Microscopic models for the effects of hydrogen on physical and chemical properties of Earth materials

Abstract

Atomistic models of interaction of hydrogen with silicates are reviewed. The most important mechanism of hydrogen dissolution in Mg (Fe)-bearing silicates is the replacement of Mg (Fe) with two protons (a vacancy at M-site trapping two protons ((2H)×M)). Influence of hydrogen on phase diagrams can be understood from this model. In addition to this fully charge-compensated defect, charged defects are also created by the ionization of a neutral defect, (2H)×M. Electrical conductivity is due to ionized defect, i.e., free proton. When the concentration of ionized, charged defects exceeds the concentration of charged defects under hydrogen-free environment, then the concentrations of defects at other sub-lattices is also modified leading to the enhancement of a range of properties that are controlled by the motion of defects at silicon or oxygen sub-lattice. The parameterization of creep law in terms of hydrogen content is summarized and applied to the estimation of upper mantle viscosity and fabric transitions. Current knowledge of influence of hydrogen on the rheological properties of deep mantle minerals is reviewed. The role of hydrogen in each mineral in the deep mantle is mineral-specific: solubility of hydrogen and its role in plastic deformation are quite different among the various minerals in the transition zone and the lower mantle. Possible roles of hydrogen in deep Earth dynamics are discussed including the effects of hydrogen on the structure of plumes. © 2007 Springer.