Intergranular water and permeability of the Lake Vostok accretion ice, Eastern Antarctica

Abstract

The relative importance of nonhydrostatic stress and lattice-rejected impurities on the phase equilibrium of intergranular liquid water in the Vostok accretion ice, Eastern Antarctica, was examined in this study. In addition, experiments were conducted to examine the influence of intergranular water in ice on the permeability of a Light Non-Aqueous Phase Liquid (LNAPL) hydrocarbon. Sub-grain scale stresses in the Vostok accretion ice were simulated with anisotropic elastic and elastocreep finite element (FE) models and compared to X-ray dislocation topographs. The phase equilibrium conditions were solved separately using stresses simulated by the FE models and ice chemistry data obtained from literature. The permeability of ice to JP-8 aviation fuel, the primary component of drilling fluid used in the Vostok borehole, was tested in three Fuel-Ice (FI) Experiments on unfractured ice in dark conditions near the melting point. The shear stresses simulated by the elastic FE model indicated plastic deformation, via basal glide, in the Vostok accretion ice. This finding was supported by observed dislocation densities exceeding 107 m-2, with higher values reported in literature. The elasto-creep FE model indicated onset of intergranular melt, at scales ₃ 1% the crystal size, in the lower few meters of the westernmost accretion ice. Model predictions of strain rate and internal melt were in reasonable agreement with literature data on polycrystalline ice. Based on an impurity model, which assumed hydrostatic stress, millimeter-size intergranular water veins were predicted in the lower few dekameters of accretion ice. The FI Experiments indicated that these water veins in ice provide conduits for rapid (> 16 cm hr-1) infiltration of JP-8 fuel in dark conditions near the melting point. This transport mechanism, referred to as fuel-tunneling, involved the formation of intergranular tubes, 1-2 mm in diameter, that were absent from experiments using ice grown from distilled water. It was concluded that intergranular water veins in ice near the melting point provide tunneling conduits for LNAPL hydrocarbons. This fuel-tunneling may be accelerated in the basal-most part of the accretion ice due to intergranular melting from both deviatoric stress and mechanical anisotropy. These results have implications for environmentally-clean penetration methods of subglacial lake exploration.