Archean geology

For the southern part of the Siberian Craton, the geological and geochronological characteristics of the reference sections of six Lower Precambrian stratoregions are presented. These sections were correlated for the first time, which served as a basis for a refined regional stratigraphic scheme. As a result, three major regional subdivisions (from bottom to top, Sahabori, Udokan, and Ulkan) were separated in the rank of suprahorizons in the regional stratigraphic scheme, as in the scheme accepted by Interdepartmental Stratigraphic Committee in 1991, in terms of their composition and age. For the first time, two regional horizons within the Udokan suprahorizon were proposed with the operating names Kodar-Chinei (lower) and Kemen (upper). These regional subdivisions are separated by unconformities and stratigraphic gaps. It can be assumed that the lower boundary age of the Kemen horizon is around 2.05 Ga; the lower boundaries of the Udokan and Ulkan suprahorizons date back to around 1.9 Ga. It is likely that these suprahorizons were separated by a long stratigraphic hiatus. The stratigraphic boundaries between the Sahabori suprahorizon and the lower Udokan suprahorizon have not been determined. According to the isotopic age data, the regional subdivisions are correlated with those in the General Stratigraphic Scale of the Lower Precambrian of Russia. The Sahabori suprahorizon undoubtedly belongs to the Upper Archean (Lopian) Eonothem, while the Udokan and Ulkan suprahorizons belong to the Lower Proterozoic (Karelian) Eonothem. The undivided Lower and Upper Karelian erathems include the rocks of the Kodar-Chinei horizon. The deposits of the Kemen horizon and Ulkan suprahorizon belong to the Upper Karelian Erathem.

We investigated microfabrics of shocked Archean gneisses from two, 10 m-deep drill cores located near the center of the Vredefort impact structure in an area that is characterized by a prominent, long-wavelength negative magnetic anomaly (< − 3000 nT) together with short-wavelength, high-amplitude anomalies attributed to lightning strikes. Planar fractures and feather features in quartz, which can be partially recrystallized, indicate shock conditions less than 20 GPa. Micrometer-sized magnetite and ilmenite along shock-related shear fractures in quartz and feldspar emanate from adjacent deformed coarse (> 100 µm) ilmenite and magnetite host grains. These fine-scaled veins suggest mobilization of magnetite and ilmenite during shear deformation of host Fe-phases and adjacent silicates, probably associated with frictional heating. Coarse ilmenite has fine-lamellar mechanical twins parallel to {10$$\overline{1}$$ 1 ¯ 1} and single (0001) twins, indicative of dislocation-glide-con...

A gold prospect on central Storø in the Nuuk region of southern West Greenland is hosted by a sequence of intensely deformed, amphibolite facies supracrustal rocks of late Mesoto Neoarchaean age. The prospect is at present being explored by the Greenlandic mining company NunaMinerals A/S. Amphibolites likely to be derived from basaltic volcanic rocks dominate, and ultrabasic to intermediate rocks are also interpreted to be derived from volcanic rocks. The sequence also contains metasedimentary rocks including quartzites and cordierite-, sillimanite-, garnet- and biotite-bearing aluminous gneisses. The metasediments contain detrital zircon from different sources indicating a maximum age of the mineralisation of c. 2.8 Ga. The original deposition of the various rock types is believed to have taken place in a back-arc setting. Gold is mainly hosted in garnet- and biotite-rich zones in amphibolites often associated with quartz veins. Gold has been found within garnets indicating that th...

High-temperature partitioning of the stable isotopes of rock-forming elements like Mg, Si, Fe, Ni and others are useful new tools in geochemistry and cosmochemistry. Understanding the fundamental driving forces for equilibrium inter-mineral fractionation comes from basic crystal chemistry and is invaluable for interpreting data from natural systems. Both charge and coordination number are key factors affecting bond length and bond stiffness and therefore the relative proclivity of a mineral phase for concentrating heavy or light isotopes. Quantitative interpretation of the plethora of new data relies on refinements of equilibrium fractionation factors through a feedback between crystal chemical reasoning, ab initio predictions, experiments, and analyses of well-characterized natural samples. This multifaceted approach is leading to a rapid rate of discovery using nontraditional stable isotopes in high temperature systems. For example, open-system mass transfer in the mantle is becoming increasingly evident from departures from equilibrium Mg and Fe isotope ratio partitioning between minerals, and differences in isotope ratios between bulk silicate Earth and meteorites are elucidating the conditions for Earth's core formation quantitatively. These applications rely critically on accurate equilibrium fractionation factors.

This study presents the first geochronological results on basement rocks from the Penguin-Bage-Webb (PBW) domain located east of the Neoarchean-Paleoproterozoic Terre Adélie craton, Antarctica. Investigated samples are paragneiss xenoliths hosted within early Paleozoic granitoids, which were emplaced during the Ross orogeny. Zircon UPb dating yielded ages ranging from the Archean to the Cambrian, with a dominant Ediacaran (550-635 Ma) population and maximum depositional ages around 570-575 Ma. U-Th-Pb analyses of monazite suggest that the metamorphic event that formed the gneiss samples occured at ca. 515 Ma, shortly prior to incorporation within the granitic magmas. The studied samples likely represent relics of the pre-Gondawana Pacific margin, which was subsequently deformed and metamorphosed during the early Paleozoic Ross orogeny. The obtained zircon UPb date distributions present similarities with those of the Kanmantoo and Nargoon sediments in Southern Australia and provide new constrains for the correlations between East Antarctica and South Australia before the opening of the Southern Ocean. ► Zircon and monazite U Pb ages of the gneiss xenoliths from eastern George V Land ► Inherited ages ranging from Archean to Cambrian with main Ediacaran population ► Relics of a pre-Gondwana pacific margin metamorphesed during the Ross orogeny Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.

