ERTH 3020

1
ERTH 3020 METAMORPHIC TEXTURES Winter Ch.23
Twiss Moores Ch.13,19 Nesse Ch.5

• offer essential clues to metamorphic and
  structural
• evolution of rock
• may allow protolith identification
• necessary for interpreting geochronological data
• necessary part of overall tectonic
  interpretation
• may have economic importance
• required for description and classification

2
ERTH 3020 METAMORPHIC TEXTURES
crystallisation - nucleation and growth of new
metamorphic minerals (associated with
reaction) recrystallisation - changes in size,
shape, internal features of pre-existing
minerals (associated with deformation and/or
recovery) BOTH are important normally occur
together
3
ERTH 3020 METAMORPHIC TEXTURES
Different types of textures   a) relict   b)
formation / growth of new phases
(crystallization)   c) recrystallization in
deviatoric stress field   d) recrystallization in
uniform stress field


we will focus on these b - reactions c d -
deformation mechanisms

4
ERTH 3020 METAMORPHIC TEXTURES
a) relict textures - inherited from
protolith given low strain and/or incomplete
reaction primary features may survive
metamorphism although normally modified in some
way e.g., sedimentary bedding, igneous
phenocrysts - inherited from earlier stage of
metamorphism and/or deformation e.g., inclusion
trails, pseudomorphs  
5
ERTH 3020 METAMORPHIC TEXTURES
a) relict textures - inherited from protolith 
MR-5 (ERTH 2002) coronitic metagabbro with
relict igneous texture (PPL fov 6.25 mm)
BRS-1 (ERTH 3020) slate with relict sedimentary
texture (PPL fov 6.25 mm)
6
ERTH 3020 METAMORPHIC TEXTURES
a) relict textures - inherited from earlier
stage of metamorphism
and/or deformation
MR-7 (ERTH 2002) inclusion trails in
staurolite relict early fabric (PPL fov 2.5 mm)
CP94-1 (ERTH 3020) relict biotite
porphyroblasts pseudomorphed by chlorite (PPL
fov 6.25 mm)
7
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
Nucleation homogeneous dominant in igneous
rocks spontaneous, random nucleation in
homogeneous medium heterogeneous dominant in
metamorphic rocks nucleation facilitated by
pre-existing structural heterogeneities
8
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
conditions needed to form stable
nucleus DGnucleus lt DGmedium a) DG products lt
DG reactants i.e., new mineral(s)
thermodynamically stable b) surface
energy below critical threshold DGs ga lt
DGscritical where g surface energy / unit
area a surface area of crystal this is related
to surface area/volume (SA/V) ratio of embryo
nucleus minimum SA/V ratio ? minimum DGs ?
maximum stability (sphere)
9
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
nucleation inhibited by need to form nuclei
large enough (low SA/V) to be stable nucleation
assisted by presence of precursor minerals with
similar crystal structures
consequences for metamorphic rocks abundant
precursor minerals with similar structure
? abundant nuclei of new phases (lots of small
crystals) absence of precursor minerals with
similar structure ? sparse nuclei that must grow
rapidly (few large crystals)
10
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
abundant precursors ? many small crystals mica
grains in matrix
no precursors ? few large crystals andalusite
porphyroblasts
OM-14 andalusite porphyroblast in schist (6.5
mm, XPL) ERTH 2001 collection
11
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
in metamorphic rocks, crystal growth is limited
by a) need to diffuse material through solid
rock, normally through grain-boundary
network b) presence of other solid phases that
interfere with growing crystal

• consequences
• products may form on/near reactants, forming
  reaction rims
• metamorphic minerals commonly have irregular
  (anhedral)
• shapes controlled by neighbouring grains
• large crystals (porphyroblasts) commonly
  overgrow smaller
• matrix grains, forming poikiloblasts with
  abundant inclusions

