1:45 PM - 2:15 PM
[SGC32-05] H/D partitioning between forsterite, wadsleyite and ringwoodite : ab initio free energy calculation
★Invited Papers
Keywords:ab initio calculation, high pressure, H/D partitioning
Deuterium is the heavy stable isotope of hydrogen. The D/H ratio shows large variation in various astronomical bodies such as protosolar nebula (2x10-5), Earth (SMOW= 1.5 x 10-4, average seawater), Venus (1.6x10-2) and carbonaceous chondrites (~2x10-4) (e.g. Saal et al. 2013). Many studies are conducted to determine the D/H ratio in various rocks with different origins of the Earth, since this may be the key to understand the evolutional history and the origin of water of the Earth.
Many studies suggest that hydrous minerals in subducting cold slabs can transport water into deep Earth’s interiors. There is a possibility that several times of sea water exist in the transition zone at depth between 410km and 660 km if the constituent minerals are largely hydrated (e.g. Smyth 1994). Recently, hydrous ringwoodite with 1 wt%H2O has been found in natural diamond, suggesting that the transition zone is at least locally hydrated (Pearson et al. 2014). Therefore, the D/H ratio may be changed by the partitioning behaviors of D and H among these mantle minerals by the circulation of water in deep interiors.
In this study, we determined the free energy of D and H bearing forsterite, wadsleyite and ringwoodite by ab initio calculation in order to determine the equilibrium constants of D and H isotopic exchange reactions between them. First, we determined the stable structures of hydrous forsterite, wadsleyite and ringwoodite with Mg vacancy with two hydrogen atoms or Si vacancy with four hydrogen atoms by first principles calculation based on density functional theory. Then, the phonon frequencies are calculated based on density functional perturbation theory (Baroni et al. 2001) and also by the finite displacement method (Parlinski et al. 1997, Togo and Tanaka 2015). Then, we used quasi-harmonic approximation to calculate the Gibbs free energy of H and D baring phases. In this presentation, we report the Gibbs free energy of isotopic exchange reaction between forsterite and wadsleyite, and also between wadsleyite and ringwoodite.
Many studies suggest that hydrous minerals in subducting cold slabs can transport water into deep Earth’s interiors. There is a possibility that several times of sea water exist in the transition zone at depth between 410km and 660 km if the constituent minerals are largely hydrated (e.g. Smyth 1994). Recently, hydrous ringwoodite with 1 wt%H2O has been found in natural diamond, suggesting that the transition zone is at least locally hydrated (Pearson et al. 2014). Therefore, the D/H ratio may be changed by the partitioning behaviors of D and H among these mantle minerals by the circulation of water in deep interiors.
In this study, we determined the free energy of D and H bearing forsterite, wadsleyite and ringwoodite by ab initio calculation in order to determine the equilibrium constants of D and H isotopic exchange reactions between them. First, we determined the stable structures of hydrous forsterite, wadsleyite and ringwoodite with Mg vacancy with two hydrogen atoms or Si vacancy with four hydrogen atoms by first principles calculation based on density functional theory. Then, the phonon frequencies are calculated based on density functional perturbation theory (Baroni et al. 2001) and also by the finite displacement method (Parlinski et al. 1997, Togo and Tanaka 2015). Then, we used quasi-harmonic approximation to calculate the Gibbs free energy of H and D baring phases. In this presentation, we report the Gibbs free energy of isotopic exchange reaction between forsterite and wadsleyite, and also between wadsleyite and ringwoodite.