Showing papers by "Donald J. Weidner published in 1999"

Thermal equation of state of aluminum-enriched silicate perovskite

Abstract: Inferences of the chemical homogeneity of Earth's mantle depend on comparing laboratory-derived equations of state of mantle phases with seismically determined properties of the material in situ. A uniform chemical composition of the entire mantle has been found to be consistent with measurements, to date, of these properties for the end-member MgSiO3 perovskite phase. New pressure-volume-temperature data for silicate perovskite containing 5 mole percent Al2O3 has yielded different values of the equation of state parameters, with the bulk modulus being significantly smaller at lower mantle conditions than for aluminum-free perovskite, thus requiring adjustments in other components to match seismic observations.

Testing plausible upper-mantle compositions using fine-scale models of the 410-km discontinuity

Abstract: We constructed models of the 410-km discontinuity, in which the shape and width of the velocity and density increase were constrained by mineral physics data on the α-β transition in (Mg,Fe)2SiO4. The transition was represented as cubic functions of depth, and its width was estimated to range between 8–24 km. Reflection coefficients were calculated for these models for competing estimates of the percentage of olivine in the mantle, using synthetic seismograms that include the finite-frequency effects of the distributed transition. Comparing the synthetic reflectivities with the average reflectivity observed in previous analyses of ScS reverberations, we conclude that the mantle composition is close to that of pyrolite (55 vol. % olivine). Furthermore, much of the range of reflectivity can be explained by temperature variation in a pyrolite mantle. An olivine-poor composition (35 vol. % olivine) marginally satisfies the seismic data only if the transition thickness averages less than 10 km.

Correction of diffraction optics and P-V-T determination using thermoelastic equations of state of multiple phases

Abstract: This report discusses a new method to determine experimental pressure–volume–temperature (P–V–T) data and to correct misalignment of diffraction optics using thermoelastic equations of state of multiple phases. Analytical approaches as well as numerical approaches are presented for the technique. A number of phase combinations yield the best experimental result and the whole-pattern Rietveld and/or Le Bail refinement of diffraction patterns of multiple phases is a significant aspect of the technique. Specific examples indicate that the corrected pressure, temperature and diffraction optics are robust. Errors are estimated from the uncertainties of the measured cell dimensions. The error propagation for different experimental cases is discussed, along with the conditions and limitations of the method. This technique for P–V–T determination and the correction of diffraction optics is crucial for experiments conducted under extreme conditions of high pressure, high temperature and dynamic reactions.