The determination of sulphate in fluid inclusions using the M.O.L.E. Raman microprobe. Application to a keuper halite and geochemical consequences

Geochimica t-1 Cosmochimica ACIC Vol. 47, pp. I - 10 0 krgamon Prcs Ltd. 1983. Printed in U.S.A.

The determination of sulphate in fluid inclusions using the M.O.L.E. Raman microprobe. Application to a Keuper halite and geochemical consequences J. DUBESSY,* D. GEISLER,** C. KOSZTOLANYI,* M. VERA** *Centre de Rechercha sur la G&logic de l’Uranium, BP 23,54501 Vandoeuvm Les Nancy Cedex, France, l*Laboratoim de !Xdimentologie, Universit6 de Nancy I, B.P. 239, 54506 Vandoeuvre Les Nancy, France, **+Centre de Recherches P6trographiques et Geochimiques, B.P. 20, 54501 Vandoeuvre Les Nancy, France (Received July 23, 198 1; accepted in revised _/km September 8, 1982) Abstract-Sulphate concentrations have been determined in fluid inclusions by Raman spectroscopy using the M.O.L.E. microprobe aBer verifying that the sulphate determination is proportional to the total sulphate in the aqueous phase. Comparison of rn,so, values in primary fluid inclusions with associated mother brine from actual solar salt works demonstrates their chemical equivalence. Keuper halite of marine origin has also been studied. The inclusions contain solid phases anhydrite and possibly gypsum and gIauberite as well. The molality of the total dissolved sulphate in the aqueous phase is lower than that obtained during present sea-water evaporation at the halite stage of precipitation. Some geochemical hypotheses are proposed to account for this anomaly.

INTRODUCTION IN order

to reconstruct

the geochemical

ture. The intensity ratio between Raman and Rayleigh scattering is about 10d to 10b9. environ-

ment of formation for halite beds, it is fundamental to be able to characterize the chemistry of the brines from which the minerals precipitated. The only available relics of these brines are located in primary fluid inclusions. However, it may be questioned whether or not the inclusions are truely representative of the original fluids. To try to resolve this problem, crystals with fluid inclusions and associated mother brines from a solar salt works have been studied. Successful resolution of this problem would enable limits to be placed on the composition of ancient evaporitic waters. ti ROSASCOand ROEDDER (1979) have shown that a Raman microprobe (ROSASCO et al., 1975) can provide an estimate of the sulphate concentration in aqueous solutions trapped in fluid inclusions. The Raman microprobe (M.O.L.E.) described by DEL HAYE and DHAMELINCOURT (1974), and DHAMELINCOURT et al. (1979), is also very well adapted to the study of polyatomic species contained in very small volumes such as fluid inclusions. After verifying the validity of the method, we have applied it to samples from a salt formation of the triassic (Lower Keuper) of Varangeville (Lorraine, France).

Thus, all polyatomic structures such as molecules, crystals or dissolved (polyatomic) ions am characterized by tiequency changes of the light given by wavenumbers U,, i2, . . . , V.. A typical Raman spectrum is shown in Fii 8. The incident laser beam is focused onto the sampk with an ob jective of a classical optical microscope (L&z otioplan). This objective also cohects the reflected light and the Rayleigh and Raman scattering. The tad&ion is analysed by a double monochromator with two holographic gratings (2000 grooves per mm). The flux is measured by a photomultiplier (RCA 3 1034) cooled to -20°C. A simplified diagramofthe micropmbe is given Fg 1. With a micropre cffsorCTracorNorthernTN-1710)itis~~edetoout some operations on the recorded spectra such as substraction of a constant, multiscale‘averaging or spectra accumulation. The principal features of this analytical instrument are: a) a spatial resolution of few pm. It is thus possible to study the inclusions separately, and in some cases even individual phases in a single inclusion; b) except for some highly coloured sample$ the method is usually non-deatruc-

