halogen-rich scapolite and biotite: implications for metamorphic fluid-rock … · 2007. 8. 29. ·...

American Mineralogist, Volume 74, pages 721-737, 1989

Halogen-rich scapolite and biotite: Implications formetamorphic fluid-rock interaction

Cr,auor.l I. Mona.r* JonN W. V,qlr,nyDepartment of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706, U.S.A.

Ansrnacr

Abundant Cl-rich scapolite and biotite indicate high Cl activities during greenschist-through amphibolite-facies regional metamorphism of metasedimentary rocks northwestof the Idaho batholith. Biotite-zone granofels contain up to 40 modal percent Cl-richscapolite. The strict stratigraphic control on the occurrence ofscapolite and the fine-scaleinterlayering of Cl-rich and Cl-poor metasedimentary rocks suggest that scapolite wasformed during regional metamorphism by reaction of plagioclase with calcite and halite.Mass-balance constraints on the formation of scapolite show that, prior to metamorphism,sedimentary units containing l0 modal percent halite along restricted sedimentary hori-zons, 0.5 to 20 cm thick, would have been sufficient to form the most scapolite-rich layers.The abundance ofCl-rich phases decreases abruptly with the appearance ofzoisite-bearingassemblages in the metasedimentary rocks, suggesting infiltration of low-Cl HrO at highergrades. Scapolite compositions range from 32 to 62 equivalent anorthite (EqAn) and 0.7 6to 0.06 Xo. There is a general increase in EqAn and decrease in X., with increasingmetamorphic grade. However, local variations in scapolite composition suggest that meta-morphic fluid compositions were heterogeneous and internally bufered. Biotites coexistingwith scapolite contain up to 0.51 wto/o Cl (biotite grade) and 2.58 wto/o F (kyanite +sillimanite grade). Biotite compositions indicate significant differences inlogffir/fr.:) val-ues of fluids in equilibrium with low-grade, biotite- and carbonate-rich granofels. Differ-ences in the inferred metamorphic fluid compositions are observed, in some cases, downt o a s c a l e o f < l c m .

Cl is strongly partitioned into an aqueous fluid relative to solid phases and is removedby metamorphic devolatilization and infiltration by dilute fluids. Therefore, the occurrenceof bufered activity gradients in Cl, as indicated by scapolite and biotite compositions,suggests that metamorphic fluid flow was highly channelized and aqueous fluid-rock in-teraction was limited. High and variable Cl activities were not smoothed out by aqueousfluid-rock interaction through two regional metamorphic events.

INrnonucrroN

Several recent studies have emphasized the role of fluidsas active agents of prograde regional metamorphism. Fluidflow during metamorphism may drive mineral reactions,enhance reaction rates and facilitate transport ofelementsand heat (Etheridge et al., 1983, 1984;Ferry, 1984;Wal-ther and Wood, 1984, 1986). Metamorphic fluid-rockinteraction can be monitored by mineral reaction prog-ress or stable-isotope data (Rumble et al., 1982; Ferry,1983, 1986;Graham et a l . , 1983;Tracy er a l . , 1983;Na-belek et al., 1984; Valley and O'Neil, 1984; Bebour andCarlson, 1986; Valley, 1986). Fluid-rock ratios thus cal-culated are time-averaged, minimum estimates of theamount offluid that has passed through a rock.

Gradients in the activities of species-such as Na*, Kt,F , or Cl -that are readily exchangeable with externally

* Present address: Department of Geological Sciences, Uni-versity of Tennessee, Knoxville, Tennessee 37996, U.S.A.

0003404x/89/07084721$02.00 721

derived fluids may further constrain metamorphic fluid-rock ratios (Wood and Walther, 1986). Cl is strongly par-titioned into aqueous fluids relative to solid phases (Mu-noz and Swenson, l98l; Volfinger et al., 1985). There-fore, minerals that concentrate Cl, such as scapolite,biotite, apatite, and amphibole, may be particularly sen-sitive indicators of fluid-rock interaction. In certain cir-cumstances, gradients in Cl activity inferred from Cl-richmicas or scapolite require low metamorphic fluid-rockratios and may thus constrain the maximum amount ofaqueous fluid added to a rock by metamorphic devola-tilization or infiltration. Water-rock ratios greater than0.01-0. I may smooth out Cl activity gradients in meta-morphic rocks (Wood and Walther, 1986). Additionally,these measurements help to delineate metamorphic fluidpathways and to define the scale ofpervasive fluid flow.

Few papers have examined Cl chemistry as an indica-tor of metamorphic fluid composition and fluid-rock in-teraction (Rich, 1979; Yardley, 1985; Oliver and Wall,1987; Sisson, 1987), and no detailed study ofCl activity

722

Fig. l. (a) Generalized geologic map of biotite- and garnet-zone Wallace Formation metasedimentary rocks north ofClarkia, Idaho (after Hietanen, 1967). Sample locations are cross-referenced in Tables 3 and 4. (b) Locations of metamorphic iso-grads (Hietanen, 1984) and higher-grade samples. WF : Wal-lace Formation (upper Belt Group), PF : Prichard Formation(ower Belt Group), BBF : Boehls Butte Formation (pre-Belt),IB : Idaho batholith.

gradients over a range of regional metamorphic pressuresand temperatures has been made. In this paper, we pre-sent compositional data for Cl-bearing scapolites andbiotites from low-variance, greenschist- through amphib-olite-facies metasedimentary rocks in the Proterozoic BeltSupergroup in northern Idaho. Moderate to high fluid-rock ratios coincident with regional metamorphism ofthese rocks have been inferred from whole-rock oxygen-isotope data (Criss et a1., 1984; Fleck and Criss, 1985;Criss and Fleck, 1987). However, oxygen-isotope valuesfor mineral separates indicate that isotopic depletions inthese scapolite-bearing rocks are the result of fluid-rockinteraction after the peak of regional metamorphism(Mora and Valley, 1988). In this study, we use experi-mentally determined mineral-fluid equilibria to estimatethe activities of NaCl, HCl, and HF in the metamorphicfluids. In particular, we establish that high, locally con-trolled Cl activities existed in the fluid compositions, re-quiring low metamorphic fluid-rock ratios duringgreenschist-facies regional metamorphism.

MORA AND VALLEY: HALOGEN-RICH SCAPOLITE AND BIOTITE

Gnor.ocrc sETTING

Scapolite-bearing rocks form part of a thick progrademetamorphic sequence exposed in the northwestern bor-der zone of the Idaho batholith (Figs. la, 1b). This meta-sedimentary sequence has been correlated with the low-ermost formations of the Proterozoic Belt Supergroup(Hietanen, 1963; Reid et al., 1981; Harrison et al., l98l).Two episodes of metamorphism have affected the rocks.The first episode, Ml, was syntectonic and resulted in theformation of a primary penetrative fabric. Equilibriummineral assemblages and the mapped isograds (Fig. lb)are attributed to M2 (Hietanen, 1984 I-ang and Rice,1985). Some flattening of micas around M2 porphyro-blasts is seen. M2 isograds increase in grade to the south,approximately concentric to the northern Idaho batholith(Fig. lb). Local development of crenulated foliation andlow-pressure, high-temperature mineral assemblages istentatively correlated with post-M2 isothermal de-compression associated with rapid uplift of the batholith(Grover et al., 1987; Ruendal and Rice, 1987). Peak M2pressures and temperatures in the study area range from480'C and 4 kbar in the biotite zone to 590 "C and 5kbar in the kyanite-sillimanite zone (Tomten and Bow-man, 1984; Mora, 1988).

In Belt Supergroup metasedimentary rocks 50 km eastofthe study area,Lang and Rice (1985) have identified arelict Ml staurolite isograd defined by pseudomorphs ofmuscovite after Ml staurolite. Hietanen (1963) also re-ported pseudomorphs after an orthorhombic mineral,possibly andalusite, occurring west of Clarkia, Idaho (Fig.lb). The lack of pseudomorphs north of the relict isogradsuggests that Ml metamorphism also increased in gradeto the south. The major focus of this study is rocks ex-posed in the biotite and garnet zones, north of Clarkia.These rocks consist predominantly of weakly foliated,thin-bedded quartzites and carbonate-bearing biotitegranofels. Sedimentary features such as mudcracks, rip-ples, cross-bedding, and channels are well-preserved inbiotite-zone rock types, and no muscovite pseudomorphshave been observed. These features suggest that the M2metamorphism was the highest-grade episode to have af-fected these rocks.

The timing of metamorphism is poorly constrained.Lang and Rice (1985) demonstrated that isograds east ofthe study area postdate thrust faulting inferred to be > 100m.y. (Harrison et al., l98l). The distribution of the M2isograds suggests that regional metamorphism may betemporally related to Cretaceous to early Tertiary em-placement of the Idaho batholith. Rb/Sr and K./Ar iso-topic studies of the batholith suggest nearly continuousigneous activity between 80 and 40 Ma, with early Ter-tiary rapid uplift (Armstrong, 1975, 1976; Criss et al.,1982; Criss and Taylor, 1983; Fleck and Criss, 1985; Crissand Fleck, 1987). Abundant kyanite in the progrademetamorphic sequence and the inferred early Tertiaryrapid uplift of the area are consistent with peak M2 meta-morphism during the earliest phase of batholith forma-tron.

i ;?s\N

WALLACE FM.

bi-corb gronofels

fh in-bedded quorfz i lei nc l . b i -mu g rono fe l s

g ronu lo r quo r l z i t e

'$''on'oo t "r--..'

