University of Nebraska - Lincoln
DigitalCommons@University of Nebraska - Lincoln
USGS Staff -- Published Research US Geological Survey
2006
Sulfur Isotope Geochemistry of Sulfide Minerals
Robert R. Seal II
U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA,
Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub
Part of the Earth Sciences Commons
Seal, Robert R. II, "Sulfur Isotope Geochemistry of Sulfide Minerals" (2006). USGS Staff -- Published
Research. 345.
https://digitalcommons.unl.edu/usgsstaffpub/345
This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of
Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized
administrator of DigitalCommons@University of Nebraska - Lincoln.
,Reviews in Mineralogy & Geochemistry
Vol. 61, pp. 633-677, 2006 12
Copyright © Mineralogical Society of America
Sulfur Isotope Geochemistry of Sulfide Minerals
Robert R. Seal, II
U.S. Geological Survey
954 National Center
Reston, Virginia, 20192, U.S.A.
e-mail:
INTRODUCTION
Sulfur, the 10 most abundant element in the universe and the 14th most abundant element
th
in the Earth’s crust, is the defining element of sulfide minerals and provides insights into the
origins of these minerals through its stable isotopes. The insights come from variations in the
isotopic composition of sulfide minerals and related compounds such as sulfate minerals or
aqueous sulfur species, caused by preferential partitioning of isotopes among sulfur-bearing
phases, known as fractionation. These variations arise from differences in temperature, or
more importantly, oxidation and reduction reactions acting upon the sulfur. The oxidation and
reduction reactions can occur at high temperature, such as in igneous systems, at intermediate
temperatures, such as in hydrothermal systems, and at low temperature during sedimentary
diagenesis. At high temperatures, the reactions tend to occur under equilibrium conditions,
whereas at low temperatures, disequilibrium is prevalent. In addition, upper atmospheric
processes also lead to isotopic fractionations that locally appear in the geologic record.
Sulfur isotope geochemistry as a subdiscipline of the geological sciences began in the late
1940s and early 1950s with early publications by Thode et al. (1949) and Szabo et al. (1950)
on natural variations of sulfur isotopes, and Macnamara and Thode (1950) on the isotopic
composition of terrestrial and meteoritic sulfur. Sakai (1957) presented an early scientific
summary of sulfur isotope geochemistry, with a particular emphasis on high-temperature
processes. Thode et al. (1961) also presented an early summary, but with an emphasis on low-
temperature processes. Both of these summaries outlined salient aspects of the global sulfur
cycle. Sulfur isotope geochemistry understandably has had a long history of application to
the study of sulfide-bearing mineral deposits. Early noteworthy papers include those by Kulp
et al. (1956) and Jensen (1957, 1959). Similarly, there is also a legacy of contributions to
understanding sedimentary diagenesis and the origin of diagenetic pyrite. The paper by Thode
et al. (1951) represents one of the earliest efforts investigating sulfur isotope fractionations
associated with bacterial sulfate reduction. Subsequent advances in the field of sulfur isotope
geochemistry have been motivated by applications to an increasing variety of geochemical
systems and by technological advances in analytical techniques. Noteworthy reviews related
to the sulfur isotope geochemistry of sulfide minerals include those of Jensen (1967), Ohmoto
and Rye (1979), and Ohmoto and Goldhaber (1997), all of which emphasize mineral deposits,
Seal et al. (2000a) which emphasized sulfate minerals and their interactions with sulfides, and
Canfield (2001) which emphasized biogeochemical aspects of sulfur isotopes.
A considerable body of knowledge exists on the metal stable isotopic composition of
sulfide minerals—a topic that will not be covered in this paper. Recent analytical advances in
plasma-source mass spectrometry have enabled precise isotopic measurements of numerous
other metals in sulfide minerals including Fe (Johnson et al. 2003; Beard and Johnson 2004),
Cu (Maréchal et al. 1999; Zhu et al. 2000; Larson et al. 2003; Albarède 2004), Zn (Maréchal et
al. 1999; Albarède 2004) and Mo (Barling et al. 2001; Anbar 2004), among others.
1529-6466/06/0061-0012$05.00 DOI: 10.2138/rmg.2006.61.12
,634 Seal
The intent of this chapter is to build upon previous reviews of sulfur isotope geochemistry
as they relate to sulfide minerals, summarize landmark studies in the field, resolve, or
at least discuss, existing controversies and summarize recent advances for a variety of
geochemical settings. The first part of this chapter is designed to provide the reader with a
basic understanding of the principles that form the foundations of stable isotope geochemistry.
