Experimental studies of alunite: I. 18O-16O and D-H fractionation factors between alunite and water at 250–450°C

Geochimicaet CosmochimicaActa, Vol. 58, No. 2, pp. 903-916, 1994 Copyright 0 1994Elsevier Science Ltd Printed in theUSA. All rights reserved 0016-7037/94$6.00 + .OO

Pergamon

Experimental

studies of alunite: I. 18O-16O and D-H fractionation factors between alunite and water at 250-450°C

ROGER E. STOFFREGEN,’ * ROBERT 0. RYE,’ and MICHAEL D. WASSERMAN’

‘Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275, USA ‘M.S. 963, U.S. Geological Survey, Denver, CO 80225, USA (Received Jmc 24, 199 1; accepted in revised,fimn July 30, 1993)

Abstract-We have determined oxygen and hydrogen isotope fractionation factors between alunite and water over a temperature range of 250-450°C by reacting synthetic natroalunite with 0.7 m K2S04 - 0.1 to 0.65 m H2S04 solutions to produce K-rich alunite. From 88 to 95% alkali and isotope exchange were observed in most of these experiments, and the partial equilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give lo3 In &lun)te(so&H@ = 3.09 ( 106/ T2 ‘80-‘60 fractionation (K)) - 2.94 and lo3 In GI~~~~~(oH)-H~o = 2.28 ( 106/ T2 (K)) - 3.90. The intramineral factor 10 3 In ~&mite (soI-OH site) is given by the expression 0.8( 106/ T2 (K)) + 0.96. The alunite-water DH fractionation factor ranges from -19 at 450°C to -6 at 250°C and does not appear to be strongly dependent on temperature. Runs with alkali exchange in the opposite direction were used to obtain ‘*O- I60 and D-H fractionation factors between natroalunite (mol% Na = 70-75) and water at 350-450°C. These indicate that mol% Na has negligible effect on the fractionation factors over this temperature range. Measured “O- 160 and D-H fractionation factors between alunite and 1.0 m KC1 - 0.5 m H,SO, fluids also agree within 2~ with the values obtained from the K2S04-H2S04 fluids. However, experiments with alunite and distilled water at 400°C gave a value of lo3 In ~~~~~~~~~~~~~~~~~ of 0.0, compared with a value of 3.9 obtained at this temperature with K2S04- and H2SOs-bearing fluids. This suggests that changes in fluid composition can affect alunite-water “O- I60 fractionation factors. Reconnaissance experiments with fine-grained natural natroalunite demonstrate that alunite-water DH exchange can occur by hydrogen diffusion, although this process is generally not significant in the experime&s with coarser-grained-synthetic alunites.

INTRODUCTION

techniques to measure the oxygen isotopic composition of the sulfate and hydroxyl sites have complete stable isotope analyses (6D, 634S, 6 ‘*Oso,, and 6 “OoH) of alunite been possible (WASSERMAN et al., 1990, 1992 ). In the last few years, a substantial body of complete stable isotope data including oxygen isotope data on both sulfate and hydroxyl sites in alunite from various environments has been obtained. Much of this work is presented and reviewed by RYE et al. ( 1990, 1992). Stable isotope data on minerals are usually interpreted in terms of the origin and history of the fluid that produced the minerals. This interpretation requires experimentally verified mineral-fluid fractionation factors over the temperature range of natural occurrences of the mineral. In this paper, we present experimentally determined alunite sulfate site- and hydroxyl site-water ‘*O- I60 fractionation factors and alunite-water DH fractionation factors obtained over a temperature range of 250-450°C. The internal fractionation of ‘8O-‘6O between the two sites in alunite has also been derived from these experiments. Results of reconnaissance experiments at temperatures as low as 55°C are also discussed. Although the experimental techniques used in this work are of limited use at temperatures of less than 25O”C, alunitewater ‘*O- I60 fractionation factors at lower temperatures may be estimated by extrapolation. In addition, the experimental data provide information about the rates of both alkali and isotope exchange reactions in alunite, as discussed in the companion paper by STOFFREGEN et al. ( 1993). These results

THE MINERALALUNITE ( KA13( SOdh( OH),)

forms in acidic, relatively sulfate-rich environments over a temperature range of 25 to at least 400°C. Alunite occurs as monomineralic veins ( CUNNINGHAM et al., 1984) or as an alteration mineral associated with kaolinite and quartz + pyrophyllite ? pyrite in a variety of hydrothermally altered and/or mineralized environments ( HEMLEY et al., 1969; FIELD and LOMBARDI, 1972: GUSTAFSON and HUNT, 1975; CUNNINGHAM et al., 1984; STOFFREGEN, 1987; JOHN, 1989; ALPERS and BRIMHALL, 1989; BOVE et al., 1990) as well as in modern geo-

thermal areas ( RAYMAHASHAY, 1968; SCHOEN et al., 1974; AOKI, 1983). It also occurs in low temperature supergene (BIRD et al., 1989; STOFFREGENand ALPERS, 1992) and sedimentary/diagenetic (ROUCHY and PIERRE, 1987; ALPERS et al.. 1992) environments. In addition to occurring in a wide variety of environments, alunite is unique among common minerals in containing four stable isotope sites. Thus, isotopic data on the sulfur, hydrogen, and both sulfate and hydroxyl oxygen in alunite can provide an exceptional amount of information about geochemical processes including the temperature of formation from internal fractionations in the two oxygen sites ( PICKTHORNand O’NEIL, 1985; RYE et al., 1990, 1992). Only recently, however, with the development of

* Present addrtw: AWK Consulting Engineers, 1225 Rodi Rd., Turtle Creek, PA 15 145, USA. 903

904

R. E. Stoffregen,

R. 0. Rye, and M. D. Wasserman

have been used to interpret isotope data from both hydrothermal and low-temperature alunite occurrences, as discussed in detail elsewhere (RYE et al., 1990, 1992; AREHART et al., 1992; ALPERS et al., 1992). EXPERIMENTAL

METHODS

Coupling of isotope exchange reactions with alkali exchange has been shown to increase significantly the rates of oxygen isotope exchange between alkali feldspar and water (O’NEIL and TAYLOR, 1967; MERIGOUX, 1968) and also between muscovite and water ( O’NEIL and TAYLOR, 1969). Our experiments demonstrate that alkali exchange also increases the rate of isotope exchange between alunite and water. However, because we were not able to obtain complete alkali exchange in any of our experiments, this method also failed to produce complete isotope exchange. As a result, the partial equilibrium technique (NORTHROP and CLAYTON, 1966; SUZUOK~ and EPSTEIN, 1976) was used to compute apparent mineral-water D-H and “O- I60 fractionation factors from the experimental data.

Table

1:

Isotopic composition of starting alunites and waters an %”

Alunites:

A (natroalunite) Et (natroalunlte) C (natraatunite) D (alunite) E (Sadler)

6.1

0.9 2.4 -0.1 2.1 16.4

a.6 6.1

a.7 4.9

-66 -46 -74 -40 -30

10 10 10 10 1

waters:

I. II. Ill IV. V. VI.

n.d.

S’BO

SD

10.8

10 -132 133 -236 -11 n.d.

