The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences (BES), of the U.S. Ultralow Velocity Zones (ULVZs) are small structures at the base of the mantle characterized by sound speeds up to 30% lower than those of surrounding cloak
Ultralow-Velocity Zones
Numerous seismic studies of the core–mantle boundary indicate that the ULVZ distribution is patchy and sometimes associated with margins or the interior of large, low shear rate provinces ( McNamara et al., 2010 ). Recent work on the equations of state for fayalite Fe2SiO4 fluid and subsequent analysis shows that partial melting of chondrite or peridotite fluid is likely not gravitationally stable at the base of the mantle (Thomas et al., 2012).
Thesis Overview
Nuclear Resonant Inelastic X-Ray Scattering (NRIXS)
Synchrotron M¨ ossbauer Spectroscopy (SMS)
Synchrotron M¨ossbauer spectroscopy measures the Rayleigh scattering of excited resonant nuclei, where a superposition of different emitted energies reflecting nuclear level splitting can be measured using detectors as a function of time. In synchrotron M¨ossbauer spectroscopy, isomer shift is determined by measuring a sample twice, with and without a reference absorber in the beam.
Experiments and Data Evaluation
The lower limit of the energy range is chosen to avoid errors arising from subtracting the elastic contributions and is mainly determined by the energy bandwidth of the X-rays. Solid lines show the best fit of the empirical function f(E), as explained in the text.
Results
At 121 GPa, the fast oscillations, hence the magnetic hyperfine fields, disappear in the time spectrum (Figure 2.6), and are consistent with the onset of a spin transition to a low-spin state of the Fe 3d electron configuration. Such a behavior is consistent with a transition to a low spin state, as similar observations have been reported for iron-poor (Mg,Fe)O in the vicinity of a spin transition (Crowhurst et al., 2008).
Ultralow-Velocity Zones
The iron-rich pockets could represent a remnant of fractional crystallization of a primordial magma ocean (Labrosse et al., 2007). Another direct result of this study is the transition pressure of the paramagnetic to antiferromagnetic transition, also known as the N´eel transition (e.g. Fujii et al., 2011). Experimental investigations of the structural transition find that an increasing FeO component corresponds to a decrease in the transition pressure (Lin et al., 2003; Shu et al., 1998b).
The transition pressure is found to be very sensitive to hydrostaticity Shu et al. (1998a), and some studies found no.
X-ray Diffraction
We will describe a synchrotron M¨ossbauer (SMS) study of (Mg0.06Fe0.94)O in the pressure range 8 to 52 GPa by in situ X-ray diffraction. Next, we will describe nuclear resonance inelastic X-ray scattering (NRIXS) measurements in the pressure range 0 to 80 GPa, also obtained by in-situ SMS and X-ray diffraction. See Duffy and Wang (1998 ) for a discussion on performing high pressure and temperature XRD experiments at the synchrotron.
Nevertheless, in-situ XRD is required for accurate sound speed determinations from NRIXS measurements, as the pressure gradients (and therefore the uncertainty in the absence of in-situ XRD) can reach up to 10 GPa above 100 µm.
Experimental Details
NRIXS with in-situ XRD
Consequently, typical XRD exposures of oxides in a diamond anvil image cell require 10 minutes at low pressure and up to 30 minutes at high pressure, while at custom beamlines for XRD at high pressure (e.g. 13-ID-D, APS) exposure is ~1 minute up to 2 minutes. At the highest compression point (#6,~82 GPa), the energy scan was measured again in October 2012 with an extended energy range.
SMS with in-situ XRD
One reflection is sufficient to determine the volume of the unit cell for cubic symmetry, which both KCl and (Mg0.06Fe0.94)O are at low pressure. At high pressure, we can estimate the rhombohedral distortion, assuming that the rhombohedral distortion is only a function of pressure. The resulting d-spacings and pressures of KCl and (Mg0.06Fe0.94)O are summarized in Table 3.2. Mg0.06Fe0.94)O d200 are interpreted as cubic volumes or as rhombohedral volumes.
