Download Toward a practical plagioclase

American Mineralogist, Volume 71, pages I I 09-1 I I 7, I 986
Toward a practical plagioclase-muscovite
thermometer
N,q,rHA.NL. GnnnN. SrnvnN I. Uso.q.Nsxv*
Department of Geology, University of Alabama, Tuscaloosa,Alabama 35487, U.S.A.
Ansrnlcr
The Na-K exchangereaction between plagioclaseand muscovite has been formulated
as a geothermometerutilizing a ternary-feldspar subregular solution model, a modified
binary white mica solution model, and available thermochemical data that account for
high and low structural statesof feldspar. The plagioclase-muscovitegeothermometerhas
been applied to pelitic metasedimentary rocks and peraluminous granitoids that have
equilibrated at temperaturesof 49G750'C and at pressuresof 2-13 kbar. The equilibration
temperaturesof pelitic rocks, predicted from compositions of coexisting plagioclaseand
muscovite, are generallysimilar to those predicted by garnet-biotite geothermometry.The
plagioclase-muscovitetemperatures of peraluminous granitoids derived using the compositions of secondarymuscovites are distinctly subsolidus, whereas temperatures estimated using primary muscovite compositions are near liquidus. Application of the plagioclase-muscovitethermometer extends the range of pelitic metasedimentaryrocks and
peraluminous granitoids for which equilibration temperaturesmay be estimated.
INrnonucrroN
Muscovite and feldsparsare important constituents of
pelitic metasedimentary rocks and peraluminous granitoids. The compositions of thesecoexistingminerals can
be related by the Na-K exchangereaction:
KAlSi3O8 + NaAlrSirAlO,o(OH),
paragonite
K-feldspar
: NaAlSi3Ot + KAlrSi3AlO,o(OH)r.
albite
muscovlte
Thompson(1974)and Chatterjeeand Froese(1975)have
provided extensivetheoretical treatments of paragonitemuscovite-alkali feldsparrelations in the systemNaAlOrKAIOr-AlrO3-SiOr-HrO. In particular, Thompson (1974)
noted the temperature dependenceofthe exchangeequilibrium and presentedfree-energyrelations basedon endmember thermodynamic data derived from both ion-exchangeexperiments (Iiyama, 1964) and from muscovite
and paragonite dehydration experiments (Day, 1973;
Chattefee, 1973;Chatterjeeand Johannes,1974).However, when theserelationshipsare applied to natural rock
systems,predictedmuscovite-alkali feldsparequilibration
temperaturesbasedon endmember thermodynamic data
are generally less than 100'C, suggestingloss of albite
component from K-feldspar during subsolidusre-equilibration (Labotkaet al., l98l).
Kotov et al. (1969) proposed a geothermometerbased
on the distribution of Na between coexising muscovite
and plagioclase.Talantsev (1971) presented a graphical
I Presentaddress:Department of Geosciences,Murray State
University, Murray, Kentucky 42071, U.S.A.
0003-004x/86/09I 0- r l 09$02.00
geothermometerbased on the Na content of muscovite
coexisting with sodic plagioclases(Ano-,r). Tracy (1978)
and Cheney and Guidotii (1979) demonstrated that the
Ca, Na, and K distributions betweenmuscovite and plagioclaseexhibit regular variations with metamorphic grade.
Cheneyand Guidotti (1979) further showedthat the compositions of coexisting muscovite and plagioclasecould
be used as a viable indicator of intensive parameter (Z
and/orfr*) variation within a single,petrographically wellcharacterizedterrane. They used the paragonitedehydration reaction involving albite, aluminosilicate,and quartz
to estimateequilibration temperaturesof lower sillimanite
through sillimanite + K-feldspar zone metapelites from
the PuzzleMountain area,Maine. Becausethe paragonite
dehydration equilibrium used by Cheney and Guidotti
requires both an estimate of/"ro and the presenceof an
aluminosilicate polymorph, this geothermometercannot
be applied to rocks in which muscovite and plagioclase
coexist in the absenceof andalusite, sillimanite, or kyanite. In order to overcome difficulties involved with subsolidusre-equilibration of alkali feldsparduring late-stage
metamorphic and magmatic processes,or the absenceof
an aluminosilicatephase,the Na-K exchangerelationship
betweenmuscovite and plagioclasehas beeninvestigated.
This paper presentsthe formulation of a geothermometer
basedsolely on the compositions of coexistingmuscovite
and plagioclase.
TxnnlronyNAMrc
BACKGRoUND
Ternary-feldsparsolution model and excessterrns
The albite and orthoclaseactivities of plagioclasefeldspars can be formulated in terms of a ternary model as
follows (Wohl, 1953;Saxena,1973):
I 109
THERMOMETER
GREENAND USDANSKY:PLAGIOCLASE-MUSCOVITE
lll0
RT ln ao,: R?" ln ao,
+ {x bII'I4N^
+ 2X',(WE"K- 'yy^)l
+ 2xo,(w?K- wc^11
+x
"lw"
+ &"xAb[0.5(w3""+ w[c^ a 14t5N'
* w\,r - w?""
