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Contrib Mineral Petrol (1990) 104:353-368 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1990

C a l c u l a t e d g r e e n s c h i s t f a c i e s m i n e r a l e q u i l i b r i a in the s y s t e m

C a O - F e O - M g O - A 1 2 0 3 - S i O 2 - C O 2 - H 2 0

Thomas M. Will ~, Roger PowelP, Tim Holland 2, and Michel Guiraud ~ * 1 Department of Geology, University of Melbourne, Parkville, Victoria 3052, Australia 2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England, United Kingdom

Abstract. The kinetic problems associated with the experi- mental determination of reactions among complex solid- solution phases at low temperatures have hindered our un- derstanding of the phase relations in greenschist facies rocks. In the absence of reliable experimental data, we have used the new, expanded internally-consistent thermody- namic dataset of Holland and Powell (1990), to present calculated phase equilibria for the system C a O - F e O - M g O - A 1 2 0 3 - S I O 2 - H 2 0 - C O 2 (CaFMASCH) with quartz in excess, in the range 4000-500 ~ C at low to interme- diate pressures, involving the minerals amphibole, chlorite, anorthite, clinozoisite, dolomite, chloritoid, garnet, margar- ite, andalusite, and calcite. By solving independent sets of non-linear equations formed from equilibrium relation- ships, we calculate not only the loci of reactions in pressure- temperature-x(CO2) space, but also the compositions of coexisting minerals in terms of the substitutions, FeMg_ 1 and (Fe,Mg)SiAI_tAI_~. Invariant, univariant and divar- iant equilibria are calculated and discussed in relation to naturally-occurring greenschist fades metabasic and sili- ceous dolomitic mineral assemblages. We thus avoid the use of activity-corrected curves so commonly presented in the literature as a substitute for genuine univariant phase diagram boundaries.

Introduction

Large areas of greenschist facies rocks of both Proterozoic and Phanerozoic age occur in almost all orogenic belts around the world. Although there is a close similarity in mineral assemblages, only few quantitative approaches have been attempted to unravel the conditions of formation of the common assemblages because: (i) the fine-grained nature of the minerals under greenschist facies conditions makes the rocks difficult to study, (ii) "green" greenschist facies rocks seem to be unattractive and less fascinating than high grade rocks to many meta- morphic petrologists (a review of the literature proves this point), (iii) the minerals involved show complex solid solutions and are commonly involved in relatively high variance assem- blages. As a consequence, direct experimental study is there-

* Present address: Laboratoire de Minrralogie, Museum National d'Histoire Naturelle, 61, rue Buffon, 75005 Paris, France

Offprint requests to: T. Will

fore more or less impossible, particularly with the pro- nounced kinetic problems at such (experimentally) low tem- peratures. Moreover, with previously available datasets, in- sufficient thermodynamic data for end-members of the usual minerals were available to say much about the mineral equilibria.

For example, metabasic rocks metamorphosed at low to intermediate pressures and temperatures display the com- mon mineral association albitic plagioclase (ab) - actinolitic hornblende (amph) - clinozoisite/epidote (cz /ep) - chlorite (chl) - quartz (q) and very often calcite (cc), with minor muscovite (mu) , biotite (b0, dolomite (dol), talc (ta), stilpno- melane (sti lp), iron oxides and sulphides in various combi- nations. With such an assemblage occurring over a wide range of pressure and temperature, there is much informa- tion to be gained from mineral compositional variations, particularly FeMg_ 1 and (Mg,Fe)Si AI_ 1AI_ 1, if the min- eral equilibria are well known. (FeMg_t and (Mg,Fe)Si AI_ 1AI_ ~ are exchange vectors in the sense of Thompson et al. 1982). Although some essentially qualitative work has been done (e.g., Brown 1977; Laird and Albee 1981), little is known about the dependence of mineral chemistry on pressure, temperature and fluid composition, nor even the stability limits of the mineral assemblages, let alone the quantitative locations of invariant points, univariant reac- tions, and divariant fields on P-T diagrams, or, more appro- priately at low-P, low-T, fluid-present conditions, on isobaric T-x(CO2) projections. Qualitative attempts at this, based on field studies, for phase relations among some min- erals forming in metabasic rocks at greenschist facies condi- tions have been made in the past (e.g. Harte and Graham 1975; Brown 1975) but were restricted to simplified systems and did not include consideration of the solid solutions which are normally encountered in metabasites and are an essential feature of these rocks.

Previously, petrogenetic grids calculated from thermo- dynamic data were restricted to systems involving phases composed of single end-members (Bucher-Nurminen et al. 1983 for margarite-bearing assemblages; Rice 1983 for rod- ingites; K/ise and Metz 1980; Metz and Trommsdorff 1968 for siliceous dolomites; Watts 1973, 1974; Barron and Bar- ron 1976 for metabasites). That no solid solutions were able to be included when the petrogenetic grids were con- structed, limits the applicability of these grids to naturally- occuring assemblages. No quantitative study has been at- tempted so far to construct a petrogenetic grid that is con- sistent in itself and covers not only most of the phases

354

relevant to rocks subjected to greenschist facies metamor- phism but also tries to establish the dependence of mineral chemistry on pressure, temperature and fluid composit ion. In other systems, for example Spear and Cheney (1989) on metapelites, l imited experimental constraints on mineral amph equilibria have been combined with observations on rocks tr in order to generate the necessary thermodynamic da ta to f ir calculate grids. Natural ly , the resulting grid is consistent hb

with geological observation, but there is the danger that fhb the features 'descr ibed ' by the grid are due to elements other than those in the system for which the grid is calcu- lated (see discussion in Powell and Hol land 1989). Others chl have tried to generate grids using individual reactions in- clin

volving end-members of minerals and an 'act ivi ty-cor- rected ' approach; this may allow an estimate of the PT daph at which a part icular mineral assemblage is stable, but it

ames is not possible to calculate the posi t ion of reactions along fame which the composi t ions of minerals change with this ap- proach. I t is much more satisfactory to use an internally- ctd

consistent thermodynamic datatset , for example the exten- mctd sire one of Hol land and Powell (1990), f rom which all the fc td equilibria of interest can be calculated, without recourse to any a priori geological data. This is the approach fol- do! lowed here. do!

