The CMB Topography Beneath Cook Inlet And Alaskan Kenai Peninsula
It has long been known that the PcP-to-P amplitude ratios demonstrate strong scatter in some regions. Rost and Revenaugh studied PcP amplitudes which sample the core-mantle boundary (CMB) beneath the Alaskan Kenai peninsula and the Cook inlet and found a ∼1° region on the CMB with very large PcP/P amplitude ratios. For some events, the ratios are several tens times larger than the theoretical ratios. After analyzing different possible mechanisms, they concluded the CMB topography is the major cause of the high amplitude ratios, but they did not give an quantitative topography model because of the lack of short period synthetic waveforms tools. We generate short period PcP synthetics using representation theorems and study the PcP phases theoretically for a core-mantle boundary (CMB) with single sinusoidal topography. After testing different combinations of the sinusoid wavelength L and amplitude H, we conclude that a dent in CMB with diameter of L=300km and height H=1∼2km best fits the observed data and can partly explain the high amplitude ratios. The PcP reflected from the CMB dip with L=300km and H=2km will be amplified by 2∼3 times which is smaller than the value the observed data needed. We primarily have three causes to determine L=300km and H=1∼2km. First, a dip with L=300km and H=1∼2km will focus the PcP significantly in a ∼1° region on the CMB which is consistent with the observed data. Certainly, the northern limit of the region is not clear, therefore more data are needed to constraint the northern limit and give a more reliable model. Second, there are not obvious travel time anomalies coupled with the high ratios in the observed PcP and our synthetic travel time anomalies are just about 0.4s too. Lastly, the strong similarities of P and PcP for some simple events excludes such large value of H. A dip with larger L and corresponding H surly produces stronger focusing effect, for example, a dip with L= 300km and H=3km will amplifies the PcP by 4∼5 times, but the waveform will be distorted seriously, contradictory to the similarity of P and PcP. So we speculate that the topography and other causes are combined to produce the high amplitude ratios together.
On the slab temperature in the deep lower mantle
Temperature of the subducted cold slab has been one of the important issues in the deep Earth dynamics because it gives thermal anomaly in the mantle. The numerical simulations can estimate the slab temperatures from a number of physical parameters of the slab. It is, however, very difficult to obtain a set of reliable parameters for the calculations. We will discuss the slab temperatures in the deep lower mantle by comparison of phase relations in cold subducted slabs with the seismic observations. In the hydrous peridotite system MgO-SiO2-H2O, seven high-pressure hydrous phases appear after serpentine dehydration (∼150-km depth). These hydrous phases carry water to the deep mantle condition. At the transition zone, a series of dehydration reactions will occur if the slab temperature is above 1300K. In the case of lower temperature, high-P hydrous phases will further carry water into the deep lower mantle. At about 1300-km depth, hydrous phase D will transfer water to high-pressure ice if the temperature is lower than 1300K. After the ice formation, no fluid-forming reaction may occur in the slab except at the core- mantle boundary where the temperature increase is expected. The depth distribution of dehydration reactions in the slab is well consistent with that of seismic event in the subduction zones if the appropriate temperature is assumed. This suggests that the deep-focus seismicity is induced by the dehydration reactions in the slab, and further suggests that the seismic event is an indicator of slab temperature and water transport into the deep mantle. CaSiO3-perovskite undergoes a structural phase transition from tetragonal to cubic symmetry at about 540 K, almost independent of pressure. This transition temperature significantly increases with increasing Al2O3 contents in Ca-perovskite. Unlike in peridotite systems, in a mid-oceanic ridge basalt (MORB) system, Ca-perovskite contains significant amounts of Al2O3 up to about 3 wt% where the structural phase transition may occur at about 1200 K. Seismological studies reported numbers of seismic scatters, reflectors, and low-V layers at a wide depth range (1100-1850-km depth) beneath Mariana subduction zone (e.g., Kaneshima and Helffrich, 2003). A key feature of these observations is a large drop in S-wave velocity without significant anomaly in P-wave velocity. The structural phase transition in Ca-perovskite may be ferroelastic type and therefore easily explain these abnormal seismic features. If these observations are indeed due to the structural phase transition in Ca-perovskite, the temperature of MORB crust of the slab at those depths should be around 1200K. If these seismic events are caused by the above phase relations, we can put constraints on the slab temperatures in the deep lower mantle. References, Kaneshima and Helffrich, 2003. JGR, 108, NO. B5, 2272, doi:10.1029/2001JB001596.
