silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

the silicon isotope record of early silica diagenesis the heavy isotopes of silicon are strongly enriched in some of the youngest early diagenetically formed porcellanite layers from the southwest indian ridge pleistocene and the maud rise pliocene these porcellanite layers are composed of opal ct and were formed by the conversion of amorphous silica opal a from siliceous sediment via dissolution reprecipitation their bulk si30 values range between 1 7 and 2 3 detritus poor siliceous sediment surrounding these layers is significantly lower at 0 3 to 1 5 sequential chemical extractions of bulk siliceous sediment show i preferential dissolution of diatoms featuring higher si30 than radiolaria and al si components the detailed investigation of porcellanite layers by micro scale si isotope and al si analyses using uv femtosecond laser ablation icp mass spectrometry show that ii precipitation of authigenic aluminum silicates enriched in light si isotopes drives pore waters to even higher si30 we suggest that the same processes redistributed stable silicon isotopes in precursor siliceous sediments of ancient chert we infer that past environmental conditions can be reconstructed with high fidelity from the stable si isotope composition of chert when initial seawater si concentrations were high such as in the precambrian exchange of si between layers during phase transformation from opal a to opal ct and from opal ct to quartz is impeded when variable amounts of detrital minerals are present because they control rates of silica phase transformation and hence the timing of dissolution reprecipitation during burial

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

reorientation of structures near granitic plutons and orthogonal lineaments russell lake supracrustal domain southwestern slave province a complex array of folds and cleavages in metasedimentary and metavolcanic rocks in the russell lake domain could reflect regional compression recurrently reoriented across crustal lineaments several generations of steeply inclined folds trending northward across the domain curve into or interfere with folds parallel to margins of bordering granite batholiths aligned orthogonally northwest lnw and northeast lne sets of later subvertical cleavages strike northward northwest parallel and oblique to lnw and northeastward subparallel to lne the cleavages extend beyond the aligned margins indicating that the association with the granites is indirect the arrangements of folds and cleavages are consistent with intermittent redirection of subhorizontal compression of cover rocks perpendicular to orthogonal steep crustal fractures that were intruded by syndeformational granitic plutons additionally overturning of folds near plutons suggests local reorientation of compression near shallow dipping margi

secular change in the precambrian silica cycle insights from chert petrology chert deposits preserve a record of secular change in the oceanic silica cycle the evolutionary radiation of silica secreting organisms resulted in a transition from abiological silica deposition characteristic of the archean and proterozoic eons to the predominantly biologically controlled silica deposition of the phanerozoic comparative petrography of phanerozoic and precambrian cherts indicates that an earlier change in chert deposition occurred toward the end of the paleoproterozoic era ca 1 8 ga in neoproterozoic and mesoproterozoic strata early diagenetic chertification is largely restricted to peritidal environments these early diagenetic cherts typically occur as nodules or discontinuous beds within carbonate deposits that have similar depositional textures the cherts formed primarily by carbonate replacement with subsidiary direct silica precipitation some of the paleoproterozoic cherts associated with iron formations however are distinctly different from younger cherts and appear to have formed largely by direct silica precipitation at or just below the seabed these primary cherts lack ghosts or inclusions of carbonate precursors have fine scale grain fracturing possibly from syneresis exhibit low grain packing densities and are not associated with unsilicified carbonate deposits of similar depositional composition cherts in some paleoproterozoic iron formations e g the gunflint formation northwestern lake superior region are composed of silica types similar to those in phanerozoic sinters e g the devonian rhynie and windyfield cherts scotland such cherts may provide evidence that basinal and perhaps global oceanic silica concentrations were higher during the paleoproterozoic era than at later times

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

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

microscale heterogeneity of fe isotopes in 3 71 ga banded iron formation from the isua greenstone belt southwest greenland we present fe isotope compositions for magnetite crystals from 3 7 ga banded iron formation bif from the isua greenstone belt and younger spatially related pyrite obtained in situ by secondary ion mass spectrometry sims across a distance of several millimeters individual magnetite crystals of bif show 56 fe values up to 2 and within single magnetite rich layers there is a 2 range in 56 fe the highest positive 56 fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric fe in aqueous solution at a likely early archean ocean temperature of 70 c and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal these high values also imply that the supply of oxidant was limited heterogeneity in 56 fe values is attributed to diagenetic reactions occurring in limited volume pore waters isolated from the bulk ocean 56 fe 0 with development of isotope reservoir effects secondary pyrite in bif and a conglomerate also show a distinct enrichment in heavy fe isotopes interpreted as redistribution and or fractionation during postformational events

the effect of early diagenesis on the fe isotope compositions of porewaters and authigenic minerals in continental margin sediments abstract iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the california continental margin where isotope compositions range from 56fe 3 0 to 0 4 at one site near monterey canyon off central california organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction dir and bacterial sulfate reduction whereas at our other site in the santa barbara basin dir appears to be comparatively small and production of sulfides fes and pyrite was extensive the largest range in fe isotope compositions is observed for fe ii aq in porewaters which generally have the lowest 56fe values minimum 3 0 near the sediment surface and increase with burial depth 56fe values for fes inferred from hcl extractions vary between 0 4 and 0 4 but pyrite is similar at both stations where an average 56fe value of 0 8 0 2 was measured we interpret variations in dissolved fe isotope compositions to be best explained by open system behavior that involves extensive recycling of feflux this study is the first to examine fe isotope variations in modern marine sediments and the results show that fe isotopes in the various reactive fe pools undergo isotopic fractionation during early diagenesis importantly processes dominated by sulfide formation produce high 56fe values for porewaters whereas the opposite occurs when fe iii oxides are present and dir is a major pathway of organic carbon respiration because shelf pore fluids may carry a negative 56fe signature it is possible that the fe isotope composition of ocean water reflects a significant contribution of shelf derived iron to the open ocean such a signature would be an important means for tracing iron sources to the ocean and water mass circulation

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

matrix independent fe isotope ratio determination in silicates using uv femtosecond laser ablation abstract uv femtosecond laser ablation coupled to mc icp ms provides a promising in situ tool to investigate elemental and isotope ratios by non matrix matched calibration in this study we investigate fe isotope composition in siliceous matrices including biotite hornblende garnet fayalite and forsterite san carlos olivine and an oceanic fe mn crust using the iron reference material irmm 014 for calibration to test the accuracy of the laser ablation data fe isotope compositions were obtained independently by solution icp ms after chromatographic separation of fe sample materials with low cr content i e biotite hornblende fayalite and the fe mn crust reveal 56 54 fe and 57 54 fe values that agree with those from solution icp ms data within the measured precision for high cr concentration 54 cr 54 fe 0 0001 i e in the garnet and forsterite sample 56 54 fe and 57 54 fe values were derived from 57 fe 56 fe ratios as correction of the isobaric interference of 54 cr on 54 fe is unsatisfactory this approach provides accurate results for both minerals moreover the garnet crystal exhibits isotopic zonation with differences of 0 3 in 56 54 fe showing that substantial heterogeneities exist in high temperature metamorphic minerals multiple analyses of homogeneous sample materials reveal a repeatability of 0 1 2 sd for 56 54 fe and 0 2 2 sd for 57 54 fe respectively this study adds to the observations of horn et al 2006 who have shown that the determination of fe isotope ratios in various matrices including iron alloys iron oxides and hydroxides iron sulfide and iron carbonates can be performed with high accuracy and precision at high spatial resolution using uv femtosecond laser ablation icp ms these results demonstrate that femtosecond laser ablation icp ms is a largely matrix independent method which provides a substantial advantage over commonly employed nanosecond laser ablation systems

