The fluid-absent melting of phlogopite * quartz: Experiments and models

17 Pages·2007·2.08 MB·English

American Mineralogist. Volume 77. pages 1206-1222. 1992
The fluid-absentm elting of phlogopite * quartz: Experiments and models
DaNrrr, Ytntznur
D6partement de G6ologie, CNRS-URA 10, 5 rue Kessler, 63038 Clermont-Ferrand, France
JonN D. Cr-nrrnNs
Department of Geology, University of Manchester,M I 3 9PL, Manchester,U .K.
Ansrnacr
Knowledge of the P-Z location and stoichiometry of the fluid-absent reaction Phl *
Qtz : En + Sa + M is required for a better understandingo f crustal anatexis and crys-
tallization and the stability of biotite in granitoid magmas. This equilibrium has been
experimentallyi nvestigatedb y Bohlen et al. (1983), Montana and Brearley( 1989),a nd
Petersona nd Newton (1988, 1989).H owever, there has been consensusn either on the
chemographic relationships among the possible phasesi nvolved in this equilibrium nor
on its precise P-I location. We have carried out experiments, at pressuresb etween 100
and 1500 MPa and temperaturesb etween 800 and 920 "C, in order to remove or explain
theseu ncertainties.T he following bracketsw ere obtained for the reaction: 100 MPa, 805-
815'C; 150M pa,798-8ll'C;200 MPa,808-821'C ;300M Pa,810-821'C ;500M Pa,
>838'C; 800 MPa,860-865" C; 1000M Pa,849-876" C, 1500M Pa,909-921' C. In
addition, we reversedt he equilibrium at 1000 MPa. Our experimentsc onfirm the presence
ofsanidine as a reaction product over the pressurer ange investigated.
Becauset he reaction Sa + Qtz + Fl : M nearly coincides with Phl * Sa * Qtz + Fl
: M and En * Sa + Qtz + Fl : M, it can be considereda s an excellent analogueo f the
solidus in the KMASH system. It is possible to calculate tlireP -T location of this solidus
as a function of anro using models for the interactions between HrO and aluminosilicate
melts. Using available thermodynamic data, we can also calculatet he shift of the subsoli-
dus reaction Phl + Qtz : En * Sa + Fl as a function ofdecreasinga "ro. The intersections
of thesec urves,a t the samec "ro, provide a theoretical location for the fluid-absentm elting
reaction. This calculatedc urve has a slope very similar to, and only l0- I 5 "C above, our
experimentalc urve. This modeling,t ogetherw ith the compositiono f the phases,a llows
us to calculate that the fluid-absent reaction will produce up to 50 + 15 wto/om elt in the
pressurer ange 100-1500 MPa.
On the basis of the experimentally determined locations of the solidus reactions( Phl +
Qtz + Sa + Fl : M and En * Sa + Qtz + Fl : M) and the subsoliduse quilibrium (Phl
+ Qtz: En * Sa + Fl), Grant (1986) concludedt hat there should be a thermal divide
on the liquidus below 500 MPa. Our experimentsa nd modeling of the solubility of HrO
in the melts suggestt hat a thermal divide may well exist but that it must be restricted to
a very narrow domain between6 5 and 100 MPa, at about 810 'C. Additional components
such as Ti and F should have the effect ofsignificantly extending such a thermal barrier.
INrnonucrroN equivalent of this reaction is phlogopite + quartz : en-
statite+ sanidine+ melt in the KMASH system( Wones,
Around 20 years ago, attention was drawn to the im- 1967;'L uth, 1967). Quantitative knowledge of the pres-
portance of fluid-absent conditions in magmatic and sure and temperaturel ocation ofthis reaction is therefore
metamorphicp rocesse(sY oder and Kushiro, 1969;R ob- required for a better understandingo fcrustal anatexis.
ertson and Wyllie, l97l; Eggler, 1973; Fyfe, 1973). In This reaction has been experimentally investigated by
such settings, melting at geologically attainable temper- Bohlen et al. (1983),M ontana and Brearley( 1989),a nd
atures most commonly dependsu pon the availability of Petersona nd Newton (1988, 1989). However, there is
HrO from crystalline hydrates such as muscovite, biotite, consensusn either on the chemographicr elationships
and hornblende. The reaction biotite * quartz: ortho- among the possible phasesi nvolved in this equilibrium,
pyroxene + potassium feldspar + melt has been used to nor on the pressure-temperaturelo cation of the reaction,
model fluid-absentm elting in common crustal rock types, in particular at 1000 MPa (T: 890-900 "C, Bohlen et
such as quartzofeldspathicm etagra)'wackes(G rant, I 973; al., 1983;8 15-850" C, Petersona nd Newton,1 988;8 35-
Clemens and Wall, l98l). The simplest end-member 850'C, Montana and Brearley, 1989).O ne purpose of
0003- oo4x/ 92 / | | | 2- | 206 $02.00 1206
VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING r20'l
this study is to remove or propose explanationsf or these (Sa)
uncertainties.
On the basis of the experimentally determined loca- Pht\ I Phr/E
\ otzlM /
tions of the solidus curves Phl + Qtz * Sa * Fl : M orz \M _ i orzl sa
sa \ | lM
and Qtz * Sa * En + Fl : M, and the subsoliduse qui-
librium Phl + Qtz : Sa * En * Fl, Grant (1986) con-
cluded that there should be a thermal divide on the liqui- o En>{
dus below 500 MPa. Below this point, there would be Sa Qtz
two fluid-absentr eactions:P hl + Sa + Qtz : En + M
and Phl + Qtz : En + M (Fig. lc). So few experiments
have been performed at low pressureo n the fluid-absent (FD MgO En
melting of phlogopite + qnarrz that it is impossible to
verify the existence and location of any such singular
points and thermal divide.
It hasb eens howne arlier( Clemensa nd Vielzeuf, 1987)
that the simple melting reactionsi n the Qtz-Ab-Sa-HrO
system and subsystemsa re good models for melting re- KAro2 en Hro
actions in more complex systems.T his is valid for sys-
tems involving MgO as an additional component (e.9.,
Phl + Qtz * Sa * Fl : M). In the simple systems,it is
possible to calculate lhe P-T location of the solidus as a
function of en,o (Burnham, 1979;B urnham and Nekva-
sil, 1986;N ekvasila nd Burnham,1 987; Nekvasil,1 988).
Using availablet hermodynamicd ata, it is also possible
to calculatet he shift of the subsolidusr eaction Phl + Qtz
: En * Sa + Fl as a function of decreasinga rro. The
intersectionso f thesec urves provide a theoreticall oca-
tion for the fluid-absent reaction which, if correct, must
tally with the experimentallyd eterminedl ocation.
