MR23A-01 INVITED
Mg-Fe partitioning between lower mantle minerals and their spin transitions
Recent experimental and theoretical studies revealed that there are spin transitions in the lower mantle minerals. Spin transition in ferropericlase occurs at 40-50 GPa at room temperature and broadening of the transition was reported at high temperature both theoretically and experimentally. Spin transition in perovskite is complicated due to existence of both ferrous and ferric iron in its structure. The spin transition pressures in perovskite varies in ferric and ferrous irons [1, 2, 3]; ferric iron shows a wide pressure interval of spin transition between about 60–160 GPa, whereas ferrous iron is in a high spin state at all mantle pressures. The theoretical studies revealed that ferric iron on the Mg-site in post-pervskite is in the high-spin state [2], and ferrous iron also remains in a high-spin state in post-perovskite at all mantle pressures [3]. These complex features of the spin transitions of the lower mantle minerals might affect the Mg-Fe partitioning behaviors of the lower mantle minerals. We have studied the Mg-Fe partitioning between ferropericlase and perovsksite and ferropericlasse, and post-perovskite and ferropericlase to the pressure of the base of the lower mantle at 2000 K . The Mg-Fe partition coefficient between Al-free perovskite and ferropericlse, KD (Pv/Fp) =(FeO/MgO) Pv /(FeO/MgO) Fp is around 0.25 at around 40-70 GPa and decreases to 0.1 at around 100 GPa. We observed a compositional dependence in KD(Pv/Fp) at 75 GPa, i.e., a gentle decrease of the partition coefficient with increasing bulk FeO content in the system, which is consistent with the compositional dependency reported by Katsura et al. at 23 GPa. The compositional and pressure dependencies of the partition coefficient of Al-free perovskite and ferropericlase are generally consistent with the partition coefficients reported by previous authors [4, 5, 6]. We also determined the Mg-Fe partition coefficient KD(PPv/Fp) between Al-free post-perovskite and ferropericlase at about 150 GPa and 2000 K. FeO is preferentially partitioned to ferropericlase, and KD (PPv/Fp) shows a strong compositional dependency, i.e., it decreases from about 0.7 to 0.1 with increasing bulk iron contents. The partitioning coefficient is consistent with the previous works by Kobayashi et al.[4], Murakami et al.[5], and Auzende et al. [6] by taking into account of the strong compositional dependency. The present observation of the pressure dependency of KD (Pv/Fp) is consistent with the spin transition in ferropericase occurring in the broad pressure range from 50-100 GPa at high temperature. On the other hand, the effect of the spin state in perovskite and post-perovskite phase on KD is not well understood due to the complex nature of the spin states in ferric and ferrous irons in these structures. [1] Li et al., 2006. Phys. Chem. Minerals. 33:575. [2] Zhang and Oganov, 2006. Earth Planet. Sci. Lett. 249, 436. [3] Stackhouse et al., 2007. Earth Planet. Sci. Lett. 253, 282. [4] Kobayashi et al., 2007. Earth Planet. Sci. Lett. 260, 564. [5] Murakami et al., 2005. Geophys. Res. Lett. 32, L03304, doi: 10.1029/2004GL021956. [6] Auzende et al., 2008. Earth Planet. Sci. Lett., 269, 164.
MR23A-02
Spin Crossover and Thermal Conductivity in the Earth's Lower Mantle
Changes in the electronic structure of iron at high pressures toward a spin-paired state in ferropericlase and silicate perovskite may directly influence the thermal conductivity of the lower mantle. The pressure dependence of optical absorption in ferropericlase, Mg(1-x)FexO (x=0.06-0.25), and silicate perovskite, Mg0.9Fe0.1SiO3, has been determined in the IR through UV regions up to 133 GPa. Whereas (Mg,Fe)O exhibits a strong pressure dependence of absorption and spectral changes associated with the high-spin (HS) to low-spin (LS) transition of Fe2+ (Goncharov et al., Science 312, 1205 (2006)), the pressure dependence of optical absorption in (Mg,Fe)SiO3 is relatively weak. We observe a moderate increase in absorption with pressure for (Mg,Fe)SiO3 in the visible and infrared spectral range due to a red-shift of absorption in ultraviolet, however the crystal-field transitions of Fe2+ become weaker with pressure and disappear above 50 GPa as a result of the HS-LS transition in (Mg,Fe)SiO3. The results on silicate perovskite show that optical absorption in the visible and near infrared spectral range is dominated by O- Fe3+ charge transfer and Fe3+- Fe2+ intervalence transitions, whereas a contribution from the Fe2+ crystal-field transitions is substantially smaller. The temperature dependence of the optical absorption of (Mg,Fe)O measured up to 65 GPa and 800 K is moderate below 30 GPa and weak above 30 GPa. The estimated total pressure dependent radiative conductivity (in approximation of a large grain size) is lower than expected from the pressure extrapolation of the ambient and low-pressure data. A new method has been developed to measure thermal diffusivity of mantle materials at high P-T using time-resolved radiometry combined with a pulsed-IR source. Here, the technique is tested on MgO to 32 GPa and Mg0.9Fe0.1SiO3 perovskite synthesized at 125 GPa. I thank B. Haugen, S. D. Jacobsen, V. V. Struzhkin, P. Beck, J. Montoya, J. Badro, D. Antonangeli, S. Kharlamova, R. Kundargi, P. Lazor, Z. Konopkova for contributing to this work. I acknowledge support of NSF-EAR-0721449, DOE/BES, DOE/NNSA (CDAC), and the W. M. Keck Foundation.
