Many of the tectonic processes expressed at Earth's surface are the manifestation of processes rooted deeper within the planet. Variations in temperatures and composition of the upper mantle (the layer below the crust) combined with the forces that drive motion contribute to the deformation patterns at the surface and/or melt generation (possibly resulting in volcanoes). Models of the mantle further our understanding of the evolution of these processes, but these models require accurate estimates of temperature and composition of the upper mantle---neither of which can be measured directly. To estimate the state of the mantle, geophysical models of a physical property sensitive to temperature and composition are typically used as a proxy (e.g. seismic velocity and electrical conductivity). Using electrical conductivity as a proxy requires laboratory measurements on minerals at a range of conditions expected for the mantle. Most interpretations of upper mantle electrical conductivity are based only on the mineral olivine, which comprises 60-70% of the mantle, and often ignore the possible influence of the remaining mineral components. While olivine typically dominates the electrical conductivity, there are cases, particularly when bound water content is high, where electrical conductivity contributions from orthopyroxene and clinopyroxene, the next most abundant upper mantle minerals, may be disproportionate to their volume. This project will involve the collection of electrical conductivity on mantle pyroxenes in a laboratory controlled setting over a wide range of physical conditions, leading to an improved model of dry pyroxene conductivity. This model will help improve current, and aid in future, interpretations of mantle conductivity models and the processes that potentially drive melt generation and surface dynamics. This project will also include the building of a conductivity apparatus than can be used in future studies as well as the training of a postdoc.

There have been few electrical conductivity studies on orthopyroxene and fewer on clinopyroxene. While they suggest anhydrous conductivity of orthopyroxene is similar to that of olivine, with clinopyroxene about one order of magnitude lower, they were conducted along predetermined oxygen fugacity paths, thus limiting their application to specific conditions. This project will focus on the collection of electrical conductivity and thermopower measurements on mantle derived pyroxenes over a wide range of temperatures and oxygen fugacities relevant to the mantle. By collecting thermopower measurements in tandem with electrical conductivity at several oxygen fugacity states and temperatures it will be possible to estimate the concentration and mobility of the various charge carriers and build point defect models that extend conductivity estimates to a much larger range of mantle conditions. A similar model for olivine has become a standard for comparison with laboratory experiments and repeatedly verified in recent experiments. In the past few years there has been a great deal of attention paid to the effect of water on olivine conductivity. However, water partitioning experiments show that pyroxenes may hold ten times as much bound H2O as olivine. Apart from contributing to bulk composition, pyroxenites (rocks with >50% pyroxene) are found regionally in veins which are important to the geochemical budget, and may be responsible for the "garnet signature" in mid-ocean ridges and ocean island basalts. If such veins form interconnected networks they would have a disproportionate effect on mantle conductivity. A reliable anhydrous pyroxene conductivity model developed as part of this study will aid interpretation of future electrical conductivity experiments on hydrous pyroxenes, as well as improve the interpretation of mantle conductivities inferred from electromagnetic sounding.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Robin Reichlin
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University of California-San Diego Scripps Inst of Oceanography
La Jolla
United States
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