Constraints on the Fe–S melt connectivity in mantle silicates from electrical impedance measurements

Abstract

The connectivity of FeS melts in olivine and in a fertile peridotite matrix has been addressed through in situ electric impedance spectroscopy (IS) measurements at 1 GPa. A first series of experiments used sintered powder samples of a fertile peridotite xenolith mixed with 5–15 vol.% Fe 70S 30 of eutectic composition. The sheared high-T garnet peridotite with Mg# ∼ 0.90 is composed of 60 vol.% olivine, 15% orthopyroxene, 5.3% clinopyroxene and 19% garnet, the powder grain size was 20–30 μm, similar to the one employed by Yoshino et al. (2003). For a second series, San Carlos olivine aggregates were used as solid matrix and 10–20 vol.% of eutectic Fe 70S 30 were added. For these, the average grain size was 3 μm, much smaller than in the experiments by Yoshino et al. (2003). The powder mixtures of peridotite + Fe 70S 30 and olivine aggregates + Fe 70S 30 were first annealed for 2–5 days in a conventional piston cylinder at 1 GPa and 950–970 °C. The electrical conductivity of samples has been measured using the impedance spectroscopy method in a BN–graphite–CaF 2 pressure cell with concentric cylindrical electrodes made from Mo- or Re-foil (the estimated oxygen fugacity was close to the IW-buffer). The results indicate that up to 15 vol.% of Fe 70S 30 the melt phase does not built a stable interconnected network in a peridotite matrix, as was recently indicated by Walte et al. (2007). The percolation threshold for a stable FeS network in olivine matrix lies at 17.5 vol.%, much higher the 6 vol.% found by Yoshino et al. (2003). Our result is in line with the high dihedral angles of typically 70–100° for Fe–S melts in mantle materials. The higher interconnectivity threshold of this study, as compared to previous studies (Yoshino et al., 2003, 2004; Roberts et al., 2007) is a result of our smaller starting grain sizes (for olivine) in combination with much longer run durations. Both these experimental conditions result in enhanced grain growth and thus to a higher degree of textural equilibration, leading to the occurrence of the time depending pinging off of Fe–S melt films in our texturally more mature experiments.