Archean Environment: the habitat of early life

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Analysing the Archean

 

Short course, Utrecht, The Netherlands

28 June - 1 July 2010

 

Abstracts

 

Arndt, Nicholas  - Early continental and oceanic crust

The Archean mantle was probably, though not certainly, much hotter than modern mantle – how can we estimate this temperature and how can we monitor its variations through Earth history? A hotter mantle melts more and produces thicker oceanic crust – would this crust have resisted subduction? If so, a mechanism other than subduction-related melting must explain the formation of Archean granitoids and growth of the continental crust – what was this process?  Jack Hill zircons with ages > 4 Ga record the presence at the surface of the very young Earth of felsic, enriched material – was this continental crust? If so, was it a small, unrepresentative mini-continent or part of a much larger volume of continental crust? The Hf isotopic compositions of the zircons indicate formation from an enriched source that had formed close to 4.5 Ga ago. Was this material a relict of the layered mantle left after solidification of the terrestrial magma ocean?

Kemp et al., 2010. Hadean crustal evolution revisited: New constraints from Pb–Hf isotope systematics of the Jack Hills zircons

 

Chardon, Dominique - Three-dimensional mass redistribution in accretionary orogens: Archean to present?

Commonly overlooked, first-order structural and metamorphic characters of Precambrian and particularly Archean accretionary orogens may be used to understand flow mode of the continental lithosphere in convergent settings involving massive juvenile magmatism. This flow mode combines distributed shortening and thickening, gravity-driven flow, lateral escape, and three-dimensional mass redistribution of buried supracrustal rocks, magmas and migmatites in a thick fluid lower crust. The combination of these processes maintains a nearly flat surface and Moho of the orogens. Such a flow mode appropriately describes the structural, metamorphic and kinematic patterns of a large number of wide hot orogens throughout earth history i.e., convergent regions of attenuated or missing mantle lithosphere that remained hot for protracted periods (> 80 Ma). The lateral component of orogenic flow essentially post-dates lateral accretion and is particularly efficient at creating the strong crustal seismic reflectivity and anisotropy of hot orogens, as opposed to commonly inferred syn-accretion, orogen normal large-scale thrusting.

Gapais et al. 2009 Mountain building and exhumation processes through time: inferences from nature and models

 

Catling, David – Early Earth’s atmosphere

The composition of the ancient atmosphere is determined by a combination of geochemical proxies and theoretical inferences. Earth’s climate has remained conducive to life since at least 3.5 Ga despite a ~25-30% increase in solar luminosity. Generally, it is hypothesized that higher concentrations of greenhouse gases (CH4 and its hydrocarbon derivatives and CO2) countered the fainter sun, perhaps assisted by a lower albedo. Geochemical constraints on the levels of greenhouse gases are limited but they generally support the interpretation that CO2 alone was insufficient to have kept the Archean Earth warm. Widespread acceptance that levels of atmospheric O2 were very low -- probably below 1 ppmv -- before 2.45 Ga implies an importance for CH4, which can attain a relatively high abundance of hundreds to thousands of ppmv in an anoxic atmosphere compared to 1.8 ppmv today. Unfortunately, the history of nitrogen, the main constituent of the atmosphere, is largely unconstrained. Pathways in the modern nitrogen cycle depend on oxidized species, which implies that the Archean nitrogen cycle should have been different. But how partial pressures of N2 were affected remains to be determined.

Sessions et al. (2009).  Review: The continuing puzzle of the Great Oxidation Event, Current Biology

 

Chauvel, Catherine - Radiogenic Isotopes

Presentation of the way radiogenic isotopes can be used to trace the early evolution of the Earth.  Radiogenic isotopes are used for dating both oceanic and continental materials but they also provide information about the time-integrated history of the source material. The data available on the oldest rocks and minerals will be used as an example to show the constraints provided on the timing of the formation of continental material from the mantle and its relationship to the evolution of the depleted mantle. The paper recently published by Kemp et al. will be used as an example.