12
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
1. nucleation 2. growth
ideal stable shape ? sphere but for crystals,
planar low Miller index faces (001, 010, etc)
are more stable than curved faces leads to
granoblastic polygonal textures
in metamorphic rocks, impingement on neighbouring
grains ? anhedral (xenoblastic) grain
shapes except where there is a strong surface
free energy advantage to growing (some) well
developed crystal faces (e.g., mica)
13
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
products may form on/near reactants, forming
reaction rims
Winter (2000) Fig. 23-53
14
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
MR-5 corona of product minerals
developed between reactant minerals in metagabbro
(6.5 mm, PPL) ERTH 2002
Winter (2000) Fig. 23-53
15
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
large crystals (porphyroblasts) commonly overgrow
smaller matrix grains, forming poikiloblasts with
abundant inclusions
cruciform inclusion pattern in chiastolite
(andalusite) formed by rapid growth in graphitic
matrix
OM-13 chiastolite in hornfels (2.5 mm, XPL) ERTH
2001 collection
Winter (2000) Fig. 23-50
16
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
poikiloblasts porphyroblasts with abundant
inclusions
matrix minerals and textures preserved as
inclusions in porphyroblasts may preserve
evidence of an earlier stage of metamorphism
and/or deformation
many porphyroblast textures indicate volume-for-vo
lume replacement of matrix minerals (especially
Al-rich reactants), and/or preferential growth
along grain boundaries, engulfing chemically
inert or insoluble grains
Winter (2000) Fig. 23-33
17
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
matrix minerals and textures preserved as
inclusions in porphyroblasts may reveal an
earlier stage of metamorphism
MR-7 staurolite overgrowing matrix fabric in
schist (6.25 mm, XPL) ERTH 2002
Winter (2000) Fig. 23-33
18
ERTH 3020 METAMORPHIC TEXTURES
b) formation / growth of new phases
(crystallization)
in the absence of deformation, growth textures
reflect a) need to diffuse material through
solid rock, normally through grain-boundary
network b) presence of other solid phases that
interfere with growing crystal
however, most metamorphic rocks have been
affected by deformation before, during, or after
metamorphism
affects grain size, grain shape, preferred
orientations, internal structures, etc. can also
influence nucleation and growth
19
ERTH 3020 METAMORPHIC TEXTURES
20
ERTH 3020 METAMORPHIC TEXTURES
Different types of textures   a) relict   b)
formation / growth of new phases
(crystallization)   c) recrystallization in
deviatoric stress field   d) recrystallization in
uniform stress field


we will focus on these b - reactions c d -
deformation mechanisms

21
ERTH 3020 METAMORPHIC TEXTURES
Different types of textures    c)
recrystallization in deviatoric stress field
- changes in internal structure, size, shape, and
orientation of pre-existing grains -
responsible for preferred orientations
(grain-scale) - ultimately responsible for
ductile deformation (outcrop to
lithospheric scale) d) recrystallization in
uniform stress field - changes in size or
shape of pre-existing grains - does not
produce new preferred orientations (but may
preserve old ones)
22
ERTH 3020 METAMORPHIC TEXTURES
c) recrystallization in deviatoric stress field
origin of planar and linear fabrics and a variety
of other metamorphic textures (descriptive
details in Lab 2)
foliation - any penetrative planar (tectonic)
fabric special types include cleavage,
schistosity, banding, gneissosity ... abbreviated
S0, S1, S2, etc. where subscript indicates
relative age lineation - any penetrative linear
(tectonic) fabric special types include
stretching, mineral, intersection
...... abbreviated L0, L1, L2, etc. where
subscript indicates relative age
how do these features form?
23
ERTH 3020 METAMORPHIC TEXTURES
c) recrystallization in deviatoric stress field
types of foliations
how do these features form?
24
ERTH 3020 METAMORPHIC TEXTURES
how do these features form?
25
ERTH 3020 METAMORPHIC TEXTURES
how do these features form?
26
ERTH 3020 METAMORPHIC TEXTURES
DEFORMATION MECHANISMS (W Ch. 23, p.
400-404)   Most metamorphic rocks contain
deformation fabrics of some kind (generally
ductile), resulting from crystallisation or
recrystallisation of metamorphic minerals
(generally in a deviatoric stress
field). brittle vs ductile deformation brittle
- rock fractures loss of cohesion cataclastic
textures ductile - rock does not fracture no
loss of cohesion wide range of ductile
deformation textures
27
ERTH 3020 METAMORPHIC TEXTURES
Important deformation mechanisms include a)
cataclastic flow b) pressure solution (diffusive
mass transfer) c) intracrystalline
deformation d) twinning e) recovery f)
recrystallisation by - grain boundary
migration - subgrain rotation g)
solid-state diffusion creep h) grain boundary
sliding and superplasticity i) grain boundary
area reduction j) static recrystallisation
28
ERTH 3020 METAMORPHIC TEXTURES
Controlling Factors - mineral (type and
composition) - some combination of T, Pl, Pf,
s, e