More than 15 years of field and laboratory investigations on samples from Terre Adélie to the western part of George V th Land (135 to 146°E) during the GEOLETA program allow a reassessment of the Terre Adélie Craton (TAC) geology. The TAC represents the largest exposed fragment of the East Antarctic Shield preserved from both Grenville and Ross tectono-metamorphic events. Therefore it corresponds to a well-preserved continental segment that developed from the Neoarchean to the Paleoproterozoic. Together with the Gawler Craton in South Australia, the TAC is considered as part of the Mawson continent, i.e. a striking piece of the Rodinia Supercontinent. However, this craton represents one of the less studied parts of the East Antarctic Shield. The three maps presented here clearly point out the extent of two distinct domains within the Terre Adélie Craton and suggest that the TAC was built up through a polyphased evolution during the Neoarchean-Siderian (c.a. 2.5Ga) and the Statherian (c.a. 1.7Ga) periods. These data support a complete re-assessment of the TAC geology and represent a valuable base for the understanding of global geodynamics changes during Paleoproterozoic times.

Plummer et al. set-up is suitable to study the induced RCA tufa precipitation. Garrels-Mackenzie model predicts the composition and mechanism of RCA tufa formation. Calcium sulfate compounds are dominant in evaporative RCA Tufa. Calcium carbonate compounds are dominant in artificially CO 2 saturated environments. Formation of evaporative tufa is more favorable for using RCA in construction.

Hydrothermal activityis considered to be responsible for ubiquitous silicification of Archean supracrustal rocks. Here, we show well-preserved evidence of hydrothermal silicification with in the Dixon Island Formation (3.2Ga) of Pilbara Craton, Australia.The formation is a part ofimmature oceanic island arc origin Cleaverville Group consists of three cycles of volcano-sedimentary sequence (Kiyokawa and Taira, in press). Each sequence consists of thick volcanics (basalt,rhyolite), black shale, ferruginous chert and BIF to the top. Especially, the Dixon Island Formation, which is the best exposed sequence, is composed of

The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 µg/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates . On average, granites are missing $60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings.

In the Halls Creek Orogen of north-western Australia, the distance of melt migration through migmatitic metasedimentary rocks and adjacent metabasites is partly constrained by relationships of leucosomes and small mafic magma veins toJrock boundaries and structural elements. Stromatic leucosomes in metasediments are cut by a network of small extensional fractures and shear zones, oriented steeply during melt migration. These shear zones allowed cm-to 10 m-scale migration of felsic magma derived by in situ anatexis. In the adjacent metabasite layers, a similar shear array allowed injection of H20-undersaturated mafic to ultramafic magma, locally dehydrating and chemically modifying these rocks. However, these mafic to ultramafic veinlets are too mafic to be explained by in situ anatexis, necessitating an external magma source. Also, the lack of felsic veinlets cutting metabasites, and mafic veinlets cutting metasediments, requires that vertical inter-connectivity of these fracture systems was restricted. We propose along-layer migration of mafic to ultramafic magma through the metabasite, assisted by horizontal connection of the shear zones. This migration occurred independantly of metre-scale felsic magma migration in the adjacent metasediments, even though these two deformation-assisted magma migration systems may have been operating at the same time.

In the Halls Creek Orogen of north-western Australia, the distance of melt migration through migmatitic metasedimentary rocks and adjacent metabasites is partly constrained by relationships of leucosomes and small mafic magma veins to rock boundaries and structural elements. Stromatic leucosomes in metasediments are cut by a network of small extensional fractures and shear zones, oriented steeply during melt migration. These shear zones allowed cm- to 10 m-scale migration of felsic magma derived by in situ anatexis. In the adjacent metabasite layers, a similar shear array allowed injection of H2O-undersaturated mafic to ultramafic magma, locally dehydrating and chemically modifying these rocks. However, these mafic to ultramafic veinlets are too mafic to be explained by in situ anatexis, necessitating an external magma source. Also, the lack of felsic veinlets cutting metabasites, and mafic veinlets cutting metasediments, requires that vertical inter-connectivity of these fracture s...

Summary In the northeastern part of the Slave Craton (Bathurst Block), the NE-striking Elu Belt (EB) is linked to the adjacent N-striking Hope Bay Belt (HBB) by an arcuate E-W connection zone termed the Elu Link (EL). The EL provides new insights into key geological relationships in the ca. 2700-2600 Ma EB and HBB. A preliminary geochemical study of mafic meta-igneous rocks from EL and HB belts shows they are similar and represent tholeiitic to calc-alkaline basalts and basaltic andesites. These rocks represent three magmatic suites, mainly Ti-depleted flat REE, Ti-enriched flat REE, and Ti+LREE enriched magmatic systems. Like mafic rocks in the Yellowknife belt, rocks in the EL and HBB are inferred to have derived from deep to shallow mantle sources and emplaced in a back-arc setting over attenuated continental crust.

A new type of gold deposit is described for the Amazon Craton, in the Carajas greenstone belt province. Instead of mesothermal mylonite-hosted lodes or exhalative deposits already recognized in the Carajas Province, the Cumuru gold deposit is hosted by a calc-alkaline 2.82 Ga granodiorite pluton of volcanic arc affiliation. The Cumaru granodiorite is intruded in the horse-tail dilation sites of a shear zone that crosscuts the volcano-sedimentary mylonitic rocks of the Archean Gradaus greenstone belt. The Cumaru gold-bearing quartz veins lie along NE-SW trending fractures at the NW edge of the granodiorite pluton where disseminated ore in a stockwork array of veinlets is associated with the main lodes in a groundmass of hydrothermal phyllic alteration. Three kinds of fluids were identified in the Cumaru quartz veins. An aquo-carbonic fluid, immiscible at the time of ore formation, is interpreted as a metamorphic fluid related to shear zone. The second type of fluid comprises brines w...

