id Assisted Recrystallization in Upper Mantle Peridotite Xenoliths

id Assisted Recrystallization in Upper Mantle Peridotite Xenoliths

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id AssistedRecrystallizationin Upper Mantle
PeridotiteXenoliths from Kimberlites
E--:
AND H. L. M. VAN ROERMUND
by M.R. DRURY*
of StructuralandAppliedGeology,Instituteof Earth Sciences,
StateUniuersityoJ
Utrecht,PO Box 80.021,3508TA Utrecht,The Netherlands
(Receiued5 January 1988;reuisedtypesuipt accepted 18 August 1988)
A BSTRA CT
development of recrystallization microstructures has been studied in some 'hot deformed'
xenolithsfrom the Thaba PutsoaKimberlitepipe in S. Africa.The xenolithsweredeformed
strainsby dislocationcreepin the uppermantleand thenannealedas theywereupliftedby the
ffuid. Static recrystallization occurs during annealing producing euhedral shaped'tablet'
Tabletgrainboundariesaresub-parallelto crystalgrowthhabitsin olivineand orthopyroxene.
grain boundarymigration,
nicrostructureis characteristicof recrystallizationby fluid-assisted
a thin fluid film is presentalong the boundary.
lhcre is microstructuralevidencefor a complexfluid infiltrationhistoryinvolvingan early Fe-Ti
ctasomatic silicatefluid and laterkimberlitefluids.Minor partial meltingof clinopyroxenecan
be inferred,which,is consistentwith infiltrationof a kimberlite-derived
C-H-O rich fluid into the
hs.Any ofthesefluidscould havebeenpresentalongthe tabletgrain boundariesduringstatic
allization.The occurrenceof tabletsrains in 'cold deformed'xenoliths,which have a simple
history,suggeststhat a C-H-O rich fluid derivedfrom kimberliteis the most probable
fluid in both the hot and cold deformedxenoliths.
Tbe occurrenceof dynamically stable semi-continuousgrain boundary fluid films during rezationindicatesthat mechanismsof fluid segregation
and transportin the uppermantleare
to be dependentupon the type of deformationand recrystallization
mechanismsoperating.In
the destabilizationof the static fluid distribution by grain boundary migration and
tion will alsb influencethe rheologyof the upper mantlewherefluids are present.
INTRODUCTION
Kimberlitepipescontainperidotitexenolithswhich representaccidentalinclusionsof the
qper mantlewall rock of the kimberlitemagmaticconduit (Ahrenset a1.,1975;Boyd &
(Z) estimates(Boyd & Nixon,
DÍeyer,
1979;Kornprost, 1984).Pressure(P) and temperature
Finnerty& Boyd, 1987)show that
1975;Carswell& Gibb, 1980,1987;Bertrandet a1.,1986;
úe xeno|ithsare derivedfrom depthsof 70-250km, so theyrepresentthe deepestsampling
ď the upper mantle that is availablefor direct study.
Many peridotitexenolithshavewell developeddeformationand recrystallizationmicrostructures(Bouillier & Nicolas, 1973, 1975).Two types of deformedxenolith can be
distinguished
on thebasisof P-Testimates,bulk compositionand mineralchemistry(Harte,
1983).The cold deformedxenolithsgiveP-T estimatesof 3G45kb and 800-1000"Cand
havesimilar(basaltdepleted)compositionsto the commoncoarsegranularxenoliths.The
hot deformed xenoliths give P-T estimatesof 50-70kb and 1200-1400"C and are
*Present address: Mineralogy Research Centre, Research School of Chemistry, Australian National University
Canberra ACT 2601. Australia.
fJournal of Petrology,Vol 30, Part l, pp 133 152,19891
o oxfoÍd Unive$ity Pres 1989
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DEF OR M ED
rBoyd & S
in tlÉ
r*siru in tbc rry
in substructurcs.
r gros'th tGu{m"
ů
in a JEOL 200C transmissionelectronmicroscope.For comparativepurposesthe
resof a numberof cold deformedxenoliths(DAC-K33,BD30l5, and BD3ó80)
Kimberley and Bultfontein mines have been studied by light microscopy.
b ..r] high srnin
d in kimberlitet
t it certainll. occuÍI
lcrcrer. 1979)but mf
rned renoliths(
I3 5
ructures in two xenoliths from the Thaba Putsoa kimberlite pipe in Lesotho,
fuJ
-tďl€ d &' BtÍi*fi| I
|ůcd a ts-o Šulc
l979l and h
X EN OL I T H S
fDawson, 1962) have been studied by light microscopy, scanning electron
ISEM)with energydispersiveX-ray micro-analysis(EDX) and by transmission
microscopy (TEM). Polished thin sections were examined at 20 kV in the
electron image mode in the SEM and ion beam thinned foils were examined at
bacn
hcÉr.
