The effect of rock particles and D2O replacement on the flow behaviour of ice

Abstract

Ice-rock mixtures are found in a range of natural terrestrial and planetary environments. To understand how flow processes occur in these environments, laboratory-derived properties can be extrapolated to natural conditions through flow laws. Here, deformation experiments have been carried out on polycrystalline samples of pure ice, ice-rock and D₂O-ice-rock mixtures at temperatures of 263, 253 and 233 K, confining pressure of 0 and 48 MPa, rock fraction of 0-50 vol.% and strain-rates of 5 × 10^-7 to 5 × 10^-5 s^-1 Both the presence of rock particles and replacement of H₂O by D₂O increase bulk strength. Calculated flow law parameters for ice and H₂O-ice-rock are similar to literature values at equivalent conditions, except for the value of the rock fraction exponent, here found to be 1. D₂O samples are 1.8 times stronger than H₂O samples, probably due to the higher mass of deuterons when compared with protons. A gradual transition between dislocation creep and grain-size-sensitive deformation at the lowest strain-rates in ice and ice-rock samples is suggested. These results demonstrate that flow laws can be found to describe ice-rock behaviour, and should be used in modelling of natural processes, but that further work is required to constrain parameters and mechanisms for the observed strength enhancement.This article is part of the themed issue 'Microdynamics of ice'.

Keywords: D2O-ice–rock rheology; ice–rock flow laws; ice–rock rheology; triaxial deformation.

Figure 1.

Microstructural observations of pre-deformation pure ice (a–c) and ice–rock (d–f) samples. Thin-section image in reflected light (a) shows a hexagonal texture with ice grains of similar sizes and smaller pores (void spaces) concentrated at grain boundary intersections. Scanning electron microscopy (SEM) images (b,c) also show this hexagonal structure, and quasi-linear grooves due to sublimation etching, which manifest in different directions in adjacent grains, indicated by arrows perpendicular to the texture in (c). Thin-section image of 10 vol.% rock sample (d) shows similar hexagonal ice grains to those seen in pure ice samples, surrounding angular fluorite particles of a similar size. (e) SEM image of unknown rock fraction; the form of etch pits (circled) in the ice grains show that grains are oriented in different directions. (f) SEM image of 50 vol.% rock sample. Fluorite particles are seen in an ice matrix with no ice grain boundaries visible. Vertical lines visible in (a) are due to irregularities in the microtome blade used for thinning and polishing the section. Fluorite particles coloured purple in post-processing.

Figure 2.

Individual differential stress (blue) and strain (red) against time for the deformation run on sample H₂O + 50% CaF₂_C, with P = 48 MPa, T = 263 K. Nominal strain-rates were 5 × 10⁻⁶, 1 × 10⁻⁵, 5 × 10⁻⁵ s⁻¹; increases in strain-rate shown by change in background colour. Features in the curves marked: A, seal friction; B, hit point; C, transient creep; D, steady-state creep; E and F, ramps in strain-rate; G, possible strain hardening in strain-rate ramp 2; H, strain hardening in strain-rate 3; I, apparent load jump probably due to friction on the ram, and unlikely to have been experienced by the sample. (Online version in colour.)

Figure 3.

Measured steady-state stresses versus imposed strain-rate for H₂O ice and ice–rock samples at a confining pressure of 48 MPa and temperature of 253 K. The measured differential stress increases with increased strain-rate. Differential stresses measured for 10 vol.% samples have similar values to pure ice, but for higher rock fractions the stress increases with rock fraction. For clarity, error bars are not shown. (Online version in colour.)

Figure 4.

Measured steady-state stresses versus imposed strain-rate and calculated flow laws for all H₂O ice and H₂O-ice–rock samples at all conditions. (a) Pure ice; (b) ice + 10 vol.% fluorite samples; (c) ice + 25 vol.% fluorite; (d) ice + 50 vol.% fluorite. As well as an increase of measured differential stress due to an increased strain-rate and higher rock fraction, the differential stress also increases with lower temperature and higher confining pressure. Higher strain-rate data show a good fit to the calculated flow laws for T = 253 K, whereas lower strain-rate data for pure ice, 25 and 50 vol.% rock may be better described by a GSS regime represented by the n = 1.8 flow law. The temperature dependence of the flow law is well described for pure ice, but for 50 vol.% rock, the flow law underestimates the measured strengths. Lighter blue diamonds are lower bounds on steady-state stresses due to extended transient creep effect discussed in the text. Errors on stresses are 1 s.d., flow law parameters are detailed in table 2. (Online version in colour.)

Figure 5.

Measured steady-state stresses versus imposed strain-rate and calculated flow laws for all D₂O ice–rock samples. Measured stress increases with strain-rate, lower temperature and increased rock fraction. (Online version in colour.)

Figure 6.

Post-deformation microstructures of ice and ice–rock samples. (a) Evidence of grain-size changes due to recrystallization, and sub-grain development and grain boundary bulging in reflected light microscope image of a pure ice sample. (b) Evidence of recrystallization of ice in a 10 vol.% rock sample in a transmitted light image. (c) Grain boundary migration and recrystallization to grains with a more rectangular habit in a SEM image of a 10 vol.% rock sample. (d) Grain boundary bulging and the possibility of grain boundary formation associated with rock particles in a SEM image of a 10 vol.% rock sample. (e) Development of a rectangular habit of ice grains, possibly associated with a rock particle, in a transmitted light thin-section image of a 25 vol.% rock sample. (f) Impaction and fragmentation of rock particles in a SEM image of a 50 vol.% rock sample.