HR: 1340h
AN: DI53A-1103    [Abstracts]
TI: Cation Disorder in Ringwoodite and its Effects on Wave Speeds in the Earth's Transition Zone
AU: * Panero, W R
EM: panero.1@osu.edu
AF: School of Earth Sciences Ohio State University, 275 Mendenhall Labs 125 S Oval Mall, Columbus, OH 43210, United States
AB: The structure and stability of the Mg₂SiO₄ polymorphs play a key role in the density and impedance contrasts across the boundaries in the Earth's transition zone. Lateral variations in temperature in the transition zone lead to wave speed variations due to structural softening of the minerals. Spinel-structured Mg₂SiO₄, ringwoodite, also can contain significant amounts of thermally activated Mg-Si disorder. First-principles, density functional theory calculations on Mg₂SiO₄ -spinel, inverse spinel, and disordered structures provide constraints on the effects of cation disorder on the elasticity and stability of ringwoodite in the Earth's transition zone. Mg-Si disorder can be described with a regular solution parameter of 14 kJ/mol, leading to the prediction of ~4% Mg-Si disorder in the mantle's transition zone, where 200 K lateral temperature variations can cause approximately 1% change in disorder. Because the base of the transition zone is likely hotter than the laboratory synthesis temperature of most ringwoodite samples, such samples likely contain a greater degree of cation ordering than is found in the mantle. Independent of temperature and compression, 1% cation disorder causes a decrease in c₁₁ and c₄₄ of 0.65% and 0.4%, respectively, leading to cation-disorder induced variation in v_p and v_s with dv_s/dv_p=1.4. Measurements made below 1000 K on the effects of temperature on elastic properties are made with approximately constant levels of disorder due to the slow kinetics of cation ordering at such temperatures. Neglecting the effects of cation disorder then leads to a 20-50% underestimate of the effect of temperature on elastic constants and wave speeds, and thereby overestimating the impedance contrast across the wadsleyite to ringwoodite transition.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3904 Defects
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting