experimental low grade alteration of biogenic magnetite indicates microbial involvement in generation of banded iron formations during the deposition of banded iron formation bif the downward flux of ferric oxyhydroxides and phytoplankton biomass should have facilitated fe iii reduction during burial with the end product being ferrous iron containing minerals including magnetite although earlier studies have attempted to quantify the significance of this pathway based on models of the ancient fe cycle the only direct evidences of a biological role in magnetite formation in bif are their iron isotope compositions and unique crystallography which are reminiscent of biologically generated magnetite however the biogenesis hypothesis lacks an explanation as to why modern biogenic magnetite crystals are generally a few hundred nm or smaller in size yet the magnetite crystals in bif are mostly tens of micrometers or larger in size in this study we demonstrate that biogenic magnetite crystals can grow in size upon reaction between oxyhydroxide and microbial biomass after compression and heating to 1 kbar and 150 1c respectively the magnetite crystals previously produced by thermoanaerobacter spp tor39 reach sizes in excess of 700 nm after the p t experiments while new magnetite grains 4 400 nm formed from the superparamagnetic magnetite dominated end product of shewanella sp culture this study indicates that the large magnetite crystals observed in bif can be derived through a three stage sequence beginning with dissimilatory iron reduction of an initial ferric iron rich sediment coupled to the oxidation of dead phytoplankton biomass followed by magnetite crystal aging and ultimately pressure temperature induced abiotic alteration of the biogenic magnetite during metamorphism

structural styles of mountain building controls of lithospheric rheologic stratification and extensional inheritance 1 the strength of the crust and the mantle lithosphere are strongly influenced by the temperature the thickness and the composition of the crust and mantle lithosphere and by inherited weaknesses it consequently strongly depends on the geodynamic setting and is then expected to be different from one orogen to another which might explain the diversity of deformation styles here we use 2d thermomechanical models at lithospheric scale to study the effect of the strength of the upper and middle crust on the geometry of contractional systems the role of extensional inheritance is included by forward modeling the formation of rift basin or passive margin and using the resulting extensional structure as initial condition for lithospheric scale inversion our results demonstrate that 1 crustal strength strongly controls the width and the elevation of orogenic wedge 2 relative weak crust facilitates efficient propagating of thick skinned crustal scale thrusts 3 contraction of a strong crust leads to the formation of long thrust sheets and an anti formal stack in the core of the orogen or the accretion of upper crust depending on the depth of the decoupling zone in the mid crust 4 erosion processes favor the localization of shortening in a narrow crustal orogen 5 rifting inheritance can explain the presence at shallower depth of lower crustal or mantle material previously upwelled and facilitates the propagation of the deformation in the external part of the chain the range of predicted behaviors is compared to first order with the zagros the alps and the pyrenees three natural examples for which the crustal structure is well constrained by geophysical data

the 13c12c ratios of siderite and organic matter of a modern metalliferous hydrothermal sediment and their implications for banded iron formations abstract a hydrothermal ferruginous sediment precipitated by the activity of the microaerophilic chemolithotrophic bacterium galionella ferruginea has been sampled in a bay of nea kameni island santorini greece due to high fugacity of carbon dioxide precipitation of siderite also occurred the 13c value pdb was determined to be 6 5 in the siderite and 26 8 in the organic matter of mainly microbiological origin the enrichment of isotopically light carbon in carbonates of banded iron formations 13 c 10 has recently been discussed in literature the dilution of these marine carbonates by isotopically light carbon is suggested to have resulted from bacterial oxidation of reduced organic carbon during early diagenesis it is further suggested that fe iii in the newly precipitated sediment served as electron acceptor during oxidation in a hydrothermal model for the origin of banded iron formations this is consistent with the data presented here it is therefore postulated from the results presented that the reduced organic carbon in banded iron formations originates from microaerophilic chemolithotrophic bacteria

could bacteria have formed the precambrian banded iron formations banded iron formations bifs are prominent sedimentary deposits of the precambrian but despite a century of endeavor the mechanisms of their deposition are still unresolved interactions between microorganisms and dissolved ferrous iron in the ancient oceans offer one plausible means of mineral precipitation in which bacteria directly generate ferric iron either by chemolithoautotrophic iron oxidation or by photoferrotrophy on the basis of chemical analyses from bif units of the 2 5 ga hamersley group western australia we show here that even during periods of maximum iron precipitation most if not all of the iron in bifs could be precipitated by iron oxidizing bacteria in cell densities considerably less than those found in modern fe rich aqueous environments those ancient microorganisms would also have been easily supported by the concentrations of nutrients p and trace metals v mn co zn and mo found within the same iron rich bands these calculations highlight the potential importance of early microbial activity on ancient metal cycling

evaluation of siderite and magnetite formation in bifs by pressure temperature experiments of fe iii minerals and microbial biomass abstract anoxygenic phototrophic fe ii oxidizing bacteria potentially contributed to the deposition of archean banded iron formations bifs before the evolution of cyanobacterially generated molecular oxygen o 2 by using sunlight to oxidize aqueous fe ii and precipitate fe iii oxyhydr oxides once deposited at the seafloor diagenetic reduction of the fe iii oxyhydr oxides by heterotrophic bacteria produced secondary fe ii bearing minerals such as siderite feco 3 and magnetite fe 3 o 4 via the oxidation of microbial organic carbon i e cellular biomass during deeper burial at temperatures above the threshold for life thermochemical fe iii reduction has the potential to form bif like minerals however the role of thermochemical fe iii reduction of primary bif minerals during metamorphism and its impact on mineralogy and geochemical signatures in bifs is poorly understood consequently we simulated the metamorphism of the precursor and diagenetic iron rich minerals ferrihydrite goethite hematite at low grade metamorphic conditions 170 c 1 2 kbar for 14 days by using 1 mixtures of abiotically synthesized fe iii minerals and either microbial biomass or glucose as a proxy for biomass and 2 using biogenic minerals formed by phototrophic fe ii oxidizing bacteria mossbauer spectroscopy and xrd showed that thermochemical magnetite formation was limited to samples containing ferrihydrite and glucose or goethite and glucose no magnetite was formed from fe iii minerals when microbial biomass was present as the carbon and electron sources for thermochemical fe iii reduction this could be due to biomass derived organic molecules binding to the mineral surfaces and preventing solid state conversion to magnetite mossbauer spectroscopy revealed siderite contents of up to 17 after only 14 days of incubation at elevated temperature and pressure for all samples with synthetic fe iii minerals and biomass whereas 6 of the initial fe iii was reduced to sideritic fe ii in biogenic fe iii minerals during incubation based on our data magnetite in bif is unlikely to be formed by thermochemical fe iii reduction in sediments of biogenic ferrihydrite hematite or goethite dominated sediments with complex microbial biomass present instead our results suggest that diagenetic magnetite in bif was either formed by microbial fe iii reduction during early diagenesis i e below 120 c or by thermochemical fe iii reduction with simple organic compounds at higher temperatures whereas siderite was formed by both microbial diagenetic fe iii reduction and thermogenic fe iii reduction with complex biomass thermochemical fe iii reduction coupled to biomass oxidation during metamorphism provides another origin for bif siderites and could have led to a significant increase in fe ii content in bif after deposition over geological timescales

