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| Titel |
Structural phase transition in wadsleyite - a MID/FAR-Infrared Study |
| VerfasserIn |
Maria Mrosko, Monika Koch-Müller, Ulrich Schade |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250045304
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| Zusammenfassung |
| The Earth‘s Upper Mantle mainly consists of (Mg,Fe)2SiO4-polymorphs. The discontinuity
in seismic velocity at app. 410 km depth is caused by the transformation of olivine to
wadsleyite (α- to β-(Mg,Fe)2SiO4). Both phases can incorporate a certain amount of water
as hydroxyl groups and it is known that water incorporation strongly affects physical
properties such as density and bulk modulus (Mao et al. 2008).
We synthesised hydrous and anhydrous wadsleyite (Mg2SiO4) in a multi-anvil-press and
measured IR-spectra on polycrystalline thin films in a diamond anvil cell.
The relatively easily accessible measurements in the MID-IR-region show the
pressure-depending behaviour of the quite incompressible SiO4-tetrahedra. However these
bands only display slight changes with increasing pressure. Therefore we developed a
THz/FIR microscope adapted to a Bruker IFS 66/v spectrometer at the BESSY Synchrotron
IR-beamline IRIS (Berlin, Adlershof). It was built to conduct in situ pressure depending
measurements in diamond anvil cells in the FAR-IR-region, where the metal-cation-involved
vibrations take place.
On the basis of the compressional behaviour of these bands we observed changes, indicating
a phase transition. These changes occur at slightly different pressures comparing hydrous (8 –
9 GPa) and anhydrous (around 10 GPa) samples.
Chopelas (1991) measured Raman spectra on wadsleyite and found a structural phase
transition at 9.2 GPa. Cynn and Hofmeister (1994) detected changes in the slope of
FAR-IR-bands around 10 GPa in hydrous β-(Mg,Fe)2SiO4.
References:
Chopelas, A. (1991) Thermal properties of β-Mg2SiO4 at mantle pressures derived from
vibrational spectroscopy: implications for the mantle at 400 km depth. Journal of
Geophysical Research, B7, 96.
Cynn, H., Hofmeister, A. M. (1994) High-pressure IR spectra of lattice modes and
OH vibrations in Fe-bearing wadsleyite. Journal of Geophysical Research, B9,
99.
Mao, Z., Jacobsen, S. D., Jiang, F., Smyth, J. R., Holl, C. M., Duffy, T. S. (2008) Elasticity of
hydrous wadsleyite to 12 GPa: Implications for Earth’s transition zone, Geophysical Research
Letters, 35, L21305. |
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