skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed phase regions in the Deep Lower Mantle?

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 per cent 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 one mole percent 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 lowermore » 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 mole percent or less), two (perched) mixed phase layers are found. This is the divariant analog to the univariant double-crosser of Hernlund, et al., 2005. 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.« less

Authors:
; ;
Publication Date:
Research Org.:
University of California , Santa Barbara, CA
Sponsoring Org.:
USDOE
OSTI Identifier:
887065
Report Number(s):
DOE/ER/15210-5
TRN: US200719%%200
DOE Contract Number:  
FG03-01ER15210
Resource Type:
Journal Article
Resource Relation:
Journal Name: Pysics of Earth and Planetary Interiors; Journal Volume: in press; Journal Issue: in press
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; FLUCTUATIONS; PEROVSKITE; SILICATES; THERMODYNAMICS; THICKNESS; TOPOGRAPHY; EARTH MANTLE; PHASE TRANSFORMATIONS; PHASE STUDIES; GEOCHEMISTRY; Tradeoffs in chemical variations; Tradeoffs in thermal variations; Post-perovskite phase transition; Mixed phase regions; Deep lower mantle

Citation Formats

Frank J Spera, David A. Yuen, and Grace Giles. Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed phase regions in the Deep Lower Mantle?. United States: N. p., 2007. Web.
Frank J Spera, David A. Yuen, & Grace Giles. Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed phase regions in the Deep Lower Mantle?. United States.
Frank J Spera, David A. Yuen, and Grace Giles. Sun . "Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed phase regions in the Deep Lower Mantle?". United States. doi:.
@article{osti_887065,
title = {Tradeoffs in Chemical and Thermal Variations in the Post-perovskite Phase Transition: Mixed phase regions in the Deep Lower Mantle?},
author = {Frank J Spera and David A. Yuen and Grace Giles},
abstractNote = {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 per cent 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 one mole percent 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 mole percent or less), two (perched) mixed phase layers are found. This is the divariant analog to the univariant double-crosser of Hernlund, et al., 2005. 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.},
doi = {},
journal = {Pysics of Earth and Planetary Interiors},
number = in press,
volume = in press,
place = {United States},
year = {Sun Apr 01 00:00:00 EDT 2007},
month = {Sun Apr 01 00:00:00 EDT 2007}
}