Jennifer Kung

Spin transition of iron in magnesiowüstite in the Earth's lower mantle.

Iron is the most abundant transition-metal element in the mantle and therefore plays an important role in the geochemistry and geodynamics of the Earths interior. Pressure-induced electronic spin transitions of iron occur in magnesiowüstite, silicate perovskite and post-perovskite. Here we have studied the spin states of iron in magnesiowüstite and the isolated effects of the electronic transitions on the elasticity of magnesiowüstite with in situ X-ray emission spectroscopy and X-ray diffraction to pressures of the lowermost mantle. An observed high-spin to low-spin transition of iron in magnesiowüstite results in an abnormal compressional behaviour between the high-spin and the low-spin states. The high-pressure, low-spin state exhibits a much higher bulk modulus and bulk sound velocity than the low-pressure, high-spin state; the bulk modulus jumps by ∼35 per cent and bulk sound velocity increases by ∼15 per cent across the transition in (Mg0.83,Fe0.17)O. Although no significant density change is observed across the electronic transition, the jump in the sound velocities and the bulk modulus across the transition provides an additional explanation for the seismic wave heterogeneity in the lowermost mantle. The transition also affects current interpretations of the geophysical and geochemical models using extrapolated or calculated thermal equation-of-state data without considering the effects of the electronic transition.

A systematic study of OH in hydrous wadsleyite from polarized FTIR spectroscopy and single-crystal X-ray diffraction: Oxygen sites for hydrogen storage in Earth∑ s interior

Abstract The incorporation of hydrogen into wadsleyite (β-Mg2SiO4) was investigated using polarized FTIR spectroscopy and X-ray diffraction on oriented single crystals. The experiments were carried out with a new suite of samples containing between ~100 and ~10 000 ppm H2O by weight (wt ppm), encompassing the H-contents most relevant to Earths potentially hydrous mantle transition zone. Attempts to synthesize anhydrous wadsleyite resulted in water contents of no less than ~50 wt ppm H2O. An empirical relation between the b/a axial ratio against estimated wt ppm concentrations of H2O in wadsleyite (CH2O) was determined: (b/a) = 2.008(1) + 1.25(3) × 10-6 · CH2O Polarized infrared absorption spectra were measured in the three orthogonal sections perpendicular to the major axes of the optical indicatrix ellipsoid and are used in concert with results from new structure refinements to place constraints on the main absorbers in the structure. All of the main bands in the O-H stretching region of the FTIR can be explained by protonation of O1, the anomalous non-silicate oxygen site. We assign the band at 3614 cm-1 to a bent hydrogen bond O1···O1 (2.887 Å) along the M3 edge in the a-c plane. The band at 3581 cm-1 is assigned to a bent hydrogen bond on O1···O3 (3.016 Å) of the M3 edge in the b-c plane. The absorption bands at 3360, 3326, and 3317 cm-1 are best explained by hydrogen bonds on O1···O4 (3.092 Å) and O1···O4 (2.795 Å) along the M3 and M2 edges with possible splitting of one band due to vacancy ordering, but we cannot rule out contributions from three other (O1···O3) edges. The broad absorption feature at 3000 cm-1 is unambiguously assigned to the O4···O4 (2.720 Å) tetrahedral edge of the Si2O7 group pointing along the [100] vector. On hydration to ~1 wt% H2O, M-site vacancies are observed exclusively at M3. A systematic shortening of several (interpreted) hydrogen bonded O···O M-site edges is attributed to reduced O-O repulsive forces on protonation in the vicinity of an M-site vacancy.

Elasticity of (Mg0.83, Fe0.17)O ferropericlase at high pressure: Ultrasonic measurements in conjunction with X-radiation techniques

Abstract The elasticity of ferropericlase with a potential mantle composition of (Mg 0.83 ,Fe 0.17 )O is determined using ultrasonic interferometry in conjunction with in situ X-radiation techniques (X-ray diffraction and X-radiography) in a DIA-type cubic anvil high-pressure apparatus to pressures of 9 GPa (NaCl pressure scale) at room temperature. In this study, we demonstrate that it is possible to directly monitor the specimen length using an X-ray image technique and show that these lengths are consistent with those derived from X-ray diffraction data when no plastic deformation of the specimen occurs during the experiment. By combining the ultrasonic and X-ray diffraction data, the adiabatic elastic bulk ( K S ) and shear ( G ) moduli and specimen volume can be measured simultaneously. This enables pressure scale-free measurements of the equation of state of the specimen using a parameterization such as the Birch–Murnaghan equation of state. The elastic moduli determined for (Mg 0.83 ,Fe 0.17 )O are K S0 =165.5(12) GPa, G 0 =112.4(4) GPa, and their pressure derivatives are K S0 ′=4.17(20) and G 0 ′=1.89(6). If these results are compared with those for MgO, they demonstrate that K S0 and K S0 ′ are insensitive to the addition of 17 mol% FeO, but G 0 and G 0 ′ are reduced by 14% and 24%, respectively. We calculate that the P and S wave velocities of a perovskite plus ferropericlase phase assemblage with a pyrolite composition at the top of the lower mantle (660 km depth) are lowered by 0.8 and 2.3%, respectively, when compared with those calculated using the elastic properties of end-member MgO. Consequently, the magnitudes of the calculated wave velocity jumps across the 660 km discontinuity are reduced by about 11% for P wave and 20% for S wave, if this discontinuity is considered as a phase transformation boundary only (ringwoodite→perovskite+ferropericlase).