Kemp et al., 2010. Hadean crustal evolution revisited: New constraints from Pb–Hf isotope systematics of the Jack Hills zircons

 

Corfu, Fernando - U-Pb geochronology of Archean rocks

The lecture will provide: (1) an outline of the basic principles of the U-Pb decay system, elaborating the main advantages and technical challenges of its application to dating; (2) an overview of the minerals that can be used, chiefly zircon, but also complementary ones such as titanite and monazite, and discuss their peculiarities as geochronometers and their behaviour in response to mineralogical processes, also dealing with the effects of radioactive damage; (3) a discussion of the type of rock and rock forming processes that can be dated: (4) a short description of the various techniques (ID-TIMS, SIMS, LA-ICP, Kober, and CHIME). The concluding part will use selected examples to demonstrate the potential of the method, to compare results by different methods, and also to show some of the pitfalls and limits of the method.

 

Derenne, Sylvie - Organic biomarkers

Bringing evidence for life in the oldest terrestrial rocks has been a challenge for many years. Some Archaean rocks are known to contain tiny amounts of organic matter which can be studied by the large array of techniques used in organic geochemistry. Two main aims drive these researches: clues for biogenicity and syngenicity. First of all, it must be known that two fractions are commonly distinguished in organic matter, the one which is soluble in the organic solvents and the insoluble one. The advantages and drawbacks in studying one fraction or the other one will be discussed. The techniques that can be used to decipher the chemical structure of the organic matter encompass microscopy, spectroscopy and degradations. In this lecture, I will present these techniques highlighting their potentials and limitations.

Derenne et al. (2008) Molecular evidence for life in the 3.5 billion year old Warrawoona chert

 

Garde, Adam - The art of observation in geology

Observations on all scales are a fundamental part of geology. All of us like to think that we are capable of making clear and unbiased observations, record them accurately, and use them to understand geological processes. However, this is an illusion. Our brains are built to sort and simplify what we see. We see what we are trained to look for, and we may overlook or neglect what we do not understand. What we see is also influenced by what we want to see, and we bias our observations to fit our favourite models ¬- both unknowingly and pressed by the need to publish.

Windley and Garde 2009. Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: Crustal growth in the Archean with modern analogues

 

Kremer, Barbara -  Geological significance of cyanobacteria and their fossil record

Cyanobacteria are among the Earth’s most ancient organisms with a unique fossil record that extends from the Archean to present. The oldest cellular remains of cyanobacteria are reported from ca. 3.5 Ga early Archean sediments in Western Australia (~3.49 Ga old Dresser Formation and cherts of the ~3.46 Ga Apex Basalt), but their identity remains controversial. Cyanobacteria are only under exceptional circumstances preserved as carbonaceous cellular microfossils. Usually, they leave their geological imprint as biosedimentary structures called microbialites (mostly referred to stromatolites). Cyanobacteria transformed the pre-2.5 Ga neutral-reducing Earth’s atmosphere into an oxygenated surface zone with a stratospheric UV screen. These conditions  opened the way for the development of aerobic, eukaryotic life. Cyanobacterial mats form in a vast variety of modern environments (marine: North Sea; quasi-marine: Indonesia; soda lake: Van, Turkey; caldera lake: Niufo’ou, Tonga) reflecting the generalist nature of these organisms. Fossil cyanobacterial mats can be used as paleo-environmental indicators by recording bathymetry, illumination, redox potential, pH-range, metal stress and eutrophication level. The wide distribution in both geography and geologic time of outcrops of different marine sediments which can be directly or indirectly attributed to the presence of benthic cyanobacterial mats demonstrates the crucial role of these microorganisms in the evolution of Earth’s biosphere. Shallow-water (intertidal) calcareous cyanobacterial microbialites from late Archean sediments of South Africa, relatively deep water siliceous cyanobacterial mats from early Silurian of southern Poland (Holy Cross Mts. and Sudetes) and several other example of modern and fossil cyanobacterial mats will be used as examples of just such dramatically different paleo-environments occupied by microbialite-forming benthic cyanobacteria over geologic time. 

