High-Pressure Mineral Physics

The fi eld of high-pressure mineral physics is highly interdisciplinary, encompassing the full range of chemical, physical, and biological processes that take place at high pressures beneath the surfaces of planets. These processes infl uence magnetism in Earth’s core at over 1.3 million atmospheres of pressure, and methane production by microbes at high pressure in ocean sediments. In the broadest sense, the goals of mineral physics studies are to understand how planetary systems operate, from the center to the surface, and to understand the processes involved in planetary evolution. This is done by examining the properties of minerals under extreme high pressure and temperature conditions, by performing computer simulations to understand the behavior of planetary materials at the most fundamental atomistic level, and by studying the interactions among the components of the entire Earth system—be they chemical reactions among minerals or biologi- cally mediated interactions. This fi eld has witnessed numerous discoveries and breakthroughs during the past decade. Along with breakthroughs come not only the ability to understand more-complex phenomena, but also the ability to confront exciting challenges. In light of these recent achievements, the challenges for the future, and, consequently, the immense prospects for discovery in the fi eld of mineral physics, it seems timely to describe them in a single document. This report is organized to take the reader on a journey from Earth’s center to its surface, and then beyond to the other planets and moons in our solar system, all from the perspective of high-pressure mineral physics. Finally, no description of high-pressure mineral physics would be complete without mentioning the role that technology plays in our research. This is a fi eld that has distin- guished itself through technological innovation and invention. In trying to understand the interiors of planetary bodies, Earth and planetary scientists have been the leaders in pushing forward the boundaries of extreme conditions that can be attained in the laboratory. They have been central players in the use of synchrotron radiation and neutron scattering for understanding the states of matter at high pressures and temperatures. High- pressure mineral physics is an area that by its very nature thrives on inventing new tools to understand Earth, and to see deeper and deeper into the interiors of planetary bodies.

Earth cover ﬁ gure courtesy of Edward Garnero, Arizona State University. Current and Future Research Directions in High-Pressure Mineral Physics

Jay Bass, Editor University of Illinois, Urbana-Champaign

August 2004 asgjkfhklgankjhgads Contents

Introduction ...... 1

Scientiﬁ c Themes ...... 3 The Core and Core-Mantle Boundary ...... 3 Subduction and Mantle Processes ...... 8 Near-Surface Processes ...... 15 Planetary Processes ...... 20

Technology ...... 25

Relationships to other Programs ...... 26

Appendix 1: Workshop Attendees ...... 28 Diamond Anvils, Multi-Anvil Apparatus, and Growing Big Diamonds: Tools of High-Pressure Science

Diamond is the hardest and toughest substance known to man. As such, it is the material of choice to use as a pressure-generating “anvil” in high-pressure devices. Diamonds have been used as anvils to attain the highest pressures ever achieved in the laboratory. However, the rarity of large natural diamonds and their great expense has limited such ultra-high-pressure experimentation to extremely small samples. Obtaining larger diamond anvils is the key to performing a range of experiments that require larger samples, such as neutron scattering experiments. Recent breakthroughs in the growth of synthetic diamonds in the laboratory indicate that a new generation of high-pressure equipment using large diamond-anvils may soon become a reality.

A research team at the Carnegie Institution of Washington and the Los Alamos National Laboratory has reported a major break- through in growing diamonds from chemical vapor deposition. They found that these high- pressure-temperature annealed crystals are the hardest diamonds ever tested. See Yan, C-s., H-k. Mao, W. Li, J. Qian, Y. Zhao, and R. J. Hemley, 2004, Phys. Stat. Solidi, (a) 201, R25.

T-25 multi-anvil, high-pressure module installed in a 1000-ton press on GSECARS beamline at the Advanced Photon Source of the Argonne National Laboratory. Using this apparatus, pressures in excess of 25 GPa and temperatures over 2000 K can be obtained in samples of 1 cubic millimeter volume. Photo courtesy of Yanbin Wang, University of Chicago.

The natural diamond anvils in these diﬀ erent styles of high-pressure cells are approximately 1/4 carat in weight. Scaled-up devices would be used with the large synthetic diamonds of over 1 carat in weight to do a new generation of high-pressure experiments using larger samples. Photo courtesy of Dmitry Lakshtanov, Stanislav Sinogeikin, and Jay Bass, University of Illinois, Urbana-Champaign. Introduction

The interiors of Earth and other planets Earth’s interior continues to affect so- outer core that can generate a magnetic cannot be directly observed. No borehole ciety in very direct and profound ways. fi eld like Earth’s have just been achieved has ever pierced Earth’s thin crust due to Earthquakes and volcanic eruptions result in recent years. We now know that Earth’s the high temperatures and pressures ex- from chemical reactions and motions in mantle likely contains far more water and isting at depth. Yet we know a great deal Earth’s mantle, the stony region from a carbon dioxide than is present in all the about Earths interior, and what we have few tens of kilometers depth beneath the world’s oceans and atmosphere combined. learned over the last 10 years in particular continents to almost 3000 km in depth. Direct evidence of carbon reservoirs in has been astonishing. The simple layered Buried at shallow levels beneath the ocean the mantle can be seen in diamonds. This models depicted in textbook diagrams fl oor and preserved by high pressures in most valuable of gemstones originates (e.g., Figure 1A) are being replaced with the sediments are the greatest known de- hundreds of kilometers beneath the sur- more sophisticated models that show the posits of hydrocarbon energy resources— face. Thus, mantle processes have probably complexity and dynamics of Earth’s inte- clathrates. Clathrates are also a potential had a strong infl uence on the quantity rior (e.g., Figure 1B). The features of this major contributor to global warming and of carbon dioxide—a major greenhouse internal system give us clues as to how the underwater landslides that can generate gas—present in the atmosphere. The com- interior works—for example how material tsunamis. At much greater depth, motions mon thread that unifi es these diverse phe- moves, how heat is transported, and what in Earth’s metallic core are responsible for nomena is that they involve processes in it is made of—and how Earth evolved to the magnetic fi eld at the surface, which materials at extremely high pressure and this state. is the basis for navigation. Computer temperature conditions. simulations of liquid metal fl ow in the

A. B. Whole Mantle Shear Velocity Heterogeneity upper mantle mantle

lower mantle 0

outer

core core

2891 Grand [2002] inner Iso-velocity contours: +/- 0.7% core

Figure 1. A. Simpliﬁ ed depiction of Earth’s layers. B. Modern view of the mantle. Seismically fast regions are shown in blue, slow regions are red. Red regions are likely hotter and more buoyant than average mantle, while blue regions are likely cold and dense. Figures courtesy of Edward Garnero, Arizona State University. B also from Grand, S.P., 2002, Mantle shear-wave tomography and the fate of subducted slabs, Phil. Trans. R. Soc. Lond. A, 360, 2475-2491.

1 Figure 2. Aerial view of the 23, 2003 in Miami, Florida. The National Advanced Photon Source Science Foundation’s (NSF) Division of at the Argonne National Earth Sciences commissioned and sup- Laboratory, Illinois, the pre- mier third-generation syn- ported this workshop, and the NSF-fund- chrotron radiation facility ed Consortium for Materials Properties in the United States. Photo Research in Earth Sciences (COMPRES) from http://aps.anl.gov/aps/ organized it. Fifty-six scientists attended frame_home.html the workshop (Appendix 1), convened by Jay Bass and Donald Wiedner. The stimulus for the workshop was the rapid growth of the fi eld of high-pressure mineral physics, the numerous scientifi c discoveries made in recent years, and the enormous prospects for future scientifi c breakthroughs. In addition, it was recognized that there is a rapidly increasing Only a few isolated pieces of deep-Earth largely stimulated by the availability of demand on centralized national facilities rock have ever been found. These are rela- U.S. national research facilities, in particu- for experimentation and computation in tively small pieces of the mantle brought lar high-energy radiation sources (e.g., high-pressure mineral physics. COMPRES to the surface over a billion years ago dur- synchrotron x-rays, infra-red radiation, was formed in part as a response to ing the most violent volcanic eruptions neutrons) that allow us to probe the prop- these developments, with the mission of in Earth history. These are the same rare erties of matter at extreme pressures and identifying promising research oppor- rocks (kimberlites) in which diamond de- temperatures. These sophisticated facilities tunities, promoting the development of posits are found. As valuable as these clues have presented extraordinary opportuni- new technology, providing coordinated are, they tell us only about a relatively ties for understanding Earth and other oversight of certain centralized facilities, shallow part of Earth’s interior. planets. The technology for performing an and providing education and outreach for Nevertheless, the last decade has entirely new generation of experiments is the high-pressure Earth and planetary sci- brought unprecedented advances in un- now within our reach. We will be able to ences community. The Miami workshop derstanding Earth’s interior and other “see” the complex interiors of planets with was held to identify the most promising planetary bodies in our solar system. a clarity that could not be imagined only a areas for future scientifi c discovery, and These advances have come through the decade ago. areas that are ripe for future technological development of instruments that simulate This report is an outgrowth of the breakthroughs. This document is intended the incredibly high pressures and tem- discussions and results of a workshop to be a statement by the high-pressure peratures existing in planetary interiors on A Vision for High Pressure Earth and Earth science community on the status of (see box opposite page 1). This growth in Planetary Sciences Research: The Planets our fi eld and some of its most exciting and the high-pressure fi eld has in turn been From Surface to Center held on March 22- challenging directions for the near future.

