Research Interests

 

My main research interests lie in high pressure and temperature experiments to simulate the conditions in the Earth's interior.  The goal is to provide experimental constraints on the thermal, chemical, and dynamic structure of the Earth and a better understanding of the physical properties of the interior. Newly developed in situ high pressure experiments, from x-ray diffraction and spectroscopy to optical and magnetic measurements, provide a wealth of information about the state of the candidate materials at the conditions of the deep Earth.  These data are crucial experimental basis for modeling the structure, composition, dynamics and evolution of the Earth’s deep interior.  

I am mostly interested in seeking answers to questions such as: what is the radial distribution of mineralogical constitution throughout the Earth? What is the thermal state of the Earth’s deep interior? How did the Earth evolve to its present state? What are the mineralogy and the change in material properties at several boundaries and related regions (670-km, core-mantle boundary, inner core-outer core boundary)?  Pursuing these studies relies largely on experimental facilities for the generation of conditions equivalent to the Earth’s interior and the measurement on samples under the in situ conditions.  Diamond anvil cell (DAC) is the unique technique for generating static high pressures for the entire range encountered in the terrestrial planetary interiors [Xu et al., 1986].  Laser heated and resistively heated DAC are powerful tools for simulating pressure-temperature conditions inside planetary interiors [Mao and Hemley, 1996; Shen et al., 2001a].  DAC sample is usually in an order of 10-100 mm in dimension, and therefore micro-probing tools are generally required, such as synchrotron radiation and highly collimated lasers.  As a faculty at MIT, I would set up a diamond cell laboratory equipped with a micro-Raman system, external heating capabilities, and a laser heating system.  A laser heating system and other heating capabilities are essential for generating high temperature conditions under high pressures equivalent to the Earth’s interior.  The high pressure-temperature facility will be also useful for material science research.  These in-house facilities will work in conjunction with synchrotron x-ray experiments for measuring the material’s properties such as crystal structure, density, equation of state, elasticity, viscosity, acoustic velocity, phase equilibrium, melting, element partitioning, electronic structure, and phonon density of states. 

 

Some interests are highlighted as follows:

Stable phases at pressure-temperature conditions of the lower mantle and the core. This is to understand what could be the candidate materials of the Earth’s deep interior.  During the last three decades, enough evidence from laboratory data has shown that, of all the possible phases under consideration for the lower mantle, the silicate perovskite phase is apparent the most abundant phase [Knittle and Jeanloz, 1987; Liu, 1976; Yagi et al., 1979].  This result has been questioned recently [Saxena et al., 1998].  The stability of the silicate perovskite at the lower mantle pressure-temperature conditions is now a subject of many articles [Fiquet et al., 2000; Serghiou et al., 1998; Shim et al., 2001].  For the core material, the debate of possible existence of the b-phase iron has been in literature for years [Andrault et al., 1997; Boehler, 1993; Saxena et al., 1993; Shen et al., 1998].  It is important to clearly answer questions for what is the stable phases of the major minerals at pressure-temperature conditions equivalent the lower mantle and the core.  Although the pressure-temperature conditions throughout the Earth are attainable with the current high pressure techniques, unambiguous determination relies on integrated effort on maximizing the sample size, minimizing both temperature and pressure gradients, and unambiguous detection with sufficient sensitivity and accuracy.  The third generation synchrotron radiation provides great opportunity to probe micro samples under extreme conditions.  With the laser heated diamond anvil cell system at the GSECARS [Shen et al., 2001a], significant effort has been put on the silicate perovskite [Shim et al., 2001] and iron [Shen et al., 2001c].  At MIT, I would be interested in continuing this project to extend candidate minerals for the lower mantle and the core.

 

Core-mantle boundary. The core-mantle boundary (CMB) region is one of the most important boundaries in the Earth.  The changes in density and viscosity across the CMB are larger than at the surface.  Molten metal contacts crystalline silicates and oxides there.  Is the core-mantle boundary a zone of intensive chemical reaction?  What is the temperature contrast across the CMB?  What is responsible for the seismological evidence [Kellogg et al., 1999; van der Hilst and Karason, 1999] for compositionally distinct regions in the deep mantle?  Many investigators [Knittle and Jeanloz, 1991; Poirier et al., 1998] concluded that the reactions between molten iron and solid oxides and silicates occur at the CMB pressure.  However, Boehler et al. [1995] claimed that no sign of reaction was visible in thoroughly dry systems.  Whether the fluid iron alloy of the core can penetrate upward is important to understand the nature of the region.  Information on the reaction of iron and oxides and silicates at the CMB condition is crucial.  The temperature contrast across the CMB may be constrained by three primary parameters: the temperature at the 670 km discontinuity; the temperature at the inner core-outer core boundary; and the adiabatic temperature gradients within the Earth’s lower mantle and the outer core.  Information of these primary parameters could be obtained through studies on phase transitions responsible for the 670-km discontinuity, high pressure melting of iron and iron alloys, and thermodynamic properties of major minerals in the lower mantle and the outer core.  The CMB is a region with interesting physics and chemistry and, importantly, a region that plays a fundamental role in Earth’s dynamics and evolution.  At MIT, I would pursue these studies by taking the advantage that the primary tools required for the research are conveniently accessible, e.g., x-rays (Advanced Photon Source, Brookhaven National Lab), Raman spectroscopy (in-house), electron microscopy (in-house).