Ferric iron in Al-free and Al-rich (Mg,Fe)SiO3 post-perovskite in the deep lower mantle conditions
We have determined the Fe3+/ΣFe ratio in both Al-free and Al-rich (Mg,Fe)SiO3 post- perovskite based on the electron energy-loss near-edge structure (ELNES) spectroscopy measurements under the transmission electron microscope (TEM). These post-perovskite samples were synthesized at 113 to 187 GPa and 1830 to 3500 K in (Mg0.9Fe0.1)2SiO4, (Mg0.8Fe0.2)2SiO4, or natural mid-oceanic ridge basalt (MORB) bulk compositions by using laser-heated diamond-anvil cell (DAC) techniques. The results demonstrate that Al-free post-perovskite contains minor amounts of ferric iron with Fe3+/ΣFe ratios of 0.11 to 0.21 (Sinmyo et al. 2008 AmMin). These values are substantially lower than those of Al-rich post-perovskite (Fe3+/ΣFe = 0.59 to 0.69) formed in the MORB material (Sinmyo et al. 2006 GRL). The observed Fe3+-Al3+ relationship in post-perovskite is very similar to that previously reported in perovskite, suggesting that the Fe3+-Al3+ coupled substitution is an important mechanism for incorporation of ferric iron into post- perovskite, similarly to the case of perovskite. While the effect of oxygen fugacity on Fe3+/ΣFe ratio has been demonstrated to be very small for perovskite, it is not known for post-perovskite. The addition of iron metal, however, did not cause a decrease in Fe3+/ΣFe ratio in our post-perovskite sample. The Al-bearing post-perovskite in a natural pyrolitic mantle composition may contain certain amount of ferric iron, which affects the various physical properties in the lowermost mantle.
Thermodynamics properties of ferropericlase
The thermodynamics properties of ferropericlase (Mg(1-x)FexO, xFe ~ 0.19), have been investigated by first principles using a combination of newly developed techniques designed to address materials of such complexity. The strongly correlated nature of ferrous iron has been successfully addressed previously in static calculations by using a first principles LDA+U approach. However, investigation of thermodynamics properties of the solid solution presents further challenges, particularly the inclusion of vibrational effects without which results are not predictive. We have developed a vibrational virtual crystal model (VVCM) to address this issue. The acoustic velocities of the VVCM are, by construction, precisely the same as those of the real solid solution. We present here unusual anomalies on the thermodynamics properties caused by the spin crossover transition.
α to β to γ transformations in Mg2SiO4 and mantle discontinuities
Phase relations in Mg2SiO4 have been investigated by first principles quasiharmonic calculations. The computed phase boundaries obtained using the local density approximation (LDA) and the generalized gradient approximation (GGA) bracket the experimental ones, with LDA (GGA) calculations giving the lowest (highest) bound, while the Clapeyron slopes are in good agreement with the experimentally determined ones. This is the same trend displayed by previous similar computations of polymorphic phase boundaries. Further analyzes reveal that despite the uncertainties in phase boundary determination, the calculated discontinuities in density, bulk modulus, and bulk sound velocity are quite insensitive to pressure and have small uncertainties and useful accuracy to discriminate potential sources of discontinuities in the mantle. We verify that ∼3% density discontinuity at 410-km depth can be produced primarily by the α to β transition in an aggregate with pyrolite composition. However, the 1.3--2.9% density discontinuity observed in some places at 520-km depth cannot be accounted for solely by the β to γ transition and requires also changes in the coexisting pyroxene/garnet/Ca-perovskite system. Research supported by NSF/EAR 013533, 0230319, 0635990, and NSF/ITR 0428774 (VLab). Computations were performed at the Minnesota Supercomputing Institute.
2D and 3D Mantle Convection Models with Temperature- and Depth-dependent Thermal Expansivity
Most previous mantle convection models have employed either a constant coefficient of thermal expansion, α, or a depth-dependent α = α(z), where z is the vertical coordinate antiparallel to gravity. In this study, the effects of both temperature- and depth-dependent thermal expansivity in 2D cylindrical shell and 3D plane layer mantle convection models have been investigated. We consider α to have the form α(z,T) = αz(z) αT(T) where αz(z) increases with height z, (or decreases with depth) and αT(T) increases with temperature, T. We present results for cases with no plate, thin and thick, small and large plates. We find that the depth dependence and temperature dependence of α each have a significant, but opposite, effect on the mean surface heat flux (or Nusselt number) and the mean surface velocity of the convecting system. For α = αz(z), a decrease of α with depth by a factor of four across the mantle causes a decrease of surface heat flux and a decrease in mean surface velocity, relative to the constant-α case. However, when the temperature dependence of α is also included, αT(T) effectively compensates for the effects of αz(z) such that the predicted decrease in heat flow and surface velocity are either eliminated or, in some cases, become increases. Consequently, previous studies that include only the effects of depth dependence of α may underestimate surface heat flow and surface velocities by a significant amount.