Investigations of Earth Electrochemistry
Abby Kavner
University of California, Los Angeles
Electrochemical reactions, which involve the transfer of charged species during a chemical reaction, play an important role in governing the evolution of the Earth and planets. For example, the evolution of the core/mantle system and ongoing reactions between the core and the overlying reactions are likely electrochemical in origin. In addition, deep Earth oxygen fugacity—which helps determine deep mantle mass transport, heat transport, and electrical conductivity—can be understood in terms of the electrochemical potential of the deep Earth. Element cycling (e.g. Fe, C, S, O) throughout the Earth’s atmosphere, hydrosphere, biosphere and tectosphere also involve charge-transfer reactions, and their behavior is therefore governed by electrochemical principles. In the laboratory, application of an electric field can be used to drive electrochemical reactions, and to manipulate oxygen fugacity in situ. The resultant chemical and physical behavior can be examined by a variety of techniques, including microscopy, mass spectrometry, X-ray diffraction and micro X-ray absorption edge spectroscopy. If electrochemical reactions generate unique signatures, then some geochemical and geophysical observations of the Earth may be shown to correspond to specific electrochemical behavior. In this poster, we present recent results from ongoing studies at UCLA examining geochemical signatures of electrochemical processes in Earth materials.
In a study on the aqueous electrodeposition of Fe metal from an Fe(II) chloride solution, a voltage-dependent stable isotope fractionation was discovered (Kavner et al., Geochim. Cosmchim. Acta, 2005). We present new results showing similar-type isotope fractionations during Zn electrodeposition. We examine the results in terms of two competing hypothesis for the isotope-selective deposition phenomenon: electron-transfer limited behavior, and mass-transfer limited behavior at an electrode.
Electrochemical reactions can also be driven at high pressures. The effect of pressure on reaction rate provides information about reaction volumes in the high pressure phases. Redox reactions in AgI, an ionic salt, can be pushed and pulled at electrodes attached to a power supply, in situ at high pressures. We show the results of recent experiments investigating the rate of Ag deposition and iodide oxidation as a function of pressure, phase, and applied voltage in the hydrothermal diamond anvil cell.