 

How hot is the Earth’s core?  This is a long-standing question that has drawn a lot attention from both theoreticians and experimentalists. Estimates of temperatures in the Earth’s core rely on the assumption that the boundary between the solid inner core and the liquid outer core is at the melting temperature of the core material.  The melting temperature of iron at or near the pressure of the inner core boundary would place a bound on core’s temperature, given that the core is mainly made of iron.  The measurements of the high pressure melting of the core material remain one of the most technically challenging aspects of high pressure geophysics.  Controversial and conflicting experimental results have been reported even within the same static technique [Boehler, 1993; Saxena et al., 1994; Shen et al., 1998; Williams et al., 1991].  The difficulties lie in the characterization of samples (e.g. unambiguous melting criteria) and in the measurement of pressure and temperature under extreme conditions.  Recently, progresses have been made in melting characterization.  Micro x-ray beam has been successfully used for recognizing melting by observing diffuse scattering signals from melts with simultaneous disappearance of all crystalline diffraction lines [Shen et al., 2001b].  The analysis on the diffuse scattering gives information on the structure of melts [Shen et al., 2001d].  This area is actually one of my current researches as reflected by a NSF project “high pressure melting of the earth’s core material” funded last year (I am the PI).  I would continue using x-ray diffraction for melting characterization.  At the same time, I am interested in using nuclear forward scattering (Advanced Photon Source) to monitor the nuclear resonance in melting studies.

 

Density of molten materials.  It is generally assumed that an addition of light elements (e.g., O, S, Si, C, H) to iron is required for the Earth’s core.  The amount of light elements required for the density deficit depends on densities of molten iron at the core pressure-temperature conditions.  It is thus desirable to directly measure the density of molten iron at the outer core pressures.  Recently, an attempt was made to measure the structure factor and density of molten indium at high pressures in a diamond anvil cell using micro x-ray beam [Shen et al., 2001d,e].  The first result is encouraging.  However, there is still a need of significant work and developments to be made for the direct measurement at the outer core condition.  I am interested in further development of heating capabilities to generate molten states of iron and iron alloys at core pressures and to be able to measure their densities.  The development will involve use of new materials for pressure cells, an area expected to have close collaboration with materials research at MIT.  

 

Chemical differentiation. Melting is the major force in chemical differentiation of the Earth. The concept is supported by evidence that early in our planet’s history the core and mantle segregated from each other from a relatively homogenous body that condensed out of the solar nebula. The crust and upper and lower mantle then further chemically segregated.  Various models of the segregation and core formation of the Earth suggest that the outer layer of the Earth was much hotter in the early stage of the Earth’s evolution than today, resulting in a formation of a primordial magma ocean [Ohtani, 1988; Ringwood, 1979].  All stratification models rely largely on experimental results of melting relations in the deep mantle and the density relationship between the liquid and minerals.  Experimental studies on these relations are now feasible at the lower mantle pressures.  I consider this is an interesting area to understand how the Earth evolved to its present form.

 

Lattice dynamics and elasticity.  Our understanding of the Earth's interior largely comes from the seismic observation with the velocity structure and elastic properties of its interior.  The elastic properties of the constituent materials provide fundamental constraints for modeling the structure, composition, dynamics and evolution of the Earth’s deep interior.  For example, during the past decade, the elastic anisotropy of the inner core established by seismological studies has emerged as a key problem of global geophysics.  This knowledge has been used to detect the inner core rotation [Song and Richards, 1996] as well as to constrain various fundamental physical and chemical properties of the deep Earth.  In order to understand the cause of the anisotropy, it is crucial to study the intrinsic elastic anisotropy of the inner core materials.  Complementary to the light scattering (Brillouin and Raman spectroscopy) used for probing the phonon structure with low frequency, the high frequency range is often studied by the neutron inelastic scattering and inelastic x-ray scattering.  The latter with x-ray is more suited for the diamond anvil cell studies since small sample is usually involved.  In recent years, nuclear resonant x-ray scattering techniques that utilize synchrotron radiation have made great progresses in the study of vibrational and magnetic properties of condensed matter under extreme conditions.  In particular, the determination of the phonon density of states with nuclear resonant inelastic x-ray scattering (NRIXS) provides unique information [Mao et al, 2001; Fiquet et al, 2001; Shen et al, 2001e].  A variety of parameters can be derived for materials at high pressures and high temperatures, such as compressional and shear velocities, Debye temperatures, Grüneisen parameters, vibrational entropy and specific heat.  Iron or iron-containing materials are well suited for such studies without other limitations in sample preparations. 

