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The distribution of Fe and Mn between chlorite and fluid: Evidence from fluid inclusions
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Gewhimrca ~1 Cosmuchimrca Actu Vol.55.pp.241-244 1991 Pewmon Press plc.Printed in U.S.A.
Copyright 0
The distribution of Fe and Mn between chlorite and fluid: Evidence from fluid inclusions SIMON
H. BOTTRELL and BRUCE W. D. YARDLEY
Department of Earth Sciences, Leeds University, Leeds, LS2 9JT, UK
(ReceivedApril 16, 1990; acceptedin revisedform October 24, I990) Abstract-The Fe/Mn ratio of fluid inclusions from low-grade metamorphic quartz veins has been analyzed by an improved crush-leach method. When compared with Fe and Mn in coexisting vein chlorites, exchange between fluid and chlorite is shown to vary with the redox state of the wall rocks, but is independent of the salinity of the fluid. Agreement between our results and theoretical predictions of Fe/ Mn partitioning between fluid and chlorite are good for veins from relatively oxidized rocks, but vein fluids from reduced hosts are enriched in Fe. INTRODUCTION
given in Table 1. Our working hypotheses are that (at least some) fluid inclusions in these veins will record the fluid present during the metamorphic event and that the vein chlorites formed in equilibrium with the same fluid.
CHLORITE IS A COMMON phase produced in low-grade metamorphism, geothermal systems, and ore deposits, and its composition may act as a sensor for the chemistry of the deep fluids from which it grew. A theoretically derived calibration for the distribution of a variety of divalent cations between chlorite and aqueous fluid has been proposed by SVERJENSKY ( 1985). Here we report analyses of coexisting metamorphic chlorites and metamorphic fluid (extracted from fluid inclusions) from the Harlech Dome, north Wales, and compare the partitioning of Fe and Mn we observe with the predictions of SVERJENSKY ( 1985). This comparison allows us to test the applicability of the theoretical method to prediction of metamorphic fluid chemistry.
ANALYTICAL METHODS Quartz vein samples were first prepared for microthermometric analysis as free-standing, doubly polished slices ca. 0.3 mm thick. Observations were made using a Linkam TH600 temperature-programmable heating-freezing stage attached to a Leitz Ortholux microscope (SHEPHERD, 198 1) and provide information on the equivalent salinity of the fluid. Fluid inclusions were then analyzed for cations using the improved crush-leach technique of BOTTRELLet al. ( 1988). Analyses of vein chlorite were made using the Electron Probe Microanalyser with EDS detector at the Department of Earth Sciences, University of Cambridge, with instrument condition as described by TRELOAR( 1981).
Sampling RESULTS The veins analyzed in this study are from the Harlech Dome in north Wales ( MATLEY and WILSON, 1943). The dome is a Caledonian structure of Cambrian sedimentary rocks which have undergone low-grade metamorphism. Metamorphic mineral assemblages are typically chlorite + phengitic muscovite + quartz + albite + paragonite + epidote t calcite ( BOTTRELL, 1986). In the lower part of the sequence (the Harlech Grits Group) there is generally no organic carbon, and pyrite is common with either magnetite or (more rarely) hematite, whereas the overlying Mawddach Group is comprised of black shales and siltstones containing graphitized organic carbon with pyrrhotine and/or pyrite. Peak metamorphic temperatures for these rocks ranged between 350 and 400°C (BOTTRELL et al., 1990) and were attained after the main period of pervasive deformation. The samples investigated were all small, isolated veins composed principally of quartz with minor chlorite and white mica, and all occurred away from faults or other possible loci of large-scale, focussed fluid flow. Thus, the fluid that formed the veins is likely to have equilibrated with local wall rocks during metamorphism. BOTTRELL et al. (1990) show that for such veins, chlorite compositions are virtually identical to metamorphic chlorites in the host rock, confirming that these veins did indeed form during the metamorphic event from a locally buffered fluid. This paper is based on results from five samples whose location and associations are
Fluid Inclusions Microthermometry Microthermometric results on fluid inclusions from quartz veins in the Harlech Dome have been reported previously by BOTTRELL ( 1986) and BOTTRELL et al. ( 1990). Inclusions are generally small ( < 10 pm), although larger inclusions < 20 pm are found in some samples. Two types can be distinguished on petrographic grounds: type I are isolated or in small groups and may be primary or secondary, while type II are of undoubted secondary origin and occur as planar arrays decorating healed fractures. As in most metamorphic quartz veins, many of the secondary inclusions are too small for microthermometric study to be practicable. In general, type I inclusions in samples from the Harlech Dome are of moderately saline brine which yields final ice melting temperatures (T,,,) in the range -1 to - lOS”C, corresponding to an equivalent salinity of 2 to 14 wt% NaCl. The onset of melting is seen at or above -2 1“C, indicating a predominantly Na-K-Cl composition. A carbonaceous phase ( COZ or CH4) is sometimes also present. Type II inclusions may include similar, but usually only weakly saline, Na-K-Cl brines (~4 wt% NaCl equivalent). and more concentrated brines with lower eutectic melting, generally < -35°C. These low eutectics suggest a significant Ca content, while equivalent sa241
242
S. H. Bottrell and B. W. D. Yardley Table
1.
