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Boundary structures and grain shape in deformed multilayered polycrystalline ice
Tectonophysics, 57 (1979) T19-T25 0 Elsevier Scientific Publishing Company,
Amsterdam,
Printed
T19
in The Netherlands
Letter Section
.-
Boundary structures and grab shape in deformed multilayered polycrystalline ice
C.J.L. WILSON Schooi
of Earth Sciences,
(Received
March 20,1979;
Uniuersity of Melbourne, Parku~Z~e,3052 ~Australia~ accepted
May 14,1979)
ABSTRACT Wilson, C.J.L., polyerystalline
1979. Boundary structures and grain ice. Tectonophysies, 57: T19-T25.
shape
in deformed
multilayered
Microstructures observed at the interface between and within quartzite layers of differing grain-sizes and compositions have been simulated using ice as an analoque for quartz. The experiments suggest that, unless an impurity is present in an individual layer or along a boundary the initial grain-size differences will not be preserved. Modification of grain shape can also be reIated to the presence of impurities.
INTRODUCTION
In the study of the deformation within a multilayered polycryst~line material, the nature and processes operating at layer boundaries are of fundamental importance. A simple situation is one in which two monomineralic polycrystalline layers having identical compositions but different initial grain-sizes are juxtaposed, as in many quartzite successions. Such a boundary may be free of other mineral phases but in general “bedding plane” boundaries in most natural quartzites contain a fine material film of another phase or occasional impurities. To model this, ice has been used as an analogue for quartz because of its hexagonal crystal structure and similarity in optical characteristics. The advantage in using ice is that, because of its low melting point and ease of deformation, expensive high-temperature-pressure deformation equipment is not required. The analogy between ice and quartz is not complete for, in terms of the crystal structure, there are significant differences, particularly the bonding, diffusion, production and mobility of dislocations, and the nature of grain nucleation. Two easily-determined characteristics that are comparable are the developed preferred orientations and the crystallographic slip systems (Table I). These similarities and the close optical analogy justify the use of polycrystalline ice to model microstructural changes in quart&e. Extensive laboratory investigations by Steinemann (1958), Rigsby (1960), Jonas and Miiller (1969), Kamb (1972), and Budd and Matsuda (1974) have
T20 TABLE I A comparison * ice
of experimentally
determined
and suggested
glide systems
and __-
Quartz
Ice
glide plane
glide direction
glide plane
glide direction
c (0001) basal m lOi prism
u Ci2io)
(0001)
(i2io)
a (i2io) C(OOO1) u + c (i2i3) u (i2io) a+~(ii23) U+ c (ii231
{ioio}
tizio) (0001) (i2i3)
(ioii}
for quartz
{oiii}
rhomb
{2ii2},(1122)
(1122)
(ii23)
trigonal dipyramid *The data for quartz is discussed by Ball and White (1978); the ice data is taken Mugurama and Higashi (1963) and Fukuda and Higashi (1969).