THE RAMAN EPEECD M.O.L.E. MICROPROBE

d&o -CVi) is the flux of the radiation with co + Vi wwo number. u& * Vi)is the relative cross-section of dilhtsion for the line emitted at Lr,& Si by the molecule under consideration. dN( v) is the number of molecules inside the difhtsing volume V, I&,) is the irradiance of the sample, and S2,is the solid angle of collection of the light. From this equation, it would be possible to determine the absolute quantity of a polyatomic structure contained in an irradiated volume if the itradiance and the diffusing volume are known. But for the inclusions, 1(a) is unknown because of the various refractions and re5ections at the difkrent phase boundaries (e.g. air-ciysml and crystal-cavity oi the inclusions), and also because the absorption differs from one

When a polyatomic structure is illuminated by a monochromatic radiation of wavenumber b0 = l/x0, many phenomena can be observed: a) the reflection, transmission and Bayleigh scattering for which the photons have the same wavenumber as the incident photons: b) the Baman scattering of photons with different wavenumbers Vo2 Vicompared with the incident photons. Raman scattering is an inelastic diffusion of the light which results from an energy exchange between the incident radiation and quantified energy levels of the polyatomic struc-

tiVC. The exciting radiation used is the 514.5 nm green line from a 5 W Spectra-Physics ionised argon laser. The experimental recording conditions are given in Table 1. For the inclusion study, we used a Leitx objective PLX 160 with 0.95 numerical aperture, while for the pmpared solution we used a Leitz objective UTKXSO with 0.63 numerical ap erture. Placaek (I 934,1962) has shown that the Raman scattered light at (co + SJ cm-’ is given by the formula:

J. Dubessy er d.

B

FIG. 1. Simplified scheme of the M.O.L.E. Raman Microprobe SC. = Screen; Sa. = Samples; ,M. = Mooochromato~ P.M. = Photomultiplier, A. = Amplifi~, R. = Recordm Ve = Exciting light.

crystal to another. Although these optical constmiots do not enable us to determine directly the absolute amount of a compound in the di&simj volume the concentmtioo of the ions dissolved in the aqueous phase can be determined because it is the ratio of ions to water molaculu within this volume. By applying the equation of Placaek to the water and to the ion we obtaim

mION( V))/&(HsO( v)) is the ratio of the number of moles of the ion to that of water in the di!Tbai~ volume. It is pmportional to the c4mceottatioo of tlte ion in the molality scale. u(HsO)/u(ION) is the ratio of the Raman cmoMccti00 of the water molecules to that of the ion for tha conaide& Raman barn& &(ION)/dE(H~O) is the ratio oftheRamanBuxemittedbytheiontothatofthewater molecules. It is the ratio of the area or height of the signals measured on the recorded spectra. Analysis of sulphae ion in the aqueous phase In the rulphue ion, the four oxygen atoms an located tatmh&onandthesuIphuratomisat attheannuaofa thecentreofgrhvityofthisstnmtum. TheRamanbandused in this study is that amociatad with the symmetric stretching vibration mode (via, = 982 cm-‘). The natnral solutions con&dared in this work are saturatedwith~tohrtite:butoth?rionrsuchasK+,Mgt+, and Ca” can be pmaent in sign&ant conoantrations. In the sir&e NaCl-NasSO,-HsO system PrnrowIaand K&gTBR(1969) have shown that the sulphate ion is distributed predominantly in the two ionic species NaSaand SOj-. The dksociatioo constants of other species like MgSO$, CaS@, KSO:, show clearly the existence of such neutral or ionic species. Furthermore, a computer simulation of sea water evapomtioo shows also that the sulphate is mainly distributedin various paired speciea and the fme sof- anion is only about 10% of tbe total dissolved sulphate (Fritz, pers. commun., 1981). Thw, it is oazssary to consider the possibiiity of a frequency variation of the symmetric stmt&mg vibration vlal as a function of the particular ionic species in which the SOf-ionislocrtedItisnecasuytodecideifthcncordad Raman spectrum in a sulpftate .3olutioo is that of the SOj- free ion, that of a paired spa&e and if so which one, or a mixture of the dilkant kinds of snlphate. It is also necemarytoeatabkhifthedifkrentpairedspecieshavethe same or diit difhuion cross-section.