-17.4 24.8 -31.0 -1.2 1.3

Not determmed

Experimental Procedures All experiments were conducted with synthetic natroalunite or alunite prepared using the method of STOFFREGEN and CYGAN ( 1990). From 60 to 225 mg of alunite and 0.9 to 2.2 grams of fluid were used in the runs, with fluid/mineral ratios of 8 to 20. This ratio was made as large as possible to minimize the shift in the isotopic composition of the water during the runs. The final 6180 and bD of the water was obtained by mass balance. In most of the experiments, the starting aqueous solution contained 0.7 m KzS04 as the source of K+. These solutions also contained from 0.1 to 0.65 m H2S04. Sulfuric acid was necessary to prevent the incongruent dissolution of alunite, which can be described by the reaction to form boehmite below 350°C KAI,(S04)2(0H)6

= 3AlO(OH) + K+ + 2SO:(boehmite)

or by the similar reaction

=

to form corundum

l.5A1203 + K+ + 2SO:-

+ 3H+,

(1)

above 350°C

+ 3H+ + 1.5H20

(2)

(STOF’FREGENand CYGAN, 1990). These reactions must be prevented from occurring, because corundum intermixed with the run product alunite would interfere with the measurement of alunite hydroxyl 6”OoH, and boehmite would interfere with both 6”Oou and 6D analyses. Because higher temperature tends to drive these reactions to the right, the molality of sulfuric acid in the starting solutions was increased from 0.1 at 250°C to 0.65 at 450°C. Several other starting solutions were also used to study the effects of solution chemistry on the measured fractionation factors, as discussed in a later section. The aqueous solutions and natroalunite were placed in gold capsules with a wall thickness of 0. IS-o.20 mm and welded shut with a conventional spot welder. As required in the partial equilibrium technique, pairs of runs containing essentially identical amounts of natroalunite and fluid were run together in the same rod bomb. The isotopic composition of these fluids, and of the starting alunites and natroalunites, are given in Table I. All runs were done at I kilobar with water used as the pressure medium with the exception of three pairs of runs conducted at one bar at 55 and 100°C. Run times ranged from 12 to 263 days. An initial set of nine runs was quenched in ice water whereas other runs were cooled to room temperature with compressed air over about 5 minutes. After quenching, the gold tubes were opened in a glove box containing humidified room air to prevent evaporation. The run product slurry of alunite + fluid was centrifuged and a portion of the fluid was drawn off by pipette for Na and K analyses by atomic absorption. The solid was washed in an ultrasonic cleaner for 5 minutes, filtered, and dried at 120°C for I h. Approximately IO mg of alunite were dissolved and analyzed for Na and K using the methods described by STOFFREGEN and CYCAN ( 1990), and the remainder was then stored prior to isotopic analysis. In some cases, the run

product alunite was not analyzed for Na or K, but its composition was estimated using mass balance based on Na and K in the aqueous phase. The methods used to obtain bD, 6”Oso1 (sulfate site), and 6”OoH (hydroxyl site) are discussed in WASSERMAN et al. ( 1990, 1992). Alkali Exchange Our initial assumption was that the % isotope exchange which occurred in the experiments would be equal to the % alkali exchange, and we therefore attempted to maximize the amount of alkali exchange subject to the constraint that the initial fluid/alunite ratio not exceed roughly 20. This ratio was limited by the minimum amount of alunite required for the three (dD, 6”OoH, 6’*Oso,) isotopic analyses (75 mg) and by the maximum amount of fluid that could be used in the charges, which varied from 2.2 to 1.O g at 250” to 450°C. Because of the preference of alunite for potassium over sodium relative to the coexisting aqueous solution over the entire temperature range of interest ( S~OFFREGEN and CYGAN, 1990), natroalunite was reacted with a potassium-bearing solution to produce a K-rich alunite in most of our experiments. The predicted final composition of alunite in these experiments, assuming the attainment of equilibrium, ranged from 99.5 mol% potassium for runs at 250°C to 95.6 mol% potassium at 450°C. The lower value at 450°C reflects the decreasing preference of alunite for potassium over sodium at higher temperatures, and also the smaller fluid/alunite ratios required at 450°C because of an increase in the molar volume of water at higher temperature. The observed composition of most run product alunites was in the range of 88-95 mol% potassium at each temperature from 250” to 450°C. Thus. although there was extensive alkali exchange. the runs failed to attain complete chemical equilibrium. In order to compare the amount of alkali exchange with the isotope exchange computed from the partial equilibrium technique, it is convenient to define a B alkali exchange in each experiment as %’alkali exchange

= 100 X (mol% Kf - molR> K,)/ (molY0 I& - mol% K,).

(3)

where the subscripts i and fdenote the initial and run product (final) alunite, and e is the potassium content at equilibrium computed with data from STOFFRECEN and CYGAN ( 1990). Because mol% K, for runs in which natroalunite was reacted with potassium-bearing solutions is essentially 100 for experiments at 250°C and below, the % alkali exchange in these runs is essentially equal to the mol% potassium in the run product alunite. However, for the natroalunite-to-alunite experiments at greater than 25O”C, as well as all experiments in which the alkali exchange reaction was run in the opposite direction, the denominator in Eqn. 3 is less than 100, and the % alkali exchange is greater than the amount of potassium added, or, for the reverse

Experimental

reaction, lost, from alunite.

puted % alkali exchange et al. (1993). Partial Equilibrium

The estimated uncertainty in the comis less than 1.5%, as discussed by STOFFREGEN

Technique

905

studies of alunite: I

ioo-

i

90

The partial equilibrium technique (NORTHROP and CLAYTON, 1966; SUZUOKI and EPSTEIN, 1976) is based on the assumption that the % isotope exchange, defined as 80.

100 x ((ui ~ oIf)/(o1j ~ a,),

(4)

is equal for sets of companion runs (O’NEIL, 1986). In this expression, (Y,is equal to ( 1 + &,,iJ lOOO)/( 1 + &,,,,,/1000) for the initial mineral and water 6 values, LYr is equal to this ratio for the final 6 values, and 01, is the equilibrium value for this ratio. The companion runs are identical in all regards except for the isotopic composition of the starting water. By using two or more companion runs the % exchange and the equilibrium fractionation factor may be determined from the relation (LYi- I) = ((u, - I) - A(Nf_

ai),

704

a0 % alkali

90 exchange

(5)

where A is the reciprocal of the % exchange / IO0 ( SUZUOKI and EPSTEIN, 1976). Based on the coupled alkali exchange and isotope exchange studies of O’NEIL and TAYLOR (l967), MERICOUX (1968), and O’NEIL and TAYLOR (1969), we expected that the amount of isotope exchange would be equal to the amount of alkali exchange, as defined above, in our experiments. This can be tested by comparing the % isotope exchange for each site obtained with the partial equilibrium technique with the percent alkali exchange observed during the runs (Fig. I a-c). The percent alkali exchange used in this comparison is the average of that observed on the two companion runs, which in all but one pair of companion runs agreed to within 2.2 mol%. The amount of ‘sO-‘6O exchange in the alunite sulfate site (Fig. la) is generally 5 to 10% less than the percent alkali exchange, whereas the “O- I60 (hydroxyl site) and D-H exchange are generally I to 5% less (Fig. 1b and c). The differences between the computed percent isotope exchange and measured percent alkali exchange are greater than their analytical uncertainty, demonstrating that the isotope exchange lags behind alkali exchange in these experiments, particularly on the sulfate site.