Details
of samples analyzed
Sample No.
Grid reference
Host rock
description
19c
SH625166
Graphitic
siltstone
35B
SH619170
Fine sandstone
36D
SH624164
Graphitic
mudstone (Clogau
124D
SH667236
Siltstone
(Hafotty
Fm.)
124E
SH667236
Siltstone
(Hafotty
Fm.)
linities are in the range 11 to 20 wt% NaCI. No examples of these low eutectic melting inclusions have been observed in the samples studied here. BOTTRELL et al. ( 1990) showed that type I inclusions formed close to peak metamorphic conditions, but many type II inclusions originated later. Detailed microthermometric studies have been carried out on three of the five samples analyzed here, although the small size of many inclusions makes melting point observations difficult. Sample I9C, from the graphitic Clogau Formation, is dominated by aqueous inclusions with equivalent salinities in the range 1.5 to 7.0 wt% NaCI, clustered about two peaks at 2.5 and 6.0 wt%. Some inclusions contain a CH, phase, whose homogenization temperature is in the range -90 to - 115”C, though few measurements were possible. The ~01% CHI is estimated at 2 to 6%, and equivalent salinities of aqueous portions of methane-bearing inclusions are in the range 2 to 6 wt% NaCI. Inclusions in samples 35B and 124D from the graphitefree Gamlan and Hafotty Formation, respectively, are very similar to one another; significantly, they also have a similar range of salinities to those in 19C. However CO2 rather than CH4 is present as an additional phase in some inclusions. Aqueous inclusions in these two samples have equivalent salinities that are mostly in the range 2.0 to 7.0 wt% NaCl, although a few lower ice melting points give salinities up to 12 wt%. Some aqueous inclusions exhibit clathrate melting behaviour, although a distinct CO2 phase is not always observed. Equivalent salinities derived from clathrate melting are in the range 2.8 to 11.3 wt% and CO* homogenisation, observed in 35B, is between -9 and - 10. which gives a CO2
Table
2.
Sample No.
Analyzed
Na
electrolyte
(Clogau
Fm.)
(Gamlan Fm.) Fm.)
density of 0.98, and hence a bulk density for the inclusions very close to 1. Chemical
analysis
Cation analyses of inclusion fluids released by crushing according to the method of BOTTRELL et al. ( 1988 ) are given in Table 2. These bulk analyses indicate that the fluids are Na-K-Ca brines with appreciable Mg, Fe, and Mn. Thus, the analyses are higher in Ca than was anticipated from the melting behaviour of the inclusions. There are three possible explanations for this. Firstly, the relatively low salinity of many of the inclusions meant that the first signs of melting were not observed until above the eutectic in the NaCl-HZ0 system. The possibility that melting began at still lower temperatures cannot be ruled out entirely. Secondly, these bulk analyses may include Ca derived from very small secondary inclusions containing a Ca-rich brine (although this has not been observed optically). Thirdly, the analyses may have been contaminated by traces of solid carbonate inclusions, although again we have checked the samples optically and such impurities are not readily apparent. We favour the first or second alternatives to explain the high Ca-levels, and if any late, lowT calcic secondary inclusions in the crushed samples are comparable to the shield types of Ca brines described by FRAPE et al. ( 1984), they are unlikely to have contributed significant amounts of any cations other than Ca to the bulk analysis. The third alternative we can rule out. Not only are solid contaminants not seen, but where analyses on other samples in our laboratory have been contaminated in this way, the effect is unambiguous.
ratios
of
fluid
K
Ca
Mg
inclusion
leachates
Fe
Mn
19c
1.00
0.19
0.50
0.040
0.045
0.011
35B
1.00
0.026
0.28
0.013
0.0027
0.022
36D
1.00
0.079
0.28
0.073
0.051
0.011
124D
1.00
0.044
0.31
0.013
0.0025
0.0041
124E
1.00
0.063
0.41
0.017
0.0077
0.010
243
Fe/Mn ratio of fluid inclusions Table 3.
