rare earth elements of carbonate rocks from the bambui group southern sao francisco basin brazil and their significance as paleoenvironmental proxies abstract rare earth elements plus y rey data obtained for carbonate rocks of the bambui group in the southern portion of the sao francisco basin match the c and sr isotope variations that subdivide the unit in three chemostratigraphic intervals ci 1 to ci 3 the lower ci 1 samples exhibit flat rey shale normalised distributions that record a high freshwater input in the basin after a glacial period the subsequent marine transgression diminished the continental influence and initiated the development of la gd and y positive anomalies in the ci 2 samples interestingly a seawater like rey pattern is present in the ci 3 samples which correspond to the closure of the sao francisco basin rather than open marine conditions the ci 3 records a decrease in the silicate chemical weathering of sources located on the marginal belts of the sao francisco craton due to higher denudation rates and a change in the dissolved influx towards carbonate rocks the resultant high concentration of carbonate ions in the restricted sea induced a rey fractionation process similarly to that of the modern ocean constant positive ce ce values throughout the shallow and deep sections suggest anoxic early diagenetic conditions inefficient circulation and stratified water column which could have worked as a barrier for life diversification in the basin at the ediacaran cambrian limit finally our data show that the rey concentrations of ancient biochemical environments are not an unequivocal tool either to track open and restricted marine settings or to distinguish proximal and distal environments

shallow marine ironstones formed by microaerophilic iron oxidizing bacteria in terminal paleoproterozoic abstract the microbial origin of precambrian iron formations is debated due to the lack of direct fossil evidence in order to reveal the genesis of ironstones under low oxygen levels integrative studies of sedimentology petrography mineralogy and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal paleoproterozoic chuanlinggou formation ca 1 65 1 64 ga of north china using microscopy sem eds icp oes icp ms and mc icp ms techniques mineralogical study shows that the fe rich mineral is predominantly hematite that resulted from dehydration of amorphous fe oxyhydroxide during diagenesis petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus basinward increases of the ironstone abundance eu anomalies from 1 39 to 1 56 and 56fe values of the ironstones from 0 5 to 1 0 suggest that the iron was mainly sourced from seafloor hydrothermal fluids and partially oxidized and precipitated in shallow subtidal to intertidal environments the common existence of fe oxide coated sheaths spiral stalks residual extracellular polymer substances eps and other biogenic fabrics indicates that microaerophilic iron oxidizing bacteria feob may have played an important role in precipitating the chuanlinggou ironstones the extremely low oxygen concentrations implied by the proliferation of microaerophilic feob in the shallow waters the weak positive ce anomalies 0 94 1 12 and low mn concentrations in the ironstones are broadly consistent with the previous result of a cr isotope study thus the establishment of a microaerophilic feob genetic model for the widespread chuanlinggou ironstones in north china provides new insight into the origin of precambrian iron formations and the redox evolution of ocean atmosphere systems during the boring billion

la ce and sm nd systematics of siliceous sedimentary rocks a clue to marine environment in their deposition la ce isotopic data together with sm nd isotopic data were determined on siliceous sedimentary rocks cherts in order to elucidate the rare earth element ree character of their sources and the nature of their depositional environments the cherts studied are a late archean chert from the gorge creek group in the pilbara block of western australia triassic cherts from central japan and cretaceous and paleogene deep sea cherts from the central pacific and the caribbean sea the archean chert from the gorge creek group shows chondritic ce and nd isotope ratios at its sedimentation age which indicate that its sources had a time integrated chondritic ree pattern triassic cherts from japan have initial ce and nd isotope ratios that show a direct derivation from their continental source on the other hand for cretaceous and paleogene deep sea cherts having negative ce anomalies in their ree patterns two different sources for ce and nd are revealed from their initial ce and nd isotope data ce from long term light ree depleted oceanic volcanic rocks and nd from light ree enriched continental rocks the reverse nature observed for deep sea cherts is considered to be a reflection of their depositional environment far from a continent these results confirm that the la ce isotopemore system is highly useful in determining the nature and cause of ce anomalies observed in marine sedimentary rocks such as chert less

rare earth patterns with positive cerium anomaly in alkaline waters from lake van turkey abstract the distribution of rare earth elements has been studied for aerobic alkaline ph 9 6 carbonate rich waters from lake van turkey shale normalized ree ree sn patterns display an almost linear increase from the light to the heavy rees and strong positive ce sn anomalies there exist neither positive nor negative gd sn anomalies the ree sn patterns are consistent with domination of ree speciation in solution by poly carbonato complexes generating enrichment of the heavy rees that is even more pronounced than that seen in seawater this is the first report of positive ce sn anomalies observed in ree sn patterns of aerobic natural waters these anomalies apparently result from stabilization of penta carbonato ce iv complexes in solution leading to enhanced abundances of ce in comparison with its trivalent ree neighbours thus in remarkable contrast to seawater positive ce sn anomalies may be general features of alkaline carbonate rich and aerobic lake waters

systematic variation of bedrock channel gradients in the central oregon coast range implications for rock uplift and shallow landsliding abstract in tectonically active regions bedrock channels play a critical role in dictating the pace of landscape evolution models of fluvial incision into bedrock provide a means of investigating relationships between gradients of bedrock channels and patterns of active deformation variations in lithology orographic precipitation sediment supply and erosional processes serve to complicate tectonic inferences derived from morphologic data yet most tectonically active landscapes are characterized by these complexities in contrast the central oregon coast range ocr which is situated above the cascadia subduction zone has experienced rock uplift for several million years did not experience pleistocene glaciation boasts a relatively uniform lithology and exhibits minor variations in precipitation although numerous process based geomorphic studies suggest that rates of erosion across the ocr are relatively constant it has not been demonstrated that bedrock channel gradients in the region exhibit spatially consistent values analysis of broadly distributed small drainage basins 5 20 km in the central ocr enables us to explore regional variability in bedrock channel gradients resulting from differential rock uplift or other sources consistent with previous studies that have documented local structural control of deformed fluvial terraces in the western portion of our study area our data reveal a roughly 20 km wide band of systematically elevated channel slopes roughly twice the background value roughly coincident with the strike of n s trending mapped folds although many factors could feasibly generate this pattern including variable rock strength precipitation gradients or temporal or spatial variations in forearc deformation the elevated bedrock channel slopes likely reflect differential rock uplift related to activity of local structures importantly our analysis suggests that rock uplift and erosion rates may vary systematically across the ocr although our calculations were focused on the fluvial dominated portion of study basins our results have implications for upstream areas including unchanneled valleys that often serve as source areas for long runout debris flows zero order basins or topographic hollows within the n s trending band of elevated channel slopes tend to be steeper than adjacent areas and may experience more frequent evacuation by shallow landsliding thus this region of the ocr may be highly sensitive to land use practices and high intensity rainstorms

