Was microbial life involved in the
reduction of sulfate in early
Archean time
(>3.3 Ga)?
Was microbial life involved in the
reduction of sulfate in early Archean
time (>3.3 Ga)?
Annemiek Asschert Student Number: 0123889 March 2007
Adress: Willem Barentszstraat 30
3572 PK Utrecht The Netherlands a.asschert@gmail.com
Institution: Utrecht University
Faculty of Earth sciences
Study: Earth sciences
Supervisors: Dr. P.R.D. Mason
“The important thing is not to stop questioning.
Curiosity has its own reason for existing”
Was early life involved in sulfate reduction in Archean time? Table of contents
Table of contents
Table of contents ... 4
1. Abstract ... 5
2. Introduction ... 6
3. Geology ... 9
3.1 General Geology
... 9
3.2 Geology Barberton Mountain land
... 9
3.2.1 The Onverwacht group ... 10
3.2.2 The Fig Tree group ... 13
3.2.3 The Moodies group ... 13
3.3 Geology of the North Pole Dome, Pilbara block, Western Australia ... 14
4. Sulfur ... 17
4.1 Sulfur isotopes
... 17
4.2 Mass dependent and mass independent sulfur isotope fractionation
... 17
4.3 Sulfate reduction and fractionation processes
... 18
4.3.1 Biogenic sulfate reduction and isotope fractionation ... 18
4.3.2 Sulfur isotope fractionation in hydrothermal systems, hydrolysis of SO
2–rich magmatic
fluids. ... 20
4.3.3 Hydrothermal sulfate reduction by ferrous iron ... 21
4.3.4 Nonbacterial (thermochemical) organic sulfate reduction ... 21
5. Methods ... 22
5.1 Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry
... 22
5.2 Chromium reduction
... 26
5.3 Microdrilling
... 27
5.4 Major and trace element determination
... 27
6. Results and discussion ... 28
6.1 Petrographic study and chemical analysis
... 28
6.1.1 Samples from the Onverwacht group, Barberton Greenstone Belt ... 28
6.1.2 Samples from the North Pole Dome, Pilbara Block ... 33
6.2 Major and trace element chemistry
... 35
6.2.1 Results ... 35
6.2.2 Interpretation ... 36
6.3 Isotopes
... 38
6.3.1 LA-MC-ICP-MS results ... 38
6.3.2 Sulfides: isotopic measurements ... 42
6.3.3 Origin of the barites: isotopic data and petrographic studies ... 44
6.3.4 Origin of barite: mass dependent versus mass Independent sulfur isotope ... 47
fractionation ... 47
6.3.5 Sulfide and sulfate isotopic data combined; Barberton models ... 49
6.4 Three Models for the barite-sulfide rocks found in the Theespruit formation in the Barberton,
South Africa.
... 51
6.4.1 Model I- Hydrothermal barite/ microbial pyrite ... 51
6.4.2 Model II- Hydrothermal barite/ later stage of hydrothermal pyrite ... 53
6.4.3 Model III- Simultaneous hydrothermal formation of barite and pyrite ... 54
6.4.4 Discussion of the Models ... 54
1. Abstract
Sulfate-rich rocks in early Archean greenstone belts can potentially provide key constraints on the evolution and activity of microorganisms that are involved in the sulfur cycle. The origin of these rocks has been variably attributed in previous studies to hydrothermal and sedimentary processes and remains controversial. In this study I concentrate on the geology and geochemistry of barite deposits from the lower Onverwacht group of the Barberton greenstone belt, South Africa with a view to constraining the role of microbial sulfate reduction to produce associated sulfide minerals, mainly pyrite. Two different models can explain the environment of deposition of the barites from this area. The δ34S sulfur isotopic signature of the barite, is relatively homogeneous, varying from +4.2‰ to 6.1‰, suggesting a seawater origin for the sulfate. Furthermore petrographic analysis suggests that the barite is sedimentary. A possible mode of formation would involve SO42- from seawater and Ba2+ from hydrothermal brines. In order to produce the sedimentary structures, the barite must have been weathered and re-deposited. The pyrites, some of which are also detrital, would have formed in the rocks at this stage, but the isotopic and trace element data show contradicting evidence for a biogenic (model I) or a hydrothermal magmatic origin (model II).
δ34S of the pyrites shows a depleted signature varying from –4.4‰ to –12.9‰, with the corresponding sulfate-sulfide fractionation (assuming equilibrium) ranging from 9.5‰ to 17.6‰. This sulfate-sulfide fractionation is as large as found anywhere else in the early Archean with the exception of a few samples from the North Pole dome in the Pilbara block.
δ33S data show a significant offset in mass independent fractionation for both barite and pyrite, which indicates an atmospheric origin for the sulfur, although the interpretation is limited by poor
measurement precision.
Since hydrothermal sulfate-sulfide δ34S fractionation normally shows a highly variable and much heavier range which cannot be found in the Barberton barites, a biogenic origin is favored for the pyrites.
The trace element chemistry of the pyrite, which shows a Co/ Ni ratio of < 1 for most of the pyrite minerals indicates a sedimentary origin of the pyrites, but does not exclude reduction of sulfate by Fe2+ bearing minerals.
Was early life involved in sulfate reduction in Archean time? Introduction
2. Introduction
The planet Earth is over 4.5 billion years old and the oldest rocks in the geological record are approximately 4 billion years old. However, the first well-preserved and large scale sequences of crustal rocks do not occur until 3.5-3.0 Ga. They are best represented in two greenstone belts: (1) the Pilbara block in Western Australia and (2) the Barberton Mountain land in South Africa at the border with Swaziland. These Archean blocks include both volcanic and sedimentary rocks and have undergone a relatively low degree of metamorphism and deformation when compared with rocks of similar or slightly older age. Deposits of sulfates, consisting predominantly of primary barite minerals, are present in both the Pilbara and Barberton regions, the first time that they appear in the geological record. In common with their host rocks they are relatively fresh and are therefore a useful window into the conditions that may have promoted the stability and extent of sulfate in Archean time. The early Archean oceans and atmosphere were most likely reduced in the period 3.5-3.0 Ga (Farquhar, 2000; Pavlov and Kasting, 2002), although some studies also suggest that oxidizing conditions reached back as far as 3.8 Ga (Ohmoto et al., 2006,). Thus the presence of an oxidized species of S (i.e. sulfate) in rocks dating from this period may provide important evidence to resolve the redox state at the surface of the Earth. Furthermore the study of Archean sulfate deposits is of key importance for studying the emergence of life on Earth. Evidence for life is very limited before approximately 2.7 Ga. The reduction of sulfate to sulfide by microorganisms has been suggested as one of the most ancient metabolic processes that could have functioned at this time. Microbial sulfate reduction is most readily identified in sedimentary rocks that have not been extensively modified by post-depositional hydrothermal alteration. This is because inorganic mechanisms can also reduce sulfate to sulfide in high
temperature magmatic systems. Thus better constraining the origin of sedimentary barite deposits from Pilbara and Barberton is of key importance for Archean geology, palaeoenvironmental and microbiological studies.
The measurement of stable isotope ratios in Archean rocks and minerals provides a useful tool for tracing early metabolic processes. Stable sulfur isotopes can be fractionated due to various
processes; both biological and non-biological, leaving a record in the ratio 34S/32S that is preserved in the sulfides and sulfates in Archean sediments. Measurements of stable isotopes of sulfur can thus potentially tell us if the rock has a biogenic signature or not. The first evidence for extensive and widespread sulfate reduction in the rock record has been found between 2.7 and 2.5 Ga. Before this time only one locality has been proposed where sulfate reduction could have occurred. This is found at the North Pole dome within the Pilbara block and at numerous scattered localities within the
Onverwacht and Fig Tree groups of the Barberton Mountain land in South Africa.