Recalculatedchloritecompositions
19c
35B
36D
124E
9
7
7
14
Si
5.10(0.08)
7.19(0.25)
5.23(0.08)
5.59(0.28)
Al
5.74tO.08)
4.39tO.16)
5.80(0.10)
5.28CO.19)
Mg
3.80(0.08)
2.28(0.10)
3.39CO.06)
4.53(0.16)
Fe
5.16(0.25)
4.32(0.15)
5.29cO.16)
4.17(0.15)
MI-I
0.19(0.02)
0.38(0.02)
0.17(0.01)
0.19(0.01)
co
0.04(0.02)
0.06(0.02)
b.d.
b.d.
K
0.01(0.01)
b.d.
b.d.
b.d.
SampleNo. No.
of analyses
Figures in bracketsgive one standarddeviation. b.d. = below detection.
Chlorites Analyses were made on polished grain mounts of chlorites separated from the quartz veins. The analytical data are presented in full by BOTTRELL ( 1986) and given as recalculated formulae in Table 3. All these compositions fall in the field of Ripidolite. OF IRON AND MANGANESE BETWEEN CHLORITE AND FLUID
PARTITIONING
The exchange of divalent Fe and Mn between chlorite and fluid is described by
for ions of such similar crystallographic radii (Mn = 0.80 A, Fe = 0.74 A). Figure 1 is a plot of the analyzed Fe/ Mn ratios for chlorite and inclusion fluid. The analyzed chlorite in 124E is taken as also in equilibrium with 124D. The data fall into two groups, for which best-fit lines of constant D are plotted. These groups correspond to rocks whose mineralogy indicates either relatively oxidized or relatively reduced conditions during metamorphism. The opaque assemblage is pyrite + magnetite + hematite in the former case, graphite with either pyrrhotine or pyrite in the latter. Also plotted are lines of D in the range of likely metamorphic temperatures, calculated from SVER-
MnSA12Si~0,~(OH)8 + 5Fe& = FesA12Si301,,(OH)8 + SMn& for which the equilibrium
(1)
constant
KM,,++ = (a~~!!-chl/aCMhfichl)/(a~zd+/a~~~+)5 (2) where a~~!:l,,,l is the activity of Fe-chlorite in chlorite, etc. In this study we have measured the concentration of Fe and Mn in chlorites and fluid, which allows calculation of a distribution coefficient for these elements; i.e.,
FO G
&n-~e = (X~~‘l’lx~1)l(M,,lM,.)5
chl
(3)
where X$’ is the mole fraction of Fe on the octahedral sites of the chlorite and MFe is the total molality of Fe in solution, etc. KM,.F, and DM”_F~are related by &I-~c
= K~,.~e/[(X~~'/X~h)/(~Fc2+/i'~n~+)~]
(4)
where @’ is the rational activity coefficient for iron in chlorite and +&f is the stoichiometric activity coefficient for aqueous Fe*+, taking account of all the complexes in solution ( HELGESON, 1969).While stoichiometric activity coefficients for the fluid phase can be estimated for some divalent cations ( HELGESON, 1969)the rational activity coefficients for chlorite cannot. Hence, the relation between KMII_Fe and DMn_Fe relies on the assumption that Xgt’/X$, = 1, not unreasonable
I
I
I
I
1
2
3
4
FO Mn
fluid
FIG. I. Plot of chlorite Fe/Mn vs. fluid Fe/Mn. Samples from relatively oxidized rocks are given as open circles and from reduced rocks as filled boxes. Broken lines are lines of constant KDfor these two groups. Solid lines give KDcalculatedat the indicated temperature using the method of SVEFUENSKY(1985).
