Dislocation microstructures of MgSiO perovskite at a high pressure and temperature condition

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high pressure and temperature condition

Nobuyoshi Miyajima, Takehiko Yagi, Masaki Ichihara

To cite this version:

Nobuyoshi Miyajima, Takehiko Yagi, Masaki Ichihara. Dislocation microstructures of MgSiO per-

ovskite at a high pressure and temperature condition. Physics of the Earth and Planetary Interiors,

Elsevier, 2009, 174 (1-4), pp.153. �10.1016/j.pepi.2008.04.004�. �hal-00533029�

Title: Dislocation microstructures of MgSiO

perovskite at a high pressure and temperature condition

Authors: Nobuyoshi Miyajima, Takehiko Yagi, Masaki Ichihara

PII: S0031-9201(08)00059-9

DOI: doi:10.1016/j.pepi.2008.04.004

Reference: PEPI 4920

To appear in: Physics of the Earth and Planetary Interiors Received date: 29-9-2007

Revised date: 11-2-2008 Accepted date: 2-4-2008

Please cite this article as: Miyajima, N., Yagi, T., Ichihara, M., Dislocation microstructures of MgSiO

perovskite at a high pressure and temperature condition, Physics of the Earth and Planetary Interiors (2007), doi:10.1016/j.pepi.2008.04.004 This is a PDF file of an unedited manuscript that has been accepted for publication.

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Accepted Manuscript

The revised Ms (PEPI –D-07-00203)

1 2

Dislocation microstructures of MgSiO

perovskite at a high pressure

3

and temperature condition

4 5

Nobuyoshi Miyajima

, Takehiko Yagi

, Masaki Ichihara

6

1

Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan

7

2

Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany

8

*

Corresponding author.

9

Tel.: +49-921-55-3728; Fax: +49-921-55-3769

10

E-mail address: [email protected] (N. Miyajima)

11 12

Abstract.

13

Dislocation microstructures of polycrystalline MgSiO

perovskite, synthesized in a multi-

14

anvil type high pressure apparatus at 26 GPa and 2023 K for 600 min, have been

15

investigated using transmission electron microscopy (TEM). The recovered MgSiO

16

perovskite displayed deformation lamellae under cross-polarized optical microscopy,

17

suggesting that the sample was deformed during the high pressure experiment. TEM

18

observations showed that curved dislocations with Burgers vectors lying in the {110}

19

plane, potentially b = <110> and [00w] (orthorhombic symmetry) were nucleated. Low-

20

angle tilt boundaries controlled by <110> dislocation climb were also activated at high

21

temperature and pressure. The potential Burgers vector of b = <110> is consistent with

22

previous results of slip to <100>

, [100]

and [010]

(cubic symmetry and pseudo-

23

Accepted Manuscript

cubic symmetry) on non-silicate perovskite analogues deformed at high temperatures and

1

ambient pressure. The microstructures of climb-accommodated dislocation creep

2

controlled by diffusion in the MgSiO

perovskite can provide useful information on the

3

plasticity at high temperatures and slow strain rates corresponding to the Earth’s lower

4

mantle.

5 6

Keywords: MgSiO

perovskite; Dislocation microstructures; Burgers vectors; TEM;

7

High pressure

8 9

Introduction

10

Dislocation textures of MgSiO

silicate perovskite under high pressures are of interest

11

in high-pressure mineral physics, seismology, and geodynamics of the Earth’s interior,

12

Since MgSiO

perovskite is the most dominant material in the Earth’s lower mantle (e.g.,

13

Ito and Takahashi, 1989), mechanical properties of this material are likely to dominate the

14

rheology of this large region of the Earth (e.g., Karato et al. 1995; Cordier et al. 2004)

15

which accounts for more than half of the total volume of our planet. In order to explain

16

the source of local anisotropy in the D” layer, deformational textures of the silicate

17

perovskite are indispensable in comparison with the CaIrO

-structured phase (“post

18

perovskite”), which is one of the strongest candidates because of its high elastic

19

anisotropy. However, no direct demonstrations on the deformational microstructures of

20

polycrystalline MgSiO

perovskite at high temperatures have been published yet.