Many studies have been conducted to show how the activity of microorganisms can influence sulfur isotope ratios during the metabolism of various sulfur species, but most have focused on sulfate reduction (Kaplan and Rittenberg, 1964, Thode et al., 1951). Overall microbial sulfate reduction is an energy yielding metabolic process, where the sulfate is reduced to sulfide. The oxidation part of the redox couple requires organic matter or hydrogen (H2). The stable isotopes 32S and 34S are
discriminated during S-O bond breaking as sulfite is reduced to sulfide within the cell. The daughter sulfides are thus isotopically fractionated with respect to the parent sulfate and are depleted in 34S which remains in the sulfate, ultimately to be mineralized into gypsum or barite (Coplen et al, 2002; Canfield 2001).
In this study I will focus on the barite-rich rocks that occur in both the Barberton and Pilbara blocks. Three main types of barite occur in the North Pole area including (1) barites in veins, (2) barite mounds and (3) stratified barites (Nijman et al, 1999). The stratified barites are considered to be of evaporitic origin. Petrographic evidence suggests replacement of the evaporitic gypsum in the North Pole area, and also for some of the deposits in the Barberton, belonging to the Fig tree group (Lambert et al, 1978).
The barite deposits of the Barberton mountain area belong to the Onverwacht group and the Fig tree group. The Fig tree barites are likely to be of evaporitic origin (Lambert et al., Reimer 1980), but an evaporitic origin of the barites of the Onverwacht group is more disputable. However, most of the barite deposits indicate a sedimentary origin or secondary reworking in a sedimentary environment (Reimer, 1980). The difficulty in interpreting sedimentary rocks of Archean age results in the fact that the rocks have had a long geological history. Both hydrothermal and diagenetic pyrite could have been introduced at different stages following deposition. Most Archean rocks are commonly affected by later hydrothermal processes (Sugitani, 1992). In order to distinguish between the pyrites that are formed by different processes, I will attempt to unravel the different characteristics of diagenetically and hydrothermally formed pyrite.
Since both sulfide and sulfate minerals coexist in the rocks selected for this study it is likely that sulfate reduction occurred in Archean times. I will describe in Chapter 4 how sulfate reduction mechanisms function and how isotopic signatures can be used to distinguish between these different sulfate reduction mechanisms.
Was early life involved in sulfate reduction in Archean time? Introduction
Petrographic studies are crucial to determine the depositional environment of the sulfates (barite) and the sulfides (mainly pyrite). The chemical composition of the sulfates and sulfides and sulfur isotopes is also of key importance. These are described in chapter 6.
The electron microprobe and 193nm laser were used in order to determine the minor elements in the barite and sulfides.
3. Geology
3.1 General Geology
Archean sedimentary sulfate deposits are rare in the geological record but have been found in several areas; including Australia, India and various parts of southern South Africa. The oldest examples are the Archean (> 3 Ga) barite deposits of the North Pole area in the Pilbara block and the Barberton mountain land in the eastern part of South African province Transvaal, at the border with Swaziland. These have both undergone relatively low degrees of metamorphism and deformation and are among the best preserved greenstone belts in the world. Therefore, these rocks are the best place to sample when studying the role of microbial life in the reduction of sulfate to sulfide on the early Earth.
3.2 Geology Barberton Mountain land
The Barberton mountain land consists of a 160 km long x 50 km wide greenstone belt, which is located along the southeast margin of the 1750 x 1650 km sized Kaapvaal craton (de Vries, 2004). It contains a well-preserved succession of low-grade metamorphosed and deformed Archean Volcano-sedimentary lithologies (Kohler et al., 2002). The rock samples of the Barberton investigated in this research belong to the Precambrian Swaziland Supergroup, which can be subdivided into three groups, from old to young, the Onverwacht, Fig Tree and Moodies groups. The age of the different sequences was established by U-Pb zircon dating, in relatively undisturbed sequences of the Swaziland Supergroup. The Lower Onverwacht group has been dated at ~3.48-3.45 Ga, the upper Onverwacht group at ~3.42-3.3 Ga, and the Moodies group at > 3.22 Ga (Kröner et al, 1996;
Armstrong et al., 1990). Figure 1 gives an overview of ages of the Swaziland super group. Armstrong argues that the sediments of the Theespruit formation could be younger than the overlying mafic and ultramafic volcanics of the Komati formation.
The Onverwacht group consists of ultramafic, mafic and felsic lavas and tuffs interlayered with chert bands up to several 100 m in thickness. Within the Onverwacht group three barite deposits have been found. The Onverwacht group is overlain conformably by carbonaceous cherts, shales, greywacke and jaspilitic banded iron formation (BIF) of the Fig Tree group (Hoffmann, 2005; Reimer, 1990). The Fig Tree group is overlain partly conformably, partly uncomformably by the Moodies group which consists of Conglomerates, sandstones, quartzites, shales, banded ironstones and subordinate lavas (Reimer, 1980). It is a 100-130 m-thick fining upwards succession with a transition from fluvial to tidal
Was early life involved in sulfate reduction in Archean time? Geology
Figure 1, Stratigraphic classification of the Swaziland Super group, Kröner et al., 1996 and Armstrong et al., 1996. Armstrong argues that a part of the Theespruit formation might be younger than the Komati formation.
3.2.1 The Onverwacht group
The Onverwacht group can be subdivided into 5 different formations. The lower Onverwacht group exists of the Sandspruit, the Theespruit and the Komati formations. The Sandspruit formation consists predominantly of serpentinized ultramafic rocks, partly of extrusive origin, with a smaller volume of meta-basalts. These rocks have been metamorphosed in the amphibolitic facies, because it was in contact with the margin of a schist belt where there were intrusive tonalitic gneisses (Reimer,
unpublished). The following formation is the Theespruit formation and this formation occurs along the whole margin of the mountain land and consists of serpentinized ultramafic rocks and pillowed meta-basalts together with felsic tuffs ands black cherts. Thus from the Sandspruit formation to the Theespruit formation the type of rock changes from dominantly mafic to felsic (Reimer, unpublished; Walsh et al, 1985). The Komati formation lies on top of the Theespruit formation and consists of an alternation of metatholeiites and extrusive komatiites of basaltic to peridotitic composition.
This study is based on a collection of samples provided by Thomas Reimer and originally collected during field studies in the 1970s (Reimer 1980). Some of the samples (TR01) were originally sampled from the lower Onverwacht group in the Barberton mountain land from the valley of the Londozi River, in the northwestern part of Swaziland close to the border of South Africa (Fig. 2). This barite deposit is situated in metamorphosed volcanic rock most probably belonging to the Theespruit formation of the lower Onverwacht group (Londozi deposit, see Fig 3a). In the barite zone itself numerous bands of fine-grained siliceous rocks occur, and can be colored bright green through inclusions of chlorite and/or fuchsite. The Barite is finely crystalline, containing a concentration of silica, along what appears to be the original bedding planes. Along these stringers, an enrichment of microcrystalline pyrite is frequently found or in some areas also some fine grained sphalerite. (Reimer, 1980)
Sandspruit
Komati
Lower
Onverwacht
group
Upper
Onverwacht
group
Theespruit
Hooggenoeg
Kromberg
Onverwacht
Group
The TR04 samples were obtained from the Stentor deposit which also belongs to the Theespruit formation and occurs in the northeastern part of the mountain land (number 2 in Figure 2). The barite bands occur together with chert bands in a sequence of quartz-sericite schists with interlayers of some chloritic portions and of sheared feldspar porphyries. The quartz-sericite schists were most probably originally felsic tuffs. The barite occurs as lenses in layers of gray-green chert and the barite bands lie parallel to the chert bands and the general schistosity developed in the area. A frequent occurrence of disseminated pyrite can be found which at some places is concentrated at the bedding places. Some of the weathered samples show indications of cross lamination, which might suggest a detrital origin of at least a part of the barite (Reimer, 1980; Reimer 1990).