geochemistry of precambrian carbonates 3 shelf seas and non marine environments of the archean a comprehensive whole rock study of mineralogical chemical and isotopic attributes of archean carbonates suggests that their lithologies and facies have been controlled by tectonic setting in the first two papers of this series we have shown that the dominant lithology of sedimentary carbonates in greenstone belt settings is limestone fe rich dolostone of ankeritic composition in older examples in this paper we suggest that the archean shelf sequences are mostly dolostone and the contemporaneous lacustrine playa lakes are characterized by limestone facies the present study is of the shelf environments of the archean represented by the pongola supergroup of south africa and the hamersley group wittenoom and carawine dolomite of australia the lacustrine playa examples have been sampled from the ventersdorp supergroup of south africa and the fortescue group of australia geological trace element and oxygen isotope considerations of the shelf carbonates suggest that their original mineralogy may have been aragonite and that the pongola dolostones probably represent a direct dolomitization product of this precursor in contrast the stabilization of the hamersley carbonates may have involved an additional step of transformation of a metastable precursor into limestone prior to dolomitization subsequently both sequences but the pongola supergroup much more so were subjected to pervasive thermal events which overprinted their chemical and isotopic signatures consideration of the best preserved samples yields 0 9 2 1 and 6 l pdb for the 13c and 18o respectively for the initial archean signature if compared to phanerozoic carbonates the archean 13c values are comparable but the oxygen isotope measurements again support the primary nature of 18o depletion in ancient marine sediments regardless of its ultimate cause the 87sr86sr of 2 5 ga old sea water was not more than 0 7033 if this value reflects the contemporaneous sea water it would indicate the commencement of a significant 87sr input from the growing and aging continents it is however entirely possible that the slight enrichment in radiogenic 87sr relative to the contemporaneous mantle represents only an unresolved artefact of post depositional alteration if so the 87sr86sr of the 2 5 ga old sea water may still have been within a mantle range the lacustrine limestones retain a strong facies control in their trace element attributes they are also 4 depleted in 13c compared to marine carbonates and strongly depleted in 18o 19 l pdb these light 18o values are somewhat enigmatic unless the lakes were of the high altitude latitude variety the 87sr86sr of 0 711 for the ventersdorp lakes suggests that their drainage basin incorporated not only volcanic rocks but also aged continental crust archean carbonates of all lithologies and facies are enriched in mn and fe compared to their phanerozoic counterparts

prestack kirchhoff depth migration of shallow seismic data prestack kirchhoff depth migration is applied successfully to shallow seismic data from a waste disposal site near arnstadt in thuringia germany the motivation behind this study was to locate an underground building buried in a waste disposal the processing sequence of the prestack migration is simplified significantly as compared to standard common cmp data processing it includes only two parts 1 velocity depth model estimation and 2 prestack depth migration in contrast to conventional cmp stacking prestack migration does not require a separation of reflections and refractions in the shot data it still provides an appropriate image our data example shows that a superior image can be achieved that would contain not just subtle improvements but a qualitative step forward in resolution and signal to noise ratio

rare earth elements in the mediterranean sea and mixing in the mediterranean outflow abstract mediterranean surface waters contain elevated levels of the rare earth elements compared to similar nutrient depleted open ocean waters application of a flux balance model shows that the mediterranean is a net sink for ce and a net source for the other ree the abundance pattern of the excess ree required for mass balance is enriched in heavy ree and has a negative ce anomaly this supports a marine possibly coastal source for the ree although the magnitudes of the fluxes involved are consistent with aeolian input an aeolian source would require large scale removal of ce and the light ree within the mediterranean sea a mixing model to compute trace metal anomalies in the mediterranean outflow predicts the observed ree concentrations well this implies that ce behaves conservatively in outflow waters over the calculated one month period for mediterranean water to reach the study site application to other trace metals gives good agreement for al cd and mn

geochemistry of precambrian carbonates i archean hydrothermal systems abstract carbonate rocks from the superior and slave provinces of canada kaapvaal craton of south africa and the pilbara block of australia considered of hydrothermal origin from field criteria have been characterized mineralogically isotopically sr o c and chemically major and trace elements in agreement with previous studies the bulk chemical composition suggests that the carbonate rocks originated by massive carbonatization silicification and k na metasomatism of intermediate to ultramafic silicate precursors the carbonate component itself defines two partially overlapping populations the ferroan dolomite breunnerite assemblage is confined mostly to shear zones conduits and stockworks while the calcite ferroan dolomite ankerite siderite mineral assemblage represents a wider regional halo this dichotomy may result from a multistory not necessarily coeval plumbing system of the volcanosedimentary piles with the regional halo population being a product of shallow circulation cells while the conduit assemblage probably formed from deeper focussed flows trace element chemistry of carbonates is consistent with their precipitation from relatively low salinity solutions 2 sea water and with derivation of solutes from the contemporaneous volcanosedimentary piles 87 sr 86 sr 0 7020 0 0008 the hydrothermal waters were derived either from metamorphic dewatering at the base of the piles or from a magmatic source with 18o of about 6 7 smow in contrast carbon dioxide in the regional and the conduit assemblages had dissimilar principal sources in the former it was derived from exogenic sources decarbonation of marine carbonates and thermal decomposition and oxidation of organic matter while in the latter the co2 was of mantle origin 13c of 2 to 10 and 4 2 pdb respectively

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

rare earth element distribution in lake tyrrell groundwaters victoria australia abstract the rare earth element ree geochemistry of the saline to hypersaline acidic brines of lake tyrrell victoria australia were analyzed in order to determine the relative importance of concentration and modification processes in the lake system several types of brines were identified on the basis of different ree patterns and concentrations regional groundwater flowing into lake tyrrell yields the highest concentrations of ree while a transitional water group is identified on the basis of lower ree concentrations a third water group the reflux brine is characterized by extremely low ree concentrations the ree patterns suggest that ree are introduced to the lake tyrrell system by the regional groundwater which has concentrated ree from the aquifer material in its acidic waters eventually ree are removed from the evolving water mass as higher levels of ph are reached the data of the study strongly suggest that ree are scavenged from solution by fe oxyhydroxide phases which precipitate as the ph of the system rises water rock interactions probably control ph in the lake system and therefore ultimately control the concentration of ree in solution in this system our results are directly correlated with studies of hydrothermal vents and geothermal solutions which have shown the concentration of ree in solution to be dominantly controlled by ph

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

rare earth elements in river waters we measured rare earth element ree concentrations in river waters to characterize the suspended and dissolved river flux of the ree to the oceans the ree pattern of river suspended materials is sensitive to drainage basin geology a positive correlation is observed between la yb ratios and nd model ages for the rivers studied major rivers have light ree enriched patterns relative to the north american shale composite nasc with la yb n 1 6 2 7 river water dissolved material 0 2 m is heavy ree enriched relative to suspended material and the most pronounced negative ce anomalies occur in rivers of high ph light ree concentrations vary by approximately 3 orders of magnitude and are inversely related to ph and major cation concentrations from these data we estimate that typical major river runoff has heavy ree depleted suspended material with la yb n 1 9 we conclude that the terrigenous input to the oceans from major rivers is heavy ree depleted relative to shales from the available data average river water dissolved material appears to be heavy ree enriched with la yb n 0 4 estuarine removal processes lower the dissolved ree river flux by approximately 60 and result in a flux that is more heavy ree enriched with la yb n 0 2 calculated oceanic residence times with respect to river input range from 2300 to 21 000 years are shortest for ce and greatest for the heavy ree and la such long residence times may suggest the presence of additional sources of ree in seawater

