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
STA
PM
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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B
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
2cm
1
2a
3a3k
3b3c
3d3f 3e
3g
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3i3j
2b4a
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11b41
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2cm
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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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B
ICE CRYSTALUNIAXIAL
COMPRESSION
SIMPLE SHEAR MULTI-MAXIMA
Storglaciären
Storglaciären 1200
1250
1300
1350
140014
501500
1550
160016
50
1550
1600
1650
N
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
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.
�ow
Slab B1 8 1 9 2 017
2 2 2 3 2 421
2 6 2 7 2 825Slab
B
Slab A
Slab A2 3 41
6 7 85
1 0 1 1 1 29
1 4 1 5 1 613
18
1615141312
17
109876
11
4321
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 Surface A, North Face
Flow
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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 (oC)
log
Pre
ssur
e (P
a)
0
-5
-150 -100 -50 0
5
Ice
Vapo
r
Sample exchange
Ice
Stag
e
Pressure Cycle
Cu braid
Cold stage
Transfer Chamber
Sliding door
gloves gloves
Doorclosed
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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0.5cm 5 1000 degrees degrees
0.5cm
0.5cm0.5cm
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
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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