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Udtdg tBudg Bk t Pt
The MaTerials science o earTh Processes
lg-rg Pg hg-Pu G WkpM 24, 2009, Tmp, az
Subduction
UpperMantle
Outer Core
Inner Core
Plu
me
Transition Zone
384 136
6370 2890
CMB
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Prepared by the Writing Group or Long-Range
Planning or High-Pressure Geosciences
Quentin Williams, Editor
J. Michael Brown, Workshop ri-Chair
James yburczy, Workshop ri-Chair
James van Orman, Workshop ri-Chair
Pamela Burnley
John Parise
Mark RiversRenata Wentzcovitch
Robert Liebermann
Tis report is drawn rom the many presentations and
discussions at the Long-Range Planning or High-
Pressure Geosciences (LRPHPG) Workshop held in
empe, Arizona, on March 24, 2009. Te workshop
was attended by 87 members o the mineral phys-
ics and geophysics research communities rom 39
institutions. Initial drats o this report were openly
available and the high-pressure geosciences commu-
nity commented on them.
Te participant list or the LRPHPG Workshop
can be ound at: http://www.compres.us/index.
php?option=com_content&task=view&id=
97&Itemid=123
Financial support or the LRPHPG Workshop
was provided by the National Science Foundation
(NSF) Division o Earth Sciences. Logistical sup-
port or the LRPHGP Workshop was provided by
the School o Earth and Space Exploration (SESE) o
the Arizona State University and the Consortium or
Materials Properties Research in the Earth Sciences
(COMPRES). COMPRES also provided support or
the preparation and dissemination o this report.Geo Prose provided editing and design assistance.
Tis nal report is being submitted to NSF and other
ederal agencies.
PreerreD ciTaTion
Williams, Q., ed. 2010. Understanding the Building
Blocks o the Planet: Te Materials Science o Earth
Processes. Report to the National Science Foundation.
COMPRES Consortium, 68 pp.
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Udtdg tBudg Bk t Pt
T Mt s et P
lg-rg Pg hg-Pu G WkpM 24, 2009, Tmp, az
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ctt
exutv summy .......................................................................................................................................................................1
cpt 1 | itdut..............................................................................................................................................................3
cpt 2 | et hbtb su: a cqu t Pt it ...............................................................8
Key Questions ............................................................................................................................................................................. 12
cpt 3 | T Mgt d, et c, d t Dp Mt................................................................................ 13
Te Magnetic Field and the Habitability o Earths Surace............................................................................................... 13
Iron AlloysTe Phase Relations o Earths Innermost Interior: Constraints on emperature,Composition, and Phase .................................................................................................................................................... 14
ransport Properties o Iron Alloys: Implications or the Sustainability and Energetics o the Geodynamo ........ 16
Te Deepest Mantle: Te Container o Earths Core .......................................................................................................... 18
Key Questions ............................................................................................................................................................................. 21
cpt 4 | T Td Dm Pt Tt........................................................................................................... 22
Termoelasticity and Seismic Mapping o the Planet ........................................................................................................ 22
Te ransition Zone and Mantle Phase ransitions ........................................................................................................... 24
Deeper ransitions? ................................................................................................................................................................... 26
Termal and Electrical Conductivity o Mantle Minerals: How Does the Mantle Homogenizeand ransport Heat and Electrons? ................................................................................................................................. 27
Chemical Diusivity and Viscosity: How Does the Mantle Mix and Flow?................................................................... 29
Properties o Planetary FluidsMagmas and Metasomatism ......................................................................................... 30
Linkages Between the Deep Earth and the Lithosphere: Deeply Derived Magmas,
Heat Sources, and Metamorphism .................................................................................................................................. 32
Key Questions ............................................................................................................................................................................. 34
cpt 5 | ot Pt, ot it ............................................................................................................................ 35
errestrial Planets and Large Moons ..................................................................................................................................... 35
Solar System Satellites and Minor Planets ............................................................................................................................ 38
Large Planets: H-rich Systems at Ultra-Extreme Conditions ........................................................................................... 39
Exoplanets: New Frontiers o Size, Termal Regime, and Composition ........................................................................ 40
Key Questions ............................................................................................................................................................................. 41
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cpt 6 | T ivb t: ctg t cdt et d Pty it.................................... 42Static, High-Pressure echniques ........................................................................................................................................... 43
Shock-Loading echniques ...................................................................................................................................................... 46
Teoretical Approaches to High-Pressure Geosciences .................................................................................................... 48
Key echnique-Oriented Goals............................................................................................................................................... 51
cpt 7 | Bd impt: nw d cmpx Mt t hg Pu............................................................... 52
Ultra-Hard Materials ................................................................................................................................................................. 52
Radioactive Waste Immobilization ........................................................................................................................................ 53
Energy Storage and Climatic Issues........................................................................................................................................ 54
Key Prospects .............................................................................................................................................................................. 56
cpt 8 | utu t d: Budg u cmmuty ................................................................................................. 57
Recommendations or New Community Experimental and Computational Inrastructure ..................................... 57
Maintaining and Enhancing Access to State-o-the-Art Beamlines ................................................................................ 60
Improving Educational Materials and Community Outreach/Recruitment ................................................................. 64
Future Educational Directions................................................................................................................................................. 65
Future Community-Building Goals ........................................................................................................................................ 66
r.................................................................................................................................................................................... 67
aym ...................................................................................................................................................................................... 68
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1
Te eld o high-pressure geosciences is dedicated toincreasing our knowledge o the materials that make
up the overwhelming majority o planet Earththose
that reside below the surace and are compressed by
the overlying burden. It is rom the interior that the
planets atmosphere and hydrosphere were originally
degassed, and melting processes at depth created
(and continue to create) our ocean basins and conti-
nents. Tus, the starting points or Earths habitable
environmentits atmosphere, its suraceoriginate
rom our planets voluminous interior. Te deepinterior produces the orces that generate virtually all
non-weather-related natural hazards: earthquakes,
volcanic eruptions, and tsunamis. Its impact on the
surace is, perhaps, best illustrated by the annihila-
tion ~251 million years ago o ~90% o Earths lie
due to the environmental consequences o a mas-
sive volcanic eruption whose outpourings covered a
sizable raction o Asia. In short, the planets interior
has been an integral and controlling inuence on
Earths evolutionand its eects are dictated bythe physical and chemical properties o the mate-
rials o the interior, which are the domain o the
high-pressure geosciences.
Te challenges associated with simulating Earths
interior through both experiment and theory are
ormidable. Probing and synthesizing materials at
the conditions o the interior, which are critical or
understanding the properties o materials at depth,
require extraordinarily high pressures and tempera-
tures. Correspondingly, state-o-the-art approaches
are necessary to theoretically calculate material
properties under these conditions. Te high-pressure
geosciences community has spearheaded the develop-
ment o new techniques to probe materials at high
pressures (and has seen its techniques adopted by a
broad range o other scientic disciplines), deployed
emergent technologies, including those developed
at national acilities, and conveyed this high-levelexpertise to new generations o students. From mak-
ing better and larger diamonds to understanding the
physical properties o hydrocarbon clathrates (which
may make up the largest natural gas reservoirs o the
planet), the high-pressure geosciences community
has also played a key role in developing and under-
standing materials o direct societal importanceand
particularly those materials that have required high
pressures to manuacture.
A 2009 workshop on rontiers in high-pressuregeosciences, unded by the National Science
Foundation (NSF), considered promising research
directions in this eld over the next decade. Tis
two-day workshop eatured nine plenary talks and
breakout discussion sessions on our themes:
1. Te Deeper Reaches o the Planet: Properties o
Iron and its Alloys and the Novel Materials o the
Deepest Mantle
2. Te Dynamic Ceramic Mantle
3. Mineral Physics and Society4. Enabling Cutting-Edge Science: ools and the
Accomplishments Tey Will Drive in the Next
Decade o Discovery.
Workshop participants reviewed the impact
the eld o high-pressure geosciences has had on
other subdisciplines o the earth sciences, including
seismology, geodynamics, and petrology. Tey also
discussed the uture o high-pressure geosciences:
what are the next major breakthroughs o our com-
munity, and what inrastructure will be necessary to
achieve them? Tis COMPRES workshop was the
second one ocusing on long-range plan or high-
pressure earth sciences. Te rst, A Vision or High-
Pressure Earth and Planetary Sciences Research: Te
Planets rom Surace to Center, was held on March
2223, 2003 in Miami, Florida, and led to the 2004
exutv summy
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2
Report on Current and Future Research Directionsin High-Pressure Mineral Physics (oten called
the Bass Report).