This study focusses on phase relationships in the system fdol

C a O - F e O - M g O - A 1 2 0 3 - S i O z - C 0 2 - H 2 0 (Ca- ta F M A S C H ) , with quartz and a H 2 0 - C 0 2 fluid in excess, ta which is applicable to, for example, siliceous dolomites and f ta metabasic rocks. C a F M A S C H can be considered as a sub- tats system of C a N K F M A S C H T O ( C a O - N a 2 0 - K 2 0 - f tat

FeO -- MgO - A1203 - SiO 2 - - C O 2 - H 2 0 -- Ti02 -- 02) which would cover the greatest major i ty of phases normal ly g encountered in greenschist facies rocks. The minerals in- alm cluded, and their component end-members, are given in gr

Table 1. Using the expanded internally-consistent thermo- el; dynamic dataset of Hol land and Powell (1990), it is now ep feasible to calculate the P-T-x locat ion of invariant points, cz univariant reactions, and divariant fields on petrogenetic grids, using the computer p rogram T H E R M O C A L C (Pow- and

ell and Hol land 1988). The results of such a study are pre- ma sented here for the C a F M A S C H system. P-T-x pseudosec- an

ab tions (Hensen 1971) are presented, and used to consider ce the influence of bulk rock composi t ions on mineral assem- q blages, as well as to generate internal and external buffering wo paths on isobaric T-x(CO2) diagrams. Fur thermore , the ex- pyhl tent of F e M g _ 1, CaFe_ 1 (Fe,Mg)SiAI_ 1A1-1 substitutions pre in the minerals are related to changes in intensive parame- di ters. fo

Table i . Mineral end-members and their compositions as used in text and figures. The abbreviations of the mineral end-members are those used in Holland and Powell (1990)

amphibole

tremolite ferro-tremolite hornblende

ferro-hornblende

chlorite

clinochlore

daphnite

amesite ferro-amesite

chloritoid

magnesium-chloritoid ferro-chloritoid

dolomite

dolomite ferro-dolomite

talc

talc ferro-talc Tschermak's talc ferro-Tschermak's talc

garnet

almandine grossular

epidote

epidote clinozoisite

andalusite margarite anorthite albite calcite quartz wollastonite phyrophyllite prehnite diopside forsterite

Ca2Mg3Mg2Si4Si4022(OH)2 CazFe3Fe2Si4Si4022(OH)2 Ca2Mga[MgxAll][Si3AI~]

Si,*O22(OH)2 Ca2Fe3[FelAll][Si3All]

Si4022(OH)2

Mg4[MglAll][Si,All] Si201o(OH)s

Fe4[Fe~AI~][SilAll] Si2Olo(OH)s

Mg4[AU[A12]Si201 o(OU)s Fe4[A12][A12]Si2Olo(OH)s

MgA12SiOs(OH)2 FeA12SiOs(OH) 2

CaMg(C03)2 CaFe(C03)2

Mg2Mg[Si2]Si201 o(OH)2 Fe2Fe[Si2]Si20 ~ o(OH) 2 Mg2AI[Si~AI~]Si201 o(OH)2 Fe2AI[Sil AlllSi201 o(OH)2

FeaAI2Si3012 Ca3A12Si3012

Ca2A12[Fe 3 +]Si3Olz(OH) Ca2A12[A1]Si3012(OH)

A12SiO5 CaAlz[A12Si2]O 10(OH)2 Ca[A12Si2]O8 Na[A1Si3]Os CaC03 SiO2 CaSiO3 A12Si4Olo(OH)2 CaA12Si3Olo(OH)2 CaMgSi206 Mg2SiO4

The grid

A 2 kbar isobaric T-x(CO2) section of CaFMASCH with excess quartz has been calculated in a temperature window from 4000-500 ~ C, Fig. 1 and Table 2. Fluid pressure has been assumed to equal total pressure. This temperature window was chosen be- cause it encompasses the reactions relevant to greenschist facies conditions at low pressures, and the transition into amphibolite conditions if the first appearance of almandine-rich garnet is taken to be indicative. As Fig. 1 is complex, initial discussion of the grid will be in terms of phase relationships with calcite also in excess, Fig. 2. This simplifies the grid considerably and, significant- ly, makes the system effectively ternary, thus allowing comprehen- sible compatibility diagrams to be drawn. Moreover the system is a good approximation for many metabasites and low grade sili- ceous dolomites under greenschist facies conditions.

In order to ensure the validity of the grid, and especially its validity at the low and high temperature boundaries of the tempera- ture window, phases additional to those appearing in the grid have been considered. These phases are wollastonite (wo), pyrophyllite (phyl), diopside (di), and prehnite (pre). In projection from quartz and calcite, the stability field of prehnite is confined to equilibrium with a very H20-rich fluid at very low temperatures. The limiting reactions for the stability of prehnite are pre + C02 = cz + cc + q + H20 and phi l+ cc =pre + 1 + C02. The stability of pyrophyllite is limited by the latter reaction and by phyl + cc = cz + q + H2 0 + C02 at very low x(CO2), p h y l + c c = m a + q + C O a + H 2 0 at x(CO2) values up to 0.7, and by p h y l = a n d + q + H 2 0 at an even higher x(CO2). At intermediate fluid compositions pyrophyllite (+ quartz, +calcite) is not stable at temperatures higher than 330~ and is restricted to even lower temperatures at low or high values of

355

T ( ~

500 ~ \ ' \ ' \ ' ~ C a F M A S C H and

�9 ~ ~ ~ 3 '~ \ / at fixed P = 2 k b a r

490 I - I \ \ . . \ ~ ~ \ \ oh, \='Zl +q

480

470

460

k I ~176 c z

c c

450

440 Idol all

430

420 IC~'~/~ f 1 / " 0 / / / / ~ d a / n ~8 ] I / m~ ~ I ~g~l I II I , , / .... ~ / ,2

410

400 / c.,~o + cC doI R

|1 / I / I l l / t / I i / [ / I I I I I i i i ~ , . . . . . . . . . II,~,l 0.1 0 .2 0.3 0.4 0.5 0.6 0 .7 0.8 0 .9

x(co2) Fig. 1. T-x(COz) petrogenetic grid for the system C a O - F e O - M g O - - A 1 2 0 3 - S iO2- C O 2 - H20 at 2 kbar projected from quartz. Bold lines are CaFMASCH reactions, except for the reaction e t d = g + a n d + c h l + H 2 0 (+quar tz) which is in FMASCH; light lines are Ca- FASCH and CaMASCH reactions; CMSCH and CFSCH reactions are given by light lines with the phases in italics. Reactions involving

f do l in CaFASCH are metastable with respect to reactions involving calcite and siderite. For labelling of invariant points see Table 2; for the abbreviations of the phases, Table 1. Tremolite labelled in italics as tr is the pure Mg-endmember, whereas tr (along the CaFMSCH reaction dol + q + H 2 0 = ta + tr + C 02) is an ( F e - Mg)-tremolite

356

Table 2. P-T-x(CO2) locations and mineral compositions of (T-x(CO2) invariant points, x, y, and z are XFe = Fe/ (Fe+ Mg) in amph, chl, dol, ctd, ta, XAI,M2 in amph, chl, ta, and Fe / (Fe+Ca) in garnet, respectively; for more details see Table 3. The numbers 1-11, ml-m8, and f l -f l7 correspond to those given in the Figures. The list of phases following the invariant point label are the phases not involved at the invariant point

CaFMASCH

1 (amph, ctd, chl, dol, ta, cz, g)

P (kbar) T (~ x (CO2) 2.0 398 0.82 5.0 496 0.717

(amph, ta, and, g, cz)

P (kbar) T (~ x (CO2) 2.0 409 0.59 5.0 metastable

x (ctd) 0.977

x (chl) 0.895

y (chl) 0.759

x (dol) 0.888

(amph, ta, g, cz, cc)

