A mineral-physics based model of mantle electrical conductivity

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Many of the tectonic processes expressed at Earth's surface are themanifestation of processes rooted deeper within the planet. Variations intemperatures and composition of the upper mantle (the layer below thecrust) combined with the forces that drive motion contribute to thedeformation patterns at the surface and/or melt generation (possiblyresulting in volcanoes). Models of the mantle further our understandingof the evolution of these processes, but these models require accurateestimates of temperature and composition of the upper mantle---neitherof which can be measured directly. To estimate the state of the mantle,geophysical models of a physical property sensitive to temperature andcomposition are typically used as a proxy (e.g. seismic velocity and electricalconductivity). Using electrical conductivity as a proxy requireslaboratory measurements on minerals at a range of conditions expectedfor the mantle. Most interpretations of upper mantleelectrical conductivity are based only on the mineral olivine,which comprises 60-70% of the mantle, and often ignore the possibleinfluence of the remaining mineral components. While olivine typicallydominates the electrical conductivity, there are cases, particularlywhen bound water content is high, where electrical conductivitycontributions from orthopyroxene and clinopyroxene, the next mostabundant upper mantle minerals, may be disproportionate to their volume.This project will involve the collection of electrical conductivity onmantle pyroxenes in a laboratory controlled setting over a wide range ofphysical conditions, leading to an improved model of dry pyroxeneconductivity. This model will help improve current,and aid in future, interpretations of mantle conductivity models and theprocesses that potentially drive melt generation and surface dynamics.This project will also include the building of a conductivity apparatusthan can be used in future studies as well as the training of a postdoc.There have been few electrical conductivity studies on orthopyroxene andfewer on clinopyroxene. While they suggest anhydrous conductivity oforthopyroxene is similar to that of olivine, with clinopyroxene about oneorder of magnitude lower, they were conducted along predetermined oxygenfugacity paths, thus limiting their application to specific conditions. This projectwill focus on the collection of electrical conductivity and thermopowermeasurements on mantle derived pyroxenes over a wide range oftemperatures and oxygen fugacities relevant to the mantle. Bycollecting thermopower measurements in tandem with electricalconductivity at several oxygen fugacity states and temperatures itwill be possible to estimate the concentration and mobility of thevarious charge carriers and build point defect models that extendconductivity estimates to a much larger range of mantle conditions. Asimilar model for olivine has become a standard for comparison withlaboratory experiments and repeatedly verified in recent experiments.In the past few years there has been a great deal of attention paid tothe effect of water on olivine conductivity. However, water partitioningexperiments show that pyroxenes may hold ten times as much bound H2O asolivine. Apart from contributing to bulk composition, pyroxenites (rockswith 50% pyroxene) are found regionally in veins which are important tothe geochemical budget, and may be responsible for the "garnetsignature" in mid-ocean ridges and ocean island basalts. If such veinsform interconnected networks they would have a disproportionate effecton mantle conductivity. A reliable anhydrous pyroxene conductivitymodel developed as part of this study will aid interpretation of futureelectrical conductivity experiments on hydrous pyroxenes, as well as improve the interpretation of mantle conductivities inferred fromelectromagnetic sounding.