Understanding the Building Blocks of the Planet: The Materials Science of Earth Processes

8/3/2019 Understanding the Building Blocks of the Planet: The Materials Science of Earth Processes

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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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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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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

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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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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-

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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

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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.

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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

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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

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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

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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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10

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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11

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.


18/76

12

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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13

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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14

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.


21/76


22/76

16

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.


23/76

17

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.


24/76

18

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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19

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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20

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

Understanding the Building Blocks of the Planet: The Materials Science of Earth Processes

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Transcript of Understanding the Building Blocks of the Planet: The Materials Science of Earth Processes