Stable isotope and major element compositions of fluid inclusions in Devonian and Cambrian dolomite cements, western Canada

Geochimica et Cosmochimica Acta, Vol. 59, No. IS, pp. 3 159-3172. 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 00 16.7037/95 $9.50 + .OO

Pergamon

0016-7037( 95)00204-9

Stable isotope and major element compositions of fluid inclusions in Devonian and Cambrian dolomite cements, western Canada WENBO

‘Department of Geology

YANG,

’RONALD

and Geophysics,

J. SPENCER,

’and H.

ROY KP.OUSE*

The University of Calgary, Calgary, Alberta T2N

lN4, Canada

‘Department of Physics and Astronomy The University of Calgary, Calgary, Alberta T2N lN4, Canada (Received

October

19, 1993; accepted in revised form May 3, 1995)

Abstract-Homogenization and melting temperatures, major element and hydrogen and oxygen isotope data for fluid inclusions, and carbon and oxygen isotope data for dolomite from the Manetoe Facies (Devonian) and Cathedral Formation (Cambrian) of western Canada indicate that the dolomite cements formed from alteration of the host limestone by heated mixtures of evaporated seawater or residual evaporite brines and meteoric water. The measured S I80 values for fluid inclusion waters are much more negative (about 8%0) than those calculated for waters in isotope equilibrium with dolomite at the moderately high temperatures of formation. Fluid inclusion waters appear to have exchanged oxygen isotopes (reequilibrated) with the host dolomite as they cooled. The data may provide low temperature isotope exchange equilibrium fractionation factors between dolomite and water. Since no hydrogen-bearing compounds other than water were found in these samples using Quadrupole Mass Spectrometry, the inclusion fluids likely retained their initial hydrogen isotope compositions. Therefore, their 6D values ( -81 to -42%0) are more useful for determining the origin of the dolomitizing fluids. 1.

INTRODUCTION

cipitated from hot ( 150-210°C) hypersaline evaporitic brine or freshwater-evaporated seawater mixtures. Using fluid inclusion and oxygen isotope data, Aulstead et al. ( 1988) found that there was little difference in Th (average, 185°C) among inclusions in dolomite buried over a range of 3000 to 5000 m depth. They also concluded that the inclusions were unlikely to have stretched during burial. Dolomite cements in the Cathedral Formation in the Canadian Rockies have been considered as forming under shallow marine conditions. Aitken and Mcllreath (1984) argued that Cathedral Formation dolomite formed a 200 m high submarine scarp. Ludvigsen ( 1989) challenged the escarpment model and suggested a ramp model based on paleontological data. Stewart ( 1991) interpreted the escarpment as a megatruncation surface.

Combined with fluid inclusion microthermometry, stable isotope data for fluid inclusions and solid dolomite are very useful in elucidating the origin of the dolomite. The microthermometric data can indicate the temperature and salinity of the associated solutions, whereas the stable isotope data can establish the origins of the dolomitizing fluids. However, the inclusion fluids can chemically and isotopically re-equilibrate with their hosts (e.g., Pecher, 1981; Gratier and Jenatton, 1984; Goldstein, 1986, 1990; Sterner et al., 1994). This can cause problems with the use of oxygen isotope data for interpreting the origin of dolomitizing fluids because of exchange of oxygen atoms between fluid inclusion waters and dolomite. The hydrogen isotope composition of fluid inclusion waters is unlikely to be altered if no other hydrogen-containing components are present. One objective of this study was to see what additional information concerning dolomitization could be obtained from the stable isotope compositions of the inclusion fluids. There have been few direct measurements of the stable isotope composition of fluid inclusions in dolomite (e.g., Ghazban et al., 1991). Accordingly, stable isotope data were obtained for fluid inclusion waters in crystalline dolomite cements in the Manetoe Facies (Middle Devonian) and Cathedral Formation (Middle Cambrian) of western Canada (Figs. 1 and 2). In addition, temperatures, salinities, and some major ion concentrations as well as carbon and oxygen isotope compositions for dolomite were determined. The Devonian-hosted Manetoe Facies is of economic importance with several large or potentially large gas fields. Garven and Freeze ( 1982) and Hitchon ( 1984) stated that dolomitization originated from early Tertiary groundwater during deep burial. Morrow et al. ( 1986) used data from Aulstead et al. ( 1988) to propose that the Manetoe Facies dolomite pre-

2. SAMPLING AND ANALYTICAL METHODS 2.1. Samples Three white crystalline Manetoe Facies dolomite samples were taken from well cores (Kotaneelee and Berry) as well as one quartz sample from outcrop (Nahanni Butte) (Fig. I). White crystalline dolomite samples were taken from the Cathedral Formation near the escarpment where the rocks are well exposed. Two samples were collected from Vermillion Pass and three from McArthur Pass (Fig. 2). 2.2.

Quadrupole Mass Spectrometry of Fluid Inclusions

Fluid inclusions in dolomite are usually very small (most less than 20 pm in diameter). Care was taken to select only samples with primary fluid inclusions trapped in bands along the crystal growth zones (Fig. 3 ) Trapped gases such as CO*, HZS, and CH, may affect the isotope compositions of fluid inclusion waters. Therefore, the fluid inclusions were examined by Quadrupole Mass Spectrometry (QMS). A steel ball was placed on one or more cleavage fragments of a crystal (dimensions, -I mm) of dolomite in a vertical Pyrex 3159

3160

W. Yang, R. J. Spencer, and H. R. Krouse

MACKENZlESHELF

130

128

126

124

122

120°w

Camsell and Tsetse

FIG. 1. Stratigraphy of the Mackenzie Shelf and locations of wells and surface exposures sampled for Manetoc Facies dolomite, Northwest and Yukon Territories, western Canada. The Manetoe Facies dolomite is a diagenetic feature which crosscuts formation boundaries.

tube which was then attached to the mass spectrometer sample handling line. After evacuation, valves were switched to connect the sample tube to the mass spectrometer to obtain a background spectrum (Fig. 4b). Then the crystal(s) were broken by moving the ball up and down with a magnet. The released gases were then admitted to the mass spectrometer. The net ion currents after subtraction of background are shown in Fig. 4c. Components other than Hz0 were not detectable above the background levels.

