To derive thermodynamic data, we evaluated the above experiments and those reported by ... series was modelled using an asymmetric Margules formalism with the following ..... The lower T and P limit is due to reactions R2 and R3 of Table 2.
LITHOS 0 ELSEVIER
Lithos 4 1 ( 1997) 229-250
Thermodynamic properties of white micas on the basis of high-pressure experiments in the systems K,0-MgO-A1,0,-SiO,-Hz0 and K,O-FeO-A1,0,-SiO,-H,0. Hans-Joachim Research
Group
L‘High-pressure
Metamorphism”
Massonne
*, Zbigniew Szpurka
at Ruhr-Uniuersitiit
Bochum,
Institutjiir Mineralogie,
D-44780
Bochum,
Germany
Abstract Experiments in the piston-cylinder apparatus were undertaken to synthesize the assemblage of phengite, garnet, kyanite and quartz or coesite. In the system K,O-MgO-Al,O,-SiO,-H,O (KMASH), compositional data of phengite were obtained for 13 experimental runs in the range of 30 to 45 kbar and 850°C to 1100°C. The same was achieved for 23 runs in the system K,O-FeO-Al,O,-SiO,-H,O (KFASH) between 600°C and 9OO”C, and 15 to 55 kbar. On the basis of the latter runs and 6 additional synthesis experiments, the dependence of the lattice dimensions on the composition of 2M, phengites in KFASH was determined. To derive thermodynamic data, we evaluated the above experiments and those reported by Massonne and Schreyer (1987, 1989) on KMASH phengites coexisting with other assemblages. Considering simple activity and Fe2+- Al-celadonite theoretical endmembers: HP = model like aMs = xMs y, we obtained for the Mg-Al-celadonite -5832415 k 3040 and -5492287 + 6267 J/mol, So = 288.527 f 4.008 and 303.148 + 6.827 J/(K mol), V” = 13.870 f 0.027 and 13.962 ? 0.067 J/(bar mol). The nonideal behavior of the muscovite(MS)-Mg-Al-celadomite series was modelled using an asymmetric Margules formalism with the following parameters: W,; = W2y = 0, W,; = 58.598 + 8.804, Wi = 15.920 + 3.393, W,; = 0.735 k 0.118, W2‘;= 0.187 + 0.026 with units as above. For the muscovite-Fe2+-Al-celadonite series we used a symmetric model with: W H = 0 , Ws = 24.083 + 3.088 J/(K . mol) and WV = 0.3394 + 0.1120 J/(bar . mol). The above data were applied to construct a petrogenetic grid. The example chosen is related to the KFASH system with potassic white mica, quartz, and H,O in excess. It covers the T range from 150°C to 550°C at a P up to 28 kbar. The new thermodynamic data are also discussed in regard to their use in geothermobarometry. 0 1997 Elsevier Science B.V.
1. Introduction Potassic white micas are widespread in metamorphic rocks. They occur mainly in metagranites and
’ Corresponding author. Present address: Institut ftir Mineralogie und Kristallchemie,
Universitkit
Stuttgart,
D-70174 Stuttgart, Germany. 0024-4937/97/$17.00
0
PII SOO24-4937(97)00023-6
1997 Elsevier
Azenbergstr.
18,
metasediments at very low to medium grades but also in metabasites particularly at high pressures. The Si content of potassic white mica is variable according to the Tschermak’s substitution (Mg, Fe’+) + Si = Alr4] + Alrsl. The solid solution range is wide, extending
from nearly
ideal muscovite
to the
theoretical Al-celadonite endmember, K(Mg, Fe2+)A1[Si,0,,](OH), (e.g. Massonne and Schreyer,
Science B.V. All rights reserved.
H.-J. Massonne, Z. Slpurka / Lithos 41 (1997) 229-250
230
1986). Phengites are intermediate members of this series. According to recent experimental P-T calibrations of the Si contents of phengite in various mineral assemblages (Massonne and Schreyer, 1987, 1989), this mica shows a high potential for geobarometry. Geobarometric approaches must preferably be based on thermodynamic calculation of concerned mineral equilibria and, therefore, require good thermodynamic data for the relevant mica endmembers and solution properties. In this paper we present experimental data in the model systems K,O-MgOAl,O,-SiO,-H,O (KMASH) and K,O-FeOAl,O,-SiO,-H,O (KFASH) on the compositional variation of phengite coexisting with garnet, kyanite and quartz/coesite as a function of P and T. These results, together with earlier experiments at high water pressures reported by Massonne and Schreyer (1987, 1989) on phengites coexisting with K-feldspar, phlogopite and quartz as well as with talc, kyanite and quartz/coesite in KMASH, were used to derive thermodynamic properties for the solid solution series muscovite-Mg-Al-celadonite and muscovite-Fe2+-Al-celadonite.
Table 1 Compositions
of solid phases and their components
Table 1 gives the structural formulae of the phases relevant to this study. Table 2 lists the equations of the various equilibria discussed later in the text.
2. Experimental 2.1. Pressure
procedures
equipment
All experimental runs were performed in a piston-cylinder apparatus similar to the Boyd and England (1960) design. The WC pressure vessels, which had a chamber of 12.7 mm (l/2 inch) diameter and 52 mm length, were loaded with NaCl pressure cells. Two different cell types were used (Massonne and Schreyer, 1986). Type I has a steel furnace with an inner diameter of 8 mm. Thus it is large enough to contain either sealed gold capsules of 2 mm diameter and up to 10 mm grouped around a thermocouple junction, or one large gold capsule of 6 mm diameter and a length up to 13 mm. For the experiments in KFASH, the latter container served as an outer capsule containing the Ni/NiO buffer + H,O and one or two inner capsules of platinum 2 mm in diameter
relevant to this study
Phase
Component
Abbreviation
Composition
Biotite (Bt)
annite phlogopite clinochlore daphnite Fe-chloritoid coesite diaspore almandine
Ann Phl
~e,[AlSi,O,,l(OH), Kk&,[AfSi,O,,l(OH), Mg,Al,Si,O,,(OH)s Fe,AI,Si,O,,(OH), FeAl,SiO,(OH), SiO, AlO Fe,Al,Si,O,,
pyrope grunerite kaolinite K-feldspar kyanite muscovite Mg-Al-celadonite Fe-Al-celadonite enstatite pyrophyllite quartz Fe-stilpnomelane talc Al-talc
PY Gru Kao Kf
Chlorite (Chl) Chloritoid Coesite Diaspore Garnet (Gt) Gnmerite Kaolinite K-feldspar Kyanite Muscovite or Phengite (WM) Orthopyroxene Pyrophyllite QUartZ Stilpnomelane Talc (Tc)
Dap Ctd cs Dsp Alm
KY MS MAC FAC En Pph
Qz Stilp Tc Al-Tc
MgsA1,SisOt, Pe,SisO,,(OH), Al,Si,O,(OH), KAlSi,O, Al,SiO, KAf,[AfSi,O,,I(OH),
KMgAl[Si,O,ol(OH), KFeAl[Si,O,,](OH), MgSiO, AI,Si,O,,(OH), SiO, K,Fe,,AI 5Si 670 168(OH),,
Mg,lSi,O,,l(OH), Mg,Al[AlSi,O,al(OH),
.36H,O
H.-J. Massonne, Z. Szpurka/Lithos Table 2 Reactions relevant to this study. Reactions of the ASH system are listed under KFASH, because reactions R15 to R39 are related to the grid of Fig. 8. The many reactions with Fe*+-Al-celadonite, considered to determine the phengite compositions along reaction curves of Fig. 8, are not listed. For abbreviations see Table 1 KMASH Rl R2 R3 R4 R.5 R6 R7 R8 R9 RlO
Tc = 3En+Cs+H,O Tc+Ky = Py+2Cs+H,O Ms+Py =Phl+2Ky+Qz 3MAC = 3Qz+2Kf+Phl+2HZ0 3MAC + 3Ky + H,O = 3Ms + Tc + 2Qz 3MAC+3Ky+H,O = 3Ms+Tc+2Cs 3MAC + 4Ky = 3Ms + Py + 4Qz 3MAC + 4Ky = 3Ms + Py + 4Cs 2Tc + 3Ky + H,O = 3Al-Tc + 2Qz 2Tc + 3Ky + H,O = 3Al-Tc + 2Cs
KFASH Rll R12 R13 R14 RI5 R16 R17 R18 R19 R20 R21 R22 R23 R24 R25 R26 R27 R28 R29 R30 R31 R32 R33 R34 R35 R36 R37 R38 R39
3FAC + 4Ky = 3Ms + Alm + 4Qz 3FAC + 4Ky = 3Ms + Alm + 4Cs 3Ctd + 2Qz = 2Ky + Alm + 3H20 7Dap + 16Qz = 2Gru + 7Alm + 26H,O 4Qz + 2Dsp = Pph Qz+2Dsp= Ky+H,O Kao = 2Qz + 2Dsp + H,O 2Kao = 2Dsp + Pph + 2H,O Kao+2Qz = Pph+H,O 6Dsp+Pph=4Ky+4HZ0 Pph = 3Qz+Ky+H,O 2Qz + 8Dsp + Dap = 5Ctd + 3H,O 4Kao + Dap = 6Qz + 5Ctd + 7H,O 4Pph+Dap = 14Qz+SCtd+3H,O 2Qz + Dap + Ctd = 2Alm + 5H,O 106Dsp + Stilp = 442 + 48Ctd + 5Ms + 60H,O 146Dsp+SStilp = 116Qz+48Dap+25Ms+ 156H,O 73Kao + 5Stilp = 262Qz + 48Dap + 25Ms + 229H,O 73Ctd+4Stilp = 122Qz+53Dap+20Ms +81H,O 63Ctd + 2Stilp = 8Cs + 53Alm+ 1OMs + 173H,O 134Qz+ 63Dap + 1OMs = 73Alm+ 2Stilp+ 142H,O 48Ms+SStilp = 116Qz+73Kf+48Dap+ 156H,O 11Kf+Stilp=52Qz+16Ann+44H20 33Ms + 8Stilp = 317Qz + 33Dap + 73Ann + 308H,O 11Ms+2Stilp=71Qz+11Alm+21Ann+110H20 3Dap + 8Kf = 9Qz + 3Ms + 5Ann + 4H,O 3Dap+Ms+3Qz=Ann+4Alm+12HZ0 Dap+Ms=Qz+2Ctd+Ann+2Hz0 342 + 3Ctd + Ann = 2Alm + MS + 3H,O
and between 3 and 4 mm long. One larger inner capsule of platinum was used for synthesis experiments in FASH. Cell type II with a graphite furnace having an inner diameter of 6 mm is able to accommodate only one gold capsule of 2 mm diameter situated laterally beside the thermocouple.
