A merican M ineralogist, Volume63, pages947-9 55, I 978
Zonedplagioclase
andperisteriteformationin phyllitesfrom
southwestern
Massachusetts
GonooN L. Nono. Jn.. JeNe HnlrueRsrnoM ANDE-nN ZsN
U.S. GeologicalSuruey
Reston,Virginia22092
Abstract
Compositionally-zoned
micropgrphyroblasts
are found in phyllitesfrom
of plagioclase
southwestern
Massachusetts.
Rocksfrom the lowestmetamorphicgradeyield unzonedalbite.
At slightlyhighergrade,albite constitutesthe core and a thin outer rim with an intervening
narrow zoneof An 13-17.At highergrades,plagioclase
is complexlyzonedin one of two
patterns:either the compositiondrops stepwisefrom an oligoclasecore to an albitic rim, or
the compositionof the coregraduallyincreases
outwardfrom An l9 to An 24, then abruptly
dropsto an albiterim. All of thesepatternsarepresentin rocksat metamorphicgradesbelow
the first appearance
of chloritoidand garnet.Abovethe garnetisograd,the zoningpatternis
simple:a coreof An 13 and a gradualoutwardincrease
to the maximumvalueof An 25.
Transmission-electron-microscopy
core,an An 9-16 intermediate
studiesof an oligoclase
zone, and an An l-5 outer albite rim show that two sets of lamellaeare presentin the
compositionrangeAn 2-25.In the rangeAn 9-16,the (1,18,T)setof lamellaeis strong,hasa
periodicityof about 122A,andgivesriseto satellites;
the otherset,(20T),is weak.In the range
An22-25 both setsare very weak,and diffusestreakingis parallelto [201]*.AII areashave
only a single lattice. The strong microstructuresare interpretedas products of spinodal
decomposition,
and the weak lamellarmicrostructures
in the nucleation-only
as fluctuations
zoneof the peristerite
solvus.
probably at about 350'C and under at most a few
The rocks were metamorphosed
kilometers
which represents
overburden,
conditionsconsiderably
belowthe peakofthe peristeritesolvus.Metastable
crystallization
is indicated.Availabledata,however,do not permit
grewduringa singleepisodeof
an unequivocal
conclusion
on whetherthe zonedplagioclase
metamorphism,or whetherthe albitic rim represents
a secondAcadian metamorphism
superposed
on preexisting
Taconianmineralassemblages.
Introduction
absence of peristerite compositions or by the presence of two coexisting plagioclases.Multiply-zoned
The composition and "structural state" of plagio- plagioclaseporphyroblasts in low-grade greenschist
clasehave beencommonly used as an indicator of the faciespelitic rocks from southwesternMassachusetts
geofogic record in many types of petrologic and geo- and adjacent areas (Zen and Hartshorn, 1966; Zen
logic problems. In detail, however, plagioclaseis ex- and Ratcliffe,l97l), however,contain compositional
ceedingly complex and contains submicroscopic in- zones that are within the peristerite field (Anr-An,.)
tergrowths and domains of distinct compositions or mantled by Ca-free albite rims. Such multiply-zoned
structural types even for sampleshomogeneouson an plagioclase is present only in a relatively narrow
optical and microprobe scale.Subsolidusdecomposi- north-northeast trending belt, coinciding with or just
tion products such as intergrowths in the composi- below the metamorphic zone where garnet makes its
tional regions of peristerite (Anr-Anru), Blggild ,,first appearancein rocks of suitable composition. At
(Anrr-Anur), and Huttenlocher (An.r-An"o) (Smith, lower metamorphic grade, the porphyroblasts of
1974,p.519) are fairly common in high-temperature plagioclaseare nearly Ca-free albite of uniform comigneousand metamorphic rocks. In low-grade meta- position, whereasat highergradesthey are gradationmorphic rocks, where plagioclaseonly forms with ally zoned, and the An content increasessteadily
compositionsin the albite-oligoclaserange, the peri- outward.
sterite two-phase field is commonly expressedby the
This paper presentsa systematic microprobe study
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947
948
NORD ET AL.: PLAGIOCLASE AND PERISTERITE
P l o g i o c l o s eZ o n i n g
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probably to the chloritoid zone, and were subsequentlyregionallymetamorphosed
again during
the Acadiandeformation(Zen andHartshorn,1966)
to gradesrangingfrom subchloritoid-subbiotite
zone
to kyanite-staurolite
zone;the metamorphicgrades
increaseto the SE and cut acrossthe regionalstructural trend.
