Electrical conductivity of siderite and its implication for high conductivity anomaly in the slab-mantle wedge interface

Abstract

Carbonate minerals as a dominant carbon host can be transported to the Earth’s deep interior via subduction of the oceanic lithosphere, and their physicochemical behavior potentially has a significant influence on the compositional heterogeneity and physical properties in the deep mantle. In this study, we measured the electrical conductivity of natural siderite at 1–3 GPa and 100–700°C using a complex impedance analyzer in a large volume multi-anvil high-pressure apparatus. A sharp increase in conductivity was observed at ∼400°C under various pressures, and subsequently, the electrical conductivity keeps anomalously high values in the whole temperature range owing to a small quantity of interconnected highly conductive phases (graphite and magnetite) produced from the low degree decarbonation of siderite. The change in electrical conductivity and activation enthalpy suggest that the conduction mechanisms before and after low degree decarbonation of siderite are the small polaron (electron hopping in Fe2+–Fe3+) and highly conductive phases, respectively. Our results indicate the incipient decarbonation temperatures at 1–3 GPa are considerably lower than the decomposition boundary of siderite determined by phase equilibrium experiments, implying the initial decarbonation reaction of Fe-bearing carbonates in the subducting oceanic crust occurs at a shallower depth. The 30 vol.% of siderite is required to enhance the electrical conductivity of (Mg, Fe)CO3 solid solutions. Magnetite and graphite generated from the decarbonation reaction of the siderite component of Fe-bearing carbonate make a significant contribution to the high conductivity anomaly observed in the slab-mantle wedge interface.

Electrical conductivity of siderite and the effect of the spin transition of iron

“…Generally, the conductivity becomes higher with increasing pressure when the small polaron hopping conduction mechanism is dominant as is the case with ferropericlase (Yoshino et al 2011;Liu et al 2014;Zhao et al 2020). This might be why the electrical conductivity of siderite is higher than that of siderite previously measurements at 1-3 GPa at 373 K (Hu et al 2022) as shown in Fig. 7.…”

“…At low temperatures, the small polaron (electron-hole hopping between Fe 2+ and Fe 3+ ions) is likely to be the predominant conduction mechanism in siderite with the characteristic of a relatively lower activation energy (Hu et al 2022). Generally, the conductivity becomes higher with increasing pressure when the small polaron hopping conduction mechanism is dominant as is the case with ferropericlase (Yoshino et al 2011;Liu et al 2014;Zhao et al 2020).…”

“…Previous studies on the dominant minerals forming the lower mantle, such as iron-bearing bridgmanite and ferropericlase, have reported clear trend changes in pressure dependence of electrical conductivity, which is likely associated with spin transition from high-spin to intermediate/low-spin states of iron (Lin et al 2007;Ohta et al 2007;2008;Yoshino et al 2011). The electrical conductivity of (Mg, Fe)CO 3 has been measured up to 3 GPa (Hu et al 2022). However, the measured pressure range is far below the pressure at which spin transition occurs, corresponding to the lower mantle.…”