% alkali

exchange

Corrections

The initial aI80 of the fluids used in the experiments must be corrected for the effects of the sulfates added to the solution, and the final 6180 and 6D in the fluids must be corrected for shifts resulting from exchange with alunite during the run. The following procedures were used to make these corrections. The initial 6’80H,o was computed by assuming that water and aqueous sulfate in the solution attained isotopic equilibrium at the run temperature prior to any exchange with the alunite. This assumption may be justified based on the work OfCHIBA and SAKAI ( 1985), which suggests that sulfatewater equilibrium should occur in minutes or less in acidic, sulfaterich solutions at 200°C and above. The initial 6’80N,o of the fluid was computed from the equation 6 ‘Qli,o.,

- 618ox,aq - (XH2S04

+ XKzS04)

X 10’ In ~~~~o~_,,~o, (6)

where 6 ‘“0 r,aq is the bulk 6 I80 value of the aqueous solution and X refers to the fraction of the total oxygen in the aqueous solution represented by the species indicated. It is assumed in this equation that the ‘80-‘60 fractionation between water and the bulk aqueous sulfate can be described using the-equation of MIZUTANI and RAFTER ( 1969) for bisulfate-water fractionation. The final 6’80H,o value for the fluid was computed with the equation 6180 H20. f = [ fi “OZ - X alunite - X alunite

(SO,-site)

(OH-site)

X 6 180t (a,UnsodJ,,e)

X 6 180e (alunOHsi,e)

70

80 %

alkali

90

100

exchange

FIG. 1. Percent alkali exchange (see text) vs. percent isotope exchange computed using the partial equilibrium technique. The percent alkali exchange is the average of the two values for each pairs of companion runs, with the errors equal to one half the difference between them. Errors for the isotope exchange are propagated from the analytical uncertainty in the isotope measurements. (a) Percent alkali exchange vs. percent sulfate 180- I60 exchange; (b) percent alkali exchange vs. percent hydroxyl ‘*O- I60 exchange; (c) percent alkali exchange vs. percent D-H exchange.

- (XH2S04

+ XK2S04) (XH*O

X 10’ In ~~~~o~.~~o]/ + XH2S04

+ XK2S0,),

(7)

where 6’8O2 is the bulk 6180 value ofthe system and the b’80fvalues for the alunite sites are those of the run product alunite. The X here refers to the fraction of the total oxygen represented by the given species or alunite site. A similar correction was made for the final dDH20 value. However, because the 6D of the sulfuric acid used in the solutions was not

906

R. E. Stoffregen. R. 0. Rye, and M. D. Wasserman

measured, its effect on the initial and final SDH20values could not be determined. Even in the most sulfuric-acid-rich solutions, the acid constituted only about 1% of the total hydrogen in the system, and so it is assumed to have had a negligible effect on 6Di,,9. RESlJLTS Experimental conditions for the partial equilibrium runs in which natroalunite was reacted with &SO4 and HzS04bearing solutions are listed in Table 2. This table also presents computed values of IO3 In Naiunite(so4)_u20r IO’ In N alUnl,C IOH)-H20,and 10 3 In &h20. ‘8O-‘6O Fractionation between Alunite ( S04) and Water Experimentally determined values for lo3 In oalunlte(So4J_HZ0 are shown vs. 106/ T2 on Fig. 2. The errors associated with each point were obtained by propagation of the analytical uncertainty for the 6 “0 measurements. The estimated uncertainty for the 6 ‘*Oso4 measurements from WASSERMAN et al. ( 1992 and unpubl. data) is 0.2%0, which yields an uncertainty of 0.4 to 0.6 for the values of IO3 computed with the partial equilibrium ln anltec~04bH2~ technique. Based on the agreement between pairs of measurements made at the same temperature in our experiments, we believe that the true analytical uncertainty for this determination is only 0. I %o. If this uncertainty is used along with a value of *0.1%0 for the 6”O of starting and final waters, an estimated error of 0.30-0.35 is obtained for lo3 ln Nalunltc(SO&H?Or as shown by the error bars on Fig. 2. The average difference between pairs of companion runs at the same temperature in Table 2 is 0.32. The only temperature where the difference between the two measurements substantially exceeds this 0.30-0.35 range is 250°C. where values of 8.1 and 8.8 were computed. The value of 8. I is considered more reliable because it is based on more complete exchange and is more consistent with the data at other temperatures. The estimated error for the single measurement at 200°C is approximately four times larger than for the measurements at higher temperature and reflects the low % exchange observed in experiments 141 and 142. A least-squares fit of the experimental data weighted by the reciprocal of the error squared gives the equation IO3 In Na,un,,r(so4)-~20= 3.09( 106/T2(K))

- 2.94.

(8)

with a correlation coefficient ( r2) of 0.998. This linear variation in IO 3 In 01with I / T2 is typical of mineral-water fractionation factors over this temperature range ( O’NEIL, I986 ). Because mineral-water “O- IhO fractionations diminish to zero at higher temperatures ( O’NEIL, 1986), this line can be expected to approach zero asymptotically at some temperature above the range of our study. This behavior is of little practical interest, however, because alunite probably does not form in nature at temperatures in excess of 450°C. Extrapolation of this equation to lower temperature yields a value of 10 3 In munlte ~S04~.H20 at 25°C of 3 1.9. This value is also of little practical interest, because the alunite sulfate site is rarely if ever in equilibrium with water in alunite formed at surficial conditions ( PICKTHORN and O’NEIL, 1985; RYE et al., 1990, 1992).

( S04) water “O- “0 fractionation factor can with bisulfate-water and mineral sulfate-water factors as shown on Fig. 3. The alunite curve the same slope as the bisulfate-water curve of MIZ~JTANI and RAFTER ( 1969) but lies slightly above it, and also lies above the curves for barite-water ( KUSAKABE and ROBINSON, 1977) and anhydrite-water ( CHIBA et al., 198 1). The limited amount of exchange in the 200°C experiments and the correspondingly large uncertainty in IO3 In 01 precludes determination of reliable fractionation factors at or below this temperature. This is even more clearly demonstrated in runs of similar duration at 15O”C, which produced less than 1%alkali exchange ( STOFFREGEN et al., 1993). Due to this small shift, no isotopic values were measured for these run product alunites. Attempts to obtain better constraints on the fractionation factors at temperatures below 200°C using fine-grained natural natroalunite as a starting material are discussed in a later section. The alunite be compared fractionation has essentially

‘*O-i60

Fractionation between Alunite (OH) and Water

Experimentally determined VdueS for IO3 ln ~~~~~~~~ (ou)_u20 are shown vs. 106/ T2 on Fig. 4. The uncertainties shown on this figure are based on an analytical precision of +O. l4%0 for 6 I80 which gives an uncertainty of about 0.4 for lo3 This value is larger than that for the sulfate ln ff,iU”ll~~“,~ )-H20. site-water fractionation because 6’*Oou is computed as the difference of the measured 6 ’80s04 and total oxygen 6 ‘*Oaiunite and its uncertainty is thus inherently larger than that of 6 “Oso, (WASSERMAN et al., 1992). Agreement between pairs of runs is within the analytical error at 250”, 300”, and 400°C but is poor between the two values obtained at 450°C. The 450°C runs 149 and 1.50 yield a value of lo3 low relative to the ln (Y&?lle(OHbH20 which is anomalously other values. It was initially suspected that this resulted from the formation of some corundum during these runs, which would interfere with the measurement of 6 “Oou but not with 6”Oso4. However, SEM examination ofboth run-product alunites failed to identify any corundum. In the subsequent pair of runs at 450°C (208 and 209), the solution chemistry was adjusted slightly in an effort to reduce the likelihood of corundum formation. This pair of runs produced a hydroxyl site-water fractionation more consistent with the lower temperature values. Only one determination of IO3 In ~~~~~~~~~~~~~~~~ was obtained at 350°C. This value is consistent with fractionation factors at the other temperatures and also with an equilibrium fractionation factor described in a later section. As with the sulfate site-water fractionation, the 200°C hydroxyl site-water fractionation has a relatively large error due to the small amount of exchange, and is too low relative to the data at other temperatures. Although the hydroxyl site-water fractionation factors show the expected monotonic increase with 106/ T2, there is more scatter than for the sulfate site-water fractionation. The straight line shown on Fig. 4 is a least-squares fit of all the data points from 200-450°C weighted by the reciprocal of their error squared, with the equation 103 ln

aalunlte(OHbH20

The correlation

coefficient

=

2.28( 106/T2(K))

- 3.90.