Interpretationof the Acadianmineralassemblages
phase-equilibrium
in light of experimental
data suggests that rocks of the kyanite-staurolitegrade,
southeastof the area shown on Figure l, recrystallizedat a temperature
of about 550oCand a pressure in excessof the triple point of the aluminum
silicatepolymorphs-at least4 kbar or about l4 km
of overburden.Yet only about 35 km to the northwest,at BecraftMountainin New York, Silurianand
Lower Devoniansedimentary
rocksare unmetamorphosedand probably were not buried under more
than about I km of stratigraphicoverburden(Z 50"C). Interpolationof pressureand temperature
valuesis uncertain.The mineral-assemblage
datafor
plagioclase
(belowchlothezoneof complexly-zoned
ritoid zone),however,suggest
temperatures
of about
Table l. Representative plagioclase analyses from specimen 3-3
3-3 TRAVERSE I
into
Distance
crystal
I
Fig. l. Geologic map of area in southwestern Massachusetts
(seeinset for location), showing sample locations and their relation
to the mapped isograds. The various types of plagioclase zoning
discussedin the text are given different symbols on the map, and
the zoning pattern is schematically shown. The symbols for 45-l
and for 693-1 are the same; however, 45-l has an oligoclase core,
and 693-l has an albite core. Cev and Ow are the pelitic units.
variationsacrossthe plagioclase
of the compositional
porphyroblasts
at differentmetamorphicgrades.We
presenttransmission
electronmicroscopy(TEM) obon onemultiply-zoned
servations
sampleand attempt
to interpret the results in terms of the plagioclase
subsolidusrelationsand the metamorphichistoryof
the area.
Geologicalsetting
The peliticrocksin which the plagioclase
porphyroblastsoccur are part of the Taconic allochthon
(Fig. l). The rockswereemplacedduring the Taconian deformation, were mildly metamorphosed,
sio2
67.7t
67.1O
65.58
66.00
64. lZ
bJ. U /
A12O3
20.55
20.O4
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21.63
23.O2
24.OO
cao
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0.78
2.O9
2.43
3.52
4.77
KzO
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0.19
0.0
0.I3
0.16
0.20
Na2O
1I.65
1I.57
10.48
10.45
9.79
8.96
t
100.25
100.28
99.15
r00.54
100.6r
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si
2.955
2.955
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ca
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0.099
0.113
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K
0.007
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0.0
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Na
0.985
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0.885
0.833
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5.010
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c r y s t a l a t s l i g h t l y d i f f e r e n t e x t i n c t i o n p o s i t i o n s t o b r i n g o u t z o n i n g f e a t u r e s .( b ) S p e c i m e n1 0 3 2 - 1 ,s h o w i n g a s i m p l e t w i n n e d a n d z o n e d
c r y s t a l ;t h e c o r e h a s t h e s a m ec o m p o s i t i o n a s t h e r i m b u t a n i n t e r m e d i a t ez o n e ( d a r k b a n d ) h a s a c o m p o s i t i o n o f A n 1 3 .( c ) S p e c i m e n6 9 3 l , s h o w i n g c o n t i n u o u s z o n i n g f r o m c o r e o u t w a r d w i t h i n c r e a s i n gA n c o n t e n t a n d n o a l b i t e r i m . ( d ) S p e c i m e n3 6 0 - 1 ,s h o w i n g m u l t i p l e
z o n i n g a s i n 3 - 3 b u t h a v i n g a c o r e l o w e r i n A n c o n t e n t t h a n t h e i n t e r m e d i a t ez o n e . P h o t o g r a p h su n d e r c r o s s e dp o l a r i z e r .