Next, an overview of analytical methods used to determine the isotope composition of sulfide
minerals is presented. This overview is followed by a discussion of geochemical processes that
determine the isotope characteristics of sulfide minerals and related compounds. The chapter
then concludes with an examination of the stable isotope geochemistry of sulfide minerals in
a variety of geochemical environments.
FUNDAMENTAL ASPECTS OF SULFUR ISOTOPE GEOCHEMISTRY
An isotope of an element is defined by the total number of protons (Z) and neutrons (N)
present, which sum together to give the atomic mass (A). For example, the element sulfur is
defined by the presence of 16 protons, but can have either 16, 17, 18, 19, or 20 neutrons, giving
atomic masses of 32, 33, 34, 35, and 36 amu, respectively. These isotopes are written as 32S,
33
S, 34S, 35S, and 36S. Four of the five naturally occurring sulfur isotopes are stable (32S, 33S,
34
S, and 36S) and one (35S) is unstable, or radiogenic. The isotope 35S is formed from cosmic
ray spallation of 40Ar in the atmosphere (Peters 1959). It undergoes beta decay with a half-life
of 87 days; therefore, it is not important from the perspective of naturally occurring sulfide
minerals. The four stable isotopes of sulfur, 32S, 33S, 34S, and 36S, have approximate terrestrial
abundances of 95.02, 0.75, 4.21, and 0.02%, respectively (Macnamara and Thode 1950).
Stable isotope geochemistry is concerned primarily with the relative partitioning of stable
isotopes among substances (i.e., changes in the ratios of isotopes), rather than their absolute
abundances. The difference in the partitioning behavior of various isotopes, otherwise known
as fractionation, is due to equilibrium and kinetic effects. In general, heavier isotopes form
more stable bonds; molecules of different masses react at different rates (O’Neil 1986).
Isotope ratios are usually expressed as the ratio of a minor isotope of an element to a major
isotope of the element. For sulfide minerals, the principal ratio of concern is 34S/32S. However,
renewed interest in 33S/32S and 36S/32S ratios has been generated by the discovery of unexpected
variations of these minor isotopes in Precambrian sulfide and sulfate minerals and in Martian
meteorites (Farquhar et al. 2000a,b; Farquhar and Wing 2003). Most fractionation processes
will typically cause variations in these ratios in the fifth or sixth decimal places. Because we are
concerned with variations in isotopic ratios that are relatively small, the isotopic composition
of substances is expressed in delta (δ) notation, as parts per thousand variation relative to a
reference material. The δ-notation for the 34S/32S composition of a substance is defined as:
⎛ ( 34
S/ 32 S) − (
34
S/ 32 S ) ⎞
δ34S = ⎜ ⎟ ×1000
sample reference
(1)
⎜
⎜
⎝
( 34
S/ 32 S)
reference
⎟
⎟
⎠
which has units of parts per thousand or permil (‰), also found in the literature spelled “per
mil,” “per mill,” and “per mille.” The values for δ33S and δ36S are similarly defined for the ratio
of 33S/32S and 36S/32S, respectively. The agreed upon reference for sulfur isotopes is Vienna
Canyon Diablo Troilite (VCDT) with δ34S = 0.0‰ by definition, which is currently defined
relative to a silver sulfide reference material IAEA-S-1 with an assigned value of −0.3‰
because the supply of the Canyon Diablo Troilite reference material has been exhausted
(Krouse and Coplen 1997). The reference was originally defined by the isotopic composition
of troilite (FeS) from the Canyon Diablo iron meteorite. The absolute 34S/32S ratio for Canyon
, Sulfur Isotope Geochemistry of Sulfide Minerals 635
Diablo Troilite is 4.50045 × 10−3 (Ault and Jensen 1963). The selection of a meteoritic sulfide
mineral as the reference for sulfur is useful because meteoritic sulfide is thought to represent
the primordial sulfur isotopic composition of Earth (Nielsen et al. 1991). Thus, any variations
in the isotopic composition of terrestrial sulfur relative to VCDT reflects differentiation since
the formation of Earth.
For sulfur, which has more than two stable isotopes, 34S/32S is the ratio most commonly
measured in studies of terrestrial systems. This ratio was chosen for two main reasons.
Firstly, it represents the most abundant isotopes of these elements, which facilitates analysis.