Tis report describes what the high-pressure geo-
sciences community does, the broad rationales or the
science done by the eld, the technical developments
that the discipline has made, and where the uture
directions o the eld likely lie. Predicting the uture
is dicult or this vibrant and ast-moving eld: the
last decade has seen new and unexpected discoveries
that have changed the views o the deep reaches oour planet, including the recognition o novel elec-
tronic and structural properties o Earth materials at
the extreme conditions o the interior. With new and
improved techniques and inrastructure, the com-
munity is poised over the next decade to continue to
produce dramatic new discoveries and truly engender
a proound understanding o the deep Earths critical
role in producing our habitable planet.
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3
When viewed rom the perspective o Earths interior,our planet is overlain by a vanishingly thin atmo-
sphere, and covered by an ocean that is tiny relative to
the massive rocky interior. Indeed, the habitable zone
o the planet occupies only the thinnest o veneers
at the surace o our planetand, like all veneers, its
existence and viability depend directly on what lies
beneath. Te discipline ohigh-pressure geosciences
is concerned with the properties o the part o our
planet that lies beneath the surace, o which almost
none is accessible to direct sampling via drilling(which is able to scratch only the uppermost ~0.2% o
the planet), and which is compressed to extraordinary
pressures by the burden o many kilometers o overly-
ing rocks. Why are geoscientists concerned with this
vast yet inaccessible region? It is the deep materials
o the planet that drive the ows that produce plate
tectonics. Our ocean and atmosphere originated rom
degassing o the deep planet and they continue to be
cycled through the interior, and the core-generated
magnetic eld protects our surace rom energeticparticle bombardment. In short, the habitable envi-
ronment o Earths surace is a direct consequence
o phenomena directly associated with Earths deep
interiorindeed, it is not an exaggeration to say that
our hydrosphere, and hence our biosphere, exists by
permission o the planets interior.
Beyond the importance o the interior to the evolu-
tion o the surace environment, the extreme pressure
and temperature conditions within the planet give
rise to a suite o phenomena that impact the dynamics
and structure o the planet that can only be under-
stood through high-pressure experiments and theory.
Materials transorm to ar denser structures under the
pressures and temperatures o the interior, including
producing economically important compounds like
diamond. Te solid interior is able to ow, generating
plate tectonics, our continents, and the topography o
the planet. Volcanism originating rom deep withinEarth is responsible or giant eruptions in Earths
history, including the massive volcanic outpourings
in Siberia 251 million years ago that killed 95% o the
planets lie and undamentally changed the nature
o the planets biota. And, our deep interior likely
contains ar more water, carbon, and certainly sulur
than exists at Earths surace. Te exchange between
the surace and interior reservoirs o volatile compo-
nents undamentally impacts our climate over short
(as was seen late last century by the eruption o Mt.Pinatubo and the associated decline in planetary aver-
age temperature o about 1C) and long time scales
(as illustrated by our planets likely uctuations rom
largely iced over to temperate ~750 million years ago),
and moderates the volume o our ocean.
Tus, the vast bulk o our planet has a proound
eect on our surace environment. It is the principal
goal o the high-pressure geosciences community to
probe the properties and processes deep within our
planet. Te knowledge that is garnered rom suchstudies o the interior has applicability across not only
the geosciences, but also through much o the physi-
cal sciences. Tese impacts extend to the neighboring
earth science disciplines o seismology, geodynam-
ics, geomagnetism, and geochemistry, and also more
broadly to materials science, condensed matter
physics, and solid-state chemistry. For example,
geoscientists are now able, using constraints on sound
speeds in Earth materials, to interpret the images o
wavespeed variations in Earths interior generated by
seismologists; the knowledge o how solid rock ows
at extreme conditions is crucial or the geodynamic
understanding o how our silicate mantle convects;
and comprehensive studies o iron and its alloys at
high pressures have illuminated the major driving
orces or the magnetic-eld-producing geodynamo
o Earths core. In short, the entire discipline o high-
cpt 1 | itdut
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4
pressure geosciences is motivated by an overarch-
ing goal o understanding the ongoing physical and
chemical evolution o our planet. Notably, the eects
o the high-pressure geosciences community are notisolated within disciplines o the earth and planetary
sciences. Many o the tools developed in the high-
pressure geosciences to examine materials at extreme
pressures have been adopted in wholesale ashion
across the scientic community as the techniques
or probing matter and synthesizing new materi-
als at extreme conditions. Hence, the high-pressure
geosciences community already exemplies one o
the primary recommendations o the 2009 NSF-GEO
Vision Report: to Reach out in bold new directions,
engaging and incorporating other disciplines.
Why have the experimental and theoretical tech-
niques o the high-pressure geosciences community
proven so valuable? Simply put, their measure-
ments and calculations are extremely challenging,
and high-pressure geoscientists have pushed the
rontiers o technique development or synthesis
and characterization o materials at extreme condi-
tions. Te vast bulk o the planet is at enormously
high pressures, and the goals o the community have
been to not only create apparatuses that simulatethe conditions o having tens, hundreds, and thou-
sands o kilometers o rock piled on our samples as
overburden (generally at extremely high tempera-
tures), but also make meaningul measurements on
samples under these conditions. Because pressure is
orce per unit area, pressures can be maximized by
making samples smallin the high-pressure com-
munity, millimeter-sized samples are considered
large-volumeand inerring the properties o a
complex aggregate o materials (sometimes called
rocks) at the multiple-micron scale under extreme
conditions requires intense and oten highly ocused
probes. Facilities at the o Department o Energys
national laboratories have enabled microsamples to
be examined with light ranging rom x-rays to the
ar-inrared, as well as intense streams o neutrons.
Alternatively, high-velocity bullets can be shot at
gu 1.1. smt t
et t, uttg mp-
t, dpt g, d pu
dt y. Dym tu
uttd, udg
ubdutg t (
bu), d upwg pum (
d). it utt t yt
tutu t p
ky t b pt t
. rvt tmptu t
dt dpt 2000 K
t t 670-km dpt m d-
tuty, 4000 K t t pt
-mt budy (cMB), d
ugy 6000 K t t t. cuty
J.. l, Uvty Tx t aut.
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5
larger samples and the properties o the shock-com-
pressed target measured very, very quickly, beore
the sample catastrophically decompresses. Here, the
challenges are primarily related to the microsecond or
lessand sometimes substantially lesstime scales
o the experiment. Finally, theoretical treatments oEarth materials require calculations on systems that
are both chemically and structurally complex, and
which oten possess dierent structures that lie close
in energy to one another. Hence, rigorous, accurate,
and oten very-large-scale theoretical calculations
are required or the systems o interest in the high-
pressure geosciences.
Te high-pressure geosciences community has
deployed its techniques to generate a broad suite
o new and unanticipated results over the lastdecade that have both illuminated the processes
and properties o materials that occur within the
planets interior, and provided insights into the
complex interactions between our surace environ-
ment and the deep planet. Te communitys recent
achievements include:
Discovered undamental pressure-induced changes
in the electronic properties o iron, one o our
planets most abundant elements; at extremeconditions, its electronic conguration shits
rom high spin to low spin. Tis shit results in
paradigm-changing eects on the density, seismic
velocity, and viscosity o the materials in Earths
deep mantle.
Constrained water and carbon sequestration deep
within the planet, with relevance to the genesis o
our planets ocean, atmosphere, and climate.
Identied a transition to a previously undiscovered
post-perovskite phase at the deepest depths oour silicate mantlea phase whose presence likely
modulates the heat ow out o Earths core and,
hence, controls the energy that produces Earths
magnetic eld.
gu 1.2 (t) smt dmd v . Tw gm-quty
dmd tutd t t tp, d mpd tgt
t mt gkt tt t mp. T d vt
utt tt t mp b pty d by vb gt,
pbd by . Vt t ptu ppxmty
8 mm. T typ pptu pb gtg pu
tt p mt t dpt t pt. img uty hw ittut Gpy. (gt) Vw g t x
tug t dmd mp (m bk wt t mp
mb, w umtd by tmttd gt, fd wt
tpt pu mdum, d uudd by mt gkt).