When two numbers are given, the ruby pressure corresponds to before and after the SMS and XRD measurement.
Results
XRD: Isothermal Equation of State
Subscripts C and R indicate volumes assuming d-mw94 spacing corresponds to cubic (200) or rhombohedral (012) reflection. The pressure was determined from the KCl equation of state (Dewaele et al., 2012), and the results are summarized in Table 3.3. In the first fit, the sample volumes were interpreted as cubic given the measured d200 (Table 3.2).
In the third pass, the room temperature rhombohedral quench data from our high pressure and.
SMS: Magnetic Ordering Transition
Green circles are volumes of rhombohedral "quench" points after each heating cycle of (Mg0.06Fe0.94)O in the buffered experiments, described in Section 4.6 and summarized in Table 4.6. As expected, there is no evidence of a spin transition in the pressure range investigated by this study.
NRIXS: Sound Velocities
The listed pressures are determined from the combined equation of state for (Mg0.06Fe0.94)O, summarized in table 3.3. In both cases, the zero-energy limit of the scaled PDOS is the Debye velocity (VD). Several of the PDOS can be included if an empirical function is used instead (blue line).
The volumes of FeO were measured at Sector 12.2.2 of the Advanced Light Source (low density) and at Sector 13-ID-D of the APS (high density). Mg0.16Fe0.84)O is not included in this figure as its volume was not measured in situ at either Sector 3-ID-B or Sector 13-ID-D.
Previous Studies
In the previous two chapters, we performed nuclear resonance spectroscopy and X-ray diffraction on iron-rich O (Mg, Fe) at 300 K and reported properties as a function of pressure and composition. This study will allow us to explore the phase diagram and density of iron-rich (Mg,Fe)O at pressures and temperatures approaching those of the Earth's core-mantle boundary region. Fischer et al., 2011b, e.g.). Non-stoichiometric, iron-rich (Mg,Fe)O studies show that both KSandKT decrease as a function of Fe concentration (Jacobsen et al., 2002; Richet et al., 1989), where KS is determined from measurements of directly of volume, composition, and of VP and VS using ultrasonic interferometry, and KT is determined in a P-V compression study.
In this study, we aim to measure the P−V −T equation of state (Mg0.06Fe0.94)O, first to constrain the thermoelasticity of rich iron (Mg,Fe)O and second to see if these trends apply to the iron-rich equation of state (Mg,Fe)O.
Experimental Details
The trend is opposite for stoichiometric samples, where ultrasound interferometry studies for iron-poor samples show a positive trend for KS with increasing iron content (Jacobsen et al., 2002). The pressures listed were determined by the equation of state for hcp-Fe (Dewaele et al., 2006; Murphy et al., 2011). The B2-NaCl thermal equation of state of Fei et al. (2007b) was used to determine the pressure in the unbuffered experiment and to compare this data set with the buffered experiment.
The difference between the two pressure scales is small, with a resulting pressure increase of 0.01 to 0.4 GPa for the values of Murphy et al. (2011).
Results
Phase Identification
To determine temperature and error in our measurements, we took the mean and standard deviation of several temperature measurements, which are known to have a precision of 100 K (Shen et al., 2001). In the buffer experiment, the discrepancy between measured upstream and downstream temperatures and the sharp diffraction peaks showing no temperature gradient made it clear that we could not assume a Gaussian distribution. In order not to place regret constraints on the assumed temperature distribution, we used a flat distribution in the error propagation.
The phase identifications presented in Figure 4.2 are consistent with previous results in that there is no detectable B8-structured (Mg0.06Fe0.94)O in the pressure and temperature range studied (Kondo et al., 2004). .
Equations of State
The range of fitted K0 includes that predicted for near-stoichiometric (Mg,Fe)O (Jacobsen et al., 2002). The limited pressure range (30 to 70 GPa) of the unbuffered data set was difficult to fit without external constraints, so we fixed ∂K/∂T to that of the buffered data set. Open circles: pre-experiment volumes at pressures determined by the equation of state of B1-NaCl (JCPDS 5-0628).