Secondly, isotopic fractionation is governed by mass balance such that different isotopic ratios
tend to vary systematically with one another in proportions that can be approximated by the
mass differences among the isotopes. In other words, the variations in the 33S/32S ratio of a
sample will be approximately half that of the 34S/32S ratio because of the relative differences
in masses. Likewise, the variations in the 36S/32S ratio of a sample will be approximately twice
that of the 34S/32S ratio. This linear fractionation trend due to physical and chemical processes
is known “mass-dependent fractionation” (Urey 1947; Hulston and Thode 1965a,b), which is
in distinct contrast to “mass-independent fractionation.” Mass-independent fractionation is
reflected by non-linear variations in isotopic fractionation with mass, and will be discussed in
more detail below.
Fractionation can be considered in terms of isotopic exchange reactions, which are driven
thermodynamically toward equilibrium. Thus, isotopic equilibrium, for example between
sphalerite (Sl) and galena (Gn), can be described by an isotopic exchange reaction such as:
Pb34S + Zn32S = Pb32S + Zn34S (2)
which is written in a form with one exchangeable atom of sulfur. The equilibrium constant (K)
for this reaction is equivalent to the isotopic fractionation factor (α):
Pb 32 S ⋅ Zn 34 S
K = 34 =
( 34
S/ 32 S )
Sl
= αSl-Gn (3)
Pb S ⋅ Zn 32 S ( 34
S/ 32
S)
Gn
where the isotopic species are meant to represent their respective chemical activities. Thus, in
a more general form, the partitioning of stable isotopes between two substances, A and B, is
quantitatively described by a fractionation factor, which is defined as:
RA
α A-B = ( 4)
RB
where R is 34S/32S. This equation can be recast in terms of δ values using Equation (1) as:
δA
1+
1000 1000 + δ A
α A-B = = (5)
δB 1000 + δ B
1+
1000
Values of α are typically near unity, with variations normally in the third decimal place
(1.00X). For example, the equilibrium 34S/32S fractionation between sphalerite and galena at
300 °C has been measured to have an αSl-Gn value of 1.0022. Thus, sphalerite is enriched in 34S
relative to galena by 2.2‰ (i.e., the fractionation equals 2.2‰). For an α value less than unity,
such as αGn-Sl, which equals 0.9978, the galena is depleted in 34S relative to sphalerite by 2.2‰
(i.e., the fractionation equals −2.2‰). In the literature, fractionation factors may be expressed
in a variety of ways including α, 1000lnα, and ∆, among others. The value ∆A-B is defined as:
∆A-B = δA − δB (6)
DigitalCommons@University of Nebraska - Lincoln
USGS Staff -- Published Research US Geological Survey
2006
Sulfur Isotope Geochemistry of Sulfide Minerals
Robert R. Seal II
U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA,
Follow this and additional works at: https://digitalcommons.unl.edu/usgsstaffpub
Part of the Earth Sciences Commons
Seal, Robert R. II, "Sulfur Isotope Geochemistry of Sulfide Minerals" (2006). USGS Staff -- Published
Research. 345.
https://digitalcommons.unl.edu/usgsstaffpub/345
This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of
Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized
administrator of DigitalCommons@University of Nebraska - Lincoln.
,Reviews in Mineralogy & Geochemistry
Vol. 61, pp. 633-677, 2006 12
Copyright © Mineralogical Society of America
Sulfur Isotope Geochemistry of Sulfide Minerals
Robert R. Seal, II
U.S. Geological Survey
954 National Center
Reston, Virginia, 20192, U.S.A.
e-mail:
INTRODUCTION
Sulfur, the 10 most abundant element in the universe and the 14th most abundant element
th
in the Earth’s crust, is the defining element of sulfide minerals and provides insights into the
origins of these minerals through its stable isotopes. The insights come from variations in the
isotopic composition of sulfide minerals and related compounds such as sulfate minerals or
aqueous sulfur species, caused by preferential partitioning of isotopes among sulfur-bearing
phases, known as fractionation. These variations arise from differences in temperature, or
more importantly, oxidation and reduction reactions acting upon the sulfur. The oxidation and
reduction reactions can occur at high temperature, such as in igneous systems, at intermediate
temperatures, such as in hydrothermal systems, and at low temperature during sedimentary
diagenesis. At high temperatures, the reactions tend to occur under equilibrium conditions,
whereas at low temperatures, disequilibrium is prevalent. In addition, upper atmospheric
processes also lead to isotopic fractionations that locally appear in the geologic record.