According to Reimer (1980) the environment of the Londozi and the Stentor barite deposit can be seen as a low-energy shallow water environment within a mainly volcanic sequence. The barite was deposited and alternated with cherty sediments which formed through silicifation of either carbonates of more likely of felsic to intermediate fine-grained tuffs.
Was early life involved in sulfate reduction in Archean time? Geology
Figure 2, General geological map of the Barberton mountain area, the Barberton greenstone belt and the
surrounding granitoid terrane. (Reimer, 1980; Kohler et al., 2002)
The Stratigraphic position of the Vergelegen deposit (sample TR02, nr 3 in Figure 2) was originally thought to belong to the upper Hooggenoeg Formation (Reimer 1980) but it has also been suggested that the sequence of the Vergelegen belongs to the lower Onverwacht group, probably the Theespruit formation, like the other two deposits described before, the Londozi deposit and the Stentor deposit (Reimer, 1990). No sulfides were found in the Vergelegen deposit and the Vergelegen barite is more intensely folded than the Londozi and Stentor barite which makes the interpretation of these rocks more difficult (Reimer, 1980). Lowe and Knauth (1977) have described white cherts in the Buck Ridge about 30 km east of the Vergelegen deposit. The White cherts are similar to the ones alternating with the barite of the Vergelegen deposit and they are described as spindle shaped silicified crystalline grains that are gypsum hosts. It might thus be possible that the barite of the Vergelegen deposit originally formed as gypsum in a shallow water sequence. However the Buck Ridge volcanic sedimentary complx shows progressively deeper water conditions from east to west towards the Vergelegen deposit (De Vries et al., 2006). But Reimer (1980) argues that it is advocated that the barite is originated from hydrothermal brines because of the close association with volcanic rocks. The
# Farm Sample
1 Londozi TR01
2 Stentor TR04
barium in the barite than most likely originated from hydrothermal solutions that ascended through the underlying volcanic rocks with sulfate that was most likely derived from seawater.
3.2.2 The Fig Tree group
The Fig Tree group contains mainly pelitic sediments (greywacke/shale) with siliceous chemical precipitates (banded ferruginous chert) and minor lavas, agglomerates and tuffs. Two barite deposits are known in this group. Beds of barite alternating with green chert, shale, and pebble conglomerate occur in a 20 m thick zone in the barite Valley syncline at the centre of the greenstone belt.
It has been suggested that cauliflower structures in the barite have morphological similarity to
structures observed in younger gypsum that are thought to represent replacement of primary gypsum by barite (Lambert et al., 1978).
Figure 3, different barite deposits within the Fig Tree group in the Onverwacht group (from Reimer, 1980).
3.2.3 The Moodies group
Was early life involved in sulfate reduction in Archean time? Geology
3.3 Geology of the North Pole Dome, Pilbara block, Western Australia
The North Pole area is situated in Northwestern Australia and belongs to the >3.46 Ga Warrawoona Group (Thorpe et al., 1992), which is a 10 km thick predominantly basaltic unit and is very well preserved considering the age of the group (Shen, 2004). (See area figure 4 and figure 5)
The supracrustal rocks at this locality have been domed upwards over a largely unexposed granitoid body into an open, doubly plunging anticline, which is called the North Pole dome. The North Pole dome is part of the Warrawoona mega sequence, which is divided into different subgroups. The first subgroup (Talga Talga group) is composed of mainly gabbro, dolerite and tholeiitic basalts. These rocks are overlain by a succession of calc-alkaline intermediate to silicic volcanic sedimentary rocks of the Duffer formation. Further upwards in the sequence are thick (>25m) chert units, which are overlain by tholeiitic and magnesian basalt interlayered with thin cherty sedimentary horizons of the Salgash subgroup (Barley, 1993; Barley et al., 1979).
The structural position of these rocks is responsible for the gentle dips, the low metamorphic grade and minor deformation, generally only low-strain brittle deformation (Buick and Dunlop, 1991). Figure 5 shows a geological map of the North Pole dome. All samples examined in this study come from the Dresser mine area.
The Bedded cherts are interpreted to have formed in a shallow water evaporate lagoon behind a barrier of volcanoclastic sandstone, which was derived from hinterland volcanism (Barley, 1993; Buick and Dunlop, 1990; Nijman et al., 1999).
Three different types of barites can be recognized in the field at North Pole; barites that occur in veins, barite mounds and bedded, stratiform barite. The vein barites are commonly associated with black chert. The barites in the mounds occur as large synsedimentary mounds of 15 m high and 50 m wide and are formed on the original shallow sub aqueous basin floor; the stratiform barites are sedimentary
Rock samples
North Pole all come
from the Dresser
mine area.
Was early life involved in sulfate reduction in Archean time? Geology
started and shortly after burial by overlying basalts because of the perfect preservation of crystal morphologies from the highly soluble gypsum (Lambert et al, 1978; Buick and Dunlop, 1990).
4. Sulfur
4.1 Sulfur isotopes
Sulfur isotopic variability has been used in diverse studies as a tracer for igneous, metamorphic, sedimentary, hydrothermal and biological processes on Earth. Sulfur isotope ratios are generally reported in standard δ-notation, given in units of per-mil (‰). Deviations of the isotope ratio are reported relative to the V-CDT (Vienna Cañon Diablo Triolite), which is calibrated against the IAEA-S1 standard (Ag2S) (Ding et al., 2001).
Delta values are expressed using the following equations.
δ34SV-CDT = [((34S/32S)sample/(34S/32S)V-CDT) – 1] x 103
δ33S
V-CDT = [((33S/32S)sample/(33S/32S)V-CDT) – 1] x 103
Sulfur has four stable isotopes, 34S, 32S, 33S and 36S. The far most abundant is 32S (95,02%), followed by 34S (4,21%), 33S (0.75%) and 36S (0.015%), which is the least abundant isotope.
4.2 Mass dependent and mass independent sulfur isotope fractionation
Most studies have measured variations in δ34S which is based on the most abundant 32 and 34 isotopes. The low abundance of 33S and 36S makes measurements less straightforward to perform and can reduce accuracy and precision (Mojzsis et al., 2003).
But there is also another reason that only δ34S and δ32S were measured in early studies, since it was expected that no new information would be gathered by measuring δ33S isotopes. Most physical, chemical and biologic processes fractionate isotopes because of the relative mass difference, and variations of δ33S should be directly correlated with those in δ34S.
This relationship can be described by the following mass dependent fractionation equation:
δ33S = 0.515 x δ34S (Hulston and Thode, 1965).