950
NORD ET AL.: PLAGIOCLASEAND PERISTEBITE
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Fig. 3. Microprobe analysesof plagioclaseporphyroblasts. The
upper part of the figure shows compositions at points along
t r a v e r s e so n l y f r o m p o r p h y r o b l a s t si n s a m p l e 3 - 3 . E a c h d o t i s a n
individual analysis and the brackets indicate the positions of
optically visible boundarieswhich correspond to compositional
breaks in the traverse. The lower part of the figure shows
h i s t o g r a m sw h i c h r e l a t et h e n u m b e r o f a n a l y s e st o t h e c o m p o s i t i o n
o f a n a l y s e sf r o m p o r p h y r o b l a s t si n e a c h o f t h e s e v e ns a m p l e s .T h e
s h a p eo f t h e g e n e r a lz o n i n g p r o f i l e f o r e a c hs a m p l e i s s h o w n o n t h e
r i g h t w i t h A n c o n t e n t i n c r e a s i n gu p w a r d s . A l t h o u g h t h e t r a v e r s e s
i n 3 - 3 i n t h e u p p e r p a r t o f t h e f i g u r e s h o w t h a t t h e p o s i t i o n si f n o t
the widths of the compositional gaps from porphyroblastsin a
single rock are reproducible,the histograms show that the gap
p o s i t i o n s a r e n o t r e p r o d u c i b l ef r o m s a m p l e t o s a m p l e .
:60-+OO"Cat a few kilometersof overburden,compatiblewith a simplestraight-line
interpolationof the
distancebetweenBecraftMtn. and the kyaniteoccurrence.The temperatureestimatesand the mineral
assemblage
both conflictwith the temperature
for the
crestof the peristerite
solvus,whichwassuggested
by
Crawford(1966a)to be well in the staurolitezoneof
regionalmetamorphism;
Crawfordbasedthissuggestion on study of compositions
of discretecoexisting
plagioclase
crystals.
The phyllitesof the EverettFormationcontaining
the plagioclaseporphyroblastsare typically green
and purplish-greenand show few visible mineral
grains in hand specimens.A conspicuous foliation
and crenulatedcleavagedue to layersof alignedmuscovite and chlorite are usually present.The porphyroblasts of plagioclase,the only feldspar phasein the
rocks, are subequantto crudely elliptic in outline and
are interfoliate; the long dimensions of the crystals
are aligned with the foliation direction. In general
each single crystal forms in a discretearea; porphyroblasts are well separated.
Reconnaissancecompositional data on the multiply-zoned plagioclase porphyroblasts showed that
the Na-rich oligoclasecoreswere continuously zoned
to more Ca-rich oligoclaseand then mantled with Cafree albite. The Na- to Ca-rich zoning of the oligoclasecore could be readily interpretedas a record of a
normal regional prograde metamorphic event. The
presenceof the reversal to a Ca-free albite rim led
Zen (1969) to the interpretation that the albite rim
records the Acadian metamorphic event but the
oligoclasecore is part of a slightly higher-gradeTaconian mineral assemblage.The detailed microprobe
and TEM observationslead to a reevaluationof the
earlier interpretation.
Optical description and microprobe analysis of the
porphyroblasts
Severalzoning patterns were discoveredin plagioclase porphyroblasts from seven sample localities
shown in Figure l. Plagioclasecompositions were
determined by analysisof porphyroblastsin thin sections using an automated ARL microprobe system
(Finger and Hadidia cos, 1972). Representativeanalysesare given in Table I for a traversein sample 3-3 in
terms of corrected oxide weight percent, calculated
cations, and sums. Four representativeporphyroblasts are shown in Figure 2, and Figure 3 provides a
Fig. 4. Reflected-light micrograph of a part of a porphyroblast
from specimen 3-3 showing two optically distinct boundaries and
An content of microorobe ooints.
NORD ET AL.: PLAGIOCLASE AND PERISTERITE
951
Fig 5. (a) Electronmicrograph
of lamellarmicrostructure
in zoneII of part of a porphyroblast
from 3-3.e : 201.(b) A selected-area
electrondiffractionpattern,showingsatellitesabout each reflection.Inset is a magnifiedview of a reflectionshowingwell-developed
satellites.
200 kV.
summary of the microprobe data and a schematic
drawing of the compositional zoning pattern across
each crystal from rim to rim.