- Wf;^c^1
+ xo,(wK - w5c"
* Wf;^x- WY^)
+ (x"" - Xoo)
'(w7*" - ttl5'91),
RT ln aoo: RZ ln aoo
+ Ixz,lwE^K+ 2x^b(wN^- tt?*)l
- w}aca)f
+ x7"1w1"."
+ 2x^b(wg^Na
+ Xo.XA"[0.5(WE^K
+ W$N^a 1YaN.
* Wf;'a - W7K - W5c^7
+ X^b(Wl'- Wf;"r
+ W?"" - Wf;"c^1
+ (x* - x"J
'(lv5c" - W*\IL
where the interaction (Margules) parameters, VIto,which
describemixing relations within the Ab-Or, An-Ab, and
An-Or binary systems,can be resolved into enthalpy, entropy, and volume terms; X-, Xor,and Xtu are mole fraction of orthoclase,albite, and anorthite; ar;.da terms represent the ideal contribution to the activity of albite
component(Price, 1985),
a"o: (&uX2 - Xot - X",)(X", * Xo),
and orthoclasecomponent,
ao,: (X)(2 - Xoo- X".X&, + X".).
Green and Usdansky (1986) have shown that theseactivity-composition expressions,combined with the An-Ab
Margules parametersof Newton et al. (1980), the Ab-Or
Margules parametersof Haselton et al. (1983), and AnOr mixing parametersderived from Seck's(197la, 197lb)
experimentalfeldsparcompositions,can be usedto model
ternary-feldsparequilibria.
Muscovite activity relations
Binary solid solution between muscovite and paragonite is nonideal,and an asymmetricbinary solution model
generally has been used to expressactivity-composition
relations of muscovite (Eugsteret al., 1972; Thompson,
1974; Chatteiee and Froese,1975; Cheneyand Guidotti,
1979),
RIln ar" : R?"ln XM" + X2p^lW"+ 2XM"(Wd - W")\
and paragonite,
RTlnar^:RIln
Xp.+ XM"lWi + 2Xp"(W - W)1,
where lQ'and Wo^arebinary excessfreeenergyofmixing
parametersthat can be expressedin enthalpy (W"), en'
tropy (l/"), and volume (Wr) terms, and the mole fractions of muscovite and paragoniteare given as
X'" : (X")(XX)'
X"" : (X""XXX)'
where
&:
X"" :
tK/(Na+K+Ca+Ba)1,
[Na/(Na * K + Ca + Ba)],
and
[AlvI/(Fe + Mg + Mn + Ti + Al"I)]
defined on an atom per formula basis for the mica. Becausemuscovite and paragonite components of a white
mica appear on opposite sides of the plagioclase-mica
Na-K exchangeequilibrium, the octahedralAl terms (i.e.,
-XX)of these mole-fraction expressionscancel. Thus, the
exchangeequilibrium would appearto be independentof
Fe-Mg (celadonite)substitution in the octahedral sites of
the white mica. Pigageand Greenwood (1982), however,
have demonstratedthat muscovite-paragoniteactivity relations are strongly affectedby the ubiquitous presenceof
celadonite and/or phengite component. They considered
a quaternary mixing model involving paragonite, muscovite, and two celadoniteendmemberswritten in terms
ofpure K and pure Na, respectively,and excessV[/Gpa'
rametersthat were assumedidentical for the muscoviteparagoniteand celadonitebinary joins. On the other hand,
severallines of evidence suggestonly minor substitution
of the Na endmember in white mica. Crystallographic
studies indicate that becauseof the sizes of the cations
involved, addition of Mg, Fe2*,or Fe3*to the octahedral
sites would causerotation and comrgation of the tetrahedral sheet, such that the interlayer site enlarges and
becomesunfavorable for holding the small Na* in place
(Bailey, 1984; Guidotti, 1984).Moreover, muscovite is
always enriched in Fe and Mg relative to coexisting paragonite (Henley,1970; Hock, 1974; Hoffer, 1978; Katagas
and Baltatzis, 1980; Grambling, 1984; Guidotti, 1984).