Was early life involved in sulfate reduction in Archean time? Sulfur
The deviation of a single data point from the mass independent fractionation line can be described by the following equation (Farquhar et al., 2000; Farquhar et al., 2003):
Δ33S = δ33S – 1000 x (1 + δ34S/1000)0.515 -1
The Δ 33S anomalies can thus reflect atmospheric fractionation processes in an oxygen free atmosphere. When these signatures are found back in the barite rocks, this might reflect an
atmospheric origin of the sulfur in the sulfate and this can be an indication of a sedimentary origin of the rocks. Seawater sulfate can have an atmospheric origin and this sulfate forms minerals with a similar S isotopic signature. The composition of the Archean ocean is still not entirely known but in general it is thought that the Archean ocean contained less than 1mM SO42- with a δ34S of
approximately +2 ‰. Sulfate minerals in 3.4 to 3.3 Ga sedimentary rocks have average δ34S of approximately 3‰ which is consistent with isotopic fractionation of 1 to 2‰ between seawater SO4 2-and SO42- minerals at a temperature of ± 25oC (Ohmoto et al., 1993).
4.3 Sulfate reduction and fractionation processes
Stable isotopes can be fractionated due to either biological or non-biological processes and the magnitude of this fractionation preserved in sediments and rocks can be used to reveal the nature and extent of these processes (Canfield, 2001). Biological fractionation is dominated by the activity of microorganisms including Bacteria and Archaea. Microbial sulfate reduction can cause large variations in δ34S in sedimentary pyrites of up to 46‰ and probably more (Brunner & Berlusconi, 2005). But non-biological sulfate reduction as hydrothermal alteration can also be a mechanism that can produce fractionations between sulfate and sulfide. It is important that both processes are given equal
consideration when investigating isotope variations in early Archean rocks. To distinguish one process from another it is necessary to combine isotopic and geochemical data and geological observations.
4.3.1 Biogenic sulfate reduction and isotope fractionation
A major process that causes variation in the isotopic composition of S in nature is the reduction of sulfate ions by anaerobic bacteria, which live in sediment deposited in both marine and fresh water settings. Sulfate reducing microbes have a large influence on the modern sulfur cycle and may be very ancient, but when they evolved is still uncertain (Canfield et al., 1999).
There are two main types of microbial sulfate reduction that can be distinguished; assimilatory sulfate reduction and dissimilatory sulfate reduction. With assimilatory sulfate reduction the sulfate is
consumed by the cell. It must be reduced to sulfide for incorporation into the principal organic compounds within a cell, and this is an energy-requiring process. The overall isotope fractionation associated with assimilatory sulfate reduction is small. Plants generally display only small isotope fractionations in organic sulfur relative to external sulfur, + 0.5‰ to –4.4‰, reviewed in Shen and Buick, 2004; Canfield 2001.
Isotopic fractionation varies over a much wider range for dissimilatory sulfate reduction than for assimilatory sulfate reduction. Dissimilatory sulfate reduction is an energy yielding reaction that is carried out by several specialized groups of prokaryotes that use it to obtain energy for growth by catalyzing exergonic chemical reactions in which organic carbon or H2 is oxidized and sulfate is reduced to sulfide.
SO42- + 2 CH2O —> H2S + 2 HCO3
-2 H+ + SO42- + 4 H2 —> H2S + H2O
At sulfate concentrations larger than 200μM, sulfides are typically isotopically fractionated by 5-46‰ compared to sulfate, but at lower concentrations the fractionation can decrease to 0‰ (Canfield et al., 1999; Kaplan et al., 1964; Habicht et al, 2005; Stam et al., 2006).
Sulfate reducing organisms are widely distributed over anoxic environments that contain sulfate, and are largely ecological tolerant. They can be found as different species at temperatures of –1.5oC to over 100oC and are found in several branches within the Bacterial domain of the phylogenetic tree. The deepest branching (oldest) sulfate reducer in the bacterial domain is the Thermodesulfobacterium and has optimum growth temperature around 80OC. In the domain of the Archaea there are also several members of the hyperthermophilic genus Archaeoglobus which includes species with a maximum growth temperature of 85oC (Canfield, 2001). Figure 7 shows a phylogenetic tree
Was early life involved in sulfate reduction in Archean time? Sulfur
Figure 7. Phylogenetic tree. (From Shen and Buick, 2004) showing groups of sulfate reducing microorganisms in bold.
4.3.2 Sulfur isotope fractionation in hydrothermal systems, hydrolysis of SO
2–rich magmatic
fluids.
Abiotic processes in hydrothermal fluids can also lead to S isotopic fractionation. In the Archean the dominant igneous activity was basaltic volcanism. At temperatures > 400oC, the dominant species in the hydrothermal fluids derived from felsic magmas is H2S.
But cooling of a SO2 rich fluid generates both H2S and SO42- this can be described by the following reaction:
4H2O + 4SO2 —> H2S + 3 H+ + 3 HSO4-
When temperature drops below 400oC the H2S isotopic signature will be 15 to 20 ‰ depleted relative to SO42- (Ohmoto and Goldhaber, 1997).
4.3.3 Hydrothermal sulfate reduction by ferrous iron
It is also possible that sulfate reduction occurs when hydrothermal fluids circulate through surrounding rocks that contain Fe2+- bearing minerals to generate H2S. Fractionations range from 20‰ at 300oC and 30‰ at 200oC. The isotopic relationship between H2S and sulfate in the system might be controlled by equilibrium fractionation as a function of temperature, rather than by kinetic isotopic effects and this will cause the ΔH2S-SO4 values to become less negative because the sulfates in this system become much heavier than the initial sulfates in the seawater (Ohmoto and Goldhaber, 1997).
4.3.4 Nonbacterial (thermochemical) organic sulfate reduction
This process is a non-biological sulfate reduction process of dissolved sulfate with solid, liquid or gaseous hydrocarbons at elevated temperatures in deeply buried sedimentary rocks. Thermochemical sulfate reduction by hydrocarbons can occur in a low temperature diagenetic environment where 0 < T < 60-80oC and high diagenetic environments with 80-100 < T < 150-200oC (Machel et al., 1995). For a temperature of 100oC the fractionation of sulfur isotopes by Thermochemical sulfate reduction can be -20‰, by 150oC -15‰ and by 200oC -10‰.
Was early life involved in sulfate reduction in Archean time? Methods
5. Methods
5.1 Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry
Several different methods are available to measure sulfur stable isotope ratios including gas source mass spectrometry and secondary ionization mass spectrometry. However, these techniques are not easily applied to in situ analysis on a scale of < 100μm. Thus in this study I have investigated a relatively new technique to perform these measurements by Laser Ablation Multi Collector Inductive Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). In situ measurements are essential to measure small, heterogeneous mineral samples in geological thin sections. In this study I have measured 32S, 33S and 34S in samples from the Barberton Greenstone belt and from the North Pole area in the Pilbara block.LA-MC-ICP-MS has been previously used for S-isotope measurements in several minerals and gives robust and accurate data. (Mason et al., 2006, Bendall et al., 2006; Rouxel et al., 2005)Depending on the material being investigated, the accuracy and precision varies and for certain minerals the
technique still has to be optimized and is still under development.
In this work I have followed the technique developed by Mason et al., (2006). This previously developed method was tested only on standards and my study represents the first time that the technique has been used for real samples from early Archean rocks. Therefore the method will be described and discussed extensively to give a good insight into the accuracy and precision of the data. I will pose the question: is this new method useful to do in situ S isotopic measurements with
sufficiently high precision and accuracy?
