The hydration of magnesium-silicate minerals of Earth's upper mantle

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. Consequently, there is a great potential for this mineral to be a reservoir for H within the Earth's upper-mantle.; 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). Two starting compositions, MWP1 and MWP2, each containing 20 wt % 2H2O, as a proxy for H2O, were prepared to yield large amounts of melt coexisting with OL, OPX, CPX, and GT. Experiments with these starting compositions have yielded mineral assemblages consisting of CPX + OPX +/- GT + melt and OL + CPX + OPX +/- GT + melt, respectively. Garnet is only present in experiments performed at 7.5 GPa.; The pressure dependence of H concentration in olivine and pyroxenes has been determined. H concentration in olivine increases with pressure while decreasing in orthopyroxene and clinopyroxene. Decreasing Al in these pyroxenes with pressure supports the hypothesis that H may become incorporated in these structures by substitution of H+ and Al3+ for Si4+ in tetrahedral sites.; H partitions more strongly to pyroxenes than olivine at low pressures (<5.0 GPa) but this relationship is reversed at high pressure (>7.5 GPa) where olivine contains higher H concentrations than pyroxenes. In any case, these minerals are considered important H reservoirs in the Earth's upper-mantle at pressure and temperature conditions beyond the stability limits of any know hydrous minerals.