Showing papers by "Donald J. Weidner published in 1998"
TL;DR: In this paper, single-crystal elastic properties of hydrous ringwoodite were reported for samples that were synthesized at 19 GPa and 1300°C, and the lattice parameter (a =8.0786±4 A) and elastic constants were determined from a single crystal with X-ray diffractometry and Brillouin scattering spectroscopy.
206 citations
TL;DR: In this article, the authors examined the impact of phase equilibria and equation of state measurements on defining the buoyancy associated with the 660 km discontinuity and concluded that there is no positive evidence for chemical-induced buoyancy across this discontinuity.
Abstract: The impact of phase equilibria and equation of state measurements on defining the buoyancy associated with the 660 km discontinuity is examined. Aluminum has a striking effect on the sequence of phase transitions expected at 660 km. With a pyrolite chemistry the sign of the phase transition contribution to buoyancy depends on temperature, with convection-assisting forces being victorious in hotter regions. Lower Al and/or temperature result in the perovskite-forming transformation resisting convection but with a much reduced magnitude compared with a pure olivine mantle. Density considerations suggest that there is no positive evidence for chemical-induced buoyancy across the 660 km discontinuity. The surface processing which forms a basaltic crust will cause a density contrast that assists convection in the transition zone but resists convection through the expanded garnet stability field of mid-ocean ridge basalt. The detailed seismic character of the 660 km depth region and its lateral variations hold the key to defining the location of the mantle state relative to the parameter space defined by Al and temperature in this study.
134 citations
TL;DR: An assumption of zero cross pressure and temperature derivatives for the elastic moduli underestimates the elastic anisotropy and overestimates the acoustic velocities of MgO at the extrapolated high-pressure and high-temperature conditions of Earth's mantle.
Abstract: The cross pressure (P) and temperature (T) dependence of the elastic moduli (Cij) of single-crystal samples of periclase (MgO) from acoustic wave travel times was measured with ultrasonic interferometry: partial differential2C11/ partial differentialP partial differentialT = (-1.3 +/- 0.4) x 10(-3) per kelvin; partial differential2C110/ partial differentialP partial differentialT = (1. 7 +/- 0.7) x 10(-3) per kelvin; and partial differential2C44/ partial differentialP partial differentialT = (-0.2 +/- 0.3) x 10(-3) per kelvin. The elastic anisotropy of MgO decreases with increasing pressure at ambient temperature, but then increases as temperature is increased at high pressure. An assumption of zero cross pressure and temperature derivatives for the elastic moduli underestimates the elastic anisotropy and overestimates the acoustic velocities of MgO at the extrapolated high-pressure and high-temperature conditions of Earth's mantle.
110 citations
TL;DR: In this paper, simultaneous sound velocity measurements and x-ray diffraction studies were made on wadsleyite (β-Mg2SiO4) to 7 gigapascals and 873 kelvin.
Abstract: Simultaneous sound velocity measurements and x-ray diffraction studies were made on wadsleyite (β-Mg2SiO4) to 7 gigapascals and 873 kelvin. The calculated adiabatic bulk ( K ) and shear ( G ) moduli yield K (at room conditions) = 172(2) gigapascals, dK / dP = 4.2(1), and dK / dT = −0.012(1) gigapascals per kelvin, and G (at room conditions) = 113(1) gigapascals, dG / dP = 1.5(1), and dG / dT = −0.017(1) gigapascals per kelvin, respectively. The data imply that the P and S wave velocity contrasts between olivine and wadsleyite require an olivine amount of 38 to 39 percent in the upper mantle to satisfy the observed 410-kilometer discontinuity, but 55 to 60 percent to account for the velocity increase through the transition zone.
108 citations
TL;DR: In this article, the x-ray diffraction peak broadening as a function of pressure, temperature and time was studied for olivine and its high pressure polymorphs, both dry and hydrous.
