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Isotope fractionation between galena and pyrite and between pyrite and elemental sulfur
EARTH AND PLANETARY SCIENCE LETTERS 11 (1971) 236-238. NORTH-HOLLAND PUBLISHING COMPANY
ISOTOPE FRACTIONATION
BETWEEN GALENA AND PYRITE AND
BETWEEN PYRITE AND ELEMENTAL
SULFUR
W. SALOMONS* Geophysical Laboratory. CarnegieInstitution of Washington, WashingtonD.C. 20008, USA Received 28 April 1971
Sulfur isotope fractionation between pyrite and galena and between pyrite and elemental sulfur was investigated. Isotope equilibrium was not achieved in 120 days at 400°C and 500°C, nor in 40 days at 600°C. Slow diffusion of sulfur isotopes in pyrite and the isotopic inhomogeneity of the pyrite crystals are probably responsible. By the techniques used so far, isotope equilibrium in systems containing pyrite cannot be reached in a reasonable amount of time at the temperatures investigated.
1. Introduction Data on sulfur isotope fractionation among coexisting sulfide minerals have been compiled by Bachinski [1]. Sakai [2] calculated the fractionation between a number of sulfide minerals. Experimentally derived fractionation between coexisting sulfide minerals [3] and between sulfide minerals and elemental sulfur [4, 5] has been reported. In these investigations [3, 4] pyrite was prepared from the elements. It has been shown, however, that the reaction of sulfur and iron to form pyrite takes a very long time to reach phase equilibrium [6], let alone isotopic equilibrium. In our investigations pyrite, therefore, was prepared from pyrrhotite and an excess of sulfur; in this reaction phase equilibrium can be achieved much faster [6] and it may be expected that the same is true of isotope equilibrium. The sulfur isotope fractionation between pyrite and galena and between pyrite and elemental sulfur was studied.
2. Experimental procedures The experiments were conducted in rigid, evacuated, * On leave from: Laboratory of Inorganic Chemistry, The University, Bloemsingel 10, Groningen, The Netherlands.
silica glass tubes as described by Kullerud and Yoder [6]. Several batches of pyrrhotite were prepared by heating iron and sulfur in a 1 : 1 atomic ratio for 7 days at 600°C. Galena was prepared by heating lead and sulfur in a 1 : I atomic ratio for 1 day at 600°C. The starting materials iron and sulfur were the same as used by Kullerud and Yoder [6], and the lead was of the material used by Kullerud [7]. The sulfur-isotope exchange experiments involving pyrite and galena were conducted by means of the tube-in-tube technique described by Kullerud and Yund [8]. The outside tube contained galena, and the inside tube contained pyrrhotite with about 2% sulfur excess in addition to that needed for complete conversion of pyrrhotite to pyrite. The sulfur-isotope exchange experiments between pyrite and elemental sulfur were conducted by heating pyrrhotite and sulfur with about 100% excess sulfur in addition to that needed for complete conversion of pyrrhotite to pyrite. On termination of each experiment the excess sulfur was separated by extraction with hot trichloroethylene. The products of the runs were identified by reflected-light study of polished sections. The 3, S/32S ratios were determined in a double-collector 60 ° mass spectrometer. The measuring gas was SF6, prepared by the method of Puchelt et al. [9]. The isotopic composition is expressed as
W. Salomons, isotope fractionation
237
Table 1 Sulfur isotope fractionation between pyrite and galena. A(% 0)
Reaction time (days)
Temperature (°C)
Phasespresent a
36 70 115
400 400 400
py + po + ga + s py + po + ga + s py+po+ga+s
0.5 0.5 0.3
36 70 115
500 500 500
py + ga + s py + ga + s py + ga+ s
1.4 1.3 1.2
70 115
600 600
py + ga + s py + ga + s
0.9 0.5
tipy-6 ga
a py, pyrite; po, pyrrhotite; ga, galena; s, sulfur.
Table 2 Sulfur isotope fractionation between pyrite and elemental sulfur. Reaction time (days)
A(°/oo)
Temperature (°C)
Phasespresent a
41
400
74 120
400 400
py+po+s py+po÷s py+s
4.0 2.0
41 74 120
500 500 500
py+s py+s py+s
0.5 0.4 0.6
41 74 120
600 600 600
py+s py+s py+s
2.0 1.4 1.5
6PY--6s
a py, pyrite; po, pyrrhotite; s, sulfur.