Pnnvrous ExPERTMENTS Fig. l. Topological relationships around invariant point (I)
in the system KAIO'-MgO-SiO,-HrO. (a) The HrO content in
In this section we recapitulate the experimental results the melt at the invariant point is greater than about 3.5 wto/0,
obtained by previous workers. Later, we will critically and phlogopite + quartz breakdownc annot provide enoughH .O
evaluatea ll experimentalw ork and presentw hat we be- to saturatet he melt (Luth, 1967).( b) The HrO content of the
lieve to be the most consistenti nterpretation. melt at the invariant point is lower than about 3.5 wto/0,a nd
Bohlen et al. (1983) reported 16 experimentso n the phlogopite + quartz is able to saturate the melt (Grant, 1986).
fluid-absent melting of phlogopite + quartz. For the Experiments and modeling indicate that the situation in b is
starting materials, Brazilian quartz was finely ground, correct in the immediate vicinity of the invariant poinr. With
increasingp ressure,t he HrO content of the melt increases,a nd
leachedin HNO,, and fired at 800'C for 48 h. Phlogopite
the relative arrangement of reactions (En), (Sa), and (Fl) must
was prepared from KrCOr, MgO, AlrO., and SiOr, react-
changef rom caseb to casea , thus creating singular points. (c)
ed hydrothermally. Actual starting materials were
Illustration of singular points resulting from a changei n reaction
mixtureso f phlogopitea nd quartz( l:5 moler atio, - 1.3:1 topologies from those in b to those in a. (d) Schematicl iquidus
by weight).T he loaded,b ut open,c apsulesw ere dried in diagrams are shown for three pressuresg iven in c. The MgO
a vacuum oven at 150'C for 30 min. After sealing,t he contents of rhe liquids are greatly exaggeratedfo r clarity. In c,
capsulesw ere placed into larger Pt capsulesw ith sintered no thermal barrier can exist above the dashed line, which cor-
hematite,t o maintain low /r, in the inner capsule.T he respondst o a melt H.O content of 3.54 wto/o(s eet ext). All these
experimentala pparatusw as a 2.54-cm piston cylinder figuresa re modified afier Grant (1986);s eea lso Eggler( 1973)
used with NaCl pressurec ells and graphite tube furnaces. and Egglera nd Holloway (1977). Abbreviations for phasesa re:
Calibration methods are detailed in Bohlen and Boettcher Phl : phlogopite, Qtz : quartz, Sa : high sanidine, En : or-
thoenstatite, Fl : supercritical aqueousf luid, and M : hydrous
(1982), and we estimateP and T uncertaintiest o have silicate melt. Other abbreviations used in the text include Ab :
beena bout +20 MPa and +5 "C. Most experimentsh ad NaAlSi.O,, Or : KAlSi.O*.
rather short durations( usually2 4 h).
Bohlen et al. proposed the following five brackets, as
indicatedi n Figure2 : 500 MPa (850-860'C), 750 MPa tergrown enstatite, sanidine, and glass. The authors ob-
(870-880" C), 1000M Pa (890-900'C), 1500M Pa (910- served that the reaction ofphlogopite + quafiz to ensta-
920'C), 2000 MPa (910-920" C). tite + sanidine * melt is rapid, with 50-700/o of the
In thesee xperiments,t he solidus was determinedb y reaction occurring within 24 h, and that the reverse re-
the abrupt appearanceo f large proportions of finely in- action is extremely sluggish. Another point of interest is
l 208 VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING
Montana and was first raised to 950 "C for -80 h and then held for
1600 Brearley -200 h at a lower temperature.
Petersona nd Newton (1989) located the melting of
1400 phlogopiteI quarlza t -810 "C at 160 MPa, <825'C at
240MPa,800-830'C at 500M Pa,8 15-850'C at 1000
12 00 Bohlen MPa, 795-805" C at 1500M Pa, and 800-810'C at 1800
MPa (Fig. 2). The criterion for melting was the presence
ofenstatite and glassi n the products. They also investi-
1000
gated the effectso f H, by isolating their samplesf rom H,
flux during the experiments. This was done by placing
6 8Oo hematite or glass around the capsules.T hey concluded
= that the temperature of the fluid-absent reaction (Fl) un-
o 600 .Bohlen et al. (1983)
M ( Peterson and Newton der fo, conditions equivalent to the hematite-magrretite
Qtz ( 19 89) buffer was consistent with the results of unbuffered ex-
Sa
Phl 0 Montana and Brearley penments.
( 19 89)
In their two-stagee xperiments at 1000 MPa Peterson
< this work
and Newton (1989) found a temperaturei nterval (815-
850 "C) over which phlogopite, quar1rzo, rthopyroxene,
and liquid coexisted.N either original nor regrown phlog-
800 850
r('c) opite was detected in the products of experiments at 7n
En Sa Fl
above 850'C. The reversal bracket reported in their Fig-
Fig. 2. Summaryo f experimentabl racketso n the fluid-ab- ure 2 indicates that fluid-absent melting occurs between
sentr eactionP hl + Qtz : En (+ Sa)+ M. The P-Z locations 790 and 850 "C, the lower limit being the maximum tem-
of reactionPs hl + Qtz: En + Sa + Fl (Wood,1 976;W ones perature for which the starting mixture of phlogopite +
andD odge,1 977P; etersoann dN ewton,1 989a) ndP hl + Sa+ quartz appearedu nreacted.W ithin the interval 815-850
Qtz + Fl : M, nearlyc oincidenwt ith Sa+ Qtz + Fl : M (Luth, 'C, they reported that the mica and coexisting pyroxene
1967;B ohlene t al.. 19831P etersona nd Newton.1 989).a re
both become less aluminous as Z increases.T heir ana-
shown.
lytical data suggestt hat the maximum extents of alumi-
nous end-member substitutions are 20 molo/oin the mica
and 7 molo/oin the orthopyroxene.
that Bohlen et al. reported the systematica ppearanceo f Sanidine was not detected in any of the experimental
sanidine in their products, though the identification products, though the samples were examined using pet-
method was not stated. rographic, scanninge lectron microscope,e lectron micro-
Some more recentd ata on phlogopite + quartz melting probe, and X-ray diffraction techniques. This led these
reactionsa re thoseo f Petersona nd Newton ( I 988, I 989). authors to conclude that the fluid-absent breakdown of
In their experiments, phlogopite was synthesized from phlogopite * quartz correspondst o a thermal divide (Phl
mixtures of KrCO, or KNOr, AlrO, orAI(OH)., and MgO + Qtz : En + M), in agreementw ith Grant's (1986)
and fused silica. Mixtures were reacted,i n cold-sealv es- topology. According to them, this thermal divide would
sels,w ith 10-20 wto/od istilled H,O, at 100 MPa and 800 extend to pressuresg reater than 1800 MPa. Thus, the
'C
for l0 d. X-ray characteization indicated that micas singular point correspondingt o the high-pressuree nd of
were slightly nonstoichiometric phlogopites [Z ranging the thermal divide would be located at an even greater
from : 495.4( + 0.2)A : ro 497.0( + 0.1)A .l, correspond- pfessure.
