Grain growth kinetics inolivine aggregates

Grain growth kinetics inolivine aggregates

168 (1989) 255-2'73
Scrence PublisheÍs B.v.' Amsterdam _ Printed in The Netherlands
Ta,w4htsics,
Eta..er
Graingrowthkineticsin olivineaggregates
S. KARATO
Ocean Research Institute, Uniuersity of Tokyo, Nakano, Tokyo 164 (Japan) *
(Received June 1ó, 1988; revised version accepted January 12' 1989)
Abstract
Karato, S., 1989. Grain growth kinetics in olivine aggÍegates' Tectonophysics, |68:255-2.73'
Grain growth kinetics were studied in hot-pressed fine-grained olivine aggregates Experimental conditions include
pressuÍesof 0.1 MP4
300 MPa and 1 GPa, and temperatures of 1473, 1573 and 16.73K, with or without the presence
of water. At the initial stage of grďn growth, growth of larger grains occurs rapidly, consuming smaller grains, which
results in significant poÍe entÍapment. Almost homogeneous grain-size distribution ís established in this process. After
this stage, grain-size distribution remains almost homogeneous in most cases. It was found that the presence of water
enhances the grain growth kinetics when the amount of water is limited. However, when the amount of water is large,
the water-containing pores significantly inhibit the grain boundary migration. Therefore the enhancement of grain
growth due to the pÍesence of water is due to the role of dissolved water in olivine rather than the mass transport
through wateÍ. AbnoÍmal grain growth was found in some samples. In these cases, normal grain growth was inhibited
by the presence of a significant amount of water-filled pores, or grains with significantly larger size than the rest
occurred in the staÍting materials. In 0.1 MPa runs, significant porosity developed and the grain growth Íates were
significantly lower than in those at high confining pressures. These results show that both water and pores have
important effects on grain growth kinetics in olivine. When the effects of pores (or secondary phases) in inlubiting grain
gÍowth are unimportant, the grain growth rate in olivine is very fast: gÍowth to 100 pm size wil| occur in 102 to 103
hours at 1500 K. Thus fine grain size in olivine will be maintained only for a short geological time, unless the grain
boundaries are effectively pinned by secondary particles.
Introduction
Microstructures of rocks at the grain scale have
an important bearing on a variety of geological
and geophysical problems. These problems include the estimation of paleostresses from recrysta|Iized grain size (Avé Lallemant et al., 1980), the
mechanisms of preferred orientation and the reSultant seismic anísotropy (Karato, 1987; Nicolas
and Christensen, 1987), and grain-size sensitive
creep (Karato et al., 1986).
Several processes may influence the grain size
of rocks (Karato, 1984). They include dynamic
* Present address: Department of Geology and Geophysics,
University of Minnesota, Minneapolis, MN 55455, U.S.A.
0040-195r,/89,/$03.50
o 1989 Elseúer Science Publishers B V.
recrystallization, grain growth and primary recrystallization. Dynamic recrystallization is a process during deformation in which the total grain
boundary energy increases (i.e. grain size decreases) at the expense of strain energy within
grains. The driving force for this process is the
strain energy stored in dislocations, and grain size
is mainly determined by the applied shear stress
(Twiss, 1977). During grain growth, the grain
boundary energy is reduced (i.e. grain size increases), the driving force being grain boundary
energy. Grain boundary migration may also occur
after deformation, being driven by dislocation energy (primary recrystallization).
The importance of these processes depends
primarily on the ratio of strain energy stored in
dislocations to grain boundary energy. Dynamic
S- K-{RATO
2)t)
recrystalfization and primary recrystallization are
important at relatively high (initial) stress and/or
coarse grain size.
Extensive studies have been carried out on dynamic recrystallization (for a review, see Urai et
a1., 1986; Karato, 1987). Toriumi (1982) studied
the primary recrystallization in olivine (see also
Morcier, 1979). However, relatively little attention
re grain growth in minerals.
t82) studied the grain growth
aggregates, and Olgaard and
the effect of secondary phases
:alcite. No experimental studken on grain growth in olivine
inary results by Karato (1984).
The purpose of this study was to obtain the first
data set on grain growth in olivine and thereby to
better understand the physical processes that may
govern the grain size of olivine in the upper mantle.
The driving force for grain growth is the grain
boundary energy and the grain growth rate decreases significantly with increase of grain size
(e.g. Kingery et a1.,7916). Therefore fine-grained
starting materials are necessary in order to observe significant grain growth within an experimental time scale. Hot-pressed fine-grained olivine
aggregates were used in this study. The starting
material was the San Carlos olivine. Its chemical
comoosition is shown in Table 1.
