The Effect of Pressure, H₂O Fugacity, and MgO/SiO₂ Activity Ratio on Hydrogen Solution in Forsterite

Darren Locke¹, Kurt Leinenweber², Rick Hervig³, and John Holloway^1,2

Trace amounts of H dissolved in olivine (50<ppmw H₂O<200), the dominant mineral of the Earth’s upper mantle, drastically affects its rheological properties (e.g. reduces creep strength). Hence, upper mantle flow dynamics and all associated processes (e.g. melting) must also be affected. Therefore, gaining knowledge of the full extent that H dissolved in olivine has on mantle processes requires determination of maximum H solubility and solution mechanisms in olivine at high pressure and temperature.

Experiments in the system SiO₂-MgO-2H₂O, between 1100-1300°C and pressures from 2.5-7.5 GPa, were conducted to investigate the effect of variable MgO/SiO₂ activity ratio on H solubility in forsterite. Equilibrating forsterite with either enstatite or periclase and a hydrous fluid varies the MgO/SiO₂ activity ratio. Enough H₂O is added to experiments to saturate the system (i.e. free fluid phase), and its fugacity at P and T is calculated using an equation of state from Saxena and Fei (1987). At all pressures studied, H solubility in forsterite is greatest when it coexists with periclase. For example, at 5 GPa forsterite equilibrated with periclase dissolves 8830±1770 ppmw H₂O compared with 504±101 ppmw H₂O in forsterite equilibrated with enstatite. The effect of pressure and H₂O fugacity on H solubility in forsterite equilibrated with enstatite at 1398K is given by:

where A = 121.815 ppmw H₂O /GPa and DV = 10.05x10^-6 m³ mol^-1.

Single crystal x-ray diffraction verifies that the structure is indeed forsterite, with no superstructure. Detailed electron probe analyses indicate that Si concentrations are lower than ideal in these forsterites (2-4%). Based on the model reaction for H substitution for Si in forsterite: Mg₂SiO4 + xH₂O(fl) = Mg₂(Si_1-x/2,H_2x)O₄ + x/2 SiO₂(fl), we have calculated H₂O concentrations that correlate well with measured concentrations.

All these results suggest that H may substitute in forsterite by the “hydrogarnet substitution”, replacing [SiO₄]^4- with [O₄H₄]^4- structural units.

¹Arizona State University, Department of Geological Sciences

²Arizona State University, Department of Chemistry and Biochemistry

³Arizona State University, Center for Solit State Science