ing to a maximum possible total excessA l content (oc- Noting the discrepancyb etween the results of Bohlen
tahedral and tetrahedral) of approximately 0.4 cations et al. (1983)a nd Petersona nd Newton (1988),M ontana
per 12 O atoms (Petersona nd Newton, 1989). Starting and Brearley (1989) decided to investigate the fluid-ab-
batches of phlogopite and quartz (2:,3o r 3:2 by weight) sent melting of phlogopite + q\arIz at 1000 MPa, in a
were ground in an agate mortar, under distilled HrO, to 2.54-cm piston-cylinder apparatus. The experimental
a fine-grainedh omogeneousm ixture. Samplesw ere dried proceduresw ere similar to those describedb y Bohlen et
at 80'C for I h, then for l-2hat ll0'C, immediately al., except that the interior of the furnace was made of
before sealingo f the Au or Pt capsules.A bove 600 MPa, BN and MgO. Type-S Pt-Rh thermocouples were used
experimentsw erec onductedi n a l.9l-cm (and more rarely and a hematite-magnetiteb uffer seemst o have been em-
in a 2.54-cm) piston-cylinder apparatus,w ith NaCl cells ployed in a double-capsulec onfiguration. Durations were
as the pressurem edia. Sample capsulesw ere embedded varied between 200 and 624 h.
either in NaCl or Ca-Na soft glass. Experiments in the These authors reported nine experimentsa t 1000 MPa
range 30-600 MPa were conducted in an internally heat- and temperatures between 825 and 950 "C and located
ed Ar pressurev essel.T he duration of thesee xperiments the reactionb etween8 35 and 850'C (Fig. 2). They at-
varied from 2 to 60 d. Aside from single-stagee xperi- tributed the disagreementw ith the bracket proposed by
ments, there were also some two-stageo nes. In these, ?r Bohlen et al. (1983) to the demonstrably sluggishk inetics
VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING 1209
of the reaction, coupled with insufrcient durations in the pressured ifferenceo f lesst han 5 MPa at 450 MPa. Tem-
Bohlen et al. experiments.M ontana and Brearley( 1989) peratures were monitored by type-B thermocouples lo-
considered the reversed experiments reported by Peter- cated immediately adjacent to each of the capsulesa nd
son and Newton (1989)t o be inadequatet o establisht he are accuratet o + l0 "C.
beginning of melting at 8 I 5 "C. The coexistenceo f phlog- Most higher pressuree xperiments were carried out in
opite and liquid in the two-stagee xperimentso f Peterson a 1.27o r l.9 I -cm piston-cylindera pparatust hat was not
and Newton between8 15 and 835'C is explaineda s the end loaded (Patera and Holloway, 1982).
metastablep ersistenceo f melt after the temperature was In the piston-cylindere xperiments,p ressurec ells 1.27
dropped from 950 'C to the final value. cm in diameter were all NaCl or NaCl-Pyrex or, more
Montana and Brearley (1989) never identified sanidine rarely, talc-pyrex, with graphite furnace tubes. The in-
with any certainty among their products. They concur sides of the graphite furnaces were filled with rods of
with Petersona nd Newton (1988)t hat potassiumf eldspar NaCl or borosilicate glass and cylinders of crushable
is "a minor or an unnecessaryp hasei n this reaction" and magnesia.T emperaturesw ere measuredw ith ft-Rh (type-
postulate the existenceo f a thermal barrier up to about S) or WRer-WReru (type-C) thermocouples, calibrated
'C,
300 MPa. against the 1000-MPa melting point of Au (1120
Akella and Kennedy, 197l). Thermal gradientso f -2 "C
acrosst he samples0 .6 mm thick are estimated from the
Trrn rr-urn-.ABSENTM ELTTNGo F pHr-ocoprrE
measurementos f Esperangaa nd Holloway (1986)a nd Ja-
+ eUARTZ REvISITED
kobssona nd Hollowav (1986)o n this type of samplea s-
Experimental techniques sembly. Reported temperatures are considered accurate
Apparatus. The lower pressuree xperiments (P - 300 to + 5 "C. Temperature was controlled to + I 'C and pres-
MPa) were conducted in cold-sealv esselsw ith vertically sureg enerallyt o + l0 MPa.
mounted furnaces( the hot spot on top ofthe vessel)a nd In additional experimentsu sing the piston cylinder, the
used N, (with trace HrO) as the pressurem edium. Tem- pressurea ssembliesl,. 9l cm in diameter,w eren earlyt he
peratures were measured with external, type-S thermo- same as those describedb y Perkins et al. (1981). They
couplesa nd are believed accuratet o a 5 'C. Thermal gra- comprised an outer NaCl bushing surrounding a glass
dients were measured with an internal type-K sleeve, which in turn surrounded the graphite furnace.
thermocouple at I atm and 800 "C and amounted to < I Inside the furnace, an additional glass sleeve, cored by
oC over 2 cm aI the sample locations in the vessels.C e- crushable magnesia,s erved as the pressurem edium. At
ramic filler rods were used to minimize the free fluid the core, the small flat Pt capsule was surrounded by
volume and, given the width of the furnace hot spots,w e crushed borosilicate glass.A type-C thermocouple, pro-
anticipate almost no convection and gradients <<I "C tected by an alumina ceramic, passed through a small
acrosst he tiny capsules.P ressurew as monitored using a hole in the magnesia.T he thermocouple tip was protect-
solid-state transducera ccuratet o +0.6 MPa. WeekJong ed by a small amount of BN powder and separated( by
experimentsa t 100 MPa and 800 "C, with Pt capsules < I mm) from the Pt capsuleb y a thin magnesiaw afer.
containing various metal oxides and HrO, showed that Independentt emperaturec alibration experimentss uggest
the intrinsic .fo,/f r, of the system is close to FerOo + that the difference between the temperature at the ther-
'C.