strontium and neodymium isotopes in hot springs on the east pacific rise and guaymas basin we have determined the concentrations and isotopic compositions of sr and nd in hydrothermal fluids from 21 n east pacific rise and guaymas basin gulf of california the purest solutions analyzed from 21 n exhibit a small range in sr concentration between individual vents from 5 8 to 8 7 ppm close to normal seawater sr concentrations they exhibit a small range in 87 sr 86 sr from e sr 0 13 4 to 17 7 corresponding to e sr 0 18 2 in the pure hydrothermal end member these results indicate extensive but not complete isotopic exchange with sr in the depleted oceanic crust e sr 0 31 8 and suggest that sr concentrations in these solutions are buffered in contrast the concentration and isotopic composition of nd in solutions show large variations between vents the concentration of nd ranges from 20 to 336 pg g 6 100 times seawater nd concentrations the isotopic composition ranges from e nd 0 3 6 similar to pacific seawater to 7 9 many samples show substantial contributions from morb but all have e nd 0 well below morb at this locality e nd 0 9 7 in spite of very large enrichments in nd concentrations while complete isotopic exchange with water rock 2 or exchange with anomalous oceanic crust can explain the sr data the nd data require exchange with a reservoir having e nd 0 pacific seawater low temperature reactions with metalliferous sediments on the ridge flanks may provide such a source both sr and nd in the guaymas basin solution are very different from21 n e sr 0 11 0 and e nd 0 11 4 and are consistent with the fluid exchanging sr and nd with heated sediments having a substantial component of old continental detritus some irregularities in the nd isotopic data reported here indicate that there must be a problem of contamination for some ultra low level trace elements during sample collection and processing which requires further attention using a simple box model the estimates for hydrothermal nd fluxes are compared with fluxes which would be required to maintain the relatively radiogenic value of e nd 0 3 in the pacific against the influx of more negative antarctic waters e nd 0 9 it is shown that the hydrothermal flux of nd from mid ocean ridges falls far short of that necessary to maintain the isotopic balance this indicates that weathered material from volcanic terranes e nd 0 7 is the most reasonable major source of radiogenic nd in the pacific

trace element and stable c o and radiogenic sr isotope geochemistry of stromatolitic carbonate rocks of the mesoarchaean pongola supergroup implications for seawater composition abstract the mesoarchaean pongola supergroup hosts one of the oldest well preserved stromatolitic carbonate successions major and trace element geochemistry along with c o and sr isotopic analyses of three stratigraphic sections of stromatolitic carbonate rocks were undertaken in order to infer the depositional environment and to unravel the chemical characteristics of contemporaneous seawater the carbonate rocks consist mainly of dolomite and to a lesser extent ankerite their variable sr concentrations of 95 to 450 mg kg are interpreted as resulting from early to late dolomitization of metastable carbonates aragonite and or high mg calcite facies control and calcitization during meteoric diagenesis 18o values range from 19 6 to 12 0 vpdb and reflect recrystallization of carbonates caused by high water rock interactions during diagenesis and metamorphism a 13c value of 2 5 vpdb was measured in carbonate samples least affected by alteration and we suggest that it approximates the composition of dissolved inorganic carbon in coeval seawater the samples display variable 87sr 86sr isotopic ratios from as low as 0 708 to 0 777 these ratios are considerably higher than those expected in archaean open ocean water masses buffered by hydrothermal interaction with the mantle we interpret the highly radiogenic sr isotopic ratios in some carbonate samples to reflect isotopic exchange with radiogenic sr from siliciclastic volcaniclastic components in the samples and fluids from clay rich layers intercalated with the carbonates the least radiogenic sr isotopic ratio may approximate the mixed isotopic composition of seawater in the epicontinental basin and of riverine freshwater draining the kaapvaal craton the dolomitic carbonate samples show shale normalized rare earth element and yttrium distribution patterns that resemble those of modern ocean water with positive lasn gdsn and ysn anomalies superchondritic y ho ratios a small negative cesn anomaly and depleted light rare earth elements relative to the heavy rare earth elements these anomalies are not as pronounced as expected for an open marine setting which together with the sr isotopic results support a more restricted setting with riverine input and fluctuations in marine ingressions a convex upward shale normalized rare earth element and yttrium pattern is probably a result of several factors including organic matter complexation and element fractionation during estuarine mixing the small negative cesn anomaly may suggest small amounts of free oxygen in shallow waters due to local oxygenic microbial activity

rare earth elements in the nw indian ocean abstract dissolved rare earth element ree distributions have been determined at three stations in the high productivity nw indian ocean nwio upwelling zone dissolved ree distributions within the well oxygenated surface waters and deep portions of the water column are similar to those reported previously from other open ocean locations at depth dissolved ree concentrations are intermediate between those of the atlantic and pacific oceans consistent with the continuous regeneration and enrichment of the strictly trivalent rees in and the progressive oxidative removal of ce from the deep ocean within the mid depth oxygen minimum zone dissolved ree concentrations exhibit a notable enrichment superimposed upon these typical oceanic profiles further dissolved ce concentrations are preferentially enriched within the suboxic horizon compared to the rest of the series indicating the reduction of insoluble ce iv to dissolved ce iii at these depths by analogy with our previous studies of ree distributions in anoxic basins we propose that the more modest enrichments of the strictly trivalent rees observed in the nwio suboxic zone must derive from uptake through scavenging within the well oxygenated portions of the water column and regeneration from sediments and settling particles within the more reducing conditions of the oxygen minimum zone

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

iron in precambrian rocks implications for the global oxygen budget of the ancient earth abstract banded iron formations bif are prominent in sediments older than 2 ga however little is known about the absolute abundance of bif in archean and early proterozoic sediments and the source of the fe is still somewhat uncertain also unknown is the role that fe may have played in the maintenance of low oxygen pressures in the archean and early proterozoic atmosphere an analysis of the chemical composition of precambrian rocks provides some insight into the role of fe in precambrian geochemical cycles the fe content of igneous rocks is well correlated with their ti content plots of fe vs ti in precambrian sandstones and graywackes fall very close to the igneous rock trend plots of fe vs ti in precambrian shales also follow this trend but show a definite scatter toward an excess of fe phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter mn ti relations show a stronger indication of precambrian mn loss perhaps due to weathering under a less oxidizing early atmosphere these data show that fe was neither substantially added to nor significantly redistributed in archean and early proterozoic sediments enough hydrothermal fe was added to these sediments to increase the average fe content of shales by at most a factor of 2 this enrichment would probably not have greatly affected the near surface redox cycle or atmospheric oxygen levels continued redistribution of fe and mixing with weathered igneous rocks during the recycling of precambrian sediments account for the excellent correlation of fe with ti in phanerozoic shales and for the similarity between their fe ti ratio and that of igneous rocks