S. H. Bottrell and B. W. D. Yardley
244
JENSKY (1985) assuming jlFe2+ = &z+ since HELGESON ( 1969) does not give data for &z+. In the case of the oxidized rocks, the analyzed Fe/Mn ratios show rather good agreement with the theoretically predicted D values computed on this basis for temperatures near the peak metamorphic values of ca. 350°C. The major errors on our analyzed D values will be associated with the inclusion Fe/Mn ratios and will be of the order of +50% ( E%OTTRELL et al., 1988). The temperaturesensitivity of the D prediction to this analytical error is variable, but for these values this error corresponds to +30/ -2O”C, meaning that the analyzed D values for the oxidized rocks are indistinguishable from values predicted for the lower estimates of peak metamorphic temperatures. In marked contrast, D values for the reduced rocks appear to indicate much lower temperatures according to Sverjensky’s method (<2OO”C). Even when the errors of ?4O”C appropriate here are taken into account, this is a significant discrepancy since the actual temperature of formation of all veins is the same. In effect, the fluid in equilibrium with a given chlorite composition has a much higher Fe/Mn ratio in the more reduced host rocks. Since we are dealing with fluid inclusion analyses produced by a bulk crush-leach method, the variation in fluid Fe/ Mn (and hence D) could stem from differences in the relative proportions of different types of fluid in the total inclusion population, rather than differences in the composition of a particular generation of inclusions. However, there is no evidence to suggest that different generations, such as any Carich brines, are more or less abundant in the reduced samples. Rather, the bulk Ca/Na ratios of populations of inclusions in all five samples show a similar range (Table 2 ). We do not believe that the existence of an “exotic” inclusion population in some samnles can exnlain the variation. Instead. it is likely that the assumption yFe2+ = &2+ is not valid in the case of fluid from the reduced lithologies. DISCUSSION
AND CONCLUSION
Although there are only a small number of samples in this study, the fluid Fe/ Mn data do fall into two distinct groups: oxidized rocks give ( Fe/Mn)auid < 0.8, while reduced rocks give (Fe/Mn)n”id > 4.2. Within the oxidized group (Fe/ Mn)““i,, is clearly related to chlorite composition (Fig. 1). The close agreement within the two groups we have identified, and the fact that data from veins in oxidized rocks correspond closely to the SVERIENSKY ( 1985 ) predictions, gives us confidence that our data truly reflect partitioning between chlorite and metamorphic fluid. In the case of fluids in more reduced lithologies, however, there is a marked discrepancy between predicted and observed D values. This appears to be caused by additional Fe in solution due to the formation of additional complexes. A par-
allel difference in Fe-Mg partitioning between analyses of veins from oxidized and reduced rocks is also seen, but we have not attempted to quantify this because the assumptions that activity coefficients would cancel would clearly be inappropriate. The difference between fluids from reduced and oxidized hosts appears to be a direct consequence of the redox state, rather than a reflection of other differences in fluid chemistry, because the microthermometry shows that fluids in all lithologies are of very similar salinity. This is in agreement with the experimental work of BOCTOR( 1985) who showed that Fe/Mn ratios in a chloride fluid equilibrated with hematite + rhodonite + quartz increase with increasing Hz fugacity.
Acknowledgments-The early parts of this work were completed while SHB was a NERC-funded research student in the School of Environmental Sciences. Universitv of East Anaha. and subseauent work supportedby NERC research-grant GR3755.27. We would like to thank Mr. F. Buckley for assistance with AAS analysis of leachates and Drs. A. Buckley and J. V. P. Long for assistance with the microprobe analyses. An earlier version of the manuscript was substantially improved by the comments of T. S. Bowers. C. A. Heinrich, and an anonymous reviewer. Editorial handling: G. Faure
REFERENCES BOCTORN. Z. (1985) Rhodonite solubility and thermodynamic properties of aqueous MnClz in the system MnO-SiOz-HCI-H20. Geochim. Cosmochim. Acta 49, 565-575. BOTTRELLS. H. ( 1986) The origin of the gold mineralization of the Dolnellau district. north Wales: the chemistrv and role of fluids. Ph.D. thesis, University of East Anglia, Norwich. BOTTRELLS. H., YARDLEYB. W. D., and BUCKLEYF. (1988) A modified crush-leach method for the analysis of fluid inclusion electrolytes. Bull Mineral. 111, 279-290. BOTTRELLS. H., GREENWOOD P. B., YARDLEYB. W. D.. SHEPHERD T. J., and SPIRO B. ( 1990) Metamorphic and post-metamorphic fluid flow in the low-grade rocks of the Harlech Dome, north Wales. J. Metamorphic Geol. 8, 13I - 143. FRAPES. K., FRITZ P., and MCNUTT R. H. ( 1984) Water-rock interaction and the chemistry of groundwaters from the Canadian Shield. Geochim. Cosmochim. Acta 48, 16 17- 1627. HELGESONH. C. ( 1969) Thermodynamics of hydrothermal systems at elevated temperatures and pressures. Amer. J. Sci. 267, 729804. MATLEYC. A. and WILSON T. S. ( 1943) The Harlech Dome, North of the Barmouth estuary. Quart. J. Geol. Sot. London 102, l-35. SHEPHERDT. J. ( 198I ) Temperature-permeable heating-freezing stage for microthermometric analysis of fluid inclusions. Econ. Geol. 75, 124441247. SVERJENSKY D. A. ( 1985) The distribution of divalent trace elements between sulphides. oxides, silicates and hydrothermal solution: I Thermodynamic basis. Geochim. Cosmochim. Acta 49,853-864. TRELOARP. J. ( 198 I ) Garnet-biotite-cordiertie thermometry and barometry in the Cashel thermal aureole, Connemara, Ireland. Mineral. Mag. 44, 183-189.