FerO, + HrO or Cu * CurO + HrO. mocouple tip and the capsulei s lesst han 5 Perkins et
All of the experimentsr eported in this study were car- al. (1981)f ound that a pressurec orrectiono f -5 to - I l0lo
ried out at Clermont-Ferrand except one 500 MPa ex- should be applied to the salt-glassa ssembly.I n order to
periment, which was performed in a small-volume, in- check this, the fayalite + anorthite : grossular * alman-
ternally heated pressure vessel in the Department of dine reaction was reversed in an internally heated gas
Chemistry at Arizona State University. The vesselu sed apparatus at 700 MPa (Perkins and Vielzeuf, 1992) and
there was held horizontally, and the pressure medium the results compared with those from the piston cylinder.
was Ar. The samplesw ereh eatedb y a single-zonew, ire- This study suggeststh at a pressurec orection ofbetween
wound resistancef urnace. Pressurew as measured by a -4 and -80/o should be applied to nominal pressures.
manganin resistancec ell, which was calibrated against a Another set of pressurec alibration experimentsw as con-
Heise gauge. Temperatures were monitored by type-K ducted by D. I-aporte at 500 and 1500 MPa, using the
thermocouplesa nd are accuratet o +2 oC. melting point of NaCl and the falling ball technique( Boh-
Experimentsc onductedi n the gasa pparatusa t 800 and len, 1984; Cemic et al., 1990).T hesee xperimentsi ndi-
1000M Pa werec arriedo ut in a 1500M Pa large-volume, cate that no pressurec orrection need be applied at 1500
internally heated vessel.T he vesselw as positioned hori- MPa, whereas a positive correction of -7o/o should be
zontally, with a pressurem edium of Nr, and the samples applied at 500 MPa. The apparent discrepancyw ith the
were heated by a single-zone,P t furnace. Pressurew as other pressurec alibration is not understood,a nd no pres-
measured by a manganin cell calibrated at the Interna- sure correction was apPlied.
tional Bureau des Poids et Mesures in Paris. A recent In piston-cylindere xperimentsa, cold pressureo f 150
comparison of the pressurem easuredb y this cell and a MPa was initially applied to the assembly.T he temper-
'C,
factory-calibrated Heise gauge( l-7000 bars) indicated a ature was then raised to approximately 510 at which
1210 VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING
point the pressurec easedt o increasea nd begant o de- give 28.686 + 0.034 wto/oS iO,. The HrO used was dis-
crease.A t that stage,t he desired final pressurec ould be tilled, deionizedH rO in all cases.
applied without shattering the cell. The starting mixture was made by grinding the pure,
Experimental procedures.F or experimentsa t P < 200 synthetic phlogopite and quartz together,i n a mechanical
MP4 thin-walledP t capsules(3 mm od x l0 mm x 0. I agate ball mill, under acetone,f or 20 min. The mixture
or 0.15 mm) were welded shut on one end and loaded contained phlogopite and quartz in equal mass propor-
with about 0.02 g of starting material. The capsulesw ere tions, so that all products were saturatedi n excessq vartz.
squashedf lat and lightly crimped, dried at 130 .C for at The mix was dried at ll0'C and stored in a vacuum
least 2 h, and then promptly sealedw ith a C arc welder. desiccatoro ver silica gel.
In the experiments at higher P, the same initial pro- Observation and analysis of products. The combined
cedurew as followed, but then the capsulesw ere squashed use of various observational and analytical techniques
flat and folded into packetsw ith approximate dimensions proved essential for the interpretation of the products.
of 3.5 x 5 x 0.6 mm. Thesep acketsl ay flaI in the pres- The contentso feach capsulew ere split into severalp or-
surec ells so that the samplesw ould experienceth e min- tions. Part was ground and used to make an optical grain
imum possiblet hermal gradients. mount. Microscopic examination of these grain mounts
All experiments were simultaneously brought to final was an important method of phase identification. X-ray
P-?"c onditions,a nd we usedt he hot piston-in technique powder diffractometry and SEM observationsw ere used
for the solid-medium experiments. Experimental condi- for (l) the detectiono f sanidine,s incel ight-opticali den-
tions were monitored and recorded severalt imes per day tification was difficult, (2) the detection of enstatite at low
over the courseo f an experiment. Reported temperatures pressure, and (3) gauging the extent of the reaction by
are the averageso f the minimum and maximum record- comparing the relative intensities of the diagnosticp hlog-
ed, and quoted uncertaintiesr epresentt he total rangeso f opite, quartz,a nd enstatitep eaks.
temperature variation during the experiments.A similar A CGR diffractometer was used with CuKa radiation
procedure was followed for the pressurem easurements. and calibrated betweene achs ample,u sing Au and quartz
To avoid vesiculationo f volatile-bearingg lassesa, ll ex- standards.
periments were quenchedi sobarically. Quench ratesw ere Some electron probe analysesw ere carried out on a
of the order of 100'C/s in the piston cylinders,3 'Cls in Cameca Camebax Micro fitted with a Link energy-dis-
the cold-sealv essels( cooledb y air jet) and I 'Cls in the persive system. For the analyses,t hree crystal spectrom-
internally heated vessels. etersw ere used,t he acceleratingp otential was l5 kV, and
After the experiment, capsulei ntegrity was checkedb y the probe current was about l0 nA with a 2-pm beam
microscopice xamination and, when possible,b y com- diameter. Specimensw ere C coated and counting times
paring post- wilh pre-experimentm asses. were l0 s per series of three elements. Standards were
Starting materials. Phlogopite was synthesized from natural minerals, and ZAF correction procedures were
KAlSi3O, gel plus MgO and excessH rO. A small aliquot applied.
of the gel-oxide mix was fused to a glass on an Ir strip The electron beam instrument at Manchester( usedf or
heater (Nicholls, 1974).E lectron probe analysiso f this examination of products, photomicrography, and some
glass showed that it has the correct stoichiometric com- analyses)is a JEOL JSM 6400, fitted with a Link EXL
position. Phlogopite synthesisc onditions were 200 MPa, energy-dispersivea nalytical system and an Oxford In-
800'C, and 122h . Optical and X-ray examinationo f the struments freezing stage.W e used ZAF4 software for the
product revealed no impurities. X-ray powder diffrac- data processing.P robe analyseso fquenched melts (glass-
tometry yieldedt he following cell constantsa: -- 5.317(6) es) were carried out using an acceleratingp otential of l5
A, b:9.215(7) A, c: 10.322(8A), p : 100.1,(4,a),n d kV and a probe current of 1.50n A. The samplet emper-
V : 498.0(5)A ., where the figures in parenthesesa re 2o ature was held at -193.5 "C and the counting time was
uncertainties.T he douov alue is 1.5364A , implying the 40 s. In order to obtain a good count rate with such a
absenceo fboth octahedrals ite vacancies( Robert, 1976) weak beam, a large-diameterB e window was fitted to the
and Fe contamination( Wones,1 963).W ithin uncertain- detector. These measurese ffectively eliminated any dif-
ties, our phlogopite is identical to that ofYoder and Eug- fusion-relatedc ountingl osseso n light elements( e.g.,M g
ster (1954) in its cell parameters.C omparedt o that of and K). Tests using a similar method on a sample of
Hewitt and Wones (1975), our phlogopitei s, again, al- NaAlSi.O, glassc ontaining 9 wto/oH rO yielded analyses
most identical; it has slightly larger c and B, but the cell that correspond closely to those obtained on the anhy-
volume is the same within error. No analysesw ere made drous equivalent,w hich wasp reparedb y l-atm fusion of
becauset he crystalsw ere much too small to probe. the same gel starting material. Figure 3 shows SEM pho-
All quartz used in our experiments was finely ground tomicrographs of some products.
