sulfur record of rising and falling marine oxygen and sulfate levels during the lomagundi event carbonates from approximately 2 3 2 1 billion years ago show markedly positive 13c values commonly reaching and sometimes exceeding 10 traditional interpretation of these positive 13c values favors greatly enhanced organic carbon burial on a global scale although other researchers have invoked widespread methanogenesis within the sediments to resolve between these competing models and more generally among the mechanisms behind earth s most dramatic carbon isotope event we obtained coupled stable isotope data for carbonate carbon and carbonate associated sulfate cas cas from the lomagundi interval shows a narrow range of 34s values and concentrations much like those of phanerozoic and modern marine carbonate rocks the 34s values are a close match to those of coeval sulfate evaporites and likely reflect seawater composition these observations are inconsistent with the idea of diagenetic carbonate formation in the methanic zone toward the end of the carbon isotope excursion there is an increase in the 34s values of cas we propose that these trends in c and s isotope values track the isotopic evolution of seawater sulfate and reflect an increase in pyrite burial and a crash in the marine sulfate reservoir during ocean deoxygenation in the waning stages of the positive carbon isotope excursion

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

element geochemistry and neodymium isotope systematics of the neoarchean banded iron formations in the qingyuan greenstone belt north china craton abstract pure marine chemical sediments such as banded iron formations bif are excellent archives of ancient seawater and may also provide constraints on the coeval hydrosphere atmosphere system however on the bases of interpreted depositional settings it has long been argued that algoma type bif merely recorded the intrabasinal volcanic or hydrothermal conditions here we couple systematic major and trace elements data with samarium sm neodymium nd isotopic analyses of 2 56 ga old algoma type bifs and associated schists from different stratigraphic horizons of the qingyuan greenstone belt qgb in the north china craton ncc to characterize the source and depositional environment over a short period 30 ma meanwhile we also compare our results with data for other archean to paleoproterozoic superior and algoma type bifs especially those near contemporaneous counterparts in the ncc to further evaluate the reliability of qgb bifs as proxies for the neoarchean seawater low al2o3

zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

trace element and stable isotope characteristics of algoma type sulfidic banded iron formations from the wutai complex central north china craton abstract the major archive of iron deposits in china occur within precambrian banded irons formations bifs here we investigate the wutai complex that represents an archean to paleoproterozoic subduction related arc system between the ordos and qianhuai micro blocks of the central north china craton ncc we report the carbon and oxygen isotopes of calcites from carbonate rich bifs and in situ trace elements la icp ms and sulfur isotopes ion probe of pyrites from sulfidic bifs and carbonate samples with an aim to characterize the nature and genesis of the iron deposits associated with these rocks the 13cv pdb 18ov pdb and 18ov smow of calcites range from 2 4 to 1 1 20 4 to 19 3 and 9 9 to 11 respectively displaying archean greenstone belt and marine carbonate settings pyrite from the bifs and carbonates are enriched ni as se te and pb their 34scdt values range from 8 8 to 7 9 with a peak at 0 0 comparing with the features of typical algoma type bifs our study provides insights into the depositional environment of the bifs in the wutai complex

evidence for the silicate source of relict soils on the edwards plateau central texas relict soils provide insights into quaternary soil formation and erosion on the edwards plateau of central texas and into soil forming processes in karst terranes late quaternary climate driven soil erosion produced a mosaic of thick and thin soils on the edwards plateau landscape thick soils on uplands of the edwards plateau are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late pleistocene to middle holocene the relict thick soils are silicate rich and most commonly overlie the relatively silicate poor cretaceous edwards limestone which supports the idea that the thick soils did not form from weathering of the underlying limestone other potential sources of silicates for the relict soils include dust alluvial sediments and the del rio clay a stratigraphically higher but locally eroded clay rich unit here we investigate the geographic distribution texture clay sized mineralogy rare earth element geochemistry and neodymium isotope composition of the relict soils we have found that the relict thick soils are deeply weathered soils that occur dominantly over the edwards limestone and have a high clay content and a neodymium isotope composition that is similar to that of the del rio clay thus we propose that in situ weathering of the del rio clay along with partial weathering of the upper portion of the underlying edwards limestone produced thick chert and clay rich soils over resistant limestone in areas like the edwards plateau where pure limestones are interbedded with clay rich strata the overlying clay rich strata must be considered as a possible silicate source to soils on pure limestone bedrock

fluvial response to late quaternary climatic and environmental change edwards plateau texas abstract radiocarbon controlled late pleistocene to modern stratigraphic frameworks for fluvial systems that drain the edwards plateau westcentral texas coupled with reconstruction of climatic and environmental changes permit development of a model for the evolution of fluvial landscapes during the past 20 000 years during this time large valleys of the edwards plateau were characterized by channel aggradation and sediment storage from 20 to 14 000 yrs b p deep excavation of bedrock valleys from ca 14 to 11 000 yr b p and deposition of extensive and complex valley fills during the last 11 000 yr valley fills contain three distinct unconformity bounded allostratigraphic units representing episodes of channel aggradation and floodplain construction during the early to middle ca 11 000 5000 yr b p and late holocene ca 5000 1000 yr b p and development of the incised channels and associated narrow floodplains of the last 1000 years early to middle holocene alluvial deposits were derived from proximal sources within the respective drainage basins whereas late holocene deposits include sediments derived from distal sources sediment supply to major valleys axes has been extremely limited during the last 1000 years alluvial sequences of the edwards plateau record fluvial responses to climatically driven changes in discharge regimes and the concentration of sediments along valley axes allostratigraphic units define time periods when the concentration of sediment exceeded stream power resulting in sediment storage whereas unconformities record widespread morphological and sedimentary adjustments unconformities between late pleistocene alluvium and holocene valley fills record bedrock valley incision in response to decreased sediment loads associated with slope stability in the uplands by contrast unconformities within the holocene valley fill record floodplain abandonment accompanied by continued channel migration and sediment storage but little additional bedrock valley cutting episodes of floodplain abandonment occurred as a result of decreased flood magnitudes following shifts to drier climatic conditions at ca 5000 and 1000 yr b p fluvial responses to climatic change were conditioned by a progressive degradation of upland soils that caused increases through time in the flashiness of flood events which in turn led to changes in processes of floodplain construction flood events on late pleistocene and early to middle holocene rivers were for the most part contained within channel perimeters and floodplains were constructed by lateral migration by contrast late holocene rivers were characterized by deep overbank flooding and floodplain construction by vertical accretion high magnitude floods were most significant from ca 2500 to 1000 yr b p when large chute channels were cut and filled on floodplain surfaces and soils developed on previously stable terrace surfaces were buried by up to 2 m of fine sands and muds

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

the late devonian frasnian famennian f f biotic crisis insights from δ13ccarb δ13corg and 87sr 86sr isotopic systematics a severe biotic crisis occurred during the late devonian frasnian famennian f f transition f367 myr here we present d 13 ccarb d 13 corg and 87 sr 86 sr isotopic systematics from identical samples of two sections across f f boundary in south china which directly demonstrate large and frequent climatic fluctuations 200 kyr from warming to cooling during the f f transition these climate fluctuations are interpreted to have been induced initially by increased volcanic outgassing and subsequent enhanced chemical weathering linked to the rapid expansion of vascular plants on land which would have increased riverine delivery to oceans and primary bioproductivity and subsequent burial of organic matter thereby resulting in climate cooling such large and frequent climatic fluctuations together with volcanic induced increases in nutrient e g biolimiting fe toxin sulfide and anoxic water supply and subsequent enhanced riverine fluxes and microbial bloom were likely responsible for the stepwise faunal demise of f f biotic crisis d 2005 elsevier b v all rights reserved

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

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

geochronological and geochemical constraints on the origin of clastic meta sedimentary rocks associated with the yuanjiacun bif from the luliang complex north china abstract the luliang complex is situated in the central part of the western margin of the trans north china orogen tnco in the north china craton ncc and consists of metamorphic volcanic and sedimentary rocks and granitoid intrusions the yuanjiacun formation metasediments occupy roughly the lowest part of the luliang group and are mainly represented by well bedded meta pelites chlorite schists and sericite chlorite phyllites banded iron formations bifs and meta arenites sericite schists which have undergone greenschist facies metamorphism the youngest group of detrital zircons from the meta arenite samples constrains their maximum depositional age at 2350 ma in combination with previous geochronological studies on meta volcanic rocks in the overlying jinzhouyu formation the depositional age of the yuanjiacun formation can be constrained between 2350 and 2215 ma the metasediments have suffered varying degrees of source weathering measured using widely employed weathering indices e g cia ciw pia and th u ratios source rocks of the low al meta pelites have undergone severe chemical weathering whereas those of the meta arenites and high al meta pelites have suffered relatively moderate chemical weathering significant secondary k addition is recognized in the a cn k diagram for most of the studied samples diagnostic geochemical features like the al 2 o 3 tio 2 values trace element ratios e g th sc and ree patterns suggest that the meta arenites and high al meta pelites are predominantly derived from felsic igneous sources whereas the low al meta pelites are sourced mainly from mafic rocks coupled with nd isotopic data it is proposed that the meta arenites and high al meta pelites were sedimentary erosion products of the less differential felsic terrain likely the old upper continental crust the low al meta pelites however have geochemical affinities with both pelite and bif like components suggesting that they were mixtures of these two sediments moreover the u pb ages of detrital zircons from the yuanjiacun metasediments yield three age populations of 2 8 2 65 ga 2 6 2 5 ga and 2 5 2 35 ga the dominant archean detrital zircons were most likely sourced from both the archean igneous rocks in the hengshan wutai fuping belt and old hidden archean basement rocks in the western block whereas the subordinate paleoproterozoic detrital zircons were probably derived from the granitoid plutons within this age range in the luliang complex taking the geochemical and geochronological data together the yuanjiacun formation is interpreted as having been deposited in a passive margin setting likely the eastern margin of the western block most probably on a stable continental shelf