ANLr-yltc,Ll, pRocEDURES

Quantitative chemical analyses of scapolite and biotite(Table 1) were made on the ARL-sEMe electron micro-probe at the University of Wisconsin, Madison. Mineralswere analyzed by wavelength-dispersive analysis using anaccelerating potential of 15 kV and a sample current of15 nA. F and Cl in biotite were analyzed at l0 kV, withthe beam slightly defocused (10 pm). Volatilization checkswere made for four times the counting time for Na, F,and Cl; no volatilization occurred. Natural, crystallinestandards were used for all elements except F, for whichsynthetic fluorphlogopite was used. Matrix correctionswere made using a modified Bence-Albee scheme (1968)or aZAF correction program (eueeon vrr; Rucklidge andGasparrini, 1969). For all elements except F and Cl, counttimes varied from l0 to 20 s per spot, with a total of 5to l0 spots analyzed per grain. Analyzed minerals arecompositionally homogeneous.

Because of their importance to this study, special carewas taken to increase the reliability of the F and Cl anal-yses. For each sample, a minimum of 10000 total countswas collected. Count time per spot was 30 to 60 s and l0to 60 spots were analyzed. Low concentrations of F andCl in some biotites required extremely long total counttimes of from 360 to 3600 s in order to accumulate thenecessary counts. Under these operating conditions, theminimum detection limit is 0.02 wto/o for Cl and 0.06wt0/o for F. For sample 84MC147, with 0.05 wto/o Cl, thepeak values were 6.9 standard deviations above back-ground (l s.d. : 1,4N;, where N : number of counts)with a peak/background ratio of 1.6. Sample 84FW70,with 0.44 wto/o F, had apeaVbackground ratio of 3.2 withthe peak value 21.7 standard deviations above back-ground. As these values represent the minimum amountofF and Cl reported in this study, it is concluded that allthe reported analyses are statistically significant.

Analyzed scapolites contain no detectable F and littleor no SOo (Mora, 1988). Scapolite was normalized to l6total cations. Normalization to 16 cations reduces theeffect ofreasonable analytical errors for Si and Al, whichis especially important for low-grade scapolites that con-tain numerous inclusions of quafiz. For the same reason,CO. in scapolite was calculated by difference (CO, : I -Cl - SO.) rather than charge balance. The sum of Ca *Na * K in scapolite averages 3.93 + 0.06 (2o), differingby less than 2o/o from the ideal formula (n : 4).Scapolitecompositions are reported as "equivalent anorthite"(EqAn, where EqAn : 100 x (Al - 3)/3 where Al iscalculated on the basis of 16 cations; Orville, 1975) andthe mole fraction of Cl [X",, where X. : Cll(Cl + CO3)].The analytical error is +3 EqAn.

Scnpolrrp rN THE W,q.LLacn FonvrattoN

Mineral assemblages

Geologic maps and a detailed description of scapolite-bearing rocks of the Wallace Formation are given by Hie-tanen (1967). In the biotite and garnet zones north of

1 )1

Clarkia (Fig. la), scapolite occurs in layers I cm to severalmeters thick composed of biotite-rich granofels, interbed-ded with fine-grained, thin-bedded biotite quartzite. Twotypes ofbiotite granofels are distinguished: a biotite-car-bonate granofels, containing 30-500/o calcite + dolomiteand no muscovite, and a biotite-muscovite granofels,containing 0-25o/o carbonate and up to 20lo muscovite(Table 2). One 30-m-thick unit of interbedded biotite-carbonate granofels and quartzite crops out discontin-uously along strike for approximately 15 km (Fig. la). Inthe kyanite-staurolite zone, scapolite occurs in well-foli-ated biotite-scapolite or actinolite-scapolite schists.Kyanite-sillimanite-zone scapolites occur in diopside-bearing layers up to l0 cm thick interbedded with mas-sive biotite quartzite east of Bovill (Fig. lb). Mineralassemblages and approximate modes for the major scap-olite-bearing rock types are listed in Table 2. Completemineral assemblages are given in Mora (1988).

In lower-grade samples, scapolite occurs as large, sub-hedral porphyroblasts within a much finer matrix. Theporphyroblasts are up to I cm in diameter and 2.5 cm inlength, although more typically less than 0.5 cm in max-imum dimension. Coexisting phases are generally less than0.2 mm in diameter. Scapolite crystals are elongate par-allel to foliation (if present). Scapolites in low-grade rocksare poikiloblastic and optically unzoned; they contain nu-merous inclusions of quartz and other matrix minerals.Compositional core-to-rim zonation is also rare, with amaximum relative difference of 4.6 EqAn and 0.06 X",;cores may be either relatively Cl rich or Cl poor. Higher-grade rocks have granoblastic-polygonal textures and rareinclusions of quartz in scapolite.

Origin of scapolite

The occurrence of Cl-rich scapolite is an indicator ofhigh NaCl activity at the time of crystallization (Orville,1975; Ell is, 1978; Vanko and Bishop, 1982). The NaClmay have been an original sedimentary constituent ormay have been introduced from an outside source. Hie-tanen (1967) noted that strict stratigraphic control on theoccurrence of scapolite in the Wallace Formation, with>900/o of the scapolite distributed parallel to bedding,suggests that scapolite crystallized during regional meta-morphism in rock layers that originally contained halite.Salt casts and large skeletal halite crystals have been de-scribed in unmetamorphosed, cyclic Wallace Formationsedimentary rocks and stratigraphic equivalents, suggest-ing evaporitic sedimentation (Fenton and Fenton, 1937;Grotzinger, 1986). In other studies, abundant Cl-richscapolite and hypersaline fluid inclusions have been in-ferred to indicate the presence of metaevaporites (Hie-tanen,1967; Ramsay and Davidson, 1970; Serdyuchen-ko, 1975; Rich, 1979; Behr and Horn, 1982; Vanko andBishop, 1982; Roedder, 1984a). Additionally, the occur-rence of abundant Mg- and Al-rich biotite with only mi-nor chlorite, muscovite, and K-feldspar may indicatemetamorphism of evaporitic argillites (Moine et al., l98l).The compositions of Cl-rich biotite coexisting with scap-


t z q MORA AND VALLEY: HALOGEN-RICH SCAPOLITE AND BIOTITE

TABLE 1A. Representative electron-microprobe analyses of scapolite from the Wallace Formation

S a m p f e : 3 4 5 6 9Zone:* biot biot biot biot biot

1 4 1 5biot biot

23bbiot

22biot

23cbiot

sio,Alro3CaONaroK.o

SO"Cor-t

I orarT

J I

AICaNaKclSOoCo.ttEqAn+X.$

54.2223.838.668.520.422.720.421.37

99.55

7.9834.1411.3632.4330.0800.6810.0440.275

38.00 .712

54.7223.278.338.530.692.870.251.27

99.25

8.0674.0461 .3192.4350.133o.717o.0270.256

34.90.737

54.4323.349.208.080.392.490.481.60

99.45

8.0674.0781.4602.3240.0710.6230.0530.324

35.90.658

55.1223.008.208.020.532.8'l0.291.26

98.60

55.2723.067.699.200.622.99b.d.1 .31

99.46

53.2123.6610.437.250.542.200.092.13

99.01

53.5823.799.777.880.502.330.072.00

99.39

7.9394.1551.5482.2600.0980.5880.0090.403

38.s0.593

53.6024.808.768.270.752.61b.d.1 7 6

99.96

7.8534.2821 3752 3490 .1410.6490.0000.351

42.70.649

Formulae based on 16 cations

54.2423.788 . 1 79.000.482.94b.d.1 .34

99 29

53.0524.479.428.330.452.57b.d.1 .79

99.50

7.955 7.8084.111 4.2441.284 1 .4862.560 2.3780.090 0.0840.732 0.6400.000 0.0000.268 0.360

37.O 41.50732 0.640

8.2244.0451 .3112.3200.1010.7100.0320.258

8.0933.9761.2052.6120 . 1 1 40.7390.0000.261

32.50 739

7.9554.1711.6722.1030.0990.5570.0090.434

39.00.562

34.80.733

Note; Additional analyses in Mora (1988). Sample numbers are cross-referenced in Tables 3 and 4 and Fig. 1. b.d. : below minimum detection..Me tamorph i czonesa f t e rH ie tanen (1984 ) i sog rads :b i o t : b i o t i t ezone ,ga r : ga rne t zone , k+s t : kyan i t e+s tau ro l i t ezone , k+s : kyan i t e

+ sillimanite zone.'- Calculated from stoichiometrv.

olite in the Wallace Formation are discussed in a latersection. Apatite and tourmaline may also incorporate For Cl. Apatite occurs in trace amounts in some samples.Tourmaline is relatively more abundant, but its occur-

rence is often restricted to scapolite-bearing horizons. Therestriction of B-rich minerals, such as tourmaline, to cer-tain sedimentary units may also indicate metaevaporitehorizons (Abraham et al., 197 2\.