Dm t mp mb ~0.1 mm. img uty
J. Jk, ct.
gu 1.3. (t) lg-vum p mby. T mp -
td wt u mby t t t t umbd
tugt bd ub; t dmd-pd g gktg.
a d t ub td tp t mby p t
mp, d t utt ub pptu t ydu-
y mpd. Dm t dg t bk
~3.5 m. P t mp, d t bk pd tp t mby. su mb uty d-
t tt pd t ~700-km dpt t pt. o t
g u mmuty t ubtty xtd t ut
pu g t pptu. Pt uty M Py
ittut, sty Bk Uvty. (gt) smt ty
dgd g-vum p mby (d D-Dia-30), dgd
ptuy dmt xpmt t g pu. T
ub mby td t t t pt t pptu,
d td dt t t mp b g-
td tug dpdt mt t ydu m. Vt
dm t pptu ppxmty 1 m. img uty
Y. Wg, Uvty cg.
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6
Determined the viscosity o solid rocks in situ
at high pressures and temperatures, providing
undamental experimental constraints on the
vigor o mantle convection and, hence, on plate
tectonics itsel.
Established the chemical systematics neces-sary to recognize rocks rom the deepest depths
ever observed, ollowed rapidly by the discovery
o such rocks.
One o the primary ocuses o the high-pressure
geosciences community lies in understanding the
complex structures o Earth and planetary materi-
als that occur at both moderate and extreme condi-
tions. Such materials oten have technologic uses
or are valuable analogues or technologic materialsand, thereore, the high-pressure geosciences com-
munity maintains a signicant materials-oriented
component. Recent economically relevant
achievements include:
Probed the properties o clathrates and hydrogen-
rich materials. Te ormer is likely one o the pri-
mary reservoirs o subsurace natural gas, and the
latter have major implications or energy storage.
Synthesized large diamonds at low pressure using
chemical vapor deposition (CVD) technology.Tis accomplishment builds on the long-standing
impact o our discipline on the synthesis o ultra-
hard materials, which has had a proound eect on
the industrial abrasives industry, and is the result
o the need or large, pure, low-cost diamonds or
high-pressure experiments. It will have applica-
tions or coatings, electronic devices, and many
other industrial applications.
Examined the capability o novel oxide structures
as media or the long-term connement o nuclear
waste. Te recognition that some minerals can
eectively retain radionuclides or long periods is
venerable, but characterizing the roles o chemis-
try, pressure, temperature, and radiation damage
on the inertness o possible conning materials has
allowed the tuning o material properties to maxi-
mize their retention ability.
Tese discoveries, which are both interdisciplinary
in their impact and which hinged on experimental
and theoretical innovations, are illustrative o a range
o uture goals o the high-pressure geosciences com-
munity. Specically, generating our science increas-
ingly requires improved collaboration, synergies,organization, and access to community acilities. Our
discipline has produced highly successul enterprises
designed to acilitate cutting-edge experimental sci-
ence or individual investigators at national par-
ticle accelerator acilities. Tese groups include the
NSF-unded COnsortium or Materials Properties
Research in the Earth Sciences (COMPRES) and, at
the Advanced Photon Source APS), portions o the
GeoSoilEnviro Consortium or Advanced Radiation
Sources (GSECARS), the High-Pressure CollaborativeAccess eam (HPCA), and the High-Pressure
Synergetic Center (HPSynC). o successully sustain
our community into the uture, we anticipate that
ensuring general access to state-o-the-art computa-
tional acilities will be a priority, as will exploring new
models or ensuring successul utilization and access
gu 1.4. (t) Pt-yd-typ pptu. T gt but
1.5 m d t mp, w typy w mmt
d, td wt t t gd-d um. T
pptu v typ pu mt tt pd t 100-kmdpt t pt, d v b xtvy ud t t
mgm mt d ud-k tt t uppmt
g t pt. img uty rkd r. (gt)
a k wv gu. T gu gt ~15 m. i t, t p
qupmt tt f g-dv pjt t
mp, d t mp tvy mtd w t m-
pd t g pu dug t k vt. Pu tt
p et tty b d by k-wv tqu.
Pt uty s. stwt, hvd Uvty.
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7
to national acilities and experimental, computational,
and analytic inrastructure that are necessary but
beyond the scope (and desirability) o a single princi-
pal investigator to maintain.
Our collective ocus on the behavior o materi-
als that make up Earth and other planets leads toan intrinsic interdisciplinarity and broadly interac-
tive character o our eld within the earth sciences.
For example, seismology, with its ocus on aulting
and wave speeds, depends upon our studies o the
mechanics o ailure and the elastic properties o
materials; geodynamics hinges on our characteriza-
tions o the viscous ow o materials; geomagnetism
depends on our determinations o the electromag-
netic properties o materials; petrology relies on char-
acterizations o mineral/melt equilibria; planetary sci-ence incorporates the equations o state and behavior
under shock-loading that we determine in modeling
the interiors and impact-history o planets; and,
ultimately, the planets climate is controlled by the
exhalations rom its interior that, modulated by the
surace environment, have generated our atmosphere.
Although we could view our discipline as central to
each o these areas o inquiry, a more accurate por-
trayal is that we provide an overarching rameworkor
how the planets materials behave, a ramework that
supports all o our adjoining disciplines within the
earth sciences.In this report, we describe both our recent achieve-
ments and the areas that we see as ripe or our com-
munity to make the next generation o advancements
in our understanding o the interiorthe very guts
o Planet Earth. Te degree o diculty associated
with probing Earth materials at pressures correspond-
ing to those generated by tens, hundreds, or thou-
sands o kilometers o piled rock, and particularly at
simultaneous temperatures o thousands o degrees
Kelvin, is extraordinary. Over the last several decades,our community has marshaled a combination o
orces, rom our innovative and continuously devel-
oping high-pressure and high-temperature experi-
mental technologies, to state-o-the-art theoretical
approaches, to the ormidable strength o national
particle accelerator acilities, to accomplish our goals
o improving understanding o
our planet. Although we have
made enormous progress, many
o the discoveries produced newquestions that we could not
have anticipated a decade ago,
and many pivotal issues remain
unsolved or controversial. Our
view is that with an integrated
approach coupled with techni-
cal advances, we can see our
way toward truly enhancing our
understanding o the deep Earth,
and ensuring that we undamen-
tally understand our piece o
the complex interrelationships
that govern the evolution o our
planets habitable environment.gu 1.5. smt kg btw g-pu g d gbg
dp. dp, t dt/ut pmy tt d vp m
g-pu g bd. at lbm (2005). og dg by
a. lttm, sty Bk Uvty, dw by J. adm, G P.
Petrology Seismology
Planetary Science
Geochemistry Geodynamics
Geomagnetism
MaterialsScience
Climate
Phase Equilibria,Magma Formation
VolatileDegassing,Retention
Interior Chemistry,Partitioning,
Diusion
Thermal andRheologicalProperties
Superhardand NovelMaterials
Elastic andAnelastic
Properties
Chemistry/Physicsof Interiors, Impact
Processes
Electromagneticand Iron Alloy
Properties
High Pressure Geosciences
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8
Te ocean basins and continents, overlain by the
ocean and atmosphere, are undamental characteris-
tics o Earths suracethey are eatures that set Earth
apart rom the other terrestrial planets. Indeed, each
o these eatures is crucial to the habitability o the
planets surace, and the deep Earth has controlled
their genesis. In the case o the ocean basins, their
depths are a result o the greater density o the oce-anic crust relative to continental crust. Te oceanic
basaltic crustal layer is produced through upwell-
ing and melting o dense mantle beneath mid-ocean
ridges. Correspondingly, the lower-density conti-
nental crust is predominantly generated by entrain-
ment o water-rich rocks o the ocean oor to depth
through subduction, ollowed by the release o water,
and comparatively low-temperature generation o
silica- and water-rich melt above the subducted slab.
It is this type o water-assisted melting that not only
generates continental crust, but also the explosive
volcanism o the Ring o Fire surrounding the Pacic.
Above the crust are the ocean and atmosphere
crucial to our planets habitability, with their exis-
tence likely being a direct consequence o degassing
o volatile materials rom Earths interior. Volcanic
degassing, which is well known to impact the sulur
content o the atmosphere on monthly and annualtime scales, has been a primary contributor to our
ocean and atmosphere. Hence, the linked processes
o silicate melting, volcanism, and volatile degassing
have played a principal role in producing the habit-
able environment o Earths surace.
