Archean Environment: the habitat of early life

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Banded Iron Formations, BIF: The Precambrian Enigma.

Abstracts IUB Bremen, 2006

 

Rare earth elements and Nd isotope systematics in 2.9 Ga-old banded iron-formation from the Kaapvaal Craton, South Africa; Constraints on the source(s) of solutes to Archean iron-formations.

B. Alexander1, M. Bau1, P. Andersson2
1 International University Bremen, Bremen, Germany
2 Swedish Museum for Natural History, Stockholm, Sweden

The Pongola Supergroup and the Pietersburg Group are contemporaneous marine sediment packages which formed on the margin of the Kaapvaal craton ~2.9 Ga ago. Siliclastic lithologies dominate the Pongola sequence, yet marine chemical sediments such as banded iron formations (IFs) are present, and along with the contemporaneous Witwatersrand Supergroup, these iron formations represent the oldest documented Lake-Superior type IFs. Deposition of the Pongola IFs should therefore contrast with the production of iron formation within the Pietersburg Group, which constitutes the Pietersburg Greenstone Belt (PGB), and represents Algoma-type IF depositional environments. Both the Pongola IF and the Pietersburg IF were subjected to low grade greenschist facies regional metamorphism, facilitating geochemical comparisons between the sequences.

The Pongola IFs are pure chemical sediments, as indicated by very low concentrations of incompatible elements (e.g., Na, K, Rb, Zr, Cs, Hf, and Th). The marine origin of these samples is supported by super-chondritic Y/Ho ratios (average Y/Ho = 42), and shale-normalized rare earths and yttrium distributions (REYSN) in the Pongola IFs exhibit positive LaSN, GdSN, and YSN anomalies, which are consistent with typical marine waters throughout the Archean and Proterozoic. The Pietersburg IFs display incompatible element concentrations that likely reflect varying proportions of clastic detritus, with Zr between 4.3 – 60ppm and Th between 0.13 – 1.2 ppm. The purest Pietersburg samples (i.e., low amounts of  Zr, Hf, Th, etc.) possess shale-normalized positive LaSN, GdSN, and YSN anomalies indicative of marine chemical sediments, and exhibit REY distributions that are similar to older Isua (~3.8 Ga) and younger Kuruman (2.5 Ga) iron-formations. The Pongola IFs are generally depleted in heavy rare earth elements (HREE) and exhibit Sm/Yb ~2.0 compared to the Pietersburg (1.0), Isua (0.9), and Kuruman (0.7) IFs. The similarities between the REY distributions of the Pietersburg, Isua, and Kuruman IFs suggests that metamorphic facies and specific depositional environments (e.g., Superior- vs. Algoma-type) exhibit little influence on REY distributions, though this relationship may primarily extend to oxide-facies IFs, which constitute the samples from this study. Distinctly different REY distributions between the Pietersburg and Pongola IFs suggests that Archean seawater influencing depositional environments on the Kaapvaal craton 3.0 Ga was heterogeneous with respect to trace element composition.

Neodymium isotope systematics are used to potentially identify sediment and solute sources within the shales and IFs of the Pongola Supergroup. The εNd(2.9 Ga) for the Pongola shales ranges from –2.7 to –4.2, whereas εNd (2.9 Ga) values for the iron-formation samples bracket the shale values (range –1.9 to –10.9). The similarity in εNd (t) values for the shales and IF samples suggests that mantle-derived Nd was not a significant source of REE within the Pongola depositional environment, though the presence of positive Eu anomalies in the IF samples (and some shale samples) indicates that high-T hydrothermal input did contribute to their REE distributions. The above observations suggest that continentally derived sources for solutes dominated the trace element budget of iron-formation produced during deposition of the Pongola Supergroup.

 

The Composition and Controls of the Oceans in the Archean and Paleoproterozoic

David A. Banks
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, U.K. D.Banks@see.leeds.ac.uk 

