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[SCG49-P01] High-temperature deformation experiments on Li-doped polycrystalline olivine: Toward understanding water weakening mechanism of olivine
Keywords:mantle rheology, diffusion creep, upper mantle, olivine, water weakening, Lithium
In order to understand the upper mantle flow, it is important to determine high-temperature deformation properties of major constituent minerals of the upper mantle such as olivine. Olivine is considered to contain water (H2O) up to 0.1 wt% in the upper mantle. The deformation properties of hydrous olivine have been experimentally investigated, which have revealed that the viscosity is greatly reduced by the presence of water [Karato et al., 1986; Mei and Kohlstedt, 2000]. Fei et al. (2013) questions the existence of water weakening or proposes very small effect of water on olivine rheology based on Si self-diffusion experiments. Recently, Yabe and Hiraga (2020) showed significant weakening of polycrystalline olivine due to grain-boundary-disordering which begins from a certain temperature near solidus. They proposed the water weakening as a result of grain-boundary-disordering in which the starting temperature is lowered by the water.
Since Li is a monovalent cation like H, it is expected to behave like H. Dohmen et al. (2010) reported that in the olivine c-axis direction, Li takes a diffusion path from interstitial sites to the metal vacancy sites, a diffusion path also found in H, and that the diffusion rates of H and Li are close in the order range. In this study, Li, which can be introduced into the olivine even under atmospheric pressure, is used as an analog of water (H).
The purpose of this study is to clarify the water weakening mechanism by comparing the high temperature deformation properties of Li-doped and Li-undoped olivine. Li-doped polycrystalline Forsterite-Diopside and Forsterite-Enstatite with an average grain size of <1 μm ware synthesized using vacuum sintering technique. Two types of samples were prepared with Li2O concentrations of 500 wt ppm and 5000 wt ppm at start material weighing. This polycrystalline material was uniaxial deformed at atmospheric pressure, temperatures of 843~1156 ℃, and stresses of 1~200 MPa. We found that the material deformed with the same strain rate at the same stress and grain size but nearly 100~200℃ lower temperature for the Lithium-undoped olivine, and a decrease in viscosity of 2~4 orders of magnitude was found. The Li content of the pre-experimental samples was analyzed by 3D atom probe analysis. The results showed that Li was contained in the olivine grains and concentrated at the grain boundary. The Li content of the post-experimental samples was analyzed by quadrupole ICP-MS. As a result, the post-experimental sample contained more than 80% of Li as weighed starting material. After experiencing sample synthesis and deformation experiments, it was confirmed that Li remained inside the sample. High temperature deformation experiments of Li-doped olivine polycrystals were successfully performed. We discuss the mechanism of Li effect on olivine rheology, which is expected to provide insight into water weakening.
Since Li is a monovalent cation like H, it is expected to behave like H. Dohmen et al. (2010) reported that in the olivine c-axis direction, Li takes a diffusion path from interstitial sites to the metal vacancy sites, a diffusion path also found in H, and that the diffusion rates of H and Li are close in the order range. In this study, Li, which can be introduced into the olivine even under atmospheric pressure, is used as an analog of water (H).
The purpose of this study is to clarify the water weakening mechanism by comparing the high temperature deformation properties of Li-doped and Li-undoped olivine. Li-doped polycrystalline Forsterite-Diopside and Forsterite-Enstatite with an average grain size of <1 μm ware synthesized using vacuum sintering technique. Two types of samples were prepared with Li2O concentrations of 500 wt ppm and 5000 wt ppm at start material weighing. This polycrystalline material was uniaxial deformed at atmospheric pressure, temperatures of 843~1156 ℃, and stresses of 1~200 MPa. We found that the material deformed with the same strain rate at the same stress and grain size but nearly 100~200℃ lower temperature for the Lithium-undoped olivine, and a decrease in viscosity of 2~4 orders of magnitude was found. The Li content of the pre-experimental samples was analyzed by 3D atom probe analysis. The results showed that Li was contained in the olivine grains and concentrated at the grain boundary. The Li content of the post-experimental samples was analyzed by quadrupole ICP-MS. As a result, the post-experimental sample contained more than 80% of Li as weighed starting material. After experiencing sample synthesis and deformation experiments, it was confirmed that Li remained inside the sample. High temperature deformation experiments of Li-doped olivine polycrystals were successfully performed. We discuss the mechanism of Li effect on olivine rheology, which is expected to provide insight into water weakening.