hydrogeological constraints on the formation of palaeoproterozoic banded iron formations banded iron formations are critical to track changes in archaean to palaeoproterozoic ocean chemistry with deposition triggered by water column iron oxidation recently however it was suggested that reduced iron minerals were the primary precipitates and these were subsequently oxidized by oxygen bearing groundwater if true this would cast doubt on our understanding of how banded iron formations were deposited and their ability to record early ocean chemistry here we present a hydrogeological box model based on the approximately 2 5 billion year old hamersley basin of western australia developed to evaluate the plausibility of secondary iron oxidation the box model calculates the time required for groundwater to flux enough oxygen through the basin to oxidize a given amount of ferrous iron less than 9 of nearly four million model iterations returned oxidation times less than the age of the basin successful simulations required simultaneously steep hydraulic gradients high permeability and elevated oxygen concentrations our simulations show that the postdepositional oxidation of large banded iron formations is unlikely except on a limited scale that is during secondary ore formation and that oxidized iron phases were probably the precursor to large palaeoproterozoic banded iron formations banded iron formations could not have formed by postdepositional oxidation according to four million hydrogeological box model iterations that failed to reproduce secondary oxidation on reasonable timescales

chemical processes affecting the mobility of major minor and trace elements during weathering of granitic rocks abstract the behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some portuguese granitic rocks element mobilities are calculated from geochemical data normalized with respect to ti in the fresh parent rock chemical elements are divided into two groups immobile and mobile on the basis of their geochemical distribution during weathering elements that are immobile during weathering are zr hf fe al th nb sc and the ree very mobile are ca na p k sr ba rb mg and si mobile elements are derived mainly from leachable minerals such as feldspars micas and apatites whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals the geochemical behaviour of mn cr v fe and ce is very dependent on redox conditions redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite the ree are mobilized or fractionated during late stages of weathering but not during moderate stages of weathering this fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing ree

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

siderite cannot be used as co2 sensor for archaean atmospheres abstract it was proposed to utilize siderite feco3 in mid to late archaean superior type banded as a proxy to constrain the co2 partial pressure of archaean atmospheres implicit in this proposition is that siderite was a primary carbonate mineral that crystallized directly from fe2 enriched archaean seawater in equilibrium with atmospheric co2 to our knowledge that proposition has not been demonstrated to be valid we test with water gas exchange experiments under controlled co2 partial pressures if siderite can be stabilized as a primary mineral in fe2 bearing seawater reduced seawater proxies enriched in fe2 and mn2 are equilibrated with reduced n2 ch4 co2 h2 gas phases with variable co2 the solid phases stabilized in fe2 enriched water compositions are amorphous ferrous iron hydroxy carbonates crystalline siderite feco3 is not found to be a stable phase the phases precipitating from mn2 enriched water include crystalline rhodochrosite mnco3 and possibly amorphous mn enriched phases based on these results we advise against using siderite in banded iron formations as a co2 sensor for the archaean atmosphere

initialization asymmetry and spindown of arctic eddies abstract initialization and development of cyclonic and anticyclonic eddies under the influence of a frictional surface as with arctic eddies under ice cover are examined both analytically and numerically using a three dimensional numerical model solutions of the linear rossby adjustment problem show that energy released from an initial density anomaly in the barotropic mode and the first few baroclinic modes destabilizes the numerical computation for small arctic eddies this result suggests the necessity of slow spinup to reduce energy release in these modes and sufficient vertical resolution to resolve higher baroclinic modes the numerical computation shows that in an initially stratified and motionless ocean a surface cyclone an anticyclone is produced by a localized salinity source sink in open waters the stable flow field consists of a vertically aligned pair of counterrotating eddies when dampened by surface friction an eddy pair produced by deep forcing has features dissimilar to the