primitive os and 2316 ma age for marine shale implications for paleoproterozoic glacial events and the rise of atmospheric oxygen abstract re os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the rooihoogte and timeball hill formations transvaal supergroup south africa provides a precise isochron age of 2316 7 ma 4 statistical uncertainty if error on decay constant is excluded and a chondritic initial 187os 188os ratio of 0 1121 0 0012 these units were deposited between what are interpreted as the second and third of three paleoproterozoic global glacial events and thus provide minimum and maximum ages respectively for these events the rooihoogte formation is correlative with the duitschland formation which records previously undated carbon isotope excursions because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative 34s values the rise of atmospheric oxygen most likely began prior to 2 32 ga the chondritic initial 187os 188os ratio requires that primitive hydrothermal sources of os dominated the os budget of seawater at 2 32 ga os is mobile only under oxidizing conditions therefore the chondritic initial 187os 188os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic os to offset the primitive hydrothermal magmatic flux even if atmospheric oxygen levels had increased significantly by 2 32 ga anoxic conditions throughout the archean most likely limited os enrichment in archean marine shales so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic os to paleoproterozoic seawater

depositional environment and metamorphism of early proterozoic iron formation in the luliangshan region shanxi province china abstract the early proterozoic iron formation of the luliangshan region is 18 km long and has undergone different degrees of metamorphism at different localities within this belt the yuanjiacun deposit is characterized by low grade metamorphism diverse iron sedimentation facies a well exposed stratigraphic section and simple structure it is an ideal case for studying the depositional environment and the metamorphic processes of precambrian banded iron ores formed in a shallow water basin of a geosyncline the yuanjiacun iron formation some 1200 m thick comprises three sedimentation ore formation cycles each of which begins with coarse to medium grained clastic rocks quartz sandstone or arkose passes through fine grained siltstone and argillitic rocks and grades into pure colloidal chemical deposits represented by thin bedded ferruginous quartzites in each cycle from the bottom upwards in general there successively occur pyrite siderite iron silicates including fe rich chlorite minnesotaite and stilpnomelane magnetite and haematite these iron mineral facies are products of sedimentation and diagenesis to very low grade metamorphism stages the spatial distribution of iron facies in stratigraphic sequence shows an increasing oxygen fugacity of the depositional environment during the course of each cycle the main factors controlling the types of iron sedimentation facies and the valence state of iron are considered to be the proportion of sandy argillitic component to colloidal chemical materials during deposition and the relative amount of organic matter in them the abundance of organic matter results in the creation of a reducing environment and the formation of principally ferrous facies whereas the lack of organic matter leads to the development of oxidizing conditions and the formation of ferric facies under a transitional oxidation reduction medium mixed ferrous ferric facies dominate the fracture related regional metamorphism yielded progressive metamorphic zonation in the luliangshan region the chlorite zone and the biotite zone of greenschist facies and the almandite zone staurolite zone and kyanite zone of the amphibolite facies these metamorphic zones cross cut banded iron ores and produce corresponding changes in the iron formation

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

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

geochronological and geochemical constraints on the luliang group in the luliang complex implications for the tectonic evolution of the trans north china orogen abstract the luliang complex is located at the western margin of the middle segment of the trans north china orogen along which the western and eastern blocks amalgamated to form the basement of the north china craton the complex consists of the late neoarchean to paleoproterozoic granitic plutons and meta supracrustal rocks of which the latter are subdivided into the jiehekou luliang yejishan and heichashan lanhe groups the luliang group is composed mainly of siliciclastic rocks in the lower part and volcanic rocks in the upper part all of which have been metamorphosed from greenschist to amphibolite facies u pb ages of detrital zircons from meta sedimentary rocks in the luliang group yield four populations of 3061 2920 ma 2790 2600 ma 2599 2280 ma and 2211 2202 ma suggesting that their provenances were derived mainly from the luliang and taihua complexes in the trans north china orogen the meta volcanic rocks give igneous zircon u pb ages of 2209 2178 ma and e hf t values of 0 8 to 3 6 geochemically they are subalkaline basalts and andesites that can be classified into two groups of which one group has flat lrees weak enrichment in lrees relative to hrees and flat hrees and displays e morb like spider diagrams with the exception of insignificantly negative nb ta zr hf and ti anomalies in contrast the other group displays stronger lree fractionation stronger enrichment in lrees relative to hrees and mildly fractionated hrees and is characterized by spiky spider diagrams similar to arc volcanic rocks the first group is interpreted to have originated from a neoarchean e morb source whereas the second group shows inputs of newly subduction derived fluids in the mantle source these geochronological and geochemical signatures indicate the existence of an 2 2 2 1 ga continental back arc system in the luliang complex of the trans north china orogen which is consistent with the model that the collision between the eastern and western blocks to form the coherent basement of the north china craton in the paleoproterozoic not at the end of neoarchean

oxygen dynamics in the aftermath of the great oxidation of earth s atmosphere the oxygen content of earth s atmosphere has varied greatly through time progressing from exceptionally low levels before about 2 3 billion years ago to much higher levels afterward in the absence of better information we usually view the progress in earth s oxygenation as a series of steps followed by periods of relative stasis in contrast to this view and as reported here a dynamic evolution of earth s oxygenation is recorded in ancient sediments from the republic of gabon from between about 2 150 and 2 080 million years ago the oldest sediments in this sequence were deposited in well oxygenated deep waters whereas the youngest were deposited in euxinic waters which were globally extensive these fluctuations in oxygenation were likely driven by the comings and goings of the lomagundi carbon isotope excursion the longest lived positive 13c excursion in earth history generating a huge oxygen source to the atmosphere as the lomagundi event waned the oxygen source became a net oxygen sink as lomagundi organic matter became oxidized driving oxygen to low levels this state may have persisted for 200 million years

decoupling photochemical fe ii oxidation from shallow water bif deposition abstract oxidized fe minerals in archean paleoproterozoic banded iron formations bifs are commonly taken to indicate the presence of biogenic o 2 or photosynthetic fe ii oxidizing bacteria in the oceans photic zone however at least one viable abiogenic oxidation mechanism has been proposed prior to the rise of atmospheric oxygen and the development of a protective ozone layer the earth s surface was subjected to high levels of ultraviolet radiation bulk ocean waters that were anoxic at this time could have supported high concentrations of dissolved fe ii under such conditions dissolved ferrous iron species such as fe 2 and fe oh would have absorbed radiation in the 200 400 nm range leading to the formation of dissolved ferric iron fe iii which in turn would have hydrolyzed to form ferric hydroxide fe oh 3 at circumneutral ph cairns smith a g 1978 precambrian solution photochemistry inverse segregation and banded iron formations nature 76 807 808 braterman p s cairns smith a g and sloper r w 1983 photo oxidation of hydrated fe 2 significance for banded iron formations nature 303 163 164 this process has been invoked to account for bif deposition without need for biology francois l m 1986 extensive deposition of banded iron formations was possible without photosynthesis nature 320 352 354 here we evaluate the potential importance of photochemical oxidation using a combination of experiments and thermodynamic models the experiments simulate the chemistry of ambient precambrian seawater mixing with fe ii rich hydrothermal fluids with and without uv irradiation we find that if fe ii was effused from relatively shallow seamount type vent systems directly into an anoxic photic zone the photochemical contribution to solid phase precipitation would have been negligible instead most of the fe ii would have precipitated rapidly as an amorphous precursor phase to the ferrous silicate mineral greenalite fe 3 si 2 o 5 oh 4 and or the ferrous carbonate siderite feco 3 depending on different simulated atmospheric pco 2 levels conversely in experiments where fe ii was exposed either to phototrophic fe ii oxidizing bacteria or to o 2 ferric hydroxide formed rapidly and the precipitation of ferrous iron phases was not observed if as suggested on mass balance grounds bif deposition requires that fe be sourced from shallow seamount type systems then we are driven to conclude that oxide facies bif are the product of a rapid non photochemical oxidative process the most likely candidates being direct or indirect biological oxidation and that a significant fraction of bif could have initially been deposited as ferrous minerals