MR23A-03
Iron partitioning in pyrolite and its relevance to the spin transition in magnesiowustite under the lower mantle P-T conditions
The partition of iron between the major phases, magnesiowustite (Mw) and MgSiO3-perovskite (Pv), in pyrolite or peridotite compositions has been a matter of debate. Multianvil (MA) experiments demonstrated relatively high Mg-Fe partition coefficients between Mw and Pv of KD=0.6-0.8 at pressures 24-28 GPa, whereas substantially lower values of 0.4-0.5 have been reported by studies using laser heated diamond anvil cell, though the pressure ranges in the latter studies are far higher than those of MA experiments. We recently developed technique to produce pressures to ~80 GPa (at room temperature) and temperatures to ~2000K (at ~50 GPa) in MA using sintered diamond anvils. Using this technique, combined with synchrotron and other radiation sources, phase transitions and chemical composition changes in a pyrolite composition have been made at pressures up to ~50 GPa along a typical geotherm. On the basis of electron microprobe analysis of the recovered samples, we noted significant increase of KD with increasing pressure to ~30 GPa, suggesting the coupled substitution of Mg2+ and Si4+ by Fe3+ and Al3+, consistent with earlier studies. In contrast, the KD value was found to substantially decrease with pressure above ~40 GPa. EELS and Mossbauer measurements on some recovered samples suggest that the Fe3+/(Fe2++Fe3+) values in both Mw and Pv do not change in this pressure range: Iron in Mw is essentially of ferrous, while ~70% of iron in Pv is of ferric under these pressures. The relative enrichment of iron in Mw above ~40 GPa may be related to the electronic high- spin to low-spin transition in ferrous iron in Mw at these pressures, as suggested by some recent theoretical and experimental studies.
MR23A-04 INVITED
On the Possible (1D) Seismological Signature of the Spin Crossover in Ferropericlase.
Recent experimental estimates of the effect of spin crossover on the elasticity and density of ferropericlase at low temperatures show strong modulus weakening during the crossover. It is also clear that spin crossover does occur at high temperatures in ferropericlase with the pressure range of the transition broadened over that at low pressures. If modulus weakening occurs at the high temperatures of the lower mantle, we might expect a measureable seismological signal even if only ferropericlase (and not perovskite) exhibits anomalous elasticity. This is particularly true of the 1D radial structure of the lower mantle which is extremely well-determined by seismology. We begin by reviewing seismological constraints on the velocity and density of the lower mantle and conclude that 1D velocities are known to 0.1 to 0.2% and 1D density is known to 0.5%. Typically, experimental mineral physics data lead to precise estimates of density, less precise estimates of bulk sound speed and poor constraints on shear velocity. We therefore find that it is best to do the comparison of mineral physics models and seismology using bulk sound speed, though density can also be a useful discriminant. For a variety of reasonable compositions and with reasonable assumptions about the temperature behavior of the effect on elasticity of the spin crossover, we find that the predicted 1D profile of bulk sound speed of the lower mantle is significantly different from the seismological models. The effect on density is at the limits of seismological resolution. Our preliminary conclusion is that the effect on elasticity of the spin crossover at high temperatures must be much less than that observed at low temperatures.
