Chemistry of fluid inclusions in halite from the Salina Group of the Michigan basin: Implications for Late Silurian seawater and the origin of sedimentary brines

(;emhimica

0016.7037/90/%3,00 + .oO

PI Cosmochmrca Acfo Vol.54,pp. 319-321 1990 fkrgamon Pressplc.Printedin U&A

Copyright 0

Chemistry of fluid inclusions in halite from the Salina Group of the Michigan Basin: Implications for Late Silurian seawater and the origin of sedimentary brines NACHIKETA DAS,* JUSKE HORITA,

and HEINRICH D. HOLLAND

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA (ReceivedAugust 3, 1989; acceptedin revisedjhm November27, 1989) A~~t-~uid was extracted from 18 fluid inclusions in halite of the Late Silurian in the Crystal Mine on the outskirts of Detroit, Michigan. Compared with modem to the same degree, the inclusion fluids are severely depleted in SO;*, somewhat Mg+‘, and greatly enriched in Ca+*. The composition of the inclusion fluids can be

Salina Group exposed seawater evaporated

depleted in Na” and derived from Silurian seawater with a composition close to that of modern seawater, if it is assumed that the composition of the Silurian seawater was modified by dolomitizing CaCO&h sediments and by albitizing silicate minerals during its evolution into evaporite brines. Since the evolution of the brines involved a number of chemical reactions, it is impossible to recover the initial concentration of all of the major ions in the parent Silurian seawater from the composition of the inclusion fluids alone. It is likely, however, that the mK+jms,- ratio and the functions

in Late Silurian seawater had values close to those of modem seawater. Measurements of the isotopic composition of sulfur and of Sr in anhydrite within and associated with the halite host of the fluid inclusions are consistent with previous measurements of 634Sin Silurian marine anhydrites and with the s7Sr/86Sr ratios of Late Silurian marine carbonates. rounded by carbonate reef complexes; the major connection to the sea was to the northeast. Similar basins, separated from each other by carbonate reefs (Fig. I), lay to the south and southeast (DELLWIG and EVANS,1969). The Upper Silurian Salina Group of the Cayugan Series (thick cyclic evaporites) was deposited confo~ably on the Middle Silurian Niagara Group of the Niagaran Series (marine carbonate reef), and was followed by the Bass Islands Group (MESOLELLA et al., 1974). The change from normal marine to hypersaline depositional environments in the basin has been interpreted to reflect a drop in regional sea level. During the Upper Silurian, the ~eolatitude ofthe basin was 20*5, the climate was hot and arid. With the exception of the Appalachians, most of the North American continent was low lying (NEWCOMBE,1932; DELLWIG, 1955; SCOTESEet al., 1979; GOODMAN,1983). All of these conditions favored the deposition of evaporites. ~dimentologi~l and paleontological studies suggest that the Saiina salt was deposited at shallow water depths (basin margin) to modest or deep water depths (basin center) in a marine environment (DELLWIGand EVANS, 1969; NURMI and FRIEDMAN,1977). VANDERWILT(1924) and LANDES(1945) have divided the Salina Group into seven units, of which A is the oldest and G the youngest (see Table I). Units B, D, and F consist of halite with layers of anhydrite, dolomite, and locally mudstone and shale. Units C, E, and G consist essentially of argillaceous dolomite and anhydrite. Unit A consists largely of halite, dolomite, and anhydrite. The central part of this unit contains a 28 m thick body of sylvite. The maximum total thickness of halite in the Michigan basin is 540 m (DELLWIG and EVANS,1969; MATTHEWSand EGLESON,1974).

HOLSER ( 1963) DEMONSTRATED that brines extracted from single fluid inclusions in halite from the Permian of New

