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

evolution of gold bearing veins in dykes of the woods point dyke swarm victoria the woods point dyke swarm comprises hundreds of narrow subparallel igneous dykes and dozens of pipe shaped dyke bulges within strongly deformed early palaeozoic turbidites of the melbourne trough porpylitic alteration accompanied dyke emplacement and was followed by microfracturing induced by high fluid pressures involving co2 of magmatic origin as the dykes solidified further stress caused through going faults having ladder and other patterns isotopic studies suggest that metamorphically or geothermally derived solutions filled the faults and other fractures with quartz and carbonate and altered immediately adjacent dyke rock however earlier formed vein and wall rock carbonates retained their magmatic isotopic composition fluid inclusions indicate vein deposition began at approximately 400 c with salinities up to 9 weight percent nacl nine sulfide minerals and gold were deposited in the veins after ankerite sericite and albite while quartz deposition continued through all stages sulfur isotopic determinations indicate the vein sulfur could not have been derived from adjacent sedimentary rocks nor exclusively from the dykes metamorphic waters of marine origin is a viable source for sulfur saline and co2 rich alkaline solutions reacted with the dyke wall rocks and probably evolved chemically prior to deposition of gold vug carbonates deposited by meteoric water that leached vein carbonates mark the end of vein formation

a giant late archean lake system the meentheena member tumbiana formation fortescue group western australia abstract the 2720 ma old meentheena member of the tumbiana formation fortescue group is a 30 50 m thick succession of alternating siliciclastic and limestone beds that crop out intermittently over a distance of 680 km across the pilbara craton western australia limestones of the member consist of flat pebble intraformational conglomerate oolite grainstone calcisiltite and calcilutite siliciclastics consist of shale siltstone and sandstone and many of these are tuffaceous sedimentary structures include abundant symmetrical ripples ripple cross stratification desiccation cracks teepee structures and planar to wavy lamination the member is superficially similar in appearance wherever it crops out however the correlation of individual beds and units over distances greater than 15 km is problematic there are no regional marker beds radiometric ages from tuffaceous units young to the west by a few million years four lithofacies are defined a flat pebble conglomerate b ripple cross laminated oolite grainstone c planar to wavy laminated calcilutite calcisiltite and d shale siltstone stromatolites are found in all three of the limestone facies evaporites are rare and include halite pseudomorphs the four lithofacies are generally found in a vertical succession that characterizes a lithofacies association and reflects a deepening upward succession associations may not have all four lithofacies but do reflect the same deepening upward pattern the uppermost portions of shale siltstone facies commonly contain symmetrical ripples and desiccation cracks indicating subaerial exposure measured sections contain as many as 25 of these parasequences indicating fluctuating water levels limestone lithology and facies commonly change abruptly laterally at the meter and greater scale and vertically centimeter to meter scale the sedimentary structures differences and abrupt changes in lithofacies patterns the inability to correlate confidently between regions and the differences in age from east to west suggest a complex diachronous depositional system of fluctuating water levels in separate bodies of water the limestone is dominated by sedimentary structures indicative of shallow water within wave base however it is devoid of evidence of any tidal influence possibilities for depositional environments include a tideless sea or a lacustrine system 87sr 86sr values from limestone differ from known archean marine carbonates ree patterns are decidedly non marine as well a lacustrine system is favored for the meentheena member based on sedimentary structures the abruptness of lithologic and facies changes the facies association lateral gradation into fluvial deposits the inability to correlate many units and beds for more than a few to several kilometers geochemical data stromatolites and comparisons with younger lacustrine basins as well as marine successions there is no single line of evidence that unequivocally establishes a lacustrine origin however multiple lines of evidence are collectively consistent with a lacustrine origin the meentheena member would therefore represent the oldest known carbonate rich lacustrine system which we call the meentheena lakes system

isotopic and geochemical evidence for crust mantle interaction during late archaean crustal growth abstract the charnockites and enderbites of the north marginal zone of the limpopo belt in southern zimbabwe represent a suite of dry magmas of tonalite trondhjemite granodiorite ttg affinity which crystallised directly in the lower crust they were emplaced between 2 6 and 2 7 ga and were derived from a source which showed remarkable lead isotope homogeneity unlike many other lower crustal rocks they have low k rb and low k u ratios and have not experienced depletion of their heat producing elements th u k and rb and thus represent an unusual suite of granulites in this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the nmz enderbites and charnockites we use a combination of high values calculated lead and strontium isotope initial ratios and old nd model ages to show that the enderbites and chamockites were derived from a protolith which was in part formed more than 500 ma before magma genesis we argue from the very high values that this protolith constituted two separate components an old high low sm nd crustal component and a younger low high sm nd mantle derived component both of which were homogenised by extensive mixing prior to 2 7 ga the presence of an older crustal component contrasts with other archaean ttg suites worldwide which show little evidence for a crustal prehistory the process of mantle crust interaction during late archaean crustal growth in southern zimbabwe invites comparison with modern convergent margins such as the andes

tectonic evolution of the bindura shamva greenstone belt northern zimbabwe progressive deformation around diapiric batholiths abstract the bindura shamva greenstone belt northern zimbabwe and adjacent batholiths are regarded as one of the type areas for granite greenstone tectonics structural observations and three dimensional strain analyses have been used to propose a model for the tectonic evolution of this late archaean greenstone belt the data indicate that pluton emplacement triggered the deformation of the greenstone sequences and that the deformation history is single phase progressive rather than polyphase they do not however lead to an unambiguous interpretation of the emplacement mechanism structural features are best explained by a combination of pluton diapirism and ballooning plutonism criteria used as evidence for pluton related deformation include the pattern of foliation and stretching lineation trajectories the variation in strain type and strain intensity fold types sense of shear at the batholith greenstone interface metamorphic zoning and timing of porphyroblast growth specific structural and strain patterns within cleavage triple points interdomal synclinoria saddle culminations and batholith margins demonstrate interference between strain fields developed around each batholith

geochemistry of vein and wallrock carbonates from the ediacaran system in south china insights into the origins of depositional and post depositional fluids abstract a combined study of carbon oxygen isotopes and major trace elements was carried out for vein and wallrock carbonates from the lantian formation of the ediacaran system in south china the results provide geochemical constraints on the origins of depositional and post depositional fluids with respect to recognition of the post depositional alteration the lantian formation was subdivided into the upper and lower carbonate units for the upper unit uu petrographic observations indicate that wallrock carbonates only experienced limited post depositional alteration the wallrock exhibits 18 o values of 19 2 to 13 1 and 13 c values of 9 6 to 8 9 both relative to vpdb the vein carbonates are divided into groups a and b based on their 18 o values and ree y patterns group a veins show nearly similar 18 o and 13 c ranges to but different ree y patterns from the wallrock these data indicate that the origin of fluids for group a veins may be similar to that of wallrock which was probably the mixture of continental and marine glacial meltwaters group b veins show both different 18 o values and ree y patterns from but similar 13 c values to the wallrock this indicates that post depositional fluids are different from the depositional fluids and thus of external origin low 18 o values 23 7 to 18 1 for group b veins indicate that the post depositional fluids were derived from continental glacial meltwater there is no correlation between 18 o and 13 c values for these vein and wallrock carbonates suggesting that the wallrock was not extensively altered by the post depositional fluids for the lower unit lu veins exhibit different c o isotope compositions and ree y patterns from wallrock carbonate indicating that post depositional fluids have different geochemical compositions from depositional fluids and thus are of external origin the simultaneous changes in 18 o and 13 c values with different laminations in the wallrock carbonates suggest the preservation of the primary geochemical compositions the 18 o values and ree y patterns for the lu veins indicate that the post depositional fluids were probably the mixtures of continental and marine glacial meltwaters therefore the lantian carbonates record the incorporation of continental glacial meltwater into the marginal sea of ediacaran age nevertheless the geochemical screening can provide constraints on the preservation of primary signatures and thus on the origins of depositional and post depositional fluids in the neoproterozoic marine carbonates