from
demonstrated in considerable detail the microstructural changes that occur during dynamic or syntectonic recrystallization of polycrystalline ice. By deforming artificial bicrystals of ice (formed by freezing two single crystals together), Rigsby (1960) also found that the boundary between the two crystals migrated and that new grains were nucleated at this site. However, neither the processes at the interface between deforming polycrystalline ice of different grain-sizes, nor the effect of large scale impurities on microstructure has been studied. Such microstructural observations are described in this letter, three sets of boundary conditions were studied: (1) An ice-ice interface, between layers of polycrystalline ice of contrasting grain-size. (2) Interfaces between layers of polycrystalline ice containing rigid objects and impurities, namely, mica plates, ink markings and cotton threads. (3) Interfaces coated with a thin plastic film of Formvar. EXPERIMENTAL
DETAILS
Cylinders of polycrystalline ice were prepared from sieved ice-crushings and distilled water. Three different grain-size fractions were used in the experiments: 1 mm (F or fine grained), 2 mm (M or medium grained) and 2 to 4 mm (C or coarse grained). Air bubbles were almost completely removed from the M and C samples. However, in the F sample, small bubbles of 0.1 to 0.2 mm diameter were irregularly distributed in grain boundary areas at an estimated concentration of 10 mm- 3 . In addition, samples of M ice containing a small concentration (7% mica by volume) of synthetic fluorine
T21
phlogopite (Synthamica 202) were prepared by mixing 0.2 to 1.0 mm sized grains to the sieved ice crushings. Each cylinder was cut into a number of plates varying in thickness from 1 mm to 6 mm, and composite blocks were prepared (Wilson and RussellHead, 1979) by freezing alternate layers together in a variety of combinations (Fig. 1A). Strain markers, either thin cotton thread or ink-filled scratch marks, had been placed at 5 mm intervals over some interfaces to record local strain variations within the specimen. In other samples, surfaces were coated (Wilson and Russell-Head, 1979) with a 5 pm film of polyvinyl formar (Formvar). All samples were finished to approximately 50 X 50 X 60 cm and were deformed in a plane strain apparatus similar to those described by Kamb (1972), and Budd and Matsuda (1974). Specimens were loaded in compression parallel to the layer boundaries, and parallel to the long axis of the prepared block. The loads on the layered blocks were either 83 kg, 100 was ty&ally kg or i50 kg. During loading and unloading the temperature - lO”C, and for the deformation it was controlled to either - 0.5 or - 1.00 +_ 0.05”C. EXPERIMENTAL
OBSERVATIONS
Ice--ice inter@es. In multilayered boundaries can be readily identified the F layer (Fig. 2B) other interfaces
1
2
3
4
5
blocks shortened > lo%, F-M or F--C by the concentration of air bubbles in cannot be recognized unless a marker is
1
2
3
4
5
Fig. 1. Thin-section mic~st~etures in multilayered polyerystalline ice. A. Undeformed sample composed of polycrystalline F(3), M(4 and 5) and C(1 and 2) layers held together by a thin film of frozen water. No ink-lines were inserted between layers 2 and 3. B. After 12% shortening at -0.5% under a load of 100 kg. Original grain size differences between individual layers have not been preserved. Ice-ice interfaces are recognized adjacent to the ink-lines (layers 4-5). Away from the ink-lines local preservation of interfaces is noted (layers 1-2 and 3-4). Interfaces free of impurities (layers 2-3) cannot be identified after deformation and extensive grain growth has occurred.
T22
Fig. 2. A. A thick section of undeformed starting material. The F layers contain hig,h bubble concentrations and are bonded with frozen water to an M layer that contair IS plates of mica. B. A thick section in plane polarized light of a sample similar to A after 12% shortenin % at -0.5”C under a load of 100 kg. The boundary between the F and M layers is on1Y identifiable from the air bubble or mica concentrations. C. The effect of mica (M) on the microstructure with the development of planer ice mica grain boundaries in the deformed sample B (crossed nicols). D. Deformed cotton threads.
T23
present. The film of frozen distilled water used to bond the layers together (Figs. 1A and 2A) is usually ~distin~ishable (Fig. 1B). Across the former interface individual grains have irregular shapes and constitute an interlocking three dimensional aggregate. Grain-size differences between layers are no longer distinct (Fig. 1B). Mica ~~~uri~~~s.The microstructure is markedly changed by the presence of mica within a layer and on the interface. I&--mica boundaries are straight (Fig. 2C) whereas ice-ice boundaries are gently curved (Fig. 1B). The average grain-size in areas where mica is present is smaller than that in areas free
Fig. 3. Multilayered sample shortened 36% at -1°C under a load of 83 kg. The initial planar surfaces were coated with Formvar and frozen together with a film of distiiled water. After deformation original grain-size differences between F and M layers were preserved across an interface. A. A thick section in plane polarized light. B. Thin section in crossed nicols illustrating the microstructure.