Many different problems can be experienced when using LA-MC-ICP-MS for measuring stable isotopes (e.g. Alberede et al., 2004) and robust and reliable corrections must be applied in order to make accurate measurements. The following are the major problems specific to S isotope analysis with laser ablation:
-Molecular interferences (also termed Spectral interference and isobaric interference), where more than one type of ion is measured at a specific mass.
-Mass bias (instrumental mass discrimination) where the measured isotopic ratio is not the true isotopic ratio but typically enriched in the heavier isotope.
These problems have to be solved or correction must be applied in order to get accurate data that can be used for further interpretation with respect to the research question.
Isotopic measurements in this study where performed on a ThermoFinnigan-Neptune multi Collector ICP-MS with a new wave laser ablation system at Bergen University. Set up parameters are listed in Table 1.
MC-ICPMS was the first instrument with the capability to achieve high mass resolution in the multi collector mode (Weyer and Schwieters, 2003). Interferences from O- and N- based polyatomic ions on S can be resolved with a mass resolution (m/Δm) of ≥ 4000. Calibration standards were available for both the sulfide measurements. The AgS standard IAEA-S3 was used for sulfide mineral calibration and NIST-127 (seawater sulfate) was used for sulfate minerals.
The measurements were performed by ablating 55μm wide and 300 to 400μm long trenches in the pyrite standards, 55μm wide and 200 to 300μm long trenches in the pyrite samples and some circular craters with a diameter of 80μm in the pyrite samples. Barite measurements were performed by ablating 80μm wide and 400μm long trenches in the standard and in the samples.
An Elemental Scientific Apex desolvating nebulizer was used to introduce a vapor of a 10ppm (three times diluted in HNO3) Si isotopic standard solution (30Si/29Si = 0.66018), which was calibrated against IRMM-018 standard reference material. The silica solution was aspirated continuously during the sessions of measurements. This solution was used for external normalization.
The original Si standard used to make the external normalization is probably not homogeneous as recently reported byReynolds et al., 2006.
MC-ICP-MS Thermo Finnigan Neptune
Cool gas flow 15.50 l/min-1
Carrier gas flow 0.54 l/min-1
Rf power 1300 W Extraction lens -2000 V Nebulizer ES Apex Collector configuration H4 34 H2 33 H1 32 L1 30 L4 29
Laser ablation system New Wave UP213 Nd:YAG
Wavelength 213 nm
Fluence Sulfides: 5-10 J/cm-2
Sulfates: 6.94- 7.36 J/cm-2
Laser crater diameter 55-80 μm
Pulse repetition rate 10 Hz
Was early life involved in sulfate reduction in Archean time? Methods
The mass bias correction involved two steps where the exponential law was used for the Si-isotope normalization with further linear interpolation using the standard sample bracketing method against the sulfide or sulfate calibration standards.
At the beginning of each measurement a 30 sec blank and base line was measured. The total time of the measurement was 140 seconds, thus 110 seconds of sample data acquisition, with, depending on the raster length of the ablated site, varying cycles of integration time.
The data was processed at the 2σ rejection level. The Si solution used for external normalization was aspirated continuously during the measurement. External analytical precision (the precision of non-adjacent replicate analysis on different sample fractions and performed at different times in the analytical run) is reported as ±2σ (2 Standard deviation). (Mason et al., 2006). ±2σ means that the measurements lie within ±2σ of the mean with a probability of 95%.
The isotopic ratios are reported in the standard delta notation (‰). Reference ratios for the IAEA-S3 silver sulfide standard reference material are 0.042745 for 34S/32S and 0.007748 for 33S/32S (Ding et al., 2001). The values taken in this study for the NIST SRM 127 standard are 0.045078 for 34S/32S and 0.007962 for 33S/32S.
The δ34S value of the sample was calculated relative to the Vienna Cañon Diablo triolite (V-CDT).
δ34S
V-CDT = [((34S/32S)sample/(34S/32S)V-CDT) – 1] x 103
(34S/32S)sample is the ratio found in the sample and /(34S/32S)V-CDT 0.04418 The general form of the formula is
δ jSV-CDT = [((jS/iS)sample/(jS/iS)V-CDT) – 1] x 103
Where i is 32 and j is 33, 34 or 36. Thus δ33S is calculated the same formula as δ34S, but δ34S replaced by δ33S . The value for /(33S/32S)
V-CDT is 0.0078796 (Ding et al., 2001)
Figure 8 shows the raw data and the data on which a mass bias correction has been applied. The green line (top line), shows the raw data before any corrections were applied. The orange line (bottom line) shows the mass bias corrected data and the blue part of that line is the interval that was selected to be used in further calculations for external correction.
060620jkc04 0.0350 0.0370 0.0390 0.0410 0.0430 0.0450 0.0470 0.0490 0 30 60 90 120 150 Run number 34 S/ 32 S raw data
mass bias corrected data data used for calculation
The isotopic values measured in the samples were corrected for blank and instrumental mass discrimination (mass bias) using the following exponential law: (Maréchal et al., 1999)
ln (r/R))/ln (r’/R’) = ΔMn/ ΔM’n
and the known 30Si/29Si isotopic ratios.
The mean values for 34S/32S and 33S/32S are calculated as arithmetic averages of the mass bias corrected sulfur ratios.
Following these two steps a standard-sample bracketing method was applied involving a linear interpolation method (Albarède, 2004), with external calibration against IAEA-S3 using the mean normalized ratios. Standard-sample bracketing is a technique mostly used in gas source spectrometry techniques. The part of the interval on which the standard sample bracketing has been applied is indicated in blue in Fig 8and the formula used for the linear interpolation method is:
Ri = (Ri)std [(ri)sample/((ri)1-θstd1 x (ri)θstd2)]
Ri stands for the true isotopic ratios and ri stands for the measured isotopic ratios.
Analytical uncertainties are displayed as the standard error of the mean of 34S/32S and 33S/32S at the 2σ level.
Figure 9 shows a schematic picture of the set up of the instrument. The Si solution was aspirated Figure 8
Was early life involved in sulfate reduction in Archean time? Methods
Figure 9, A schematic picture of the sample introduction system of the MC-ICP-MS instrument.
5.2 Chromium reduction
The sulfides in the crushed rock samples were separated from the sulfate and other mineralogical components in the rock using a chromium reduction technique. The crushed rock samples were first put into a roundbottomed flask and 4ml of water was added. The system was then flushed with Argon for about 5 minutes to deoxygenate the system. 8 ml 6M HCl was added to volatilize the sulfide component. The sulfide was trapped in a tube containing water and 250μl of 1M AgNO3, where it precipitated as Ag2S. If necessary more AgNO3 was added to the solution to ensure that 100% of the sulfide precipitated.
If no Ag2S precipitated then the chromium solution was added after about 15 minutes. If precipitation did occur in the first step, then distillation was allowed to proceed for about an hour to precipitate all Ag2S for this reaction step. When all Ag2S of this step precipitated, the traps had to be replaced before the chromium solution could be added to the samples in order to separate the sulfide from ZnS, which volatizes already without chromium reduction, from the sulfides from pyrite, which only volatizes with the chromium added to the system. 16 ml of chromium solution was added and the sample was heated with cooling water flowing through the distillation column. The distillation continued for ~ 60 minutes, and extra AgNO3 was added if necessary as described above. Once the precipitation reaction had ceased the precipitate was collected on 250μm filter paper and allowed to dry overnight. Samples were loaded into tin cups for elemental analyzer gas source mass spectrometry at Utrecht University. A more extensive description of the method for chromium reduction can be found in Appendix I.
