Silicon and oxygen self-diffusion in forsterite and implications to upper-mantle rheology

(1) Silicon lattice diffusion coefficient in dry forsterite The high temperature creep of olivine is believed to be controlled by self-diffusion of olivine. However, the experimentally measured silicon diffusion coefficients (DSi) [Dohmen et al., 2002; Jaoul et al., 1981] were about 2-3 orders of magnitude lower than those estimated from dislocation creep rates by deformation experiments [Durham and Goetze, 1977a; Goetze and Kohlstedt, 1973]. In order to resolve this discrepancy, we measured DSi in a dry forsterite single crystal at 1600-1800 K, 1 atm -13 GPa using an ambient pressure furnace and Kawai-type multi-anvil apparatus. The water contents in the samples were carefully controlled at <1 wt. ppm. The results of DSi showed small negative pressure dependence with an activation volume of 1.7±0.4 cm3/mol. The activation energy is found to be 410±30 kJ/mol. LogDSi at 1600 and 1800 K at ambient pressure are -19.7±0.4 and -18.1±0.3 (DSi in m2/s), respectively, which are ~2.4 orders of magnitude higher than those reported by Jaoul et al. [1981]. Their low DSi might reflect the effects of a horizontal migration of the isotopically enriched thin films applied on the sample surfaces, which may inhibit diffusion into the substrate during annealing. Our results resolved the discrepancy of DSi measured in diffusion experiments with those deduced from creep rates measured in deformation experiments. (2) Effect of water on silicon self-diffusion coefficient in forsterite Water has been considered to largely affect geodynamical processes in the Earth’s interior. In particular, experimental deformation studies suggested that even several tens wt. ppm of water can enhanced creep in olivine by several orders of magnitude. However, those deformation results are doubtful because of the experimental limitations, e.g., considering only a limited range of water content and very high stresses applied to the samples. Because the high temperature creep of silicate minerals is controlled by silicon self-diffusion, we systematically measured DSi in iron-free forsterite at 8 GPa, 1600 - 1800 K, and water content (CH2O) from <1 up to ~800 wt. ppm, showing a relationship, DSi ∝ (CH2O)0.32±0.07. This CH2O exponent is strikingly lower than 1.2, which has been obtained by deformation experiments [Hirth and Kohlstedt, 2003]. The high nominal creep rates in the deformation studies under wet conditions may be caused by excess grain boundary water. Thus, the effect of water on olivine rheology is much smaller than that it has been considered before and many geodynamic problems should be reconsidered. The viscosity in the upper mantle calculated from DSi continuously decreases with increasing depth without appearing a minimum zone by mineral hydration, and therefore, the asthenosphere softening cannot be caused by water effect. The CH2O differences between the source of hotspots and their surrounding regions only causes a viscosity contrast by a factor of two, which is rather small in comparison with that caused by temperature differences. Therefore, CH2O differences cannot be the major reason that leads to the immobility of hotspots. (3) Effect of water on oxygen self-diffusion coefficient in forsterite Oxygen is the second slowest diffusion species in olivine with similar diffusion coefficients as silicon. Therefore, oxygen diffusion also plays essential role in rock deformation as well as silicon diffusion. In order to examine the effects of water on creep reported by rock deformation experiments, we also measured oxygen self-diffusion coefficient (DO) in forsterite at a pressure of 8 GPa and temperatures of 1600 - 1800 K as a function of CH2O from <1 up to ~800 wt. ppm. The experimental results showed DO ∝ (CH2O)0.06±0.1 ≈ (CH2O)0. Namely, water has no effect on DO. Together with the small effect of water on silicon self-diffusion coefficient, we conclude that the role of water on upper mantle rheology is insignificant. (4) Silicon grain boundary diffusion coefficient in forstetrite Dislocation creep causes non-Newtonian viscosity and seismic anisotropy whereas diffusion creep doesn’t. Determination of deformation mechanism in Earth’s interior is thus essential to understand mantle dynamics. We have measured silicon grain-boundary diffusion coefficient in forsterite as a function of pressure, temperature, and water content. The activation volume, activation energy, and water exponent are found to be 1.8±0.2 cm3/mol, 245±12 kJ/mol, and 0.22±0.05, respectively. The rates of dislocation creep, Coble diffusion creep, and Nabarro-Herring diffusion creep calculated from silicon lattice and grain-boundary diffusion coefficients suggest dominant diffusion creep in cold mantles and mantle wedges. In the asthenosphere, dislocation creep always dominates because of the high temperature. The deformation mechanism transition does not occur in the asthenosphere. In the lithosphere, diffusion creep dominates in shallow regions and dislocation creep dominates in lower regions. In mantle wedges, diffusion creep dominates and therefore olivine does not form lattice-preferred orientation: their strong anisotropy is caused not by olivine but by serpentine. The Newtonian rheology suggested by postglacial rebound and the seismically observed mid-lithospheric discontinuity should be attributed to the diffusion creep dominated cold continental lithosphere.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Fei, Hongzhan

Contributors dc:contributor

• Katsura, Tomoo

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/107/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:107