Pergramon Scripta Materialia, Vol. 37, No. 9, pp. 1399-1406, 1997 Elsevier Science Ltd Copyright 8 1997 Acta Metallurgica Inc. Printed in the USA. All rights resewed 1359-6462/97 $17.00 + .OO PI1 S1359-6462(97)00246-7 GRAIN BOUNDARY SLIDING IN FINE-GRAINED Ice I D.L. Goldsby and D.L. Kohlstedt Department of Geology and Geophysics University of Minnesota Minneapolis, MN 55405 (Received December 18, 1996) (Accepted June 11, 1997) Introduction Although dislocation creep in ice has been reasonably well-investigated at both ambient (e.g., l-6) and high pressures (e.g., 7), diffusion creep and other grain size sensitive creep mechanisms in ice have not been reliably identified for two primary reasons: 1) Natural and synthetic ice samples typically have large grain sizes of -1 mm, which favor dislocation creep over grain size sensitive creep mechanisms. 2) At a given ‘homologous temperature, diffusion in ice is slow compared to diffusion in most other materials (S), so that the transition from dislocation creep to diffusion creep occurs at impractically slow strain rates. Yet, diffusion creep and other grain size sensitive creep mechanisms may well domi- nate the rheology of ice at the low differential stresses, expected within icy planetary bodies of the outer solar system, as well as at sufficiently low stresses recorded for some glaciers. A handful of studies on ice show creep results consistent with Newtonian flow (3, 9-12); all of these experiments, however, were conducted at strain rates <l x l@’ s-l, so that true diffusional flow may have been masked by transient effects. We therefore developed and employed new techniques for fabricating ice samples with grain sizes zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA d in the range 3 I d I 90 pm, in order to explore grain size sensitive creep of ice at practical laboratory strain rates (i.e., 21 x 10-8s~‘). Experimental Procedure Samples were prepared by pressure-sintering ice powders formed by misting distilled water from a two-phase mixing nozzle into a reservoir of liquid nitrogen. Powders were sieved to 525 pm and uniaxially hot-pressed at 195 K and 100 MPa for 2-4 h, yielding average ‘baseline’ grain sizes of 25-40 pm. Grain sizes >40 pm were obtained by grain growing samples at T 2 253 K. Smaller grain sizes were obtained by pressurizing baseline samples into the Ice II stability field (2200 MPa at 195 K) and then quenching in pressure back into the Ice I stability field, causing homogeneous nucleation to pro- duce 3-5 pm Ice I grains (13). Samples were deformed in compression at one atmosphere under constant load in a high-resolution creep apparatus (14) fitted with a cryogenic cell to control sample temperature. Most experiments were conducted at nominally constant stress due to the small strains, usually 10.03, incurred per stress step. 1399