P (kbar) T (~ x (CO2) 2.0 412 0.74 5.0 515 0.61

x (ctd) 0.918 0.836

x (chl) 0.692 0.554

y (chl) 0.775 0.720

x (dol) 0.678 0.545

(amph, ta, ma, cz, cc)

P (kbar) T (~ x (CO2) 2.0 431 0.82 5.0 517 0.625

x (ctd) 0.949 0.841

x (chl) 0.794 0.564

y (chl) 0.727 0.716

z (g) 0.902 0.840

x (dol) 0.785 0.556

(amph, ctd, dol, chl, ta, and, g)

P (kbar) T (~ x (CO2) 2.0 391 0.06 5.0 499 0.308

(amph, ta, and, g, an)

P (kbar) T (~ x (CO2) x (ctd) x (chl) y (chl) 2.0 metastable 5.0 490 0.225 0.945 0.800 0.698

x (dol)

0.793

(amph, ta, an, ma, and)

P (kbar) r (~ x (CO2) x (ctd) x (chl) y (chl) 2.0 metastable 5.0 495 0.24 0.959 0.848 0.689

z (g)

0.761

x (dol)

0.842

(amph, ta, chl, an, and)

P (kbar) T (~ x (CO2) x (ctd) z (g) x (dol) 2.0 metastable 5.0 498 0.295 0.964 0.766 0.861

(amph, ctd, ta, ma, and)

P (kbar) T (~ x (CO2) x (chl) y (chl) z (g) 2.0 metastable 5.0 508 0.268 0.736 0.661 0.740

x (dol)

0.729

10 (dol, ctd, ta, ma, and)

P (kbar) T (~ x (CO2) x (amph) y (amph) x (chl) 2.0 metastable 5.0 516 0.235 0.715 0.160 0.683

y (chl)

0.628

z (g)

0.713

11 (ctd, ta, ma, and, cz)

P (kbar) T (~ x (CO2) x (amph) y (amph) x (chl) 2.0 metastable 5.0 519 0.29 0.692 0.161 0.659

y (chl)

0.629

z (g)

0.725

x (dol)

0.652

12 (ctd, g, ma, and, cz) {for activity of anorthite = 0.40}

P (kbar) T (~ x (CO2) x (amph) y (amph) x (ta) 2.0 451 0.58 0.0728 0.0827 0.039

y (ta) x (chl) y (chl) x (dol) 0.054 0.060 0.460 0.058

Table 2 (continued)

CaFASCH

fl (fta, fctd, fdol, and, ma, g)

P (kbar) T (~ x (COz) y (amph) 2.0 419 0.049 0.115 5.0 metastable

f2

f3

(amph, fta, and, ma, g, cz)

P (kbar) T (~ x (CO2) y (chl) 2.0 413 0.600 0.742 5.0 metastable

(fctd, fta, and, ma, g, cz)

P (kbar) r (~ x (CO2) y (amph) 2.0 448 0.61 0.133 5.0 metastable

f4 (ha, amph, chl, g, cz, cc)

P (kbar) T (~ x (CO2) 2.0 406 0.770 5.0 505 0.673

f5

f6

f7

(amph, chl, fta, ma, cz, cc)

P (kbar) T (~ x (CO2) z (g) 2.0 424 0.83 0.900 5.0 metastable

(fta, amph, and, ma, cz, cc) P (kbar) T (~ x (CO2) y (chl) 2.0 431 0.75 0.712 5.0 metastable

(fta, fctd, and, ma, cz, cc)

P (kbar) T (~ x (CO2) y (amph) 2.0 450 0.63 0.135 5.0 metastable

f8 (fta, fctd, chl, and, ma, cz)

P (kbar) T (~ x (CO2) y (amph) 2.0 458 0.70 0.128 5.0 metastable

(amph, fta, fdol, and, g, cz)

P (kbar) T (~ x (CO2) y (chl) 2.0 410 0.41 0.753 5.0 metastable

(amph, fta, chl, and, g, cz)

P (kbar) T (~ x (CO2) 2.0 409 0.66 5.0 metastable

f9

y (chl) 0.580

fl0

f l l

y (chl) 0.614

z (g) 0.879

t"12

y (ch0 0.617

z (g) 0.817

f13

z (g) 0.808

(amph, fctd, fta, chl, and, cz)

P (kbar) T (~ x (CO2) z (g) 2.0 metastable 5.0 501 0.51 0.793

(amph, fta, chl, and, an, cz)

P (kbar) T (~ x (COz) z (g) 2.0 metastable 5.0 496 0.43 0.791

(amph, fdol, fta, chl, and, an)

P (kbar) T (~ x (CO2) z (g) 2.0 metastable 5.0 495 0.265 0.766

357

358

Table 2 (continued)

f14 (fctd, fta, and, ma, an, cz)

P (kbar) T (~ x (CO2) 2.0 metastable 5,0 495 0.15

f15

y (amph)

0.109

f16

1"17

y (chl)

0.537

(amph, fta, and, ma, an, cz)

P (kbar) r (~ x (CO2) y (chl) z (g) 2.0 metastable 5.0 484 0.18 0.670 0.785

(amph, fdol, fta, and, ma, an)

P (kbar) T (~ x (CO2) y (chl) 2.0 metastable 5.0 482 0.12 0.672

(fdol, fctd, fta, and, ma, an)

P (kbar) T (~ x (COz) 2.0 metastable 5.0 479 0.056

CaMASCH

ml (ta, mctd, dol, and, ma, g)

P (kbar) T (~ x (CO2) 2.0 427 0.046 5.0 520 0.21

m2

m3

m4

m5

m6

z (g)

0.791

m7

z (g)

0.747

m8

y (amph)

0.127

y (chl)

0.579

z (g)

0.671

y (amph) 0.106 0.152

y (ch0 0.533 0.602

(mctd, an, ma, and, g, cz)

P (kbar) T(~ x (CO2) 2.0 457 0.56 5.0 496 0.12

y (amph) 0.107 0.086

y (ta) 0.0705 0.0605

y (chl) 0.527 0.451

(mctd, ta, ma, and, g, cz)

P (kbar) T (~ x (CO2) 2.0 461 0.600 5.0 525 0.295

y (amph) 0.125 0.155

y (chl) 0.567 0.606

(mctd, ma, and, g, cz, cc)

P (kbar) T (~ x (CO2) 2.0 463 0.63 5.0 536 0.45

y (amph) 0.127 0.161

y (ta) 0.0845 0.123

y (chl) 0.571 0.614

(mctd, amph, ta, g, cz, co)

P (kbar) T (~ x (CO2) 2.0 413 0.735 5.0 514 0.608

y (chl) 0.775 0.721

(amph, mctd, ta, and, g, cz)

P (kbar) r (~162 x (CO2) 2.0 410 0.555 5.0 rnetastable

y (chl) 0.757

(amph, mctd, ta, and, g, ma)

P (kbar) T (~ x (CO2) 2.0 metastable 5.0 511 0.257

y (chl)

0.649

(amph, mctd, ta, and, g, an)

P (kbar) T (~ x (CO2) 2.0 metastable 5.0 485 0.185

y (chl)