(34S/3~S) x _ (34S/32 S) s 6 3 4 S 0 / 0 0 "=
- - X
1000,
(a, S/3: S) s where x is the unknown and s the standard. The precision of the measurements was always better +- 0 . 2 % 0 .
3. Experimental results and discussion The results of the sulfur-isotope exchange experiments between pyrite and galena are given in table 1, and the results of the sulfur-isotope exchange experiments between pyrite and elemental sulfur are compiled in table 2. Phase equilibrium was apparently
achieved in the runs at 500°C and 600°C. In the 400°C runs, observations of polished sections revealed in most experiments that small patches of pyrrhotite remain inside pyrite grains, indicating that phase equilibrium was not achieved. Although in some runs an almost constant fractionation with time was obtained (within experimental error), isotope equilibrium was not achieved within 120 days in the runs at 400°C and 500°C; nor within 40 days for the runs at 600°C (we cannot decide whether isotope equilibrium is established at 600°C after 120 days). After the sulfur has reacted and phase equilibrium achieved, a process of homogenization follows, leading toward isotope equilibrium, an occurrence which has been observed in the fractionation be-
238
w. Salomons. Isotope fractionation
ween galena and elemental sulfur [5] and between silver sulfide and elemental sulfur [10]. Pyrite equilibrates very slowly [ 11 ], and the diffusion of sulfur isotopes in pyrite is consequently very slow. In the exchange experiments, the sulfur phase 'recognizes', as a result of the low diffusion rate, only the outer layers of the pyrite crystals, and is in equilibrium with these outer layers only. Because of the low diffusion rate, the process of homogenization takes a long time, which also results in an almost constant fractionation with time, although equilibrium is not attained. Isotopic inhomogeneity in the synthetic pyrite, used as a starting material, may also explain why equilibrium was not achieved. During the formation of pyrite, an added inhomogeneity occurs through the faster diffusion rate of the 32S isotope in comparison with the S4S isotope. Since sulfur isotope equilibrium was not attained in most of our experiments, a comparison of our results with calculated isotope fractionation [2] is not meaningful. Owing to the low rate of sulfur isotope equilibration in pyrite the results of previous sulfur-isotope fractionation studies involving pyrite [3, 4] should be regarded as doubtful, where lower temperatures and/ or shorter times of equilibration were applied. The sluggish rate of sulfur isotope equilibration may have significant geological implications. During metamorphism, pyrite has a tendency to retain its original isotopic composition, whereas other sulfides readily reequilibrate. Under conditions of changing temperature, only the outer layers of pyrite may reequilibrate, resulting in isotopically zoned crystals.
Acknowledgements I wish to thank Drs. T.C. Hoering and G. Kullerud for the use of their laboratories and for their many helpful suggestions in improving the manuscript. Thanks are due to the Netherlands Organization for the Advancement of Pure Research (ZWO) for providing the fellowship which made possible my stay at the Geophysical Laboratory.
References [ 1] D.J. Bachinski, Bond strength and sulfur isotope fractionation in coexisting sulfides, Econ. Geol. 64 (1969) 56. [2] H. Sakai, Isotopic properties of sulfur compounds in hydrothermal processes, Geochem. J. 2 (1968) 29. [31 Y. Kajiwara, H.R. Krouse and A. Sakai, Experimental study of sulfur isotope fractionation between coexisting sulfide minerals, Earth Planet. Sci. Letters 7 (1969) 271. [4] J. Grootenboer and H.P. Schwarcz, Experimentally determined sulfur isotope fractionations between sulfide minerals, Earth Planet. Sci. Letters 7 (1969) 162. [5] H. Puchelt and G. Kullerud, Sulfur isotope fractionation in the Pb-S system, Earth Planet. Sci. Letters 7 (1970) 301. [6] G. Kullerud and H.S. Yoder, Jr., Pyrite stability relations in the Fe-S system, Econ. Geol. 54 (1959) 533. [7] G. Kullerud, The lead-sulfur system, Am. J. Sci., Schairer Vol. 267A (1969) 233. [81 G. Kullerud and R.A. Yund, The Ni-S system and related minerals, J. Petrol. 3 (1962) 126. [9] H. Puchelt, B. Sabels and T.C. Hoering, Preparation of Sulfur-hexa-fluoride for isotope geochemical analysis, Geochim. Cosmochim. Acta (1971) in press. [ 10] W. Salomons, unpublished results. [ 11 ] H. Puchelt, Sulfur isotope fractionation in minerals, Carnegie Inst. Wash. Yearbook 1967-1968 (1969) 192.