micro and nanobands in late archean and palaeoproterozoic banded iron formations as possible mineral records of annual and diurnal depositions abstract the microbands in precambrian banded iron formations bifs have been conjectured to record annual or even diurnal depositions however these bands have rarely been observed in high resolution at their true micro scale here i suggest that nanobands of fine grained hematite represent possible diurnal depositions and that microbands of chert jasper represent possible annual depositions in three sets of bifs 2460 myr bifs from the kuruman iron formation transvaal supergroup of south africa 2480 myr bifs from the dales gorge member of the brockman iron formation western australia and 2728 myr bifs from the hunter mine group abitibi greenstone belt canada observations made using scanning electron microscopy indicate that hematite and chert were syngenetic and that there was a hiatus between their precipitation and the genesis of the remainder of the minerals containing structural fe ii spindle like grains of hematite monocrystals of magnetite and ferro dolomite formed microbands of 30 70 m in thickness which appear cyclically in the matrix of the chert neither the band bound magnetite and dolomite nor the linear formations of the hematite spindles represent annual depositions due to their diagenetic features the thinnest microbands 3 12 m were observed in the chert and jasper and indicate depositional rates of 6 6 22 2 m myr in the bifs these rates are consistent with the integrated deposition rates calculated by geochronologic methods for the bifs if annual deposition is assumed the 26 nm nanobands observed only in hematite grains reflect an annual deposition of 18 6 m or 18 6 m myr which is also consistent with the depositional rate calculated by geochronologic methods it is tentatively suggested that these 26 nm nanobands were formed from the diurnal precipitation of fe iii resulting from the circadian metabolism of fe ii oxidizing or oxygen evolving photosynthetic microorganisms which slowed down the rise of atmospheric oxygen the diurnal precipitation of fe iii as hematite and the annual deposition of silica as chert jasper in the bifs provide internal clocks that may facilitate the examination of short term processes such as ecological oceanographic and climatic cycles that are recorded by the mineral or chemical compositions of bifs

evidence for exclusively inorganic formation of magnetite in martian meteorite alh84001 magnetite crystals produced by terrestrial magnetotactic bacterium mv 1 are elongated on a 111 crystallographic axis in a so called truncated hexa octahedral shape this morphology has been proposed to constitute a biomarker i e formed only in biogenic processes a subpopulation of magnetite crystals associated with carbonate globules in martian meteorite alh84001 is reported to have this morphology and the observation has been taken as evidence for biological activity on mars in this study we present evidence for the exclusively inorganic origin of 111 elongated magnetite crystals in alh84001 we report three dimensional 3 d morphologies for 1000 magnetite crystals extracted from 1 thermal decomposition products of fe rich carbonate produced by inorganic hydrothermal precipitation in laboratory experiments 2 carbonate globules in martian meteorite alh84001 and 3 cells of magnetotactic bacterial strain mv 1 the 3 d morphologies were derived by fitting 3 d shape models to two dimensional bright field transmission electron microscope tem images obtained at a series of viewing angles the view down the 110 axes closest to the 111 elongation axis of magnetite crystals 111 110 0 provides a 2 d projection that uniquely discriminates among the three 111 elongated magnetite morphologies found in these samples 111 elongated truncated hexa octahedron 111 tho 111 elongated cubo octahedron 111 eco and 111 elongated simple octahedron 111 eso all 111 elongated morphologies are present in the three types of sample but in different proportions in the alh84001 martian meteorite and in our inorganic laboratory products the most common 111 elongated magnetite crystal morphology is 111 eco in contrast the most common morphology for magnetotactic bacterial strain mv 1 is 111 tho these results show that 1 the morphology of 111 elongated magnetite crystals associated with the carbonate globules in martian meteorite alh84001 is replicated by an inorganic process and 2 the most common crystal morphology for biogenic mv 1 magnetite is distinctly different from that in both alh84001 and our inorganic laboratory products therefore 111 elongated magnetite crystals in alh84001 do not constitute as previously claimed a robust biosignature and in fact an exclusively inorganic origin for the magnetite is fully consistent with our results furthermore the inorganic synthesis method i e the thermal decomposition of hydrothermally precipitated fe rich carbonate is a process analogue for formation of the magnetite on mars namely precipitation of carbonate globules from carbonate rich hydrothermal solutions followed at some later time by a thermal pulse perhaps in association with meteoritic impact or volcanic processes on the martian surface

formation of single domain magnetite by a thermophilic bacterium magnetite is a common product of bacterial iron reduction and may serve as a potential physical indicator of biological activity in geological settings here the authors report the formation of single domain magnetite under laboratory conditions by a thermophilic fermentative bacterial strain tor 39 that was isolated from the deep subsurface time course analyses were performed at 65 c to study the effect of bacterial activity on solution chemistry and magnetite formation during the growth of tor 39 run products were examined by transmission electron microscopy magnetite particles formed exclusively outside of bacterial cells and exhibited octahedral shapes having relatively equal length and width 15 difference tiny magnetite particles 12 nm nucleated between 10 and 11 h of incubation and increased to average lengths of 55 4 26 8 nm after 24 h of incubation between 24 h and 22 d of incubation magnetite particles maintained average lengths of 56 2 24 8 nm based on size constraints greater than 85 of the particles observed fell within the magnetic single domain little to no magnetite was detected in abiotic controls at 65 or 95 c or in tor 39 cultures whose activity was suppressed unlike mesophilic iron reducing bacteria e g gs 15 tor 39 produced crystals having shapes and sizesmore similar to some particles produced intracellularly by magnetotactic bacteria thus the single domain magnetite produced by thermophiles such as tor 39 may represent a heretofore unrecognized biological contribution to natural remanent magnetization in sedimentary basins and other geothermal environments less