2 Scientiﬁ c Themes The Core and Core-Mantle Boundary

One of the greatest achievements of high- at Earth’s center is anisotropic, with faster tive of large-scale chemical reactions and pressure science has been the ability to seismic velocities in N-S directions than the presence of iron-rich silicate liquids reach pressures and temperatures of along equatorial paths. Earth’s inner core where the core and mantle are in contact. Earth’s core in the laboratory (Figure 3). may also be “super-rotating,” with a speed Alternatively, the properties of the CMB This pressure regime is from 1.3 million that is faster than the surrounding mantle may also reﬂ ect a phase transition, or new atmospheres (1.3 Mbar) at the bound- and crust. If so, this superrotation may be atomic structures for phases with differ- ary between the mantle and core, to 3.3 key to understanding how magnetic forces ent properties than those of the overlying Mbar at Earth’s center. The magnitude of couple with the iron-rich core and how mantle. The CMB region may be as com- such pressures far exceed anything known the geomagnetic ﬁ eld is generated. plicated as the interface between the solid through human experience. At the upper boundary between the Earth and the atmosphere. Understanding Although Earth’s core is not directly rocky mantle and the liquid outer core the cause of the CMB structure appears to accessible to us, a detailed and fascinating (the core-mantle boundary or CMB), be crucial for understanding the dynam- picture of this region has been revealed scientists have detected a far more compli- ics and evolution of our planet. Due to during the last decade. Seismological stud- cated structure than previously thought. possible chemical and physical interac- ies have shown that the solid inner core This CMB structure is, in fact, sugges- tion between the mantle and core, and/or

Figure 3. Summary of the pressures and temperatures that can be attained by “static” experimental techniques, compared with the expected pressures and temperatures in Earth (the geotherm). With static techniques, samples are compressed by two or more anvils, as in the Large Volume Press (LVP, six anvils), or the Diamond Anvil Cell (DAC, two diamond anvils). A goal for the next decade is to simultaneously attain the pressures and temperatures at Earth’s center and at the deeper portions of the giant planets. Figure courtesy of Guoyin Shen, University of Chicago.

3 crystallization of the inner core, it is now thought possible that the liquid outer core may be chemically heterogeneous, Simulating Core Pressures and Temperatures with rather than being a well-stirred liquid. It the Laser-Heated Diamond Cell may also contain much larger amounts of radioactive elements than previously Advanced technologies of high-pressure experimentation have made it possible to thought. simulate material properties and reactions under extreme conditions equivalent While seismologists have been mak- to those in Earth’s core. Rapid progress in computational mineral physics has also ing observations on the properties of the contributed to a better understanding of materials properties under these condi- core, high-pressure mineral physicists have tions. The laser-heated diamond anvil cell is one of the main experimental tools been conducting experiments that tell us for reaching the pressures and temperatures conditions in Earth’s core for long why the core has such peculiar properties time periods, allowing a variety of measurements to be made. High-powered la- (see box, right). The ability to recreate the sers produce the greatest temperatures under static, high-pressure conditions. pressure and temperature conditions in the core has come through advances in the Major questions about the core that are being pursued with the diamond anvil cell diamond anvil high-pressure cell, and par- and theoretical calculations include: allel advances in theoretical calculations on core materials (primarily iron). A wide Heating Up The Core. One of the fi rst quantum mechanical calculations on min- variety of experiments are now possible eral properties conducted ~30 years ago suggested a signifi cant solubility of potas- using the diamond anvil cell. Not only is sium in iron under core conditions. Recent experimental studies under high pres- it possible to reach pressures exceeding sure support this notion.These results suggest that substantial heating in Earth’s those in Earth’s center, but samples can core is possible, which affects the history of Earth’s magnetic fi eld. be heated to core temperatures using focused laser beams. The atomic structure of Core Anisotropy. Both theoretical and experimental studies have been performed materials can be determined using x-ray on the elastic anisotropy of solid iron under Earth’s inner core conditions. Results diffraction techniques. The texturing of indicate substantial elastic anisotropy, but the nature of elastic anisotropy appears metal grains under non-hydrostatic stress to be sensitive to temperature. can be measured, simulating possible iron textures that may produce seismic Core-Mantle Boundary. The properties of silicate minerals such as perovskite at anisotropy in the inner core. Sophisticated core-mantle boundary layer conditions have been investigated both experimental- quantum mechanical calculations can be ly and theoretically. Both results show that a phase transformation in this mineral performed to simulate the behavior of to a highly anisotropic structure provides a possible explanation of the properties iron under pressures and temperatures of the core-mantle boundary. of the core, giving insight on the ultimate causes of the core’s fi ne structure and properties. Any realistic interpretation of the geophysical observations requires accurate knowledge of the material properties of candidate core materials under extremely

4 To simulate conditions in Earth’s core, the sophisticated laser-heating system at the Advanced Photon Source, Argonne National Laboratory, heats to high temperatures (up to thousands of degrees Celsius) samples that are compressed to high pressure in diamond anvil cells (left). This particular system is used for Nuclear Inelastic X-ray Scattering (NRIXS) experiments to measure the physical properties of deep-Earth materials containing iron atoms. It is being used extensively to study the properties of possible core materials.

In this Nuclear Inelastic X-ray Scattering (NRIXS) experiment with laser heating, x-rays come from the right and hit the sample at high pressure and temperature (left). The sample is buried within the copper cooling block near the center of the photo. From this experiment, which is only possible with the most modern synchrotron facilities, seismic velocities on core materials can be measured. This new type of experiment is being carried out at the Advanced Photon Source. (Photos courtesy of Wolfgang Sturhahn, Argonne National Laboratory)

5 high pressures and temperatures. For ex- plausibility arguments. We are now in a light elements such as sulfur, oxygen, and ample, to understand the cause of seismic position to directly obtain a new genera- hydrogen, which are likely present in the anisotropy of the inner core and how core tion of physical property and geochemical core. material deforms in response to various measurements on core materials under ac- forces, we need to know the elastic con- tual core pressures and temperatures. Structure and Dynamics stants of iron under inner core conditions. of the CMB These have been measured and calculated Key Questions for the fi rst time only within the last few 1. How much potassium and other radio- The core undoubtedly has some inter- years. Theoretical studies on the energy active elements can be present in the action with the overlying mantle at the budget of the core-mantle system suggest core, and where does all the heat ema- CMB. Yet it is not known whether the that a large amount of heat is expected to nating from the core come from? strange seismic properties of isolated be generated in the core. Indeed, it is this 2. How old is the inner core and how patches in the bottom 200 km of the man- heat that drives fl uid motion in the outer quickly is it growing? tle and the CMB (known as the D” region) core and generates Earth’s magnetic fi eld. 3. What is the energy source that drives are due to reaction of mantle material Both theoretical and experimental studies the geomagnetic dynamo? with the core below, the sinking of dense now suggest that the quantity of radio- 4. Did the core segregate from the mantle materials through the mantle to pile up active elements that can be dissolved in early in Earth history or did it evolve at the CMB, or phase transformations in iron may be very different at deep-Earth separately? What effect did this possible silicate minerals under the pressures and conditions. A better understanding of this core segregation have on the chemical temperatures of the CMB. Depending on problem is critical for understanding the evolution of the mantle? the melting point of iron alloys and the generation of the geomagnetic fi eld as well 5. Did the cores of other planetary bod- concentration of radioactive elements in as the history of this planet as a whole. ies such as Mars and Venus evolve in a the core, there may be a huge change in different way, due to differences in core temperature at the CMB, which stimulates Core Energetics compositions and melting points? convective upwellings and hotspot plumes 6. How does the melting temperature, Tm, like Hawaii. Heat from the core drives the geodynamo, vary with composition, and how did generates the magnetic fi eld, and is largely differences in core melting tempera- Key Questions responsible for much of the tectonic activ- tures affect the evolution of other plan- 1. Is D” a graveyard of cold, dense materi- ity and movement of continents on the ets, such as Mars and Venus? al that was once at the surface and then surface. The ultimate source of all this These questions can now be addressed by was subducted back into the mantle, heat is not completely understood, but doing measurements under core condi- sinking to the CMB? Or is D” a region much of it comes from radioactive decay. tions of pressure and temperature in the of the mantle that has reacted chemi- At the same time, we suspect that the core diamond anvil cell, and with dynamic cally with core materials? What phase is cooling, and that as a result the inner shock wave techniques. Of major im- transformations in mantle minerals core has crystallized and is growing larger portance is measuring the solubilities of occur at CMB conditions, and what are with time. In many cases this view of core radioactive elements (such as potassium) the seismic properties of these phases? evolution is based on enormous extrapo- in iron and iron alloys, and measuring 2. How hot is the molten outer core, and lations of data obtained at much lower the melting points of iron diluted with how big is the temperature jump at the pressures than are found in the core, and CMB? The answer to these questions in other cases our views are shaped by will tell us the heat fl ux out of the core.

6 3. How big is the lateral variation in heat Is light material crystallizing out of 4. What is the crystal structure of the fl ux from the core, and how does it af- the core and rising to the core-mantle solid inner core phase? We have learned fect the dynamo action (geomagnetic boundary? much about the properties and crystal reversals)? 2. How do light elements affect the physi- structures of iron at core conditions, 4. Is the mantle partially molten at the cal and chemical properties of the outer but we are just starting to appreci- CMB? core, in particular the melting point, ate how small amounts of impurities • What are the melting temperatures density, elasticity, seismic velocities, and mixed with iron can change both its of lower mantle rocks? electrical conductivity? properties and structure. It has long • What would be the effects of temper- been presumed that a simple hexagonal ature and compositional variations Inner Core Structure form of iron (hexagonal closest-packed be on lateral heterogeneity in seismic iron) is its stable structure in the inner wave velocities and density? That is, The inner core has crystallized out of a core. However, other structures are pos- how does seismic structure depend cooling molten iron alloy, and is growing sible (such as the body-centered cubic, on chemical composition and tem- with time. This growth has likely affected or BCC structure that is stable under perature at CMB conditions? the properties of the magnetic fi eld over normal conditions), especially for the • What is the thermal conductivity Earth history. In fact, magnetic forces may actual alloy of the inner core. The most of the mantle, and how much is the actually speed up the rotation of the inner pressing questions to be answered next fl ow of heat out of the core impeded core with respect to the rest of Earth. are: at the D” layer? • Is a BCC body-centered phase of Key Questions iron alloy stable in the inner core? Composition and Structure of 1. How does the inner core affect the • What effect will the crystal structure the Outer Core geodynamo? It appears that the core have on observable properties of the and Earth’s magnetic fi eld are strongly core, like the elastic anisotropy of It is well known from shock wave and dia- linked through the observation of in- iron under inner core conditions? mond anvil experiments that the core is ner core superrotation. Little is known 5. How large are the grains of inner core not dense enough to be made of pure iron. beyond that, although this interaction material? This will be important for Some unknown light elements must be al- certainly has an effect on the magnetic understanding seismic attenuation and loyed with iron, and these will likely have fi eld through time. Electrical conduc- fl ow within the inner core. a strong effect on the melting point of the tivity and viscosity (strength) measure- 6. How great is the inner core’s viscosity core. Even the solid inner core seems to ments on iron will be important in this and how strongly coupled are the in- have some lighter material dissolved in it. regard. ner core and the mantle (can one rotate 2. Is there any signifi cant heat fl ux from with respect to the other)? Key Questions the inner core? 7. Is the inner core convecting or fl owing? 1. What light elements are present in the 3. Is there partial melting (a mushy zone) outer core, and how are they distribut- in the inner core? We do not know if ed? Is there any variation in the concen- the inner core is completely solid or tration of the light elements between whether it includes some molten mate- the bottom and top of the outer core? rial between grains. Matching seismic velocities and seismic constraints on attenuation is very important.