Liquid water existed on the planet some 4.3 Ga ago and sedimentary successions show that by 3.8 Ga this was widespread. The initial fluids came from a combination of condensing volcanic gasses and comets and although highly acidic would have been neutralized by water rock interactions. No evidence of the early oceans exist, the oldest preserved fluids may be those found in fluid inclusions in pillow lavas from Barberton and Pilbara at c.3.5Ga.  The nature of the oceans after 4 Ga has largely been inferred from a variety of indirect evidence, from sedimentary and evaporitic sequences and from chemical modeling. Only a few limited studies have directly measured the composition of fluid inclusions believed to have trapped Archean and Proterozoic seawater. The oldest fluid inclusions from Isua and the Caozhuang formation in China (c. 3.7 Ga)  that showed seawater was highly saline have now been shown to be metamorphic fluids. The best examples of ancient seawater are found as fluid inclusions hosted by quartz infilling vesicles, cooling cracks and the space between pillow lavas from Barberton, Pilbara and Abitibi ranging in age from c. 3.5 to 2.2 Ga. The salinities are generally between 8 and 12 wt.% NaCl for water depths probably greater than 200m. Fluids from shallower, probably evaporitic, environments can be highly saline c. 25 wt.%. The fluid inclusions represent a mixture of seawater and seawater altered by WRI and can be compared with modern low temperature seafloor hydrothermal systems, the less energetic off-axis almost passive style of WRI where similar deposition of quartz is observed. Extracting the contents of the inclusions by crushing and leaching, releases the two fluids and the analyses represents a mixture of the two in unknown proportions. Therefore, the approach taken is the same as that used for samples of present-day seafloor hydrothermal fluids where sampling collects both ambient seawater and seawater after WRI. As the objective in modern systems is to exclude the seawater component the data are regressed to zero Mg which is taken to represent the hydrothermal end member fluid. SO4 is also quantitatively removed by WRI. This has allowed estimates of elements such as Na, K, Ca, Mg, Fe and SO4 in seawater through the Archean and Paleoproterozoic. Relative to Na, levels of K, Mg and Ca are higher than the present. There is no evidence for a dominantly carbonate ocean in the oldest fluids, by c.3.5 Ga the oceans were Cl-dominated. Of particular interest are the levels of Fe and SO4 in the seawater end-member fluids. SO4 would have been at higher concentrations than was thought in a dominantly anoxic ocean and the Fe concentrations would have been between 10’s to 100’s of ppm.  Whereas cations are affected by WRI, Cl and Br are largely unaffected and their analyses do not depend on the proportions of seawater and altered seawater analysed. The Cl/Br of inclusions thought to have trapped deep seawater is approximately half of the current value. Inclusions thought to have trapped shallower seawater have ratios almost the same as the present and this is believed to show a near surface biomass removing Br from solution. Cl-isotopes of the inclusion fluids are distinctly different to current values and independent of the Cl/Br ratio and water depth. Values of37Cl are between +3 and +4 per mil until about 2 Ga when there is a sudden change to values similar to the present (0 per mil). The heavy values are close to the published value for the mantle and one interpretation is that this represents the period when the oceans stopped being buffered by the mantle. The hydrothermal activity on the seafloor was much reduced and this coincides with the period when BIF’s were rapidly diminishing and the oxygen levels of the atmosphere were increasing, “the Great Oxidation Event”.

 

Rare-Earths and Yttrium in BIFs, MnFs, and Early Precambrian Seawater

Michael Bau1
1Geosciences & Astrophysics, International University Bremen, Germany

In aqueous solutions, element fractionation within the group of the rare earths and yttrium (REY) is controlled by the systematic decrease of the ionic radii of the isovalent REY(III), by the redox-sensitivity of Ce(III) and Eu(III) that may occur as Ce(IV) and Eu(II), respectively, and by the specific electron structures of the REY(III). These parameters may cause anything from complete decoupling of an element from its REY neighbours to subtle differences in the stability of chemical complexes of the REY. The overall result is the shale-normalized REY pattern of modern seawater, for example, that is characterized by strong enrichment of HREE over LREE, negative Ce anomaly, positive anomalies of La, Gd and possibly Lu, and super-chondritic Y/Ho ratio (i.e., a positive Y anomaly). Marine chemical sediments that scavenge REY from seawater without major fractionation, such as Ca and Mg carbonates and Ca phosphates, for example, preserve the REY distribution of the water from which they formed and may, therefore, provide information on the redox level and on REY speciation. In marked contrast to modern hydrogenetic Fe-Mn oxide precipitates (and because of unknown reasons), the Early Precambrian BIFs and MnFs show a REY distribution that is similar to that of contemporaneous limestone and dolomite, suggesting that they also preserved the seawater REY signature.

       The presence of positive La, Gd, Lu and Y anomalies is clear evidence for a marine origin of the BIFs and MnFs. The similar degree of HREE enrichment in Archean BIFs and carbonates and in modern seawater suggests that the speciation of dissolved REY in seawater was rather similar, which argues against an alkaline Archean ocean.

            With the exception of redox-sensitive Ce and Eu, even detailed features of the REY patterns of Archean marine chemical sediments bear close resemblence to that of modern seawater. They do, however, show positive Eu anomalies which indicate the presence of a significant high-temperature hydrothermal component in Early Precambrian seawater. This anomaly is somewhat larger in BIFs than in contemporaneous carbonates, suggesting a higher hydrothermal contribution to the Fe-rich deep water than to Fe-poorer shallower waters. This also suggests that the Fe in BIFs is predominantly mantle Fe transported by hydrothermal solutions and not upper-crustal Fe transported by rivers.

            Archean BIFs and marine sedimentary carbonates do not show any Ce anomaly, indicating that Ce was not oxidized in the surface system of Archean Earth. The oldest BIFs or MnFs that display Ce anomalies are about 2.3 Ga old and coincide with the formation of the first large-scale Mn oxide deposit in Earth history. The onset of the “modern” redox-cycle of Ce in the Paleoproterozoic agrees well with the postulated “Great Oxidation Event” between 2.4 and 2.3 Ga ago.