TABLE 1
Chemical composition of the starting materiď
wt.Vo
FeO
Nio
Mgo
CaO
Total
Sample
Method of grair
Average grain size
size separation "
(p m)
Powder suspended in
1.3
(cs)"o
(cs),"
alcohol after 20 hours
Powders suspended in
1.0
alcohol after 2 hours
but deposited after
20 hours
Powders suspended in
2.3
).1
,a
8.3
alcohol after 10 min.
Powder suspended in
alcohol after 20 min.
but deposited after
t hour
u The powder samples and alcohol were kept in a 50 cm height
cylinder for graín síze separation'
o (GS)" refers to the mean grain size defined by number
" (G,S), refers to the mean grain size defined by volume
fraction, i.e. (GS), : Iť,so) dr, where g(r) dr is the
volume fraction of grains the grain size of which is r - r { dr.
Specimen Jabrication
sio2
Methods of grain size separation and average grain sizes of
olivine powders
de nsity, i'e . (Gs)" : I ť, ÍQ) d r, whe re f (r)d r is the nu m ber fraction of grains the grain size of which ís r - r ldr.
Experimental procedure
Al 2 o 3
TABLE 2
41.23
0.00
7.83
0.26
50.72
0.00
100.04
About 3-5 mm size optically clear olivine grains
were hand-picked, crushed in a tungsten carbide
rock crusher and then ground in an alumina or an
agate mortar for 30 to 200 min. The optical microscope observation on decorated powders showed
that these processes introduced few dislocations.
Powders of particular grain sizes were separated
by sieves for grain sizes larger than 38 pm, and by
the sedimentation method (in alcohol) for the
smaller grains (see Table 2). After separation of
grain size, the samples were dried ín a Co/Co,
K for
gas mixture (Ío,:10-5 Pa) at 73.73_14,73
10-20 hours. In order to avoid the precipitation of
carbon, argon was flushed after the temperature
dropped below ca. 1000 K. The grain-size distribution of the starting powders was measuÍed by the
Elzon Particle Counter and the results are shown
in Table 2 and in Fig. 1.
Hot-pressing was carried out with or without
the addition of water using a gas medium apparatus described by Paterson (1970). The thermodynamic conditions and the details of the hot-
S KAR.\TO
rain
,M"{tr\ GROWTH
IN OLIVINE
AGGREGATES
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892
259
N OX-TTT\-E {rcREGATES
toqu0
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.
Temper-
Pressure
atule
(K)
(MPa)
r573
r4'73
r4'73
Water
o
Duration
Average
Density c
(hours)
grďn size
(E/"^. )
(pm)
0.5
45
187
46
90
r32
200
10
18
to /J
300
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0 .1
0.1
dry
dry
dry
dry
dry
dry
dry
dry
dry
dry
300
300
300
300
wet
wet
I
1
2
wet
4
l) /J
l5'r3
7573
1573
7673
7673
ÚHil
m;il
Í1)1
rÍ]'01
{nn
A
flg
4914
{Dtr
1924
14'73
I4'.73
7473
1473
{NM
m
ffi
{I8!r
A
4918
4923
4925
rs'73
15'73
15',t3
7573
300
300
300
300
wet
1
wet
1
s.mr
c
D
t5'73
14'73
300
300
wet
#.,8F
{5ó
160
{m4
{EI
{SE3
s3D
4891
{lm
B
B
C
B
A
c
75'73
1573
1573
75'13
14'73
1413
1513
1.573
15'73
300
300
300
300
300
300
300
300
300
B
A
7s73
75'73
300
1000
sz1
1929
lokb
A
A
Comments
wet
wet
wet
wet
dry
wet
5ó
2.6
3.6
5.3
'7.9
10.8
11.8
15.5
r23
18.3
z 5 .2
'7.8
2
Á
4.5
3
40;r
25.5
3.2'l
8
1
5
22.0
3.33
3.30
3.27
2r.8
wet
wet
J
dry
dry
3
20
11.1
37.2
wet
3.29
3.24
2.5
2.5
1
2
wet
3.17'r
3 . 1 1d
3.21d
3 . 1 3d
3.04d
7'7.3
24.5
35.0
44.1
dry
wet
3 . 3 1d
3.30d
3.30d
3.22d
10.7
14.8
21,.0
45.2
13.1
18.3
20.5
3.6
t7.7
29.8
wet
J.JI
).21
Deformation
(abnormal grain growth)
J.JJ
Deformation
Deformation
2.6-2;7
3.2'7
3.26
Deformation
3.30
3.33
(abnormalgrain growth)
Deformation
' For explanation of A, B, C and D, see Table 2.
o *DrY'' indicates water-free conďtions; ..wet'' indicates water-added conditions'
' Density of olivine grain is 3.33 g/cm3.
d Estimated from point counting on thin sections.
paratus as for hot-pressing was used, but in a 1
GPa run, a piston cylinder type solid-medium
apparatus was used.