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

effects of mass transfer compaction and secondary porosity on hydrothermal upgrading of paleoproterozoic sedimentary manganese ore in the kalahari manganese field south africa in the northwestern part of the kalahari manganese field low grade carbonate rich mamatwan type ore is altered to high grade oxide rich wessels type ore in association with normal faults mass balance calculations based on the assumption that manganese was geochemically immobile suggest that upgrading of the manganese ore can be attributed to leaching of mg ca co2 and sio2 from the sedimentary ore with residual enrichment of mn hydrothermal alteration resulted in development of about 10 to 20 of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness

the archaean and early proterozoic banded iron formations of north china their characteristics geotectonic relations chemistry and implications for crustal growth abstract bif occurs extensively throughout the archaean and early proterozoic terrains of northern china there are four main iron metallogenic provinces anshan in n e china e hebei and wutai in n china and qingling dengfeng taihua on the south margin of the north china platform all bifs have undergone varying degrees of regional metamorphism from subgreenschist to granulite facies and deformation the chinese bifs have three marked characteristics 1 most bif are archaean in age their formation reaching a peak during the late archaean 2 bif in china occur in both high grade regions and greenstone belts the former are more significant their reserves accounting for more than three quarters of those of the precambrian bif in china 3 oxide iron formation is predominant in nearly all archaean bif silicate and carbonate iron formations are rare the few carbonate iron formations occur in lower proterozoic sediments there is no difference in chemical composition between bif in high grade regions and in greenstone belts however there are geochemical and petrological differences between their associated supracrustal sequences the presence of such large amounts of bif within high grade regions is unusual from their regional geology rock association and from the geochemistry of associated metavolcanics and orthogneisses we suggest that the high grade type of bif formed in arcs or back arc basins which were intruded by voluminous tonalites and granodiorites later converted by deformation and high grade metamorphism into orthogneisses such processes of accretion and crustal growth are different in kind but not in principle from those operating today

primary and diagenetic controls of isotopic compositions of iron formation carbonates mineralogic chemical and isotopic compositions have been determined for 97 carbonate microbands in five core segments from the early proterozoic 2 5 ga dales gorge member of the brockman iron formation hamersley basin western australia samples were obtained both from banded iron formation bif macrobands 9 12 at paraburdoo on the southern margin of the basin and from bif macroband 13 at wittenoom 130 km nw at paraburdoo oxygen isotopic compositions of coexisting chert and magnetite microbands were measured and indicated final equilibration temperatures ranging from 60 160 degrees c this range is consistent with observed mineral assemblages and indicates a considerable temperature gradient across the basin cf t approximately 300 degrees c at wittenoom becker and clayton 1976 carbon isotopic compositions of carbonates are near 7 vs pdb at paraburdoo and 10 5 at wittenoom but the greater isotopic depletion at wittenoom appears related to primary or diagenetic processes not metamorphism contents of organic carbon are consistently low isotopic depletion is roughly correlated with iron abundance and together with petrographic observations and chemical balances is consistent with the model of bif deposition introduced by beukes et al 1990 primary siderite delta approximately 5 precipitated from an anoxic water column depleted in 13c additional depletion of 13c is associated with coprecipitation of iron oxides and organic carbon oxygen isotopic abundances of microbanded carbonates are similar to those under and overlying massive marine carbonates ranging from 17 6 to 21 0 vs smow 9 6 to 12 9 vs pdb millimeter scale variations in abundances of 13c and 18o are associated with diagenetic replacement of primary siderite by secondary ankerite and or magnetite it is shown that these isotopic variations cannot result from mineral dependent fractionations metamorphism or the influence of large volumes of water in an open system

redistribution of rare earth elements thorium and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism the role of apatite and monazite in orthogneisses from southwestern norway the amphibolite to granulite facies transition has been studied in a high k calc alkaline hornblende biotite k feldspar megacrystic augen gneiss series from the rogaland vest agder rog va sector of the sveconorwegian province southwest norway hornblende begins to break down mainly to clinopyroxene at the cpx in isograd and biotite mainly to orthopyroxene at the opx in isograd abbreviations for minerals following kretz 1983 the magmatic accessory mineral association comprises titanite and allanite which begin to break down before the cpx in isograd titanite is preserved as relict inclusions in other minerals at higher grade monazite and thorite formed in the breakdown of allanite monazite abundance reaches a maximum between the cpx in and opx in isograds the middle and heavy rare earth element m hrees except eu content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite hornblende and biotite the u contents of apatite are low and do not increase with metamorphic grade the light rare earth elements lrees and th content of apatite are not correlated to the breakdown of allanite around the cpx in isograd but increase around the opx in isograd a simplified equation is proposed for monazite crystallisation in the vicinity of the cpx in isograd that accounts for the allanite titanite and hornblende breakdowns and the m hree substitution in apatite 3 m hree 2o3in hornblende and litanite 3 lree 2in allaniteo3 2ca5 po4 3apatite f oh 6sio2quartz 6 lree pmonaziteo4 2ca2 m hree 3 sio4 3 f oh lessingite in apatite 6caoin plagioclase at the opx in isograd the increase of lrees and th content of apatite results either from the breakdown of some monazite 3 lree po4monazite 3sio2quartz 4caoin plagioclase f2 h2o fluid ca5 po4 3 f oh apatiteca2 lree 3 sio4 3 f oh less ingite in apatite or the breakdown of the remaining allanite 3 lree 2o3in allanite 6sio2quartz 4caoin plagioclase f2 h2o fluid 2ca2 lree 3 sio4 3 f oh lessingite in apatite the release of fluorine from the breakdown of biotite at the opx in isograd may increase apatite stability relatively to monazite in the granulite facies when compared to amphibolite facies the granulite facies augen gneisses do not show any u or th depletion this means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution the coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that high k calc alkaline orthogneisses are a suitable material to date with the u pb method prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

formation of meteorite hydrocarbons from thermal decomposition of siderite feco3 thermal decomposition of siderite has been proposed as a source of magnetite in martian meteorites laboratory experiments were conducted to evaluate the possibility that this process might also result in abiotic synthesis of organic compounds siderite decomposition in the presence of water vapor at 300 c generated a variety of organic products dominated by alkylated and hydroxylated aromatic compounds the results suggest that formation of magnetite by thermal decomposition of siderite on the precursor rock of the martian meteorite alh84001 would have been accompanied by formation of organic compounds and may represent a source of extraterrestrial organic matter in the meteorite and on mars the results also suggest that thermal decomposition of siderite during metamorphism could account for some of the reduced carbon observed in metasedimentary rocks from the early earth

carbon isotopes and the rise of atmospheric oxygen new data for the isotopic composition of carbon in carbonate sediments deposited between 2 6 and 1 6 ga indicate that the value of 13c in these sediments underwent a very large positive excursion between 2 22 and 2 06 ga a reassessment of the earlier 13c data for carbonate sediments shows that this excursion was probably worldwide and that it was preceded and followed by several hundred million years during which the 13c of carbonate sediments differed little from that of modern carbonates the large 13c excursion between 2 22 and 2 06 ga was probably related to an abnormally high rate of organic carbon deposition which generated an abnormally high rate of o2 production we estimate that the total excess o2 produced during the excursion was between 12 and 22 times the present atmospheric o2 inventory the 13c data therefore suggest that the o2 content of the atmosphere increased very significantly between 2 22 and 2 06 ga this inference is supported strongly by several other lines of evidence

petrology and geochemistry of the 2 9ga itilliarsuk banded iron formation and associated supracrustal rocks west greenland source characteristics and depositional environment abstract here we present new field petrographic and geochemical data from the 2 9 ga itilliarsuk banded iron formation bif and associated lithologies within the itilliarsuk supracrustal belt south eastern nussuuaq west greenland the supracrustals represent a volcanic sedimentary sequence which rests unconformably on a basement of tonalite trondhjemite granodiorite ttg lithologies felsic metagreywackes meta semipelites and thinly bedded ferruginous shales were identified intercalated with the itilliarsuk bif other associated rocks include metapelites acidic metavolcanics and metagabbroic sills the supracrustals have experienced amphibolite facies metamorphism which has resulted in complete resetting of the u pb system with an apparent age of 1895 48 ma this tectono metamorphic event corresponds well with the paleoproterozoic rinkian orogeny known from this region the itilliarsuk oxide facies bif has been divided into two segments on the basis of major and trace elements chemistry a shaley bif with a strong clastic component and a more chemically pure bif the shaley bif contains high terrigenous influx as reflected by elevated al 2 o 3 up to 12 wt tio 2 high field strength elements hfse and transition metals the chemically pure bif is characterised by alternating high iron 68 wt and high silica 64 wt bands with low total rare earths and yttrium rey al 2 o 3 tio 2 and hfse contents suggesting a low detrital component the least altered bands of the bif record diagnostic archaean seawater features with post archaean average shale paas normalised positive la and eu anomalies enrichment in heavy rare earth elements hree relative to light rare earth elements lree pr yb paas t dm model ages of 3 23 2 85 ga and initial nd nd i values in the range of 0 43 to 4 05 average of 1 35 in contrast the chemically pure fe rich bands yield t dm model ages of 3 61 3 22 ga and nd i values from 2 87 to 0 09 average of 1 29 the associated supracrustal rocks in the study area have significantly higher positive nd i values the 143 nd 144 nd in the itilliarsuk bif therefore contrasts world bifs by exhibiting radiogenic positive nd i values in shallow seawaters where the rey s were controlled by a local depleted continental crust whereas the negative nd i values found in the iron rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle possibly an older subcontinental lithospheric segment the felsic metagreywackes are immature first cycle sio 2 al 2 o 3 4 4 la yb 1 chon metasediments with affinities to ttg suites primarily extrusives whereas the meta semipelites and metapelites contain a larger mafic contribution with higher content of fe 2 o 3 mgo cr ni and hrees this suggests that the bif was deposited in a highly unstable basin presumably in a palaeo continental slope or outer shelf environment with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources the t dm model ages and the use of th sc zr and la th sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting

using in situ monazite and xenotime u pb geochronology to resolve the fate of the missing banded iron formation hosted high grade hematite ores of the north china craton

rise in seawater sulphate concentration associated with the paleoproterozoic positive carbon isotope excursion evidence from sulphate evaporites in the 2 2 2 1 gyr shallow marine lucknow formation south africa past oceanic sulphate concentration is important for under standing how the oceansredox state responded to atmospheric oxygen levels the absence of extensive marine sulphate evaporites before 1 2 gyr probably reflects low seawater sulphate and or higher carbonate concentrations sulphate evaporites formed locally during the 2 22 2 06 gyr lomagundi positive d 13 c excursion however the 2 2 2 1 gyr lucknow formation south africa provides the first direct evidence for seawater sulphate precipitation on a carbonate platform with open ocean access and limited terrestrial input these marginal marine deposits contain evidence for evaporite molds pseudomorphs after selenite gypsum and solid inclusions of ca sulphate in quartz carbon and sulphur isotope data match the global record and indicate a marine source of the evaporitic brines the apparent precipitation of gypsum before halite requires 2 5 mm l 1 sulphate concentration higher than cur rent estimates for the paleoproterozoic during the lomagundi event which postdates the 2 32 gyr initial rise in atmospheric oxygen seawater sulphate concentration rose from archean values of 200 lm l 1 but dropped subsequently because of higher pyrite burial rates and a lower oceanic redox state

use of snow cover derived from satellite passive microwave data as an indicator of climate change to assess future global change monitoring of the climate system through observation and analysis of seasonal and interannual fluctuations of climate variables is necessary cryospheric elements such as snow cover are often seen as sensitive indicators and integrators of basic climate conditions and hence an indicator of regional and global change snow cover elements which may serve as signals of variability and change are discussed with respect to the effective use of conventional and remotely sensed information conventional data are shown to be effective for assessing questions of temporal variability but are limited for spatial variability passive microwave satellite data make an important contribution by providing spatial and temporal information on snow water equivalent swe and the regional distribution of snowpack extent and state use of nimbus 7 smmr scanning multichannel microwave radiometer data to develop a time series of swe is assessed as a complement to conventional data limitations of smmr coverage compared to dmsp ssm i special sensor microwave imager coverage for production of swe maps for climate change analysis are discussed although there are limitations during early season snow cover information derived from passive microwave data is shown to be able to map and compute the areal coverage of swe allowing interannual comparison of the amount of water available the date of peak accumulation and the associated spatial distribution however the satellite data record is still too short to establish any definitive trend in snow cover variability

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

an iron shuttle for deepwater silica in late archean and early paleoproterozoic iron formation iron formations are typically thinly bedded or laminated sedimentary rocks containing 15 or more of iron and a large proportion of silica commonly 40 in the ca 2590 2460 ma campbellrand kuruman complex transvaal supergroup south africa iron formation occurs as a sediment starved deepwater facies distal to carbonates and shales iron minerals primarily siderite define the lamination the silica primarily occurs as thin beds and nodules of diagenetic chert now microcrystalline quartz filling pore space and replacing iron formation minerals and co occurring deepwater lithologies mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production anoxygenic photosynthesis abiotic photochemistry and chemoautotrophy using fe ii as an electron donor the origin and mechanism of silica precipitation in these deposits have received less attention here we present a conceptual model of iron formation that offers insight into the deposition of silica the model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides which precipitate fe oh 3 binds silica and sinks from the surface ocean along with organic matter shuttling silica to basinal waters and sediments fe iii respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters driving precipitation of siderite from fe2 and respiration influenced co2 silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases this model explains many of the mineralogical textural and environmental features of late archean and earliest paleo proterozoic iron formation

correlation of paleoproterozoic glaciations based on u pb zircon ages for tuff beds in the transvaal and huronian supergroups abstract the rise in atmospheric oxygen between 2 45 and 2 2 ga has been linked to the demise of a methane rich atmosphere and the onset of multiple glaciations culminating in a possible snowball earth the glacial deposits represent possible global marker horizons however their correlation is uncertain because key sedimentary successions are undated hampering paleoenvironmental reconstructions three potentially global glaciations have been proposed based on the correlation of glacial deposits in southern africa with those in north america however it has also been suggested that there were four glaciations and that the youngest was a snowball earth event recorded only in south african successions in situ u pb zircon ages for tuffs in southern african and north american successions establish the existence of four glaciations between 2 45 and 2 22 ga geochronological and stratigraphic data demonstrate that the three oldest glaciations predate 2 31 ga and that the final glaciation which is only recognized in south africa occurred between 2 26 and 2 22 ga the new age calibrated correlations show that a rise in atmospheric oxygen inferred from sulfur isotope data occurred between the second and third glaciations at 2 31 ga the first marine sulfate evaporites were deposited contemporaneously with 13 c enriched carbonates indicating a direct link between perturbations of the carbon and sulfur cycles and rising atmospheric oxygen the appearance of red beds and oxidized palesols after the third glaciation signals a major increase in atmospheric oxygen levels which culminated with the fourth glaciation between 2 26 and 2 22 ga after which the atmosphere remained irreversibly oxygenated

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

the composition and depositional environments of mesoarchean iron formations of the west rand group of the witwatersrand supergroup south africa this paper documents the sedimentological setting mineralogy and geochemistry of several iron formation units interbedded with siliciclastic strata of the mesoarchean witwatersrand supergroup well known for its world class conglomerate hosted au u deposits four major iron formation beds with associated magnetic mudstones are present in two distinctly different lithostratigraphic associations namely shale and diamictiteassociated iron formation the shale association is represented by the water tower and contorted bed iron formations in the parktown formation of the hospital hill subgroup in the lower part of the succession and the diamictite association by the promise and silverfield iron formations in the overlying government subgroup the iron formation units have been subjected to lower greenschist facies metamorphism oxide magnetite and limited hematite carbonate and silicate facies iron formations are recognized the iron formations typically overlie major transgressive flooding surfaces in the succession and in turn form the base of progradational coarsening upward increments of sedimentation comprising magnetic mudstone nonmagnetic shale and interbedded siltstone quartzite the upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite magnetite iron formation at the base in the most distal setting through magnetite siderite and siderite facies iron formation in the transition zone to magnetic mudstone the siderite with associated ankerite displays highly depleted 13c values suggesting crystallization via iron respiration in presence of organic carbon the iron formations display positive postarchean australian shale normalized eu and y anomalies with depletion in light rare earth elements relative to heavy rare earth elements indicating precipitation from marine water with a high temperature hydrothermal component integration of sedimentological petrographic and geochemical results indicates that the shale associated iron formation was deposited during the peak of transgression when reduced iron rich hydrothermal waters entered the witwatersrand basin over a limited vertical extent due to neutral buoyancy with the top of the plume occurring below the photic zone it is suggested that chemolithoautotrophic iron oxidizing bacteria which would have been able to exploit the difference in chemistry between the iron enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen were responsible for precipitation of initial ferric iron oxyhydroxides the vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume with from distal to proximal hematite preserved where the base of the plume was not in contact with the basin floor magnetite where the plume water was in contact with bottom sediment iron rich carbonates where organic carbon input was high iron rich alumosilicates where siliciclastic input became significant in more proximal settings and iron poor sediment above the top of the plume diamictite associated iron formations in the witwatersrand are inferred to have been deposited in a fashion similar to the shale associated iron formations with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover the climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the witwatersrand supergroup during deposition of the glacially associated iron formations

iron silicate microgranules as precursor sediments to 2 5 billion year old banded iron formations banded iron formations bifs are chemical sedimentary rocks comprising alternating layers of iron rich and silica rich minerals that have been used to infer the composition of the early precambrian ocean and ancient microbial processes however the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues petrographic examination of the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia reveals the presence of abundant silt sized microgranules composed of stilpnomelane the microgranules are most common in the least altered bif where they define sedimentary laminations implying a depositional origin we suggest that the precursor mineral was an iron rich silicate that formed either in the water column or on the seafloor the microgranular texture may have developed due to clumping of amorphous mud forming silt sized floccules the microgranules were resedimented by dilute density currents and deposited in lamina sets comprising a basal microgranular rich lamina overlain by amorphous mud with dispersed microgranules the lamina sets collectively define plane lamination structure probably of the lower flow regime the microgranular textures are preserved only where early diagenetic silica prevented the compaction of lamina sets episodic resedimentation of iron silicates alternating with periods of nondeposition and seafloor silicification provides an explanation for some of the characteristic banding in bif we propose that for most of the early precambrian the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation

anoxic to suboxic mesoproterozoic ocean evidence from iron isotope and geochemistry of siderite in the banded iron formations from north qilian nw china abstract banded iron formations bifs provide important constraints on geochemical cycling of fe and ancient marine environment globally the formation of bifs peaked during late archean and early paleoproterozoic with limited reappearance during neoproterozoic here we investigate a rare example of mesoproterozoic ca 1 3 ga bifs from jingtieshan in the north qilian region of nw china we present rare earth element and yttrium ree y compositions together with iron isotope features of siderite from the bifs and integrate these with our published data on carbon isotope compositions from the siderite rich bifs including carbonate facies bifs and mixed carbonate oxide facies bifs siderite from the bifs shows ree y characteristics that are consistent with submarine hydrothermal fluids mixed with seawater including positive eu eu anomalies and high y ho ratios the siderite also displays homogeneous iron isotope compositions 56fe range from 0 71 to 0 41 one sample down to 1 62 and moderate negative carbon isotope compositions 13c values 8 40 to 3 0 these findings argue against the traditional view that siderite associated with bifs was formed through microbial dissimilatory iron reduction dir instead siderite from jingtieshan bifs is in equilibrium with submarine hydrothermal fluids and seawater suggesting an inorganic origin and was directly precipitated from the co2 and fe oversaturated water column with only insignificant role for dir these mesoproterozoic bifs are also characterized by ce anomalies and the siderite falls within the pe log s t and po2 pco2 stability fields suggesting that the bifs were deposited in an anoxic to suboxic ocean the results from our study suggest the presence of iron rich mesoproterozoic deep ocean in the north qilian area

geochemical constraints on the genesis of the ekou banded iron formation shanxi province north china the ekou banded iron formation bif in the wutai area is hosted within the late archean baizhiyan formation the mineral assemblage is used to identify oxide and silicate facies the oxide facies is composed of magnetite and quartz and the silicate facies is characterized by the presence of silicate minerals a geochemical analysis shows that the major elemental compositions are dominantly sio2 and fe2o3t with very little al2o3 and tio2 and minor abundances of incompatible elements and transition elements these results indicate that negligible terrigenous materials were involved in the bif deposition the rare earth elements rees normalized by post archean australian shale exhibit the characteristics of light ree lree depletion heavy ree hree enrichment and positive la y and eu anomalies the y ho ratios are superchondritic these results indicate that the material that formed the ekou bif originated from the mixing of seawater and submarine hydrothermal fluids a ce anomaly deficiency and heavy fe isotope enrichment indicate that the ekou bif formed in an anoxic marine environment the 30sinbs 28 values for quartz in the ekou bif are similar to those of other bifs with distribution ranges of modern sinters and black smokers the 18ov smow distribution is similar to that of hydrothermal sedimentary siliceous rocks these results suggest that the formation of the ekou bif was closely connected to submarine volcanic exhalation activity

uranium in iron formations and the rise of atmospheric oxygen abstract the concept of the great oxidation event goe during which atmospheric oxygen rose precipitously and perhaps to near modern levels around 2 4 2 1 billion years ago ga has become entrenched in our views on secular atmospheric evolution multiple proxies confirm a permanent shift towards more oxygenated conditions at some time near the archean proterozoic boundary however it remains unclear precisely when this transition occurred due in part to the likely temporal variability in those early levels and different sensitivities of the proxies utilized to track atmospheric oxygen partial pressures here we provide a new look at the timing and magnitude of early atmospheric oxygenation through the record of uranium u concentrations in iron formations if just as if are important archives of the redox state of seawater concentrations of redox sensitive u in if are faithful proxies for oxidative continental weathering and associated delivery of dissolved u to seawater our dataset suggests that there was an increase in u redox cycling and transport at ca 2 47 ga just before the permanent loss of mass independent sedimentary sulfur isotope anomalies traditionally used to define the onset of the goe further there is significant temporal variability in the if u record that we propose reflects dynamic precambrian redox conditions we provide additional support for earlier suggestions that the goe was a protracted event marked by vacillating oxygen levels

nonredox transformations of magnetite hematite in hydrothermal systems the transformation of magnetite to hematite or hematite to magnetite in nature has generally been considered a redox reaction and linked to a specific redox state of fluid however a nonredox reaction fe2o3 hm fe2 h2o fe3o4 mt 2h may have been the principal mechanism for the transformations of iron oxides in nature especially in hydrothermal environments for example the transformation of goethite and or hematite primary precipitates to magnetite in banded iron formations bifs probably occurred through nonredox reactions with fe2 bearing hydrothermal fluids during the accumulation of a bif sequence rather than through redox reactions involving organic matter during and or after the bif deposition the proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite rich secondary ores extending the target for orebodies to much deeper zones below the paleosurface another important implication of the proposed mechanism is that the presence or absence of magnetite and or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid

decoupled sources of the 2 3 2 2ga yuanjiacun banded iron formation implications for the nd cycle in earth s early oceans abstract the recognized worldwide gap in bif deposition between 2 4 and 2 0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time however the recently dated 2 3 2 2 ga yuanjiacun banded iron formation bif in the north china craton ncc offers a possibility to redress these uncertainties shale normalized rare earth element yttrium ree y patterns of the bif and interlayered meta chert samples show features characteristic of other archean and paleoproterozoic bifs with hree enrichment relative to lree positive la and eu anomalies and superchondritic y ho ratios comparable to modern seawater very low al 2 o 3 e nd t 3 5 derived from interaction with a depleted mantle source and associated with high fe fluxes the second is ambient surface seawater e nd t 2 4 which obtained its signature through weathering of the nearby landmasses and associated with high si fluxes our findings suggest that the ree budget of the oceans prior to 2 3 ga was generally dominated by hydrothermal circulation of seawater through depleted mantle derived source rocks however where evolved local continental crust and or an enriched mantle source were present this positive mantle nd signal became discernible in the bifs by comparing the nd isotopic features of similar aged bifs and marine carbonates it is concluded that similar to modern oceans the early precambrian ocean was not well mixed with respect to its nd isotopic composition

the geochemistry of rare earth elements in the seasonally anoxic water column and porewaters of chesapeake bay a twelve cruise time series study of a seasonally anoxic basin in chesapeake bay was carried out between february 1988 and february 1989 data from filtered bottom water and upper 0 1 cm porewater samples are presented this is the first study in which time series measurements have been employed to understand the low temperature geochemistry of rare earth elements rees the focus is on the coupling between redox reduction oxidation variations and ree cycling fractionation of trivalent rees and development of cerium anomalies the time series results demonstrate that rees have large seasonal cycles in both the water column and porewaters in response to the development of anoxia in the spring and reoxygenation in the fall the transition from oxic to suboxic to anoxic conditions results in the release of rees into the upper porewaters and bottom waters the release of rees and fe are coincident in the bottom waters while the release of rees lags fe by approximately 50 days in the upper porewaters this decoupling is explained in terms of a redox front which moves upward from the sediments to bottom waters during the development of seasonal anoxia the release of rees to the upper porewaters and bottom water are accompanied by 1 1 fractionation across the trivalent ree series and by 2 2 the preferential input of ce relative to its trivalent only ree neighbors ree iii fractionation is such that the relative rate of release increases from lu to la light ree middleree heavyree during reoxygenation removal of dissolved rees occurs from both the water column and upper porewaters and follows the same relative rates lree mree hree cerium removal occurs faster than its neighbor ree iii indicating a rapid oxidation of ce iii to ce iv

hydrothermal nontronite formation at eolo seamount aeolian volcanic arc tyrrhenian sea abstract a sediment core containing a yellowish green clay bed was recovered from an area of extensive hydrothermal deposition at the se slope of the eolo seamount tyrrhenian sea the clay bed is composed of pure nontronite described for the first time in the tyrrhenian sea which appears to be the most aluminous nontronite ever found among the seafloor hydrothermal deposits the high al content suggests precipitation from al containing hydrothermal solutions the ree distribution of the eolo nontronite has a v shape pattern the heavy ree enrichment is in part due to their preferential partitioning in the nontronite structure this enrichment was possibly further enhanced by the hree preferential sorption on bacterial cell walls the light ree enrichment is the result of scavenging uptake by one of the nontronite precursors i e poorly ordered fe oxyhydroxides from the hydrothermal fluids oxygen isotopic composition of the nontronite yields a formation temperature of 30 c consistent with a low temperature hydrothermal origin the relatively radiogenic nd isotopic signature of the nontronite compared to the present day mediterranean seawater indicates that approximately half of nd and presumably the rest of the lree are derived from local volcanic sources on the other hand 87 sr 86 sr is dominated by present day seawater sr scanning electron microscopy investigation revealed that the nontronite is composed of aggregates of lepispheres and tube like filaments which are indicative of bacteria assisted precipitation bacteria inhabiting this hydrothermal site likely acted as reactive geochemical surfaces on which poorly ordered hydrothermal fe oxyhydroxides and silica precipitated upon aging the interactions of these primary hydrothermal precipitates coating bacterial filaments and cell walls likely led to the formation of nontronite finally the well balanced interlayer and layer charges of the crystal lattice of seafloor hydrothermal nontronite decrease its sorption capacity to zero thus the ubiquitous nontronite precipitation along the active plate boundaries and around the hot spots has no significant impact on oceanic trace element chemistry