MR23A-05
Implication of a high viscosity hill in the mid-lower Mantle on the transient dynamics of the subduction of Kula-Farallon plate shown by slab morphology in global tomography
Many global tomography models display a remarkably stable view of a complex morphology of the remnant slab after the subduction of the Kula-Farallon plates system below the North American Continent 60 million years ago. Tomography reveals a wide slab (order of 10000km) penetrating directly into the lower mantle without apparent interaction at all with the 660 km discontinuity. In the lower mantle, however, the same slab is characterized by a complex morphology composed by two different types of bending: 1) a horizontal flattening at depths in the range 1500km – 2000km. 2) a radial "cusp", or "spikey", shape at its center, more prominent at higher depth. While the presence of bending (2) has been exploited by Bunge and Grand (Nature, 2000) to constrain the paleo-tectonic position of the ridge separating Kula and Farallon plates during the Laramide orogeny 40 million years ago , the nature of the first bend and the links between first and second have not been explored in depth. We will show here how a viscosity peak at depths of about 2000 km or deeper can justify both observed bends (1) and (2). In order to study such system we employ a recently developed Multipole-accelerated Boundary Element Method (FMM-BEM) approach for modeling global scale geodynamics, which allow the simulation of a subducting plate at unprecedented spatial resolutions, 50 km in lateral resolution, never reached before in a global spherical frame (Morra et al, ICCS, 2007). This approach had been already exploited in order to show how a very wide (9000km) homogeneous plate will shorten at depths typical of the lower mantle when encountering a viscosity discontinuity at depths of the mid-lower mantle, displaying a bend of the type (1) described above and folding in the plate center (Morra et al, 2008). We present here, instead, a more realistic model in which the uniform plate is replaced with an analogue of the Kula-Farallon plates system, i.e. two plates divided by a ridge, subducting below a trench orthogonal to the ridge itself. We find that in this case the flattening of type (1) is accompanied by a "cusp", whose strong prominence is due to the intrinsic strength of the subducted slab. Our dynamical numerical models therefore go beyond associating a ridge with angular bend at depth. In fact we propose that the cusp is not a buoyancy feature but more a combination of plate rheology (weaker at the plate boundary) and lateral shortening of the lithospheric sheet caused by a combination of Earth's basic curvature, lack of space at depth and the presence of a viscosity peak at depths of 1800-2000km, as predicted in 1991 by Ricard and Wuming (1991), confirmed by Forte and Mitrovica (2001) and correlated in the pressure or depth with studies on the high-spin to low-spin transition of Fe ++ in periclase (Lin et al, 2004). Below are shown a sequence of snapshots showing the descending slab interacting with the high viscosity hill in the mid-lower mantle under North America in the past 60 million years.
MR23A-06
The Effect of Pressure and Spin on the Isotopic Composition of Ferrous Iron Dissolved in Periclase and Silicate Perovskite
The distribution of iron-isotopes between and within planetary bodies provides constraints on their histories of accretion and differentiation. Equilibrium isotopic signatures should become heavier with increasing pressure as bonds are compressed and vibrational frequencies increase. Because the ionic radius of low-spin iron is smaller than high-spin iron, iron-isotope signatures also may be made heavier by the spin transition for Fe2+, which, in ferropericlase, is predicted to occur within the Earth's mantle. We perform B3LYP density functional calculations of the equilibrium 57Fe/54Fe ratios for ferrous iron dissolved in periclase and MgSiO3 perovskite at the pressures and temperatures of the Earth's lower mantle. The coordination environments for iron are represented with autocompensated clusters terminated to conserve Pauling bond strength on the outer rind of oxygen atoms. The vibrational modes of the core atoms within the cavity that displace the iron center are assumed to approximate the vibrational modes that govern equilibrium isotopic fractionation in the crystal. Pressure increases the partitioning of 57Fe into both phases by a factor of three from the Earth's surface to the core-mantle boundary. In ferropericlase, a large contribution to this increase comes from the electronic transition from high-spin to low-spin iron. Our calculations do not indicate a spin-crossover for Fe2+ in ferroperovskite. Although heavy iron is partitioned into Fe-perovskite more strongly than Fe-periclase below the spin transition in Fe-periclase, at pressures above the spin transition, the equilibrium isotopic composition of the two phases should be approximately equal. Our calculations demonstrate that the spin transition can play an important role in determining planetary iron-isotope composition, potentially creating a reservior of isotopically heavy iron below the spin transition. Such a reservior could shield the heavy iron from dispersal during impact events, leaving larger planets with heavier iron isotope signatures.
