Title:
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Electrical conductivity of rock samples subjected to high temperatures and pressures
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The field determinations of crustal electrical conductivity/depth
profiles show anomalously high conductivities in the lower crust.
This has yet to be explained by a consistent theory and
interpretation of field results is difficult due to the lack of
laboratory conductivity measurements of saturated rocks at lower
crustal temperatures.
A cell was designed to measure the electrical conductivity of
saturated rocks up to the lower crustal conditions of; confining
pressures of 1 GPa, pore-fluid pressures of 1 GPa and
temperatures of 900°C. This complex exercise required the use of
a metal sleeve and the use of guard-ring techniques to remove
leakage currents induced by the sleeve. The development of the
cell involved several years work and is a breakthrough in
measurement techniques as it has enabled the measurement of
saturated rock conductivities at lower crustal temperatures and
high pressures for the first time.
The conductivity of 14 samples of acidic and metabasic rocks was
measured at a variety of confining pressures (<0.2 GPa), porefluid
pressures (<0.2 GPa), temperatures (<900°C) and saturation
fluids.
The pressure variations showed that the basic rocks had a
conductivity too high to be explained by conduction through
saturating electrolyte alone. The temperature variation showed a
dramatic difference between acidic and basic rocks. The acidic
rocks showed large decreases in conductivity above 350°C after
initial rapid increases in conductivity. The basic rocks showed
no such reduction in conductivity indicating that a conduction
mechanism in addition to pore-fluid conduction was present.
Results indicate that saturated -rocks at high temperatures have
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conductivities sufficient to explain the high conductivity layers
in the upper lower crust whereas acidic rocks do not. The results
also demonstrate that graphite may be an important additional
conduction mechanism possibly accounting for the moderately high
conductivities in the lower lower crust even in the absence of
electrolyte conduction
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