Um novo tipo de depósito aurífero foi descrito para o Craton Amazônico, nos greenstone belts da Província Carajás. Em vez de veios mesotermais, hospedados em milonitos, ou depósitos exalativos, já reconhecidos na Província Carajás, o depósito auríferó do Cumaru é hospedado em um pluton granodiorítico cálcio-alcalino arqueano, típico de arcos vulcânicos. O Granodiorito Cumaru colocou-se em domínios transtensivos de uma zona de cisalhamento tipo rabo de cavalo, cortando rochas vulcano-sedimentares miloníticas do greenstone belt arqueano de Gradaús. Os veios de quartzo auríferos do Cumaru ocorrem em fraturas com direção preferencial NE/SW, na borda NW do pluton do Cumaru, envolvidos por um enxame de filonetes em padrão stockwork, com mineralização disseminada em uma matriz de alteração fflica. Três tipos de fluidos foram identificados nos veios de quartzo do Cumaru. Fluidos aquo-carbônicos, imiscíveis no momento de seu aprisionamento, foram interpretados como fluidos metamórficos relacionados à zona de cisalhamento. O segundo tipo de fluido é representado por inclusões de salmouras do sistema H 2 O-NaCl-KCl-CaCl 2 , interpretadas como derivadas dos fluidos residuais do magma granodiorítico. Inclusões aquosas tardias de baixa salinidade correspondem ao terceiro tipo de fluido, considerado como água meteórica. As condições de T-P para a deposição do ouro, estabelecidas pelo geotermômetro da clorita e isócoras calculadas a partir dos dados microtermométricos das inclusões fluidas, situam-se entre 300-350°C e l ,3-3,8Kb. Os baixos valores da fO 2 do fluido mineralizante e a paragênese sulfetada do minério aurífero sugerem que o enxofre encontrava-se em estado reduzido no momento da formação do depósito, condições que favorecem o transporte do ouro como tiocomplexos. Dois eventos de deposição do ouro foram identificados no depósito do Cumaru, ambos relacionados com oxidação do fluido mineralizante. Inicialmente a oxidação foi causada pela imiscibilidade do fluido aquo-carbônico e pelo abaixamento da f$2 (sulfetação) nas rochas hospedeiras. A mistura tardia do fluido aquo-carbônico com as salmouras também provocou oxidação do fluido mineralizante, com aumento da fO2 e diminuição do pH, favorecendo a deposição do ouro. Análises de isótopos estáveis forneceram importantes indicações sobre as fontes dos fluidos mineralizantes. Os valores de δ 18 O e δD sugerem mistura de águas metamórficas e magmáticas, enquanto que os dados de δ 13 C são compatíveis com fluidos carbônicos de derivação mantélica. As características geológicas, estruturais e geoquímicas, assim como os dados de inclusões fluidas e isotópicos do depósito do Cumaru são consistentes com depósitos auríferos arqueanos tipo Iode, hospedados em greenstone belts, que ocorrem em zonas de cisalhamento, como também com mineralizações de estilo porfirítico típicas de arcos magmáticos fanerozóicos. Tal situação, sugere que tanto a zona de cisalhamento como a intrusão granodiorítica atuaram na gênese do depósito de ouro mesotermal do Cumaru, incluído em uma nova classe de depósito aurífero denominada aqui de "Iode porfirítico".

We present detailed petrographic surveys of apatite grains in association with carbonaceous material (CM) in two banded iron formations (BIFs) from the Paleoproterozoic of Uruguay and Michigan for comparison with similar mineral associations in the highly debated Akilia Quartz-pyroxene (Qp) rock. Petrographic and Raman spectroscopic surveys of these Paleoproterozoic BIFs show that apatite grains typically occur in bands parallel to bedding and are more often associated with CM when concentrations of organic matter are high. Carbonaceous material in the Vichadero BIF from Uruguay is generally wellcrystallized graphite and occurs in concentrations around 0.01 wt% with an average d 13 C gra value of À28.6 ± 4.4& (1r). In this BIF, only about 5% of apatite grains are associated with graphite. In comparison, CM in the Bijiki BIF from Michigan is also graphitic, but occurs in concentrations around 2.4 wt% with d 13 C gra values around À24.0 ± 0.3& (1r). In the Bijiki BIF, more than 78% of apatite grains are associated with CM. Given the geologic context and high levels of CM in the Bijiki BIF, the significantly higher proportion of apatite grains associated with CM in this rock is interpreted to represent diagenetically altered biomass and shows that such diagenetic mineral associations can survive metamorphism up to the amphibolite facies. Isotope compositions of CM in muffled acidified whole-rock powders from the Akilia Qp rock have average d 13 C gra values of À17.5 ± 2.5& (1r), while d 13 C carb values in whole-rock powders average À4.0 ± 1.0& (1r). Carbon isotope compositions of graphite associated with apatite and other minerals in the Akilia Qp rock were also measured with the NanoSIMS to have similar ranges of d 13 C gra values averaging À13.8 ± 5.6& (1r). The NanoSIMS was also used to semi-quantitatively map the distributions of H, N, O, P, and S in graphite from the Akilia Qp rock, and relative abundances were found to be similar for graphite associated with apatite or with hornblende, calcite, and sulfides. These analyses revealed generally lower abundances of trace elements in the Akilia graphite compared to graphite associated with apatite from Paleoproterozoic BIFs. Graphite associated with hornblende, calcite, and sulfides in the Akilia Qp rock was fluid-deposited at high-temperature from carbon-bearing fluids, and since this graphite has similar ranges of d 13 C gra values and of trace elements compared to graphite associated with apatite, we conclude that the Akilia graphite in different mineral associations formed from the same source(s) of CM. Collectively our results do not exclude a biogenic origin of the carbon in the Akilia graphite, but because some observations can not exclude graphitization of abiogenic carbon from CO 2 -and CH 4 -bearing mantle fluids, there remain ambiguities with respect to the exact origin of carbon in this ancient metasedimentary rock. Accordingly, there

The oldest putative fossils occur as hematite filaments and tubes in jasper-carbonate banded iron formations from the 4280- to 3750-Ma Nuvvuagittuq Supracrustal Belt, Québec. If biological in origin, these filaments might have affinities with modern descendants; however, if abiotic, they could indicate complex prebiotic forms on early Earth. Here, we report images of centimeter-size, autochthonous hematite filaments that are pectinate-branching, parallel-aligned, undulated, and containing Fe 2+ -oxides. These microstructures are considered microfossils because of their mineral associations and resemblance to younger microfossils, modern Fe-bacteria from hydrothermal environments, and the experimental products of heated Fe-oxidizing bacteria. Additional clusters of irregular hematite ellipsoids could reflect abiotic processes of silicification, producing similar structures and thus yielding an uncertain origin. Millimeter-sized chalcopyrite grains within the jasper-carbonate rocks ha...