Mexico and the Silurian of Ontario have Mg’2,fCI- and Br-/ Cl- ratios similar to those of evaporated modern seawater, and that the brines in fluid inclusions in halite provide a good basis for reconstructing the composition of ancient seawater. Almost no additional work has been done since then due to technical and analytical difficulties. Recently, LAZAR and HOLLAND (1988) improved HOLSER’S (1963) techniques for extracting and analyzing fluid inclusions in halite. With these new techniques, it is possible to measure the concentration of all of the major and some of the minor elements in fluid inclusion brines in halite with a precision of a few percent. In this study we present analyses of fluid inclusions in halite from the Silurian Salina Group of the Michigan Basin to constrain the composition of Late Silurian seawater and to explore the relationship between the inclusion fluids and subsurface brines in the basin. GEOLOGY OF THE MICHIGAN BASIN The Michigan Basin in the eastern midcontinent area was originally some 700 km in diameter and slightly elongated in a northwestsoutheast direction. During the Silurian, the basin was periodically downwarped, and the sediments in the central portion were thickened (DELLWIG,1955). The basin had a gentle slope, on the order of 1: 1000, and a water depth perhaps as great as 200 m (MATTHEWSand EGLESON,1974). During the Silurian, the Michigan Basin was sur-

PETROGRAPHY

OF THE SAMPLES

Unit F of the Salina Group is present at a depth of about 370 m below the surface in the Crystal Mine on the outskirts of Detroit at the margin of the Michigan Basin. Sampling was confined to subunit F-l, the lowermost part of the F unit (Table I). The halite beds are flat lying, laterally persistent (KUNASZ, 1970), and do not show flow-. age. The thickness of the individual halite beds ranges from 10 to 40 cm, and averages ca. 20 cm. The halite beds alternate with OS-4 cm bands of anhydrite. The regular interlayering of halite and anhydrite,

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PALEOGEOGRAPHY

Rc;. 1. Paleogeographic map of the Michigan Basin and its environs during the Niagaran and Cayugan Epochs. Mine locations indicated are D-Detroit, Michigan; G-Goderich, Ontario; C-Cleveland, Ohio: F-Fairport Harbor, Ohio: R-Retsof, New York: B-Glen Bradley No. 4 well. (DELI.WIGand EVANS. 1969). which has been attributed to seasonal changes in the depositional environment and to the influx of seawater, is interrupted by numerous large pods of coarsely crystalline, white halite. DELLWIGand EVANS TABLE 1 STRATIGRAPHYOF THE UTE SILURIAN SALINA GROUP IN THE MCHIGAN BASIN (Vandemilt 1924, Landes 1945) Formation

Group

l4ambar

Cayugan Bass Island Series Group Salina

Group

G F

F F F F F

Salt Salt Salt Salt Salt

F Salt E D

A

Niagaran

A2

Guclph-Lockport

Series * Member Unit from which the samples were collected

6 5 4 3 2 l*

( 1969) pointed out that the halite pods contain bedded dolomite and anhydrite with faint, virtually horizontal bedding. They suggested that the dissolution of small patches of bedded halite is unlikely to have formed these pods, and proposed that water moving through cavities or channelways in the salt subsequent to at least partial lithification was responsible for pod formation. The bedded halite consists of grayish crystalline salt (cm in grain size) with yellowish (clay) and black (pyrite) spots. Small (cl00 pm) anhydrite needles occur dispersed and in aggregates in the halite. The halite beds also contain small amounts of clay, pyrite, and dolomite. DELLWIG(1955) reported the presence of a few anhydrite pseudomorphs after gypsum and grains of polyhalite, pyrite, and some rounded grains of dolomite with cores of calcite in the anhydrite bands. The bedded halite samples contain abundant chevron structures with zones of small (
ANALYSIS OF BRINE INCLUSIONS

Brines were extracted from I8 fluid inclusions and were analyzed following the techniques of LAzAR and HOLLAND(1988): individual

321

Fluid inclusions in halite of the Salina Group, Michigan

fluid inclusions (500 to 1000 grn) were opened with a micro-drill, and brine in the inclusions was extracted with a micropipette. After the volume of the extracted brine had been measured, the concentrations of the major elements and of Br- in the brine were determined by ion chromatography (I.C.); the precision of the IC analyses was ca. +2%. The results are shown in Table 2. The nine analyses marked with an asterisk are of brines from the clear part of two primary, bedded halites (Fig. 3). The other nine brines were extracted from fluid inclusions in two coarsely crystalline pod halites. The estimated densities of the Silurian Sahna brines are based on density and concentration data for solutions of single salts (CRC Handbook oJ‘Chemistry and Physics). All our brines are essentially Na+-K+-Ca+Z-Mg+2-C1-brines. In our calculations we assumed that the partial molal densities of the constituents of these complex brines are additive. Densities for synthetic brines calculated in this fashion agree within ca. 2% with their measured densities (WEINBERG,1987). One additional source of error in the concentration of the inclusion brines was the uncertaintv, fca. . _ of the _ 3.5%) in the volume caoacitv

micropipette which was used to measure the volume of the extracted inclusion fluids (LAZAR and HOLLAND, 1988). Charge balance calculations show that 13 of the analyses had charge imbalances I 3%. Four of the remaining five brine samples had charge

FIG. 2. Photograph of Salina halite 86ND103 showing typical chevron structures and fluid inclusions; the length of the bar is 1 mm.