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

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

use of multi proxy approaches to determine the origin and depositional processes in modern lacustrine sediments carajas plateau southeastern amazon brazil abstract geochemical and isotopic compositions of surficial sediments from a plateau lake in carajas southeastern amazon region were investigated to understand the spatial distribution of major and trace elements 13 c 15 n and c n ratio depositional processes and the origin of inorganic and organic fractions the 13 c 15 n and c n ratio indicate mainly an autochthonous source with siliceous sponge spicules and algae being the major source of organic matter in the center of the lake sector 3 while an allochthonous source mainly derived from c3 vascular forest plants dominates in the shallowest portion of the lake sector 1 consequently there was an apparent dilution of c4 plants montane savanna in the sediment water interface among major elements fe 2 o 3 is highly enriched in sector 3 which is controlled by the erosion of catchment laterites and underwater topography while al 2 o 3 and p 2 o 5 enrichment near the northern border of the lake is controlled by the weathering of mafic rocks similar spatial distribution of sio 2 with total organic carbon toc and isotopic evidence indicate that si distribution is partially controlled by organic components such as siliceous sponge spicules and algae the occurrence of most of the trace and rare earth elements ree is independent of fe 2 o 3 and toc but controlled by detrital aluminum silicates and heavy minerals indicating a lack of post depositional diagenetic control on their distribution the distribution of as and mo are possibly controlled by organic matter mineralization during early diagenesis and subsequently precipitation of fe phases the values of the chemical index of alteration cia in the sediments are very high 94 99 and similar to those of the source rocks suggesting that sediment composition is mainly controlled by mechanical weathering rather than chemical weathering geochemical indices al k ti k al ti la th ti zr zr hf hf nb la al co th ba sr and th sc together with the a cn k plot suggest that the provenance of the inorganic sediments remained relatively uniform or constant during the depositional period and that they were mainly derived from laterite crusts and subordinately from mafic soils this inference was further substantiated by the chondrite normalized ree patterns and discrimination plots geochemical indices such as u th authigenic u mo al and v cr indicate that the sediments were deposited under an oxic environment

iron isotope and ree y composition of the caue banded iron formation and related iron ores of the quadrilatero ferrifero brazil the minas supergroup banded iron formations bifs of the brazilian quadrilatero ferrifero qf mineral province experienced multiple deformational events synchronous with hypogene mineralization which resulted in the metamorphism of bifs to itabirites and their upgrade to high grade iron ore here we present rare earth element and yttrium ree y compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during bif deposition and the effects of hypogene iron enrichment the ree y characteristics of itabirites positive eu anomaly and lree depletion indicate hydrothermal iron contribution to the minas basin iron isotope data and ce anomalies suggest bifs were precipitated by a combination of anoxic biological mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen such as an oxygen oasis perhaps related to increase in oxygen concentrations before the great oxidation event goe the similarity of the ree y composition of the itabirites from the different qf deformational domains as well as to other superior type bifs indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original bifs however iron isotope compositions of iron ore vary systematically between deformational domains of the qf likely reflecting the specific mineralization features in each domain

rare earth elements in the indian banded iron formation abstract the normalised patterns of rare earth elements ree of the banded iron formation bif associated rocks and ores from eastern india show similarities to the patterns of the bif from other parts of the world but their absolute content is rather low for the chemically precipitated rocks of india the process of formation appears to have influenced the relative concentration as the clastic phases are richer in ree compared to the chemically precipitated rocks the observed ce and eu anomalies are similar to those reported for such rocks and have been ascribed to mineralogy and process of formation rather than to their temporal position or the source rock the bif of precambrian and younger age could very well be discriminated by the eu sm ratio the ree are selectively enriched in the fe oxides compared to silica during diagenetic differentiation but neither diagenesis nor metamorphism affects the distribution patterns of the ree as diagenetically unaffected supergene products and highly metamorphosed bif exhibit analogous normalised patterns

sources of iron formations in the archean isua and malene supracrustals west greenland evidence from la ce and sm nd isotopic data and ree abundances abstract isotopic ratios of ce and nd and abundances of rare earth elements rees and ba are reported for the iron formations of the archean isua and malene supracrustals west greenland it is suggested that the sm nd system of the isua iron formations was disturbed by metamorphisms at 1680 ma oxide facies iron formation and 2740 ma carbonate facies iron formation however the malene iron formation samples show similar nd isotopic ratios and ree patterns indicating that they suffered little disturbance after deposition at 2800 ma initial ce and nd values estimated at 2800 ma for the malene iron formation are close to chur chondritic uniform reservoir of 3 samples two show negative ce and positive nd and the other gives chondritic ce and negative nd these initial values suggest that the sources for the malene iron formation had a chiefly chondritic or rather mantle like depleted nature without a notable long term negative ce anomaly similar ree features are indicated for the sources of the isua iron formation by the estimated initial ce and nd values at 3760 ma of these samples

paleobasinal to band scale ree y distribution in iron formations from carajas amazon craton brazil abstract banded iron formations bifs from the carajas mineral province cmp southeastern amazonian craton are the protolith of the world class iron deposits of the cmp besides being one of the largest iron ore deposits in the world they recorded important paleoenvironmental and regional geodynamic markers across the neoarchean paleoproterozoic transition we investigate the secular variation of geochemical signature from several deposit of carajas province the low contents of al2o3 and of 19 other immobile trace elements e g zr th hf ti in 26 high resolution detritus free bif samples indicate their detritus free nature their positive shale normalized subscript sn la and gd anomalies are similar to those of modern seawater whereas their y ho ratios are higher these bif samples also display large positive chondrite normalized subscript cn eu anomalies reflecting the influence of high temperature hydrothermal vents and bottom solutions in contrast to modern seawater and to most other neoarchean paleoproterozoic detritus free bifs the carajas bifs do not show the typical heavy rare earth element ree enrichment relative to light ree instead they do show variable light to heavy ree fractionation trends ybsn prsn ratios 0 05 4 91 the th u ratios of both bulk bifs and mesobands are generally lower than the upper crustal ratio although the bulk bifs show mostly chondritic values the individual fe and si bands exhibit distinctly higher ratios the general absence of negative cesn anomalies attests for rather reducing paleoenvironmental conditions unlike the great majority of the data which lack negative anomalies of ce a few samples do display such anomalies which may be compatible to the oxidizing conditions shown by the low th u ratios of most the carajas bifs such low th u ratios may register terrestrial weathering conditions which cause preferential u input into the oceans in response to the oxidation of immobile u4 to more mobile u6

ree in the great whale river estuary northwest quebec we report rare earth element ree concentrations of a longitudinal profile within the estuary of the great whale river in northwest quebec and in hudson bay all of the measured ree have concentrations less than those predicted by conservative mixing of seawater and river water demonstrating removal of the ree from solution ree removal is rapid occurring primarily at salinities less than 2 removal of the ree is greatest for the light ree and ranges from about 7 for the light ree to no more than 40 for the heavy ree fe removal is essentially complete at low salinity the shape of the fe and ree vs salinity profiles is not consistent with a simple model of destabilization and coagulation of iron and ree bearing colloidal material a linear relationship between the activity of free ion ree3 and ph is consistent with a simple ion exchange model for ree removal surface and subsurface samples of hudson bay seawater are characterized by high ree concentrations and high la yb relative to average seawater the subsurface sample has a nd concentration of 100 pmol kg and an end of 29 3 0 3 these characteristics are consistent with the high ree concentration high la yb and low end of river inputs into hudson bay these results indicate that rivers draining the canadian shield are a major source of non radiogenic nd and ree to the atlantic ocean we estimate that outflow of water from hudson bay to the labrador sea could supply 30 of the non radiogenic nd in north atlantic deep water

redox condition in the nanhua basin during the waning of the sturtian glaciation a chromium isotope perspective abstract the neoproterozoic oxygenation event noe is often considered to be synchronously and possibly causally associated with the cryogenian glaciations reconstructing early cryogenian redox conditions is therefore essential for understanding the onset of the noe we analyzed cr isotope compositions 53cr as well as major and rare earth element ree concentrations of the banded iron formations bifs in the basal fulu formation of the xiajiajiang section nanhua basin which are thought to have been deposited during the waning stage of the sturtian glaciation strongly positively fractionated authigenic 53cr values were obtained after detrital correction ranging from 0 64 to 1 22 and averaging at 0 91 0 40 2 n 10 or after detrital evaluation 0 71 this suggests extensive oxidative cr cycling occurred at the earth s surface and implies that the po2 was probably higher than 1 pal during this period the lack of ce anomalies 0 90