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

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

iron isotopes in the early marine diagenetic iron cycle measurements of fe oxyhydroxides reactive fe iii in two shallow sediment profiles from reducing and oxic environments on the arabian sea margin demonstrate fe isotope fractionation during early marine diagenesis reactive fe iii has 56 fe values between 0 77 and 0 19 values are lowest at the top of the sediment profile and considerably lower than bulk sediment fe 56 fe 0 preferential reduction and dissolution of light fe isotopes during diagenesis leaves behind an increasingly smaller and heavier reactive fe iii residual initially the isotopic composition of reactive fe iii evolves down core according to the fractionation factor typical of microbial dissimilatory fe reduction deeper in the profile 56 fe values remain unchanged despite further fe reduction here another process with a different fractionation factor becomes dominant probably reduction by dissolved sulfide the 56 fe of the residual reactive fe iii suggests that 25 of the initially present reactive fe iii was reduced by microbial fe reduction when fe is diagenetically recycled between reducing sediments at depth and an oxic top layer the process depending on recycling efficiency may result in the accumulation of light fe in the top layer while the complementary heavier residual is buried fe diffusing from the seafloor back into the ocean should reflect the low 56 fe diagenetic source of dissolved fe

iron isotopes in an archean ocean analogue abstract iron isotopes have been extensively used to trace the history of microbial metabolisms and the redox evolution of the oceans archean sedimentary rocks display greater variability in iron isotope ratios and more markedly negative values than those deposited in the proterozoic and phanerozoic this increased variability has been linked to changes in either water column iron cycling or the extent of benthic microbial iron reduction through time we tested these contrasting scenarios through a detailed study of anoxic and ferruginous lac pavin france which can serve as a modern analogue of the archean ocean a depth profile in the water column of lac pavin shows a remarkable increase in dissolved fe concentration 0 1 1200 m and 56fe values 2 14 to 0 31 across the oxic anoxic boundary to the lake bottom the largest fe isotope variability is found at the redox boundary and is related to partial oxidation of dissolved ferrous iron leaving the residual fe enriched in light isotopes the analysis of four sediment cores collected along a lateral profile one in the oxic layer one at the redox boundary one in the anoxic zone and one at the bottom of the lake indicates that bulk sediments porewaters and reactive fe mostly have 56fe values near 0 0 0 2 similar to detrital iron in contrast pyrite 56fe values in sub chemocline cores 60 65 and 92 m are highly variable and show significant deviations from the detrital iron isotope composition 56fepyrite between 1 51 and 0 09 average 0 93 importantly the pyrite 56fe values mirror the 56fe of dissolved iron at the redox boundary where near quantitative sulfate and sulfide drawdown occurs suggesting limited iron isotope fractionation during iron sulfide formation this finding has important implications for the archean environment specifically this work suggests that in a ferruginous system most of the fe isotope variability observed in sedimentary pyrites can be tied to water column cycling foremost to the oxidation of dissolved ferrous iron this supports previous suggestions that enhanced iron isotope variability in the archean may record a unique stage in earth s history where partial ferrous iron oxidation in upwelling water masses was a common process probably linked to oxygenic or anoxygenic photosynthesis

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

natural variations of δ30si ratios during progressive basalt weathering hawaiian islands abstract silicon stable isotopes can be used to trace the biogeochemical pathways of si as it moves from its continental sources to its sink in ocean sediments along the way si is incorporated into clay minerals taken up by plants where it forms plant opal and leached into rivers the major land to ocean conduit compared to igneous rocks the waters that drain continents are enriched in heavy si isotopes but the mechanisms that control fractionation have not been elucidated we studied si isotope fractionation along a 4 million yr basaltic soil chronosequence on the hawaiian islands using the natural context of these samples in combination with laboratory experiments we demonstrate that the isotopic composition of dissolved si in weathering systems is determined by the combined effects of rock disintegration clay mineral neosynthesis and si biocycling weathering preferentially releases 28si into solution whereas secondary mineral formation preferentially removes 28si from solution in humid environments leached soils have lost large amounts of this soluble si thus creating a net loss of 30si from the entire soil system as soils develop and greater fractions of si reside in neoformed clay minerals 30sibulk soil values change progressively toward more negative values basalt 30si values are about 0 5 but older soils have 30si values up to 2 5 the difference between the solid and solution 30si values remains more or less constant with progressive weathering and therefore soil water from older soils has a more negative 30si composition in the upper horizons of the hawaiian soils this weathering driven 30si shift is modified by the addition of unweathered primary minerals via dust carrying 30si values of about 0 5 and by biocycling of si via plants producing negative 30si values in phytoliths and positive 30si values in soil solutions derived from upper horizons due to the high concentrations of dissolved si in these near surface layers rivers have more positive 30si values than predicted based on the weathering status of the lower horizons when combined with published 30si values from large rivers worldwide we find that the results from hawaii point to weathering control of si isotopes delivered to the oceans and thus to an important continent ocean linkage that warrants further investigation

effect of diagenetic phase transformation on the silicon isotope composition of opaline sinter deposits of geysir iceland detectible 30si variations in present day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents interpretations of 30si signatures remain problematic in view of potential post depositional changes of which magnitudes and underlying mechanisms are largely unknown a critical issue in the interpretation of isotope data from cherts concerns the extent to which early diagenetic processes modify original 30si signatures here we report 30si variations in opal a opal a ct and opal ct from fossil sinter deposits in an active discharge apron in the geysir geothermal field iceland opal a samples show an average 30si of 0 7 0 2 while opal ct samples are isotopically lighter with an average 30si of 2 0 0 4 implying a sizable shift of approximately 1 3 between the different phases this shift can be explained by repetitive dissolution re precipitation processes diffusive transport or temperature differences during phase transitions on average the fossil opal a tends to be less negative in 30si than amorphous silica that recently precipitated from the hydrothermal water the difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface or to a post depositional release of surface 28si at the onset of diagenetic formation our results corroborate the perception that original silicon isotope signatures of silica acquired during chemical precipitation from a saturated solution may not be preserved in the geological record and that post depositional changes must be taken into account when interpreting data from ancient chert deposits

precambrian si isotope mass balance weathering and the significance of the authigenic clay silica sink abstract the geological cycles of carbon and silicon are linked via the silicate weathering feedback but growing understanding of reverse weathering the formation of authigenic clay minerals via back reaction of silica with cations indicates that the nature of the connection between these two elemental cycles may be more complex than previously assumed recent carbon cycle modeling has suggested that enhanced reverse weathering during precambrian time could have played an important role in regulating climate the topology of the precambrian silica cycle including the size of the authigenic clay sink and its influence on the carbon cycle is challenging to reconstruct due to inherent preservation biases in the rock record here we used an alternative approach using si isotope values to invert for the magnitudes of precambrian silica fluxes by assuming that the silica cycle must be in mass balance with the bulk silicate earth over geological time scales with respect to si isotopes to estimate uncertainties associated with these flux reconstructions we used monte carlo simulations based on the natural variation exhibited in si isotope ratio data from the precambrian sedimentary record n 2118 spanning 3 8 ga to 525 ma and modern authigenic clays n 123 the perspective of si isotope mass balance shows that the previously observed increase in chert 30si values across precambrian time is a signal of an evolving silica cycle rather than a secular change in seawater temperature results yielded a secular increase in the relative contribution of the authigenic clay sink across most of precambrian time this view is consistent with previous interpretations of a concomitant decrease in the concentration of dissolved silica in seawater this result is consistent among a variety of model versions incorporating different assumptions about the abundance of iron formations and possible relationships between authigenic clay formation and concepts that invoke slow growth in the absolute amount of the continental crust this approach also highlights key areas where improving si isotope records could further enhance our constraints on the precambrian silica cycle and its relationship with the carbon cycle and climate stability