mesoproterozoic chronostratigraphy of the southeastern llano uplift central texas the llano uplift of central texas exposes mesoproterozoic crystalline rocks affected by a grenvillian orogeny between 1 2 and 1 1 ga we report four new u pb zircon protolith ages of 1247 4 1257 3 1272 8 5 and 1366 3 ma for four quartzofeldspathic rocks from the regionally defined packsaddle schist and valley spring gneiss in the southeastern part of the uplift the 1366 3 ma gneiss also yields a metamorphic age of 1325 5 ma these new u pb ages in conjunction with previous u pb age data demonstrate that several regionally defined map units contain constituents with widely disparate ages we have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the llano uplift that appear to represent rock packages of distinct tectonic origins 1 the northern 1366 1272 ma felsic gneisses of volcanic plutonic and continentally derived sedimentary origin valley spring gneiss and an older gneiss component 2 the geographically intermediate ca 1257 1247 ma and possibly older basinal sequence that formed along a continental shelf and slope near an arc packsaddle schist 3 the southern 1326 1275 ma remnant of an exotic ensimatic arc complex big branch gneiss and coal creek serpentinite and plutonic complex and 4 the 1239 1232 ma tectonized felsic rocks that intruded the first two suites the first three suites of rocks were structurally imbricated during grenville orogenesis and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the packsaddle schist in addition these new data show that contrary to previous reports younger packsaddle schist lies in structural contact above older components of the valley spring gneiss moreover one of the dated meta igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most llano uplift rocks its 1366 3 ma protolith age is coeval with rocks of the western granite rhyolite terrane age and geochemical similarities link this unit of unknown regional extent with the western granite rhyolite terrane of known laurentian affinity thus this component of the llano uplift may be the basement on which the younger components of the valley spring gneiss and packsaddle schist formed in a tectonic setting proximal to laurentia and south of the western granite rhyolite terrane

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

207pb 206pb zircon ages and the evolution of the singhbhum craton eastern india an ion microprobe study1this paper is dedicated to prof a k saha one of the senior co authors who passed away after submission of the paper 1 abstract we used an ion microprobe to measure 207 pb 206 pb ages of single zircons from four key lithologies from the singhbhum craton of eastern india detrital zircons from the oldest recognized sedimentary sequence of the craton the older metamorphic group yielded 3 5 3 6 ga ages indicating that crustal formation had already been initiated by 3 6 ga in this region this data also established an older limit of 3 5 ga for the age of sediment deposition the first appearance of extensive continental crust in the singhbhum region marked by the emplacement of the older metamorphic tonalite gneisses is dated at 3 44 ga our data from both of these units reveal that a 3 2 ga event induced overgrowths within each of these pre existing zircon populations we determine an age of 3328 7 ma for the singhbhum granite phase ii the most widespread of this granite s three phases the emplacement of the two phases of the mayurbhanj granite marking the youngest major episode of acid plutonism in this craton took place in rapid succession at 3 1 ga marking the broad scale stabilization of this terrain

mesoscale eddies of the arctic ocean subsurface baroclinic eddies in the arctic ocean have a diameter of 10 20 km and are confined in depth between 50 and 300m during the arctic ice dynamics joint experiment aidjex in the central beaufort sea between march 1975 and may 1976 extensive information on eddy characteristics statistics and behavior was collected from four manned drifting ice camps during this 14 month period 146 eddies were encountered of which 19 were found to be repeated crossings making a total of 127 separate eddies observed during this period on the basis of the 1975 1976 aidjex data set it is concluded that eddies are 1 prevalent in the amerasia basin of the arctic ocean and in particular the beaufort sea where they may occupy up to a quarter of the available surface area 2 located in the depth range of 50 300 m although a few deeper eddies are also present 3 the second largest producers 32 of kinetic energy found within the upper 200 m of the beaufort sea and 4 predominantly anticyclonic in rotational tendency these eddies apparently originate north of point barrow alaska as a result of instability in the eastward flowing alaskan coastal current and transfer seasonally varying water properties from the chukchi sea and alaskan shelf into the arctic ocean they appear to translate in response to barotropic forcing over short time scales and over longer time periods move with the mean geostrophic field the season of formation for an eddy may be identified by the anomalous thermal properties still residing in its core

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

generation and separation of mesoscale eddies from surface ocean fronts abstractthis study is motivated by arctic ocean observations of sub mixed layer eddies found at large distances from their assumed formation region of a surface ocean front eddy formation is explored through high resolution numerical simulations of surface fronts separating two mixed layers with a range of configurations similar to those observed in the arctic ocean this study finds that frontal instabilities lead to the development of self propagating dipoles which have the potential to propagate far from the front if interactions with other eddies are avoided however most dipoles are unbalanced consisting of a dominating surface cyclone and a weaker anticyclone below and thus propagate on curved trajectories with eventual recirculation back to the front their maximum separation distance from the front depends on the ratio of self advecting velocities balanced dipoles that have 1 and the ability to propagate far from the front for dipoles generated numerically this study estimates u

shallow convection and buoyancy equilibration in an idealized coastal polynya abstract the recent theoretical approach of visbeck marshall and jones is used to examine shallow convection and offshore transport of dense water from an idealized coastal polynya a constant negative buoyancy flux is applied in a half elliptical region adjacent to a coastal boundary surrounded by a forcing decay region with uniform width w over which the imposed buoyancy flux decreases smoothly to zero initially the density beneath the forcing increases linearly with time a baroclinically unstable front forms at the edge of the forcing region the width of the front is imposed by the width of the forcing decay region provided this distance is larger than the baroclinic rossby radius baroclinic eddies whose velocities are inversely proportional to w develop along the front and exchange dense water from the forcing region with ambient water eventually reaching an equilibrium in which the lateral buoyancy flux by eddies balances the prescribed surface buoyancy flux the time to reach equilibrium

authigenic precipitation of ferric oxyhydroxides and greenalite in archean oceans abstract the initial mineral phases of banded iron formations bifs were mostly altered during diagenesis and metamorphism the initial iron mineral whether it was hydrous ferric oxides hfo or hydrous ferrous silicates hfs is a matter of debate each of those being the primary iron mineral offers us considering completely different biogeochemical conditions of archean oceans some archean greenstone bifs of badampahar 3 2 ga singhbhum craton and nuvvuagittuq 3 8 ga northern quebec consist alternating grunerite magnetite and quartz rich authigenic sedimentary bands the minerals appear to be co crystallized during metamorphism and show evidence of being originated from different precursor minerals e g magnetite from hfo and grunerite from hfs the process of such episodic hfo and hfs precipitations apparently related to two different fe si cycles in the archean oceans is not understood yet in situ trace element geochemistry of the minerals proves their authigenic origin and shows some characteristic geochemical differences e g grunerite having higher contents of hydrothermally sourced trace elements whereas magnetite being rich in elements involved in marine biogenic cycles the origin of mineralogically different e g quartz grunerite and magnetite rich bands of the studied bifs is explained with a model of hfs rich precipitation during episodic hydothermalism subsequent hfo rich precipitation by microbial oxidation and silica rich precipitation during no hydrothermal activity or non depositional period of iron minerals

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

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

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

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

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

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

a numerical study of a shallow sea front generated by buoyancy flux generation mechanism abstract we studied numerically the frontogenesis of shallow sea fronts such as are observed in the kii channel japan during winter under conditions of sea surface cooling and buoyancy influx from coast and open ocean numerical experiments were carried out in a vertically two dimensional basin with a new model nh model without using the hydrostatic approximation and the convective adjustment considering the vertical acceleration term in the momentum equation intermittent gravitational convections with a large aspect ratio were produced in the frontal region to intensify the horizontal convergence and to strengthen the horizontal density gradient consequently a front in the tracer distribution had a sharpness comparable to the observed front in the kii channel and 3 6 times the sharpness in an h model using the hydrostatic approximation and the convective adjustment in the present model situation this effect of gravitational convections on sharpening a front in the nh model is equivalent to t