41 (1997) 229-250
231
The temperature was measured with mantled chromel-alumel thermocouples. T corrections were made following Massonne and Schreyer (19861, including an emf correction for P according to Getting and Kennedy (1970). However, recent calibration measurements have shown that, for the steel furnace assemblage with two gold containers, the sample temperature is about 1% lower than the temperature of the thermocouple junction. The 2~ temperature uncertainty is estimated to be k 2% for the buffered experiments and &-1% for other experiments. The hydraulic pressure read from the gauge was converted into sample pressure based on a forceper-area calibration related to the piston area at 1 bar. Small pressure corrections were carried out according to Massonne and Schreyer (1986). A 2a uncertainty of f 1% is assumed. The overall temperature and pressure error is probably twice as large as the mentioned experimental uncertainty. 2.2. Characterization
of run products
Most of the run products were inspected with the polarizing microscope and a scanning electron microscope. In general, the products were characterized with an X-ray powder diffrattometer using copper radiation (CuK LY,= 1S4050 A). For the determination of lattice constants, pure Si (a, = 5.43088 A, U.S. Department of Commerce, National Bureau of Standards reference material 640) or phases already present in the run products were used as internal standards. For the latter the following unit cell pa,rameters were considered: pyrope a, = 11.4255 A (Massonne, 1997); almandine a, = 11.5350 A (this work); kJanite a,, = 7.1217 A, b, = 7.8476 A,, c,, = 5.5730 A, (Y= 89.945’, ,0 = 101.192”, y = iO6.006” (Massonne, 1989a); quartz a, = 4.9133 A, c,, = 5.4053 A. Run products obtained in KFASH and some of those in KMASH were embedded in araldite and polished for investigation with an electron microprobe (Camebax of CAMECA) to determine the composition of the synthetic phases. As standards, a synthetic K-bearing glass, a glass of andradite composition, and relatively large crystals of pure synthetic pyrope were used. Conditions for the measurements with the wavelength dispersive system were 15 kV accelerating potential, 14 nA beam current,
232
H.-J. Massonne, Z. Szpurka / Lithos 41 (1997) 229-250
well focussed beam, and 20 s counting time per element. The PaP correction was applied. Structural formulae were calculated on the following basis. For garnet, the sum of the six and eight-fold coordinated cations was set to five. The amount of trivalent iron was calculated by subtracting the Al content per formula unit (p.f.u.) from two. For phengite, a cation charge of 21 valencies was taken neglecting K. The OH content was assumed to be 2 p.f.u. 2.3. Starting materials 2.3.1. KMASH Pure pyrope with traces of enstatite and corundum was crystallized from a gel at 30 kbar and 990°C (V580; Massonne, 1997). To form a mixture of pyrope, kyanite and coesite, a glass of composition MgO . Al,O, .5SiO, already used by Massonne (1989a) was crystallized at 50 kbar and 1160°C or at 37.5 kbar and 1000°C (V672 and V654; see also Massonne, 1997) in the presence of water. 2.3.2. KFASH Gels with the compositions K,O .3Al,O, .6SiO, and K,O . Al,O, . 8Si0, were prepared according to the method of Luth and Ingamells (1965). Chemicals used were powdered aluminum (99.99% purity, Aldrich), K,CO, (> 99%, heating loss about I%, Merck) and Si(C,H5)4 (99.99%, Fluka). These gels were mixed together with metallic iron powder (> 99.5%, Fluka) and Fe,O, (99.8%, Baker) by grinding in an agate mortar in order to finally synthesize pure KFASH micas (runs SO6-SO9). Iron powder, Fe,O,, y-Al,O, prepared from aluminium band (99.99%, Merck), and silica glass (> 99.99%, Heraeus) were mixed and crystallized in the presence of H,O by a buffered experiment at 800°C to form almandine, kyanite, and quartz at 20 kbar. An experiment at 30 kbar was performed to produce the same paragenesis with coesite instead of quartz. Electron microprobe analyses showed a virtually ideal composition of almandine. The lattice dimensions of this almandine were determined to be a, = 11.5349 + 0.0005 A. However, this value is identical to that of almandine with 5% skiagite component synthesized in the presence of the hematite-magnetite buffer by Harlov and Newton (1992).
2.4. Synthesis experiments for phengites kyanite + SiO,
+ garnet +
2.4.1. KMASH Various gels used in previous experiments (see Massonne and Schreyer, 1989) were mixed together with SiO, gel (> 99.999% purity, K and K) and a few percent of synthetic pyrope. Only for runs V914 and V915, a precrystallized mixture of pyrope, kyanite and coesite was used for seeding instead of pure pyrope. These mixtures showed compositions close to that for run V925 (= 63 wt% SiO,, 22 wt% Al,O,, 8.4 wt% MgO and 6.4 wt% K,O). In order to drive off any adsorbed CO, and H,O, the starting materials were heated inside the open gold capsules to 700°C before adding some water. Afterwards, the capsules were immediately sealed by arc welding. The runs were conducted in the pressure cell of type I and type II. The compositions of phengites in the run products were determined from X-ray powder diffractograms following the method described by Massonne and Schreyer (1987). According to these authors, the 2 (T uncertainties of the Si content and octahedral occupancy per formula unit (p.f.u.) are kO.04 and kO.015, respectively. In a few cases, several phengite grains of one run product each were analyzed with the electron microprobe. Because of the minuscule grain size, we neglected mica analyses that showed either clearly too low potassium contents, e.g. below 9 wt%, or oxide sums without H,O less than 93 wt% or any significant deviation from the average mica composition obtained. 2.4.2. KFASH Gel K,O . 3Al,O, .6SiO, was also mixed with slightly variable amounts of iron powder, Fe,O, , and silica glass. These mixtures showed an average composition of 9.4 wt% K,O, 30.6 wt% Al,O,, 45.6 wt% SiO, and 14.4 wt% Fe0 before seeding with a few percent of the almandine-kyanitequartz/coesite mixture synthesized previously. As for KMASH, adsorbed molecules were driven off, but by heating to only 350°C for some hours. Subsequently, ca. 10 wt% of water were injected into the platinum capsule which was then sealed by cold pressing. For runs at about 900°C and at higher Z’, only a few wt% of H,O were added to the starting material after heating. All experiments were buffered.
H.-J. Massonne, 2. Szpurka / Lithos 41 (1997) 229-250
Compositions of the phases in the run products were analyzed with the electron microprobe. As for the KMASH micas, some analyses were ignored due to poor analytical results. In addition, the lattice dimensions of phengites were determined.
3. Synthesis
products
3.1. The assemblage coesite in KMASH
phengite + pyrope + kyanite +
Phengites, mainly of the Md polytype, were obtained in KMASH in the presence of pyrope, kyanite and coesite f additional phases in the P-T range of 30 to 45 kbar and 830” to almost 1100°C (Table 3). Si contents p.f.u. of such phengites lie between 3.38 and 3.58 (Fig. 1). Decreasing Si with rising T but still more clearly with falling P is discernible. Octahedral occupancies of the phengites are elevated with respect to ideally dioctahedral micas, as also observed previously for phengites coexisting with Kfeldspar, quartz and phlogopite (Massonne and
Phengite
233
1987) or with kyanite, talc and Schreyer, quartz/coesite (Massonne and Schreyer, 1989). In addition, the trioctahedral content of phengite correlates with its Si content. Highest octahedral occupancies were observed for phengites with Si contents close to 3.4 p.f.u. that formed at a temperature near 1000°C (Table 3). These could even exceed 2.1 p.f.u. and thus indicate increasing solid solution towards phlogopite with rising T, when compared with data of Massonne and Schreyer (1986). Besides Md micas, phengites of the 2M, polytype appeared. According to the method of Massonne and Schreyer (1986), their contents were estimated to be as high as 40-55% for phengites from all runs at 45 kbar and also runs at temperatures close to 1000°C at lower pressures. For these phengites, the lattice dimensions for the 2M, polytype could be determined, while for the others the a, and c0 of the Md polytype were obtained. The composition of the phengites were determined from their lattice dimensions (see Massonne and Schreyer, 1986) except for those of run products V1007, VlOlO and V1012. Phengites formed in these runs and in runs V934 and
+ Pyrope-Kyanite-SiOz
P/ kbar
T/OC
1
I
700
900
1100
Fig. I. (Si-3) p.f.u. of phengite coexisting with SiO,, kyanite and pyrope in KMASH according to experimental data (m) and the thermodynamic recalculation (isopleths: Si/octahedral occupancy p.f.u.). The lower T and P limit is due to reactions R2 and R3 of Table 2 (see also text). The coesite-quartz transition is that of Minvald and Massonne (1980).
H.-J. Massonne, Z. Szpurka/Lithos
234
41 11997) 229-250
Table 3 Experimental and analytical results on the syntheses of KMASH phengites in various parageneses. Data on top of the upper part are from Massonne and Schreyer (19871, data at the bottom of the upper part are from Massonne and Schreyer (1989) and data in the lower part are selected runs related to this work Run No.