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

iron isotope fractionation during microbial reduction of iron the importance of adsorption in experiments investigating the causes of fe isotope fractionation the d 56 54 fe value of fe ii remaining in solution fe ii aq after reduction of fe iii goethite by shewanella putrefaciens is 21 2 relative to the goethite in agreement with previous research the addition of an electron shuttle did not affect fractionation suggesting that fe isotope fractionation may not be related to the kinetics of the electron transfer furthermore in abiotic anaerobic fecl2 aq experiments in which approximately one third of fe ii aq is lost from solution due to adsorption of fe ii onto goethite the d 56 54 fe value of fe ii aq remaining in solution is shifted by 20 8 relative to fecl 2 this finding demonstrates that anaerobic nonbiological interaction between fe ii and goethite can generate signif icant fe isotope fractionation acid extraction of sorbed fe ii from goethite in experi ments reveals that heavy fe preferentially sorbs to goethite simple mass balance modeling indicates that the isotopic composition of the sorbed fe ii pool is 11 5 to 12 5 heavier than fe in the goethite 2 7 3 7 heavier than aqueous fe ii mass balance is also consistent with a pool of heavy fe that is not released to solution during acid extraction

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

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

hydrothermal fe fluxes during the precambrian effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers abstract modern mid ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved h2s in excess of fe these fluids are the consequence of high temperature interactions between sulfate rich seawater and mid ocean ridge basalt at conditions near the critical point for seawater the precipitation of fes and fes2 during venting titrates most of the fe from the fluid significantly reducing the net flux of fe to the open ocean here we suggest that hydrothermal fluids emanating from precambrian seafloor systems older than 1 8 ga had fe h2s ratios 1 and with fh2 higher than today because seawater lacked its primary oxidant dissolved sulfate ion this predominance of fe over h2s would have promoted the establishment of an iron rich deep ocean and the deposition of banded iron formations bif accordingly the end of bif deposition at 1 8 ga was the result of the buildup of sulfate in seawater from oxidative weathering and its return at 750 ma the result of reductions in seawater sulfate concentrations during snowball earth episodes enhanced by elevated fe concentrations during depressurization of hydrothermal systems by large eustatic sea level falls moreover precambrian chemosynthetic vent communities may have been based on h2 synthesis rather than on h2s oxidation as they largely are today

chemostratigraphy of the paleoproterozoic duitschland formation south africa implications for coupled climate change and carbon cycling the paleoproterozoic duitschland formation lies stratigraphically beneath the timeball hill formation which contains the only unequivocal glacial unit of this era in the transvaal basin south africa lithologic evidence in paleoproterozoic successions of north america however indicates the existence of three discrete and potentially global ice ages within this 300 my interval carbonates of the duitschland formation are significantly enriched in 13c up to 10 1 permil in the upper part of the succession above a notable sequence boundary in contrast the lower part of this unit contains carbonates with consistently negative 13c values trace and major element compositions of these carbonates as well as carbon isotopic compositions of coexisting organic matter support a primary origin for the markedly positive carbon isotope anomaly the stratigraphic constraints indicate that 13c enriched carbonates were deposited prior to paleoproterozoic glaciation in southern africa similar to carbonates stratigraphically beneath neoproterozoic glacial diamictites worldwide also mirroring the neoproterozoic record are strongly negative 13c values in cap carbonates atop glacial diamictites in paleoproterozoic strata of wyoming and ontario the litho and chemostratigraphic constraints indicate that the interval of negative carbon isotope values in well preserved carbonates of the lower duitschland formation may reflect a second paleoproterozoic ice age in the transvaal succession this interpretation is further supported by recently discovered bullet shaped clasts with striations in diamictite from the basal part of the succession thus the emerging temporal pattern of carbon isotope variations and glaciation in the paleoproterozoic has a close analogue to neoproterozoic events suggesting a coupling of climatic and biogeochemical changes at both ends of the eon

u pb and hf isotopic study of detrital zircons from the yejishan group of the luliang complex constraints on the timing of collision between the eastern and western blocks north china craton abstract the trans north china orogen is a recently recognized continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the coherent basement of the north china craton however there is a hot debate on the timing of the collision between the two blocks with one school of thought arguing the collision occurred at 2 5 ga whereas others believe it took place at 1 85 ga u pb ages and hf isotopic data of detrital zircons from foreland basins in the trans north china orogen can provide rigorous constraints on this controversial issue one of such foreland basins is represented by the yejishan group in the luliang complex in the middle sector of the western margin of the trans north china orogen the group consists of conglomerates quartz wackes sandstones and minor limestones most of which contain large amounts of detrital zircons cl images reveal that nearly all detrital zircons from the yejishan group possess oscillatory zoning with th u ratios of 0 2 2 34 typical of an igneous origin the 207 pb 206 pb ages of detrital zircons range between 3 24 and 1 81 ga with most having paleoproterozoic ages of which the youngest group ages are around 1 87 1 81 ga this indicates that the foreland basin in which the yejishan group was deposited must have formed not earlier than 1 87 1 81 ga thus supporting the collision between the eastern and western blocks at 1 85 ga not at 2 5 ga

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

carbonate petrography kerogen distribution and carbon and oxygen isotope variations in an early proterozoic transition from limestone to iron formation deposition transvaal supergroup south africa the transition zone comprises campbellrand microbialaminated replacing cryptalgalaminate limestone and shale with minor dolomite conformably overlain by the kuruman iron formation of which the basal part is characterized by siderite rich microbanded iron formation with minor magnetite and some hematite containing units the iron formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones the sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees c and pressures of 2 kbars carbonate minerals in the different rock types are represented by primary micritic precipitates now recrystallized to microsparite early precompactional sparry cements and concretions deep burial limpid euhedral sparites and spar cements precipitated from metamorphic fluids in close contact with diabase sills paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them kerogen occurs as pigmentation in carbonate crystals as reworked organic detritus in clastic textured carbonate units and as segregations of kerogen pigment around late diagenetic carbonate crystals locally kerogen may also be replaced by carbonate spar carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies integration of sedimentary petrographic geochemical and isotopic results makes it possible to distinguish between depositional early diagenetic deep burial and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence major conclusions are that deep burial thermal decarboxylation led to 13c depletion in euhedral ferroan sparites and 13c enrichment in kerogen organic carbon metamorphic sparites are most depleted in 13c carbonates in oxide rich iron formations are more depleted in 13c than those in siderite rich iron formation whereas the kerogens in oxide banded iron formations bif are more enriched this implies that the siderite rich iron formations were not derived from oxide rich iron formation through reduction of ferric iron by organic matter organic matter oxidation by ferric iron did however decrease the abundance of kerogen in oxide rich iron formation and led to the formation of isotopically very light sparry carbonates siderite and calcmicrosparite both represent recrystallized primary micritic precipitates but differ in their 13c composition with the siderites depleted in 13c by 4 6 per mil on average relative to calcmicrosparite this means that the siderites were precipitated from water with dissolved inorganic carbon depleted in 13c by about 9 per mil relative to that from which the limestones precipitated this implies an ocean system stratified with regard to total carbonate with the deeper water from which siderite rich iron formation formed depleted in 13c iron formations were deposited in areas of very low organic matter supply depletion of 13c may therefore derive not from degradation of organic matter but from hydrothermal activity a conclusion which is supported by 18o composition of the carbonate minerals and trace element and rare earth element ree compositions of the iron formations

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

isotopic constraints on the late archean carbon cycle from the transvaal supergroup along the western margin of the kaapvaal craton south africa few existing studies illuminate the operation of the carbon cycle before the rise of atmospheric oxygen circa 2400 million years ago stable carbon isotopic measurements of shallow stromatolitic carbonates 0 vpdb and basinal carbonate minerals 6 in iron formation have been used to infer a strong isotopic depth gradient in archean ocean basins from new diamond drill cores obtained by the agouron drilling project from the griqualand west structural basin in the northern cape province south africa we present 13 c data from carbonates and organic matter that offer fresh insights into the late archean carbon cycle three drill cores cover the development progradation and ultimate demise by drowning of the campbellrand carbonate platform ca 2590 2500 ma one captures the platform top shallow marine and intertidal paleoenvironments the other two run through slope and basinal sections deposited adjacent to the platform margin increasing inwater depth likely to 1 km both shallowand deep water carbonates precipitated on the seafloor consistently show 13 c values around 0 5 incompatible with a strong late archean isotopic depth gradient a mathematical model suggests that these isotopic data are consistent with a reduced biological pump increased dissolved inorganic carbon in seawater due to higher atmospheric p co2 or both certain horizons do show distinct isotopic variability such areas are commonly shaly and they tend to be organic and or iron rich strong c isotopic variations occur on a cm scale and most likely stem from diagenetic remineralization of organic matter in sediment starved areas where iron formation developed siderite tends to be 13 c depleted sometimes by as much as 14 these observations suggest a carbon cycle in which iron respiration played a conspicuous role carbon isotope ratios from organic matter in shales are commonly 1 lighter than stratigraphically contiguous carbonates but there is no clear water depth trend in the organic carbon isotopic data taken as a whole the 13 c of organic matter can be explained by several non unique sets of processes including different autotrophic mechanisms of carbon fixation heterotrophic recycling including fermentation and methanotrophy and post depositional diagenesis the most striking feature is the occurrence of organic 13 c values 40 a feature that appears to be commonplace in late archean successions framed in the context of carbon cycle isotopic mass balance both organic and carbonate carbon isotopic data suggest that the proportion of carbon buried as organic matter was not radically different before the appearance of free environmental oxygen