 

Diagenesis simulation of Banded Iron Formation minerals

Bernd Binder, Nicole Posth, C. Berthold, Andreas Kappler 
Geomicrobiology Group, Center for Applied Geosciences (ZAG), University of Tuebingen Sigwartstrasse 10, 72076 Tuebingen, Germany Phone: +49-7071-29-76801, fax +49-7071-29-3060 e-mail: bernd.binder@uni-tuebingen.de, andreas.kappler@uni-tuebingen.de 

The minerals that we observe in BIFs today are a direct consequence of diagenetic processes. One theory of the primary source of BIF minerals are precipitates produced by anoxygenic phototrophs. In addition to analyzing these primary mineralogical precipitates, it is essential to follow the transformation of such precipitates under P/T-conditions characteristic of those to which BIFs have been exposed over geologic time. After their deposition on the ocean floor, iron and silica minerals undergo diagenetic transformations that strongly depend on pressure and temperature (P/T) conditions over time. Late Archean to Early Proterozoic BIFs were exposed to different P/T-conditions depending on their location. The Griqualand West sequence was generally heated to temperatures of 110-170 °C (Myano & Beukes, 1984). Very little is known about diagenesis of biogenic iron minerals. In particular, the fate of the organic matter associated with such aggregates is unclear. Kennedy et al. (2004) showed that synthetic ferrihydrite transforms to hematite at 80°C and atmospheric pressure within two days, whereas biogenic ferrihydrite showed no phase transition under these conditions. Therefore our main research questions are:  - What diagenetic transformations undergo the primarily precipitated iron minerals after deposition at the ocean floor? In particular, what are the mineral products of diagenetic transformations in the presence of various amounts of organic matter?   In preliminary experiments, we exposed synthetic ferric iron hydroxide (ferrihydrite) and ferrihydrite/glucose mixtures (glucose as a proxy for biomass) to 170°C and 1.2 kbar for different time periods. The samples were filled into gold capsules that were welded closed. We used rapid quench autoclaves to expose the samples to high pressures and temperatures (170°C and 1.2 kbar). In experiments with synthetic ferrihydrite in the absence of organic matter, we observed (as expected) a transformation to hematite (identified by micro Raman- and X-ray analysis). First experiments with ferrihydrite in the presence of excess glucose yielded siderite and magnetite despite an access of reducing equivalents in the form of glucose. Glucose was used as a model compound for organic matter; in later experiments dried microbial cells will be added as an organic matter source.  To identify and quantify gaseous products that might have been formed during P/T-treatment of mineral-Corg mixtures (e.g. methane, CH4) by gas chromatography, the closed Au-capsules will be frozen in liquid N2, opened in the frozen state, and then thawed in a closed glass vial. Total Fe(II), total Fe(III), and remaining organic carbon will be quantified with a spectrophotometric assay and by TOC analysis after complete dissolution of the capsule content in HCl (6 M). Minerals are analyzed and identified by µ-XRD, RAMAN spectroscopy and reflected light microscopy. Finally, an overall electron balance considering the amounts of Fe(II), Fe(III) and CH4 (and remaining Corg) will show whether Fe(II) found in BIFs can be used as a proxy for organic carbon that was originally present in the BIFs.

References:

Kennedy C.B., Scott S.D., Ferris F.G. (2004): Hydrothermal phase stabilization of 2-line ferrihydrite by bacteria. Chemical Geology 212, 269-277. 

Miyano T., Beukes N.J. (1984): Phase relations of stilpnomelane, ferri-annite, and riebeckite in very low grade metamorphosed iron formations. Transactions of Geological Society of South Africa 87, 111-124.

 

New data about ultrahigh-temperature/high pressure metamorphism of Archean BIF of the Voronezh Crystalline Massif (Russia); N2-CH4 - rich fluid inclusions as Precambrian biomarker.

Fonarev Vyacheslav

Institute of Experimental Mineralogy, Russian Academy of Sciences,
E-mail: fonarev@iem.ac.ru