( r2) for this fit is 0.95.

(9)

400

400

25

12

12

259

259

206

209

96

97

350

300

300

67

30

30

263

263

113

113

259

259

120

120

174

174

136

203

205

93

94

133

134

91

92

143

144

141

142

150.2(B)

150.0(B)

150.9(B)

149.4(B)

194.1(A)

194.3(A)

121.7(C)

121.6(C)

197.3(A)

200.6(A)

62.1(S)

62.0(E)

146.2(B)

1.9176(IV)

1.9099(111)

1.9117(IV)

1.9051(111)

1.9490(l)

2.2393(11)

1.7594(111)

1.7494(lV)

1.7313(l)

1.7336(11)

l.l005(Vl)

1.1106(V)

1.5974(lV)

1.6003(111)

1.1932(l)

1.1693(11)

l.l075(IV)

1.1040(111)

1..3320(11)

1.7843(l)

0.9197(lV)

0.9306(111)

0.9376(1'//3

experiments.

94.4 93.9

22.2

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

0.7

20.1 -17.1

25.1 -11.4

07

0.1

01

0.1

-79

-16 3.5

-1.7

6.4

12.4

16.2

-10

0.1

12.9

-10.1

-5.6

-124

0.1

-196

22.1

25.3

0.1

-20.3

-16.6

104

12.0

15.0

-217

-10.7

-7.3

0.3

-6

0.3

0.1

-130

4.2

6.6

n.d.

0.5

0.1

1.9

4.6

n.d.

5

23

1

20.9

21.2

61.6

62.5

66.0

69

20.9

21.2

62.0

62.9

66.5

22.6

23.6 -29.3

-29.0

23.3

-28.1

23.7 -29.2

9.6

-15.6

22.6

-27.6

9.7

-15.2

10.3

-16.3

23.5

91.3

69.6

-26.6

91.1

90.4

10.4

-16.2

93.9

1.6

-0.4

-0.5 1.8

-26.5

9

9

-236

133

-236

131

-131

131

-236

-131

n.d.

n.d.

-235

131

n.d.

10.3 22.3

n.d.

-15.6

-26.0

23.4

10.6

-16.2

92.5

91.9

66

96.2

96.2

n.d.

n.d.

132 -236

22.7 -26.7

23.5 -27.9

9

96.3

-131

9.9 -14.3

96.6

10.5

n.d.

-15.4

n.d.

22.7

97.6

65.1

90.6

131 -236

-26.6

1

22.6 -26

final bDtisc

23.5

4

@0+20~

fluid (Y&) tinal

-27.0

-27.5

95.4

94

92

96.7

94

00

90.5

alk

exchg

%

86.6

94.1

-21.3

-16.6

-227

0.5

0.7 0.5

94.1

21.6

24.5

0.7

105

n.d.

0.5

n.d.

n.d.

13.5

n.d.

0

n.d.

-6.7

n.d.

0

-16.6

0.7

-231

91.3

0.7

0.5

24.2 -23.2

0.7

0.7

97

0.5 0.5

92.9

10.3 -11.6

13.0 -6.7

-4 -129

0.5

0.7

god

0.7

66d

21.7 -23.7

23.2 -20.2

n.d.

0.6

n.d.

94.2

0.65

90.4

20.8 -24.2

0.65

23.5 -16.7

100 -237

0.65

K

mol%

alunlte (S in 5,)

BisOm4 b’%

0.65

IH;SO4

run Droduct

0.6

0.7

0.7

KZS04

molalitv

exchange

All runs were done at a pressure of 1.0 kilobars a see Table 1 for isotopic composition of alunites A-C b see Table 1 for isotopic compostion of waters I-VI c calculated (see text) d calculated from analysis of fluid chemistry by mass balance B “equilibrium” value (see text) n.d. not determined

200

200

250

250

250

250

300

300

350

350

146.5(B)

350

67

60.6(A)

96

135

63.6(A)

350

350

96

58

59

74.5(B)

74.7(B)

400

223.0(A)

400

41

41

147

450

146

61.9(A)

60.2(A)

450

195.6(A)

73.0(B)

0.9360(111)

(9)

(mg)

70.6(B)

tluidb

atunitea

startino

alkali and isotope

450

450

25

150

N)

149

T

darj

Coupled

Run

Table 2: alunite

alunIte

72.2

87.1

83.7

9.2

6.6

8.1

6.5

6.4

4.v

n.d.

89.7

5.09

5.0

5.3

3.6

3.9

n.d.

65.0

77.5

67.2

90.0

2.6

16.1

73.0

69.3

64.9

91.4

4.9

3.6

4.2

3.6

4.4

5.0

3.9

2.9

16.6

70.3

67.6

-6

2.9 3.4

69.6

n.d. n.d. n.d.

2.4

2.3e 2.4e

-11

n.d

-16

-6

n.d

-19

n.d.

91.8

n.d.

91.0

90.2

n.d.

93.7

I" a

n.d.

3.1

n.d.

2.5

2.5

2.0

% exchg

103

alunite (D)-Hz0

2.7

1.9

".d.

1.3

1.4

0.8

-0.3

n.d.

66.3

n.d.

92.1

91.0

92.3

92.6

3.2

67.1

66.3

exchg

io3 I” 01

40

(OH)-HLJO

exchg

iO

~S04)-HP0

G

Ez 2

s

5

w

B 1.

R. E. Stoffregen, R 0. Rye, and M. D. Wasserman

908

T (“Cl 450 II

350 I

200

250

I

t

IO-

6-

6-

4-

4

3

5

lO?T*(K) 2, 1

2

3

4

106/T2

5

(K)

FIG _2. Experimental vatues of 103 In aslunite(s04~_H 0 obtained with the partial equilibrium technique plotted against lOa/ TZ (K). Error bars were computed assuming an analytical uncertainty of jrO.1%s for each isotope measurement. The line is a least-squares fit weighted by the reciprocal of error squared (see text).

‘sO-‘6O Fractionation between Alunite ( S04) and Alunite (OH) The ‘8O-16O fractionation between alunite (Sod) and alunite (OH) may be obtained by subtracting Eqn. 9 from Eqn. 8, which gives 10 3 In %unitr (So.,-oHsite) = O.S( ~O’/~‘(K)}

+ 0.95.

f i0)

This line is shown on Fig. 5 along with the sulfate-OH fractionation factors obtained from each pair of experiments. These points define a linear trend with the exception of runs 149-50 at 45O’C and 143-144 at 250°C. In the case of the first pair, the disagreement refiects an anomalously low value for the OH-water fractionation factor, whereas for the 250°C runs it reflects an anomalously high value for the sulfate site-

FIG. 4. Experimental values of IO3 In cyaluni,~~oN~_~po obtained with the partial equilibrium technique plotted against 106/ T2 (K). Errors are computed assuming an analytical unce~ainty of %O.14%~~for the alunite OH site and &O.1%O for water. The line is a least-squares ht weighted by the reciprocal of error squared (see text).

water fractionation factor. Although the plot suggests that inclusion of these two points may shift the fit line to an incorrect position relative to the remaining points, their exclusion from the fit changes the equation very little. The above equation is, therefore, retained as the best description of the experimental data. Although the sulfate and hydroxyl site-water fractionation factors at 200°C are too small relative to the other temperatures, the experimental value of lo3 In a,lunire(soaou site) at 200°C is consistent with values at other tem~~tures. This probably rest&s from a cancellation of errors in the partial equilibrium technique when lo3 In &rUnIte(oH)_u20is subtracted from lo3 In ~~~~~~~~~~~~~~~~~~ Also shown on Fig. 5 is the range of IO3 In o&&(son_oH sire)observed in the starting synthetic dunites and natroalunites, which were initially synthesized at about 150°C. These values range from 5.2 to

T (‘(=) c

450 * !B

250

350

t

6_ lo-

.