contrasting behavior of oxygen and iron isotopes in banded iron formations revealed by in situ isotopic analysis abstract in situ o and fe isotope measurements of magnetite and hematite in banded iron formations bifs from the 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia document distinct fine scale isotopic zonation for hematite o 18 values vsmow range from 7 1 to 0 6 and fe 56 values range from 0 50 to 1 53 magnetite has a o 18 range of 5 6 to 7 0 and a fe 56 range of 0 76 to 1 33 notably magnetite shows significant o isotope variability at a o 18 values exists between low si core and si rich overgrowth iron isotope compositions are homogeneous to 0 2 in fe 56 values within individual grains or layers hematite is always isotopically heavier than co existing magnetite by 0 1 0 4 in fe 56 within individual samples and there is a large variation 2 in fe 56 values between bif samples at the m scale e g between meso or macro bands of the dales gorge member in situ isotopic results highlight the distinct behavior of o and fe isotopes during interaction with post deposition diagenetic or metamorphic fluids the large variations in o 18 values of iron oxides likely reflect exchange with fluids during post depositional events at temperatures up to ca 280 c and the lower o 18 values of fe oxides are interpreted to reflect less isotopic modification and lower temperatures o 18 variations support a model where initial fe oh 3 precipitates that formed in the photic zone were converted to hematite during very early diagenesis followed by production of magnetite over a range of temperatures during later diagenesis metamorphism in contrast magnetite and hematite seem to have preserved the initial fe 56 values of early fe oh 3 precipitates despite significant changes in o isotope compositions in magnetite the consistency in apparent fe isotope fractionation between co existing hematite and magnetite suggests that they share a common precursor where magnetite was formed in soft sediment through in situ reduction by microbial processes and possibly including reaction with aqueous fe ii combined in situ fe o isotope analyses allow distinctions to be made between near primary and post depositional signatures in bifs which bears on the use of bifs as paleo environmental proxies and recorders of microbial processes

questioning fe isotopes as a provenance tool insights from bog iron ores and alternative applications in archeometry abstract provenancing metal artifacts with scientific methods is an established tool in archeometry to identify the metal deposits which were exploited for the production of ancient metal objects it is thus an important method to reconstruct ancient exchange networks and the socio economic organization of past societies previously introduced isotope based approaches pb sr os and chemical analyses of slag inclusions have severe limitations concerning their application or the amount of sample required fe isotopes were previously suggested in a quite early stage of investigation as a potential provenance tracer and it was postulated that they would not fractionate during the smelting procedure however previously published analytical data from iron ores indicate a wide overlap between deposits further the earlier studies on fe isotopes did not included bog iron ores despite their high importance in ancient metallurgy as geochemical reactions during ore formation are complex and fractionation cannot be generally excluded the applicability of fe isotopes as an alternative provenance tracer still asks for further investigation this purely methodological study focuses on specimens from two sites of the formerly mined bog iron ore deposit eyller bruch germany which are analyzed together with products of a smelting experiment based on these ores the fe isotopic composition of the bog iron ore from the investigated region suggests an intra deposit zonation caused by environmental parameters and its overall variation is comparable with that of other deposits the bog iron ore isotope signature largely overlaps with the isotope range of mineralizations in other regions as a consequence of this and although the absence of fe isotope fractionation during the smelting procedure is confirmed the study demonstrates the lack of discriminatory power of fe isotopes for provenance studies potential applications for archeometry can rather be found in the environmental parameters especially organic matter which seems to have a strong influence on fe isotope compositions of bog iron ores zonations within deposits might be identified and could help to reconstruct the exploitation history of the deposit or to reconstruct past bog landscapes

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

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

stratigraphic changes of ge si ree y and silicon isotopes as insights into the deposition of a mesoarchaean banded iron formation in order to determine the origin of silicon si in banded iron formation bif we have undertaken a multi tracer study combining ree y data ge si ratios and si isotopes d 30 si on stratigraphically resolved layers from a 2 95 ga bif from the pongola supergroup south africa si in both si rich and fe rich layers has a common origin represented by a seawater reservoir strongly influenced by continent derived freshwaters 10 and very limited o 0 1 high t hydrothermal fluids as indicated by eu anomalies and y ho ratios the coevolution of d 30 si signatures of si and fe rich

sims analyses of silicon and oxygen isotope ratios for quartz from archean and paleoproterozoic banded iron formations banded iron formations bifs are chemical marine sediments dominantly composed of alternating iron rich oxide carbonate sulfide and silicon rich chert jasper layers isotope ratios of iron carbon and sulfur in bif iron bearing minerals are biosignatures that reflect microbial cycling for these elements in bifs while much attention has focused on iron banded iron formations are equally banded silica formations thus silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in bifs bifs are banded by definition and microlaminae or sub mm banding are characteristic of many bifs in situ microanalysis including secondary ion mass spectrometry is well suited for analyzing such small features in this study we used a cameca ims 1280 ion microprobe to obtain highly accurate 0 3 and spatially resolved 10 lm spot size analyses of silicon and oxygen isotope ratios for quartz from several well known bifs isua southwest greenland 3 8 ga hamersley group western australia 2 5 ga transvaal group south africa 2 5 ga and biwabik iron formation minnesota usa 1 9 ga values of d 18 o range from 7 9 to 27 5 and include the highest reported d 18 o values for bif quartz values of d 30 si have a range of 5 from 3 7 to 1 2 and extend to the lowest d 30 si values for precambrian cherts isua bif samples are homogeneous in d 18 o to 0 3 at mm to cm scale but are heterogeneous in d 30 si up to 3 similar to the range in d 30 si found in bifs that have not experienced high temperature metamorphism up to 300 c values of d 30 si for quartz are homogeneous to 0 3 in individual sub mm laminae but vary by up to 3 between multiple laminae over mm to cm of vertical banding the scale of exchange for si in quartz in bifs is thus limited to the size of microlaminae or less than 1 mm we interpret differences in d 30 si between microlaminae as preserved from primary deposition silicon in bif quartz is mostly of marine hydrothermal origin d 30 si 0 5 but silicon from continental weathering d 30 si 1 was an important source as early as 3 8 ga

experimental determination of fe isotope fractionation between aqueous fe ii siderite and green rust in abiotic systems abstract iron isotope fractionation between aqueous iron and siderite has been measured using abiotic synthesis experiments at 20 c applying a rayleigh distillation model to three experiments in which siderite feco3 was continuously precipitated from solutions over the course of tens of hours the derived fractionation factor is 103ln fe ii siderite 0 48 0 22 2 this experimentally determined fractionation factor is significantly smaller by 1 5 than that calculated from vibrational frequencies and mossbauer shifts geochim cosmochim acta 64 2000 849 865 geochim cosmochim acta 65 2001 2487 2497 and lies on the high end of that inferred from natural mineral assemblages where the fe ii aq siderite fractionation factor was inferred to lie between 1 7 and 0 3 contrib mineral petrol 144 2003 523 547 the measured fe ii aq siderite fractionation from this study suggests that siderite from archean and proterozoic rocks which have moderately negative 56fe values 1 5 to 0 5 may reflect precipitation from fe ii aq whose origin was hydrothermal in contrast natural siderites that have higher 56fe values apparently require non hydrothermal sources of fe and or different pathways of formation or may reflect the effects of cation substitution on the fe ii aq carbonate fractionation factor

