HR: 1340h
AN: MR23B-0058    [Abstracts]
TI: Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed Phase Regions in the Deep Lower Mantle?
AU: * Giles, G F
EM: giles@umail.ucsb.edu
AF: Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106 United States
AU: Spera, F J
EM: spera@geol.ucsb.edu
AF: Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106 United States
AU: Yuen, D A
EM: davey@krissy.geo.umn.edu
AF: Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455
AB: The recent discovery of a phase-transition in Mg-rich perovskite (Pv) to a post-perovskite (pPv) phase at lower mantle depths and its relationship to D", lower mantle heterogeneity and iron content prompted an investigation of the relative importance of lower mantle (LM) compositional and temperature fluctuations in creating topographic undulations on mixed phase regions. Above the transition, Mg-rich Pv makes up ~70 percent by mass of the LM. Using results from experimental phase equilibria, first-principles computations and thermodynamic relations for Fe²⁺-Mg mixing in silicates, a preliminary thermodynamic model for the perovskite to post-perovskite phase transition in the divariant system MgSiO₃-FeSiO₃ is developed. Complexities associated with components Fe₂O₃ and Al₂O₃ and other phases (Ca-Pv, magnesiowustite) are neglected. The model predicts phase transition pressures are sensitive to the FeSiO₃ content of perovskite (~-1.5 GPa per one mole percent FeSiO₃). This leads to considerable topography along the top boundary of the mixed phase region. The Clapeyron slope for the Pv to pPv transition at X_FeSiO3=0.1 is +11 MPa/K about 20% higher than for pure Mg-Pv. Increasing bulk concentration of iron elevates the mixed (two-phase) layer above the core-mantle boundary (CMB); increasing temperature acts to push the mixed layer deeper into the LM into the D" thermal boundary layer resting upon the (CMB). For various LM geotherms and CMB temperatures, a single mixed layer of thickness ~300 km lies within the bottom 40% of the lower mantle. For low iron contents (X_FeSiO3 ~5 mole percent or less), two perched layers are found. This is the divariant analog to the univariant double-crosser. The hotter the mantle, the deeper the mixed phase layer; the more iron-rich the LM, the higher the mixed phase layer. In a hotter Hadean Earth with interior temperatures everywhere 200-500 K warmer pPv is not stable unless the LM bulk composition is Fe-enriched compared to the present upper mantle.
DE: 3919 Equations of state
DE: 3924 High-pressure behavior
DE: 3939 Physical thermodynamics
DE: 4465 Phase transitions
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005