High Electrical Conductivity of Olivine at Oxidizing Conditions of the Shallow Mantle and Geophysical Implications

Abstract

High electrical conductivity in the shallow mantle has long been recognized by electromagnetic depth soundings, but the origin remains debated. Various candidates, of which the two most popular are water enrichment in olivine (the dominant mineral in the upper mantle) and partial melt, have been proposed to explain the electrical anomalies. These models nearly exclusively assume a very low conductivity for dry or water‐poor olivine. Here, we show by experimental work that, under well‐controlled conditions of pressure, temperature, oxygen fugacity (fO2), and water inventory, the conductivity of olivine is in fact insensitive to water content at mantle temperature (e.g., >1,000°C), although the enhancement is substantial at relatively low temperature. In particular, the conductivity of olivine at temperature beyond ∼1,000°C is strikingly high, about 0.01–0.1 S/m, at the oxidizing conditions of the shallow mantle. The high conductivity is comparable to the geophysically measured high conductivity in many conductive regions of the shallow mantle. The results provide a new framework for understanding the electrical anomalies and electrical structure of the shallow mantle.

Electrical Conductivity of Eclogitic Omphacite and Garnet at Water-Rich Conditions

“…The obtained activation enthalpy is ~70 and 84 kJ/mol for omphacite and garnet, respectively (Table 2). The conductivity of both minerals increases with temperature and water content at otherwise identical conditions, which is in line with previous work on nominally anhydrous minerals (NAMs) (Dai and Karato, 2009;Yoshino et al, 2009;Poe et al, 2010;Yang et al, 2011;Zhao and Yoshino, 2016;Liu et al, 2019;Liu et al, 2021a;Liu et al, 2021b;Zhang et al, 2019).…”

“…Omp, omphacite; Grt, garnet. arc and a low-frequency short tail in each spectrum, resembling those reported for silicate minerals (Huebner and Dillenburg, 1995;Dai and Karato, 2009;Yoshino et al, 2009;Poe et al, 2010;Yang et al, 2011;Zhao and Yoshino, 2016;Li et al, 2017;Zhang et al, 2019;Liu et al, 2021b). The arc is commonly attributed to grain interior conduction and tail to sample-electrode reactions (or electrode effects).…”

“…In brief, BN-Ni double capsules were used, in which the sample, two Pt electrodes, Ni-NiO buffer pairs, and a type-S thermocouple were located. The assembly yields a relatively sealed chamber, which helps maintain sample geometry and buffer redox state, and sample Fe loss to Pt electrodes is negligible, as documented previously (Yang et al, 2011Liu et al, 2021b).…”

“…Conductivity in the first heating cycle was usually subjected to residual moistures in the system, yielding data not following Eq. 1 as documented in similar work (Yoshino et al, 2009;Li et al, 2016;Li et al, 2017;Liu et al, 2019;Liu et al, 2021a;Liu et al, 2021b), and relevant data were excluded. The conductivity of samples and their fittings are plotted in Figure 3, and parameters by fitting the data to Eq.…”

“…Impedance was measured with a Solartron 1260 Impedance/Gain-Phase Analyzer, with frequency sweeping at 10 6 1 Hz and 0.5 V applied voltage. The assembly design follows that in our previous studies (Yang et al, 2011Yang, 2012;Li et al, 2016;Li et al, 2017;Liu et al, 2019;Liu et al, 2021a;Liu et al, 2021b). In brief, BN-Ni double capsules were used, in which the sample, two Pt electrodes, Ni-NiO buffer pairs, and a type-S thermocouple were located.…”