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

drag measurements for first year sea ice over a shallow sea for a free drift case a method is developed for simultaneously determining air ca and water cw drag coefficients by solving the vertically integrated stress balance equation for sea ice the method allows the algebraic determination of ca and cw from measurements of relative wind and relative current at single reference levels from estimates of the densities of all three fluids and ice thickness and from estimates of the accelerations caused by the nonsurface stress terms derived from position measurements the method treats the ice as a natural drag plate and therefore includes contributions from both skin and form drag one shortcoming of the technique is that if the relative wind and current are colinear then only the ratio of ca and cw can be determined another is that the accelerations caused by other forces need to be independently determined an experiment was conducted on a single floe in the eastern bering sea over the continental shelf during march 1981 the site was about 80 km from the ice edge and was occupied for approximately 3 days following the passage of a storm which had broken the pack into 10 20 m diameter floes both profile and slab inversion estimates of drag were made from the current meter and anemometer measurements at the site ca at 3 m was 4 55 10 3 by the profile method compared to 3 63 4 39 10 3 for slab depending on assumptions cw at 1 1 m by profile was 24 2 10 3 compared to 18 3 22 0 10 3 for slab the two methods gave results which were not statistically separable and which were among the highest drags observed for sea ice the instantaneous stress balance on the floe included contributions from material acceleration coriolis force the two surface stresses and sea surface tilt relative to mean current and tide

comparison of calcite dolomite thermometry and carbonate silicate equilibria constraints on the conditions of metamorphism of the llano uplift central texas u s a temperatures based on the composition of calcite coexisting with dolomite calcite dolomite thermometry range from 475 to 600 c for 63 marbles from the llano uplift of central texas the highest temperatures 600 c were obtained by carefullyreintegrating calcite containing exsolved lamellae of dolomite in some cases these high temperatures were determined for marbles that contain an isobarically invariant assemblage consisting of calcite dolomite tremolite diopside forsterite at a pressure of 3 kbar these five minerals are stable at 630 ocand xco 0 62 in contrast relatively low calcite dolomite temperatures of 475 480 cwere obtained for marbles containing the assemblage calcite dolomite tremolite talc this talc bearing assemblage is stable at 475 oc depending on fluid composition at a pressure of 3 kbar additional isobarically univariant equilibria are stable at intermediate temperatures generallybetween 535 and 630 0q and these are also generallyconsistent with results obtained from calcite dolomite thermometry agreement between the calcite dolomite temperatures and those inferred from silicate carbonate equilibria in the marbles indicates that the temperatures generally reflect peak conditions of metamorphism although some resetting has occurred the marbles having the highest calcite dolomite temperatures as well as those containing the hightemperature isobarically invariant assemblage are generally found close to post tectonic pluton contacts indicating that some of the amphibolite facies assemblagesare related to the emplacement ofthe granitic intrusions relatively low temperatures are recorded within approximately 2 km of pluton contacts suggestinga possible thermal aureole however relatively high temperatures of 550 600 c are recorded in marbles that are not spatially related to post tectonic granites 2 km from pluton contacts these temperatures may be relicts of an earlier metamorphic event although they could be related to granites that were emplaced above or below the current erosional level

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

episodic negative anomalies of cerium at the depositional onset of the 2 65 ga itabira iron formation quadrilatero ferrifero of minas gerais brazil abstract magnetite bands in sericitic phyllite of the batatal formation mark the transition to the overlying 2 65 ga itabira iron formation or caue itabirite in the quadrilatero ferrifero of minas gerais brazil the magnetite bands have low non crustal th u ratios suprachondritic y ho ratios and sporadic anomalies of ce that are characterised as truly negative the data point to mild oxygenation of sea water at the depositional onset of the itabira iron formation some 200 ma before the great oxidation event