Mesoarchean BIF of the Voronezh crystalline massif contains pyroxenes with exsolution textures. Using the reintegrated compositions of the primary clinopyroxene and pigeonite the ultrahigh-temperature (about 1000°C) of the peak metamorphism was found (Fonarev et al., 2006). Such metamorphic conditions of Precambrian BIF have previously been reported only for Archean meta-ironstones from Scourie, Scotland (Barnicoat, O’Hara, 1979) and Enderby Land, Antarctica (Sandiford, Powell, 1986; Harley, 1987). Fluid inclusions in the samples studied are mostly CO2-rich with subordinate amounts of nitrogen-methane, nitrogen-aqueous, and aqueous-salt varieties. The magnetite quartzites, iron-silicate rocks, metapelites and meta-clinopyroxenites of the region contain CO2-rich inclusions of different generations. The densest inclusions with Thmin= -49.2°C occur in the magnetite quartzites only with relicts of the exsolved pyroxenes and are related to the peak metamorphic conditions. Primary nitrogen-methane (CH4 from 8 to 39 mol. %) inclusions are associated with them. The peak metamorphic pressure of the BIF was estimated as 10-11 kbar at depth 36-40 km with using of isochores of the densest CO2-rich inclusions and calculated temperature. We infer that formation of such thick sequences of volcano-sedimentary rocks (including BIF) in the Mesoarchean of the VCM occurred in rift-like interplate (interdomain) conditions during prolonged and slow spreading of the ocean basin. N2-CH4 - rich fluid inclusions occur only in the investigated samples of the BIF. Other types of rocks (metapelites, metabasites) do not contain such inclusions. Similar data were obtained early for the Archean metamorphic rocks of the Central Kola granulite area of the Baltic Shield (Fonarev, Touret, Kotelnikova, 1998). These results can indicate that nitrogen is connected with a sedimentary protolith and characterizes the ancient (Archean) conditions of the sedimentation. It can be interesting also from the point of view of the hypothesis about “ammonium as a biomarker in precambrian metasedimants” (Boyd, 2001) and the indicator (together with methane) of an organic life on planets of solar system. The work has been supported by Grants RFBR № 04-05-64585 and № 04-05-65109.

 

Precambrian ironformations: More facts – more problems?

Jens Gutzmer and Nic Beukes 
Paleoproteozoic Mineralization Research Group, Department of Geology, University of Johannesburg, P.O. Box, Auckland Park 2006, South Africa 

Banded iron formations (BIF’s) are common to many marine sedimentary successions of Precambrian age. Four principal types of BIF are distinguished and these are generally thought to be bound to specific time intervals, including Algoma-type (Archean), Hamersley-Transvaal-type (early Paleoproterozoic), Superior-type (late Paleoproterozoic) and Rapitan-type (Neoproterozoic). Although BIF’s are associated with various lithologies and often display a complex facies architecture, they all mark distinct events of precipitation of vast volumes of iron and silica in continental shelf environments. These events, common only until the end of the Paleoproterozoic (with a minor resurgence in the Neoproterozoic), must be of considerable paleoenvironmental significance. However, hampered by the absence of modern examples of iron formation deposition, and by uncertainty surrounding their primary mineralogical/ geochemical composition, the origin of BIF’s has remained a contentious topic.  Important attributes of the deposition of BIF’s, irrespective of their age and type, include (i) hydrothermal alteration of oceanic crust constitutes the most important source of iron that was supplied to an anoxic deep oceanic water mass; (ii) precipitation of iron took place in response to mixing of the iron-rich anoxic deep marine water mass with oxygenated shallow marine water along a chemocline that at least episodically intersected the continental shelf; (iii) chert precipitation was a continuous process in shallow environments of the Precambrian Ocean. The accumulation of BIF as a distinct litho-type, composed essentially of chert with variable amounts of intercalated iron minerals, thus required environmental conditions suited to establish and maintain a density-stratified ocean and continental shelf areas starved of siliciclastic influx for the duration of iron formation deposition. Unfortunately, such simple preconditions may be met by a large number of possible scenarios - they do not suffice to constrain the paleoenvironmental significance of BIF’s. An improved understanding of the parameters controlling the formation BIF’s may, however, be gained by studying their relationship to spatially and genetically related sedimentary rock suites. Such associated lithologies include manganese formations (MnF), shallow marine platform carbonates, and glaciogenic diamictites. The association of BIF’s with such lithologies is especially well documented in the Transvaal Supergroup, South Africa. Close examination of the observed geological and geochemical relationships reveal important new constraints for the deposition of iron formations.

 

The microbial role in banded iron formation

Kurt Konhauser
University of Alberta, Email: kurtk@ualberta.ca

Banded iron formations have occurred throughout much of geological time. Their presence has been used as a means of arguing for specific geochemical and environmental conditions in the Precambrian, but after decades of intense research, there are still many unresolved issues, including (i) by what mechanism(s) was Fe(II) oxidized (photochemical or biological, the latter using either O2 or light), if indeed an oxidative process was required; (ii) were BIFs formed over the shelf vs. open ocean; (iii) the amount of phytoplankton biomass, which relates to the nutrient status of the surface waters; and (iv) the relative importance of Fe(III) reduction vs. the other types of metabolic pathways utilized by seafloor microbial communities. In terms of oxidative mechanisms, recent experimental studies have shown that both anoxygenic photosynthesis and ultraviolet light could account for the Fe(III) component in BIF, but that available Fe(II) concentrations in surface waters were likely constrained by mineral saturation states and upwelling rates. During diagenesis, microbial communities may also have played a substantive role in Fe cycling, coupling the reduction of ferric hydroxide to plankton biomass oxidation. By estimating the potential amount of biomass generated in the water column with the reducing equivalents required for magnetite formation, it is also likely that other anaerobic metabolic pathways were used, such as methanogenesis, and maybe even methanotrophs that employed Fe(III) reduction.

