THE ROLE OF DEEP, REDUCING CRUSTAL AND UPPER MANTLE FLUIDS IN THE MOBILITY OF CHROMIUM AND THE EVOLUTION OF HYDROCARBONS

Fluids have played important roles at deep crustal and upper mantle conditions, especially in subduction zones. Fluid-rock interactions are vital for the cycles of inorganic and organic species in the deep Earth. The mobility of species should be in principle dependent on temperature, pressure, chloride concentration, and oxidation state. However, most studies of subduction zone fluids have focused on oxidizing conditions. Here, it is proposed that metals, especially chromium, and carbon have drastically different behavior in aqueous fluids at lower oxidation states compared to higher oxidation states. For chromium, it is suggested to be mobile as Cr(II) in aqueous solutions at high temperatures, rather than Cr(III) under ambient conditions. In the case of aqueous carbon, a wide range of species with oxidation states from C(IV) to C(-IV) can exist at elevated temperatures and pressures. Furthermore, there is the potential for the development of separate C-rich fluids immiscible with an aqueous phase. This dissertation is motivated by studying the chemical evolution of deep, reducing crustal and upper mantle fluids, focusing on the mobility of inorganic Cr species and the formation of immiscible hydrocarbon fluids. This is done through three specific objectives. In the first study, a new Cr(II) complex CrCl(OH)0 was regressed in the Deep Earth Water model using experimental solubility data for Cr2O3. A high solubility was predicted in saline, peridotitic diamond-forming fluid, which implicated that diamond formation may involve simultaneous oxidation of Cr(II) to Cr(III) and reduction of C(IV) to C(0). In the second study, in situ experiments were performed using a diamond anvil cell to investigate the effect of pressure on the formation of immiscible hydrocarbon fluids at subduction zone conditions. Hydrocarbon fluids could perhaps play an important role in the deep carbon cycle. In the third study, an irreversible chemical mass transfer model was carried out to simulate the formation of immiscible CH4-rich fluids in the alteration of ophicarbonates in the Lanzo Massif, previously described by Vitale Brovarone et al. (2017). This work provides insight into the potential importance of reducing conditions in subduction zone systems