Altermann, W. & Kazmierczak, J. 2003 Archean microfossils: a reappraisal of early life on Earth. Research in Microbiology 154: 611–617

 

Lowe, Don -  Techniques for the study of Archean greenstone belts

Archean greenstone belts are structurally complex, volcanic-sedimentary sequences embedded within the basement of most large Archean blocks of continental crust. They have provided most of the direct information that we have about the Earth's early surface system and life. The interpretation of greenstone belts is complicated by a number of factors: (1) The rocks are stratigraphically and structurally complex and there is a paucity of reliable geochronological data over large areas. A basic knowledge of lithology and age, with or without stratigraphic and structural controls, often provides some basis for interpretations, such as the existence and implications of ultramafic lavas (komatiites) and possible microfossils in cherty sedimentary layers. However, many interpretations involve concepts of variation in time and/or space and the relative arrangement of rock types. These studies require maps of rock distribution, structure, and stratigraphy. The results of these studies, because of greenstone belt complexity and the paucity of geochronological tools, are inevitably highly interpretative. Pictures of Barberton greenstone stratigraphy have ranged a more-or-less intact 10-km-thick stack of volcanic and sedimentary rocks to a 500-1000 m thick sequence repeated by bedding-parallel thrust faults.  High-resolution zircon geochronology has helped to resolve key issues of structure and stratigraphy, but is expensive and limited to sections where appropriate felsic volcanic or detrital units are available. (2) The Archean was a non-uniformitarianistic world. Water still ran downhill but many of the surface environments and deposits were quite different from those recognized in younger geologic sequences. Many sedimentary and some igneous rock types, such as banded iron formation, komatiites, and many chert types, are rare or absent in the Proterozoic and Phanerozoic or may have different origins today than in the ancient past. Basic field observations and stratigraphic relations often provide key clues about the origins and implications of these rocks. The use of analogs and appeals to features and processes of younger geologic time are risky and often lead to problematic outcomes, as illustrated by the controversy surrounding the biological versus abiological interpretation of the C-isotopic composition of Archean carbonaceous matter. (3) While textural features are commonly well preserved in areas of low strain, profound early diagenetic and metasomatic alteration have compromised the original mineralogy and geochemistry of many rocks. Fine-grained volcaniclastic layers that occur throughout pre-3.0 Ga greenstone belt sequences and silica- and potash-rich zones capping many ultramafic volcanic units have been interpreted, based on major element geochemistry, as felsic tuffs and felsic caps to mafic-to-felsic volcanic cycles, respectively.  Subsequent field observations, such as ghost spinifex textures in the volcanic units, and trace element and REE geochemistry show that these are both metasomatically altered ultramafic rocks. Key to the successful study of greenstone belt rocks is the selection of the appropriate combination of field and laboratory methodologies, often involving collaborations of investigators with a wide range of skill sets. Silver bullets, single techniques that will provide unique, unambiguous, correct answers to major questions, are elusive.

Lowe et al, 2003 Rubey Colloquium Paper Spherule Beds 3.47–3.24 Billion Years Old in the Barberton Greenstone Belt, South Africa: A Record of Large Meteorite Impacts and Their Influence on Early Crustal and Biological Evolution

 

Mojzsis, Stephen - Continental Crust Evolution and its Relation to the Early Surface Environment