Figure 10, experimental set up, used in the chromium reduction technique
5.3 Microdrilling
Approximately 300µg barite was drilled out of the rock using a 1 mm microdrill. The sample powder was then dried in the oven and 242-292µg of barite was weighed and put into tin cups. δ34S was measured using elemental analyzer gas source mass spectrometry at Utrecht University
5.4 Major and trace element determination
The chemical composition of the sulfate and sulfide minerals was determined by electron microprobe microanalysis using the Jeol JXA-8600 superprobe at Utrecht University.
Trace elements were measured in the pyrites by LA-ICP-MS for samples TR01S1, TR03-3 and TR04-1 at Utrecht University. Most of the trace elements in the pyrite were present below the detection limit of the microprobe.
Heater
Refraction column, cooling water
Round bottom flask Trap attachment
Was early life involved in sulfate reduction in Archean time? Results and discussion
6. Results and discussion
6.1 Petrographic study and chemical analysis
6.1.1 Samples from the Onverwacht group, Barberton Greenstone Belt
Samples TR01, TR02, and TR04 were sampled by Thomas Reimer at the farms Londozi, Vergelegen and Stentor respectively and all belong to the Theespruit formation of the Lower Onverwacht group. This group has an age of approximately ± 3.45 Ga. (Figure 3, Geology chapter)
TR03 belongs to the Amo Group, the stratigraphical position of this group is uncertain but it most likely belongs definitely to the Onverwacht group.
The TR01 samples from the Londozi area were collected from the main mine, at a depth of 150m deep. The material is relatively fresh.
TR01S1, Londozi deposit, Theespruit formation, Lower Onverwacht group.
Euhedral pyrite crystals are disseminated throughout the thin-section, mainly in chert material but also at the edges with of barite sand layer. The pyrite crystals in the barite are smaller than those in the cherts. Barite sand is present at several areas in the thin section and the grains have rough fractured edges. They do not show any preferred orientation, and can be reworked barites (picture 1). Some small barite grains occur in the chert layers. The pyrite crystals consist stoichiometrically of Fe and S but some contain up to 0.5 Atom % of Co. Most of the barite grains contain minor Al.
Picture 1, Sample TR01S1
The origin of the scattered euhedral pyrites is not clear. The pyrite is most likely precipitated in situ or was transported over very small distances.
TR01V1, Londozi deposit, Theespruit formation, Lower Onverwacht group.
Many structures can be interpreted as sedimentary in this thin-section. From bottom to top within the section the following features were observed: (1) cavity infill structures. In some spots of yellow/brown siderite fills the cavity. This filing is most likely to be a late stage process.
Barite sand
(2) Mass loading. This was observed in the large semi horizontal grey/ brown sediment layer. However if the thin-section is inverted, this can also be interpreted as a drape structure. (3) Barite crystals and chert fragments are laminated and show a fining upward sequence (or coarsening upwards if top-bottom is reverse), in which some layers can be distinguished (Picture 2). (4) Some chert minerals look like bird eye structures that are common in modern carbonates, the bottom is rounded and the top irregular. If this is the case the top of the sequence would be the other way around than shown on this photo. (5) Syn-sedimentary deformation might have taken place whilst a hydrothermal fluid was injected along the brown sediment layer. The sulfide mineralization along the brown layer might have precipitated from a hydrothermal fluid that was introduced just before lithification of the entire rock. The sulfide minerals in this sample show a variety of chemical compositions, depending on which mineral surrounds the sulfide. The sulfide minerals within the chert layer are almost pure Fe and S (in one spot a small amount of Ni can be found, 0.09 atom%).
The sulfides in contact with barite all have some trace amount of two other elements; these elements are Ni and Co, with concentrations up to 0.241 atom % and 0.424 atom % respectively. In general the pyrites have low trace element content. There is one larger sulfide mineral (several mm in diameter) and this mineral is sphalerite, ZnS.
The barites examined in this thin-section all contain Al and Sr, except for only one spot. The concentrations Al and Sr are max. 0.396 element % and 0.928 element % respectively.
Picture 2, Sedimentary features in sample TR01V1.
The silica minerals show a fining sequence. Bird eye structures that typical for modern carbonates have a rounded bottom and irregular top. If this interpretation is correct the sedimentary sequence would be at the bottom of the photo. It is possible that a hydrothermal fluid has percolated through the thin section, because of mineralization of some sulfides and barite surrounding the brown layer. This must have happened before silicification of the entire rock.
Fining upwards Recrystallized laminated sediment Cavity infill? Soft Sediment deformation
Mass load, or drape structure, if it is a drape structure top and bottom are reversed.
Was early life involved in sulfate reduction in Archean time? Results and discussion
TR01-S4, Londozi deposit, Theespruit formation, Onverwacht group.
The top and bottom of the sequence are readily identified in this thin-section. A clear contact is visible at the base, which looks like a stylolitic contact, and original sedimentary layers can be recognized. Internal lamination can be seen in the transparent chert layer. No barite and sulfides are present in this section. Chert with an Al (10 atom %) is present above the sharp stylolitic contact, see figure 3.
It can be concluded from this interpretations that we are looking at original sedimentary layering in this section. The lack of sulfate and sulfide minerals in the section means that it was not used for further study. However, this sample was collected in very close proximity from the other Londozi samples, supporting a general sedimentary setting for these rocks.
Picture 3, TR01-S4
TR01-S3, Londozi deposit, Theespruit formation, Onverwacht group.
This section contains many finely divided green colored minerals which are probably fuchsite. No barite minerals were observed in this sample. Sulfide minerals are scattered throughout the section (Picture 4). Some sedimentary layering was observed. The measured sulfides were all pyrite and contain besides Fe and S some Co, up to 0.603 atom %. This sample was not used for further interpretation due to the lack of an obvious connection between barite and sulfides.
Picture 4, TR01-S3
Top
Internal lamination, tiny grains Stylolitic contact?
2 cm
TR04-1, Stentor deposit, Theespruit formation, Onverwacht group.
This sample was found in the north of the mountain area. Sedimentary structures are recognizable in this section. The large vein like structure could be a deformed sedimentary layer but it might also be a hydrothermal vein. Most of the pyrite is concentrated near the bigger pyrite structure. Many barite crystals are also present in this area. The major matrix component is chert, but in some areas barite is also present. Smaller pyrite crystals are scattered through the section. At the base of the sequence under the larger vein like structure, a deformed zone of chert and barite was observed. Many of the pyrites appear slightly corroded, suggesting they were deposited in a detrital sedimentary environment under anaerobic conditions. Some of the pyrites form small aggregates of several grains joined together but all with rounded edges.
Picture 5, TR04-1
Summary of Petrographic observations of rocks from the Londozi and Stentor deposits
The sections of the Londozi deposit show many sedimentary structures, some with possible evidence for later stage hydrothermal fluid flow. The sulfides are found scattered through the sections, in both chert and barite layers. The Londozi sample TR01V1 is most likely to have been affected by
hydrothermal fluids along the brow layer. The sample from the Stentor deposit does not give a clear structure but it is most likely a sedimentary deformed rock.
If we place the sample in a wider context (described by Reimer 1980), the barite is usually finely crystalline, with pyrite that frequently occurs along what appear to be original bedding planes. This further supports a sedimentary origin of the barite, which might have been reworked from deposits that originally formed hydrothermally.
