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Optical spectroscopy of the upper-mantle high-pressure minerals and their synthesized analoguses with using an automized single-beam optical microspectrometer

ABSTRACT

 

 

              Report on the SRW: 70 p., 30 figs., 5 tables, 68 references.

 

              The object of investigation – natural and synthesized high-pressure β- and γ- modifications of olivine (Mg, Fe)₂SiO₄.

The aim of the work is a study of the peculiarities of isomorphic substitution ofiron ions into the crystal structures of wadsleyite and ringwoodite.

The methods of investigations are optical absorption spectroscopy at various temperatures and pressures, Fourier-transform infraredspectroscopy (FTIR), Mössbauer spectroscopy and microprobe analysis.

 

By a complex of spectroscopic methods, predominantly by optical absorption and in a lesser extent by FTI Rand Mössbauer spectroscopies, the natural and synthesized high-pressure β- and γ- modifications of olivine (Mg, Fe)₂SiO₄, the minerals, which, in accordance with modern theories, are the main constituents of the Transition zone between the Upper and the Lower Mantles of the Earth (the depth between410 and 660 kilometers), were studied.Particularly, ringwoodite, γ- (Mg, Fe)₂SiO₄, is considered as the most probable constituent of the lower part of the Transition zone. Therefore, investigation of this mineral is important for understanding and forecasting of the geophysical characteristics of the Earth depths. It is established that hydrostatic compression up to ~13 GPa and the consequent decompression cause changes in optical spectra and, as a result, in the coloration of ringwoodite.That is an evident indicationthat a part of Fe²⁺ ions in its structure oxidizes to Fe³⁺. The spectra of both ringwoodite andwadsleyitechange at annealing of the samples at temperature up to 300 Cin air. The intensity of the spin-allowed absorption bands of Fe²⁺ decreases, whereas the intensity of the UV bands, caused by electronic O^2– → Fe³⁺ charge-transfer band and seen in the spectra as the intense UV-absorption edge, increases. These crystal chemical changes are seen as a weakening of blue (ringwoodite) and green (wadsleyite) coloration and a concomitant increase of yellowish tints. The effects of oxidation of Fe²⁺ to Fe³⁺ under decompression and annealing at relatively low temperatures might be the reason of disintegrationof these phases in processes of deep kimberlitic volcanism or at exhumation of ringwoodite- and wadsleyite-bearing rocks at plate tectonics. As the result, these minerals of the terrestrial originare not found so far.

For the investigations by the method of optical absorption spectroscopy we have constructed a single-beam microspectrometer, which allows to not only measure spectra of microscopic objects (grains of natural and synthesizes minerals, ringwoodite and wadsleyite, particularly) at ambient conditions, but to record their spectra at different temperature and pressure. The constructing of such equipment was an important component of the project.

Authors:

Taran Mykhailo Mykolaiovych

Tarashchan Arkadii Mykolaiovych

Khomenko Volodymyr Mykhailovych

Other authors:

Kulishova Ye.P., Manziuk L.P., Steshyn V.A., Tarashchan A.Ya.

Keywords:

Mantle minerals, ringwoodite, wadsleyite, spectroscopy, isomorphism, structural modifications of (Mg, Fe)₂SiO₄, optical, infrared and Mössbauer spectra, ions of iron, temperature and pressure

Head of the research:

Taran Mykhailo Mykolaiovych

Registration Card (RC):

0120U000193

Execution period of the research:

2020 - 2021