• type of microstructure observed reflects a
• combination of these factors
• microstructures evolve over time and may
• overprint each other
• different minerals in the same rock may display
• different microstructures for the same ambient
  
• conditions

29
ERTH 3020 METAMORPHIC TEXTURES
deformation style varies in crust as T, Pl, Pf, s
vary
Winter Fig. 23-2
30
ERTH 3020 METAMORPHIC TEXTURES
Important deformation mechanisms include a)
cataclastic flow b) pressure solution (diffusive
mass transfer) c) intracrystalline
deformation d) twinning e) recovery f)
recrystallisation by - grain boundary
migration - subgrain rotation g)
solid-state diffusion creep h) grain boundary
sliding and superplasticity i) grain boundary
area reduction j) static recrystallisation
31
ERTH 3020 METAMORPHIC TEXTURES
WHAT IS DIFFUSION? (Winter, Ch. 3)   - process
by which atoms, ions, molecules are transported
through matter   volume diffusion - random
motions within chemically homogeneous
crystal (self-diffusion) - transport in
response to a chemical gradient within
crystal (inter-diffusion) grain-boundary
diffusion - transport along grain-boundary
network - can be dry or wet
32
ERTH 3020 METAMORPHIC TEXTURES
rate of diffusion flux (J) J ? dC/dx
(dC/dx concentration gradient) J -D
(dC/dx) (D diffusion coefficient)
dC/dx changes with time
t0 - initial concentration gradient (e.g.,
grain boundary) tn - intermediate stage t8 -
final gradient (flat) (chemical equilibrium)
t8
tn
C
t0
x
33
ERTH 3020 METAMORPHIC TEXTURES
Amount/rate of diffusion depends on   D -
diffusion coefficient - depends on type of
material varies exponentially with 1/T T -
diffusion rates increase exponentially with T
time - rate of diffusion depends on D, dC/dx,
T mechanism - volume or grain boundary
diffusion? - volume diffusion enhanced by
high T, vacancy density - grain boundary
diffusion enhanced by fluid   For a given T, D,
which is more efficient?  volume vs.
grain-boundary diffusion? wet or dry
grain-boundary diffusion?
34
ERTH 3020 METAMORPHIC TEXTURES
DIFFUSIVE MASS TRANSFER (Winter, Ch. 23, p.
401)   - involves transport of material by
diffusion along the grain-boundary network of the
rock - results in physical redistribution of
material (i.e., detectable!) - driven by
deviatoric stress (sets up activity
gradients) results in material transport
from high ?n ----gt low ?n relative
compression ----gt relative extension (with
respect to ?mean)
35
ERTH 3020 METAMORPHIC TEXTURES
dissolution
diffusive mass transfer (pressure solution) high
?n ---gt low ?n
preciptation
original grain boundary
high ?n
low ?n
Winter Fig. 23-2
36
ERTH 3020 METAMORPHIC TEXTURES
DIFFUSIVE MASS TRANSFER (Winter, Ch. 23, p.
401)   ENHANCED BY   1. High Temperature -
increases diffusion rates 2. Small Grain Size -
increases surface area, path length  3.
Fluid - enhances diffusion rates allows
solution /precipitation reactions to take
place   process commonly referred to as pressure
solution because material moves in response to
?n
37
ERTH 3020 METAMORPHIC TEXTURES
D.M.T. TEXTURES (/- other mechanisms)   1.
stylolites, corroded detrital grains and fossils
(dissolution)  2. strain shadows, pressure
fringes (precipitation)  3. slaty cleavage (
rotation, new grain growth)  4. crenulation
cleavage ( rotation, new grain growth)  5.
metamorphic segregation layering (
transposition, rotation, new grain growth)  6.
mylonitic banding ( grain size reduction,
transposition)  7. gneissic layering ( grain
growth, transposition)  8. veins, low T pegmatites
38
ERTH 3020 METAMORPHIC TEXTURES
D.M.T. TEXTURES (/- other mechanisms) 
fov 6.25 mm
slaty cleavage (BRS-1, PPL)
?1
?1
fov 1.25 mm
material dissolved - s1 insoluble residue
(mica graphite) defines cleavage grain
rotation also involved
39
ERTH 3020 METAMORPHIC TEXTURES
D.M.T. TEXTURES (/- other mechanisms) 
?1
strain shadow (BRS-1, XPL, 6.25 mm)
quartz dissolved from high ?n domains (mica-rich
schistosity) repreciptated in low ?n domains
("lee" of strong garnet p'blast)
?1
40
ERTH 3020 METAMORPHIC TEXTURES
D.M.T. TEXTURES (/- other mechanisms) 
?1
tension gashes (MGS-01, PPL, 6.25 mm)
?3
?3
?1
material transport from domains of relative
compression (?1) to domains of relative extension
(?3)
41
(No Transcript)
42
ERTH 3020 METAMORPHIC TEXTURES
Important deformation mechanisms include a)
cataclastic flow ? b) pressure solution
(diffusive mass transfer) c) intracrystalline
deformation d) twinning e) recovery f)
recrystallisation by - grain boundary
migration - subgrain rotation g)
solid-state diffusion creep h) grain boundary
sliding and superplasticity i) grain boundary
area reduction j) static recrystallisation
43
ERTH 3020 METAMORPHIC TEXTURES