Gewhimrca ~1 Cosmuchimrca Actu Vol.55.pp.241-244 1991 Pewmon Press plc.Printed in U.S.A.
Copyright 0
The distribution of Fe and Mn between chlorite and fluid: Evidence from fluid inclusions SIMON
H. BOTTRELL and BRUCE W. D. YARDLEY
Department of Earth Sciences, Leeds University, Leeds, LS2 9JT, UK
(ReceivedApril 16, 1990; acceptedin revisedform October 24, I990) Abstract-The Fe/Mn ratio of fluid inclusions from low-grade metamorphic quartz veins has been analyzed by an improved crush-leach method. When compared with Fe and Mn in coexisting vein chlorites, exchange between fluid and chlorite is shown to vary with the redox state of the wall rocks, but is independent of the salinity of the fluid. Agreement between our results and theoretical predictions of Fe/ Mn partitioning between fluid and chlorite are good for veins from relatively oxidized rocks, but vein fluids from reduced hosts are enriched in Fe. INTRODUCTION
given in Table 1. Our working hypotheses are that (at least some) fluid inclusions in these veins will record the fluid present during the metamorphic event and that the vein chlorites formed in equilibrium with the same fluid.
CHLORITE IS A COMMON phase produced in low-grade metamorphism, geothermal systems, and ore deposits, and its composition may act as a sensor for the chemistry of the deep fluids from which it grew. A theoretically derived calibration for the distribution of a variety of divalent cations between chlorite and aqueous fluid has been proposed by SVERJENSKY ( 1985). Here we report analyses of coexisting metamorphic chlorites and metamorphic fluid (extracted from fluid inclusions) from the Harlech Dome, north Wales, and compare the partitioning of Fe and Mn we observe with the predictions of SVERJENSKY ( 1985). This comparison allows us to test the applicability of the theoretical method to prediction of metamorphic fluid chemistry.
ANALYTICAL METHODS Quartz vein samples were first prepared for microthermometric analysis as free-standing, doubly polished slices ca. 0.3 mm thick. Observations were made using a Linkam TH600 temperature-programmable heating-freezing stage attached to a Leitz Ortholux microscope (SHEPHERD, 198 1) and provide information on the equivalent salinity of the fluid. Fluid inclusions were then analyzed for cations using the improved crush-leach technique of BOTTRELLet al. ( 1988). Analyses of vein chlorite were made using the Electron Probe Microanalyser with EDS detector at the Department of Earth Sciences, University of Cambridge, with instrument condition as described by TRELOAR( 1981).