Three specimensfrom the lowest-gradepart of the
s t u d y a r e a , 1 0 3 2 - 1 ,l 1 0 2 - 1 , a n d l l l 4 - 1 , h a v e a l b i t e
c o r e s a n d r i m s ( F i g . 3 ) . S p e c i m e nl l 0 2 - l c o n t a i n s
slightly rounded plagioclase porphyroblasts which
are virtually unzoned (An 0-3). Porphyroblasts in
both 1032-l and I I l4-l have a 40- to 70-micron-wide
and optically distinct band of peristeritecomposition,
which separatesthe albite rim from the albite core
(Fig. 2b). Maximum An content in these crystalsis
An l3 in the center of this band, and the An content
drops off symmetrically towards the rim and core.
Plagioclasein specimen 693-1 (Fig. 2c) contains
cores of albite (An 1-4). The An content increases
outward from the core through peristerite compositions to a maximum of An l5-17 at the crystal margin; no discreterim was observed.Without its albitic
r i m , 1 0 3 2 - l w o u l d b e c l o s e l ys i m i l a r t o 6 9 3 - 1 .
The areacontainingspecimens
3-3,360-1,and 45-l
marks a changefrom porphyroblasts
havingalbitic
with morecalcic
corecompositions
to porphyroblasts
cores.Specimen
360-l (Fig. 2d) hasa coreof An 18
whichincreases
outwardto a maximumof An 23 and
then trails off through peristeritecompositionsto a
distinctalbite rim. It also has a well-developed
rim
separated
from theinnerpartsofthe crystalbyazone
of alteration;the inclusionsare alignedparallelto
crystallengthand are moreconcentrated
in the core
than in the rim. At highergrades,asin specimen45-1,
zoningis continuous,
typicallyfrom An l3 at thecore
to An 25 at the rim.
Specimen3-3 has beenstudiedin detail and was
also used for the TEM observations.
It is a conspicuouslyfoliated chlorite-plagioclase-muscoviteparagonite-quartzphyllite containingoptically- and
porphyroblasts
(Fig. 2a).The
compositionally-zoned
boundariesof thecrystalsarelocallycomplex,having
jaggedor embayedrims enclosing
singleor composite
Fig. 6. (a) The coarserlamellarmicrostructurein zone II (Fig. 5) changesto a finer lamellarmicrostructurein zone III. (b) Weak
l a m e l l a e i n c e n t e r o f t h i s c o r e a r e a . T h i s f i n e m i c r o s t r u c t u r e g i v e s r i s e t o d i f f u s e s c a t t e r i n g i n t h e d i f f r a c t i o n p a t t e r!n=( i n s e t i n F i g
201,200 kv.
952
NORD ET AL.: PLAGIOCLASE AND PERISTERITE
rims tend to have fewer inclusions,and the interface
between the rim and the transition zone may show
concentrationsof inclusions.The oriented inclusions
of the rims could have been inherited from a preexisting mineral fabric originally in the matrix.
TEM observations
Fig. 7. Stereographic projection of low albite, showing the
orientation of the satellite vector which is within the triangular
a r e a ;t h e h i g h - c o n t r a s tl a m e l l a en e a r ( l , l 8 , l ) ; a n d t h e l o w - c o n t r a s t
l a m e l l a en e a r ( 2 0 1 ) .
core crystals.Boundariesbetweenzonesare readily
seenbecauseof the differentextinctionpositionsof
arenearlyend-member
the zones.Rim compositions
steadily
albite (An 0-5). The An contentincreases
acrossthe first optical boundary into a transition
zoneof compositionAn 9-16.Numerouspoint analyses,closelyspacedstep traverses,and backscatter
imagesfailed to reveal the existenceof any abrupt
in the transitionzone.The
boundaries
compositional
An 22oligoclase,
coreof the crystalis homogeneous
zone
by
the
transition
from
25.The coreis separated
jump
in
marks
a
composiwhich
an opticalboundary
tion from An 16to 22.