Te continent-ocean basin dichotomy o Earths
surace hinges on both the melting processes that
occur at depth within the planet, and the ability o
Earths interior to retainand, under some circum-
stances, releasewater. Indeed, despite our planets
cpt 2 | et hbtb su:a cqu t Pt it
gu 2.1. ud dym md t
tm gtu mt upwg
m t -mt budy, w
ud b td t d bt g-
t. T pt tmptu
bv t t uudg mt.
h, t pt dm xpd
mutp 2885 km, t tk
t mt. T pdut d
bt wud u t g pumd pp et u (t 2.2
t vt x). m g-pu g-
pptv, t ky u t
qutty d tu t mt gtd
t dt dpt t ut dt
tmptu wt tm upwg.
rptd by pm m Mm
Pub ltd: Nature, l d v Kk
(2005), pygt 2005.
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9
average land-surace elevation o over 2 km below sea
level, there is an abundant portion o the planet that
sits above the waterline. Tus, the principal topo-
graphic eatures o our planetour high-standing,
exposed continentsare generated by deep melting
o the planet. Te eects o melting on the suraceenvironment are not solely conned to the generation
o continents and ocean basins. Dramatic volcanic
events that involve immense outpourings o magma
rom Earths mantle have occurred sporadically in
Earth history; perhaps the best-known o these ood
basalts are the Siberian raps. Tis set o erup-
tions ~251 million years ago released ~4 million
cubic kilometers o basaltic magma onto the planets
surace, a volume more than sucient to cover the
combined areas o Alaska, exas, and Caliornia withlava to depths o more than a kilometer. Te eects
o such a massive eruption on the surace environ-
ment are still poorly understood, but a clear indica-
tion o its impact is derived rom the synchronous
extinction o 95% o marine species and 70% o all
terrestrial vertebrate species. Te mechanism or the
extinctions likely resides in the voluminous amount o
gasesprincipally sulur- and carbon-bearingthat
would have accompanied such an eruption. Beyond
atmospheric climatic changes, a large inux o suchgases to the atmosphere would have also probably
acidied the near-surace ocean. Te Siberian raps,
which came as close as any known event in Earth
history to destroying lie on the planet, may appropri-
ately be viewed asDeath rom the Deep Earthand
understanding o the physical and chemical processes
that give rise to deeply derived magmatism is thus o
major interest. Indeed, melting is the primary means
not only by which the planets crust ormed, but also
by which the planet segregated into dierent compo-
sitional layers. Hence, understanding the process and
eects o melting at depth within the planet is one o
the key goals and major recurrent themes o the high-
pressure geosciences community.
With respect to the ocean and atmosphere, both
water and carbon dioxide share a common trait : each
can be stably bound into rocks and thus transported
into, or retained within, the planet. In the case o
water, a broad suite o hydrated phases, rom low-
pressure clays and layered structures such as talc and
micas, to more exotic hydrated phases stable only at
high pressures, and even water stably dissolved into
the structure o normally water-ree phases, each can
provide stable, solid hosts or water over a range o
pressure and temperature conditions. Te recogni-tion that amounts o water equivalent to that o the
ocean or more could be sequestered in nominally
water-ree minerals within Earths deep interior
represents an unexpected discovery o the high-
pressure community.
Te amount o atmospheric carbon dioxide repre-
sents the main dierence between the atmospheres
o Mars, Earth, and Venus. Venus has an atmosphere
that is ~96% carbon dioxide and a suocating atmo-
spheric pressure o about 93 atmospheres, while Mars
has about the same percentage o atmospheric carbon
dioxide, but a pressure o only ~0.007 atmospheres.
For comparison, Earth has an atmospheric partial
pressure o carbon dioxide o about 0.0004 atmo-
spheres. Te contrasts between the terrestrial planets
indicate that they have markedly dierent degrees to
which carbon dioxide is retained within, and cycled
gu 2.2. d bt xpu t t sk rv cy, Wa,
wg qut v w mpd v but 1 km vt-
. T t bt w pt t cumb rv
Bt, w wdy tugt t v b pdud by t
m mt upwg tt uty pdu t ubt
t t Ywt g. Pt uty V. cmp, s
Dg stt Uvty.
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into, each planets interior. Tese dierences among
planetary carbon cycles exercise a undamental
control on the atmosphere, climate, and habitability
o each o the terrestrial planets, and probing the por-
tions o the carbon cycle that reside within the solid
portion o the planets represents a primary goal o thehigh-pressure geosciences community.
In contrast to the many possible orms in which
water can be stored at depth, carbon dioxide may be
sequestered at depth within the planet primarily as
CO3-bearing carbonatesin essence, the equivalent
o deep-Earth limestones. Te ambiguity that emerges
with carbon storage at depth involves the degree o
oxidization o the planets interior. Te existence o
diamonds and (possibly) abiogenic methane within
Earths mantle each show that the degree o oxidationo the mantle undamentally inuences how carbon
is stored deep within the planet. Te usual viewpoint
is that zones that have been aected by subduction
(and hence that have indirectly interacted with the
surace) are likely more oxidized, while regions that
retain a chemical signature o core ormation are
more reduced. Hence, the oxidation state o a par-
cel o material in Earths mantle likely reects the
processes and chemical interactions to which it has
been exposedand producing means or determiningthe oxidation state at depth could provide a valuable
orensic tool to illuminate the chemical history o
our planets interior. Indeed, the oxidation state o
dierent mantle regions likely controls the genesis o
perhaps our planets most aesthetic major economic
mineraldiamond.
Te precise amounts o water that are retained
at depth is uncertain but, as long as the eects o
water on mineral properties are well characterized,
they can be inerred or dierent regions based on
observations o seismic wave velocities or electrical
conductivity at depth. Indications are that at least the
equivalent o an ocean o water is likely sequestered
at depth within the planet, and perhaps substantially
more. Because the ocean accounts or only ~0.025%
o Earths mass, even a relatively small amount o
water retention at depth within solid crystalline
phases can yield a reservoir that dwars our near-sur-
ace hydrosphere. Te net observation here is that the
deep Earths likely storage capacity or water is large
relative to the size o the ocean.
Deriving constraints on how water can be stored
within Earths mantle, through both theory andcrystallographic and spectroscopic experiments on
materials synthesized in wet environments at the con-
ditions o Earths interior, has been an area o major
advances or the high-pressure geosciences commu-
nity over the last decade. Te key unknown param-
eters have been the amounts o water delivered to the
surace (through volcanism) relative to rewatering o
the interior (through subduction o water-rich mate-
rials) throughout Earth history. Te balance between
these two uxes exercises a undamental control onthe volume o water at the surace, and determina-
tion o their relative rates is crucial or understanding
the geologic history o water at the planets surace.
Hence, our community is producing data that address
one o the most long-standing questions o not simply
science, but also humanity: why do we have an ocean?
As with water, the amount o carbon present at
depth is dicult to determine, but is certainly ar
greater than the modest amount present within our
atmosphere. Te carbon dioxide reservoir o thenear surace is dominantly sequestered in carbonate
rocksrocks that have taken up the slowly released,
over geologic time, voluminous amount o volcanic
carbon dioxide degassing rom the planets interior.
Te interest in this slow bleed o carbon dioxide rom
the interior is not casual: it is the steady accumula-
tion o deep-Earth-generated carbon dioxide within
the atmosphere ~750 million years ago that pushed
Earth out o the so-called snowball Earth climate
that appears to have produced global glaciations,
including sea ice at equatorial latitudes. Hence, the
deep Earth carbon reservoir has been responsible
or keeping our planet rom remaining an iced-over
planet with lie conned to small and peculiar niches.
Te Cambrian evolutionary explosion o multicel-
lular organisms 540 million years ago was likely made
possible by the equable climate produced by steady
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greenhouse degassing. Among the most critical roles
played by the surace environment (when it is noticed over) has been the sequestration o degassed
carbon dioxide rom the interior, and the conver-
sion o a small portion to atmospheric oxygen (and
complementary organic carbon). In this sense, there
is a direct eed-through o deep Earth carbon into the
planets biosphere and, ater decay, eventually back
into the rock record.
Te eects o the planets interior on lie, and
surace habitability, are clear. Yet, the issue o to what
depth lie can exist within Earths interior is an area o
active inquiry. Lie has long been known to persist to
the pressures present in the deepest ocean: at the base
o the Marianas rench, pressures exceed 0.1 GPa
(1 kbar), but lie appears to thrive at these depths.
