doi:10.1016/j.pepi.2006.07.007
Copyright © 2006 Elsevier B.V. All rights reserved.
Tradeoffs in chemical and thermal variations in the post-perovskite phase transition: Mixed phase regions in the deep lower mantle?
References and further reading may be available for this article. To view references and further reading you must
purchase this article.
Frank J. Speraa, b, 
, 
, David A. Yuenc and Grace Gilesa
aDepartment of Earth Science, University of California, Santa Barbara, CA 93106, United States
bInstitute for Crustal Studies, University of California, Santa Barbara, CA 93106, United States
cDepartment of Geology and Geophysics and Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455, United States
Received 12 April 2006;
revised 7 July 2006;
accepted 26 July 2006.
Available online 27 September 2006.
Abstract
The 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 compositional and temperature fluctuations in creating topographic undulations on mixed phase regions. Above the transition, Mg-rich Pv makes up 
70% by mass of the lower mantle. Using results from experimental phase equilibria, first-principles computations and empirical scaling relations for Fe2+–Mg mixing in silicates, a preliminary thermodynamic model for the Pv to pPv 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 1 mol% FeSiO3). This leads to considerable topography along the top boundary of the mixed phase region. The Clapeyron slope for the Pv → pPv transition at XFeSiO3=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 in the lower mantle perhaps into the D″ thermal-compositional boundary layer resting upon the CMB. For various lower mantle 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 (XFeSiO3 5 mol% or less), two (perched) mixed phase layers are found. This is the divariant analog to the univariant double-crosser of Hernlund et al., 2005 [Hernlund, J., Thomas, C., Tackley, P.J., 2005. A doubling of the post-perovskite phase boundary and structure of the Earth's lowermost mantle. Nature 434, 882–886.]. The hotter the mantle, the deeper the mixed phase layer; the more iron-rich the lower mantle, the shallower the mixed phase layer. In a younger and hotter Hadean Earth with interior temperatures everywhere 200–500 K warmer, pPv is not stable unless the lower mantle bulk composition is Fe-enriched compared to the present-day upper mantle. The interplay of temperature and Fe-content of the lower mantle has important implications for lower mantle dynamics.
Keywords: Tradeoffs in chemical variations; Tradeoffs in thermal variations; Post-perovskite phase transition; Mixed phase regions; Deep lower mantle
Fig. 1. Stability diagram showing the effects of fluctuations in temperature (δT) and composition (δXFeSiO3) on the topography of the phase boundary (i.e., top or bottom of mixed phase region). Upper panel shows case where thermal perturbations are dominant. This leads to upwelling lower mantle. The phase boundary is depressed because the Clapeyron slope of the Pv → pPv is +10 MPa/K. If the inversion pressure increases with increasing Fe then the boundary is further depressed. If the pressure–composition derivative is <0 then the phase boundary is less depressed. Lower panel shows case when fluctuations in composition Fe/Mg ratio dominate the local buoyancy. These are regions of downwelling. If the pressure–composition derivative is positive, the phase boundary is depressed. However, for negative ∂p/∂X, the boundary may be elevated because increasing the Fe content lowers the phase transition pressure along an isotherm.
Fig. 2. Energy–momentum equation feedbacks in divariant mixed phase equilibria and mantle flow. Buoyancy is a function of bulk composition, phase abundance and temperature. Reaction liberates enthalpy, a heat source. Viscosity depends on phase abundance, temperature and pressure as does isobaric heat capacity and thermal conductivity.
Fig. 3. Schematic isothermal pressure–Composition (P–X) section illustrating divariant phase relations in system MgSiO3–FeSiO3. At some pressure indicated by the dashed horizontal line and for any bulk composition between the vertical dashed lines, two phases (Pv + pPv) are present. The abundance depends on the bulk composition. The pressure–composition derivative defining the top of a mixed phase region (at low pressure) is less steep than that for the Pv-out boundary. These thermodynamic derivatives are useful in estimating mixed phase region topography and are calculated from data in Table 1 (see text).
Fig. 4. The composition-derivative of the phase transition pressure in terms of the mole fraction of FeSiO3 component dissolved in Mg-rich perovskite (Pv) vs. the mole fraction of FeSiO3 component in Pv at T = 2500 K, 3500 K and 4000 K.
Fig. 5. Isothermal P–X sections showing the region of mixed phases as a function of P, T and bulk composition: (a) 2500 K; (b) 3500 K; (c) 4000 K.
Fig. 6. Schematic depiction of the thickness and height above the CMB of the mixed (Pv + pPv) phase layer on the 2500 K, 3500 K and 4000 K isotherms for lower mantle bulk compositions of 0.1 and 0.2 mole fraction Fe/(Fe + Mg). The effect of increasing temperature at fixed bulk composition is to make the mixed phase layer thinner and deeper. The effect of increasing the iron content at fixed temperature is to make the layer shallower and thicker. The modal amount of pPv increases monotonically (but not linearly) with increasing depth.
Fig. 7. Phase relations in the lower mantle for non-isothermal conditions. CMB temperature is 4400 K, thickness of D″ thermal boundary layer is 190 km. Hot (3400 K adiabat) and cold (2400 K adiabat) geotherms are shown. The adiabat outside the mixed phase region is 0.1 K/km in the lower mantle. In the mixed phase region, the geotherm becomes steeper due to the release of enthalpy for the exothermic reaction Pv → pPv using parameters of the thermochemical model in Table 1.
Fig. 8. Thickness and height above the CMB for the 2400 K and 3400 K adiabat with fixed CMB temperature of 4400 K as a function of the lower mantle bulk composition molar ratio Fe/Fe + Mg. In the 3400 adiabat case at XFesiO3=0.1, the top and bottom of the mixed phase layers lies 400 km and 200 km above the CMB, respectively. For XFeSiO3 less than 2 mol% there is no mixed layer and Pv is a stable at the CMB. For a cooler mantle (2400 K adiabat), two mixed phase regions can develop when Fe/Fe + Mg < 5 mol%. The top of the mixed phase region moves to substantially greater hights above the CMB as the iron content increases. For XFeSiO3=0.2, the top of the mixed phase region for a cool mantle reaches 700–900 km above the CMB with a thickness approaching 500 km.
Table 1.
Corresponding author. Tel.: +1 8058934880; fax: +1 8058932314.
Abstract
Purchase PDF (408 K)
E-mail Article
Add to my Quick Links
Cited By in Scopus (7)
Purchase PDF (767 K)