statistically learning archean carbonate diagenesis abstract geochemical data are often very noisy due to the large natural heterogeneity of the geological materials as well as oversimplification of rock classification this has serious repercussions on the precision of our knowledge of the deep past as we often rely solely on geochemical proxies to investigate the geological evolution of archean and proterozoic environments here statistical learning procedures were applied to achieve unbiased classification of neoarchean stromatolitic dolostone textures on the basis of the distribution of their trace elements and rare earth elements ree investigated through laser ablation induced coupled plasma mass spectrometry la icp ms multivariate statistical analyses and supervised statistical learning have revealed that different dolomite fabrics thought as products of aggrading diagenesis and recrystallization are in fact chemically indistinguishable the diagenetic processes that cause the re crystallization of dolomite and the consequent change of textures is not affecting the distribution of major and trace elements inherited by the depositional environment or during early stages of diagenesis at the same time the algorithm has revealed that an optically homogeneous microcrystalline dolomite sample may in fact be geochemically inhomogeneous because of processes of ripening and recrystallization occurred at an early stage of marine diagenesis and that have contributed to element mobilization statistical learning has succeeded in recognizing chemofacies which not always overlap with dolomite textures and fabrics highlighting the importance of crystallographic and diagenetic studies before any study of carbonates as geochemical proxies

recognition of diagenetic contribution to the formation of limestone marl alternations a case study from permian of south china abstract limestone marl alternations lmas have been extensively used in studies of sedimentology cyclostratigraphy and paleoclimatology however it has long been debated whether lmas are derived from primary sedimentation or from diagenetic alteration and the markers to distinguish one source from the other remain ambiguous in this study we investigate widespread permian lmas from various depositional settings in south china using traditional petrographic mineralogical and major element al2o3 vs tio2 methods as well as emerging trace and rare earth element ree analyses performed using solution based and in situ techniques the original depositional differences of the limestone and its coupled marl beds can be identified based on 1 the high volumes of siliciclastic minerals in the marls 2 unremarkable differential compaction 3 the recognizable signatures of the detrital admixture i e the al zr and th characteristics and 4 flattened shale normalized ree patterns diagenetic lmas are characterized by the compaction of fossils rare aragonite skeletons and abundant diagenetic clays sepiolite and talc preserved in the marl beds we also identified fully diagenetic lmas originating from primary carbonate sedimentation based on two almost completely consistent seawater like ree patterns with diagnostic positive la and negative ce anomalies la la sn 1 64 0 17 ce ce sn 0 73 0 05 superchondritic y ho 47 4 and depleted light rees relative to high rees pr yb sn 0 39 0 09 diagenesis also dominates the formation of lmas with limited terrigenous input in shallow water carbonate depositional environments it is likely that both the active metabolic processes in early diagenesis especially aerobic sulfide oxidation and the temporal palaeoceanographic conditions e g high levels of dissolved oxygen and sulfate play crucial roles in the genesis of the diagenetic lmas in the permian strata of south china

behavior of rare earth elements in seawater at the ocean margin a study along the slopes of the sagami and nankai troughs near japan using a flow injection icp ms method yttrium and all the lanthanides in seawater were determined in the coastal offshore mixing regime near japan marked enrichments in the trivalent rare earth elements rees in the surface waters are ascribed to the influence of coastal waters from tokyo bay in march and perhaps somewhere else in september and october to this region at less than half of the stations where the detailed vertical profiles were obtained throughout the water column we found marked excess ree iii concentrations near the bottom these bottom concentration anomalies are probably caused by resuspension of underlying sediments excluding those surface and bottom anomalous concentrations the distribution of ree iii s in the midwater column are very similar to those of the open ocean and are highly correlated with each other r2 0 95 this suggests that the boundary effects on the chemical fractionation of ree iii s are small we find no effect of the boundary scavenging known for 210pb and 231pa on the ree distribution whereas there may be preferential release of light and middle ree iii s over heavy rees from slope sediments to the seawater much like 227ac from the shale normalized ree patterns we obtained up to 90 negative ce and 10 positive gd anomalies in the water column

dynamic controls on accretion and lithification of modern gypsum dominated thrombolites los roques venezuela meter sized thrombolites coated by well developed zonally differentiated microbial mats have been found growing in the shallow waters depth 1 m of a restricted hypersaline lagoon on the archipelago los roques in venezuela by contrast within the deeper parts of the studied lagoon sedimentation is characterized by several decimeters of organic rich material containing gypsum granules lacking carbonate cementation the lithification of the thrombolites is thought to have proceeded as follows first extracellular polymeric substances eps comprising the microbial mat concentrate ca2 and other metal cations by adsorption from the hypersaline waters second some of these bound metals then serve as nucleation sites for primary calcium carbonate caco3 precipitation third while carbonate phases are forming in some zones of the mat in others zones they are being re dissolved due to the acidity generated through the metabolism of sulfide oxidizing bacteria fourth as the dissolved sulfide is oxidized into sulfate the pore water become saturated with respect to gypsum caso4 2h2o fifth as primary gypsum precipitates within the structures endolithic sulfate reducing bacteria metabolize the sulfate moiety in the mineral phase while simultaneously oxidizing the eps trapped during accretion sixth as microbial eps degradation proceeds the anaerobic oxidation of specific protein fractions of the eps matrix leads to increased alkalinity the partial dissolution of gypsum supersaturation with respect to calcium carbonate and ultimately pseudomorphic aragonite replacement this differs from secondary calcite cements in being enriched in 12c and depleted in minor and trace metals initially associated with the eps the biogeochemical processes occurring in this thrombolite constructing lagoon represent a novel field site for studying the chemical and isotopic processes characterizing early diagenetic gypsum and the role microbes play in its precipitation dissolution and calcification in this regard insights gained from this modern field site will help to better understand mechanisms by which some precambrian microbialites were lithified

archaean stromatolites from the ngesi group belingwe greenstone belt zimbabwe preservation and stable isotopes preliminary results abstract the petrology and stable isotope chemistry of cyanobacterial stromatolites of archaean age 2 7 ga from the cheshire and manjeri formations of the belingwe greenstone belt in zimbabwe have been examined palaeomagnetic data suggest that the stromatolites formed in tropical to subtropical latitudes the cheshire formation shows little evidence of either anion or cation exchange during metamorphism and the stable carbon and oxygen isotope ratios suggest a formation at temperatures perhaps considerably below 80 c the manjeri formation only slightly older but overlain by a thick volcanic sequence shows a low grade of metamorphism and isotope ratios that are consistent with a metamorphic temperature of around 200 c

