Tracking sodium in Earths deep mantle

Michigan State University, East Lansing MI

Investigators

Abstract

Sodium is concentrated in regions of the Earth where fluids cycle within the planet as well as in oceanic crust sinking and mixing into the mantle. Because sodium affects melting of rocks and increases electrical conductivity of fluids, its chemistry at high pressures and temperatures of Earth’s interior is important to understanding deep Earth carbon and water cycles. This study will observe the quantity and mineral forms of sodium in the Earth’s largest layer, the lower mantle. Experiments and computer simulations will test sodium storage in the most abundant oxide mineral in the planet, which has the same atomic structure as rock salt. The project will support training for a graduate student and undergraduate assistant and a partnership with the MSU Museum to produce, present, and evaluate modular exhibits on salt geology and crystal structures for a public audience. The host phase and capacity for sodium in Earth’s mantle are important for modeling deep Earth cycles of essential volatiles and reading the mantle record of plate tectonics. (Mg,Fe)O ferropericlase may be the major host for sodium in Earth’s interior. Systematic experiments using laser-heated diamond anvil cells will measure the concentration of sodium in (Mg,Fe)O under sodium-saturated conditions and in equilibrium with the other major lower mantle phases bridgmanite and/or davemaoite. Complementary density functional theory calculations will test the relative favorability of mechanisms for dissolving sodium in lower mantle phases. In combination these methods will be used to determine the dominant lower-mantle host phase for sodium and the seismic properties of Na-bearing lower mantle. Improved constraints on the high-pressure mineralogy of sodium can be applied to update thermodynamic and geophysical modeling of the mantle for more accurate determination of mantle composition and heterogeneity. The incorporation of archetypal alkali metal sodium into (Mg,Fe)O oxide will also produce insights into systematics of salts and bonding at high pressure. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.