21

Unfortunately, the MgSiO

silicate perovskite phase is unstable under ambient pressures

22

(e.g., Wang et al. 1992). It is not possible to study the mechanical properties and lattice

23

Accepted Manuscript

defects of the phase under usual laboratory conditions, such as high temperature around

1

1000-2000 K at an ambient pressure. Plastic properties of the analogue oxide phases with

2

the perovskite structure (non-silicate perovskite) at ambient conditions have been

3

investigated in order to help constrain those of the silicate perovskite, but there is no

4

systematic relation of their high temperature creep behaviour (e.g., Poirier et al., 1989;

5

Wright et al.. 1992; Wang et al., 1999). One of the reasons for the divergence of the

6

results (e.g., slip systems) of analogue studies may be due to the difference in symmetries

7

(cubic and tetragonal in addition to orthorhombic) in some oxide and fluoride perovskites

8

which undergo phase transitions at high temperatures in deformation experiments, which

9

stabilize a higher symmetry rather than the orthorhombic symmetry (Space group, Pbnm)

10

of MgSiO

. For an orthorhombic perovskite analogue, Wang et al. (1999) reported a

11

dominant (001)[010] slip system with the minor (012)[100] slip system in YAlO

at high

12

temperatures of 1763-1883 K (0.82-0.88 T/T

, where T

is the melting temperature at 0.1

13

MPa) and an ambient pressure. Walte et al. (2007) have recently performed coaxial

14

shortening experiments in the orthorhombic CaIrO

perovskite at 1GPa and 1450°C [T/T

15

= 0.8-0.9, where T

is about 1550 °C at 1-3 GPa, according to Hirose and Fujita (2005)].

16

They have observed curved [100] and [010] dislocations as well as <110> screw

17

dislocations in the recovered sample. But no crystallographic preferred orientation could

18

be detected in their deformed samples, suggesting that dislocation glide was not the main

19

deformation mechanism.

20

For MgSiO

perovskite, Cordier et al. (2004) reported that Burgers vectors, b =

21

[100]

and [010]

in the orthorhombic symmetry (S.G., Pbnm) were activated in

22

deformation experiments at 25 GPa and 1673 K (~0.6 T/T

) for 1 h, whereas no

23

Accepted Manuscript

preference of b = <110>

was reported, which is expected to become dominant under

1

high temperatures from studies of various perovskite analogues (e.g., Poirier et al. 1989). ,

2

Carrez et al. (2007) and Ferre et al. (2007) presented a numerical modelling of

3

dislocations for possible slip systems of MgSiO

at 30 GPa and 0 K. They demonstrated

4

that the orthorhombic distortion has a significant influence on the dislocation core fine

5

structure, which suggests perovskite-structured materials having various symmetries do

6

not constitute an isomechanical group.

7

In the present study we report a transmission electron microscopy (TEM)

8

investigation of a type of dislocation and subgrain boundary consisted of a dislocation

9

array in a synthetic MgSiO

phase. With a view to understanding better the rheology in

10

the Earth’s lower mantle, the first report on the deformation microtextures of the MgSiO

11

phase can provide us with useful information.

12 13

Experimental procedure

14

Polycrystalline MgSiO

perovskite was synthesized from synthetic MgSiO

15

clinopyroxene at 23 GPa and 2073 K for 152 min using a Kawai-type multi-anvil

16

apparatus (1200 ton press) with a 10/4-type assembly at Bayerisches Geoinstitut (BGI),

17

University of Bayreuth. Subsequently, the recovered silicate perovskite was annealed as

18

part of a diffusion couple at 26 GPa and 2023 K for 600 min using the same high pressure

19

apparatus with a 7/3-type assembly (Miyajima et al. 2004). The temperature corresponds

20

to ~0.7 T/T

(where T

is the melting temperature at 23 GPa) (Ito and Katsura 1992). The

21

recovered sample was cut and polished to 30-m thin sections and then mounted on a 3-