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

pb and nd isotope systematics of stromatolitic limestones from the 2 7 ga ngezi group of the belingwe greenstone belt constraints on timing of deposition and provenance abstract pb pb isochrons have been obtained for stromatolitic limestones from the late archaean belingwe greenstone belt of zimbabwe providing direct age constraints on the deposition of these shallow water marine sediments samples from the manjeri formation and stratigraphically higher cheshire formation yield age estimates of 2706 49 ma mswd 11 5 and 2601 49 ma mswd 0 93 respectively these data are in agreement with published u pb zircon and pb pb whole rock ages of associated volcanics and we therefore interpret our pb pb ages as representing the timing of early diagenesis thus providing a minimum age for carbonate precipitation a 2543 70 ma age mswd 5 1 for one sample from the cheshire formation is considered to reflect either late stage diagenesis a craton wide thermal chemical disturbance or tectonic activity along the crustal scale mtshingwe fault calculated model 1 values for manjeri and cheshire limestones are 8 40 0 02 and 9 02 0 01 similar to values for approximately 3 5 and 2 9 ga archaean basement units adjacent to the belingwe belt negative end tdeposition and fsm nd suggest derivation of the ree from old lree enriched continental crust two stage nd model ages for the carbonates indicate that precursor rocks were extracted from the mantle at around 3 5 ga cheshire formation and 3 3 ga manjeri formation in good agreement with mantle extraction ages for local basement units model tdm 3 5 3 3 ga

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

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

remnants of an archean oceanic plateau belingwe greenstone belt zimbabwe stratigraphic and structural data from the archean zimbabwe craton suggest that a major detachment surface exists within the belingwe greenstone belt the surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex older volcanic sedimentary rocks and a shallow water sedimentary sequence in the footwall rocks dated at ca 2 7 ga above the detachment surface form the proposed mberengwa allochthon the regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets not indigenous magmas erupted within autochthonous continental rifts magmatic rocks of the mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts comparison of the zimbabwe craton with the proterozoic birrimian terranes of west africa leads us to suggest that precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere

rare earth element geochemistry of late devonian reefal carbonates canning basin western australia confirmation of a seawater ree proxy in ancient limestones rare earth element and yttrium ree y concentrations were determined in 49 late devonian reefal carbonates from the lennard shelf canning basin western australia shale normalized sn ree y patterns of the late devonian samples display features consistent with the geochemistry of well oxygenated shallow seawater a variety of different ancient limestone components including microbialites some skeletal carbonates stromatoporoids and cements record seawater like ree y signatures contamination associated with phosphate fe oxides and shale was tested quantitatively and can be discounted as the source of the ree y patterns co occurring carbonate components that presumably precipitated from the same seawater have different relative ree concentrations but consistent ree y patterns clean devonian early marine cements n 3 display ree y signatures most like that of modern open ocean seawater and the highest y ho ratios e g 59 and greatest light ree lree depletion average nd sn yb sn 0 413 sd 0 076 however synsedimentary cements have the lowest ree concentrations e g 405 ppb non contaminated devonian microbialite samples containing a mixture of the calcimicrobe renalcis and micritic thrombolite aggregates in early marine cement n 11 have the highest relative ree concentrations of tested carbonates average total ree 11 3 ppm stromatoporoid skeletons unlike modern corals algae and molluscs also contain well developed seawater like ree patterns samples from an estuarine fringing reef have very different ree y patterns with lree enrichment nd sn yb sn 1 possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged lree from a nearby riverine input source hence devonian limestones provide a proxy for marine ree geochemistry and allow the differentiation of co occurring water masses on the ancient lennard shelf although appropriate partition coefficients for quantification of devonian seawater ree concentrations from out data are unknown hypothetical devonian canning basin seawater ree patterns were obtained with coefficients derived from modern natural proxies and experimental values resulting devonian seawater patterns are slightly enriched in lree compared to most modem seawaters and suggest higher overall ree concentrations but are very similar to seawaters from regions with high terrigenous inputs our results suggest that most limestones should record important aspects of the ree geochemistry of the waters in which they precipitated provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high non seawater like ree contents hence we expect that many other ancient limestones will serve as seawater ree proxies and thereby provide information on paleoceanography paleogeography and geochemical evolution of the oceans copyright c 2004 elsevier ltd

stromatolites at 3 500 myr and a greenstone granite unconformity in the zimbabwean archaean two controversial areas of geological endeavour are the establishment of the antiquity of life and the tectonic setting of greenstone sequences we record here the recent discoveries in the fort victoria greenstone belt of stromatolites in limestones assigned to 3 500 myr minimum age sebakwian group rocks of the rhodesian archaean craton within zimbabwe and a nearby outcrop of a thin sedimentary formation basal to a thick 2 700 myr volcanic pile resting with definite unconformity on 3 500 myr mushandike granite

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

rare earth and trace elements of microbialites in upper jurassic coral and sponge microbialite reefs major trace y zr pb th and u rare earth element ree and sm nd isotope analyses were carried out on microbialites and detrital carbonate sediments collected in upper jurassic coral and sponge microbialite reef settings selected bioconstructions developed in various palaeogeographic settings and depositional environments on northwestern tethys and eastern atlantic margins pure carbonate lagoon carbonate dominated deep shelf to epicontinental basin and mixed carbonate siliciclastic shallow ramp and lagoon the proximity to terrigenous sources directly controlled ree patterns of carbonate samples lateral and intrareef sediments and to a lesser degree stromatolitic microbial crusts are more susceptible to contamination by terrigenous material than are microbialites of thrombolitic fabric in mixed carbonate siliciclastic settings microbialites display a flat shale normalized ree pattern in pure carbonate lagoonal settings microbialites have shale normalized ree patterns similar to those of modern seawaters with a negative ce anomaly ce ce up to 0 14 positive la and gd anomalies and light ree depletion relative to heavy ree these data imply that similar ree chemistry than modern seawater already occurred on the northwestern tethys margin at the upper jurassic strong negative ce anomalies b0 4 imply microbialite formation in well oxygenated seawaters both in coral reefs of pure carbonate lagoonal environments pagny sur meuse and sponge bioherms of deep shelf settings plettenberg absence of positive ce anomalies rules out an influence of alkaline and high ph seawaters thus ree patterns and nd isotopic signatures which are in perfect agreement with existing results for tethysian seawater confirm that microbialites can be used as proxies of palaeoenvironmental seawater chemical characteristics