TABLE 2 COMPOSITIONOF BRINES FROM FLUID INCLUSIONS IN HALITE FROMTHE SALINA GROUP, AND SUBSURFACEBRINES (mmol/kB of H20)

Fluid IncluSample

sion

No.

No.

86NDlOl n n n

n n 86ND103 ” II n ” ” 86ND104 * n 86ND105 ” I, Subsurface

Li+

Na+

K+ NB+2 c&2

Cl‘

Br’

Charge* Balance SO4’2 Error(*)

ND1

1970

350

1560

730

6530

31.3

<4

-2.3

ND2 ND3 ND4

1480 1050 1390 750 1320 960 880 950 1000 860 1040 1000 960 850 1830 1650 1780

410 370 370 460 490 490 490 500 540 510 540 460 480 460 460 420 410

1840 2050 1960 2340 1470 1750 1990 2070 2220 2220 2220 2130 2030 1630 1280 1520 1590

930 950 920

7770 7550

31.5 46.2

<3
2.5 1.2

1100

7170 7200

45.0 43.9

<2 <2

-1.9 -5.3

1410 1310 1020 1040 1050 1040 970 1110 920 1910 740 830 890

7840 7500 7690 7460 8060 7700 8000 7200 7000 9580 6210 6220 7380

48.7 53.3 40.4 44.2 51.9 43.6 41.8 46.5 40.2 41.3 35.0 31.3 34.9

<4
2.2 0.1 2.4 -1.0 0.4 -0.8 3.2 -4.3 -2.2 6.8 -0.5 -3.7 1.8

ND5 ND6 ND7* ND8* ND9* NDlO* NDll* ND12* ND13* ND14* ND18* ND15 ND16 ND17

Denlity gn/cc

1.235 1.248 1.248 1.254 1.277 1.256 1.257 1.246 1.260 1.269 1.256 1.251 1.271 1.250 1.275 1.218 1.234 1.241

Degree6 of Evaporation

36.0 36.2 53.2 51.8 50.5 56.0 61.3 46.5 50.9 59.7 50.2 48.1 53.5 46.3 47.5 40.3 36.0 40.2

brine

CM-l CM-2 CM-3

41.2 28.4 25.4

944 1860 1830

289 226 195

568 535 449

2940 2030 1830

7980 7290 6600

10.6

565

-1.5 0.7 0.4

60.1 44.8 39.8

1.291 1.257 1.254

Modern seawater 0.027

485 10.6

55.1

0.87

29.2

1S Calculated degree of saturation with respect to halite * Primary bedded halite * [(Canions-Ccations)/(Zanions+Ccations)]xlOO d Br concentration of inclusion brine/ Br concentration of modern

seawater

69.1 51.5 45.7

1S

0.97 0.96

0.91 0.98 0.95 0.94 0.90 0.87 0.93 0.97 0.93 0.91 0.98 0.90 0.93 0.87 0.92 0.96

U. Dus. J. Horita. and H. D. Holland

32’

sand). The sample mixtures wcrc packed in glass capsuies and loaded in the furnace together with a piece of rolled copper wire. The SO? gas obtained by directly reducing the CaSO, was collected and purified following the technique OfCOI.EMAN and MOORE(1978). The isotopic composition of sulfur in the purified SOZgas was determined with d VG Micromass mass spectrometer at the Scottish Universities Kcsearch and Reactor Centre (SIJRRC) at East Kilbride, Scotland. The isotopic composition of Sr in anhydrite was determined b) preparing 5 to 10 mg ofdry. sieved, dolomite-free anhydrite powders. spiking these with Sr. and dissolving them in 15 ml of doubly deionized water. The samples were loaded onto ion exchange columns. and Sr was collected following standard ion exchange procedures. The samples were analyzed on the VG Micromass mass spectrometer of the SURRC. calibrated against NBS Standard 987. The measurcrments of h3% and of R7Sr/86Srare summarized in Tahlt~ ; DISCIJSSION FIG. 3. Photograph of fluid inclusions in Salina halite 8ANDIO3. A fluid inclusion from which fluid was extracted is located in the clear part of the halite and is surrounded by chevron structures: the length of the bar is I mm.