TABLE 18. Representative electron-microprobe analyses of biotite from the Wallace Formation

Sample: 1 5Zone: biot biot

28gar

27gar

24gar

6 1 1 1 4biot biot biot

1 7 1 9biot biot

23c 23bbiot biot

sio,Tio,Alr03FeOMnOMgoCaONaroKrOH,O.'Fcl

Totalt

5 lratAl

rstAl

TiFe.-

MnMg

NaKoH+F

cl

40.18 40.202.16 1 .28

15.05 16.739.37 7.73b d. b.d.

19.07 19.570.08 b.db.d. 0.089.88 9.883.55 3.910.94 0.630.43 0.21

100.22 99.91

2.935 2.9151 .065 1.0850.231 0.3450.1 19 0.0700.572 0.4690.000 0.0002.078 2 .1160.006 0.0000.000 0.0120.920 0.9141.729 1.8350.217 0.1400.054 0.025

37.08 37.87't .07 1.1019.05 18.9611.83 12.130.09 0.08

15.55 15.350.13 0.080.07 0.099.93 9.613.64 3.680.68 0.650.17 0 .16

98.97 99.47

38.25 38.87 40.011 .95 1.20 1 .81

18.08 17 .64 17 .0012.90 7.19 7.080j2 b.d. b.d.

14.70 19.38 19.940.07 0.11 0.070.09 0 .10 0 .13

10j2 10.20 9.743.60 3.69 3.770.74 0.82 0.79o.22 0.05 0.06

1 00.48 98.89 100.05

2.854 2.850 2.886't .146 1 .150 1 .1140.444 0.374 0.3310.109 0.066 0.0980.805 0.441 0.4270.007 0.000 0.0001 .635 2.1 19 2.1440.006 0.009 0.0050.013 0.013 0.0180.963 0.956 0.8961.797 1 .804 1.8120.175 0.190 0.1800.028 0.006 0.008

40.331 . 3 1

16.337.040.05

20.360.050.129.553.860.54o.17

99.44

40.20 37.82 37.03 37 .452.12 0 .93 2 .17 1 .87

14.12 19.57 17 .94 18.559.13 11.42 14.27 14.270.06 0.10 0.10 0.07

19.26 1 6.10 13.1 1 13.45b.d. 0.17 0.10 b.d.b . d . 0 . 1 8 0 . 1 1 0 1 8

1 0.49 9.99 10.1 1 10.253.38 3.74 3.67 3.531.09 0.58 0.44 0 760.41 0.24 0.24 0.36

99.71 100.54 99.90 1 00.34

Formulae based on 7 tetrahedral + octahedral cations2.916 2.967 2.781 2.815 2.826 2.776 2.8161 .084 1 .033 1.219 1 .185 1 .174 1 .224 1 .1840.307 0.196 0.476 0.423 0.476 0.457 0.4770.071 0.177 0,051 0.124 0.106 0.060 0.0620.425 0.564 0.702 0.961 0.901 0.741 0.7540.003 0.003 0 006 0.007 0.004 0.006 0.0062.194 2.120 1 .765 1 .485 1 .513 1 .736 1 .7010.004 0.000 0.013 0.008 0.000 0.010 0.0060.017 0.000 0.025 0.016 0.026 0.010 0.0130.881 0.988 0.937 0.980 0.986 0.948 0.9121 .856 1 .690 1 .835 1 .863 1.773 1.817 1.8270.123 0.258 0.135 0.106 0.181 0.161 0.1530.021 0.052 0.030 0.031 0.046 0.022 0.020

Note: Additional analyses in Mora (1988). Sample numbers are cross-referenced in Tables 3 and 4 and Fig. 1. Metamorphic zones defined in Table 1a.

.- Calculated.f Adiusted for F and Cl.i o H : 2 . 0 0 - F - c t .

b.d. : below minimum detection.

MORA AND VALLEY: HALOGEN-RICH SCAPOLITE AND BIOTITE 725

Taete 1t-Continued

24gar

29 30gar gar

27gar

26gar

32k + s t

33k + s t K + S

37k + s

38K + S

54.O423.738.468.690.692.940.171.24

99.30

7.9434.1171.3342.4730.1330.7330.0180.249

37.2o.746

52.4624.7610.877.250.482 1 20 . 1 42.22

99.82

7.7734.3271.7282.0830.0890.5340 0 1 80.448

44.20.544

5 1 . 1 424.2512.875.800.471 .420.592.74

98.94

t . I o o

4.3402.0941.7080 0920.3650.0670 568

44.70.391

3 Z . O I

24.349.847.88n 0 7

2.38b.d.2.01

99.55

7.7734.2331.5562.2550 1830.5940.0000.406

41 10.594

48.4825.6216.034.45n 2 e

0.65b.d.4.02

99.43

/ . JVC

4.6102.6211.3200.0640.1650.0000.835

53.70 .165

48.5125.9516.584.200.470.51b.d.4.23

100.33

7.3444.6332.6941.2380.0910.1270.0000.873

54.40 j27

52.4124.2912.286.460.681.70b.d.2.82

100.26

7.7944.2631 9581.8600.1 250.4290.0000.571

42.10.429

47.2726.88't7.423.540.200.24b.d.4.50

r 00.00

7.2254.8372.8551.0460.0370.0640.0000.936

61.20.064

47.01 49.5527.00 25.6816.45 14.084.07 5.480.28 0.440.40 1.08b.d. b.d.4.33 3.51

99.45 99.58

7.184 7.4754.863 4.5652.694 2.2751.206 1 .6010.053 0.0840.104 0.2760.000 0.0000.896 0.724

62.1 52.20.104 0276

Formulae based on 16 cations

l Adjusted for Cl.t f c o " : 1 - c t - s o 4t EqAn : 100(Al - 3y3.$ cr/(cr + co3).

38.70 38.02 38.39 37.701 . 53 1 .23 1 .17 0 .81

17 .25 ' t7 .48 16.62 1 6.4611 .75 14 .27 11 .31 11 .64b d . 0 12 0 .18 0 .07

16.34 14.52 17.05 17.85b.d. b.d. 0.05 b.d.b .d . b .d . 0 .13 0 .11

10 35 10 .00 10 .16 10 .633.72 3.57 3.44 3.270.61 0.83 1.17 1 .550.07 0.14 0.11 0.07

r00.0s 99.80 99 26 99.49

An alternative source of high salinity might be provid-ed if the interbedded metasedimentary rocks had beeninfiltrated by Cl-rich brines ofeither an igneous or sedi-mentary origin. However, we consider that such brines

Ttate la-Continued

29 30 31 3s 37 38g a r g a r g a r k + s k + s k + s

are unlikely to have been the major source of saline porefluids for several reasons. The distribution of Cl-richscapolite is not spatially related to igneous intrusives inthe area. In fact, the highest salinities are found in thebiotite zone, farthest from the Idaho batholith. Further-more, fine-scale interbedding of compositionally similarscapolite-bearing and scapolite-free layers, which differonly in their Cl contents, argues strongly for an in situsedimentary origin of the salt and against brine infiltra-tion (Hietanen, 1967). For example, scapolite in sample84FW8 I A occurs along a single biotite-rich layer < 2 mmthick within a cross-bedded biotite qtrartzite (Fig. 2). Otherbiotite-rich layers contain plagioclase and calcite, but noscapolite. Finally, scapolite-bearing layers are widespreadin low-grade Wallace Formation rocks throughout a largearea. Abundant Cl-rich scapolite is found in stratigraph-ically identical, low-grade rocks 50 km east ofthe studyarea (Hietanen, 1967; King and Rice, 1984), and scapoliteis also reported in equivalent low-grade Wallace For-mation argillites in Montana (Ackerman, 1959). Theseobservations are consistent with a model of scapolite for-mation by reaction of plagioclase with calcite and halitepresent in the metasedimentary rocks rather than by in-filtration of plagioclase-bearing metasedimentary rocks byan externally derived brine.