After thorough crushing of the dolomite, the alcohol and water solution was decanted/filtered and the crushed dolomite was rinsed with an additional 5 mL of ethanol. The ethanol solution was allowed to evaporate. The precipitated salts were dissolved in 10 mL of doubly distilled water. The solution was then analyzed by Atomic Absorption Spectrophotometer. In order to test the analytical procedures, triplicate and blank tests were performed for each sample. The analytical precision of the concentrations was better than + 1.O%.

2.3. Micrnthermometry

2.5. Stable Isotope Analyses

Small dolomite cleavage fragments were examined for fluid inclusions. Prior to crushing, homogenization and melting temperatures were obtained using a microscope equipped with a calibrated Fluid Inc.-modified USGS heating-cooling stage. Measurements were duplicated for heating and cooling. Data were reproducible to t2”C. The homogenization temperatures (Th) of primary fluid inclusions determine the isochore along which the mineral grew. For shallow burial diagenetic phases, T/I is close to the temperature of mineral formation (Roedder, 1984). Otherwise, a pressure correction may be required to determine the entrapment temperature (Roedder, 1984; Goldstein and Reynolds, 1994). The final ice melting temperature (Tm,,,) is a direct measure of a nZo of the fluid and can be used to measure the salinity of aqueous inclusions (e.g., Roedder, 1984; Shepherd et al., 1985; Spencer et al., 1990; Johnson and Goldstein, 1993). 2.4. Major Element Chemistry About 1.0 g of dolomite was crushed in 5 mL of 100% ethyl alcohol using an agate mortar to liberate the fluid inclusion waters.

2.5.1. Fluid Inclusion Waters The methods used for determining hydrogen and oxygen isotope compositions of the fluid inclusion waters are similar to those used for halite (Yang, 1993; Yang et al., 1995) with the exception that extraction could not involve heating because dolomite begins to degas CO, at about 200°C and oxygen isotope exchange between HZ0 and carbonate is promoted. The sequence was as follows:

1) About 6 g of dolomite from each sample (enough to expel 4-6 PL H,O) from fluid inclusions) were placed in a stainless steel container with ball bearings; 2) The sample container was attached through a stainless steel valve to a vacuum line. After the internal pressure was reduced to 10-j Torr, the valve was closed; 3) The container was removed (with the closed S.S. valve) and shaken by hand for about 10 min to crush the sample; 4) The container was reattached to the vacuum line and the released water was cryogenically transferred into two parallel pyrex tubes immersed in liquid N?. After pumping to remove any noncon-

Geochemical and isotopic study of dolomite formation

3161

Cana ian ‘6(

0

5Okm

FIG. 2. Stratigraphy of Middle Cambrian deposits of the Rocky Mountains and sample locations at McArthur and Vermillion Passes, western Canada. Dolomite samples were collected along the Ricking Horse Rim, which forms the western margin of the Cambrian platform sediments.

densable gases, the Pyrex tubes were collapsed and sealed with a C&-air torch and removed; and 5) Water in one Pyrex tube was reacted with zinc to produce HZ for hydrogen isotope analysis (modified from Coleman et al., 1982), whereas that in other was reacted with guanidine hydrochloride (GuHCl) to produce CO? for oxygen isotope analysis (modified from Boyer et al., 1961, and Dugan et al., 1985). The 6D and 6”O values were measured with gas-source isotope ratio mass spectrometers built using Micromass 602 and 903 major components. Results were reported with respect to the SMOW standard. The overall precisions were found to be ?0.6%0 and 5 1%0for 5 “0 and 6D, respectively. To test whether extraction occurred without significant isotope fractionation, the procedure was carried out on mixtures of 5 PL of a laboratory standard water and 4-6 g of degassed powdered dolomite. The agreement between the accepted S-values and those for the extracted HZ0 are within the analytical precisions (Table 1)

2.5.2. Dolomite About 20 mg of powdered dolomite, taken from each crushed sample after water extraction, were reacted with H,PO, to generate CO2 gas for carbon and oxygen isotope analyses (McCrea, 1950). The 6 “C and 6 “0 values are reported relative to the PDB standard, the precisions for both being better than t0.2%0.

3. RESULTS

3.1. Devonian Manetoe Facies Homogenization temperatures of fluid inclusions from well core samples of the Manetoe Facies dolomite ranged from 163 to 175°C (Table 2). Final ice melting temperatures ranged from -24 to -18°C indicating salinities about 10 times modem seawater (Table 3 ) . Final ice melting temperatures were combined with major element concentrations in the thermochemical model of Spencer et al. ( 1990) to calculate the chloride and sulfate concentrations of the inclusion fluids (Tables 3 and 4). As shown in Fig. 5, the Na+ , K+ ,and Cl- contents in the fluid inclusion waters from the dolomite cements are very similar to those of many Devonian formation waters and indicate the same parentage (Hitchon et al., 1971; Spencer, 1987). The S “0 and S “C values of the Manetoe Facies dolomite ranged from -12.1 to -11.3%0 and -3.2 to -1.1%0, respectively (Table 2). Measured S “0 values of the fluid inclusion waters from cores in the Manetoe Facies dolomite cements ranged from -5.7 to - 1.7%0 and SD values from -76 to -42%0. Measured S”O and 6D values of the Nahanni Butte

3162

W. Yang, R. J. Spencer, and H. R. Krause

(4

03

FIG. 3. Thin section photomicrographs of dolomite cements, Cathedral Formation. (a) Primary fluid inclusion bands along crystal growth zones (scale is 0.30 mm). (b) Higher magnification of (a) illustrating two-phase (liquid-vapour) fluid inclusions (scale is 0.01 mm).

sample (mainly quartz) tively (Table 2).