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

growth of subcontinental lithospheric mantle beneath zimbabwe started at or before 3 8 ga re os study on chromites re os isotope results on chromites separated from age constrained archean 2 7 to 3 5 ga ultramafic intrusions in the zimbabwe craton show that chromites average a present day 187os 188os ratio of 0 1066 significantly below the bulk silicate earth value the source reservoir is interpreted to reflect the zimbabwean subcontinental lithospheric mantle sclm archean re depletion of the sclm has previously been postulated on the basis of xenolith studies from southern africa siberia and the wyoming craton because our samples are age constrained however the re os data also shed light on the evolution of the archean sclm through time they indicate that the zimbabwean sclm began to be separated from asthenospheric mantle before 3 8 ga and grew quasi continuously throughout the archean this result is reconciled with present day sclm data in forward modeling

petrography and stable isotope ratios from archaean stromatolites mushandike formation zimbabwe abstract the mushandike sanctuary near masvingo fort victoria zimbabwe contains well preserved but metamorphosed stromatolitic limestones of archaean age despite the metamorphism textural preservation is excellent to a scale as fine as c 100 200 cores recovered from the stromatolites have 18 o 15 pdb approximately and 13c 0 0 to 0 5 pdb in less weathered samples the results are consistent with a metamorphic history involving volatile loss at 200 c or less from an original source rock which may have been isotopically similar to the cheshire stromatolites belingwe

reconstructing paleoceanographic conditions during the middle ediacaran evidence from giant ooids in south china abstract the ediacaran was a period of intense environmental change in earth history transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies including rare earth elements and yttrium rey but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder in this study we explore the possibility that a newly discovered deposit of giant ooids in the nanshanping outcrop in south china contains a relatively intact record of the ree composition of ediacaran seawater providing an important window into the redox state of the surface ocean at this time these ooids are relatively large in size 1 6 mm and exhibit densely banded laminae a regional chemostratigraphic correlation shows that the giant ooid bearing succession in south china is in the upper part of the second ediacaran positive carbon isotopic shift ep2 and thus its occurrence was not associated with the shuram excursion additionally the occurrences of giant ooid bearing oolites in different blocks were not synchronous during the middle ediacaran in situ laser ablation analysis of giant ooid cortices reveals that the shale normalized rey patterns are similar to those of modern seawater including depleted light rees superchondritic y ho ratios and positive la anomalies yet no ce anomalies 1 0 0 15 in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in south china at that time the petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial they formed in open ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and rey signatures from further diagenetic overprinting the characteristics of ce anomalies and rey compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in south china during the late stage of ep2 implying the very low atmospheric o2 level at that time our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early earth history

geochemical evidence from marine carbonate for enhanced terrigenous input into seawater during the ediacaran cambrian transition in south china abstract high resolution carbon and oxygen analyses along with residue contents major and trace element concentrations were carried out for marine dolostone from a profile through the upper ediacaran to the lower cambrian strata in south china the results provide geochemical evidence for enhanced input of terrigenous material into seawater in this period four positive and four negative carbon isotope excursions are identified in this profile the first remarkably negative 13c excursion exhibits the lowest value down to 2 at the ediacaran to cambrian boundary ecb in agreement with the global 13c and stratigraphy correlation the second remarkably negative 13ccarb excursion is as low as 1 81 subsequent to a positive 13ccarb excursion as high as 1 74 in the lower cambrian the ecb position in the stratigraphic profile is constrained by carbon isotope correlations the decreases of dolostone 18o and 13c values are synchronous across the ecb suggesting a change of the seawater composition at that time the increase of residue contents is concordant with decreases of y ho ratios and 18o values for the dolostone across the ecb suggesting the increase of terrigenous input in this period the relatively smooth ree y patterns of dolostone suggest its deposition in the environment of continental platform an integrated interpretation of these geochemical data including the major and trace element concentrations mn sr and mg ca ratios and the well preserved dolostone 13c and 18o values indicates that the terrigenous material from the continental weathering is rich in nutrients and 13c depleted organic matter which would have contributed to the negative 13ccarb excursion of dolostone at the stratigraphic boundaries the strong positive 13ccarb excursion in the lower cambrian may be linked to the biological radiation with lots of organic matter buried from the seawater in this period the fall and rise of sea level during different periods in the same region may result in differential input of terrigenous materials including freshwater into marine carbonates recording the coupling between regional tectonism and paleoclimatic change on the continental platform

the mushandike granite further evidence for 3 4 ga magmatism in the zimbabwe craton the conflict between independently published ages for the mushandike granite zimbabwe 2 92 0 17 ga and 3 45 0 13 ga has been resolved in favour of the older age by shrimp u pb analyses of zircon two samples yield indistinguishable estimates of 3374 7 and 3368 11 ma for the crystallization age of the magma together with published data from elsewhere in southern zimbabwe the results imply a widespread magmatic event at about 3 35 ga a single zircon core giving 3 46 ga together with the granite s previously measured nd model age suggests that the mushandike magma could have incorporated remobilized basement similar to the c 3 5 ga tokwe gneisses which crop out 30 km to the west the published rb sr and pb pb datasets show evidence of late archaean disturbance of sr and pb isotope systematics in the absence of exposed contacts between the mushandike granite and the neighbouring mushandike stromatolitic limestone the new u pb emplacement age suggests that the limestone is unconformable on the granite

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

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

continental extensional setting for the archean belingwe greenstone belt zimbabwe the tectonic setting of the archean ngezi group greenstones in the belingwe greenstone belt zimbabwe is controversial both autochthonous continental and oceanic settings have been proposed the sedimentary facies of the basal manjeri formation have been logged in surface sections and in drill cores deposition occurred under fluviatile and shallow marine conditions followed by alluvial fans and fan deltas detrital mineralogy rare earth element patterns and sm nd depleted mantle model ages between 2 9 and 3 7 ga are consistent with derivation from very local sources the facies and geochemical association imply sedimentation in an extensional continental setting

chemical evolution of seawater in the transvaal ocean between 2426 ma ongeluk large igneous province and 2413 ma ago kalahari manganese field abstract drill core and outcrop samples of pure marine chemical sediments banded iron formation bif manganese formation mnf jaspilites lutites and cherts from the transition of the 2426 ma old ongeluk formation into the 2413 ma old hotazel formation transvaal supergroup south africa reveal remarkable changes of seawater chemistry in the transvaal ocean similar to pre ongeluk chemical sediments the shale normalized rare earths and yttrium reysn patterns of jaspilites intercalated with the volcanic rocks of the ongeluk large igneous province and directly overlying cherts do not show positive eusn anomalies indicating that high temperature 250 c hydrothermal fluids did not contribute significantly to the rey budget of ambient waters however a 10 cm drill core section in the lower hotazel formation is characterized by conspicuous positive eusn anomalies revealing temporary inflow of water masses strongly affected by high temperature hydrothermal fluids after this short episode the reysn pattern of transvaal seawater returned to that of pre ongeluk times showing heavy reysn enrichment positive lasn gdsn and ysn anomalies but no cesn or eusn anomalies higher up in the stratigraphy the hotazel formation shows negative cesn anomalies in some of the lutites bifs and mnfs reflecting ce depletion in ambient seawater all hotazel lutite bif and mnf samples studied show unradiogenic end t values 0 5 0 2 to 2 4 0 2 indicating a mostly continental rey source the rey distribution and nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved rey to local transvaal seawater precipitation of the hotazel lutites bifs and mnfs with negative cesn anomalies therefore suggests that oxic conditions prevailed on the kaapvaal craton and in hotazel seawater already at 2 413 ga i e 80 m y before the disappearance of mass independent sulfur isotope fractionation mif s that defines the great oxidation event at 2 33 ga

