Stress-induced proton disorder in hydrous ringwoodite |
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| Authors: | Monika Koch-Müller Sergio Speziale Fiorenza Deon Maria Mrosko Ulrich Schade |
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| Institution: | (1) Deutsches GeoForschungsZentrum Potsdam, Department 3, Telegrafenberg, 14473 Potsdam, Germany;(2) Helmholtz Zentrum BerlinGmbH, Albert-Einstein-Str. 15, 12489 Berlin, Germany |
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| Abstract: | We have measured in situ high-pressure IR absorption of synthetic hydrous (MgxFe1−x)2SiO4 ringwoodites (x = 0.00 to 0.61) up to a maximum pressure of 30 GPa. In our study, we combined the megabar-type diamond-anvil cell (DAC) with
conventional and synchrotron FTIR spectroscopy. The high-pressure measurements were performed in three different pressure-transmitting
environments: (1) CsI powder, (2) cryogenically loaded liquid argon, and (3) cryogenically loaded liquid argon annealed at
8.6 GPa at temperature of 120°C before further pressure increase. Between 10 and 12 GPa, all the samples loaded with methods
(1) and (2), independent on composition, showed a sudden disappearance of the prominent OH-stretching feature and simultaneous
discontinuities and/or changes in the pressure dependence of lattice vibrations compared with spectra of samples loaded with
method (3). In experiments performed with method (3) the OH-stretching vibrations as well as lattice vibrations could be observed
up to 30 GPa and their pressure behavior (dν/dP) can well be described by linear fits. Molecular vibrations, such as the OH stretching, are sensitive to non-hydrostatic
conditions, especially in minerals with highly symmetric structures. We interpret the disappearance of the OH bands using
methods (1) and (2) as a stress-induced proton disordering in hydrous ringwoodite. Our results confirm that argon pressure
medium produces strongly non-hydrostatic conditions comparable to CsI or KBr, if it is not thermally annealed at pressures
above 8 GPa. Our results suggest that the transition observed in hydrous Mg-ringwoodite end member is not present in compositions
containing Fe. By comparing the behavior of samples compressed in different environments, we suggest that sudden disappearance
of the OH-stretching band in hydrous ringwoodite could be driven by deterioration of the quasi-hydrostatic stress condition
instead of a pressure-induced effect. |
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