Sulfur isotope geochemistry as a subdiscipline of the geological sciences began in the late
1940s and early 1950s with early publications by Thode et al. (1949) and Szabo et al. (1950)
on natural variations of sulfur isotopes, and Macnamara and Thode (1950) on the isotopic
composition of terrestrial and meteoritic sulfur. Sakai (1957) presented an early scientific
summary of sulfur isotope geochemistry, with a particular emphasis on high-temperature
processes. Thode et al. (1961) also presented an early summary, but with an emphasis on low-
temperature processes. Both of these summaries outlined salient aspects of the global sulfur
cycle. Sulfur isotope geochemistry understandably has had a long history of application to
the study of sulfide-bearing mineral deposits. Early noteworthy papers include those by Kulp
et al. (1956) and Jensen (1957, 1959). Similarly, there is also a legacy of contributions to
understanding sedimentary diagenesis and the origin of diagenetic pyrite. The paper by Thode
et al. (1951) represents one of the earliest efforts investigating sulfur isotope fractionations
associated with bacterial sulfate reduction. Subsequent advances in the field of sulfur isotope
geochemistry have been motivated by applications to an increasing variety of geochemical
systems and by technological advances in analytical techniques. Noteworthy reviews related
to the sulfur isotope geochemistry of sulfide minerals include those of Jensen (1967), Ohmoto
and Rye (1979), and Ohmoto and Goldhaber (1997), all of which emphasize mineral deposits,
Seal et al. (2000a) which emphasized sulfate minerals and their interactions with sulfides, and
Canfield (2001) which emphasized biogeochemical aspects of sulfur isotopes.
A considerable body of knowledge exists on the metal stable isotopic composition of
sulfide minerals—a topic that will not be covered in this paper. Recent analytical advances in
plasma-source mass spectrometry have enabled precise isotopic measurements of numerous
other metals in sulfide minerals including Fe (Johnson et al. 2003; Beard and Johnson 2004),
Cu (Maréchal et al. 1999; Zhu et al. 2000; Larson et al. 2003; Albarède 2004), Zn (Maréchal et
al. 1999; Albarède 2004) and Mo (Barling et al. 2001; Anbar 2004), among others.
1529-6466/06/0061-0012$05.00 DOI: 10.2138/rmg.2006.61.12
,634 Seal
The intent of this chapter is to build upon previous reviews of sulfur isotope geochemistry
as they relate to sulfide minerals, summarize landmark studies in the field, resolve, or
at least discuss, existing controversies and summarize recent advances for a variety of
geochemical settings. The first part of this chapter is designed to provide the reader with a
basic understanding of the principles that form the foundations of stable isotope geochemistry.
Next, an overview of analytical methods used to determine the isotope composition of sulfide
minerals is presented. This overview is followed by a discussion of geochemical processes that
determine the isotope characteristics of sulfide minerals and related compounds. The chapter
then concludes with an examination of the stable isotope geochemistry of sulfide minerals in
a variety of geochemical environments.
FUNDAMENTAL ASPECTS OF SULFUR ISOTOPE GEOCHEMISTRY
An isotope of an element is defined by the total number of protons (Z) and neutrons (N)
present, which sum together to give the atomic mass (A). For example, the element sulfur is
defined by the presence of 16 protons, but can have either 16, 17, 18, 19, or 20 neutrons, giving
atomic masses of 32, 33, 34, 35, and 36 amu, respectively. These isotopes are written as 32S,
33
S, 34S, 35S, and 36S. Four of the five naturally occurring sulfur isotopes are stable (32S, 33S,
34
S, and 36S) and one (35S) is unstable, or radiogenic. The isotope 35S is formed from cosmic
ray spallation of 40Ar in the atmosphere (Peters 1959). It undergoes beta decay with a half-life
of 87 days; therefore, it is not important from the perspective of naturally occurring sulfide
minerals. The four stable isotopes of sulfur, 32S, 33S, 34S, and 36S, have approximate terrestrial
abundances of 95.02, 0.75, 4.21, and 0.02%, respectively (Macnamara and Thode 1950).
Stable isotope geochemistry is concerned primarily with the relative partitioning of stable
isotopes among substances (i.e., changes in the ratios of isotopes), rather than their absolute
abundances. The difference in the partitioning behavior of various isotopes, otherwise known
as fractionation, is due to equilibrium and kinetic effects. In general, heavier isotopes form
more stable bonds; molecules of different masses react at different rates (O’Neil 1986).