the iron isotope composition of microbial carbonate abstract in a comparative study of fossil and recent stromatolites and microbialites we have investigated whether reefal stromatolitic limestones are able to preserve fe isotope compositions that potentially serve as proxies of seawater chemistry at the time of their formation it was found that 56 fe values of archean stromatolites vary between 2 1 and 0 5 devonian and carboniferous microbiallites vary between 0 and 1 and modern microbialites from the great barrier reef show a range from 0 12 to 0 15 five lines of evidence support the possibility that these compositions are potentially pristine 1 fe concentrations and fe isotope ratios are not correlated 2 the concentrations of other elements that are potentially mobile during carbonate diagenesis mg sr do not correlate with 56 fe 3 mn concentrations are inversely correlated with 56 fe which might hint at meteoric alteration however if mn concentrations and fe isotope signatures were pristine features they might serve as a record of the reduction of these two metals in seawater 4 dolomitisation of two devonian limestones does not shift their fe isotope compositions 5 rare earth element and yttrium rey patterns in all limestones are similar to seawater rey sources this suggests that the bulk of the fe in the samples occurs in hydrogenous carbonates not in primary or secondary fe and mn hydroxides because the latter have different rey patterns the modern microbialites are unfractionated in fe relative to crustal rocks thus these hydrogenous precipitates either incorporated unfractionated fe of the ambient seawater composition or they modified the fe composition upon incorporation from a fractionated ocean fe reservoir in either case provided diagenetic alteration has not modified the fe isotope composition in a major way a potential open ocean fe isotope proxy may exist that may allow reconstruction of secular variations in seawater fe isotope compositions our preliminary results suggest that neoarchean microbial carbonate fe may have been lighter than today s microbialite fe and that the carbonates fe isotopes potentially record the efficiency of transition metal reduction in ocean basins through geologic time

zircon geochronology of archaean felsic sequences in the zimbabwe craton a revision of greenstone stratigraphy and a model for crustal growth u pb ion microprobe shrimp work on zircon populations from 13 zimbabwean archaean felsic rocks are presented and interpreted samples were extracted from felsic volcanic sequences from most of the major greenstone belts and represent the first zircon geochronological data from within the greenstone belts themselves the data demonstrate a late archaean volcanicity spanning 250 ma which began at least 2900 ma ago and ended at 2650 ma the intrusion of extensive granitoid sills of the chilimanzi suite at c 2 6 ga marks the widespread stabilization of the craton based on the new zircon data and a re evaluation of published mapping a new stratigraphic subdivision is presented for the late 3 3 ga fragment of archaean crust in zimbabwe these data suggest that the early archaean crust is restricted to the tokwe segment in the south of the country since even the oldest of the felsic volcanics 2 90 ga lower belingwean have zircons which are 50 ma older it is suggested that remnants of earlier cyclic greenstone granitoid events must underlie the present craton and that all of the currently exposed greenstone belts of zimbabwe were developed on sialic crust

iron stable isotopes beyond biosignatures abstract the stable isotope geochemistry of fe has attracted intense interest in the past five years this interest was originally motivated by the possible use of fe isotopes in biosignature applications particularly in sediments from the ancient earth or mars this application is still being developed with particular attention to fractionation mechanisms understanding such mechanisms should also provide new insights into the environmental biogeochemistry of fe at the same time the fe isotope system holds promise for other exciting frontiers including applications in oceanography solid earth geochemistry and biomedicine such applications will be increasingly attractive as fe isotope analysis becomes routine

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

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

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

fraction specific controls on the trace element distribution in iron formations implications for trace metal stable isotope proxies iron formations ifs are important geochemical repositories that provide constraints on atmospheric and ocean chemistry prior to and during the onset of the great oxidation event trace metal abundances and their mo cr u isotopic ratios have been widely used for investigating ocean redox processes through the archean and paleoproterozoic mineralogically ifs consist of three main fe bearing fractions 1 fe ca mg mn carbonates 2 magnetite and or hematite and 3 fe silicates these fractions are typically fine grained on a sub m scale and their co occurrence in varying amounts means that bulk rock or microanalytical geochemical and stable isotope data can be influenced by cryptic changes in mineralogy fraction specific geochemical analysis has the potential to resolve mineralogical controls and reveal diagenetic versus primary precipitative controls on if mineralogy here we adapt an existing sequential extraction scheme for fe phases poulton and canfield 2005 to the high fe content in if and the specific three fraction mineralogy we optimized the scheme for magnetite dominated archean ifs using samples from the hematite poor asbestos hills subgroup if transvaal supergroup south africa previously commonly used hydroxylamine hcl and dithionite leaches were omitted since ferric oxides are quantitatively insignificant in these if samples the acetate leach was tested at variable temperatures reaction times and under different atmospheres in order to ensure that all micro crystalline fe carbonates were effectively dissolved resulting in an optimum extraction for 48 h at 50 c under anoxic conditions the dissolution of magnetite by nh4 oxalate was also tested resulting in an optimum extraction for 24 h under an ambient atmosphere finally a hf hclo4 hno3 leach was used to dissolve the residual silicate fraction which has to date not been considered in detail in if accuracy of the extraction technique was generally excellent as verified using 1 elemental recoveries 2 comparison of major and trace element distributions against mineralogy and 3 comparison to results from microanalytical techniques this study focuses on the distribution of three frequently used geochemical proxies in if u mo and cr molybdenum abundances in the kuruman and griquatown if are low and show an apparent correlation with mineralogical variability as determined by the sequential extraction this suggests that changes in bulk rock mineralogy rather than redox chemistry might significantly affect mo stable isotopes for cr a minor bulk rock stratigraphic increase can be related to the oxide and silicate fraction however a positive relationship with zr indicates that this was also controlled by detrital or volcanic ash input uranium is predominantly bound to the silicate fraction and shows clear correlations with zr and sc implying detrital reworking under anoxic conditions the discrepant behaviour of these three proxies indicate that mineralogy should be taken into account when interpreting heterogeneous bulk rock samples and that fraction specific techniques will provide new insights into the evolution of atmosphere and ocean chemistry

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

abiologic silicon isotope fractionation between aqueous si and fe iii si gel in simulated archean seawater implications for si isotope records in precambrian sedimentary rocks abstract precambrian si rich sedimentary rocks including cherts and banded iron formations bifs record a 7 spread in 30si 28si ratios 30si values yet interpretation of this large variability has been hindered by the paucity of data on si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to precambrian marine conditions using the three isotope method and an enriched 29si tracer a series of experiments were conducted to constrain si isotope exchange kinetics and fractionation factors between amorphous fe iii si gel a likely precursor to precambrian jaspers and bifs and aqueous si in artificial archean seawater under anoxic conditions experiments were conducted at room temperature and in the presence and absence of aqueous fe ii fe ii aq results of this study demonstrate that si solubility is significantly lower for fe si gel than that of amorphous si indicating that seawater si concentrations in the precambrian may have been lower than previous estimates the experiments reached 70 90 si isotope exchange after a period of 53 126 days and the highest extents of exchange were obtained where fe ii aq was present suggesting that fe ii fe iii electron transfer and atom exchange reactions catalyze si isotope exchange through breakage of fe si bonds all experiments except one showed little change in the instantaneous solid aqueous si isotope fractionation factor with time allowing extraction of equilibrium si isotope fractionation factors through extrapolation to 100 isotope exchange the equilibrium 30si 28si fractionation between fe iii si gel and aqueous si 30sigel aqueous is 2 30 0 25 2 in the absence of fe ii aq in the case where fe ii aq was present which resulted in addition of 10 fe ii in the final solid creating a mixed fe ii fe iii si gel the equilibrium fractionation between fe ii fe iii si gel and aqueous si 30sigel aqueous is 3 23 0 37 2 equilibrium si isotope fractionation for fe si gel systems is significantly larger in magnitude than estimates of a near zero solid aqueous fractionation factor between pure si gel and aqueous si indicating a major influence of fe atoms on si o bonds and hence the isotopic properties of fe si gel larger si isotope fractionation in the fe ii bearing systems may be caused by incorporation of fe ii into the solid structure which may further weaken fe si bonds and thus change the si isotope fractionation factor the relatively large si isotope fractionation for fe si gel relative to pure si gel provides a new explanation for the observed contrast in 30si values in the precambrian bifs and cherts as well as an explanation for the relatively negative 30si values in bifs in contrast to previous proposals that the more negative 30si values in bifs reflect hydrothermal sources of si or sorption to fe oxides hydroxides