mineralogical and microfabric characteristics of magnetite in the wuyang precambrian bifs southern north china craton implications for genesis and depositional processes of the associated bifs abstract precambrian banded iron formations bifs are widely distributed in the north china craton ncc among them the wuyang bifs located in the southern margin of ncc occur in the late archaean tieshanmiao formation and can be subdivided in two different sub types i quartz magnetite bifs qmb consisting of magnetite fine microcrystalline quartz and minor calcite and ii pyroxene magnetite bifs pmb composed of pyroxene fine microcrystalline quartz and subordinate feldspars both sub types display apparent discrepancies in terms of petrography and mineral composition as shown in electron backscattered diffraction ebsd images and micrographs magnetite grains from the qmb range in size from tens up to hundreds of m whereas magnetite crystals from the pmb can be up to a few tens of m across the x ray diffraction xrd structural data indicate that magnetite from both bif sub types is equiaxed cubic and was generated by sedimentary metamorphic processes the cell parameters of magnetite in the qmb are a b c 8 396 a and z 8 which deviate slightly from these of magnetite in the pmb a b c 8 394 a and z 8 the analytical results of raman spectroscopy analysis revealed micro structural signatures of both magnetite raman shifts near 552 cm 1 and 673 cm 1 and hematite raman shifts near 227 cm 1 295 cm 1 and 413 cm 1 in magnetite from both qmb and pmb the crystallinity degree is similar for magnetite micro structures but varies significantly for hematite micro structures oxygen fugacity fo2 conditions fluctuated during the recrystallization of magnetite in the qmb whereas no evident variation of fo2 occurred during the formation of magnetite in the pmb analytical results of laser ablation inductively coupled plasma mass spectrometry la icp ms show that the si al and mg abundances are higher in magnetite from the qmb whereas the ti and mn contents are more elevated in magnetite from the pmb magnetite composition also denotes that both bif sub types are sedimentary metamorphic origin whereas the deposition of pmb was also affected by volcanic activities overall data indicate that the differences in the depositional environment of each bif sub type are due to the involvement of volcanic eruption processes in the genesis of the pmb thus this paper indicated that the qmb was deposited by chemical deposition at the long term interval of volcanic eruptions and the pmb were the product of chemical deposition affected by the volcanic eruption

iron mineralization and taphonomy of microfossils of the 2 45 2 21 ga turee creek group western australia abstract we report a new assemblage of carbonaceous microfossils intimately associated with siderite and fe silicates from a black chert nodule included in iron formation of the ca 2 45 2 21 ga turee creek group western australia this chert comprises microbial fabrics dominated by filaments preserved in matrix of nano to micrometric quartz filaments occur in clumps and in a cobweb like fabric interspersed with coarse crystalline void filling quartz granules we studied this chert with optical microscopy combined with scanning transmission electron microscope observations of focused ion beam sections of microfossils this distinguished three types of well preserved fossil interpreted as polysaccharide sheaths that usually do not preserve chains of cells trichomes type 1 comprises narrow filaments with thin continuous kerogen sheaths type 2 comprises narrow filaments with thick granular sheaths and type 3 comprises broad filaments with thin sheaths type 4 filaments are poorly preserved as granular kerogen organic ultrastructures of type 2 4 microfossils are variably replaced by siderite crystals associated with minor fe silicates iron isotope analyses on bulk powder and reactive iron fraction show indistinguishable and highly positive 56 fe values 1 45 relative to the reference irmm 014 indicating that the bulk of siderite derives from reduction of fe iii oxides this provides indirect evidence that the microbial community was originally associated with fe iii oxides siderite and fe silicates are found with type 2 4 but not type 1 filaments suggesting that only the former were encrusted by fe iii oxides which may have been reduced in situ siderite and fe silicates could result from oxidation of organic matter in filaments coupled with microbial and or thermal reduction of fe iii bio minerals the increasing abundance of siderite correlated with decreasing organic matter preservation in filaments supports that this reaction occurred to variable extents in situ on each microfossil type 2 4 microfossils may thus represent iron oxidizing bacteria these microbial mats display strong similarities with those associated with immediately overlying carbonate rocks of the turee creek group where filaments were interpreted as sulfur oxidizing bacteria some filamentous bacteria can oxidize both iron and sulfur such metabolic versatility could have enabled benthic microbial mats to thrive in the drastically changing chemical conditions of the great oxidation event

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

microstructures crystallography and geochemistry of magnetite in 2500 to 2200 million year old banded iron formations from south africa western australia and north china abstract as a major mineral in banded iron formation bif magnetite can provide information on depositional environment and mechanism of bif it can provide insights into precambrian biological process the crystallography and microstructures of magnetite in bif have been poorly studied due to its simple chemical composition here we report an integrated crystallographic and geochemical study on various magnetites from four bifs of 2500 to 2200 million year old for a better understanding on their geneses and alterations euhedral magnetite crystals from the 2480 ma dales gorge and 2460 ma kuruman bifs all show pure and homogeneous colors in their ebsd phase euler and ipf maps suggesting they are well crystallized and devoid of inclusions or fractures these crystals are recognized as silician magnetite with appreciable structural si ave 1 22 and 2 05 wt respectively but rare al ave 0 27 and