geochemistry and si o fe isotope constraints on the origin of banded iron formations of the yuanjiacun formation lvliang group shanxi china abstract banded iron formations bifs within the lvliang region of shanxi province china are hosted by sediments of the yuanjiacun formation part of the paleoproterozoic lvliang group these bifs are located in a zone where sedimentation changed from clastic to chemical deposition indicating that these are superior type bifs here we present new major trace and rare earth element ree data along with fe si and o isotope data for the bifs in the yuanjiacun within the fe deposits at yuanjiacun jianshan and hugushan when compared with post archean australian shale paas these bifs are dominated by iron oxides and quartz contain low concentrations of al 2 o 3 tio 2 trace elements and the ree and are light rare earth element lree depleted and heavy rare earth element hree enriched the bifs also display positive la y and eu anomalies high y ho ratios and contain 30 si depleted quartz with high 18 o values that are similar to quartz within siliceous units formed during hydrothermal activity these data indicate that the bifs within the yuanjiacun formation were precipitated from submarine hydrothermal fluids with only negligible detrital contribution none of the bif samples analyzed during this study have negative ce anomalies although a few have a positive ce anomaly that may indicate that the bifs within the yuanjiacun formation formed during the great oxidation event goe within a redox stratified ocean the positive ce anomalies associated with some of these bifs are a consequence of oxidization and the formation of surficial manganese oxide that have preferentially adsorbed ho lree and ce 4 these deposits formed during reductive dissolution at the oxidation reduction transition zone or in deeper level reducing seawater the loss of ce lree and ho to seawater and the deposition of these elements with iron hydroxides caused the positive ce anomalies observed in some of the bif samples although the limited oxidizing ability of surface seawater at this time meant that y ho and lree hree ratios were not substantially modified unlike similar situations within stratified ocean water during the late paleoproterozoic magnetite and hematite within the bifs in the study area contain heavy fe isotopes 56 fe values of 0 24 1 27 resulting from the partial oxidation and precipitation of fe 2 to fe 3 in seawater in addition mass independent fractionation of sulfur isotopes within pyrite indicates that these bifs were deposited within an oxygen deficient ocean associated with a similarly oxygen deficient atmosphere even though the bifs within the yuanjiacun formation formed after initiation of the goe

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

results of a stratified snow emissivity model based on the wave approach application to the antarctic ice sheet the results from a snow emissivity model are used to help with the interpretation of the scanning multichannel microwave radiometer data aboard the satellite nimbus 7 1978 1986 over antarctica the model is based on the solution of maxwell s equations through strong fluctuation theory stogryn 1986 the snow is considered isothermal with horizontal stratification isotropic inside each stratum and with smooth interfaces to wipe out the interference pattern created by plane stratification and in order to take into account the spatial variability of the snow characteristics the final result is a mean value of many computations done with many stratigraphies randomly distributed around a mean profile the spatial distribution of the microwave signatures over antarctica reveals three areas with typical spectral signatures these areas are identified from the point of view of snow characteristics through a ground data set surdyk and fiiy 1993 the model reproduces well the effect of the stratification on the polarization differences as observed the strong gradient of emissivity versus frequency observed on one of the test sites is not completely understood depth hoar layers characterized by faceted and cup shaped coarse snow grains seem to have a particular scattering behavior compared with that of the usual grains when those coarse grains are replaced by fine grains the model results are closer to the measured data the presence of crusts the roughness at the snow ice interfaces and crystal orientation may also have significant effects

carbon isotope record for the onset of the lomagundi carbon isotope excursion in the great lakes area north america carbonate units of the chocolay group in the lake superior area usa and the gordon lake formation of the correlative upper huronian supergroup ontario canada were deposited after the last paleoproterozoic glacial event and an episode of intense chemical weathering the huronian supergroup contains at the base 2 45 ga volcanics and is intruded by the 2 22 ga nipissing diabase dykes and sills while the chocolay group is bracketed in age between 2 29 and 2 20 ga the lomagundi 2 22 2 1 ga carbon isotope excursion started after the paleoproterozoic glacial epoch and before a plume breakout event at 2 22 ga therefore the chocolay and upper huronian carbonates were deposited either before or during the onset of the lomagundi event notably thin carbonates of the basal gordon lake formation and thick carbonate succession of the kona dolomite in the northeastern exposures of the chocolay group record 13 c values as high as 9 5 versus v pdb similar to other successions deposited during the lomagundi event both units contain pseudomorphs and molds after sulfates this observation suggests that seawater sulfate contents rose dramatically in association with the onset of the lomagundi event and the rise of atmospheric oxygen carbonates in the western and southern exposures of the chocolay group randville and bad river dolomites and saunders formation previously assumed to be equivalent to the gordon lake formation and kona dolomite have carbon isotope values close to 0 based on basin analysis we infer that these carbonate units were deposited during a negative carbon isotope excursion after the lomagundi event started and are slightly younger than the kona dolomite and gordon lake formation this interpretation implies that the carbonate platform in the great lakes area transgressed to the west over shallow marine and fluvial deposits the negative carbon isotope excursion in the lake superior area might correspond to similar 13 c values of the mooidraai dolomite in the griqualand west basin south africa supporting correlation between paleoproterozoic successions of north america and south africa and the notion of three global glaciations in the paleoproterozoic era carbonates of the mille lacs group trout lake glen township and denham formations in minnesota have 13 c values ranging from 1 2 to 2 5 combined with geochronologic constraints these data suggest that these units were deposited after the lomagundi excursion and are related to the rifting event that led to development of the so far unrecognized 2 0 ga passive margin in the lake superior area 2006 elsevier b v all rights reserved

pyrite multiple sulfur isotope evidence for rapid expansion and contraction of the early paleoproterozoic seawater sulfate reservoir abstract earth s oxygenation is often described in terms of two unidirectional steps at the beginning and end of the proterozoic eon separated by a long lived intermediate redox state recent work defines a more complicated path to oxygenation exemplified by an apparent drop in oxidation state following the early paleoproterozoic lomagundi carbon isotope excursion the timing of this proposed drop in oxidation state is not well constrained and it is not clear how it relates to redox conditions during the remainder of the proterozoic here we present a study of pyrite multiple sulfur isotopes supported by fe speciation and organic carbon isotopes from early paleoproterozoic black shales we find evidence for the rapid expansion of the seawater sulfate reservoir during the great oxidation event at ca 2 3 ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca 2 05 ga this scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early paleoproterozoic comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early paleoproterozoic did not return until the late neoproterozoic

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

metallogeny of the north china craton link with secular changes in the evolving earth abstract the north china craton ncc has experienced a complex geological evolution since the early precambrian and carries important records of secular changes in tectonics and metallogeny here we synthesize the salient geological and tectonic features of the evolution and destruction of the ncc vis a vis major metallogenic events and the formation of potential ore deposits we identify a close relationship between the major geological events in the ncc and those reported elsewhere on the globe we trace the records of a regular change in the pattern of metallogeny mineral deposit character spatial distribution and genetic mechanisms which closely match the timing and styles of the major geological and tectonic events in this craton the ncc went through five major tectonic cycles 1 neoarchean crustal growth and stabilization 2 paleoproterozoic rifting subduction accretion collision with imprints of the great oxidation event goe 3 late paleoproterozoic neoproterozoic multi stage rifting 4 paleozoic orogenesis at the margins of the craton and 5 mesozoic extensional tectonics associated with lithospheric thinning and decratonization coinciding with these major geological events are five major metallogenic systems identified as follows 1 an archean bif system 2 paleoproterozoic cu pb zn and mg b systems 3 a mesoproterozoic ree fe pb zn system 4 a paleozoic orogenic cu mo system and 5 mesozoic intracontinental au and ag pb zn and mo systems the ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities from early precambrian through late precambrian to paleozoic and mesozoic the ncc records a transition from primitive to modern style plate tectonics evidence for imbricated oceanic plate stratigraphy in a subduction accretion setting and collisional orogenesis along at least three major zones of ocean closure are documented major transitions in tectonic style and surface environmental changes recorded in other parts of the world are also reflected in the geological history and metallogenic events in the ncc large scale gold deposits formed through intraplate tectonics during the mesozoic provide important insights into mantle dynamics and crust mantle interaction associated with lithospheric thinning and craton destruction the ncc provides one of the best examples for documenting secular changes in the geological history and metallogenic epochs of an evolving earth