Results
Conditions P (kbar)
T (“C)
v155 V380 V164 v379 V160 V378 v102 v103 V389 V269 v515 V248 V338 V198 v113 V275 v251 v714
3.0 3.5 5.0 7.0 7.0 7.0 7.0 7.0 10.75 10.6 10.9 10.9 14.6 15.0 15.0 20.0 20.0 23.55
650 450 700 450 450 550 550 650 493 590 593 689 450 546 649 544 642 663
V863 V800 V352 V902 V238 V837 V746 v75 V646 V647 v747 v745 v793 V916 V840 v799 v900
14.8 19.9 25.1 19.9 19.95 25.05 25.0 25.1 29.8 30.2 30.1 35.0 34.95 34.95 39.95 40.2 39.75
663 728 796 576 647 573 656 742 601 685 754 587 666 770 587 669 750
phases observed
(Kf),@hl),qz,WM Kf,phl,qz,WM Kf,(Phl),(Qz),WM Kf,phl,qz,WM (Kf),phl,(Qz),WM (Kf),phl,(Qz),WM &f),phl,qz,WM
(Kf),pU(Qd,WM Kf,(Phl),Qz,WM (Kf),phl,(Qz),WM Kf,(Phl),Qz,WM (Kf),(Phl),(Qz),WM Kf,phL(Qz),WM @f),phl,(Qz),WM (Kf),Phl,(Qz),WM Kf,(Phl),(Qz),WM Kf,(Phl),(Qz),WM Kf,Phl,Qz,WM
Ky,(Phl),Qz,WM Ky,Phl,Qz,WM Ky,Phl,(Qz),WM
Ky,Qz,(Tc),WM (KY),(Qz),Tc,WM
Ky,Qz,(Td,WM (KY),(Qz),Tc,WM (KY),(Qz),Tc,WM (CS),(KY),TC,WM (CS),&Y),TC,WM GJ,(KY),Tc,WM (CS),(KY),TC,WM GMKY),Tc,WM (CS),WY),TC,WM
Cs,Ky,Tc,WM Cs,Ky,Tc,WM (CSHKY),TC,WM
V940 were analyzed with the electron microprobe (see Table 3). The average from at least 8 single spot analyses per sample was calculated. 1 a-uncertainties were less than 0.01 Si p.f.u. and 0.005 Ott (= sum of octahedrally coordinated cations) p.f.u. In addition, the average compositions of phengites from run V934 and V940 were close to those determined with the powder diffractometer. In run V940 at 990°C instead of coesite quartz
composition
of phengite
activity
Si/f.u.
Oct/f.u.
Phl
3.053 3.152 3.075 3.256 3.276 3.160 3.202 3.156 3.391 3.371 3.314 3.336 3.610 3.584 3.495 3.782 3.675 3.776
2.05 1 2.056 2.056 2.067 2.078 2.087 2.065 2.073 2.040 2.059 2.067 2.071 2.051 2.028 2.042 2.008 2.038 2.014
0.395 0.45 0.41 0.55 0.55 0.50 0.50 0.45 0.63 0.58 0.58 0.52 0.88 0.74 0.65 0.90 0.85 0.90
3.194 3.302 3.383 3.369 3.370 3.455 3.398 3.456 3.504 3.542 3.484 3.508 3.569 3.529 3.584 3.600 3.593
2.163 2.098 2.097 2.039 2.041 2.010 2.065 2.046 2.007 2.010 2.040 2.004 2.009 2.045 2.002 2.011 2.019
Tc
0.905 0.900 0.895 0.935 0.920 0.945 0.925 0.910 0.945 0.930 0.915 0.950 0.940 0.920 0.955 0.945 0.930
appeared, despite seeding with that phase. This is in agreement with the low-quartz stability (Fig. 1) reported by Mirwald and Massonne (19801, Bohlen and Boettcher (1982) and recently by Bose and Ganguly (1995) for runs below 30 kbar. In the product of run V935, both quartz and coesite, were present. Despite the P-T conditions of 30.3 kbar and 845°C quartz might have spontaneously formed from the gel and was afterwards not completely
235
H.-J. Massonne, Z. Szpurka / Lithos 41 (19971229-250 Table 3 (continued) Run No.
Results
Conditions t (days)
P (kbar)
T CC)
wt% (H,O)
added
Phases observed
Composition
of phengite
XRD
v935 v934 v940 V925 v915 V936 V926 v937 v914 v1012 V1007 V1006 VlOlO
8 4.5 5.5 4 3 2.5 2.5 4 1.5 5.5 6 3.5 2.5
30.3 30.55 29.7 35.05 35.2 35.0 40.05 39.95 39.85 45.1 45.0 45.05 45.1
845 929 990 849 937 1024 847 931 1002 853 940 1017 1089
15 3.5 3.7 14 4.1 3.8 15 4.1 3.8 10 4.4 4.5 4.1
(Cs),en,Ky,Phl,Py,qz,WM Cs,en,Ky,(L),Py,WM Ky,(L),(Phl)Py,Qz.WM (Cs),@n),Ky,Py,WM Cs,(En),Ky,(L),(Py),WM Cs,(Ky),H%‘y,WM Cs,(En),Ky,Py,WM Cs,(Ky),(L),Py,WM (CSMY,OJ,PY,WM
Cs,Ky,(Py).WM Cs,(Ky),(Py),WM Cs,(Ky),(L),Py,WM Cs,(Ky),(L),Py,WM
For abbreviations see Table 1. Ott = octahedra1 occupancy. Phases in parentheses case letter at the beginning indicate small amounts in the products.
transformed to coesite as the quartz-coesite transition curve is close to the experimental conditions. Probably the appearance of some phlogopite in the products of runs V93.5 and V940 must be explained in the same way, because according to Massonne (1995a), the curve of the reaction pyrope + phengite = phlogopite + kyanite + quartz lies about 1 kbar below the quartz-coesite transition curve (Fig. 1). Also, enstatite was present as a minor constituent of some synthesis runs obtained particularly when lower P and T were applied. This was taken as an indication for the metastable formation of kyanite + enstatite, instead of pyrope + coesite/quartz, at an early stage in some of the experiments in spite of seeding with pyrope. However, it is likely that pyrope + coesite/quartz grew later during these runs. No talc was present in any run products. This is in agreement with the stability of talc (Rl of Table 2) at temperatures below 830°C (see discussion in Massonne, 1995a) and, more relevantly, with the stability of pyrope + coesite (R2 of Table 2). The alternative assemblage in the presence of H,O, talc + kyanite, forms at a somewhat lower temperature according to the experiments of Chopin (1984). Even lower reaction temperatures were calculated on the basis of thermodynamic data sets, for instance, by Guiraud et al. (1990) and Massonne (1995a). The calculated curve for the talc + kyanite breakdown reaction of
EMS
Si/f.u.
Oct/f.u.
3.414 3.383 3.398 3.524 3.448 3.397 3.525 3.490 3.428
2.070 2.116 2.142 2.058 2.082 2.069 2.047 2.034 2.080
3.490
Si/f.u.
Oct/f.u.
3.420 3.344
2.080 2.143
3.580 3.495
2.007 2.04 1
3.395
2.052
2.037
refer to minor constituents
or traces, those with lower
the latter author is given in Fig. 1. In the products of the four runs in part B of Table 3 at temperatures around 850°C and run V1007 at 940°C and 45 kbar, no glass was detected. In all other runs in this part of Table 3, melt appeared. The quenched products had a glass-like rather than powdery consistency, the background of the X-ray powder diffractograms was slightly higher at relatively low 20 and the amount of phengite obtained was smaller in comparison to the other run products. Sometimes very low contents of phengites were observed in products of runs at temperatures above 1000°C. These were repeated with only 3.5 to 4 wt% H,O added to the heated starting material before sealing the capsule. Then the amount of melt formed was relatively small. Because in the runs at the highest temperature, shown in Fig. 1, the stability field of phengite with excess SiO, was not overstepped, it can reach about 1100°C at 45 kbar and about 1000°C at 30 kbar (see Massonne, 1995a). These high stability temperatures are compatible with earlier results of Massonne and Schreyer (1980) on the stabilization of potassic white micas by the inverse Tschermak’s substitution. 3.2. KFASH In several high-pressure runs KFASH phengites were obtained coexisting with garnet, kyanite and
236
H.-J. Massonne, Z. Szpurka/Lithos
quartz/coesite, although the amount of phengite was sometimes very low. Such runs, as well as those that did not yield the correct assemblage were generally not further studied. In many cases, these runs were repeated by changing the starting composition somewhat or by reducing the amount of water added. Finally, the assemblage garnet, kyanite and quartz/coesite with significant amounts of phengite
41 (1997) 229-250
was successfully synthesized in the P-T range 15 to 55 kbar and 600” to 900°C in 23 runs (Fig. 2). This range probably does not cover the stability field of this paragenesis which is assumed to be larger. Nevertheless, extending the experimental P-T conditions seemed to make less sense for the following reasons. Various attempts to obtain the desired assemblage at 1000°C and different pressures above or
A P/kbar
50
LO
Phengite + Almandine
- Kyanite - Si02 Tl°C
600 Fig. 2. (Si-3) p.f.u. of phengite coexisting with SiO,, thermodynamic recalculation (isopleths as Fig. 1).
700 almandine
800 and kyanite
900 in KFASH
resulting
I
1000 from experimental
data (m) and the
H.-J. Massonne, Z. Szpurka / Lithos 41 11997) 229-250
equal to 35 kbar were unsuccessful probably because larger amounts of melt was formed in spite of relatively small amounts of water added to the dry starting material. Therefore, a suitable starting composition could not be selected in order to obtain the desired assemblage with significant amounts of phengite. Already at 600°C the quality of phengite was relatively poor, because of broad X-ray reflections and tiny grain size, so that we did not dare to apply lower experimental temperatures. At 50 and particularly at 55 kbar several runs could not be
237
finished due to breakage of the WC parts of the piston-cylinder apparatus. Most of the phengites synthesized crystallized predominantly as the 2M, polytype. Only the product of run GV60 showed phengite with the Md polytype clearly dominating. Therefore, the lattice dimensions of numerous 2M, phengites could be obtained (Table 4). In many cases, their composition could also be determined with the electron microprobe, because the KFASH phengites were generally larger than those synthesized in KMASH at compa-
Table 4 Experimental conditions of selected runs and analytical results on KFASH phengites of the 2M, polytype kyanite and quartz/coesite (at the top) or with one or all of these latter phases lacking (at the bottom) Run No.
