Japan Geoscience Union Meeting 2016

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-CG Complex & General

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

Mon. May 23, 2016 10:45 AM - 12:15 PM 201B (2F)

Convener:*Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Toru Inoue(Geodynamics Research Center, Ehime University), Chair:Mysen Bjorn(Geophysical Laboratory, Carnegie Inst. Washington)

11:15 AM - 11:30 AM

[SCG19-09] Hydrogen mobility in transition zone silicates

★Invited papers

*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)

Keywords:diffusion, molecular dynamics, mantle silicates

Hydrogen defects in mantle silicates adopt a variety of charge-balanced defects, including VMg’’+2(H*), VSi’’’’+4(H*), and VSi’+(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 VMg’’+2H* defects than as hydrogarnet defects and that VMg’’+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 VSi’’’’+4(H*) defect, where the H atoms remain inside the octahedron from which they replaced the Si.