Earth-related projects

Electrical properties of iron sulfides in mantle xenoliths

S. Saxena, A. Pommier, & M.J. Tauber (2021, JGR-Solid Earth); M.J. Tauber, S. Saxena, E.S. Bullock, H. Ginestet, & A. Pommier (2023, American Mineralogist).

The detection and quantification of metal sulfides in host rocks by electrical measurements have been priorities for field and laboratory studies, motivated by mineral prospecting, fundamental interest in the mantle structure or core/mantle differentiation, among other reasons. In particular, some regions in the cold and stable parts of the Earth's upper mantle have exceptional electrical properties; they are very conductive. What causes these anomalies is not well understood, and one explanation could be the presence of iron-sulfur compounds.

In Saxena et al., 2021 (JGR), we tested this hypothesis by performing electrical experiments under pressure and temperature on natural and synthetic mantle rocks. The amount of iron-sulfur compound was varied to understand its effect on the electrical properties. Our results show that a small amount of iron-sulfur compound can affect the electrical properties of the upper mantle dramatically. 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 concluded that the low-resistivity anomalies in Tanzania, Kaapvaal, and Gawler cratons can be explained by the presence of a few vol.% of solid sulfide.

Recently (Tauber et al., 2023), we reanalyzed electrical data for a dunite host with added FeS or Fe-S-Ni (Saxena et al., 2021), and report additional experimental runs along with electron microprobe analyses. The applied pressure was 2 GPa; impedance spectra were acquired while annealing at 1023 K (below the metal-sulfide solidus), and while varying temperature from 570 to 1650 K. Addition of 6.5- or 18-vol.% FeS strongly enhances conductivity of the bulk sample compared with that of the dunite host, though values are 100 – 100,000 times less than those of pure FeS. These results indicate that most metal sulfide content is not part of a viable conductive path, even for the 18-vol.% quantity. Nevertheless, the relatively high conductivity and weak temperature dependence of the 18-vol.% sample reveal that contiguous paths of solid or molten FeS span the electrodes. The sample with 6.5-vol.% sulfide also exceeds the percolation threshold for temperatures as low as ~100 K below the eutectic melting point, likely because FeS softens. Conductivity is nearly unchanged upon crossing the eutectic temperature, however a decline over 1400 – 1500 K reveals that the 6.5 vol.% molten FeS forms a fragile electrical network in dunite. Samples with Fe50S40Ni10 or Fe40S40Ni20(at.%) are less conductive than pure dunite at temperatures below ~1450 K. This surprising result, likely caused by a reducing influence of Fe or Ni metal, does not support the use of FeS as an analog for compositions with nickel or excess metal. Our findings suggest that probing the electrical network of metal sulfides as solids complements other studies focused on connectivity of molten metal sulfides.

Support: This work has been funded by NSF-CAREER and the Carnegie Endowment. The multi-anvil cell assemblies used for electrical measurements are available to the scientific community via ASU.

Back-scattered electron image of FeS sulfide (white) in dunite aggregate (grey); sample retrieved from an experiment at 2 GPa and quenched at 1620 K.

Probing volatiles at depth

Pommier et al. (2019, JGR-Solid Earth); Yoshino, Manthilake & Pommier (2024, Elements); Hao et al. (2024, JGR-Solid Earth); Codillo et al. (submitted and in prep.); Backhouse et al. (in prep.).

Magnetotelluric data have been increasingly used to image subduction zones. Models of electrical resistivity commonly show features related to the release of fluids at several depths through the systems imaged, consistent with thermal and petrologic models of dehydration of the downgoing slab. In Pommier & Evans (2017), we hypothesized that regions where very strong conductive anomalies are observed in the mantle wedge at depths ~80-100km are related to the subduction of anomalous seafloor, either related to excessive fracturing of the crust (e.g., fracture zones), subduction of seamounts or other ridges and areas of high relief. These features deform the seafloor prior to entering the trench, permitting more widespread serpentinization of the mantle than would otherwise occur. An alternative explanation is that the large conductors represent melts with higher contents of crustal-derived volatiles (such as C and H) , suggesting in particular locally higher fluxes of carbon into the mantle wedge, perhaps also associated with subduction of anomalous seafloor structures with greater degrees of hydrothermal alteration.

In Pommier et al. (2019), our electrical experiments up to 10 GPa showed that lawsonite dehydration could contribute to (but not solely explain) high conductivity anomalies observed in the Cascades by releasing aqueous fluid at a depth (~50 km) consistent with the basalt-eclogite transition. In subduction settings where the incoming plate is older and cooler (e.g., Japan), lawsonite remains stable to great depth. In these cooler settings, lawsonite could represent a vehicle for deep water transport and the subsequent triggering of melt that would appear electrically conductive, though it is difficult to uniquely identify the contributions from lawsonite on field electrical profiles in these more deep-seated domains.

We are now combining electrical and acoustic measurements in the lab to explore the electrical and seismic properties of subduction minerals. As a recent example, in Hao et al. (2024), we conducted acoustic and electrical experiments on natural talc aggregates at relevant pressure-temperature conditions. Compressional wave velocity (Vp) was measured using ultrasonic interferometry in a Paris-Edinburgh press at pressures up to 3.4 GPa and temperatures up to 873 K. Similar Vp values are obtained regardless of the initial crystallographic preferred orientation of the samples, which can be explained by talc grain reorientation during the experiment, with the (001) plane becoming perpendicular to the uniaxial compression axis. Electrical conductivity of the same starting material was determined using impedance spectroscopy in a multi-anvil press up to 6 GPa and 1263 K. Two conductivity jumps are observed, at ∼860–1025 K and ∼940–1080 K, depending on pressure, and interpreted as talc dehydroxylation and decomposition, respectively. Electrical anisotropy is observed at low temperature and decreases with increasing pressure (∼10 at 1.5 GPa and ∼2 at 3.5 GPa). Comparison of acoustic and electrical results with geophysical observations in central Mexico supports the presence of a talc-bearing layer atop the subducted Cocos plate.

Support: NSF-CAREER award, the Carnegie Endowment.

Schematic diagram of the talc‐bearing subducted Cocos plate under central Mexico (Hao et al., 2024). The hypothetical talc‐bearing layer at the top of the slab is indicated with pink dashes. The seismic (Vp) profile of the flat slab is from Kim et al. (2010). The electrical conductivity‐depth map under central Mexico is modified after Jödicke et al. (2006). The horizontal color scale bars correspond to Vp velocity values (left) and electrical conductivity (right). The temperature profile is modified after Syracuse et al. (2010) and van Keken and Wilson (2023b). Our Vp measurements of talc with the (001) cleavage parallel to the slab surface reproduce field velocity values for a 3 km talc‐bearing layer with <∼30 vol.% talc and >∼50 vol.% chlorite. Talc dehydration is expected to occur at 70–80 km depth, from which fluids with a salinity equivalent to 0.1 M are released. Our electrical results are consistent with ∼2 vol.% aqueous fluids at ∼70 km depth. The conductive anomaly located at 20–50 km depth is compatible with ∼1–17 vol.% hydrous melt containing 1 to 4 wt.% water.

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