play an essential role in understanding the dynamics, evolu­
model can be constructed if (i) one relaxes the assumption tion, and structure of Earth's mantle. In particular, quantita­
that the mantle is in steady state and (ii) the noble gas con­ tive geodynamics provides a framework for the integration of
centration (e.g., 3He) in the convecting mantle is higher—by results from different disciplines (e.g., seismology, mineral
a factor of 3.5—than hitherto assumed. Albarede (this vol­ physics, geochemistry, and geodesy), and owing to vastly
ume) also uses characteristic time scales—here of residence increased computation power, the field of quantitative geo-
times of incompatible lithophile elements—to question the dynamical modeling of mantle convection has taken flight in
basic assumption of steady state and argues that most of the the past 15 years. The mounting evidence for compositional
noble gas and heat flux paradoxes disappear if one allows heterogeneity implies that traditional views of mantle pro­
that a significant fraction (~50%) of the "missing" heat is cesses based on thermal convection may be in error, however,
still trapped in Earth's deep interior, perhaps in small-scale and strongly suggests the need for explicit thermo-chemi­
heterogeneity scattered throughout Earth's mantle. cal convection modeling. The capabilities, remaining chal­
The type of geochemical analysis presented here by lenges, and future of numerical modeling of thermo-chemical
Harrison andBallentine and Albarede can provide constraints convection are reviewed by Tackley et al (this volume). In
on isotope and trace element concentrations and on character­ addition to past successes of numerical modeling of deep
istic time scales of mixing processes, but it does not constrain mantle processes, Tackley et al also discuss the challenge to
the location of anomalous materials. Seismology provides deal with uncertainty. Fundamental uncertainties in the key
the most direct probe for constraining deep mantle structure physical parameters can be so large that conflicting convec­
(through the mapping of 3-D variation in (an)elastic wave tion behaviors seem possible. It is therefore of key importance
propagation) and on the spatial distribution of major elements that predictions from numerical models are tested against
(through their influence on the elastic constants and on mass independent data (e.g., seismic models, geochemical records,
density). Trampert and van der Hilst (this volume) review per­ geodynamo history). Another source of uncertainty concerns
tinent results of seismic tomography, with emphasis on lateral the model simplifications which reflect imperfect knowledge
deviations from radially stratified (1-D) reference Earth mod­ of many key physical processes or which are necessary to
els. Because of the non-uniqueness of joint interpretations of render problems tractable. This includes processes that operate
tomographic images along with mineral physics data Trampert on small scales but which may be key controls for the large-
et al (2004) performed a statistical analysis of a wide range scale dynamical behavior. Examples include slab processes
of possible solutions. This approach confirms and quantifies (such as hydration), melting, and plate formation. Others are
earlier indications that the propagation behavior of seismic too complicated or still too poorly understood, for instance,
waves is different from what can be expected from a thermal continent formation and evolution and the effect of volatiles
origin and shows that the emerging evidence for lateral varia­ and grain size on transport properties.
tions in bulk composition is robust. The analysis of body wave Inspired by Kellogg et al. (1999), Samuel et al. and van
travel times, surface wave phase velocities, and spectra of Thienen et al. investigate mantle dynamics in the presence of
Earth's free oscillation modes suggests that lateral variations compositionally distinct domains in the deep mantle. Samuel
in temperature, iron, and silicon (through perovskite/mag- et al. (this volume) assume that such a deep structure exists
nesiowiistite partitioning) occur globally at a wide range of and explore the range of compositions that are consistent
spatial scales and throughout the mantle, but appear stronger with the emerging evidence for compositional heterogeneity
in the bottom 1000 km or so. In a detailed regional study based from seismic imaging. In agreement with earlier findings
on a careful analysis of broad-band waveforms, Helmberger (e.g., Kellogg et al, 1999; Saltzer et al, 2004; Trampert et
and Ni (this volume) describe rather dramatic lateral varia­ al, 2004) they conclude that deep mantle lateral variations in
tions in shear wave speed (relative to PREM) in the deep iron and silicon must be invoked to explain the seismologi­
mantle beneath southern Africa. Interestingly, the variations cal observations. Realizing that the present-day existence
in P-wave speed are not nearly as large. Combined with the of such domains begs the question as to their origin—see
shape of the deep structure and its remarkably sharp bound­ also Righter (this volume)—van Thienen et al. (this volume)
aries, this observation strongly argues for a thermo-chemical explore possibilities for the early origin and evolution of
origin. Detailed studies of this kind are restricted to regions such domains. Their analysis shows that the formation of
of sufficiently high sampling density, and ongoing efforts to deep, enriched, and gravitationally stable mantle domains
install arrays of seismograph stations in new locations may by convective instabilities and resurfacing before the onset
soon lead to more discoveries of thermo-chemical structures of modern-style plate tectonics is dynamically plausible and
in the deep mantle. consistent with the geochemical record.