22

mm Mo grid with a 0.3 mm hole. Electron transparent foils were produced by Ar-ion

23

Accepted Manuscript

milling at 4.0 kV under a low beam angle of 13° at lower temperatures using an Ar

1

milling attached with a liquid-nitrogen cooling system. TEM characterization was carried

2

at the Institute for Solid State Physics, University of Tokyo using a transmission electron

3

microscope (JEOL 2010F) operated at 200 kV. In the TEM observations, we used a liquid

4

nitrogen cooling holder in order to reduce the amorphization process during electron

5

irradiations (Miyajima et al. 2004). The conventional determination of Burgers vectors of

6

the dislocations, based on the g ▪ b = 0 and g ▪ (b x u) = 0 invisibility criteria (Edington,

7

1976), was used with Bright field (BF) and Dark Field (DF) images in TEM.

8

In this study, the convention of the orthorhombic symmetry (Space Group, Pbnm) and

9

the unit cell setting (the c-axis > the b-axis > the a-axis) is used for the crystallographic

10

direction, [uvw] or < uvw> and plane, (hkl) or {hkl} of the MgSiO

perovskite phase. In

11

case of a different symmetry, the corresponding subscript is additionally noted after those

12

crystallographic directions and planes (e.g, < uvw>

and {hkl}

for cubic symmetry;

13

<uvw>

and {hkl}

for pseudo-cubic symmetry, where an unit cell with a

= 2 x a

=

14

2 x a

, b

= 2 x a

= 2 x b

and c

= c

, followed in Appendix A in Wang

15

et al. 1999).

16 17

TEM results and interpretations

18

Optical image of the recovered specimen shows four single crystal domains of silicate

19

perovskite, which appears with crossed polarizers (Domain 1-4 in Figure 1). The whole

20

specimen exhibits a barrel shape, a characteristic shape for samples deformed plastically

21

in compression (Li et al. 2003), indicating that the sample was plastically deformed

22

during the experiment. In one of the middle domains (Domain 2 in Figure 1), high

23

Accepted Manuscript

density of bands of birefringence is visible as black lines in the images. The birefringence

1

bands can therefore be interpreted as elongate cells (Boland et al. 1971) and deformation

2

lamellae (McLaren et al. 1970; McLaren and Hobbs, 1972; Hobbs et al. 1976), on which

3

plastic deformation occurred. Their crystallographic orientation is nearly parallel to the

4

domain walls that are nearly parallel to the {110} twin plane. This parallel relation

5

between the birefringence band and the twins is different from the relation that the {110}

6

twins are sheared by deformation bands in Cordier et al (2004). We examined the

7

dislocation texture of the domain with the conventional contrast method in TEM. Typical

8

dislocation microstructures of a synthetic MgSiO

silicate perovskite are shown in Figure

9

2. In the TEM image along the 1 1 1 zone axis, most of free dislocations do not form

10

systematic arrays in any particular crystallographic orientations and are not straight

11

enough to determine the direction of the dislocation lines and the slip systems. However

12

some dislocation lines are nearly parallel to the [110] or   ^{1 10} directions, corresponding to

13

[100]

or [010]

directions in case of the pseudo-cubic symmetry (Stereographic

14

projection on the zone axis in Figure 2(d)). These dislocation lines fulfil the invisibility

15

criterion with g = 110 (arrows in Figure 2(c)), i.e. the Burgers vectors lies in the (110)

16

plane. It is reasonable to assume that the Burgers vectors are b =   ^{1 1 0} , consistent with a

17

potential slip system of (001)<110> or   ^{1 1 0} <110> found in previous studies of non-

18

silicate perovskites. Figure 3 shows a typical result of contrast experiments on some free

19

dislocations. Here, the specimen was viewed along the 11 3 zone axis with different

20

diffraction vectors (g). Almost all dislocations except for one labelled by D are visible

21

and the contrasts are not largely changed with g = 121, 112, 02 and 3 2 1 . The result 0