Conditions r (d)
GV18 GVlO GV54 GV53 a GV16 GV12 GV56 GV9 GV8 GV70 GV7 GV34 GV1.5 GV50 GV192 GV20 GV51 GV52 GV57 a GV312 GV361 GV37 GV60 b GV32 GV35 SO6 so7 SO8 so9
3 2.5 3 2.5 2 3 2 2.5 2 2.5 2 2.5 2.5 2.5 1.5 3 3 2.5 4 0.5 0.5 0.5 3 3 2.5 2 2 2 2
coexisting
either with almandine,
Results P (kbar)
T (“C)
14.6 15.15 20.9 20.1 24.95 24.9 25.1 34.85 34.85 35.05 34.85 34.85 39.85 41.5 39.85 39.65 45.8 44.95 50.5 49.4 49.8 49.8 54.9 15.0 39.85 25.1 25.1 25.1 25.1
602 698 652 751 601 700 803 597 701 753 800 899 602 753 804 903 754 802 599 700 801 900 603 803 1002 804 804 804 804
lattice dimensions
composition
of phengite
da/a0
d, (A)
cb (A)
Vce,, (A3)
Si/f.u.
Oct/f.u.
1.00034 1.00061 1.00084 1.00104 1BOO8 1 1.00065 1.00027 1.00036 1.00035 1.0003 1 1.00064 1.00023 1.00040 1.00013 1.00049
5.2142(08) 5.2148(06) 5.2216(09) 5.2138(16) 5.2259(09) 5.2194(11) 5.2116(09) 5.2265(14) 5.2165(07) 5.2169(07) 5.2179(08) 5.2181(16) 5.2316(10) 5.2176(07) 5.2207(06)
19.9690(18) 19.9731(21) 19.9228(22) 19.9472(37) 19.8958(24) 19.9269(27) 19.9431(24) 19.8569(41) 19.8958(16) 19.9202(20) 19.9035(18) 19.9061(30) 19.8460(17) 19.8882(18) 19.8848(22)
940.56 940.76 940.86
1.0006 1 1.00055 1.00048 1.00102 1.00080 1.00106
5.2227(06) 5.2228(12) 5.2267(08) 5.2306(15) 5.2245(11) 5.2225(09) 5.2278 5.2192(12) 5.2239(20) 5.2147(10) 5.2225(09) 5.2321(12) 5.2369(11)
19.8753(18) 19X710(30) 19.8292(14) 19.8285(41) 19X551(23) 19.X797(12) 19.7952 19.8838(21) 19.8984(40) 19.9867(25) 19.9892(19) 19.9322(35) 19.9993(25)
3.1163(157) 3.1016(416) 3.1749(083) 3.1659 3.2893(190) 3.2131(117) 3.19X2(077) 3.3938(274) 3.3060(297) 3.2802(138) 3.2752(211) 3.2814(130) 3.4123(142) 3.3047(074) 3.2496(188) 3.1719(403) 3.3675(117) 3.3372(060) 3.4489 3.4229(145) 3.3521(187) 3.2776(104) 3.5932(066) 3.2852(220) 3.3325(500) 3.0706(074) 3.0746(092) 3.1248(105) 3.1715(148)
2.0580(054) 2.0702(093) 2.0745(05 1) 2.0393 2.0321(071) 2.0391(042) 2.0220(035) 2.0341(098) 2.0023(081) 2.0022(040) 2.0055(072) 2.0003(060) 2.0264(079) 2.0196(031) 2.0235(072) 2.0133(110) 2.0235(045) 2.0055(025)
1.00089 1.00131 1.00062 1.00206 1.00121 1.00107
941.11 940.24 938.20 939.49 937.72 939.05 938.60 938.79 940.82 937.76 938.72 939.00 938.84 93964 938.69 939.14 937.03 938.16 940.51 941.38 944.31 945.09 950.01
Numbers in parentheses refer to the 1~ error. For 2M, micas, a’, is equal to (a, + b,/fi)/2 and c’,, is c0 sin p. a A run with a phengite composition determined from its lattice dimensions. In case of GV57, the octahedral occupancy assumed to be 2.00. b A run that has led to phengite with dominating
Md polytype,
for which a, and 2c, (hexagonal
2.0395(059) 2.0215(072) 2.0212(041) 2.0156(025) 2.0307(051) 2.0202(120) 2.0589(043) 2.0587(062) 2.1095(096) 2.1331(082)
(Oct/f.u.)
metrics) is given in this table.
was
238
H.-J. Massonne, Z. Szpur!sa/Lithos 41 (1997) 229-250
rable P-T conditions. Frequently the mica platelets were as thick as 1 pm or even thicker. The diameter often exceeded 2 to 3 pm. Altogether, we obtained compositional and metric data from twenty-one 2M, phengites coexisting with additional phases and from four single phase 2M, phengites that were also synthesized in the piston-cylinder apparatus in this study (Table 4). Moreover, such data were determined for one Md mica, which turned out to be the phengite with the highest Si content obtained in our runs. The Md mica was synthesized together with almandine, kyanite and coesite at the highest pressure applied, that is 55 kbar. With decreasing P a significant lowering of Si was observed for phengites in the above assemblage (Fig. 2). The phengite Si content also decreased slightly with rising T. The same tendencies with P and T were observed for KMASH phengites coexisting with the same paragenesis. Slightly elevated octahedral occupancies up to 2.07 p.f.u. were found, but not as large as in KMASH (compare Tables 3 and 4). Therefore, unless there is some oxymica component involved, it is likely that the iron in phengite is by far divalent as assumed for the calculation of the structural formula. If some iron would have been trivalent, neglecting the oxymica component, the corresponding calculation procedure would, apparently, have led to clearly higher octahedral occupancies. Almandine and kyanite often appeared in the run products as crystals being clearly larger than the coexisting phengites. Their composition did not change significantly with variable P-T conditions. Kyanites contain some iron close to 1 mol% of the theoretical Fe, SiO, endmember. Garnet compositions showed Al to Fe ratios close to 213 and are, thus, assumed to be virtually ideal almandine. The cb parameter that is identical to the double mica sheet thickness clearly changes with variable Si p.f.u. for the 26 KFASH phengites given in Table 4 (without GV53, GV57, and GV60). As for KMASH phengites (Massonne and Schreyer, 19861, no dependence of cb with variable octahedral occupancy was discernible for the many phengites having Si contents above 3.2 p.f.u. However, two KFASH micas with octahedral occupancies above 2.1 p.f.u. and relatively low Si contents demonstrate that increasing octahedral occupancies lead to slightly increasing
Fig. 3. cb( = double mica sheet thickness = cO’sin p) versus Si content p.f.u. of KFASH 2M, phengites (and one Md mica with 2c, = cb) with octahedral occupancies of 2.00 p.f.u. and slightly above. The estimated average 2u error range is indicated by the box. The broken line shows the behavior of cb of KMASH 2M, phengites with ideally dioctahedral occupancy (Massonne and Schreyer, 1986) for comparison.
cb at least for low Si. Neglecting these two micas, a straight line was fitted showing a clear decrease of CL with rising Si similar to KMASH phengites (Fig. 3). Because this line must meet with the KMASH line at Si = 3.0 p.f.u., that is ideal muscovite, a slight change of the slope of the KFASH curve can be predicted. A similar change was also observed for KMASH (Massonne and Schreyer, 1986). To determine the compositional dependence of d,( = (a, + b,/ fi)/2 for 2M, micas), again the 26 phengites of Table 4 were considered. Because a clear dependence on both Si content and octahedral occupancy is discernible, u’,, was fitted to a plane (Fig. 4). For this purpose, the ub value of the Si richest phengite obtained in run GV60 was neglected, because this value showed a significant deviation from the plane finally calculated. With rising Si and octahedral occupancy of phengite ah increases in both systems (KMASH and KFASH). However, in KFASH the increase is stronger than in KMASH, both with rising octahedral occupancy and Si content. In fact, in this system the u’,, parameter increases with rising Si of phengite but reaches more or less constant values above Si = 3.5 p.f.u. The same might be true for KFASH, because this would fit the do value of the phengite with Si close to 3.6
H.-J. Massonne, Z. Szpurka / Lithos 41 (1997) 229-250
Si /f.u 30
32
3L
36
w
Fig. 4. d,( = (a, + b, /fi)/2) versus Si content p.f.u. of KFASH ZM, phengites shown for octahedral occupancies of 2.00 and 2.08 p.f.u. The a’,, value of phengite obtained in run GV60 ( = a0 of a Md mica) including an estimated 3~7 error range is shown by the box. The behavior of d, of KMASH 2M, phengites (Massonne and Schreyer, 1986) is indicated by the broken lines.
p.f.u. (Fig. 4). The u’,,/u, ratios of the KFASH phengites, with an average around 1.0007, are higher than those of the KMASH micas (see Massonne and Schreyer, 1986). The unit cell volumes of the potassic white micas given in Table 4 are clearly above that of ideal 2M, muscovite with about 935 A3 (Massonne and Schreyer, 1986). In particular, increasing octahedral occupancy, which is due to the substitution of 3Fe2+ for 2A1, is expected to lead to significantly rising cell volumes. Increasing Si contents, according to the inverse Tschermak’s substitution Fe’+ + Si for 2A1, result in increasing cell volumes, as well. This will be demonstrated in more detail in the subsequent chapter.