Abstract: Rheological properties of olivine and its high pressure polymorphs, both dry and hydrous, have been studied by monitoring x-ray diffraction peak broadening as a function of pressure, temperature and time. The measurements were carried out up to a pressure of 10 GPa and a temperature of 600°C for olivine and wadsleyite, and a pressure of 20 GPa and a temperature of 1000°C for ringwoodite. The observations indicate that olivine is much weaker than the other two minerals for this range of conditions, and furthermore, water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite. When the temperature increases from 25°C to 400°C the yield strength in the olivine drops by 39% without water and 62% with water, however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite.
108 citations
TL;DR: In this paper, the authors measured the acoustic velocities of brucite at room pressure in over 48 directions from Brillouin spectroscopy using a natural sample and supplemented with volume measurements as a function of pressure and temperature that range from ambient conditions to 11 GPa and 873 K.
Abstract: Acoustic velocities of brucite were measured at room pressure in over 48 directions from Brillouin spectroscopy using a natural sample. These data are supplemented with volume measurements as a function of pressure and temperature that range from ambient conditions to 11 GPa and 873 K using synchrotron X-ray radiation at the National Synchrotron Light Source (NSLS) in a cubic-anvil apparatus (SAM-85) with a synthetic polycrystalline sample. The diffraction patterns are collected during cooling cycles to minimize the effect of deviatoric stress on the measurements. These data yield internally consistent thermoelastic parameters defining the equation of state of brucite along with the single-crystal elastic moduli. The Brillouin spectroscopy measurements are best fit with the following elastic model: C11 5 156.7(8), C33 5 46.3(8), C44 5 21.7(5), C12 5 44.4(10), C13 5 12.0(15), and C14 5 0.2(8) GPa. The resultant linear compressibilities of the a and c axes are 3.8(1) 3 10 23 and 19.6(6) 3 10 23 (GPa 21 ), respectively, with the Reuss bound for the bulk modulus, KR 5 36.7(10) GPa and the Hill average, KH 5 46(1) GPa. The unitcell parameters (a, c, and volume) determined from the diffraction measurements were fit with a Birch-Murnaghan equation of state, yielding K0 5 39.6(14) Gpa, K 95 6.7(7), (]KT/ ]T)P 52 0.0114(16) GPa/K, and a5 5.0(7) 3 10 25 /K. The bulk modulus and linear compressibilities from X-ray diffraction are in agreement with those from Brillouin spectroscopy. The ratio of linear compressibility of the a to c axes is about five times at ambient conditions and reduces to almost unity by 10 GPa. The axial thermal expansions reflect a similar pressure dependence. The ambient shear anisotropy (C44/C66) is about 2.5.
100 citations
TL;DR: In this paper, two garnet samples along the pyrope majorite join (PynMj1−n) were measured at pressure and temperature conditions up to 11 GPa and 1163 K, respectively, with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil, DIA-6 type apparatus.
90 citations
TL;DR: The T-cup is a miniature 6-8 multianvil apparatus designed to fit in a 250 ton hydraulic press, and modified to provide x-ray access as discussed by the authors, which can reach internal pressures of 22 GPa, and can be heated to at least 1500°C.
Abstract: T-cup is a miniature 6-8 multianvil apparatus designed to fit in a 250 ton hydraulic press, and modified to provide x-ray access. When used with eight 10 cm tungsten carbide cubes with 2mm truncation, it has reached internal pressures of 22 GPa, and can be heated to at least 1500°C. It is a work in progress.
48 citations
TL;DR: In this paper, a modified Rietveld profile refinement program has been applied to refine the diffraction spectra of low symmetry and multiple phases observed in energy dispersive mode, and the P-V-T data were fitted to the high-T Birch-Murnaghan equation of state.