In somecrystals,the boundarybetweenthe oligoclasecoreand transitionzoneis partiallyor entirely
mixtureof mica,
occupiedby a zoneof fine-grained
and quartz.Thesemay be zonesof alterazoisite(?),
tion, and are most readily explainedif they representeda zoneof maximum anorthitecontent suband if the outer rims are due
sequentlyretrograded,
to metamorphismat a later time. Such an interpretationwould be compatiblewith the observation
of rimming of apparentlyembayedcoresmentioned
before.In favor of a singleepisodeof crystalgrowth
of mineralinclusions
is the fact that the orientations
in the crystals(especiallyilmenite)seemto persist
throughall the zonesof a givencrystal,althoughthe
A l-mm-long porphyroblast was chosen from
sample 3-3 for the TEM study, becausethe compositional zones were optically distinct and the crystal
was large and relatively unfractured. Three detailed
microprobe traverseswere made acrossit. The TEM
sampleswere then preparedby ion-thinning two fragments, each containing a microprobe traverse.Figure
4 shows one fragment, with the microprobe analyses
indicated, before thinning. The optically-visible
boundaries marking compositional breaks divide the
crystal into three zones: the rim (I), the transition
zone (II), and the core(III).
In the TEM, the very albitic edge of zone I is
featurelessexceptfor an occasionalmicrotwin. In the
part of zone I having the composition of peristerite
(An 2-16), a very faint lamellar microstructure is
visible, with a wavelength of about 100A. The electron diffraction pattern has only sharp (a) reflections.
The compositional gap between zones I and II is
unrecognizablein the electron microscope;the only
changeis a continued gradual increasein black-white
contrast of the lamellar microstructure. Once in the
composition range of zone II, however, the lamellar
microstructureis easilyimaged (Fig. 5a), and appears
to be developedevenly acrossthis zone. The lamella
split and recombine,exhibiting a connectiveness.
The contrast of the lamellaeis strong for operating
Orville(1974)
()
o
o
Crowford(1966o)
=
o
o
o
E
o
F
Studiedby TEM
o5r0152025
An Confent
Fig. 8. A phase diagram showing the peristerite solvus of
C r a w f o r d ( 1 9 6 6 a ) a n d t w o - p h a s ef i e l d o f O r v i l l e ( 1 9 7 4 ) , T h e 3 - 3
porphyroblast compositions studied in TEM are shown between
300'C and 400'C, the estimated temperature of crystallization.
NORD ET AL.: PLAGIOCLASE AND PERISTERITE
o
o
o
=
Spinodol
o
o
o
E
o
F
Weoklo
Slrong
Conlrosl
Lomello€
Slrono
Conlrdsf
Lomelloe
Weok
Conlrosf
Lomellos
o5tot52025
An Content
Fig. 9. The strain-free solvus and estimated coherent spinodal
are shown with the TEM observations from the various portions of
p o r p h y r o b l a s t 3 - 3 . S p i n o d a l d e c o m p o s i t i o nc a n o c c u r o n l y u n d e r
the spinodal curve whereas nucleation can occur anywhere under
the strain-free solvus. Thus both mechanisms comDete within the
coherent spinodal.
in the planeof the
I vectorscontainingcomponents
lamellae(201,021,001)and weak for thosevectors
containingcomponentsnormal to the plane of the
famellae(020)(seealsoMclaren,1974,and Nord e/
al., 1974).In addition,when! : 020 and the strong
setof lamellaevisiblein Figure5ais out of contrast,a
faint setof lamellaenormalto thestrongsetis visible.
The selected-area
diffraction pattern for this area
(Fig. 5b) showsthat the lamellaehavea high degree
of periodicity,because
satellitereflections
are found
about all the main reflections.
The averagelamellae
wavelengthcan be calculatedfrom the pattern by
dividing the satellite-to-main
spot spacinginto the
appropriatecameraconstantK. The averagerealspacespacing is 122A,althougha diffusestreakconnectsthe satellites
and evenextendsslightlybeyond,
indicating a rangeof wavelengths.In all diffraction
patternstakenof the modulatedstructures,
all main
reflections
weresharpand no splittingwasobserved.
This is surprisingbecause
7* is the reciprocallattice
parameterthat undergoesthe greatestchangebetweenthe exsolvedphases
(1.7obetween
in peristerite
An 0 and 4n25, Ribbe,1960);evenin a*b* diffraction patterns,only singlediffraction spotswerepresent from all threezones.Satellites
werefound only in
zoneII.