• INTRACRYSTALLINE (plastic) DEFORMATION
• set of processes whereby crystals undergo changes
  in size and /or shape via internal deformation
  without loss of cohesion or change in crystal
  structure
• - involves migration of defects within crystal
  lattice
• point defects
• linear defects
• planar defects

44
ERTH 3020 METAMORPHIC TEXTURES
Problem
progressive strain
granite
how are undeformed protoliths transformed into
fine-grained, strongly foliated, layered rocks?
mylonite
GZ-1, GZ-3, GZ-6 (fov 6.25 mm)
45
ERTH 3020 METAMORPHIC TEXTURES
What are defects?
mistakes in the crystal lattice naturally
occurring, present in all crystals density,
forms depend on T, P, s, type of crystal, etc...
What forms do they take?
classified according to geometry point ( 0D)
line ( 1D) planar ( 2D) different types of
each
What is their significance?
control diffusion within the crystal
lattice control deformation within the crystal
lattice important in reactions, strain, other
solid-state processes
46
ERTH 3020 METAMORPHIC TEXTURES
POINT DEFECTS
mistakes in arrangement of atoms (points, 0D) in
lattice
impurity defects
Schottky (vacancy) ve -ve
Frenkel (misplaced)
interstitial (extra)
substitution (wrong kind)
Nesse, 2000 Fig. 5.11
47
ERTH 3020 METAMORPHIC TEXTURES
interstitial
vacancy
Twiss Moores, 2000 Fig. 19.4
vacancies are particularly important in
controlling atomic-scale motions within the
crystal lattice
48
ERTH 3020 METAMORPHIC TEXTURES
motion of a vacancy
Twiss Moores, 2000 Fig. 19.5
by exchanging places with atoms (randomly, or
driven by s) vacancies contribute to changing the
shapes of crystals
49
ERTH 3020 METAMORPHIC TEXTURES
motion of a vacancy
by exchanging places with atoms (randomly, or
driven by s) vacancies contribute to changing the
shapes of crystals
Winter Fig. 23-2
50
ERTH 3020 METAMORPHIC TEXTURES
motion of a vacancy
Winter Fig. 23-3
by exchanging places with atoms (randomly, or
driven by s) vacancies contribute to changing the
shapes of crystals
51
ERTH 3020 METAMORPHIC TEXTURES
example vacancy migration and creep creep
slow, time-dependent strain diffusion creep
transfer of material from areas of
high compressive stress to areas of low
compressive stress
? diffusion of point defects through a crystal
lattice ? diffusion of atoms or ions along grain
boundaries ? diffusion of dissolved components
in grain boundary fluid
volume diffusion, grain boundary diffusion
52
ERTH 3020 METAMORPHIC TEXTURES
POINT DEFECTS
volume diffusion diffusion of material through
the crystal lattice also termed Nabarro-Herring
creep controlled largely by vacancy
migration driven by gradients in stress or
composition favoured by high T
Twiss Moores, 2000 Fig. 19.6
53
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS linear (1D) misfits in crystal
lattice dislocation line separating adjacent,
slightly mismatched parts of the same crystal
2 types
edge dislocations
screw dislocations
Nesse (2000) Fig. 5.13
54
ERTH 3020 METAMORPHIC TEXTURES
edge dislocation extra half plane (EHP) of atoms
in lattice dislocation migrates by exchanging
bonds at end of EHP dislocation migrates in same
direction as lattice offset net
result displacement of lattice by one full unit
cell analogy movement of caterpillar or moving
rug from under piano
van der Pluijm Marshak (1997) Fig. 9.12a
55
ERTH 3020 METAMORPHIC TEXTURES
Twiss Moores, 2000 Fig. 19.9
56
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
screw dislocation lateral displacement of atoms
in lattice wedge-shaped zone of
offset dislocation migrates at right angles to
lattice offset net result displacement of
lattice by one full unit cell analogy tearing
a piece of paper
van der Pluijm Marshak (1997) Fig. 9.12b
57