Sound velocity measurement using transfer function method

The transfer function of a piezoelectric transducer, buffer rod and sample assembly is used to measure the sound velocity of solid materials. From the recorded transfer function, pulse echo patterns at frequencies of the passband of the input signal are reproduced after convoluting with monochromatic RF input signals. The time delay is obtained by performing pulse echo overlap and phase comparison measurements on reproduced signals. Results for a single crystal of MgO along the [100] direction from this study are in good agreement with previous measurements but have the advantage of offline data analysis and fast data acquisition.

In‐situ elasticity measurement for the unquenchable high‐pressure clinopyroxene phase: Implication for the upper mantle

@KS/@ T= � 0.017(1) GPa K � 1 and (@G/@T) = � 0.015(1) GPa K � 1 . Our results show that the elastic velocities of HP-CEN are not only higher than those of OEN but also higher than olivine at upper mantle conditions. According to these new data, the elastic velocities for the ‘‘depleted’’ mantle composition, with � 20 vol% of HP-CEN, would result in � 0.8% and � 1.6% in difference for P and S waves, respectively, when OEN data were used in previous calculations. Newly measured results also suggest that it is possible to distinguish ‘‘undepleted’’ and ‘‘depleted’’ composition models in the upper mantle. Citation: Kung, J., B. Li, T. Uchida, and Y. Wang (2005), In-situ elasticity measurement for the unquenchable high-pressure clinopyroxene phase: Implication for the upper mantle, Geophys. Res. Lett., 32, L01307, doi:10.1029/2004GL021661.

Pressure calibration to 20 GPa by simultaneous use of ultrasonic and x-ray techniques

Simultaneous measurements of elastic P- and S-wave travel times, density (specific volume), and sample length using a multianvil apparatus interfaced with ultrasonic interferometry and x-ray diffraction and radiography have enabled simultaneous determination of elastic properties and absolute pressure at high pressures and high temperatures. Experimental procedures and data analyses are demonstrated using data collected on polycrystalline samples of ferropericalse (MgFe) and wadsleyite Mg2SiO4 under quasihydrostatic conditions up to 20GPa. Compared with those derived from the internal pressure standard in the same experiments based on equations of state of NaCl, the directly determined pressure at 20GPa is ∼8% and ∼12% higher than those inferred from the Brown and Decker pressure scales, respectively. The discrepancy cannot be reconciled by uncertainties in the NaCl pressure scale and current experimental data. Until further results are available, the pressure determined in this study is believed to be a ...

Nanoprobe measurements of materials at megabar pressures

The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up to multimegabar (> 200) pressures, namely, the spatial resolution of measurements of multiple samples, stress gradients, and crystal domains in micron to submicron size samples in diamond-anvil cells. Mixtures of Fe, Pt, and W were studied up to 282 GPa with 250–600 nm size synchrotron x-ray absorption and diffraction probes. The probes readily resolve signals from individual materials, between sample and gasket, and peak pressures, in contrast to the 5-μm-sized x-ray beams that are now becoming routine. The use of nanoscale x-ray beams also enables single-crystal x-ray diffraction studies in nominally polycrystalline samples at ultrahigh pressures, as demonstrated in measurements of (Mg,Fe)SiO3 postperovskite. These capabilities have potential for driving a push toward higher maximum pressures and further miniaturization of high-pressure devices, in the process advancing studies at extreme conditions.

Elasticity of ScAlO3 at high pressure

Abstract ScAlO 3 is a close structural analogue for MgSiO 3 -perovskite in terms of its molar volume, structural distortion and compressional behavior. Polycrystalline specimens of ScAlO 3 were synthesized in a 1000-ton multi anvil press at 10 GPa, 1470 K from two different starting materials; amorphous ScAlO 3 and mixed oxides. Two well-sintered and fully-transformed specimens were recovered with low porosities of 0.5 and 1.7%. Elastic wave-speeds of these two specimens have been determined up to 3 GPa at room temperature. The results indicate that for these specimens, ∂ K /∂ P =3.8 and ∂ G /∂ P =1.9. The pressure derivatives of elastic moduli for the polycrystalline ScAlO 3 specimens are expected to be reliable within 10% using this well-established ultrasonic technique. This is demonstrated by comparing determinations of the elasticity of polycrystalline Al 2 O 3 with the available single crystal data. The values of ∂M/∂ P of ScAlO 3 are consistent with those determined for MgSiO 3 perovskite from X-ray P – V study and ultrasonic measurement.