These relationships imply an antipathy between Fe-Mg
substitution in the octahedral sites and Na occupancyof
neighboringinterlayer sitesof white micas. It is therefore
possiblethat most Na substitution in natural muscovites
occursas a paragonitecomponent. This suggestionis supported by the generallylow Na contents(0.02-0.04 atoms)
of natural celadonites(Wise and Eugster,1964; Boles and
Coombs, 1975; Odom, 1984). Accordingly, the binary
white mica solution model has been modified to reflect
the possibility that Na enters the lattice of natural muscovites only as the paragonite endmember, whereas K
would be present in both the muscovite and celadonite
components.The muscovite mole-fraction terms (X'") in
the muscovite and paragoniteactivity expressions,given
previously, have been modified by reducing X'" empirically as
4r:
X,":[K/(Na+K+Ca+Ba)]
.(Al"' - Na)/(Fe + Mg + Mn + Ti
+ Alu')l
GREEN AND USDANSKY: PLAGIOCLASE-MUSCOVITE THERMOMETER
on an atoms per formula basis, where by virtue of the
(Alu' - Na) term, only the K associatedwith the muscovite component is considered. The paragonite molefraction terms remain
X". : [Na/(Na + K + Ca + Ba)],
where all Na is present as paragonite endmember. This
asymmetricformulation ofXr. and X"" is compatible with
Grambling's (1984) suggestionthat increasedceladonite
substitution shifts the position of the muscovite limb of
the white mica solvus markedly toward higher K/(Na *
K) ratios, but has limited efect on the compositions of
coexistingparagonite(i.e.,paragonitebecomesonly slightly less K-rich).
In addition, muscovite and paragoniteendmembershave
beenassumedto interact with all other componentsin the
same manner as they interact with each other. In the
absenceof appropriate Wo parametersfor celadonite or
phengitecomponents,the activity expressionshave therefore been taken as
R?" ln cr" : RZ ln XM" + (l - X*")t
.IW" +2XM"(W;
- W)l
action, however,is complicatedby differencesin the structural statesof the low albite-microcline and high albitesanidine solid solutions. The free-energyrelationships of
the reaction have therefore been calculated from the internally consistentthermodynamic data of Helgesonet al.
(1978), which take into account the structural state of
plagioclaseand alkali feldspars(Table l). The variation
of RZ ln KD with temperature is illustrated in Figure l.
The equilibrium condition that the Gibbs free-energy
change(in joules per mole) is zero at a given temperature,
T, and pressure,P, results in the following expression:
- 0.043rP.
AGo: -7.58052 - 2087.6587
This free-energyrelation, combined with the ternary-feldspar and modified binary white mica activity models, leads
to the following plagioclase-muscoviteNa-K geothermometric expression,with f in kelvins and P in bars:
T : 0 9 4 5 6 A + 1 2 2 3 0 8 + 2 7 3 2 0 C+ l 8 8 l O D
+ 84738 + 28226F - 65407.0G
+ 65305.4H - 2087.6s87)
+ p ( - 0 . 0 4 3 1 - 0 . 4 5 6 A+ 0 . 6 6 5 3 8+ 0 . 3 6 4 C
+0.364D
+ 2.tt2tc + 0.9699m/
- 8.3147
(7.5805
ln K" - 1.6544A
- 0.71048
+ 10.3c+ 10.3D
- lr4.r040c+ 12.5365rn.
and
R?nln a"" : RZ ln Xr" + (l - Xr^),
.lW€+2X,^(W["-W.)]
with (XM"+ Xp) <1.
Muscovite-paragoniteMargules parametershave been
determined experimentally by Eugster et al. (1972) and
Blencoe and Luth (1973). Chatterjee and Froese (1975)
adjusted the pressure dependenceof the Eugster et al.
Margulesparametersbasedon a review of published data
on the molar volumes of muscovite-paragonitesolid solutions (injoules, kelvins, and bars):
vlft" :
wo^ :
t9 456.0+ 1.65437- 0.456tP
12230.3 + 0.71047 + 0.6653P.
Pigageand Greenwood (1982) subsequentlymodified the
Eugsteret al. interaction parametersto take into account
excessvolumes of mixing determined experimentally by
Blencoe (1977). Although the various Wrval.ues lead to
significantly different equilibrium solvus compositions of
muscovite-paragonite solid solutions at high pressures
(Blencoe,1977),Ihereis very little differencein calculated
equilibrium curves using the various W[" and Wo" parameters(Cheneyand Guidotii,1979; Pigageand Greenwood, 1982).The Margules parametersof Chatterjeeand
Froese (1975) have been adopted here, but the revised
constants based on Blencoe's (1977) experimental data
yield similar results when used to describe plagioclasemuscovite Na-K exchangeequilibria.