3.2. Cambrian

were - 18.3%0 and - 163%0, respec-

Cathedral Formation

Homogenization temperatures of fluid inclusions in the dolomite cements ranged from 138 to 152°C (Table 2; Fig. 6). Final ice melting temperatures ranged from -12 to -9”C, indicating fluids about 5-6 times the salinity of modern seawater (Table 3). Fluid inclusions from the Cathedral Formation were found to have lower Cl -, Na +, and Mg 2+ contents than those in the Manetoe Facies (Fig. 5a and c). The S IgO values of the Cathedral dolomite ranged from -18.4to-17.3%0and6”Cvaluesfrom-3.2to-l.%~(Table 2). The 6D and 6 “0 values for the extracted water ranged from -81 to -72%0 (average, -75%0) and -3.7 to -2.%0 (average, -3.4%0), respectively (Table 2).

4. DISCUSSION 4.1. Devonian Manetoe Facies The moderately high homogenization temperatures for fluid inclusions from the Manetoe Facies indicate the dolomite cements formed from a hydrothermal system. These samples were selected from the larger group studied by Aulstead et al. (1988) because of the uniform homogenization temperatures. Aulstead et al. ( 1988) and Aulstead and Spencer ( 1985) interpret the dolomitization to have occurred at shallow burial depth. These authors give evidence that early diagenesis, including the formation of dolomite, proceeded along a path near the liquid-vapor curve. Therefore, we believe that the homogenization temperatures are very close to the entrapment temperatures for these fluid inclusions and have not adjusted the temperatures for pressure. The salinity and major dissolved ions in the fluid inclusions from the Manetoe Facies dolomite are very similar to some

Geochemical

(4

g

3163

and isotopic study of dolomite formation

33.09 10273

t a 5

5cKlO-

<

b E

0

I

10

15

I

I

I

I

I

I

I

I

I

I

20

25

30

35

40

45

50

55

60

65

Seconds of Scan 04

Background:

368

H20

co2 N2 I

I

20

25

’

I,,

I

30

35

II

I

I

I

40

45

50

I

I

&

45

50

I I

55

I.

60

Mass Number (4

Scan at 33.09: -6553

H20

I

I

I

I

20

25

30

I

35

I

I

55

60

Mass Number FIG. 4. Quadrupole mass spectrometer spectrum of fluid inclusions in dolomite. (a) Total ion current with time, background was scanned for about 32 s. The sample was crushed yielding the peak at 33.09 s as result of fluid inclusions breaking. (b) Background spectrum showing average ion currents up to mass 60 prior to breaking dolomite crystals (the first 32 s). The background consists of water, carbon dioxide, nitrogen gas, and hydrocarbons (small amounts of vacuum pump oil). (c) Net ion currents at 33.09 s, with background subtracted, resulting from crushing the sample. Only water was detectable above the background.

formation waters. The Mg*‘/Ca*’ values of the fluid inclusion waters in the Manetoe Facies dolomite are much higher than those of Devonian formation waters (Table 4 and Fig. 5e). However, the sums of [Ca”] + [Mg”] are similar (Columns 5 and 6, Table 3). The higher Mg2+ and lower Ca*+ concentrations can be attributed to dolomitization (Spencer, 1987). Published data for S “C and S I80 of Devonian marine carbonates range from 0 to +3%0 PDB and -5 to -2%0 PDB (Gao, 1993; Lindth et al., 1981; Keith and Weber, 1964). Lower 6°C values from the Manetoe Facies dolomite than generally found for Devonian marine carbonates may indicate some contribution from oxidized organic carbon (Fig. 7). However, the 6 “C data for the Manetoe Facies dolomite are consistent with those found for host limestone by Aulstead et al. ( 1988). This suggests that the white dolomite cement and dolomitized host rock inherited the 6 ‘?C values of the host limestone. The more negative 6180 values of the Manetoe Devonian

Facies dolomite relative to Devonian marine carbonates are attributed primarily to the elevated temperature of dolomitization. Homogenization temperatures can be used along with S “0 of the crystalline dolomite to calculate the 6 I80 values of waters in equilibrium with dolomite from dolomite-water fractionation factors such as those of Sheppard and Schwartz (1970): lo’ ln

~dolom~te-water

=

3.23 x 106Tm2 - 3.29 (lOO-650°C).

(1)

Calculated values for waters using this equation are given in Table 2. There is about an 8%0 difference in S180 values between directly measured fluid inclusion waters and those calculated from 6 I80 of the dolomite and the homogenization temperatures (except for the Nahanni Butte sample, which is mainly

W. Yang, R. J. Spencer, and H. R. Krouse

3164

Table 1. Isotope compositions of Calgary water (C.W.) extracted after mixing with 4-6mg powdered dolomite for 30 minutes.

Salllple

Volume (pl)

6D (So SMOW)

Table 2. Sample location, average homogenization temperature (Tb) and isotope data for dolomite and fluid inclusions.

8’80 ((960 SMOW) Location

C.W. C.W. C.W. C.W. C.W. C.W. C.W.

5 5 5 5 5 5 10

Mean values Standarddeviation

Recommxded values of C.W.