4. Thermodynamic
evaluation
From the experimental data given above and those reported in the literature an attempt was made to derive the thermodynamic properties of the celadonite endmembers and the mixing properties of ternary mica solid solution series in KMASH and KFASH. To model the nonideal behavior of these series, the following equation for asymmetric solid solutions
239
was applied, although in some cases symmetric solutions were considered: (a) G”” = x,x2(x,W2~ + x,W,;) + X, a+,(x,W3y + x3W,Y) + x2 x3(x$5; + +w$ + xi x2 x3w,;,> (b) Margules parameter W G = W H - TWs + PW “, same for other endmember Cc> aMs = xM, Y? components, The base of the thermodynamic evaluation is the software package of Brown et al. (1988) that includes the internally consistent data set of Berman (1988) and new data for almandine (Berman, 1990). Data for quartz, kyanite, K-feldspar, phlogopite and H,O were thus directly used. Those for coesite, muscovite, pyrope, and talc had been previously changed slightly (Massonne, 1995a). Data for annite are given in Massonne (1995b). cP functions of the Mg-Al-celadonite and Fe’+Al-celadonite endmembers were estimated according to Berman and Brown (1985). Compressibility and expansivity data (e.g. Massonne, 1992a) as well as some of the Margules parameters that show a minor effect on equilibria with mica components considered here were also estimated. The important parameters, however, were determined using least square fits considering the relation: AGReac.P.T = 0 = AGReac,P.T.unknown+ AGReac.P.T,known. (1) For that purpose, a computer program was used written by Y. Oka. For instance, this program was applied by Oka et al. (1984) to derive thermodynamic mixing properties. Molar volumes of the celadonite endmembers and WV parameters were determined from the lattice dimensions of phengites using Oka’s program prior to the calculation of- Hfo and So of the celadonite endmembers and of the corresponding W H and Ws parameters. 4.1. Thermodynamic
properties
in KMASH
Volume data for ideally dioctahedral KMASH micas are reported by Massonne and Schreyer (1986). These data of the binary muscovite-Mg-Al-celadonite series were fitted to the above type of Margules relation: Vex = x,&,Wbvu + x,w,;>
240
H.-J. Massonne, Z. Szpurka /Lithos
The results given in Table 5 clearly support the asymmetric model selected. The recalculated KMASH curve of Fig. 5 is nearly identical to the corresponding curve of Massonne and Schreyer (1986). According to Holland and Powell (1990) the molar volume of Mg-Al-celadonite should be, in fact, clearly lower than that of ideal muscovite, which is 14.080 J/bar (Massonne, 1992a), but our value is even lower than the estimate of Holland and Powell. For the muscovite-phlogopite and Mg-Al-celadonite-phlogopite series negative values for WV . are given m Table 5. W& was set to zero. However, this volume behavior was not mathematically calculated but roughly estimated on the basis of a molar volume of phlogopite of 14.977 J/bar (Berman, 1988) and of volume data of KMASH white micas with octahedral occupancies of up to 2.1 p.f.u. (Massonne and Schreyer, 1986). Subsequently, HP and So of Mg-Al-celadonite as well as W H and Ws parameters for the muscoviteMg-Al-celadonite series were simultaneously determined from the experimental data given in Table 3. However, before starting the least square fit several assumptions were made besides those mentioned above. Because phengites considered for the fit show some amounts of phlogopite component, albeit often very small, we had to take into account the limited miscibility towards the trioctahedral mica endmember. Some information on the extent of this solid solution range are given by Massonne and Schreyer
Table 5 Estimated (‘..‘I and covite-trioctahedral End member data Holland and Powell
calculated volume data of the ternary musmica-celadonite solid solution range in J/bar. for celadonite are compared with those of (1990) KMASH
KFASH
Vl” (Holland and Powell, 1990)
13.96
14.00
V3” w;=w;
13.870*0.027 ‘-0.135’ 0.735*0.118 0.187*0.026 ‘ - 0.20’
13.962 kO.067 -0.1341+0.1148 0.3394+0.1120 0.3394+0.1120 ‘0’
Wtl; W3‘: w*; = w;:
Components: (1) ideal muscovite, (2) either ideal phlogopite or ideal annite, (3) either ideal Mg-Al-celadonite or ideal Fe’+-Alceladonite.
41 (1997) 229-250
t
V”/ Joule
bar-’
\_ \
13 90
X AI-M I
Musccwte O2
04
\
06
08
‘I AI-Celadcmte
Fig. 5. Volume behavior of the binary muscovite Al-celadonite series. Solid portions of the curves are underlaid by volume data determined (see text).
(1986). According to these authors, phengites with relatively low Si contents can contain up to 10 mol% of phlogopite component when coexisting with phlogopite. Clearly lower maximum phlogopite contents were observed for phengites with high Si contents. With regard to the new data presented in Table 3 (e.g. V940), a slight increase in the solid solution range towards phlogopite is indicated to occur with rising T. Margules parameters for the solid solution series muscovite-phlogopite and Mg-Alceladonite-phlogopite were, thus, introduced to model the limited miscibility range. To account for the relatively slight temperature dependence, both W” and Ws had to be considered. The quantities of both parameters must be relatively high, but they should be higher for the Mg-Al-celadonite-phlogopite join than for the muscovite-phlogopite series, because of the relatively low maximum phlogopite contents in phengite coexisting with phlogopite, that decrease with rising Si contents. The roughly estimated values for W” and Ws of the above series (Table 6) lead to activities often around 0.7 for the phlogopite component in phengites coexisting with phlogopite. To evaluate the previous experimental data on reactions R4 to R6 by Massonne and Schreyer (1987, 1989) we made the assumption that, besides quartz, coesite, K-feldspar and kyanite, even H,O is a pure
H.-J. Massonne, Z. Szpurka / Lithos 41 (19971229-250 Table 6 Estimated (‘..‘) and calculated thermochemical data of the ternary muscovite-trioctahedral (1 + v,(P - 1) + uZ(P - 1)’ + U&T- 298) + C.&T- 298)*). cp = k, + k,T-’ 5 + k,T-* nents etc. see Table 5
H& (Holland and Powell, 1990) H/3
k
WI2
=
w27
w,‘: = wsy w*‘: = w3y St (Holland and Powell, 1990) s,”
w,:=w;
W,s w: w; = w; c,,3 IO6 L’a,s 10’2 c3,3 lo6 c4.s k 0.3 k 1.3 k z.3 k 3,3
241
mica-celadonite solid solution range. V,;, = V” + k,Tm3. Units are in J, bar and K. For compo-
KMASH
KFASH
- 5834270 - 5832415 i 3040 ‘5000’ 0 ‘15000’ 297 288.527 + 4.008 ‘-10’ 58.598 f 8.804 15.920 * 3.393 ‘-30’ ‘- 1.7169’ ‘4.295’ ‘33.527’ ‘0’ ‘645.915’
- 5484960 - 5492287 + 6267 ‘5000’ 0 ‘15000’ 328 303.148 f 6.827 ‘-10’ 24.083 + 3.088 24.083 + 3.088 ‘-30’ ‘-1.7’ ‘4.0’ ‘33’ ‘0’ ‘664.755’ %-4553.11’ ‘ - 12459890’ ‘1871056000’
‘-4129.54’
13864470’ ‘1978171000’
‘-
phase with an activity equal to unity. In contrast, phlogopite and talc are solid solution phases in KMASH and, thus, the P-T relation of the activity of phlogopite and talc endmember components for R4 to R6 must be known. In case of talc, we used the data of Massonne (1991), who reported Al contents of this phase coexisting with kyanite, quartz or coesite, and H,O (reactions R9 and RlO). The Al contents at given P and T calculated from data of Holland and Powell (1990) are, however, clearly higher, whereas those reported by Hoschek (1995) from his experimental study are only slightly lower than the results of Massonne. According to the activity model for talc in talc: uTc = (Si/4>4 (Massonne, 1995a), the talc activities in Table 3 were obtained. Only rough compositional data exist for phlogopite coexisting with phengite, K-feldspar, quartz and H 20 (Massonne, 1991). Here we assume that the Si isopleths for phlogopite of the above assemblage run parallel to the ones of phengite in the P-T field. The phlogopite isopleths for Si = 2.65 and 3.2 p.f.u. should be more or less identical to the phengite isopleths for Si = 3.1 and 3.7 p.f.u., respectively.
The octahedral occupancy of such phlogopites is probably close to 2.8 p.f.u. (Massonne and Schreyer, 1987). Only those with Si higher than 3.2 p.f.u. show still lower octahedral occupancies. In addition, we considered the following activity relation for phlogopite in phlogopite: aPi,, = (Mg/3)’ . y. An activity coefficient of 1.3 was assumed for phlogopite with compositions across the solvus gap coexisting with phengite. The above assumptions led to the phlogopite activities given in Table 3. For the assemblage phengite, talc, kyanite, quartz or coesite, and H,O, we used all the data of Massonne and Schreyer (19891, but for phengite, phlogopite, K-feldspar, quartz and H,O we ignored the experimental data obtained by Massonne and Schreyer (1987) at T below 450°C and one experiment at 490°C and 10.6 kbar, because of the poor quality of the corresponding experimental products. Altogether with the one experiment on phengite coexisting with pure pyrope, kyanite and quartz (R7 of Table 2) and the 12 experiments on R8 with coesite instead of quartz reported here (Table 3), 48 data points and, thus, equivalent expressions for Eq.