 

Cycling of Mn and Fe in the modern marine environment

Andrea Koschinsky

International University Bremen GmbH, a.koschinsky@iu-bremen.de

Iron and manganese in the modern ocean are mostly bound into oxic precipitates on the seafloor or are present in the water column as colloids or fine oxic particles, which play an important role for the scavenging of trace elements from seawater. Hydrogenetic marine Fe-Mn crusts are formed by these scavenging processes and consist of Fe-Mn oxide layers precipitated onto hard-rock substrates throughout the ocean basins at water depths of about 400 to 4000 m.  They grow at very slow rates (1-10 mm/Myr), have high porosity and enormous specific surface area.  Mineralogically, they consist of amorphous FeOOH and ?-MnO2 (vernadite) intergrown at a Fe/Mn ratio around 1. These characteristics promote the acquisition from seawater of significant quantities of metals, including Co, Ni, Cu, and trace metals such as PGEs and Te. Of special interest is the enrichment of redox-sensitive trace metals, such as Co, Ce, Te, and Pt, which provides information on redox conditions during the time of formation. The older parts of most thick Fe-Mn crusts were impregnated by carbonate fluorapatite (CFA) during the Middle Cenozoic. This phosphatization had a secondary diagenetic effect on Mn-Fe phases and led to the partial redissolution and redistribution of some elements. In the vicinity of hydrothermally active areas, Fe-Mn crusts can form by both contributions of metals from ambient seawater, and hydrothermal fluids, or can even form exclusively from hydrothermal emanations. Besides mixed Fe-Mn crusts, also pure Fe-crusts or Mn-crusts have been found. Hydrothermal crusts have distinctly different compositions than hydrogenetic crusts, which is usually expressed by lower heavy metal concentrations, different REE patterns, and a higher variability in composition. The composition of hydrothermal crusts reflects the composition of rocks leached during hydrothermal circulation, temperature and composition of the hydrothermal fluids, and mixing processes with seawater. Fine-scale analysis of hydrothermal crusts can provide information on the variability of hydrothermal activity in the respective area. In the deep-sea basins, Fe and Mn are present in the surface sediment and in the form of ferromanganese nodules, which are mostly diagenetic, i.e. they form by recycling of elements in the pore waters of the sediment. Hydrogenetic contribution to the metal content of the nodules is also known, however, hydrogenetic crusts are much more suitable for interpretation of paleoceanographic information stored in the Fe-Mn layers than nodules. In summary, knowledge gained on the composition and conditions of formation of recent marine Fe-Mn precipitates enables scientists to use them as analog for ancient Fe-Mn formations, for which the respective information on formation conditions may not be well constrained.  

 

Geo- and cosmochemistry of Y and Ho: The Solar System Y/Ho with implications for the formation of rocks from Akilia

Andreas Pack

Institut für Mineralogie, Universität Hannover, E-mail: a.pack@mineralogie.uni-hannover.de

We report a new value for the Solar System Y/Ho ratio. Our data were obtained on high-T inclusions of chondritic meteoorites by means of LA-ICPMS. Most inclusions show an excellent correlation between Y and Ho. Rare refractory inclusions and one chondrule show significant deviation from the canonical Y/Ho ratio. This indicates that Y and Ho have different volatility and that refractory high-T components in chondrites formed by fractional condensation. We have compared our chondritic Y/Ho with data obtained from BCR-2G basalt and compiled literature data. Basalt shows only a very small deviation from the chondritic value. This suggests that Y and Ho were well mixed on planetary scale (Earth, Asteroids). More differentiated siliceous rocks from Earth show a trend toward higher Y/Ho. We suggest that the activity coefficient ratio of the Y- and Ho-bearing components in the silicate melt changes with silica content. We compare our Y/Ho ratio with literature data obtained from the rocks from Akilia, which were suggested of hosting the Earth's oldest traces of life. Implications for metasedimentary or igneous origin of these rocks will be discussed.

 

BIFs: An Archive of an Ancient Biosphere?

Nicole Posth 

Geomicrobiology, Center for Applied Geoscience, University of Tuebingen
E-mail: nicole.posth@uni-tuebingen.de

