The effect of pressure on the electrical conductivity of olivine under the hydrogen-rich conditions

https://doi.org/10.1016/j.pepi.2014.03.010Get rights and content

Highlights

  • Pressure dependence of hydrogen-assisted electrical conductivity in olivine was determined.

  • Various results on electrical conductivity in olivine agree well except for Yoshino et al. (2009).

  • There should be a substantial drop in conductivity at the 410-km discontinuity if water content is constant.

Abstract

The influence of pressure on the hydrogen-assisted electrical conductivity in olivine has been studied under conditions of 4–10 GPa and 873–1273 K. Synthetic polycrystalline olivine samples with the water content of 160 ppm wt (Paterson calibration) were used and the electrical conductivity was determined from the results of the impedance spectroscopy. We found that the pressure reduces the hydrogen-assisted electrical conductivity, but its magnitude is small: between 4 and 10 GPa, the difference is a factor of ∼2.5 for the same water content and temperature. The pressure dependence is characterized by a negative activation volume and the negative dependence of the pre-exponential factor on pressure. Such anomalous behavior is explained by a model where hydrogen-related defect is considered as a weak inclusion in a strong matrix.

We use these new data to calculate the electrical conductivity contrast at the 410-km discontinuity. We found that if the water content of the mantle does not change with depth, there should be a large (a factor of ∼10) drop in conductivity from above to below the 410-km discontinuity. In contrast, geophysically inferred electrical conductivity increases at 410-km suggesting the increase in water content from above to below the 410-km discontinuity.

The present results are used to compare several previous results obtained at different pressures. It was shown that our results agree well with most of previous studies but not with (Yoshino et al., 2009). Possible causes for this discrepancy are discussed.

Introduction

Although the role of hydrogen in electrical conductivity in olivine has been extensively studied (e.g., (Poe et al., 2010, Wang et al., 2006, Yang, 2012, Yoshino et al., 2009, Yoshino et al., 2006)), the influence of pressure on hydrogen-assisted electrical conductivity has not been investigated. Consequently, it was difficult to compare experimental results on olivine obtained at different pressures. For example, (Karato, 2011) noted that the results by Yoshino et al. (2009) are systematically different from those by Wang et al. (2006) and discussed that the difference between the results by Wang et al. (2006) and Yoshino et al. (2009) might be caused by the pressure effect (the former study was at 4 GPa shows higher conductivity than the latter measured was at 10 GPa). Similarly, in order to compare previous studies on olivine conductivity (Poe et al., 2010, Wang et al., 2006, Yang, 2012, Yoshino et al., 2009), one needs to know the pressure effects, because these studies were performed at a broad range of pressure (1–10 GPa).

In addition, the estimation of the conductivity-depth profile has been hampered by the lack of the data on the pressure effect on hydrogen-assisted electrical conductivity in olivine. For example, (Karato, 2011) used two different models to estimate the electrical conductivity profiles for the whole upper mantle, one with no pressure effect, and another with a large pressure effect assuming that the difference between Wang et al. (2006) and Yoshino et al. (2009) was due to the pressure effect. The difference between these two models is large making it difficult to estimate the water distribution across the 410-km discontinuity.

The aims of the present study are to address these two issues, i.e., (1) to evaluate the expected conductivity jump at the “410-km” discontinuity for a constant water content model, and (2) to compare various studies normalized to the same pressure (of 8.0 GPa) to identify the source of discrepancy noted in the previous studies.

Section snippets

Sample preparation

The hydrous polycrystalline olivine samples were synthesized from San Carlos olivine powders with the grain size less than 5 micron by hydrothermal annealing experiments at P = 4 GPa and T = 1473 K for 3 hrs. About 1.85 wt% of San Carlos orthopyroxene was added to buffer the oxide activity. Powder samples of olivine and orthopyroxene mixture were sealed into a nickel capsule. Both synthesis and conductivity measurements were performed using the same Ni-NiO solid oxygen buffer. Our previous experiences

Results

The representative impedance spectroscopy results at conditions of 7 GPa, 873–1273 K and the water content of ∼160 ppm wt is shown in Fig. 3. Results obtained under other conditions are qualitatively similar to those illustrated here. We observed two circles at high temperatures, whereas only one circle was observed at low temperatures. The presence of two circles in the impedance spectroscopy implies two processes of charge transfer and blocking. The first circle originated at the origin (Z′ = Z″ = 0)

Comparison with previous studies

There are some studies on the influence of pressure on electrical conductivity. The influence of pressure on the polaron conduction in San Carlos olivine single crystal was studied by Xu et al. (2000) under conditions of 4–10 GPa, 1273–1673 K and a Mo-MoO2 oxygen fugacity buffer. In addition, the influence of pressure on both proton and hopping conduction was studied for pyrope garnet by Dai and Karato (2009a). In all cases, the influence of pressure is relatively small: a change in pressure by ΔP

Acknowledgements

Robert Farla, George Amulele and Zhenting Jiang kindly provided technical guidance and assistance in the FT-IR measurement. This research was financially supported by the “135” Program of Institute of Geochemistry, CAS and NSF of China (41174079) and partly by NSF of United States.

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