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

geochemical evolution of the banded iron formation hosted high grade iron ore system in the koolyanobbing greenstone belt western australia the banded iron formation bif hosted iron ore deposits in the lower greenstone succession of the koolyanobbing greenstone belt 50 km north of southern cross in western australia are a 200 mt high grade fe 58 pre mining resource and represents one of the most important iron ore districts in the yilgarn craton four hypogene alteration ore forming stages and one supergene upgrading event took place 1 during ore stage 1 lree depleted transition metal enriched mg fe ca carbonates replaced quartz in bifs the deposit scale alteration was most likely induced by devolatilization of sea floor altered ca si depleted mafic rocks in the vicinity of the bif during early regional syn d 1 very low to low grade metamorphism and was most strongly developed on reactivated bif basalt contacts 2 ore stage 2 involved the formation of patchy magnetite ore by a syn d 2 to d 4 dissolution of early carbonate enrichment of fe 2o 3total in magnetite iron ore was by a factor of 2 to 2 4 and compatible trace elements in magnetite such as ga v and al were immobile a subdeposit scale ferroan talc footprint proximal to magnetite iron ore in the largest deposit k deposit was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the bif under greenschist facies conditions and potentially high fluid rock ratio 3 magnetite growth during ore stage 3 forming granular magnetite martite ore is related to a subsequent hydrothermal event occurring locally throughout the belt especially in d 2b faults 4 ore stage 4 was associated with fe ca p l ree y enriched hydrothermal fluids possibly from a magmatic source such as the postmetamorphic lake seabrook granite that crops out about 10 km west of the koolyanobbing deposits and at the southern margin of the greenstone belt these ca enriched fluids interacted with distal metamorphosed mafic rock and influenced the bif ore system in a small number of deposits they were channelled through regional d 4 faults and caused specularitedolomite quartz alteration resulting in fe grades of up to 68 5 supergene upgrade ore stage 5 by further gangue leaching in the weathering zone was most effective in carbonate altered bifs and magnetite ore this process together with supergene martitization and goethite replacement of magnetite led to the formation of high grade locally at k deposit high p goethite martite ore at koolyanobbing the two geochemically distinct stages of archean carbonate alteration clearly controlled the formation of hypogene magnetite specularite martite rich ore and recent supergene modification including the further upgrade of fe ore 2012 by economic geology

rare earth element systematics in hydrothermal fluids abstract rare earth element concentrations have been measured in hydrothermal solutions from geothermal fields in italy dominica valles caldera salton sea and the mid atlantic ridge the measured abundances show that hydrothermal activity is not expected to affect the ree balance of either continental or oceanic rocks the ree enrichment of the solutions increases when the ph decreases high temperature solutions 230 c percolating through different rock types may show similar ree patterns

sulfur isotopes in early proterozoic volcanogenic massive sulfide deposits new data from arizona and implications for ocean chemistry the 34s range of sulfides from early proterozoic volcanogenic massive sulfides of arizona is narrow 129 out of 133 measurements lying within a range of 2 8 to 3 2 other early proterozoic and archean deposits of comparable type and mineralogy have similar s isotope characteristics these deposits unlike phanerozoic analogues do not appear to have incorporated reduced sulfur derived from marine sulfate early proterozoic oceans are known to have contained sulfate in this paper it is a major tenet that sulfate was confined to an upper water layer it is proposed that most volcanogenic massive sulfide hydrothermal systems prior to 1 7 ga were recharged with sulfate free water from an anoxic lower layer and in general discharged into reducing environments rare deposits with exceptional mineralogy or s isotope compositions may reflect hydrothermal recharge from the oxidized upper layer or deposition from fluids discharging into and mixing with the upper water layer

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

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

geochemistry and mineralogy of a silica chimney from an inactive seafloor hydrothermal field east pacific rise 18 s an inactive vent field comprised of dead chimneys was discovered on the ultrafast east pacific rise epr at 18 degrees s during the research campaign naudur with the r v le nadir in december 1993 one of these chimneys was sampled studied and found to be largely composed of silica mineralized bacterial like filaments the filaments are inferred to be the result of microbial activity leading to silica fe oxyhydroxide precipitation the chimney grew from the most external layer precipitated 226 4 yr b p towards the central chimney conduit hydrothermal activity ceased 154 13 yr b p and the chimney conduit was completely sealed mixing between an end member hydrothermal fluid and seawater explains the sr nd isotopic composition of the chimney seawater was the major source of sr to the chimney whereas the dominant nd source was the local mid ocean ridge basalt morb leached by the hydrothermal fluids the mixing scenarios point to a dynamic hydrothermal system with fluctuating fluid compositions the proportion of seawater within the venting fluid responsible for the precipitation of the silica chimney layers varied between 94 and 85 pb isotope data indicates that all of the pb in the chimney was derived from the underlying morb the precipitation temperatures of the chimney layers varied between 55 and 71 degrees c and were a function of the seawater end member hydrothermal fluid mixing ratio delta si 30 correlates with the temperature of precipitation implying that temperature is one of the major controls of the si isotope composition of the chimney concentrations of elements across the chimney wall were a function of this mixing ratio and the composition of the end member hydrothermal fluid the inward growth of the chimney wall and accompanying decrease in wall permeability resulted in an inward decrease in the seawater hydrothermal fluid mixing ratio which in turn exerted a control on the concentrations of the elements supplied mainly by the hydrothermal fluids the silica chimney is significantly enriched in u likely a result of bacterial concentration of u from the seawater dominated vent fluid the chimney is poor in rare earth elements ree it inherited its ree distribution patterns from the parent end member hydrothermal fluids the dilution of the hydrothermal fluid with over 85 seawater could not obliterate the particular ree features positive eu anomaly of the hydrothermal fluids

geochemistry u pb zircon geochronology and sm nd isotopes of the xincai banded iron formation in the southern margin of the north china craton implications on neoarchean seawater compositions and solute sources abstract banded iron formations bifs that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition here we discuss the major and trace elements and sm nd isotopic geochemistry of the poorly studied xincai bif located west of the superior type huoqiu bif along a nw se trending supracrustal belt in the southern margin of the north china craton to understand the compositions and solute sources of seawater during the xincai bif precipitation u pb zircon dating of granitic intrusive rock and biotite schist interlayer of the bif constrains the depositional age of the xincai bif to 2 5 2 7 ga for the first time indicating that the xincai bif could be part of the huoqiu bif or consist of contemporaneous sediments formed in a similar tectonic depositional environment shale normalized rare earth element and yttrium rey patterns of the bulk and individual band samples display features that are characteristic of other archean bifs with light ree depletion relative to heavy ree positive lasn gdsn and ysn anomalies indicative of pure chemical sedimentation the marine origin of these samples is further supported by their super chondritic y ho ratios average y ho 47 the very low al ti and high field strength element contents of the xincai bif indicate an essentially detritus free component the bulk bif samples show a narrow range of end t values from 1 5 to 1 0 and most of the samples have negative end t values 0 3 on average which is different from archean bifs with mantle derived depleted nd isotopes deviations toward negative end t values of the iron bands 0 5 to 0 1 and the silica bands 4 7 to 0 5 further indicate a continental nd contribution in addition positive eusn anomalies that were found in all of the bulk and individual band samples indicate that a high t hydrothermal input also contributed to their rey signatures it is considered that both high t hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their rey signatures