Scapolite chemistry

Scapolite in the Wallace Formation shows a range ofcompositions from Cl-rich to almost fully carbonated withlittle or no SOo solid solution (Table 3; complete analysesin Mora, 1988). In Figure 3, the equivalent anorthite con-tent and the mole fraction of Cl in scapolite are plotted.The dashed lines show the stoichiometry of commonlyproposed solid solutions (Shaw, 1960; Evans etal.,1969)

39 51 39.210.87 0.26

14.31 15.3612.47 10.52b.d. 0.09

17 .92 19.39b .d . 0 .170.05 0.07

10.50 9.382.95 3.202.24 1.860.09 b.d

100 10 98.73

Formulae based on 7 tetrahedral + octahedral cations2.871 2.85s 2.860 2.804 2.935 2.8811 . 1 2 9 1 . 1 4 5 1 . 1 4 0 1 . 1 9 6 1 . 0 6 5 1 . 1 1 90.379 0.402 0.325 0.247 0.187 0.2110.085 0.069 0.066 0.045 0.049 0.0140.729 0.896 0.704 0.724 0.774 0.6460.000 0.008 0.011 0.004 0.000 0.0061 .807 1 .625 1 .894 1.980 1.984 2.1230.000 0.000 0 000 0.000 0.000 0.0140.000 0.000 0.019 0.01 6 0.008 0.01 00.979 0.957 0.966 1.008 0.995 0.8791 .841 1 .785 1.710 1 .626 1 .462 1 .5680.143 0197 0.276 0.365 0.527 0.4320.016 0.018 0.014 0.009 0.01 1 0.000

726 MORA AND VALLEY: HALOGEN-RICH SCAPOLITE AND BIOTITE

Fig.2. Hand-sample 84FW8IA showing interlayered scapolite-bearing biotite-carbonate granofels and cross-bedded biotitequartzite. Granofels contains white, porphyroblastic scapolite and fine-grained calcite, dolomite, plagioclase, and tourmaline. Quartzitealso contains minor calcite and plagioclase. SC : porphlroblastic scapolite growing along a single biotite-rich bed in quartzite.This mode of occurrence suggests scapolite formation in sedimentary horizons that originally contained halite. Scale bar is 1 cm.

between marialite (Mar), mizzonite (Miz), and meionite(Mei). Many scapolites analyzed in this study are signif-icantly more Cl-rich than scapolites on the Mar-Mei join,as the marialitic scapolites (12-3'7 EqAn, 0.72-O.96 Xc,)reported by Vanko and Bishop (1982).

Scapolite compositions vary according to rock type andmetamorphic grade. Compositions of biotite-zone scapo-lites fall in the range 3243 EqAn and 0.594.76 Xct.

These values are close to the Al-Si-ordered scapolitecomposition (33 EqAn) predicted by crystal chemistry tohave the lowest temperature stability (Oterdoom andWenk, 1983; Sheriffet al., 1987). Although there is someoverlap, scapolite compositions in biotite-carbonategranofels are more NaCl-rich than those in biotite-mus-covite granofels. Biotite-carbonate granofels average 34. 8+ 1.5 (20) EqAn and 0.71 + 0.04 xct (Table 2). Biotite-

At6 s i6

Fig. 3. Compositions of coexisting scapolite and plagioclase in the Wallace Formation. Plagioclase compositions are plottedalong the horizontal axis. Dashed lines are solid solutions between marialite (Mar) and mtzzonite (Miz) or meionite (Mei). Ingeneral, Cl in scapolite decreases with increasing metamorphic gtade; however, compositional variations within each grade suggestlocal control of fluid and scapolite compositions.

1,ooo+6

6

SCAPOLITE COMPOSITIONSo Bl ZONE, bi-corb gronofals. B l ZONE, b i -mu gronofe lsA GAR ZONE, bl-corb g.onofolsA G A R Z O N E , o l l o t h o r+ KY-STAUR ZONEO KY-SILL ZONE

PLAGIOCLASE COMPOSITIONSI ALL ZONES

30 40 50 60

Eq An


TABLE 2. Approximate modes of scapolite-bearing rock types of the Wallace Formation

727

Rock type Carb Bt Qtz Scp Pl Kfs Ms Act Di Czo Other

Bt-Carb granofelsBt-Ms granotelsBt schistAct schistDi gneiss

30-50 10-30 10-250-25 30-40 10-250-10 2040 20-400-5 0-10 10-155-10 0-5 5-10

2040 0-5 0-210-40 0-5 0J5-20 0-10 0-55-20 0-5 0-55-10 5-10 10-20

0-5 00 0

0-5 010-40 0

0 30-40

0-1 0-20-1 0-20-5 0-23-5 0-21-2 0-5

00-20-500

Note :Ca rb : ca l c i t e+do lom i t e ,B t : b i o t i t e ,Q tz :qua r t z , scp : scapo l i t e ,P l : p l ag ioc l ase ,K f s :K je l dspa r ,Ms :muscov i t e ,Ac t : ac t i no l i t e ,Di : Diopside, Czo : clinozoisite. Other includes sphene, apatite, tourmaline, pyrite, zircon, allanite, and ilmenite.

Taele 3. Compositions of coexisting scapolite and plagioclase and estimated metamorphic fluid salinities (Eq. 4)

Sample EqAn ct/(ct + co3) Xoo' In ,G" Xl,:€,t

Biotite-carbonate granofels(21 84MC73(3) 84MC74(4) 84MC7s(5) 84HUK77(6) 84MC150(7) 84FW79(8) 84FW80(9) 84FW81A

(11) 84FW84B(12) 84FW96(13) 84FW97

Average

Biotite-muscovite granofels(14) 84FW98(15) 84FW95(19) 84HUK86(20) 84HUK88(21) 84HUK90(22) 84HUK91(23 ) 86HUK111c(23 ) 86HUK111b(23) 86HUK108c(23) 86HUK108b

Average

Biotite-carbonate granof els(26) 84MC67(27) 84MC147

Biotite-muscovite granotels(24) 84MC149(25) 84MC66B(29) 86MC99B(30) 86MC120

Average

(31) 84MC59(32) 84MC64(33) 83MC95

Average

(3s) 848049(36) 84BO51B(37) 848052(38) 84BO53B(39) 848055

Average

3293 8 034.9J 5 . 9

3 4 834.3J 4 . C

32.5J J . O

33.5JC. /

34.8(1.5)

39.038.543.63 9 338.742.737.04 1 . 532.433.539.6(3.6)

44.244.7

4 1 . 837.262.152.247.0(8.8)

42.04'l.153.746.0(6.7)

54.445.442.161.2s8.452.7(7.9)

- 1 .0970-1.2417-1.2061- 1 .3351-1 2105- 1.1 878- 1 3408-1 . 1938- 1 .2990-1.2163

- 1 .0548-1 .0619

$

$-0 .9153-1.1122-0.9258- 1 .3533- 1 .2895

-0.8499-0.8381

-0.9070- 1 . 1 0 0 1-0.4334-0.6139

-o 5720

-0.5492-0.7837-0.9052-0.4496-0.4765

Biotite zone

0.751o.7120.7370.6580.7330.7090.6790.7390 7470.6410.6530.71(0.04)

u.coz0.5930.6410.6060.7390.6490.7320 640U / C J

0.7090.66(0.07)

Garnet zone

0.5440.391

0.5730.7460 104o.2760.44(0.23)

0.5020.5940.1650.42(0.23},

0 1270.352o.4290.0640.0650.21(0.17)

+0.960.980.97, 0.87o.970.970.990.980.980.96,0.850.96, 0.87

0.91, 0.83

0.76+0.980.96n.a.n.a.n.a.n.a

U . J /

0.62

i 1 t

0.980 6 2U . O J

++0.60

0.620 6 9

0.570.70

0.780.770.550.680.680.620.700.850.460.520.66(0.1 2)

0.430.47

0.70o.870.680.780.660.67(0.1 5)

0.480.26

0.510.960.070.200.41(0.32)

0 . 1 1

0.080.240.300.040.040.1 4(0.1 2)

Kyanite-staurolite zone

Kyanite-sillimanite zone

/Vote; Complete analyses in Mora (1988). Prefix to sample number (in parentheses) refers to sample location (Fig. 1); c and b following samplenumbers refer to carbonate-rich and biotite-rich layers, respectively; n.a : not analyzed 2d deviations given in parentheses after average values.

. Composition of plagioclase; two values indicate coexisting plagioclases." Calculated from Eo. 3t Calculated from Eo. 2.+ No plagioclase present.$ No calcite present.


muscovite granofels average 39.6 + 3.6 EqAn and 0.66+ 0.07 X.,. Similar NaCl enrichment of scapolite in low-grade calcareous metapelites is observed at Notch Peak,Utah, and Mary Kathleen, Australia (Ramsay and Da-vidson, 1970; Hover Granath et al., 1983). At highergrade, Wallace Formation scapolites ratge from 37-62EqAn and 0.75-0.06 X.,. There is a general decrease inthe X., and an increase in EqAn with increasing meta-morphic grade (Fig. 3), as is found in metamorphic rocksin many regions (Oterdoom and Wenk, 1983). However,compositional variations within each metamorphic gradesuggest that local rock and/or local metamorphic fluidcompositions exert an important control on scapolitecompositions.

Plagioclase coexists with scapolite in nearly all sam-ples, and scapolite-plagioclase relations are also shown inFigure 3. Scapolite is always more calcic than coexistingplagioclase. TieJine slopes in Figure 3 are similar overmost of the compositional range but begin to rotate forscapolite compositions greater than 50 EqAn. In severalbiotite-zone samples (Z: 480 "C), scapolite coexists withtwo plagioclases, the compositions of which roughly de-fine the peristerite solvus. Similar scapolite-plagioclaseKlu-c" values are reported for other metamorphosed Wal-lace Formation samples (Hietanen, 1967; King and Rice,I 984). Scapolite-plagioclase relations are consistent withexperimental (Orville, 1975; Ellis, 1978) and natural(Shaw, 1960) observations of plagioclase-scapolite rela-tions in systems with relatively sodic plagioclase (<Anoo),indicating that Wallace Formation mineral compositionsare in chemical equilibrium.