Te question that we pose is how deeply might lie
extend to substantially greater pressures within the
solid Earth in places such as between grains, or in
ractures. Te techniques developed by the high-pres-
sure geosciences communitywhich include opti-cal access to high-pressure cells and spectroscopic
techniques that can detect metabolic productshave
proved particularly valuable in this quest or the
deepest possible organism. Although lie clearly can-
not exist at the temperatures and pressures present
in the vast majority o the planet, there are indica-
tions that some single-celled organisms can survive
and even conduct metabolic processes at pressures
corresponding to depths o ~30 km. Te ability o lie
to persistand perhaps thriveat moderately high
pressures also has implications or the possibility that
lie could exist in protected locations in other parts
o the solar system. Possible locales include the water
layer that lies beneath the ice o the Jovian moon
Europa, or within deep aquiers on Mars. Te study
o lie under such extreme conditions hence incorpo-
rates not only geobiology, but also planetary science.
gu 2.3. cyt tutu t tw yt p tt ky t b t gt t wt
d b dxd pt et. (t) cyt tutu ydu-(Mg,)2so
4-wdyt, w t
dmt p wt et mt btw 400- d 520-km dpt. lg b pt dt
xyg t, d d d g td typy upd by mgum t, w bu
ttd t . Pk b w t pb t ydg ubttut t p. T
p w pdtd t b mj wt tt gud tw dd g, wt t pd-
t ubquty bg vfd by xpmt; t mut t tu t b ddtd tpbg t ppt t mt. (gt) cyt tutu Mgco
3-mgt. T gy b dt
b, d t td pt mgum tt tdy dtd by xyg. T
p , wt dmd (w pt ud -xdzg dt), t pbb t b
m w dpt t 2000-km dpt wt et mt. rt wk dt tt t
dpt, mgt tm t d p tt t b wt u gbg xyg; t
gf t tmt t pt b budgt tp tv quy. img u-
ty J. smyt, Uvty cd.
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Te high-pressure geosciences community has been
critically concerned with how carbon (in its many pos-
sible chemical orms) is retained and processed within
the deep Earth. Among the primary results is that,
under oxidizing conditions, carbon can be retained
in carbonate phases throughout the depth range oEarths mantle, while more reducing conditions result
in diamonds. Te widespread appreciation that deep
carbon represents a major (and, by mass, the domi-
nant) player in our planets carbon cycles represents
one o the true achievements o our feld. But, the
chemistry and phase equilibria o carbon at depth are
complex, and we have not yet approached a ull under-
standing o this critical carbon reservoir.
Key Questions
How has Earths interior controlled the sur-
ace budget o carbon and water through the
planets history?
Are there hidden reservoirs o hydrogen and car-bon at depth?
What is the oxidation state o Earths interior?
What are the properties o the molecular u-
ids CO2, H
2O, and CH
4at high pressures and
temperatures?
What are the melting relations and phase equi-
libria o hydrated and carbonated materials at all
mantle conditions?
How does carbon behave over a wide range o deep
Earth conditions, including as a unction o pres-sure, temperature, and oxidation?
o what depths within the planet can single-celled
lie persist, and to what conditions can it thrive?
gu 2.4. Tw pb w qut gt wb et mgt v ud btw
716.5 d 620 m y g. i t tp , g mp bt t p d mt t
uptt rd tw t , wt tt (d mty -) pd wg t g pd. i t
w , vg fd t t qut tt d t p g dug t dt
pd. i t , - d uu-m m v vt gt ttp mt g,
w g-tm umut b dxd m v dgg utmty pdu uft gu
wmg t dt t pt. cdt: Z Dtky, nt s udt.
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T Mgt d d thbtbty et su
Earths magnetic eld is oten thought o simply as a
navigational tool, producing reliable compass direc-
tions at the planets surace. However, the eects o
the presence o a magnetic eld on biologic systems
are proound. Te energetic cosmic ray ux at Earths
surace is dramatically reduced by having a eldandEarths is the strongest among the terrestrial planets.
Tis reduction in energetic particle ux decreases
the mutation rate rom charged particles that are
deected by the planets dipolar magnetic eld. Tus,
the magnetic eld contributes signicantly to the
habitability o the planets surace, and has or at least
the last 3.5 billion years. From a technologic perspec-
tive, the magnetic eld provides protection rom what
can be an electromagnetically harsh solar environ-
ment. Frequently, the eects o solar ares disruptthe Canadian and Scandinavian electrical distribution
systemsa direct consequence o the orientation o
eld lines near Earths poles. But, such disruptions
are minor compared to more extreme solar events.
For example, the largest coronal mass ejection on
record, the 1859 Carrington event, was suciently
severe that it generated auroras at the equator and
induced res in telegraph oces. Te magnetic eld
acts as a protector against both major and minor solar
events, with their prospectively proound eects on
our electrical inrastructure. Indeed, the magnetic
elds eect on extraterrestrial energetic particles can
be viewed as similar to that o the ozone layers role in
screening damaging ultraviolet radiation.
It has long been appreciated that the magnetic eld
is generated by uid motion within Earths electri-
cally conductive, iron-rich liquid outer core (conven-
tional solid-state erromagnets are annihilated at the
high temperatures o the core). Although the dipolar
character o the eld is clearly produced by the eects
o rotation on the uid core, rotation alone is insu-
cient to drive a long-term magnetic eld. Improving
our insights into the composition and dynamics o
Earths core, and hence the energetics and drivers o
Earths geodynamowhich includes how our mag-
netic eld is producedis among the primary goals o
high-pressure geosciences.
cpt 3 | T Mgt d,et c, d t Dp Mt
gu 3.1. cmpt utt t tt -
m jt wt et mgt fd. T tt
u jt wt et fd dpd t pty tv t
tt et, d pt dt t fd by pvu -
gt vt. Dt t t . cuty s. h, nasa.
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i ayT P rt et imt it: ctt Tmptu, cmpt, d P
Te properties o Earths core materials are pivotal in
understanding the magnetic eld generation pro-
cess. Seismologic observations yield a strong start-ing point or inerences about the state o material
within the planets core. Te recognition o the core
as being composed o a central solid inner core and
liquid outer core provides a compelling case that
liquid-solid phase equilibria are critical or the cores
evolution. Moreover, the combination o high-
pressure measurements o elastic properties and
densities, the velocities o seismic waves and densi-
ties in the core, and the cosmochemical abundance
o elements leads to the robust conclusion that thecore is an iron-nickel alloy. Yet, it is an impure alloy,
with about 10 wt% o a lighter alloying component in
the outer core and roughly 5 wt% o a lighter mate-
rial in the solid inner core. Te precise identity o
these lighter alloying components has been a major
unsolved question in geophysics, as well as one o the
primary sources o uncertainty in our knowledge o
the bulk composition o our planet. Te most likely
major components o the light-alloying component
are sulur, oxygen, silicon, carbon, and hydrogen,
with minor roles likely being played by elements suchas phosphorus and nitrogen. Moreover, i elements
with long-lived radioactive isotopes (potassium is
the most common suggestion) are present in even
minor abundance in the core, then the magnetic
dynamo could be partially driven by radioactive
heating. Our knowledge o the elastic and chemical
properties o alloys o each o these elements with
iron has mushroomed over the last decade, but these
properties are oten characterized at pressures and/
or temperatures that all substantially short o thosepresent within the core. Indeed, measurements o
properties at the extraordinary pressures and tem-
peratures o Earths core remain among the scarcest
and most challenging experiments and calculations
in the earth sciences.
Even knowledge o the temperature o
Earths core remains uncertain. Although the
top o Earths core is generally thought to be
around 4500 K with the central temperature
o the planet near 6000 K, these values hingeon both the identity o the lighter alloying
component(s) and the melting relations o
this composition at Earths core conditions,
and might dier by ~1000 K rom these
estimates, generating a large uncertainty in
our knowledge o the overall heat budget o
our planet. In concept, the interace between
the solid inner core and liquid outer core
should represent a xed point in pressure
and temperature spacebut the location o
that xed point depends on the composition
o the coexisting liquid outer and solid inner
core, and hence the compositional uncer-
tainty associated with the core maps directly
into a range o possible temperatures or this
solid-liquid interace. Tus, our constraints
on the thermal state o this region o the
gu 3.2. iutt t qumt gt yg m-
pt wt bt t ut d . Bw pt
t tmtd dty xgy -pkd t dt
tmptu, d t bu d d dt w t my dvd
(Pmy r et Md, PreM) tt dty
wt t d ut . ioB dt -ut
budy. m: gu 2 qut t . (2008); pdud wt
pm m t Mg sty am.