One intriguing and unresolved aspect of Banded Iron Formations is the mechanism(s) behind their deposition. As BIFs were deposited throughout the Precambrian, the debate over the emergence of oxygen on Earth lies at the heart of this question. Consideration of the microbial, as well as the chemical and geological systems present on Early Earth is the key to clarification of early BIF deposition (ca.3.8-2.2 Ga).  The dominant theory of biotic BIF precipitation is the production of oxygen via cyanobacteria, which then reacts with dissolved iron to form Fe (III) oxides. The feasibility of BIF precipitation via Fe (II)-oxidizing photoautotrophic bacteria was first proposed 30 years ago (Garrels, 1973, Hartman, 1984). The recent discovery of strains capable of such a metabolism (Widdel, 1993) bolstered the suggestion and has made experimental work with model organisms possible.  We are interested in understanding iron cycling in the early Earth system and the viability of Archean BIF deposition via anoxygenic Fe (II) oxidizing photoautotrophy.  Here we present calculations of iron and silica mineral deposition founded on experiments with modern Fe (II)-oxidizing photoautotrophic organisms under light intensity, temperature conditions, silica and iron concentrations based on estimates of Precambrian water conditions. Recent work on cell-mineral interactions between Fe (II)-oxidizing photoautotrophic and both silica and iron minerals is shown.  Additionally, mineral diagenesis simulations (pressure-temperature) performed on chemically synthesized and biogenic minerals will be introduced. A new geochemical-microbial model supported by this data will be presented which clarifies the deposition of alternating mineral layers in early BIFs.  Our experimental and theoretical work attempts to explain an Early Earth structure by considering the most likely ecological setting at the time of deposition (3.8-2.2 Ga). We suggest that in the Precambrian ocean, a layer of anoxygenic phototrophic microbes living beneath the wind-mixed surface stratum was the most likely catalyst for BIF deposition, even in the presence of cyanobacteria. 

References:

Garrels R.M., Perry, E.A., and MacKenzie, F.T. (1973) Genesis of Precambrian Iron Formations and the Development of Atmospheric Oxygen. Economic Geology 68, 1173-1179.  

Hartman, H. (1984) The evolution of photosynthesis and microbial mats: A speculation on banded iron format  ions. In Microbial Mats: Stromatolites (ed. Y. Cohen, R.W. Castenholz, and H. O. Halvorson), pp. 451-453.

Alan Liss, N.Y.  Widdel, F., Schnell, S., Heising, S., Ehrenreich, A., Assmus, B., and Schink, B.  (1993) Ferrous iron oxidation by anoxygenic phototrophic bacteria. Nature 362, 834-836. 

 

Comparison of Hydrothermal Systems in Different Geological Settings

Katja Schmidt

International University Bremen (IUB)

Hydrothermal activity is known to take place in very different settings at the seafloor and plays a key role in buffering the chemical and isotopic composition of seawater. High-temperature hydrothermal systems are known to occur along mid-ocean ridges, in island arc settings (both in back-arc basins and along submarine volcanic chains) and at intraplate volcanoes. The fluid geochemistry varies strongly between these settings and is controled by a variety of factors, with host rock composition and reaction temperatures as the main primary parameters. Secondary processes like phase separation, which is most likely to take place in areas with active magmatic activity, can modifiy the composition of the emanating fluid significantly. The hydrothermal systems act as sinks and sources for different elements depending on their setting. Pervasive low-temperature hydrothermal circulation at ridge flanks (<65°C) plays a major role regarding the elemental exchange between lithosphere and hydrosphere.

The known hydrothermal systems at mid-ocean ridges are mainly located in mafic rock environments and have been studied in detail. However, an increasing number of active vent fields have been found in ultramafic provinces on slow and ultra-slow spreading axes. This clearly shows, that ultramafic-hosted hydrothermal activity may contribute significantly to lithosphere-hydrosphere elemental exchange budgets. In contrast to basaltic systems generally higher disolved gas concentrations like methane and hydrogen are produced. The pH of the hydrothermal solutions varies between pH 3 and pH 11, which is directly related to the reaction temperature and strongly influences the fluid geochemistry. Additionally, low-temperature weathering of abyssal peridotites differs strongly from that of basaltic rocks. In particular, it leads to a 5% flux of Mg to the oceans in contrast to Mg uptake in basaltic environments. As ultramafic rocks are known to be a common part of the oceanic lithosphere in the early earth, knowledge of elemental fluxes in hydrothermal systems in these environments are of special interest.

In contrast to the well-known hydrothermal systems in MOR environments hydrothermal activity in island arc settings (backarc basins and submarine arc volcanoes) and at intraplate volcanoes exhibit a much wider spectrum of vent site characteristics, emanation temperatures and fluid geochemistry. The subduction-related host rock composition in island arc settings varies between basaltic and felsic composition. Generally, high-temperature fluids in backarc settings show relative enrichments in Pb, Zn, Cd, Mn, Au and Ag, and volatiles like CO2 compared to MOR fluids and lower Fe/Mn ratios. In hydrothermal systems along island arc chains,phase separation is a common phenomena, triggered by frequent magmatic activity at shallow depth.High volatile contents are very typical both in island arc and intraplate settings and are either caused by higher concentrations in leached rocks or result from a direct input of magmatic volatiles from a degassing magma (CO2 and SO2). The continuous contribution of high amounts of magmatic volatiles can lead to very specific fluid compositions. Warm fluids (<120°C) sampled from shallow island-arc or hotspot volcanoes are very acidic and gas-rich and act as a source of Mg. The low pH results from the disproportionation of magmatic SO2 gas. In extreme cases deposits of native sulfur and bubbles of liquid CO2 have been observed.