The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the evolution of Earth's hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz-carbonate pods that formed during oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry and synchrotron radiation X-ray microfluorescence (A-SR-XRF) techniques. Bulk chemical analyses were also performed using crush-leach method. Microthermometric data combined with crush-leach and A-SR-XRF analyses yielded a model composition of 1100 mM Na, 2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration (ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin environment. Individual fluid inclusion analysis using A-SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe-S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions ( ) is similar to the modern seawater value (649). By contrast, Ba-and Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metaldepleted fluid displays low sulfate concentration (0-8 mM compared to 28 mM in present-day ocean). Sulfur content of the Ferich fluids ranges between 41 and 82 mM.

Almost all evidence for the oldest traces of life on Earth rely on particles of graphitic carbon preserved in rocks of sedimentary protolith. Yet, the source of carbon in such ancient graphite is debated, as it could possibly be non-biological and/or non-indigenous in origin. Here we describe the co-occurrence of poorly crystalline and crystalline varieties of graphitic carbon with apatite in ten different and variably metamorphosed banded iron formations (BIF) ranging in age from 1,800 to >3,800 Myr. In Neoarchean to Palaeoproterozoic BIF subjected to low-grade metamorphism, 13 C-depleted graphitic carbon occurs as inclusions in apatite, and carbonate and arguably represents the remineralisation of syngenetic biomass. In BIF subjected to high-grade metamorphism, 13 C-depleted graphite co-occurs with poorly crystalline graphite (PCG), as well as apatite, carbonate, pyrite, amphibole and greenalite. Retrograde minerals such as greenalite, and veins cross-cutting magnetite layers contain PCG. Crystalline graphite can occur with apatite and orthopyroxene, and sometimes it has PCG coatings. Crystalline graphite is interpreted to represent the metamorphosed product of syngenetic organic carbon deposited in BIF, while poorly crystalline graphite was precipitated from C-O-H fluids partially sourced from the syngenetic carbon, along with fluid-deposited apatite and carbonate. The isotopic signature of the graphitic carbon and the distribution of fluiddeposited graphite in highly metamorphosed BIF is consistent with carbon in the fluids being derived from the thermal cracking of syngenetic biomass deposited in BIF, but, extraneous sources of carbon cannot be ruled out as a source for PCG. The results here show that apatite + graphite is a common mineral assemblage in metamorphosed BIF. The mode of formation of this assemblage is, however, variable, which has important implications for the timing of life's emergence on Earth.

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Our data show that, applied to exceedingly ancient and geochemically highly altered (graphitic, carbonized) kerogenous microscopic fossils, Raman imagery can be used to correlate directly chemical composition with optically discernible morphology and to prove the presence of crucial indicators of biogenicity, such as kerogenous cell walls. When applied to particularly poorly preserved fossil-like microscopic ®laments, for which optical microscopy can only hint at cellularity (Fig. ), this highly sensitive technique can yield convincing evidence of biogenicity. Most notably, by providing a means by which to correlate biologically characteristic (kerogenous) composition with features of morphology that typify ®lamentous microbesÐsuch as cylindrical (Fig. i), sinuous (Fig. ) and tapering (Fig. ) ®lament form, and the presence of transverse cell walls (Fig. , e±i) and distinctively shaped terminal cells (Fig. )Ðoptical microscopy (Fig. ) coupled with laser±Raman imagery (Fig. ) and measurement of Raman point spectra (Fig. ) substantiates the biological origin of the oldest putative fossils now known (Fig. ). By the time of preservation of the diverse, exceptionally ancient, microscopic ®laments of the Apex chert 4,5 , ,3,500 Myr ago, microbial life was ¯ourishing and presumably widespread. We acquired the optical photomicrographs (Fig. ) in transmitted white light with a Leitz Orthoplan 2 automatic photomicroscope. Laser±Raman spectroscopic data (Figs 2 and 3) were obtained with a Dilor XY (formerly Instruments S.A., now J.Y. Horiba) 0.8-m triple-stage system that has macro-, micro-and confocal line-scan imaging options. This system permitted acquisition of both individual point spectra, typically over a period of 100 s, and true Raman images that show the spatial distribution of molecular components. The confocal capability of the system was such that we could use a 100´objective without immersion oil (having an extended working distance of 3.4 mm and a numerical aperture of 0.8) to obtain all Raman data with a planar resolution of , 1 mm and a vertical resolution of 1±3 mm. Laser wavelengths ranging from blue to infrared were provided by a coherent krypton ion laser equipped with appropriate optics. We used a wavelength of 531 nm (in the green portion of the spectrum) typically at a laser power of less than 8 mW over a 1-mm spotÐan instrumental con®guration that we showed to be well below the threshold that results in radiation damage to the specimens analysed. Fossil specimens situated in the uppermost ,65 mm of petrographic thin sections were centred in the path of the laser beam projected through an Olympus BX40 microscope. The upper surfaces of thin sections were either polished (with a paste containing 1-mmsized diamonds) or unpolished and ®nished by a slurry of 600 mesh carborundum. To enhance the optical image of specimens situated more than ,10 mm below the upper surface of unpolished sections, we covered the area imaged by a thin veneer (,1-mm thick) of type B non-drying microscopy immersion oil (R. P. Cargille Laboratories). Not only have appropriate analyses (see above) shown that the presence of this thin veneer has no appreciable effect on the Raman spectra acquired, but the point spectra of specimens in polished sections (for example, Fig. ) and of most specimens (12/23) analysed in unpolished sections (for examples, Fig. ) were obtained without this veneer. To acquire Raman images (Fig. ), a rectangular area enclosing a part of a fossil was selected for imaging, the backscattered Raman spectra obtained in each rectangle were collected through the optical system described above along micrometre-resolution scan lines, and their x±y registrations were automatically recorded to provide a pixel-assigned array of spectral elements (`spexels') 11 . A typical Raman image comprised 25 ´25 spectral elements, each analysed for 5±10 s, resulting in a total data collection time of 50±100 min for each image. We processed the several hundred spexels for each specimen by constructing a map of the intensity in the spectral window corresponding either to the `D' band (at ,1,350 cm -1 ) or the `G' band (,1,600 cm -1 ) of the kerogenous material; maps made by the `G' band were generally of relatively higher Raman image quality. The resulting `chemical image' shows the areal distribution of the fossil structures that produced the Raman spectral bands originating from speci®c molecular components.