the nature of mesoproterozoic central indian tectonic zone with exhumed and reworked older granulites abstract the ese wnw trending central indian tectonic zone citz divides the indian subcontinent into southern and northern crustal provinces its narrow northern belt consists of closed early proterozoic mahakoshal rift zone which is presently confined between two moho reaching faults that characterize the son narmada sona lineament the mesoproterozoic sausar mobile belt smb the chhotanagpur granite gneiss complex cggc and the gneissic complex of northeast india represent the wider southern belt the mahakoshal rift belt has a thick pile of mafic ultramafic rocks and possibly oceanic components the basin closed due to a south directed subduction creating a magmatic arc that was sutured to its southern boundary during 1 8 1 7 ga a south dipping ductile shear zone known as the son narmada south fault affects both its supracrustals and intrusive granitoids the son narmada south fault generally delimits the northern boundary of the gondwana basins whereas the son narmada north fault delineates the northern boundary of the mahakoshal mobile belt and the southern boundary of the meso neoproterozoic vindhyan basin the smb representing the southern belt of the citz in the central sector comprises strongly folded and metamorphosed non volcanic and mn rich sausar sediments intermixed with reworked basement gneisses migmatites and granulites representing the pre sausar assemblage the sausar orogeny closed during terminal mesoproterozoic ca 1 0 ga on the other hand metamorphism in the basement gneiss migmatites and northern and southern granulite belts pre dates the sausar orogeny the earliest structure of the smb truncates and rotates the n s oriented structures of the southward located sakoli and the dongargarh belts of neoarchaean palaeoproterozoic age the northern granulite belt of the smb and the cggc and the high grade supracrustals from the sonapahar area in ne india all indicate very high p t conditions during 1 7 1 5 ga peak metamorphism which was followed by decompressional cooling the setting resembles continental collision which was followed by tectonic exhumation and denudation on the other hand the southern granulite belt from the smb documents a late archaean 2 6 ga event and a later 1 4 ga event the dalma volcanic belt located close to the southern tectonized boundary of the cggc belt and possibly the assemblage of copious mafic enclaves within the ca 550 ma granite plutons from ne india represent back arc like setting during 1 6 1 5 ga the setting is caused due to the southward subduction of the north indian block beneath the southern block leading to continental collision the metasedimentary belt located further to the south of the dalma volcanic belt and flanking the archaean singbhum craton to the north on the other hand is palaeoproterozoic in age but has been metamorphosed during ca 1 6 ga because of the northern collision related orogeny reworking and incorporation of basement gneisses and granulites from the mid crust level to the younger sausar cover sediments denote renewed spell of thrust tectonics that remobilized the collision zone rocks of the earlier orogeny the tectonic transport was from the north to the south with north dipping thrusts thus there was a reversal in the direction of convergence from the preceding orogeny the sausar orogeny closed during ca 1 0 ga it caused anatectic event producing granitoid plutons migmatites and charnockites in the cggc belt and migmatites in the gneissic complex in ne india

two configurations of the western arctic shelfbreak current in summer abstractdata from a closely spaced array of moorings situated across the beaufort sea shelfbreak at 152 w are used to study the western arctic shelfbreak current with emphasis on its configuration during the summer season two dynamically distinct states of the current are revealed in the absence of wind with each lasting approximately one month the first is a surface intensified shelfbreak jet transporting warm and buoyant alaskan coastal water in late summer this is the eastward continuation of the alaskan coastal current it is both baroclinically and barotropically unstable and hence capable of forming the surface intensified warm core eddies observed in the southern beaufort sea the second configuration present during early summer is a bottom intensified shelfbreak current advecting weakly stratified chukchi summer water it is baroclinically unstable and likely forms the middepth warm core eddies present in the interior basin the mesoscale instabilities extract energy from the mean flow such

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

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

kinematic constraints on rodinia reconstructions from the core of the texas grenville orogen abstract mesoproterozoic metamorphic rocks exposed in the llano uplift central texas were involved in a 1 15 1 12 ga grenville age collisional orogenic event along the southern margin of laurentia this event is characterized by polyphase ductile deformation synchronous with transitional amphibolite granulite to eclogite facies metamorphism results of detailed structural mapping in the southeastern part of the llano uplift show that these rocks record a deformational history involving northeast directed ductile thrusting and regional scale polyphase folding all related to northward collision of a southern continent and an arc with the southern margin of laurentia we use the structural metamorphic and kinematic history of the southeastern llano uplift to critically evaluate collisional models for the grenville orogen in laurentia and discuss the implications for reconstructions of rodinia