The compositional
breakbetweenzonesII and III
is manifested
as an abruptchangein lamellacontrast
from relativelystrong contrastin zone II to weak
contrastin zone III (Fig. 6a). The sizeand orientation of the lamellaeappearconstant.Within zoneIIl,
weak lamellaecan be observedevenat the centerof
the core area (Fig. 6b). Diffuse scatteringseenin
953
Figure6b is parallelto the row [201]*in diffraction
patternsfrom zoneIII. Type(e)reflections
characteristic of intermediateplagioclasewould, if present,
occupythe positionmidwaybetweenthe 201 reflections [20] would be indexed202 for the l4,{ cell and
(b) reflections
would be l0l, 303,etc.l.
The orientation of the two sets of lamellae,one
strong and one weak, is shown in Figure 7 in a
projectionof the crystallographicaxes
stereographic
patternswereusedfor
of low albite.Many zone-axis
the analysis;threewhich nearlycontain the vector
(001),(100),and (102),are
connectingthe satellites,
plotted.The satellitevectorwas locatedat the intersectionof great circlescontainingthe zone Rormal
and the vectortrace.Thesatellitevectorfalls6o from
(081)(c : 7A) to near(l,l8,l) plottedas a heavy
great circle. The orientationof the weak lamellae
near (20T)was determinedby traceanalysisfrom the
imageplates,as no streakingfrom this lamellarset
was in the diffraction pattern.
Discussion.and conclusions
Peristeritephaserelations
The peristeritetwo-phasefield (recentlyreviewed
by Smith, 1975)hasbeendescribed
aseithera solvus
or as a two-phase
binary"loop." Both typesof phase
relationsare shownin Figure8. The solvusis asymmetrical;Crawford (1966a)estimatedthe low-temperature endmembersfrom a study of coexisting
metamorphicplagioclasesto be about An, and
Anrn.Sheplacedthe crestof the solvusbetweenthe
almandineand stauroliteisogradsat 450'-500oC.
More recentdata on staurolitestability suggestthat
the stauroliteisogradshould be placedat leastat
550'C (Thompson,1976),raisingthe crestnearerto
the high albite-low albite transition (575-625'C,
McConnell and McKie, 1960).In contrastto the
solvus model, a two-phasefleld consistingof low
albiteand high- to intermediate-albite
solid solution
is preferredby Smith (1972)and Orville (1974).Orville (1974)rejecteda solvus,on the basisof experiideal solution behaviorwhich
mentally-determined
range
An' to Anro-uuat 700oC
over
the
occurred
(Orville, 1972)and at 600'C (Orville, 1974).
The maximumtemperatureof crystallizationof the
porphyroblasts
is estimated
to be between300oCand
400oC,as discussedpreviously,and this is indicated
the compositions
in Figure 8 by areasrepresenting
found in the porphyroblast
3-3,studiedby TEM. The
compositionalbreaks,however,haveno significance,
by all the
sincethe total compositionsrepresented
954
NORD ET AL.: PLAGIOCLASE AND PERISTERITE
analysedporphyroblastsfrom all the samplescompetelyspanthe rangeAnoto Anr' (Fig.3). Thebreaks
probablyrepresent
polyphase
or local
metamorphism
changesin growth rate or reactingphases.
The homogeneousdistribution,diffuseinterfaces,
and connective
andwavelikemorphologyof the peristeritelamellae,as well as the formation of satellites
in the strongcontrastlaabout the main reflections
mellae,suggestthat the exsolutionmechanismis
This conclusionis consisspinodaldecomposition.
tent with thoseof previousauthorswho havestudied
peristeritemicrostructures(Christie, 1968;Weber,
1972;Lorimer et al., 1974).lt should be pointedout,
however,that the spinodaldecompositionmechanism can only be provenby observingthe progressof
with time (Laughlinand Cahn, 1975;
decomposition
thereYund, 1975b).The peristeritemicrostructure,
fore, could possiblyhave formed by homogenous
nucleationand growth,but this is unlikely,in viewof
the difficulty of forming a critical nucleusin feld(Yund, p. Y-35,
at low temperatures
spars,especially
with
ease
1975b),contrasted
the relative
of spinodal
no
nucleation.
which
requires
decomposition,
Unmixing in the two-phasefield of the binary
"loop" model can proceedonly by a nucleationand
growth mechanism,since the free energycurvesfor
the two phasesare discrete,and thereforespinodal
is not possible(Champness
and Loridecomposition
mer,p. 175,1976).Nucleationandgrowth,aswell as
spinodal decomposition,is possiblein the solvus
model.
Perist erit e microst ructure
are summarizedin
The types of microstructures
Figure9 on a phasediagramshowingan asymmetric
strain-freesolvusand an asymmetriccoherentspinodal curve (Christie,1968)(seeYund, 1975a,for a
detailed discussionof the curves).In the region
differences
exist
Anr-ruof our sample,compositional
betweenthe coherentlamellae,as indicated by the
strongimagecontrastobtainedusingoperatingvectors for which the structurefactor differencebetween
albite and oligoclaseis large [i.e., for € : 021,
: 20l.This area lies
Fc(albite): 0, Fc(oligoclase)
within the coherentspinodal.
Spinodal theory predictsa critical wavelengthtrc
whileall other
that is amplifiedduringdecomposition
decay(Cahn,1968).Oncethe decompowavelengths
sitionis complete,the lamellaewill coarsenandother
waveformswill grow at the expenseof l,c, eliminating
the sinewaveformand periodicquality of the original
microstructure.We suggestthat the sharp satellitesin
the Anr-,u region and small -122A wavelengthof the
lamellae indicate that the microstructure has not
coarsenedto any appreciable extent. Therefore, trc is
-120A for a composition Anr-t6 at an undercooling
of 100 to 200'C (Cahn, 1968).
Crosshatched lamellar microstructures which exhibited only weak contrast were also present outside
the estimated spinodal curve in the Anrr-ru core (Fig.
7). Mclaren (1974) found similar weak microstructuresin both An,3 and Anru.Although the wavelengths are similar to those in the Anr-t. region, no
satelliteis present and the contrast is weak. We suggest that this microstructureis in the nucleation-only
field and consists of minor fluctuations (1-2 mole
percent An) in composition or structure of variable
wavelength.Thesecompositional fluctuationsare the
precursorsto decomposition;they can amplify in the
spinodal field about trc but can only decay in the
nucleation-only field. The diffuse scattering in the
diffraction pattern from this region may represent an
attempt to form the intermediate plagioclasesuperlattice structure.The two setsof lamellaefound in the
porphyroblasts lie very close to the predicted positions of strain energyminima calculatedby Willaime
and Brown (1974).
Geologic inferences
Our data do not permit us to decide whether the
zoning pattern of the plagioclaseis the product of a
single or two discrete episodes of regional metamorphism. Becausethe TEM observationsshow that
the continuous microstructure is superimposed
across the already-formed core-rim boundary, the
data are compatible with either hypothesis for the
formation of the entire porphyroblasts.In any event,
however,the various rimmed porphyroblastsindicate
a complexity in spatial and sequentialarrangment of
metamorphism not revealedby conventional microscopicpetrography.
The rocks were metamorphosedprobably at temperatures no higher than about 350'C and under at
most a few kilometers of overburden. These conditions are considerably below estimatesof the peristeritesolvus (- 550'C). The areasof peristeritecomposition therefore crystallized metastably, and those
compositions within the coherent spinodal subsequently exsolved by spinodal decomposition. The
low temperature of formation apparently suppressed
coarsening of the microstructure and therefore preservedthe high degreeof lamellar periodicity.
NORD ET AL.: PLAGIOCLASEAND PERISTERITE
Acknowledgments
-
955
(1974) The peristeritegap as an equilibrium between
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Ribbe, P. H. (1960)An X-ray and optical investigationof peristeriteplagioclases.
A m. M ineral., 45, 626-644.
Smith,J. V. (1972)Critical reviewof synthesisand occurrenceof
plagioclase
feldsparsand a possiblephasediagram.J. Geol.,80,
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for publication,May 25, 1978.
We thank PeterT. Lyttle,PaulH. Ribbe,RichardA. Robie,and
Malcolm Rossfor reviewingthe manuscriptand givingus helpful
suggestions.
We alsothank Lovell Wigginsfor assistance
with the
microprobe.