7 Subduction and Mantle Processes

Subduction is the process by which cold composition and structure of the mantle; als at high pressure, runaway heating from slabs of brittle oceanic lithosphere plunge convective fl ow of material in the mantle; a central nucleation site of failure, or the down into Earth’s interior. These subduct- and core-mantle boundary interactions. sudden dehydration of hydrous (water- ing slabs consist of old basaltic oceanic bearing) minerals, such as serpentine, crust that has been altered and hydrated, Deep Earthquakes under high-pressure conditions. None of with cold and chemically depleted mantle these mechanisms satisfactorily explains underlying the crust, and a thin veneer of To generate earthquakes, large differential all of the observational information on sediments. Subduction is the mechanism stresses must be exerted on brittle, cold deep-focus earthquakes; however, the by which crustal and upper mantle mate- material. For these reasons, most earth- technology needed to generate differential rial is recycled back into the mantle, and quakes occur at relatively shallow levels. As stresses and deform rocks under pres- this process expresses itself in ways that lithosphere is subducted to greater depths sures and temperatures for intermedi- greatly impact life on the surface. Most and is heated by the surrounding mantle, ate-focus earthquakes has just recently earthquakes and subaerial volcanoes oc- its strength decreases as it warms, becom- been developed. A great deal of additional cur at convergent plate margins where ing softer and ductile. Seismicity should laboratory study on the deformation and subduction is active. While plate tectonics cease when slabs are hot enough to fl ow failure of rocks is required before deep- provides a general framework for under- like heated plastic instead of rupturing Earth seismicity can be fully understood. standing how subduction relates to vol- in a brittle fashion. Yet earthquake activ- Developing such instrumentation is a canoes and earthquakes, the mechanisms ity persists to depths of about 700 km in fi rst-order priority for the high-pressure by which they are generated at subduction the mantle, much deeper than calcula- community. zones are just beginning to emerge. tions and laboratory experiments would Studies of subduction processes and predict. The problem of why intermediate Volcanism and Melts the mantle into which subducted mate- and deep-focus earthquakes occur is one rial is delivered will continue to be a focus of the great unsolved problems in geo- Aside from presenting a serious natural of high-pressure studies over the next physics today (Figure 4). hazard, volcanism is the mechanism by decade. Trying to understand the origin A number of hypotheses have been pro- which new crust is formed, and it has been of intermediate- (70-300 km depth) and posed for the origin of deep earthquakes. perhaps the most important process in- deep-focus (300-700 km depth) earth- The interiors of slabs may remain cold volved with formation of the atmospheres quakes, and developing models of subduc- enough that the mineral olivine persists and oceans. It has also likely exerted a tion-related volcanism, will be primary metastably to depths below 400 km. The signifi cant infl uence on climate change goals. Some of the key related issues to atoms in this metastable olivine may sud- through time by delivering greenhouse be addressed include: the effi ciency of denly rearrange to form the stable β (beta) gases to the atmosphere. Most volcanism recycling volatiles like water and carbon and γ (gamma) high-pressure minerals occurs at mid-ocean ridges on the seafl oor dioxide into the mantle; identifying min- (called wadsleyite and ringwoodite, re- far beneath the ocean surface, and this eral reservoirs for volatiles in the deep spectively), thereby generating an earth- is the mechanism by which new oceanic mantle; magma generation and migration; quake in the process. Alternate hypotheses crust is created. On the surface of the the ultimate fate of subducted slabs; the include shear instabilities in stable miner- continents, most present-day volcanism

8 is related to subduction, occurring along few years, the development of new genera- ally sink rather than fl oat to the surface. continental margins near subduction tions of large-volume diamond anvil cells This curious concept could have profound zones and on numerous island arcs such and multi-anvil high-pressure devices for consequences for the evolution of the as Japan. neutron-scattering experiments could lead early Earth, its possible chemical stratifi ca- Ocean water alters the basalt of the up- to important breakthroughs in research tion, and for the sequestering of elements per crust, forming hydrated minerals such on magmas. and volatile compounds at depth. There as serpentine, which contain abundant Interestingly, not all magmas may rise are at present very few experimental stud- water. Along with entrained sediments, to the surface. This counterintuitive situ- ies of the properties of silicate melts at these wet subducting materials are the fi rst ation results from the fact that melts are high pressures—especially their density to melt as they are warmed by the sur- very compressible and their density in- and elastic properties. However, the devel- rounding mantle. The water and carbon creases more rapidly with pressure than opment of new tools for conducting such dioxide in volcanic gasses clearly show for most crystalline solids. Thus, below a studies promises to be an area of emphasis that they result from the formation of wet critical depth, silicate magmas may actu- in the coming decade. subduction-zone magmas. Exactly how these melts make their way to the surface, how quickly they travel, and how their chemistry is altered along the way are poorly constrained. Yet all of these things affect the properties of the magmas we Figure 4. ultimately see on the surface, the eruptive Generalized view of a lithospheric styles of volcanoes, and the hazards they slab subducting pose to society. The atomic structures of through the up- silicate melts are key to understanding per mantle, into their behavior, physical properties, their the lower mantle. The distribution of volatile content, and the migration of earthquakes in the melts from their point of origin at high subduction zone and pressure to the surface. slab are shown by High-pressure experiments and com- red and black fi lled circles. Figure modifi ed puter simulations to determine the prop- from Green, H., 1994, erties of silicate melts will be an active area Solving the paradox of of investigation in the future. Neutron deep earthquakes, Sci scattering is a particularly effective experi- Amer., September 1994, p. 65. Original fi gure mental tool for investigating melt struc- © Roberto Osti tures and properties. As more intense neu- Illustrations. tron sources such as the SNS (Spallation Neutron Source) come on line in the next

9 Volatiles at Depth at great depth. Thus, the mantle may well 3. What subduction-zone minerals can be Earth’s largest reservoir of water and retain water and carbon until they are Water has a great effect on mineral carbon. in the transition zone where they can be properties, such as seismic velocities, Where would this water reside? We now readily absorbed by minerals there? anisotropy, strength, and viscosity, to know that minerals we normally think of 4. When mantle rocks rise above 410 km name a few. As a potentially vast source as “dry,” so-called nominally anhydrous depth, do saturated minerals dehydrate, of water and CO2, volatiles in Earth’s minerals, can in fact accommodate signiﬁ - thus releasing dense aqueous melts that mantle may have had a huge impact on cant quantities of hydrogen (water) locked are recycled to depth at mantle down- climate change throughout geologic time. up in their crystal structures. The upper- wellings? That is, is the 410 km seismic Therefore the presence or absence of wa- most mantle minerals, garnet and pyrox- discontinuity a dehydration boundary ter and carbon in Earth’s deep interior has ene, could accommodate multiple oceans or “water ﬁ lter”? become a central issue in high-pressure of water in their structures. Even more 5. Can we identify volatiles in the mantle research, and will be an area of intense in- water could be present in the transition using seismic prospecting techniques? vestigation for the foreseeable future. zone between 410 and 660 km depth. A Answering this question will require In its earliest history, 4.5 billion years major recent discovery is that the primary knowledge of the elastic properties of ago, Earth was hotter than at present. As minerals in this depth range, the high- hydrous phases, especially at high pres- a result, convection of mantle rocks was pressure forms of olivine named wads- sures and temperatures. extremely rapid and it is likely that a large leyite and ringwoodite, can accommodate 6. Do volatiles affect the properties of portion of the initial volatile inventory of up to three weight percent of structurally seismic discontinuities, such as their the deep Earth was lost early on through bound water. Even the small amounts of widths, depths, and the velocity jumps volcanism. The impact of a planet-sized water in the densest phases of the lower across them? body with Earth in a moon-forming event mantle (perovskites and magnesiowustite) 7. What are the densities and elastic prop- would have produced additional heat and could amount to several oceans worth of erties (including seismic properties) of loss of volatiles from the interior. Yet we H2O. aqueous silicate melts or solutions at know that volatile elements such as hy- high pressures? drogen and carbon still reside at depth. Key Questions 8. How do small amounts of water affect Rocks from the interior such as kimberlites A number of intriguing questions are the rheologic properties of high-pres- contain diamonds (made entirely of car- raised if the mantle is a reservoir for po- sure minerals and the patterns of con- bon), as well as hydrous phases. There is tentially vast quantities of volatiles. vection in the deep Earth? also abundant water and carbon dioxide in 1. What are all the possible phases in volcanic gasses. These volatiles may be re- which water could be retained at depth? The Mantle sidual from Earth’s early accretion, or they The answer to this question is of the may have been recycled into the mantle most fundamental importance, and will Earth’s mantle, a nearly 3000-km-thick through subduction. We do not know involve phase equilibrium experiments shell of stony material between the crust what fraction of the subducted volatile over a broad range of pressures, tem- and the metallic core, accounts for ap- inventory quickly returns to the surface at peratures, and compositions. proximately seven-eighths of Earth’s vol- volcanoes, but current estimates indicate 2. If there is water in the mantle, is it from ume. Knowledge of the composition and that it is low, about 15 percent. The re- subduction recycling, or is it primor- mineralogy of the mantle is therefore es- mainder may be redeposited in the mantle dial? sential for understanding Earth’s total inventory of elements, and the evolution of Earth through time.