0.700

CaMSCH

m' (mctd, clin, an, ma, and, g, cz)

P (kbar) T (~ x (CO2) 2.0 463 0.59 5.0 503 0.128

T(~ 5O0

4 9 0

4 8 0

470

460

4 5 0

4 4 0

430

420

410

400

CaFMASCH

a t f i x e d P = 2 k b a r

+q + C C

,m3 ',\ _ / \

/

m2

11,

m6

0 .8 0 .9

x(CO2)

c h~h~t d m/~ ~ct fdol

L i I 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Fig. 2. T-x(CO2) petrogenetic grid at 2 kbar as projected from quartz and calcite. The traverse is discussed in the text

359

x(CO2). Wollastonite is stable in the window but is restricted to fluid compositions of x(CO2)<0.005. Diopside is also stable in the temperature window but occurs only at high temperatures and very H20-rich fluids. The same is also true for grossular (gr), which enters into the phase relations at temperatures slightly less than 500~ and x(COz)<0.01. The phase relations among minerals

typical of ultramafic bulk rock compositions such as magnesite, chrysotile etc. are outside the scope of this study (i.e. in not involv- ing quartz) and will be covered separately.

The limiting reactions and stability fields for phases involving single end-members are now discussed; the stability of solid solu- tions depends on bulk composition as well as P-T-x(CO2) as dis-

360

cussed below. The reaction cz + C 0 2 = an + cc + Ha 0 has a slightly negative slope in the 4000-500 ~ C window and restricts all clinozo- isite-bearing assemblages to low x(COa) fluids for ferric iron-free clinozoisite and sodium-free plagioclase end-members. However, with an increase in pressure and/or the amount of ferric iron in clinozoisite, this reaction shifts to more COz-rich fluid composi- tions. The opposite effect is achieved when anorthite is diluted by sodium. For the phases not involving solid solutions in the HzO-rich part of Fig. 1, the topology of the grid presented here is identical with that of Rice (1983, Fig. 6). Our reactions, however, involve more H20-rich fluid compositions and/or higher tempera- tures. For instance, the clinozoisite-anorthite reaction is stable at x(COz)'S of 0.06-0.15 in Rice's diagram but is a more HaO-rich fluid compositions, never exceeding 0.06, in our diagram. Ob- viously, this discrepancy is caused by the use of different thermody- namic data, primarily those relating to non-ideal mixing of HaO-- CO2 in the fluid.

Without calcite, the upper stability limit of margarite is deter- mined by the reaction ma + q = and+ an + H z O running through the CaFMASCH invariant points 1 and 3 for most x(COa)-rich fluids, by the reactions ma + CO z = and+ ce + HEO. Since margarite is often restricted to calcareous rocks, it is more relevant to consider that margarite + calcite-bearing assemblages are confined to tem- peratures below 410 ~ C over a wide range of fluid compositions. This is in agreement with previous studies (e.g., Bucher-Nurminen et al. 1983). In calcite-bearing rocks the stability of andalusite is restricted to very COa-rich fluids and low temperatures.

In order to look at the stability of assemblages involving min- erals which are complex solid solutions, it is important to determine the changes in mineral composition along the reaction lines, for example on Figs. 1 and 2. The next section focusses on an impor- tant reaction for metabasie mineral equilibria at greenschist facies conditions to illustrate this. Firstly, the method of calculation of the equilibria is outlined.

The reaction chlorite + calcite + C O 2

= amphibole + anorthite + dolomite + quartz + H 2 0

M i n e r a l equ i l ibr ia c a l cu l a t i ons

To illustrate the way in which the mineral equilibria are calculated, we examine the process for the C a F M A S C H T-x(CO2) univariant reaction:

ehl + ec + C 0 2 = a m p h + do l + an + q + H 2 0

which connects the CaMASCH invariant point m 3 with its CaFASCH equivalent f 3 , Figs. 1 and 2. For this reac- tion, at constant x(CO2) and pressure, we have as variables temperature, x and y in chlorite, x in dolomite, and x and y in amphibole (with x meaning Xv~=Fe/ (Fe+Mg) and y meaning Tschermak's substitution). Thus there are 6 variables to consider, and these can be calculated by solv- ing the set of simultaneous equations provided by the equi- librium relationships for any independent set of reactions between the mineral end-members in these phases. For each reaction, the relationship 0 = A G ~ RT In K must be sat- isfied, with AG ~ being just a function of pressure and tem- perature; In K is a function of x(COz) [with x(HaO)= 1 - x(CO2)], x and y in chlorite, x in dolomite, and x and y in amphibole. The activities of the end-members of the minerals in these equations can be expressed in terms of the x's and y's for ideal mixing on sites, see Table 3. Non- ideal mixing in C 0 2 - H 2 0 fluids is accounted for using the approach of Powell and Holland (1985). All the calcula- tions were undertaken using the computer program, T H E R M O C A L C (Powell and Holland 1988).

Table 3. Activity-composition relationships for the phases which are solid solutions. In amphiboles, since we impose a vacant A-site and no cations other than Ca in the M4-site, Xv,~,A = 1 and Xca, M4 = 1 and these sites are therefore not involved in the following activity expressions. Likewise, we impose Xc,,m = 1 in dolomites. In amphi- boles, chlorites and talcs we assume that only the M2 sites are capable of accepting A1. Garnets are assumed to be almandine- grossular solid solutions. For each mineral group x is defined as the ratio Fe/(Fe + Mg)

Amphiboles A(M4)2(M13)3(M2)2(TI)4(T2)4Oaa(OH)2 :(y = XAt,M2)

Tremolite a(tr) = (XMg,M 13) 3 (XMg.M2) 2 (XSi,T1) 4" = 1/16 (l-x) 5 (l-y) a (2-y) 4

Ferro-tremolite a(ftr) =(xF~,m3) 3 (XF.,M2) z (XsI,T1) 4 = 1/16 X 5 (l-y) 2 (2-y) 4

Hornblende a(hb) = 37.93 (XMg,m3) 3 (XMg'Ma) (XAl,r~z) (Xsi,T1) 3 (XAI,T1)

= 64/27 (l-x) # (l-y)y2 (2_y)3

Chlorites (M1) , . (M2)2(T1)2(T2)zOIo(OH)s : ( y= xAI.Mz)

Clinochlore a(elin) = 16 (XMg,M1) 4 (XMg,M2) (XAI,M2) (Xsi,T J.) (XAI,T1)

= 16 (l-x) s (l-y) 2 y2

Daphnite a(daph) = 16 (XFe,~I) 4 (XVe.Me) (XAI,MZ) (Xsi,T0 (xnl,~l)

= 16 x 5 (l-y) 2 y2

Amesite a(ames) = (XMg,m) 4 (XAI,Ma) z (XAI,T1) 2 = (l-x) 4 y4

Talc (M1)2M2(T 1)2(T2)201 o(OH)2 : (.v = XA1.M2)

talc a(ta) = (XMg,M1) 2 (XMg,M2) (Xsi,Ti) 2 = 1/4 (l-x) 3 (l-y) (2-y) 2

ferro-talc a(fta) = (XF~,m) 2 (XFe,M2) (XsI,T1) 2 = 1/4 X 3 (l-y) (2-y) 2

Tschermak's talc a(tats) =4 (XMg,M02 (XAI,M2) (XsI.T~) (XA1,T0

= (l_x)Z yZ (2-y)