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Tpt Ppt i ay:impt t sutbty degt t Gdym
Te power requirement to produce our planetary-
scale magnetic eld is about 1 erawatt (or about
a actor o ten less than that o all power used byhumans on Earth). Tis gure is a lower bound, as the
eciency o the generation process is ill constrained.
Te energy required to run the geodynamo has
several likely sources: (1) outer core uid ow driven
by cooling rom the mantle above; (2) presuming
that the solid inner core is growing with time, release
o latent heat o usion at the inner core-outer core
boundary; (3) exclusion o a light alloying compo-
nent during solidication o the iron-enriched inner
core, and buoyant rise o the light material; and (4)heat generated by any radioactive elements dissolved
within the core (with 40K being the most commonly
suggested candidate).
Te rst o these sources, heat ow out o the
top o the core, is largely controlled by the thermal
conductivity o the core and mantlein essence, the
rate at which heat can be delivered into the overlying
mantle rom the core. Not only does this core-
derived heat help drive the geodynamo, but it also
plays a key role in driving mantle convection (and
hence plate tectonics), through heating the convect-
ing mantle rom below. Moreover, the magnitude
o irons thermal conductivity controls the sustain-ability o Earths magnetic eld given that uid ow
is required to generate the geodynamo. Termal
conductivity, while a conceptually simple parameter,
is dicult to measure at extreme conditions. One o
the goals o the high-pressure community over the
next decade is to improve our ability to both measure
and theoretically calculate this parameter at deep-
Earth-relevant conditions.
wo o the other possible energy sources or the
geodynamo, latent heat release and buoyant rise ocrystallization-excluded lighter-alloying enriched
material, each depend on the growth o the solid
inner core with time. Seismologic studies have
demonstrated that the material o the inner core is
textured, and may be zonally heterogeneous. How
such structures might arise hinges both on the physi-
cal and chemical processes occurring during core
crystallization, and on any convective stirring that
occurs in the solid inner core. Te ormer processes
depend entirely on the phase equilibria o the corealloy (which in turn is dictated by its composition),
and on the nucleation and growth o iron crystallites
at the ultra-high pressure and temperature condi-
tions o Earths inner core. In comparison, inner core
convection is controlled by the competing transport
properties o viscosity and thermal conductivity o
solid iron alloys at core conditions.
Tus, we see that the thermal conductivity o core
materialshow eciently these iron-rich materi-
als conduct heatis crucial or constraining a range
o broad-reaching problems, including Earths heat
ow budget, the convective vigor in the outer and
inner cores, and the timing o the ormation o the
inner core. Measurements o thermal conductivity at
extreme conditions are very challenging. Although
a ew measurements have been conducted using
the coupling o light with thermal waves within the
gu 3.4. iutt t tm d m
buyy t vt. cuty
B. Butt, Uvty c t Bky.
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sample (such as impulsive stimulated scattering), a
range o other time-resolved measurements, includ-
ing time-domain thermoreectance and emtosecond
broadband optical spectroscopy, will likely be increas-
ingly deployed over the next several years. Te stakes
in accurately constraining thermal conductivity arehigh. From the perspective o the history o Earths
magnetic eld, the key question that will be deter-
mined by accurate thermal conductivity measure-
ments is: how long has Earth had a solid inner core?
Because the inner core plays a key role in determining
the convective style o the core and in generating the
driving orces o the geodynamo, its growth history is
critical or understanding how our magnetic eld has
evolved through time. Our present uncertainty o a
actor o two or three in the thermal conductivity atcore conditions (which, in light o the extreme condi-
tions and diculty o the measurement, is an excel-
lent achievement) causes undamental dierences in
our models o the evolution and timing o inner core
ormation, with estimates varying between ages o
~1.5 billion years to ~4 billion years or the onset o
inner core crystallization.
Te possible presence o radioactive elements
(especially potassium-40, but less plausibly uranium
and/or thorium) within the core, whose decay wouldgenerate heat that would contribute to the geody-
namo, also lies in the rmly testable domain. Both
experiments and theory have provided some tanta-
lizing hints that, in marked contrast to their lower-
pressure behavior, radioactive elements may dissolve
in iron alloys at high pressures. I this is the case, then
the undamental process o radioactive decay o long-
lived radionuclides may play a key role in producing
Earths magnetic eld. Te presence (or absence) o
radioactive elements within the core may ultimately
be observationally determined through a ortuitous
synergy between the elds o astrophysics and geo-
physics. Radioactive decays produce neutrinos, thus,
neutrino detectors, which are usually deployed within
mines near Earths surace, should be able to resolve
whether there is a signicant source o geoneutri-
nosand hence radioactive elementswithin Earths
deepest interior.
Te need or more inormation on the properties o
iron alloys at core conditionstheir phase equilibria,
their melting (and crystallization) behavior, their solid
and liquid viscosities, and their thermal and electrical
conductivitiesis thus driven by a desire to under-
stand the chemical and physical properties o Earths
core, and hence the planets geodynamo. Signicantnew constraints on core alloys have emerged over the
last decade, but the underpinning questions require
suites o experiments and complex calculations at the
conditions o Earths deepest interiorand these are
among the most challenging o enterprises.
It is not solely with an eye to constraining the
genesis o Earths magnetic eld that the physical and
chemical properties o the outer core are o interest.
Te chemistry o the core likely reects the manner in
which the earliest Earth accreted rom smaller bodies.
Te key questions here include: (1) how much did
Earths core material react with the silicate portion o
the planet? and (2) what raction o the bodies that
accreted to orm the bulk o Earth were themselves
dierentiated into core and mantle? In this sense,
gu 3.5. (t) su gut t t Kmd ut
dtt Kmk, Jp (dd dt tt g
ux ut dttd m t g), juxtpd
wt mg my gtd dp et tutu (gt).
rptd by pm m Mm Pub ltd: Nature,ak t . (2005), pygt 2005.
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the composition o Earths core may provide one o
the ew records o the long-ago objects that merged
together to orm our current planet.
Te reaction o iron-rich material with the silicate
mantle would cause water within the silicates to react
with iron, orming rust at low pressure, and iron oxideand iron hydride at higher pressures. Each o these
processes would lead to water loss rom the silicate
Earth-ocean-atmosphere-climate system, either
through hydrogen release and escape rom the atmo-
sphere, or through sequestration o hydrogen within
Earths core. Hence, the process o core ormation
controlled the water budget that the earliest Earth
retained, and thus the reservoir o water available or
ormation o the planets ocean.
T Dpt Mt:T ct et c
Te composition o Earths core may also evolve
through time by interactions with the mantle. In
eect, the mantle acts as a ceramic thermos around
the corebut the degree to which the ceramic
interacts with the molten iron o the core is unclear.
Certainly, the lowermost ~300 km o the mantle is
among its most structurally complex regions, andhence, mirrors the complexity o the uppermost ew
hundred kilometers (which are aected both by plate
tectonics and the deep roots o continents). Because
o its distinctive and complex character, this zone
is distinguished rom the overlying mantle, and is
reerred to as the D (D double-prime) layer. Te ea-
tures that are likely present in the lowermost mantle
include a new high-pressure silicate phase known as
post-perovskitea phase that only becomes stable
within the deepest lowermost mantle, and whose
existence may explain a long-enigmatic discontinu-
ity in seismic wave velocities a ew hundred kilome-
ters above the core-mantle boundary. Additionally,
the crystal structure o this phase could explain the
robust anisotropy o seismic wave propagation (in
which waves propagating with dierent orientations
travel with dierent velocities) observed within D.
Although weve come a long way in our under-
standing o the deep mantle, major issues remain. Te
current estimates o the dependence o the pres-
sure/depth o the phase transition on temperature
indicate that in hot regions, this transition may not
occur, and lower-density perovskite could be juxta-posed with cooler post-perovskite. Such a scenario
explains the regionally variable character o the
seismic discontinuity that has been associated with
the post-perovskite transition; but it also poses a suite
o dynamical issues. In particular, the role that hot
regions containing the less-dense perovskite phase
play in driving mantle upwellings is a topic o intense
scrutiny. Moreover, the temperature at the top o the
outer core could be suciently high that a lens o
perovskite may exist directly above the core, under-lying the denser post-perovsite phase. Te dynamic
implications o such an inverted-density scenario
are also complex, and their exploration will require a
cross-disciplinary eort that incorporates improved
constraints on the conditions under which this transi-
tion might occur within the planet.
gu 3.6. cyt tutu t (Mg,)so3-pvkt d
pt-pvkt p. T bu tm , w t d
d yw pyd t xyg gb t mgum
, d t d tm pt-pvkt xyg. Wt
pt-pvkt, m wv ppgt m pdy g t
dt dfd by t y yw pyd, d m
wy ppdu t t y. rptd by pm m
Mm Pub ltd: Nature, Duy (2008), pygt 2008.