The traditional notion of the Hadean – Eoarchean transition on Earth (ca. 4.1 – 3.8 Ga) held by geologists and biologists is that this was an unfavorable time for the emergence of life, or that life could have been extinguished and was forced to reappear when conditions became more favorable. Knowledge of key constraints on earliest surface environments of Earth was lacking. Numerous changes to this perspective have occurred in the last decade because of geochemical studies of the oldest zircons, conjectures based on daughter products of both long- (e.g. 176Lu-176Hf) and short- (e.g. 146Sm-142Nd) lived radionuclides preserved in the oldest rocks and minerals, and increasingly more sophisticated geophysical and geochemical modeling studies. Present thought places the establishment of the lithosphere, appearance of the granite-basalt dichotomy and generation of a sedimentary cycle well before the end of the Hadean. Evidence has also mounted to the degree that it can be said that there existed a hydrosphere of some kind which (i) hydrated the crust, (ii) facilitated a mature rock cycle and (iii) initiated continental crust formation since within ~150 Myr of the accretion of the planets (and perhaps earlier). Yet, with the exception of Hadean zircons (≤4.38 Ga) found in younger sediments, a geological record from actual rocks of the first 500 Myr is apparently absent. It may simply be that the rocks have not been discovered yet, or more pessimistically, that they were destroyed long ago. It is not until the oldest granitoid gneiss complexes appear with igneous (~4.02 Ga) and sedimentary (≥3.83 Ga) protoliths that some direct information about potentially habitable planetary conditions arises. At the terminus of the Hadean eon, Earth may have experienced the “Late Heavy Bombardment” (LHB) of comets and asteroids. Almost immediately after the LHB, metamorphic equivalents of marine volcano-sedimentary rocks are preserved within granite- and granitoid gneiss complexes which probably represent small continents.

Harrison (2009) The Hadean Crust: Evidence from >4 Ga Zircons

 

Mojzsis, Stephen - Early Life: Stable Isotope Paleontology

The quest for a record of early biological activity on Earth is hampered by the rarity of a sedimentary rock record from before about 3.5 Ga; those rocks which do exist from that earlier time have all been transformed by metamorphism.  The result is the absence of morphological features that could bear on the presence of an early biome (stromatolites, microfossils). Hence, we are left with chemical and isotopic indicators of past life as our sole direct resource to search for the most ancient biological activity. It helps that elements common to life (with the exception of P), have multiple stable isotopes that tend to be strongly fractionated relative to inorganic reservoirs (e.g. mantle, atmosphere, oceans) by metabolic activity. The concept of stable isotope “paleontology” through the use of light elements (principally C, N, S) has a long history, and work in the last decade on the so-called “non-traditional” stable isotopes of the transition metals (Fe, Ni) has been used to probe biogeochemical processes in modern and ancient rocks. I will provide a brief review of the history of isotope biosignatures and explore some past criteria for better or for worse that has been used to test for the presence of early life in the oldest rocks. For example, although a several possible abiotic pathways exist that lead to the synthesis of various organic and reduced carbon compounds depleted in 13C, the abiotic explanation is less satisfactory if fractionated C isotopes can be correlated with other stable isotope indicators (N, S, Fe, Ni, etc.) in the same rock that also show isotopic discriminations consistent with life. It makes sense now to integrate different stable isotopic systems for constraining the role (if any) of biology at the time of the formation of the oldest sedimentary systems for which we have a record.

Thomazo et al. 2009 Biological activity and the Earth’s surface evolution: Insights from carbon, sulfur, nitrogen and iron stable isotopes in the rock record.

 

Nisbet, Euan – Early life/ early environments

Modern air is a biological creation, excepting Helium. Nitrogen, oxygen, carbon dioxide, and methane are all dominantly biologically cycled, and water vapour is controlled by greenhouse feedbacks. When did biological control begin? The Hadean atmosphere must have been inorganic: accretion from space, degassing from the interior, modified by chemical and photochemical processes. Key steps in biological control of the air must have occurred in the Archaean. Then, biological activity remade the air, which was probably initially anoxic. Archeal methanogenesis may be very ancient. In anoxic air, ethane's atmospheric  lifetime would have been long, and methane flux from methanogenic archaea not necessarily greater than today. Rubisco II anoxygenic and then rubisco I oxygenic photosynthesis likely evolved in the Archaean, remaking the atmosphere. Release of free O2- into the water would have created oxic surface waters, challenging but not necessarily overwhelming the methane greenhouse until atmospheric lifetimes changed. After the Great Oxidation Event around 2.3 to 2.4 billion years ago, perhaps linked to glaciation, the atmosphere itself became oxic and eukaryote evolution possible. By the early Proterozoic, all the key biochemical processes that maintain the modern atmosphere were probably present in the microbial community.

