title: Silicon and Oxygen volume diffusion in Wadsleyite and implications to mantle transition zone rheology
Creators: name: Druzhbin, Dmitry
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Diffusion
subject: Wadsleyite
subject: silicon
subject: oxygen
subject: rheology,
subject: mantle transition zone
subject: 550

description: Silicon is the slowest diffusing species in the majority of mantle minerals, and therefore provide the rate-limiting step for plastic deformation at high temperatures (Costa &amp; Chakraborty, 2008; Dohmen, 2002; H. Fei et al., 2012; H. Fei, Wiedenbeck, Yamazaki, &amp; Katsura, 2013; Shimojuku et al., 2009). Wadsleyite is one of the most abundant minerals in Earth’s mantle upper transition zone (MTZ) and the main constituent mineral of subducting slabs in this region. Robust evaluation of the effect of H2O content on diffusion properties in wadsleyite has not been considered (Shimojuku, Kubo, Ohtani, Nakamura, &amp; Okazaki, 2010; Shimojuku et al., 2009; Shimojuku, Kubo, Ohtani, &amp; Yurimoto, 2004). As observed for olivine in silicon diffusion experiments (H. Fei et al., 2012) water has a less significant effect on its rheological properties than proposed based on deformation experiments (G. Hirth &amp; Kohlstedt, 2003). However, this may not be the case for wadsleyite since it has a different structure and defect chemistry. Furthermore, previous experiments have showen that wadsleyite can accommodate a significant amount of water (1 – 3 wt.%) (Jacobsen, Demouchy, Frost, Ballaran, &amp; Kung, 2005; Kohlstedt, Keppler, &amp; Rubie, 1996) in comparison to olivine (Bali, Bolfan-Casanova, &amp; Koga, 2008; Smyth &amp; Jacobsen, 2006). So far there is no study of anisotropy of silicon and oxygen self-diffusion in wadsleyite. Its orthorhombic structure raises the possibility that diffusion rates may be anisotropic (Holl, Smyth, Jacobsen, &amp; Frost, 2008). In this project, I focused on measuring the volume diffusion coefficients of silicon and oxygen in Mg2SiO4 wadsleyite single crystals as a function of temperature, and water content, and investigated the upper MTZ rheology based on silicon and oxygen diffusivities. Silicon volume diffusion activation enthalpy was obtained from to kJ/mol, with average kJ/mol. This activation enthalpy is a half of that obtained by Shimojuku et al., 2010 of 342(143) kJ/mol using polycrystalline samples. A possible explanation of this disagreement may be two orders of magnitude difference in the water content in the samples between this study and Shimojuku et al., 2010. The present study shows, that Si is the slowest atomic species in wadsleyite along a mantle geotherm at water contents lower than 1500 wt. ppm. Si diffusivity is strongly water content dependent and anisotropic. The water dependence of Si volume self-diffusivity along the a-direction is , b-direction: , and c-direction: . The water-content exponents obtained in this study in b- and c- directions are significantly different from that r = 0.6 reported by Shimojuku et al., 2010 from polycrystalline samples. The large water content dependence implies that not only Mg-sites are substituted by protons (Smyth, 1987) but the Si-sites are also substitute by protons. Si diffusion in wadsleyite occurs by a vacancy mechanism. Moreover, water exponents suggest that Si-site and O-site vacancies are associated, given that O vacancies produce corridors for Si migration. The large water dependence in the c-direction implies Si migration through the Mg-site vacancies in M3 octahedra. The obtained activation enthalpy for oxygen diffusion ranges from kJ/mol to kJ/mol, which are smaller than those obtained for Fe-bearing wadsleyite by (Shimojuku et al., 2009) with . The effect of oxygen fugacity on diffusion rates in wadsleyite, however, was not studied so far. O diffusion in Mg2SiO4 wadsleyite is found in this study to be strongly anisotropic and water dependent in b- and c-directions. The water dependence of O volume self-diffusivity along main crystallographic directions is , , . At water content higher than 1500 wt. ppm O diffusivity is the lowest in Mg2SiO4 wadsleyite at all MTZ temperatures. O diffusion in wadsleyite occurs by a vacancy mechanism. The large water dependence in the c-direction implies O, like Si, migrates through Mg-site vacancies in the M3 octahedra. The estimated strain rates for Mg2SiO4 wadsleyite demonstrate three regions with different water exponents indicated by the superscript: (1) dry wadsleyite with less than 1000 wt. ppm H2O has where the water exponent is strongly affected by diffusion along the a-direction of the Si atom, (2) intermediate 1000 – 5000 wt. ppm with and (3) hydrous wadsleyite at more than 5000 wt.ppm H2O where is mainly determined by O diffusion along the c-direction. Based on Si and O diffusivities in wadsleyite, deformation maps at different water contents and grain sizes were constructed. Additionally, viscosity as a function of temperature for both diffusion and dislocation creep regimes were estimated. According to this study, previous MTZ stress estimations of 1 – 10 MPa (Shimojuku et al., 2009; Vassiliou, Hager, &amp; Raefsky, 1984) together with geophysical observations of its viscosity of 1019.5 to 1021.5 Pa s (Peltier, 1998; Soldati, Boschi, Deschamps, &amp; Giardini, 2009) indicate that the MTZ should be drier then tens wt. ppm H2O. A hydrous MTZ of 1 wt.% H2O, however, is proposed at lower stresses from 0.05 to 0.1 MPa, in order to match the observed viscosity range in the MTZ.