-149.2 -146.2 -148.8 -149.7 -149.4 -148.6 -149.3

-18.88 -19.55 -18.89 -19.80 -20.21 -19.10 -20.36

-148.7 f1.09

-19.54 M.57

-149.0

-19.50

quartz, Fig. 8). The difference is in the direction expected if oxygen atoms exchanged with the host dolomite during cooling. Calculated 6”O values for initial fluid inclusion waters in the Manetoe Facies are about 5%0 more positive than SMOW. This, coupled with the measured 6D values (-76 and -42%0) and high salinities, suggest that these waters are mixtures of hypersaline evaporated seawater and meteoric water which was subjected to water-rock interactions with oxygen isotope exchange between HZ0 and O-bearing minerals (carbonates, silicates). Hypersaline evaporated seawater satisfies the requirement of a Mg2+ source of dolomitization. Aulstead et al. ( 1988) reported homogenization temperatures ranging from 130 to 210°C for primary fluid inclusions in Manetoe Facies quartz with salinities between 1.5 and 20 eq. wt% NaCl. They interpreted the quartz to have formed as meteoric water migrated through the system. Near meteoric 6D and 6 ‘*O values of - 163%0 and - 18.3%0 found for fluid inclusion water in the Nahanni Butte quartz sample are consistent with this interpretation (Fig. 8). 4.2. Cambrian Cathedral Formation Homogenization temperatures from the Cathedral Formation dolomite samples used here are about midway in the range of temperatures measured from sparry dolomite cements along the Ricking Horse Rim (range of several hundred samples is from 90 to 210°C; R. J. Spencer, unpubl. data; Moore, 1994). We have no direct indication of the timing or depth of formation of the dolomite samples analyzed here. However, Moore ( 1994) interprets dolomite cements from the overlying Eldon Formation to have formed over an extended time, from syndepositional to moderate or deep burial. She presents evidence that dolomitizing fluids in the Eldon Formation boiled periodically, including those at about 130°C. Based on this work, we believe that the dolomite samples used here, which yield fluid inclusion homogenization temperatures between 138 and 152”C, formed at shallow burial depths. We have chosen to use the homogenization temperatures as temperatures of formation without adjustment for pressure. Any pressure adjustment is likely to be small and result in temperatures only slightly (probably
Th

&so,,, (%a SMOW) ---.______------________ 8D, (%o SMOW) Meas. talc.

ason 6’3C (%oPDB) ’

(@C)

DEVONIAN MANETGE Kotaneelee. Y .T. H-38 1700 Berry F-71 1750 F-71 166.4’ Nahanni Butte Amica Fm 163.3’

FACIES: -11.25

-3.21

-65.1

-3.74

5.69

-11.36 -12.08

-2.69 -1.14

-42.0 -75.6

-1.66 -5.72

6.55 4.55

-11.28

-1.09

-162.7b

-18.32b

5.67

-72.0 -73.2

-11.21 -11.42

-3.52 -3.73

-80.6 -77.0 -74.3

-11.08 -13.72 -11.63

-3.31 -3.40 -2.92

CAMBRlAN CATHEDRAL FORMATION: Vermillion Pass D-2 145.0 -18.17 -1.85 D-4 145.6 -18.42 -2.59 McArthur Pass D-l 142.3 -17.73 -3.21 D-3 142.3 -17.81 -2.98 D-5 141.5 -17.27 -2.36

Mea. - measured data, Calc. - calculated data. Subscripts: D - dolomite, W - water, Superscripts: a - Data from Aulstead et al. (1988). b - Data for the mixture of quartz (major) with dolomite (minor).

The salinity of the fluid inclusions from the Cathedral Formation dolomite studied here is lower than that of the Devonian samples. Ratios of major dissolved ions are similar, with Na+ and Cl- as the major constituents (Fig. 5). Magnesium

Table 3. Chemical compositions of fluid inclusion waters from dolomite cements and formation waters given in molality units (formation water data converted from Hitchon et al., 1971).

Sample

Tm(oC) Na+

K+

Gas+

Mg*+

CI-

so,s-

UPPER DEVONIAN Woodbend Group 3* 19 16’ w Wabamun Group 17s

FORMATION WATERS:

FLUID INCLUSION Kotaneelee Y.T. 3879m -21.1s Berry F-7 1 71Orn -24.lb 721m -18.lb

WATERS, MANETGE FACIES:

FLUID INCLUSION Vermillion Pass D-2 -12.1 D-4 -11.1 McArthur Pass D-l -9.1 D-3 -9.1 D-5 -11.1

WATERS, CATHEDRAL FORMATION:

2.5 3.5 4.5 3.1

0.12 0.21 0.15 0.07

0.86 0.72 0.68 0.68

0.18 0.12 0.10 0.15

4.9 5.3 4.9 4.8

0.003 0.003 0.003 0.006

3.5

0.16

0.64

0.11

5.2

0.003

2.364

0.665

0.529

0.490

5.068c

0.m

2.323 3.022

0.289 0.216

1.035 0.404

0.372 0.222

5.433’ 4.489

0.m O.Of@

2.331 1.790

0.068 0.097

0.172 0.337

0.253 0.248

3.250c 0.m 3.058’ O.Of@

1.695 1.549 2.304

0.132 0.111 0.126

0.153 0.273 0.125

0.223 0.197 0.179

2.579 2.6m 3.037c

0.m O.OO@ O.Om

Superscripts: a _ Sample number used by Hitchon et al. (1971). b - Data from Aulstead et al. (1988). and c - Calculated data.

Geochemical and isotopic study of dolomite formation

-

3 c..

0

3165

-Q-l3

Cl3

cl ,

I

I

_

I

I

0.2

4

2

Na’(m)

I

I

0.4

ok

W-n)

1

b

:8

0

loo

211 0.0

I 0.4

0.8

1 1.2

-i

0.0

Ca2+(m)

0.2

0.4

0.6

Mg"(m)

(e) 0.6 /

0.0

1

0.0

I

I

0,4

1

On8

I

Devonian formation waters

9

Manetoe Facies inclusion waters

0

Cathedral Formation inclusion waters

I

1.2

Ca2+(m) FIG. 5. Major element chemical compositions of Devonian formation waters (Hitchon et al., 1971) and fluid inclusion waters in the Manetoe Facies and Cathedral Formation dolomites. All waters are Na-Cl-rich, the fluid inclusion waters have similar chemistry, but the Cambrian waters are much less concentrated than the Devonian waters,

to calcium ratios of the fluids are relatively high, indicating the fluids still had potential for dolomitization when trapped.

