[S-CG19] Hydrogen in the Earth's interior from the crust to the core

[SCG19-09] Hydrogen mobility in transition zone silicates

★招待講演

*Razvan Caracas¹、Wendy Panero² (1.CNRS, Ecole Normale Superieure de Lyon, University of Lyon, Laboratoire de Geologie de Lyon, Lyon, France、2.Ohio State University, School of Earth Sciences, Columbus OH USA)

キーワード：diffusion, molecular dynamics, mantle silicates

Hydrogen defects in mantle silicates adopt a variety of charge-balanced defects, including V_Mg’’+2(H*), V_Si’’’’+4(H*), and V_Si’+(Mg+2H*). Constraining the defect mechanism experimentally can be quite difficult, as it relies almost entirely on vibrational spectroscopy whose interpretation can often be controversial. Here we use a computational alternative: we study the above-mentioned defect mechanisms using molecular dynamics simulations based on the density-functional theory, in the VASP implementation. We perform isokinetical NVT simulations at 2000 and 2500 K using supercells containing 16 equivalent formula units of Mg₂SiO₄.
Our results show that temperature has a tremendous effect on mobility. H is significantly more mobile when incorporated as V_Mg’’+2H* defects than as hydrogarnet defects and that V_Mg’’+2H* defects are more mobile in wadsleyite than ringwoodite. This result is the opposite from the proton conductivity inferences of Yoshino et al. [2008] and Huang et al [2006], as well as the observed increase in electrical conductivity with depth through the transition zone [e.g. Kuvshinov et al, 2005; Olsen 1998].
Over the simulation time of several tens of picoseconds the H travel over several lattice sites. However, during its path it spends a considerable amount of time pinned in the defect sites. The lowest mobility is for the V_Si’’’’+4(H*) defect, where the H atoms remain inside the octahedron from which they replaced the Si.