Abby Kavner

High‐pressure transformations in MgAl2O4

X ray diffraction and transmission electron microscopy on laser-heated diamond cell samples show that with increasing pressure MgAl2O4 spinel transforms first to Al2O3 corundum + MgO periclase, then to the CaFe2O4-structured phase, and finally to a new phase having the CaTi2O4 structure above ∼40 GPa. The CaFe2O4 and the CaTi2O4 structures are closely related and have almost the same densities and bulk moduli. Transformation from the CaFe2O4 to the CaTi2O4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle.

Melting Criteria and Imaging Spectroradiometry in Laser-Heated Diamond-Cell Experiments [and Discussion Comment]

As a step toward resolving current discrepancies among ultrahigh-pressure melting curves obtained with the laser-heated diamond cell, we critically evaluate two aspects of the experiments that require further examination: (i) the criteria used to detect that melting has taken place, and (ii) the methods employed for measuring spatially variable temperatures. A review of recent efforts illustrates how defining reliable melting criteria remains problematical in many experiments, whereas current and prospective advances in imaging spectroradiometry can yield robust methods for determining the temperature distribution within the laser-heated diamond cell.

Elasticity and strength of hydrous ringwoodite at high pressure

OH−-bearing (hydrous) ringwoodite compressed non-hydrostatically in a diamond anvil cell supports a differential stress that increases from 2.9 to 4.5 GPa over the pressure range of 6.7–13.2 GPa at room temperature. This result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart [Kaver and Duffy, Geophys. Res. Lett. 28 (2001) 2691–2694]. The elastic anisotropy (=2C44/(C11−C12) of hydrous ringwoodite is measured to be 0.87(7) throughout this pressure range, similar to measured values for anhydrous ringwoodite [Kaver and Duffy, Geophys. Res. Lett. 28 (2001) 2691–2694]. This lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure. These results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature, resulting in stronger preferred lattice orientation. This, in turn, may be seismically detectable, which opens the possibility of using seismic anisotropy as a marker for local volatile-containing areas within the upper mantle and transition zone.

Strength and elasticity of ringwoodite at upper mantle pressures

The differential stress supported by a natural iron-bearing ringwoodite was determined using energy-dispersive synchrotron x-ray diffraction in a radial geometry in a diamond anvil cell. The yield strength of ringwoodite was found to increase nearly linearly from 5.8(5) GPa at a pressure of 6.2 GPa to 10.2(9) GPa at a pressure of 27.0 GPa. When scaled by the shear modulus, silicate yield strengths are systematically higher than metals, oxides, and halides at pressures up to 30 GPa. For silicates, the yield strength is 5–7% of the shear modulus in this pressure range, whereas for the other classes of solids the yield strength is typically less than 2% of the shear modulus. The results show that ringwoodite has a small postive elastic anisotropy over this pressure range.

Phase stability and density of FeS at high pressures and temperatures: implications for the interior structure of Mars

The phase diagram of stoichiometric iron sulfide (FeS) was investigated at high pressures and temperatures (15^35 GPa, 1400^2200 K) with a laser-heated diamond anvil cell and synchrotron X-ray diffraction. The NiAs-structured polymorph of FeS is found to be stable within the P-T range of the Martian core. The density of FeS at 1600 K is measured to be 5.96 g/cm 3 at 17 GPa and 6.65 g/cm 3 at 35 GPa. The density measurements are used to evaluate structural models of Mars containing a core within the Fe^FeS system. The models that satisfy the geophysical constraints proscribe limits on the thickness of the Martian crust, the size and composition of the core, and the thickness of a perovskite-bearing layer close to the Martian core^mantle boundary. fl 2000 Elsevier Science B.V. All rights reserved.

Toward inexpensive superhard materials: tungsten tetraboride-based solid solutions.