Sc,q.pot,r:rr srABrr-rry AND FLUrD coMposrrroNs

The stability of scapolite in the system Ab-An-CaCO.-NaCI has been experimentally determined by severalstudies (Orville, 1975; Goldsmith and Newton, 1977;El-lis, 1978; Vanko and Bishop, 1982; Aitken, 1983). Thesestudies indicate (l) that increasing the concentration ofNaCl greatly expands the stability of scapolite and therange of stably coexisting scapolite + plagioclase com-positions and (2) that concentrations of NaCl are high influids in equilibrium with Cl-rich scapolite. Ellis (1978)investigated the reaction

NaCl- **,,," * (CaCOr)," -"1. : (CaCOr)io ""opure+ NaCl"uro (l)

at 7 50 'C and 4 kbar. For this reaction,

6o - (]3?3"'Xf,R*9(rR:'sJ(xE388i) Q)

where X1 is the mole fraction of NaCl or CaCO, (l) in therespective phases (7). From measured values of Ko forReaction l, run at different (fixed) values of EqAn, Ellis(1978) derived the expression

ln K" : -0.0028(Xo';-sssao, (3)

where Xo, is the atomic ratio All(Al + Si) in scapolite.

This expression can be used to estimate the NaCl contentof fluid coexisting with scapolite of a given compositionat 750 'C and 4 kbar.

It is important to evaluate the assumptions that mustbe made in order to apply Equation 3 to naturally occur-ring scapolites and the limitations these assumptions im-pose on estimates of the metamorphic fluid composi-tions. Fluid activities are strongly temperature dependent;therefore, the accuracy of data derived from Equation 3is diminished for scapolites crystallized at other than theexperimental conditions of 750 'C and 4 kbar. Equation3 assumes that NaCl and CaCO, mix ideally in scapoliteand NaCl and H,O mix ideally in the fluid. Ideal mixingin scapolite cannot be evaluated, as the activity-compo-sition relations for Cl-rich scapolite have not yet beendetermined. The assumption of ideal mixing in the fluidmust be viewed with caution. Fluid compositions in ex-periments of Ellis (1978) are in the system HrO-COr-NaCl; however, Equation 2 considers XFfd: NaCV(NaCl+ HrO). Significant nonideality exists in the ternary sys-tem (Bowers and Helgeson, 1983). High concentrationsof NaCl may result in immiscibility and separation of thefluid into an aqueous brine and a relatively COr-rich va-por. However, the equation of state for HrO-COr-NaClfluids is not yet established over the pressure and tem-perature ranges of metamorphism for the Idaho rocks(480-590'C,4-6 kbar), and Ellis's expressions cannot bereformulated at present.

Calculated values of ln K" and XSp! for Wallace For-mation metasedimentary rocks are given in Table 3. Val-ues for X$,i! decrease with metamorphic grade and rangefrom 0.46 to 0.85 in biotite-zone biotite-carbonate gran-ofels, 0.47 to 0.87 in biotite-zone biotite-muscovite gran-ofels, 0.07 to 0.96 in garnet-zone samples, and 0.04 to0.30 in the kyanite-sillimanite zone. Equilibrium com-positions of metamorphic fluids estimated from calc-sil-icate equilibria in these samples indicate the presence ofCO2 (Xco, < 0.7) as well as HrO and NaCl (Mora, 1988);therefore, the reported values of X*,., are maximum val-ues.

The higher values of -(,';6j are clearly unrealistic. Val-ues calculated for -Xfl,ij in biotite-zone samples corre-spond to fluid salinities of 62 to > 90 wt0/o NaCl equiva-lent. The maximum solubility of NaCl in water along thethree-phase boundary vaporJiquid-NaCl is about 55 wto/oat 500'C (Keevil, 1942). Solubility of NaCl decreases asCO, is added to the system (Takenouchi and Kennedy,1965). Hydrothermal experiments by Vanko and Bishop(1982) show that at 750 "C and 2.8 kbar and with excesscalcite, scapolite with composition 28 EqAn and 0.63 Xclis stabilized by a fluid with X^"", : NaCl/(NaCl + HrO): 0.5-0.7. Fluid-inclusion studies indicate that meta-morphic fluids may contain significant amounts of NaCl.These studies indicate that metamorphic fluid salinitiesrange from 0 to 6 wto/o NaCl equivalent in pelitic schistsand gneisses (Poty et al., 1974; Hollister and Bumrss,197 6) and, from 20 to 25 wto/o NaCl equivalent in calcar-eous rocks (Crawford et al., 1979a,1979b; Kreulen, I 980;

Sisson et al., l98l). Higher salinities (up ro 50 wto/o NaClequivalent) are noted where evaporites are suspected(Rich, 1979; Behr and Horn, 1982; Roedder, 1984a). Fluidinclusions associated with porphyry ore deposits maycontain as much as 70 wto/o NaCl equivalent (Roedder,l 984b).

Reconnaissance study of fluid inclusions in scapolite-bearing Wallace Formation metasedimentary rocks in-dicates that small (<10 pm), isolated saline inclusionsoccur in matrix quartz in all metamorphic zones. Daugh-ter minerals were not observed in inclusions at any grade.Liquid + vapor inclusions in two kyanite-sillimanite-zonesamples had melting temperatures (f-) in the range -5to - l5 'C, indicating moderate salinities (10-20 wto/o NaClequivalent; Potter et al., 1978). Attempts to determineZ- for two-phase (liquid + vapor) inclusions in severallow-grade samples were unsuccessful as no freezing andlor melting behavior was observed between 25 and - 150'C. Crawford et al. (1979a, 1979b) observed similar be-havior in metamorphosed carbonate-rich metasedimen-tary rocks. Cl-rich fluids that are high in Ca and Mg maycontain no daughter minerals and be very difficult to freezebecause of the high viscosities of such brines at low tem-perature (Roedder, 1984a). Thus, preliminary examina-tion of fluid inclusions suggests the presence of meta-morphic brines.

Cl-rich scapolite compositions clearly indicate highlysaline fluids during crystallization, although more accu-rate estimates of XNucr must await calibration of ReactionI at lower temperatures. Despite these limitations on theaccuracy of quantitative estimation of fluid compositionscoexisting with scapolite, Ilrre precision of these estimatesis sufficiently high to be useful for qualitative compari-sons of fluid-rock interaction among our metasedimen-tary samples, especially among samples of similar meta-morphic grade and bulk composition.

Prograde metamorphic fluid compositions

Several lines of evidence suggest that scapolite is a sen-sitive indicator of metamorphic fluid composition. Scap-olite modal abundance in the Wallace Formation de-creases from up to 40o/o in scapolite-bearing layers ofbiotite-zone rocks to less than l0o/o at higher grade (Table2). The abrupt decrease in the abundance of scapolitemay in part reflect a facies change in the sedimentaryprotolith in which NaCl and/or plagioclase componentsdecrease in the direction ofincreasing grade. Alternative-ly, prograde metamorphic fluid compositions affect thestability of scapolite. The percentage of scapolite de-creases approximately coincident with the appearance oflow-variance, zoisite-bearing assemblages in the calc-schists. These assemblages require water-rich metamor-phic fluids even though volatilization products are CO,rich. This situation can only be satisfied by combinedbuffering and infiltration ofan aqueous fluid (Xco, < 0. I 5)during metamorphism (Mora and Valley, 1986a). Infil-tration of water-rich fluid would dilute highly saline porefluids, reducing the stability of Cl-rich scapolite. For ex-

729

ample, scapolite compositions in garnet-zone, zoisite-bearing samples 86MC99B and 86MCl20 reflect equili-bration with a relatively Cl-poor fluid (.fR*g : 0.07-0.20)by comparison with zoisite-free samples of similar grade(Table 3).

In general, X. decreases with increasing grade (Fig. 3)indicating that scapolite composition, and thus the in-fened -lRlil, changes with prograde metamorphic devola-tilization and/or infiltration. For example, scapolite com-positions show a decrease from 0.71 to 0.45 X. betweenbiotite- and garnet-zone samples in the same rock type(biotite-carbonate granofels; Table 3). The mineral as-semblage dolomite + qtafiz + tremolite + calcite ingarnet-zone sample 84MCl47 (Table 3) suggests dilutionof metamorphic salinity by the prograde reaction dolo-mite + quartz + HrO: tremolite * calcite + COr. Thecomposition of scapolite in this sample rs 44.7 EqAn 0.39X.,; the inferred XRli! : 0.26 (Table 3). By comparison,scapolite in sample 84MC67 (Table 3), which containsdolomite + quartz + calcite unreacted to tremolite, in-dicates significantly less dilute metamorphic fluids (44.2EqAn 0.54 X.,; fT;ij : 0.48).

