[S-GC56_30PM1] 固体地球化学・惑星化学

[SGC56-05] 氷多結晶体の粒界拡散の可視化 ~氷の塑性流動則の解明に向けて~

野口 直樹¹、*久保 友明²、Durham William³、清水 以知子⁴、鍵 裕之⁵ (1.広島大学大学院工学研究科応用化学専攻、2.九州大学大学院理学研究院地球惑星科学部門、3.マサチューセッツ工科大学地球大気惑星科学部、4.東京大学大学院理学系研究科地球惑星科学専攻、5.東京大学大学院理学系研究科附属地殻化学実験施設)

キーワード：氷, 拡散, 粒界拡散, 水素同位体, ラマン分光, レオロジー

Ice I_h is a primary constituent of surfaces of the icy Galilean satellites and ice sheets at the Antarctica. Thus understanding rheological behavior of ice Ih will contribute to better knowledge about the dynamics and tectonics of the surface of the icy satellites and ice sheets. Stress applied by the tidal deformation to the surfaces of the icy satellites was predicted to be very low, ~1 MPa (Sotin and Tobie 2004), and diffusion and grain-size-sensitive creeps probably control the deformation of the surfaces of the ice satellites. The deformation map under such a low stress condition can be constructed, based on two diffusion constants, grain boundary and volume diffusion coefficients, of polycrystalline ice I_h. The volume diffusion coefficient of ice I_h was determined from experiments using a single crystal of ice I_h (Ramseier 1967; Itagaki 1967), while the grain boundary diffusion coefficient has not been determined yet. Thus we carried out experiments to determine directly the grain boundary diffusion coefficient of polycrystalline ice I_h.The diffusion couples have been composed of a pair of disks of pore-free polycrystalline H₂O and D₂O ices. The glass beads with a diameter of 2 μm were doped in the diffusion couples to prevent the grain growth during diffusion experiments by Zener pinning effect. The volume fraction of the glass beads was approximately 1 %. The diffusion experiments were carried out under the confining pressure of 100 MPa using a gas apparatus surrounded with a cryostat (Durham et al. 2001). Temperatures were set in the range from 235 K to 256 K. After keeping the diffusion couples in the deformation instrument for 20 ~ 94 hours, we shaved off thin sections from the diffusion couples. Two-dimensional diffusion profiles of the thin section were determined with micro- and imaging- Raman spectroscopes and a cryo-stage. The Raman mapping or Raman imaging measurements were carried out with keeping the thin sections at -90℃. The concentration of deuteron can be determined from the relative intensity of Raman band of OH stretching mode to that of OD stretching mode using a quantitative curve. The two-dimensional distribution showed enrichment of deuterium at the grain boundaries near the H₂O/D₂O boundary. It indicates that grain boundary diffusion of ice I_h is rapid. The diffusion profiles obtained by the experiments enable to estimate the grain boundary diffusion coefficient of ice I_h which is essential to construct the deformation map.