10 It is now well established that with in- are the most prominent seismic features discontinuity, whereas others seem to pen- creasing pressure, upper mantle minerals between the shallow crust-mantle bound- etrate directly into the lower mantle. Thus, such as (Mg,Fe)2SiO4 olivine, garnet, and ary (Moho) and the core-mantle bound- the patterns of fl ow, recycling, and mixing pyroxene transform to denser mineral ary at about 3000 km depth. They provide in the mantle seem to be uneven and com- phases. These transformations are associ- major clues as to the chemical, thermal, plex (Figure 5). Whether such complexity ated with major jumps in seismic velocity, and dynamic state of the mantle. extends to yet greater depths is highly called discontinuities, at depths of 410 and What is more diffi cult to ascertain is uncertain. We do not at present know 660 km. Additional subtle discontinuities the degree to which the mantle might be whether seismic velocity discontinuities with smaller velocity jumps may occur chemically heterogeneous, with an onion- involve changes in chemical composition at other depths as well. Transformations like structure of chemically distinct lay- as well as changes of phase, or whether to a perovskite-structured phase with the ers. Yet the extent and nature of chemical the mantle is roughly homogeneous on a formula (Mg,Fe)SiO3 is generally associ- heterogeneity in the mantle is crucial for gross scale. Even more diffi cult to discern ated with the major seismic discontinuity understanding the patterns of convection are lateral variations in composition and at 660 km depth. This phase, along with that drive plate tectonics, Earth’s internal temperature. Seismic studies have indicat- (Mg,Fe)O and perovskite-structured thermal structure, and the planet’s evo- ed the presence of large superplume struc-

CaSiO3, are thought to account for the lution through time. Seismic studies of tures extending upwards over a thousand bulk of the lower mantle deeper than 660 the upper mantle reveal a highly hetero- kilometers or more from the core-mantle km. This basic sequence of phase transfor- geneous region in terms of temperature boundary. Whether these structures result mations is now well established and pro- and composition (Figure 1B). Continents from temperature or a chemically distinct vides a basis for understanding the promi- appear to have massive roots of cold, diapir is as yet unknown. A new genera- nent seismic structures of the mantle. The chemically distinct material that extends tion of velocity measurements on lower 410 and 660 km discontinuities are often perhaps hundreds of kilometers beneath mantle phases at the extreme high pres- taken as the boundaries of the “transition the surface. Some subducting slabs appear sure-temperature conditions of the lower zone” in the mantle. These discontinuities to be trapped and ﬂ oating on the 660 km mantle will be necessary to answer these questions (see box on p. 12-13).

Figure 5. A conceptual view of a stratifi ed Earth’s mantle. Two cold blue slabs penetrate the 660 km discontinuity (dashed line), and sink through the lower mantle. They deform a dense layer in the lowermost mantle without penetrating through it. Other subducting slabs, such as that beneath the back arc on the right, do not penetrate into the lower mantle and fl oat on top of the 670 km discontinuity. See Kellogg, L.H., B.H. Hager, R.D. van de Hilst, 1999, Compositional stratifi cation in the deep mantle, Science, 283(5409), 1881-1884. Figure from http:// www-geology.ucdavis.edu/%7Ekellogg/mantle.jpegs courtesy of Louise Kellogg.

11 The Mantle Transition Zone and Deep Earth Chemistry

Perhaps the most prominent features within the mantle are rapid When the crystal structure of a mineral changes due to high-pres- jumps in seismic velocities—or discontinuities—at 410 and sure phase transitions, the characteristic seismic wave speeds of 660 km depth. These discontinuities mark the upper and lower the minerals change as well. What we do not know is whether boundaries of the “transition zone,” a region in which seismic transition-zone discontinuities are associated with changes in velocity and density increase very rapidly with depth. Various chemical composition, or just changes in crystal structures to features of the transition-zone discontinuities, such as their size, high-pressure forms. sharpness, and variation in depth around the globe, are among Key to understanding the seismic structure of the transition the most important clues available on the chemistry of the deep zone and, indeed, the entire mantle, is measuring seismic wave mantle. Mineral physicists are obtaining the type of information speeds of minerals at high pressures and temperatures. Today, that is needed to interpret the seismic structure of the transition measurements of wave speeds (or elastic properties) are done zone. Indeed, the use of high-pressure laboratory data to analyze using a variety of sophisticated techniques, for example: (1) seismic features is one of our most powerful tools for under- Brillouin laser light scattering from atomic vibrations (fi gures standing the current state and evolution of the deep Earth. right-hand page), (2) generating ultrasonic waves in crystals Laboratory experiments have established that transition-zone (fi gures below), and (3) by synchrotron x-ray techniques (fi gures discontinuities are associated with phase transitions in the major below and on p. 5). Extending these techniques and others to minerals of the upper mantle: olivine, pyroxene, and garnet. higher temperature and pressure conditions is a primary goal for the future.

Techniques for Studying the Mantle

CCD Camera

Slits Synchrotron YAG Source

2θ X-ray

To Ultrasonic Incident X-ray Transducer Interferometer SSD/CCD Ultrasonic generator and control T-25 multi-anvil apparatus at the GSECARS beamline at the Advanced Photon Source, Argonne Synchrotron x-ray experiments with a multi-anvil high-pressure press like the T-25 National Laboratory, as used for ultrasonic velocity shown at right. Eight cubic anvils compress the a sample at their center (shown measurements at the pressures and temperatures in red). X-rays are used to monitor the sample size and for x-ray diﬀ raction, while characteristic of Earth’s transition zone. Photo a transducer transmits sound waves into the sample for measuring their speed. courtesy of Yanbin Wang, University of Chicago, Figure courtesy of Baosheng Li, Stony Brook University. and Baosheng Li, Stony Brook University.

12 θ CO2 Laser laser beam in

Metal Laser light going into a diamond anvil cell to measure velocities by Brillouin scatter- gasket ing. Photo courtesy of Jennifer Jackson and Jay Bass, University of Illinois, Urbana- Champaign.

Pressure marker Sample (e.g. ruby)

Goniometer Polarization Controller Rotator Ar laser Polarizer PC Sample Scattered Laser light M 1 mm beam out Detector Laser heating in the diamond cell while measur- PR 6-pass tandem ing seismic velocities using Brillouin scattering. Fabry-Perot Figure courtesy of Stanislav Sinogeikin and Jay Bass, M Interferometer University of Illinois, Urbana-Champaign. M R < 6 M < 2 > M 3 L 5 > 1 > M 4 < M

Schematic diagram of a Brillouin spectrometer used to measure sound

velocities VP and VS in the laboratory using laser light scattering. Figure courtesy of Stanislav Sinogeikin, University of Illinois, Urbana-Champaign.

13 Figure 7. Emerging view of Earth’s interior. Deep-mantle complexities are illustrated for three regions: (a) Beneath Central America, the deepest mantle contains a high-velocity D’’ refl ector, and localized scatterers of seismic energy that may be related to pockets of ultralow velocities and possibly plume formation, and strong anisotropy. (b) Beneath the central Pacifi c, which underlies surface hotspot volcanism, abundant evidence exists for D’’ anisotropy and ultralow-velocity zones, as well as possible plume genesis, a mild D’’ high-velocity refl ector, and core-ri- gidity zones. (c) Beneath the South Atlantic and southern Africa, a large-scale low-velocity structure with sharp edges extends upward into the lower mantle, and may exist independently of neighboring D’’ material. These three locales exemplify the new CMB paradigm of a structurally and dynamically Figure 6. Crystal structure of the newly discovered rich CMB region. Reprinted with permission, post-perovskite phase of MgSiO3 (M. Murakami, Garnero, E., 2004, A New Paradigm for Earth’s Hirose, K., Kawamura, K., Sata, N., and Ohishi, Core-Mantle Boundary, Science, 304, 834-836. Y., 2004, Science, 304, 855-858). This phase may Copyright 2004 AAAS. explain the anomalous seismic properties of the core-mantle boundary region. See also related work reported by Iitaka et al. (Nature, 430, 442- 445, 2004), by Oganov & Ono (Nature, 430, 445- 448, 2004), and by Tsuchiya et al. (Earth Planet. Sci. gion where slabs ultimately descend and Letters, 224, 241-248, 2004). Reprinted with per- are reassimilated into the mantle, making mission, M. Murakami, 2004, Science, 304 (5672), the CMB a “slab graveyard” (Figure 7). cover image. Copyright 2004 AAAS. Alternatively, core-mantle interactions may lead to pockets of iron-rich silicate melts that are too dense to rise very far Recent seismic studies have shown that upwards. High-pressure studies of the the lowermost 200 km of the mantle, the stable phases that may exist in D’’ and the so-called D’’ region, is among the most chemical reactions between silicates and heterogeneous regions of the planet. This metals are in their infancy, but will do thin layer above the core-mantle bound- much to shed light on the possible reasons ary (CMB) displays a high degree of for the existence of D’’. For example, re- lateral variability in its velocity structure. searchers in Japan have recently discovered Isolated patches of lens-shaped low-veloc- a post-perovskite phase that exists under ity material seem to fl oat on top of the D’’ conditions (Figure 6). Measurements core-mantle boundary. It seems possible of the elastic properties, densities, attenu- that chemical reactions are taking place ation, and viscosity of both crystalline in D’’ between the silicate mantle and the phases and melts appropriate to the D’’ iron-rich metallic core. This may be a re- region are of the utmost importance.

14 Near-Surface Processes

The near-surface environment—primarily rocks show clear signs that liquid water Carbon Dioxide the atmosphere, hydrosphere, biosphere, was present at Earth’s surface. Current and deeper levels that directly interact theories for how our planet generated an Like the oceans, Earth’s initial atmosphere with them—is affected to a surprising ocean involves degassing of water from was degassed from the interior, probably degree by deep-Earth and high-pressure Earth’s interior. Indeed, it appears likely very early in Earth history. The composi- processes. In a practical sense, we might that the amount of water/hydrogen bound tion, and even the pressure, of this earliest deﬁ ne the relevant environments as those into Earth’s interior is at least equivalent atmosphere are poorly constrained. The that we experience on the surface or that to, and may dwarf, that within the planet’s best guesses are that the Hadean atmo- can be directly sampled, for example, by oceans. How that water (or hydrogen) is sphere was dominated by CO2, perhaps drilling. Over the past several years there retained in the interior, and how abundant even more than one atmosphere of this has been increasing awareness of the con- it is, are issues that can only be resolved well-known greenhouse gas. For compari- tinuous interaction between Earth’s deep through work on materials at high pres- son, today’s atmosphere has an abundance interior and its oceans and atmospheres. sures. of CO2 of only about 0.0003 atmospheres.

