| Magnesium-silicate minerals such as olivine (Mg2SiO 4), orthopyroxene (MgSiO3), and clinopyroxene (CaMgSi 2O6) are potential reservoirs for hydrogen (H) in the Earth's upper-mantle. Previous experimental results demonstrate that the strength of these minerals is significantly decreased by addition of small amounts of H. This reduction in strength will have a large effect on mantle rheology. However, interpreting the extent that H in these minerals can affect mantle rheology is dependent on knowledge of H solubility in the minerals at the conditions within the mantle, and the H incorporation mechanism. Few experimental data are available for mantle minerals and this has served as impetus for this dissertation.; Experiments in the system SiO2-MgO-2H 2O, between 1125--1300°C and pressures from 2.5--10 GPa, were conducted to investigate the effect of variable MgO and SiO2 activity ratios on H solubility in forsterite. The MgO/SiO2 activity ratio in forsterite was varied by equilibrating it with either enstatite or periclase.; It was found that H solubility in forsterite is greater when it coexists with the mineral periclase (MgO) than with enstatite (MgSiO3). For example, at 5 GPa pressure 8800 +/- 414 ppmw H2O can be dissolved in forsterite equilibrated with periclase versus only 450 +/- 90 ppmw in forsterite equilibrated with enstatite. Single crystal X-ray diffraction shows that the hydrous component occupies the forsterite structure without any modifications. Based on the model reaction, Mg2SiO4 + xH2O = Mg2Si1-x/2O 4-2x(OH)2x + x/2 SiO2, combined with Si and Mg stoichiometry data from the electron probe, it was determined that calculated H2O concentrations compare well with measured concentrations.; The data also show that H solubility in forsterite is strongly pressure dependent with concentrations generally increasing exponentially with pressure.; Supersolidus experiments in the system SiO2-TiO2-Al 2O3-MgO-CaO-Na2O-2H2O were performed to investigate the partitioning of hydrogen (H) between olivine (OL. Mg2SiO4), orthopyroxene (OPX, MgSiO3), clinopyroxene (CPX, CaMgSi2O6), and garnet (GT,Mg 3Al2Si3O12) at pressures relevant to the Earth's upper mantle (2.5 to 7.5 GPa). (Abstract shortened by UMI.)... |