In rare cases where hydrothermal fluids are interacting with sedimentary deposits they get enriched in B, NH4, halogenides, alkali letals and hydrocarbon compounds and exhibit higher pH and alkalinity values.

 

AquaHab – Biomonitoring, Biomimetics and Evolution for Space Exploration

K. Slenzka 1), M. Dünne 1), U. Johanningmeier 2), K. Rischka 3)  
1) OHB-System AG, Dept. Life Science Universitätsallee 27 -29, 28359 Bremen, Germany
2) Martin-Luther-University Halle, Plantphysiology, Weinbergweg 10, 06120 Halle (Saale), Germany
3) Fraunhofer Institute for Manufacturing Technology and Applied Materials Research, Department Adhesive Bonding Technology and Surfaces, Wiener Str. 12, 28359 Bremen, Germany 

Bioregenerative closed ecological life support systems (CELSS) will be necessary in the exploration context revitalizing atmosphere and waste water, producing food for the human CELSS mates, reducing the general stress level as well as biomonitoring the quality of the limited resource water. Human exploratory missions will require a set-up of human habitable units. From the actual point of view a transfer of whole units to the outposts seems not being possible to be realized. Thus a step-by-step development and evolution is necessary taking into account transfer systems capable to grow to larger units before arrival of the crew. Exploratory expeditions will require new technologies since a point of no return will be reached and no emergency return missions can be done. One of a possible technology and research field enabling this is Bionic or Biomimetic.

Two actual projects will be presented - a biomimetic adhesive as well as biomimetic strategies for antimicrobial surface treatments. The experiences gained during the last years in our research group lead to the development of a closed aquatic habitat, called AquaHab, which serves as a test system for prospective ecotoxicological risk assessment of chemicals and drugs. AquaHab can use the cabin air for CO2 removal enhancing the water plant growth. In parallel oxygen is produced to be used in the system as well as for enrichment of the air. In addition culturing fish is a primary food source. In an additional step the incorporation of an algal photobioreactor is under development. Genetically engineering of the D1 subunit of PS II Photosystem 2 - to be adapted to space radiation by a strategy of directed evolution is planned, by generating a pool of randomly mutated DNA fragments by error prone PCR and transforming the mutant strain with this pool, it is possible to expose the resulting transformants to selective conditions like space radiation. Mutants adapted best to such conditions can then be identified and analyzed.  All the data presented are in the context of AquaHab and a perspective for the use of such a microcosm investigating extreme environments will be discussed.

 

The Superior Type Iron Formations and Iron Rich Shales of the Mesoarchean Wiwatersrand and Pongola Supergroups of South Africa

Bertus Smith

University of Johannesburg,  glgy12@rau.ac.za

Deposited soon after the consolidation of the Kaapvaal Craton, the Mesoarchean (~3.0-2.8 Ga) Witwatersrand and laterally correlatable Pongola Supergroups are the oldest well-preserved laterally extensive cratonic cover successions worldwide.  The thick successions (Witwatersrand up to 10.4 km thick, Pongola up to 14 km thick) are predominantly siliciclastic and host economically very significant Au and U deposits.  The occurrence of iron formation in thin (2 to 20 m) but laterally extensive units is well known (some are used as marker beds) but their nature and distribution have not been investigated in any detail.  The present study was based on the detailed description of all the iron formation units based on numerous deep diamond drill core intersections.  Associated iron rich shales were studied for comparison. The mineralogy of the banded iron formations reflects a lower greenschist facies metamorphic overprint, with magnetite and quartz dominating, suggesting the deposition as oxide facies iron formations.  Locally some thin intercalations of carbonate (in the Water Tower and Contorted Bed banded iron formations) and iron rich silicate (in the Scotts Hill Member) occur.  Grunerite occurs in close contact to mafic sills and dykes, reflecting a slight contact metamorphic overprint. The major element geochemistry of the iron formations correspond well to expected values, with a range of 10-50 weight percent Fe2O3 and 40-80 weight percent SiO2.  Al2O3 concentrations range from 0.4-5 weight percent and indicate a considerable detrital contribution into most of the iron formations.  This influence is also illustrated by the rare earth element distributions which are PAAS-like where the detrital component is significant.  However, rare earth element distributions also show a clear positive Eu-anomaly (EuSN/EuSN* of 1.17-1.98) and a mild enrichment of HREE to LREE ((Yb/Sm)SN of ±1.2 and (Sm/La)SN of ±1.1). Biogenic influence on the origin of the iron formations is indicated by negative &#948;13C values (-9.12 to -15.70) for carbonate minerals as well as oncolitic textures present in the iron formation of the Nconga Formation of the Pongola Supergroup. From the currently available evidence it is concluded that the iron formations in the Witwatersrand and Pongola Supergroups show all the typical characteristics for Superior type iron formations including their lithostratigraphic associations, mineralogy and geochemistry.  They are laterally of similar extent to the much better known Paleoproterozoic examples, though much less voluminous and much older (ca. 500 Ma).  The high Al2O3 content, the presence of Al-bearing silicate minerals and the strong association with and interlayering of iron rich shale suggests that these iron formations were never completely cut off from detrital input.  These observations may suggest that stable environments for iron formation deposition did not persist long enough for stratigraphically thick successions to form.

