The high-pressure dimension in earth andplanetary scienceHo-kwang Mao and Russell J. Hemley*Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015

By far the bulk of our planet ishidden from view, within theearth, under high pressures andtemperatures. The behavior of

this material dictates the formation, evo-lution, and present state of the solidearth. Recent geophysical and geo-chemical studies of the planet present uswith a rich array of large-scale processesand phenomena that are not fully un-derstood. These range from the fate ofdeeply subducted slabs and the origin ofplumes, to the nature of the core–mantle boundary; the differentiation ofmaterials to form the present-day crust,mantle, and core; the distribution oftrace elements; and the uptake and re-cycling of volatiles throughout earth’shistory.

Addressing these questions experi-mentally has a long history (1), but it isonly recently that the entire range ofpressures that prevail within the earthcould be produced in the laboratory andthe materials probed with the necessarytools (Fig. 1). Experiments have dem-onstrated that, under these extremeconditions, the physical and chemicalbehavior of materials can be profoundlyaltered, causing new and unforeseen re-actions and giving rise to structural,elastic, electronic, and magnetic transi-tions not observed in rocks and mineralsin the near-surface environment. Re-solving the new issues that have arisenrequires an integrated approach involv-ing subfields that include seismology,geochemistry, petrology, and geodynam-ics, as well as theoretical and experi-mental high-pressure mineral sciences.The collection of feature articles thatfollows, which were presented at a re-cent symposium,† highlights an array ofnew developments in high-pressuregeoscience.

In ultrahigh-pressure metamorphicrocks, solid and fluid inclusions in phe-nocrysts contain rich information ondeep-mantle processes. The structure,texture, strain, chemistry, and exsolutionof these micrometer- to nanometer-sizedinclusions indicate the formation envi-ronment of these rocks and contain richinformation about the relevant physicaland chemical processes. Diamonds,coesite, and new minerals have beendiscovered as micro–nano inclusions, inconcert with in situ high pressure–temperature (P–T) experiments that de-

fine the conditions of formation. Theemerging field of micro-to-nano miner-alogy may take center stage in thegeosciences with new enabling micro-analytical techniques, including synchro-tron x-ray nanoprobes, synchrotroninfrared probes, electron microscopy,focused ion beam methods, and nano-secondary ion microscopy for analyzingexperimental and natural specimens.

‘‘Ultrahigh pressure,’’ in the contextof metamorphism, refers to the previ-ously unexpectedly high pressure signa-ture in recovered rocks on the earth’ssurface. However, at greater depths inthe mantle the pressure is much higher,but no samples are available. Instead,studies must be conducted in situ byseismology and other geophysical obser-vations, and the results must be com-pared with mineral data obtained eitherin the laboratory or from computationaltheory. There is growing evidence forlateral variations in temperature andhydration in the upper mantle, gainedfrom using long-period seismic wave-forms together with new, physically con-strained inversion methods. In addition,systematic ultrasonic measurements ofmantle minerals as a function of pres-

sure and temperature are providingcrucial wave velocity information forcomparison. The origin of the paradoxi-cal deep-focus earthquake is examinedfrom the standpoint of the experimentalfindings of shear instability induced byphase transitions and dehydration.

Deeper in the mantle, many long-standing seismic anomalies in the D�layer immediately above the core–mantle boundary can now be under-stood with the discovery of MgSiO3post-perovskite at pressures �100 GPa(2). The post-perovskite phase providesnew insight for modeling the metastablesuperplume recently inferred from seis-mology and for understanding the topol-ogy of the top of the D� layer by usingthree-dimensional simulations of mantleconvection. The earth’s core plays a cen-

Author contributions: H.-k.M and R.J.H wrote the paper.

The authors declare no conflict of interest.

*To whom correspondence should be addressed. E-mail:hemley@gl.ciw.edu.

†Workshop on Synergy of 21st Century High-Pressure Sci-ence and Technology, April 29–May 1, 2006, AdvancedPhoton Source, Argonne National Laboratory, Argonne, IL(www.hpcat.aps.anl.gov/2006workshop1/Home.htm).

© 2007 by The National Academy of Sciences of the USA

Fig. 1. Range of pressures and temperatures now accessible with static compression techniques in thelaboratory. (Left) Graphical representation of accessible pressures and temperatures, specifically, thoseattainable with diamond anvil cell methods, which are used to generate the most extreme conditions. Thefields include so-called resistive-heating and laser-heating methods, as described in articles in this issue.Estimated variations in pressure and temperature with depth in various planets are indicated. (Right)Cutaway of the earth’s interior, showing the pressures at the various boundaries within the planet (i.e.,upper and lower mantle, core–mantle boundary, and inner–outer core).

9114–9115 � PNAS � May 29, 2007 � vol. 104 � no. 22 www.pnas.org�cgi�doi�10.1073�pnas.0703653104

tral role in the evolution and dynamicprocesses within the planet; however,the origin of some of its most funda-mental properties, for example, the tem-perature, chemical composition, mineralphases, elasticity, magnetism, and for-mation of the core, remains elusive.Indeed, new study of earth’s core isuniting observational, theoretical, andexperimental geophysics, thereby enrich-ing each discipline through interactionsand feedback. For instance, geophysicalobservations are uncovering surprisinginner-core properties such as seismicanisotropy, layering, and superrotation.Studies of candidate component materi-als of the core include first-principlescalculations and direct experiments

on iron and its alloys. High energy-resolution, nuclear-resonant x-rayspectroscopy provides information onmagnetism, as well as on the phonondensity of states, bulk longitudinal andshear wave velocities, and thermody-namic properties (3).

Laboratory techniques continue tobe developed. It should be possible toproduce a very broad range of P–Tconditions by using laser shocks in pre-compressed samples. These develop-ments have the potential to provideinformation essential for addressingnumerous questions concerning extra-solar planets, including proposed ‘‘su-per earths.’’ Mechanically pulsing theload of a diamond cell bridges the time

domain between conventional dynamicand static compression conditions.Pressure calibration is improving withadvances in high P–T x-ray techniques.New methodologies are being devel-oped for calorimetry and determina-tion of thermal diffusivity: two chal-lenging measurements on small high-pressure samples. This integratedapproach, using a diversity of experi-mental, observational, and simulationmethods, is providing a new window onplanetary interiors. Fully exploiting thepressure variable promises to add anew dimension to scientific explorationof a broad range of other fields, to behighlighted in subsequent PNAS Spe-cial Features.

1. Hemley RJ (2006) Phys Today 59:50–56. 2. Murakami M, Hirose K, Kawamura K, Sata N,Ohishi Y (2004) Science 304:855–858.

3. Lin J-F, Sturhahn W, Zhao J, Shen G, Mao HK,Hemley RJ (2005) Science 308:1892–1894.

Mao and Hemley PNAS � May 29, 2007 � vol. 104 � no. 22 � 9115

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