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Shear Waves in the Diamond-Anvil Cell Reveal Pressure-Induced Instability in (Mg,Fe)O

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Shear Waves in the Diamond-Anvil Cell Reveal Pressure-Induced Instability in (Mg,Fe)O Shear waves in the diamond-anvil cell reveal pressure-induced instability in (Mg,Fe)O Steven D. Jacobsen*†, Hartmut Spetzler‡§, Hans J. Reichmann¶, and Joseph R. Smyth§ *Bayerisches Geoinstitut, Universita¨t Bayreuth, 95440 Bayreuth, Germany; ‡Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-0216; §Department of Geological Sciences, University of Colorado, Boulder, CO 80309-0399; and ¶GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Communicated by Russell J. Hemley, Carnegie Institution of Washington, Washington, DC, March 5, 2004 (received for review February 15, 2004) The emerging picture of Earth’s deep interior from seismic tomog- compositional variation or phase changes in (Mg,Fe)O are raphy indicates more complexity than previously thought. The ongoing (16). presence of lateral anisotropy and heterogeneity in Earth’s mantle Megahertz-frequency ultrasonics measurements in the mul- highlights the need for fully anisotropic elasticity data from min- tianvil press are a leading resource for experimental thermoelas- eral physics. A breakthrough in high-frequency (gigahertz) ultra- tic data on polycrystalline samples at simultaneous high pres- sound has resulted in transmission of pure-mode elastic shear sures and temperatures relevant to Earth’s mantle (17). waves into a high-pressure diamond-anvil cell using a P-to-S However, single-crystal experiments giving elastic anisotropy elastic-wave conversion. The full elastic tensor (cij) of high-pressure (cij) in the multianvil press are relatively few (18). Also at minerals or metals can be measured at extreme conditions without megahertz frequencies, ultrasonic interferometry has been ap- optical constraints. Here we report the effects of pressure and plied successfully to single-crystal samples in the gas- or liquid- composition on shear-wave velocities in the major lower-mantle pressurized piston-cylinder apparatus (19) and the Paris– oxide, magnesiowu¨stite-(Mg,Fe)O. Magnesiowu¨stite containing Edinburgh cell (20), but these frequencies still limit the Ϸ more than 50% iron exhibits pressure-induced c44 shear-mode minimum sample size to Ϸ1 mm. For microcrystals less than softening, indicating an instability in the rocksalt structure. The Ϸ0.5 mm in size, as most available synthetic high-pressure Ϸ oxide closer to expected lower-mantle compositions ( 20% iron) mantle phases are, the DAC is the preferred pressure cell, shows increasing shear velocities more similar to MgO, indicating capable of hydrostatic pressures to Ϸ10 GPa with liquid pressure that it also should have a wide pressure-stability field. A complete media, and to much higher pressures when loaded with a gas such sign reversal in the c44 pressure derivative points to a change in the as helium or neon. Brillouin scattering in the DAC is a powerful Ϸ topology of the (Mg,Fe)O phase diagram at 50–60% iron. The method for measuring single-crystal elasticity to pressures now relative stability of Mg-rich (Mg,Fe)O and the strong compositional in excess of 50 GPa (21) and temperatures to 1,500 K (22), but ٢͞ ٢ dependence of shear-wave velocities (and c44 P) in (Mg,Fe)O as an optical method it is limited to fairly transparent single implies that seismic heterogeneity in Earth’s lower mantle may crystals. Another novel method, impulse-stimulated scattering result from compositional variations rather than phase changes in (23), has been extended recently to include acoustic surface- (Mg,Fe)O. wave measurements (24) from which shear velocities in opaque minerals or metals are obtained but without P-wave velocities dense oxide of iron and magnesium is expected to coexist needed for the complete elastic tensor. Awith magnesium silicate perovskite-(Mg,Fe)SiO3 in Earth’s By extending the ultrasound to gigahertz frequencies, we lower mantle at 660- to 2,900-km depth (1). The physical reduce acoustic wavelengths in minerals to Ϸ1–10 ␮m. Pure- properties of (Mg,Fe)O have been studied extensively by using mode ultrasonic shear waves have been transmitted into a both static (2–5) and dynamic methods (5–9), but there remains high-pressure DAC. To resolve some of the questions surround- considerable uncertainty surrounding the structure and behavior ing (Mg,Fe)O stability, we measured the pressure dependence of of (Mg,Fe)O in the lower mantle. Periclase (MgO) has the S-wave travel times in (Mg,Fe)O crystals ultrasonically to ex- rocksalt structure to at least 227 GPa (10). On the other hand, ϭ␳ 2 amine the behavior of the c44 elastic constant ( VS͓100͔)asa wu¨stite (FeO) undergoes a displacive phase transition to a Ϸ function of composition. This pure-shear mode is the elastic rhombohedral distorted B1 structure at 17 GPa