(<10 pm), acid-washed,B razilian, optical-grader ock Identification and descriptiono f phasesi n the products.
crystal. In all experimental products, most phlogopite forms very
The tetraethyl orthosilicate (TEOS) used as a sourceo f fine-grainedm assesin which individual crystalsa re com-
SiO, in the sanidine gel was standardized,i n triplicate, to monly difficult to distinguish. However, some products
VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING t2lr
Fig. 3. SEM secondarye lectron photomicrographs of some experimental products. The scaleb ars represent I pm in all cases.
(a) Unreacted phlogopite + quartz from experiment K-44 (800 MPa, 850 "C, 312 h). (b) Matted enstatiten eedlesa nd glassf ragments
produced in experiment K-17 (200 MPa, 820'C, 280 h), in which the fluid-absentm elting reaction has gone almost to completion.
(c) Relict phlogopite + new enstatite and sanidine crystals in a matrix of quench glassf rom low-pressuree xperiment K-30 (150
MPa, 838 "C,241h). (d) Sanidinec rystala nd glassf ormed in high-pressureex perimentK -34 (1500M P4920 "C, 130 h).
contain a few crystals that attain a respectables ize (= l0 the subsolidus region. The products of these low-P ex-
pm). These larger grains are colorlessa nd have medium periments are, in fact, so fine-grained that we were able
relief that varies with crystal orientation, high birefrin- to identify the phasesp resento nly with X-ray diffractom-
gence,a nd straighte xtinction. The 001 reflection,a t about etry or SEM examination. Figure 3b showst he extremely
10.10A , was usedt o identify this phaseo n XRD traces. small and fine enstatite needlesf ormed in one of these
Quartz grains in supersolidusp roducts are commonly low-pressuree xperiments.T he 610 reflection,a t about
larger (=25 pcm)t han in the starting material. The larger 2.57 A, was used to positively identify this phaseu sing
grains have a rounded, subhedral,0 habit. XRD.
The forms of enstatite crystals vary dramatically. At Sanidine was not identified optically, but its presence
1000a nd 1500M Pa, enstatiteu suallyf orms stubbyp risms was detected through characteristic X-ray reflections
with length/width ratios averaging <3. All of these have (corresponding to 20 l, 13 0,2 02, 002, l 3 l, and 060 planes)
straight extinction, and none attains great size.O ne ofthe and its crystal form and composition, as seeni n the SEM
great surprises in our experiments was the form of the and microprobe. This potassium feldspar is always in the
enstatitei n experimentsa t low P (=300 MPa). Here, the high structural state, as confirmed by the X-ray methods
enstatite is optically indistinguishablef rom the very fine- of Wright (1968),a nd forms rhombiclooking prisms <5
grained, equigranular phlogopite aggregatesth at exist in pm acrossG eeF ig. 3c, 3d). The 040 reflection,a t 3.23 A,
t2t2 VIELZEUFAND CLEMENS: PHLOGOPITE + QUARTZ MELTING
Trau 1 Experimentarel sults
Ther-
Expt. Machine and mo- Dura-
no. assembly couple P (MPa) rfo) tion( h) Assemblage Remarks
K-64 EHPV s 100+ 0.5 806 + 1 240 Phl + Otz unconsolidated powder
K.65 EHPV S 100+ 0.5 814+ 1 351 (Phl +) Qtz + En (+ Sa) + Gl hard and brittle, strong
decreaseo f Phl and
Qtz, sanidine
present
K-32 EHPV s 151.3+ 0.7 800 + 2 263 Phl + Qtz soft and crumbly
K-33 EHPV s 151.3+ 0.7 810+ 1 263 (Phl +) Otz + En + Gl hard and brittle,
pseudomorphso f
En after Phl
K.28 EHPV S 150.2+ 1.1 819+ 2 265 (Phl +) Qtz + En + Gl
K-29 EHPV S 150.2+ 1.1 830 + 1 235 Otz+En+Gl hard and brittle
K-30 EHPV S 150.2+ 1.1 838 + 4 241 ((Phl))+ atz + En ( + Sa) + Gl hard and brittle,
sanidinep resent
K-12 EHPV s 200.2+ 0.9 800+1 263 Phl + Qtz
K-13 EHPV s 200.2+ 0.9 812+ 4 242 Phl + Otz
K.17 EHPV S 198.6+ 1.1 820+1 280 (Phl +) Otz + En + Gl hard
K.18 EHPV S 199.3+ 0.8 830+1 140 (Phl +) Qtz + En + Gl hard
K.35 EHPV s 299.7+ 1.2 811r 1 238 Phl + Qtz
K-36 EHPV s 299.7+ 1.2 82O+2 238 (Phl +) Qtz + En + Gl hard and brittle
K-62 EHPV s 300+5 820+1 352 (Phl +) Qtz + En + Gl
K-63 EHPV s 300+2 820+1 470 Phl + Qtz (+ En) + Gl
K-43 IHPV K 503.3 841+3 168 Phl + Qtz
K-44 IHPV B 800+ 7.5 850+7 312 Phl + Qtz unconsolidatedp owder
K-48 IHPV B 800+ 2.5 845+5 306 Phl + Qtz
K-47 IHPV B 800+ 2.5 865+5 306 Phl + Otz
K-46 IHPV B 800+ 2.5 875+5 306 Phl + Qtz + En (+ Gl) hard and brittle
K-51 |HPV B 800+5 863+2 358 ((Phl))+Otz+En+Gl
K-7 PC 7z in., all salt c 999+23 830 + 215 Phl + Qtz
K-15 PC 7z in., all satt c 1004+ 21 840 + 193 Phl + Qtz
K-25 PC % in., all satt c 1011+ 19 850 + 208 Phl + Qtz
K-6 PC 72 in., alt satt C 1005+ 14 860 + 156 Qtz+En+Sa+Gl sanidinep resent
K-4 PC y2 in., all salt C 1008+ 17 880 + 113 Qtz+En+Gl
K-50 PC 72 in., att satt C 996+23 880 + 18 Phl + Qtz short duration equiva-
lent of K-4
K-38 PC 7z in., all satt s 1012+ 13 860 + 168 Phl + Qtz
K39 PC Y2i n., all salt S 1010+ 14 880+ 't76 (Phl +) Qtz + En + Gl?