ISOTOPE FRACTIONATION
BETWEEN GALENA AND PYRITE AND
BETWEEN PYRITE AND ELEMENTAL
SULFUR
W. SALOMONS* Geophysical Laboratory. CarnegieInstitution of Washington, WashingtonD.C. 20008, USA Received 28 April 1971
Sulfur isotope fractionation between pyrite and galena and between pyrite and elemental sulfur was investigated. Isotope equilibrium was not achieved in 120 days at 400°C and 500°C, nor in 40 days at 600°C. Slow diffusion of sulfur isotopes in pyrite and the isotopic inhomogeneity of the pyrite crystals are probably responsible. By the techniques used so far, isotope equilibrium in systems containing pyrite cannot be reached in a reasonable amount of time at the temperatures investigated.
1. Introduction Data on sulfur isotope fractionation among coexisting sulfide minerals have been compiled by Bachinski [1]. Sakai [2] calculated the fractionation between a number of sulfide minerals. Experimentally derived fractionation between coexisting sulfide minerals [3] and between sulfide minerals and elemental sulfur [4, 5] has been reported. In these investigations [3, 4] pyrite was prepared from the elements. It has been shown, however, that the reaction of sulfur and iron to form pyrite takes a very long time to reach phase equilibrium [6], let alone isotopic equilibrium. In our investigations pyrite, therefore, was prepared from pyrrhotite and an excess of sulfur; in this reaction phase equilibrium can be achieved much faster [6] and it may be expected that the same is true of isotope equilibrium. The sulfur isotope fractionation between pyrite and galena and between pyrite and elemental sulfur was studied.
2. Experimental procedures The experiments were conducted in rigid, evacuated, * On leave from: Laboratory of Inorganic Chemistry, The University, Bloemsingel 10, Groningen, The Netherlands.
silica glass tubes as described by Kullerud and Yoder [6]. Several batches of pyrrhotite were prepared by heating iron and sulfur in a 1 : 1 atomic ratio for 7 days at 600°C. Galena was prepared by heating lead and sulfur in a 1 : I atomic ratio for 1 day at 600°C. The starting materials iron and sulfur were the same as used by Kullerud and Yoder [6], and the lead was of the material used by Kullerud [7]. The sulfur-isotope exchange experiments involving pyrite and galena were conducted by means of the tube-in-tube technique described by Kullerud and Yund [8]. The outside tube contained galena, and the inside tube contained pyrrhotite with about 2% sulfur excess in addition to that needed for complete conversion of pyrrhotite to pyrite. The sulfur-isotope exchange experiments between pyrite and elemental sulfur were conducted by heating pyrrhotite and sulfur with about 100% excess sulfur in addition to that needed for complete conversion of pyrrhotite to pyrite. On termination of each experiment the excess sulfur was separated by extraction with hot trichloroethylene. The products of the runs were identified by reflected-light study of polished sections. The 3, S/32S ratios were determined in a double-collector 60 ° mass spectrometer. The measuring gas was SF6, prepared by the method of Puchelt et al. [9]. The isotopic composition is expressed as
W. Salomons, isotope fractionation
237
Table 1 Sulfur isotope fractionation between pyrite and galena. A(% 0)
Reaction time (days)
Temperature (°C)
Phasespresent a
36 70 115
400 400 400
py + po + ga + s py + po + ga + s py+po+ga+s
0.5 0.5 0.3
36 70 115
500 500 500
py + ga + s py + ga + s py + ga+ s
1.4 1.3 1.2
70 115
600 600
py + ga + s py + ga + s
0.9 0.5
tipy-6 ga
a py, pyrite; po, pyrrhotite; ga, galena; s, sulfur.
Table 2 Sulfur isotope fractionation between pyrite and elemental sulfur. Reaction time (days)
A(°/oo)
Temperature (°C)
Phasespresent a
41
400
74 120
400 400
py+po+s py+po÷s py+s
4.0 2.0
41 74 120
500 500 500
py+s py+s py+s
0.5 0.4 0.6
41 74 120
600 600 600
py+s py+s py+s
2.0 1.4 1.5
6PY--6s
a py, pyrite; po, pyrrhotite; s, sulfur.
(34S/3~S) x _ (34S/32 S) s 6 3 4 S 0 / 0 0 "=
- - X
1000,
(a, S/3: S) s where x is the unknown and s the standard. The precision of the measurements was always better +- 0 . 2 % 0 .