in situ fe isotope ratio determination in fe ti oxides and sulfides from drilled gabbros and basalt from the iodp hole 1256d in the eastern equatorial pacific abstract in situ fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the fe isotopic composition of fe ti oxides and fe sulfides of the different lithologies of the drilled rocks from iodp hole 1256d eastern equatorial pacific 15 ma crust formed at the east pacific rise most igneous rocks normally have a very restricted range in their 56fe 54fe ratio in contrast fe isotope compositions of hot fluids 300 c from mid ocean ridge spreading centers define a narrow range that is shifted to lower 56fe values by 0 2 0 5 as compared to igneous rocks therefore it is expected that mineral phases that contain large amounts of fe are especially affected by the interaction with a fluid that fractionates fe isotopes during exsolution precipitation of those minerals we have used a femtosecond uv laser ablation system to determine mineral 56fe 54fe ratios of selected samples with a precision of one key observation is that the temperature dependent inter mineral fractionations of fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values while synthesized well equilibrated magnetite ilmenite pairs are compatible with the theoretical predictions theoretical considerations including factors of different aqueous fe chlorides and rayleigh type fractionations in the presence of a hydrous chlorine bearing fluid can explain this observation the disagreement between observed and theoretical equilibrium fractionation the fact that magnetite in contrast to ilmenite shows a slight downhole trend in the 56fe values and the observation of small scale heterogeneities within single mineral grains imply that a general re equilibration of the magnetite ilmenite pairs is overprinted by kinetic fractionation effects caused by the interaction of magnetite ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling or incomplete re equilibration at low temperatures furthermore the observation of significant small scale variations in the 56fe 54fe ratios of single minerals in this study highlights the importance of high spatial resolution analyses of stable isotope ratios for further investigations

geological mineralogical and geochemical aspects of archean banded iron formation hosted gold deposits some examples from southern africa this paper reports on different styles of gold mineralization observed in archean gold deposits hosted by algoma type banded iron formations bif in southern africa genetic aspects of various occurrences are discussed in the context of mineralogical as well as geochemical data of bifs from the greenstone terranes of the zimbabwe and kaapvaal cratons the study revealed that in spite of their different provenance and age 3 5 to 2 6 ga the bifs are geochemically similar whereas observed mineralogical differences reflect various degrees of metamorphic overprint generally the bifs belong to mixed oxide carbonate sulfide facies ree distribution patterns of the investigated archean bif samples exhibit positive eu anomalies which suggest a strongly reducing nature of the solutions which also provided the distinctive element contents now present in the chemical sediments irrespective of their formation gold enrichment in bif only occurs if the s and or as contents of the bifs exceed specific threshold values i e gold mineralization is always associated with increased contents of the iron sulfides pyrite arsenopyrite and pyrrhotite the studies indicate that bif hosted gold occurrences are not products of a single universal metallogenic process but may be explained by several different genetic processes such as primary syn sedimentary formation diagenetic changes metamorphic remobilization and epigenetic hydrothermal emplacement

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

in situ iron isotope ratio determination using uv femtosecond laser ablation with application to hydrothermal ore formation processes abstract the feasibility of in situ stable fe isotope ratio measurements using uv femtosecond laser ablation connected to a multiple collector inductively coupled plasma mass spectrometer mc icp ms has been investigated different types of matrices independently determined by solution mc icp ms after chromatographic separation of fe have been analysed by laser ablation using the isotopically certified iron reference material irmm 014 as the bracketing standard the samples have been pure iron metal jm puratronic fe meteorites north chile glenormiston and toluca the meteorite phases kamacite and taenite in toluca and fe sulphides furthermore fe isotope ratios from hydrothermal hematite siderite and goethite from an old mining area in the schwarzwald germany and of magnetite from the metamorphic biwabik iron formation have been determined the results show that a precision of better than 0 1 2 sigma can be achieved with laser ablation and that all the results obtained agree with those determined by solution icp to better than 0 1 this precision and accuracy is achievable in both raster and spot ablation mode a matrix matched bracketing standard is not required and all these materials can be measured accurately against a metal standard the hydrothermal minerals show significant fe isotope zonations in some samples the range of 56fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district for example isotopic zonations found in secondary fibrous hematites show a continuous change in the 56fe values from 0 5 in the core to 1 8 in the rim primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim while the siderite is thought to record primary fluid histories the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re precipitation of the oxidized fe abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0 4 lighter than the core of the grain if indeed fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals and these in turn record hydrothermal fluid histories then the tools are in place for a detailed reconstruction of the deposit s genesis we expect similar observations from other fe rich deposits formed at intermediate and low temperatures e g banded iron formations laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable fe isotope fractionation

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

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

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

the bulawayan supergroup a late archaean passive margin related large igneous province in the zimbabwe craton the four principal lithostratigraphic units koodoovale manjeri reliance and zeederbergs of the widespread late archaean bulawayan supergroup in the zimbabwe craton record the early to middle stage development of a major c 250 000 km 2 volcanosedimentary basin the craton wide c 2 70 ga manjeri unit varies systematically from a tens of metres thick dominantly chemogenic basinal sediment in gradational conformable contact with the underlying several kilometres thick diachronous 2 83 2 71 ga felsic koodoovale unit in the north and west to an up to hundreds of metres thick dominantly siliciclastic retrogradational platformal sediment unconformably overlying protocratonic basement in the south and east the intermittent c 2 70 ga reliance unit consists of the flow fields of several discrete komatiitic sill flow complexes scattered throughout the craton and fed by an interlinked system of mid to upper crustal flow through feeder sills the c 2 70 ga zeederbergs unit comprises a submarine flood basalt sequence of several kilometres thickness that erupted from ene trending parallel mafic dykes preserved in the south and east the basalts form a craton wide blanket to the reliance flow fields and intervening manjeri sediments certain structural stratigraphic features rule out primary construction of the bulawayan supergroup by early lateral accretion as proposed by several previous workers instead evolution of the bulawayan basin can best be explained by vertical accretion involving 1 abundant episodic and bimodal plume related volcanism and 2 extension and rapid subsidence of continental lithosphere many aspects of the bulawayan supergroup provide important insights into archaean tectonics mantle plume activity magma generation and lithospheric plumbing

