Tradeoffs in chemical and thermal variations in the post-perovskite phase transition: Mixed phase regions in the deep lower mantle? (original) (raw)
2006, Physics of the Earth and Planetary Interiors
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, Mgrich Pv makes up ∼70% by mass of the lower mantle. Using results from experimental phase equilibria, first-principles computations and empirical scaling relations for Fe 2+ -Mg mixing in silicates, a preliminary thermodynamic model for the Pv to pPv phase transition in the divariant system MgSiO 3 -FeSiO 3 is developed. Complexities associated with components Fe 2 O 3 and Al 2 O 3 and other phases (Ca-Pv, magnesiowustite) are neglected. The model predicts phase transition pressures are sensitive to the FeSiO 3 content of perovskite (∼ −1.5 GPa per 1 mol% FeSiO 3 ). This leads to considerable topography along the top boundary of the mixed phase region. The Clapeyron slope for the Pv → pPv transition at X FeSiO 3 = 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 (X FeSiO3 ∼ 5 mol% or less), two (perched) mixed phase layers are found. This is the divariant analog to the univariant double-crosser of . 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.
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Structure, mineralogy and dynamics of the lowermost mantle
Mineralogy and Petrology, 2010
The 2004-discovery of the post-perovskite transition initiated a vigorous effort in high-pressure, high-temperature mineralogy and mineral physics, seismology and geodynamics aimed at an improved understanding of the structure and dynamics of the D"-zone. The phase transitions in basaltic and peridotitic lithologies under pT-conditions of the lowermost mantle can explain a series of previously enigmatic seismic discontinuities. Some of the other seismic properties of the lowermost mantle are also consistent with the changes in physical properties related to the perovskite (pv) to post-perovskite (ppv) transition. After more than 25 years of seismic tomography, the lowermost mantle structure involving the sub-Pacific and sub-African Large Low Shear-Velocity Provinces (LLSVPs) has become a robust feature. The two large antipodal LLSVPs are surrounded by wide zones of high Vs under the regions characterized by Mesozoic to recent subduction. The D" is further characterized by a negative correlation between shear and bulk sound velocity which could be partly related to an uneven distribution of pv and ppv. Ppv has higher VS and lower \( V_{\Phi } \) (bulk sound speed) than pv and may be present in thicker layers in the colder regions of D". Seismic observations and geodynamic modelling indicate relatively steep and sharp boundaries of the 200-500 km thick LLSVPs. These features, as well as independent evidence for their long-term stability, indicate that they are intrinsically denser than the surrounding mantle. Mineral physics data demonstrate that basaltic lithologies are denser than peridotite throughout the lowermost mantle and undergo incremental densification due to the pvppv- transition at slightly shallower levels than peridotite. The density contrasts may facilitate the partial separation and accumulation of basaltic patches and slivers at the margins of the thermochemical piles (LLSVPs). The slopes of these relatively steep margins towards the adjacent horizontal core-mantle boundary (CMB) constitute a curved (concave) thermal boundary layer, favourable for the episodic generation of large mantle plumes. Reconstruction of the original positions of large igneous provinces formed during the last 300 Ma, using a paleomagnetic global reference frame, indicates that nearly all of them erupted above the margins of the LLSVPs. Fe/Mg-partitioning between pv, ppv and ferropericlase (fp) is important for the phase and density relations of the lower mantle. Electronic spin transition of Fe2+ and Fe3+ in the different phases may influence the Fe/Mg-partitioning and the radiative thermal conductivity in the lowermost mantle. The experimental determination of the \( {K_D}{^{Fe/Mg}_{pv/fp}}\left[ { = {{\left( {Fe/Mg} \right)}_{pv}}/{{\left( {Fe/Mg} \right)}_{fp}}} \right] \) and \( {K_D}{^{Fe/Mg}_{ppv/fp}} \) is technologically challenging. Most studies have found a \( {K_D}{^{Fe/Mg}_{pv/fp}} \) of 0.1-0.3 and a higher Fe/Mg-ratio in ppv than in pv. The experimental temperature is important, with the partitioning approaching unity with increasing temperature. Although charge-coupled substitutions of the trivalent cations Al and Fe3+ seem to be important in both pv and ppv (especially in basaltic compositions), the complicating crystal-chemistry effects of these cations are not fully clarified. The two anti-podal thermochemical piles as well as the thin ultra-low velocity zones next to the CMB may represent geochemically enriched reservoirs that have remained largely isolated from the convecting mantle through a major part of Earth history. The existence of such “hidden” reservoirs have previously been suggested in order to account for the imbalance between the inferred composition of the geochemically accessible convecting mantle and the observed heat flow from the Earth and chondritic models for the bulk Earth.