chemistry and stromatolites of the sea linked satonda crater lake indonesia a recent model for the precambrian sea abstract the abundance of in situ calcified stromatolites in the precambrian paleontological record and their absence from modern seas is still an enigma we report on the discovery of the first recent in situ calcified stromatolites growing in a sea linked environment the stromatolites are produced by coccoid cyanobacteria and started to grow 4000 yr ago in the crater lake of satonda island indonesia compared to seawater the lake has gained alkalinity and lost ca its ph 8 45 as well as its saturation with respect to calcite is significantly higher than in seawater the additional alkalinity is provided by biogenic co2 and subsequent weathering of volcanic silicates satonda crater lake may provide an analogue to the stromatolite sustaining precambrian marine environments which therefore may have been more alkaline than present day seawater this conclusion is congruent with the recently advanced hypothesis of an early soda ocean

geochemistry and provenance of metasedimentary rocks from the archean golden pond sequence casa berardi mining district abitibi subprovince the archean golden pond sequence is made up of deformed and metamorphosed conglomerates greywackes and mafic volcanic rocks and is overlain by ferrugineous metasedimentary rocks of the north iron formation the clastic rocks were derived mainly from a volcanic source that had undergone weak chemical weathering their source area was dominated by the presence of 60 80 high al2o3 felsic volcanics having strongly fractionated la sm n 3 7 0 3 and very low ta th ratios 0 09 0 02 with lesser proportions of basaltic 10 30 and ultramafic volcanic rocks 1 10 the ferrugineous metasedimentary rocks can be modelled by mixing 20 40 siliciclastic material of the composition of the average golden pond greywacke with an fe and si rich precipitate molecular fe si 0 6 0 2 the high al2o3 felsic source rocks were most likely produced by subduction processes within an oceanic arc environment but the mafic and ultramafic volcanic rocks were derived by different processes from an asthenosphe

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

rare earth elements in the early archaean isua iron formation west greenland abstract the rare earth element ree patterns in the 3 8 ga old isua iron formation are generally flat resembling those of some primitive basalts samples with positive negative or no europium anomaly were found it is shown that diagenesis and metamorphism did not significantly change the ree patterns the presence or absence of europium anomalies in iron formations cannot be used as an indicator of the presence or absence of oxygen in the atmosphere during the archaean and precambrian the ree contents cannot be used to distinguish algoma type from superior type iron formations there appears to be a striking similarity between the archaean submarine exhalations and modern submarine hydrothermal systems it is considered likely that archaean and early precambrian seawater had a chondritic ree pattern with a slight enrichment of light ree

mineralogy bulk and rare earth element geochemistry of massive sulphide associated hydrothermal sediments of the brunswick horizon bathurst mining camp new brunswick massive sulphides are spatially and temporally associated with iron formation if and other hydrothermal sedimentary rocks in the vicinity of the brunswick no 12 brunswick no 6 and austin brook deposits bathurst mining camp sulphide carbonate oxide and silicate predominant if is present carbonate predominant if is best developed in and around the brunswick no 12 deposit whereas hematite bearing if is absent here but prominent in the austin brook brunswick no 6 area the if is composed dominantly of si co2 fe mn and ca minor constituents include mg p ti al and s statistically significant interelement correlations between eu fe mn pb zn cd au ca sr ba p co2 and s indicate that these elements were precipitated from hydrothermal fluids vented onto the seafloor positive interelement correlations between si ti al mg k zr rare earth elements ree s except eu se v y yb co ni and cr reflect the presence of detrital clastic mafic and aluminosilicate minerals and

marine cements reveal the structure of an anoxic ferruginous neoproterozoic ocean neoproterozoic oceans provided the setting for the rise of animals yet little is known of their chemical composition marine carbonates from the cryogenian oodnaminta reef complex south australia reveal the chemical structure of a neoproterozoic ocean pseudo depth profiles from shallow to deep water reef facies have been constructed from geochemical and sedimentological analysis of marine cements evidence suggests that under a peritidal oxic anoxic chemocline the water column was largely anoxic strongly ferruginous and had a chemistry profoundly different from that of modern seawater these geochemical data suggest early archaean like conditions for this late cryogenian ocean posing problems for metazoan evolution in extremely anoxic conditions supplementary material detailed methods and data tables are available at www geolsoc org uk sup18764

an early soda ocean in analogy to modern soda lakes commonly associated with volcanic regions it is postulated that the ancient sea had a high alkalinity a high ph and low ca and mg concentrations the change towards nacl dominance as observed in the present day ocean can best be explained by a series of mineral equilibria crustal differentiation and life processes in principle hydrothermal leaching of chlorine from the oceanic crust caused this ion to accumulate in the sea while at the same time dissolved carbonates became gradually removed by organisms and carbonates the switch from a soda to a halite ocean was accomplished 1 ga ago critical biochemical events marking the chemical evolution of the precambrian sea and their implications for the pco2 in air will be discussed

chemical sedimentary protoliths in the 3 75 ga nuvvuagittuq supracrustal belt quebec canada abstract analyses of chemical sedimentary precipitates such as banded iron formation bif provide a direct means to explore the nature and composition of the early hydrosphere the recently discovered 3750 myr old nuvvuagittuq supracrustal belt nsb in the northeast superior province quebec canada hosts a suite of iron oxide rich pyroxene and amphibole units that are interpreted to be the metamorphosed equivalents of fe oxide facies bif and a collection of bif like ca fe mg silicate rocks the nsb rocks provide a rare glimpse of trace metal availability in eoarchean ca 3800 ma seawater as they may be contemporaneous with the relatively well studied isua supracrustal belt of southern west greenland their comparison provides an opportunity to enhance our basic understanding of the eoarchean oceans at a global scale work since the initial discovery of the nsb in 2001 has established the basic lithological geochemical and petrographic characteristics of these bif and bif like rocks here we review the current state of knowledge of nsb rocks of probable chemical sedimentary origin including aspects of their geology likely origin and age we conclude by examining the implications of results thus far for our understanding of early seawater compositions and for the emergence of life in the context of early metallo enzyme evolution

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

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

biologically recycled continental iron is a major component in banded iron formations banded iron formations bifs record a time of extensive fe deposition in the precambrian oceans but the sources and pathways for metals in bifs remain controversial here we present fe and nd isotope data that indicate two sources of fe for the large bif units deposited 2 5 billion y ago high end and 56fe signatures in some bif samples record a hydrothermal component but correlated decreases in end and 56fe values reflect contributions from a continental component the continental fe source is best explained by fe mobilization on the continental margin by microbial dissimilatory iron reduction dir and confirms for the first time to our knowledge a microbially driven fe shuttle for the largest bifs on earth detailed sampling at various scales shows that the proportions of hydrothermal and continental fe sources were invariant over periods of 100 103 y indicating that there was no seasonal control although fe sources varied on longer timescales of 105 106 y suggesting a control by marine basin circulation these results show that fe sources and pathways for bifs reflect the interplay between abiologic hydrothermal and biologic processes where the latter reflects dir that operated on a basin wide scale in the archean

theoretical constraints on oxygen and carbon dioxide concentrations in the precambrian atmosphere simple one dimensional climate models suggest that carbon dioxide concentrations during the archean must have been at least 100 1000 times the present level to keep the earth s surface temperature above freezing in the face of decreased solar luminosity such models provide only lower bounds on co2 so it is possible that co2 levels were substantially higher than this and that the archean climate was much warmer than today periods of extensive glaciation during the early and late proterozoic on the other hand indicate that the climate at these times was relatively cool to be consistent with climate models co2 partial pressures must have declined from approximately 0 03 to 0 3 bar around 2 5 ga ago to between 10 3 and 10 2 bar at 0 8 ga ago this steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data which implies that pco2 did not change appreciably during that time oxygen was essentially absent from the earth s atmosphere and oceans prior to the emergence of a photosynthetic source probably during the late archean during the early proterozoic the atmosphere and surface ocean were apparently oxidizing while the deep ocean remained reducing an upper limit of 6 x 10 3 bar for po2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean the establishment of oxidizing conditions in the deep ocean marked by the disappearance of banded iron formations approximately 1 7 ga ago permitted atmospheric oxygen to climb to its present level o2 concentrations may have remained substantially lower than today however until well into the phanerozoic