Published data for 6 “C and 6 I80 of Cambrian marine carbonates range from -3 to +2%0 PDB and - 11 to -3%0 PDB (Gao and Land, 1991; Lindtb et al., 1981; Keith and Weber, 1964). The 6°C values from the Cathedral Formation dolomites are within this range (Fig. 7). These data are a few permil more negative than Cambrian marine limestones reported

by Gao and Land ( 199 1) and those found in the Eldon Formation limestones (+O.l to +OS) by Moore ( 1994). The bulk of the carbon in the dolomite appears to be contributed by the host limestone, with a lesser contribution from an organic carbon source. The oxygen isotopes from the Cathedral Formation dolomite are depleted with respect to Cambrian marine carbonates, primarily as a result of the higher temperature of dolomitization.

W. Yang, R. J. Spencer, and H. R. Krause

3166

20

20

D-l

1

130

140

130

lS0

Temperature

20

s + k

wall dolomite will not likely have the same oxygen isotope composition as the bulk material for which measurements were made. A relevant exercise is to calculate temperatures using published oxygen isotope fractionation curves and measured 6”O values for dolomite and fluid inclusion water. These give temperatures near 75°C in contrast to the current ambient temperature (about 25°C) expected if the fluid inclusion water had isotopically re-equilibrated under conditions of a high dolomite/water ratio. Possibilities for the disagreement in temperatures are

D-2 1

eC)

140

lso

Temperature

20

D-3

(“C)

1) The 6 “0 value of the inclusion wall dolomite is higher than that of the bulk dolomite. This is in the direction expected for an effective dolomite/water mass ratio < 1 since 10’ In (Yd,,lomit_lerincreases with decreasing temperature; 2) Some oxygen isotope exchange occurred during cooling, but equilibrium at current ambient temperature has not been attained; and 3) The oxygen isotope data are consistent with equilibrium exchange conditions at current temperature, but calculations based on extrapolation of the higher temperature experimental data give values of T which are high.

D-4

1

10

130

140

130

Ii0

Temperature

PC)

lb0

150

Temperature

PC]

20

D-5 s

r!z 0 IL ii 10 130

140

150

Temperature

(“Cl

FIG. 6. Distribution of homogenization temperatures from primary fluid inclusions in the Cathedral Formation dolomite cements.

In order to assess the validity of the low temperature partitioning coefficient determined for dolomite and water, we need to understand if the water in the fluid inclusions is in equilibrium with the bulk dolomite, or with an alteration halo around the fluid inclusion, which has a different composition from the bulk dolomite. A series of schematic plots of “0 concentration in the dolomite with distance, going from the wall of a fluid inclusion to the bulk dolomite away from the fluid inclusion are shown in Fig. 9. Initially, at time t,, the fluid inclusion and dolomite crystal are at high temperature,

4.3. Low Temperature Oxygen Isotope Exchange between

Dolomite and Water

+2.0

Based on the homogenization temperatures microthermometrically measured from fluid inclusions, 10” In (YdOl,,mlle_water values of the Manetoe Facies and Cathedral Formation dolomites were calculated using several equations (Table 5 )_ 6values are related to (Y-and A-values by ~dolomte-water

=

(~dalom,te

A dolomite-water

(so)

=

+

l”‘)~(k”at~r

~adalom~te-water

+

-

1 I

lo’),

X

lo’,

(2) (3)

s g

0

0 0 5 0

-2.0

f,a

when 10’ In (1.00X) = X,

1

0

0

I

$ (4)

These samples had been at or near Eartb surface temperature for years (cores) or much longer (outcrops). Hence, a relevant question is the extent to which oxygen isotopes in the fluid inclusion waters have “re-equilibrated” with those in dolomite. Addressing this question is not straightforward, one problem being the uncertainty of the effective (in contrast to the bulk) dolomite to inclusion water mass ratio. In other words, how many “molecular layers” of dolomite in the inclusion wall can exchange O-atoms with the water? If the corresponding effective mass of carbonate oxygen is low, the

A

-4.0 I

-20

I

-16

I

I

I

-12

,

-8

I

-4

S”O (o/o0 PDB) 0

Cambmn

Cathedral

A

Dmnhn

Manetoe

Formation Fades

dolomite

dolotite

FIG. 7.6’% vs. 6”O values for dolomite from Cathedral Formation and Manetoe Facies. Data for Devonian and Cambrian marine car-

bonates are from Keith and Weber (1964), Lindtb et al. (1981), Gao and Land (1991), and Gao (1993).

Geochemical and isotopic study of dolomite formation

3167

Table 4. Major element molalityconcenuationratios for

fluid inclusion waters in dolomite and formation waters, western Canada (Devonian formation water data converted from Hitchon et al., 1971).

