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3.10 h. Complex electrical impedance measurements in the multi-anvil press (B.T. Poe and D.C. Rubie)

In-situ high-pressure high-temperature complex electrical impedance spectroscopy has been developed for the multi-anvil press. These experiments are likely to place tighter constraints on such parameters as the temperature distribution, fluid content and mineralogy of the Earth's interior when compared with geophysical observations. We expect this method of determining the electrical properties of minerals at extreme conditions to be complementary to diamond anvil cell techniques, and should resolve many of the controversies brought about by conflicting data sets. The advantages of the multi-anvil method include a more accurate control and measurement of experimental conditions such as temperature and chemical environment as well as large sample volumes for quality sample characterization. For example, trace amounts of water present in polycrystalline olivine can account for large increases in its conductivity. This is likely to explain the differences between high pressure data and 1 atmosphere data obtained in controlled atmosphere furnaces (Fig. 3.10-7). An important feature in the experimental design is a sample geometry which accommodates coaxial electrodes. The sample, 2.5 mm in length, is contained in a 1.8 mm diameter molybdenum capsule (outer electrode) which is open at both ends. A 0.5 mm molybdenum wire is inserted through a hole down the cylindrical axis of the sample as the inner electrode. This geometry minimizes leakage and maintains a nearly fixed cell constant during compression. An MgO sleeve insulates the sample from a low voltage (0-1 V), stepped graphite furnace capable of heating the sample to 1200 °C. Interference from the AC furnace on the impedance measurements is only observable near 50 Hz. We have made no additional attempts to shield the sample from the furnace. The complex impedance can be measured over a frequency range of 10-5 to 107 Hz, making it possible to distinguish and quantify contributions from both grain interior and grain boundary conduction mechanisms.

Fig. 3.10-7: Temperature dependence of the electrical conductivity of polycrystalline (Mg,Fe)2SiO4 olivine determined at different pressures. The high P studies report significantly higher conductivities than the 1 atmosphere studies due to the inability to keep the sample completely anhydrous at high pressure and temperature.

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