TR02-2, Vergelegen deposit, Theespruit formation, Onverwacht group
Chert is the main mineral present in this section but a small amount of barite can be found, especially along the edges. Some sedimentary structures are visible in this thin-section, in the left lower corner. Fractured brecciated clasts can be seen at the top of the section and a boundary, which could be a
Deformed sedimentary layer?
Deformed zone
Layering
Was early life involved in sulfate reduction in Archean time? Results and discussion
Picture 6, TR02-2
TR03-3, Amo deposit, Onverwacht group.
Three different domains can be recognized in this thin section: (1) Intact lamination, (2) a chert layer and (3) chaotic lamination. There are clear sedimentary structures visible in this thin section but we cannot exclude the possibility that some barite occurs in veinlets. Pyrite crystals are very abundant in the mainly barite matrix. The pyrite crystals are not cubic but have an irregular to rounded shapes indicating deposition as detrital grains under anoxic conditions. In the left upper part of the section, an intact lamination can be seen, in contrary to the major right part of the thin section, where the
lamination is chaotic (picture 7). This indicates reworking of the barite. The pyrite seems to follow possible bedding planes in many parts.
Picture 7, TR03-3 Sedimentary sequence Fractured, brecciated clasts Breccia, Boundary/Fault Intact lamination Chaotic lamination
Remnant of crystalline barite? or barite vein? 2 cm
Summary of petrographic observations of the rocks from Vergelegen and Amo deposits
The Vergelegen sample and Amo sample both show sedimentary structures. The brecciation of the clasts in the Vergelegen samples and the chaotic lamination in the Amo samples indicate reworking of the barite. The irregular to rounded shapes of the pyrite minerals indicate deposition as detrital grains under anoxic conditions and the pyrite minerals seem to follow possible bedding planes. These features also indicate a sedimentary origin of the barite that might have been reworked from barite originally hydrothermally formed.6.1.2 Samples from the North Pole Dome, Pilbara Block
DB 4
A very small amount of sulfide minerals are present in this thin-section with a grainsize of < 50μm, Large bladed crystals of barite are present in the bottom and top part of the section. In the middle of the section thick layers of mainly iron oxide minerals can be seen and in between these layers very small irregular shaped barite crystals were observed. Some small barite and quartz crystals occur in between the large barite blades.
Barites can be classified into two types: (1) large bladed crystals up to 5 mm in length and (2) small to very small more tabular crystals. The large bladed barite crystals can be found at the top and bottom of the section with various orientations. On top of the larger euhedral barite there is a reworked zone of broken small barite and quartz crystal together with the magnetite. The bigger barite crystals contain fluid inclusions.
Picture 8, DB4
Iron oxide layers Large bladed barite crystal
Was early life involved in sulfate reduction in Archean time? Results and discussion
EPB-4
This section contains large barite crystals, with abundant inclusions. Crystals are bounded by fine-grained veins of quartz. In part of the thin-section some of the barite crystals are fractured. Within some barite crystals opaque minerals can be seen, and a very small amount is pyrite. Some opaque iron oxide minerals are also present within the quartz veins.
Picture 9, EPB4.
Picture 10, photo of part of the
Section EPB4 where the pyrite occurs
.
In summary for the North Pole samples there are many sedimentary structures including fining upward structures, syn-sedimentary deformation and reworking. Thus most of the sections described probably have a sedimentary origin. This does not automatically mean that all pyrites in the thin sections are sedimentary. Some pyrites have euhedral shapes and are therefore not likely to be reworked like the barite. They could be formed during later stage fluid percolation.
Samples TR01S1, TR01V1, TR03-3 and TR04-1 from the Onverwacht group in the Barberton were used for isotopic measurements on the coexisting pyrite and barite minerals. Samples TR02-2 from the Barberton and DB4 from the North Pole, Australia were also used for isotopic measurements but only for the barite minerals because the pyrite grains were absent or too small for isotopic
measurements. 2 cm
6.2 Major and trace element chemistry
6.2.1 Results
The mineral chemistry was determined by the electron microprobe (Jeol JXA-8600 superprobe) at Utrecht University. The chemical composition of the Sulfate and sulfide minerals was determined for samples from both the Barberton and the North Pole region. Because most of the trace elements occur in concentrations below the detection limits of the microprobe, samples TR01S1, TR03-3 and TR04-1 were also measured with the 193 nm laser ablation ICP-MS at Utrecht University, which has a sufficiently low detection limit to determine elements in the ppm-ppb range. Results are show in Tables 2 and 3 below.
The Cobalt -Nickel ratio of the samples measured with the microprobe are shown here. The Co/Ni ratio can give an indication of the origin of the pyrite as discussed below in Section 6.2.2.
North Pole EPB4 Co/Ni
1 0.21 2 0.31 3 0.25 4 0.24 5 0.19 Londozi TR01V1 3 0.55 6 0.34 7 1.98 Londozi TR01S1 14 0.45
Table 2. Co/Ni ration of first run at the microprobe.
Microprobe data
0 0.5 1 1.5 2 2.5 0 20 40 60 80 100 Sample number C o /N i rat io TR01S1 TR03-3 TR04-1Was early life involved in sulfate reduction in Archean time? Results and discussion 1 10 100 1000 10000 10 100 1000 10000 Ni, ppm C o , p p m TR01S1 TR03-3 TR04-1 Co/Ni 0.1 Co/Ni 1 Co/Ni 10 Co/Ni 100 TR03-01 TR03-02 TR03-03 TR04-01 TR04-02 TR04-03 TR01-03 TR01-04 TR01-05 S 460312 460312 460312 527024 527024 527024 527024 527024 527024 Co 3430 3662 3509 13149 11313 12459 9493 17774 9688 Ni 308 475 341 2164 1950 2066 197 194 75 Cu 116 205 302 206 116 196 135 236 140 Zn 7.2 83 11 58 12 31 6 10 5.6 As 7.1 8.5 9.5 41 40 64 3 123 9.1 Se 16 16 28 15 12 52 8 15 11 Co/Ni 11 7.7 10 6.1 5.8 6.0 48 92 129 Se/S 3.5 x 10-5 3.4 x 10-5 6.1 x 10-5 2.8 x 10-5 2.2 x 10-5 9.8 x 10-5 1.5 x 10-5 2.8 x 10-5 2.0 x 10-5 Table 3, laser ablation results of trace elements.
Table 3 shows the trace element chemistry of the Barberton samples measured by laser ablation ICP-MS. The Co/Ni ratio, which is indicated as well as the S/Se ratio, both of which can give an indication of the environment of deposition.
The Ni concentrations measured with laser ablation are consistent with the Ni concentrations
measured with the microprobe. Unfortunately the Co concentration shows significant differences. The Co concentrations measured with laser ablation are significantly higher than the Co concentration measured with the microprobe, where the Co concentration is below detection limit for a considerable part of the measured pyrites. This indicates that the laser ablation data are, at least for the Co data, not accurate.
6.2.2 Interpretation
The trace element chemistry can be used to constrain the environment of formation of the pyrites. The Co/Ni ratio and S/Se ratios are particularly useful discriminators between magmatic-hydrothermal and
sedimentary environments, although, they do not give conclusive answers (Loftus-Hill and Solomon, 1967; Bajwah et al., 1987). Trace elements enter a crystallizing mineral from the fluid phase by solid solution, adsorption, or by incorporation as independent phases. Pyrites can be divided into different groups based on the Co/Ni ratio.
