MR22A-01 10:20h
Mineralogy of the Earth's lowermost mantle
A recent discovery of post-perovskite phase (MgPP) in pure MgSiO3 composition suggests that a primary lower mantle mineral of MgSiO3-rich perovskite (MgPv) undergoes structural phase transition near the base of the mantle. However, the effects of other major elements in the mantle such as iron and aluminum on the stability of MgPP are not known yet. In addition, knowledge of the element partitioning between MgPP and coexisting phases is also of great importance because it strongly affects the geophysical and geochemical properties of the lowermost mantle. Here we report the post-perovskite phase transition in a natural primitive mantle composition and the phase chemistry of the lowermost mantle by a combination of in-situ x-ray diffraction measurements in a laser-heated diamond anvil cell and chemical analyses on recovered samples using transmission electron microscope (TEM). Nine separate experiments were conducted at pressures from 38 to 126 GPa and temperatures form 1950 to 2550 K along the typical temperature profile in the lower mantle. Starting material was a gel with a chemical composition of KLB-1 peridotite, which has a primitive mantle composition. The sample was covered with a thin film of gold for both sides that served as an internal pressure standard and a laser absorber. It was loaded into a rhenium gasket, together with insulation layers of NaCl except one experiment at 126 GPa in which the sample was sandwiched by pure KLB-1 gel layers. Heating was achieved by a focused multimode continuous wave Nd:YAG laser using the double-sided heating technique. We confirmed three-phase assemblage of MgPv + (Mg,Fe)O magnesiowustite (Mw) + CaSiO3-rich perovskite (CaPv) up to 92 GPa. At higher pressures above 115 GPa, the mineral assemblage changed to MgPP + Mw + CaPv, and MgPv was not observed. Minor modification of perovskite structure in MgPv proposed by Shim et al., (2001) was not found. Mw retains a rocksalt structure throughout the P-T conditions in the present study. CaPv adopts cubic structure at high temperatures and a distortion to tetragonal structure of CaPv at room temperature was clearly recognized above 114 GPa. Chemical analyses of these coexisting phases show that distribution of iron significantly changes at the post-perovskite phase transition. The Fe-Mg partition coefficient between MgPP and Mw shows that iron partitions predominantly into Mw. This also shows that MgPP contains small amount of iron much less than MgPv. These results demonstrates that post-perovskite phase transition occurs at about 2500-km depth in the mantle (about 400-km above the core-mantle boundary) and that the primitive lowermost mantle consists of 72% iron-poor MgPP, 21% iron-rich Mw, and 7% CaPv. A strong partitioning of iron into Mw causes unique geophysical and geochemical properties such as viscosity, electrical conductivity or melting reaction in the lowermost mantle.
MR22A-02 10:35h
Post-Perovskite Double-Crossing, Partial Melting, and the Thermal Structure of Earth's D" Layer
The recently discovered post-perovskite (post-Pv) phase transition has been proposed to explain the occurrence of a seismic discontinuity at the top of D". Recent seismic migration techniques have revealed the presence of an even deeper discontinuity that accompanies the discontinuity at the top of D" beneath Eurasia and the Caribbean region. We show that both discontinuities can be explained by the post-Pv phase transition as the result of a double-crossing between the geotherm in the mantle's lower thermal boundary layer and the post-Pv phase boundary, consistent with current estimates for the phase diagram of MgSiO3 and higher estimates of core-mantle boundary (CMB) temperatures. This double-crossing model predicts that perovskite (Pv), rather than post-Pv, is stable in the lowermost D" layer, while post-Pv can only exist as a layer that does not extend to the CMB. Furthermore, the thickness of such a post-Pv layer will be greatest in cool regions of D", and can become entirely absent for high enough mantle temperatures. Thus the model can explain the lack of detection of a D" discontinuity in some regions as the result of a geotherm that is not cool enough to dip into the post-Pv stability field. Additionally, any partial melting of the lowermost D" layer takes place from the Pv stability field, and implies an intersection between the mantle solidus with the post-Pv phase boundary at outer core pressures. The proximity to this triple point provides a viable explanation for seismic and experimental inferences of dense melting that does not solely depend on partitioning of heavy elements into the melt phase. Using numerical models, we show that dense partial melting in a thermally convecting mantle can explain the observed features of ultra-low velocity zones, and combined with porous flow/segregation implies the existence of a silicate compaction boundary layer at the CMB interface that might explain a core-mantle transition zone, or "fuzzy CMB." Thus a simple model of a phase boundary double-crossing and dense partial melting can explain a wide variety of features in D". Finally, applying the model to the thermal boundary layer structure, current quantitative constraints for the post-Pv Clapeyron slope imply a minimum heat flux of 50 to 80 mW/$m^{2}$ where a D" discontinuity exists, and a global CMB heat flow on the order of 10 TW or greater.