PucrocusE-MuscovrrE
THERMOMETER
The ternary albite and orthoclase solution models, white
mica activity relations,and high-temperatureendmember
thermodynamic data can be combined to formulate aplagioclase-muscovitethermometer. The Na-K exchangere-
1l1l
where
A:l-4XM"+5Xh"-2xil"
- 2X,^ + 4X?^ - 2Xl^,
B : 2X-" - 4Xh" + 2I/3M"+ 4XP^
- 5X7" + 2X7" - r,
C : 2XooX[, + 0.5Xo,X^, - Y"o + 2Xo,P^b
- 0.5X^^X^b+ zXo,XA"X^b,
D:
X6, - 2XooX6,+ 0.5Xo.XA"- zXo,XAb
- O.sxA^x^b- 2xo,x^nx^b,
E:2XooX'z[^ + O.sxo.xA, + Xo,x^"X^b
+ O.sxA.xAb- x^"x^b(xAn- xno),
F: )fo^- 2X^bX1^+ 0.5Xo,X^^- X^nXobXo,
G:
+ O.sxA"xAb+ x^^x^b(x^" - xoo)'
-0.5Xo,X^" - XA,Xo,(Xo,- Xo^)
- 2Xo,X2^,- o.sxo^X^b- Xo,X^nX^b,
H : -}.SXo,Xo^ * X^"Xo,(Xo, - Xo,)
- x7^+ 2X"J&" - 0.5X",&o
+ xo,x^nx^b,
ln Ko : ln X'. + ln[X"o(2 - Xoo- Xo)
'(&o + X*)l
- ln Xr^ - lnlX",(2 - Xoo- Xo)
(x^b + xo)],
ll12
GREEN AND USDANSKY: PLAGIOCLASE-MUSCOVITE THERMOMETER
Table 1. Thermodynamicdata (Helgesonet al., 1978)
O
|H 146,t)
(kJ/mol)
w
a
v{48,t)
(J/mol.K)
Muscovite
Albite
-5972.275
-3931.621
287.859
207j50
Paragonite
Alkali feldspar
-s928.573
-3971.403
277.818
213.928
cx105
408.191
258.153
342.586
407 647
320.566
't10.374 -106.440"
-62.802b
58.158
14.870 -209.844.
102.508 -110.625"
18.037 -125.29e
(cm3/
mol)
140.71
100.25
132.53
108.87
Nofej Upper temperature limits for the heat-capacitypowerJunction coefficientsare (a) 1000'K,
(b) 479'K, and (c) 140tr K.
X*, Xoo,and XA, are mole fractions of orthoclase,albite,
and anorthite in plagioclase,and XM. and Xp" are mole
fractions of muscovite and paragonitein white mica. The
application ofthis equation requires an independent estimate or a geobarometric determination of P, because
the pressure dependenceof the calculated temperature
varies from ll to 21" per kilobar depending upon the
compositions of the coexisingminerals.
TrsrrNc oF GEoTHERMoMETER
There are various ways in which the plagioclase-muscovite geothermometercould give spurious results. Systematic errors might easily result from erroneous assumptionsconcerningthe mixing relations in feldsparand
mica solid solutions. Moreover, it is likely that the geothermometer results will be extremely sensitive to analytical uncertaintiesin the Or composition of plagioclase
feldspars.In view of these potential difficulties, it is important to show that temperatures calculated with the
geothermometerare geologicallyreasonable.As the logical way of testing the plagioclase-muscovitegeothermometer, temperatures have been calculated utilizing
compositionsof coexistingfeldspar-micapairs from pelitic metasedimentaryrocks and peraluminous granitoids,
for which independent temperature determinations and
complete mineral analysesare available.
Pelitic metasedimentaryrocks
Plagioclaseand muscovite commonly coexist with garnet and biotite in thesepelitic assemblages;
thus, the plagioclase-muscovitetemperatures may be compared directly to those calculated with garnet-biotite thermometry.
Results of application of the plagioclase-muscovitethermometer to five metapelite suites are presentedin Table
2. Unfortunately, severalmodifications ofthe experimentally calibrated garnet-biotite thermometer of Ferry and
Spear(1978) have been proposedto model nonideal mixing of garnet solid solutions (Holdaway andl;ue, 1977;
Hodges and Spear, 1982; Pigageand Greenwood, 1982;
Ganguly and Saxena,1984). In some rocks, the different
garnet-biotite calibrations yield somewhat varied temperatures,so that it is difficult to determine,a priori, which
calibration is appropriate. As a result, temperaturespredicted by all these calibrations are presentedin Table 2
for comparison.
The Penfold Creek occurrence represents complexly
folded garnet, kyanite-staurolite, and sillimanite zone rocks
that outcrop within the Omineca geanticline and at the
northern extremity of the Shuswap melamorphic complex, British Columbia (Fletcher and Greenwood, 1979).
The pelites are characterizedby some variation in composition between plagioclase grains within individual
specimens,both primary and secondarymuscovite, and
two generationsof garnet that may representtwo pulses
of the same metamorphism. The various garnet-biotite
thermometersyield temperaturesthat range from 383 to
7 l2C at an estimated pressureof 7000 bars; some samples are characterizedby a bimodal distribution of calculated temperatures,399 to 538"C and 599 to 695"C.