Isotope ratios are usually expressed as the ratio of a minor isotope of an element to a major
isotope of the element. For sulfide minerals, the principal ratio of concern is 34S/32S. However,
renewed interest in 33S/32S and 36S/32S ratios has been generated by the discovery of unexpected
variations of these minor isotopes in Precambrian sulfide and sulfate minerals and in Martian
meteorites (Farquhar et al. 2000a,b; Farquhar and Wing 2003). Most fractionation processes
will typically cause variations in these ratios in the fifth or sixth decimal places. Because we are
concerned with variations in isotopic ratios that are relatively small, the isotopic composition
of substances is expressed in delta (δ) notation, as parts per thousand variation relative to a
reference material. The δ-notation for the 34S/32S composition of a substance is defined as:
⎛ ( 34
S/ 32 S) − (
34
S/ 32 S ) ⎞
δ34S = ⎜ ⎟ ×1000
sample reference
(1)
⎜
⎜
⎝
( 34
S/ 32 S)
reference
⎟
⎟
⎠
which has units of parts per thousand or permil (‰), also found in the literature spelled “per
mil,” “per mill,” and “per mille.” The values for δ33S and δ36S are similarly defined for the ratio
of 33S/32S and 36S/32S, respectively. The agreed upon reference for sulfur isotopes is Vienna
Canyon Diablo Troilite (VCDT) with δ34S = 0.0‰ by definition, which is currently defined
relative to a silver sulfide reference material IAEA-S-1 with an assigned value of −0.3‰
because the supply of the Canyon Diablo Troilite reference material has been exhausted
(Krouse and Coplen 1997). The reference was originally defined by the isotopic composition
of troilite (FeS) from the Canyon Diablo iron meteorite. The absolute 34S/32S ratio for Canyon
, Sulfur Isotope Geochemistry of Sulfide Minerals 635
Diablo Troilite is 4.50045 × 10−3 (Ault and Jensen 1963). The selection of a meteoritic sulfide
mineral as the reference for sulfur is useful because meteoritic sulfide is thought to represent
the primordial sulfur isotopic composition of Earth (Nielsen et al. 1991). Thus, any variations
in the isotopic composition of terrestrial sulfur relative to VCDT reflects differentiation since
the formation of Earth.
For sulfur, which has more than two stable isotopes, 34S/32S is the ratio most commonly
measured in studies of terrestrial systems. This ratio was chosen for two main reasons.
Firstly, it represents the most abundant isotopes of these elements, which facilitates analysis.
Secondly, isotopic fractionation is governed by mass balance such that different isotopic ratios
tend to vary systematically with one another in proportions that can be approximated by the
mass differences among the isotopes. In other words, the variations in the 33S/32S ratio of a
sample will be approximately half that of the 34S/32S ratio because of the relative differences
in masses. Likewise, the variations in the 36S/32S ratio of a sample will be approximately twice
that of the 34S/32S ratio. This linear fractionation trend due to physical and chemical processes
is known “mass-dependent fractionation” (Urey 1947; Hulston and Thode 1965a,b), which is
in distinct contrast to “mass-independent fractionation.” Mass-independent fractionation is
reflected by non-linear variations in isotopic fractionation with mass, and will be discussed in
more detail below.
Fractionation can be considered in terms of isotopic exchange reactions, which are driven
thermodynamically toward equilibrium. Thus, isotopic equilibrium, for example between
sphalerite (Sl) and galena (Gn), can be described by an isotopic exchange reaction such as:
Pb34S + Zn32S = Pb32S + Zn34S (2)
which is written in a form with one exchangeable atom of sulfur. The equilibrium constant (K)
for this reaction is equivalent to the isotopic fractionation factor (α):
Pb 32 S ⋅ Zn 34 S
K = 34 =
( 34
S/ 32 S )
Sl
= αSl-Gn (3)
Pb S ⋅ Zn 32 S ( 34
S/ 32
S)
Gn
where the isotopic species are meant to represent their respective chemical activities. Thus, in
a more general form, the partitioning of stable isotopes between two substances, A and B, is
quantitatively described by a fractionation factor, which is defined as:
RA
α A-B = ( 4)
RB
where R is 34S/32S. This equation can be recast in terms of δ values using Equation (1) as:
δA
1+
1000 1000 + δ A
α A-B = = (5)
δB 1000 + δ B
1+
1000
Values of α are typically near unity, with variations normally in the third decimal place
(1.00X). For example, the equilibrium 34S/32S fractionation between sphalerite and galena at
300 °C has been measured to have an αSl-Gn value of 1.0022. Thus, sphalerite is enriched in 34S
relative to galena by 2.2‰ (i.e., the fractionation equals 2.2‰). For an α value less than unity,
such as αGn-Sl, which equals 0.9978, the galena is depleted in 34S relative to sphalerite by 2.2‰
(i.e., the fractionation equals −2.2‰). In the literature, fractionation factors may be expressed
in a variety of ways including α, 1000lnα, and ∆, among others. The value ∆A-B is defined as:
∆A-B = δA − δB (6)
