Chemistry and Dynamic Implications of Heterogeneous Fe and Si in the Deep Lower Mantle


Champagne time! Image credit: Niels Noordhoek (Wikimedia Commons)

We have just received our first NSF grant for our new project, “Chemistry and Dynamic Implications of Heterogeneous Fe and Si in the Deep Lower Mantle.” This award to PIs Susannah Dorfman (MSU), Allen McNamara (MSU) and Catherine Macris (IUPUI) will be funded by the Collaborative Studies of Earth’s Deep Interior program. Here’s the public abstract:

The composition of Earth’s rocky mantle is fundamentally linked to dynamic processes responsible for the composition and motion of tectonic plates on Earth’s surface. Although the mantle is stirred by convection, it is not well-mixed: images of the mantle obtained from seismology reveal enigmatic regions of unknown composition and age. Progress towards identifying these regions and their role in the history of the Earth has been based on determining the range of compositions and physical properties consistent with remote geophysical observations. Physical properties of mantle minerals can be measured in experiments that mimic the high pressures and temperatures of the deep Earth. Effects of contrast in temperature, density, and viscosity on shapes and dynamics of mantle structures can be tested by simulations of convecting rock. The team combines complementary expertise in experimental mineralogy and computational geodynamics and will train students in both fields, resulting in not only new constraints on the compositions and origins of mantle heterogeneities, but also well-rounded, multidisciplinary young scientists.

Attempts at explaining large low shear velocity provinces (LLSVPs) and ultralow velocity zones (ULVZs) identified by seismology at the base of the mantle involve various combinations of contrasting temperature and composition relative to the surrounding mantle. Possible mechanisms for generating compositional differences have dramatically different implications for the age of these regions, including subduction of modern basalt and segregation of iron-rich mantle during the formation of the planet. Towards understanding these provinces, the project will systematically investigate the effects of variable amounts of iron and silicon on observable properties of the lower mantle phase assemblage such as density and viscosity. A series of well-characterized samples with variable amounts of ferrous iron, ferric iron, and silicon will be compressed to lower mantle conditions to synthesize assemblages of bridgmanite, ferropericlase, and at the highest pressures, post-perovskite. Effects of iron and silicon content on mantle density will be obtained from equations of state. Effects of variable silicate fraction on strength and viscosity of the mantle will be evaluated from observed shear strain. Resulting constraints from experiments on density and viscosity will be incorporated in geodynamic simulations of effects of compositional differences between background mantle and heterogeneous provinces on resulting morphology of thermochemical piles. The results will be shared widely at conferences and on the web and will be of interest to a broad community of solid Earth geochemists and geophysicists.

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