The plagioclase-muscovitethermometer indicates equilibration temperaturesbetween 466 and 674{. The temperaturesofPenfold Creek rocks are in generalagreement
with those predicted using the Hodges-Spear,Holdawayke, and Ganguly-Saxenaformulations of the garnet-biotite thermometers, but there is no apparent correlation
betweenthe plagioclase-muscovitetemperaturesand those
derived from any specificgarnet-biotitethermometer (Table 2).
Pigage(1982) described metamorphic mineral assemblagesin the Azure Lake area of the Shuswapmetamorphic complex, British Columbia, that are characteristicof
the kyanite, kyanite-sillimanite, and sillimanite zones of
the Barrovian series.Pigageand Greenwood (1982) concluded that observedmineral assemblagesrepresenta close
approachto equilibrium and used the intersection ofseveral mineral equilibria with the kyanite-sillimanite reaction curve to estimate metamorphic pressuresof 5133 to
6731 bars. Their revised Ferry and Spear(1978) garnetbiotite geothermometeryields temperaturesup to 100 degreeshigher than those calculatedwith the other calibrations (579-603'C). The calculated plagioclase-muscovite
temperaturesof the Azure Lake rocks rangebetween 500
and 601"C and, typically, are generally intermediate between those predicted by the Holdaway-ke and PigageGreenwood garnet-biotite thermometers.
Hodgesand Spear(1982) used pelitic schistsof the Mt.
Moosilauke region, New Hampshire, to evaluate the internal consistency of a variety of published geothermometers and geobarometerswith the AlrSiO, invariant
point of Holdaway (1971). The rocks studied by Hodges
GREEN AND USDANSKY: PLAGIOCLASE-MUSCOVITE THERMOMETER
and Spear contain andalusite andJor sillimanite, but do
not contain kyanite. Sillimanite occurseither as subhedral
prismatic grainsor as fibrolite intergrown with muscovite;
where present,andalusiteoccursas subhedralto anhedral
porphyroblasts. The described textures suggestthat all
samples equilibrated at temperatures above the aluminosilicate triple point (501'C) and that the two samples
containingboth andalusiteand fibrolite (1468, 146D) may
have equilibrated near the intersection of the muscovite
breakdown and andalusite-sillimanite reaction curves
(545-590"C). In this case,only the calibration ofthe garnet-biotite thermometerby Pigageand Greenwood(1982)
producescomparable temperatures(561 and 621"C); all
other calibrations generallyyield temperaturesof the aluminosilicate triple point or lower. Application of the plagioclase-garnet-AlrSiO,
geobarometer(Ghent, I 976; Ghent
and Stout, l98l) produced preferred pressureestimates
that rangefrom 2677 to 2918 bars.Two Mt. Moosilauke
rocks have plagioclase-muscovitetemperatures(422 and,
441"C) that suggestsubsolidusre-equilibration, but four
samplesyield temperaturesabove the triple point (5 lzl588t) that are consistentwith the observedaluminosilicate minerals. The compositions of plagioclaseand muscovite in Mt. Moosilauke specimen 90A define a temperatureof 801"C,which reflectsthe anomaloustyhigh Or
content ofplagioclase porphyroblasts in this rock.
Zen ( I 98 I ) describedmineral assemblages
from slightly
calcic, chloritoid, garnet, and kyanite-staurolite zone pelitic rocks from the Taconic Rangein southwesternMassachusettsand adjacent areas of Connecticut and New
York. Comparison of the mineral assemblageswith hydrothermal phase-equilibrium data suggeststhat the approximate range of metamorphic conditions was 400600{ at pressuresprobably about 4 kbar. Garnet-biotite
thermometry yields a similar rangeof temperatures(416607"C). Plagioclase-muscovitethermometry yields temperaturesof 438-567t, which are in generalagreement
with garnet-biotite temperatures.
Hodges and Royden (1984) described kyanite-grade
pelitic schistsfrom the Efiord area,northern Norway, that
exhibit abundant textural evidenceof retrogradere-equilibration under nonuniform pressure-temperatureconditions. The presenceof second-stagekyanite in all retrogradesamplesindicatesthat the P-f path oftectonic uplift
occurred entirely within the kyanite stability field. The
plagioclase-garnet-AlrSiO,geobarometer (Ghent, 1976;
Ghent and Stout, l98l), coupled with garnet-biotite thermometry, yields estimatedpressuresthat rangefrom 3 to
14 kbar at temperatures of 471 to 853'C. Plagioclasemuscovite temperatures (423-8i3C) exhibit the same
variation with pressureasthat shown by the garnet-biotite
temperatures(Hodgesand Royden, 1984; Fig. 8).