To enhance the hardness of tungsten tetraboride (WB(4)), a notable lower cost member of the late transition-metal borides, we have synthesized and characterized solid solutions of this material with tantalum (Ta), manganese (Mn), and chromium (Cr). Various concentrations of these transition-metal elements, ranging from 0.0 to 50.0 at. %, on a metals basis, were made. Arc melting was used to synthesize these refractory compounds from the pure elements. Elemental and phase purity of the samples were examined using energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), and microindentation was utilized to measure the Vickers hardness under applied loads of 0.49-4.9 N. XRD results indicate that the solubility limit is below 10 at. % for Cr and below 20 at. % for Mn, while Ta is soluble in WB(4) above 20 at. %. Optimized Vickers hardness values of 52.8 ± 2.2, 53.7 ± 1.8, and 53.5 ± 1.9 GPa were achieved, under an applied load of 0.49 N, when ~2.0, 4.0, and 10.0 at. % Ta, Mn, and Cr were added to WB(4) on a metals basis, respectively. Motivated by these results, ternary solid solutions of WB(4) were produced, keeping the concentration of Ta in WB(4) fixed at 2.0 at. % and varying the concentration of Mn or Cr. This led to hardness values of 55.8 ± 2.3 and 57.3 ± 1.9 GPa (under a load of 0.49 N) for the combinations W(0.94)Ta(0.02)Mn(0.04)B(4) and W(0.93)Ta(0.02)Cr(0.05)B(4), respectively. In situ high-pressure XRD measurements collected up to ~65 GPa generated a bulk modulus of 335 ± 3 GPa for the hardest WB(4) solid solution, W(0.93)Ta(0.02)Cr(0.05)B(4), and showed suppression of a pressure-induced phase transition previously observed in pure WB(4).

Pressure–volume–temperature paths in the laser-heated diamond anvil cell

The temperature, pressure, and stress conditions in the diamond anvil cell sample chamber before, during, and after laser heating are mapped by employing standard materials as in situ pressure markers. Unit cell volumes of Pt, MgO, and NaCl were monitored by synchrotron-based x-ray diffraction at temperatures between 300 and 2290 K and pressures ranging from 14 to 53 GPa. To aid in interpreting the resulting pressure–volume–temperature paths, we perform a series of model calculations of the high-temperature, high-pressure x-ray diffraction behavior of platinum subjected to a general stress state. Thermal pressure and thermal expansion effects within the laser-heated volume are observed but are not sufficient to fully explain the measured paths. Large apparent pressure changes can also result from relaxation of deviatoric stresses during heating and partial reintroduction of those stresses during quench. Deviatoric stresses, monitored from both diffraction peak widths and lattice parameter shifts as a func...

Ultralow viscosity of carbonate melts at high pressures

Knowledge of the occurrence and mobility of carbonate-rich melts in the Earths mantle is important for understanding the deep carbon cycle and related geochemical and geophysical processes. However, our understanding of the mobility of carbonate-rich melts remains poor. Here we report viscosities of carbonate melts up to 6.2 GPa using a newly developed technique of ultrafast synchrotron X-ray imaging. These carbonate melts display ultralow viscosities, much lower than previously thought, in the range of 0.006-0.010 Pa s, which are ~2 to 3 orders of magnitude lower than those of basaltic melts in the upper mantle. As a result, the mobility of carbonate melts (defined as the ratio of melt-solid density contrast to melt viscosity) is ~2 to 3 orders of magnitude higher than that of basaltic melts. Such high mobility has significant influence on several magmatic processes, such as fast melt migration and effective melt extraction beneath mid-ocean ridges.

High-pressure melting curve of platinum

The melting curve of platinum, determined to 70 GPa by spectroradiometry and visual observation through the laser-heated diamond cell, is described by Tm(P)=2057+27.2×P−0.1497×P2 K, where Tm is the melting temperature in K and P is pressure in GPa. This expression is valid to a precision of ±97 K in the pressure range 10 to 70 GPa.

Elasticity of natural majorite and ringwoodite from the catherwood meteorite

Sound velocities and elastic moduli of natural polycrystalline majorite (Mg0.78Fe0.21Ca0.01)SiO3, and ringwoodite (Mg0.75Fe0.25)2SiO4 from the Catherwood meteorite were measured by Brillouin spectroscopy. These are the first acoustic measurements for such natural high-pressure phases with Fe contents comparable to that of the Earths mantle. The adiabatic bulk modulus of majorite is Ks=164(4) GPa, and the shear modulus is μ=87(2) GPa, which are typical values for garnet-structured silicates. Both the elastic moduli and the sound velocities of natural cubic majorite are slightly lower than values for aluminosilicate garnets with comparable Fe contents. Moreover, the bulk and shear moduli of natural majorite are indistinguishable from those of pure Mg majorite, and the dependence of these moduli on Fe content is small. In contrast, ringwoodite (Ks=193(3), μ=113(2)) exhibits a pronounced change in the aggregate elastic moduli and sound velocities with increasing Fe content. The addition of 10% Fe to γ-phase changes the velocity by an amount comparable to the velocity change across the β→γ transition in Mg2SiO4. It is therefore necessary to account for the effect of Fe in constructing mineralogical models of the transition zone.