0,

1 1

2

3

4

5

106/T2(K)

FIG. 3. Comparison of 10’ In ~~~~~~~~~~~~~~~~~ with hisulfate-water ( MIZUTANIand RAFTER,1969), barite-water ( KUSAKABEand ROBINSON, 1977) and anhydrite-water (CHIBAet al., 1981) fractionations.

,

,

,

,

2

3

4

5

. 6

106/T2(K)

Fto. 5. “O- I60 fractionation between the alunite sulfate and hydroxyl sites obtained with the partial equilibrium technique (solid circles) plotted against 106/ T2 (K). The range in this fractionation observed for alunite and natroalunite by the open circles.

synthesized

at 150°C is shown

909

Experimental studies of alunite: I 6.6 and are broadly consistent higher temperature data.

T ( “‘3

with the trend defined by the

D-H Fractionation between Alunite and Water Values of 10 3 In a~~~lte_H20obtained from the experiments in Table 2 are shown in Fig. 6. An estimated analytical precision of 2.0%0 (u) for each alunite dD measurement results in uncertainties in IO3 In ~~~~~,~~_u~o generally between 4.5 and 6.5. The two estimates of lo3 In c~$~~,,~_u~o at 250°C are in excellent agreement, and the pair of values at 300°C agree to within the estimated analytical uncertainty. In contrast, the two measurements at 400°C differ by 12. The 400°C value of - 18 from runs 147 and 148 is considered more reliable than the value of -6 from runs 96 and 97 because it is more consistent with results of other 400°C experiments discussed in a later section. As in the results for 180- I60 exchange, the value of lo3 In a$&_uIo at 200°C has a substantially larger error than the other data points due to the low % D-H exchange. The measured value of -9 nevertheless appears plausible in comparison with the higher temperature results. In Fig. 7 the alunite-water D-H fractionation factors are compared with the boehmite-water curve (GRAHAM et al., 1980) and the kaolinite-water curve ( LIU and EPSTEIN, 1984). The value of lo3 In ol!~,$e_H20is relatively constant near -20 from 450” to 400°C increases by roughly 10 between 400°C and 350°C and remains in the range of - 10 to -5 down to at least 200°C. This pattern may be compared with the DH fractionation factor between water and boehmite, with which alunite has chemical and structural similarities. Although more negative by 30, the boehmite curve is similar in that it appears to be relatively constant over a range of temperatures then increases by 10. For boehmite this shift occurs at 200-250°C. The alunite-water fractionation factors are similar to kaolinite-water D-H fractionation factors over the temperature range where we have measured them. However, the 25°C alunite-water fractionation from BIRD et al. ( 1989) suggests that the mineral-water fractionation curves

T (“Cl 450

-50

350

250

200

6 1

2

3

4

-. *. kaolinite *. _.

0

2

4

6

6

10

12

106/T2(K) FIG. 7. Comparison of alunite-water D-H fractionation curve obtained in this study with D-H fractionations for boehmite-water (GRAHAMet al.. 1980) and kaolinite-water (Lru and EPSTEIN,1984). The open circle at 25°C is the alunite-water D-H fractionation from BIRD et al. (1989).

of the two minerals diverge at lower temperature. The relationship between these two fractionation factors is especially important because alunite and kaolinite are commonly associated both as alteration minerals in hydrothermal ore deposits and in low temperature settings (e.g., RYE et al., 1990, 1992). Equilibrium Experiments To corroborate the results from the partial equilibrium technique and to obtain true equilibrium fractionation factors experimentally, we conducted two additional runs at 350°C in which 6’80u,o of the starting fluids were within ?3.5%0 of expected equilibrium with the oxygen sites in alunite. These reactions also used coupled alkali exchange, and are listed with the partial equilibrium runs in Table 2. They were designed to constrain the equilibrium fractionation factors to within 0.2 for both sulfate and hydroxyl sites. Both of these runs showed roughly 85% alkali exchange. The sulfate site-water fractionation factors for experiments 203 and 205, both of which started with lo3 In 01 greater than the inferred equilibrium value, are 5.0 and 4.8, respectively, in reasonable agreement with the partial equilibrium value of 5.1. Run 203 gave a minimum value for IO3 In (Yalun,,e(oHj_NZoof 2.3 and run 205 gave a maximum value of 2.4, thus providing an equilibrium fractionation factor of 2.35 + 0.14. This is in reasonable agreement with the value of I .9 It_0.38 obtained using the partial equilibrium technique in experiments 135 and 136.

5

lo6 I T*(K) FIG. 6. Experimental values of lo3 In c~$&.u o obtained with the partial equilibrium technique plotted against lo’/ T2 (K). Errors are computed assuming an analytical uncertainty of -+2.0%0for alunite 6D.

EFFECTS

OF NATROALUNITE COMPONENT FRACTIONATION FACTORS

ON

O’NEIL and TAYLOR ( 1967, 1969) demonstrated that oxygen isotope fractionation factors for alkali feldspars and micas are not affected by mol% Na. In order to test the effect