H.-J. Massonne, Z. Szpurka /Lithos
242
41 (1997) 229-250
(1) were used for the final fit. An example is given below for run V647 (Table 3) setting W&, as for all relations, to zero:
+.x;
- 2ax,x,2 + 2bx,x,
(~ux,x, +b.(H;,
- 2ax;x,
1 AGE;,
z9sK/( kJ . mol) ’
- 2bx,x,“)
+ bxf - 2bx;x,)
- TS:) + Qwwn
with stoichiometric coefficients a = - 3 and b = 3 (R6 of Table 2), x, = 0.448, x3 = 0.542, T = 958.15 K, AGknown = 18353692 J. Because both sets of results, obtained either by fitting WI: and Wsy or by setting these parameters to zero, showed almost the same overall error, we learned that they must be small and deviate insignificantly from zero. Therefore, we finally decided to present here only the results for WI: and W3y = 0 in Table 6. The new value for the molar entropy at standard state for Mg-Al-celadonite is close to the estimated value of Holland and Powell (1990) although our value is lower, whereas HP for this mica endmember is nearly identical to that of Holland and Powell. WA and W,“, differ clearly from each other and, thus, confirm the asymmetric solid solution characteristics for the muscovite-Mg-Al-celadonite series, as was already demonstrated by the volume behavior. In order to better understand why the obtained values for the Ws parameters are relatively high, we have plotted the free energy of mixing versus the binary muscovite-Al-celadonite solid solution series at 1 bar and 298 K in Fig. 6, also to discuss various mixing models. As mentioned before, the deviation from ideal mixing related to our activity model that suggests molecular mixing of the mica endmember components is obvious. The same is true for the mixing model (B of Fig. 6) related to ideal mixing on sites as already introduced by Powell and Evans (1983). However, the model of Powell and Evans considers Al-Si mixing on all tetrahedral sites, whereas the corresponding model of Holland and Powell (1990), to which curve B of Fig. 6 is related, takes mixing into account only on half of these sites. Nevertheless, both models for ideal mixing on sites
XAI-M
-5 0
02
OL
06
06 Al-Celadonlte
MUSCOVlk
Fig. 6. Free energy of mixing at 1 bar and 298 K for the binary muscovite-Al-celadonite solid solution series as determined here (thick lines) and calculated considering various activity models (thin lines), which are: (A) aMs = x~~; (B) aMs = 4,(~~,,~*)‘. xS,,T2 for muscovite = KAl,,,,(Al,Si),, ,Si,O,,(OH), %I,T2 (Holland and Powell, 1990); (C) aMs = (Al,, - ij2 = ,I&,; (D) a Ms = (Al,, /2)“AlT2 and ai\l_celadonlte= (Mg or Fe’+ It2 .(l Al,,) (Massonne, 1991).
are nearly equivalent. Mixing models C and D (see Massonne, 1991) that consider the coupled Tschermak’s substitution are much closer to the mixing properties determined in this study. Model C is almost identical to our result for muscovite to low Si phengites. Model D shows higher amounts of AG”‘” at 1 bar and 298 K for low Si phengites but lower amounts for high Si phengites than determined here. Therefore, model D, as suggested by Massonne (1991), leads already to a good approach to the experimental data, whereas the ideal mixing models A and B do not. 4.2. Thermodynamic
properties
in KFASH
In order to describe the volume behavior of the KFASH white micas, we fitted the mica volume data of Table 4 to the following equations including a symmetric Margules model: V = x,VMs + x2VAnn + x3V,,, + Vex with Vex = x, x,w,; + x, x,w,;.
H.-J. Massonne, Z. Szpurka / Lithos 41 (1997) 229-250
The above symmetric approach to the muscoviteFe”+-Al-celadonite series was selected because most of our data relate to low Si phengites. To justify a fit to an asymmetric model more data for Si rich phengites are necessary. For the same reason WZx was set to zero. The ternary Margules parameter W,& was also assumed to be zero. We finally obtained data for the molar volume of Fe’+-Al-celadonite and two Margules parameters given in Table 5. It was surprising that W,: is identical to that of the KMASH micas and W,: is near to the average of W,: and W3y of the muscovite-Mg-Al-celadonite series. In addition, the molar volume of Fe’+-Al-celadonite is close to the estimated value of Holland and Powell (1990). HP and So of Fe2+-Al-celadonite as well as W H and Ws parameters for the muscovite-Fe’+-Al-celadonite series were determined from the experimental data shown in Fig. 2. The following assumptions were made, before starting the least square fit. According to the microprobe analyses, the almandine activity was set to unity, whereas for kyanite we adopted an activity of 0.98, based on the simple activity model uKy = xi,, for all data of the 23 experimental runs. The Margules parameters W H and Ws for the solid solution towards annite, less extensive than in KMASH (see Section 3.21, were assumed to be identical to those of the KMASH micas. As for the volume fit, we used a symmetric model lacking data for Si rich phengites. In addition, the ternary Margules parameter W,,, was again set to zero. We give below an example for one of the 23 relations for the final fit that is related to run GV50: 0 = (w,‘: - 7’W,s,) . (ux; + bxf) + b . (Hfl: T.S:) + AGknown with a = - 3, b = 3 (R12 of Table x3 = 0.3047, T = 1026.15 K, 2), xi = 0.6757, AG known= 17435189 J. As in KMASH, both fits with W,; equal and unequal to zero resulted in almost the same overall error, so that we again present only the data for W,! = 0 (Table 6). The new data for Fe’+-Al-celadonite at standard state show the same characteristics as those for Mg-Al-celadonite. The entropy is significantly lower than the value estimated by Holland and Powell (1990) and HP is, within the given error, identical to that of these authors. WI; lies between Wi and Wz of the KMASH white micas but is closer to Wi and, thus, shows a behavior
243
similar to W,y of KFASH phengites. In regard to the free energy of mixing for the binary muscovite-Alceladonite solid solution series at I bar and 298 K (Fig. 6), the Si poor members of KFASH and KMASH are identical. Towards higher Si contents, the amount of AG”“’ is smaller for KFASH phengites and, thus, similar to the mixing model C. 4.3. Discussion of the new thermodynamic data with respect to the P-T position of Si isopleths of phengites The derived values can be used, among other things, to recalculate the P-T isopleths of Si contents of phengites coexisting with various assemblages including those that were experimentally investigated. For the KMASH paragenesis talc-phengite-phlogopite-quartz with H,O in excess, the phengite Si isopleths recalculated with talc and phlogopite activities as suggested above are very close to those given by Massonne and Schreyer (1989) on the basis of a few experiments not used for the mathematical fit here. Fig. 1 shows Si isopleths that are in good agreement with the experimental data in KMASH for the assemblage phengite, kyanite, pyrope and coesite. The same is true for phengites coexisting with talc, kyanite, H,O and quartz or coesite. The reason for that is certainly that the underlying data were used for the mathematical fit. However, as demonstrated by the recalculated Si isopleths for KMASH phengites coexisting with Kfeldspar, phlogopite, quartz and H,O in Fig. 7, this must not be necessarily so, because the fit is poor for phengites richest in Si, and also for those obtained at low experimental temperatures. For the latter, the compositional phengite data reported by Massonne and Schreyer (1987) might be somewhat uncertain. In case of the Si rich phengites, we have to consider observations by Massonne (1992b), who reported significant dissolution by the hydrous fluid at the corresponding high pressures. Therefore, the water activity might be significantly below unity. However, it must be already in the range of 0.5 to account for the deviations from the recalculated Si isopleths mentioned above (see Fig. 7). A further reason for that might be an inappropriate fit of the thermodynamic data to high Si KMASH phengites. According to Massonne and Schreyer (19861, it is
H. -.I. Massonne, Z. Szpurka / Lithos 41 (1997) 229-250
244
2c
IE
I(
I 5 -
T/OC O-
I
300
I
400
w
I
I
500
600
700
Fig. 7. Si content p.f.u. of phengite coexisting with quartz, K-feldspar, phlogopite and H,O according to experimental data (boxes) and the thermodvnamic recalculation (isopleths: %/octahedral occupancy) with phlogopite activities as indicated or shown in Table 3. (- - -) curve for aPh, 1 0.80 related to aHzO of only 0.7.
impossible to synthesize pure Mg-Al-celadonite and micas close to this ideal composition for structural reasons related to a misfit of octahedral and tetrahedral layers in such micas. Our solution model would be incompatible with this observation. At the moment, the derived thermodynamic properties are thus only reasonable for the series muscovite-Mg-Al-celadonite with Si values below 3.6 p.f.u., albeit the extrapolation to endmember Mg-Al-celadonite yields thermodynamic data that are at least close to those estimated previously, as discussed above. This fact and the observation that the experimental results obtained for various assemblages with phengite can be described by the new thermodynamic data support the view of Massonne and Schreyer (1987, 1989) that the experiments had led to stable and not to metastable products. Further support comes from the
application of the new data to natural assemblages as discussed in Section 5.2. The phengite Si isopleths of Fig. 2 fit the experimental data well up to contents of 3.45 p.f.u. At higher Si contents, the new thermodynamic data for KFASH phengites are questionable, because only one experiment had led to a high Si phengite. Further experimentation will be necessary to refine our thermodynamic data for such micas. For geobarometric purposes, P-T diagrams with Si or corresponding In or log K isopleths have been presented by various authors for phengite coexisting with different assemblages. In this respect, the paragenesis chlorite-biotite( = phlogopite)-muscovite(or phengite) in presence of quartz and H,O was subject of a study by Powell and Evans (19831, BucherNurminen (1987) as well as Massonne (1989b). The
H.-J. Massonne, Z. Szpurka/
relations of P, T and phengite compositions given by these authors differ clearly from each other. The phengite Si isopleths recalculated in KMASH with the new thermodynamic data using phlogopite activities as suggested above and clinochlore activities resulting from thermodynamic data for the clinochlore-amesite solid solution series as reported by Massonne (1995a) are only close to those of Massonne (1989b) and show solely a somewhat higher dP/dT slope. In fact, Powell and Evans (1983) as well as Bucher-Nurminen (1987) tried to corroborate their results by the consideration of natural chlorite-biotite-phengite assemblages formed at known metamorphic P-T conditions, however, we suggest not to apply their data for geothermobarometry. As discussed in Section 5, isopleths diagrams should not be used for geothermobarometry, at all. A striking argument comes from our investigation of the assemblage phengite, garnet, kyanite, and SiO,, because at fixed P and T the Si content of KFASH phengite is significantly lower than that in KMASH (compare Figs. 1 and 2). This is in clear contrast to a prediction of Bucher and Frey (1994; Fig. 7.15b). In regard of the assemblage phengite, quartz, K-feldspar, and biotite, the P-T positions of isopleths for identical Si in KMASH and KFASH phengites are, however, very similar.