 

References

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Boehler, R., Temperatures in the Earth's core from melting-point measurements of iron at high static pressures, Nature, 363, 534-536, 1993.

Boehler, R., A. Chopelas, and A. Zerr, Temperature and chemistry of the core-mantle boundary, Chem. Geology, 120, 199-205, 1995.

Fiquet, G., A. Dewaele, D. Andrault, M. Kunz, and T. Le Bihan, Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions, Geophys. Res. Lett., 27, 21-24, 2000.

Fiquet, G., J. Badro, F. Guyot, H. Requardt, and M. Krisch,  Sound velocities in iron to 110 gigapascals, Science, 291, 468-471, 2001.

Garnero, E.J., and D.V. Helmberger, Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central-Pacific, Geophys. Res. Lett., 97, 4477-4487, 1996.

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Knittle, E., and R. Jeanloz, Synthesis and equation of state of (Mg,Fe)SiO3 perovskite to over 100 gigpascals, Science, 235, 668-670, 1987.

Knittle, E., and R. Jeanloz, Earth's core-mantle boundary: Results of experiments at high pressures and high temperatures, Science, 251, 1438-1443, 1991.

Liu, L., Orthorhombic perovskite phases observed in olivine, pyroxene, and garnet at high pressures and temperatures, Phys. Earth Planet. Inter., 11, 289-298, 1976.

Mao, H.K., and R.J. Hemley, Experimental studies of Earth's deep interior: Accuracy and versatility of diamond-anvil cells, Phil. Trans. R. Soc. Lond. A, 354, 1315-1332, 1996.

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Saxena, S.K., L.S. Dubrovinsky, P. Lazor, Y. Cerenius, P. Häggkvist, M. Hanfland, and J. Hu, Stability of Perovskite (MgSiO3) in the Earth's Mantle, Science, 274, 1357-1359, 1998.

Saxena, S.K., G. Shen, and P. Lazor, Experimental evidence for a new iron phase and implications for Earth’s core, Science,  260, 1312-1314, 1993.

Saxena, S.K., G. Shen, and P. Lazor, Temperatures in Earth’s core based on melting and phase transformation experiments on iron, Science, 264,  405-407, 1994.

Serghiou, G., A. Zerr, and R. Boehler, (Mg,Fe)SiO₃-perovskite stability under lower mantle conditions, Science, 280, 2093-2095, 1998.

Shen, G., H.K. Mao, R.J. Hemley, T.S. Duffy, and M.L. Rivers, Melting and crystal structure of iron at high pressures, Geophy. Res. Lett., 25, 373-376, 1998.

Shen, G., M.L. Rivers, Y. Wang, and S.J. Sutton, A laser heated diamond cell system at the Advanced Photon Source for in situ x-ray measurements at high pressure and temperature, Rev. Sci. Instrum., 72, 1273-1282, 2001a.

Shen, G., N. Sata, M.L. Rivers, and S.R. Sutton, Melting of Indium at High Pressure Determined by Monochromatic X-ray Diffraction in an Externally-Heated Diamond Anvil Cell, Appl. Phys. Lett., 78, 3208-3210, 2001b.

Shen, G., N. Sata, M.L. Rivers, and S.R. Sutton, Stable phases of iron at high pressure and temperature, in preparation, 2001c.

Shen, G., N. Sata, N. Taberlet, M. Newville, M.L. Rivers, and S.R. Sutton, Melting studies of indium: Determination of structure and density of melts at high pressures and high temperatures, in International Conference of High Pressure Science and Technology (AIRAPT), Beijing, in press, 2001d.

Shen, G., N. Sata, M.L. Rivers, and S.R. Sutton, Molar volumes of liquids measured in a diamond anvil cell, submitted to Phys. Rev. Lett., 2001e.

Shen, G., V. Prakapenka, H.K. Mao, Y. Meng, E.E. Alp, W. Sturhahn, T. Toellner, and J. Zhao, Experimental study on the lattice dynamics of iron at high pressures and high temperatures (abstract), EOS, 82, F942, 2001f.

Shim, S.H., T.S. Duffy, and G. Shen, Stability and Structure of MgSiO₃ Perovskite to 2300-km Depth in the Earth’s Mantle, Science, 293, 2437-2440, 2001.

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Williams, Q., E. Knittle, and R. Jeanloz, The high pressure melting curve of iron: A technical discussion, J. Geophys. Res., 96,  2171-2184, 1991.

Xu, J., H.K. Mao, and P.M. Bell, High pressure ruby and diamond fluorescence: Observations at 0.21 to 0.55 TPa, Science, 232, 1404-1406, 1986.

Yagi, T., P.M. Bell, and H.K. Mao, Phase relations in the system magnesium oxide-iron(II) oxide-silicon dioxide between 150 and 700 kbar at 1000 C, Carnegie Institution Washington Yearbook, 78, 614-616, 1979.