Garnet is a major rock-forming mineral that is stable over a wide range of pressures and temperatures, from the Earth’s crust to the lower mantle. Garnet occurs widely in the earth’s interior, such as in mantle peridotites and eclogite—the high-pressure equivalent of basalt in subducted oceanic crust. Garnet typically contains 80% pyrope in the utramafic rocks of peridotite xenoliths, while it is rich in the almandine and grossular components with several percent of the uvarovite component.
The electrical properties of minerals and rocks are influenced mainly by temperature, pressure, oxygen fugacity, water content, grain boundary, point-defect chemistry, chemical composition, distribution of partial melting, and electronic spin-state transitions. Oxygen fugacity not only drives redox reactions, element partitioning, and structural phase transitions, but also controls transport properties, especially in minerals such as silicates and oxides which contain multi-valent elements.
Previous measurements of the electrical conductivity of garnet only involve the effect of chemical composition or water content. Recently, Prof. LI Heping’s group from Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) has investigated the high-pressure electrical property of garnet using in situ Solartron-1260 impedance/gain phase analyzer and YJ-3000t large-volume multi-anvil press. The scientists have published their results on Contributions to Mineralogy and Petrology (Volume 163, Number 4 (2012), 689-700). In their paper, the electrical conductivityof garnet was determined under various conditions of temperature, pressure,oxygen fugacity (Fe2O3 + Fe3O4, Ni + NiO, Fe + Fe3O4,Fe + FeO, and Mo + MoO2), and chemical composition within thefrequency range from 10-2 to 106 Hz. They modeled the dependences of electrical conductivity on the various influence factors above and discussed the conduction mechanism.Based on the effective medium theory, they also discussed the geophysical implications of their experimental results and established the electrical conductivity model of eclogite with different mineral contents at high temperatures and high pressures, thereby providing constraints on the inversion of field magnetotelluric sounding results in future studies.
This research was financially supported by the Knowledge-Innovation Key Orientation Project of CAS (Grant Nos. KZCX2-YW-Q08-3-4 and KZCX2-YWQN110) and by NSF of China (Grant Nos. 41174079 and 40974051), and its publication is available from the website http://www.springerlink.com/content/d8453u2561568202/ .
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| Figure Electrical conductivity of dry and wet Py73Alm14Grs13garnet for five oxygen buffers (Fe2O3+ Fe3O4, Ni + NiO, Fe + Fe3O4, Fe + FeO, and Mo + MoO2) at 2.0 GPa and 873–1273 K. The water contents of the dry and wet samples are 0.13 and 465 ppm, respectively. (Image by IGCAS) |
(By DAI Lidong)