imbalances between 3.0 and 5.3%. The sample with the largest charge imbalance (6.8%) was diluted very strongly (by a factor of4034): this probably explains the relatively poor quality of the analysis. A cross-check on the precision of the brine analyses was obtained by calculating their degree of saturation with respect to halite with the computer program developed by HARVIE and WEARE (1980). The Cl. concentrations were adjusted to satisfy the requirement of electrical neutrality of the computer model. According to the computer model, all of the brine analyses in Table 2 are slightly undersaturated with respect to halite. Their calculated degree of undersaturation, defined as the fraction by which the water content of the brines had to be reduced to reach halite saturation. is shown in the last column of Table 2. The agreement with the Harvie-Weare model is considered reasonably satisfactory. considering the analytical errors involved in the analysis of the brine inclusions and the uncertainties in the computational scheme. R. J. SPENCER(pers. comm.) has pointed out that the degree of undersaturation with respect to halite in Table Z is the same if the Cl- concentrations arc not adjusted for inclusion fluids with excellent charge balance. but that this is not true for inclusion fluids with poor charge balance. The last three analyses in Table 2 are ofbrines that had accumulated in puddles on the floor of the single level in the Crystal Mine. These brines had apparently leaked from nearby salt. Their composition is similar to those of the individual brine inclusions. This suggests that the inclusion fluids which we analyzed are not very different from average fluids in this section of the Salina Group in the Crystal Mine. SULFUR

AND

STRONTIUM

ISOTOPE

ANALYSIS

To check the possibility ofsulfate reduction and non-marine water contamination on the composition of our inclusion fluids, we determined the isotopic composition of sulfur and strontium in anhydrites coexisting with halite in the Salina F unit from which the inclusion fluids were extracted. Anhydrite from the thin bands which alternate with halite and anhydrite dispersed in halite were obtained by dissolving whole rocks in deionized water. The anhydrite residues were collected on filters, dried at 105°C and examined by X-ray diffractometry and light microscopy. No silicate minerals were found, but some of the anhydrite samples were found to contain minor quantities of dolomite. Samples containing dolomite were sliced and stained using the technique described by FRIEDMAN(1959). Staining identified irregular patches of dolomite; these were avoided when the anhydrite was collected for analysis. The purity of the samples was confirmed by high yields of SOI during the extraction process. The purified anhydrite samples were crushed in a stainless steel pulverisette. The resulting powders were sieved through a 250 mesh nylon cloth sieve and oven dried at 105°C. Sieved, dolomite-free anhydrite powders (ca. I S-20 mg) were mixed thoroughly with 200 mg of CuO and 600 mg of dry silica (quartz