Sample

Cl-Ma+ Cl-/K+

UPPER DEVONIAN Woodbend Group: 3 2.0 19 1.5 16 1.1 90 1.5 Wabamun Group: 17 1.5

Cl-/Ca*+

C1-/Mg2+

Na+iK+

Ca*+/K+ Mgz+/K+

Ca*+lNa+

MgZ+/Na+

MgZ+/Ca2+

FORMATION WATERS: 40.8 25.2 32.7 68.6

5.7 7.4 7.2 7.1

27.2 44.2 49.0 32.0

20.8 16.7 30.0 44.3

7.2 3.4 4.5 9.7

1.5 0.6 0.7 2.1

0.34 0.21 0.15 0.22

0.07 0.03 0.02 0.05

0.21 0.17 0.15 0.22

32.5

8.1

47.3

21.9

4.0

0.7

0.18

0.03

0.17

10.3

3.6

0.8

0.7

0.22

0.21

0.93

14.6 20.2

8.0 14.0

3.6 1.9

1.3 1.0

0.45 0.13

0.16 0.07

0.36 0.55

34.3 18.5

2.5 3.5

3.7 2.6

0.07 0.19

0.11 0.14

1.47 0.74

12.8 14.0 18.3

1.2 2.5 1.0

1.7 1.8 1.4

0.09 0.18 0.05

0.13 0.13 0.08

1.46 0.72 1.43

DEVONIAN MANETOE FACIES: Kotaneelee Y.T. H-38 2.1 7.6 9.6 Berry F-7 1 710m 2.3 18.8 5.2 721m 1.5 20.8 11.1

CAMBRIAN CATHEDRAL FORMATION: Vermillion Pass D-2 1.4 47.8 18.9 12.8 D-4 1.7 31.5 9.1 12.3 McArihor Pass D-l 1.5 19.5 16.9 11.6 D-3 1.7 23.4 9.5 13.2 D-5 1.3 24.1 24.3 17.0

in isotope equilibrium, and there is no gradient in “0 from the wall of the fluid inclusion outward. At t2 the temperature is lowered. If we assume that the reaction of the oxygen isotopes in the fluid inclusion waters with the wall dolomite is rapid, relative to diffusion of isotopes in the crystal lattice, a gradient in I80 will be established as shown in Fig. 9. With

time, diffusion of oxygen isotopes through the crystal lattice will result in some sort of distribution such as that shown at tl in Fig. 9, and eventually with a smooth distribution as shown at t. If there is a distribution such as that at t3 in the dolomite measured here, then we can say nothing about the values calculated by several equations at the Table 5. lO%a dol0nuls-w*l homogenization temperaturesof the dolomite cements.

Location

Th (OC)

1031na*lmnllc-wPler values _____......._..____________._..__...__________.__.. A

DEVONIAN MANETOE FACIES: Kotaneelee Y.T. H-3 170.0 14.81 Berry F-71 710m 175.0 14.44 721m 166.4 15.07 Nahanni Butte Amica Fm 163.3 15.31

-80 Solid symbols: Open symbols:

measured calculated

/

-160 !/*

Q”ART2 I

i/

r

-25

I

l

I

I

I

I

-15 -5 +5 PO (0100 SMOW)

I

I

+15

FIG. 8. Measured 6D vs. 6’*0 values for fluid inclusion waters in (solid circles) and Manetoe Facies (solid squares) dolomite cements; and Nahanni Butte quartz (triangle). Val-

the Cathedral Formation

ues of 5”O calculated using the isotope composition of solid dolomite and homogenization temperatures, assuming isotope equilibrium are plotted along with the measured 6D values as open symbols. Devonian residual evaporite brines (Spencer, 1987) (rhomb), MWL-Meteoric Water Line, and SMOW-Standard Mean Ocean Water are shown for reference.

B

C

D

E

13.68

13.17

14.28

12.35

13.30 13.96

12.80 13.44

13.96 14.51

12.01 12.61

14.21

13.68

14.71

12.84

CAMBRIAN CATHEDRAL FORMATION: Vermillion Pass D-2 145.0 16.80 15.76 15.18 16.01 14.26 D-4 145.6 16.75 15.71 15.13 15.96 14.21 McArthor Pass D-l 142.3 17.04 16.01 15.42 16.22 14.94 D-3 142.3 17.04 16.01 15.42 16.22 14.94 D-5 141.5 17.11 16.09 15.50 16.28 14.56 _______________________._..._______________________________________._~__._______________ Temperature of 25°C

A - Based on the equation of BCDE-

34.49

34.23

33.05

31.38

Northrop and Clayton (1966); O’Neil and Epstein (1966); Sheppard and Schwarcz (1970); Fritz and Smith (1970); Matthews and Katz (1977).

31.18

W. Yang, R. J. Spencer, and H. R. Krouse

3168

11 % 08

Tl

T2

m e

In.

Distance

Inclusion wall

Distance

BUlk

BUlk

Inclusion wall

Distance

Inclusion wall

Distance

BUlk

FIG. 9. A series of schematic plots of b’“0 value in the dolomite with distance, going from the inclusion wall dolomite to the bulk dolomite away from a fluid inclusion. Initially, at time t,, the fluid inclusion and dolomite crystal are at high temperature, in isotope equilibrium, and there is no gradient in “0 from the inclusion wall dolomite outward. At rz the temperature is lowered. Assuming that the oxygen isotope exchange reaction between the inclusion wall dolomite and the fluid inclusion waters is rapid, relative to oxygen isotope diffusion in the crystal lattice, a gradient in “0 will be established. At r3 the diffusion of oxygen isotopes through the crystal lattice will result in some sort of distribution with time, and eventually with a smooth distribution as shown at t4.

low temperature partitioning of oxygen isotopes between dolomite and water, because we have measured the isotope composition of the bulk dolomite and the inclusion water, but not the dolomite in contact with the water. However, if there is a distribution such as that shown at t4 in Fig. 9, then we can determine the low temperature equilibrium partitioning of dolomite and water from our data. The dolomite samples analyzed here are “rock” dominated systems. We were able to extract about 1 pg of water per gram of dolomite. Visual estimates of the amount of water in fluid inclusions in these dolomites range from 0.01 to 0.001%. We do not believe we were able to remove all of the water from the fluid inclusions using our crushing techniques so that the visual estimates are consistent with the amount extracted. Most of the fluid inclusions in these samples are less than 20 /L across and average about 10 /I across. For the calculations below we have assumed that the “typical” fluid inclusion is a sphere, 10 p in diameter and far (more than 10 ,u) from its nearest neighbor. The volume of a 10 /I diameter sphere is about 5.25 X 10 I” cubic centimeters, and assuming a density of the water near one, contains about 1.75 x 10 ” atoms of oxygen. Assuming an initial fluid with 6 I80 of -3%0 and final 6 “0 of - 11%~