The deep Earth is indeed the source of At plate margins, water is reintroduced Part of this early CO2 atmosphere was the planet’s oceans and primordial atmo- into the planet by subducting water-con- fi xed into rocky form through either sphere, and continues to affect climate on taining minerals formed in the oceanic weathering or biologic processes, while many time scales. Understanding the pre- crust and overlying wet sediments. Water another portion was converted (through cise nature of these interactions and the vapor is degassed from the interior in ex- photosynthesis) into the present-day oxy- rates at which they occur will be an area plosive volcanic events at these plate mar- gen within our atmosphere. As with water of intense activity within the high-pres- gins (e.g., Mount St. Helens, Figure 8) and and the genesis of Earth’s oceans, one of sure community for the foreseeable future. at hot springs and fumaroles associated the basic scientifi c challenges for high- Closer to the surface, modest pressures with volcanic activity. Taken together, de- pressure Earth sciences is to determine the stabilize what is likely the planet’s largest gassing and subduction form a large-scale recycling and sequestration processes of hydrocarbon reservoir, and one which has hydrologic cycle that has been pumping carbon within the deep Earth. not yet been exploited. water and gases onto the surface and then The importance of carbon dioxide Earth’s oceans, while appearing vast at recycling them. The underlying question degassing from Earth’s interior extends the surface, only represent a small feature that emerges from studying these process- well beyond the fundamentally important on the skin of the planet: in total, the es is whether Earth’s oceans are decreasing problem of the genesis of Earth’s atmo- oceans amount to less than 1/1000th of or increasing in volume with time. Such sphere. Even the small (and growing) Earth’s mass. How did the planet’s oceans changes in ocean water volume are antici- amount of carbon dioxide in the pres- arise? It is likely that much of Earth’s earli- pated to take place over long time scales ent-day atmosphere has a profound effect est atmosphere and water was lost about (tens to hundreds of millions of years), on Earth’s climate due to the effi ciency

4.5 billion years ago when the impact of a but are fundamental for interpreting the of CO2 as a greenhouse gas. It has been Mars-sized body upon Earth formed the sedimentary geologic record. estimated that over most of the previous

Moon, melting and vaporizing a large por- 600 million years, the CO2 content of the tion of the planet. Yet 3.7 billion-year-old atmosphere has been 4-15 times the present value.

15 Figure 8. Oblique aerial view of High-pressure studies will provide the Mount St. Helens eruption of insight into long-term planetary climatic May 18, 1980, which sent volcanic ash, steam, water, and debris to changes through studies of the behavior a height of 60,000 feet. This is an of greenhouse gasses such as CO2 within excellent example of how volcanic Earth’s deep interior. eruptions serve as a mechanism for transferring volatiles like water and CO2 from Earth’s interior Magmas: Tapping Deep to the atmosphere. Photo by Reservoirs of Water and Carbon Austin Post, Skamania County, Washington. May 18, 1980, U.S. Our oceans and atmospheres, and indeed Geological Survey Photo Archive. the carbon present in life itself, is ultimately derived from Earth’s interior—delivered by magma to the surface. While there are strong indications that magmas dramatically change their structure, density, and chemical properties at high pressures, our knowledge of the interplays between the microscopic structure of magmas and the solubility of gasses at depth is in its infancy. How microstructural changes depend on the temperature, pressure, and chemical composition of the magma (including the presence of carbon and water) remains ill-deﬁ ned. Understanding these relationships is important for un-

derstanding how water and CO2 are delivered to the surface, the rate at which this The most prominent mechanisms for Key Questions delivery occurs, and whether it is likely to release of this CO2 involve the deep Earth: 1. What is the efﬁ ciency of CO2 degassing increase or decrease in the future. either enhanced volcanic degassing and/or from different types of subducted ma- It has long been assumed that the mag- greater subduction of carbonate-rich sedi- terials? mas we see at Earth’s surface reﬂ ect the ments into the deep Earth. Subduction 2. To what degree do different types of composition of Earth’s outermost skin. puts carbonate sediments in proximity to major volcanic events carry carbon In some cases this is clearly not true. For hot mantle material where decarbonation from Earth’s deep interior and inject it example, diamonds (composed entirely of reactions release CO2. This general con- into the atmosphere? carbon) were formed deep within Earth’s ceptual framework immediately raises a 3. What is the distribution, manner of mantle and carried rapidly to the surface number of critical issues for understand- sequestration, and average content of in rare, unusually violent eruptions. Under ing interactions between the deep Earth carbon at depth? What minerals hold some conditions, nearly pure carbonate and the atmosphere. carbon in the deep Earth and where? magmas, called carbonatites, may erupt. Carbonatites resemble molten washing

16 soda more than molten rock. These mag- such, they will continue to be a subject of substance at low temperatures and high mas are of considerable geologic interest future study by the high-pressure com- pressures. The water molecules in clath- because they may be revealing vast res- munity. rate hydrate form an ice-like structure ervoirs of volatile material at depth, and that traps small molecules (guests) such as because they provide most of the planet’s Key Questions methane in nearly spherical cavities. The economic deposits of several technologi- 1. What circumstances lead to the erup- interest in clathrates is motivated by an cally important elements such as tantalum tion of kimberlites? increasing recognition of its abundance in and niobium. 2. Do kimberlites represent a large region Earth’s subsurface and on icy bodies of the The conventional wisdom is that most of the mantle with kimberlite composi- solar system, its possible economic impor- magmas originate in Earth’s uppermost tion, or are they isolated anomalies? tance as a source of fuel, and its potential mantle and lower crust. Whether the The answer to this question will have role in both climate change and natural more common (and less violent) magmas far-reaching implications for under- hazards. Large volumes of methane are erupting at the surface today have some standing mantle composition and the sequestered by this solid along deep conti- link with the deepest parts of the mantle possibility of chemical stratiﬁ cation nental margins and below regions of per- is, however, not known. Experiments at within the mantle. mafrost (Figure 9). high pressure may well show that magmas 3. From what depths did kimberlites Global estimates of the methane in (and thus, new crustal material) can, in originate, and have they resided over clathrates indicate that it may be the larg- fact, be derived from throughout Earth’s a range of depths before rising explo- est sources of hydrocarbon on Earth. The silicate mantle, dramatically altering our sively to the surface? release of methane from this clathrate ideas about Earth’s average composition 4. At what rates have kimberlites risen? reservoir has been suggested as a source and deep-seated heterogeneity. Has their speed of ascent changed with of variation in atmospheric methane dur- Kimberlites deserve additional com- depth, or are they restricted to only the ing glacial/interglacial cycles in the recent ment as an extreme example of how rocks upper couple of hundreds of kilometers past. These suggestions have fueled specu- from Earth’s interior can be emplaced at as often thought? lation about future releases of methane in the surface. In the case of these diamond- Answers to these questions will tell us response to global warming. rich deposits, they are driven upward by much about the composition of the interi-

CO2 at velocities of perhaps hundreds or, and in particular the size of the mantle Key Questions of kilometers per hour, cooling as they reservoir of CO2 and water. Among the critical questions about clath- rise and expand. Through this process, rates to be addressed in the coming years diamond is preserved instead of revert- Clathrates are: ing to graphite as it normally would at 1. What are the stability ﬁ elds of clath- magmatic temperatures. Kimberlites are Degassing of CO2 from Earth’s interior rates, especially methane and CO2 of interest not only because they are the may have played a signiﬁ cant role in clathrates? The answer to this question primary source of diamonds (they are in warming Earth’s climate, but it is not the will help predict how much of a change fact named after the Kimberly diamond only greenhouse gas that can be retained in sea level (and hence a drop in pres- mines in South Africa), but because they in geologic materials. There has recently sure at the ocean bottom) or change represent the deepest magmatic materials been a growing interest in clathrates, in ocean temperature would lead to a known. However, their chemical com- or solid gas hydrates. Clathrates can be release of greenhouse gasses into the position is very different from common thought of as combination of a gas and notions of what the mantle is made of. As normal water ice that forms a single solid

17 essential in prospecting for clathrates Earth’s subsurface is critical to under- using seismic techniques, just as the standing global carbon cycling. Although petroleum industry prospects for oil the extent of near-surface carbon cycling and gas. has been extensively studied, the deep 4. Are there further high-pressure phase subsurface contributions to the hydrocar- transitions in clathrates? Phase transi- bon content of the planet remain uncon- tions would change all of the physical strained. and chemical properties of clathrates, The existence of a deep biosphere has affecting calculations of their abun- been hypothesized since very early times, dance, stability, and release into the but only since the 1950s, with the isolation environment. of deep-sea microbes specially adapted Experimental studies using neutron for growth at high pressures, were the ﬁ rst sources, such as the Spallation Neutron direct clues to the existence of deep life