DAVIS et (II. (1974). IKAWA et ai. (1977) have shown that the symmetric stretching frequency in the HSOi ion is 1OSI cm-‘, so no confusion is possible between HSOi and SOi-. But for electrolyte solutions at high ionic strength. the various authors who have studied the influence of for= matioo of paired cation -Saspecies on the Y,U,frequent> disagree. HESTER and PLANE (1964) found no significam differences in the Y,U, frequencies of concentrated sulphate solutions with different cations Na? 980 cm-‘. Mg”: 985 cm-“, Cu2+: 980 cm-‘). DALY ef al. (1972) showed that the three species a-, NaSOi and M& cannot be distinguished by Raman spectroscopy. On the contrary, DAVIS and OUVER ( 1973) demonstrate that the MgSO, species can be characterized by a weak band located at 995 cm-‘. Furthermore, they have compared Mm concentration determinations made by Raman spectroscopy and by uitrasonic sound in sohstions from 0.1 to 2.5 l@SO, mole/l. For 1.41mole/l MgSO,, the cottoentratioo of contact ion-paired wy is 0.: 5 mole/l MsS04daermklbyRamPln and 0.14 mole/l determined by ultrasonic sound. So, in the pure HsO-MgSO, system, the rn-easured mtensity of Ulphate can be undema&ated by a factor around 10%. But in a more complex solution, the existence of a great number of other species wilI probably decrease the sulphate proportion in the ion-paired MN, species, and so the underestimation will tend to be negligible. In order to confirm this assumption, we have studied solutions of variable composition. Because the host crystal of the indusions is halite, the pmpared solutions were first saturated with respect to halite. Then sulphate was added by the dissolution of di&ant kinds of sulphatea, in order to test the eventual contact ion-paired effect. In each prepared solution, total sulphate concentration was determined by the gravimatk method (Cl+_, 1974). The typical Raman spectrum of water is pmsented in Fig. 2. Three kinds of bands can be distinguished:

a) intta-molecuiar

vibrations of three types:

~rf.r~ : bending uIul : symmetric stretching & : antisymmetric stretching bJ inter-moiecular vibraaons: Y;, pi c) combination bands: 2v2a,

The inter-molecular vibrations or l&ration bands are a of the formation of water dimers and water polymers by hydmgen bonding (bo, 1972). EIectrostatic fOrccSare at the Origin ofthese hydmotn bonds. Some electrolytes are able to ma these hydrogen bonds (WALRAVEN, 1962, 1964, 1966). So we will consider only the Raman bands comsponding to the iotra-molecular vibrations. But hydrogen bonds also mrxiify the intramolecular vibrations @cl-ttJLn and HORNU;, 1%1;BUSINGand HORNIG, 1961; LILLEY, 1973). In patticular, the halogens I-. COnaSqUCUCc

Table i.

Exberlmental recordingconditions of Raman spectra

0 Sl 52 so:Hz0

8.9.

F1

100-1200 950-1050

400

950-1010 1500-1800

300 400

2

200

/

CIW

20

3.7 3.5 ll.7 'I .5 O.!

:0 : 2

20 20

*i

3.87 2

+ = studiedphases; S.R.= spectralrange snalysed(cm-'); Fi = slitwidth (rmI . ; SpectralresolutionIon-!)d = scanning SpCed icn-'!mn); du = dwell-tinr of sampling the signal laser !YJ

channel(5) , -1 a ~C+W 3f tne Sl = solidphase . $2 solidDhar?

per

3

Sulphate in fluid inclusions

H20 25 ‘C

FREOUENCY cm” FIG. 2. Raman spectrum of pure water (WALRAFEN,

Br- and to a less important extent, Cl- are strong breakers of these hydrogen bonds. Because the cross-section u(H20) can change as a function of the nature and concentration of the dissolved electrolytes, it is necesaaty to construct a calibration curve from solutions with chemistry close to that of the studied inclusion. A calibration curve has been constructed on prepared solutions saturated with respect to halite in which the sulphate has been adjusted by dissolving the salts Na2S04, K2S04, CaS04. 2H20., MgSO, +7HrO. These brines were injected into a small glass cell. A peristaltic pump maintained a continual movement of the solution in order to optimize its homogeneity. Raman spectra recorded on a prepared solution and on an inclusion are given in Figs. 3a and 3b respectively. The diffused Raman flux (I) is assumed to be proportional to the height (h) of the signal on the spectrum. The experimental data are given in Fig. 4. The calculated parameters of the linear regression are: I(SOi-)/I(H*O)

= 23.1843

X mrSOI -

1962: 1964).