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
in combination, edge screw dislocations form
2-D loops allow displacement (slip or glide) of
lattice segments in any direction on slip (or
glide) plane
Buergers vector (b) direction and amount of
misfit (traverse error)
Twiss Moores, 2000 Fig. 19.11
58
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
in combination, edge screw dislocations form
2-D loops allow displacement (slip or glide) of
lattice segments in any direction on slip (or
glide) plane analogous to slipped part of fault
plane
dislocation loop
Nesse (2000) Fig. 5.14
59
ERTH 3020 METAMORPHIC TEXTURES
dislocations in undeformed olivine (PPL, 2.5 mm)
decorated by oxidation
60
ERTH 3020 METAMORPHIC TEXTURES
edge screw dislocations in undeformed olivine
(PPL, 0.25 mm) decorated by oxidation
61
What is the total length of dislocations in 1
cm3if they were strung end to end?
62
What is the total length of dislocations in 1
cm3if they were strung end to end?
100,000 km Two and half times around the world!
63
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
dislocation slip (glide) migration of
dislocations along slip (glide)
plane accomplished by switching bonds at end of
dislocation one atom at a time
slip system slip (glide) plane direction
within plane
Twiss Moores, 2000 Fig. 19.14
64
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
dislocation slip (glide) migration of
dislocations along slip (glide)
plane accomplished by switching bonds at end of
dislocation one atom at a time
slip system slip (glide) plane direction
within plane (described using Miller indices)
Nesse (2000) Fig. 5.12
65
ERTH 3020 METAMORPHIC TEXTURES
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles
66
ERTH 3020 METAMORPHIC TEXTURES
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles strain
hardening increased resistance to
deformation in materials with high dislocation
density effect increase stress to continue
deformation change deformation
mechanism ? brittle failure dislocation
must migrate around the tangle
67
ERTH 3020 METAMORPHIC TEXTURES
LINE DEFECTS
dislocation climb migration of dislocations out
of slip (glide) plane requires diffusion of
atoms (or vacancies) from one lattice plane to
another facilitated by high T, high vacancy
concentration resulting offsets termed jogs
Twiss Moores, 2000 Fig. 19.15
68
ERTH 3020 METAMORPHIC TEXTURES
dislocation slip changes shape of crystal
lattice by incremental displacements problem
migrating dislocations can interfere with
each other, creating tangles strain
hardening increased resistance to
deformation in materials with high dislocation
density effect increase stress to continue
deformation change deformation
mechanism ? brittle failure dislocation
must migrate around the tangle dislocation
creep dislocation slip climb allows
deformation to continue most effective at high T
69
ERTH 3020 METAMORPHIC TEXTURES
PLANAR DEFECTS
2-D (planar) arrays of dislocations
form during crystal growth by phase
transformations by dislocation
migration include grain boundaries twin
planes stacking faults antiphase
boundaries dislocation walls (low-angle tilt
boundaries)
70
ERTH 3020 METAMORPHIC TEXTURES
PLANAR DEFECTS
2-D (planar) arrays of dislocations
antiphase boundary in clinopyroxene formed by
augite ? pigeonite transformation on cooling
Nesse (2000) Fig. 5.15
71
ERTH 3020 METAMORPHIC TEXTURES
PLANAR DEFECTS
2-D (planar) arrays of dislocations
dislocation wall (low-angle tilt boundary) formed
by accumulation of edge dislocations
Twiss Moores, 2000 Fig. 19.17
72
ERTH 3020 METAMORPHIC TEXTURES
dislocation walls in undeformed olivine (PPL,
0.25 mm) decorated by oxidation
73
ERTH 3020 METAMORPHIC TEXTURES