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Te discovery o this new phase ranks as one o the
major advances in the high-pressure geosciences over
the last decade. Its discovery has galvanized interest
across the elds o geodynamics, seismology, and the
high-pressure geosciences community. Te synergies
that have arisen between these elds in conjunctionwith the possible presence o the post-perovskite
phase o (Mg,Fe)(Si,Al)O3
has provided constraints
on the possible heat ow out o the core (and thus the
driving orce or the geodynamo), as well as produc-
ing new paradigms or how mantle plumesthe
isolated upwellings that give rise to non-tectonic vol-
canic eatures such as the Hawaiian Islandsmightorm in the lowermost mantle.
gu 3.7. Dgm t pb t vb tmptu t u t pt-pvkt p
tt. (tp) Tt p budy btw pvkt d pt-pvkt, mpd t t tmp-
tu dtbut m, vtg mt (mt dbt). T tm budy y t -mt
budy pdu t, dwwd dt t budy, w d g w b td wt
upwpd budy (gt d t p w fgu). i t gt p t w fgu, t d, gd d
bu pt t, m d d tmptu dtbut t dp mt, d t bk p-
t t pu-tmptu p t pvkt t pt-pvkt tt. nt tt bu t tp t
ut y tm, t pbty xt tt t y pvkt pt v t t u
t -mt budy. Tp fgu m: ly t . (2005). Bttm p m: gu 5 sm (2008).
Subduction
UpperMantle
UpperMantle
Outer Core
Inner Core
Plu
me
Transition Zone
384 136 23.5 13.5 0
0
Pressure (GPa)
6370 2890 660 410 Depth (km)
Core-Mantle Boundary
D Layer
Temperature
Depth
CMB
CMB
PvLower Mantle
Pv
PPv
Pv
PPv
WarmerColder
PPv
Outer Core
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Temperature(K)
70 80 90 100 110 120 130 140 150Pressure (GPa)
Hill Top
Error ~ 5 GPa
7.5 MPa/K
Post-PvOrthorhombic
Perovskite
MantleAdiab
at
ValleyBottom
~ 8 GPa
~ 250 km
CMB
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However, the post-perovskite phase, and its likely
seismic signature, is ar rom the only anomalous
eatures near the base o the mantle. For example,
two large, low-shear velocity provinces (LLSVPs)lie in almost antipodal positions beneath Arica and
the Southwest Pacic. Tese eatures, which extend
~1000 km above the core-mantle boundary are zones
characterized by seismic shear wave velocities that are
a ew percent lower than the surrounding normal
mantle. Teir compressional velocities are slightly
depressed as well, but are ar short o their shear
velocity anomaly. Te current understanding o the
change o seismic velocities with temperature implies
that, i we attribute the shear wave velocity depression
o these eatures simply to them being hotter than
their surroundings, then their temperature would
be elevated by order 1000 K. Not only would such a
dramatic temperature anomaly be expected to have
a larger signature within the compressional wave
velocity o these eatures, but i LLVSPs were purely
thermal eatures, they would be expected to dominate
mantle convective ow, producing massive upwellings
and perhaps associated volcanism. Rather, it seems
that these eatureseach roughly the size o the larg-
est asteroid, Ceresdier in composition rom their
surrounding material. Indeed, current indications
are that they are may be slightly denser than theirsurroundings. Tis dierence in composition is con-
sistent with the seismic observation that the sides o
these eatures, where they can be interrogated, appear
to be airly sharp, a probable signature o a chemical,
rather than solely thermal, dierence. But, we do not
yet understand either the chemistry o these eatures,
or how they might have arisenand these are major
challenges or our community to constrain. Are they
primordial, dating rom Earths earliest ormation, or
have they been generated over time by a yet-unrec-ognized deep Earth process? And, however they were
generated, what does their presence imply or our
canonical view o mantle convection?
Not all velocity changes near the core-mantle
boundary span such large regions. Ultra-low velocity
zones (ULVZs) are ound in the lowermost 1025 km
o the mantlethese regions involve decreases in
seismic velocity o 1030%. Such strongly depressed
velocities are ound essentially nowhere else in Earths
mantle (indeed, they are comparable to the dierencein velocity between Earths highly silicic, buoyant
crust and its underlying mantle), and our community
has aggressively launched eorts to explain ULVZs.
wo primary options exist: these zones could be areas
o partial melting o the mantle, or they could be
areas where the iron content is dramatically increased
above that o normal mantle. In the ormer case, these
regions would represent the largest silicate magma
chambers on the planet, existing directly above the
core, and characterized by melts that, unlike near-sur-
ace magmas, are so dense that they sink rather than
rise. In the case o iron enrichment, ULVZs would be
eatures that are transitional in composition between
the core and mantle. It has long been appreciated that
chemical reactions occur between silicates and mol-
ten iron, but the length scale over which these reac-
tions operate is uncertain, and hinges on the diusion
gu 3.8. ctud mg m vty m t
wmt mt; t d g t g, w- vty
pv (llsVP) bw t ct Pf, d t bu g my t z tt my b td wt t ubdu-
t ut. T w t g udyg
b. T yw pt pt tvd by tquk wv
m -u u (d dt) tt tgt t llsVP.
cuty e. G, az stt Uvty.
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rates at this hot, ceramic-molten metal interace. Our
community is addressing these challenges through
a tandem approach using both high-level theory
and state-o-the-art high-pressure experiments. Te
goal is to determine both the behavior o iron within
silicates at extreme conditions and the properties osilicate liquids at ultra-high pressure conditions.
Ky Qut
What are the light-alloying components o
Earths core?
What is the temperature o Earths core?
What are the transport properties (particularly
thermal conductivity and viscosity) o iron alloys
at core conditions, and hence what is the likely ageand growth rate o the inner core?
What are the magnitudes o dierent heat sources
within Earths core, including its radioactive ele-
ment content?
gu 3.9. (tp) ctud gb
m vty m.nt t g, w- v-
ty pv (llVsP) tu bt
t sut Pf d a. cuty
c. hu, Uvty c,
st cuz. (bttm) c t
tug m tmgp
md (gt) d tptv dw-
g pb tu pt,
w ud llsVP, pt-pvkt
t b t mt (pPv),
t ut-w vty z (UlVZ),
d t p-tt z
(sTZ). m: gu 1
G d Mnm(2008). rptd
wt pm
m aaas.
How do variations in chemistry and tempera-
ture aect the depth (or pressure), thickness, and
amplitude o the post-perovskite transition in the
lowermost mantle?
What are the temperatures and chemistries o the
two large, low-shear velocity provinces in the deepmantle?
With respect to ultra-low velocity zones, what
are the properties o silicate melts at core-mantle
boundary conditions, and how, and at what level,
can the core enrich the mantle in iron at Earths
core-mantle boundary?
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Development o the plate tectonic paradigm is surely
the prime achievement o the geosciences in the
20th century. It provides a comprehensive understand-
ing o how the surace o our planet evolves, and
explains processes ranging rom the driving orce
o seismic ailure, to mountain building, to most
volcanism. Te surcial lateral motion o plates is
well understood rom a wide range o geodetic andgeologic measurements. What remains an enigma,
however, is the third dimension o plate tectonics.
Tis third dimension includes the ultimate ate o
subducting slabs, eatures that exert a downward pull
that translates to lateral orces, and their interaction
with Earths internal system. By the same token, the
manner in which upwellingsmid-ocean ridges and
non-tectonic volcanismare sourced rom great
depths lies also at the cutting edge o our understand-
ing o the deep planet. Moreover, how the planetprogressed rom having an early, likely largely molten
uppermost ew hundred kilometers (or more) o the
mantle 4.5 billion years ago to its current, gener-
ally well-characterized near-surace system in which
plates overlie a mostly solid (but actively convect-
ing) mantle remains raught with uncertainties.