Samples from the kyanite-sillimanite-zone in the Wal-lace Formation were collected from thin (< l0 cm) diop-side-rich layers in massive quartzite and contain nearlyidentical zoisite-bearing assemblages (phlogopite + cal-cite + quartz + diopside + K-feldspar + zoisite + pla-gioclase * scapolite; sample 84BO55 has no phlogopite;Table 2). These assemblages indicate combined bufferingand infiltration of an aqueous fluid, with equilibrium X"ro> 0.90 (Mora and Valley, 1986a). Despite these similar-ities, X., in scapolite varies from 0.06 to 0.43 (xR*1 :

0.04 to 0.30: Table 3). Differences in the amount or com-position of fluids infiltrated into these layers may accountfor the range of scapolite compositions observed. Alter-natively, variable amounts of halite contained along theselayers in the protolith may also have contributed to dif-ferences in scapolite compositions. It is important to notethat aqueous infiltration did not overwhelm the butreringcapacity of the scapolite-bearing layers, nor did aqueousfluid flow smooth compositional heterogeneities betweenrock layers.

Local gradients in metamorphic salinities

Local gradients in the Cl content of metamorphic fluidsare inferred between thin (<2 mm), plagioclase * calcite,scapolite-free and scapolite-bearing layers, as seen in Fig-ure 2. Gradients are also observed between adjacent sca-polite-bearing layers in samples 86HUKI I I and86HUK108 (Table 3). These rocks consist of carbonate-rich and biotite-rich layers intimately mixed on a scale of<2 cm (Fig. a). The carbonate-rich layers contain abun-dant blue-gray euhedral scapolite up to 7 mm in lengthand 2 mm in cross section. Scapolite in the fine-grainedbiotite-rich layers forms white, round crystals <4 mm indiameter. Differences in fR# of up to 0. 19 are inferredfrom scapolite compositions (37.0 vs. 41.5 EqAn) in ad-jacent (within I cm) carbonate- and biotite-rich layers.


730 MORA AND VALLEY: HAL,OGEN-RICH SCAPOUTE AND BIOTITE

f ,

Fig. 4. Mixed carbonate-rich and biotite-rich layers (sample 86HUKI08). Light gray, carbonate-rich layers contain dark gray,euhedral scapolites (A) whereas dark brown, biotite-rich layers have rounded, gray scapolites (B). Sharp gradients in .1R*1 of up to0.19 per I cm are inferred from scapolite compositions in this sample and others from this outcrop (sample 86HUK1 l l; Table 3).Scale bar is I cm.

*l'*F

The compositional differences between adjacent scapo-lites are much greater than the analytical error (allowing+ l0o/o relative Cl) and indicate that significant differencesin fluid compositions existed in these rocks on a scale of< I cm. The finely interbedded nature of the Wallace For-mation metasedimentary rocks suggests that other ex-amples of fine-scale gradients in metamorphic salinitiesmay be widespread in the study area.

. o BIOTITE ZONE

A A GARNET ZONE

I tr KYANITE.SILLIMANITEZONE

Op€n symbols=Cl do ioClosod symbols=Fdq lo

o.z o.4 0.6 0.8XMg,B io t i r "

Fig. 5. Halogen chemistry, wto/o Cl or F, plotted against themole fraction of Mg in biotite. Open symbols are Cl data andclosed symbols are F data. In general, Cl decreases and F in-creases with increasing metamorphic grade. The high and vari-able Cl content ofbiotites in low-grade biotite-carbonate gran-ofels (stippled) indicates local differences in metamorphic fluidcompositions and internal buffering of satne metamorphic fluids.

H.q.LocrN coMposrrroNs oF BrorrrEs

Biotite coexists with scapolile at all metamorphicgrades, and biotite compositions yield independent in-formation on the metamorphic fluid compositions. Ex-periments indicate that the F + OH and Cl + OH ex-change in biotite is governed by several independentfactors including temperature, the activities of halogenacids, and the compositions of biotite (Munoz and Lu-dington, L9741,Lludington and Munoz, 1975; Munoz andSwenson, 1981; Volfinger et al., 1985). Experimentalstudies indicate that F is preferentially incorporated intobiotites with high Mg/Fe ratios. "Fe-F avoidance" (Ram-berg, 1952) has also been extensively documented in nat-ural systems (Ekstrom, 1972; Zaw and Clark, 19781, Ja-cobs and Parry,1979; Valley and Essene, 1980; Valley etal., 1982). The compositional control of Cl in biotite isnot as well understood. Experimental studies suggest "Mg-Cl avoidance," the preferential incorporation of Cl intoFe-rich biotites (Munoz and Swenson, 198 I ; Volfinger eta l . , 1985).

The relation of Mg/Fe and halogen content in biotitevaries in metamorphic and hydrothermal environmentsas a function oftemperature, pressure, and halogen activ-ities (cf. Fig. 4 of Sisson, 1987). Compositions of biotitescoexisting with scapolite in the Wallace Formation aresummarized in Table 4. The wto/o of Cl and F have beenplotted versus Xr, (X*": Mg + total octahedral cations)in Figure 5. The percentage ofCl decreases with increas-ing metamorphic grade, from 0. 16-0.51 wto/o in the bio-tite zone to 0.00-O.09 wto/o in the kyanite-sillimanite zone,whereas F shows the opposite trend, increasing from 0.44-I .09 wto/o to I .5 5-2.5 8 wt0l0. Most samples reflect the pre-

olr t.sde+3


TABLE 4. Biotite compositions and log fugacity ratios of halogen acids

731

Sample Xuo" X"o-- X*t (vt%)F

(v'/t%),"s* ,"s*

Biot i tezone: p:4.5kbaL f :480.CBiotite-carbonate granof els(1) 84MC69A(1) 84MC69B(s) 84HUK77(6) 84MC150

(10) 84FW83(11) 84FW84

Average

0.6880.6930.7050 7310.7120.7070.706(0.02)

0.1520.1420.2030.1650.1 020 . 1 1 5

o.4260.3810.3930.4710.4120.3910.3910.3250.384o 412o.427

0.1840.163

0.3130.2490.3020.219

0.1 600.1650.0920.1040.1860.178

0.0790.0840 0150.02s0.0200.0940.0210 0470.0370.0210.008

0 . 1 1 00.122

0.1420.1490.1 560.149

0.1660.2120.153

0.840.94U.OJ

0.541 0 41 .090.85(0.22)

0.440 6 3o.730.760.620.690.580.520.680.650.580.63(0.09)

2.582.241.862.06(0.4s)

0.420.430.210.170.470.410.35(0.1 3)

o.240.250.070.360.18o.47o.240.16o.170.160.230.23(0.11)

0.050.060.05(0.00)

0.220.070.140 . 1 10.1 4(0.06)

0.070.09+0.06(0.04)

2.O32.002.322.351.911.982.1 0(0.1 9)

2.642.552.952.442.622.272.482.592.632.70z-J5

2.58(0.17)

2.732.s92 66(0.1 0)

2.372.512.562482 48(0.08)

1 .901 8 31.770.00

4.894.845.065 . 1 54.814.784 92(0.15)

4.904.804.804 6 54.844.724.905.024.814.824.864.83(0.1 0)

4.834.874.85(0.03)

4.644.824.594.554.65(0.12)

4 .193.973.994 .16

Biotite-muscovite granofels(16) 84FW70(171 84FW71(18) 84FW72(19) 84HUK86(20) 84HUK87(21) 84HUK90(14) 84FW98(1s) 84FW95 .

0.4950.5350.s920.5040.5680.5150.s880.628

(23) 86HUK111c 0.579(23) 86HUK111b 0.s67(23) 86HUK113A 0.565

Average 0.557(0.04)

Biotite-carbonate granof els(27)- 84MC147(28) 84MC148

Average

Biotite-muscovite granofels

Gamet zone: P : 5 kbar. f: 505'C

0.820.790.81(0.02)

0.740.61n a a

1 . 1 70.84(0.24)

Kyanite-sillimanite zone: P: 6 kbar, f : 590'C0.205 0.135 1 .55 0.07

0.7060.7150.71 1(0.01)

(24) 84MC149(29) 86MC99B(30) 86MC120(31) 84MC146

Average

(35) 848049(36) 84BO51B(37) 848052(38) 84BO53B

Average

0.5450.602o.5420 6320.580(0.04)

U.OOU

0.7100.6610.7080.685(0.03)

0.1240.1270.1 39

Note.' Complete analyses in Mora (1988). Prefix to sample number (in parentheses) refers to sample (Fig. 1); c and b following samplenumbers refer to carbonate-rich and biotite-rich layers, respectively. 2r deviations given in parentheses after average values. Metamorphic P and ffrom mineral thermobarometers (see text).