Chloritoid �89 T20lo(OH)~}

Mg-chloritoid a(mctd) = XMg,m = (l-x) Fe-chloritoid a(fctd) = XF~,M 1 = X

Dolomite M2MI(CO3)z

dolomite a(dol) = XMg,m = (l-x) Fe-dolomite a(fdol) = XF~,Ul = X

Garnet (MI)3(M2)z T3 O12 :(z = XFe,M1)

almandine a(alm) = ( xv~,m ) 3 = z 3 grossular a(gr) = (Xca.M1) 3 = (t-z) 3

P s e u d o s e c t i o n s

This reaction, referred to above, is focussed on in Fig. 3, in projection from calcite and quartz; on Fig. 3 a, qualita- tive compatibility triangles, using a projection from calcite, quartz and fluid onto AFM, show the change in mineral assemblages across the reaction line. The way the XFe of the minerals change along the reaction line in Fig. 3 a is shown in Fig. 3 b. These mineral composition changes can be thought of in terms of the quadrilateral composition volume, defined by the compositions of the phases involved in the reaction, swinging through the A F M projection from Fe-rich towards Mg-rich mineral compositions, with in- creasing temperature along the reaction line.

T (~

a

+q +CC

m3 amph an..__..__... M

chllE

r chl amp

dol

dol amph do1

chl

a x(CO 2) D

~ N ~ ph

dol

0 X b Fe

Fig. 3. a The reaction ehl + cc + CO~ = amph + dol + an + H2 0 ex- tending from the CaFASCH invariant point, f3, to the CaMASCH invariant point, m3, with the qualitative AFM diagrams showing the tie line changes across the reaction, b Variations in xve along this reaction

An informative way of visualising the dependence of mineral assemblage on bulk composition, as implied by the composition volume of reaction changing with P-T-x(CO2), is to use T-x(CO2) pseudosections (eg. Hensen 1971). These are Y-x(CO2) diagrams drawn for just one bulk composi- tion, in contrast to T-x(COz) diagrams like Figs. 1 and 2 which, being projections, include phase relationships for all bulk compositions in the system. Figure 4 shows a calcu- lated P-T-x pseudosection for the bulk compositions indi- cated on Fig. 3 a. On such diagrams, the divariant (three phase) fields have the same shape as the corresponding reac- tions in CaMASCH and CaFASCH, and lie in an interme- diate position between them, controlled by the Fe/Mg ratio of the bulk composition. The amphibole-chlorite-dolomite

T (~

460

450

440

430

420

0.0

361

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

x(CO 2) Fig. 4. Pseudosections for a bulk composition of intermediate Fe/ Mg, and of an alumina content lying between amphibole and chlo- rite in AFM, in CaFMASCH projected from quartz and calcite

divariant field is sub-paralM to the CaFMASCH reaction dol+ chl= amph + ta (+ q, + ce); it narrows towards lower temperatures, and finally terminates in a chlorite field. This occurs because the composition of the chlorite involved in this reaction becomes progressively less aluminous with de- creasing temperature, until the chlorite one-phase field in AFM expands across the chosen bulk composition; thus a quadrivariant chlorite field occurs at lower temperatures on the T-x(CO2) pseudo-section. This feature is in agree- ment with certain natural low-pressure assemblages in which, at low temperatures, chlorite is stable without am- phibole, dolomite or talc. The very narrow divariant fields are a reflection of the near collinearity of amphibole + chlo- rite + dolomite + anorthite in CaFMASCH, in projection from calcite, quartz and fluid.

Internal and external buffering paths

Having drawn pseudosections, the diagrams can only be used to understand metamorphic rocks if possible paths can be envisaged. The limiting situations involve internal buffering, in which the mineral assemblage controls its fluid composition, and external buffering, in which the fluid com- position is externally controlled, for example by infiltration processes. External buffering is straightforward to visualise because, in the extreme case, a vertical path will be followed on a T-x(CO2) diagram, at the x(CO2) of the infiltrating fluid. Any fluid required for reaction, or any fluid generated by reaction, is dominated by the infiltrating fluid.

For internal buffering, an assemblage starting within

362

T(~ 53O

520

510

5OO

490 -

4 8 0 - -

ampr g

CZ

i

t f17

/ 1

m3

ml

~ "/~., / Y / V/_/~_ I;~

f13 f 1 4

m8

J 1 I

0..1 0.2 0.3

11

0.4

at f i x e d

C a F M A S C H

ma

Fig. 5. T-x(CO2) diagram for CaFMASCH ( + calcite + quartz) at 5 kbar. The traverse is discussed in the text

f12

P = 5 k b a r

+q

+ c o

f l l

0.5

x(CO2)

363

a divariant field stays within it until it reaches a univariant reaction (or until it reaches a maximun on the divariant field). This is because any reaction involved in crossing a divariant field inevitably changes the fluid composition (un- less near a maximum). Therefore internal buffering paths within divariant fields tend to parallel their boundaries. Clearly trivial reaction will be involved in "passage up" a divariant field. In contrast, substantial reaction is involved in passage along a univariant reaction, with the composi- tions of the minerals changing along the reaction. Depar- ture from the reaction into the appropriate divariant field occurs when a phase involved in the reaction is consumed, as a consequence of the compositional volume of reaction swinging through the bulk composition of the mineral as- semblage. For example, consider a rock with bulk rock composition " A " in Fig. 3 b, in terms of the pseudosection Fig. 4. Assume that rock A has the starting assemblage amphibole-chlorite-anorthite (+ quartz, + calcite). During prograde metamorphism the internal buffering path will stay within the amphibole-chlorite-anorthite divariant field and will be subparallel to its boundaries. Trivial fluid-pro- ducing reaction takes place within the divariant field until the assemblage hits the reaction chl + cc + COz = amph + do t+an+q+H~O at about 455~ and x(CO2)=0.60. There, substantial reaction will occur while the assemblage follows the reaction line until one phase which is chlorite (or calcite or quartz) is reacted out and thus requires an internally buffered path to follow up into the high-T, high- x(CO2) amphibole-dolomite-anorthite divariant field.

Given the fact that the common assemblage quartz-pla- gioclase-actinolite-chlorite-clinozoisite/epidote-calcite is the typical assemblage of greenschist facies metabasites, it is indeed surprising that the divariant field of this assemblage is restricted to a very narrow field. Even taking into account the addition of Na20, primarily into plagioclase, and F%O3 primarily into epidote, the field is narrow. For external buffering this assemblage would occur only as a very nar- row section of a prograde sequence, corresponding to the narrow temperature width of the field, but for internal buf- fering, together with the fact that the divariant field amphi- bole-chlorite-anorthite (+ quartz, + calcite) extends from low x(CO2)'s to 0.60, the widespread occurrence of this assemblage is accounted for easily.

