Neutron diffraction data on D2O and H2O ice analysed at ANSTO

This dataset was collected as part of AAS project 4581 - Microstructure and rheological changes in experimentally deformed ice from the Law Dome, Antarctica."Laboratory experiments, using ice from Law Dome ice cores, will replicate the microstructural changes previously identified in fast flowing ice streams. We will systematically measure the effects of deformation on natural polycrystalline ice and compare this to patterns observed in ice cores. Using a fabric analyser microscope we are able to continuously record stress strain distributions, crystallographic preferred orientation(CPO)and grain size evolution at incremental strain steps up to 50% shortening. Such data sets have the advantage that they allow for the continuous assessment of the relationships between the changing microstructure, CPO and rheology in a material that traditionally has only been experimentally deformed to less than 20% shortening. Comparisons will also be made to neutron diffraction observations obtained from synthetic ice experimentally deformed at the Australian Nuclear Science and Technology Organisation (ANSTO). Results will significantly advance our understanding of constraints on strain localization in dynamic ice systems".Taken from the abstracts of the two referenced papers:Wilson, C.J.L., Peternell, M., Salvemini, F., Luzin, V., Enzmann, F., Moravcova, O., and Hunter, N.:J.R. Partial melting in polycrystalline ice: Pathways identified in 3D neutron tomographic images, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-70, 2023.Hunter, N. J. R., Wilson C. J. L. and Luzin, V. Crystallographic preferred orientation (CPO) patterns in axially compressed deuterated ice: quantitative analysis of historical data. J. Glaciol., doi.org/10.1017/jog.2022.95, 2022.Strain, temperature and strain rate are crucial factors governing the development of crystallographic preferred orientations (CPO) in ice. To better understand how CPO patterns change in response to these variables, we performed quantitative analyses on neutron diffraction data between 2010 and 2019, collected in situ during uniaxial compression experiments on deuterium ice. At strains greater than 10% and temperatures less than -10°C, the c-axis pattern switches from a single maximum (‘cluster’) to small circle (‘cone’), both oriented parallel to shortening. The diameter and mean width of the cone pattern decrease as strain and/or strain rate increases. Prismatic axis (a and m) patterns are characterised by great circles parallel to the pole figure margin and may be distinguishable from the patterns in ice deformed under simple shear. While strain has the main influence on the degree of preferred orientation (or CPO ‘strength’), both temperature and strain rate have minor influences, which limits the extent to which CPOs can be used to measure strain. As cluster patterns can be observed in the c-axes of ice deformed under both pure and simple shear settings, this may complicate interpretations of flow geometry in terrestrial ice unless the prismatic axis patterns are also considered.In frozen cylinders composed of deuterium ice (Tm+3.8 ºC) and 10 % water ice (Tm 0 ºC) it is possible to track melt pathways produced by increasing the temperature during deformation. Raising the temperature to +2 ºC produces water (H2O) which combines with the D2O ice to form mixtures of HDO. As a consequence of deformation, HDO and H2O meltwater are expelled along conjugate shear bands and as compactional melt segregations. Melt segregations are also associated with high porosity networks related to the location of transient reaction fronts where the passage of melt-enriched fluids is controlled by the localized ductile yielding and lowering of the effective viscosity. Accompanying the softening, the meltwater also changes and weakens the crystallographic fabric development of the ice. Our observations suggest meltwater-enriched compaction and shear band initiation provides instabilities and the driving force for an enhancement of permeability in terrestrial ice sheets and glaciers.