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

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

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

texture specific si isotope variations in barberton greenstone belt cherts record low temperature fractionations in early archean seawater sedimentary cherts are unusually abundant in early archean pre 3 0 ga sequences suggesting a silica cycle that was profoundly different than the modern system previously applied for the purpose of paleothermometry si isotopes in ancient cherts can offer broader insight into mass fluxes and mechanisms associated with silica concentration precipitation diagenesis and metamorphism early archean cherts contain a rich suite of sedimentological and petrographic textures that document a history of silica deposition cementation silicification and recrystallization to add a new layer of insight into the chemistry of early cherts we have used wavelength dispersive spectroscopy and then secondary ion mass spectrometry sims to produce elemental and si and o isotope ratio data from banded black and white cherts from the onverwacht group of the barberton greenstone belt south africa this geochemical data is then interpreted in the framework of depositional and diagenetic timing of silica precipitation provided by geological observations sims allows the comparison of si and o isotope ratios of distinct silica phases including black carbonaceous chert beds and bands many including well defined sedimentary grains white relatively pure chert bands including primary silica granules early cavity filling cements and later quartz filled veins including all chert types and textures analyzed the 30 si dataset spans a range from 4 78 to 3 74 with overall mean 0 20 median 0 51 and standard deviation 1 30 n 1087 most samples have broadly similar 30 si distributions but systematic texture specific 30 si differences are observed between white chert bands mean 0 60 n 750 which contain textures that represent primary and earliest diagenetic silica phases and later cavity filling cements mean 1 41 n 198 we observed variations at a 100 m scale indicating a lack of si isotope homogenization at this scale during diagenesis and metamorphism although fractionations during diagenetic phase transformations may have affected certain textures we interpret these systematic variations to reflect fractionation during silica precipitation as well as isotopically distinct fluids from which later phases originated sims 18 o values fall in a range from 16 39 to 23 39 n 381 similar to previously published data from bulk gas source mass spectrometry of onverwacht cherts we observed only limited examples of texture related variation in 18 o and did not observe correlation of 18 o with 30 si trends this is consistent with hypotheses that si isotope ratios are more resistant to alteration under conditions of rock buffered diagenesis marin carbonne et al 2011 our results indicate that low temperature processes fractionated silicon isotopes in early archean marine basins a behavior that probably precludes the application of chert 30 si as a robust paleothermometer the values we observe for facies that sedimentological and petrographic observations indicate formed as primary and earliest diagenetic silica precipitates from seawater are more 30 si rich than that expected for bulk silicate earth this is consistent with the hypothesis that the silicon isotope budget is balanced by the coeval deposition of 30 si enriched cherts and 30 si depleted iron formation lithologies precipitation of authigenic clay minerals in both terrestrial and marine settings may have also comprised a large 30 si depleted sink with the corollary of an important non carbonate alkalinity sink consuming cations released by silicate weathering

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

silician magnetite from the dales gorge member of the brockman iron formation hamersley group western australia we report silician magnetite from banded iron formation bif in the dales gorge member of the brockman iron formation hamersley group western australia magnetite mesobands typically consisting of individual 100 m microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite silician magnetite overgrowths contain from 1 to 3 wt sio 2 whereas low si magnetite domains contain less than 1 wt sio 2 silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro x ray diffraction and high resolution transmission electron microscopy three textures are distinguished in magnetite mesobands 1 magnetite sub microlaminae with silician magnetite overgrowths 2 recrystallized magnetite fragments with silician magnetite overgrowths and 3 a complex intergrowth of magnetite and silician magnetite all three textures are found in magnetite mesobands from the bif4 5 and bif12 16 macrobands of the dales gorge type section drill core ddh 47a from wittenoom western australia magnetite domains contain numerous submicrometer to micrometer inclusions of quartz carbonate stilpnomelane and apatite whereas silician magnetite overgrowths are devoid of mineral inclusions the presence of mineral inclusions in magnetite indicates the bif oxide precipitate was not chemically pure iron oxyhydroxide oxide magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this bif silician magnetite is the dominant iron oxide in the dales gorge bif and is present in many other sub greenschist facies bifs worldwide we suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite thus silician magnetite is a potential biosignature in bifs

the effects of metamorphism on o and fe isotope compositions in the biwabik iron formation northern minnesota the biwabik iron formation of minnesota 1 9 ga underwent contact metamorphism by intrusion of the duluth complex 1 1 ga apparent quartz magnetite oxygen isotope temperatures decrease from 700 c at the contact to 375 c at 2 6 km distance normal to the contact in 3d metamorphic pigeonite at the contact however indicates that peak temperatures were greater than 825 c the apparent o isotope temperatures therefore reflect cooling and not peak metamorphic conditions magnetite was reset in d 18 o as a function of grain size indicating that isotopic exchange was controlled by diffusion of oxygen in magnetite for samples from above the grunerite isograd apparent quartz magnetite o isotope temperatures are similar to calculated closure temperatures for oxygen diffusion in magnetite at a cooling rate of 5 6 c kyr which suggests that the biwabik iron formation cooled from 825 to 400 ci n 75 kyr at the contact with the duluth complex isotopic exchange during metamorphism also occurred for fe where magnetite fe silicate fractio nations decrease with increasing metamorphic grade correlations between quartz magnetite o isotope fractio nations and magnetite iron silicate fe isotope fractionations suggest that both reflect cooling where the closure temper ature for fe was higher than for o the net effect of metamorphism on d 18 o d 56 fe variations in magnetite is a strong increase in d 18 omt and a mild decrease in d 56 fe with increasing metamorphic grade relative to the isotopic compositions that are expected at the low temperatures of initial magnetite formation if metamorphism of iron formations occurs in a closed system bulk o and fe isotope compositions may be preserved although re equilibration among the minerals may occur for both o and fe isotopes

improved in situ sr isotopic analysis by a 257 nm femtosecond laser in combination with the addition of nitrogen for geological minerals abstract in situ sr isotope analysis of geological materials by laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms is a powerful tracer technique for tracking magmatic source components and geological processes however the accuracy and precision of the 87sr 86sr ratio are limited in the analysis of natural minerals because of the low sr concentration the isobaric interference or small grains with complex textural contexts especially for transparent minerals such as feldspars in this study analytical results demonstrated that ablation rates in fs laser ablation were consistent for various samples 0 08 0 11 m per pulse but those in ns laser ablation were obviously material properties dependent such as the rates of 0 026 m per pulse and 0 144 m per pulse for feldspar and pyrite respectively in addition at similar energy fluences the sensitivities of sr in feldspars analyzed by the fs laser were 3 4 times higher than those analyzed by the ns laser due to the higher ablation efficiency of the fs pulse these advantages of the fs laser not only offer the benefit of eliminating or weakening the matrix effect during the laser ablation processes but also help to improve the analytical precision for transparent minerals we also demonstrated that the isobaric interferences of calcium dimers and argides caar caca and kr were dramatically reduced by factors of 6 5 11 7 and 5 12 5 in the presence of 6 12 ml min 1 n2 respectively furthermore with the addition of n2 12 ml min 1 the sensitivity of rb was inhibited resulting in a decrease of 1 47 times in rb sr signal ratios due to the effect of suppressing interferences by adding n2 both the stability and accuracy of the 87sr 86sr and 84sr 86sr ratios show improvement especially for the rb rich feldspars combining the advantages of the fs laser system with the addition of nitrogen an improved in situ sr isotope analytical method is then developed the satisfactory accuracy and precision of the 87sr 86sr ratio from natural plagioclases a k feldspar with high rb sr ratios 0 46 and a low sr clinopyroxene were obtained demonstrating the reliability of the proposed method four feldspars which have different contents of the major elements sr and rb showed homogeneous sr isotope compositions and were recommended as potential suitable reference materials for in situ sr isotope analysis as an application two plagioclases in mafic microgranular enclaves mmes with small grain sizes 200 300 m and wide ranges of rb sr ratios were analyzed and showed obvious variations of the 87sr 86sr ratios from core to rim which indicated that the proposed method in this study can provide high spatial resolution geochemical information for a single mineral