At this stage, it is important to note that for many reactions there is a marked difference between the externally and internally buffered reaction on T-x(CO2) and P-x(CO2) projections. Externally and internally buffered reactions may correspond to each other but this is not necessarily always the case. For example, consider the CaFMASCH reaction ch l+g=amph+cz (+q, +cc) emanating from in- variant point 10 on Fig. 5. This reaction, as written on the diagram, is the reaction given by the compositions of the phases on a compatibility diagram for any position along the reaction line; it is also obtained from Schreinemaker's rule at invariant point 10. Quite obviously, this reaction seems to be very odd as it consumes garnet instead of pro- ducing it, with increasing temperature. Whereas this is the reaction a rock will experience along a vertical, i.e. exter- nally buffered path on the T-x(CO2) diagram, the inter- nally-buffered reaction is rather different. This is because the internally-buffered reaction is controlled by the chang- ing compositions of the phases along the reaction line. This reaction may be obtained from the out-phases of the reac- tions making up the sub-system invariant point at which

the full system reaction terminates (here the CaFASCH in- variant point f17). This leads to the more plausible inter- nally-buffered reaction amph+chl+cz=g. The reaction considered in the first part of this section is an example of one for which the two ways of writing the reaction are the same.

C a F M A S C H + quartz + calcite + ( H z O - COz) fluid

Having established in the last section the importance of thinking about T-x(CO2) diagrams like Figs. 1 and 2 in terms ofpseudosections, it is possible to draw compatibility diagrams for the fields in a T-x(CO2) diagram, as long as it is realised that it is the topology that is being represented, not the actual mineral compositions, given that these will change continuously over each field in a T-x(CO2) diagram. On this basis, the compatibility triangles along a traverse as indicated on Fig. 2 are presented in Fig. 6. Some aspects of these are discussed in the remainder of this section.

The nature of the change in composition of amphibole and talc with P-T-x(C02) at 2 kbar

These compatibility diagrams can be useful for examining the stabilisation and compositional changes of amphibole, talc and dolomite as a function of temperature (and x(CO2)). Starting in field I in Fig. 2 and following traverse I (fields 1-11) shows the changes in compositional range of amphibole with increasing temperature. Field 1 lies below the stability field of amphibole. When the CaFASCH reac- tion fdol + an + q + HzO = chl + cc + C02 is crossed, F e - A1 amphibole appears in the compatibility triangle (field 2). Hence, the first amphibole to appear is not a member of the alumina-free tremolite-actinolite series, Ca2(Mg, Fe)sSisO22(OH)2, but a ferro-aluminous amphibole. Pro- gressively, with increase in temperature, an AlzO3-free Fe- end-member amphibole becomes stable (field 3), then an A 1 - M g amphibole (field 9) and finally the pure tremolite- endmember (field 10). By this stage amphibole is stable over the whole lower part of AFM.

The progressive changes in the composition of talc can also be documented by following this traverse. The first talc to appear has a Fe-free, aluminous composition (field 5), and with temperature increase the pure Mg-end- member talc becomes stable (field 7). The now-established compositional range of talc remains stable in field 8 until in field 9 only the Mg-endmember talc is still stable before it disappears and breaks down to tremolite (field 10).

At low temperatures the full compositional range of dolomite is stable (fields 1 and 2) until Fe-dolomite breaks down to Fe-amphibole (field 3). By crossing from field 6 into field 7 Mg-dolomite gives way to Mg-talc until, finally, F e - M g dolomite disappears to form F e - M g amphibole and talc (field 7 to field 8).

The aluminium-content o f phases in siliceous dolomites at 2 kbar

Siliceous dolomites are commonly represented in the system C a t - MgO - Sit2 - CO2 - H20. FeO and A1203 are nor- mally considered to be present in only small amounts (e.g., Winkler 1979). Our T-x(CO2) invariant point m' is located in this CaMSCH system and controls the geometry of the low-grade reactions in siliceous dolomites. Adding iron to

364

~h

v,o dol M~O FeO dol MgO

(7) ~ (8) ~

+cc+q ~ ~ +cc+q

�9 1 ~ 1

FeO d o l MgO FeO MgO

ph ph [ ph

F~o dol UgO F~o dol u~o ~o MgO FeO MgO

1 1

wo d o l MgO FeO d o l MgO VeO

ph

MgO

Fig. 6. AFM compatibility diagrams in CaFMASCH ( + calcite + quartz) at 2 kbar for the fields labelled 1-11 in Fig. 2

the system shifts the reactions around m' to higher tempera- tures and stabilizes the CaFMSCH reaction d o l + c c + q + H20 = A1203-free amph + A1203-free ta + CO2, which runs from m' to lower temperatures and lower x(CO2). The ge- ometry of these reactions restricts the stability field of talc in the presence of quartz and calcite to a very narrow tem- perature range of less than 20 ~ C, Fig. 2. The introduction of A1203 into the system leads to a reaction leading away from rn' towards rn2, ta + cc = amph + dol, along which A1203 increases in the talc and tremolite until these minerals are saturated with A1203 and chlorite is stabilised. Emanat- ing from m2 are the chlorite-bearing equivalent reactions in CaMASCH and CaFMASCH to ones in CaMSCH and CaFMSCH around m'. These CaMASCH reactions lie at slightly lower temperatures (less than 5 ~ C) than the CaMSCH reactions and, at very low or very high x(CO2), they beome asymptotic to each other. This is because the phases involved in these CaMASCH reactions are Al-poor and are most aluminous at the invariant point m2. There- fore, along these reactions, the phases becomes less and less aluminous (compared to the relevant reactions emanat- ing from m2 in Fig. 8 b, c) and come close to and finally asymptotic to the CaMSCH reactions. The same is true for the CaFMASCH reaction c h l + d o l + c c + q + H 2 0 = amph + ta + CO2 which becomes asymptotic to the CaFMSCH dol + cc + q + H20 = A1203-free amph + A1203- free ta + CO2 and the CaFSCH f dol + f t a + q = f i r + CO2 re- actions. From this, it is clear that the first talc to appear with increasing grade is one with a low but significant alu- mina content.

Dependence on pressure

At intermediate pressures, Fig. 5, the topology of the grid in projection from quartz and calcite changes drastically compared to the 2 kbar projection. The clinozoisite field is enhanced, the margarite and chlorite stability fields are reduced and almandine-rich garnet comes in at low x(CO2)'s. The clinozoisite-anorthite-grossular-calcite invariant point lies outside of the temperature window drawn and occurs at 630 ~ C and an x(CO2) of 0.08. The CaFMASCH reaction emanating from the 2 kbar invariant point j~ now intersects the rn3-f3 join and stabilises a new CaFMASCH invariant point, 11. This point, together with the invariant points 9 and 10, is involved in the mineral equilibria in the transition from greenschist to amphibolite facies. Qualitative compatibility diagrams along the traverse on Fig. 5 are shown on Fig. 7; they show the change in mineral assemblages around these invariant points.