The compositions of the inclusion fluids differ greatly from that of simply evaporated modern seawater. The concentration of SOi* in the inclusion fluids. for example. is very low, and that of Ca” is very high compared to the concentrations of these ions in modern seawater that has been simply evaporated to the same degree (M~‘C‘AVFKIJY et al.. 1987). It could, therefore, be argued that these brines are unrelated to Silurian seawater. This seems unlikely. however. Geological and sedimentological studies ( DELLWK; and EVANS. 1069: NCrRMl and FRIEDMAN. 1977) indicate that evaporites in the Michigan Basin were almost certainly derived from Silurian seawater. This is consistent with the data in Table 3 for the isotopic composition of sulfur and strontium in anhydrite dispersed in the bedded halite. from thin anhydritc beds alternating with bedded halite. and from anhydrite bands within coarsely crystalline halite pods. The fiY4Svalues agree well with the literature values for Silurian seawater. However, all but one of these values are for anhydrites in the basal parts of the Salina Group. The one that is not. the 6% value of +28.2%0 for the Silurian IHenryhouse shalt of Oklahoma (CLAYPOOL et al.. 1980), is in good agreement with our measurement from the Salina Group. The X7Sr/86Srvalues for the anhydrite samples fall between 0.708615 and 0.708675. with a mean value of 0.708653 f 0.000065 (2~). The Sr content of the anhydrites ranged from 1485 to 2803 ppm. There is no indication of extensive diagenetic alteration in the F unit of the Salina Group. The isotopic composition of Sr in our anhydrites is. therefore, probably the same as or verk close to that in the brines from which they precipitated. Our values in Table 3 agree with those of BURKE et al. ( 1982) for Late Silurian seawater. Their data have a considerably larger range than our>. Much of this range is probably the result of diagenetic alteration. which tends to raise the X7Sr/8hSr ratio of carbonate rocks (VEIZER and COMPSTON, 1973). Our values fall in the lower part of their range. and are probably good indicators of the “‘Sr/*%r ratio of Late Silurian seawater. Present rivers have a mean *‘Sr/“?jr ratio of 0.7 1I + 0.00 I (PALMER and EDMOND. 1989). A mass-balance calculation shows that a 10%’contribution of riverine strontium to the Sr budget in the Michigan Basin could raise the “Sr/%r ratio of the brine by about 0.0002. They show no signs of admixture with non-marine sulfur and strontium, but the data do not rule out the passibility that non-marine sulfur and Sr of isotopic composition similar to those of Upper Silurian seawater were mixed with the sulfur and Sr of marine origin in the Michigan Basin.

Fluid inclusions

in halite of the Salina Group, Michigan TABLE 3

THE ISOTOPIC CONPOSITIONOF SULFUR AND STRONTIUMIN ANHYDRITEASSOCIATFJ WITH HALITE FROM THE SALINA CROUP

Sample No. SO2 Yield 86ND102f 86ND103g S6ND104f 86ND10Sf 86ND10gh E6NDl10h Wean Silurian

a b c d e f g h

07% 91% 93% 93% 73% 88%

Seawater

634S (‘/**) +28.6 +27.0 +27.5 +27.6 +26.9 +27.4 +27.5

f 0.6

(lo)

+28.2a i23.5 6 26.3b +24.2 to +28.8= (n-31) +24.3 6 25.2d

g’sr/%r

20

Sr content

0.708675

(24)

1485

0.708668

(35)

2100

0.700615 0.700653

(29) (65)

2803

0.7086

(ppm)

to 0.J08ge

Claypool et al. (1980) Holser & Kaplan (1966) Thode and Monster (1965) Ault 6 Kulp (1959) Burke et al. (1982) Anhydrite from bands in bedded halite Anhydrite dispersed in bedded halite Anhydrite from bands in halite pods

A marine origin of the Salina Group evaporites would not guarantee a marine origin for the brines in our fluid inclusions. The brines which we analyzed were extracted from inclusions in clear halite in the bedded and pod halites, and may be samples of brines that were present during the recrystallization of the primary halite. The composition of brines from inclusions in clear halite very close to chevron structures in primary bedded halite is similar to the composition of inclusion brines in halite from halite pods (Figs. 4 to 7), although the variability of the composition of the inclusion fluids in the pods is somewhat greater than that of the inclusion fluids in bedded halite. Recrystallization which might have formed clear halite in the primary bedded halite was probably a micro-scale phenomenon; this notion is supported by the preservation of bedding structures and of many small chevron structures. The pods are not interconnected, and their origin is still obscure. Their isolation suggests that they were not formed by brines which permeated this part of the Salina Group much later than the deposition of the primary salt. It is likely that the inclusion fluids in the clear parts of the bedded halite and those in the halite pods are part of the seawater-derived brines from which the Salina evaporites were deposited. We can now inquire whether these brines could have been derived from seawater with a composition comparable to that of modem seawater, or whether we must call on a different composition for seawater during the Silurian to explain their composition. Unfortunately, so much has happened to Silurian seawater during its transformation into the inclusion brines, that a great deal of information has been lost en route.

The first compound to precipitate from evaporating seawater during the Phanerozoic has always been CaCOs (HOL LAND, 1984). The precipitation reaction is dominantly Ca+* + 2HCO; + CaC03 + CO2 + H20.

