I. The Heterogeneities at the Core-Mantle and Inner-Core Boundaries from PKP Phases. II. The Static and Dynamic Behavior of Silica at High Pressures

Author: Luo, Sheng-Nian

Year: 2003

Degree: Dissertation (Ph.D.)

Advisor: Clayton, Robert W.

Committee Members: Clayton, Robert W.; Helmberger, Donald V.; Ravichandran, Guruswami; Asimow, Paul David; Ahrens, Thomas J.

Option: Geophysics

DOI: 10.7907/D8HB-A439

Abstract

Waveform and differential travel-time (DTT) of various PKP phases have been utilized to study the velocity variations at the core-mantle boundary (CMB) and the inner-core boundary (ICB). The spatial concentration of events and stations, and the significant variations in PKPab-PKPdf DTT and waveform of PKPab, indicate localized sharp lateral variation of velocity at the CMB as supported by simulations. Modeling of DTT's among PKiKP, PKIKP and PKP-B-diffracted (Bdiff) phases, and waveform of Bdiff supports that the ratio of relative velocity variations of S- and P-wave at the CMB is larger than 2, and that hemispheric P-wave velocity variations exist at the top of the inner core, and that D' structure is related to the ICB via core dynamics.

The equation of state of stishovite is obtained by direct shock wave loading up to 235 GPa as K0T=306 ± 5 GPa and K0T' = 5.0 ± 0.2 where K0T is ambient bulk modulus and K0T' its pressure derivative. Phase diagram of silica (including melting curve) up to megabar pressure regime is established based on molecular dynamics (MD) simulations and dynamic and static experiments. Calculations show that perovskite is thermodynamically stable relative to the stishovite and periclase assemblage at lower mantle conditions. A detailed and quantitative examination is conducted on the thermodynamics and phase change mechanisms (including amorphization) that occur upon shock wave loading and unloading of silica. The systematics of maximum undercooling and superheating, are established by incorporating normalized energy barrier for nucleation and heating (cooling) rate, and validated at the atomic level with systematic MD simulations. By considering superheating in shock wave experiments, high-pressure melting curves for silica, alkali halides and transition metals are constructed based on the Lindemann law and the $ln2$ rule for the entropy of melting.

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