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| Titel |
IR Absorption Coefficients for the Quantification of Water in Hydrous Ringwoodite |
| VerfasserIn |
Sylvia-Monique Thomas, Steven D. Jacobsen, Craig R. Bina, Joseph R. Smyth, Daniel J. Frost |
| 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 |
250044399
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| Zusammenfassung |
| Raman spectroscopy, combined with the ‘Comparator technique’ has been developed
to determine water contents ranging from a few wt ppm to wt% in glasses and
nominally anhydrous minerals including garnets, olivine, and SiO2 polymorphs
(Thomas et al. 2009). The routine is one promising example of quantification tools to
determine mineral specific molar absorption coefficients (É) for IR spectroscopy.
Mineral specific absorption coefficients are required because general IR calibrations
do not necessarily apply to minerals with water incorporated as hydroxyl point
defects.
Here we utilize the ‘Comparator technique’ to provide É-values for a set of synthetic
Fe-free (Fo100) and Fe-bearing (Fo90, Fo87, Fo83, Fo60) ringwoodites, as well as for
γ-Mg2GeO4. Ringwoodite is considered one of the major phases of the Earth’s
lower transition zone (520-660 km depth) and the knowledge of its absolute water
storage capacity is essential for modeling the Earth’s deep water cycle. Samples were
synthesized at variable P-T conditions in a multi-anvil press and cover a range of
OH contents. Single-crystals were characterized using X-ray diffraction and IR
spectroscopy. Mineral specific IR absorption coefficients were calculated from
independently determined water contents from Raman spectroscopy. Unpolarized IR
spectra of Mg-ringwoodite show broad absorption features in the OH region with
band maxima at ~2350, 2538, 3130, 3172, 3598 and 3688 cm-1. In the spectra of
Fe-bearing ringwoodite and γ-Mg2GeO4 the maxima of the main OH band are
shifted to 3244 cm-1 (Fo60) and 3207 cm-1, respectively. For Mg-ringwoodite
with the mean wavenumber (area-weighted average of the peak position) of 3170
cm-1 an É-value of 191500 ± 38300 L cm-2/ molH2O was determined. For the
ringwoodites with Fo90, Fo87 and Fo83 composition and the mean wavenumbers of 3229
cm-1, 3252 cm-1 and 3163 cm-1 values of 123600 ± 24700 L cm-2/ molH2O,
176300 ± 52900 L cm-2/ molH2O and 155000 ± 46500 L cm-2/ molH2O were
computed.
Our value for pure Mg-ringwoodite is in very good agreement with the value according to
Libowitzky & Rossman (1997) and the absorption coefficient proposed by Balan et al.
(2008), but is higher than the extrapolated value from Koch-Müller & Rhede (2010).
However, in case of the sample with Fo60 composition water content and É-value determined
here are in excellent agreement with those calculated by Koch-Müller & Rhede
(2010).
Here, we will further discuss general IR calibrations and the dependence of É on structure,
composition and frequency for the (Mg,Fe)2SiO4 polymorphs in the mantle. We agree with
the findings of Koch-Müller & Rhede (2010), which report that using the calibrations
according to Paterson (1982) and Libowitzky & Rossman (1997) leads to a water content
underestimation in case of Fe-rich (Fay-Fo60) samples. At this point this cannot be
generalized for Mg-rich ringwoodite.
References
Thomas et al. (2009), Phys. Chem. Mineral., 36, 489-509.
Libowitzky & Rossman (1997), Am. Mineral., 82, 1111-1115.
Koch-Müller & Rhede (2010), Am. Mineral., in press.
Paterson (1982), Bull. Mineral. (Paris), 105, 20-29. |
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