Figure 9. Burning methane clathrate, perhaps the Source now under construction at Oak recognized. The physical limits (pressure most abundant energy source in the world. Figure Ridge National Laboratory (Figure 10), and temperature) on the existence of a from http://www.gashydrate.de/. will be invaluable for determining the biosphere remain open questions, as these properties of clathrates. This is because hold important clues to the extent of car- these water-based compounds are made bon reservoirs within Earth’s subsurface, atmosphere. It would also help to pre- of light elements, in particular hydrogen, as well as clues to the viability of life in dict where underwater avalanches in making them extremely diffi cult to study planets and planetary bodies. There is now the ocean fl oor might occur. using x-rays. Neutrons are far more sensi- a growing awareness that life can exist un- 2. What are the kinetics of clathrate for- tive to the hydrogen atoms in a material der extreme high-pressure conditions that mation and decomposition, and under and interact strongly with them. Until were previously thought to be unimagi- what conditions would methane be re- now, the use of neutron scattering under nable, extending the depth range of what leased into the atmosphere? This affects high-pressure conditions has been quite we think of as the biosphere. calculations of global warming and the limited. The mineral physics community Recent fi eld studies have pointed rates at which atmospheric gasses can is already developing new types of dia- towards the presence of a signifi cant di- make their way back into ocean sedi- mond-anvil and multi-anvil high-pressure versity and mass of biology in the deep ments. devices that will be suitable for use with subsurface. However, most interpretations 3. What are the physical properties of neutron scattering. on the extent of biologic interactions clathrates, in particular their elastic with geologic systems are at best guesses. properties? This information will de- High-Pressure Geobiology To get a clearer picture of the extent of termine the speed of seismic waves in geobiological interaction as well as bet- clathrates, which are needed to inter- The origin of the methane within clath- ter constrain the kinetics and viability of pret geophysical surveys for regional rates is likely biologic. Yet, until the recog- these biological processes, a focus towards inventories of clathrate or to model the nition of these deposits over the last ~15 laboratory-based, high-pressure geobiol- response of the clathrate reservoir to years, mankind was unaware of this large ogy studies is essential. With the expertise changing environmental conditions. reservoir of organic carbon. Indeed, the available to study materials and their Knowledge of the elastic properties is distribution of organics and biota within properties at extreme pressures, the high-

18 pressure community is well poised to not • Determine the non-biologic origins of only open new frontiers in high-pressure organic material. geobiology, but also take a lead in the fur- • Determine the ways in which diamonds ther development of this aspect of high- grow within Earth. pressure science. • Determine the properties of natural ﬂ uids at high pressures. Goals of the High-Pressure • Simulate in the laboratory the ﬂ uids Community Relevant to Near that are characteristic of the subduc- Surface Processes tion process, and compare them with exhumed natural samples. • Constrain the rates at which water and • Improve constraints on the genesis of carbon are cycled into, and out of, the carbon-rich magmas. Earth. • Determine how the structural proper- • Determine how the mantle releases ties of melts control their properties at carbon to the atmosphere/hydrosphere, high pressures. and what climatic effects it might have. • Determine the properties of the clath- • Determine the effect of pressure on life, rate hydrocarbon reservoir. mechanisms of life’s survival at high • Determine the interrelationship be- pressure, and the metabolic processes of tween the deep biosphere and the clath- life at high pressure. rate reservoir.

Figure 10. Spallation Neutron Source (SNS). Photo courtesy of Oak Ridge National Laboratory.

19 Planetary Processes

The overarching intellectual issue and From this “elemental” grouping we can The fi rst question is perhaps most central challenge for planetary scientists is to un- identify four kinds of bodies in the solar because we believe that the presence of derstand how planetary bodies form and system: The gas giants with their strong a core is related to the process whereby evolve, and to determine their current gravitational fi elds (Jupiter and Saturn), planets form. This leads immediately to structures and dynamics. Specifi c ques- the ice giants (Uranus and Neptune), the the problem of how to detect a core from tions and many of the experimental chal- solid ice/rock bodies (Europa, Ganymede, our remote vantage point. Observations lenges differ from those posed for Earth Callisto, Titan, Triton, Pluto, and many by spacecraft missions and telescopes studies because the range of compositions smaller bodies), and the terrestrial bod- give us information on atmospheres and and thermodynamic conditions in the so- ies (Mercury, Venus Earth, Moon, Mars, the near surface environment, while ce- lar system is much greater than for Earth. and Io). Below we discuss the intellectual lestial mechanics tell us their total mass, The data returned from spacecraft, and challenges in understanding each of these average densities, and the distribution the ongoing explosion of new information classes of bodies. of mass inside a planet. If we know how about extrasolar planets, enable us to ad- the density of surfi cial material increases dress fundamental questions, but only if Gas Giants with pressure and temperature inside we can improve our understanding of ma- the planet (that is, if we know the pres- terial properties and processes at relevant The gas giants (Jupiter and Saturn) are sure (P) – volume (V) – temperature (T) temperatures and pressures. fl uid, predominantly hydrogen planets. All equation of state), then we can tell how The universe consists mostly of hy- of the recently discovered extrasolar plan- much core material would be needed to drogen and helium, with minor amounts ets are comparable in mass to Jupiter and obtain the right planetary mass and den- of heavier elements created through nu- are, accordingly, almost certainly of simi- sity. Therefore, detection of a core requires cleosynthesis in massive stars. However, lar composition to Jupiter, since no mate- knowledge of the behavior of the overly- the most abundant elements also tend to rial other than hydrogen is of suffi cient ing hydrogen. We believe that the mass of be the most volatile, and the weak gravi- abundance. At the outset, we pose central the Jovian core is only ~ 3 percent or less tational fi elds of smaller bodies (such as questions for these bodies. of Jupiter’s mass, but this estimate is sensi- Earth) make it diffi cult to retain these tive to the equation of state of hydrogen. light elements. It is convenient to divide Key Questions Consequently, how well the internal struc- the elements into three groups: 1. How do gas giants form? Do they have ture of the gas giants is understood relies • Gases are those that do not condense rock/ice cores and if so, how large are primarily on a better understanding of (i.e., do not form solids or liquids) un- their cores? hydrogen and hydrogen-helium mixtures. der conditions plausibly reached when 2. What is their radial structure? Are The thermodynamic conditions of great- planets formed; there fi rst-order phase transitions (e.g., est interest for these planets are probably • Ices are those that form volatile com- metallization of hydrogen or a plasma P ~0.3 to 10 megabars and temperatures pounds and condense, but only at low phase transition)? of a few thousand Kelvin. Much of this re- temperatures (beyond the asteroid belt); 3. What are the dynamics of these planets gime is colder than the conditions probed • Rocks are those that condense at high (heat fl ow, magnetic fi eld, convective by conventional shock-wave experiments, temperatures and provide the building state)? but hotter than that achieved in static ex- blocks for the terrestrial planets. periments thus far. Nearly isentropic (at

20 constant entropy) compression experi- plicated because all the cosmically abun- P<2 kilobars). We identify three areas of ments using shock reverberation tech- dant classes of materials are signifi cantly greatest interest and relevance to the high- niques, or magnetic compression tech- represented. The greatest need is a better pressure community. niques, show much promise for determin- understanding of water and its mixing ing material properties under the relevant properties with other materials (hydrogen, Water Ices P-T conditions. other ices, especially methane, and rock). Most of the moons of the outer planets, The conditions of interest are slightly Jupiter, Saturn, Uranus, and Neptune, lack Key Experiments lower pressure (0.1 to several megabars) a rocky crust but are surfaced by hard, 1. Improve determination of P-V-T but at temperatures of several thousand K rigid ices of various kinds that behave like equations of state for hydrogen. Phase (well into the fl uid regime, even if water rocks. These planet-sized ice/rock satel- transitions can cause a sudden increase forms a high-melting-point ionic solid). lites are the most water-rich solid bodies in density, so they are important to The questions posed and experimental in the solar system. For example, Jupiter’s identify, as well as the mixing properties techniques used are the same as those for moon, Ganymede, is larger than Mercury with other cosmically abundant materi- the giant gaseous planets, but the range of and the pressure at the base of its up- als, especially helium and water. interesting compositions is broader. permost ~1000 km-thick ice layer is ~15 2. Determine derivatives of the equation times the pressure in the deepest parts of of state. Of particular importance is the Solid Ice/Rock Bodies Earth’s oceans. Therefore, the high-pres- Gruneisen parameter, which determines sure properties of water, ices, and their how temperature increases with pres- The solid ice/rock bodies (large icy moons, interactions with rocky material are of sure (or depth). These properties are but also Pluto) pose a different set of critical importance to the chemical evolu- essential for determining the thermal questions than for the gas and ice giants. tion of ice-dominated bodies in outer so- structure of the interior and its convec- The lower temperatures and pressures of lar system. Models of the present structure tive state. their interiors, in combination with their of these bodies are uncertain. For some, 3. Determine transport properties, espe- watery compositions, lead to somewhat whether they are differentiated bodies cially electrical conductivity, for a broad different structures and internal dynamics or not depends on whether radiogenic range of pressure, temperature, and compared with their much larger neigh- and accretional heat were suffi cient to al- compositions. bors. Accretional heating early on or the low differentiation into a rocky core and Key experimental approaches: presence of antifreeze (i.e., salt) can lead an icy mantle. The depth and thickness 1. Extend diamond anvil cell techniques to subterranean liquid layers, and the pos- of their internal layers depend on these to higher temperatures and pressures, sibility of extensive water-rock interac- thermodynamic phase boundaries among

which is especially difﬁ cult for hydrogen. tions. We have magnetic ﬁ eld evidence for various forms of H2O-ice (e.g., I, II, VI, 2. Further use and develop shock-wave water oceans in Europa, Ganymede, and VIII), which are unknown in most cases. techniques, which can produce isentro- Callisto, and suspect oceans in Titan, and Therefore, a starting point for understand- pic or nearly isentropic compression. perhaps even Triton and Pluto. Pressures ing the inner structure of icy satellites like in the ice/rock bodies are modest, extend- Titan is a full understanding of the phase

Ice Giants ing up to 10 GPa or less, and many of diagram of H2O-ice. the interesting questions are therefore at The ice giants (Uranus and Neptune) relatively low pressures (e.g., down to and Key Questions are less well understood than Jupiter and including the stability ﬁ eld of normal, 1. What are the phase boundaries between Saturn and are also probably more com- low-pressure ice on Earth’s surface [ice I], the low-temperature (i.e., ~70 K at 0 GPa, ~300 K at 10 GPa), high-pressure

21 phases, including hydrogen-disordered to or not much larger than the melting of the dynamics and evolution of plan- phases. These boundaries are still un- point of water ice, it is unlikely that they etary interiors have now reached a stage known. are present in gas giants or ice giants, but where one can incorporate some aspects 2. What are the energy barriers and tran- they could be important for the large icy of realistic rheologic laws as constrained sition rates for structural transitions? moons. Perhaps of greatest interest is the by laboratory experiments and theoreti- Because metastability is the norm in “high” pressure (pressures of a few GPa or cal mineral physics. These models show