Earth's oldest preserved granitoid crust dates back to the Paleoarchean and consists predominantly of sodic tonalite-trondhjemite-granodiorite (TTG) granitoids that arose through the partial melting of hydrated metabasalts. In contrast, granites (sensu stricto) typically postdate the TTG and appear late in the plutonic record of the old cratons. However, the existence of Hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust; although we have direct evidence, this earliest felsic crust is not preserved. In this PhD I present an unusual variety of markedly CaO-poor, K 2 O-rich, rutile-bearing, peraluminous granite and rhyolite that are located in the basal conglomerate of the Moodies Group (South Africa). These rocks challenge the common view of the Archean craton evolution as they were produced concurrently with TTG magmas during three magmatic cycles in the Barberton Greenstone Belt (BGB) and were later emplaced, as clasts, in a younger conglomerate. The study of mineral inclusions located in the zircons present within the granites and rhyolites, shows that alkali feldspar inclusions are abundant relative to plagioclase inclusions and demonstrates that the main characteristics of these granites, i.e. they are K-rich and Ca-poor, are a magmatic signature. The oxygen isotope signature of these zircon grains reveals that the zircons have preserved the δ 18 O value of the magma from which the granites originated and that the source of the granites had a magmatic oxygen isotope value close to the one of the regional coeval TTG. Further study of the zircons shows that their Lu-Hf isotopic system reflects the crustal signature of the magma into which they grew. Sm-Nd study of the granites and rhyolites whole rock indicates that the minimum age of the source's protolith of the granites and rhyolites is close to 3.9 billion years, which is in agreement with the zircons' Lu-Hf signature. Additionally I show in this thesis that the peraluminous character of the granites and rhyolites, along with their high Sr and low Ca content associated to their Eu/ Eu* ~ 1 is a consequence of phengite melting in a metagreywacke source at pressures in excess of plagioclase stability. My work therefore illustrates that K-rich, Ca-poor peraluminous granites were generated in the Paleo and Meso Archean, alongside with the sodic TTG, through partial melting of sediments at high pressures. Not only has this process demonstrated the ability of the early Earth to recycle relatively young material since 3.9 billions years ago, but it has also contributed to each episode of continental crustal growth through the Paleoarchean to Mesoarchean in the BGB, despite leaving no plutonic record at the typical mid-crustal level of exposure that the TTG plutons around the belt represent.

Carbonyl sulfide (OCS) is the most abundant gas containing sulfur in the atmosphere, with an average mixing ratio of 500 p.p.t.v. in the troposphere. OCS is suggested as a sulfur source of the stratospheric sulfate aerosols (SSA) which plays an important role for Earth of radiation budget and for ozone depletion. OCS, therefore, should be validated for prediction of climate change, but the global OCS budget is imbalanced. It is known that some microorganisms in soil can degrade OCS, but the mechanism and the contribution to the OCS in the air are still uncertain. Isotopic compositions and isotopic fractionation factors are used to trace the sources and transformations of atmospheric trace gases. Recently, we developed new method measuring sulfur isotopic composition of OCS using fragmentation ions S , and this method can be used to investigate its sources and sinks in the troposphere. In order to determine sulfur isotopic fractionation factors for OCS during degradation via microorg...

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With the surge of interest in understanding the evolution of environments and ecosystems on the early Earth, many proxies are being applied to ancient sedimentary rocks. The isotope composition of nitrogen recorded in sedimentary rocks ( 15 N sed) is one of these proxies. Nitrogen isotopes are now routinely used as a tracer of the global and regional marine Nbiogeochemical cycle during the Cenozoic and are increasingly being applied to the ancient rock record, including the Precambrian. The objectives of this review are (i) to articulate guidelines for using  15 N sed as a proxy for the past marine global N-biogeochemical cycle with an emphasis on the Precambrian and (ii) to develop a broad framework for interpreting the Precambrian  15 N sed record. Based on the isotopic pattern displayed by the present day Nbiogeochemical cycle, significant  15 N sed spatial variability is expected for most of the ocean redox structures envisaged for the Precambrian. Furthermore, fundamentally different Ncycling processes may give rise to only subtly different  15 N sed signatures, which themselves may be masked or accentuated by post-depositional processes. Consequently,  15 N sed profiles from individual basins alone are insufficient for inferring behavior of the global nitrogen cycle. Rather,  15 N sed distributions based on data from multiple basins are essential. Furthermore, in order to interpret  15 N sed data from Precambrian sedimentary rocks with more confidence, several avenues of focused research are required. The effects of diagenesis and metamorphism and their manifestation in the  15 N compositions of both bulk and kerogen records need to be better understood. Much more data are required in order to apply statistical approaches to interpreting  15 N sed variability within given geological time intervals. Finally, numerical modeling of the  15 N sed distributions expected from different redox scenarios envisaged for the Precambrian environment is necessary to establish a predictive template for interpreting the ancient nitrogen isotope record. In spite of the intrinsic complexity of the  15 N sed proxy and the great deal of work still required to realize its full potential, the available Precambrian  15 N sed record shows several intuitive features within the context of the inferred evolution of the marine N-biogeochemical cycle. The  15 N sed distributions are roughly similar in shape and amplitude to that of the present ocean, with a change in mode from +1 to +3‰ at ca. 2.7 Ga and transient excursions to 15 N-enriched values at 2.7 and 1.9 Ga. Fundamental shifts in global marine N-cycling, perhaps related to stepwise oxygenation of the surface environment, are inferred, highlighting the potential of nitrogen isotopes to reveal clues about the evolution of early Earth.