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

meso and neoarchean banded iron formations and genesis of high grade magnetite ores in the anshan benxi area north china craton a discussion

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

fe c and o isotope compositions of banded iron formation carbonates demonstrate a major role for dissimilatory iron reduction in 2 5 ga marine environments abstract combined fe c and o isotope measurements of 2 5 ga banded iron formation bif carbonates from the kuruman iron formation and underlying bif and platform ca mg carbonates of the gamohaan formation south africa constrain the biologic and abiologic formation pathways in these extensive bif deposits vertical intervals of up to 100 m were sampled in three cores that cover a lateral extent of 250 km bif fe carbonates have significant fe isotope variability 56fe 1 to 1 and relatively low 13c down to 12 and 18o values 18o 21 in contrast gamohaan and stratigraphically equivalent campbellrand ca mg carbonates have near zero 13c values and higher 18o values these findings argue against siderite precipitation from seawater as the origin of bif fe rich carbonates instead the c o and fe isotope compositions of bif fe carbonates reflect authigenic pathways of formation in the sedimentary pile prior to lithification where microbial dissimilatory iron reduction dir was the major process that controlled the c o and fe isotope compositions of siderite isotope mass balance reactions indicate that the low 13c and low 18o values of bif siderite relative to those expected for precipitation from seawater reflect inheritance of c and o isotope compositions of precursor organic carbon and ferric hydroxide that were generated in the photic zone and deposited on the seafloor carbon fe isotope relations suggest that bif fe carbonates formed through two end member pathways low 13c low 56fe fe carbonates formed from remobilized low 56fe aqueous fe2 produced by partial dir of iron oxide whereas low 13c high 56fe fe carbonates formed by near complete dir of high 56fe iron oxides that were residual from prior partial dir an important observation is the common occurrence of iron oxide inclusions in the high 56fe siderite supporting a model where such compositions reflect dir in place in the soft sediment in contrast the isotopic composition of low fe carbonates in limestone dolomite may constitute a record of seawater environments although our petrographic studies indicate that the presence of pyrite in most low fe carbonates may influence the fe isotope compositions the combined fe c and o isotope data from kuruman bif carbonates indicate that bif siderites that have negative near zero or positive 56fe values may all record biological fe cycling where the range in 56fe values records differential fe mobilization via dir in the sediment prior to lithification our results demonstrate that the inventory of low 56fe marine sedimentary rocks of neoarchean to paleoproterozoic age although impressive in volume may represent only a minimum of the total inventory of fe that was cycled by bacteria

petrogenesis of mid proterozoic granitic magmas examples from central and west texas abstract circa 1 1 ga granitic magmatism in texas was manifested as two compositional groups 1 the 1 12 ga red bluff granitic suite in west texas and 2 1 12 1 07 ga granites of the llano uplift of central texas both suites share some characteristics typical of anorogenic granites e g potassium and iron rich bulk compositions fe rich hydrous silicates emplacement conditions involving low oxygen fugacities and water contents and exhibit similar isotopic characteristics however rock associations mineral chemistries and trace element compositions of the two suites are distinct and no single petrogenetic model for the two suites is possible the red bluff granitic suite includes cogenetic syenites quartz syenites and granites transitional ferrobasaltic dikes are also present in contrast syenitic and mafic rocks are not associated with the llano granites the llano granites contain biotite and calcic amphibole with lower fe fe mg ratios compared to those occurring in the red bluff rocks alkali amphiboles e g arfvedsonite occur in the red bluff granites but not in the llano granites the red bluff granitoids are characterized by high feo t mgo ratios high na2o k2o high concentrations of hfse and rare earth elements ree and other features typical of a type within plate granites e g the pikes peak batholith ppb the llano granites are geochemically distinct with generally higher p2o5 and sr lower na2o feo t mgo zr y and ree and much lower ta and nb nd isotopic data overlap between the two granite suites and have juvenile signatures however trace element data suggest different petrogeneses for the two suites the red bluff suite is interpreted as having a direct derivation from mantle sources via extended fractional crystallization of basaltic parental magmas with minor crustal assimilation the llano granites appear to represent anatectic melts derived from slightly older juvenile crustal sources some melts underwent fractional crystallization controlled by feldspar and accessory minerals the petrology and geochemistry of 1 1 ga granites in texas indicate that they should not be considered as part of a single anorogenic magmatic event the red bluff granitic suite was emplaced into a shelf sequence north of the grenville front within a broad zone characterized by mild extension in contrast llano granites are late stage intrusions emplaced into multiply deformed and metamorphosed crust south of the grenville front during or after waning stages of grenville orogenesis

oxygen overshoot and recovery during the early paleoproterozoic abstract during the lomagundi event ca 2 22 to 2 06 ga marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion the earliest extensive marine sulfate evaporites were deposited and the average ferric iron to total iron expressed as fe 2 o 3 fe fe2o3 ratio of shales increased dramatically at the end of the lomagundi event the first economic sedimentary phosphorites were deposited and the carbon isotope values of marine carbonates returned to 0 vpdb thereafter marine sulfate evaporites and phosphorites again became scarce while the average fe 2 o 3 fe fe2o3 ratio of shales decreased to values intermediate between those of the archean and lomagundi age shales we propose that the large isotopic and chemical excursions during the lomagundi event were caused by a positive feedback between the rise of atmospheric o 2 the weathering of sulfides in the pre 2 3 ga continental crust and the flux of phosphate to the oceans cf holland 2002 the rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle the end of the lomagundi event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low pyrite sediments that were deposited during the lomagundi event the rate of deposition of organic matter and the precipitation of sulfate evaporites decreased the isotopic and chemical excesses of the lomagundi event were eliminated and the ocean atmosphere system entered the period frequently called the boring billion

changes of ge si ree y and smnd isotopes in alternating fe and si rich mesobands reveal source heterogeneity of the 2 54 ga sijiaying banded iron formation in eastern hebei china abstract the north china craton ncc is one of the most important regions hosting abundant banded iron formations bifs the 2 54 ga sijiaying bif the best preserved and most extensive deposit in eastern hebei is intercalated and closely associated with meta volcanic rocks of the luanxian group in this context major and trace element and sm nd isotopic analyses of individual mesobands of a sijiaying bif specimen were conducted to characterize the source and depositional environment over a transient period low al 2 o 3 tio 2 and high field strength elements hfses concentrations show that the bif is relatively detritus free shale normalized rare earth and yttrium distributions ree y of individual bif mesobands exhibit positive la anomalies enrichment in hree relative to lree and mree and suprachondritic y ho ratios which are typical features of marine waters throughout the archean and proterozoic the presence of consistently positive eu anomalies indicates a significant high t hydrothermal input while the absence of true ce anomalies suggests deposition from an anoxic water column by comparison with other typical bifs e g the isua bif from greenland the kuruman bif from south africa the sijiaying bif is depleted in hree and appears to record variations in solute fluxes related to changing intensities of hydrothermal activity these features coupled with sm nd isotopic relations and ge si distributional patterns point to two decoupled sources controlling the depositional environment of the bif and thus reveal source heterogeneity for silica and iron of the sijiaying bif high fe fluxes were associated with seafloor vented hydrothermal fluids which received their sm nd isotopic signature from alteration of depleted oceanic crust whereas significant amounts of silica were associated with ambient seawater whose ree signature was controlled by solutes derived from weathering of nearby mesoarchean continental landmasses the old up to 3 0 ga nd t dm model ages of si rich mesobands of the bif support such a scenario

petrography and geochemistry of the paleoproterozoic hotazel iron formation kalahari manganese field south africa implications for precambrian manganese metallogenesis new data are presented concerning the petrography and geochemistry of the paleoproterozoic hotazel iron formation of the kalahari manganese field south africa mineralogically and texturally the hotazel iron formation is very similar to paleoproterozoic banded iron formations of the superior type comprising laminae of chert iron oxides magnetite hematite iron silicates greenalite minnesotaitc stilpnomelane riebeckite fe mica carbonates calcite ankerite minor siderite and pyrite the major chemical constituents of the hotazel rocks are sio 2 fe oxides cao and mgo whereas mno al 2 o 3 na 2 o and k 2 o contents are negligible below 1 wt elements of detrital derivation ti zr rb and transition metals such as zn cu ni co and v are present in low concentrations on the order of a few tens of parts per million stratigraphically higher the hotazel banded iron formation shows a conspicuous increase in cao contents that is coupled with a subtle decrease in bulk rock fe number ratios fe super 2 fe super 2 fe super 3 implying a gradual transition to a more oxidizing environment shale normalized rare earth element ree patterns are similar to those of older banded iron formations of the transvaal supergroup exhibiting light ree depletion and negative ce anomalies comparable to modern shallow level seawater occasional weak positive eu anomalies in iron silicate rich rocks indicate minor admixtures of a hydrothermal component below the chemocline the similarities between the hotazel iron formation and most superior type banded iron formations the lack of transition metal enrichments and the absence of strongly positive eu anomalies in the hotazel rocks challenge volcanogenic exhalative models proposed previously for the genesis of the interbedded kalahari mn deposits

meso and neoarchean banded iron formations and genesis of high grade magnetite ores in the anshan benxi area north china craton a reply