In two runS (NoS. 4821. and 4929), grain gÍowth
occurTed during deformation. The ďfferential
stress ransed from 25 to 72 MPa.
0.1 MPa pressure "dry" runs.
The starting material was hot-pressed (at 300
MPa) without water. Thin slices with ca. 0.5 mm
thickness were cut from the hot-pressed sample
and annealed at 0.1 MPa in a CO/CO'
gas mixture (/o,: 10-s Pa) at 1473-1673 K.
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.:i';;;ň;;#;' "*;;
tru:5..:.! Fig 3). Most of the pores inside
Úll[/M.
tu 5:"::
fl03lij!-r-:-::a:llialh in zones surrounding the angulúi.ur-.'T-!:c.
-...res. in which no pores are found.
'Det*'een
-:;
ll'lc
, :-s
the zone and the grain
.]1Í|lllirillLi]ú].i.:.r e a relatively small number of pores.
.tl!5
' -.::epores on grain boundaries are de]|lÍÍ]TTu:':
i"3. .l3. C). Deformed pores are associL..-ú..ť'
::ned grain boundaries, and the shape
l, |1ť::: : :Ínedpores Suggeststhat grain boundary
E:E;- --.in these cases is driven by the grain
!í r|]'::.. Čnergy(Karato' 1988). In Some cases,
[!É:.:. bl moving boundaries is so marked that
l r::
torn and trapped into the grains (Fig.
-.
'{-- ]-:-: srze of pores inside the grains appears to
Ír-::s- irom the center to the outer part (see also
;r i
-,'r. This suggests an increase in pore size
-:
;-:.rain growth.
Fig.
4 are shown the dislocation structures of
-.siine
sample
as shown in Fig. 3. Three regions
^rr.
:.early recognized. The central portion, often
":.
r---.:lar shaped, has relatively low dislocation den-
very high dislocňion deisities which coincides
with the pore-rich zone. The dislocations in the
outer rim between the dislocation and pore-ricl
zone and grain boundaries are mostly straight and
perpendicular to the grain boundaries. These dis.
locations tend to be parallel to the [100] or [001
orrentatron.
Initial rapid grain growth consuming fine grainr
was also observed with the other starting materi.
als. However, the kinetics of this initial stage ol
grain growth was found to depend on the initial
grain-size distribution. The wider the starting grair
distribution, the faster the initial stage grair
growth. The pore-rich zone was not clearly seer
when the starting powder had a relatively uniforn
and fine grain size. Entrapped porosity was gener
ally larger in samples hot-pressed from powders
with wider grain-size distribution (see Table 3).
The SEM micrographs of sample No. 4891, the
densification of which was imperfect due to the
effective sealing of water, are shown in Fig. 5. Thr
rounded shape of grains clearly indicates signifi.
cant mass transport driven by surface energy. Thr
30 pm
Fig
4. Díslocations in a hot-pressed and annealed sample revealed by oxidation
j : . 1^^^+: ^.
decoration. Note the dislocation-rich
zone
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{C'GREGATES
263
tormal gÍ-.rr
9i7).
size is shor:
powder hac
:ig. 1,A.)an;
a neglig:bn'"
L1nrema1nď
n-s.The dau
(1 r
grarn SŽe aI
l Á. is a rate
lce of grain
Temperature
ad from this
ent mns was
vJnatlon ln
pendence is
\2 )
t_
tBt
E* is the
and ?" is
; :1 .6 x
gro\\'thrates
t rnechanism
reep (Karato
(.}ninvolúng
aťfectgrain
n in most of
o".rmal (Fig.
gÍerns grew
Fig. 10). In
rrarnedmany
frrund in the
s. -{bnormal
Ltl \\aS large
J sÍains with
annealed in
rng pressure
ater had few
2 hours; (B)
at 1573 K, 300 MPa at wateÍ-added conditions' (A) After
Fig. ó. Time variation of microstructures in samples annealed
after 4 hours; (C) after 8 hours'
uo Á1ululrrpeJJncco seJod eql .(srnoq 00I < )
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