Iron Sulfides and Anomalous Electrical Resistivity in Cratonic Environments

Abstract

The interpretation of low‐resistivity anomalies in the lithospheric mantle of several cratonic regions has invoked hydrogen, or connected networks of graphite with iron‐rich silicates, and/or metal sulfides. Electrical laboratory measurements are a powerful approach for exploring these alternatives. We report electrical measurements of two xenoliths (pyroxenite and dunite) from Tanzania; two metal sulfides (FeS and Fe‐S‐Ni); and several mixtures of metal sulfides (3.4–18.2 vol.%) with xenolith. A multi‐anvil press was employed to maintain a 2 GPa pressure and temperatures up to 1,627 K. The addition of 3.4 vol.% FeS to the pyroxenite or dunite matrix has little effect on bulk resistivity, particularly for T > 800 K. However, the resistivity drops dramatically–by factors of up to 1,000, depending on temperature–upon addition of 6.5 vol.% FeS in the dunite. Addition of 18.2% FeS causes a further decrease of the same magnitude relative to the 6.5% sample. Scanning electron microscope images do not reveal the formation of a connected FeS network as part of the decreased resistivity. Possible explanations for the apparently conflicting results include connections of the sulfide that are not imaged in the back‐scattered images, either because of limited resolution, or perhaps the inherent limitations of the 2‐D perspective. The complete data set (xenoliths, metal sulfides, and mixtures) was modeled with a modified version of Archie's law, and we find satisfactory agreement over a truncated temperature range. We conclude that the low‐resistivity anomalies in Tanzania, Kaapvaal, and Gawler cratons can be explained by the presence of a few vol.% of solid sulfide.

Lithospheric Conductors Shed Light on the Non‐Uniform Destruction of North China Craton

“…During subduction of an oceanic plate, sulfides at a convergent margin could be carried to great depth (B. Yang et al., 2015). Measured data show the resistivity of FeS achieve $${10}^{-5}\,$$ Ωm in a wide range of temperature (Saxena et al., 2021). In addition, graphite is a strong contender in explaining low resistivity of the conductor C1.…”

“…Yang et al, 2015). Measured data show the resistivity of FeS achieve 𝐴𝐴 10 −5 Ωm in a wide range of temperature (Saxena et al, 2021). In addition, graphite is a strong contender in explaining low resistivity of the conductor C1.…”

“…When the temperature of the crustal base reaches 700°C–1,000°C, the bulk resistivity of common minerals in the lower crust can reach 10 Ωm if they contain high iron or water content (X. Yang, 2011). However, when the rocks contain a small amount of (∼1 wt%) sulfide, due to the resistivity of these minerals can be as low as <0.00001 Ωm at temperature of 427°C–1,000°C (Saxena et al., 2021) or at temperature of 350°C–1,200°C (Watson et al., 2010), the bulk resistivity of the rocks will be less than 10 Ωm. Of course, when the temperature is higher than the solidus of the lower crust rocks, silicate melts appear, and the resistivity will decrease significantly with the degree related to the volume content, properties, and connectivity of the melts (Pommier et al., 2010).…”

A Review of Subsurface Electrical Conductivity Anomalies in Magnetotelluric Imaging

“…Recent electrical laboratory measurements of xenoliths containing sulfides from several cratons reveal that a few vol.% sulfides can be used to explain their high-conductivity anomalies ( Figure 10 ), as indicated by Saxena et al [ 143 ]. However, a much lower content of sulfides could explain these anomalies if the mixing models in Section 4 were used as the resistivity of the sulfides is very low.…”

“…However, a much lower content of sulfides could explain these anomalies if the mixing models in Section 4 were used as the resistivity of the sulfides is very low. The discrepancy between the measurement and the calculation is caused by the fact that the sulfides were not well connected according to the measurement [ 143 ], while fully connected might be assumed by calculation. Similar caution should be paid to the graphite content estimate due to the lack of related electrical laboratory measurements.…”