Sampling RESULTS The veins analyzed in this study are from the Harlech Dome in north Wales ( MATLEY and WILSON, 1943). The dome is a Caledonian structure of Cambrian sedimentary rocks which have undergone low-grade metamorphism. Metamorphic mineral assemblages are typically chlorite + phengitic muscovite + quartz + albite + paragonite + epidote t calcite ( BOTTRELL, 1986). In the lower part of the sequence (the Harlech Grits Group) there is generally no organic carbon, and pyrite is common with either magnetite or (more rarely) hematite, whereas the overlying Mawddach Group is comprised of black shales and siltstones containing graphitized organic carbon with pyrrhotine and/or pyrite. Peak metamorphic temperatures for these rocks ranged between 350 and 400°C (BOTTRELL et al., 1990) and were attained after the main period of pervasive deformation. The samples investigated were all small, isolated veins composed principally of quartz with minor chlorite and white mica, and all occurred away from faults or other possible loci of large-scale, focussed fluid flow. Thus, the fluid that formed the veins is likely to have equilibrated with local wall rocks during metamorphism. BOTTRELL et al. (1990) show that for such veins, chlorite compositions are virtually identical to metamorphic chlorites in the host rock, confirming that these veins did indeed form during the metamorphic event from a locally buffered fluid. This paper is based on results from five samples whose location and associations are
Fluid Inclusions Microthermometry Microthermometric results on fluid inclusions from quartz veins in the Harlech Dome have been reported previously by BOTTRELL ( 1986) and BOTTRELL et al. ( 1990). Inclusions are generally small ( < 10 pm), although larger inclusions < 20 pm are found in some samples. Two types can be distinguished on petrographic grounds: type I are isolated or in small groups and may be primary or secondary, while type II are of undoubted secondary origin and occur as planar arrays decorating healed fractures. As in most metamorphic quartz veins, many of the secondary inclusions are too small for microthermometric study to be practicable. In general, type I inclusions in samples from the Harlech Dome are of moderately saline brine which yields final ice melting temperatures (T,,,) in the range -1 to - lOS”C, corresponding to an equivalent salinity of 2 to 14 wt% NaCl. The onset of melting is seen at or above -2 1“C, indicating a predominantly Na-K-Cl composition. A carbonaceous phase ( COZ or CH4) is sometimes also present. Type II inclusions may include similar, but usually only weakly saline, Na-K-Cl brines (~4 wt% NaCl equivalent). and more concentrated brines with lower eutectic melting, generally < -35°C. These low eutectics suggest a significant Ca content, while equivalent sa241
242
S. H. Bottrell and B. W. D. Yardley Table
1.
Details
of samples analyzed
Sample No.
Grid reference
Host rock
description
19c
SH625166
Graphitic
siltstone
35B
SH619170
Fine sandstone
36D
SH624164
Graphitic
mudstone (Clogau
124D
SH667236
Siltstone
(Hafotty
Fm.)
124E
SH667236
Siltstone
(Hafotty
Fm.)
linities are in the range 11 to 20 wt% NaCI. No examples of these low eutectic melting inclusions have been observed in the samples studied here. BOTTRELL et al. ( 1990) showed that type I inclusions formed close to peak metamorphic conditions, but many type II inclusions originated later. Detailed microthermometric studies have been carried out on three of the five samples analyzed here, although the small size of many inclusions makes melting point observations difficult. Sample I9C, from the graphitic Clogau Formation, is dominated by aqueous inclusions with equivalent salinities in the range 1.5 to 7.0 wt% NaCI, clustered about two peaks at 2.5 and 6.0 wt%. Some inclusions contain a CH, phase, whose homogenization temperature is in the range -90 to - 115”C, though few measurements were possible. The ~01% CHI is estimated at 2 to 6%, and equivalent salinities of aqueous portions of methane-bearing inclusions are in the range 2 to 6 wt% NaCI. Inclusions in samples 35B and 124D from the graphitefree Gamlan and Hafotty Formation, respectively, are very similar to one another; significantly, they also have a similar range of salinities to those in 19C. However CO2 rather than CH4 is present as an additional phase in some inclusions. Aqueous inclusions in these two samples have equivalent salinities that are mostly in the range 2.0 to 7.0 wt% NaCl, although a few lower ice melting points give salinities up to 12 wt%. Some aqueous inclusions exhibit clathrate melting behaviour, although a distinct CO2 phase is not always observed. Equivalent salinities derived from clathrate melting are in the range 2.8 to 11.3 wt% and CO* homogenisation, observed in 35B, is between -9 and - 10. which gives a CO2
Table
2.
Sample No.
Analyzed
Na
electrolyte
(Clogau
Fm.)