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

neoarchaean banded iron formation within a 2620 ma turbidite dominated deep water basin slave craton nw canada abstract banded iron formations bifs are iron and silica rich chemical metasedimentary rocks that span much of the archaean and early palaeoproterozoic rock record given that bifs precipitated from ambient seawater they provide an important archive of earth s ancient marine environments within the slave craton occurs one of the largest and well preserved achaean turbidite basins in the world interstratified within the greywacke mudstone turbidites is bif that is commonly associated with ferruginous sediments and locally with a felsic tuff bed field investigations were conducted at four bif locations from west to east across the slave craton these bifs are composed of micro and mesobands of quartz and magnetite with various amounts of metamorphic grunerite ferro actinolite ferro hornblende as well as greenalite and stilpnomelane the latter two are interpreted as retrograde metamorphic phases as the bif was deposited in a deep water turbidite dominated basin s the chemical component was occasionally diluted by clastic detritus as illustrated by highly variable al 2 o 3 content and al 2 o 3 tio 2 and cr ni that are similar to the clastic sediments on a layer by layer basis the iron content in the bif ranges from as low as 4 wt and up to 75 wt fe 2 o 3 whereas the sio 2 content ranges from 20 wt to as high as 94 wt this together with a hyperbolic germanium silicon distribution suggests that continentally sourced silica comprised the background precipitation which was periodically interrupted by pulses of hydrothermally sourced iron high temperature hydrothermal fluids contributed with an average shale normalized positive eu anomaly of 1 92 which indicates input of reduced eu 2 to the ambient seawater the bif shows characteristic aqueous fluid derived rare earths yttrium fractionation pr yb 3 to ce 4 the age and trace element signature of the felsic tuff is similar to arc type volcanic rocks potentially linking the bif and turbidite basin s to the defeat suite plutons ca 2630 2620 ma the slow chemical rainout behind bif deposition presumably took place on the basin plain adjacent to a steep submarine ramp where turbidite currents and slump generated debris flows sourced and triggered by the tectonically active arc terrain episodically delivered clastic sediment

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

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

geochronology and geochemistry of the nanfen iron deposit in the anshan benxi area north china craton implications for 2 55 ga crustal growth and the genesis of high grade iron ores abstract the nanfen iron deposit situated in the anshan benxi area of the north china craton ncc hosts scattered banded iron formations bifs and bif hosted high grade iron orebodies defined by feot 80 wt the associated roof and bottom rocks which occur in parallel to orebodies are mica quartz schist and chlorite hornblende schist locally interlayers respectively the paas normalized rare earth element ree patterns indicate that quartz magnetite banded iron formations bifs are enriched in heavy rees with positive la eu and y anomalies indicating that the nanfen bif precipitated from a mixture of ambient seawater and submarine high t hydrothermal fluids 0 1 in addition the bif exhibits low concentrations of al2o3 and tio2 with depletion in hfses high field strength elements e g nb ta zr hf and ti suggesting little contamination with continental detritus excepting lower eu eu values the high grade iron ores display similar geochemical features to the bif likely implying that they have a same iron source but did not form synchronously electron microprobe analyses empa and in situ la icp ms analyses reveal that in relative to the magnetite grains in high grade ores the magnetite grains in the bif contain higher mno mgo and zn concentrations and lower al2o3 and ree abundances implying different formation conditions thus we argue that high grade ores are likely to be alteration products of primary bifs chlorite hornblende schist meta basalt samples exhibit low sio2 contents 43 78 50 30 wt with flat chondrite normalized ree and n morb normalized trace element patterns which are similar to those of babb back arc basin basalt the zircon u pb dating results indicate that magmatic and metamorphic zircons from chlorite hornblende schist formed at approximately 2552 ma and 2482 ma respectively the former volcanic age could constrain the deposition age of the nanfen bif and the latter reflects a subsequent metamorphic event which is also reflected by the 2480 ma hydrothermal zircons selected from the bif moreover most zircons from chlorite hornblende schist have positive hf t values 0 22 7 96 close to the depleted mantle evolutionary curve implying that the source magma were derived from a depleted mantle with some contamination from ancient crust hence we propose that the nanfen bif is a neoarchean algoma type bif and its deposition was most likely benefited due to crustal growth

rare earth elements in saanich inlet british columbia a seasonally anoxic basin rare earth element and manganese distributions have been investigated in saanich inlet british columbia a seasonally anoxic basin redox cycling of ce and mn takes place across an oxic suboxic boundary approximately 20 m shallower than the o2h2s interface the remaining strictly trivalent ree undergo scavenging above this oxic suboxic boundary and regeneration from dissolving host phases below it these processes result in pronounced removal of ce from solution above the oxic suboxic boundary due to oxidative precipitation along with preferential uptake of the lighter trivalent ree during scavenging below the oxic suboxic boundary regeneration of particulate ree into solution leads to a less pronounced ce depletion and a less pronounced hree enrichment in the anoxic bottom waters dissolved ree concentrations in the sediment pore waters exhibit an approximately ten fold increase across the seawater sediment interface but then decrease with increasing depth indicating removal from solution during later diagenesis calculated diffusive fluxes of the ree and mn from the sediments into the overlying anoxic water column are insufficient to account for the accumulated trace metal enrichments observed in the water column we propose an annual redox cycle for saanich inlet in which flushing causes extensive oxidative precipitation and scavenging of trace elements from the water column each autumn the same material is rapidly regenerated from the underlying near surface sediments when the basin next turns anoxic