K-40 PC 7z in., all salt J 993+6 900+ 32 Qtz+En+Gl
K-41 PC Y2 in., TP 987 + 16 890 + 166 Otz+En+Gl
K-49 PC Vzi n., TP 1002+ 16 880 + 168 Phl + Otz (+ En) + Gl
K-52 PC 1/zi n.,TP 998 + 23 870 + 2't8 (Phl?+ ) Qtz + En + Gl
K-53 IHPV B 1002+ 2 872+ 4 284 Phl + Otz (+ En) + Gl
K-54 IHPV B 1002+ 2 888 + 2U Phl + otz (+ En)( + Gt?)
K-55 PC % in., SP c 995+6 870+ 168 Phl + Otz (+ En) + Gl
K-56 PC 7ni n., SP 1001+9 890+ 156 Phl + Qtz (+ En) + Sa? + Gl probable sanidine
detected by xRD
K-57 PC % in., SP 998+7 900+ 168 (Phl +) Otz * En * Gr
K-58 PC 7+i n., SP c 1003+7 910+ 214 ((Phl))+ Otz + En + Sa + Gl sanidinep resent
K-59 PC 7c in., SP c 1002+9 920+ 183 ((Phl))+ Qtz + En + Gl
K-60 PC 7+i n., SP 999+7 9t0+ 168 (Phl)) + Otz + En + Sa + Gl pseudoreversal,la rge
crystals of En
875 + 167 sanidinep resent
K-61 PC 7+i n., SP 995+7 910+ 184 Phl + Otz (+ En) + Gl pseudoreversal,
850 + 159 regrowth of Phl
K-26 PC Y2i n., all salt c 1506 + 26 870 + 192 Phl + Otz
K-27 PC 7z in., all satt c 1502+ 37 890 + 160 Phl + Qtz
K-31 PC /z in., atts att c 1501+ 25 910 + 165 Phl + Qtz
K-34 PC Y2i n., all satt c 1491 + 36 920 + 130 (Phl +) Qtz + En + Sa + cl sanidinep resent
(. )/V : omtei.n'EoHr qPuVa: nteitxyt eorfn pahllays hee,a ((te )d) p: retrsascuer.e vessel, IHPV : internallyh eated pressurev essel, PC : piston cylinder,T p: talc-pyrex, Sp : salt-pyrex;
was used to positively identify this phase on the diflrac- identifiable by optical means. Its presencew as inferred
tograms. from the brittleness and hardnesso f the products when
In experiments at P > 500 MPa, colorless, isotropic they wereb eing crushedi n an agatem ortar. In somel ow-P
glass formed easily recognizable patches, interstitial to experimentsw ith small extents of reaction, glassw as not
enstatitec rystals.A t lower pressuresg, lassw as not readily identified at all; its presencew as infened from the oc-
VIELZEUFAND CLEMENS: PHLOGOPITE + QUARTZ MELTING t2t3
currence of traces of more easily identifiable enstatite Tmu 2. Summaroyf bracketsfo rt heP hl+ Otz : En + Sa + M
needles.T his difficulty results from the sluggishnesosf between1 00a nd1 500M Pa
phlogopite breakdown at P < 500 MPa and f < 800 "C. flMpa) r(rc) Exptn. o.
Results 100 805-81 5 K-64, K-65
150 798-81 1 K-32, K-33
About 50 experiments were performed on the fluid- 200 808-821 K-13,K -17
absentm elting of phlogopite + qvaftz at pressureso f 100, 300 810 -821 K-35, K-63
500 >838 K-43
150,2 00, 300, 500, 800, 1000,a nd 1500M Pa. In order 800 860-865 K-47, K-51
to understand the behavior of the system, various tech- 10 00 849-876 K-60,K -61-
15 00 909-921 K-31,K -34
niques, sample arrangements,t hermocouples,a nd dura-
tions were used.T his was done to demonstratec onsis- ' Pseudoreversael xperiments'
tency in the results, despite changesi n methodology that
involve different error margins on variabless uch as P and
?".T hese experiments are reported in Tables I and 2. from the products in the other. There is no requirement
Experimental brackets.S ix serieso f experiments,u sing for complete conversion in either direction. If we dem-
variable configurations, were performed at 1000 MPa. onstrate that we grew En + Sa + M from Phl + Qtz and
The first seriesw as made in a 1.27- cm piston cylinder that we see an increasei n Phl * Qtz plus a decreasei n
with type-C thermocouples,u sing all-salt assemblies.N o En * Sa * M when such products are held at alowet T,
enstatiteg rew at ?" < 850 "C. The bracketw e obtainedi s we have demonstrated reversibility. This will never be
849-861 "C (experimentsK -25, K-6). Note that, at 860 proof of equilibrium, however. The "pseudo" character
'C, sanidine was detectedb y X-ray and confirmed under ofour reversal arisesf rom the fact that we did not actu-
the electron microprobe. In the same experiment, both ally open the capsulea nd prove that the particular prod-
the sanidine and the glass had significant Na contents, uct had En * Sa * M in it before we lowered the Z,
though no Cl was detected.T his indicatesp ossiblec on- though it is very likely, consideringt he number of exper-
tamination by Na diffusion through the Pt capsules.A iments we performed at high Z and the reproducibility
short-duratione xperiment( K-50, l8 h) at 880 "C showed of the results. Pseudoreversails our term for these two-
no growth of enstatite, confirming sluggish reaction ki- stage experiments, and we consider that they represent
netics, in agreementw ith the observations of Montana the best way to change only one parameter without af-
and Brearley( 1989)a nd Petersona nd Newton (1989).A fecting the others.
second seriesw as performed with type-S thermocouples In our pseudoreversals,t he temperature was brought
and similar assemblies.T his provided a bracket between up to 910 "C for 7 or 8 d and then dropped to the desired
859 and 879 "C (experimentsK -38, K-39). The third se- value. At 875 "C, there was no optical evidenceo f phlog-
ries used 1.27- cm talc-Pyrex-magnesiaas sembliesw ith opite regrowth, though some rare crystals were detected
type-C thermocouplesT. his seriesi ndicatest hat the re- under the SEM. Microprobe work also indicated the pres-
action beginsa t T < 87 | "C. Additional experimentsw ere enceo f sanidine.I n the experiment where I was dropped
performed in an internally heatedp ressurev essel.A t 888 to 850'C, extensivep hlogopite regrowth occurred;e nsta-
"C, reaction is extensive.R are crystalso f enstatitew ere tite was detected neither optically nor by XRD, though
observeda t T : 872 + 4'C, indicatingp roximity to the it was found under the SEM. Unless there are kinetic
solidus.T he fifth seriesw as performedi n a l.9l-cm pis- problems preventing the nucleation of the phlogopite, the
ton cylinderu singN aCl-glassa ssemblieas nd type-Ct her- reaction must be located between 849 and 876 'C (ex-
mocouples. The study of the products indicated clearly perimentsK -60, K-61). In summary, at 1000 MPa, the
that X-ray examination is insufficiently sensitive to de- bracketf or the reactioni s 849-876 "C and possibly8 59-
termine the beginning of melting. The characteristicp eak 861 'C (if we are optimistic). The full set of brackets
of enstatite,a t 31" 20, did not appear at T < 900 'C, between1 00 and 1500M Pa is given in Table 2.