910

R. E. Stoffregen, R. 0. Rye, and M. D. Wasserman

of mol% Na on alunite-water fractionation factors, we conducted four pairs of experiments designed to produce natroalunite from alunite by reaction with NazSO1-bearing solutions. Because of the preference of alunite for K over Na, this reaction could not be driven as far towards complete exchange on the alkali site as the initial natroalunite-to-alunite experiments. For mineral/fluid ratios comparable to those used in the initial experiments, the predicted equilibrium composition of the run product is in the range of only 7377 mol% natroalunite, compared to 95-100 mol% alunite predicted for runs in the opposite direction. Experiments 156 and 157 (Table 3) were run for 30 days and produced alunite with 72 mol% Na, which is equal to approximately 93% alkali exchange. This was accompanied by 88% ‘80-‘h0 exchange in the sulfate site and 92% “OI60 exchange in the OH site. The extent of alkali and isotope exchange was similar to that observed in experiments 147 and 148 (Table 2), which were run for 41 days at 400°C with the alkali exchange reaction going in the opposite direction. Similar amounts of alkali and isotope exchange were obtained for the alunite to natroalunite experiments 194 and 195 (35O”C), 184 and 185 (4OO”C), and 206 and 207 (450°C). Fractionation factors obtained with these experiments are compared in Table 4 with the values obtained from the fit equations presented above. At 350°C fractionation factors between water and both the sulfate site and hydroxyl site in natroalunite are within 20 of the previous values. In runs 156 and 157 at 400°C IO3 In ~~~~~~~~~~~~~ (so4).n20 is in excellent agreement with the value computed with Eqn. 8, whereas 103 In ~n,troa~unlte cO~j-~20is 1.3 more negative. However, a second pair of runs at 400°C yielded a value of lo3 In CY na,rDalUn,,e (oHJ_HZoof 1.3, in excellent agreement with the value from Eqn. 9 of 1.1. At 450°C both the natroalunite (OH )-Hz0 and ( Sod)-Hz0 fractionation factors deviate by more than 0.5 from the values for alunite-H20. If the natroalunite (OH )-Hz0 fractionation from experiments 156-157 is discounted, the remaining data at 350400°C indicate that the mol% natroalunite has a minimal effect on both ‘80-‘60 fractionation factors in this temperature range. This observation, coupled with the fact that isotope fractionations generally decrease with increasing temperature, suggests that the larger differences observed in the fractionation factors at 450°C are not accurate. This is supported by the poor reproducibility of the alunite ( OH)-HI0 fractionation factors at 450°C observed in the initial experiments (Table 2). Based on these arguments, we reject the results at 450°C and tentatively conclude that the mol% natroalunite has little effect on either the sulfate site- or the hydroxyl site-water fractionation, at least above 350°C. The D-H fractionation factor between natroalunite and water was determined only for runs 184 and 185 at 400°C. These give 10 3 In (Y!,~!oalunite_HZo equal to -30 + 5, compared with a preferred value of - 18 from the coupled alkali and isotope exchange in alunite. This difference is greater than 2~ of the estimated analytical error and indicates that lo3 In ~$$~~_n~o probably does vary as a function of mol% Na at 400°C. However, this variation is not large enough to affect geological interpretations based on alunite 6D measurements in most cases. BIRD et al. ( 1989) report no detectable effect of the mol% Na on lo3 In aLEii,,.n,o at 25°C.

EFFECTS OF SOLUTION

CHEMISTRY

Exchange experiments were conducted between alunite and pure water and also between natroalunite and KCI-H2S04 bearing solutions in order to study the effect of aqueous solution chemistry on alunite-water isotope fractionation factors. Run conditions and results of these experiments are given in Table 5. Table 6 provides a comparison of lo3 In ~l~r~~,,~(so~)_~~o and lo3 In ~~$$,~_n~o obtained from these experiments with the values obtained from the fit equations presented above. In runs I54 and 155 alunite was reacted with distilled water at 400°C. Corundum was produced in these two experiments due to incongruent alunite dissolution (cf. STOFFREGEN and CYGAN, 1990). Although this corundum was not taken into account in the isotopic mass balance equation, the amount produced was sufficiently small to have negligible effect on the computed fractionation factors. The value of lo3 154 and 155 was 0.0 In &lunlte(SO&H20 based on experiments f 0.7, which is substantially different from the value of 3.9 computed with Eqn. 8 (Table 6). In contrast, lo3 182 and 183. In aalunltccS04j-~z0 obtained from experiments in which natroalunite was reacted with a I.0 m KCl-0.5 m H,SO, fluid at 4OO”C, is within 2a of 3.9. Similarly, IO’ 180 and 18 1, In &lunlte(SO&H20 obtained from experiments in which natroalunite was reacted with a 1.O m KCI-0.1 m HISO fluid at 25O”C, is within 2n of the value of 8.4 computed with Eqn. 8. Values of lo3 ln ~~/$$e_n20 obtained at 400°C were -25 f 6 using distilled water as a starting solution and -13 f 5 with the 1.0 m KU-O.5 m HzS04 solution (Table 6). Both are within u of the initial value of - 18, and suggest a discernible but not substantial effect of the fluid chemistry on IO3 In m(D). alun,te_H20.The 250°C value of lo3 ln a!~~ite_n20 obtained with the 1.O m KCI-0.1 m H2S04 solution of - 14 f 5 is within 2u of the initial value (Table 6). These results can be compared with previous work on the effects of fluid chemistry on “O- “0 and D-H fractionation factors. CHIBA et al. ( 198 1) observed no change in the measured value of lo3 In LY~~,+,,~(so&&o for 1.O m NaCl, 1.O m H2S04, and 1.O m HCI solutions at 350°C. Similarly, values 0flO’lnoc bar,te(soI)_n20reported by KUSAKABE and ROBINSON ( 1977) for 1.O m NaCl solutions and for 1.0 m NaCl-1.0 m HzS04 solutions agree within 0.5 over a temperature range of 250 to 350°C. LICANG et al. ( 1989) found that variations in the amount of dissolved NaCl, KCl, and NaF did not change measured values of lo3 In aquatiz_n20by more than 0.5 between 250 and 450°C. The total salt content in the experiments of LIGANG et al. (1989) ranged from 5 to 40 wt%. These results are consistent with experiments 180- 18 1 and 182- 183, which also imply that changes in fluid chemistry do not significantly affect mineral-water “O- rhO fractionation factors. However, none of these experiments provide information about the fractionation of “O- ‘(‘0 between minerals and dilute fluids. As such they cannot be compared directly with the results of experiments 154 and 155, which imply that these fractionation factors are significantly different for dilute fluids than for saline fluids. Information about mineral-pure water 180- I60 fractionation factors is difficult to obtain experimentally because of slow exchange rates. In the absence of such data, these frac-

Experimental studies of alunite: I

Table 3: Isotope exchange experiments between alunite and sodium-bearing solutions. __________________~~~___________________~~~~~~~~~__________~~~_____~~~~~~_~__~~~~~~~_____~~_-_____ starting

run

molality

product

alumte

(6 in ‘:,) fluid

Run

#

days

T (“C)

alunitea

ffuidb

(mg)

(9)

Na2S04

6D

f-l@04

F1eOm4

Pa@+

mol% KC

% alk ex.

initial s’eo&oc

(a, ) final

final

F’eO&oc

~f&Oc

206 207

10 10

450 450

72.2(D) 71.5(D)

0.86Oljfll) 0.8551(1\1)

0.6 0.6

0.65 0.65

n.d. n.d.

22.9 -21.0

22.9 -24.0

28 28

98 97

23.8 -28.2

22.6 -26.6

132 -236

156 157

30 30

400 400

75.0(D) 74.3(D)

1.1019(111) 3.0944(1\1)

0.7 0.7

0.5 0.5

n.d. n.d.

24.7 -19.5

21.2 -25.2

29 28

93 94

23.8 -28.4

23.0 -27.2

132 -236

184

34

400

75.6(D)

1 .1110(111)

0.7

0.5

21.7

185

34

400

74.8(D)

l.l045(IV)

0.7

0.5

194 195

27 27

350 350

69.9(D) 67.1(D)

1.2372(111) 1.2055(lV)

0.7 0.7

0.5 0.5

70 -214

n.d. n.d.

n.d. n.d.

25.0 -18.7

25

99

-22.7

27

95

-28.4

-27.2

-236

22.3 -22.9

29 28

96 96

23.7 -28.5

23.1 -27.5

132 -236

---_--------------------------------------------------------------------------------All runs were done at a pressure of 1.0 kilobars a see Table 1 for isotopic composition of alunite D b see Table 1 for isotopic compostion of waters III-W C calculated from analysis of fluid chemistry by mass balance n.d. not determined

Table 3. (Continued). aluntte

alunite (OH)-HP0

(=4)-H20

alunite S04-OH

atunite (W-H20

103 In a

% exchg

103 In a

103

%

exchg

In a

exchg

89.1

1.9

95.0

1.2

0.6

n.d.

nd.

88.2

3.8

92.7

4.1

n.d.

n.d.

n.d.

n.d.

86.7

1.3

n.d.