about 2.8 X 10’ ‘*O atoms were transferred from the fluid to the solid. If we assume that the 6180 value of the dolomite can not excede the low temperature equilibrium value (that is, if the laws of thermodynamics are valid), using the low temperature partitioning value from Sheppard and Schwartz ( 1970)) 6 I80 of the dolomite can not excede - 8%0. Based on the number of I80 atoms transfered from the fluid, these atoms must be distributed over a minimum of 8 X lo-” cc of the dolomite, or a distance of 0.8 /.L(assuming any gradient in I80 such as that in Fig. 9 at t3 this distance will double). Therefore, it appears that the oxygen isotopes must have diffused at least this far through the dolomite crystals, through tens of thousands of molecular layers around each fluid inclusion. The Devonian dolomite samples have been at surface temperatures from a few tens of years, since the cores were drilled. Prior to this they were at temperatures in excess of one hundred degrees celcius for tens to hundreds of millions of years. It appears that significant diffusion of I80 through the dolomite crystal has occurred during this time. Assuming a gradient such as that at t3 (Fig. 9) exists for these dolomite crystals, then we expect significantly more diffusion and a value for 10’ In (Ymuch closer to the true equilibrium value, for the Cambrian dolomite samples. The Cambrian samples

Geochemical and isotopic study of dolomite formation

have been at low temperatures for a minimum of several tens of thousands of years, and possibly for tens of millions of years. Since the 10” In LYvalues obtained for both sets of samples overlap, we interpret the I80 distribution in both dolomites to be similar to what is shown at L,in Fig. 9. Therefore, we interpret the 10’ In cr values determined here to represent oxygen isotope equilibria between the fluid inclusion waters and the bulk dolomite. Determining the problem of bulk or local equilibria remains a challenge. It is of interest to note that Ghazban et al. ( 1991) values ranging from 17 to 27%0, which are found &orom,rc-watrr similar to those found in the current study. In contrast to our interpretation, they deduced that these data were generally consistent with temperatures of deposition determined by other techniques. Microthermometric data were not presented for individual samples in the report of Ghazban et al. ( 1991) . The current study shows that the large differences (about 8%0) between measured and calculated S I80 values of fluid inclusion waters strongly reflect oxygen isotope exchange between dolomite and the inclusion water during cooling. Hence, measured S’*O values of fluid inclusion waters in dolomite can not be used for calculation of the mineral formation temperatures. It is interesting that if the measured 10’ In ~~~~~~~~~~~~~~~ values are combined with the high temperature experimental data of Northrop and Clayton ( 1966) (Table 6), they fit the relationship

Table 6. dolomite

10%~ do!uau~-w.~rvalues based on measured &So,,,,, for fluid inclusions in at ambient temperature for current study, and experimental and SW,,,,,

values at high temperatures from Nortluop and Clayton (1966). 1OJlna dolanile-u.tcr

Location

6’80~,rmilc (%

DEVONIAN MANETOE Kotancelee Y.T.

PDB)

8’sO,,,

10Qa *!.anilr-w*r

(5% SMOW)

FACIES:

H-38

-11.25

-3.14

22.75

Berry F-7 1 710m

-11.36

-1.66

721m

-12.08

-5.72

20.81 24.12

Nahanni Butte

CAMBRIAN Vcrmillion

_.___

-11.28

Amica Fm

CATHEDRAL

FORMATION:

Pass

D-2

-18.17

-11.21

23.34

D-4

-18.42

-11.42

23.29

D-l

-17.73

-11.08

23.66

D-3

-17.81

-13.72

26.22

D-5

-17.27

-11.63

24.69

3169

.~ -25

I_

-25

-15 -5 +5 6”O (o/o0 SMOW)

I

~~1

I

+15

__!~

-15 -5 +5 6”O (o/o0 SMOW)

t15

FIG. 10. Two possible sequences for generating fluid inclusion waters in the dolomite cements from Cathedral Formation (open circles) and Manetoe Facies (open squares), and in the Nahanni Butte quartz (triangle) from Manetoe Facies, western Canada. DREB (rhomb) stands for Devonian residual evaporite brines (data from Spencer, 1987), WRl (horizontal arrows) for water-rock interactions, MWL for Meteoric Water Line, and SMOW for Standard Mean Ocean Water. (a) Evaporated seawater is mixed with meteoric water along the dotted lines in proportions to give the 6D values and salinities for the fluid inclusions. This mixture then reacts with either silicates or carbonates at elevated temperature enriching the fluids in ‘“0. These fluids then react with the limestone to form dolomite. (b) Evaporated seawater and meteoric water react with silicates or carbonates at elevated temperature and become enriched in “0. These fluids then mix and react with the host limestone to form dolomite.

McArthur Pass

Mean value of 1O’lna dolamilr-w.lcr values

High Temperature (‘JC)

23.61

~03W+Z.hi#~.W*,

2%

6.75

300

8.18 6.91

510

Natural hydrothermal dolomites

r2 = 0.983

(5)

where T is the absolute temperature in Kelvin (OK).

----- _____________________________ __.____.______ B-dolomite’

350 410

lo3 In adolomite-water = 2.0 x 10YZ + 0.74,

8.38 1.81 6.29

4.95 2.96

from ‘Bamle, Norway; %x.,

L-dolomitez

4.59 3.78

Mass.

5. CONCLUSIONS The relatively high homogenization temperatures and low melting temperatures of fluid inclusions in the Manetoe Facies

and Cathedral Formation dolomites indicate an origin from hot, hypersaline water. The 6°C and S I80 values of the Manetoe Facies and Cathedral Formation dolomites are lower than those of the

3170

W. Yang, R. J. Spencer, and H. R. Krouse

+50 I ,

Devonian formation waters (from Krause. 1980)

0 I 6

-50 1

2 m

(

8 2 g

-100

t I

7.