impact of biomineralization on the preservation of microorganisms during fossilization an experimental perspective abstract the biogenicity of fossil microbial biomorphs is often debated because their morphologies are poorly informative and the chemical structural and isotopic signatures of putative biogenic organic molecules have been altered during their incorporation into the sediments and the geological history of the host rock here we investigated the effect of encrustation by biominerals on the morphological and chemical degradation of escherichia coli cells during experimental thermal treatments non calcified e coli cells and e coli cells encrusted by calcium phosphates were exposed to heating under an argon atmosphere at two different temperatures 300 c 600 c for 20 h two additional experiments were performed on non calcified e coli cells at 300 c for 2 h and 100 h to discuss the influence of experiment duration organic residues of all experiments were characterized at a multiple length scale using a combination of scanning electron microscopy transmission electron microscopy raman microspectroscopy electron paramagnetic resonance epr spectroscopy fourier transform infrared ft ir spectroscopy and x ray absorption near edge structure spectroscopy xanes at the carbon k edge in the absence of encrusting biominerals the morphological structure of the organic residues of e coli cells was completely lost after heating at 300 or 600 c even after short 2 h long heating experiments the content of aromatic functional groups of the organic residues of non calcified e coli cells increased during heat treatment at 600 c for 20 h while the amide functional groups were lost as indicated by ft ir spectroscopy consistently the epr spectrum of these organic residues indicated important transformation as a comparison this spectrum appeared similar to epr spectra of ancient organic carbons such as carbons from the apex chert ca 3460 myr indicating a similar concentration of aromatic moieties in contrast calcified e coli exposed to the same conditions showed only limited morphological alteration as observed by electron microscopy as well as lower chemical transformation as detected by ft ir and epr spectroscopies despite the difficulties to relate experimental conditions directly to geological conditions these experiments evidence the influence of cell encrustation by minerals on their chemical and morphological preservation potential during fossilization processes

structure and morphology of magnetite anaerobically produced by a marine magnetotactic bacterium and a dissimilatory iron reducing bacterium intracellular crystals of magnetite synthesized by cells of the magnetotactic vibroid organism mv 1 and extracellular crystals of magnetite produced by the non magnetotactic dissimilatory iron reducing bacterium strain gs 15 were examined using high resolution transmission electron microscopy electron diffraction and57fe mo ssbauer spectroscopy the magnetotactic bacterium contained a single chain of approximately 10 crystals aligned along the long axis of the cell the crystals were essentially pure stoichiometric magnetite when viewed along the crystal long axis the particles had a hexagonal cross section whereas side on they appeared as rectangules or truncated rectangles of average dimension 53 35 nm these findings are explained in terms of a three dimensional morphology comprising a hexagonal prism of 110 faces which are capped and truncated by 111 end faces electron diffraction and lattice imaging studies indicated that the particles were structurally well defined single crystals in contrast magnetite particles produced by the strain gs 15 were irregular in shape and had smaller mean dimensions 14 nm single crystals were imaged but these were not of high structural perfection these results highlight the influence of intracellular control on the crystallochemical specificity of bacterial magnetites the characterization of these crystals is important in aiding the identification of biogenic magnetic materials in paleomagnetism and in studies of sediment magnetization

shrimp u pb zircon ages for the palaeoproterozoic kuruman iron formation northern cape province south africa evidence for simultaneous bif deposition on kaapvaal and pilbara cratons abstract stilpnomelane rich tuffaceous mudrocks are an important component of the palaeoproterozoic kuruman and griquatown iron formations in the northern cape province they document volcanic activity during the deposition of these iron formations and the zircon populations that they contain record syn depositional volcanism resedimentation and post depositional isotopic disturbance the trace element signatures of tuffaceous samples suggest that although zircons are felsic volcanogenic in origin mixing of felsic ash with basaltic ash and or muds in the water column was also likely in addition these rocks record minor terrigenous input and some samples indicate mixing with iron rich clays oxides and smectites moreover they reflect a shift from mafic volcanism e g mafic tuffs in the underlying gamohaan formation towards more felsic volcanism that is a trend also recorded for tuffaceous rocks in the brockman iron formation the best estimate of the depositional age for the top of the riries member and hence the kuruman iron formation is 2460 ma where most of the zircon populations were derived from volcanism coeval with bif deposition ages obtained in this study that are older than the 2465 5 ma age for the lower riries member probably represent resedimented pyroclastic deposits i e give eruptive ages from earlier eruptions that contain an unimodal zircon population and as such only provide maximum depositional ages although a compacted sedimentation rate csr for the entire kuruman iron formation is 3 4 m per million years a csr of about 22 m per million years is deduced for the riries member suggesting that there are cryptic gaps within the kuruman stratigraphic record the latter is comparable to the csr of 33 m per million years calculated for hamersley province bifs ages of the tuffaceous horizons in the kuruman iron formation are comparable to those from the brockman iron formation of the hamersley province the 2460 5 ma age for the top of the riries member corresponds to the 2459 3 ma age for the base of the joffre member and the 2463 5 ma whaleback shale member of the brockman iron formation although ca 2468 2478 ma ages reflect detrital or syn depositional zircon populations they are comparable to the 2470 4 ma s13 and the 2479 3 ma s9 macrobands of the dales gorge member brockman iron formation moreover the zircon age spectra and pb loss histories of both the kuruman and brockman iron formation tuffaceous sediments are strikingly similar without doubt the kuruman and brockman iron formations were deposited synchronously whether they were a response to global magmatic events or were deposited in the same basin are questions still being debated

banded iron formations through much of precambrian time similarities between proterozoic 1 8 2 5 gyr and archean 2 5 gyr banded iron formations are probably more significant than their differences the contrasts largely reflect differences in the tectonic settings of proterozoic and archean terrains archean banded iron formations are not as thick nor laterally as extensive as the major proterozoic iron formations nevertheless some archean iron formations have strike lengths of over 150 200 km and may have been quite extensive prior to the deformation that has affected most archean terrains stratigraphic sequences in which iron formations occur are highly variable and indicate that iron formations formed in many depositional environments sedimentary textures in the iron formations are dominated either by granules and oolites or laminations including microbanding reflecting differences in their physical conditions of deposition granular and oolitic textures are abundant in only three proterozoic depositional basins and most precambrian iron forma

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

topo europe understanding of the coupling between the deep earth and continental topography linking different spatial and temporal scales in coupled deep earth and surface processes is a prime objective of the multidisciplinary international research program topo europe the research approach of topo europe integrates active collection of new data reconstruction of the geological record and numerical and analog modeling the results of the program presented in this special volume focus on four closely interrelated topics crustal and upper mantle structures lithosphere geodynamics sedimentary basin dynamics and surface processes quantitative understanding of topographic evolution in space and time requires study of processes from the upper mantle through the lithosphere and crust and acting on the earth s surface the results presented here demonstrate the opportunities to further understanding of topography through integrated studies of the full earth system across space and timescales