Earth and Planetary Science Letters, 2005
In situ observations of the perovskite-CaIrO 3 phase transition in MgSiO 3 and in pyrolitic compositions were carried out using a laser-heated diamond anvil cell interfaced with a synchrotron radiation source. For pure MgSiO 3 , the phase boundary between the orthorhombic Mg-perovskite and CaIrO 3 -type phases in the temperature range of 1300-3100 K was determined to be P (GPa) = 130 (F 3) + 0.0070 (F 0.0030) Â (T À 2500) (K) using platinum as a pressure calibrant. We confirmed that the CaIrO 3 -type phase remained stable up to pressures of at least 156 GPa and temperatures of 2600 K. The consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the CaIrO 3 -type phase. The phase change from an orthorhombic Mgperovskite to a CaIrO 3 -type bearing assemblage in a pyrolitic mantle composition was also observed at P = 125 GPa, which corresponds to the same mantle depth as the seismic discontinuity. The phase boundary between the orthorhombic Mgperovskite and CaIrO 3 -type bearing assemblage was determined to be P (GPa) = 124 (F 4) + 0.008 (F 0.005) Â (T À 2500) (K) using gold as a pressure calibrant. This transition boundary indicates that the temperature at a depth of 2700 km is about 2600 K, and the adiabatic temperature gradient in the lower mantle is estimated to be 0.31 K/km. The partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic MgSiO 3 perovskite and CaIrO 3 -type MgSiO 3 were estimated from ab initio calculations. Our experimental and theoretical results indicate that the DW layer consists of a CaIrO 3 -type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the Earth's mantle. D
Geophysical Research Letters, 2005
We report here new data on pressure dependence of Fe-Mg partitioning between (Mg, Fe)SiO 3 perovskite (Pv) and magnesiowüstite (Mw), K Pv/Mw , and (Mg, Fe)SiO 3 postperovskite (PPv) and Mw, K PPv/Mw , up to 123.6 GPa at 1600 K measured by synchrotron X-ray diffraction method and analytical transmission electron microscopy (ATEM). We observed a high FeO content in PPv coexisting with Mw [K PPv/Mw = (FeO/MgO) PPv /(FeO/MgO) Mw = 0.30] compared to that in Pv [K Pv/Mw = (FeO/MgO) Pv /(FeO/ MgO) Mw = 0.12] observed from 23.0 to 95.4 GPa. K Pv/Mw keeps a constant value of 0.12 up to the PPv phase boundary. Our results also support the possibility that a metallic phase may form in the lower mantle. The assemblage of PPv and Mw is 1.5-1.7% denser than the Pv bearing assemblage, which results in a gravitational stabilization of the lowermost mantle.
Discovery of Post-Perovskite and New Views on the Core-Mantle Boundary Region
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A phase transition of MgSiO 3 perovskite, the most abundant component of the lower mantle, to a higher-pressure form called post-perovskite was recently discovered for pressure and temperature conditions in the vicinity of the Earth's core-mantle boundary. This discovery has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle called the D" region. Several major seismological characteristics of the D" region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. Here we discuss the current understanding of the core-mantle boundary region.
Influence of the post-perovskite transition on thermal and thermo-chemical mantle convection
Geophysical Monograph Series, 2007
Since the discovery of the post-perovskite (PPV) transition, several studies have focused on its possible effect on the dynamics of the deep mantle, as well as the complex seismological structures that may arise through the interplay of variations in temperature, composition and the PPV phase transition. Various constraints indicate that the most likely explanation of deep-mantle structure involves a combination of all three. Here these issues are explored using numerical models of thermal convection then thermo-chemical convection in various geometries including a three-dimensional spherical shell, followed by a discussion of structures that may arise when a more realistic, composition-dependent manifestation of the PPV transition is considered. A zero-, single-or doublecrossing of the PPV phase boundary is observed depending on the temperatures of the CMB and deep mantle, and this evolves with time as the core and mantle cool. The post-perovskite transition has a minor but measurable effect on the dynamics and mantle temperature, acting to mildly destabilize the lower boundary layer and slightly increase mantle temperature, consistent with other modeling studies for Earth and Mars. This appears to hold regardless of model geometry. However, the influence of the PPV transition depends on its depth relative to the thermal boundary layer thickness, and is typically overestimated using a fixed depth parameterization of the transition. If piles of dense subducted MORB accumulate above the CMB and the PPV transition is independent of composition, then there is an anticorrelation between regions with a thick PPV layer and hot dense piles, but with a composition-dependent PPV transition this can change. Lateral variations in the occurrence of PPV are the dominant contributor to long-wavelength lateral shear-wave velocity heterogeneity in the deepest mantle, although this depends on some uncertain scaling parameters. The different contributions to seismic heterogeneity have different spectral slopes: temperature has a "red" lateral temperature spectrum, the spectrum of composition is relatively flat, while the contribution of PPV variations is in between these slopes. Theoretical considerations suggest that when compositional effects on the stability of PPV are taken into account, a large potential variety of complex behavior could occur, generating structures such as discontinuities, gaps or holes, and multiple (i.e., >2) crossings. Such effects, as well as more realistic variations in viscosity and other physical properties, must be addressed in future modeling efforts coupled to seismological investigations.
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