Abstract
Abstract. The electrical conductivity of gneiss samples with different chemical compositions (WA = Na2O + K2O + CaO = 7.12, 7.27 and 7.64 % weight percent) was measured using a complex impedance spectroscopic technique at 623–1073 K and 1.5 GPa and a frequency range of 10−1 to 106 Hz. Simultaneously, a pressure effect on the electrical conductivity was also determined for the WA = 7.12 % gneiss. The results indicated that the gneiss conductivities markedly increase with total alkali and calcium ion content. The sample conductivity and temperature conform to an Arrhenius relationship within a certain temperature range. The influence of pressure on gneiss conductivity is weaker than temperature, although conductivity still increases with pressure. According to various ranges of activation enthalpy (0.35–0.52 and 0.76–0.87 eV) at 1.5 GPa, two main conduction mechanisms are suggested that dominate the electrical conductivity of gneiss: impurity conduction in the lower-temperature region and ionic conduction (charge carriers are K+, Na+ and Ca2+) in the higher-temperature region. The electrical conductivity of gneiss with various chemical compositions cannot be used to interpret the high conductivity anomalies in the Dabie–Sulu ultrahigh-pressure metamorphic belt. However, the conductivity–depth profiles for gneiss may provide an important constraint on the interpretation of field magnetotelluric conductivity results in the regional metamorphic belt.
Highlights
According to magnetotelluric (MT) and geomagnetic depthsounding results, the electrical conductivity of geological samples at high temperature and pressure can be used to extrapolate the mineralogical composition and thermodynamic state in the Earth’s interior (Maumus et al, 2005; Dai et al, 2008; Hui et al, 2015; Manthilake et al, 2015; Li et al, 2016; Hu et al, 2017)
The electrical conductivity of most metamorphic rocks has not been explored at high temperature and pressure, and the interpretation of high conductivity anomalies distributed in representative regional metamorphic belts is still not comprehensive
Electrical conductivity of the gneiss samples significantly increased with temperature and weakly increased with pressure
Summary
According to magnetotelluric (MT) and geomagnetic depthsounding results, the electrical conductivity of geological samples at high temperature and pressure can be used to extrapolate the mineralogical composition and thermodynamic state in the Earth’s interior (Maumus et al, 2005; Dai et al, 2008; Hui et al, 2015; Manthilake et al, 2015; Li et al, 2016; Hu et al, 2017). High conductivity anomalies are widely distributed in the middle to lower crust and upper mantle, and there are various causes of these anomalies in different regions (Xiao et al, 2007, 2011; Pape et al, 2015; Novella et al, 2017). The electrical conductivity of most metamorphic rocks has not been explored at high temperature and pressure, and the interpretation of high conductivity anomalies distributed in representative regional metamorphic belts is still not comprehensive. Geophysical exploration results confirmed that a large number of high conductivity anomalies have been observed in metamorphic belts (Xiao et al, 2007; Wannamaker et al, 2009; Zeng et al, 2015). Metamorphic rocks (e.g., slate, schist, gneiss, granulite and eclogite) with different degrees of metamorphism play an important role
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