22

Accepted Manuscript

suggests that the Burgers vectors are not b = [u00] and [0v0], i.e. [100] and [010] but b =

1

<uv0> and [00w], i.e. <110> and [001]. Some subgrain boundaries with an array of

2

parallel dislocations are also observed in bright field image with g = 020 (Figure. 4(a)),

3

which may correspond to deformation lamellae of slightly different crystallographic

4

orientations in the optical image (Figure 1). The tilt boundary consists of parallel edge

5

dislocations forming a tilt boundary and a mixed dislocation (labelled ABC in Figure

6

4(a)), which might be a remnant of glide processes before the formation of the tilt

7

boundary. The texture in the orthorhombic perovskite is related with the climb motion of

8

edge dislocations controlled by diffusion. These dislocation microstructures are

9

potentially consistent with <110> dislocations (comparable direction to [100]

and

10

[010]

) of analogue perovskites in previous studies at high temperature corresponding to

11

~0.7 T/T

(Doukhan and Doukhan, 1986; Poirier et al., 1989; Besson et al., 1996) and

12

contradistinctive with [100]

and [010]

dislocations in MgSiO

perovskite (Cordier

13

et al., 2004).

14 15

Discussion and conclusion

16

The observed dislocation microstructures of silicate perovskite are more complex

17

than those of the post-perovskite phase with CaIrO

composition (i.e. Figure 2 in

18

Miyajima et al. 2006; Walte et al. 2007). On scanning the thin region by using TEM, the

19

dislocation lines do not appear to be aligned to any specific crystallographic directions

20

and the length of dislocation lines is shorter and curved in the foil oriented nearly to the

21

1 1

1 zone axis. Some longer dislocation lines exhibit parallelity with the projected [100]

22

and <110> directions, but the lines spread over various directions. This may be due to the

23

Accepted Manuscript

higher pseudo-symmetric multiplicity of the perovskite structure in comparison to the

1

post-perovskite structure. The present observation is consistent with co-activation of the

2

multiple slip systems, such as (010)[100], (100)[010], (001)[100], {110}<110> and

3

(001)<110> (e.g., Wenk et al. 2006), in perovskite structures, and as previously reported

4

in a variety of analogues of silicate perovskite (e.g., Poirier et al., 1989, Karato and Li,

5

1992, Wang et al., 1999) as well as in the MgSiO

silicate perovskite phase itself (Karato

6

et al., 1990; Cordier et al. 2004). It is noted that most dislocation lines are curly and not

7

confined in the above-mentioned glide planes, which indicates that not only dislocation

8

glide but also diffusion-assisted dislocation creep (climb-accommodated dislocation

9

creep) occurred during the dynamical recovery process assisted by diffusion at high

10

temperature. Therefore, the texture of a simple tilt boundary consisting of parallel edge

11

dislocations with possible Burgers vectors of b = <110> implies that dislocation climb

12

related with atomic diffusion becomes a dominate process at the high temperature of

13

2023 K, corresponding to ~0.7 T/T

. The interpretation of the deformation

14

microstructures is consistent with the dominant slip direction of [100]

and [010]

in

15

pseudo-cubic symmetry (equivalent to <110> in orthorhombic symmetry) at high

16

temperature in several analogue materials (Poirier et al., 1989).

17

In summary, dislocation microtextures of a synthetic high-pressure sample of MgSiO

18

are documented for the first time directly by using TEM. Potential dislocations with b =

19

<110> were nucleated and the microstructures indicated that dislocation creep assisted by

20

climb of the dislocation becomes dominant at the higher temperature of 2023 K,

21

corresponding to ~0.7 T/T

, which is contradistinctive with b = [100] (equivalent to

22

1/2<110>

) of the previous study at 1673 K, ~0.6 T/T

(Cordier et al. 2004). We

23

Accepted Manuscript

speculate that at the high temperature <110> dislocations are activated with [100] and

1

[010] dislocations, which are related with {110}<110> and (001)<110> slip system, as

2

expected from results of high temperature deformations with other non-silicate

3

perovskites. Further studies on the systematic deformation experiments and determination

4

of the Burgers vectors of the silicate perovskite are needed in discussing a strong contrast

5

of observed seismic anisotropy in the lowermost of the Earth’s lower mantle, as recently

6

discussed in an analogue study of perovskite and post perovskite phases in CaIrO

7

composition (Walte et al. 2007). The climb-accommodated dislocation creep controlled

8

by diffusion in the MgSiO

perovskite can play an important role in the plasticity of the

9

constituent materials at high temperatures and slow strain rates, corresponding to the

10

Earth’s lower mantle.