Hence, deriving constraints on how the interaction
between the deep Earth and the plate tectonic system
has evolved throughout Earth history represents
one o the high-pressure geoscience communitys
principal challenges.
In tandem with seismic probes o the planets
interior, geochemical examination o rocks gener-
ated at and extracted rom depth, and geodynamic
simulations, the tasks or the eld o high-pressure
geosciences involve constraining the composition,
minerals, viscosity, density, and temperature at
depth, which in turn map into the buoyancy orces
that drive upwellings and downwellings within the
planet. Ultimately, the most basic reason that cold
slabs sink and hot materials rise involves the ther-
mal expansion o Earth materialstheir decrease in
density with increased temperature. Te rate at which
they move is, in turn, also controlled by the viscos-
ity o the material through which the upwellings anddownwellings migrate. But, temperature is not the
only property that produces positive and negative
buoyancy. Compositional shits, such as iron enrich-
ment or depletion, and changes in phase (which are
oten correlated with composition) can also control
whether material is buoyant. A classic example o
such eects involves the mineral garnet. At pressures
corresponding to depths o ~50 km, this mineral
becomes abundant within subducted oceanic crust,
at which point this ormerly buoyant crustal mate-rial becomes denser than its surrounding mantle
an eect produced by the abundance o aluminum
within basaltic oceanic crust.
Tmtty d smMppg t Pt
Te primary evidence that can be brought to bear
to understand the nature o the mantle convection,
which ultimately drive plate tectonics, involves varia-
tions in seismic wave velocities, and discontinuities
in seismic wave velocities within Earths interior.
Such seismic data on Earths interior usually include
the velocity o both compressional and shear waves
(so-called body waves, as opposed to surace waves,
which primarily sample the uppermost ew hun-
dred kilometers o the planet). Compressional waves
cpt 4 | T Td Dm Pt Tt
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have particle motions in the same direction as wave
propagation (akin to sound waves), while shear waves
have particle motions perpendicular to the direction
o propagation.
One o the major challenges or high-pressure
geosciences involves determining the dependenceo seismic wave velocity on pressure, temperature,
composition, requency, and even crystal orienta-
tion o the materials o Earths interior. Seismic waves
typically have requencies o about 1 Hertz and
wavelengths on the order o kilometers, ar in excess
o the dimensions o high-pressure samples. Hence,
experiments that directly
constrain wave velocities
are conducted using either
ultrasonic sound waves (otenin the megahertz range), or
laser light (Brillouin spectros-
copy, usually in the gigahertz
range). But, velocities also
depend on the elastic proper-
ties o Earth materialshow
materials respond to com-
pression or shearing while
held at high pressures. So,
measurements o density asa unction o pressure and/
or temperature (under static,
or zero requency conditions)
can yield elastic moduli that
can be compared with either
seismic measurements or
the results o high-requency
experiments. Tus, such
experiments can be used to
validate that high-requency
results can be extrapolated to
seismic requencies.
In instances where there
is a requency dependence o
velocity (i.e., dispersion), or
where attenuation o waves
can be measured, then these
provide prima acie evidence that anelastic behavior
is occurring within the materialthat is, a portion
o the energy o the waves is being absorbed as they
travel through the material. Such anelasticity can be
constrained in the seismic requency band rom the
attenuation o seismic waves, and strong attenuationprovides an indication that grain size or partial melt
eects are present within the mantle. Evaluating the
relative magnitude o these dierent eects requires
a sequence o challenging measurements, oten in a
range that approaches the low requency o seismic
waves. While progress has been made in this area,
gu 4.1. (t) lty vgd dty (r),
vty (V), d mp wv
vty (Vp) ut dpt wt
et. P tt pdu t dt-
ut bvd w t 700-km dpt.cuty J.M. Jk, ct. (gt) T
ptt vty vt et
mt t qu x dpt, m jut
bw t ut (125 km) t jut bv t
-mt budy (2850 km). T d -
pt w vt t t v-
g, w t bu t. T mxmum
vt 4% dpt xpt
t wt dpt, w t vt
15%. m: rtm t . (2004).
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the ability to ully interpret seismic maps o mantle
attenuation remains incomplete, and a primary uture
goal is to better constrain anelastic eects within
Earths mantle.
Within the elastic regime, the challenges are to
map out the relative variations o compressional andshear wave velocity so that, or example, regions in
the planet o higher temperature (and hence lowered
wave velocities) can be distinguished rom zones
with enriched iron contents (which would also be
characterized by lower wave velocities, but oten
have dierent ratios o shear to compressional wave
velocity depression). Obviously, tradeos exist with
respect to enrichments in other elements as well
(e.g., water content), and distinguishing dierent
compositional signatures rom one another and romthe characteristics o thermal anomalies represents
a primary challenge. In this sense, the overall eld o
thermoelasticity provides the basis or determining
what the complex variations in seismic velocities in
the deep Earth mean, and how they map into varia-
tions in temperature or chemistry at depth. Tis
eectively provides the means by which the charac-
teristics o rockswhat types o rocks they might be
and how hot they arecan be determined remotely
through a combination o seismic observations andhigh-pressure measurements and calculations. It is
this ability to meaningully probe and interpret results
rom regions o the planet that we can never view
or sample that has enabled high-pressure geoscien-
tists to geologically map the third dimension o plate
tectonics, resolving the motion o solids at depth
that drives our plate tectonic system, and composi-
tional variations that have a complex interplay with
the uid dynamics o the deep Earth system. Te
capability to describe the orces that drive mantle
convection allows the time dependence o mantle
ow to be addressed, and hence permit constraints
on the likely history (or ourth dimension) o plate
tectonics on Earth. Indeed, the results that we derive,
which can constrain both the chemical and thermal
buoyancy (whether positive or negative) o regions o
the deep Earth, ultimately eed directly back into our
understanding o the history and dynamics o Earths
interiorthe dynamics that, rom beneath, continue
to drive our plate tectonic engine.
T Tt Z dMt P Tt
Earths mantle is divided into two main parts: the
upper mantle, extending down to ~400-km depth,
and the lower mantle, which begins near 700-km
depth. Te transition zone lies between these two
depths. Te samples that we have rom these regions
decrease progressively in abundance with depth.
From the upper mantle, we have abundant samples
that were entrained in volcanic upwellings, while
rom the massive lower mantle, there might exista ew isolated samples o a ew tens o microns in
dimensions embedded within diamonds. From the
transition zone, occasional rock ragments have made
their way to Earths surace. Within the transition
zone, the common minerals o the upper mantle
olivine (also known as the gemstone peridot) and
pyroxenesconvert to a suite o spinel- and garnet-
related as well as more complex crystal structures
(dubbed ringwoodite, majorite, and wadsleyite,
respectively) beore ultimately converting to silicateperovskites and a simple magnesium-iron oxide at the
top o the lower mantle. Te pressures at which these
transitions initiate, and the width o the pressure
interval required or them to proceed to completion,
depend on both temperature and, to a lesser extent,
composition. Tereore, seismic characterizations
o the depth and sharpness o these discontinuities
can, when coupled with accurate laboratory mea-
surements o these transitions, provide a particularly
accurate gauge o the temperature and composition
through this critical region o the planet. It is in the
transition zone region that downwelling slabs can
be markedly deected, with some even becoming
nearly horizontal, and where their seismic signature
appears to dramatically broadena likely indicator o
increased viscosity at depth.
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Te underpinning causes o deep earthquakes,
those that occur between ~350- and 684-km depth
(the depth o the deepest earthquake ever recorded),
may well be connected to the phase transitions that
occur within the transition zone. Tese events are
always associated with ancient subducted material.However, they occur at depths below which the nor-
mal brittle racture that generates near-surace seismic
events is likely completely suppressed by pressure.
Tereore, mechanisms dierent rom standard low-
pressure aulting are likely required to explain why
these deep events occur. Te idea that these events are
associated with how phase transitions proceed within
subducted slabs (with the transitions likely impeded
by the low temperatures present in these environ-
ments) has provided a possible suite o solutions tothis long-standing dilemma (aulting within meta-
stable material that is converting to its high-pressure/
temperature phases). Hence, exploration o the link-
ages between phase transitions and seismic ailure is
motivated by the importance o understanding the
deeper portion o the planets seismicity.
Although the general nature o the transitions that
give rise to the seismic discontinuities within the
transition zone are known, their precise tempera-
ture and composition dependences remain areas o
extraordinarily active inquiry. Future progress on
this topic awa