T24
of mica. The mica appears to preserve original grain-size differences between adjacent layers. This strongly suggests that the presence of mica inhibits grain-boundary migration and hence an increase in grain-size. Ink impurities. After deformation of the sample ink-lines have become discontinuous and are located along grain boundaries (Fig. 1B). Near the inklines grain shape is locally modified enabling the former interface to be discerned. Away from the ink-lines the microstructure is an interlocking grain aggregate identical to a deformed ice-ice interface. Cotton threads. These deform with the ice aggregate and develop a series of irregular small-amplitudes folds (Fig. 2D). The amplitude and wavelength of the folds are similar in size to the average recrystallized grain. They are non-penetrative in the third dimension and appear unrelated to the buckling, which has a large wavelength and small amplitude, observed on the outer surface of the deformed block. Grain shape and size is modified as it was in the vicinity of the ink-lines. The rest of the interface cannot be distinguished. Ice-plastic interface. Relative grain-size differences have been preserved across interfaces coated with a plastic film. However, close examination of the interface reveals that, where multilayer boundaries were firmly frozen together, small undulations develop in the plastic film (Fig. 3). The scale of these undulations is similar to the grain size of the finer layer. Where boundaries were initially unattached (that is, imperfectly frozen together), layers were able to flex, forming voids, and interfaces remain relatively smooth. CONCLUSIONS
In the artificial ice-rocks described here, deformation and syntectonic recrystallization obliterate any interface which was characterized only by an initial grain-size difference. The presence of linear objects and fine impurities on the interface results in local grain-shape modification and local preservation of the interface. When a thin material film entirely covers an interface, grain-size differences across layer boundaries or “pseudo-bedding planes” are clearly preserved. Grain-boundary migration is inhibited in layers containing a platy mineral such as mica, and hence differences between initial layers are retained (Fig. 2B). Local grain shape modification is marked in such layers and the microstructure of the ice aggregate is almost identical to many natural quartzites (cf. Wilson, 1973, Fig. 8). Therefore, in pure metamorphic quartzites where boundary markers and impurities do not exist, former grain-size differences that define bedding planes or other interfaces, such as current bedding, may be difficult to recognize. ACKNOWLEDGEMENTS
This research has been supported by the University of Melbourne, the Australian Antarctic Division and the Australian Research Grants Commit-
T25
tee. L.B. Harris is thanked for his help in setting up the initial experiments and D.S. Russell-Head for his able assistance with the later experiments. W.D. Means is thanked for the supply of the synthetic fluorine phlogopite mica used in some of these experiments.
REFERENCES Ball, A. and White, S., 1978. On the deformation of quart&e. Phys. Chem. Miner., 3: 163-172. Budd, W.F. and Matsuda, M., 1974. On preferred orientation of polycrystalline ice by bi-axial Creep test. Low Temperature Sci. Ser., A 32: 261-265. Fukuda, A. and Higashi, A., 1969. X-ray diffraction topographic studies of the deformation behaviour of single crystals. In: N. Riehl, B. Bullerner and H. Engelhardt (Editors), Physics of Ice. Plenum, New York, N.Y., pp. 239-250. Jonas, J.J. and Miiller, F., 1969. Deformation of ice under high internal shear stresses. Canad. J. Earth Sci., 6: 963-968. Kamb, B.W., 1972. Experimental recrystallization of ice under stress. Am. Geophys Union, Monogr., 16: 211-241. Mu~~rna, J. and Higashi, A., 1963. Non-basal glide bands in ice crystals. Nature, 198: 573. Rigsby, G.P., 1960. Crystal orientation in glaciers and experimentally deformed ice. J. Glacial., 3: 509-606. Steinemann, S., 1958. Experimentelle Untersuchungen zur PlastizitLt von Eis. Beitr. Geol. Schweiz, Hydrologie, 10: 72 pp. Wilson, C.J.L., 1973. The prograde microfabric in a deformed quartzite sequence, Mount Isa, Australia. Tectonophysics, 19: 39-81. Wilson, C.J.L. and Russell-Head, D.S., 1979. Experimental folding in ice and the resultant c axis fabrics. Nature, 279: 49-51.
