Using electron backscatter diffraction (EBSD) to characterize microstructures in coarse-grained ice 1MONZ, Morgan E., 1HUDLESTON, Peter J., and 2PRIOR, David J.

1Department of Earth Sciences, University of Minnesota, Minneapolis, MN 554552Department of Geology, University of Otago, Dunedin, New Zealand

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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

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Background of CPO in Ice• Dominant deformation mechanism in ice is glide on the basal crystallographic plane1,2

• Typical fabrics: small circle girdle, single maximum and deep in ice sheets and valley glaciers multiple maxima3

• Issues with coarse-grained ice: 1) U-stage is low accuracy and only measures c-axes, and 2) branching crystals cause a probable bias

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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

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ICE CRYSTALUNIAXIAL

COMPRESSION

SIMPLE SHEAR MULTI-MAXIMA

Storglaciären

Storglaciären 1200

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SG9

SG18

SG23

SG24

SG16

SG26

SG19

SG12

SG11-BSG21SG20

SG15SG4(2)

SG28SG27

SG2-C

SG6-B SG5SG2-A

SG7-C

0 250 500 750 1000 m

Locations from 2015 and 2016

Locations from 2018

Trend of marginal foliation

Stratification, SoApproximate 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

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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.

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Slab B1 8 1 9 2 017

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2 6 2 7 2 825Slab

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1 4 1 5 1 613

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COMPOSITE SECTION

Top of sample

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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 Surface A, North Face

Flow

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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

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gloves gloves

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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

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Working Results

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One point per grain c-axes

One point per grain c-axes contoured

One point per grain a-axes contoured

SG26

SG20

SG28

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composite 1

composite 1

composite 2

composite 2

composite 3

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SG26 WholeSection 3

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• 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

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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