5. Concluding
remarks
To demonstrate further applications of the new thermodynamic data, examples are presented in the subsequent sections. One is related to the construction of petrogenetic grids which is important for the understanding of paragenetic relations in metamorphic rocks. In the section on geothermobarometry, we give examples for the use of the new thermodynamic data to decipher the P-T evolution of metamorphic rocks. 5.1. Petrogenetic grids As demonstrated by Massonne (1995a), petrogenetic grids can be constructed applying the GeO-Calc software package of Brown et al. (1988) with the data base of Berman (1988) augmented by data for several new phases and solid solution series, which are compatible to Berman’s data set. The example
Lithos 41 (1997) 229-250
245
given here is related to the KFASH system and, thus, an application of the new data for the muscoviteFe*+-Al-celadonite solid solution series. It concerns very-low up to medium grade metamorphic conditions also for the range of high-pressures. Further restrictions are the presence of quartz, phengite, and a hydrous fluid phase with a H,O activity equal to unity. Natural rocks related to these conditions are mainly metapelites. According to natural rocks, we considered as Fe-bearing phases garnet, chlorite, and chloritoid but ignored Fe-carpholite (cf. Theye et al., 1992). In addition, we took into account phases with additional potassium, which are stilpnomelane and biotite. For corresponding endmembers, thermodynamic data are not involved in Berman’s (1988) data set augmented by Massonne (1995a). Therefore, data for endmembers almandine and annite were taken from the literature (see Table 7). For Fe-chloritoid we used data of Berman (pers. comm. by file feb89.rgb) resulting, for instance, in a calculated stability of Fe-chloritoid + quartz by R13 to form almandine + kyanite + H,O at higher Pm0 around 57O”C, which is compatible with experimental data (see Massonne and Schreyer, 1989). For the chlorite endmember daphnite, the values for the molar entropy and volume at standard state were taken from Holland and Powell (1990). The cp function was formulated according to Berman and Brown (1985) and the expansivity and compressibility was assumed to be identical to clinochlore as given by Berman (1988). HP of daphnite was estimated as given in Table 7. The recalculated reaction R14 of daphnite + quartz to form grunerite + almandine + H,O using thermodynamic data for grunerite reported by Lattard and Evans (1992) and a daphnite activity of 0.7, which is close to that of clinochlore in chlorite with Si N 2.85 p.f.u. decomposing with quartz at corresponding water pressures, lies at P H20 = 3 kbar and 538°C. This is compatible with the experimental results of Hsu (1968). According to Eggleton (1972), the ideal formula of stilpnomelane (Table 1) was given for a Fe2+ endmember fully occupied with 36 H,O but with less Al than reported by this author. We considered only 5 Al p.f.u., because some more Al in natural stilpnomelanes could be due to the Tschermak’s substitution. The molar volume of the stilpnomelane endmember was estimated to be 3037 cm3 on the basis of natural
H.-J. Massonne, Z. Szpurka / Lithos 41 (1997) 229-250
246 Table I Thermochemical
data for KFASH phases Phase almandine a
annite b
daphnite ’
Fe-chloritoid
gnmerite e
Fe-stilpnomelane
so
- 5261216
- 5148200
- 7142000
- 3201624
- 9623000
- 99933000
340.007
415.5
559.0
173.712
725.0
7364
V0 0,
11.511
15.432
12.34
6.961
27.80
303.70
- 0.5578
- 1.697
- 1.8195
- 0.7894
- 1.5
- 1.9
0.3211
0
0
1.966
0
0
HP IO6
LIZ 10’2
d
18.613
34.441
26.452
24.401
28.4
26
uq IO’O
32.11
0
0
18.66
0
0
ko
573.962
731.046
1229.233
444.37
1347.83
15936.5 1
k,
- 1483.13
- 5219.94
- 10256.49
- 3880.21
- 9356.910
- 122444
k, k,
-29291968
-9931330
- 12276980
- 4869293
- 20228480
-181141800
5022076928
1518465000
2121507000
916326240
3039190000
28600910000
u,.
lo6
’
a Berman (1990). b Massonne (1995b). ’ See text. d Berman (pers. comm.). e Lattard and Evans (1992).
stilpnomelanes reported by Eggleton (1972). An about 1% smaller value was given for a Fe’+ endmember by Miyano and Klein (1989) however, with only 12H,O and 45(OH) but with SAl p.f.u. According to Holland (1989) So of Fe’+-stilpnomelane was estimated to 7364 J/(mol . K). Miyano and Klein (1989) gave 7220 J/(mol . K) for their stilpnomelane endmember. The cp function was calculated according to Berman and Brown (1985). Expansivity and compressibility of stilpnomelane were roughly estimated (Table 7) by comparison with data of other layer silicates given by Berman (1988). HP was obtained by shifting the stilpnomelane + phengite stability field to its final position shown in Fig. 8. A discussion of the upper thermal stability limits will follow. To calculate the P-T position of reaction curves for the KFASH grid of Fig. 8, we considered activities for the participating phases to be unity except for daphnite in chlorite that was fixed to 0.7 as justified above. Further exceptions concerned annite component in biotite and muscovite and Fe’+-Al-celadonite components in phengite. For the latter, the thermodynamic relations given here for the muscovite-annite-Fe*‘-Al-celadonite solid solution were used also to calculate the variable Si contents of phengites along the reactions curves considering equilibria of the type such as Rl 1 (Table 2). Octahedral occupan-
ties of phengites were fixed, for instance to 2.07 for those with Si contents of 3.2 and 3.15 p.f.u. or to 2.02 for a phengite with Si = 3.6 p.f.u. Maximum Si contents were assumed to be 3.733 p.f.u. for an ideal dioctahedral mica, according to KMASH phengites investigated by Massonne and Schreyer (1986). Biotite compositions and, thus, annite activities were related to phengite compositions as done here for experimental results in KMASH (e.g. see Table 3). With the above provisions, the KFASH grid of Fig. 8 resulted. It shows the subsequent important features. The chlorite + K-feldspar stability extends to about 350°C at very low pressures. Towards higher temperatures biotite appears according to R36 of Table 2. The maximum pressure for chlorite + Kfeldspar is 4.1 kbar. The corresponding invariant point lies at 316°C. Phengite shows a composition of 3.5 Si and 2.035 Ott p.f.u. and annite activity was assumed to be 0.68 at this invariant point. For KFASH, Massonne (1991) argued for a similar chlorite + K-feldspar stability considering natural occurrences in low-temperature metamorphic rocks, although Bucher and Frey (1994) assumed a nearly 100°C higher temperature stability. For the Mgbearing system KFMASH, chlorite + K-feldspar shows a significantly extended stability mainly to high pressures (Massonne, 1991). In contrast to calculations of Miyano and Klein
H.-J. Massonne, 2. Szpurka / Lithos 41 (19971 229-250
25
20
15
10
5
w
200
300
LOO
500
Fig. 8. Petrogenetic grid for the system KFASH with SiO,, phengite and H,O in excess. For abbreviations see Table 1. Reactions arc listed in Table 2. The broken lines refer to a reduced activity of almandine in garnet.
(1989), stilpnomelane can be stable at high pressures. Even in presence of phengite, the breakdown of stilpnomelane occurs with rising water pressures at increasingly higher temperatures up to 500°C and more due to R34. Therefore, at the conditions of blueschist metamorphism, there is a significant T interval for the stability of phengite-biotite-stilpnomelane, e.g. 50°C at Pm0 = 15 kbar. Phengitegarnet-stilpnomelane become stable in KFASH at P H20 > 21.5 kbar. The Si content of phengite at this pressure shows already the maximum assumed. With respect to natural rocks, the assemblage phengitegarnet-stilpnomelane is probably stable also at lower P. A reason for that are natural low temperature garnets containing large amounts of grossular and spessartine component that reduce the almandine activity significantly. For a*,,,, = 0.2, the minimum water pressure in KFASH lies already at about 16.5
241
kbar. In any case, phengite-garnet-stilpnomelane should be a high pressure assemblage according to Massonne (1991). Therefore its appearance, for instance, reported by Black (1977) and Cotkin (19871, is clearly related to blueschists. Phengites show typically high Si contents. Another interesting feature is the stability of stilpnomelane with phases rich in Al, which are either kaolinite, diaspore or chloritoid. The reactions R27 to R30 for the decomposition of the corresponding assemblages take place at relatively low temperatures and high pressures. If such assemblages would occur in nature, they should point to metamorphic rocks formed at low geothermal gradients. Indeed, stilpnomelane + chloritoid was reported by Triboulet (1974) in metapelites from the paleozoic blueschist terrane of the Ile de Groix, Brittany. According to our grid, the minimum pressure for this assemblage is 15.2 kbar at 294°C. The Si content of phengite coexisting at the corresponding invariant point of Fig. 8 with chloritoid, stilpnom:lane, quartz, and H,O is 3.31 p.f.u. In case of the Be de Groix, the Si content of phengite reported by Triboulet (1974) is only somewhat lower. The temperatures for the appearance of chloritoid and garnet in our grid are around 3OO”C, as calculated previously by Theye et al. (1992), and 500°C respectively. These are compatible with the occurrence of both phases compositionally close to the Fe-bearing endmembers in natural metapelites (e.g. Massonne, 1991). The lowering of the almandine activity by the introduction of Ca, that is not introduced in the other phases participating in R25, leads to a significant decrease of the temperatures for garnet formation (Fig. 8). The stability field of chlorite + SiO, even in presence of phengite is extended to higher P than expected by Massonne (1989a). In particular, because intermediate Fe-Mg chlorites should show an enlarged stability field with respect to the Fe and Mg chlorite endmembers, a revision of it is necessary. The stability field of biotite + chloritoid in presence of the excess phases, due to R38 and R39, is very small. Its maximum pressure is 2.0 kbar. The Si content of muscovite at the corresponding invariant point, which lies at 514°C is about 3.07. For intermediate Fe-Mg biotite, the stability field with chloritoid could be clearly enlarged. However, if it could reach 9 kbar in KFMASH as
248
H.-J. Massonne, Z. Szpurka/
Lithos 41 (1997) 229-250
assumed by Massonne (1991) is not clear. Powell and Holland (1990) predicted a maximum pressure of 6 kbar, but only if some amounts of trivalent iron are introduced in biotite and chloritoid. In any case, biotite + chloritoid will not be a high-pressure assemblage forming at pressures of 20 kbar or even more as calculated by Spear and Cheney (1989). 5.2. Geothermobarometry For geothermobarometry, the knowledge of the P-T shift of a univariant reaction with the chemical variation of the participating phases is essential. With the new thermodynamic data, we are now able to calculate this for many equilibria with participating muscovite and Mg-Al-celadonite or Fe’+-Al-celadonite. However, in order to apply the new thermodynamic data to metamorphic rocks, information on the influence of other elements introduced in phengite, but not considered by the experiments, on such equilibria are important. For instance, potassic white micas can contain significant amounts of sodium. Fortunately, good experimental data exist for the muscovite-paragonite series and, thus, we know about the nonideal behavior of this binary solid solution (Chatterjee and Flux, 1986). Furthermore, we learn from coexisting paragonite and phengite in nature about the nonideal behavior of the paragonite-celadonite solid solution. Massonne (1992a) used such information to derive a mixing model based on the above Margules formulation for natural white micas composed of six endmembers among them are muscovite, Mg-Al-celadonite, and Fe’+Al-celadonite. His assumption that the muscoviteFe*+-Al-celadonite series behaves like muscoviteMg-Al-celadonite turned out to be fairly realistic with respect to our new data, but Massonne’s phengite mixing model can now be improved. However, when we apply the old and the improved mixing model to phengite-bearing eclogites, e.g. reported by Massonne (1995a), calculating the P-T positions of equilibria shown in Fig. 9 and, thus, the invariant point for a selected metamorphic stage, the results are very similar. Under these circumstances, there is no revision necessary with respect to the ultra-high pressure nature of eclogites from the Franciscan Complex, California, detected by applying this new type of phengite barometry (Massonne, 1995a). For
Fig. 9. Relative P-T positions of equilibria, in which the garnet components almandine, grossular (Gross), and pyrope, the clinopyroxene components diopside (Diop) and hedenbergite (Hed), and the phengite components muscovite and Mg-Al-celadonite are involved. The reaction curves intersect in an invariant point.