though enstatite was optically identified at lower 7n Pe- Sanidine was detected in various products at different
terson and Newton (1989) also observedt hat the 610 pressures( experimentK -34 at 1500 MPa; experiments
peak of enstatite was suppresseda nd that the 420 peak K-6, K-58, K-60 at 1000M Pa; experimenKt -30 at 150
at 28.2 20 proved more reliable (J. W. Peterson,w ritten MPa; and experimentK -65 at 100 MPa), clearlyi ndicat-
communicatiorl., 1992). These experiments indicate that ing that the melting reaction is Phl + Qtz : En + Sa +
the fluid-absent melting of phlogopite + quartz begins at M. Consequencesfo r the topology of phaser elationships
T < 871"C. will be discussedl ater.
In order to constrain the location of the reaction more All thesee xperimental bracketsa re shown in Figure 2.
accurately,w e performed pseudoreversalssi milar to those They imply a slightly curved reaction boundary for the
describedb y Petersona nd Newton (1989).A definition fluid-absent breakdown of phlogopite + quartz. Taking
of a reversal is that the equilibrium can be approached into account all additional uncertainties and the various
from both sides.T his meanst hat the product phasesg row experimentalt echniquesa nd proceduresu sedi n this study,
from the reactantsi n one direction, and the reactantsg row we find that the entire length of the reaction approximates
t2t4 VIELZEUF AND CLEMENS:PHLOGOPITE + QUARTZ MELTING
TABLE3. Compositionso f phasesa nalyzedi n productso f 1000
and 1500 MPa exoeriments
ExperimentK -61 (1000 MPa, 91G850 C)
Phl( n: 3)' Pure Phl
Phl /=n sio, 44.77(1.14) 43.19
Otz Sa Al,o3 12.10(0.41) 12.22
Mgo 27.24(0.141 28.98
M KrO 11 . 57(0.14 ) 11.29
HrO 4.32 4.32
ExperimenKt -60( 1000M Pa,9 10-875e C)
Starting
Pure Pure Glass mate-
d
En En Sa Sa (n: 15) rial-'
o
sio, 60.43 59.85 64.89 U.76 79.38(0.71)71.60
Alro3 0.09 0 18.00 18.32 11.55(0.,14)6.11
P2 prrre tz= En r,t Mgo 39.4840.1s000.81(1.02) 14.49
GO 0 0 17. 11 16.92 8.26(0.26) 5.64
H.o 00000 2.16
ExperimenKt -56( 1001M Pa,8 90'rC)
En( n:7) Sa (r:3) Glass( n: 14)
=n. " oE Asliroo,3 602..2O12((0O.3.238)) 6165..'2t46((10.2.57)1) 797..3621((00..74611)
En sa Mgo 37.46(0.41) 0.73(0.46) 0.74(0.81)
KrO 0.31(0.20) 17.87(0.441 12.e{0.46)
HrO 0 0 0
ExperimentK -34 (1491 MPa,9 20 €)
En( n: 3) Sa (n:4) Glass( n: 13)
sio, 61.33(0.27) 64.92(0.91) 77.33(0.701
Alros 0.0q0.09) 16.77(0.61) 10.8s(0.38)
Mgo 38.60(0.18) 0.50(0.36) 0.30(0.24)
K,O 0 17.82(O.731 11.52(0.34)
T(oC) HrO 0 0 0
Fig. 4. Enlargemenot f the relationsi n the vicinity of the Note: The compositionso f pure Phl, En, Sa, and the starting material
invariantp oint.N ote the highlym agnifiedte mperatursec aleA. are given for comparison. All compositions are normalizedt o 100% an-
hydrous except those of Phl and the starting material. The abbreviation
decreasien temperaturoef the invariantp oint (I) oflesst han 10
'C n: number of individualp robe analyses. Figures in parentheses are 10
would be sufficientto rendert he slopeo f the (Fl) reaction values. ExperimentsK -60, K-61 analyzedo n the Clermont-Ferrandp robe;
positivea ll the wayt hrough. experiments K€4, K-56 analyzed on the Manchester SEM/probe with
freezings tage at -193'C.
a straightl ine [P(MPa) : 14.06f fq - I1305], provid- -''NNoorrmmaalliizzeeddtt oo 110000%% wwiitthh 42..3126%0/ HH" ,,OO..
ed we accepta n error not greatert han + l0'C. However,
there is no reason why this reaction should be linear in
the P-Zplane, nor is it necessarilya simple curve. Indeed,
the changef rom a positive to a negative slope, at about persist metastably to rather high temperatures.W e thus
150 MPa (Fig. 4), seemst o be rather well constraineda nd believe that the appearanceo f enstatitei s the best marker
consistent with our preferred location of the invariant of the initiation of the reaction. We attribute the coexis-
point at 815 "C and 65 MPa, basedo n the availabled ata tence of phlogopite + qvafiz + melt + enstatite + san-
on the solidus and subsoliduse quilibria. However, con- idine to kinetic problems.
sidering the errors involved in all the experimental data Phasec ompositions.C ompositions of phases,a s deter-
used to locate the invariant point, the slope ofthe fluid- mined by electron probe, are shown in Table 3. Phlogo-
absentr eaction could remain positive all the way through, pite was analyzed in the 1000-MPa pseudoreversale x-
providing that we select the low-Z extreme for the loca- periment K-61, and enstatite, sanidine, and glass in the
tion ofthe invariant point. A decreaseo fabout l0 "C of 1000-MPa pseudoreversael xperiment K-60, the 1000-
the temperatureo f this invariant point would be sufrcient MPa experiment K-56, and the 1500 MPa experiment
(Fig. a). K-34. All of the analyzed crystalline phasesa re close in
It could be arguedt hat the disappearanceo fphlogopite composition to the pure theoreticalp hases,in dicating that
should be considereda s indicative ofthe location ofthe the fluid-absent reaction is a truly univariant reaction.
