ABR: Studies of Partial Melting of the Mantle and Deep Earth Volatile Cycles

In this Accomplishment Based Renewal, two projects will be executed that both relate to the chemical dynamics of melting in Earth's interior. One project aims to understand the relationship between molten sulfides and carbon in the mantle. A significant portion of Earth's carbon may be dissolved in molten sulfide in the mantle, and so it may play an important role in cycling of carbon between different Earth reservoirs. It may also play an important role in the origin of diamonds, as well as affect but geophysical properties of the mantle. The second project investigates the relationship between magma formation and the oxidation state of Fe in Earth's mantle. Subtle differences in the proportions of oxidized (Fe3+) and reduced (Fe2+) iron in Earth's mantle could have profound effects on melting behavior. For example, mantle enriched in Fe3+ may melt more readily, thereby affecting the relationship between mantle temperature and the volume of volcanic material erupted at mid-ocean ridges or oceanic islands such as Hawaii. More oxidized mantle also initiates melting at greater depth, which determines how the mantle and crust change composition through time and the masses of volatiles such as H2O, CO2, and sulfur that are liberated from the mantle during volcanic activity. This also influences geophysical properties, including the mantle's resistance to convection, and how both seismic waves and electric currents propagate through Earth's interior. Finally, inferences about the oxidation state of Earth's interior based on compositions of volcanic rocks require knowledge of the melting behavior of Fe3+ during melting. Experimental studies will be conducted to improve understanding of both of these topics.

Preliminary data indicates that the melting point of sulfide is diminished significantly by the presence of carbon, and new experiments will be conducted to explore how this relationship changes with pressure and sulfide composition, as well as to determine the C solubility in sulfide melts. The experiments will take into account the changing activity of metal that occurs with increasing pressure in the mantle, and in particular how the ratio of metal to sulfide changes. Experiments will be conducted with piston cylinder and multianvil devices up to 18 GPa and will be analyzed by electron microprobe and SIMS. Investigation of the behavior of Fe3+ during partial melting of the mantle will focus on the behavior in pyroxene, as this is the chief reservoir of oxidized iron in the mantle and there are few previous measurements. Experiments and X-ray spectroscopy will determine the partitioning of Fe3+ between pyroxene and silicate melt during partial melting to quantify the relationship between melting and mantle oxidation state. Additional experiments will explore the relationship between oxidation state of peridotite and partial melting behavior.