Experimental Study On The Electrical Conductivity Of Mantle Minerals At High Temperatures And High Pressures

Electrical conductivity is an important geophysical parameter. Measurements of the electrical conductivity of minerals and rocks provide a powerful tool for probing temperature, chemical composition, mineralogy, oxygen fugacity, water content of the Earth's interior, and help geomagnetists to constrain the magnetotelluric results obtained from the inversion of field data. In this thesis, FeTiO3 ilmenite, periclase, enstatite and orthopyroxene were selected to be our research objects. Based on the electrical conductivity experiments under high-temperature and high-pressure, we discuss the Arrhenius parameter, conduction mechanism, influence factor of the electrical conductivity of mantle's minerals and rocks, and also construct a conductivity-deep model in the Earth mantle derived from the present data. The main results are summarized as follows.1. The electrical conductivity of synthetic FeTiO3 ilmenite was measured under the conditions of 8-16 GPa, 300-600 K and the fO2 of Fe-FeO with a Kawai-type multianvil high-pressure apparatus. The present experimental results show that the electrical conductivity of FeTiO3 ilmenite increases greatly with temperature but increases slightly with pressure. The low activation energy of 0.21 eV, the small negative activation volume of -0.22 cm3/mol, a weak pressure effect and theoretical calculation of polaron radius are consistent with conduction by hopping of small polaron, suggested to be an intervalence charge transfer Fe2+ + Ti4+→Fe3+ + Ti3+. On the basis of our results, some abnormal phenomena were well explained for the electrical conductivity of a structural analogue (Mg0.93, Fe0.07)SiO3 ilmenite at high temperature and high pressure.2. Under the conditions of lower mantle (20 GPa and temperature up to 1700 K) and the fO2 of Fe-FeO, we have measured the electrical conductivity of sintered polycrystalline periclase using a Kawai-type multianvil high-pressure apparatus. Arrhenius plot of the electrical conductivity shows two linear regions, implying a change of charge transport mechanism with temperature in this experiment. At low temperatures region (<700 K), the small activation energy of 0.2 eV is compatible with small polaron conduction. At higher temperature region (700-1700 K),both the relatively high activation energy (1.18 eV) and the theoretically calculated mobility of charge carrier suggest the high-temperature conduction is attributed to a large polaron process with magnesium vacancy trapping hole on oxygen. For the first time, a new large polaron model was proposed for periclase. In comparison with the electrical conductivity of (Mg,Fe)O magnesiowüstite with different iron content, dependence of the chemical composition on the electrical conductivity of magnesiowüstite was also discussed in this study.3. The electrical conductivity of (Mg0.9Fe0.1)SiO3 enstatite was measured for the first time under physical conditions of the mantle transition zone (10-20 GPa, 750-1600 K) and the fO2 of Fe-FeO in a Kawai-type multianvil high-pressure apparatus. The experimental results demonstrate the effect of pressure on the electrical conductivity of enstatite is weak, and small polaron is the dominant mechanism in the high temperature regions. On the other hand, both the small activation enthalpies and water content determined in the recovered sample imply that proton is in charge of the low temperature regions. It is also found that pressure induced phase transition from enstatite to ringwoodite under pressure of 20 GPa by X-ray diffraction, and the electrical conductivity of hydrous ringwoodite is in good agreement with previous data.4. For the first time, we investigated systematically the effect of Al2O3 content on the electrical conductivities of orthopyroxene [(Mg,Fe,Al)(Si,Al)O3: XFe = 0.1] at the conditions of the upper mantle (3 GPa and temperature up to 1800 K), frequency range from 10-1 to 106 Hz and the fO2 of Mo-MoO2 using complex Impedance Spectroscopy. The Fe3+ content of samples determined from M?ssbauer spectra both before and after complex impedance measurements linearly increases with Al content in orthopyroxene, hence the conductivity increases with increasing Fe3+ content. The present experimental results demonstrate the electrical conductivity of Al-bearing orthopyroxene was essentially determined by the concentration of Fe3+. At low temperature (<1350 K), the activation enthalpies decreased from 1.65 to 1.26 eV with increasing Fe3+ content and pre-exponential factor increases with Fe3+ content, these variations suggest Fe2+-Fe3+ electron hopping (small polaron) is the dominant conduction mechanism. At higher temperatures (>1350 K), high activation enthalpies (2.26-2.74eV) indicate that Mg vacancy ionic conduction is the dominant conduction mechanism. A conductivity-depth profile of the upper mantle was constructed by extrapolating current data and combining previous conductivity data of anhydrous olivine. Moreover, we estimate the possible Al2O3 content and Fe3+/ΣFe ratio in the upper mantle, and concluded that aluminous orthopyroxene cannot account for the conductivity anomaly obtained from the geophysical observations at the top of the asthenosphere.5. At the upper mantle conditions of 3 GPa, 500-1000 K, frequency range from 10-1 to 106 Hz and the fO2 of Mo-MoO2, for the first time, the complex electrical properties of hydrous orthopyroxene were systematically measured using complex Impedance Spectroscopy. The present experimental results indicate that the electrical conductivity of hydrous orthopyroxene was completely determined by water content; the conductivity of orthopyroxene systematically increases with water content increasing from 0.005 to 0.641 wt.%, whereas the activation enthalpy decreases from 0.95 to 0.59 eV. The low activation enthalpies and water content dependence of the conductivity suggest that the dominant mechanism of charge transport is proton (H+) conduction in hydrous orthopyroxene below 1000 K. Extrapolation of the present experimental data to the top of asthenosphere and transition zone, and combination of the previous results from hydrous olivine and wadsleyite, a laboratory-based conductivity structure model was constructed in the Earth's mantle. Comparison of our model with the currently available conductivity-depth profiles derived from the geophysical observations beneath the Pacific shows that hydrous orthopyroxene cannot account for the high conductivity anomaly in the upper mantle, which might be attributed to the presence of partial melt at the corresponding region.