 

Fe and Si isotopic composition of banded iron formation determined by femtosecond laser ablation

Grit Steinhoefel, Ingo Horn, Friedhelm von Blanckenburg and Jérôme Chmeleff

Institut für Mineralogie, Universität Hannover, Callinstr. 3, D-30167 Hannover

The investigation of stable Fe and Si isotopes in banded iron formations (BIFs) can refine our understanding of the Precambrian Fe and Si cycle in the ancient ocean . BIFs are the product of initial precipitation from seawater and subsequent depositional, diagenetic and metamorphic processes. In order to study these processes we have determined Fe isotopes ratios in Fe-oxides and Si isotopes ratios in cherts from two BIFs of different ages and composition.  The development of a UV-femtosecond laser ablation system coupled to a MC-ICP-MS provides us now with a tool to determine d56 Fe  and d30 Si values in situ at a spatial resolution of 30mm and less with a precision of 0.1‰.

The formation of BIFs involves redox reactions, dissolution and subsequent precipitation of Fe resulting in the fractionation of Fe isotopes. The hematite-chert BIF of the P roterozoic Transvaal Supergroup shows a maximal range in d56Fe from -1.17 to -0.27 for hematite in a single microband within the same stratigraphical level. A s imilar range is observed in several sequenced microbands while average Fe isotopic composition remains surprisingly constant with d56Fe of -0.41±0.13‰ (2s). The carbonate-magnetite-chert BIF of the Archean Shurugwi Greenstone exhibits a similar  heterogeneity in the Fe isotopic composition for magnetite in single magnetite-rich bands showing  a variation of up to 0.40‰ in d56Fe while the average of the bands is again uniform with d56Fe +0.73±0.10‰ (2s). The observed isotopic inhomogeneities are in part the result of isotopic zonation  of the individually analysed m agnetite crystals   . The small-scale variability in the Fe isotope composition suggests relocation of Fe during diagenesis and metamorphism associated with recrystallization and crystal growth. This process took place locally and varied in both intensity and reaction rate at a sub-millimetre scale. However the overall process and the Fe sources remained steady over at least several periods of microband formation as no significant secular change in the Fe isotope composition is observed.

The Si isotopic composition of chert layers interspacing the magnetite or hematite microbands seems to be robust to post-depositional processes and may carry primary signatures [1]. First results indicate an overall homogeneous Si isotopic composition of the chert-bands in both samples. Chert-bands in the Proterozoic sample appear to be homogeneous themselves as well as to each other over several sequenced bands. The Archean sample  exhibits a similar picture but is in average 0.6‰ lighter in d30Si when compared to the Proterozoic sample. Some single bands differ from the average by up to 0.6‰.  However, the Si source and the overall process seem to be steady over the formation of several sequenced chert-bands.

These results show that the study of Fe and Si isotopes on a high-spatial resolution can discover sources and diagenetic effects in BIFs and enhance our understanding of P recambrian Fe and Si cycles.

[1] André, L. et al. (2006) EPSL 245, 162-173.

 

Micro-Scale Heterogeneity and Redistribution of Iron Isotopes in Earth's Oldest Banded Iron Formation Revealed by Secondary Ion Mass Spectrometry

Martin Whitehouse

Swedish Museum of Natural History, E-mail: martin.whitehouse@nrm.se

Deformed and metamorphosed rocks from the > 3.7 Ga Isua greenstone belt, southwest Greenland, represent Earth's oldest known supracrustal succession. The section is dominated by abundant mafic volcanic rocks, with less abundant sedimentary rocks such as BIF and conglomerate, sandstone, and mudstone of volcaniclastic origin. Here we present iron isotopic compositions determined by secondary ion mass spectrometry (SIMS) for magnetite crystals comprising the BIF and pyrite from younger, cross-cutting veins and clusters from BIF and conglomerate. In three lithologic variants of BIF (quartz+magnetite; quartz+magnetite+minor amphibole; amphibole+magnetite+quartz), magnetite crystals span a range of &#948;56Fe from -0.7 to +2.7 ‰ over a sub-millimeter scale, although many values range between +1 and +2 ‰. Such values are unpredicted at such a small scale and potentially consistent with an abiotic origin for the BIF. Secondary pyrite veinlets in BIF and cubes of metamorphic pyrite from a conglomerate also show a similar distinct enrichment in heavy Fe isotopes, suggesting that such fractionations are not unique to the original environment as has been assumed, but can be redistributed during subsequent post-formational processes.