geochemistry of late archaean stromatolites from zimbabwe evidence for microbial life in restricted epicontinental seas we report comprehensive trace element and sr isotope data for microbial carbonates from the archaean mushandike limestone masvingo greenstone belt zimbabwe the stromatolites have very coherent ree y patterns and share the essential shale normalised characteristics of other archaean marine precipitates positive la and gd anomalies absence of a negative cc anomaly and a strongly superchondritic y ho ratio mixing models constrain the maximum amount of shale contamination to 0 25 1 and calculated detritus free carbonate ree y systematics require precipitation from seawater in terms of light ree over heavy ree depletion however the studied samples are very different from all other known archaean marine precipitates in shale normalised plots the mushandike samples yield a negative slope a very restricted regional input source of the dissolved load is indicated because normalisation with locally occurring tonalite gneiss ree y data yields a pattern closely resembling typical shale normalised archaean marine chemical sediments the disappearance of a negative eu anomaly when patterns are normalised with local tonalite gneiss strengthens this interpretation sr isotope ratios are strongly correlated with trace element contents and ratios which explains the modest scatter in sr isotope ratios as representing minor clastic contamination importantly even the least contaminated samples have very radiogenic initial sr 87 sr 86 ratios 0 7184 implying sr input from an ancient high rb sr source such as the early archaean gneisses of south central zimbabwe a local ancient 3 5 3 8 ga source is also indicated by previously published pb isotope datasets for the mushandike stromatolites this is entirely compatible with the occurrence of 3 7 3 8 ga zircons in quartzites and metapelites from comparably old greenstone belts within less than 150 km of the studied locality comparison of the pb isotope ratios of the mushandike stromatolites with 2 7 and 2 6 ga old stromatolites from the neighbouring belingwe greenstone belt demonstrates differences in initial isotope composition that relate to the extent of exchange with the open ocean the development of numerous basins on old continental crust with water masses variably restricted from the open ocean suggests a lack of strong vertical topography on this late archaean craton c 2004 elsevier b v all rights reserved

the europium anomaly of seawater implications for fluvial versus hydrothermal ree inputs to the oceans olivarez a m and owen r m 1991 the europium anomaly of seawater implications for fluvial versus hydrothcrmal ree inputs to the oceans chem geol 92 317 328 seafloor hydrothermal vent fluids are characterized by rare earth element ree enrichments and large positive eu anomalies however this input is not recorded by seawater hydrothermal sediments from the southeast pacific dsdp site 598 were examined to address this discrepancy between the observed and predicted ree pattern of seawater scavenging indices which reflect long term ree scavenging by iron rich hydrothermal precipitates were calculated for each of eight ree s comparisons between these indices and the ree content of seawater indicate that the intensity of ree scavenging by hydrothermal precipitates is proportional to the concentration of ree s in the fluid phase we predict that ree s are rapidly scavenged from hydrothermal solutions and that eu is removed disproportionately faster relative to its neighbors eu anomaly values for site 598 and other hydrothermal sediments support this prediction a vent signature is recorded in sediments closest to the ridge axis and the eu anomaly values become more seawater like with increasing distance and scavenging this implies that the ree content of seawater cannot be used to constrain hydrothermal versus fluvial fluxes to the oceans

the ree content of some hydrothermal fluids ree concentrations were determined in solutions from submarine hydrothermal vents on the east pacific rise and for the first time from continental geothermal fields in tibet and bulgaria they indicate that in hydrothermal systems ree are immobile at temperatures below 350 c and for water rock ratios of 105 nd values of submarine solutions and anhydrites 4 7 to 6 2 suggests recharge on the sedimentcovered flanks of the ridge carbonate anions as measured by alkalinity appear to increase the solubility of heavy ree somewhat but not to the extent that substantial amounts of ree could be driven out from rocks by hydrothermal activity

microbiological processes in banded iron formation deposition banded iron formations have been studied for decades particularly regarding their potential as archives of the precambrian environment in spite of this effort the mechanism of their deposition and specifically the role that microbes played in the precipitation of banded iron formation minerals remains unresolved evidence of an anoxic earth with only localized oxic areas until the great oxidation event ca 2 45 to 2 32 ga makes the investigation of o2 independent mechanisms for banded iron formation deposition relevant recent studies have explored the long standing proposition that archean banded iron formations may have been formed and diagenetically modified by anaerobic microbial metabolisms these efforts encompass a wide array of approaches including isotope ecophysiological and phylogeny studies molecular and mineral marker analysis and sedimentological reconstructions herein the current theories of microbial processes in banded iron formation mineral deposition with particular regard to the mechanisms of chemical sedimentation and post depositional alteration are described the main findings of recent years are summarized and compared here and suggestions are made regarding cross disciplinary information still required to constrain the role of the biosphere in banded iron formation deposition

ree fe variations in hydrothermal sediments implications for the ree content of seawater seafloor hydrothermal vent solutions exhibit rare earth element ree enrichments ranging between one to three orders of magnitude greater than average seawater to assess the impact of these hydrothermal inputs on ocean chemistry we have examined the behavior of rees for hydrothermal sediments collected adjacent to two pacific spreading ridge sites the east pacific rise at 19 s and the southern juan de fuca ridge at 4s n in general the ree fe ratios for both proximal and distal hydrothermal sediments are greater than vent solutions by a factor of 2 to 500 and these ratios increase with increasing distance away from the ridge axis an evaluation of these results in the context of previous models of ree behavior indicates that in fact seawater experiences a net depletion in rees as a result of hydrothermal activity this is due primarily to the large scavenging capacity of iron oxyhydroxides which precipitate from these solutions such an interpretation explains why the ree content of seawater collected in the vicinity of hydrothermal vents is anomalously lower than normal seawater sampled from a comparable depth

origin and depositional environment of ordovician stratiform iron mineralization from zamora nw iberian peninsula the ordovician stratiform iron deposits at zamora nw iberia are arranged in several levels ranging between 0 2 and 1 5 m in thickness which are interstratified in the upper member of the pielgo quartzites formation arenig the sandy nature and trace fossils corresponding to the ichnogenus cruziana and daedalus together with major and trace element contents of this formation suggest an inter and subtidal shallow marine depositional environment which on a global scale formed part of a broad shelf situated in the northern margin of the gondwana continent the iron mineralization displays a foliated and banded structure due to the alternation of quartzitic phosphatic apatite chamositic chamositic biotitic and ferriferous magnetite and hematite beds they have high tio2 ta sc v nb co zn and y contents the magnetite contains unusually large amounts of tio2 v cr and ni there is also a clear depletion in eu and the eu sm cn ratio is 1 the chamosite contains high concentrations of cr and v these results suggest that iron was supplied from the weathering of a continental source in combination with volcanic activity such as within basin basic volcanism or the presence of basic volcanic rocks in the exposed land the physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following log f o2 38 8 to 30 7 log f s2 13 2 to 9 5 and t 200 to 330 c these results together with the 18o value 2 of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low grade metamorphism under redox conditions below the magnetite hematite buffer

geochemical constraints on the genesis of the algoma type banded iron formation bif in yishui county western shandong province north china craton abstract the yishui iron deposit is a typical algoma type banded iron formation bif located in yishui county taishan group shandong province in the eastern block eb of the north china craton ncc the bif is interlayered with amphibolite migmatitic granite gneiss and schist integrated petrological mineralogical and geochemical interpretations of the iron ores were used to reconstruct the genesis of the yishui bifs and their tectonic evolution the iron ore samples were collected from surface outcrops banding textures are not always present in the iron ores because the bif experienced complex high grade metamorphism after precipitation interpretations of the major elements and rare earth elements plus yttrium rey data of the iron ores suggests that the ore forming materials of the yishui bif precipitated from a mixture of seawater and high temperature hydrothermal fluids 2 and fe 2 o 3 t sio 2 fe 2 o 3 t 85 8 95 8 wt are consistent with the major minerals such as quartz and iron oxide which represent chemical precipitates chlorite amphibole plagioclase biotite and calcite are present as minor minerals the low contents of tio 2 0 01 0 09 wt al 2 o 3 0 42 1 18 wt and high field strength elements hfses indicate little to no detrital assimilation which implies that the iron ores preserved their original geochemical signatures based on geochemical interpretations we conclude that the yishui bif can be explained by a mantle plume model that describes not only the tectonic evolution but also the sources of iron and silica of the yishui bif