a petrographic and geochemical study of carbonate and silica phases from the ediacaran doushantuo formation in the three gorges area of south china implications for diagenetic conditions abstract the carbonate of the ediacaran doushantuo formation contains various silica phases in the three gorges area of south china to better understand the diagenetic conditions under which these silica phases formed we carried out a petrographic and geochemical study of the silica phases and the coadjacent carbonate phases at the xiaofenghe section xfh in the three gorges area of south china five types of silica phases can be distinguished based on petrographic characteristics s1 and s2 are composed of microcrystalline quartz in chert nodules and disseminated in the host dolostone respectively s3 is composed of the megaquartz while s4 and s5 are microcrystalline quartz and fibrous silica respectively both replacing calcite cement six distinct types of carbonate can be recognized including the host matrix carbonate c1 sparry calcite nodules c2 calcite around chert nodules c3 dispersed calcite cement within chert nodules c4 fine micropeloidal calcite cements c5 and calcite veins c6 sedimentary structures suggest that formation of the s1 through s5 silica phases and c2 through c5 carbonate phases occurred during early burial diagenesis and therefore their formation recorded early diagenetic conditions assuming that ediacaran surface seawater temperature ranged from 28 to 39 c surface seawater 18o values can be calculated based on the oxygen isotopic composition of the host dolostone matrix c1 which range from 6 to 0 4 to 0 and 2 to 0 for the d2 d3 and d4 subunits of the doushantuo formation respectively based on the 18o values of chert nodules and beds mainly phase s1 diagenetic temperatures can be constrained to between 27 and 60 c under such low diagenetic temperatures the primary geochemical signatures of the host dolostone such as ree y patterns 13ccarb and 34spy values are likely to be preserved those silica phases without evidence of contamination of continental silicate detritus show characteristic ree y patterns with minor light ree depletions positive la eu and gd anomalies negative ce anomalies and slightly super chondritic y ho ratios implying that the source of silica may be derived from seawater negative ce anomalies in the silica phases in d2 through d4 subunits indicate that the bottom seawater or early diagenetic porewater was oxic at the time of silica precipitation

rare earth element and nd isotopic variations in regionally extensive dolomites from the burlington keokuk formation mississippian implications for ree mobility during carbonate diagenesis abstract marine carbonates of the burlington keokuk fm miss of the midcontinent region have been subject to two major episodes of regionally extensive 100 000 km2 dolomitization previous studies of the cathodoluminescent petrography sr c and o isotopic compositions and sr fe mn and ca concentrations of burlington keokuk dolostones outline a diagenetic history in which lime mud was pervasively dolomitized by a seawater dominated fluid at many localities this early dolomite generation dolomite i was recrystallized by nonmarine diagenetic fluids at shallow burial depths to produce dolomite ii dolomites i and ii have similar rare earth element ree patterns and abundances and a narrow range of initial 143nd 144nd isotope ratios of 0 511906 0 000025 1 this consistency in the nd isotope ratios and ree patterns along with the presence of anomalously low abundances of cerium in the dolomites cherts glauconite pellets fish apatite and crinoids suggests that 1 seawater derived ree dominated the system and 2 the extensive water rock interaction that produced profound textural and compositional changes during the formation of dolomite ii did not significantly alter the ree patterns or nd isotopic signatures of the dolomites the results of quantitative modeling of isotopic and trace lement exchange during water rock interaction in carbonate systems indicate that extremely large water rock ratios are required to alter the ree signatures of diagenetic carbonates such large water rock ratios can reflect either 1 large extents of fluid flow during carbonate recrystallization or 2 minimal interaction between diagenetic fluids and host rocks during carbonate cement crystallization late vug filling calcite and dolomite have low sm nd and high ce nd n ratios relative to the dolostones such distinct ree signatures probably reflect limited interaction between late diagenetic fluids and burlington keokuk host rocks penecontemporaneous clastic sediments have similar initial 143nd 144nd isotope ratios 0 511857 0 511877 and lower sm nd ratios compared to the dolomites suggesting 1 a common crustal source of ree for the clastics and carbonates and 2 ree fractionation during the formation of carbonate apatite glauconite and other authigenic phases in the marine environment

shallow water redox conditions from the permian triassic boundary microbialite the rare earth element and iodine geochemistry of carbonates from turkey and south china redox sensitive elements serve as useful proxies of the oxygenation state of ancient environments but their interpretation may not always be straightforward to evaluate the inherent complexities rare earths and yttrium rey iodine and major element concentrations are determined in two carbonate sections spanning the permian triassic p tr transition in demirtas turkey and cili south china we use major oxides to identify non seawater rey sources such as siliciclastics fe oxides phosphates and diagenetic fluids additionally we employ y ho ratio la anomaly and light rare earth element depletion to identify which samples preserve a seawater like reysn distribution in contrast to past interpretations we find that the p tr boundary microbialites in both sections contain rey signatures indicative of deposition in an oxic environment these boundary microbialites have their base at the extinction horizon and are widespread within the tethyan region in the cili section the underlying permian limestone also preserves a seawater like rey signature with a negative ce anomaly this indicates that the water column was oxygenated both before and after the extinction event the permian limestone in the demirtas section does not preserve a seawater like reysn distribution so the absence of a ce anomaly cannot be used to distinguish prevailing redox conditions during deposition however in these samples we find the presence of a diverse permian benthic community sufficient to identify deposition in an oxic environment the geochemical evidence for a continuously oxic environment during the deposition of the boundary microbialite presented in this study strongly supports work done using ostracods as redox indicators within the boundary microbialite in south china the microbialite has been proposed as a disaster facies in part resulting from the exclusion of grazers although anoxia is one of the suggested mechanisms for the exclusion of grazers we find that it is not supported by geochemical and biotic evidence

continental setting inferred for emplacement of the 2 9 2 7 ga belingwe greenstone belt zimbabwe comment and reply comment the tectonic evolution of the belingwe greenstone belt in zimbabwe has been a matter of great controversy in the most recent contribution to this debate bolhar et al 2003 1 reported major element trace element and nd isotope data for mafic volcanic rocks of different stratigraphic units of

depth dependent δ 13 c trends in platform and slope settings of the campbellrand malmani carbonate platform and possible implications for early earth oxygenation highlights carbon cycle of neoarchean carbonate platform and potential oxygen oasis carbon isotopes reveal a shift to aerobic biosphere and increasing oxidation state rare earth element patterns reveal decrease in open ocean water influx rimmed margin architecture was crucial for evolution of aerobic ecosystems abstract the evolution of oxygenic photosynthesis is widely seen as the major biological factor for the profound shift from reducing to slightly oxidizing conditions in earth s atmosphere during the archean proterozoic transition period the delay from the first biogenic production of oxygen and the permanent oxidation of earth s atmosphere during the early paleoproteorozoic great oxidation event goe indicates that significant environmental modifications were necessary for an effective accumulation of metabolically produced oxygen here we report a distinct temporal shift to heavier carbon isotope signatures in lagoonal and intertidal carbonates 13ccarb from 1 6 to 0 2 relative to vpdb and organic matter 13corg from about 40 to 25 relative to vpdb from the 2 58 2 50 gy old shallow marine campbellrand malmani carbonate platform south africa this indicates an increase in the burial rate of organic matter caused by enhanced primary production as well as a change from an anaerobic to an aerobic ecosystem trace element data indicate limited influx of reducing species from deep open ocean water into the platform and an increased supply of nutrients from the continent both supporting primary production and an increasing oxidation state of the platform interior these restricted conditions allowed that the dissolved inorganic carbon dic pool in the platform interior developed differently than the open ocean this is supported by coeval carbonates from the marginal slope setting which had a higher interaction with open ocean water and do not record a comparable shift in 13ccarb throughout the sequence we propose that the emergence of stable shallow water carbonate platforms in the neoarchean provided ideal conditions for the evolution of early aerobic ecosystems which finally led to the full oxidation of earth s atmosphere during the goe