Amsterdam,
Printed
T19
in The Netherlands
Letter Section
.-
Boundary structures and grab shape in deformed multilayered polycrystalline ice
C.J.L. WILSON Schooi
of Earth Sciences,
(Received
March 20,1979;
Uniuersity of Melbourne, Parku~Z~e,3052 ~Australia~ accepted
May 14,1979)
ABSTRACT Wilson, C.J.L., polyerystalline
1979. Boundary structures and grain ice. Tectonophysies, 57: T19-T25.
shape
in deformed
multilayered
Microstructures observed at the interface between and within quartzite layers of differing grain-sizes and compositions have been simulated using ice as an analoque for quartz. The experiments suggest that, unless an impurity is present in an individual layer or along a boundary the initial grain-size differences will not be preserved. Modification of grain shape can also be reIated to the presence of impurities.
INTRODUCTION
In the study of the deformation within a multilayered polycryst~line material, the nature and processes operating at layer boundaries are of fundamental importance. A simple situation is one in which two monomineralic polycrystalline layers having identical compositions but different initial grain-sizes are juxtaposed, as in many quartzite successions. Such a boundary may be free of other mineral phases but in general “bedding plane” boundaries in most natural quartzites contain a fine material film of another phase or occasional impurities. To model this, ice has been used as an analogue for quartz because of its hexagonal crystal structure and similarity in optical characteristics. The advantage in using ice is that, because of its low melting point and ease of deformation, expensive high-temperature-pressure deformation equipment is not required. The analogy between ice and quartz is not complete for, in terms of the crystal structure, there are significant differences, particularly the bonding, diffusion, production and mobility of dislocations, and the nature of grain nucleation. Two easily-determined characteristics that are comparable are the developed preferred orientations and the crystallographic slip systems (Table I). These similarities and the close optical analogy justify the use of polycrystalline ice to model microstructural changes in quart&e. Extensive laboratory investigations by Steinemann (1958), Rigsby (1960), Jonas and Miiller (1969), Kamb (1972), and Budd and Matsuda (1974) have
T20 TABLE I A comparison * ice
of experimentally
determined
and suggested
glide systems
and __-
Quartz
Ice
glide plane
glide direction
glide plane
glide direction
c (0001) basal m lOi prism
u Ci2io)
(0001)
(i2io)
a (i2io) C(OOO1) u + c (i2i3) u (i2io) a+~(ii23) U+ c (ii231
{ioio}
tizio) (0001) (i2i3)
(ioii}
for quartz
{oiii}
rhomb
{2ii2},(1122)
(1122)
(ii23)
trigonal dipyramid *The data for quartz is discussed by Ball and White (1978); the ice data is taken Mugurama and Higashi (1963) and Fukuda and Higashi (1969).