Nisbet, E.G., Grassineau, N.V., Howe, C.J., Abell, P.I.,  Regelous, M., and Nisbet, R. E. R.  (2007) The age of Rubisco: the evolution of oxygenic photosynthesis Geobiology. 5, 311-335.

 

Ohmoto, Hiroshi – Oxygenation of the atmosphere and oceans on early Earth

Current popular theory for the evolution of oxygen in the atmosphere and oceans postulates the following scenario: (1) oxygenic photoautotrophs evolved by ~2.7 Ga but the atmospheric pO2 level remained below ~1 ppm (10-6 atm) until ~2.4 Ga; (2) the atmospheric pO2 rose to ~10% PAL and shallow oceans became oxygenated at ~2.4 Ga, but deep oceans remained anoxic until ~600 Ma; and (3) the atmospheric pO2 level rose to ≥~50% PAL and the entire oceans became ~600 Ma and remained the same since then. Major lines of evidence cited to support this theory include (but are not restricted to) the geologic records of: (a) MIF-S (D33S and D36S); (b) d34S of sedimentary rocks; (c) BIFs; (d) detrital grains of uraninite and pyrite; (e) U and Mo in black shales; and (f) Fe in paleosols. I will explain why these mineralogical/geochemical data may not be linked to the evolution of atmospheric O2. I will also present various types of mineralogical and geochemical data that support a theory postulating the early (>3.5 Ga) development of a fully oxygenated atmosphere-ocean system. They include (but are not restricted to): (a) the abundance of primary ferric (hyd)roxides (goethite, hematite) and the enrichments of Mn, U, V, Mo, Cr, and Ce with the ferric (hyd)oxides in deep-sea Archean hydrothermal systems; (b) the behaviors of redox sensitive elements in ~3.4 Ga paleosols; (c) the abundance of organic C, pyrite and barite in Archean sedimentary rocks; (d) the d13C record of sedimentary carbonates and organic C. The atmospheric O2 level has been regulated to within ±50% of the present level since >3.5 Ga through two negative feedback mechanisms: (1) changes in the burial flux of organic C in response to changes in atmospheric pO2; and changes in the O2 consumption flux by soil formation in response to changes in atmospheric pO2.

Ohmoto et al. 2008 Biosignatures in Ancient Rocks: A Summary of Discussions at a Field Workshop on Biosignatures in Ancient Rocks

 

Poulton, Simon - Ocean Redox: Reading the Archean Rock Record

The Archean ocean has traditionally been assumed to have been anoxic and rich in dissolved iron (ferruginous). This assumption is, in large part, based on the processes thought to have been responsible for the deposition of banded iron formations. Over the last few years, however, the development of new and refined techniques for evaluating paleodepositional redox conditions has opened up the possibility for more detailed investigation of the temporal and spatial evolution of Archean ocean chemistry. Techniques such as Fe speciation and trace metal concentrations and ratios have been used to track both the onset of oxygenated or partially-oxygenated surface waters, and the occurrence of euxinic conditions within an otherwise ferruginous ocean. Our understanding of the most robust paleoredox indicators is firmly rooted in biogeochemical studies of modern environments, and while significant progress have been made with regard to their application to the ancient rock record, there remains a tendency to apply paleoredox indicators either blindly or incorrectly. This talk will provide an overview of the process controls underpinning some of the more useful paleoredox indicators, and will highlight both positive and negative aspects of their application to the Archean rock record.

Anbar, A.D. et al., (2007) A whiff of oxygen before the Great Oxidation Event.