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

a multistage origin for neoarchean layered hematite magnetite iron formation from the weld range yilgarn craton western australia abstract the origins of iron formations remain somewhat enigmatic despite much progress over the past decades this is due in part to continuing research demonstrating that these deposits do not have a single origin but rather can be formed under various and variable conditions this study describes a formation pathway for an algoma type banded iron formation bif from the 2 75 billion year old weld range of western australia based on petrographic and fe isotope analyses of drill core samples the bif is composed of alternating layers of jaspilitic and fe poor chert magnetite layers of varying thickness and abundance and rare pyrite petrographic analysis indicates the minerals formed in the order hematite magnetite and then pyrite with the latter two clearly replacive of primary bedded hematite hematite 56fe values from all portions of the drill core are positive and essentially identical 56fe 0 61 0 05 magnetite 56fe values are positive as well 56fe 0 51 0 07 but vary systematically with total magnetite content of the core samples pyrite 56fe values are positive and higher than both hematite and magnetite 56fe 1 03 0 05 combined these analyses reveal a multistage formational model for the weld range bif initial deposition occurred by the partial oxidation of hydrothermally sourced aqueous fe ii to form a fe iii si co precipitate probably by anoxygenic photosynthetic iron oxidizers this material would have been converted to hematite under equilibrium conditions with excess aqueous fe ii magnetite formed by a later intrusion of reducing fluids mostly along bedding planes but also as cross cutting veins this produced large quantities of magnetite layers parallel and sub parallel to the original bedding but that do not represent a primary depositional feature pyrite was produced in some portions of the unit likely under equilibrium conditions by later infiltration of reducing fluids containing sulfide the entirely positive 56fe values for all fe phases in the weld range bif stands in stark contrast to the more massive superior type bifs of the paleoproterozoic hamersley and transvaal basins of western australia and south africa respectively which have 56fe values that average close to 0 but vary over nearly the entire range of 56fe values measured in nature 56fe 2 5 to 2 6 the weld range bif 56fe values are similar to those of older algoma type bifs such as the paleoarchean isua bif of greenland but are less positive on average this overall trend of decreasing average 56fe values through time likely records an overall increasing trend of oxygenation of the archean surface ocean from 3 8 to 2 45 billion years ago

geochemical and mineralogic effects of contact metamorphism on banded iron formation an example from the transvaal basin south africa intrusion of a diabase sill into the paleoproterozoic kuruman iron formation transvaal basin south africa makes it possible to distinguish between effects of contact and regional metamorphism on mineralogic elemental and isotopic compositions of banded iron formation bif lithofacies temperatures within the aureole of contact metamorphism likely did not significantly exceed 450 c as suggested by the presence of early diagenetic carbonates greenalite and stilpnomelane formed prior to intrusion in chert rich facies as close as 4 m from the sill in sediments 32 m away from the intrusion a temperature of 280 c is indicated by the magnitude of 18o fractionation between successive chert and siderite microbands in unmetamorphosed equivalents similar isotopic analyses on whole rock samples yield temperature estimates between 150 and 230 c in comparison with samples of similar lithofacies from the underlying campbellrand kuruman transition zone carbonates from within the metamorphic aureole are depleted in 13c and 18o the abundance and c isotopic composition of total organic c toc in whole rock samples is lower and more enriched in 13c than those in samples outside of the zone of contact metamorphism within 5 m of the diabase sill the abundance of fe increases dramatically and the oxidation state of the sediments fluctuates considerably more than in sediments outside of the metamorphic aureole isotopic and mineralogic systematics in the zone of contact metamorphism in the kuruman iron formation are similar to those in siliceous dolomites and limestones altered by similar thermal processes as the most distinct isotopic and elemental variations occur within 10 m of the diabase sill it is suggested that volatilization and fluid infiltration associated with the intrusion were the main processes of alteration consideration of the effects of metamorphism both contact and regional as well as early diagenesis on the mineralogy and isotopic chemistry of bif sediments may be a key to our understanding of primary mineralogies and oxidation state of these enigmatic deposits

magnetite biomineralization in ferruginous waters and early earth evolution abstract burial of large quantities of magnetite fe ii fe iii 2o4 in iron formations ifs likely contributed to the protracted oxidation of earth s surface during the precambrian eons magnetite can form through a diversity of biological and abiotic pathways and its preservation in ifs may thus be variably interpreted as the result of some combination of these processes such interpretations give rise to divergent pictures of the precambrian earth system and models for its evolution through time new knowledge on the contribution of specific magnetite formation pathways is therefore needed to accurately tether our conceptual and numerical models to the geologic record to constrain pathways of magnetite formation under ferruginous conditions we conducted geochemical and multi method microspectroscopic analyses on particles obtained from the water columns and sediments of ferruginous lakes matano and towuti in indonesia we find that biologically reactive fe iii mineral phases are reduced in the anoxic waters of both lakes causing the formation of primary authigenic magnetite directly in the water column this water column magnetite often takes conspicuous framboidal forms which given the link to microbial fe iii reduction may provide a biological signature on early earth and by extension other planetary bodies the consumption of more biologically reactive forms of fe iii and the resulting delivery of primary magnetite to underlying sediments promotes the burial of oxidized equivalents and implies that primary magnetite formation could have been a principal pathway of fe delivery to ifs combined the removal of fe from earth s surface through biologically induced magnetite formation and subsequent burial in ifs suggests that seawater chemistry and the microbially mediated reactions that cause magnetite formation played key roles in earth system evolution and in setting the pace for planetary oxidation through the precambrian eons

spatial and temporal distribution of sand containing basin fills in the holocene rhine meuse delta the netherlands the quantitative significance of coarse grained deposits in the overbank realm such as crevasse splay deposits has not been studied at the delta scale or at the holocene timescale such knowledge would be beneficial for understanding and explaining sediment distribution in delta plains this study addresses delta scale distribution of sand containing basin fills and their sand body proportion variability based on eight valleywide cross sections in the holocene rhine meuse delta in the netherlands we found that sand containing basin fills form 7 1 of the fluviodeltaic wedge of which splay deposits are most frequently observed midway between the delta apex and the coast organic clastic lake fills and bay head delta deposits in contrast are limited to the distal delta plain over four successive periods between 9000 and 800 cal yr bp the largest proportions of splay deposits remain at 50 150 km downstream of the upstream shifting delta apex we show that intermediate floodbasin widths between 3 1 and 3 6 km in the holocene rhine meuse delta yield the highest proportions of splay deposits high rates of base level rise and wide floodplains both facilitate the creation of accommodation which in turn provides conditions for peat forming wetlands in which organic clastic lake fills can develop the results show that sand bodies form 26 30 of sand containing basin fills this proportion is shown to be controlled by among other variables channel planform and superelevation of the trunk channel and substrate composition we conclude that potentially large volumes of nonchannel sand bodies exist in distal delta plains they constitute up to 39 of the reservoir volume in the distal rhine meuse delta and yield relatively high connectedness ratios 2011 by the university of chicago all rights reserved

