Using electron backscatter diffraction (EBSD) to characterize microstructures in coarse-grained ice 1 MONZ, Morgan E., 1 HUDLESTON, Peter J., and 2 PRIOR, David J. 1 Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455 2 Department of Geology, University of Otago, Dunedin, New Zealand STA P M Introduction • Microstructural processes and their relationship to flow are important for understanding the mechanical behaviour of ice • Crystallographic preferred orientation (CPO) develops as ice deforms by plastic flow, and this influences the strength of the ice • Microstructural work on coarse-grained ice has been limited due to techniques that are limited by grain size • We developed a new sample preparation method to obtain representative bulk CPO on coarse- grained samples collected from Storglaciären 2cm A) Polished north face of sample SG21, with individual crystals visible due to low angle light B) Slices aligned during composite sample preparation A B Background of CPO in Ice • Dominant deformation mechanism in ice is glide on the basal crystallographic plane 1,2 • Typical fabrics: small circle girdle, single maximum and deep in ice sheets and valley glaciers multiple maxima 3 • Issues with coarse-grained ice: 1) U-stage is low accuracy and only measures c-axes, and 2) branching crystals cause a probable bias 2cm 1 2a 3a 3k 3b 3c 3d 3f 3e 3g 3h 3i 3j 2b 4a 6 5a 5b 5c 10 4c 4d 8a 8b 9 7 11a 13 12 4b 2c 11b 41 14 15 24 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 35 38 40 37 39 36 34 2cm 1 1 2 2 4 4 5 5 3 3 y x z y x z y x z C a1 a2 a3 A) Ice crystal highlighting the c-axis and schematic, common c-axis CPO plots. In all cases, z is normal to foliation and x is parallel to the shear direction. B) Branching crystal and associated 2D thin section representation A B ICE CRYSTAL UNIAXIAL COMPRESSION SIMPLE SHEAR MULTI-MAXIMA Storglaciären Storglaciären 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1550 1600 1650 N SG9 SG18 SG23 SG24 SG16 SG26 SG19 SG12 SG11-B SG21 SG20 SG15 SG4(2) SG28 SG27 SG2-C SG6-B SG5 SG2-A SG7-C 0 250 500 750 1000 m Locations from 2015 and 2016 Locations from 2018 Trend of marginal foliation Stratification, So Approximate equilibrium line Glacier surface 50m contour intervals • Polythermal valley glacier characterized by a cold surface layer in the ablation zone • Margins and terminus are frozen to the overlying rock; most of the deformation in these areas is due to creep A B A) View up glacier (west) of the deformed southern margin of Storglaciären. B) Map of Storglaciären with sample locations. Samples collected in 2018 were analyzed using EBSD. flow Slab B 18 19 20 17 22 23 24 21 26 27 28 25 Slab B Slab A Slab A 2 3 4 1 6 7 8 5 10 11 12 9 14 15 16 13 18 16 15 14 13 12 17 10 9 8 7 6 11 4 3 2 1 5 COMPOSITE SECTION Top of sample North Bottom of slabs N Top of sample N Bottom of slabs Sample Preparation • 18 composite sections made for bulk CPO and 39 whole sections made for microstructures • Composites made to maximize number of grains collected and minimize number of repeated grains SG19, Polished Flow A B C A) Polished north face of SG19 with crystals intersecting the surface outlined, highlighting the coarse grain size and irregular shapes. B) Schematic composite sample preparation. Composites are cut perpendicular to fabric (foliation) and perpendicular to flow (lineation), stacked and glued using wet paper towels before being mounted and polished. C) sample prep illustrated using sample SG28 Cryo-EBSD Temperature ( o C) log Pressure (Pa) 0 -5 -150 -100 -50 0 5 Ice Vapor Sample exchange Ice Stage Pressure Cycle Cu braid Cold stage Transfer Chamber Sliding door gloves gloves Door closed A) Nitrogen glove box for sample transfer and cold stage fitted to the SEM. B) Schematic P-T diagram modified from Prior et al., 2015, to explain sublimation process. 4 A B Working Results X Y X Y X Y X Y X Y X Y X Y X Y X Y 0.5cm 5 10 0 0 degrees degrees 0.5cm 0.5cm 0.5cm One point per grain c-axes One point per grain c-axes contoured One point per grain a-axes contoured SG26 SG20 SG28 composite 1 composite 1 composite 1 composite 2 composite 2 composite 3 1.5cm 1.5cm 1.5cm 1.5cm 1.5cm 1cm SG26 WholeSection 3 x y • In all composite sections, x is the flow direction. Y is geographically horizontal in composites from the margin and y is geographically vertical in composite from front • Whole sections are not large enough to define crystal size, but capture the complexity of crystal shapes • Subgrains are present, but most large crystals show little lattice distortion • Multi-maxima fabrics are present, however, clustered c-axes that are only 2-3 degrees apart likely represent one crystal • It is likely that some point maxima are a result of sampling the same crystal A B 1 2 3 4 A) Three sets of composite sections representing ice from the northern margin, southern margin and the front of the glacier. Data from composites are stacked to give a bulk CPO for each sample. B) (1) Image of SG26, and whole section 3 in the cold room (2) EBSD map of whole section 3 (3) Nearest neighbour pixel misorientations within grains, highlighting subgrains in the section, and (4) Kernel Average Misorientation (KAM) map References (1) Faria, S.H., Weikusat, I., Azuma, N., 2014. The microstructure of polar ice. Part II: State of the art. Journal of Structural Geology 61, 21-49. (2) Wilson, C.J.L., Peternell, M., Piazolo, S., Luzin, V., 2014. Microstructure and fabric development in ice: Lessons learned from in situ experiments and implications for understanding rock evolution. Journal of Structural Geology 61, 50-77. (3) Hudleston, P.J., 2015. Structures and fabrics in glacial ice: A review. Journal of Structural Geology 81, 1-27. (4) Prior, D.J., Lilli, K., Seidemann, M., Vaughan, M., Becroft, L., Easingwood, R., Diebold, S., Obbard, R., Daghlian, C., Baker, I., Caswell, T., Golding, N., Goldsby, D., Durham, W.B., Piazolo, S., Wilson, C.J.L. 2015. Making EBSD on water ice routine. Journal of Microscopy 259, 237- 256. Acknowledgements • Sheng Fan, Marianne Negrini, Pat Langhorne, Nathaniel Parsons and Rilee Thomas at the University of Otago for use of their facilities and help with sample preparation and analyses • Hannah Blatchford for her help transporting samples to NZ, and for her help with sample preparation • Cameron Meyers for all of his help with MTEX • Troy Zimmerman as a field assistant • The University of Stockholm and the staff at the Tarfala Research Station for their hospitality and assistance