The absolute and relative uncertainties about I@@-)/ Z(H20) = h(SO,)/h(HrO) ratios as a function of the sulphate molality are given in Table 2. The correlative uncertainties on the determined sulphate concentrations are also calculated The average of (A!Sa-)/Seis 11%. APPLICATION

TO SOLAR SALT WORKS

From the geochemical point of view, it is important to check if the aqueous phase trapped inside the

0.1049

where mrso, is the molahty of the total dissolved sulphate. The correlation coefficient for the line is: R = 0.985. From these results we can draw several conclusions. All tbe 22 experimental data, corresponding to various associated cations, Na+, K’, Ca*‘, Me, define only one straight line. This result confirms that the cation effect can be considered as negligible. The scattered Raman light from the sulphate is thus proportional to the total sulphute dissolved in the aqueous phase. We can therefore assert that we analyse the total sulphate, mrSor. This straight calibration line can be used for the determination of total sulphate molality in saturated halite brines, whatever the nature and concentrations of other cations. The regression curve can be considered as a straight line. At these concentration levels, the number of water molecules can be regarded as constant. The sulphate ions do not appreciably change the structure and intensity of the water Raman band. This observation confirms that the Cl- is predominant and that it is the only anion which is able to modify the intra-molecular vibrations as previously shown by SCHULTZ and HORNIG ( 1961); BUSING and HORNIG (1961). and LILLEY (1973). The signal to noise ratio for rn,so, = 6.3 X 10e3 mole/kg Hz0 standard solution is 2.7 for nine accumulated spectra. However the signal to noise ratio from an inclusion is two times less than that from a free brine. By using spectra accumulation, it is reasonable to expect that a signal three times smaller can be identified. So, the limit of detection can be estimated to be mm. = 2 X IO-‘/Kg H20 or 220 ppm.

4

2x’

ml

WAVENUMBER

l500

0M

(Cm-')

FIG. 3. Raman spectra of sulphate (~,a,) and water (Y~Q,)

from A) prepared solutions and B) the aqueous phase of a natural inclusion (Keuper, Varangeville salt mine, France).

J. Dutxssy et al.

n

1

,,,/“’

NiS’+

K'

x

Hg+*r

+

cat-*

4

/’ ;

,‘

FIG. 4. Calibration curve for sulphate determination in solutions saturated with respect to halite. I(sof-): Raman intensity of the ~~(1,symmetric stretching line of sulphate, I(Hz0): Raman intensity of the yu, bending line of water ’ m-1 molality of all the dissolved sulphate.

inclusions is representative of the solution from which the host crystal has precipitated. Therefore we selected a solar salt pan in the Camargue (Salin-deGiraud, Southern France) and sampled it during its active period. The Mediterranean water, pumped into the solar salt works during its active period, traveis about fifty km. As a consequence of progmssive evaporation, carbonates and gypsum precipitate successively before saturation of the solution with respect to halite. The salt pan studied (A) is one of those in which halite first precipitates. At the sampling location, the

brine layer in the pan was about 50 cm deep wtth ;1 temperature of 28°C. The bottom was covered with a single layer of halite cubes with edges a few mm long deposited on a crust of gypsum about a millimeter thick. The halite crystals and the brine were sampled exactly at the same location to avoid any possible salinity evolution inside the salt pan. The small amount of precipitated halite showed that only a small volume of water had evaporated since the gypsum precipitated. Therefore it is reasonable to consider that the brine precipitating the last gypsum crystals and the brine above the halite are very similar

SO'-mole/k9 H 0 4 2

'I.;31 O.:ll

0.082

0.051

0.032

0.023

0.020

0.013

0.0094

3.0063

WSOLH ~A1S02‘1,1S0 01 4 -2

j.33

0.04

0.04

il.37

0.09

0.08

0.09

0.10

3.12

>.:4

4 2 ~:50~-/!H*01

0.38

0.09

0.09

0.12

0.14

o.ii

O.i4

0.15

3.17

,:.20

i(ISO~~lIh20)

1.26

O.Zi

9.23

0 'C

ii.10

0.05

3.05

3.04

0.02

j.9;

2.012

0.0095 *3.:c

ri.OO5 J.005

0.002

3.002

0.002

O.')Ol

1.005

2.5

9.8

Y.!