(1)

The concentration of Ca+* in evaporated seawater after the end of CaC03 precipitation and before the beginning of gypsum and/or anhydrite precipitation is close to mC&

0

10

=

iu

~(O~CP~

30

40

-

OmHCO#

!I0

60

(2)

70

a0

90

%icm~‘kw) FIG. 4. A plot of m- vs. msr- in inclusion brines from Salina halite, and in brines from the CrystalMine, compared with evaporated modem seawateron Greet Inagua Island (MCCAFFREY et al., 1987).

“\i. Das, J. Horita. and H. D. Holland 800

700

o

primry

hatite

/, 600

0

to

20

30

LO mgi

50

*

60

(mmot/lPgi$O)

FIG. 5. A plot of mNa+vs. msr- in inclusion brines from Salina halite, and in brines from the Crystal Mine, compared with evaporated modem seawater on Great Inagua Island (MCCAEREY et al., 1987).

0

10

20

30 mt+-Cmmd./k$$N u)

60

70

80

FIG. 7. A plot of mK+vs. MB<-in inclusion brines from Sahna halite and in brines from the Crystal Mine, compared with evaporated modern seawater on Great lnagua Island (MCCAFFREYet al., 1987).

where omc;i+2 = initial Ca+2 concentration 0 r%Ko)_ = initial HCO; concentration

in seawater

Since omso,;2 > (ornfa+2- 0mnoo;/2f, Ca+’ is nearly exhausted during CaSOd precipitation. The concentration of SOi’ at the end of the gypsum and/or anhydrite facies and before the precipitation of other sulfate minerals is therefore approximately

in seawater

and mBr= degree of evaporation of seawater 6= 0%,-

(3) mso;= = Nom&

where

- 0m0+2 f 0mnco,/2)

(5)

where

mBr- = concentration of Br- in the evaporated seawater and omsr- = initial Br- concentration in seawater. The next mineral phases that have always precipitated from seawater during the Phanerozoic (HOLLAND, 1984) are gypsum and/or anhydrite. The precipitation reaction ofanhydrite is Ca” + SOT2 + CaS04.

(4)

mso;2 = concentration of SO;’ in the evaporated seawater, and umso;2 = initiai concentration of SO;;* in seawater. The Br- content of seawater during the Phanerozoic has probably been close to that of present-day seawater (HOLSER, 1965; HOLLAND, 1978); the degree of evaporation of brines can, therefore, be calculated with some assurance: hence the value of the sum &,rnso;2- OmCa+z + omHco;/2) in the initial seawater can be determined. This sum cannot, however, be decomposed without additional information. Many marine evaporites lack MgS04 minerals. The absence of MgS04 minerals in marine evaporites and of SO;* in seawater-derived brines is usually considered to be due to the dolomitization of limestones. In the course of dolomitization, Mg+’ in seawater reacts with CXOJ and releases Ca”

Mgi2 + 2CaCOr --* CaMg(CO& + Cai2.

(6)

The liberated Ca+2 reacts with SO;* to form gypsum and/or anhydrite. As long as SOz2 is present in a brine, the overafl reaction is Mg+’ + SO;* + 2CaCQ -t CaMg(CO& + CaS04. FIG. 6. A plot of the C.F./mB,- vs. the A F. in inclusion brines from Salina halite, and brines from the Crystal Mine. Comparable datum for unevapomted modern seawater (solid circle) and the range of these functions in evaporated modern seawater on Great Inagua Wand (MCCAFFREYet al., 1987) are ako plotted.

(7)

After essentially all of the SO;* has been removed from seawater by this process, the concentration of Cas2 rises again until the brines are in equilibrium or in near-equilibrium with calcite and dolomite. The brines are then essentially concentrated Naf-K+-Ca+2-Mg+2-Cl- solutions, Dolomiti-

325

Fluid inclusions in halite of the Salina Group, Michigan zation can change the composition of seawaterderived brines quite drastically, and it is helpful to define a function that is invariant not only with respect to CaC03 and CaSO.,precip nation, but also with respect to dolomitization. The “Carpenter Function” (C.F.) (CARPENTER, 1978) divided by mgr-satisfies these requirements: C.F.

-=

mB1-

mMe+Z+ m&+2 -

ms0;2

-

mHc0;/2 1.

(8)

mk-

Sulfate reduction is often advocated as an alternative mechanism to explain the absence of MgS04 minerals in some marine evaporites. Since HCOS and CO?’ produced by sulfate reduction are normally precipitated as carbonates, the value of C.F. is also independent of sulfate reduction. Inclusion brines in halite are saturated with respect to NaCl and have generally lost NaCl during evaporation prior to trapping. During NaCl precipitation, Cl- and Na+ are removed in a I: 1 molar ratio. The difference (mcl-- mNa+)is therefore unaffected by NaCl precipitation, and the function %--mNa+

1

(9)

is constant in brines from which Cl- and Na+ have only been subtracted by the precipitation of NaCl. Like the Carpenter Function, this function cannot be decomposed without additional information. Figures 4 and 5 are plots of the Cl- and Na+ concentration as a function of the Br- concentration in the inclusion fluids from the Salina halite. The Cl--Br- and the Na+-Br- trends during the evaporation of modem seawater on Great Inagua Island in the Bahamas (MCCAFFREYet al., 1987) are added for comparison. The Salina brines all contain more Cl- and less Na+ than evaporated modem seawater of the same Brconcentration. The difference between the two data sets is probably not due to the differences in the Br- concentration of Silurian and modem seawater, since the data for Salina brines are not offset systematically from the data for brines from modem seawater. The inclusion fluids have probably gained Cl- and/or lost Na+ during their evolution from Silurian seawater by reactions other than halite precipitation and/or dissolution. The scatter in the inclusion brine data in Figs. 4 and 5 implies that the history of Cl- gain and/or Na+ loss was quite variable during the evolution of these brines. This variability is well illustrated in Fig. 6, a plot of C.F./ mBr_vs. A F. The values of these functions for modem seawater and the range of these functions observed for evaporated seawater on Great Inagua Island (MCCAFFREYet al., 1987) are also plotted in Fig. 6. The Great Inagua Island data plot within a small area, close to the point for modern seawater. The Salina brine data scatter close to a line which extends to values of A F. and Cf./m,,-somewhat more than twice those of modem seawater. The brine samples from the floor of the Crystal Mine plot closest to the point for modem seawater. The inclusion brines have probably gained Cl- and/or lost Na+. Both processes increase A F.; it is likely, therefore, that the A F. value of Silurian seawater was 5 120, a value about 30% greater than that of modem seawater. The reactions which increased A F. in the inclusion brines

are difficult to define. A sufficiently large gain of Cl- by the brines due to the solution of halide minerals other than NaCl is possible, but unlikely. Camallite and sylvite arc the most likely candidates for this process in the Salina basin, but sylvite is reported only at the basin center in Unit A, the lowermost unit of the Salina Group. Sodium loss by reactions other than NaCl precipitation is more likely than Cl- gain. The precipitation of glaubetite, Na$Za(SO&, which is predicted to replace anhydrite during the equilibrium evaporation of modem seawater (EUGSTER et al., 1980) is possible, but would not reduce the Na+ concentration significantly. Cation exchange reactions on clays, the incorporation of Na+ into CaC@ (see for instance ISHIKAWA and ICHIKUNI, 1984), and the conversion of clay minerals and other silicates into albite are more attractive alternatives (CARPENTER, 1978; LAND and PREZBINDOWSKI, 198 1). The loss of Na+ from brines by such reactions involves the replacement of this cation, largely by Ca+*, Mg+*, and K+. The replacement can be either direct or indirect. Direct replacement of Na+ by K+ would, for instance, accompany the replacement of orthoclase by albite. Na+ could be replaced indirectly by Ca+’ by the conversion of clays and SiOz to albite followed by the reaction of H+ released during such reactions with CaCO,. Albite is a common, but minor constituent of many carbonate rocks. The total amount of authigenic feldspar present in the carbonate sediments studied by KASTNER(1971) was at maximum 2 wt%, and generally less than 0.5 wt%. The brines in the Michigan Basin migrated across a large exposure of carbonate rocks during the Silurian, but it remains to be seen whether the extensive Na+ loss suffered by at least some of the brines was matched by a corresponding Na+ gain due to the formation of authigenic albite in the carbonate reefs which the brines encountered en route. The correlation between the A F. and C.F./mBr- in Fig. 6 is to be expected. Charge balance in the brines requires that

N 2mso;2+ mcl-+ mnco;. (10) This can be rearranged into the form 2[mca+z + mMg+2- rn& - mHco;/2] = ma- - m&f - mK+

(11)

and if both sides are divided by mBr-,Eqn. (11) reduces to the form 2 C.F.fmB,- = A F. - mK+/mBr-.