In both cases, ∂K/∂T of the unbuffered data set is fixed to that of the buffered one.
Discussion
Effect of Buffering on Equation of State
The unbuffered (squares, solid lines) and buffered (circles, dashed lines) data sets are in good agreement where they overlap under pressure–temperature conditions. If this were true in our case, it would also explain the similarity between the buffered and unbuffered data sets at high pressure and temperature.
Effect of Composition on the Thermal Equation of State of (Mg,Fe)O
Dotted lines show areas of extrapolated curves, and the error bars shown are the 1σ error in the thermal expansion coefficient. In Figure 4.7 we plot the thermal expansion of various members of the (Mg,Fe)O solid solution as a function of pressure, at 1900 K. At ambient pressure, the thermal expansion of MgO is (Mg0.64Fe0.36 )O , and FeO vary as a function of composition.
Within our experimental uncertainties, we cannot resolve the effect of composition on the thermal expansion of (Mg,Fe)O at high pressures.
- Extrapolation of Magnesiow¨ ustite properties to the CMB
- Mixture of Magnesiow¨ ustite and Ambient Mantle
- Mixture of Magnesiow¨ ustite and Silicate Perovskite
- Dynamics of a Solid-state ULVZ
When combined with a geodynamic model of a solid ULVZ ( Bower et al., 2011 ), we can directly correlate inferred sound speeds with mineralogy and predicted ULVZ shapes. Previous versions of this model were published in Wicks et al. 2011), and mixes iron-rich oxide with surrounding lower mantle represented by the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981). We then calculate the Voigt and Reuss bounds for VP, VS and density (Watt et al., 1976) for a.
Another mixture model whose earlier version is published in Bower et al. 2011), again starts with the assumption that iron-rich oxide is present in a ULVZ and explores a possible equilibrium.
Conclusions
Jeanloz (1987), Temperature measurements in a laser-heated diamond cell, High Pressure Research in Mineral Physics, p. Zhao (2007), Seismological constraints on ultralow velocity regions in the lowermost mantle due to core-reflected waves, Phys. Bando (2005), O and Si solubility in liquid iron in equilibrium with (Mg,Fe)SiO3 perovskite and light elements in the core, Geophys.
In June 2012, re-measurement of the sample composition revealed the presence of Ti contamination in the sample, which was previously missed.
Fe 1−x O
Microprobe analysis of the synthesized material gives a sample composition of (Mg.0580(9)Fe.9420(9)Si.0021(9))O, assuming the oxygen is stoichiometric. A conventional M¨ossbauer spectrum (Figure 2.3) taken of the synthesized sample is consistent with divalent iron and shows no indication of Fe3+. A secondary electron image (Figure A.1) taken of the sample before microprobe analysis shows no indication of compositional heterogeneities.
Additional Thermodynamic Parameters
Sound Velocities of FeO from Nuclear Resonant Inelastic X-ray Scattering
- Summary of experiments presented in this thesis
- Summary of pressure, density, Debye sound velocity (V D ), and compressional (V P ) and
- Details of the (Mg 0.06 Fe 0.94 )O NRIXS experiment
- XRD results of the (Mg 0.06 Fe 0.94 )O SMS experiment
- Equations of state fit to the (Mg 0.06 Fe 0.94 )O dataset
- Debye Velocity (V D ) of (Mg 0.06 Fe 0.94 )O as a function of in-situ density
- Summary of sound velocities of (Mg 0.06 Fe 0.94 )O as a function of pressure
- Pressure-volume-temperature data for the buffered experiment
- Pressure-volume-temperature data for the unbuffered experiment
- Equation of state parameters using B2-NaCl as a pressure marker, with K 0T fixed to
- Pressure-volume data for the buffered experiment at 300 K
- Model parameters for mixing model “Mw+PREM”
- Model parameters for mixing model “Mw+Pv”
There is a marked difference in the distribution of vibrations between the ambient pressure spectrum and the high pressure spectrum, which is also reflected in the calculated sound velocities. The Debye sound speeds determined from the low-energy region of these PDOS are shown in Figure 3.12.
