First-principles study of hydrogen incorporation mechanism in Mg2SiO4

Most of the geoscientists believe that olivine-based minerals form the major constituent in the upper mantle, which extends to a depth of 660km. The Earth's upper mantle consists mainly of following three phases, alpha-, beta- and gamma-Mg₂SiO₄. Pressure induced phase transitions occur at about 10 GPa and 15 GPa under low temperature condition from alpha- to beta-Mg₂SiO₄, and from beta- to gamma-Mg₂SiO₄, respectively. The existence of beta-Mg₂SiO₄ gives rise to the discontinuity in seismic velocities at a depth of 410 km. It is widely accepted that the atmosphere and the oceans of the Earth are formed by degassing of the Earth's mantle. Most of the water and/or hydrogen may have been lost or it may still be stored in the Earth's mantle. If considerable amounts of hydrogen are present in the Earth's mantle, such hydrogen plays a key role in the geodynamics of the Earth's interior, because it affects the melting temperature and the transport properties of minerals as well as their elastic properties. Earlier high-pressure experiments suggested that main components of the transition zone of the Earth's mantle, wadsleyite and ringwoodite, can store significant amount of hydrogen [1-4]. More recently, it was reported that the lower mantle minerals, consisting of Mg-perovskite, magnesiowüstite and Ca-perovskite [5], can potentially store considerable amounts of hydrogen. However the effects of hydrogen solution on their physical properties and substitution mechanism of hydrogen in mantle minerals have not yet been fully understood. In the present study, the first-principles Density Functional Theory (DFT) calculations have been performed to investigate the influence of hydrogen incorporations on the compressional mechanism of the major components of upper mantle minerals in the Earth, i.e., forsterite (alpha-Mg₂SiO₄), wadsleyite (beta-Mg₂SiO₄) and ringwoodite (gamma-Mg₂SiO₄), and the mechanism of hydrogen incorporation in these minerals. Observed equilibrium volumes of anhydrous and hydrous wadsleyite and ringwoodite at ambient conditions were well reproduced by our present calculations. The calculated bulk moduli of the hydrous wadsleyite and ringwoodite have become significantly lower than those of hydrogen-free ones, as suggested by the high-pressure experiments [6]. The incorporation mechanisms of hydrogen in these minerals were thoroughly caluculated by the first-principles DFT method changing the positions of hydrogen. The transition pressures from beta- to gamma-Mg₂SiO₄ were also estimated for both anhydrous and hydrous Mg₂SiO₄, in which hydrous Mg₂SiO₄ showed higher transition pressure than anhydrous one by approximately 6 GPa. [1] T. Inoue et al., Geophys. Res. Lett. 22 (1995) 117. [2] Y. Kudou and T. Inoue, Phys. Chem. Minerals (1999) 382. [3] D.L. Kohlstedt et al., Contrib. Mineral. Petrol. 123(1996) 345. [4] H. Yusa and T. Inoue, Geohpys. Res. Lett. 24 (1997) 1831. [5] M. Murakami et al., Science 295 (2002) 1885. [6] T. Inoue et al., Earth Planet. Sci. Lett. 160 (1998) 107.