MR23A-07
Elastic Wave Velocities of Lower Mantle Perovskite With Intermediate-spin Iron and Consequences for Mantle Properties and Dynamics
The recent discovery that the bulk of the lower mantle contains Fe2+ in the intermediate-spin (IS) state raises important issues with regard to modelling of lower mantle properties using mineral physics and seismological data. So far there are no elastic wave velocity measurements of lower mantle perovskite containing intermediate-spin iron, and the currently used experimental constraints from the MgSiO3 endmember are likely not realistic due to the inferred coupling between IS state stability and structure distortion combined with the large observed difference in anisotropic compression between MgSiO3 and (Mg,Fe)SiO3 perovskite. We therefore undertook a nuclear inelastic scattering (NIS) study of (Mg,Fe)SiO3 perovskite at beamline ID18 at the European Synchrotron Radiation Facility to address this problem. We collected room temperature NIS data for Mg0.88Fe0.12SiO3 perovskite at pressures from 0 to 115 GPa using a specially designed panoramic diamond anvil cell, and we made nuclear forward scattering measurements to confirm the spin state, and X-ray diffraction measurements for pressure calibration and to verify the structural state of the sample. We performed annealing using an infrared laser at low power after nearly all increases or decreases of pressure above 30 GPa to relieve stresses in the sample. Vp and Vs were calculated from the NIS data using the Debye model, and both show a marked deviation from the trend for the MgSiO3 endmember, which is likely related to iron spin crossover in the perovskite structure. We will present the implications for modelling of lower mantle compositions and temperatures, and discuss the consequences of the heterogeneous spin region for perovskite at the top of the lower mantle for dynamic mantle processes.
MR23A-08
The behavior of iron in (Mg,Fe)SiO3 post-perovskite at megabar pressures
Ferromagnesium silicate post-perovskite (PPv) is suggested to co-exist with CaSiO3 perovskite and (Mg,Fe)O in Earth's D" layer. The electronic charge and spin state of iron in these phases may influence its hosts' physical and chemical properties. We present measurements of the electronic environment of the iron sites in post-perovskite (PPv) structured (57Fe,Mg)SiO3, which have been measured in-situ at 1.12 and 1.19 Mbar at room temperature, using 57Fe synchrotron Mössbauer spectroscopy. Evaluation of the time spectra reveals two distinct iron sites, which are well distinguished by their hyperfine fields. The dominant site is consistent with an Fe3+-like site in a high spin state. The second site is characterized by a small negative isomer shift with respect to α-iron and no quadrupole splitting, consistent with a metallic iron phase which has disassociated from PPv. Our in-situ Mössbauer measurements and SEM/EDS analyses of the quenched assemblage strongly supports the presence of a metallic iron phase co-existing with a ferric-rich PPv. If such a reaction occurs at the core-mantle-boundary region within Earth, the resulting phase assemblage would have unique physical and chemical properties, consisting primarily of PPv, (Mg,Fe)O, CaSiO3 perovskite, α-PbO2-type structured SiO2, and a metallic iron phase.
MR23A-09
(Mg,Fe)SiO3 in the lower mantle: phase equilibria and stability, influence of spin on compressibility and elasticity
We studied Fe-bearing magnesium silicate perovskite and post-perovskite over a range of thermodynamic conditions corresponding to the Earth's mantle using computational (density-functional theory) techniques. We analyzed the crystallochemical effects induced by the presence of Fe in the structure and we monitored the compressibility as a function of Fe spin state and Fe content. At 0K in static calculations the high spin states are more stable over low spin states over the whole mantle pressure range for both perovskite and post-perovskite. We find that the compressibility of the structure is slightly anisotropic and depends on the spin state of Fe. The mechanism responsible for this is the tilt of octahedra that act as nearly rigid bodies. We observed that low-spin Fe2+ changes less the crystallochemical properties of MgSiO3 than the intermediate- or high-spin Fe2+ do. We derived the seismic properties of (Mg,Fe)SiO3 perovskite and post-perovskite as a function of Fe2+ spin state and Fe distribution and observed that the effects of Fe2+ spin state are rather small on the compressional seismic wave velocities and larger on the shear seismic wave velocities. However, these variations might not be easily observable in the seismic profiles of the mantle because of other factors, like temperature, spin transitions in magnesiowüstite or the presence of other minor elements in perovskite. During the calculation of elastic moduli from density function perturbation theory, we found several cases where intermediate compositions on the Mg-Fe solid solution are dynamically unstable. We discuss in detail the implications of such instabilities.
MR23A-10
Potential effects of the spin crossover transition in ferropericlase on mantle velocities
The thermoelastic properties of ferropericlase Mg1-xFexO (x = 0.1875) throughout the iron spin crossover have been investigated by first principles at Earth's lower mantle conditions. The transition has important consequences for the elasticity such as a substantial bulk modulus reduction. At room temperature the transition is quite sharp in pressure but broadens with increasing temperature. Along a typical geotherm the transition should occur across most of the lower mantle with a more significant bulk modulus reduction around 1400-1600 km depth. This crossover transition is yet another ingredient, in addition to changes in temperature, chemical composition, and mineralogy that can introduce velocity heterogeneities in the mantle. We compare predictions of the effect of this spin transition alone on the elastic properties of homogeneous aggregates with the elastic properties of the lower mantle extracted from seismic tomography and PREM. Research supported by NSF/EAR 0635990, and NSF/ITR 0428774 (VLab). Computations were performed at the Minnesota Supercomputing Institute.