6180 values of unaltered, primary minerals from two Cretaceous rocks of komatiitic affinity and from associated rocks from Gorgona Island, Colombia, indicate that the komatiitic samples , have 5180 values > 6.0 whereas the associated tholeiitic igneous rocks have greater (Ce/Sm)N ratios and 6180 values near 5.5. 5180 values of the komatiitic rocks, in conjunction with their low LREE contents and high ENd values , suggest derivation from sources that were extensively depleted by prior melting events and different from those of some of the associated rocks. The divergence between the 6180 values of primary chromite , olivine and pyroxene in the komatiitic rocks is in accordance with petrographic relations which suggest substantial undercooling took place during their emplacement. 6D and 6180 values of chrysotile from the komatlitic samples are characteristic of those for serpentines formed from meteoric water at Gorgona rather than from seawater. This post emplacement alteration has also affected the 87Sr/86Sr ratios. GEOLOGIC CONSIDERATIONS Komatiites and komatiitic basalts record information about both the composition and the physical state of the upper mantle. Problems abound, however, because most komatiitic lavas are Archean in age and are so severely altered that their original compositions must be recon structed using a mixture of petrographic inter

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

This field guide describes the geology and landscape along the disused Railway Reserve Heritage Geotrail, in John Forrest National Park that is located on the Darling Scarp about 22 km east of Perth in Western Australia.  It includes explanations of the geological features, their genesis and how they lead to the landforms that attract the public;s interest.  It is a key geotrail in support of geotourism.

Silica-replaced microfeatures in a well-developed, Upper Mississippian paleosol from north-central Arizona, were examined by scanning electron microscopy using backscattered electron imagery. Preserved microfeatures include hollow and solid tubiform filaments and mycelium-like stringers which radiate from problematic (biogenic?) soil structures. Preservation of these features suggest that microstructures in the soil zone are not uniformly destroyed during post-diagenetic silica replacement and that biological soil symbionts may have occurred as early as the Upper Mississippian (-280 Mya).

In this work the theoretical relationship between the clear-sky outgoing infrared radiation and the surface upward radiative flux is explored by using a realistic finite semi-transparent atmospheric model. We show that the fundamental relationship between the optical depth and source function contains real boundary condition parameters. We also show that the radiative equilibrium is controlled by a special atmospheric transfer function and requires the continuity of the temperature at the ground surface. The long standing misinterpretation of the classic semi-infinite Eddington solution has been resolved. Compared to the semi-infinite model the finite semi-transparent model predicts much smaller ground surface temperature and a larger surface air temperature. The new equation proves that the classic solution significantly overestimates the sensitivity of greenhouse forcing to optical depth perturbations. In Earth-type atmospheres sustained planetary greenhouse effect with a stable ground surface temperature can only exist at a particular planetary average flux optical depth of 1.841. Simulation results show that the Earth maintains a controlled greenhouse effect with a global average optical depth kept close to this critical value. The broadband radiative transfer in the clear Martian atmosphere follows different principle resulting in different analytical relationships among the fluxes. Applying the virial theorem to the radiative balance equation we present a coherent picture of the planetary greenhouse effect.

The ca. 2.45-2.22 Ga Turee Creek Group, Western Australia, contains carbonaterich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2-2.05 Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation. Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn-and post-depositional remineralization of organic matter. In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate-siliciclastic shelf. These carbonates exhibit moderate δ 13 C values of-2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss 4 of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

The study of tidal rhythmites, involving sedimentology, time-series analysis and astronomy, can contribute to understanding the history of the Earth's rotation and the Moon's orbit. It is regrettable, therefore, that the paper by Mazumder and Arima (2005) [Mazumder R., Arima M., 2005. Tidal rhythmites and their implications. Earth-Sci. Rev. 69, 79-95.] on tidal rhythmites and their implications contains errors of methodology and unsubstantiated speculation that could confuse and mislead.

Dr. George Williams (Williams, 2005) provided me an opportunity to clarify the misunderstandings and misconceptions prevailing among some so-called tidal rhythmite researchers. Ancient tidal rhythmites, like their modern counterparts, can preserve a record of astronomically induced tidal periods. Unlike modern tide and tidal deposits, analysis of ancient tidal rhythmites, however, is not straightforward. This is simply because spectral analysis of ancient tidal cycles is not an actual time series, but is in fact a thickness series. Many geologists use standard timeseries techniques while analyzing ancient tidal rhythmites without concern to this basic fact! Complications are thus inherent with the analysis of ancient tidal rhythmite. In discrete-signal records the relationship between sediment thickness and time is more complex than for continuous signal records (Weedon, 2003, p. 28-32) Understandably, quality of rhythmite raw data and application of basic astronomical laws, if applicable at all, has nothing to do with these inherent complications. One of the major objectives

The concentration of halogens (Cl, Br) and sulfate in seawater during the Archaean eon have important implications for the evolution of Earth's hydrosphere and atmosphere and the development of early life. Insights into the composition of Archaean seawater and hydrothermal fluids can be obtained by direct analysis of fluid inclusions preserved in Archaean sediments and hydrothermal systems. Here, we investigated a suite of well-preserved intrapillow quartz-carbonate pods that formed during oceanic hydrothermal alteration of the 3.49 Ga Dresser Formation, North Pole Dome, Western Australia. Texturally, the pods seems to contain a unique population of primary fluid inclusions which were analyzed individually using microthermometry and synchrotron radiation X-ray microfluorescence (A-SR-XRF) techniques. Bulk chemical analyses were also performed using crush-leach method. Microthermometric data combined with crush-leach and A-SR-XRF analyses yielded a model composition of 1100 mM Na, 2250 mM Cl., and 375 mM Ca, which corresponds to a bulk fluid salinity of 12 wt.% salt equivalent. This high Cl concentration (ca. four-times present-day value) reflects a typical modern-day seawater evaporation trend in a shallow marine, closed basin environment. Individual fluid inclusion analysis using A-SR-XRF revealed the presence of three main fluid populations: a metal-depleted fluid, a Ba-rich and S-depleted fluid, and a Fe-S-rich end-member. The Cl/Br ratio of metal-depleted fluid inclusions (630) is similar to the modern seawater value (649). By contrast, Ba-and Fe-rich brines have Cl/Br ratios (350 and 390) close to bulk Earth value (420), hence arguing for a mantle buffering and a hydrothermal origin of these fluids. The metaldepleted fluid displays low sulfate concentration (0-8 mM compared to 28 mM in present-day ocean). Sulfur content of the Ferich fluids ranges between 41 and 82 mM.

