June K. Wicks
Email: wicks "at" caltech . edu
Office: 062b Arms
June K. WicksEmail: wicks "at" caltech . edu Office: 062b Arms |
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High pressure sound velocity measurements using NRIXS | A geodynamic and mineral physics model of a solid ULVZ |
| Nuclear Resonant
Inelastic X-ray Scattering (NRIXS) is a powerful
technique that gives us information about the vibrational properties of
a crystal lattice. We take advantage of the accessible nuclear resonant
transition of 57Fe,
probing how excitation of the nucleus is modified by available
vibrations (phonons). Using this method, we can measure properties like
the Debye sound velocity (think of it as an average sound speed
measurement for a crystal), the Lamb-Mössbauer factor (which
describes the probability of
a nucleus to undergo recoil-less absorption), the vibrational kinetic
energy, and more. Geologically, we care about sound speeds at high pressure because sound velocities of seismic waves that travel through the earth are the only direct information we have of the earth's interior. Direct comparisons of what we measure in lab to what we measure in seismometers gives us better insight of what's actually down there! My first project involved measuring the Debye sound velocities of iron-rich (Mg,Fe)O, a possible phase down at the core-mantle boundary. Using good estimates from similar studies for the density and bulk modulus of each pressure point, we can calculate the seismically relevant VP and VS. It turns out that the characteristic sound speeds of this dense oxide are low enough to potentially cause the low velocities in Ultra Low-Velocity Zones (ULVZs), which are these curious patches of slow mantle that sit above the core. |
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Here is a figure showing the calculated VP and VS
of our sample. We tracked the magnetic state at each pressure
using Synchrotron Mössbauer Spectroscopy, and found that the
observed velocity dips in both VS
and VP
directly precede magnetic transitions! These dips have been noted before in other studies. For example, Jacobsen2004 has shown that the shear wave velocities (VS) of iron-rich oxide decrease over at least the pressure range 0-10 GPa, and that their behavior is related to the cubic to rhombohedral distortion. Studies such as Crowhurst2008 show that the compressional wave velocities (VP) of (Mg,Fe)O decrease in the vicinity of the spin transition. |
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Here
is a figure comparing our iron-rich (Mg,Fe)O sample velocities to those
of the Preliminary Reference Earth Model (PREM) and of the ULVZs
(plotted at 135 GPa). PREM
is a one-dimensional average of earth's properties as a function of
depth, plotted here as a function of pressure. ULVZs tend to be described as having VP drops of ~5-20% and VS drops ~10-30% compared to PREM, but they vary. There is a good summary figure of the spread of velocities and thicknesses reported by seismology in Bower2011. In the black x's, I show the velocities predicted for this composition using the relations given in Jacobsen2002. The basic punchline of this figure is that the velocities of this material are ridiculously low compared to even ULVZs! We can imagine, then, that mixing a small amount of this material with ambient mantle can match the properties of a ULVZ. Or, maybe the oxide doesn't have be nearly as iron-rich as the sample we studied. We can explore these scenarios as thought exercises if we make (large) assumptions about the partitioning behavior of iron or about the high-temperature properties of these phases, which we try out in Bower2011. |
Relevant Publications |
| Wicks,
J. K., J. M. Jackson, and W.
Sturhahn (2010). Very low sound velocities in iron-rich
(Mg,Fe)O:
Implications for the core-mantle boundary region. Geophys Res. Lett.
37, L15304,
doi:10.1029/2010GL043689. (pdf
reprint) Sturhahn, W. and J. M. Jackson (2007). Geophysical applications of nuclear resonant scattering, in Ohtani, E., ed., Advances in High-Pressure Mineralogy: GSA Special Paper 421, 157-174, doi:10.1130/2007.2421(09). (pdf reprint) |