close interactions between two pairs of heton like vortices under sea ice close interactions between two pairs of vertically stacked counterrotating eddies under sea ice were investigated in numerical experiments the numerical model contains a stratified ocean capped by an ice layer under the ice layer a shallow brine source produces a top cyclone and a submerged anticyclone while a shallow freshening source generates a top anticyclone and a submerged cyclone ice exerted friction dissipates the top eddies in time leaving submerged ones in lone existence in this work winning vortices are sought from a brine brine pair a freshening brine pair and a freshening freshening pair brine and freshening sources are made equal in strength to level the playing field when closely paired fission an increase in the number of subsurface vortices occurs from a brine brine pair and a freshening brine pair but not from a freshening freshening pair consequences of fission invariably increase the number of submerged anticyclones a heuristic argument is given to explain this polarity the strong interaction among closely packed eddies operates in timescales of tens of days helping to explain the predominance of submerged anticyclones under the arctic sea ice

eddy maintenance and attrition in a vertically sheared current under arctic ice abstract steady drift of an ice cover produces a vertically sheared current in the upper ocean of the arctic under the ice cover mesoscale shallow brine and freshening sources generate submerged anticyclones and cyclones respectively a submerged eddy extending deep into the water column experiences differential advections by the vertically sheared current interaction between subsurface eddies and the sheared current is examined using a three dimensional numerical model in a coordinate system moving with the ice the initial salinity field is in geostrophic balance with the sheared current and a pulse of brine or freshening forcing produces an anticyclone or a cyclone in a coordinate system moving with the ice eddies are in a vertically sheared backward ambient current to an observer looking into the direction of the backward ambient current eddies move with the current and deflect to the right left for counterclockwise clockwise rotating eddies in both hemispheres the lower half of the eddy

the archaean nucleus of singhbhum the present state of knowledge abstract the supracrustal rocks of the older metamorphic group omg consisting of metasediments and ortho amphibolite constitute the oldest unit in the archaean nucleus of singhbhum however there are indications that still older 3 4 3 8 ga crust of both sialic and mafic composition existed in this region the omg ortho amphibolites were formed by partial melting of mantle with near chondritic composition ca 3 3 ga ago probably as a result of plume activity shortly afterwards partial melting of the underplated mafic material produced a tonalitic melt older metamorphic tonalitic gneiss omtg which intruded the omg supracrustals and the entire suite was deformed and metamorphosed to upper amphibolite facies subsequent to this melting of the omg ortho amphibolites and the lower crustal material of probable andesitic composition produced melts varying in composition from tonalite to granite and these intruded in different phases to produce plutons of singhbhum granite bonai granite and kaptipada granite which together form volumetrically the major part of the archaean nucleus the older omg and omtg occur as enclaves within these younger granitoids the time difference between the emplacements of the omtg and the early phases of younger granitic intrusion was of the order of 100 200 ma thus serial additions of juvenile material led to the formation of a stable microcontinent by 3 2 ga thermally triggered stretching in this microcontinent produced basins peripheral to the present day singhbhum granite pluton and in these basins the younger supracrustal rocks of the iron ore group iog consisting of bif associated argillaceous and subordinate arenaceous rocks and mafic lavas were laid down there is inadequate field or geochronological evidence to resolve the issue of whether the different iron ore basins were coeval or not meagre geochronological data suggest that some of the bifs are older than ca 3 1 ga post iog activity is confined to the intrusion of mafic dyke swarms and formation of intracratonic basins the ages of both being uncertain

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

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

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

online preconcentration icp ms analysis of rare earth elements in seawater the rare earth elements rees with their systematically varying properties are powerful tracers of continental inputs particle scavenging intensity and the oxidation state of seawater however their generally low pmol kg concentrations in seawater and fractionation potential during chemical treatment makes them difficult to measure here we report a technique using an automated preconcentration system which efficiently separates seawater matrix elements and elutes the preconcentrated sample directly into the spray chamber of an icp ms instrument the commercially available seafast system elemental scientific inc makes use of a resin with ethylenediaminetriacetic acid and iminodiacetic acid functional groups to preconcentrate rees and other metals while anions and alkali and alkaline earth cations are washed out repeated measurements of seawater from 2000 m water depth in the southern ocean allows the external precision 2 of the technique to be estimated at 23 for all rees and 15 for most comparison of nd concentrations with isotope dilution measurements for 69 samples demonstrates that the two techniques generally agree within 15 accuracy was found to be good for all rees by using a five point standard addition analysis of one sample and comparing measurements of mine water reference materials diluted with a nacl matrix with recommended values in the literature this makes the online preconcentration icp ms technique advantageous for the minimal sample preparation required and the relatively small sample volume consumed 7 ml thus enabling large data sets for the rees in seawater to be rapidly acquired