 

Rosing, Minik – Crustal growth rates and models

MISSING

 

Simionovici, Alex – Synchotron techniques. X-ray methods in Earth and Planetary Sciences

Since the advent of 3rd generation synchrotrons (ALS Berkeley, ESRF Grenoble, APS Argonne, Spring 8 Japan…) X-ray methods have been optimized to analyze ever smaller, more diluted or more complex samples. X-rays are natural probes for the least-destructive, yet penetrative, sensitive analysis methods. X-rays can be combined to yield simultaneous diagnostics of complex samples. Furthermore, X-rays can be focused  to perform high resolution imaging of samples using two or more X-ray probes, spanning the scales from tens of  nanometers to hundreds of micrometers. Last but not least, high intensity (1012 ph/s) , monochromatic energy beams can be produced for analyzing trace and ultratrace elements.

Fluorescence spectroscopy can be used to for quantitative element analysis of samples at concentrations down to ppb and absolute detection limits of few ag for Z≥ 10. Mineral assemblages of crystalline powders with up to few hundred nm crystallites can be identified and quantified by WAXS (Wide Angle X-ray Diffraction) methods. Speciation is used in either absorption respectively fluorescence modes for dilute or concentrated samples to retrieve the chemical environment and symmetry of molecules using the XANES (X-ray Near Edge Absorption Spectroscopy) method.

Finally, tomography is used to image the sample absorption coefficient, directly linked to the sample density in 2D or 3D modes.

Combining all these probes, hyperspectral imaging is performed with focused beams to simultaneously scan samples using for example fluorescence and diffraction or fluorescence and tomography. A 2D/3D image of the sample can then be produced using sophisticated reconstruction methods, in the scale ranges mentioned before.

Thus, an unrivalled rich rendering of the sample composition, mineralogy, morphology and chemistry can be obtained in a relatively short time and samples can then be used for highly sensitive destructive analyses such as isotopic ratios, chronology, etc.

The presentation will show basics as well as examples of these applications on Planetary Sciences samples. The tutorial will be based on the European Synchrotron Radiation Facility PyMCA free software to perform qualitative and quantitative X-ray fluorescence analysis of a chondrite meteorite. Finally, the imaging of sub-assemblages of a chondrule at a resolution of one micron will be shown.

Software : http://sourceforge.net/projects/pymca/files/

V.A. Solé, E. Papillon, M. Cotte, Ph. Walter, J. Susini, A multiplatform code for the analysis of energy-dispersive X-ray fluorescence spectra, Spectrochim. Acta Part B 62 (2007) 63-68. 

 

 

Sleep, Norm - Temperature of the early Earth, faint young sun paradox

Astrophysical results indicate that the Sun was ~70% has bright has present in the Hadean and ~80% as bright in the Archaean. Climatic modeling indicates that the Earth would have been ice covered had its atmosphere and albedo been the same at present. Yet the geological record indicates that liquid water was present as soon as we have a record that might record it, ~4.4 Ga and that glaciation was rare until the end of the Archaean. Archaean sediments provide constrains but little agreement. We do not know the temperature of the Archaean climate, the total atmospheric pressure, the greenhouse gas, nor the chemistry of the oceans. The straightforward interpretation of oxygen and silicon isotopes in Archaean chert is indicates hot ~70°C climate. The tendency for quartz to weather mechanically indicates clement conditions. The existence of banded iron formation (BIF) provides direct constraints on atmospheric pCO2. The formation and persistence of magnetite-siderite requires aPCO2 a few times present level. Fe++ is insoluble at high pCO2 and oceanic pH precluding open water BIF and reef carbonates in the first place. A lower albedo from fewer continents and clouds, a ~2-3 bar N2 atmosphere, and black daisy Gaia effects from biota fresh basalt on land all contributed to maintaining clement Archaean conditions at modest pCO2 and pCH4.

