Modeling Deep Earth Fluids and Diamond Formation

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The deep Earth is inaccessible for direct observation, but most people are well aware of its importance when earthquakes strike and volcanoes erupt. Less appreciated is the fact that the deep Earth has a role in regulating the Earth's atmosphere over the enormous span of geologic time. Fluids and volcanoes linking the deep Earth and the near-surface environment have played a major role in keeping our planet habitable over billions of years. For example, carbon dioxide in the atmosphere is cycled over time down into the Earth and back up through volcanic emissions via deep fluids and their chemical reactions with rocks. However, remarkably little is known about these fluids. Laboratory experimental studies seek to simulate them, geologists study rock samples for clues about them, and theoretical modeling is needed to synthesize a consistent picture of what is happening at depth. The current project seeks to advance such a synthesis. It is expected that advancing our understanding of fluids in the deep Earth will also help in building models for the evolution of and the search for life on other planets, particularly for the huge number of new planets being discovered outside our solar system. Previous models of fluids in the deep Earth have assumed they are Carbon-Oxygen-Hydrogen fluids containing molecular species (e.g. CO2, CH4, H2, and H2O) without aqueous ions. No quantitative models exist for the ionic speciation and reactivity of these fluids with their silicate environment at upper mantle conditions, which severely hampers understanding of planetary volatile evolution. The goal of this project is to develop a new approach to modeling the chemistry of deep fluids calibrated on experimental silicate and carbonate solubility measurements at high pressures, one that can be extrapolated to predict fluid-rock interactions under upper mantle conditions. It is proposed to develop predictive theoretical models for investigating the nature and reactivity of deep fluids in the Earth with specific application to the deep carbon cycle. A key issue to be investigated is the nature of the species in deep fluids. In particular, metal-silicate and carbonate complexes will be studied by modeling published experimental solubilities of important upper mantle mineral assemblages and new experimental solubilities of aragonite at elevated temperatures and pressures. The model will then be tested with published fluid inclusion compositions in diamonds and coexisting mineral compositions to develop aqueous speciation models for the end-member fluids thought to form diamonds. Building on these results, chemical mass transfer models will be developed to enable a quantitative understanding of the chemical processes involved in the formation of diamonds from metasomatic fluids in the peridotitic environment of the subcratonic lithospheric mantle.