Following the Earth's formation about 4.6 Ga, its initial basaltic outer layer underwent differentiation, generating the oldest continental crust. However, how and when this differentiation took place and what processes led to the formation of early silicic continental crust are controversial topics, which are rooted in a bigger question of when modern-style plate tectonics started operating in Earth's history. In general, there is a well-accepted consensus that the Earth transitioned from single lid (squishy, stagnant, and/or sluggish lid?) to plate tectonics at some point, largely as a result of the establishment of the first subduction zone(s). Members of the geoscience community are divided in answering the question of the timing of the onset of plate tectonics, as reflected in the four perspective papers by different international research teams here. Using different proxies and datasets, the proposed models range from the Late Archean to Paleoproterozoic, and Neoproterozoic for the beginning of a plate tectonic convective regime. Another interpretation states that we do not know when plate tectonics might have started based on the extant observations and data. Although the beginning of plate tectonics is still a subject of debate, most researchers agree on the definition of plate tectonics as a globally networked system of laterally moving rigid plates riding on the aesthenosphere and mantle convection cells, which facilitated recycling of the top boundary layer through time. Testing the proposed models and concepts in these perspective papers with new data, observations and geodynamic modelling efforts will be critical to better understanding the Earth's thermal history and evolution since 4.4 Ga.

The biogenicity of stromatolitic features in the Early Archaean Strelley Pool Chert have recently been the subject of some debate. If biogenic, these features would be a valuable source of evidence for Earth's earliest biosphere. If abiogenic, the features may still be able to provide information about distinguishing biogenic from abiogenic stromatoloids. A new method developed for the remote identification of stromatolites and subsequent quantification of their likely biogenicity is herein described. Initial results and future directions are presented. Further applications to planetary missions are outlined. Strelley Pool Chert: The 3.45 Ga Strelley Pool Chert (SPC) was first described by Lowe [1]. The SPC succession is one of several volumetrically small volcaniclastic sedimentary units within the primarily volcanic Early Archaean Warrawoona Group [2]. Stromatoloids at the Trendall Locality have been described by Hoffman et al [3] and meter scale mapping and a geochemical study of the area have been conducted by Van Kranendonk [4, 5]. More recently a photo montage and geological map have been collated by Brown [6]. Biogenicity of Stromatolitic Features: Lindsey et al [7] have recently disputed the biogenicity of stromatolites at the Trendall Locality of the Strelley Pool Chert. They propose that the stromatoloidal carbonate and silicified carbonate layers of the Trendall locality were "generated by direct precipitation of carbonate from hydrothermal fluids in a relatively deep marine setting". They state that they "find no direct evidence of biogenic mediation" and point out one of their observations is that the structures are "conspicuously isopachous". This provides an opportunity to apply texturally based complexity analysis to assess the biogenicity of the stromatoloidal structures. The term "stromatolite" will be used rather than "stromatoloid" [8] to describe the laminar features within the SPC within this paper, even though their biogenicity is still being determined. Complexity Analysis: Storrie-Lombardi et al [9] recently described a method by which the biogenicity of stromatoloids may be judged using complexity analysis. The complexity of a digital file can be quantified using lossless compression algorithms which reduce file size by identifying redundant information. Decompression of the compressed file returns the original file unaltered. Both biological and physicochemical structures can exhibit "complexity". The ap-

Ancient crustal rocks provide the only direct evidence for the processes and products of early Earth di€erentiation. SHRIMP zircon U-Th-Pb dating has identi®ed, amongst the Acasta gneisses of the western Slave Province, Canada, two metatonalites and a metagranodiorite that have igneous ages of 4002 ‹ 4, 4012 ‹ 6 and 4031 ‹ 3 Ga respectively. These are the ®rst identi®ed Priscoan terrestrial rocks. A record of metamorphic events at 3.75, 3.6 and 1.7 Ga also is preserved. These discoveries approximately double, to 40 km, the area over which 4.0 Ga gneisses are known to occur. A single older zircon core in one sample suggests that rocks as old as 4.06 Ga might yet be found in the region. As early as 4.03 Ga, terrestrial di€erentiation was already producing tonalitic magmas, probably by partial melting of pre-existing, less di€erentiated crust.