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

the composition of earth s oldest iron formations the nuvvuagittuq supracrustal belt quebec canada abstract the composition of iron formations in the 3 75 ga yr old nuvvuagittuq supracrustal belt in northern quebec provides a proxy for seawater composition of the eoarchean and perhaps hadean oceans as well as constraints on the types of nutrients available to earth s earliest life forms integrated petrologic and geochemical relationships mapped between mineral phases in thin section and whole rock chemistry provide a framework for interpreting bulk and micro scale variations in these chemical sedimentary precipitates results show that there are two distinct chemical sedimentary units in the nuvvuagittuq belt i a banded iron formation bif consisting of alternating micro bands of magnetite ca mg fe silicates and quartz and ii a more silicate rich fe poor unit the banded silicate formation bsf of alternating micro bands of quartz and ca mg fe silicates precursor bif and bsf deposits were likely layered amorphous silica and ferric oxyhydroxides fine grained carbonate oozes and or ca mg fe rich silicate gels deposited in a marine setting low al2o3 tio2 and hfse concentrations show that they are relatively detritus free with distinctively seawater like ree y profiles and consistently positive eu anomalies these features suggest that the rocks preserved their seawater like compositions despite metamorphic overprinting the most significant trace elements in the sediments are ni and zn experimentally derived partitioning coefficients show that ni was enriched in eoarchean seawater as compared to today up to 300 nm while zn was fairly similar up to 20 nm compositional resemblances between the nuvvuagittuq sediments and those documented in the ca 3 8 ga isua supracrustals west greenland provide a plausible case that global ocean processes in terms of trace metal abundances had reached steady state by the eoarchean

p t x conditions of calc silicate formation evidence from fluid inclusions and phase equilibria llano uplift central texas usa the llano uplift in central texas is a grenville aged c 1 1 ga metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages the formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area two types of granitic intrusion have previously been recognized 1 town mountain granites which occut as relatively large circular shaped bodies of coarse grained granite and 2 younger granites which are present as smaller and more irregular bodies of finer grained granite in the central part of the uplift wollastonite bearing calc silicate rocks occur within the valley spring gneiss the development of these calc silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated younger granites the stability of coexisting quartz calcite wollastonite grossular and anorthite and coexisting quartz calcite wollastonite andradite and hedenbergite shows that the calc silicate rocks equilibrated under h20 rich conditions with xcs 0 10 fluid inclusions present within the calc silicate minerals are h o rich with salinities of 17wt equivalent naci the absence of any detectable co in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more h20 rich conditions compared to the stability of the peak metamorphic mineral assemblage homogenization temperatures measured for texturally primary inclusions range from 360 to 368 c corresponding to a density range from 0 53 to 0 82g cm3 isochores for these fluid inclusions when combined with the stability of the solid solid equilibria grs qtz wo an yield formation conditions of xhl 550 c at 1 2 kbar this indicates that the granitic intrusions involved in the formation of the blount mountain calc silicates were emplaced at a pressure of at least 1 2 kbar