SOf‘ rasot-,/so?-r

:0.9

12.2

15.6

10.0

:5.5

10.6

.i

Sulphate in fluid inclusions

5

FIG. 5. A: Analyzed inclusions with shape of negative cubes in halite crystals of salt pan A (solar salt works Sabn-de-Giraud, Camargue, France). B: Chevron structure made out of fluid inclusions in a halite crystal. The inclusions are deveioped parallel to the halite cube face (Varang&ille salt mine, France). C: Detail of the boundary between chevron and transparent halite (Varangkville salt mine, France) D: Chevron and transparent halite crossed by a fracture characterized by a line of fluid inclusions (Varangeville salt mine, France) E: Detail of the distribution tluid inclusions in a halite crystal. Some inclusions show a solid phase (Varat&ville salt mine, France) F: Inclusions No 1 and 2 pointed out by an arrow, which were analyzed for their solid phases (S). chemically. The only important changes in chemical composition are related to the Na+ and Cl- ions, as halite is the only mineral precipitating. The inclusions in the halite have rectangular to square sections ranging in size from a few to some tenth of micrometers. Parallel to the crystal’s growing faces. they show chevron structures on cross-sections. They are single phase aqueous sometimes with a small vapour bubble (Fig. 5A). The sulphate concentration of the sampled brine has been analysed by classical chemical methods and Raman spectroscopy (Fig. 6). The results are given

in Table 3. The close similarity of the analytical results obtained on the brine of the pan by Raman spectroscopy and classical chemical methods corroborates the fact that the formation of the different ion pairs has no influence on the analysis of the total sulphate. From the geochemical point of view, the identity of the concentration of sulphate in the brine trapped inside the inclusions with the one sampled in the pan, confirms that primary inclusions in such an environment trap a fluid representative of the mother brine of the growing crystal. Ancient primary fluid inclusions can thus be assumed to be represen-

3. Dubrssy

et ai.

FtfA 6. Sul&ute molality (P&QJ determination in brioes trapped in inclusions. Salt pan A: first halite p&p&ion. *: @O&/$Hfl) mwwed in tbc free brine with the M.O.L.E., it: !&l&ate wnccntration in the fres brine dacrrmned bv chemhai analvsis, Salt pan B: end of halite precipitation, Keuper: %uanghk mIt mine,France. -

To follow the evobtion of the sulphate concentration in p-t day sea water, we samplod another salt pan (B) at the end of the halite ptipittstion stage. Seven halite inclusions with the same shape and size have been analyacd and showed ma values ranging from 0,427 to 0.522 mole/kg Hz0 (Fig. 61.

analysed crys& am in a halite layer which is 10 cm thick and da& colourcd because of a high clay content located at the top of the mine. A paiynological dating of this level givesalowefcarnian age (GWLER et ol., 19781. The bromine content of the halite layers in the mine varies from 70 to 200 ppm (GU%E~ 1979). These values am consistent with a marine origin (BRAITSCH,1962). The Table 3. Maiality

KEUPER IiALlTEs

Jf rulphate

A. Comparison

sam~k

I" flutd inclusionr. with the Wine

Chemcai

Geological setting and deseriprion oj*the studied sample The salifbmus basin itself extends to the center of the Paris Basin (MAIAUX and MENILLET, 1980) and is limited

,

totheEa&inLolraioe,byanoutcroppingborder.The amdyaaJympks come from Varan&W salt mine, located around IO km to the East of Nancy, on the eastern boundary of the basin. The maximum overburdeo thickness was I.500m. Since thcParisBnsinisin trscmtonic, the gtothermal gradient is expected to be around 3O”C/km. Therefore the Keuper formation probably reached a maximum temperature of 50 to WC. The salt mine is operated at a depth of 160 m, at the bottom of the third saliferous level (MAU~~~JGE, 1950). The

SM)DIE.I'

in the S~IE pan.

LORRAINE

1

0.168

3

3.71

a.165

4

3.82

0.169

5

i.80

0.168

3.82

0.169

F.B.n.f

c INC

0.172

1 3.80

2 1%

‘

3.89

3.168

'I,174

1 = inclusion

red :A 5 average

0.1101

> i72

;

F.S. = free brine

. d = standard

; n.f. = non
dev~atton.

St&hate in fluid inclusions studied halite layer has a bromine content of 126 ppm. which is well within the range observed for halite precipitated from modem sea-water (BRAITXX, 1962; HOLSER, i 966). Two types Of hi&e cryat& arc ~sting~~~, chevron and transparent. Chevron halite is inch&on-rich with very distinct chevron structures related to the traces ofcube faces (GEISLER,1979). The inclusions characterize growing face of halite cubes 8s obscrvcd in Camaque. Depending on crystal orientation, they show the section of one or more faces, sometimes even one section of an apex (Fig. SB). The inclusions are negative cubes ranging in size from a few, to hundred micrometers at the most. Except for rare inclusions, very close to the cleavage surface of the sample, the lack of vapour bubbles speciahy in huger fluid inclusions (ROEDIXR and BASSETT, 1981) and the preservation of the chevron structures (ROEDDER and BELKJN,1980) indicates no important recrystallization related to the overburden. All these data caabie us to characterizethese inclusions as well preservedprimary inclusions (DELLWIG,1953;GOTTESMANN, t963). However the filling of the inclusions is not uniform. Some of them are just monophase aqueous inciusions wbile others are more comphcated. In a single growing layer, some inclusions contain one or more daughter crystals (Fig. SE), especialiy these with edges over 20 micrometers.Sometimes it is possible to follow the solid particie into the host halite crystal. Heterogeneous trapping is probable for the origin of these so&is. The transparent halite shows no specific structure, and carries very few inclusions. If present, they are located along former fractures which am now healed They have the same shape and size 85 those from chevron halite but never contain solid phases. Betweenboth halite types, the boundary is always cbaracterkxl by the sudden disappearance of the inclusions (Fig. SC). otherwise, the concavity of the boundary is often turned towards the transparent halite. Finally, some fmctums cut through both halite. types (Fig. 5D). Recent halites in Mexico show early dissolution cavities possibly C&d later on with tmnsparent halite, between the chevron structures (SHEARMAN, 1970).As far as Keupcr haiites are concern& the concavity of the boundary between both cqstal types turned towards the tmmpatent balk is in good agreement with such a diihition phenomenon. Thus, the chevrons are relic strucWes of primary habte preserved inspite of some iater tilution. On the other hand, trampatent halite without inclusions is diagenetic in origin,growing slowly at equiiibrium from brines filling the dissohnion cavities between the primary inclusion-rich halite crystais (SHEARMAN, 1978), which have grown rapidly, first as hoppers at the water surface and then on the bottom after sedimenting out (DELLWIG,1953). Therefore,we consider that the inclusions in the chevron halite have trapped the brine from which halite was precipitated at the time of mother brine evaporation.

Results from the spectroscopic study a) Solid phases. in inclusion 1, (Fig. 5F), the unique strongest band at IO 19 cm-‘, Fig. 7, ciearly indicates a sufphate in which this ion is probabiy located in a unique symmetry site. The data on the other bands given by KRISHNAN and SHANTA KUMARI ( 1950) enables us to characterize this suiphate crystal as anhydrite. Even though two crystaIs can be recognized with the optical microscope, the recorded Raman spectrum shows that they are both anhydrite. In inclusion 2, (Fig. SF) the optical observation shows a few associated cry&&. The general spectrum (Fig. 8) with its strongest line at 1019 cm-’ also indicates

1

409Bm

2044 FS

i

FIG. 7. Raman spectra of the solid phase in inclusion no 1 in Keuper halite: anhydrite. a stdphate. But a detailed recording with an improved spectral resolution establishes the existence of three bandsat iOO1,1009 and 1018 cm-’ mspectively. The hiser beam was focused at different points on these so&is. No change in the relative intensity of these three bands was observed. Thus it is either the spectrum of a single crystal or the spectrum of multiple associated solids. Because the lateral spatial resoiution is around 1 pm, the size of the eventual cr’ystahtes forming the solids is in the same range or less. If we assume this last hypothesis, after a comparison with the %Xature (BERENBLUTet al., 197 I; tiHNAMURTHYand !4ooTs, 197 1) and spectra we recorded of ah the common sulphates from evaporitic environments (gypsum, bassanite, anhydrite, dauberite, thenardite, polyhahte, bloedite, syngenite, glaserite, iangbeinite, kainite) it seems hkeiy that these soI& are an association of gypsum, anhydrite and gIauberite. These same spectra have been found in some other inclusions with solid phases. However it is also well known that a polyatomic species within a crystal generahy has a lower symmetry which yields severai Raman active lines. For instance the analysis of the symmetric stretching (vi) of the sulphate ion by the correiation method (FATELEY et al., 1971; WILSONd ai., 1980) shows four vl Raman active lines for epsommite and two for glauberite. But only one is typically observed for this fast mineral because the spectral resohttion is lower than the sphtting effect due to the symmetry site effect. Thus because of the lack of extensive Raman spectra and detailed cristallographic structure, one cannot exclude a single crystal with three Raman active lines.

J. Dubessy et ai.

8

AllKIm /

8lafs

planes. They are clearly secondary. Their sulphate concentration is noticeably lower than those of ?he primary inclusions. Moreover, two families of setondary inclusions can be distinguished. The solutions trapped in F2 fracture have higher sulphate concentration than those from the F 1 fracture. This indicates two distinct diagenetic events. DISCLJSSION

FIG. 8. Raman spectra of the solid phases in inclusions no 2 in Keuper halite: 1001 cm-‘: giauberite (Gl), 1009 cm-‘: gypsum (Gy), 1018 cm-‘: anhydrite (An).

b) Aqueous phase. AU data from the Keuper inclusions are presented as a w (Fig. 9). The inclusions, located in a single chevron structure show good homogeneity in their dissolved sulphate content. Among the 17 primary inclusions, 11 (65%) have sulphate concentrations between 45 and 55 X IO-’ mole/kg HrO. The mean and standard deviation respectively for inclusions without solid phases are m = 50.9 X lo-’ mole/kg Hz0 o = 5.4 X lo-’ mole/kg Hz0 (excluding inclusion 7), and for inclusions with solids m, = 50.9 X IO-’ mole/kg HrO, us = 2.4 X lo-’ mole/kg HrO. This confirms the heterogeneous trapping of solids. Inclusions 16 and I3 are deep located and have a low signal to noise ratio. No analytical reason (Raman spectra recorded twice) nor special location can explain fhe high concentration of sulphate in inclusion 7. It might be related to a local supersaturation. Inclusions F, and Fr occur on sealed fracture

20

30

40

FIG.9. Histogram of sulphate concentration (mm: F2) inclusions in a Keuper halite crystal.

The comparison of the results in primary flurd inclusions in modern solar salt works and in a halite layer of the Keuper saliferous formation of Lorraine shows a much lower sulphate content in the Keuper trapped brine. This is clearly demonstrated by the relation between sulphate and bromine. Using the partition coefficient of bromine between halite crystals and brines (b = 0.14 to 0.073, Bra&h and Herrmann. 1963) we have calculated the possible range of Brcontent in the Keuper brines (900 to 1,800 ppm for 126 ppm in the crystal). On the diagram of rneo, versus Br- content in the brines of modem solar saltworks (HERRMANN et al., 1973) the Keuper brine plots well away from the trend of a modem sea-water during evaporation (Fg IO). The analysed Keuper halite crystal, part of a halite layer, is taken as representative of the basin chemistr) at a given moment. This is possible because the single halite layers can be followed throughout the mine. On the scale of the Paris Basin, the halite units are sharply defined by use of logging techniques, and show a wide lateral extension of over a hundred km (MARCHAL, 1982). It is also necessary to keep in mind two facts: i i the water which precipitated the salts of the Keuper Basin of Lorraine (France) has a marine origin as demonstrated by the high bromine content of the primary crystals of halite; 2) according to BRAITSCH (1962) the mineralogy of marine evaporites has not changed from late Precambrian to the present time. Thus the possible association anhydrite, glauberite and halite is normal. The amount of precipitated