development of an assessment system for the blastability of rock masses intact rock properties and the discontinuity structure of a rock mass are among the most important variables influencing blasting results this influence is considered to be a composite intrinsic property of a rock mass and is referred to as the blastability of a rock mass it represents the ease with which a rock mass can be fragmented by blasting this paper outlines an energy block transition model recently proposed by the authors for characterising the blast process a preliminary validation of this model using two sets of field data from the literature is briefly outlined the model is comparable or better than bond s comminution theory at predicting blasting results for cases where intrinsic rock properties are relatively constant to generate a predictive capability for the model a blastability designation bd is designed which reflects the intrinsic resistance of the rock mass to blasting the quantification of bd based on rock engineering systems approaches and consideration of a comprehensive range of intact rock properties and discontinuity structures is illustrated a case study is given which applies the model and the associated assessment system to a highway cutting site confidence as to the potential value of the assessment system and the model is obtained since refinement and improvement on pre existing models can be seen from the new preliminary results

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

biogenic magnetite in stromatolites ii occurrence in ancient sedimentary environments abstract in this paper we report the discovery of fossil bacterial single domain magnetite particles in ancient stromatolites the biogenicity of the crystals was determined by the following criteria 1 distinctive morphology and habit 2 composition and 3 environment of deposition stromatolites ranging in age from the middle archean to pleistocene composed of both carbonate and chert were analyzed for the presence of single domain magnetite using rock magnetic methods the granulometry and composition of the ultra fine grained magnetite crystals extracted were determined by transmission electron microscopy and electron diffraction the oldest magnetofossils were extracted from stromatolitic chert of the gunflint iron formation which is approximately 2000 ma old the implications of these findings and the potential uses of fossil bacterial magnetite in studies of the evolution of biomineralization and prokaryotic metabolic processes paleomagnetism and as an indicator of ancient oxygen levels are discussed bacterial magnetite represents the oldest evidence of biomineralization yet discovered in the fossil record

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

new insights into the genesis of iocg deposits from a case study of the yinachang deposit in sw china abstract the yinachang fe cu au u ree deposit is hosted by the lower part of the paleoproterozoic kunyang group in the central yunnan province of sw china in an area known to contain iron oxide cu au mineralisation the deposit includes i the sulphide assemblage chalcopyrite pyrite magnetite cobaltite molybdenite cassiterite ii rare earth element assemblage bastnaesite parasite fergusonite xenotime monazite iii the uranyl hydroxide mineral vandendriesscheite iv gold commonly hosted by ankerite and v the gangue mineral assemblage apatite biotite fluorspar quartz calcite the ore is commonly massive disseminated banded or in calcite veins the banded mineralisation contains high concentration of au co mo u fe cu and ree and the vein type mineralisation has abundant ree minerals of bastnaesite parisite fergusonite and xenotime monazite exists as inclusions in apatite or as discrete grains on the surface of magnetite and the uranium is associated with chalcopyrite the mineralising process can be divided into an initial na alteration followed by the ore forming assemblages of fe ree and cu au u ree mineralisation the average 18oh2o values in the quartz and calcite change are between 3 5 and 10 7 with dv smow values ranging between 98 2 and 47 7 the calcite siderite and ankerite have 13c isotope composition ranges from 14 to 1 and 18o compositions of 6 9 to 22 the h o c isotope systematics indicate that the mineralising fluids have mixed magmatic and metamorphic sources whereas the carbon is sourced from marine carbonate and the low temperature alternation of polyphase mantle material in situ 34s sulphide value ranges between 1 1 and 11 9 and the bulk 34s value ranges between 4 7 and 10 5 in situ pb isotopes of sulphide samples have 206pb 204pb values of 18 58 18 76 207pb 204pb values of 15 51 15 66 and 208pb 204pb values of 37 47 38 06 these values plot in the upper crust pb evolution curve indicative of a complex source of pb in the deposit the pb is interpreted to have a deep seated magmatic source with a contribution from the crust it is also proposed that the ree mineralised fluids are derived from a late stage of alkaline magma differentiation the 40ar 39ar biotite date indicates that the precambrian rocks in the kangdian region were regionally metamorphosed at ca 900 ma the yinachang fe cu au u ree deposit is interpreted as an iocg deposit based on geological and isotopic studies the banded ore and vein type ore are the main exploration targets for uranium gold and ree this study helps us better understand iocg deposits in china

experimental constraints on kinetic and equilibrium silicon isotope fractionation during the formation of non biogenic chert deposits silicon isotopic compositions 30si of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments but interpretations are hampered by the lack of experimentally validated fractionation factors here we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch reactors at low temperature 10 35 c and near neutral ph 7 5 8 5 as analogue for non biogenic chert formation instantaneous fractionation factors derived from 30si values of the total dissolved sitd silica and mass balance computations with inst 30sippt 1000 30sitd 1000 decrease with progressive precipitation and reduced reaction rates this suggests that silica deposition in the batch reactors is kinetically dominated at the start of the experiments but approaches a metastable equilibrium after ca 400hours modelled kinetic fractionation factors range from 0 9965 at 10 c to 0 9976 at 20 c and 0 9993 at 35 c and ph8 5 whereas equilibrium isotope effects are smaller and range from 0 9995 at 10 c to 1 000 at 20 c and 1 0005 at 35 c our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated implying that the kinetic conditions of non biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks with particular relevance for those preserved in the archean chert record

iron and sulphur isotopes from the carajas mining province para brazil implications for the oxidation of the ocean and the atmosphere across the archaean proterozoic transition abstract mineralogical analyses and major and trace element geochemistry combined with iron and sulfur isotopes were applied to banded iron formations bifs and pyrite bearing sandstones from the carajas mineral province brazil the bifs belong to the 2 7 ga old grao para group and show a rather simple mineralogy hematite and quartz or cryptocrystalline silica their ree y patterns suggest deposition under marine sub oxic conditions fe isotopes show highly positive values 57 fe irmm 14 2 06 0 34 in average which can be explained by the partial oxidation of fe 2 aq in the surface ocean subsequent to the upwelling of deep anoxic seawater the resulting iron oxy hydroxides likely formed in the shallow photic zone as a seasonal process and acquired highly positive iron isotope signature values consistent with a rayleigh distillation model the pyrite bearing sandstones belong to the aguas claras formation of palaeoproterozoic younger age and were cored at the base of the giant azul manganese deposit diagenetic pyrite is characterized by positive 57 fe irmm 14 1 02 0 2 and 34 s vctd 11 97 0 12 average values that are different from values of modern marine biogenic pyrite results likely indicate that dissolved sulfate was the limiting factor controlling biogenic pyrite formation at that time near zero 33 s values 0 013 0 003 indicate minimal mass independent sulfur isotope fractionation mif s consistent with the 2 1 ga age previously obtained from the aguas claras pyrites this result suggests deposition at a time when the oxygen partial pressure in the atmosphere was already high enough 10 5 to 10 2 pal or present atmospheric level to hinder mif s by photochemical reactions incidentally this also confirms that the giant mn deposit of carajas is in the same age range 2 1 0 2 ga as other mn ores in gabon and south africa pointing to the importance of mn precipitation in paleoproterozoic times