from
demonstrated in considerable detail the microstructural changes that occur during dynamic or syntectonic recrystallization of polycrystalline ice. By deforming artificial bicrystals of ice (formed by freezing two single crystals together), Rigsby (1960) also found that the boundary between the two crystals migrated and that new grains were nucleated at this site. However, neither the processes at the interface between deforming polycrystalline ice of different grain-sizes, nor the effect of large scale impurities on microstructure has been studied. Such microstructural observations are described in this letter, three sets of boundary conditions were studied: (1) An ice-ice interface, between layers of polycrystalline ice of contrasting grain-size. (2) Interfaces between layers of polycrystalline ice containing rigid objects and impurities, namely, mica plates, ink markings and cotton threads. (3) Interfaces coated with a thin plastic film of Formvar. EXPERIMENTAL
DETAILS
Cylinders of polycrystalline ice were prepared from sieved ice-crushings and distilled water. Three different grain-size fractions were used in the experiments: 1 mm (F or fine grained), 2 mm (M or medium grained) and 2 to 4 mm (C or coarse grained). Air bubbles were almost completely removed from the M and C samples. However, in the F sample, small bubbles of 0.1 to 0.2 mm diameter were irregularly distributed in grain boundary areas at an estimated concentration of 10 mm- 3 . In addition, samples of M ice containing a small concentration (7% mica by volume) of synthetic fluorine
T21
phlogopite (Synthamica 202) were prepared by mixing 0.2 to 1.0 mm sized grains to the sieved ice crushings. Each cylinder was cut into a number of plates varying in thickness from 1 mm to 6 mm, and composite blocks were prepared (Wilson and RussellHead, 1979) by freezing alternate layers together in a variety of combinations (Fig. 1A). Strain markers, either thin cotton thread or ink-filled scratch marks, had been placed at 5 mm intervals over some interfaces to record local strain variations within the specimen. In other samples, surfaces were coated (Wilson and Russell-Head, 1979) with a 5 pm film of polyvinyl formar (Formvar). All samples were finished to approximately 50 X 50 X 60 cm and were deformed in a plane strain apparatus similar to those described by Kamb (1972), and Budd and Matsuda (1974). Specimens were loaded in compression parallel to the layer boundaries, and parallel to the long axis of the prepared block. The loads on the layered blocks were either 83 kg, 100 was ty&ally kg or i50 kg. During loading and unloading the temperature - lO”C, and for the deformation it was controlled to either - 0.5 or - 1.00 +_ 0.05”C. EXPERIMENTAL
OBSERVATIONS
Ice--ice inter@es. In multilayered boundaries can be readily identified the F layer (Fig. 2B) other interfaces
1
2
3
4
5
blocks shortened > lo%, F-M or F--C by the concentration of air bubbles in cannot be recognized unless a marker is
1
2
3
4
5
Fig. 1. Thin-section mic~st~etures in multilayered polyerystalline ice. A. Undeformed sample composed of polycrystalline F(3), M(4 and 5) and C(1 and 2) layers held together by a thin film of frozen water. No ink-lines were inserted between layers 2 and 3. B. After 12% shortening at -0.5% under a load of 100 kg. Original grain size differences between individual layers have not been preserved. Ice-ice interfaces are recognized adjacent to the ink-lines (layers 4-5). Away from the ink-lines local preservation of interfaces is noted (layers 1-2 and 3-4). Interfaces free of impurities (layers 2-3) cannot be identified after deformation and extensive grain growth has occurred.
T22
Fig. 2. A. A thick section of undeformed starting material. The F layers contain hig,h bubble concentrations and are bonded with frozen water to an M layer that contair IS plates of mica. B. A thick section in plane polarized light of a sample similar to A after 12% shortenin % at -0.5”C under a load of 100 kg. The boundary between the F and M layers is on1Y identifiable from the air bubble or mica concentrations. C. The effect of mica (M) on the microstructure with the development of planer ice mica grain boundaries in the deformed sample B (crossed nicols). D. Deformed cotton threads.
T23
present. The film of frozen distilled water used to bond the layers together (Figs. 1A and 2A) is usually ~distin~ishable (Fig. 1B). Across the former interface individual grains have irregular shapes and constitute an interlocking three dimensional aggregate. Grain-size differences between layers are no longer distinct (Fig. 1B). Mica ~~~uri~~~s.The microstructure is markedly changed by the presence of mica within a layer and on the interface. I&--mica boundaries are straight (Fig. 2C) whereas ice-ice boundaries are gently curved (Fig. 1B). The average grain-size in areas where mica is present is smaller than that in areas free
Fig. 3. Multilayered sample shortened 36% at -1°C under a load of 83 kg. The initial planar surfaces were coated with Formvar and frozen together with a film of distiiled water. After deformation original grain-size differences between F and M layers were preserved across an interface. A. A thick section in plane polarized light. B. Thin section in crossed nicols illustrating the microstructure.