P ER I DOT I T E
P ETROGRA P H Y
bot deformedsamplesstudied include a partially recrystallizedporphyroclastic
mosaicporphyroclasticxenolith(DACIDAC-L22)and a completelyrecrystallized
Tbe nomenclatureusedis that óf Harte (|977\.
bolith DAC-L2? has a porphyroclastictexture(Fig. 1A) with a weak lamination
by modal variations of orthopyroxene,clinopyroxene and garnet, and also by
of the amount of olivine recrystallization.Bands with higher content of other
have more extensive olivine recrystallization. A foliation is defined by the modal
ion and the preferredshape orientation of inequant orthopyroxene and olivine.
r the occurrence ď
r,rhich grow into tbc
lber. 1976;Gueguen
19.-ireuhedralrecrlhadon
of
Srcchronometer', to
!Bó: Urai et al.,l98ful
h recrystallization
lL l98óa). In this
y. and migration occurt
d precipitation of the
ůkh aims to investigate .
rat of the tablet grainr d
tsallization mechanisms fo
ms can have an imporrat
nd grain-size palaeopio
D to the upper mantle (Ar
atge of the recrystallizatim
t_ l98s).
nribution of fluid phasesir
,ohlstedt,1982;Cooper &
:nan. 1987).The physical
I the rheologicalproperties
m studieshave considered
& Bulau. 1979),or during
L 1984).Far less is knswn
plasticity or undergoing
relativelywell constrained
: peridotitesin a fluid-rich
C
-I
,&-
Ftc. l. Microstructures of peridotite xenoliths. (a) Partially recrystallized microstructure of porphyroclastic
xenolith, DAC-L22. Scale bar =2mm (b) Completely recrystallized microstructure of mosaic-porphyrpclastic
xenolith, DAC-L?7. Scale bar: 2 mm. (c) Primary orthopyroxene grain in DAC-LZ2, showing rim of secondary
clinopyroxene Scale bar:400pm. (d) Corroded margins of primary clinopyroxene grains. Scale bar:400 lm
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IilL\D
dasts which contair
rocrur in bands
I mm) euhedral to
roblast and tablet
P ER I DOT I T E
Í3.1
X EN OL I T H S
;l$
Fig lB). Olivine
nt- large (1 5 mm)
r Éatures and exteffic
lt t}.p€ orthopyroxer
2-l0 mm in size'whiÓ
tn-subgrain boundari
ůe primary
SP i n e l
Al
Cr
Fe
e obtained on this rim
d Na/Si ratios compard
n1- clinopyroxene occutr
ccrvstallization. These
d electron images are li
Ti
mber. This is consistent
r of Na and Al in
polyphasereactionrim. Tb
D thosedescribedby Reid &
ústsofeuhedralspinel,Ti rkh
rerpentine. In the partially
he olivine tablet grains.Tbc
frases (Fig.2B)'eitherspinď
nd amphibole.The grain sizr
iling into a very fine grained
spinelshave lower
Tmplectite
rmphiboleshavelower Ti, IL
nhs are qualitativelysimilar
neoblasts,and
orphyroclasts,
much smallerthan in the hot
res showsstrong undulatory
Bands of
Ď|ast development.
bng the foliation defininga
cscriptionsof cold deformed
IN OL IV IN E
n the porphyroclastic xenolith
aight sub-boundaries parallel
of cumulative misorientation
res developed are described in
ure consists of closely spaced
attice tilt rotations (tilt walls),
c
iffira a Garnet reaction rims (a) Light micrograph oÍ polyphase reaction rim' showing decrease of grain size
mr*ards the garnet interface.Scale bar = 400 pm (b) Line drawing of SEM backscatteredelectron image, showing
!@qÍs€ outer zone of spinel+amphibole+phlogophite and finer grained middle zone of spinel+orthopyroxene
+anher clinopyroxene or amphibole. Middle zone clinopyroxene and amphibole can only be distinguished by
ffi\ spectra.In the middle zone of individual reaction rims the occurrence of clinopyroxene is mutually exclusive
m m ,u n ph ibolea n dviceversa. S cale bar:60p m. (c) Qualitative EDXspe ctraob taine donthe S EM, ofthe pha ses
shown in (a) and (b).
uith a moderate to high density of free dislocations (1011-10t2m-2) between the walls
iFig.4A). In addition to this 'background' structureother structuresare locally developed
{Drury & van Roermund, in press) including healed micro-fractures which are defined by
planar arrays of multiphase inclusions.