(Gamlan Fm.) Fm.)
density of 0.98, and hence a bulk density for the inclusions very close to 1. Chemical
analysis
Cation analyses of inclusion fluids released by crushing according to the method of BOTTRELL et al. ( 1988 ) are given in Table 2. These bulk analyses indicate that the fluids are Na-K-Ca brines with appreciable Mg, Fe, and Mn. Thus, the analyses are higher in Ca than was anticipated from the melting behaviour of the inclusions. There are three possible explanations for this. Firstly, the relatively low salinity of many of the inclusions meant that the first signs of melting were not observed until above the eutectic in the NaCl-HZ0 system. The possibility that melting began at still lower temperatures cannot be ruled out entirely. Secondly, these bulk analyses may include Ca derived from very small secondary inclusions containing a Ca-rich brine (although this has not been observed optically). Thirdly, the analyses may have been contaminated by traces of solid carbonate inclusions, although again we have checked the samples optically and such impurities are not readily apparent. We favour the first or second alternatives to explain the high Ca-levels, and if any late, lowT calcic secondary inclusions in the crushed samples are comparable to the shield types of Ca brines described by FRAPE et al. ( 1984), they are unlikely to have contributed significant amounts of any cations other than Ca to the bulk analysis. The third alternative we can rule out. Not only are solid contaminants not seen, but where analyses on other samples in our laboratory have been contaminated in this way, the effect is unambiguous.
ratios
of
fluid
K
Ca
Mg
inclusion
leachates
Fe
Mn
19c
1.00
0.19
0.50
0.040
0.045
0.011
35B
1.00
0.026
0.28
0.013
0.0027
0.022
36D
1.00
0.079
0.28
0.073
0.051
0.011
124D
1.00
0.044
0.31
0.013
0.0025
0.0041
124E
1.00
0.063
0.41
0.017
0.0077
0.010
243
Fe/Mn ratio of fluid inclusions Table 3.
Recalculatedchloritecompositions
19c
35B
36D
124E
9
7
7
14
Si
5.10(0.08)
7.19(0.25)
5.23(0.08)
5.59(0.28)
Al
5.74tO.08)
4.39tO.16)
5.80(0.10)
5.28CO.19)
Mg
3.80(0.08)
2.28(0.10)
3.39CO.06)
4.53(0.16)
Fe
5.16(0.25)
4.32(0.15)
5.29cO.16)
4.17(0.15)
MI-I
0.19(0.02)
0.38(0.02)
0.17(0.01)
0.19(0.01)
co
0.04(0.02)
0.06(0.02)
b.d.
b.d.
K
0.01(0.01)
b.d.
b.d.
b.d.
SampleNo. No.
of analyses
Figures in bracketsgive one standarddeviation. b.d. = below detection.
Chlorites Analyses were made on polished grain mounts of chlorites separated from the quartz veins. The analytical data are presented in full by BOTTRELL ( 1986) and given as recalculated formulae in Table 3. All these compositions fall in the field of Ripidolite. OF IRON AND MANGANESE BETWEEN CHLORITE AND FLUID
PARTITIONING
The exchange of divalent Fe and Mn between chlorite and fluid is described by
for ions of such similar crystallographic radii (Mn = 0.80 A, Fe = 0.74 A). Figure 1 is a plot of the analyzed Fe/ Mn ratios for chlorite and inclusion fluid. The analyzed chlorite in 124E is taken as also in equilibrium with 124D. The data fall into two groups, for which best-fit lines of constant D are plotted. These groups correspond to rocks whose mineralogy indicates either relatively oxidized or relatively reduced conditions during metamorphism. The opaque assemblage is pyrite + magnetite + hematite in the former case, graphite with either pyrrhotine or pyrite in the latter. Also plotted are lines of D in the range of likely metamorphic temperatures, calculated from SVER-
MnSA12Si~0,~(OH)8 + 5Fe& = FesA12Si301,,(OH)8 + SMn& for which the equilibrium
(1)
constant
KM,,++ = (a~~!!-chl/aCMhfichl)/(a~zd+/a~~~+)5 (2) where a~~!:l,,,l is the activity of Fe-chlorite in chlorite, etc. In this study we have measured the concentration of Fe and Mn in chlorites and fluid, which allows calculation of a distribution coefficient for these elements; i.e.,
FO G
&n-~e = (X~~‘l’lx~1)l(M,,lM,.)5
chl
(3)
where X$’ is the mole fraction of Fe on the octahedral sites of the chlorite and MFe is the total molality of Fe in solution, etc. KM,.F, and DM”_F~are related by &I-~c
= K~,.~e/[(X~~'/X~h)/(~Fc2+/i'~n~+)~]
(4)
where @’ is the rational activity coefficient for iron in chlorite and +&f is the stoichiometric activity coefficient for aqueous Fe*+, taking account of all the complexes in solution ( HELGESON, 1969).While stoichiometric activity coefficients for the fluid phase can be estimated for some divalent cations ( HELGESON, 1969)the rational activity coefficients for chlorite cannot. Hence, the relation between KMII_Fe and DMn_Fe relies on the assumption that Xgt’/X$, = 1, not unreasonable
I
I
I
I
1
2
3
4
FO Mn
fluid
FIG. I. Plot of chlorite Fe/Mn vs. fluid Fe/Mn. Samples from relatively oxidized rocks are given as open circles and from reduced rocks as filled boxes. Broken lines are lines of constant KDfor these two groups. Solid lines give KDcalculatedat the indicated temperature using the method of SVEFUENSKY(1985).