The following is an account o f the stratigraphy, structure, metamorphism, and mineralization found in tha Jamestown and Sheba Hills areas, situated on the northwest flank of the Barberton Mountain Land. The area embraces one of the most complexly deformed and highly m e t a mo r p h o s e d regions in the early Precambrian Barberton greenstone belt. A brief account of the history and mining associated with the e arly discovery o f gold in the area is outlined and a fairly c o m p r e h e n sive account is given of previous geological work done in the Barberton region. Particular emphasis is placed on previous work applicable to the area under discussion. The general geology, including an overall structural synthesis and the recently revised stratigraphy of the Barberton Mountain Land, is also summarized. The g e o l o g y of the Jamestown Hillc area is discussed in detail and the revised stratigraphy is presented. Two formations of the O n v erwacht Group, viz. the Theespruit and Komati Formations, are r e c o g n i z e d in the area. The Theespruit Formation consists of a wide v a r i e t y o f therm a l l y and d ynamically m e i amorphosed mafic and ultramafic r o c k s together with minor siliceous schist and chert horizons. New chemical data re l a t i n g to the amphibolites and siliceous schists is presented and discussed. The Komati F o m a t i o n consists essentially of metamo r p h o s e d mafic and ultramafic rocks together with associated quartz and felspar porphyries. A series of d ifferentiated ultramafij bodies, which probably represent sills, occur ma i n l y within the Komati Formation assemb l a g e s but some differentiated ultramafic bodies, elsewhere in the Jair.estown Schist Belt, may represent sills in the Theespruit Formation. Rocks o f the Fig Tree and Moodies Groups are developed in the ea s tern part of the Jamestown Hills area, particula rly in the n e i g h b o u r h o o d of the New Consort Gold Mine-These predom i n a n t l y sedimentary successions are complexly deformed and have been th ermally metamorphosed by the Nelspruit Granite border phase and a s s o ciate d pegmatites. The structure of the Jamestown Schist 3elt and the Consort Mine area is explained in detail, use having been made of m i nor structural t e c hniques to unravel the tectonic history of the area which displays several phases of deformation. , mineralization of the Jamestown Schist Belt, which includes gold, asbestos, talc, magnesite, nickel, verdite, and buddstone is described and the controlling influence of structure in the formation and localization of the mineralization is emphasized. The Handsup-Mundt's Concession differentiated ultramafic igneous bodies, at the eastern end of the Jamastown Schist 5elt, are described in detail. At least four separate cycles or pulses are evident, each beginning at the base with a serpentinized olivine peridotite, and followed, in turn, by serpentinized pyroxene peridotite, pyroxenite, and meta-gabbro. The petrography of these rocks is g iven and new chemical data r e lating to the differentiated ultramafic bodies is presented and discussed. The primary magma type is considered to have been originally o f ultrabasic composition. The differentiated ultramafic bodies are c o n s idered to represent the intrusive equivalents of the volcanic u l t r a m a f i c and mafic successions of the Kor.ati River ValJ^y. A detailed accounv o f the geology of the Eureka Syncline, in tae Sheba Hills area of the Mountain Land, embraces a description of the st r a t igraphy and structure of the area. Although Onverwacht and Fig Tree Group r o cks occur in the Eureka Syncline, the Moodies Group assemblages are best developed and represent the most complete succession of Moodies rocks in the Barberton Mountain Land. Three main pulses of sedimentation C3 n be recognized. The first of these began with the deposition of the basal conglomerate, followed b y calcareous o rthoquartzites and felspathic quartzites, and ended wi t h shales and banded magnetic jaspilites. The next pulse began with quartzites and conglomerates and was followed by a m i n o r period of v o l c anism and a second banded magnetic jasper horizon, together with shales. The third and last pulse began with minor c o n g l o m e r a t e and quartzite development and was followed by an alternating sequence of shales, s u b g r e y w a c k e s , grits, and, finally, a small-pebble conglomerate. Each pulse appears to have started with an influx of coarse detritus and gradu a l l y the depositional pattern trended towards an environment that allowed for the settling out of shales and chemically pre c i p i t a t e d jaspilites and banded ironstones. The sedimentary structures displayed in the Moodies rocks of the Eureka Syncline ar<s indicative of d e p osition in a shallow water environment.

Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that 2 ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous-ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semirestricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth.

The article was written 24 years ago! At that time there were no known reliable Archean ophiolite complexes on Earth.  The absence of Archean ophiolites has been used by many to deny the mechanisms of plate tectonics in the Archean. A detailed geochemical study of magmatic complexes of Mesozoic and Paleozoic ophiolites has shown that fragments of oceanic crustal sections formed in suprasubduction environments predominate among them. These sections have absolutely certain indicative features, such as the presence of boninites and their derivatives among the dyke and lava complexes, which sharply distinguishes them from the sections of the oceanic crust born in ocean ridges. Field studies in 1999 in North Karelia in the Irin-mountain structure allowed us to identify and describe fragments of an ophiolitic section with a complex of parallel dikes and lavas with boninite specification. The age was determined as 2.782 billion years. This allows us to assert the existence of subduction conditions in the Early Precambrian.

This study defines a comprehensive chemostratigraphic framework for the 2820-2738 Ma magmatic rocks of the northwestern Youanmi Terrane of the Yilgarn Craton. Sixteen chemostratigraphically distinct units can be correlated between greenstone belts that are separated by later voluminous granitic plutons. On a regional scale, the chemostratigraphic units define discontinuous, broadly NNE striking zones up to 350 km long and 50 km wide (including later granitic plutons). Neodymium isotope and trace element systematics of magmatic rocks require an increasing component of metasomatised mantle from c. 2820 to 2738 Ma and variable recycling of a 3050-2920 Ma crustal component. We interpret the first magmatic episode at 2820-2782 Ma to reflect lithospheric thinning and asthenosphere upwelling in a back-arc-like setting and attribute the metasomatism of sources for multiple mafic units to subduction of 3050-2920 Ma, predominantly isotopically juvenile, mafic crust. Following an approximate 16 Ma hiatus, a second magmatic episode at 2766-2738 Ma is dominated by crustal-derived transitional TTGs with lower volumes of mantle-derived shoshonitic and sanukitoid-like rocks, and LILE-LREE enriched tholeiitic mafic to felsic rocks. The compositions of the transitional TTG and shoshonitic rocks in this study area are similar to potassic granites and shoshonitic rocks (including shoshonitic lamprophyres) from post-orogenic continental collision settings, such as the Mesozoic Tibetan Plateau. We interpret this later episode as reflecting post-orogenic melting of crustal and upper (modified) mantle sources caused by upwelling mantle related to delamination of arc-crust or slab breakoff. The northwestern Youanmi Terrane preserves an extensive package of Archean magmatic rocks that can be confidently attributed to subductionmagmatism and provides a reference point to show that some form of subduction occurred in the Meso-Neoarchean.

Point counting of this thin section indicates that it has approximately 9% carbonate, most of which is associated with the crush zone (Figure 1). Carbonates show the globular, vein/fracture filling and interstitial textures reported by [3] with some distinctive variations. Fibrous and granular carbonate is observed in a number of places growing into void spaces (Figure 2). Carbonate is observed in association with maskelynite (first noted by [4]), but here it forms tubular to globular shapes within the feldspathic glass; in some cases, isolated globules appear to be the ends of tubes (Figure 3). Carbonate globules/tubes show subparallel alignment in the glass. Zoned and unzoned carbonate globules