Rosing et al. 2010 No climate paradox under the faint early Sun

 

Stevens, Gary - Metamorphic analysis of Archean terranes

Despite the fact that many Archean sections are dominated by highly deformed rocks, there is little consensus on the crustal scale processes that drive this deformation.  Well constrained studies of pressure-temperature-time evolution of relevant mineral assemblages can provide definitive answers; however, such studies are very rare in the Archean record. Pressure determination during crustal metamorphism commonly relies on the presence of garnet in the rocks and garnet is only produced under very high pressure conditions in the typical metamafic rocks that commonly constitute much of the greenstone belt record. Where garnet can be found, often within rare layers of more Al- and Fe-rich composition, much can be done to constrain the detailed pressure-temperature evolution of the rocks. This contribution examines the fields of garnet stability within various metamafic rocks of the lower portions of the stratigraphy of the Barberton greenstone belt. Best strategies for the construction of relevant pseudosections will be investigated and the results, in terms of garnet mode and compositional isopleths, will be compared with garnet proportion, composition and compositional zoning information, from the amphibolite facies, Stolzburg block that forms the southern boundary of the Barberton greenstone belt. Different interpretations of this information will be presented and analyzed, with a view to exploring the constraints the metamorphic information provides on the crustal scale processes that have shaped the Barberton greenstone belt.

Brown (2006) Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean. Geology; 34;961-964.

 

 

Van Zuilen, Mark - Carbon in Archean rocks

The search for traces of early life has been hampered by poor preservation of organic structures in the Archean rock record. Sedimentary biologic material preserved in greenstone belts has been subject to diagenesis, hydrothermal alteration and metamorphism. The combined effect of these processes has slowly converted original organic structures, such as bacterial cell walls, to an insoluble macromolecular carbonaceous material (kerogen) that has lost most of its original functional groups. In highly metamorphosed terrains this kerogen has ultimately converted to crystalline graphite. For our understanding of Archean life the important questions to be answered are: To what extent can we still recognize a biologic origin for carbonaceous matter in metamorphosed terrains? Are there abiologic processes that lead to the formation of carbonaceous matter (kerogen, graphite)? Can these abiologic processes be recognized and excluded? How do we exclude modern artifacts, such as extant microbiota that live in the rocks we study? A range of in-situ analytical techniques is now available for detailed, structural, chemical and isotopic characterization of microscopic organic structures. In this presentation I will focus on two of these techniques; Raman spectroscopy and secondary ion mass spectrometry. Raman spectroscopy is a highly effective technique for identification of indigenous organic microstructures in Archean terrains and for ruling out opaque mineral inclusions and post-metamorphic organic contamination. Secondary Ion Mass Spectrometry (SIMS) utilizes a Cs+ beam to ablate small 10-50 μm pits in polished rock samples, and is therefore highly suitable for in situ carbon isotope analysis or elemental mapping of organic microfossil structures. Some examples of the application of these techniques to the study of organic structures in Archean greenstone belts will be discussed.

Tice et al. 2004 Thermal history of the 3.5–3.2 Ga Onverwacht and Fig Tree Groups, Barberton greenstone belt, South Africa, inferred by Raman microspectroscopy of carbonaceous material

Oehler et al. 2009 NanoSIMS: Insights to biogenicity and syngeneity of Archaean carbonaceous structures

 

 

Windley, Brian - Tectonic Style and Evolution of the Archaean Lower Crust

The lower crust of Archaean orogenic belts worldwide mostly consists of homogeneous, foliated gneisses in the granulite or high amphibolite facies, for which it has been difficult to apply plate tectonic models. What can you do with just a bunch of homogeneous gneisses? The answer lies in the other rocks that occur as relicts in the gneisses, such as meta-volcanic amphibolites derived from island arcs or the ocean floor, and of layered complexes with anorthosites and gabbros that formed in the magma chambers of island arcs. Deformation and intercalation of these rocks led to a characteristic tectonic style, as in West Greenland, which is amenable to plate tectonic analysis. A remaining question is: can similar rocks and tectonic style be found in younger orogenic belts?

Windley and Garde 2009. Arc-generated blocks with crustal sections in the North Atlantic craton of West Greenland: Crustal growth in the Archean with modern analogues