11 12

Acknowledgements

13

We thank D. Rubie, D. Frost and H. Schulze for his support of high pressure experiments

14

and preparation of the thin section of the recovered sample, respectively. We also

15

appreciate valuable discussions with P. Cordier, F. Heidelbach and F. Langenhorst for

16

valuable discussion and constructive comments from two anonymous reviewers. The

17

TEM analysis was done using facilities of the Materials Design and Characterization

18

Laboratory (MDCL), Institute for Solid State Physics, University of Tokyo. This work

19

was supported by the Visiting Scientists’ Program of Bayerisches Geoinstitut, Germany

20

and the Grant-in-aid for scientific research from Ministry of Education, Culture, Science,

21

Sport and Technology of Japanese Government (15740313) to N. Miyajima.

22

23

Accepted Manuscript

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Accepted Manuscript

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Accepted Manuscript

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Accepted Manuscript

Figure Captions

1

Figure 1. Optical photomicrograph of four single crystal domains of the synthetic

2

MgSiO

silicate perovskite phase under crossed polarizers. The specimen exhibits a

3

barrel shape, which usually results from high-temperature uniaxial compression

4

experiment. In the middle domain, label “D2”, deformation lamellae are visible as

5

parallel black linear lines by associated birefringence between crossed polarizers. Note:

6

The size of the single crystal domain is about 150 m length and 50 m width.

7 8

Figure 2. (a) TEM image showing dislocation microstructures in a single crystal domain

9

of the polycrystalline MgSiO

silicate perovskite phase. Some of the dislocations exist as

10

dislocation loop and split. The dislocation density is in the order of 10

m

. (b) A 1 1 1

11

zone axis image corresponding to lower right region (white circle) in (a), (c) Bright-field

12

image of the same region, some dislocations (open arrows) become invisible with the

13

diffraction condition of g = 110, while the other ones (e.g., grey arrow) are visible. (d)

14

Stereographic projection oriented with respect to zone axis images of (a) and (b). Some

15

elongate dislocations are approximately parallel to the trace of (100), (010) and (110),

16

which means the dislocation lines lie in the corresponding planes. Indexing of crystal

17

directions is for the upper hemisphere of the projection.

18 19

Figure 3. Dislocation analysis of dislocations in the MgSiO

perovskite. (a) Bright field

20

TEM image with g = 121, showing some free dislocations with label from A to G. (b)-(d)

21

Dark field TEM images with g = 121, g = 112, g = 02 2 and g = 1 0 3 , respectively. (e) The

22

nearest zone axis pattern along the 11 3 . The presence of the spot 01 was confirmed in 1

23

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terms of double diffraction, because of the extinction rule, (0kl): k = 2n in the space

1

group of Pbnm. (f) Stereographic projection oriented with respect to the nearest main

2

zone axis of (e).

3 4

Figure 4. (a) Bright field TEM image with g = 020, showing a subgrain boundary

5

consisting of parallel edge dislocations in the MgSiO

silicate perovskite. The array of

6

dislocations forming the boundary is approximately parallel to the trace of   ^{1 1 2 . A mixed}

7

dislocation is labelled as ABC. Insets indicate two beam condition of g = 020 (upper

8

right) for the bright filed image and the nearest main zone axis (lower left). (b) Dark field

9

TEM image with g = 020, showing the same subgrain boundary, corresponding to the

10

upper part of image (a). (c) Stereographic projection oriented with respect to the nearest

11

main zone axis (lower left inset in (a)).

12

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