Because the mineral compositions at these invariant points are very ferrous (see Table 2), and become more ferrous along all the CaFMASCH reactions towards lower temperatures, metabasic rocks with an average bulk compo- sition (as in Fig. 4) will n o t " see" all of the possible mineral assemblages. Only along chlorite-out and garnet-out (ter- minated by m3) emanating from 11 are the tie-triangles swung through this bulk composition, stabilising the divar- iant and trivariant equilibria. Thus a pseudosection for the same bulk composition as used in Fig. 4 will be essentially identical to .Fig. 4, but with the equilibria displaced to higher x(CO2).

365

v0o dol Mgo

(2) A I 2 0 ~ ? n

+CO+

F~o dol Mgo

(7) AI2~

1

am ana l

~eo dol MgO FeO dol Mgo

(3) aI2~ an (4) '%~ an +CC +ce+

amp am

Feo dol Mgo veo dol ~go

am 1

FeO

am]

MgO FeO MgO

am]

FeO dol Mgo

(6) ~ 2 ~ an

auil

~o do/ ~go

Fig. 7. AFM compatibility diagrams in CaFMASCH (+cal- cite+quartz) at 5 kbar for the fields labelled 1-10 in Fig. 5

C a F M A S C H + quartz + ( n 2 0 - - C O 2 ) fluid

It is more difficult to portray the stability relationships for this system without calcite in excess, because compatibility diagrams would take the form of rather incomprehensible tetrahedra. Noting the importance of bulk composition, it is possible to make some general comments on mineral sta- bility. For example, the high-T, high-x(CO2) FMASCH re- action ctd= g + and+ chl+ Hz 0 ( + quartz) (Fig. 1) deter- mines the upper stability limit of chloritoid. At even higher temperatures and x(COz) chlorite breaks down (not shown in Fig. 1) by the FMASCH reaction chl=g+and+ta+ H20 (+quartz). Garnet first appears in CaFASCH with increasing temperature, along reactions running through g6 and J7 from low temperature and high x(CO2) to high tem- perature and x(CO2). Rocks do not seem to occur which might see the reactions around invariant point 3 in Fig. 1, controlling margarite and andalusite-dolomite stability. Moreover, the equilibria around invariant point 4, will only be sampled exceptionally, requiring relatively aluminous rocks to be carbonate-bearing and to have a high x(CO2) fluid.

Progressive changes in mineral chemistry with prograde metamorphism

The variations in xv~ along CaFMASCH reactions in min- erals subject to a FeMg-1 substitution are shown in Fig. 8 a. In most cases there are only minor changes in xve along the reactions. This is either because the mineral com- positions at the invariant points are already close to the Mg- or Fe-systems and some of the reactions emanating

from the invariant points are terminated in the CaFASCH or CaMASCH sub-systems of the respective systems (e.g., [margarite] from invariant point 2 is terminated by f2, [chlo- rite] and [andalusite] from 4 by f5 and f6) or XFe seems to be rather insensitive to changes in temperatures at these conditions (e.g. along [dolomite[ and [anorthite] emanating from invariant point 4). It is only in a few cases that rapid changes in XF~ take place along the CaFMASCH reac- tions (e.g. [chloritoid] from 4, along the trace of f7 in CaFMASCH and the CaFMASCH reaction connecting m3 and f3). In general, the calculated chlorite compositions are slightly more iron-rich than coexisting amphibole com- positions, as documented by natural chlorite-amphibole pairs (Laird 1980, Fig. 4a; 1988).

The extent of the Tschermak's substitution in amphibole and chlorite in CaFMASCH and Fe- and Mg-sub-system reactions is shown in Fig. 8b, c. Generally, the A12Os-con- tent in amphiboles increases slightly with increasing temper- ature but never becomes more hornblende-rich than about y = 1/5. Although no obvious trend is discernible, the Mg- end-member may accept slightly more alumina than the Fe-end-member. This would correspond to natural occur- rences of amphiboles in greenschists from Knapdale, Scot- land (Harte and Graham 1975), where the more aluminous amphiboles are more iron-rich. Talc is very Al-poor and its composition is always close to being AlzO3-free. Chlorite is more complicated: the Fe-end-member daphnite is more susceptible to Tschermak's-substitution than the Mg-end- member clinochlore, except for the high x(COz), low T in- variant points m5 and m6 which will only rarely be sampled in nature. This behaviour has been described from silicic volcanics and pelitic rocks (Stewart and Flohr 1984), and

366

T(~

480

f8 460 (

f7 f3

440

f6 f5

420 f2

400 Fe 0.8 0.6 0.4 0.2 Mg

XFe

480 - - CaFMASCH ~ 480 [~ ~ CaFMASCH ]~ - - C a F A S C H / ~ | [ l - CaFASCH/ ] |

CaMASCH ~\~/1 ] l CaMASCH / | \ \ \ / I / m4 r n 4 ~ . m3 m4

m3 460 m2 460 m2 __ ~ ' 7 m2

440 440

4 2 0 7 / / ; f l 420 nl~ 400 [ I I 400

0.0 0.05 0.i0 0.15 0.5 0.6 0.7 0.8 b XAI,M2 c XA1,M2

Fig. 8a-e. Mineral composition changes along reactions at 2 kbar. a A T-Xve diagram showing the change in xve in phases involving FeMg_ 1 solid solutions along the respective reactions. XFe values of i and 0 correspond to ferro-tremolite and daphnite and to tremolite and clinochlore, respectively. T'XA~.M2 diagrams for amphiboles (b) and chlorites (e). Heavy lines show the extent of the Tschermak's substitution in chlorites and amphiboles along CaFMASCH reactions, light lines denote compositional changes in the CaFASCH and CaMASCH systems. In b, XAI,M 2 of 0 corresponds to tremolite and ferro-tremolite, and 0.5 to hornblende and ferro-hornbtende; whereas, in e, an XA~.M2 of 0.5 corresponds to clinochlore and daphnite, and 1 to amesite and ferro-amesite. The arrows mark the positions where the corresponding reactions leave the T-x(COa) window of Fig. I

seems plausible for naturally-occurring chlorites in meta- basic rocks (Laird 1988, Fig. 2 and comment on p 430).

Down temperature, chlorites involved in the [dolomite], [amphibole] and CaFMASCH reaction ch l+do l+q= amph + ta + cc + H 2 0 + COz emanating from m2 become progressively Al-depleted with respect to the clinochlore end-member and have compositions towards Miyashiro and Shido's (1985) extreme end-members. The sub-alumi- nous composition of these chlorites may be an artefact of the ideal mixing assumption made, but sub-aluminous chlo- rites have been described in the literature (Laird 1988, and references therein).

