The role of grain-scale non-equilibrium thermodynamics in the production and evolution of oceanic crust and lithosphere

California Institute Of Technology, Pasadena CA

Investigators

Abstract

One of the most powerful approaches that scientists have for understanding chemical systems, both natural and synthetic, is equilibrium thermodynamics. This approach allows us to predict, in detail, the state that systems will settle into if given enough time to react completely. In studying the melting and crystallization of the magmas that erupt at mid-ocean ridges and form new ocean floor, equilibrium has been the basic assumption and tool that scientists have long relied on, on the basis of the reasonable assumption that temperatures are quite high in magmatic systems and melt production and migration is happening reasonably slowly. Much has been learned from work based on this idea. However, it has limitations. Some reactions are very slow. Exposed samples of the mantle may have a texture like "marble cake" and, if the regions of different composition are big enough, they cannot react with each other completely. Diffusion of elements through large crystals limits the rate at which they can reach equilibrium. This reasoning leads to the conclusion that a framework for thinking about melting, melt migration, and crystallization that addresses the approach to equilibrium (rather than just the end state) is necessary to test these assumptions, address harder problems, and gain a full understanding of the origin of the seafloor and the information about Earth's deep interior that can be gained by picking up rocks there. This is a challenging endeavor because it requires new categories of numerical models built on entirely different equations, and able to follow systems through time. This work will borrow numerical approaches from materials engineering (fields like metallurgy) that explicitly include a description of a parcel of Earth's mantle at the scale of individual mineral grains, and tracks how those grains grow or shrink, react with one another, and contribute atoms to the liquid phase as melting proceeds. This tool will be applicable to numerous Earth science problems and will enable solid Earth scientists to think about volcanism, mid-ocean ridges, and subduction zones in entirely new ways. This grant will support a multi-scale computational study of melting processes in Earth's mantle, specifically focusing on the role of non-equilibrium thermodynamics in determining the chemical and textural evolution of the melting source and the composition of melts. The numerical framework for grain-scale non-equilibrium thermodynamics under development can self-consistently simulate processes such as coarsening, phase transformation, major and trace element diffusion, reactions, and melting of minerals or rocks. The work will begin by constraining model parameters and validating the model's explanatory power against kinetic laboratory experiments. That will set the stage for applying the model to the decompression melting of oceanic mantle beneath spreading centers. Developments to be studied include the use of phase-field techniques to describe interfacial dynamics, adding grain-boundary diffusion, developing an algorithm for melt extraction, and adopting a thermodynamic database for sub-solidus and magmatic phase relations. The first task will proceed via application of the model to coarsening of solid and melt-bearing assemblages, characterizing the roles of chemical and interfacial mobilities, volumetric free energies, bulk composition, and grain boundary diffusion on coarsening and phase transformation rates. Experimental validation will examine reaction rates and textures between periclase, quartz, enstatite, and forsterite. The main application will then be to investigate the microstructural and chemical evolution of mantle peridotite during decompression melting beneath mid-ocean ridge spreading centers. This phase will study the topology of melt during production and migration, textural evolution due to melting and phase transformations (e.g., garnet to spinel), and the effect of grain size and decompression rates on composition. Investigations will consider two scenarios: a semi-closed-system, near-fractional melting model and an open-system, reactive flow melting model. The project supports training a postdoctoral researcher, developing grain-scale non-equilibrium thermodynamic modeling in the Earth sciences, and expanding that development to numerous problems in material physics and engineering related to coarsening, diffusion, and phase transformation. 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.