a key role for green rust in the precambrian oceans and the genesis of iron formations iron formations deposited in marine settings during the precambrian represent large sinks of iron and silica and have been used to reconstruct environmental conditions at the time of their formation however the observed mineralogy in iron formations which consists of iron oxides silicates carbonates and sulfides is generally thought to have arisen from diagenesis of one or more mineral precursors ferric iron hydroxides and ferrous carbonates and silicates have been identified as prime candidates here we investigate the potential role of green rust a ferrous ferric hydroxy salt in the genesis of iron formations our laboratory experiments show that green rust readily forms in early seawater analogue solutions as predicted by thermodynamic calculations and that it ages into minerals observed in iron formations dynamic models of the iron cycle further indicate that green rust would have precipitated near the iron redoxcline and it is expected that when the green rust sank it transformed into stable phases within the water column and sediments we suggest therefore that the precipitation and transformation of green rust was a key process in the iron cycle and that the interaction of green rust with various elements should be included in any consideration of precambrian biogeochemical cycles large iron deposits formed episodically in the archaean oceans experimental data and geochemical modelling suggest that green rust was an important contributor to the formation of these deposits and the archaean iron cycle in general

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

greenalite precipitation linked to the deposition of banded iron formations downslope from a late archean carbonate platform banded iron formations bifs were deposited as deep water facies distal to the late archean campbellrand carbonate platform transvaal supergroup south africa they are traditionally interpreted to have formed from iron oxides hydroxides and silica that precipitated when upwelling hydrothermal water enriched in fe2 and silica mixed with cooler shallow seawater enriched in dissolved oxygen the ferric oxides hydroxides were then converted to hematite or in the presence of organic matter reduced and incorporated into siderite new high resolution microscopy of bifs from distal facies of the campbellrand platform questions that interpretation revealing the presence of abundant greenalite nanoparticles interpreted to represent primary precipitates from ferruginous seawater consistent with recent results from equivalent aged bifs of the hamersley group western australia the particles locally define primary sedimentary lamination and are engulfed in diagenetic chert that preserves shrinkage structures considered to have formed during dehydration and recrystallization of amorphous silica cement the replacement of greenalite nanoparticles by coarser grained oxide silicate and carbonate minerals with destruction of primary textures and structures indicates that iron silicate muds were originally much more widespread based on our findings we propose a new model for the deposition of the bifs involving iron silicate precipitation from seawater followed by diagenetic silica cementation on the seafloor we suggest that ferrous iron and silica were transported in hydrothermal plumes sourced from acidic vent fluids upon mixing with cooler more alkaline seawater the solubility of ferrous iron and silica fell to levels that favored rapid nucleation and the precipitation of iron silicate nanoparticles over vast areas of the seafloor the close association between iron and silica during the deposition of bifs may be explained by the precipitation of iron silicate nanoparticles and possible sorption of silica onto the surfaces of sinking nanoparticles furthermore on the seafloor iron silicate nanoparticles presumably acted as nucleation sites for dissolved silica promoting early diagenetic silica precipitation and cementation of the iron rich muds our results indicate that iron silicates were important primary precipitates and suggest that changes in alkalinity rather than redox state played a key role in the precipitation of the precursor sediments of bifs

generation of submesoscale vortices a new mechanism small scale intense vortices composed of water with anomalous values of quasi conservative tracers have been observed in many oceans they are particularly numerous in the beaufort gyre of the arctic ocean possible mechanisms responsible for the generation of these features are examined conservation of temperature salinity and nutrients requires that beaufort sea vortices be created from water that flows into the arctic ocean from the pacific ocean across the broad shallow chukchi sea much of this water passes through barrow canyon near point barrow alaska observations in the canyon show the generation of vortices of approximately the same size as those observed in the beaufort sea barrow canyon is therefore a likely site for the generation of beaufort sea vortices most observed beaufort sea vortices have a central potential vorticity near zero because of a relative vorticity close to their chukchi sea source water is typically stratified with little relative vorticity so that its potential vorticity is nonzero a key step in the generation of the vortices is therefore the reduction of the potential vorticity of chukchi sea water to nearly zero it is proposed that this occurs through the action of frictional torques occurring as the chukchi sea water flows through barrow canyon rather than througji the action of vertical mixing because this flow is driven by a downstream pressure gradient it is concentrated on the right hand wall of the canyon by the coriolis force the frictional torques thus preferentially reduce the potential vorticity of the fluid anticyclonic vortices are therefore preferentially generated it is suggested that a similar scenario may play a role in the generation of submesoscale vortices containing water from the mediterranean and labrador seas

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered