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.

Methods of data processing and analysis: 

The data (electrical resistivity ρ) were calculated from the measured resistance values (R) at each temperature using an impedance spectrometer (1260 Solartron Impedance/Gain Phase Analyze). The pressure is 2 GPa. A DC potential of 1 V with 100 mV AC amplitude was applied over a range of frequencies from ~5 MHz to 10 Hz for silicate-bearing samples, and from ~50 to 0.1 Hz for pure metal sulfides. The following equation is used to compute electrical resistivity ρ from R:

ρ=R⨯G 

where G is the geometric factor