UCLA

Aqueous fluids rich in CO2 and NaCl play an important role in the geochemical evolution of mafic and ultramafic rocks in the upper mantle and lower crust, where they act as strong agents of metasomatism and are capable of transporting significant mass and modifying bulkrock composition. Keys to understanding fluid-rock interaction in these settings are constraints on the solubilities of silica and magnesium in mixed fluids at high pressure and temperature. This dissertation outlines three experimental projects which provide new data and improve thermodynamic modeling on mineral dissolution in the system MgO-SiO2-H2O-CO2-NaCl. The first investigation focused on the solubility of magnesite (MgCO3) in H2O-NaCl-CO2 solutions at 10 kbar, from 600-800 C, at a mole fraction of CO2 (XCO2) of 0.05 and XNaCl from 0 to halite saturation. These experiments show that the solubility of MgO derived from magnesite in H2O-NaCl-CO2 fluids is significantly lower than that of Ca derived from calcite (CaCO3). The measurements improve thermodynamic modeling to allow prediction of magnesite or dolomite CaMg(CO3)2 solubility in H2O-CO2-NaCl fluids at high pressure and temperature. The second study investigated the solubility of quartz in H2O-CO2 fluids at 500-800 C and 10 kbar. My results show that, at constant P and T, quartz solubility declines with increasing XCO2 at all temperatures investigated. Critical to quantifying silica dissolution is understating its interactions with the H2O component of any mixed crustal fluid. This can be modeled by accounting for the hydrous components of dissolved silica via the equilibrium

〖SiO〗_(2,s)+nH_2 O=Si(OH)_4∙(n-2)H_2 O_((aq)) quartz hydrated monomer

where n is the “hydration number”, which corresponds to the total number of H2O molecules consumed by the transfer of SiO2 from quartz to the fluid. Previous studies argue that n is a constant independent of P and T. My results require that n varies with P and T, and I show that the variation can be linked to changes in the dielectric constant of H2O. The final study investigated the concentration of dissolved SiO2 in H2O-CO2 fluids in equilibrium with quartz + magnesite, talc + magnesite, and forsterite + magnesite at 500-800 �C and 10 kbar. Results show that total Si concentration in equilibrium with quartz + magnesite is identical to that in equilibrium with quartz alone at the same P, T, and XCO2. In contrast to changes in quartz solubility, Si concentration buffered by talc + magnesite and forsterite + magnesite decrease with rising temperature. Comparison to thermodynamic models reveals that the models are generally accurate, provided that silica hydration is taken into account, though I find that a revised model based on Newton and Manning (2002, 2008) yields better agreement with experiment than that of the Deep Earth Water model (Sverjensky et al., 2014; Fang and Sverjensky 2019). I show how my results can be incorporated into models of Si metasomatism during infiltration of H2O-CO2 fluids, as in the mantle wedge above subduction zones. Taken together, the experimental and modeling results will provide new insights into SiO2 and MgO mobility in H2O-CO2-NaCl fluids in deep crustal and upper mantle settings.