79.8

66.4

4.5

89.6

1.7

2.8

%

-0.2

n.d.

103 In a

-30

n.d.

23.8

‘23.0

132

VI2

R. E. Stoffregen, R. 0. Rye, and M. D. Wasserman Table

4:

factors

Comparison

with

of experimental

alunite-water

fractionation

run # (pairs)

T (“C)

206 and 207

450

1.9kO.3 (3.0f0.3)

156 and 157

400

3.8f0.3

184 and 185

400

194 and 195

350

1 031nanatroalunite

fractionation

factors

103h

(S04)-H20’

lOsIn CL(~)

%atroalunite

(OH).H20”

natroahmie-Hz0

1.2f0.4

(0.5*0.4)

n.d

(3.9kO.3) -0.2f0.4

(l.lf0.4)

n.d.

1.3kO.4(1 .lf0.4)

n.d.

4.5+0.3

natroalunite-water

(5.OkO.3) 1.7f0.4

-3Ok5

@

(-18+5)

(2.OkO.4)

* Values in parantheses are computed with equation 8. ** Values in parantheses are computed with equation 9. @ Values in Darantheses are oreferred values based on data in Table 2 (see text). n.d. Not determined tionation

factors

have

been

calculated

using data

from

also

of temperature

( TRUESDELL,

1974).

As

noted

by

of 1*O-‘6O between

TRUESDELL ( 1974) and COLE and WESOLOWSKI ( 1989), this

aqueous solutions and CO2 at up to 250°C (e.g., KUSAKABE

effect of fluid chemistry on isotope fractionation factors may need to be considered in the interpretation of some natural 6180 and 6D data. In the case of alunite. which occurs in sulfuric-acid rich environments ( HEMLEY et al., 1969), additional data on the effect of alkali sulfate and sulfuric acid on mineral-water isotope fractionation factors will be required for a quantitative understanding of the impact of fluid composition on the interpretation of natural alunite data.

TRUESDELL ( 1974) on the partitioning

and ROBINSON, 1977; COLE and WESOLOWSKI,

1989). The data of TRUESDELL ( 1974) imply that 10 3 ln (Y,,,,r&H20 is 3.0 larger at 250°C for a 4.0 m NaCl solution than for pure water (COLE and WESOLOWSKI, 1989). This is similar to the difference of 3.9 between lo3 ln &lun~te_,+Ofor pure water and for a 0.7 m K2S04-0.5 m H2S04 solution at 400°C inferred from experiments 154 and 155. However, since no data are available on the effect of alkali sulfate or sulfuric acid on ‘8O-‘6O fractionation at any temperature, the value of lo3 In ~~~~~~~~~~~~ for pure water obtained from experiments 154 and 155 cannot be directly corroborated. As such this value should be considered tentative. Moreover, this fractionation factor is inherently less accurate than the others reported in Table 6 because it is based on only 43.7% exchange vs. more than 80% exchange for the other pairs of experiments. Mineral-water D-H fractionations have been shown to be affected by fluid chemistry. COLE and WESOLOWSKI ( 1989) report that apparent mineral-water D-H fractionation factors are more negative for pure water than for solutions with 1.O m to 4.0 m NaCl by up to 11. Similarly, GRAHAM and SHEPPARD ( 1980) report that epidote-solution D-H fractionation factors are twelve larger for seawater than for pure water at 350°C. Our results are broadly consistent with these data in that values of lo3 In a@). &n,te_H20measured using chloride and sulfate-bearing fluids are seven to twelve greater than IO3 In a!k&,o measured with pure water. Changes in mineral-water “O- I60 and D-H fractionation factors as a function of fluid composition are generally ascribed to partitioning of oxygen and hydrogen isotopes between bulk water and waters of hydration around aqueous species ( TAUBE, 1954; BOPP et al., 1977; TRUESDELL, 1974). The extent of such fractionation is known to be a complex function of the type and concentration of dissolved salts and

EXPERIMENTS

WITH NATURAL ALUNITE

Alunite commonly occurs as 1 pm or smaller pseudocubic grains in low-temperature natural settings ( STOFFREGEN and ALPERS, 1992). These alunites have a surface to volume ratio roughly one order of magnitude greater than the synthetic alunites used in our experiments. Their large surface area suggests that they are more reactive than the synthetic alunites and could be used to measure alunite-water isotope fractionation factors at temperatures below 250°C. We conducted four pairs of experiments at 55” to 150°C using a natroalunite from Sadler, Texas. This natroalunite occurs in 0.5-1.0 pm pseudocubic grains, has a mole fraction sodium of 0.82 and contains appreciable nonstoichiometric water ( STOFFRECEN and ALPERS, 1992). It also contains traces of calcium, strontium, and phosphorous (ROSS et al., 1968). Run conditions for these experiments are summarized in Table 7. Experiments 164 and 165 were run at 150°C and 1000 bars, whereas the other six runs were at one bar. The isotopic exchange was coupled with alkali exchange in the 150°C experiments and in one of the two pairs of runs at 1oo”c. Only 4% alkali exchange was observed after 84 days at 150°C. In contrast, the partial equilibrium technique yielded a value of 58% D exchange with lo3 In ~~~~jt&,Zo equal to

12

14 14

183

180 181

T

250 250

400

400

400 400

W)

atunitea (mg)

81.9(B] 83.1(B)

74.3(B)

74.2(B)

75.1(D) 75.1(D)

experiments

1.0 1.0

1.0 1.0

1.0651(111)

l.O708(IV)

l.~463(lll~ l.O515(lV)

__--L_-__x_-_-I__-P

0 0

KU

0.1 0.1

0.5

0.5

a a

W4

molality

with different

l.loll(lv) 1.1022(ltl)

wt (91

fiuidb

starting

Isotope exchange

24.8 -21.5

28.2 -17.3

100 -225

86 -209

-8.1 15.4

-215 f7

6D

initial

24.3 -29.8

24.7 -30.9

95 98

92 90

-31.0 24.8

n-a. n.a.

&‘8sl20

23.8 -29.5

-28.5

23.4

-30.5 24.5

final S’aq^lzo

fluidc (%0)

alunite

131 -236

85.7

89.4

131

7.8

3.3

0.0

43.7

-238 132

-236

103 In a

fS~4bW

% exchg

final w-t20

---__--1----1_1_--~--~~~~~~~~~~~-~~~-~~~~~~-~~~~~

gld 8gd

g6d

g3d

n.a. n.a.

run praduct alunite (6 in G) tY8ost~4 mol% % alk K exchg

fluid chemistries

All runs were done at a pressure of 1.0 kilobars a see Table 1 for isotopic composition of alunites 6 and D b see Table 1 for isotopic compostion of waters Ill-IV C calculated (see text) d calculated from analysis of fluid chemistry by mass balance n.a. not applicable

--_-

12

182

days

85 80

#

154 155

Run

Table 5:

81.6

89.8

81.4

% @x&g

z2. R

2 S E B E? 8 0,

-13

-14

G a 3.