The free dislocations and some healed microfractures are decorated by three phase
inclusions,0'2-0'1 pm in size,which contain ilmenite,an amorphous phaserich in Si, Al, and
Fe with minor quantitiesof Mg, Ca, and Na, and a portion that is a void or a fluid filled
bubble (Drury & van Roermund, in press).Some healed microfracturesare decorated by
another type of inclusion,which is usually filled by an amorphous single phase.This phase
did not generateany additional strong characteristic EDX peaks different from the matrix
olivine although there were traces of K, S, Ti, Cr, Ca, and Cl. A few of these inclusions
contained a crystallinephase rich in K, S, and Ti+(Mg, Fe, Si).
Numerous larger scale healed fractureswere visible by light microscopy. Many of the
fractureswere decoratedby strings ofopaque blebs parallel to the trace of(100) planes in
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I D lc
DEF OR M ED
P ER I DOT I T E
X EN OL I T H S
139
,Jicatingthat they locally decorate(100)sub-boundaries.The fracturesoftenhave a
r,::irinsence than the rest of the olivine.
R E CRYSTA LLI Z A TI ON
M I CROSTR UCTUR ES
N eoblasts
TÍe
2 O OFm
L :. rlet gr ai ns d ev e l o p e di n d e : : - m u u t
'.lírineneoblastsare polygonal,optically strain free small grains that occur in bands
iiuillr'rir".urrl,;
rhe porphyroclasts of the partially recrystallized xenolith (DAC L22). The grains
llllllltlur',Lr: boundarieswith numerous near 120"triple points.TEM observationsshowed
'::aight
[lWj]I
u]rČdislocation structures (Fig. aB). Some grains contain similar Structuresto the
with the additional developmentof (001)tilt walls made up by dislocations
1pr'":niroclasts,
Lwrttr:ltt
B"rrsersvector b:[001] (Fig. aC). Other neoblastsare almost defectfree or have an
111p11;l:gr$€ n substructure.
eous
Figure 48 shows an examp|ewherea portion of the grain with
mmll.r
defect density is separated from a strain free portion by a curved, moderatelnml'r.entationangle (0: 5') subgrain boundary.
-:-,entationmeasurementsin bands of neoblastsrevealedstrong local crystallographic
r"mr:rc:red
orientations.Individual bands containing 20-100 grains are made up of only 3-5
iřE:tetions' some of which are close to orientationsof adjacent porphyroclasts.
Tablet qrams
Tre most distinctive featureof the partially recrystallizedmicrostructuresis the develirr:int of the euhedral to subhedral shapes of the tablet grains (Fig. 5). Universal stage
.lr]Iřůsurements
show that the orientationof many grain boundariesis relatedto the |atticeof
Lrne
tablet grain. The most commonly formed orientationsin olivine are {010} and {110}
:;-.:. showing (100) tilt walls with a
]€ : .]eous Substructure in neoblast A
: - - ' :na por t i o n w i t h h i g h d i s l o c a ti o n
, l . :ilt w al l (h o r i zo n ta l )a n d two ( 1 0 0 )
L].ťi sraln
Ftc. 5. Light micrograph showing cuhedral olivine tablet grain growing into a deformed porphyroclast.
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t41
X EN OL I T H S
P ER I DOT I T E
oo
\
?4 n
}.ý
^\.
010
llr'=:e
lst
..f the tab|el gratE- ÍúŤ
slpi*1t1 1u61.,
I
cl iattice and the tablet
lrrr-e o rientation relati
lncl for [001] in the tabb
riousll by Bouillier & \i
! alw have boundaries ru&
r l u de l0l1 ), {2 10} ,{21l : d
d a ren'low density(lessthmr
ďl1 no subgrain developrntmniled into 2-5 large subgranr
Gnt to that developed in h.
s and sub-micron multiphas
zlaj fractures and the tabltr
g through porphyroclastsinto
caled fractures.
mpletely replaced by a rer microstructure is identical to
t the completely recrystallizď
*ď!
->:1_
"4
.-
Ó
(
D-
!L
n thick film of polycrystalline
nII quantities of K, S, and Ti
or
of crystala|9 91ie,nja]191s
projections
Lowerhemisphere
orientations.
preferred
Ftc.7. Crystallographic
per
4,8,12,|6'20o/,
porphyroclasts
levels:
iontoui
orientation.
foliation
tr'.
great
.i,"ů,^t'o*'
Thedotted
olivine.
lobarea(25grains)Jtablet
e,^;ii1,1,Á,e,*'iu'ío#.iil',í:.area(100grains);neoblasts,|,2,4,6,8,10oÁperIoÁ
II E \ T ATIONS
r in the partially recrystallized
g a universal stage(Fig. 7).The
anal."-sisof the porphyroclasts.
ll to be sub-parallel and [010]
tablets have similar preferred
The recrystallized
orientations with the main maxima close to those of the porphyroclasts.
as found
porphyroclasts,
the
than
preferred
orientation
diffuse
grains appear to have a more
effect owirig to the small
Ěy souiilie. & Nico1as (I913).This may, however, be a samp1ing
number of porphyroclasts measured'
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