Abstract: We have conducted an in situ synchrotron x-ray diffraction study on a monoclinic pyroxene: CaMgSi2O6 diopside at simultaneous high pressures and high temperatures up to 8. 2 GPa and 1280 K. A modified Rietveld profile refinement program has been applied to refine the diffraction spectra of low symmetry and multiple phases observed in energy dispersive mode. Thermoelastic parameters for diopside CaMgSi2O6 are derived by fitting the P-V-T data to the high-T Birch-Murnaghan equation of state. We obtained isothermal bulk modulus KTO =109. 1 GPa with a pressure derivative of bulk modulus K'=∂K/∂P=4. 84, temperature derivative of bulk modulus K=∂K/∂T=-2. 05×10-2 GPa/K, volumetric thermal expansivity α=a+ bT with values of a=2. 32×10-5K-1 and b=1. 88×10-8K-2.
46 citations
Journal Article•
32 citations
TL;DR: In this article, high-precision crystal structure determination and refinement from powder X-ray diffraction data, determination of kinetic parameters and structure from time-resolved diffraction, and determination of absolute pressures by the combined use of ultrasonic techniques at high pressures and temperatures with simultaneous monitoring of X ray diffraction are presented.
Abstract: ▪ Abstract Increased access to multi-anvil high-pressure devices interfaced to synchrotron X-ray radiation sources has led to a new class of experiments. These new capabilities include (a) high-precision crystal structure determination and refinement from powder X-ray diffraction data; (b) the determination of kinetic parameters and structure from time-resolved diffraction data; (c) the determination of absolute pressures by the combined use of ultrasonic techniques at high pressures and temperatures with simultaneous monitoring of X-ray diffraction; and (d) the determination of the strength and rheological properties of materials through the monitoring of the relaxation of broadened diffraction peak widths in the presence of a well-characterized deviatoric stress field generated in the multi-anvil high-pressure apparatus.
TL;DR: In this paper, sound wave velocities and lattice parameters have been measured on samples of polycrystalline alumina (GE Lucalox and Coors 998) to simultaneous pressures of 9 GPa and 800 K.
Abstract: Ultrasonic techniques for multi-anvil high-pressure apparatus are adapted for use in a DIA-type cubic-anvil apparatus (SAM 85) and installed on the superconducing wiggler beamline (X17B) at the National Synchrotron Light Source of the Brookhaven National Laboratory. Sound wave velocities and lattice parameters have been measured on samples of polycrystalline alumina (GE Lucalox and Coors 998) to simultaneous pressures of 9 GPa and 800 K
TL;DR: In this paper, the imaging plate diffraction system at beamline X17B1 of the NSLS at Brookhaven National Laboratory for high pressure studies was used to observe the olivine-spinel phase transformation in fayalite.
Abstract: New developments of the imaging plate diffraction system at beamline X17B1 of the NSLS at Brookhaven National Laboratory for high pressure studies are reported here. Time resolved diffraction patterns can be recorded on an imaging plate by translating the plate in this system. This system was used to observe the olivine-spinel phase transformation in fayalite. The result demonstrates that the phase transformation relaxes stress stored in the sample. Some diffraction peaks of spinel phase, e. g. (400) and (440), were observed to appear prior to the others.
Journal Article•
TL;DR: The full set of single-crystal elastic moduli of forsterite and wadsleyite were measured at pressures of 3-16 GPa and 0-14 GPa, respectively.
Abstract: The full set of single-crystal elastic moduli of forsterite (α-Mg 2 SiO 4 ) and wadsleyite (β-Mg 2 SiO 4 ) were measured at pressures of 3-16 GPa and 0-14 GPa, respectively. For forsterite, the pressure derivatives of the bulk (K 0 's) and shear (G 0 ') modulus are 4.2 ± 0.2 and 1.4 ± 0.1, respectively. For wadsleyite, the corresponding pressure derivatives are 4.3 ± 0.2 and 1.4 ± 0.2. These values are much lower than those reported in earlier, low-pressure studies for both materials. At the pressure of the 410-km seismic discontinuity, the roomtemperature velocity increase across the α-β transition is 9.8% for compressional waves and 12.4% for shear waves. The results are consistent with an olivine fraction in the upper mantle of 30-50%.