High Pressure Minerals Physics

Understanding of the elastic behavior of minerals under high pressure is a crucial factor for developing a model of the Earth structure since the information about Earth interior comes mainly from seismological data. Brillouin scattering can be used for measuring elastic properties of non-transparent materials at ambient conditions and transparent materials at pressures up to 130 GPa. Conventional way to measure elastic properties of non-transparent materials at high pressures is to use large volume press (LVP). Large volume press technique has two main disadvantages: (a) it does not allow measuring of the sound velocities in melts and (b) its pressure range is limited to 20-25 GPa. It is fairly recently, impulsive stimulated light scattering (ISLS) have been applied for measuring velocities of surface acoustic wave in non-transparent specimens under high pressure and high temperature. Obviously ISLS cannot be used for measuring elastic properties of non-transparent melts and it does not allow determination of shear and longitudinal velocities of the solid since velocity of the surface acoustic wave is a combination of the longitudinal and shear velocities in solid. The next step in measuring sound velocities under high pressure by combining the diamond anvil cell device with the technique of picosecond laser ultrasonics in plane wave configuration was proposed recently (Decremps, Belliard, Perrin et al., Phys. Rev. Lett. 1, 3, 2008). The main disadvantage of these technique is that the thickness of the studied layered specimen at high pressures is unknown and it can be estimated only from additional measurements of the equation of state using for instance x-rays. Certainly such a technique cannot be used for elastic characterization of melts and amorphous materials. With my colleagues from University du du Maine, Le mans, France, we demonstrate that using laser induced ultrasound in diamond anvil cells in point source point receiver configuration (LIU-DAC) it is possible to detect and measure both longitudinal and shear velocities of the non-transparent iron layer in DAC at pressures up to 23 GPa (Chigarev, Zinin, Ming, Amulele, Bulou, Gusev.  App. Phys. Lett. 93, 181905, 2008). LIU-DAC technique does not require any additional information (such as equation of state) for determining both shear and longitudinal velocities of the material loaded into DAC. This system can be used for measuring anisotropy of the elastic properties of solids and elasticity of melts under high pressure.

Sketch of the sound waves propagation in the DAC. The following notations are introduced in the sketch: h is the thickness of the layer; d is the distance between the pump and a probe laser beams; x and z are Cartesian coordinates.

The signals measured at different distances d. The step of the scan is 7.3 m. The top signal was measured when d = 16 m. The lowest signal was measured when d = 95.6 m (From Chigarev, Zinin et al. App. Phys. Lett. 93, 181905, 2008).

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n 2012, e developed a unique and multifunctional in-situ measurement system under high pressure equipped with laser ultrasonics system1, Raman device, and laser heating system (LH-LU-DAC) at the University of Hawaii (Fig. 1). The sketch of the LH-LU-DAC system is shown in Fig. 1b. The system consists of four components: (1) LU-DAC system (probe and pump lasers, photodetector, and oscilloscope); (2) a fiber laser (100 W, 1064 nm, yttrium fiber laser, IPG Photonics, YLR-100-AC-Y11IPG), which is designed to allow precise control of the total power in the range from 2 to 100W by changing the diode current, for heating samples; (3) a spectrometer (Andor, Shamrock 303i with CCD camera, DU401A-BV) for measuring the temperature of the sample (using Black body radiation), fluorescence spectrum (spectrum of the ruby for pressure measurement), and Raman scattering measurements inside DAC under high pressure and high temperature (HPHT) conditions; and (4) an optical system for focusing laser beams (pump, probe, and 100W CW lasers) on the sample in DAC and for imaging a sample inside the DAC.

Fig. 1. (a) Image of the combined LU-DAC, laser heating, and Raman scattering system at the University of Hawaii; (b) Sketch of the LH-LU-DAC system

The imaging optical system includes power regulators, combining optics, laser power detector, beam splitter, and focusing system (Fig. 1b). The system allows us to: (a) measure acoustical properties of materials under HPHT; (b) synthesize new phases under HPHT; and (c) measure Raman scattering under HPHT conditions for detection of phase transition.

The measurements of the longitudinal velocity in iron in the pressure range 1 to 30 GPa have been conducted earlier, however, shear wave velocity has been measured only at 16 GPa15. Within the experimental error, results obtained with the point-source-point-receiver LU technique are in good agreement with available experimental data.

Emerging view of Earth's interior. Deep-mantle complexities beneath the central Pacific: surface hotspot volcanism; D'' anisotropy and ultralow-velocity zones; and possible plume genesis. (From Garnero, E., Science, 304, 834, 2004).

Experimentally measured velocities of longitudinal (filled squares) and shear (filled rhombi) waves in pure iron layer as a function of pressure and those taken from literature (open circles and open triangle). Lines are added to guide the eyes.

The presented work is a starting point in a new direction: study of elasticity of the non-transparent iron rich minerals under pressures above 30 GPa. Experimental data thus obtained using LU-DAC will shed light on the nature and composition of the D layer and possibly on the formation of the Hawaiian Islands through the hot spot as shown in the figure above.

. N. Chigarev, P. Zinin, M. Li-Chung, G. Amulele, A. Bulou, V. Gusev, Appl. Phys. Lett. 93, 181905 (2008).