ancient geochemical cycling in the earth as inferred from fe isotope studies of banded iron formations from the transvaal craton variations in the isotopic composition of fe in late archean to early proterozoic banded iron formations bifs from the transvaal supergroup south africa span nearly the entire range yet measured on earth from 2 5 to 1 0 in 56fe 54fe ratios relative to the bulk earth with a current state of the art precision of 0 05 for the 56fe 54fe ratio this range is 70 times analytical error demonstrating that significant fe isotope variations can be preserved in ancient rocks significant variation in fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments and the range measured for bifs stands in marked contrast to the isotopic homogeneity of igneous rocks which have 56fe 0 00 0 05 as well as the majority of modern loess aerosols riverine loads marine sediments and proterozoic shales the fe isotope compositions of hematite magnetite fe carbonate and pyrite measured in bifs appears to reflect a combination of 1 mineral specific equilibrium isotope fractionation 2 variations in the isotope compositions of the fluids from which they were precipitated and 3 the effects of metabolic processing of fe by bacteria for minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis the relative order of 56fe values appears to decrease in the order magnetite siderite ankerite similar to that estimated from spectroscopic data although the measured isotopic differences are much smaller than those predicted at low temperature in combination with on going experimental determinations of equilibrium fe isotope fractionation factors the data for bif minerals place additional constraints on the equilibrium fe isotope fractionation factors for the system fe iii fe ii hematite magnetite fe carbonate 56fe values for pyrite are the lowest yet measured for natural minerals and stand in marked contrast to the high 56fe values that are predicted from spectroscopic data some samples contain hematite and magnetite and have positive 56fe values these seem best explained through production of high 56fe 54fe reservoirs by photosynthetic fe oxidation it is not yet clear if the low 56fe values measured for some oxides as well as fe carbonates reflect biologic processes or inorganic precipitation from low 56fe ferrous fe rich fluids however the present results demonstrate the great potential for fe isotopes in tracing the geochemical cycling of fe and highlight the need for an extensive experimental program for determining equilibrium fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments

deposition of banded iron formations by anoxygenic phototrophic fe ii oxidizing bacteria the mechanism of banded iron formation bif deposition is controversial but classically has been interpreted to reflect ferrous iron fe ii oxidation by molecular oxygen after cyanobacteria evolved on earth anoxygenic photoautotrophic bacteria can also catalyze fe ii oxidation under anoxic conditions calculations based on experimentally determined fe ii oxidation rates by these organisms under light regimes representative of ocean water at depths of a few hundred meters suggest that even in the presence of cyanobacteria anoxygenic phototrophs living beneath a wind mixed surface layer provide the most likely explanation for bif deposition in a stratified ancient ocean and the absence of fe in precambrian surface waters

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

geneses and evolutions of iron bearing minerals in banded iron formations of 3760 to ca 2200 million year old constraints from electron microscopic x ray diffraction and mossbauer spectroscopic investigations abstract minerals preserved in banded iron formations bifs are diagenetic and or metamorphic rather than primary knowing their geneses and evolutions are essential for understanding the interactions between the atmosphere ocean and biosphere in the first two billion years of life history on earth here we use high resolution electron microscopy x ray diffraction and mossbauer spectroscopy to investigate the mineralogy of six bifs aged from 3760 ma to ca 2200 ma with a variety of genetic conditions the results reveal that each major iron mineral magnetite hematite carbonate and silicate has a range of structural and morphological features that represent different geneses and evolutionary histories euhedral magnetite crystals with homogeneous internal structures and compositions 3 5 nm hematite particles aggregated in iron rich bands and submicrometer sized euhedral hematite in chert matrix of fe si transitional bands are identified to have transformed from precursors directly precipitated from seawaters these minerals retain some primary environmental information when bifs are deposited the other mineral phases display features of alteration due to multi redox reactions hydrothermal or supergene processes such as crosscutting mineral replacement or overgrowth deformation fracture and porous structures that have overprinted the primary signatures such minerals include 1 massive magnetite with fragmentation structures 2 partly to completely oxidized magnetite crystals 3 acicular fibrous hematite in fractures or iron and silica layering 4 microplaty hematite 5 hematite oxidized from other fe ii bearing minerals e g magnetite and stilpnomelane 6 carbonates with heterogeneous chemical compositions 7 carbonates replacing other minerals e g siderite thin film coating the euhedral magnetite crystals 8 irregular carbonate cementing other minerals e g magnetite 9 preferentially orientated fibrous spindle like silicates cutting iron and silica rich bands and 10 massive silicate pieces this study provides detailed mineralogical and petrologic characterizations of bif minerals that can serve as a reference for future in situ geochemical measurements by recent developed techniques

alternating si and fe deposition caused by temperature fluctuations in precambrian oceans precambrian banded iron formations contain thin alternating layers of iron and silica rich minerals microbial culture experiments show that these alternations could be caused by changes in the relative dominance of microbial iron oxidation and abiotic silica precipitation driven by fluctuating ocean temperature

iron formation in neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts abstract neoarchean deltaic deposits present in superior province canada contain an excellent record of shallow water siliciclastic and iron formation if deposition the deltas were fed by braided fluvial systems which commonly resulted in the development of small active lobes that repeatedly cycled through progradation and abandonment although a thick progradational deltaic assemblage was also investigated magnetite rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore proximal fluvial channels and distributary mouth bars of the braid delta deposits magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in ripples to barforms thicker accumulations of if up to 73 m form portions of transgressive systems tracts where chemical sedimentation dominated offshore equivalents of these successions do not contain chemical sediments sedimentologic data imply that if deposition in this well mixed nearshore water mass was not compatible with slow rates of constant iron compound precipitation in the world ocean combined with siliciclastic sediment starvation the model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore stimulating photosynthetically induced oxidation of fe2 under these conditions elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts

primary hematite in neoarchean to paleoproterozoic oceans banded iron formations bifs are ironand silica rich chemical sedimentary rocks formed throughout the archean and paleoproterozoic eras the presence of hematite fe2o3 and magnetite fe3o4 in bifs has led to the widespread assumption that fe ii oxidation must have occurred in the ancient oceans via either a biological or chemical mechanism however it is unclear whether the ferric iron now present in bif represents the original ferric oxyhydroxide e g ferrihydrite fe oh 3 precipitated in the water column or if it is the result of later stage circulation of oxidizing fl uids through the sediment pile in this study we conducted high resolution microscopic investigations on bif from the 2728 ma abitibi greenstone belt located in the superior province of the canadian shield and the 2460 ma kuruman iron formation in south africa to ascertain the timing and paragenesis of the hematite three types of hematite are identifi ed by high resolution electron microscopic characterization and selected area electron diffraction 1 3 5 nm ultrafi ne hematite particles in the iron oxide rich bands h1 2 submicrometer subhedral to euhedral hema tite crystals randomly distributed in the chert matrix of transitional zones between iron oxide and chert rich bands h2 and 3 needle like radial and fi brous hematite that replaced stilpnomelane or carbonates and is distributed along fractures or layer boundaries h3 we interpret the fi rst two types as primary minerals dehydrated from precursor ferric oxyhydroxides h1 remains ultrafi ne in size while h2 has undergone an ostwald coarsening process facilitated by internal fl uids produced during amorphous silica to quartz transformation h3 is a later stage mineral formed by external fl uid mediated replacement of iron silicates or carbonates these results indicate that a signifi cant fraction of the hematite in the bif originated from ferric oxyhydroxide precursors importantly this implies that photosynthetic fe ii oxidation by either a direct or indirect biological mechanism did exist in seawaters from which some bif material was deposited

biogenic fe iii minerals from formation to diagenesis and preservation in the rock record abstract fe metabolizing bacteria are intimately linked to the cycling of fe in modern environments and have likely been key players in the evolution of the earth s biogeosphere fe minerals have also been suggested as a key preservative of cell organic matter in sediments keeping otherwise labile phases conserved at least on time scales of 100 000 years the interpretation of a biological influence on the fe rock record is difficult without a deeper understanding of the mechanisms of biogenic fe iii and fe ii mineral formation the character of these minerals and their diagenesis over short and long time scales here we present the recent advances in the study of abiogenic and biogenic fe iii minerals in particular we focus on the role of fe ii oxidizing bacteria in the deposition of ancient banded iron formations bif we discuss this work within the framework of the main challenge separating biogenic from abiogenic processes over deep time we describe how efforts in isotope geochemistry biomarker research mineral analysis and biogeochemistry are helping to establish a window to the past finally we present some new approaches that help investigate the main processes leading to the formation and potential fate of fe organic matter aggregates

conditions of riebeckite formation in the iron formation of the dales gorge member hamersley group western australia stability relations in the system na fe mg si c o h at low temperature and pressure at constant temperature riebeckite is stable to lower fo 2 conditions than hematite and higher ph values than magnetite or hematite where riebeckite coexists with carbonates its stability field is enlarged by a decrease of total carbonate at constant ph riebeckite assemblages in the dales gorge member can be divided into two groups carbonate free and carbonate bearing modified journal abstract

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

the formation of magnetite in the early archean oceans abstract banded iron formations bifs are iron and silica rich chemical sedimentary rocks that were deposited throughout much of the precambrian the biological oxidation of dissolved fe ii led to the precipitation of a ferric oxyhydroxide phase such as ferrihydrite in the marine photic zone upon burial ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological fe iii reduction coupled to the oxidation of buried microbial biomass however it has always been intriguing as to why the oldest bifs are characteristically magnetite rich while bifs formed after the neoarchean are dominated by hematite here we propose that some magnetite in early archean bif could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot fe ii rich hydrothermal fluids that existed in the deeper waters we conducted experiments that showed the reaction of fe ii with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite our model further posits that with the progressive cooling and oxidation of the earth s oceans the above reaction shuts off and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism

depositional environment of the paleoproterozoic yuanjiacun banded iron formation in shanxi province china the paleoproterozoic 2 38 2 21 ga yuanjiacun banded iron formation bif located in shanxi province is a superior type bif in the north china craton this bif is within a metasedimentary rock succession of the yuanjiacun formation in the lower luliang group which has undergone lower greenschist facies metamor phism iron oxide magnetite and hematite carbonate and silicate facies are all present within the iron rich layers the eastward transition from carbonate into oxide facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite ankerite and magnetite stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east these distinct lateral facies are also observed vertically within the bif i e the iron mineral assemblage changes upsection from sider ite through magnetite into hematite rich iron formation the oxide facies bif formed near shore whereas carbonate siderite and silicate facies assemblages formed in deeper waters based on detailed analyses of these variations on a basinal scale the bif precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters the bif samples display distinctively seawater like rees y profiles that are characterized by positive la and y anomalies and hrees enrichment relative to lrees in post archean australian shale normalized diagrams consistently positive eu anomalies are also observed which are typical of reduced high temperature hydrothermal fluids in addition slightly negative to positive ce anomalies and a large range in ratios of light to heavy rees are present in the oxide facies bif these characteristics in combination with consistently positive 56fe values suggest that deposition of the bif took place along the chemocline where upwelling of deep anoxic iron and silicarich hydrothermal fluids mixed with shallower and slightly oxygenated seawater the ankerite displays highly depleted 13c values and the carbonate rich bif has a high content of organic carbon suggesting dissimilatory fe iii reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column that same biomass was likely tied to the original oxidation of dissolved fe ii the fact that the more ferric bif facies formed in shallower waters suggests that river sourced nutrients would have been minimal thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory fe iii reduction by contrast in deeper waters more proximal to the hydrothermal vents nutrients were abundant and high biomass productivity was coupled to increased carbon burial leading to the deposition of iron rich carbonates the deposition of the yuanjiacun bif during the onset of the great oxidation event goe ca 2 4 2 2 ga confirms that deep marine waters during this time period were still episodically ferruginous but that shallow waters were sufficiently oxygenated that fe ii oxidation no longer needed to be tied directly to proximal cyanobacterial activity

simulating precambrian banded iron formation diagenesis abstract post depositional diagenetic alteration makes the accurate interpretation of key precipitation processes in ancient sediments such as precambrian banded iron formations bifs difficult while microorganisms are proposed as key contributors to bif deposition the diagenetic transformation of precursor fe iii minerals associated with microbial biomass had not been experimentally tested we incubated mixtures of ferrihydrite proxy for biogenic ferric oxyhydroxide minerals and glucose proxy for microbial biomass in gold capsules at 1 2 kbar and 170 c both wet chemical analysis and mineralogical methods microscopy x ray diffraction and mossbauer spectroscopy were used to analyze the reaction products under these conditions ferrihydrite fe iii oh 3 transforms to hematite fe 2 iii o 3 magnetite fe ii fe 2 iii o 4 and siderite fe ii co 3 silica coated ferrihydrite prepared at conservative si fe ratios as predicted for the precambrian oceans and mixed with glucose yielded hematite and siderite whereas magnetite could not be identified microscopically our results show that electron transfer from organic carbon to fe iii minerals during temperature pressure diagenesis can drive the production of key bif minerals our results also demonstrate that the post depositional mineralogy of bif does not directly archive the oceanic or atmospheric conditions present on earth during their lithification as a consequence atmospheric composition regarding concentrations of methane and co 2 during the time of bif mineral deposition cannot be directly inferred from bif mineralogical data alone

preservation of fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation we present the iron isotope composition of primary diagenetic and metamorphic minerals in five samples from the contact metamorphosed biwabik iron formation these samples attained peak meta morphic temperatures of 1 on the millimeter scale minerals in the lowest grade samples consistently show a sequence in which d 56 fe of magnetite silicate carbonate the inter mineral fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature dependent fractionation factors textural evidence re veals that most if not all magnetite in the biwabik formation is diagenetic not primary and that there was tremendous element mobility during diagenesis the short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine magnetite compositions there fore act as an isotope record of the environment in which these sediments formed larger scale fluid flow and longer timescales may allow equilibration of fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environ ments but weakly regionally metamorphosed rocks may preserve small scale fe isotopic heterogeneities like those observed in the biwabik iron formation importantly fe isotope compositions that are charac teristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter mineral variations and at high grade in the form of unique bulk rock compositions this observa tion confirms earlier work that has suggested that fe isotopes can be used to identify sedimentary processes in the precambrian rock record

mineral ecophysiological data provide growing evidence for microbial activity in banded iron formations the phosphorus composition of banded iron formations bifs has been used as a proxy for precambrian seawater composition and the paleoeredox state of earth s surface environment however it is unclear whether the phosphorus in bifs originally entered the sediment as a sorbed component of the iron oxyhydroxide particles or whether it was incorporated into the biomass of marine phytoplankton we conducted high resolution mineral analyses and report here the first detection of an fe iii acetate salt as well as nanocrystals of apatite in association with magnetite in the 2 48 ga dales gorge member of the brockman iron formation a bif hamersley western australia the clusters of apatite are similar in size and morphology to biogenic apatite crystals resulting from biomass decay in phanerozoic marine sediments while the formation of an fe iii acetate salt and magnetite not only implies the original presence of biomass in the bif sediments but also that organic carbon likely served as an electron donor during bacterial fe iii reduction this study is important because it suggests that phytoplankton may have played a key role in the transfer of phosphorus and other trace elements from the photic zone to the seafloor

the mineralogy mineral chemistry and origin of the wuyang banded iron formations north china craton abstract several banded iron formations bifs occur in precambrian strata of the north china craton ncc two sub types of bif occur in the wuyang area quartz magnetite qmb and pyroxene magnetite bifs pmb however the genesis of pyroxene and the how the bifs formed are not well understood magnetite grains in qmb are relatively large with a straight boundary and sharp corners and display a pronounced alignment whereas magnetite in pmb is disseminated and has a smaller grain size x ray diffraction xrd analyses indicate that the crystal parameters of magnetite in qmb are a b c 8 396 a and z 8 whereas those of the magnetite in pmb are a b c 8 394 a and z 8 these parameters are consistent with magnetite forming in sedimentary iron formation based on scanning electron microscopy and energy dispersive spectroscopy eds results magnetite of both bifs possesses high stabilities of geochemical property compared with magnetite in pmb magnetite in qmb has higher concentrations of sio2 na2o al2o3 cao k2o mgo sc co y nb ta and u the enrichment of these elements is consistence with their derivation from seawater due to the strong adsorption capability of precursor iron hydroxide a spider diagram of trace elements reveals that magnetite in qmb is enriched in ta whereas magnetite of pmb is depleted in ce based on raman and electron probe micro analysis of pyroxene in pmb synchronous and compositional zoning are found in the degrees of crystallinity fwhm values of gaussian fitting for raman shift next to 1014 cm 1 and the concentration of sio2 al2o3 mgo mno cao and feo of pyroxene from core to rim the two types of bif formed in different sedimentary environments from different precursors the precursor to the qmb bif is almost pure chemical sediment whereas a that for pmb contains a volcanic ash component the qmb was likely deposited during a period of hydrothermal sedimentation in the wuyang basin that was associated with neoarchean magmatism activity while the pmb was deposited during periods of volcanic paroxysm

the link between biomineralization and fossilization of bacteria insights from field and experimental studies abstract fossil biominerals and fossils of microorganisms in ancient rocks contain important biogeochemical signals decoding this record may reveal crucial information about the evolution of life on earth and terrestrial paleoenvironments however the identification of traces of life especially in very old rocks is extremely challenging because the morphological and chemical signatures of biominerals and microfossils are subtle of microscopic size and inevitably altered with aging in this review we stress on the fact that biomineralization is often the first step of fossilization and produces particular chemical structural and morphological features that can be preserved in fossil biominerals or microfossils with a special focus on fe biomineralization the taphonomic processes affecting biominerals and microfossils and altering their morphology and or chemical composition over time are then discussed in light of experimental fossilization simulations and field sample analyses we suggest that taxonomic biases observed in the fossil record may be related to differential abilities of species to trigger biomineralization this calls for studies of the effective biomineralizing activity and fossilization potential of each species present in highly diverse natural microbial communities finally recent analytical developments leave little doubt that very substantial progress in the study of biomineralization processes and ancient biostructures will be achieved in the near future

extending the upper temperature limit for life the upper temperature limit for life is a key parameter for delimiting when and where life might have evolved on a hot early earth the depth to which life exists in the earth s subsurface and the potential for life in hot extraterrestrial environments a combination of geological and