fluid-absent melting reaction. The presenceo f some en- Products of K-60 and K-61 were analyzed at Clermont-
statite, at lower ?", would then be viewed as a result of Ferrand using standard techniques.T he glassa nalysisi n
the presenceo f moisture, which is impossible to remove K-60 is corundum-normative( C : 2.610/o)i,n dicating
from the starting material, and thus the result of the in- some counting losseso n KrO. The products of K-34 and
cipient reaction Phl + Qtz + HrO : En * M. However, K-56 were analyzed at Manchester using the sample-
our experiments, together with the data from Montana freezing technique described above. The glass analyses,
and Brearley (1989), clearly show that phlogopite may in these cases,a re not peraluminous, demonstrating es-
VIELZEUF AND CLEMENS: PHLOGOPITE + QUARTZ MELTING t2t5
sentiallyn o loss of K,O. The solubility of MgO in these peared in some of their products at temperatures well
melts, in equilibrium with enstatite,s anidine,a nd quartz, below their inferred position for reaction (Fl). Our ex-
is less than I wto/0,a veraging 0.630/oin the three experi- periments indicate that durations in excesso f 100 h are
mentsa nalyzed. required for significant reaction at 1000 MPa. The Boh-
We performed mass-balancec alculations,b asedo n the len et al. experimentsw ere only around 24 h long. In
fact that the bulk starting composition must equal a linear agreemenwt ith Montana and Brearley( 1989)a nd Peter-
combination of the compositions of the products. For son and Newton (1989),w e suggestth at the sluggishness
these, we used the compositions of the glass formed in ofthe reactione xplainsw hy Bohlen et al. had to greatly
the experimenta t 1000 MPa, 890 'C. The mixing pro- overshoot the equilibrium Z, in order to get detectable
gram includes an error-propagation routine (Provost, reaction in very short durations. Confirmation of this is
1989).E nstatite,s anidine,a nd quartz werec onsidereda s seen in comparing the results of our experiments K-49
oC,
pure phases,w ith 0oloe rror (all errors quoted in this para- and K-50. Thesew ereb oth at 1000M Pa and 880 but
graph are given in relative percent). A +2o/oe rror was K-50 (showingn o reaction)l astedf or only l8 h, whereas
assumedf or all the oxides in the calculated composition K-49 (with a definite reaction) lasted for 168 h. This lack
of the starting material. The greatestu ncertainty concerns of equilibrium would explain the incomplete decompo-
the composition of the glass (quenched melt), and the sition of the phlogopite, the reported detection of trace
following compositionw as used:S iO, : 73.46, AlrOr: glass at much lower temperatures, and the inability of
9.13,M gO : 0.70,K ,O : I1.71,H ,O : 5.00.T he H'O Bohlen et al. to obtain a reversal on (Fl). Attempts at
content was calculated from a solubility model (seeb e- reversal probably failed becausea ll of the experiments
low), and an appropriately largeu ncertainty of + 300/ow as would have been carried out at temperaturesi n excesso f
attributed to its value. A +2o/oe rror was assumedf or all the true equilibrium value. Note that our results at 1500
the remaining oxides. The following results were ob- MPa agree with the data of Bohlen et al. This suggests
tained: quartz : 21.2 -r 5.8 wt0/0e, nstatite : 35.8 + 2.7, that the reaction kinetics improve at higher pressure.
sanidine: 19.4+ 7.5,m elt :23.7 + 12.4.O n the sup- Montana and Brearley( 1989)l ocatet he reactioni n the
position that sanidine might disappears oon after the be- interval 835-850 oC, about 30'C below our preferred
ginning of melting, a similar calculation was performed location at 1000 MPa (but only 14 "C below the lower
without this phase, yielding etrar1:z: 8.3 + 6.2 wto/o, value of our bracket). Examination of their table of ex-
enstatite: 35.0 + 3.4, melt : 56.6 + 5.4. From these perimental results indicates that there are two possible
numbers, we can balance the following two reactions: bracketsf or this reaction( 835-850 or 860-890'C). Se-
63.49P hl + 36.6eQtz: 45.4gBn + 24.69S a + 30.09 lecting the bracket at 835-850 "C, as they did, is incon-
M and 54.59P hl + 45.59Q tz : 38.2eE n + 61.8gM . It sistent with two of their own experiments (nos. I 18 and
is clear that the quality ofthese results dependsc ritically 126),w hich showedn o enstatiteg rowth at T > 850'C.
on the composition of the melt, which is difficult to ob- Selectingt he secondb racket( 860-890'C) would be in-
tain. In particular, the amount of melt is directly related consistentw ith only one of their experiments( no. 216),
to its HrO content, which has not been determined di- which produced enstatite at T : 850 "C. Montana and
rectly. However, on the basis of our secondm ass-balance Brearley ascribedt he absenceo fenstatite in experiments
equation, we agree with Peterson and Newton's (1989) I 18 and 126 to insufficientd uration (-200 h compared
statementt hat, to a first approximation, the volume per- to 600 h for no.216) and disregardedt he results.A c-
centageo f melt in the products is close to that of phlog- cording to our study, such a duration is sufficient to ini-
opite entering the reaction. We draw the reader's atten- tiate melting:f or instance,1 40 h at 830'C and 200 MPa
tion to the fact that this conclusion cannot be simply (experiment K-18) were sufrcient to promote extensive
extendedt o most natural rocks, in particular becauset he melting. Thus, we prefer the higher temperature bracket
effecto fplagioclasei s not considered. of Montana and Brearley becausei t agreesw ith the re-
growth ofphlogopite at 850 "C in both our and Peterson
and Newton's (1989)t wo-stagee xperimentsT. aking this
Drscussrox
into consideration, there would be no discrepancyw ith
Critical comparisonw ith previous work our results at 1000 MPa. Note that, at 2000 MPa, their
At P > 250 MPa, there are major differencesi n the bracket is consistentw ith the Bohlen et al. result.
location of the fluid-absent melting reaction (Fl), accord- In order to explain the higher temperatures reported
ing to various workers (see Fig. 2). Below, we consider by Bohlen etal. atP = 1000M Pa, Montana and Brearley
interpretationso f the resultso f previous experiments,a nd suggestt hat Bohlen et al. traced the metastablee xtension
how the various discrepanciesm ay be explained. of the thermal barrier. However, this doesn ot explain the
The data ofBohlen et al. (1983)p lace( Fl) between8 87 reported systematicp resenceo f sanidine in the products
and 903'C at 1000M Pa, which is about 25,50, and 70 of Bohlen et al., the reaction correspondingt o the thermal
oCa bove the preferredl ocation for the reaction according barrier being both sanidine- and fluid-absent'
to us, Montana and Brearley( 1989), and Petersona nd The data and interpretations of Petersona nd Newton
Newton (l 989), respectively. (1989) apparently disagreew ith all other studies,a nd this
Bohlen et al. noted that small amounts of glass ap- problem becomesm ore pronounced at higher P. At 1500