Rheology of ice I at low stress and elevated confining pressure

Triaxial compression testing of pure, polycrystalline water ice I at condit ions relevant to planetary interiors and near-surface environments (differe ntial stresses 0.45 to 10 MPa, temperatures 200 to 250 K, confining pressur e 50 MPa) reveals that a complex variety of theologies and grain structures may exist for ice and that theology of ice appears to depend strongly on t he grain structures. The creep of polycrystalline ice I with average grain size of 0.25 mm and larger is consistent with previously published dislocat ion creep laws, which are now extended to strain rates as low as 2 x 10(-8) s(-1) When ice I is reduced to very fine and uniform grain size by rapid p ressure release from the ice II stability field, the theology changes drama tically. At 200 and 220 K the theology matches the grain-size-sensitive the ology measured by Goldsby and Kohlstedt [1997, this issue] at 1 atm. This f inding dispels concerns that the Goldsby and Kohlstedt results were influen ced by mechanisms such as microfracturing and cavitation, processes not exp ected to operate at elevated pressures in planetary interiors. At 233 K and above, grain growth causes the fine-grained ice to become more creep resis tant. Scanning electron microscopy investigation of some of these deformed samples shows that grains have markedly coarsened and the strain hardening can be modeled by normal grain growth and the Goldsby and Kohlstedt theolog y. Several samples also displayed very heterogeneous grain sizes and high a spect ratio grain shapes. Grain-size-sensitive creep and dislocation creep coincidentally contribute roughly equal amounts of strain rate at condition s of stress, temperature, and grain size that are typical of terrestrial an d planetary settings, so modeling ice dynamics in these settings must inclu de both mechanisms.