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

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

marine carbonate records of terrigenous input into paleotethyan seawater geochemical constraints from carboniferous limestones a combined study of trace elements carbon and oxygen isotopes was carried out for carboniferous marine limestones from the lower yangtze platform in south china the results provide insights into the influence of terrigenous input to paleotethyan seawater from which pure and impure carbonates precipitated in terms of the correlations between the concentrations of elements with different properties variable extents of two component mixing are evident between seawater and terrigenous detritus this is quantitatively dictated by such parameters as the calcite seawater partition coefficients dx and the seawater continental upper crust partition coefficients kx water soluble elements with high and medium kx values such as na mg p v cr ni cu zn sr mo and u are susceptible to incorporation into carbonate lattices and thus their concentrations represent the geochemical composition of paleoseawater in contrast water insoluble elements with low kx values such as be al sc co ga cs ree hf and th cannot be incorporated into carbonate lattices thus providing a proxy for contributions from terrigenous material the carboniferous marine limestones from a stratigraphic profile exhibit three positive and three negative d 13 c excursions from the late visean to gzhelian these d 13 c excursions generally match those of contemporaneous carbonates elsewhere in the world which can be linked to the alternative occurrences of cold and warm paleoclimates in the carboniferous the lowest d 13 c values in the profile are generally associated with the lowest values of y ho and the highest values of nd yb paas th sc and insoluble residue this indicates a significant contribution from the terrigenous material to the negative d 13 c excursions the effect from the warm paleoclimate is also indicated by high chemical indices of alteration for the insoluble residue from the three intervals of high terrigenous input on the other hand the positive d 13 c excursions are associated with the decreases of terrigenous input therefore the incorporation of terrigenous material into the epicontinental seawater is temporally linked to the paleoclimatic change on continental margins adjacent to the paleotethyan seawater

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

tectonic controls on atmospheric climatic and biological evolution 3 5 2 4ga abstract this paper presents a speculative scenario for the evolution of the earth s surface environment and biological community 3 5 2 4 ga based on the geologic record and its interpretation available geologic evidence suggests that the earth s climate before 3 2 ga was hot probably 60 73 c as a result of a co 2 and ch 4 greenhouse the early biological community was probably dominated by anoxygenic photosynthetic thermophiles cyanobacteria if they evolved before 3 1 ga would have struggled to survive at such high temperatures as a result the atmosphere contained extremely low levels of o 2 and 33 s shows wide departures from 0 indicating mass independent fractionation of s isotopes in the upper atmosphere this hot early climate collapsed 3 1 3 0 ga due to growth of large new blocks of continental crust cratons the weathering of which resulted in the depletion of atmospheric co 2 and eventual drawdown of ch 4 due to formation and rainout of methane aerosols cooling may have culminated in glaciation about 3 0 2 9 ga this climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3 0 2 7 ga characterized by the emergence of new low temperature taxa including cyanobacteria and their spread throughout surface environments at the expense of extreme thermophiles including methanogens during a period of general tectonic stability 3 0 2 7 ga the earth s surface was characterized by overall clement conditions with temperatures 2 after 2 75 ga and by 2 7 ga surface temperatures had returned to 60 c cyanobacteria were again suppressed o 2 production waned and atmospheric mass independent fractionation is again indicated by extreme variations in 33 s these conditions persisted perhaps intermittently until about 2 5 2 4 ga when weathering and erosion of vast new blocks of continental crust formed about 2 65 2 6 ga caused the second and probably last collapse of a 60 c surface climate broad tectonic climatic and biological events 3 5 2 9 ga are remarkably parallel to those of the late archean and paleoproterozoic 2 75 2 2 ga and neoproterozoic 1 0 0 50 ga these 500 myr long cycles of greenstone volcanism and crustal generation climatic and atmospheric instability and biological innovation reflect the long term interaction of the tectonic atmospheric climatic and biological components of the earth s surface system they suggest that the evolution of the earth s interior expressed through its control on the formation of large blocks of continental crust has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history

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

establishing criteria to distinguish oil seep from methane seep carbonates hydrocarbon seeps harbor copious chemosynthesis dependent life the traces of which are preserved in the fossil record within authigenic carbonates these environments are mostly characterized by seepage of methane rich fluids yet numerous crude oil dominated seeps have been discovered in recent years oil seepage has a profound influence on the local fauna but recognizing such seeps in the rock record remains elusive this study presents new geochemical data that will allow for a more confident identification of ancient oil seep deposits geochemical data from modern and ancient seep limestones reveal that oil dominated seep carbonates are enriched in rare earth elements and uranium compared to their methane dominated counterparts these trace element patterns have the potential to serve as a basis for an improved understanding of the adaptation of chemosynthetic life to oil seepage and to better constrain the marine carbon cycle in the geologic past

juvenile crust formation in the zimbabwe craton deduced from the o hf isotopic record of 3 8 3 1 ga detrital zircons hafnium and oxygen isotopic compositions measured in situ on u pb dated zircon from archaean sedimentary successions belonging to the 2 9 2 8 ga belingwean bulawayan groups and previously undated sebakwian group are used to characterize the crustal evolution of the zimbabwe craton prior to 3 0 ga microstructural and compositional criteria were used to minimize effects arising from pb loss due to metamorphic overprinting and interaction with low temperature fluids 207 pb 206 pb age spectra concordance 90 reveal prominent peaks at 3 8 3 6 3 5 and 3 35 ga corresponding to documented geological events both globally and within the zimbabwe craton zircon delta o 18 values from 4 to 10 point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near surface environments in epsilon hf time space 3 8 3 6 ga grains define an array consistent with reworking of a mafic reservoir 176 lu 177 hf similar to 0 015 that separated from chondritic mantle at similar to 3 9 ga crustal domains formed after 3 6 ga depict a more complex evolution involving contribution from chondritic mantle sources and to a lesser extent reworking of pre existing crust protracted remelting was not accompanied by significant mantle depletion prior to 3 35 ga this implies that early crust production in the zimbabwe craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas possibly because the volume of crust extracted and stabilised was too small to influence asthenospheric mantle isotopic evolution growth of continental crust through pulsed emplacement of juvenile chondritic mantle derived melts into and onto the existing cratonic nucleus however involved formation of complementary depleted subcontinental lithospheric mantle since the early archaean indicative of strongly coupled evolutionary histories of both reservoirs with limited evidence for recycling and lateral accretion of arc related crustal blocks until 3 35 ga c 2017 elsevier ltd all rights reserved

