UC San Diego

Electrical measurements in the field are one of the most important methods for probing the structure and dynamics of the Earth’s mantle. Certain geographical regions, such as those around cratons, have anomalously high conductivity, and have drawn great interest in the geophysical community. The presence of iron sulfide minerals is one explanation for the unusual conductivities. The validation of this proposal or others depends on measurements in the laboratory, conducted at temperatures and pressures relevant to the mantle.Although electrical properties of pure iron sulfides are a well-studied topic in the laboratory, much less is known about the properties of sulfides as they exist in the mantle. For example, the networks that allow a relatively minor (~5 vol.%) amount of metal sulfide to strongly impact conductivity of a rock are not yet understood. The impact of other elements that are prevalent in the upper mantle, such as oxygen or nickel, have largely been unexplored. Likewise, the effect of annealing sulfide-silicate mixtures on their electrical properties is unknown. The research presented in this dissertation represents progress towards understanding the topics above. Mixtures of iron sulfides in two natural mantle rocks are presented. Impedance spectra from 1 – 106 Hz were acquired, for samples under pressures from 2 – 6 GPa (equivalent to roughly 60 – 360 km depth) over temperatures from 300 – 1400 °C. Large changes in conductivity values, and changes in Arrhenius activation energies, occur when the quantity of iron sulfide increases from 3 to 6.5 vol.%. The duration of an annealing (dwell) was found to have a significant impact on conduction, particularly at temperatures lower than the annealing temperature. Oxygen or nickel alters the conductivity significantly when added to the sulfide phase. The frequency dependence of these data was analyzed and led to a conclusion that instrumental aspects dominated the imaginary response (capacitance). The impedance spectra acquired in situ were complemented with ex situ electron microprobe analyses and images which advances our understanding of grain interior / grain boundary structure and overall conductivity. Applied to the field, the conductivity results were incorporated into an electrical model of sulfide-bearing mantle rocks. This model suggests that a small amount of iron sulfide is consistent with some electromagnetic anomalies around the cratons of Tanzania, Australia, and southern Africa.