H2O-ice, rather than the exception, less) stability of various clathrate phases that mantle dynamics and the evolution of transition rates are important for de- involving methane and other gases. These Earth and other terrestrial planets depend termining the lifetime of metastable may be relevant to understanding the ori- critically on such laws. Realistic models phases that might be abundant in these gin of methane on Titan. The rheology of thus need accurate rheologic proper- objects. clathrates is also of interest because they ties that take into account, for example, 3. What are the properties of aqueous will affect convective fl ow and heat trans- mineralogy, water content, and grain fl uids at pressures of 1-2 GPa and port (they appear to be signifi cantly stiffer size. Thus far, experimental studies of the temperatures of hundreds of degrees than water ice). rheology of mantle materials have been Celsius, and what chemical reactions focused mainly on olivine at low pressure. occur between these fl uids and rocks of Terrestrial Bodies Although olivine may indeed be the major meteoritic chemistry? These are each phase in Earth’s upper mantle, the min- critical areas of experimentation for As with ice/rock bodies, the evolution of eralogy of other planets might be domi- understanding the internal evolution of the terrestrial planets depends on knowing nated by other minerals (e.g., pyroxenes), some of the most novel bodies in our the phase diagrams (e.g., stability fi elds which could have very different rheologic solar system. of different phases, melting curves) and behavior. Moreover, virtually no quanti- the rheologic or fl ow behavior of relevant tative data are available for the rheologic Clathrate Studies materials. The needed phase diagrams properties of deep mantle minerals, except Clathrates on solid ice/rock bodies are the that remain to be worked out experimen- for some very recent pioneering efforts. same type of clathrates that exist in Earth’s tally must cover a much wider range of The rheologic properties of more than ocean sediments. They consist of modi- compositions than those for Earth (e.g., 90 percent of Earth’s mantle are uncon- fi ed water-ice structures in which there including relatively sulfur-rich mixtures). strained by laboratory studies. Techniques are available sites for other molecules. Perhaps the biggest gap in knowledge con- are now being developed for quantita- However, on the ice/rock bodies the most cerns the rheology of relevant materials tive rheologic experiments under pres- interesting possible molecules are hydro- and mixtures of phases as in the case of sures and temperatures equivalent to the gen, carbon monoxide, carbon dioxide, the ice bodies; this gap is likely to be fi lled mantle transition zone and deeper. These and nitrogen, in addition to methane as in the near future by experiments using techniques will enable us to explore the on Earth. Noble gases are also of interest new high-pressure deformation apparatus rheologic properties of the entire mantle, (though not highly abundant) because such as the D-DIA (Figure 11). providing insight into the dynamics of the they may be detected in atmospheres and whole Earth. Phases likely to exist at great thereby provide diagnostics of formation Rheology depth, such as perovskites and magnesio- and evolution involving clathrate decom- wustite, urgently need to be investigated. position and outgassing. Because clathrate One of the central issues for all planetary The rheology of materials relevant to oth- stability fi elds are typically bounded above bodies is the rheologic properties of the er planetary bodies is similarly crucial yet by temperatures that are comparable materials within their interiors. Models poorly constrained.

22 Figure 11. A sketch of D-DIA, a systems is where one phase is undeform- new high-pressure deformation able. Likely important mixtures are water apparatus capable of generating pressures of up to 15 GPa and ice with ammonia hydrate ices, or water temperatures of up to 2000 K, in ice and methane clathrate. conjunction with synchrotron x-radiation. This apparatus makes Two-Phase Systems it possible to perform quantita- tive rheologic experiments under Subduction of oceanic lithosphere is dif- the pressure and temperature fi cult to initiate on Earth, and subduc- conditions of the transition zone tion is even more diffi cult to initiate on of Earth’s mantle, as well as for planets that do not have plate tectonics. other planetary bodies (see Wang, Y., W.B., Durham, I.C. Getting, and Laboratory experiments on dry rocks D.J. Weidner, 2003, Rev. Sci. Instr., predict a lithospheric strength that is too 74, 3002-3011) . Figure courtesy high to allow subduction. Small stresses of William Durham, Lawrence on the San Andreas Fault and small stress Livermore National Laboratory. drops during earthquakes are also diffi - cult to reconcile with these experiments. Water or melt are factors that are believed to be responsible for the reduction in rock strength. The rheology of two-phase systems (rock-water, rock-melt) and as- Ices ing on pressure and temperature, and GSS sociated processes such as dehydration The rheology of water ice and related ices creep in many of these water ice phases embrittlement and shear localization are remains imperfectly understood yet of also needs to be measured in the lab. An critical for understanding subduction. The great importance for understanding con- analogy may exist between ice I and oliv- rheology of two-phase systems is also im- vection and tidal dissipation in these solid ine, where very important viscosity dif- portant for understanding core formation ice/rock bodies. In particular, the competi- ferences exist between GSS and disloca- and extraction of magma onto planetary tion and balance between grain-size-sen- tion creep. Measuring GSS creep at more surfaces. We need to continue studying sitive (GSS) and dislocation creep in ice extreme conditions is a monumental task the rheology and dynamics of two-phase needs more consideration. Grain growth technically, but it is a long-range vision for systems both experimentally and theoreti- is promoted in GSS creep and grain-size high-pressure planetary sciences. cally. reduction is promoted in dislocation The deformation of multi-phase sys- creep. Therefore, for materials where both tems, such as ice-clathrate mixtures, is Microstructures mechanisms produce measurable strain, emerging as an important void in our Increasing attention is being given to it is entirely possible that steady-state knowledge of planetary ices. To solve these rock microstructures because they affect deformation in some portions of the ice/ ice problems requires a combined labora- rheology and also because deformation rock bodies (e.g., the ductile layer of the tory/theoretical approach. Given the com- modifi es them. Particularly important are Europan ice shell) there will be signifi cant plexity introduced by a second phase in grain size and the preferred orientation contributions (>10 percent) to the strain addition to ice we will likely need to study of crystallographic axes (lattice-preferred rate from both mechanisms. H2O exists in many individual systems and discover sys- orientation). Grain size strongly infl uenc- a number of distinct ice phases, depend- tematic trends. A special case of two-phase es rock viscosity whereas lattice-preferred

23 orientation controls the nature of seismic Impacts in the Solar System anisotropy from which the geometry of fl ow can be inferred. The importance of Impacts and collisions are major processes grain-size evolution on convection pat- in the formation and evolution of plan- terns has been studied through numeri- etary bodies. Collisions have generated cal modeling. We now have evidence of a wide range of pressures and tempera- seismic anisotropy from the crust to the tures throughout the solar system’s his- inner core, but its geodynamic signifi cance tory, from early accretionary encounters can only be understood when the factors to present-day events, resulting in cata- controlling anisotropic structure forma- strophic disruption. Recently, craters have tion are known. Recent laboratory studies been used as a powerful probe of material have shown that microstructure develop- properties. The morphology of impact ment is highly sensitive to stress state and structures refl ects properties beneath chemical conditions (e.g., water content). planetary surfaces, providing precious in- Currently available data on microstruc- formation about internal composition and tural evolution are limited to low-pressure structure, such as the depth to the subsur- Figure 12. Pwyll crater on Europa. The bright ejecta conditions and most data are for single- face ocean on the Jovian moon, Europa rays indicate that this is a relatively young impact crater. The central peak in this 26 km diameter phase materials. (Figure 12). crater has been used to constrain the depth of the Grain growth is now recognized as an Current investigations focus on the cu- brittle crust overlying the subsurface liquid ocean. important process in mantles of terrestrial mulative chemical and physical alteration Image credit: NASA/JPL. planets. Grain growth affects planetary of planetary bodies as a result of billions evolution through the feedback among of years of collisional evolution. Recent grain size, viscosity, and mantle convec- studies have shown how energy deposited ments that measure the high-pressure and tion, and can also control the existence or from impact events on Mars and Titan has high-temperature material properties absence of convection in planetary interi- melted ground ice and possibly formed of rocks and minerals. Increasingly, data ors. Although grain growth in one-phase transient habitats containing liquid water. obtained from both shock and static com- systems has been studied to a limited Advances in understanding all plan- pression experiments are combined to extent, grain growth in multi-phase sys- etary bodies are constrained by the limited formulate the necessary equations of state tems has not been well understood either measurements of the phase diagrams and and dynamic rheology (strength) models. for low- or high-pressure phases. Some thermodynamic properties of the large Further developments of in situ measure- aspects of microstructural evolution such range of chemical compositions found ments of shock processes are required, as grain-growth kinetics are totally differ- in the solar system. Critical aspects of such as x-ray and neutron diffraction and ent between single-phase and multi-phase fundamental processes remain poorly un- Raman spectroscopy, which are under ac- materials. Further progress on these issues derstood, including the kinetics of shock- tive development using static techniques. is needed to improve understanding of induced melting and solid-solid phase Invention of new experimental techniques terrestrial planet dynamics. Both experi- transformations. that enable widespread use of isentropic mental and theoretical studies of coars- To understand the evolution of the or arbitrary compression paths will bridge ening in realistic mineral assemblages is solar system and to utilize craters as a the fi elds of shock and static compression, needed. remote-sensing tool, planetary scientists providing access to phase spaces previous- rely upon dynamic, high-pressure experi- ly unattainable, such as the second critical point of water.