proterozoic transpressive deformation in the northern marginal zone limpopo belt zimbabwe a major tectono metamorphic event affected the southern half of the northern marginal zone of the limpopo belt at 2 0 ga structural analysis along a 43 km long profile perpendicular to strike shows extension inclined toward the northeast extension parallel to axes of folds that uplift the southeast of the section i e the central zone of the limpopo belt and dextral shear these deformation patterns developed coevally the lineations form distinct tightly clustered groups that occur together in the profile there is no evidence for superimposed deformation or deformed lineations nor are lineations observed cross cutting each other this single episode of deformation can therefore be called transpression petrographic and thermobarometric evidence support this kinematic picture in that decreases of both pressure 2 5 kb and temperature 150 c are documented from southeast to northwest across strike 39 ar 40 ar step heating of metamorphic hornblende from amphibolites has yielded ages of

differentiating marine vs hydrothermal processes in devonian carbonatemounds using rare earth elements kess kess mounds anti atlas morocco abstract petrological and geochemical analyses were carried out on early devonian kess kess mound limestones of the seheb el rhassel group exposed in the hamar laghdad ridge tafilalt platform eastern anti atlas morocco in order to evaluate marine vs hydrothermal processes for the origin of the mounds hydrothermal fluid circulation affected limestone deposition resulting in the formation of a plumbing system preserved in the mound facies and inter mound facies as cavities veins and dykes shale normalized rare earth element ree patterns for limestones display pronounced light ree depletion compared to middle ree average prsn dysn 0 46 and heavy ree average prsn ybsn 0 55 a super chondritic y ho ratio up to 87 and positive la anomaly consistent with precipitation from normal marine seawater within our data set the ce anomaly as well as u and other trace element concentrations vary with facies providing evidence for variable redox conditions the presence of positive ce anomaly in carbonates of the plumbing system is consistent with precipitation under anoxic condition samples from mound facies and quartz veinlets in the upper part of the group show ree patterns consistent with precipitation from hydrothermal fluids early devonian hypothetical seawater ree patterns were calculated from samples showing normal marine ree patterns i e inter mound facies and from samples with weaker marine signature i e fossiliferous mound limestones and plumbing system deposits hypothetical patterns are slightly enriched in ree compared to modern open ocean seawater this study reveals that ree analysis is a powerful tool for understanding polygenetic carbonate systems it sheds light into the genesis of the kess kess mounds by reconstructing fluid pathways and palaeo redox conditions

trace element and nd isotope composition of shallow seawater prior to the great oxidation event evidence from stromatolitic bioherms in the paleoproterozoic rooinekke and nelani formations south africa abstract pure stromatolitic limestones from the 2 44 to 2 43 ga old rooinekke formation transvaal supergroup south africa provide unique insight into the geochemistry of paleoproterozoic shallow seawater and therefore into the evolution of ambient redox levels of the earth s atmosphere hydrosphere system in contrast to the stromatolitic klipput dolostones from the overlying nelani iron formation if the stromatolitic rooinekke limestones do not show post depositional alteration and are virtually free of detrital contamination this makes them excellent geochemical archives of proxies of ambient shallow marine transvaal seawater shale normalized sn rare earth element and yttrium rey patterns of pure rooinekke limestones show seawater like rey distributions with positive lasn and gdsn anomalies super chondritic y ho ratios and a depletion of the light lrey relative to the heavy reysn hrey however the lack of negative cesn anomalies implies that redox conditions at the earth s surface were not oxidizing enough to stabilize significant amounts of ce iv in contrast to the majority of archean marine chemical sediments the pure early paleoproterozoic rooinekke limestones lack positive eusn anomalies suggesting that the fraction of rey derived from high temperature 250 c hydrothermal fluids in shallow rooinekke seawater was negligible the predominance of continental sources in the geochemical rey budget of rooinekke seawater is corroborated by unradiogenic end 2 44 ga values close to those of contemporaneous upper crustal material the rooinekke limestones therefore reveal the absence of a high temperature hydrothermal component from shallow seawater already at least 100 million years prior to the great oxidation event in contrast older chemical sediments from the transvaal supergroup such as the campbellrand carbonates and the penge and kuruman ifs show positive eusn anomalies and more radiogenic end t values indicating a significant contribution of mantle derived rey via high temperature hydrothermal fluids to the late neoarchean shallow marine rey inventory on the kaapvaal craton the declining impact of high temperature hydrothermal fluids on surface seawater that is recorded by the late neoarchean to paleoproterozoic shallow water transvaal carbonates is similar to the general trend of decreasing positive eusn anomalies with decreasing depositional age that is observed in eoarchean to proterozoic ifs which recorded the composition of deeper marine waters

trace elements and ree geochemistry of middle devonian carbonate mounds maider basin eastern anti atlas morocco implications for early diagenetic processes abstract trace and rare earth elements rees have proven their utility as tools for assessing the genesis and early diagenesis of widespread geological bodies such as carbonate mounds whose genetic processes are not yet fully understood carbonates from the middle devonian conical mud mounds of the maider basin eastern anti atlas morocco have been analysed for their ree and trace element distribution collectively the carbonates from the maider basin mud mounds appear to display coherent ree patterns three different geochemical patterns possibly related with three different diagenetic events include i dyke fills with a normal marine ree pattern probably precipitated in equilibrium with seawater ii mound micrite with a particular enrichment of overall ree contents and variable ce anomaly probably related to variation of ph increase of alkalinity or dissolution remineralization of organic matter during early diagenesis and iii haematite rich vein fills precipitated from venting fluids of probable hydrothermal origin our results reinforce the hypothesis that these mounds were probably affected by an early diagenesis induced by microbial activity and triggered by abundance of dispersed organic matter whilst venting may have affected the mounds during a later diagenetic phase

northwest european palaeoclimate as indicated by growth frequency variations of secondary calcite deposits a compilationn for the northwest of europe of over 500 uranium series speleothen and travertine dates is presented using a cumulative distributed error frequency approach these secondary carbonate deposits require both significant groundwater supply and a biogenic soil carbon dioxide source for their growth during glacial periods soil co2 production is inhibited and the water supply is ice locked therefore growth is slowed significantly if not stopped thus deposits can be used on a chronological basis to give a signal of glacial and non glacial periods however sensitivity to the groundwater supply modifies this signal with growth also limited in times of high aridity the palaeoclimatic signal is thus complex the compilation presented is used both as a chronology for comparison with the orbitally tuned marine oxygen isotope record and as a palaeoclimatic indicator attention is focused on two periods of the last glacial interglacial cycle where the record provides a significantly different palaeoclimatic record to other terrestrial and oceanic records these are the isotope stages 5 4 transition for which a relatively low cumulative growth frequency indicates an earlier increase in aridity than observed elsewhere and isotope stage 3 the pleniglacial where both statistically significant high 49 62 ka and low 22 35 44 46 ka levels of growth are observed and can be used to constrain the timing of many of the interstadial events within this period