these eclogites peak pressure conditions are even almost as high as 50 kbar (Nowlan and Massonne, 1995). This result is a step forward for understanding processes that are related to the subduction of oceanic crust and the subsequent exhumation of the corresponding metamorphic rocks. The same is true for phengite-bearing high-pressure gneisses and eclogites to which the above phengite mixing model was applied by Massonne (1995b). This particular work was undertaken to answer the important questions, if eclogites and surrounding gneisses would have formed by in situ metamorphism and if the depths of burial of continental crust can exceed 70 km, with the aid of phengite geothermobarometry. Altogether, the application of phengite barometry using good thermodynamic data will certainly help in the near future to understand various geodynamic processes mainly related to high and ultra-high pressure metamorphism.
Acknowledgements The manuscript benefitted from critical reviews by B.W. Evans and T. Gottschalk. Thanks are also due to G. Andersen, H.-J. Bemhardt, A. Fischer and R. Lehmann for technical assistance. This work was financially supported by Deutsche Forschungsgemeinschaft.
H.-J. Massonne, Z. Szpurka/
References Berman, R.G., 1988. Internally-consistent thermodynamic data for minerals in the system Na,O-KzO-CaO-MgO-FeOFe,0,-A1,0,-Si0,-Ti0,-H,O-CO,. _I. Petrol. 29, 445522. Berman, R.G., 1990. Mixing properties of Ca-Mg-Fe-Mn garnets. Am. Mineral. 75, 328-344. Berman, R.G., Brown, T.H., 1985. Heat capacity in the system Na,O-K,O-CaO-MgO-FeO-Fe20, -AlaO, -SiO, -TiO, H,O-CO,: Representation, estimation and high temperature extrapolation. Contrib. Mineral. Petrol. 89, 168-183. Black, P.M., 1977. Regional high pressure metamorphism in New Caledonia: Phase equilibria in the Ouegoa District. Tectonophysics 43, 89-107. Bohlen, S.R., Boettcher, A.L., 1982. The quartz-coesite transformation: A precise determination and the effects of other components. J. Geophys. Res. 87, 7073-7078. Bose, E.K., Ganguly, J., 1995. Quartz-coesite transition revisited: Reversed experimental determination at 500-1200°C and retrieved thermochemical properties. Am. Mineral. 80, 231-238. Boyd, F.R., England, J.L., 1960. Apparatus for phase equilibrium measurements at pressures up to 50 kbar and temperatures up to 1750°C. J. Geophys. Res. 65, 741-748. Brown, T.H., Berman, R.G., Perkins, E.H., 1988. GeO-Calc: Software package for calculation and display of pressure-temperature-composition phase diagrams using an IBM or compatible Personal Computer. Comput. Geosci. 14, 279-289. Bucher, K., Frey, M., 1994. Petrogenesis of metamorphic rocks. Springer, Berlin, 318 pp. Bucher-Nurminen, K., 1987. A recalibration of the chlorite-biotite-muscovite-geobarometer. Contrib. Mineral. Petrol. 96, 5 19-522. Chattejee, N.D., Flux, S., 1986. Thermodynamic mixing properties of muscovite-paragonite crystalline solutions at high temperatures and pressures and their geological application, J. Petrol. 27, 677-693. Chopin, C., 1984. Coesite and pure pyrope in high grade blueschists of the western Alps: A first record and some consequences. Contrib. Mineral. Petrol. 86, 107-I 18. Cotkin, S.J., 1987. Conditions of metamorphism in an Early Paleozoic blueschist, Schist of Skookum Gulch, northern California. Contrib. Mineral. Petrol. 96, 192-200. Eggleton, R.A., 1972. The crystal structure of stilpnomelane. Part II. The full cell. Mineral. Mag. 38, 6933711. Getting, I.C., Kennedy, G.C., 1970. Effect of pressure on the emf of chromel-alumel and platinum-platinum 10% rhodium thermocouples. J. Appl. Phys. 41, 4552-4562. Guiraud, M., Holland, T., Powell, R., 1990. Calculated mineral equilibria in the greenschist-blueschist-eclogite facies in Na,O-FeO-MgO-Al,O, SiO, -H,O. Methods, results and geological applications. Contrib. Mineral. Petrol. 104, 85-98. Harlov, D.E., Newton, R.C., 1992. Experimental determination of the reaction 2 magnetite + 2 kyanite + 4 quartz = 2 almandine + 0, at high pressure on the magnetite-hematite buffer. Am. Mineral. 77, 558-564. Holland, T.J.B., 1989. Dependence of entropy on volume for
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silicate and oxide minerals: A review and predictive model. Am. Mineral. 74, 5-13. Holland, T.J.B., Powell, R., 1990. An enlarged and updated internally consistent thermodynamic dataset with uncertainties and correlations: The system KzO-NazO-CaO-MgO-MnOFeO-Fe,O, -Al,O, -TiO, -SiO, -C-H, -0,. J. Metamorph. Geol. 8, 89-124. Hoschek, G., 1995. Stability relations and Al content of tremolite and talc in CMASH assemblages with kyanite+zoisite+ quartz+H,O. Eur. J. Mineral. 7, 353-362. Hsu, L.C., 1968. Selected phase relationships in the system AlMn-Fe-Si-O-H: A model for garnet equilibria. J. Petrol. 9, 40-83. Lattard, D., Evans, B.W., 1992. New experiments on the stability of grunerite. Eur. J. Mineral. 4, 219-238. Luth, W.C., Ingamells, O., 1965. Gel preparation of starting materials for hydrothermal experimentation. Am. Mineral. 50, 2555258. Massonne, H.-J., 1989a. The upper thermal stability of magnesian chlorite + quartz: an experimental study in the system MgOAI,O,-SiOz-H,O. J. Metamorph. Geol. 7, 567-581. Massonne, H.-J., 1989b. Extreme high-pressure, low-temperature metamorphism of pelitic and acidic protoliths based on experiments in the system K,O-MgO-AlaO,-SiO,-HzO, 28th Int. Geol. Congr. Washington, Abstr. vol. 2, pp. 383-384. Massonne, H.-J., 1991. High-pressure, low-temperature metamorphism of pelitic and other protoliths based on experiments in the system K,O-MgO-Al,O, -SiO, -HzO. Habilitation thesis, Rum-Univ. Bochum, 172 pp. Massonne, H.-J., 1992a. Thermochemical determination of water activities relevant to eclogitic rocks. In: Kharaka, Y.K., Maest, A. (Eds.), Water-rock interaction WRI-7, Proc. 7th Int. Symp., Park City, Utah, USA, pp. 1523-1526. Massonne, H.-J., 1992b. Evidence for low-temperature ultrapotassic siliceous fluids in subduction zone environments from experiments in the system K,O-MgO-AlzO,-SiO,-Hz0 (KMASH). Lithos 28, 421-434. Massonne, H.-J., 1995a. Experimental and petrogenetic study of UHPM. In: Coleman, R.G., Wang, X. (Eds.), Ultrahigh Pressure Metamorphism. Cambridge Univ. Press, pp. 33-95. Massonne, H.-J., 1995b. Is the concept of in situ metamorphism applicable to deeply buried continental crust with lenses of eclogites and garnet peridotites?. Chin. Sci. Bull. 40 (Suppl.), 145-147. Massonne, H.-J., 1997. High-pressure experimentation on the stability of pyrope + quartz and reevaluated thermodynamic properties of pyrope. Neues Jahrb. Miner. Abh., submitted. Massonne, H.-J., Schreyer, W., 1980. Erhohung der Muscovitstabilitat durch MgSi-Einbau im Bereich von 3 bis 35 kbar PHZO. Fortschr. Miner. 58 (Beih. 11, 88-90. Massonne, H.-J., Schreyer, W., 1986. High-pressure syntheses and X-ray properties of white micas in the system K,O-MgOAl,O,-SiO,-HzO. Neues Jahrb. Miner. Abh. 153, 177-215. Massonne, H.-J., Schreyer, W., 1987. Phengite geobarometry based on the limiting assemblage with K-feldspar, phlogopite and quartz. Contrib. Mineral. Petrol. 96, 212-224. Massonne, H.-J., Schreyer, W., 1989. Stability field of the high-
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