50

60 70 a0 10-l mole/kg H20) in primary and secondary (f

9

Sulphate in fluid inclusions

glauberite is negligible. So the sulphate concentration in the remaining waters may have been essentially controlled in the stage of gypsum precipitation. When a mineral is forming from a solution during evaporation, the chemical evolution of this solution is strictly determined by the initial ratio of the anions and cations relative to the considered mineral in the solution (AL-DROUBI, 1976; AL-DROUBI ef al., 1980). So, during the precipitation of gypsum or anhydrite, if m&m< 1, the m&mm will decrease, while it will increase- if nircl/rnrso, > 1. This is illustrated by the diagram log mlSOIversus log ma (Fig. 11) constructed from the data of HERRMANN er al.(1973). fn this d&ram one can see the evoiution of log mm and log me along a straight line of slope unity at the beginning of the evaporation before any precipitation of minerals occurs, a depletion of calcium once calcite precipitates and a continuous depletion of calcium and increase of sulphate as gypsum precipitates. So a possible explanation of this abnormally low sulphate ~ncen~tion can be an initial mrcJmisor close to one at the beginning of gypsum precipitation. A possible enrichment in calcium may be related to dolomitization (HOLLAND, 19’78) which was very active at that time on the southern border of the German Basin in the Alps area, more precisely in the Dolomites (TRUMPY, 1980; JANOSCHEK and MATURA 1980). Alternatively, the irn~ve~~rnent of sulphate could be related to bacterial reducing activity. Except for one very iimited exception (SCHRODE% 1977) no sulphide rich levels are known. But this does not exclude such a reduction obscured by a loss of HIS.

f%0* SO

~

i *

400}

300

:

.

I

KEUPER

t

o*B-

FIG. 10. Diagram qso. versus Br- in brines of modem solar saltworks in Yugoslavia (HERRMANNet al., 1973). Kcuper brine with Br- = 900 1800 ppm (b, = 0.14 to b2 = 0.073).

103nbok

-3

-2

0

-1 ,

I

c*tclte

mtC. ->1

m* El

/

,/I 9R*+ / a*+

0 $$‘&,+

/

--’

YpMl

g

--2

+

log mtel

-3

1

FIG. 11. Diagram log me versus log me constructed fmm the data of HERRMANNa al. (1973). The calcite and gypsum precipitation are located. S.W. = modern sea-water.

Finally a m&me raiio close to one in the Keuper sea-water would be inside the limits defmed by HOLLAND( 1972) for the variation of the composition of sea-water since the Phanerozoic. But a more extensive study would be necessary to check these different hypothesis. CONCLWSKINS I. It is possible to determine ~~ti~tively the total sulphate concentration in the aqueous phase of fluid inclusions using the M.O.L.E. Raman microprobe. 2. This analytical method has been applied to the study of a modem salt pan. Good agreement has been found between the value of sulphate concentration inside primary fiuid inclusions in halite crystals, and the free brines from which they have formed. 3. The sulphate concentration determined in the primary inclusion fluids of Keuper is abnormally low (mm = 0.05 mole/kg H20) compared to the sulphate concentration of a modem sea-water at the w of halite precipitation (msc* = 0.2 mole/kg H20). 4. This limited study shows clearly a new direction of research into evaporitic environments based on the study of solids and aqueous phases in primary indusions with the M.O.L.E. Ramah microprobe. Because the chemistry of the solutions and solid phases can be determined, it will be possible to define more precisely the geochemistry of the evaporitic waters and their evolution through space and time. Acknowledgements-It is a pleasure to acknowledge Professor H. D. Holtand, B. Fritz and M. Arnold for fruitful discussions. The suggestions of P. Dhamclincourt with regard to the spectroscopic study are gratefully acknowledged. We would like to thank S. M. F. Sheppard and R. Wilkins for helpful suggestions during preparation of the text. E. Rceddcr and W. T. Holser greatly improved the final version of our paper.

3. Dubessv et ai!

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