'X"n: M9 + total octahedral cations.-'&,0: (X,uro/o.167X1 - XMJ (J. Munoz, personal communication).t & " " : 1 - X " n - X " 6t Cl below minimum detection.

dicted compositional trends of Fe-F and Mg-Cl avoid-ance. Biotites shown in the stippled pattern in Figure 5occur within a single rock type (biotite-carbonate grano-fels; Fig. la) and equilibrated at the same metamorphicpressure and temperature. The biotites have approxi-mately the same Mg-rich compositions (Table 4). Theirhigh and variable halogen contents must therefore reflectlocal variations in pore-fluid composition that have notbeen homogenized by metamorphic fluid flow.

tr'lCl ratios and fluid compositions

Experimental studies have determined the temperatureand compositional (Fe-Mg-Al) dependence of biotite F +OH and Cl = OH exchange reactions (Munoz and Lu-dington, 1974; Ludington and Munoz, 1975; Munoz and

Swenson, l98l). On the basis of these data, Munoz (1984)derived the following expresstons:

logffi,o/fno): 2100/T + 1.52X^E + 0.42X^""+ 0.20xsid - log(Xo/X"") (4)

logffi 'o/f^.,): 5l5l/T - 5'01 - I '93XME- log(X.,/Xor). (5)

The variables XME, X^nn, and Xr,o are defined in Table 4.OH in biotite is usually determined from mineral stoi-chiometry and is thus subject to greater uncrrtainties thanF or Cl, which are measured directly by electron micro-probe. Equations 4 and 5 can be combined to eliminatethe hydroxyl variable:

log(f",/f".'): 3051/T - 5.01 - 3.45Xw - 0.42XA*,- 0.20xstd + log(Xr/X). (6)


2 . O

B l ZONE, b i - ca rb g rano f e l s

B l ZONE, b i -mu g rano f e l e

GAR ZONE, b i - ca rb g rano fe l s

GAR ZONE, b i o r . amph sch i s t

KY-S ILL ZONE, d i gne i ss

ra)

o.oo . 4 0 . 6 o.8

^ Mg, Biotite

Fig. 6. I-ag(X"/X) in biotites plotted against the mole fraction of Mg. Diagonal contours are logffi"/fs,) in fluid calculatedfrom Eq. 6 for the estimated metamorphic temperatures of the biotite, garnet, and kyanite-sillimanite zones. The data indicate thatthe two biotite-zone rock types equilibrated with metamorphic fluids of significantly different composition.

oaAAt

C)><

LxtrDI

Equation 6 permits evaluation of the halogen contentof biotite as a function of biotite composition, equilibra-tion temperature, and fluid compositions. In Figure 6, log(XF/Xc) in Wallace Formation biotites is plotted againstX.r. Biotites with varying Xna' which equilibrated at thesame temperature and with the same fluid composition(fno/.fr.,: constant) form an approximately linear arrayon this type of diagram. The arrays lie along isopleths ofthe Iogffio/f".,)H-d. The slopes of the sample arrays de-pend, to a small degree, on the annite and siderophyllitecontent of the biotites. Values of Xo," and X'o are verysimilar within each rock type and metamorphic zone inthe Idaho samples (Table 4). The positions of the fluidisopleths in Figure 6 were calculated using average valuesof X""" and Xsid for each group of samples and the appro-priate metamorphic temperatures (Tomten and Bowman,1984; Mora, 1988). Higher temperatures shift the iso-pleths upward.

Of particular interest are the biotite-zone samples, whichform two distinct groups in Figure 6. Biotites with highX*" are from biotite-carbonate granofels (closed circles),and biotites in the other group are from biotite-muscovitegranofels (open circles). These rock types are interbeddedon the scale of I cm to l0 m. Calcite-dolomite thermom-etry in 12 samples indicates that all of the biotite-zonesamples equilibrated at 480 + ll 'C (2o) (Mora and Val-ley, 1986b). Therefore, the bimodal distribution does notresult from variable temperature between the two rock

types. Instead, the data suggest that these rock typesequilibrated with metamorphic fluids having substantial-ly different relative halogen activities. Halogen activitygradients between the rock types were not smoothed outby fluid circulation.

It is important to recognize the opposing behavior ofCl and F in hydrous minerals during prograde metamor-phism. F is increasingly concentrated in the residual hy-drous phases as metamorphic devolatilization proceedsbecause it is so strongly partitioned into solid phases. Forthis reason, F is relatively insensitive to infiltration ofexternally derived aqueous fluids. Cl, on the other hand,is strongly partitioned into an aqueous fluid, and the Clcontent of metamorphic minerals will be increasingly di-luted by exchange with aqueous fluids produced inter-nally or infiltrated into the rocks. Calculated log fugacityratios ffiro/f'r) and (ft o/fr",) are listed in Table 4 andplotted (as crosses) against the equilibration temperaturein Figures 7a and 7b. Error bars are shown for a relativeanalytical uncertainty of + l0o/o for F and Cl and +25 "Cfor temperature. Halogen enrichment increases to the leftin both figures. Shown for comparison (dots) are valuesof the log ratios calculated from biotite compositions re-ported in other studies of regionally metamorphosedmetasedimentary rocks (Tyler, 19791, Yalley and Essene,1980; Hoschek, 1980; Valley et al.,1982; Yardley, 1985;Sisson, 1987). Values of logffi,o/f^.).'ro in Wallace For-mation samples are comparable to values reported for

other amphibolite-facies metasedimentary rocks (Fig. 7a).In contrast, logffiro/fr",)o",0 values are much lower in theIdaho samples than in amphibolite-facies pelitic schists(Yardley, 1985), probably reflecting the evaporitic pro-tolith of the Wallace Formation calc-silicates (Fig. 7b).

Values of logffiro/fr.)o,. in the Idaho samples de-crease with increasing metamorphic grade, reflecting con-centration ofF in hydrous phases during prograde meta-morphism (Fig. 7a). An increase in logffi,o/fr.).',. withincreasing grade would be the predicted result of meta-morphic devolatilization or aqueous infiltration. Such anincrease is not seen in Figure 7b, suggesting that the meta-morphic fluids were not substantially diluted by progradevolatilization products or aqueous infiltration, or thatbiotites in these samples have interacted with an exter-nally derived Cl-rich fluid. However, the predicted trendof Cl dilution strictly applies only in the case of rockswith the same initial degree of Cl enrichment. If high-grade samples were initially enriched in Cl relative tolow-grade samples, prograde Cl dilution may not be rec-ognized on a plot such as Figure 7b, although it may infact have occurred.

Prograde changes in the buffered halogen contents ofmetamorphic fluids may be reflected in Cl partitioningbetween biotite and scapolite. Although scapolite com-positions are a function of the cSSl whereas biotite com-positions are a function of the /EHd, both should reflectthe same relative trends of Cl enrichment or depletion.Coexisting scapolite-biotite pairs are plotted in Figure 8with,l2;'o versus IV(CI)*.,, where IV(Cl)'t", is a measureof Cl enrichment in biotite [IV(Cl)'i", : -5.01 - l.93XME- log(X.,/Xor); Munoz, 19841. High and variable valuesof both 4:"o and IV(Cl)Bt",, especially in low-grade rocks,reflect the heterogeneous metamorphic fluid composi-tions. By comparison, zoisite-bearing samples from thegarnet and kyanite-sillimanite zones have nearly con-stant values of IV(CI)"., and markedly lower values ofXS"e (Fig. 8). Prograde zoisite formation requires infiltra-tion of an aqueous fluid (Hewitt, 19731' Ferry, 1976).Aqueous infiltration results in smoothing of values ofIV(Cl)Bt., and reducing -4:"o at higher grade. Thus, al-though infiltration and consequent Cl dilution are notidentified in the general trend in Figure 7b, they are in-ferred from the compositional trends of coexisting scapo-lite and biotite in Figure 8.

CoNsrn c.rNTS oN METAMoRpHTc FLUrD-RocKRATIOS AND FLUID FLOW

Metamorphic fluid-rock ratios can be estimated on thebasis of the reaction progress of fluid-buffering mineralequilibria. Biotite-zone assemblages of calcite + dolo-mite + qLrartz + biotite + K-feldspar + tremolite (Table2) restrict the equilibria to lie along or between the tworeactions

3 Dolomite + K-feldspar + H,O: Phlogopite + 3 Calcite + 7COr. (7)

t J 5

los tf^"otfrrl

9ool/r"or fr"rl

Fig. 7. (a) l-og(f","/f",) determined from biotite composition(Eq. 4) plotted against metamorphic temperature for regionallymetamorphosed samples. Data from this study are shown withan M and diagonal lines. Crosses indicate individual data points.Other data sets are (V) Valley and Essene (1980) and Valley etal. (1982), (H) Hoschek (1980), and (S) Sisson (1987; correctedvalues; Sisson, personal communication). Data for (Y) Yardley(1985) are estimates based on apatite compositions. Error barsare shown for t25 "C and +100/o F (relative). Decreased logratios with higher metamorphic grade reflect the relative im-mobility of F during devolatilization or infiltration. b) LoEUiH,o//"o) determined from biotite composition (Eq. 5) plotted againstmetamorphic temperature. All labels for references are as in Fig.7a, except (T) Tyler (1979). Decreased log ratios with highermetamorphic grade suggest that pore fluids were not substantiallydiluted by devolatilization or aqueous infiltration or have inter-acted with an externally derived Cl-rich fluid.

