Constraints on the Fe-S melt connectivity in mantle silicates from electrical impedance measurements
Résumé
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 1GPa. A first series of experiments used sintered powder samples of a fertile peridotite xenolith mixed with 5-15 vol% FeS 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 20-30μm, similar to the one employed by Yoshino et al. [2004]. For a second series, San Carlos olivine aggregates were used as solid matrix and 10-20 vol.% of eutectic FeS were added. For these, the average grain size was 3μm, much smaller than in the experiments by Yoshino et al. [2003]. The powder mixtures of peridotite+FeS and olivine aggregates+FeS were first annealed for 2-5 days in a conventional piston-cylinder at 1GPa and 950-970°C. The electrical conductivity of samples has been measured using the impedance spectroscopy method in a BN-graphite-CaF pressure cell with concentric cylindrical electrodes made from Mo- or Re-foil (which corresponds to an oxygen fugacity close to the IW buffer). The results indicate that up to 15 vol% of FeS 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.
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