24 Technology

Progress in high-pressure Earth and plan- • High-pressure devices for synthesizing • Improved x-ray tomography equipment etary sciences is closely tied to technologi- large quantities of samples at >30 GPa. for use with high-pressure samples. cal innovation and infrastructure develop- This will involve the development of ment. Researchers are continually pushing new anvils for multi-anvil presses with Properties of Core Materials – The forward the limits of pressures and tem- superhard materials. Ultimate in Extreme Earthly Conditions: peratures that can be achieved in the labo- • Computer simulations of the proper- The development of experimental facili- ratory, adapting new types of experimental ties of deep Earth crystalline phases and ties for measuring the physical and chemi- probes for high-pressure experiments. melts. cal properties of Earth materials under Examples of the latter are the recent devel- core conditions (P>135 GPa, T>3000 K) opments of using inelastic x-ray scatter- New X-Ray Probes: High priority areas are among the most difﬁ cult technical ing techniques to determine the electronic are the development of inelastic x-ray scat- challenges in our community because the structures and sound velocities of core tering to measure sound velocities at ultra- pressures involved are so high. Properties and mantle materials at high pressures. high pressures, free-electron laser x-rays that are key include the densities, equa- Inventing technology to perform new under shock conditions, and the develop- tions of state, thermal conductivity, ki- types of experiments in higher pressure- ment of optics for inelastic x-ray scatter- netics of chemical reactions, and plastic temperature regimes is the lifeblood of our ing with multi-anvil apparatus. Dedicated ﬂ ow. Relatively large sample volumes will science. Listed below are some technolo- beam-lines to perform state-of-the-art be needed to characterize many of these gies and experimental capabilities that are inelastic x-ray scattering experiments are properties. New techniques for measuring critical to develop in the coming decade, necessary at national facilities. these properties will need to be developed, along with some experiments that need to including the measurement of rheologic be performed to advance understanding of Neutrons: Neutrons are the best way to and elastic properties under these extreme planetary interiors. probe the structures of hydrous phases, conditions. and to determine the structural proper- Needed Technology ties of silicate melts and aqueous solutions. Computational Mineral Physics: Development New high-pressure equipment must be Investments in computational infrastruc- developed to utilize the next generation ture must parallel other areas of infrastruc- • Experimental equipment and tech- of neutron scattering facilities, such as the ture development. Advances in compu- niques for measuring the crystal SNS. Ultra-high-pressure experimentation tational hardware and software continue structures and seismic properties (or at SNS will involve scaling up the types of at a fast pace, presenting our community equivalently, elastic properties) of deep- equipment used for x-ray experiments, with great opportunities for address- Earth crystalline phases and melts under including techniques for growing large dia- ing new problems with greater accuracy. pressure and temperature conditions monds. High-priority objectives include: Challenging issues in this area include spanning the mantle and core. • Large-volume diamond-anvil cells for incorporating the effects of chemical envi- • Development of large-volume devices determining the structures of minerals ronment, such as oxygen fugacity, on the for determining phase equilibria at pres- by neutron scattering to 100 GPa. solubility of elements in iron, and calculat- sures to 60 GPa. This would expand the • Paris-Edinburgh class high-pressure ing transport properties such as diffusion current capabilities by approximately a cells for pressures to 20 GPa and high rates and thermal conductivity. As com- factor of two. temperatures of 2500° C for x-ray and putational technology advances, accurate • Apparatus to perform high-pressure neutron beam-lines. calculations of these properties will be rheology experiments to 30 GPa and • Focused ion beam instruments for pre- possible via simulations with large number beyond. paring precision electron microscopy of atoms and the proper treatment of in- samples and nano-machining. teratomic forces.

25 Relationships to Other Programs

Much of the fundamental EarthScope is an The National Aero- research in high-pressure integrated geo- nautics and Space mineral physics is sup- logical, geophysi- Administration (NASA) ported by the National cal, and geochemical investigation of the missions to various Science Foundation Division of Earth North American continent. One of the parts of our solar system return diverse Sciences. The Earth and planetary scienc- three EarthScope components is USArray, geophysical, chemical, spectroscopic, and es communities make observations round a dense network of permanent and porta- photographic data sets on planetary bod- the clock, 365 days per year. Seismometers ble seismometers that will provide images ies. As soon as one tries to infer what the are monitoring ground motion and of Earth’s interior with unprecedented interiors of these bodies are like, using pri- earthquake activity, while space probes, clarity. USArray will be a major break- marily remote-sensing data obtained from satellites, and telescopes are constantly re- through in our ability to precisely deﬁ ne planetary surfaces and atmospheres, input cording diverse sets of data on planetary Earth structure at depth: its composition, from the high-pressure materials prop- bodies. Interpreting these vast quantities thermal structure, and dynamic state. This erties community is needed. Everything of information to form a physical picture is a key element of understanding the from determining the size of rocky or me- of what Earth (or other planetary body) forces that drive plate tectonics, the depths tallic cores of planets, to understanding is like inside requires input from high- from which magmas are derived, and the the surface fractures and topography on pressure scientists doing experiments in evolution of North America. Knowledge giant ice bodies, to understanding the ori- their laboratories or on their computers. of mineral properties under the high pres- gin of sulfurous volcanism on Io depends Information on the properties of Earth sures and temperatures of Earth’s interior on knowing the properties of planetary and planetary materials at high pressures will be required to make such interpreta- materials at high pressure. Understanding and temperatures is indeed the key that tions from USArray data. In particular, the conditions under which life forms allows us to see inside planetary bodies, to studies of seismic velocities of minerals might be hidden below the surface, per- understand what deep interiors are made under the full range of pressure-tempera- haps even thriving at high pressures, is an- of, and to decipher how they work. In ad- ture conditions inside Earth will be neces- other question that high-pressure experi- dition, many industrial or other processes sary. There is a natural synergy between mentalists will address in the next decade. of practical importance involve high pres- EarthScope’s goals and those of the high- sures. Therefore, very natural connections pressure Earth and planetary sciences exist between the activities of high-pres- community. sure researchers and other scientiﬁ c programs or agencies. Several examples of such relationships are given in the follow- ing columns.

26 A major goal of the The Department of Research on the equations Integrated Ocean Energy (DOE) operates of state and properties of Drilling Program (IODP) the synchrotron and materials at high pressures is to understand the na- neutron facilities that are and temperatures is cen- ture of the seafl oor and crust beneath at the heart of much of the research done tral to the mission of the Department of the oceans. Among their specifi c goals is by the high-pressure materials properties Defense (DOD). Many of the issues that characterizing the distribution and na- community. The high-pressure commu- arise in designing materials suitable for ture of clathrates within ocean sediments nity has worked with DOE to develop a specialized military applications and in because these compounds are a vast po- number of beamlines for state-of-the-art predicting the performance of hardware tential energy resource and they likely high-pressure studies. These facilities are involves the behavior of matter under ex- play a role in climate change. Sampling used not only by researchers in the Earth tremely high pressures and temperatures. clathrates is a diffi cult task because they and planetary sciences, but also for funda- The experimental techniques developed are only stable at high pressure and they mental research in physics, chemistry, and by the Earth and planetary sciences com- quickly decompose when drilling cores materials sciences. In addition to a num- munity and the training that it provides to are recovered to the surface. Designing ber of synchrotron beamlines dedicated to young scientists are of direct relevance to drilling experiments and analyzing the high-pressure research, a new high-pres- DOD research. results will depend on knowledge of the sure beamline for neutron scattering stud- pressure-temperature conditions under ies is being developed at the Spallation which clathrates are stable. Measurements Neutron Source currently under construc- of the seismic velocity properties of clath- tion at Oak Ridge National Laboratory. rates will likely play a large role in future attempts to prospect for clathrates using seismic techniques. Other areas of interface include measurements of the thermal conductivity of materials at high pressures, which are needed to interpret heat- fl ow measurements at the ocean fl oor.

27 Appendix One

Attendees of the March 22-23, 2003 Workshop

A Vision for High Pressure Earth and Planetary Sciences Research: The Planets from Surface to Center

Rob Abbott Sandia National Laboratory Charles Prewitt Carnegie Institution of Washington Ross Angel Virginia Polytechnic Institute and Glenn Richard Stony Brook University State University Mark Rivers University of Chicago Jay Bass University of Illinois, Urbana- Nancy Ross Virginia Polytechnic Institute and Champaign State University Bruce Buffett University of British Columbia Surendra Saxena Florida International University Pamela Burnley Georgia State University Anurag Sharma Carnegie Institution of Washington Jiuhua Chen Stony Brook University Thomas Sharp Arizona State University Hyunchae Cynn Lawrence Livermore National Guoyin Shen University of Chicago Laboratory Sang-Heon Dan Shim University of California, Berkeley Przemek Dera Carnegie Institution of Washington Joseph Smyth University of Colorado Robert Downs University of Arizona Viatcheslav Solomatov New Mexico State University Michael Drake University of Arizona Maddury Somayazulu Carnegie Institution of Washington Thomas Duffy Princeton University David Stevenson California Institute of Technology William Durham Lawrence Livermore National Sarah Stewart- Harvard University Laboratory Mukhopadhyay Michael Furnish Sandia National Laboratory Lars Stixrude University of Michigan Ivan Getting University of Colorado, Boulder James Tyburczy Arizona State University Harry Green University of California, Riverside Michael Vaughan Stony Brook University Russell Hemley Carnegie Institution of Washington David Walker Columbia University Donald Isaak University of California, Los Angeles Yanbin Wang University of Chicago Chi-Chang Kao Brookhaven National Laboratory Donald Weidner Stony Brook University Shun Karato Yale University Renata Wentzcovitch University of Minnesota Abby Kavner University of California, Los Angeles Quentin Williams University of California, Santa Cruz David Lambert National Science Foundation Bernard Wood University of Bristol Kurt Leinenweber Arizona State University Takehiko Yagi University of Tokyo Baosheng Li Stony Brook University Chang-Sheng Zha Cornell University Ho-kwang Mao Carnegie Institution of Washington Jianzhong Zhang Los Alamos National Laboratory Guy Masters University of California, San Diego Yusheng Zhao Los Alamos National Laboratory Li-chung Ming University of Hawaii Richard O’Connell Harvard University Eiji Ohtani Tohoku University Dean Presnall University of Texas, Dallas

28 Suggested Reading

Karato S., A. M. Forte, R. C. Liebermann, G. Masters, and L. Stixrude, eds., 2000, Earth’s Deep Interior: Mineral Physics and Tomography from the Atomic to the Global Scale, Geophysical Monograph Series, 117, 289 pp. Rubie, D.C., T. Duffy, and E. Ohtani, eds., 2004, New Developments in High Pressure Mineral Physics and Applications to the Earth’s Interior, Physics of the Earth and Planetary Interiors, 616 pp.

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