comparative fe and sr isotope study of nephrite deposits hosted in dolomitic marbles and serpentinites from the sudetes sw poland implications for fe as au bearing skarn formation and post obduction evolution of the oceanic lithosphere abstract nephrites belonging to both the dolomite related and serpentinite related genetic types from the zloty stok deposit and jordanow l ski and naslawice prospects in the leza ophiolite respectively were analyzed for fe and sr isotope compositions these deposits in the central sudetes sw poland are unusually situated relatively close to each other this study is a first attempt to perform combined fe and sr isotope study of nephrites our results show that fe in the fe as au bearing skarn samples from zloty stok 2 77 7 00 wt fe 56fe 0 07 to 0 14 and 57fe 0 12 to 0 21 to which the dolomite related nephrite belongs is not inherited from the host dolomitic marbles ca 0 4 wt fe 56fe 0 08 to 0 07 and 57fe 0 12 to 0 09 but derived from the nearby ca 340 ma granite intrusion 2 64 5 64 wt fe in contrast iron in the serpentinite related nephrites 2 06 3 88 wt fe 56fe 0 41 to 0 45 and 57fe 0 59 to 0 66 was likely inherited from host serpentinites 5 27 4 81 wt fe rather than metasomatically derived from adjacent granite intrusions 0 13 0 45 wt fe moreover both the marble transformation into a dolomite related nephrite and fe as au bearing skarn and the abyssal serpentinite through ophiolite towards serpentinite related nephrite transformation are accompanied by a progressive increase of 56fe and 57fe this increase is regardless of changes in the bulk fe content increasing during the dolomite related nephrite formation and decreasing during serpentinite related nephrite formation this phenomena may be caused by one of the below listed factors or combinations a preferential incorporation of heavy iron by tremolite and diopside b preferential incorporation of the heavy iron in the fe3 bearing phases crystallizing during nephrite formation c preferential incorporation of the heavy fe from migrating granite derived fluids positive fractionation between aqueous fe bearing fluid and tremolite diopside d positive isotope fractionation between dolomite antigorite precursor and newly formed amphibole clinopyroxene e change of the fe oxidation state furthermore fe in the rock forming tremolite from the serpentinite related nephrites is isotopically heavier than fe in tremolite from the dolomite related ones in contrast to the iron the isotopic composition of strontium combined with its content and structural evidence suggest multiple sources for this element in the case of the dolomite related nephrite ca 3 4 ppm sr initial 87sr 86sr 0 7085 sr inheritance from the host dolomitic marble ca 90 ppm sr initial 87sr 86sr 0 7081 as well as its derivation from intrusions of both the syn to late tectonic ca 340 ma granites ca 400 440 ppm sr and post tectonic 305 ma granites 86 4 ppm sr is suggested in the case of the serpentinite related nephrite 6 3 61 7 ppm sr initial 87sr 86sr 0 7037 to 0 7081 the situation is similar i e sr in the most primitive samples tends to be sourced from ophiolitic rocks up to 2 4 ppm sr in serpentinite whereas in the more evolved types it was likely delivered from the adjacent ca 340 ma 8 8 102 6 ppm sr partially rodingitized granites as well as 305 ma granites ca 80 ppm sr moreover these granites caused the fluid flow responsible for sr bearing clinozoisite vein formation these veins are likely related with nephrite formation and with a metallic mineralization

ge si in hamersley bif as tracer of hydrothermal si and ge inputs to the paleoproterozoic ocean abstract ge si and ge fe ratios have been measured by laser ablation icp ms in selected sections of the hamersley banded iron formation bif including chert siderite bands i e carbonate and silicate facies bif the major ge carriers are magnetite and hematite in the oxide facies and the fe rich silicates greenalite minnesotaite and stilpnomelane in chert siderite bands while quartz has the lowest ge si ratios 0 15 0 7 mol mol in accord with discrimination against ge in quartz the homogeneous si fe ratio of 1 25 for dales gorge oxide facies bif is consistent with si ferrihydrite as precursor phase this si concentration is sufficiently high for formation of iron free silica clusters at the surface of ferrihydrite so that only a minor fractionation of the ge si ratio should result from preferential ge adsorption relative to si hematite chert bands that have been minimally altered by metamorphic reactions and or fluid metasomatism are the preferred material to determine the sources of silica in hamersley bif variations of fe si and ge si ratios in chert mesobands are interpreted as the result of diagenetic mineral segregation a consistent ge si 3 mol mol is then derived by extrapolation of measured ge si ratios to the average fe si composition of dales gorge bif simple mass balance calculations indicate hydrothermal inputs to the ocean of the order of 25 for si and 90 for ge assuming ge si 10 mol mol for high temperature hydrothermal fluids the high si concentration in siderite chert bands si fe 4 demands only a small amount of si ferrihydrite precursor and some excess silica likely precipitated in pore water oversaturated with regard to amorphous silica the average ge si 0 80 0 1 mol mol for bulk chert siderite samples including some stilpnomelane rich cherts is within the compositional range of modern riverine and aeolian inputs pointing to a continental si source during their deposition

the effect of plume processes on the fe isotope composition of hydrothermally derived fe in the deep ocean as inferred from the rainbow vent site mid atlantic ridge 36 14 n abstract the rainbow hydrothermal vent site which is the largest known point source for dissolved fe delivered to the deep north atlantic ocean has remained invariant in its fe isotope composition over at least the past 16 000 years based on analysis of metalliferous sediments beneath the plume because of the conservative behavior of fe in the rainbow plume 56 fe values of particles in the neutrally buoyant plume 0 18 0 05 and underlying sediments 0 19 0 05 are indistinguishable from the 56 fe values of the high temperature fluid sources 0 23 0 04 particles from the near vent buoyant stage of the plume however have higher 56 fe values 0 15 to 1 20 relative to the original vent fluid consistent with fractionation during oxidation of fe ii aq to fe iii aq isotope compositions become invariant in the plume once all fe ii aq is fully oxidized preserving the original composition of the vent fluid the constant fe isotope compositions of the vent fluids over time implies that changes in seawater fe isotope composition of the north atlantic ocean as they are recorded in fe mn crusts requires changes in the relative fluxes of fe to the ocean

silicon isotopes in 3 8 ga west greenland rocks as clues to the eoarchaean supracrustal si cycle we report mc icp ms si isotopic measurements 29si recalculated as 30si on micro subsamples at 500 m scale from several major rock types of the 3 8 ga old isua greenstone belt igb southern west greenland and surrounding eoarchaean terrains with a large overall range of variations 2 80 30si 0 68 they demonstrate strong involvement of surface fluids enriched with dissolved si the resistance of si isotopes to metamorphic resettings and metasomatic overprints is also established metabasaltic pillows and metasediments display similar 30si enriched signatures suggesting that emergent surfaces of the eoarchaean protocrust were composed of slightly weathered hydrothermally altered mafic ultramafic bodies isua magnetite quartz banded iron formation bif is strongly depleted in 30si relative to all coeval rocks this depletion supports rayleigh controlled precipitation from seafloor vented hydrothermal fluids in contrast banded quartz pyroxene rocks from akilia island some 150 km southwest of the igb which some authors have identified as bif related yield quartz with si isotopic composition 30si 0 36 similar to metamorphic derived quartz 30si 0 50 this supports their derivation from tectonic reworking of ultramafic protoliths penetrated by metamorphic silica and is at variance with their proposed role as harboring earliest biogenic tracers

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