and subse- parameter most sensitive to the rhombohedral distortion ob- quently to the B8 or NiAs-type structure above 100 GPa (11). served in FeO before the B1–B8 phase transition (11) and Differences in the high-pressure behavior of MgO and FeO therefore should be a useful predictor of (Mg,Fe)O stability. imply a change in topology of the (Mg,Fe)O phase diagram, Shear-mode softening (i.e., decreasing shear velocities with GEOPHYSICS requiring there to be a two-phase field or exsolution gap, but increasing pressure) in FeO (25) is the result of strong magne- where in composition between ferropericlase (Mg-rich) and toelastic coupling driving the B1 (paramagnetic) to rhombohe- magnesiowu¨stite (Fe-rich) this should occur is not clear yet. dral (antiferromagnetic) phase transition (26). Mode softening Iron-rich compositions (Mg0.1Fe0.9)O and (Mg0.25Fe0.75)O dis- in FeO has also been deduced from lattice strain in nonhydro- play the rhombohedral distortion above 20 and 60 GPa, respec- static x-ray diffraction experiments (27), but uncertainties in the tively (12, 13). At 80–90 GPa and 1,000 K, (Mg Fe )O and 0.50 0.50 magnitude of c from such methods are on the order of 15–20%. (Mg Fe )O were reported to separate into ferropericlase ij 0.60 0.40 Our direct (ultrasonic) observations of shear elasticity in plus FeO in the externally heated diamond-anvil cell (DAC) (Mg,Fe)O were carried out by using an acoustic shear-wave (14). But in a similar study using a laser-heated DAC, both interferometer for the DAC. The methodological advance not (Mg Fe )O and (Mg Fe )O were stable in the B1 struc- 0.39 0.61 0.25 0.75 only has broad application to high-pressure research in Earth ture at 100 GPa and 2,500 K (13). A high-spin to low-spin Ϸ transition in Fe was reported for (Mg0.83Fe0.17)O between 50 and 60 GPa (15) but without evidence for a volume change or Abbreviation: DAC, diamond-anvil cell. phase stability. Thus, despite concerted efforts to elucidate the †To whom correspondence should be addressed at: Carnegie Institution of Washington, structure and stability of (Mg,Fe)O at lower mantle conditions, Geophysical Laboratory, 5251 Broad Branch Road NW, Washington, DC 20015. E-mail: experiments thus far have led to conflicting results. The search [email protected]. for wave-speed discontinuities in Earth’s lower mantle related to © 2004 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0401564101 PNAS ͉ April 20, 2004 ͉ vol. 101 ͉ no. 16 ͉ 5867–5871 Downloaded by guest on September 24, 2021 Fig. 1. Making waves in the DAC. (A) The P-to-S acoustic wave converter (between a pair of tweezers) before sputtering the P-transducer. (Scale bar, 3 mm.) The gem is an oriented single crystal of yttrium aluminum garnet (Y3Al5O12), hand-faceted to tolerances of Ϯ0.1° (H. Schulze, Bayerisches Geoinstitut). It produces pure-mode elastic shear energy with 1- to 10-␮m wavelengths and well defined polarization direction for high-pressure elasticity experiments in the DAC. (B) Shear waves are introduced into the DAC through one of the anvils (shown schematically, not to scale). Echoes are produced at each major impedance contrast and labeled as follows: the buffer rod echo, diamond echo, and sample echo. S-wave travel times are determined from acoustic interface spectra produced in the combined diamond and sample echoes by scanning the frequency (29). and materials sciences but also for development of an absolute transducer. The entire round trip through the system is typically pressure scale (21, 28). Ϸ3 ␮s. The time difference between the diamond and the first sample echo is usually very short (8–20 ns) compared with the Experimental Methods width of the input signal (Ϸ100 ns). An interference pattern is The acoustic interferometer is based on our first-generation produced by measuring the amplitude of the combined signal at gigahertz-ultrasonic delay line (29). In its initial form, gigahertz- a position where there is first-order interference (between the ultrasonic interferometry was well suited for high-pressure ex- diamond and sample echoes) and scanning the frequency (Fig. periments in the DAC (30, 31) but limited to P-wave experiments 2). Shear-wave travel times are determined from each fitted by the absence of commercially available shear-wave transducers frequency maxima and minima of interference (29). operating at gigahertz frequencies. Here we describe an acoustic The samples are prepared as oriented single-crystal plates: P-to-S conversion buffer rod for high-pressure shear elasticity polished with parallel faces and flat to Ϸ1͞10␭ with thicknesses experiments in the DAC, pictured in Fig. 1. The buffer rod works ranging from Ϸ20 to 40 ␮m. The plates have a finishing polish on the principle of Snell’s Law. A P-wave traveling in the [100] of optical quality and are placed directly on the culet of the direction of an yttrium aluminum garnet crystal buffer rod (Fig.
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