High-temperature elasticity of polycrystalline orthoenstatite (MgSiO3)

Abstract Compressional and shear wave velocities of a polycrystalline specimen of MgSiO3 orthoenstatite have been measured by ultrasonic interferometry to 1373 K at 300 MPa in an internally heated gas-medium apparatus. The elastic wave velocities and bulk and shear moduli vary linearly with temperature to 1073 K. Below 1073 K, the temperature derivatives of the elastic moduli [(∂KS/∂T)P = -28.3(7) MPa/K and (∂G/∂T)P = -14.5(1) MPa/K, respectively] determined in this study are consistent with averages of single-crystal elastic constants measured using Brillouin spectroscopy by Jackson et al. (2007). The measured temperature dependence of elastic moduli, along with pressure dependence of elastic moduli, thermal expansion and calorimetric data have been assimilated into a finite-strain equation of state of the type proposed by Stixrude and Lithgow-Bertelloni (2005). This analysis suggests significant revisions to the optimal values of the zero-pressure Grüneisen parameter γ0 and its zero-pressure logarithmic volume derivative q0. The unusually high absolute values of (∂K/∂P)T and (∂K/∂T)P are related through the extrinsic part of the temperature derivative. Above 1073 K, a pronounced softening of the elastic wave velocities is observed, which is plausibly associated with a phase transformation for which there is microstructural evidence: The recovered specimen was found to have transformed to the low-pressure clinoenstatite polymorph.

Silicate perovskite analogue ScAlO3: temperature dependence of elastic moduli

Abstract The elastic moduli of ScAlO 3 perovskite, a very close structural analogue for MgSiO 3 perovskite, have been measured between 300 and 600 K using high precision ultrasonic interferometry in an internally heated gas-charged pressure vessel. This new capability for high temperature measurement of elastic wave speeds has been demonstrated on polycrystalline alumina. The temperature derivatives of elastic moduli of Al 2 O 3 measured in this study agree within 15% with expectations based on published single-crystal data. For ScAlO 3 perovskite, the value of (∂ K S /∂ T ) P is −0.033 GPa K −1 and (∂ G /∂ T ) P is −0.015 GPa K −1 . The relative magnitudes of these derivatives agree with the observation in Duffy and Anderson [Duffy, T.S., Anderson, D.L., 1989. Seismic velocities in mantle minerals and the mineralogy of the upper mantle. J. Geophys. Res. 94, 1895–1912.] that |(∂ K S /∂ T ) P | is typically about twice |(∂ G /∂ T ) P |. The value of (∂ K S /∂ T ) P for ScAlO 3 is intermediate between those inferred less directly from V ( P , T ) studies of Fe-free and Fe- and Al-bearing MgSiO 3 perovskites [Wang, Y., Weidner, D.J., Liebermann, R.C., Zhao, Y., 1994. P – V – T equation of state of (Mg,Fe)SiO 3 perovskite: constraints on composition of the lower mantle. Phys. Earth Planet. Inter. 83, 13–40; Mao, H.K., Hemley, R.J., Shu, J., Chen, L., Jephcoat, A.P., Wu, Y., Bassett, W.A., 1991. Effect of pressure, temperature and composition on the lattice parameters and density of (Mg,Fe) SiO 3 perovskite to 30 GPa. J. Geophys. Res. 91, 8069–8079; Zhang, Weidner, D., 1999. Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782–784]. The value of |(∂ G /∂ T )| P for ScAlO 3 is similar to those of most other mantle silicate phases but lower than the recent determination for MgSiO 3 perovskite [Sinelnikov, Y., Chen, G., Neuville, D.R., Vaughan, M.T., Liebermann, R.C., 1998. Ultrasonic shear wave velocities of MgSiO 3 perovskite at 8 GPa and 800K and lower mantle composition. Science 281, 677–679]. Combining the results from the previous studies and current measurements on ScAlO 3 perovskite, we extracted the parameters ( q and γ 0 ) needed to fully specify its Mie–Gruneisen–Debye equation-of-state. In this study, we have demonstrated that acoustic measurements of K S ( T ), unlike V ( P , T ) data, tightly constrain the value of q . It is concluded that ScAlO 3 has ‘normal’ γ 0 (∼1.3) and high q (∼3.6). The high value of q indicates that ScAlO 3 has very strong intrinsic temperature dependence of the bulk modulus; similar behaviour has been observed in measurements on Fe- and Al-bearing silicate perovskites (Mao et al., 1991; Zhang and Weidner, 1999).