(12)

The data in Fig. 7 show that mK+/mBr-N 12 for most of the inclusion fluids. The scatter of the inclusion data in Fig. 7 is perhaps due to the uptake of K+ by polyhalite formation and illitization. This value is very close to that in modem seawater. As expected, therefore, the line 2 C.F.fmr,- = A F. - 12

(13)

N. Das. J. Horita, and H. D. Holland

326

passes through the point representing modern seawater, and passes satisfactorily close to most of the analyses of the inclusion brines. Since the concentration of K+ in the inclusion brines is not correlated with the value of the A F., the loss of Na’ from the brines appears to have been compensated largely by gains in Ca+* and Mg+‘. This is consistent with the consequences of the albitization of clays followed by the dissolution of calcite and dolomite. Other inte~retations are possible. Silurian seawater could have had a composition such that its A F. and C.F.Jmn,-m values were well above those of modern seawater. Variably extensive admixture with river waters, which generally have largely negative values of A F. and C.F./mB,- values (--244 and - 166, respectively; HOLLAND, 1978; MEYBECK, 1979), could then be invoked to account for the scatter of the Salina analyses toward lower values of A F. and C.F./ma,-. However, several lines of evidence argue against this interaction: little terrestrial elastic material has been reported in the sequence, and there is no detectable difference between the isotopic composition of sulfur and Sr in the Salina anhydrites and in time equivalent marine carbonates. CONCLUSION The fluids which we extracted from inclusions in bedded halite and halite pods in the F unit of the Salina Group in the Michigan Basin are probably syndepositional or early diagenetic brines. The isotopic composition of S and Sr in anhydrite within the bedded salt and the halite pods is consistent with a marine origin of the brines. Their evolution from Silurian seawater has clearly been complex. In addition to modification by evaporation, and by the deposition of CaC03. gypsum and/or anhydrite, and halite, the composition of the brines was almost certainly affected by the dolomitization of limestone and by one or more reactions that involved considerable Na+* loss and Cat2 gain. The albitization of clays in the limestones across which the brines passed from the open sea to the sites of evaporite deposition is an attractive but unproven mechanism of Na’ loss from the brines. The chemical evolution of the brines has been so complex that rather little info~ation can be extracted from their composition regarding the composition of Silurian seawater. The K’ content of the brines seems to have been affected largely by evaporation, and it is likely that the concentration of K+ in Silurian seawater was nearly the same as that of modern seawater. The plot of the CF./ma,- vs. the A F. values of the inclusion fluids defines a line with a slope close to the theoretical value of 0.50. Since the numerical values of these two functions almost certainly increased during the evolution of the brines, their values in Silurian seawater were less than or equal to their lowest values in the inclusions fluids. If they were equal to the lowest values, they exceed those of modern seawater by ca. 30%. The composition of the inclusion brines does not, however, demand that the com~sition of Silurian seawater differed at all from the composition of seawater today. Aeknowfedgments-Tie S and Sr isotope analyses were carried out at the Scottish Universities Research and Reactor Center at East Kil-

bride, Scotland, UK. A. E. Falfick, P. J. Hamihon, T. Donnelly, A. Boyce, and P. Ainsworths of the SURRC are thanked for their help. H. Poloni, manager of the Crystal Mine in Detroit, is thanked for his help in obtaining the samples. Thanks go to B. Lazar ofthe Hebrew University of Jerusalem, Israel. for introducing the senior author to ion chromatog~phy, and to J. If. Weare and J. P. Greenberg of the University of California at San Diego for computing the degree of halite saturation of our brine analyses. Thanks are due to R. J. Spencer, S. Brantley, and C. R. Feakes for their very helpful comments on this paper, and to P. Solomon and C. Cezeaux for typing the manuscript. The research was funded by NSFgrant No. 527-7683-1-30 to Harvard University. Editorial handling: G. Faure

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