S. H. Bottrell and B. W. D. Yardley
244
JENSKY (1985) assuming jlFe2+ = &z+ since HELGESON ( 1969) does not give data for &z+. In the case of the oxidized rocks, the analyzed Fe/Mn ratios show rather good agreement with the theoretically predicted D values computed on this basis for temperatures near the peak metamorphic values of ca. 350°C. The major errors on our analyzed D values will be associated with the inclusion Fe/Mn ratios and will be of the order of +50% ( E%OTTRELL et al., 1988). The temperaturesensitivity of the D prediction to this analytical error is variable, but for these values this error corresponds to +30/ -2O”C, meaning that the analyzed D values for the oxidized rocks are indistinguishable from values predicted for the lower estimates of peak metamorphic temperatures. In marked contrast, D values for the reduced rocks appear to indicate much lower temperatures according to Sverjensky’s method (<2OO”C). Even when the errors of ?4O”C appropriate here are taken into account, this is a significant discrepancy since the actual temperature of formation of all veins is the same. In effect, the fluid in equilibrium with a given chlorite composition has a much higher Fe/Mn ratio in the more reduced host rocks. Since we are dealing with fluid inclusion analyses produced by a bulk crush-leach method, the variation in fluid Fe/ Mn (and hence D) could stem from differences in the relative proportions of different types of fluid in the total inclusion population, rather than differences in the composition of a particular generation of inclusions. However, there is no evidence to suggest that different generations, such as any Carich brines, are more or less abundant in the reduced samples. Rather, the bulk Ca/Na ratios of populations of inclusions in all five samples show a similar range (Table 2 ). We do not believe that the existence of an “exotic” inclusion population in some samnles can exnlain the variation. Instead. it is likely that the assumption yFe2+ = &2+ is not valid in the case of fluid from the reduced lithologies. DISCUSSION
AND CONCLUSION
Although there are only a small number of samples in this study, the fluid Fe/ Mn data do fall into two distinct groups: oxidized rocks give ( Fe/Mn)auid < 0.8, while reduced rocks give (Fe/Mn)n”id > 4.2. Within the oxidized group (Fe/ Mn)““i,, is clearly related to chlorite composition (Fig. 1). The close agreement within the two groups we have identified, and the fact that data from veins in oxidized rocks correspond closely to the SVERIENSKY ( 1985 ) predictions, gives us confidence that our data truly reflect partitioning between chlorite and metamorphic fluid. In the case of fluids in more reduced lithologies, however, there is a marked discrepancy between predicted and observed D values. This appears to be caused by additional Fe in solution due to the formation of additional complexes. A par-
allel difference in Fe-Mg partitioning between analyses of veins from oxidized and reduced rocks is also seen, but we have not attempted to quantify this because the assumptions that activity coefficients would cancel would clearly be inappropriate. The difference between fluids from reduced and oxidized hosts appears to be a direct consequence of the redox state, rather than a reflection of other differences in fluid chemistry, because the microthermometry shows that fluids in all lithologies are of very similar salinity. This is in agreement with the experimental work of BOCTOR( 1985) who showed that Fe/Mn ratios in a chloride fluid equilibrated with hematite + rhodonite + quartz increase with increasing Hz fugacity.
Acknowledgments-The early parts of this work were completed while SHB was a NERC-funded research student in the School of Environmental Sciences. Universitv of East Anaha. and subseauent work supportedby NERC research-grant GR3755.27. We would like to thank Mr. F. Buckley for assistance with AAS analysis of leachates and Drs. A. Buckley and J. V. P. Long for assistance with the microprobe analyses. An earlier version of the manuscript was substantially improved by the comments of T. S. Bowers. C. A. Heinrich, and an anonymous reviewer. Editorial handling: G. Faure
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