3. Experimental results and discussion The results of the sulfur-isotope exchange experiments between pyrite and galena are given in table 1, and the results of the sulfur-isotope exchange experiments between pyrite and elemental sulfur are compiled in table 2. Phase equilibrium was apparently
achieved in the runs at 500°C and 600°C. In the 400°C runs, observations of polished sections revealed in most experiments that small patches of pyrrhotite remain inside pyrite grains, indicating that phase equilibrium was not achieved. Although in some runs an almost constant fractionation with time was obtained (within experimental error), isotope equilibrium was not achieved within 120 days in the runs at 400°C and 500°C; nor within 40 days for the runs at 600°C (we cannot decide whether isotope equilibrium is established at 600°C after 120 days). After the sulfur has reacted and phase equilibrium achieved, a process of homogenization follows, leading toward isotope equilibrium, an occurrence which has been observed in the fractionation be-
238
w. Salomons. Isotope fractionation
ween galena and elemental sulfur [5] and between silver sulfide and elemental sulfur [10]. Pyrite equilibrates very slowly [ 11 ], and the diffusion of sulfur isotopes in pyrite is consequently very slow. In the exchange experiments, the sulfur phase 'recognizes', as a result of the low diffusion rate, only the outer layers of the pyrite crystals, and is in equilibrium with these outer layers only. Because of the low diffusion rate, the process of homogenization takes a long time, which also results in an almost constant fractionation with time, although equilibrium is not attained. Isotopic inhomogeneity in the synthetic pyrite, used as a starting material, may also explain why equilibrium was not achieved. During the formation of pyrite, an added inhomogeneity occurs through the faster diffusion rate of the 32S isotope in comparison with the S4S isotope. Since sulfur isotope equilibrium was not attained in most of our experiments, a comparison of our results with calculated isotope fractionation [2] is not meaningful. Owing to the low rate of sulfur isotope equilibration in pyrite the results of previous sulfur-isotope fractionation studies involving pyrite [3, 4] should be regarded as doubtful, where lower temperatures and/ or shorter times of equilibration were applied. The sluggish rate of sulfur isotope equilibration may have significant geological implications. During metamorphism, pyrite has a tendency to retain its original isotopic composition, whereas other sulfides readily reequilibrate. Under conditions of changing temperature, only the outer layers of pyrite may reequilibrate, resulting in isotopically zoned crystals.
Acknowledgements I wish to thank Drs. T.C. Hoering and G. Kullerud for the use of their laboratories and for their many helpful suggestions in improving the manuscript. Thanks are due to the Netherlands Organization for the Advancement of Pure Research (ZWO) for providing the fellowship which made possible my stay at the Geophysical Laboratory.
References [ 1] D.J. Bachinski, Bond strength and sulfur isotope fractionation in coexisting sulfides, Econ. Geol. 64 (1969) 56. [2] H. Sakai, Isotopic properties of sulfur compounds in hydrothermal processes, Geochem. J. 2 (1968) 29. [31 Y. Kajiwara, H.R. Krouse and A. Sakai, Experimental study of sulfur isotope fractionation between coexisting sulfide minerals, Earth Planet. Sci. Letters 7 (1969) 271. [4] J. Grootenboer and H.P. Schwarcz, Experimentally determined sulfur isotope fractionations between sulfide minerals, Earth Planet. Sci. Letters 7 (1969) 162. [5] H. Puchelt and G. Kullerud, Sulfur isotope fractionation in the Pb-S system, Earth Planet. Sci. Letters 7 (1970) 301. [6] G. Kullerud and H.S. Yoder, Jr., Pyrite stability relations in the Fe-S system, Econ. Geol. 54 (1959) 533. [7] G. Kullerud, The lead-sulfur system, Am. J. Sci., Schairer Vol. 267A (1969) 233. [81 G. Kullerud and R.A. Yund, The Ni-S system and related minerals, J. Petrol. 3 (1962) 126. [9] H. Puchelt, B. Sabels and T.C. Hoering, Preparation of Sulfur-hexa-fluoride for isotope geochemical analysis, Geochim. Cosmochim. Acta (1971) in press. [ 10] W. Salomons, unpublished results. [ 11 ] H. Puchelt, Sulfur isotope fractionation in minerals, Carnegie Inst. Wash. Yearbook 1967-1968 (1969) 192.