Peraluminousgranitoids
Plagioclaseand muscovite are also important constituents ofthese granitic rocks, but only locally coexist with
mineral pairs that have compositions applicable to geothermometry. As a result, most available temperaturees-
l1l3
o
Y
c
F
(r
Albite-K Feldspar
AG= -7.58057
- 2 0 8 7 . 65 8 7
I
600
800
1000
1200
Temperature(oK)
for the Na-K
Fig. l. Variationof RZln KDwith temperature
exchangereactionbetweenplagioclase(all structuralstates)and
dataof Helgemuscovite(heavyline)basedon thermodynamic
reactioncalsonet al. (1978).Reactioncurvesfor the exchange
culatedusingthermodynamicdata of sanidine-highalbite and
microcline-lowalbitesolidsolutionsarealsoshown.
timates are basedupon phase-equilibriumconsiderations,
such as biotite-iron oxide stability relations. Five peraluminous granitoid suites,chosento test the thermometer,
are discussedbelow, and estimatedeqirilibration temperatures are presentedin Table 3.
Green and Usdansky (1984) described the petrology,
mineralogy, and crystallization conditions of two-mica
epidote-bearinggranitoids of the Alabama tin belt. Garnet-biotite and apatite-biotite F-OH geothermometrysuggested temperaturesof 630 to 740C, whereas ilmenite
and biotite stability relations suggestedtemperaturesup
to 770"C. Calculatedplagioclase-muscovitetemperatures
rangefrom distinctly subsolidus(414C) to near liquidus
(740"C).
Kistler et al. (1981) describedtwo-mica granitesin the
Ruby Mountains, Nevada, and reported garnet-biotite
equilibration temperaturesbetween 365 and 505qCbased
on the Goldman and Albee (1977) calibration. Other calibrations of the garnet-biotitethermometer,however,give
temperaturesof 527-725'C. The calculated plagioclasemuscovite temperaturesare higher than those predicted
by all but the Pigage-Greenwoodgarnet-biotite thermometer (Table 3) and reflect the coexistenceof K-rich muscovite with K-rich plagioclasein thesegranites.
Czamanskeet al. (1981) detailed the chemistry of rockforming minerals of the Cretaceous-Paleocene
batholith
in southwesternJapan. They noted significant differences
in the chemistry of primary and secondarymuscovites,
in agreementwith the characteristicsoutlined by Miller
et al. (1981). Temperaturesbased on two-feldspargeothermometry indicated that none of the suites faithfully
retains solidus feldspar relations; perthitic alkali feldspar
in all sampleshas lost Ab. Ilmenite and biotite stability
relations in for-T space, however, suggesteqrilibration
temperaturesof 68O-8l0'C. Plagioclase-muscovitetemperatures calculated using primary and secondary mica
compositions are given in Table 3.
Bradfish (1979) investigated conditions of muscovite
crystallization in the Teacup Granodiorite, Arizona, and
concludedthat the mica in all faciesof the intrusion was
THERMOMETER
GREENAND USDANSKY:PLAGIOCLASE-MUSCOVITE
ll14
rocks
of peliticmetamorphic
temperatures
Table2. Plagioclase-muscovite
Sample
Xo^
Xoo
Xo,
xru
Xu"
P (bars)
G-S
F-S
Penfold Creek (Fletcherand Greenwood, 1979)
5
o
7
8
10
11
12
13
14
15
373
121
367
82
398
492
223
2-376
2-13
74
5V
40
0.359
0.292
0.302
0.260
0.290
0.310
0.290
0.202
0.206
0.282
0.129
0.635
0.705
0.694
0.738
0.708
0.685
0.706
0.794
0.789
0.712
0.868
0.005
0.003
0.003
0.002
0.002
0.003
0.003
0.003
0.005
0.005
0.003
0.146
0.238
0.167
0.238
0.241
0.202
0.186
0.222
0.236
0.163
0.202
0.277
0.314
0.394
0.278
0.207
0.227
0.337
0.277
0.309
0.358
0.256
0.240
0.718
0.681
0.602
0.716
0.788
0.766
0.658
0.716
0.685
0.638
0.738
0.758
0.004
0.004
0.003
0.004
0.005
0.003
0.004
0.005
0.005
0.003
0.005
0.003
0.170
0.160
0.140
0.160
0.209
0.209
0.150
0.150
0.140
0.120
0.150
0.179
560
596
676
628
577
594
599
630
677
700
674
668
553
663
600
538
551
573
617
640
659
650
617
549
626
576
536
546
553
596
613
617
621
599
523
577
545
510
514
525
568
582
619
623
674
565
561
488
505
574