• distortion of crystal lattice by dislocation
  migration
• ? misorientations of crystallographic axes
• (and therefore optical directions)
• between different parts of the same crystal

undulose extinction in quartz
74
ERTH 3020 METAMORPHIC TEXTURES

• distortion of crystal lattice by dislocation
  migration
• ? misorientations of crystallographic axes
• (and therefore optical directions)
• between different parts of the same crystal

eventually accumulated dislocations
inhibit further deformation by dislocation
slip lattice distortion ? high internal strain
energy how can crystals get rid of dislocations?
75
ERTH 3020 METAMORPHIC TEXTURES
Important deformation mechanisms include a)
cataclastic flow ? b) pressure solution
(diffusive mass transfer) ? c)
intracrystalline deformation d) twinning e)
recovery f) recrystallisation by - grain
boundary migration - subgrain rotation g)
solid-state diffusion creep h) grain boundary
sliding and superplasticity i) grain boundary
area reduction j) static recrystallisation
76
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
recovery set of processes leading to reduction
in dislocation density and overall strain
energy recrystallisation creation of new
grain(s) from pre-existing grain(s) of same
mineral involves changes in grain size and
grain shape
77
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
dislocation wall (low-angle tilt boundary) formed
by accumulation of edge dislocations
separate misoriented lattice segments continued
migration of dislocations into low angle tilt
boundaries ? progressive misorientation, eventuall
y forming subgrains
Twiss Moores, 2000 Fig. 19.17
78
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
continued dislocation migration into low angle
tilt boundaries ? progressive misorientation, even
tually forming subgrains (lattice misorientation
lt10o)
subgrains initially strain-free since
dislocations have migrated into their boundaries
Winter Fig. 23-5
79
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
continued migration of dislocations into low
angle tilt boundaries ? progressively more
misorientation, eventually forming
subgrains (lattice misorientation lt10o)
subgrains initially strain-free since
dislocations have migrated into their boundaries
formation of subgrains is therefore a recovery
process
with increasing misorientation (gt10o) subgrains ?
new grains
Passchier Trouw, 1996 Fig. 3.14
80
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
creation of subgrains ? new grains ? grain size
reduction (polygonisation) facilitates
deformation because new grains can now rotate,
move past each other, and/or change shape
independently facilitates diffusion (and thus
both reaction and deformation) by increasing
grain boundary path length and surface area
81
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
recrystallisation mechanisms
Winter Fig. 23-6
82
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
undulose extinction and subgrains in quartz
creation of subgrains ? new grains ? grain size
reduction (polygonisation)
83
ERTH 3020 METAMORPHIC TEXTURES
undulose extinction ? subgrains ? new
grains grain size reduction by polygonisation a
type of recrystallisation
84
ERTH 3020 METAMORPHIC TEXTURES
undulose extinction ? subgrains ? new
grains grain size reduction by polygonisation a
type of recrystallisation
85
ERTH 3020 METAMORPHIC TEXTURES
where recovery keeps pace with grain size
reduction and strain, equilibrium grain size (for
given T, s) develops during deformation dynamic
recrystallisation polygonal or somewhat
elongated grains that may define strong fabrics
86
ERTH 3020 METAMORPHIC TEXTURES
granoblastic polygonal texture strong foliation
developed during high T deformation
fov 6.25 mm
fov 2.5 mm
RJ97-2 dynamically recrystallised mafic
granulite, western Grenville
fov 2.5 mm
where recovery keeps pace with grain size
reduction and strain, equilibrium grain size (for
given T, s) develops during deformation dynamic
recrystallisation polygonal or somewhat
elongated grains that may define strong fabrics
87
ERTH 3020 METAMORPHIC TEXTURES
deformed quartz (fov 2.5 mm) showing different
degrees of recovery and recrystallisation
low T and/or high strain rate
high T and/or low strain rate
88
ERTH 3020 METAMORPHIC TEXTURES
Reference Collection samples showing different
degrees of recovery and recrystallisation in
quartz and feldspar
fov 5 mm
low T and/or high strain rate (GFC-1 cataclasite)
high T and/or low strain rate (G89-14 high T
mylonite)
fov 6.25 mm
89
ERTH 3020 METAMORPHIC TEXTURES
Different types of textures   a) relict   b)
formation / growth of new phases
(crystallization)   c) recrystallization in
deviatoric stress field   d) recrystallization in
uniform stress field