These groups are:
-Sedimentary or diagenetic origin, is usually associated with a high nickel content and Co/Ni < 1 However cobaltite is often associated with framboidal pyrite and tiny inclusions of Co may have affected the data collected here.
-A volcanic origin, without lead and zinc minerals, is usually indicated by Co/Ni >1 -A volcanic origin in the presence of lead and zinc minerals, has a Co/Ni < 1
Pyrites associated with volcanic-hosted deposits are characterized by Co/Ni > 5 and often > 10.
In Australian ore deposits, pyrites of magmatic-hydrothermal origin generally have Se/S ratios of more than 6.7*10-5 while those of sedimentary origin have ratios of less than 3.3*10-5. It is important that high ratios can indicate magmatic-hydrothermal origin whilst, low ratios do not necessarily prove a sedimentary origin. Most sediments and sedimentary rocks contain only a few ppm selenium. Pyrites in sediments usually contain between 1 and 11ppm Se, but values up to 28ppm have been recorded (Loftus-Hill and Solomon, 1979).
Samples TR03-3 and TR04-1 have a low Se content, mostly within the range observed in sedimentary pyrite, and also the Se/S ratio falls within the sedimentary ratio. Sample TR01S1 has a relatively high Se content and the Se/S ratio is also higher and plots within the hydrothermal range.
The laser ablation data all show a Co/Ni ratio >1, with a large difference between the samples. TR03-3 gives a Co/Ni ratio between 7.7 and 11.1, TR04-1 gives a Co/Ni ratio between 5.8 and 6.1 and
TR01S1 gives a Co/Ni ratio between 48.2 and 129.2, which is significantly higher than samples TR03-4 and TR0TR03-4-1. These ratios indicate a volcanic-hydrothermal origin of the mineralization. But as discussed above the data for Co of the laser ablation data and microprobe data are inconsistent. The laser ablation data indicate that the Co concentration is far above the detection limit of the Microprobe, but this is not the case. The points where the Ni and Co concentration were above the detection limit of the microprobe, the data show for a large part of the data a Co/Ni ratio between 0.1 and 1, which indicates a sedimentary origin of the pyrites. TR01V1 was also measured with the microprobe, but unfortunately the Ni and Co content was high enough to be detected in only three grains. Two grains show a Co/Ni ratio < 1 and one grain shows a Co/Ni ratio >1, which might indicate different
generations of pyrite formation.
As with many chemical variations, the Co/Ni data alone are not enough to conclude a definite
Was early life involved in sulfate reduction in Archean time? Results and discussion
6.3 Isotopes
6.3.1 LA-MC-ICP-MS results
Three different thin-sections were analyzed with the LA-MC-ICP-MS, two from the Onverwacht group (TR01S1 and TR04-1), and one from the North Pole area (DB4). Only the S isotopes of the sulfides were measured within the South African samples and only the S isotopes of the barites were
measured for the North Pole sample because the sulfides at the North Pole were not large enough to perform isotopic measurements. Figure 13 shows all data measured for the standards plotted together in one figure. At the left site of the diagram are the values measured for the sulfide standard (IAEA-S3) and at the right site the values for the barite standard (NIST SRM 127).
External corrections were made on the different isotopic ratios to correct for instrumental mass bias and molecular interferences; as described in section 5.1. As can be seen from the figure the data are not precise. Run 6 shows a large scatter especially in δ33S. Just before run 6 a vacuum pump had broken down and was replaced. This is most likely the cause of the very scattered signal which can be seen in the figure, and all data from this analytical session were rejected.
All other sulfide data (run 1, 4 and 2) cluster together in a much smaller area ranging for from –36,2 to –29,03‰ for δ34S and from –13,07 to –25,8‰ for δ33S. Run 5 also shows wide range but only for the δ33S, a variation of 20.12 to 1.54‰ for δ34S and from 32.03 to –9.57‰ for δ33S.
Standards, all S isotope data of sulfides and sulfates
-60.00 -50.00 -40.00 -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 40.00 -50.00 -40.00 -30.00 -20.00 -10.00 0.00 10.00 20.00 30.00 34 S 33 S
run 1,2, 4 run 3 run 6 run 5
Figure 13, all data on the standards. Sulfides Sulfates
Sample data sulfides, Barberton
-12
-10
-8
-6
-4
-2
0
2
4
-20
-15
-10
-5
0
34S
33S
run 1, TR01S1 run 2, TR01-S1 run 4, TR04-1 MDF
Figure 14, all data from the sample measurements of sulfides in the sample.
Figure 14 shows the sample data collected during the runs that had been accepted on the basis of the standard data (see above). Further shows run 2 a wide spread for δ34S but a narrower range for δ33S. Even though extensive corrections for mass bias and instrument instability have been applied
significant drift remained between standard measurements, further data reduction was thus necessary. Data of the standards and the samples are presented separately in two different tables, in Appendix I. The standards are put together in one table to investigate remaining drift between bracketing
standards. If the variation between two successively measured standards was larger than 1.5‰ for δ34S and larger than 2‰ for δ33S then the sample data were rejected.
The following figure (Figure 15) shows the sulfide and sulfate data that remain after all corrections and filtering for drift had been completed. Run 6 and run 5 were excluded from further interpretation because all standards showed differences between successive standards that were larger than 1.5‰ for δ34S and larger than 2‰ for δ33S. A total of 65% of all data were rejected, thus only a small proportion could be used for further interpretation.
Was early life involved in sulfate reduction in Archean time? Results and discussion -15 -10 -5 0 5 10 15 -15 -10 -5 0 5 10 15 34 S 33 S
TR01-S1, sulfides TR04-1, sulfides DB4, barite MDF
Figure 15, Final sample data after all data reduction was completed.
In general the range in δ33S is larger than that for δ 34S. Barite values for sample DB4 range from 1.43‰ to 5.58‰ for δ 34S whilst δ33S varied between –1.34‰ to –13.13‰. The isotopic values of the sulfides ranged for TR01-S1 from –10.2‰ to –7.21‰ for δ34S and –7.55‰ to –3.52‰ for δ33S and range for TR04-1 from –12.86‰ to –12.46‰ for δ 34S and –8.07‰ to –6.32‰ for δ33S.
The following table shows the natural abundance of sulfur isotopes for Vienna Cañon Diablo Troilite, to which all data are normalized in this study.
Isotope Abundance %
34S 4.20
32S 95.04
33S 0.748
36S 0.0146
Table 4, natural abundance of sulfur isotopes for Vienna Cañon Diablo Troilite
Because the abundance of 34S is much higher than the abundance of 33S, in all measurements the precision of 34S is always higher than that for 33S. It is difficult to get precise and accurate data for 33S and therefore we are limited in the interpretation of the δ33S and Δ33S data that were collected with the LA-MC-ICP-MS technique.
Another limitation was the fact that the barite did not ablate as well as the pyrite using the 213 nm laser (see Picture 11). During the ablation of barite, it was possible to see large particles removed from the sample. Large particles tend to remain on the surface of the sample and are not transported to the plasma (Mason et al., 2006). Therefore long raster lines were used for performing the measurement and a larger width of the beam was used in order to increase the mass of the material per second reaching the plasma. Barite did not ablate well because of the relative transparency of the mineral to 213nm photons.
Picture 11. 11a. Ablated barite, 11b ablated pyrite.
The shape of the ablated pyrite raster line is much better shaped than the barite raster line. When the grainsize of the pyrites was large enough raster lines were ablated.