558
508
585
625
466
7 000
7 000
7 000
7 000
7 000
7 000
7 000
7 000
7 000
7 000
7 000
AzureLake(Pigage,1982)
602
545
0.763
651
535
0.772
608
0.791
533
626
525
0.772
652
541
0.734
618
543
0.734
621
533
0.782
641
555
0.782
656
523
0.791
658
581
0.792
682
557
0.782
580
540
0.754
573
603
571
549
557
579
570
585
552
612
581
557
578
548
542
553
569
556
564
542
582
565
545
535
535
524
544
534
529
547
535
578
549
565
581
580
552
562
505
601
592
600
569
579
ss8
5s3
530
500
5 678
5 546
5 783
5 133
5 528
5 979
6 035
5 828
s 396
6 731
5 828
5 264
499
518
513
518
524
540
498
488
513
504
508
524
527
486
514
588
801
422
441
531
532
3 918
3 444
3 801
3 493
3 167
2722
2677
524
447
492
475
545
473
460
520
530
461
501
495
531
489
482
527
544
458
479
5't7
561
479
469
516
538
547
567
484
548
472
438
535
4 000
4 000
4 000
4 000
4 000
4 000
4 000
4 000
775
507
503
523
499
597
584
673
516
512
507
503
565
565
653
466
475
492
489
533
526
873
423
451
546
524
675
546
13000
3 600
3 300
6 000
4 900
I 000
7 000
0.786
0.700
0.766
0.695
0.693
0.721
0.742
0.714
0.704
0.762
0.726
596
s34
648
572
521
529
539
602
625
631
640
788
80D
90A
92D
145E
1468
146D
0.116
0.280
0.141
0.108
0.089
o.24't
0.243
0.876
0.714
0.831
0.888
0.90s
0.753
0.750
0.006
0.006
0.026
0.003
0.004
0.005
0.005
(Hodges
andSpear,1982)
Mt. Moosilauke
477
533
468
0.245
0.713
512
565
489
0.776
0.172
497
486
568
0.237
0.721
501
491
557
0.252
0.701
507
498
567
0.727
0.231
538
621
519
0.767
0.190
480
561
464
0.762
0.190
463-1
339-1
356
103-1
103-2
234-1
14-1
1052-2
0.190
0.266
0.138
0.120
0.300
0.368
0.247
0.059
0.804
0.729
0.853
0.875
0.694
0.630
0.750
0.930
0.005
0.004
0.008
0.005
0.004
0.002
0.002
0.010
0.280
0.237
0.225
0.187
0.170
0.133
0.200
0.187
8B
8C
12E
18A
21A
21C
21F
0.541
0.143
0.207
0.29s
0.302
0.312
0.252
0.454
0.853
0.788
0.699
0.694
0.680
0.744
0.004
0.003
0.004
0.004
0.003
0.008
0.004
Efjord(HodgesandRoyden,1984)
853
712
0.120
0.785
491
527
0.152
0.737
528
485
0.776
0.101
573
476
0.751
0.101
471
634
0.703
0.166
640
0.780
554
0.075
615
557
0.111
0.781
TaconicRange(zen,1981)
568
509
0.687
500
416
0.721
469
508
0.727
515
460
0.733
607
0.761
510
495
452
0.815
474
443
0.763
548
0.780
505
Notes: Garnet-biotitethermometer calibrations as follows: F-S, Ferry and Spear (1978); P-G, Pigage and Greenwood (1982); H-S' Hodges and
Spear (1982); H-L, Holdaway and Lee (1977); and G-S, Ganguly and Saxena (1984). f = plagioclase-muscovitegeothermometer.
probably secondary and resulted from subsolidus reactions involving a coexistingaqueousphase.Predictedplagioclase-muscovitetemperatures (478 and 530'C) are
compatible with such an interpretation and with temperatures predicted by some but not all garnet-biotite thermometers (Table 3).
Ferry (1978, 1979)discussedthe origin ofhydrothermally altered feldspars and mica in samples of granitic
rock from south-central Maine. He reported alteration
assemblagesconsisting of muscovite, calcite, quartz, epidote, plagioclase,and alkali feldspar that record temperaturesof4l5 to 435t at 3500 bars. The measuredfractionation ofAb componentbetweencoexistingplagioclase
and microcline yields temperaturesof 41 5 to 454qCat the
same pressure,although Ferry (1978, 1979) did not present data that would p€rmit evaluation of the other two
feldspar endmember equilibria in order to determine
whether stableequilibrium has been achievedbetweenall
THERMOMETER
GREENAND USDANSKY:PLAGIOCLASE-MUSCOVITE
lll5
granitoids
Table3. Plagioclase-muscovite
ot peraluminous
temperatures
Sample
xh
xe
Xo,
X""
Xt"
F-S
P (bars)
P-G
ATB.67
ATB-58
RF-20s
RF-3s
0.337
0.354
0.017
o.224
0.644
0.635
0.969
0.754
0.008
0.010
0.006
0.008
1984)
Alabamatin belt(Greenand Usdansky,
63s
863
689
0.070
0.753
n.a.
n.a.
n.a.
0.070
0.753
n.a.
n.a.
n.a.
0.060
0.780
n.a.
n.a.
n.a.
0.060
0.785
618
n.a.
n.a.
n.a.
740
n.a.
n.a.
n.a.
n.a.