Discussion

Qualitatively, the calculated phase relationships in CaFMASCH correspond to observations on siliceous dolo- mites and metabasic rocks. This correspondence is a vindi- cation of the internally consistent dataset approach used here, and to the quality of the dataset. For a detailed com- parison it is necessary to envisage extensions into the full system CaNKFMASCHTO (CaO-- Na20 - K20 - FeO-- M g O - A1203 - Si02 - C02 - H 2 0 - TiO2 - 02). The ex- tension into the K20-bearing system will little affect the grid presented here given that the minerals included involve small or negligible substitutions of K. With addition of a small amount of K20 to CaFMASCH, a K-rich phase is stabilised. As a consequence the grid can be considered as a projection from an unspecified K-rich phase: variously, biotite, muscovite or stilpnomelane. Whereas biotite and muscovite could be added to the grid now, inclusion of stilpnomelane is not yet possible owing to the lack of ther- modynamic data. As with KzO, the effect of adding T i O 2

to CaFMASCH should be small since TiO2 is incorporated in small or trivial amounts in the CaFMASCH minerals. Whenever sufficient TiO2 is present a Ti-rich phase will be stabilised, which will be sphene, or ilmenite under more oxidizing conditions. Therefore the grids presented can also be considered as a projection from a Ti-rich phase.

In contrast to K20 and TiO2, the effect of addition of Na20 and F%O3 to CaFMASCH is more profound because these elements enter CaFMASCH phases; F%O3 stabilises epidote, and to a lesser extent amphibole, and NazO stabilises plagioclase, and to a lesser extent amphi- bole. The calculation of mineral equilibria in the full system awaits the generation of thermodynamic data for the end- members of the appropriate phases in the full system, and the establishment of reliable activity-composition relation- ships in, for example, plagioclase and amphibole. However, qualitative to semi-quantitative extensions of the grids in Figs. 1 and 2 can be made.

The presence of sodium can lead to the stabilization of the white mice paragonite and will, at higher tempera- tures, introduce a small amount of the edenite end-member into amphibole, but the primary importance of Na20, how- ever, is in the stabilisation of plagioclase. The critical fea- ture of plagioclase solid solutions at the conditions of inter- est here is the peristerite gap, between albitic plagioclase (say an2, henceforth called albite) and oligoclase (say an2o). On addition of Na20 to CaFMASCH, univariant assem- blages occurring along reaction lines in Figs. 1 and 2 be- come divariant. The resulting T-x(CO2) fields are wide for reactions involving an, but will be much narrower for reac- tions not involving an, corresponding to the limited substi- tution of edenite in amphibole at these conditions. With continuing addition of Na20, plag-bearing divariant field will involve plag of decreasing proportions of an until the peristerite gap is encountered. Then the divariant fields will be terminated by NCaFMASCH univariant reactions in- volving ab and otig. Similarly, divariant fields correspond- ing to reactions not involving an will terminate at NCaFMASCH univariant reactions involving ab or olig. NCaFMASCH grids result which correspond to Figs. 1-2, each original an-bearing reaction involving ab and olig, dis- placed according to the anorthite activity in ab and olig at the peristerite gap, and each an-free reaction involving ab or olig displaced only trivially. So that semi-quantiative calculations could be undertaken, the anorthite activity

367

T (~

450

445

a

+q activity of ano~thite = 0.40 _ +CC

+ab i ~ / ~ ~

amph olig ta 7 ~ ' ~ Z Q

ch1s / o,V _

maph chl dol

olig

I I 0.4 0.5

x(CO 2)

+ q

+CC

olig

10'

10

m3'

chl ab'

11'

11

T(~

b x(CO2) Fig. 9. a Calculated univariant reactions in NCaFMASCH around intersection 12 for an activity of anorthite of 0.4 at 2 kbars, consid- ered to be appropriate for albite + oligoclase across the peristerite gap (see text), and no edenite substitution in amphibole, b A pseu- dosection for the same bulk composition as used for Fig. 4

(aan) at the peristerite gap is taken to be 0.40, corresponding to 7,n=2 for olig (xa,= 0.2) and 7an=20 for ab (Xa.=0.02).

Some phase relationships for a,n = 0.40 for NCaFMASCH in projection from albite, calcite and quartz are shown in Fig. 9. The CaFMASCH reaction c h l = a m p h + d o l + a n (+ cc + q) becomes chl = amph + dol + olig ( + ab + cc + q) in NCaFMASCH, displaced to lower temperature and x(CO2). The CaFMASCH reaction chl + dol= amph + ta, in not involving an, is trivially displaced before the appearance of ab, to balance the small substitution of edenite in the amphibole. The relative displacement of these reactions re- sults in a new invariant point, 12, Fig. 9. The phases are already extremely magnesian at the invariant point (see Ta- ble 2) and become more iron-rich only along the talc-out and oligoclase-out reactions. Therefore, just as in the case of the 5 kbar pseudosection, the majority of metabasic rocks will only "see" divariant assemblages emanating from these two reactions. The topology of the pseudosec- tion, Fig. 9b, is identical to the one presented in Fig. 4,

, - - CaFMASCH

NCaMASCH

r b ~ x(CO2) NCaFMASCH

Fig. 10. Qualitative T-x(CO2) diagram showing the phase relation- ships in NCaFMASCH at 5 kbar in the H20-rich part of the dia- gram

but with projection from quartz, calcite and albite, and the plagioclase is oligoclase instead of anorthite. Again, chlorite becomes less aluminous than an average metaba- saltic bulk composition and a quadrivariant chlorite field is established at low temperatures and low x(CO2)'s.

A qualitative T-x(CO2) diagram to illustrate the conse- quences of the addition of Na20 to CaFMASCH is given in Fig. 10 for the mineral equilibria around invariant points 9, lO and 11 in Fig. 5. The addition of Na20 leads to the stabilisation of albite, with the plagioclase becoming more albitic along the reactions leading from invariant points 9 to 9', 10 to 10' and l l to 11'. The degenerate reaction c z + a b = p l a g + cc will be at a lower temperature and x(CO2) than for the reaction cz = an + cc, but probably not much displaced from it. Moreover, the other reactions around 2' will be little displaced from their equivalents around 9', 10' and 11' because the proportion of plagioclase involved in these reactions is small owing to the approxi- mate coplanarity of the mafic minerals in CaFMASCH.

Adding ferric iron to the system will stabilize epidote and, to a lesser extent, amphibole. Although it is feasible to sketch the consequences of adding ferric iron to, for example, Figs. 9 and 10, the possibilities multiply. At this

368

stage we prefer to note that these two Figures are topologi- cally appropriate for epidote in excess, with the min imum epidote end-member content of the epidote being progres- sively larger for higher x(CO2) beyond cz+ab=plag+ cc. At some ferric iron content, when epidote becomes satu- rated, magnetite is stabilised. At this stage, a net of magne- tite-involving reactions, paralleling the reactions in Figs. 9 and 10, occur. We are not yet in a position to calculate these equilibria but we think that the reactions may be at low to intermediate x(CO2) as magnetite-bearing meta- basic assemblages occur commonly. Further work and re- finement, especially with respect to the activity-composition relationships in plagioclase and epidote is required in order to quantify the exact P-T-x locations of these controlling reactions.

Acknowledgements. We thank J. Connolly for a helpful review and J. Baker for a valuable discovery. This paper is testament to the conversion of an experimental structural geologist, and some good timing in the generation of the appropriate thermodynamic data. TMW acknowledges financial support from the Studienstiftung des Deutsehen Volkes. Cambridge Earth Science contribution no. 1386.

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Received February 16, 1989 / Accepted October 10, 1989 Editorial responsibility: V. Trommsdorff