http://geodyn.ess.ucla.edu/~hernlund/double-cross.html
MR22A-03 INVITED 10:50h
Electronic Transitions in (Mg,Fe)SiO$_3$ Perovskite and Post-perovskite phases: Implications for Deep-Earth Dynamics
It was recently reported that perovskite undergoes a crystallographic phase transition at D'' conditions (Murakami {\em et al.}) above 120 GPa. We measured the spin state of iron in magnesium silicate perovskite (Mg$_{0.9}$ Fe$_{0.1}$)SiO$_3$ at high pressure and found two electronic transitions occurring at 70 GPa and at 120 GPa, corresponding to partial and full electron pairing in iron, respectively. The pressure range of the first transition (70 GPa) is consistent with the depth (1700 km) at which lower mantle chemical heterogeneities have been proposed, as well as with the transition pressure for iron in magnesiow\"ustite, the other main iron-bearing mineral of the lower mantle. The second transition pressure (120 GPa, corresponding to a depth of 2600 km), at which the HS-LS transition is completed in perovskite, is consistent with that of the D" layer; at higher pressure (or below this depth) all lower-mantle minerals consist of LS iron. An important characteristic of this assemblage would be an increased radiative conductivity. The transitions should have a strong dynamical signature as inferred from geodynamical modeling, because an increase in thermal conductivity will result in a decrease of the Rayleigh number, and hence may hinder convection and favor layering. The proportion of iron in the low spin state thus grows with depth, increasing the transparency of the mantle in the infrared region, with a maximum at pressures consistent with the D" layer above the core-mantle boundary. Concerning the second transition, one can conjecture here that it is linked to, or could even be a driving force for, the crystallographic phase transformation. This claim is actually strengthened by our finding of a Clapeyron slope of 120 K.GPa$^{-1}$ for the transition, in excellent agreement with the results from two recent and independent theoretical studies of the crystallographic transition.
MR22A-04 INVITED 11:05h
OBSERVATIONS AND MODELING OF EARTH'S DEEP MANTLE BOUNDARY LAYERS
Traditional ideas regarding Earth's core-mantle interface as a simple division between solid silicate rock lower mantle and liquid iron alloy outer core material are significantly challenged by a broad range of recent discoveries. These include the recent evidence for a lower mantle phase change from perovskite (Pv) to a post-perovskite phase (PPv), lowermost mantle thermo-chemical layering at multiple scales, partial melting, seismic anisotropy from mineralogical texture or fabric development, and small-scale convection with whole mantle plume genesis. Recent seismological analyses have imaged strong topographical variability in the D" discontinuity: up to 100 km over short lateral scales. Published values for the PPv Clapeyron slope imply lateral temperature variations of 600-700 K to induce such topography, if due to a phase change alone. Thus a chemical component to D" and/or strong short scale circulation above D" may be required. Recent analyses of seismic wavespeed anisotropy in D" have similarly provided a richness of results. These include abundant evidence for SV waves slower and faster than SH, as well as coupling between the SH and SV components that implies azimuthal anisotropy. These findings can be fit with tilting of the anisotropy symmetry systems (e.g., the vertical axis of symmetry in vertical transverse isotropy) to varying degrees, as a result of deep mantle dynamics. In addition, more careful analysis of the vertical extent of anisotropy may be used to search for the proposed Pv-to-PPv phase change. As material passes through the phase change, preexisting anisotropic fabric is likely reset, and only after a finite degree of strain will a new anisotropic fabric be developed. One prediction is that if the D'' layer is indeed due to the PPV phase, the site at which material passes into should have a region of very little seismic anisotropy adjacent to (i.e., just below) the transition. Ultra-low velocity zones (ULVZ) may be isolated partially molten pockets at the CMB with significant iron enrichment to account for anomalously high density; while ULVZ layering is an order of magnitude thinner than the proposed PPv layer, better understanding the role of Fe in the PPv phase is paramount. It is also possible that ULVZ may be composed of another material all together such as CMB sediments or subducted crustal material. In this presentation we will summarize several seismological and geodynamical findings, especially as they relate to the PPv discovery.
