EVOLUTION OF AQUEOUS FLUIDS, HYDROCARBONS, AND DIAMOND FORMATION IN THE UPPER MANTLE

Fluids in the deep crust and upper mantle appear to have played roles in the longterm evolution of the subcratonic lithospheric mantle and therefore the stabilities of the continents, in the geochemical cycles of the elements, particularly the volatile elements, from subduction zones to Earth’s surface environment, and in the formation of diamonds. Much evidence of the chemistry of deep fluids has accumulated from studies of xenoliths and fluid inclusions in diamonds. But the origins of the fluids and their behavior are still unclear. In part, this is due to the lack of a comprehensive theoretical model of aqueous, high-pressure fluids. Moreover, the deep and surficial carbon cycle governs the long-term habitability of the Earth, but deep part is not well understood. In particular, the existence and importance of hydrocarbon fluids in the deep Earth have been debated for the past a few decades. The principal discoveries of this thesis provide a new window into upper mantle processes. With the extended DEW model, it is now possible to compute predictive models of upper mantle metasomatism and diamond formation involving a wide range of fluid compositions quantitatively. The diamond formation model helps understand the sources and speciation of upper mantle fluids and the metasomatic history of minerals. This study opens up the possibility of studying the co-evolution both fluids and minerals from different cratons to provide information about the differences in compositional, thermal and barometric profiles, and tectonic activity in the lithospheric mantle. Last but not least, aqueous and fluid hydrocarbon species in the deep Earth could participate in mantle metasomatism, diamond formation and migration of carbon in the deep Earth. If ascending to shallower depths, these hydrocarbons could possibly provide a source for deep natural gases and energy for a deep biosphere under proper conditions. Both water and hydrocarbon fluids could change the physical properties of the upper mantle, influencing interpretations of mantle history