760"
463b
584b
698
740
414
536
8000
8000
6000
6000
11-66A
31-66D
0.008
0.010
0.866
0.905
0.125
0.08s
(Kistler
RubyMountains
et al.,1981)
725
548
0.068
0.828
538
527
0.064
0.836
514
607
550
533
630
609
n.a.
n.a.
718
67'l
5000
5000
LN.1
SN-19
0.126
0.146
0.863
0.844
0.012
0.008
(Bradfish,1979)
TeaCupGranodiorite
774
512
0.065
0.776
501
607
0.055
0.770
590
916
s25
586
615
720
n.a.
n.a.
530
478
5000
5000
6510s
T25s
T173bc
T173Br
T76
T133r
0.245
0.017
0.224
0.354
0.017
0.224
0.354
0.739
0.969
0.754
0.635
0.969
0.754
0.635
0.014
0.006
0.008
0.010
0.006
0.008
0.010
(Czamanske
Japanese
batholiths
et al.,1981)
n.a.
n.a.
n.a.
0.020
0.769
n.a.
n.a.
n.a.
0.060
0.780
n.a.
n.a.
n.a.
0.060
0.785
n.a.
n.a.
n.a.
0.070
0.753
n.a.
n.a.
0.060
0.780
n.a.
n.a.
n.a.
n.a.
0.060
0.785
n.a.
n.a.
0.070
0.753
n.a.
n.a.
n.a.
n.a,
n.a.
n.a.
n,a,
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
485
556
669
651
669
678
616
s000
5000
5000
5000
5000
5000
s000
694s
698s
821s
838s
276s
787s
0.122
0.137
0.170
0.i73
0.213
0.096
0.871
0.8s8
0.817
0.816
0.780
0.900
0.005
0.004
0.012
0.009
0.006
0.003
Augusta,
Maine(Ferry,1978,1979)
n.a.
n.a.
n.a.
0.040
0.U2
n.a.
n.a.
n.a.
0.040
0.816
n.a.
n.a.
0.040
0.791
n.a.
n.a.
n.a.
n.a.
0.040 0.816
n.a.
n.a.
n.a.
0.030
0.798
n.a.
n.a.
0.030
0.798
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
403"
447"
376
364
474
446
401
31s
3500
3500
3500
3500
3500
3s00
415d
423d
435d
442.
Notesj Garnet-biotitethermometer calibrations as follows: F-S, Ferry and Spear (1978); P-G, Pigage and Greenwood (1982); H-S, Hodges and
Spear (1982); H-L, Holdaway and Lee (1977); and G-S, Ganguly and Saxena (1984). Other temperature estimates (7-) based on (a) biotite-apatite
F-OHexchangeequilibria,(b) biotite-ilmenitestability relations,(c) twGfeldspar albite geothermometer,and (d) muscovite-4alcite-epidote-plagioclasealkali leldspar equilibria; n.a., not applicable. Where indicated, analyses represent (c) core or (r) rim of primary (p) muscovite, or secondary (s)
geothermometer.
muscovite. I plagioclase-muscovite
feldspar endmembers. The calculated plagioclase-mus- covite geothennometer extends the range of pelitic and
covite temperaturesare compatible with those predicted peraluminous rocks for which equilibration temperatures
by other geothemometric techniques(Table 3).
may be estimated.
Suprulnv
AND coNcLUsroNS
Available thermochemical data have been used to calibrate the Na-K exchangereaction between plagioclase
and muscovite as a geothermometer.The formulation of
the plagioclase-muscovitegeothernometer accounts for
high and low structural statesof feldspar and utilizes plagioclaseactivity-composition relations expressedin tenns
of a ternary subregularsolution model. Binary mica solution modelshave beenmodified to accountfor increased
K/(K + Na) in the muscovite component associatedwith
increased celadonite andlor phengite substitution. The
geothermometer has been applied to pelitic metasedimentary rocks and peraluminous granitoids that crystallized at temperaturesof 4 | 5-7 25"C and pressuresof 2.513.0kbar. The predicted equilibration temperaturesusing
plagioclase-muscovite pairs generally fall within the range
of equilibration temperaturesestimated using other geothermometric techniques(e.g.,garnet-biotite thermometry). Observed discrepanciesbetween calculated temperaturesappear to be related to re-equilibration of muscovite
and/or plagioclase.Application of the plagioclase-mus-
AcxNowr,nncMENTs
We are grateful to J. M. Rice, S. K. Saxena,and J. C. Stormer,
Jr., for their critical reading ofour manuscript and valuable constructive suggestionsfor its improvement. Stimulating discussions with R. J. Donahoe and C. M. Lesher about the thermodynamic modeling are greatly appreciated.We also thank S. Miller
and S. Simpson who helped in the preparation of the manuscript.
This work was supported by a gant from the School of Mines
and Energy Development at the University of Alabama.
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MeNuscnrprREcETVED
JuNs 6, 1985
Meuuscnrpr ACCEPTED
Mav 16, 1986