MR22A-05 11:20h
Ferromagnesian Post-Perovskite Silicates in the D" Conditions
The D" boundary layer between the crystalline silicate mantle and molten iron core displays the largest contrast in physical and chemical properties of all the regions in the Earth, and undoubtedly plays a pivotal role in global dynamics and evolution. Recent high pressure-temperature {\it (P-T)} experiments and {\it ab-initio} theoretical calculations indicate that the pure end-member MgSiO$_{3}$ transforms to a post-perovskite (ppv) phase with the CaIrO$_{3}$ structure. Natural olivine with 12 mole percent Fe$_{2}$SiO$_{4}$ and synthetic orthopyroxenes with 20 and 40 percent FeSiO$_{3}$ were studied at high pressure-temperature to explore the possible incorporation of iron into ppv. The studies were conducted at Sector 13 (GSECARS) and 16 (HPCAT) of the Advanced Photon Source, Argonne National Laboratory. Samples were compressed in a diamond anvil cell and laser-heated. All samples were found to convert entirely or partially into the ppv structure, which was recently reported for pure MgSiO$_{3}$. With the liquid core as an unlimited reservoir of iron, core-mantle reactions could further enrich the iron content in this phase and explain the intriguing seismic signatures observed in the D" layer.
MR22A-06 11:35h
Strong chemistry dependence of the perovskite - post-perovskite phase transition boundary and of the post-perovskite structure
We show that MgSiO3 of representative mantle composition transforms to the post-perovskite structure at a noticeable lower pressure than pure MgSiO3. We also show that there is a further transition to a different post-perovskite than previously reported. Together, they indicate that minor-element chemistry, in particular the presence of Al, is highly influential on structure and stability of post-perovskite phases and puts constrains on rock composition and temperature profile in the Earth's lowermost mantle. We further present the first Raman spectrum of a post-perovskite phase which indicates a strong polarized bonding between the Mg- and the silicate-sublattice.
MR22A-07 11:50h
Crystal Chemistry and High P,T Behavior of the Post-Perovskite Phase of MgSiO$_{3}$
A new polymorph of MgSiO$_{3}$ more stable than the {\it Pbnm} perovskite phase has recently been identified at pressures and temperatures corresponding to the lowermost part of the Earth's mantle by both experimental and theoretical techniques [1,2]. The post-perovskite phase of MgSiO$_{3}$ has been identified as being isostructural with CaIrO$_{3}$. The post perovskite structure belongs to space group {\it Cmcm} and consists of layers of SiO$_{6}$ octahedra alternating with layers of Mg atoms in approximate 8-fold coordination. The SiO$_{6}$ octahedra share edges that form chains running parallel to the {\it a}-axis that are interconnected by corner-sharing apical oxygen atoms along the {\it c}-axis. Magnesium atoms are located between the SiO$_{6}$ layers at the centers of bicapped trigonal bipyramids sharing faces that run parallel to the {\it a}-axis. In this presentation we compare the crystal chemistry of this new phase with that of MgSiO$_{3}$ perovskite. In particular, we will discuss how the differences in the structure may influence trace element partitioning and transport properties. We will also present results from first principles calculations of the post perovskite phase carried out to high pressures (up to 180 GPa) and temperatures (up to 4000K) using a similar technique recently applied to high P,T calculations of MgSiO$_{3}$ perovskite [3]. Compression of the post perovskite structure is anisotropic with the {\it b}-axis (perpendicular to the layers) about 30% more compressible than {\it a} and {\it c}, which have very similar compressibilities. The major structural change with increasing pressure is the decrease of the Mg-O and Si-O bond lengths; the Si-O1-Si angle that connects the SiO$_{6}$ octahedra along [001] shows a very slight increase from 136.8$^{o}$ at 30 GPa (and 300K) to 138.8$^{o}$ at 180 GPa (and 300K). The effect of temperature on the structure appears to be inversely related to that of pressure. References: [1] Murakami M., Hirose K., Kawamura K., Sata N. Ohishi Y. (2004) Science, 855;[2] Oganov, A.R., Ono S. (2004) Nature, 445; [3] Wentzcovitch R.M., Karki B.B., Cococcioni M., de Gironcoli S. (2004) Phys.Rev. Lett. 92 018501.
MR22A-08 12:05h
Aluminum incorporation in post-perovskite from first principles
The phase transition of pure MgSiO$_3$ perovskite ({\em Pbnm}) structure to the post-perovskite ({\em Cmcm}) structure has been reported recently at pressures corresponding to the Earth's lowermost mantle. The resulting Clapeyron slopes suggest that the transition pressure may be very sensitive to the presence of impurities (Al, Fe, Ca); it is therefore necessary to determine whether this phase transition survives for realistic mantle compositions. We use ab initio calculations to investigate the effects of aluminum on the transition pressure. We examine three substitution mechanisms of Al in the two structures: Mg + Si $\rightarrow$ 2Al; 2Si + O $\rightarrow$ 2Al + O-vacancy; and Si $\rightarrow$ Al + H. Furthermore, we determine the solubility of aluminum in post-perovskite, and the partitioning of aluminum between the two phases at the phase boundary. Our static phase transition pressure of 106 GPa agrees well with previous studies. Preliminary calculations indicate that charge coupled substitution lowers the transition pressure by approximately 5 GPa.