we will focus on these b - reactions c d -
deformation mechanisms

90
ERTH 3020 METAMORPHIC TEXTURES
Important deformation mechanisms include a)
cataclastic flow ? b) pressure solution
(diffusive mass transfer) ? c)
intracrystalline deformation d) twinning ?
e) recovery ? f) recrystallisation by -
grain boundary migration - subgrain
rotation g) solid-state diffusion creep h)
grain boundary sliding and superplasticity i)
grain boundary area reduction j) static
recrystallisation
91
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
Passchier Trouw, 1996 Fig. 3.25
grain boundary migration another type of
recrystallisation driven by need to reduce grain
boundary surface energy
92
ERTH 3020 METAMORPHIC TEXTURES
quartz mica
quartz only
under strain-free conditions, deformed crystals
in mono-mineralic aggregates will tend to
increase grain size (reduce SA/V) by grain
boundary area reduction (a form of
recrystallisation)
93
ERTH 3020 METAMORPHIC TEXTURES
grain boundary migration another type of
recrystallisation driven by need to reduce grain
boundary surface energy
94
ERTH 3020 METAMORPHIC TEXTURES
"foam structure" (bimineralic)
B
A
B
dihedral angle
B
A
A

• stable grain shapes
• (minimum SA/V)
• polygonal outlines
• straight grain boundaries
• grains of given mineral
• roughly the same size

dihedral angle
95
ERTH 3020 METAMORPHIC TEXTURES
"foam structure" (bimineralic)
B
A
B
dihedral angle
B
A
A

• stable grain shapes
• (minimum SA/V)
• polygonal outlines
• straight grain boundaries
• grains of given mineral
• roughly the same size

dihedral angle
RJ97-2 (2.5 mm)
96
ERTH 3020 METAMORPHIC TEXTURES
close to contact
away from contact
RECOVERY RECRYSTALLISATION
annealing static recrystallisation driven by
heating in strain-free environment (one type of
recovery process)
Nesse (2000) Fig. 5.22
97
ERTH 3020 METAMORPHIC TEXTURES
close to contact
away from contact
RECOVERY RECRYSTALLISATION
annealing static recrystallisation driven by
heating in strain-free environment (one type of
recovery process)
BRS-1 (6.25 mm)
71-111 (6.25 mm)
Nesse (2000) Fig. 5.22
98
ERTH 3020 METAMORPHIC TEXTURES
granoblastic polygonal (foam) texture stable
microstructure can be produced by either dynamic
recrystallisation (strain recovery) or static
recrystallisation (no strain)
99
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
MR-13 polygonal texture in amphibole (2.5 mm)
granoblastic polygonal (foam) texture stable
microstructure can be produced by either dynamic
recrystallisation (strain recovery) or static
recrystallisation (no strain)
100
ERTH 3020 METAMORPHIC TEXTURES
RECOVERY RECRYSTALLISATION
JR94-4 (fov 2.5 mm) granoblastic polygonal
texture in plagioclase
G-10 (fov 2.5 mm) granoblastic polygonal texture
in plagioclase
granoblastic polygonal (foam) texture stable
microstructure can be produced by either dynamic
recrystallisation (strain recovery) or static
recrystallisation (no strain)
101
ERTH 3020 METAMORPHIC TEXTURES
What are defects?
naturally occurring imperfections in the crystal
lattice
What forms do they take?
point ( 0D) e.g. vacancies, impurities line
( 1D) edge and screw dislocations planar
( 2D) e.g., low angle tilt boundaries, grain
boundaries
What is their significance?
control diffusion and deformation within the
crystal lattice dislocation slip and/or climb,
recovery, recrystallisation important in
reactions, strain, other solid-state processes
102
ERTH 3020 METAMORPHIC TEXTURES
deformation style varies in crust as T, Pl, Pf, s
vary
Winter Fig. 23-2
103
ERTH 3020 METAMORPHIC TEXTURES
deformation style varies in crust as T, Pl, Pf, s
vary
Winter Fig. 23-2
104
ERTH 3020 METAMORPHIC TEXTURES
105
ERTH 3020 METAMORPHIC TEXTURES
cataclasite
mylonite
well foliated granulite
106
ERTH 3020 METAMORPHIC TEXTURES
107
ERTH 3020 METAMORPHIC TEXTURES
Introduction to Lab 3