-25

In a

103

alunite (W-M’

914

R. E. Stoffregen, R. 0. Rye, and M. D. Wasserman

Table 6: Effect of fluid chemistry on fractionation ________________________---___-------------------_----__-solution run # T(C) 1031nWmite (pairs) (SO4)-H2O’ 154

and

factors 1 Osln(r(D) alunite-H20”

pure Hz0

O.Of0.7

(3.9+0.3)

-25f6

(-18f5)

182 and 183 400

1 .Om KCI, 0.5m HaSO

3.3f0.3

(3.9kO.3)

-13f5

(-18*5)

180 and 250 181

l.Om KCI, O.lm H2SO4 7.8f0.3

(8.4f0.3)

-14*5

(-7i5)

155

400

’ Values in parantheses

are computed

** Values in parantheses 2 (see text).

are preferred values based on data in Table

-24 + 6.0. Because of the presence of appreciable excess water and minor amounts of other elements in this natural alunite, this fractionation factor may not pertain to pure alunite or natroalunite. These experiments nevertheless provide unequivocal evidence that the rate of D exchange at 150°C is substantially faster than the rate of alkali exchange, in contrast to the higher temperature experiments with coarser-grained synthetic alunites. The shifts in 6 ’80s04 and 6 ‘*Oon during the 150°C runs were too small to compute meaningful fractionation factors, and as a result neither value was measured in the experiments at lower temperatures. The two pairs of 100°C experiments are more difficult to interpret. Runs 168 and 169 used a 0.7 m KzS04-0. 1 m HzS04 solution but showed minimal change in mol% Na. In run 168, the natroalunite 6D was shifted from -30%0 to -19%0 by exchange with water III (6D = 133%0), but run 169 showed a substantially larger shift to -95%0 during reaction with water IV (6D = -238%0). In runs 166 and 167, which used pure water as a starting fluid, the relative amounts of D-H exchange were reversed. In run 166 natroalunite was reacted with water III and its 6D value increased to 4 I %o,whereas exchange with water IV in reaction 167 produced only a 15%0 shift in 6D to -45%0. Such inconsistent results are typical of experiments with low % isotope exchange, as noted by O’NEIL and KHARAKA ( 1976). Because of these inconsistencies, no value of lo3 In acD). alun,te_Htowas computed from the 100°C data. No D exchange was detected in either of the two runs at 55°C.

with equation

8.

The experiments with Sadler natroalunite indicate that even with fine-grained natural alunites, exchange reactions are too slow at 150°C and below to measure 1*O-‘6O fractionation factors between alunite and water. In contrast, D exchange is sufficiently rapid to allow measurement of 10’ In CY!~&~ at 150°C and to produce significant D exchange down to 100°C in experiments of less then three months duration. These experiments demonstrate that at low temperature, rates of D exchange are substantially higher than both oxygen and alkali exchange, indicating that transfer of protons into alunite occurs independently of any oxygen or alkali exchange. Similar results were obtained by O’NEIL and KHARAKA

( 1976) on clay-water

isotope

exchange.

CONCLUSIONS We have used coupled alkali and isotope exchange experiments together with the partial equilibrium technique to determine alunite-water l8O- I60 and D-H fractionation factors from 250 to 450°C. The temperature dependence of these fractionation factors is given by the equations

I O3In aahnlteCSO~)-H~O = 3.09(106/7’*(K))

- 2.94

(8)

- 3.90.

(9)

and IO3 In

%hn~te(OH)-H20

=

2.28( 106/T2(K))

915

Experimental studies of alunite: I The intramineral fractionation of ‘*O- I60 between the sulfate and hydroxyl sites in alunite is given by the equation 103 In

Wunite

(sOA-OH

site) =

0.8( 106/ T2(K))

+ 0.96.

( 10)

This equation predicts a variation in lo3 ln %lunite (S04.OH site) of from 2.5 at 450°C to 10.0 at 25”C, and suggests that oxygen isotope fractionation between the sulfate and hydroxyl sites in alunite could provide a useful single mineral geothermometer. This geothermometer has been applied successfully to alunite from a number of hydrothermal environments, as discussed by RYE et al. ( 1992). However, many natural alunites are not in internal ‘*O- I60 equilibrium ( PICKTHORN and O’NEIL, 1985; RYE et al., 1990, 1992), and as such are unsuitable for application of this geothermometer. The alunite-water D-H fractionation factor ranges from - 19 at 450°C to -6 at 250°C and does not appear to be strongly dependent on temperature. The alunite-water H and 0 isotope fractionation factors have broad application to the study of natural alunites from a variety of environments. As with any experimental data, however, care must be taken in applying them to natural systems. Where isotopic equilibrium is not attained during alunite formation, or where stable isotopes in alunite are altered by post-depositional mineral-water exchange, the measured isotope values will not provide accurate information about initial formation conditions. The likelihood of such exchange can be evaluated using rate data as discussed in ST~FFREGEN et al. ( 1993).

BOVE D. J., RYE R. O., and HON K. ( 1990) Evolution of the Red Mountain alunite deposits, Lake City, Colo. USGS Open File Repf. 90-0235. CHIBA H. and SAKA~H. ( 1985) Oxygen isotope exchange rate between dissolved sulfate and water at hydrothermal temperatures. Geochim.

Cosmochim. Acta 49,993-1000. CHIBA H., KUSAKABE M., HIRANO S-I., and SOMIYA S. ( 198 1) Oxygen isotope fractionation factors between anhydrite and water from 100 to 550°C. Earth Planet. Sci. Lett. 53, 55-62. COLE D. R. and WESOLOWSIU D. J. ( 1989) Influence of NaCl aqueous solutions on isotopic equilibria and rates of exchange in mineralfluid svstems. Geotherm. Res. Council. Trans. 13. 227-234. CUNNINGHAM C. G., RYE, R. O., STEVEN T. A.,‘and MEHNERT H. H. ( 1984) Origins and exploration significance of replacement and vein-type alunitedeposits in the Marysvale volcanic field, west central Utah. Econ. Geol. 79, 50-7 I. FIELD C. W. and LOMBARDI G. ( 1972) Sulfur isotopic evidence for the supergene origin of alunite deposits, Tolfa, district, Italy. Min-

eral. Deposita 8, I33- 125. GRAHAM C. M. and SHEPPARD S. M. F. ( 1980) Experimental hydrogen isotope studies II. Fractionations in the systems epidoteNaCI-H20, epidote-CaC12-Hz0 and epidote-seawater. and the hydrogen isotope composition of natural epidotes. Earth Pkanet. Sci

Let/. 49, 237-25 1. GRAHAM C. M., SHEPPARD S. M. F.. and HEATON T. H. E. (1980) Experimental hydrogen isotope studies I. Systematics of hydrogen isotope fractionation in the systems epidote-HZO, zoisite-Hi20 and AlO( OH)-H*O. Geochim. Cosmochim. Acta 44, 353-364. GUSTAFSON L. B. and HUNT J. P. ( 1975) The porphyry copper deuosit at El Salvador Chile. Econ. Geol. 70. 857-9 12. H&EY J. J.. HOSTETLER P. B., GUDE A. J., and MOUNTJOY W. T. ( 1969) Some stability relations of alunite. Econ. Geol. 64,

599-612. JOHN D. A. ( 1989) Evolution

Premier Stock, central Wasatch

of hydrothermal fluids in the Park Mountains. Utah. Econ. Geol. 84,

879-902. Acknclwledgments-This study benefited from the assistance of Dr. Phil Bethke in its early stages. We are also grateful to Drs. J. J. Hemley and M. J. Holdaway for use of experimental facilities at the U.S. Geological Survey and at Southern Methodist University, respectively, and to Ellen Johnston for supplying us with a sample of Antarctic water. Helpful reviews were provided by C. N. Alpers and Hitoshi Chiba. The work was supported by NSF grant EAR-88 16706 to Stoffregen and Rye. XRD patterns of run products were obtained on equipment purchased with NSF equipment grant EAR-8721093 to M. J. Holdaway.

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916

R. E. Stoffregen, R. 0. Rye, and M. D. Wasserman

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