T24
of mica. The mica appears to preserve original grain-size differences between adjacent layers. This strongly suggests that the presence of mica inhibits grain-boundary migration and hence an increase in grain-size. Ink impurities. After deformation of the sample ink-lines have become discontinuous and are located along grain boundaries (Fig. 1B). Near the inklines grain shape is locally modified enabling the former interface to be discerned. Away from the ink-lines the microstructure is an interlocking grain aggregate identical to a deformed ice-ice interface. Cotton threads. These deform with the ice aggregate and develop a series of irregular small-amplitudes folds (Fig. 2D). The amplitude and wavelength of the folds are similar in size to the average recrystallized grain. They are non-penetrative in the third dimension and appear unrelated to the buckling, which has a large wavelength and small amplitude, observed on the outer surface of the deformed block. Grain shape and size is modified as it was in the vicinity of the ink-lines. The rest of the interface cannot be distinguished. Ice-plastic interface. Relative grain-size differences have been preserved across interfaces coated with a plastic film. However, close examination of the interface reveals that, where multilayer boundaries were firmly frozen together, small undulations develop in the plastic film (Fig. 3). The scale of these undulations is similar to the grain size of the finer layer. Where boundaries were initially unattached (that is, imperfectly frozen together), layers were able to flex, forming voids, and interfaces remain relatively smooth. CONCLUSIONS
In the artificial ice-rocks described here, deformation and syntectonic recrystallization obliterate any interface which was characterized only by an initial grain-size difference. The presence of linear objects and fine impurities on the interface results in local grain-shape modification and local preservation of the interface. When a thin material film entirely covers an interface, grain-size differences across layer boundaries or “pseudo-bedding planes” are clearly preserved. Grain-boundary migration is inhibited in layers containing a platy mineral such as mica, and hence differences between initial layers are retained (Fig. 2B). Local grain shape modification is marked in such layers and the microstructure of the ice aggregate is almost identical to many natural quartzites (cf. Wilson, 1973, Fig. 8). Therefore, in pure metamorphic quartzites where boundary markers and impurities do not exist, former grain-size differences that define bedding planes or other interfaces, such as current bedding, may be difficult to recognize. ACKNOWLEDGEMENTS
This research has been supported by the University of Melbourne, the Australian Antarctic Division and the Australian Research Grants Commit-
T25
tee. L.B. Harris is thanked for his help in setting up the initial experiments and D.S. Russell-Head for his able assistance with the later experiments. W.D. Means is thanked for the supply of the synthetic fluorine phlogopite mica used in some of these experiments.
REFERENCES Ball, A. and White, S., 1978. On the deformation of quart&e. Phys. Chem. Miner., 3: 163-172. Budd, W.F. and Matsuda, M., 1974. On preferred orientation of polycrystalline ice by bi-axial Creep test. Low Temperature Sci. Ser., A 32: 261-265. Fukuda, A. and Higashi, A., 1969. X-ray diffraction topographic studies of the deformation behaviour of single crystals. In: N. Riehl, B. Bullerner and H. Engelhardt (Editors), Physics of Ice. Plenum, New York, N.Y., pp. 239-250. Jonas, J.J. and Miiller, F., 1969. Deformation of ice under high internal shear stresses. Canad. J. Earth Sci., 6: 963-968. Kamb, B.W., 1972. Experimental recrystallization of ice under stress. Am. Geophys Union, Monogr., 16: 211-241. Mu~~rna, J. and Higashi, A., 1963. Non-basal glide bands in ice crystals. Nature, 198: 573. Rigsby, G.P., 1960. Crystal orientation in glaciers and experimentally deformed ice. J. Glacial., 3: 509-606. Steinemann, S., 1958. Experimentelle Untersuchungen zur PlastizitLt von Eis. Beitr. Geol. Schweiz, Hydrologie, 10: 72 pp. Wilson, C.J.L., 1973. The prograde microfabric in a deformed quartzite sequence, Mount Isa, Australia. Tectonophysics, 19: 39-81. Wilson, C.J.L. and Russell-Head, D.S., 1979. Experimental folding in ice and the resultant c axis fabrics. Nature, 279: 49-51.