5 Dolomite * 8 Quartz + HrO: Tremolite + 3 Calcite + 7CO2. (8)

At 500 'C and 4.5 kbar, these reactions restrict X"o, :CO,/(CO, + H,O) to -0.40 (Mora, 1988). If solid solu-tion and a +20 oC error in temperature are taken intoaccount, 0.20 < X.o, 3 0.75. Mineral assemblages inkyanite-sillimanite-zone samples are located at the inter-section of the devolatilization reactions


3.O

2.O

Zoiei te

734

Bl ZONE, b i -carb granofs ls

B l ZONE, b i -mu gnanof . l s

GAR ZONE, b i -c r rb g ranof r l sGAR ZONE, b l -nu grano lc ts

KY-SILL ZONE, d l gnc ie r

5 .O 4 .5 4 .O

-lV(cl)sb1

Fig. 8. Cl enrichment of biotite [IV(CI)",.,; see rext] and molefraction of Cl in coexisting scapolite. High and variable valuesofboth parameters in low-grade rocks reflect Cl-rich, heteroge-neous metamorphic fluid compositions. Values are smoothed orreduced in higher-grade, zoisite-bearing samples. Arrow showsdirection of Cl enrichment.

Phlogopite + 3 Calcite + 6 Quartz: 3 Diopside + K-feldspar + HrO + 3COr. (9)

2Zoisite + COr: 3 Anorthite + Calcite + H,O. (10)

At 590 oC and 6 kbar, the equilibrium fluid compositionfor this intersection is X.o, : 0. 10, if the effects of solidsolution on the location of the invariant equilibria aremodeled (Mora, 1988).

A typical mode for biotite-zone samples is 20o/o biotite,20o/o quartz, 250lo scapolite, 25o/o calcite, 50/o dolomite,and 50/o plagioclase + K-feldspar. If it is assumed that allof the biotite was formed by Reaction 7 and that theequilibrium X"o, : 0.40, the conditions would requiremetamorphic infiltration of the metasedimentary rocksby at least 0.734 mol of pure HrO per 100 cm3 of rock.This corresponds to a minimum fluid-rock ratio of 0.09,on an atomic oxygen basis (volumetric fluid-rock ratio :

0. 15, assuming pure HrO infiltration at 480 'C and 4.5kbar, molar volume data of Burnham et al., 1969). Forthe minimum X"or: 0.20, similar calculations indicatea fluid-rock ratio of -0.35 for biotite-zone samples. Min-eral assemblages in the kyanite-sillimanite samples indi-cate invariant equilibria at the intersection of Reactions9 and l0 with equilibrium X.o, : 0.10. For a rock con-


adIa- o.4

5><

taining 39.70lo diopside and l.4o/o zoisite (sample 84BO49),the conditions require infiltration of 5.25 mol of pureHrO per 100 cm3 of rock, or a fluid-rock ratio of 0.64(atomic oxygen basis; volumetric fluid-rock ratio : l.l0at 590 "C and 6 kbar). It must be emphasized that fluid-rock ratios are minimum estimates that depend on theinferred mineral reaction, the rock mode, the tempera-ture, and the composition of the infiltrating fluid. Fluid-rock ratios in the biotite-zone rocks are also sensitive tothe choice of biotite-forming reaction. For example, bio-tite formation involving dehydration of clay minerals mayrequire much less aqueous infiltration than biotite for-mation by Reaction 7.

These calculated fluid-rock ratios can be considered inlight of mass-balance constraints on the formation ofscapolite. Scapolite in low-grade Wallace Formationmetasedimentary rocks may have formed during meta-morphism of a halite-bearing protolith by the reaction 3Albite + Anorthite + 0.667NaCl + 0.333 Calcite :

Scapolite (EqAn 33). Bv this reaction, it requires only 1.5modal percent halite to form a rock with 25 modal per-cent scapolite, assuming a closed system. In an open sys-tem, fluid escape during diagenesis, metamorphic dehy-dration, and infiltration will remove halite in solutionand increase the necessary amount of halite in the sedi-mentary protolith. Halite will be dissolved into infiltratedaqueous fluid in order to maintain the highly saline fluidin equilibrium with Cl-rich scapolite. To maintain -ln;5{: NaCl/(NaCl + H,O) : 0.5 in biotite-zone metasedi-mentary rocks infiltrated by 0.734 mol of pure HrO (as-sumes X.o, : 0.40; see above) requires dissolution of0.734 mol NaCl (21 cm3 per 100 cm3 of rock) into thefluid. Infiltration of 5.25 mol of pure H,O into kyanite-sillimanite-zone metasedimentary rocks (see above) re-quires dissolution of 23 cm3 of NaCl per 100 cm3 of rockto preserve a fluid composition with XR*l : 0.14 (Table2). Infiltration of substantially larger amounts of water(fluid-rock ratios > 0.4 in biotite-zone samples; atomicoxygen basis) would require amounts of halite (30-1000/0)for which there is no evidence in unmetamorphosedequivalent sedimentary rocks.

These mass-balance calculations consider only themineral equilibria present in scapolite-bearing layers anddo not apply to the metasedimentary sequence as a whole.The requirement that -200lo halite is necessary in theprotolith to stabilize abundant, Cl-rich scapolite is lim-ited to the relatively thin and volumetrically minor sed-imentary horizons that contain scapolite. The abundanceof halite required, if scapolite-free layers within the Wal-lace Formation are included in the mass balance, is manytimes less. If XR;i1 is reduced by one-half (to 0.25 for thebiotite-zone example and 0.07 for the kyanite-silliman-ite-zone example), only l0 modal percent halite is re-quired. Immiscibility in the NaCl-rich metamorphic fluidmay concentrate NaCl in an aqueous brine and reducethe necessary amount of halite. Finally, although the dis-tribution of Cl-rich minerals suggests very limited fluidexchange between scapolite-bearing and scapolite-free

oaAAI

MORA AND VALLEY: HALOGEN-RICH SCAPOLITE AND BIOTITE 735

layers, some bedding-parallel flow of saline fluid may haveoccurred. Such fluid flow may also reduce the amount ofhalite required to be present in the protolith.

DrscussrouIn all Wallace Formation scapolite-bearing rock types

and at all metamorphic grades, scapolite and biotite com-positions indicate considerable variation in the Cl con-tent of the metamorphic fluid with heterogeneous fluidcompositions preserved on a scale of < I cm. Metamor-phic fluid flow is thus inferred to have been channelized,and/or fluid-rock interaction was limited. Cl present inthe Wallace protolith would have been sensitive to dilu-tion or removal by aqueous fluid-rock interaction duringthe entire metamorphic history of the rock. It is thereforevery significant that high and variable Cl activities havenot been smoothed out by fluid-rock interaction throughtwo regional metamorphic events.

It is possible that metamorphic fluids were predomi-nantly channelized into the adjacent impure and massivequartzites rather than into the scapolite-bearing layers. Itis difficult to mineralogically document fluid-rock inter-action in the quartzites because of the lack of reactivemineral assemblages. However, the fine-scale interlayer-ing of low-grade, Cl-rich granofels with quartzite and thehigh and variable Cl activities recorded in the thin scapo-lite-bearing layers argue against extensive interaction ofthe adjacent quartzites with externally derived aqueousfluids.

Thus, Cl-rich, but heterogeneous fluid compositions re-ported here support low fluid-rock ratios throughout thisportion of the metasedimentary pile. This conclusion dif-fers from that in recent oxygen-isotope studies that haveproposed large-scale, pervasive circulation of moderateto high fluid-rock ratios of aqueous fluid through theWallace Formation in the study area, occurring approx-imately concurrently with regional metamorphism (Crisset al., 1984; Fleck and Criss, 1985). Furthermore, anyfluid present at the peak of regional metamorphism wouldhave been at lithostatic pressure (4-6 kbar), on the basisof geobarometry in the metapelites, and the viability oflithostatically pressured fluid convection is controversialat present (e.g., Valley, 1986). Oxygen-isotope analyses ofmineral separates from the scapolite-bearing metasedi-mentary rocks examined in this study show isotopic de-pletions that are due to interaction with hydrostaticallypressured meteoric-hj'drothermal flutds after the peak ofregional metamorphism (Mora and Valley, 1988). It isclear that the details ofany large-scale fluid event in theserocks are yet to be elucidated.

AcrNowr.nncMENTS

Research was supponed by the National Science Foundation (EAR85-08102), the Gas Research Institute 5086-260-1425, GSA Penrose grants,and Sigma Xi. Fellowship support to C.I M. was provided by a FordFoundation Minority Dissertation Fellowship, the American GeologicalInstitute Minority Participation Program, and the University of Wiscon-sin. We thank J. Munoz, B. Wood, I. Cartwright, R. Criss, S. Dunn, S.Komor, D. Moecher, L. Riciputi, and V. Sisson for helpful reviews of this

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MaNuscnrm RECETVED JuNs 23, 1988MnNuscnrrr AccEprED Mencs 6. 1989


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