Fluid inclusion waters in Manetoe Facies dolomite

Fluid inclusion waters in cathedral Formation dolomite

-150

Devonian formation waters (from Hitchon et at., 1969)

Surface and near-surface waters (from Hitchon et al., 1969)

FIG. 11. 6D vs. 6”O values for surface and near-surface waters, Devonian formation waters from Alberta and fluid inclusion waters

in dolomite cements from Cathedral Formation and Manetoe Facies. general Devonian and Cambrian marine carbonates. The 6°C values of these dolomites show that the source of carbon during dolomitization is the host limestone. The slight depletion in “C may indicate some CO:- is derived from organic matter. The depletion in I80 results from dolomitization at moderately high temperatures. The measured 6’*0 values for fluid inclusion waters are much more negative (about 8%0) than those calculated for waters in isotope equilibrium with dolomite at the moderately high temperatures determined by microthermometry. This is explained by subsequent lower temperature oxygen isotope reequilibration between dolomite and the fluid inclusion water. Therefore, the measured oxygen isotope compositions for the fluid inclusion waters do not reflect those of the dolomitizing fluids and can not be used to calculate the temperatures of dolomite formation or composition of dolomitizing fluids. Isotopic alteration of the fluid inclusions seems unlikely by diffusion through crystals. Recent research indicates that the diffusion mechanism is insufficient to significantly change the mass of the fluid (Sinogeikin et al., 1994). Kronenberg et al. ( 1986) reported that for pressures up to 15OOMPa and temperatures of 700 to 900°C diffusion of molecular water into millimeter scale single crystals is too slow for any significant penetration. They found that the oxygen of the fluid could not have exchanged with the bulk of oxygen in the crystals by diffusive transport and hydrogen could not diffuse in the form of OH or H,O under these conditions. Further, SD values of the fluid inclusion waters in dolomite could not have been altered by exchange reactions because of the absence of other

hydrogen-containing compounds. Therefore, 6D values can be used in combination with the calculated oxygen isotope data to infer the origin of the fluids. The 6D and calculated S I80 values of fluid inclusion waters in dolomite from both Manetoe Facies and Cathedral Formation reveal that these dolomites were formed neither from simple nor evaporated seawater (Fig. 8). The possibility that high salinities of the dolomitizing Auids are due to salt dissolution by meteoric water poses the problem of how to obtain the Mg” needed to form the dolomite. Soluble salts in the basin are high in Na’ and Cl _, but low in Mg2+ . Of many possible sequences, two are shown in Fig. 10. In Fig. lOa, water-rock interaction occurred after mixing of meteoric water with either evaporated seawater or a residual evaporite brine. Fig. lob shows how the data for the Devonian Manetoe Facies dolomite inclusions might be explained by having both brine and meteoric water undergoing significant water-rock interaction prior to mixing. The more negative SD and 6180 values of fluid inclusion waters in the Cambrian Cathedral Formation dolomite (an average of -75%0 for 6D and -3.4%0 for S ‘“0) suggest similar mixing phenomena, but with evaporated seawater of lower salinity and/or more meteoric water (Fig. 10). It is significant that the isotope data for fluid inclusion waters in both the Devonian Manetoe Facies and Cambrian Cathedral Formation dolomites follow the formation water trend for Alberta (Fig. 11) . Also on the plot of 6D vs. mcl , these data lie on the mixing line between meteoric water and the Devonian residual evaporite brines (Fig. 12). In brief, the SD, 6 “0 (calculated), salinity, and major element content of fluid inclusion waters in the Manetoe Facies dolomite are similar to many of the more concentrated De-

n a

-100

iI I

/

7.5

FIG. 12. 6D vs. Q,- values for sea water, Devonian formation waters, residual evaporite brines and fluid inclusion waters in the Cathedral Formation and Manetoe Facies dolomite cements. Both 6D and Cl- are likely to behave as conservative components in this systern. Note that the fluid inclusion waters appear to be mixtures of residual evaporite brines and meteoric water. Data for seawater, Devonian formation waters, and residual evaporite brines are from Spencer (1987).

Geochemical

and isotopic

vonian formation waters in the basin. These waters appear to be evaporated seawater/residual evaporite brines mixed with meteoric water. The Cathedral Formation dolomite may have had a similar origin to the Manetoe Facies, but with more influence from meteoric water and/or less intensive evaporation. Acknowledgmenrs-We thank Robert H. Goldstein, Jay M. Gregg, and Isabel Montanez for reviewing the manuscript. Their many thoughtful suggestions improved the quality and organization of this paper. Funding for this work was provided by Operating and Stable Isotope Laboratory Infrastructure Grants to RJS and HRK from the Natural Sciences and Engineering Research Council of Canada (NSERC) . We thank technicians in the stable isotope and geochemical research laboratories at the University of Calgary for their assistance. Thanks also to Dr. Ian Hutcheon (Head) and Dr. N. C. Wardlaw at the Department of Geology and Geophysics, the University of Calgary, for their supervision and encouragement to WY during his MSc study.

Editorial handling: M. A. McKibben

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and H. R. Krause eastern Canadian Rocky Mountains. Ph.D. thesis, OCGC, Univ. Ottawa. Yang W. ( 1993) Improved techniques for stable isotope analyses of microlitre quantities of water and applications to paleoclimate and diagenesis using fluid inclusions in halite and dolomite. MSc thesis, Univ. Calgary. Yang W., Spencer R. J., Krause H. R., and Lowenstein T. K. ( 1995) Stable hydrogen and oxygen isotope techniques for studying arid basin hydrology. In Tracer Techniques for Hydrological Systems, IAHS Publication No. 43 (in press).