geology and lithogeochemistry of hydrothermal mudstones from the upper block near the duck pond volcanogenic massive sulfide vms deposit newfoundland canada evidence for low temperature venting into oxygenated mid cambrian seawater pyrite and pyrrhotite rich mudstones are spatially associated with cambrian 512 509 ma volcanogenic massive sulfide vms deposits throughout the tally pond group central newfoundland canada at the duck pond mine sulfide rich mudstones are hosted within a weakly mineralized upper block that structurally overlies the deposit but is older 513 versus 509 ma the mudstones are laminated 10 30 cm thick and pyrite and pyrrhotite rich and occur along pillow lava selvages or in between pillow lavas rhyolite flows and volcaniclastic rocks the mudstones are laterally extensive and proximal to the mudstone host rocks are hydrothermally altered to epidote quartz chlorite basalt host and sericite quartz rhyolite host lithogeochemical data for the sulfide rich mudstones reflect the varying contributions of elements from sedimentary detritus hydrothermal discharge and hydrogenous scavenging from middle cambrian seawater the mudstones have minor detrital element abundances and significant hydrothermal element enrichments i e elevated fe2o3 s pb zn cu and ba concentrations high fe al ratios the hydrothermal mudstones are also enriched in oxyanions i e p2o5 u v cr ni co and hg interpreted to have been enriched via oxidative scavenging from seawater by fe oxide oxyhydroxide particles the mudstones also have ree y signatures similar to modern oxygenated seawater with high y ho and negative ce anomalies ce ce 0 40 0 86 average 0 58 which correlate with adsorbed oxyanion concentrations the low eu eu 1 02 1 86 average 1 22 in the mudstones suggest that they were deposited from low temperature 250 c fe rich hydrothermal fluids that likely formed a buoyant plume into an oxygenated water column the ree y oxyanion signatures suggest that the particles within the hydrothermal plume had sufficient residence time to scavenge oxyanions from seawater and inherit a middle cambrian seawater signature the predominant seawater ree y oxyanion signature in the duck pond upper block sulfide rich mudstones suggests that they are distal hydrothermal sedimentary rocks that could have formed up to 10 km from their original vent sources correspondingly to utilize hydrothermal mudstones as vectors to mineralization in the tally pond belt and similar belts globally it is critical to identify vent proximal samples that have hydrothermal signatures i e high fe al base metals ba s with subdued seawater and adsorption signatures i e chondritic y ho low p2o5 ni u co cr v and hg indicating minimal residence time in the water column and deposition proximal to the vent

ca carbonates precipitation and limestone genesis the microbiogeologist point of view abstract experiments show that the production of carbonate particles by heterotrophic bacteria follows different ways in heterotrophy the passive carbonatogenesis is generated by modifications of the medium that lead to the accumulation of carbonate and bicarbonate ions and to the precipitation of solid particles it is induced by several metabolic pathways of the nitrogen cycle ammonification of amino acids degradation of urea and uric acid dissimilatory reduction of nitrates and of the sulphur cycle dissimilatory reduction of sulphates the active carbonatogenesis is independent of the mentioned metabolic pathways the carbonate particles are produced by ionic exchanges through the cell membrane following still poorly known mechanisms in autotrophy non methylotrophic methanogenesis and cyanobacterial photosynthesis also may contribute to the precipitation of carbonates autotrophic carbonates as carbonatogenesis is neither restricted to particular taxonomic groups of bacteria nor to specific environments it has been an ubiquitous phenomenon since precambrian times carbonatogenesis is the response of heterotrophic bacterial communities to an enrichment of the milieu in organic matter after a phase of latency there is an exponential increase of bacterial numbers together with the accumulation of metabolic end products these induce a ph increase and an accumulation of carbonate and hydrogenocarbonate ions in the medium this phase ends into a steady state when most part of the initial enrichment is consumed and there is a balance between death and growth in bacterial populations particulate carbonatogenesis occurs during the exponential phase and ends more or less after the beginning of the steady state the active carbonatogenesis seems to start first and to be followed by the passive one which induces the growth of initially produced particles in eutrophic conditions the first solid products are patches that appear on the surface of the bacterial bodies and coalesce until forming a rigid coating and or particles excreted from the cell all these tiny particles assemble into biomineral aggregates which often display precrystalline structures these aggregates grow and form biocrystalline build ups which progressively display more crystalline structures with growth in oligotrophic conditions the primary solid products are rapidly smoothed in the crystalline structure and leave no trace in present aqueous environments apart from deep ocean the potential efficiency of heterotrophic bacterial carbonatogenesis in ca carbonate sedimentation is much higher than autotrophic or abiotic processes it much more likely accounts for extensive apparently abiotic limestone formation than any of the latter as far as biodetrital particles are concerned it may be observed that the shells and tests of organisms are built from the activity of cellular organites which are nowadays considered by a number of biologists as endosymbiotic bacteria thus apart from probably mythical purely evaporitic and autotrophic ones most limestones must be considered as principally of heterotrophic bacterial origin as the carbon of limestones is issued from organic matter bacterial heterotrophic carbonatogenesis appears as a fundamental phenomenon in the relationships between atmosphere and lithosphere during the biogeological evolution of the earth

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

geochemical stratigraphy sedimentology and mo isotope systematics of the ca 2 58 2 50 ga old transvaal supergroup carbonate platform south africa abstract the neoarchean paleoproterozoic transvaal supergroup in south africa contains the well preserved stromatolitic campbellrand malmani carbonate platform which was deposited in shallow seawater shortly before the 2 40 2 32 ga great oxidation event goe this platform is composed of alternating stromatolitic carbonates and mudstones and is a prominent candidate for isotope geochemical mapping to investigate the appearance of very small amounts of free oxygen that accumulated in shallow waters preceding the goe mo isotopes in sedimentary archives are widely used as a proxy for redox changes in modern and ancient environments and recent evidence suggests that oxy molybdate moo 4 2 is directly transferred from ocean water to inorganic carbonates with negligible fractionation thus reflecting oceanic mo isotope signatures in this study we analyzed major and trace element compositions as well as mo isotopic compositions of carbonate and mudstone samples from the kmf 5 drill core geochemical indicators such as fe and mn concentrations and fe to mn abundance ratios reveal the preservation of some geochemical indicators despite the widespread silicification and dolomitization of the platform heavy 30 si values of silicified carbonates between 0 53 and 2 35 point to si precipitation from surface water during early diagenesis rather than to a later hydrothermal overprint this assessment is supported by the frequent observation of rip up structures of silica chert layers within the entire sedimentary succession the 98 mo values of whole rock samples throughout the malmani campbellrand platform range between 0 82 and 1 40 similar to values reported for deeper slope carbonates from the griqualand west area but variations are independent from lithology or depositional water depth these large variations in 98 mo values indicate molybdenum redox cycling and thus the presence of free oxygen in the atmosphere ocean system at that time in agreement with earlier mo isotopic studies on campbellrand carbonates and shales a similar range in 98 mo values for carbonates between 0 40 and 0 87 however was also found on the hand specimen scale indicating the danger of a sample bias on the mo isotopic stratigraphy of this carbonate platform results of previously unpublished adsorption experiments of mo on caco 3 clearly indicate that the mo inventory of malmani campbellrand carbonates was not only influenced by primary adsorption from seawater but to a much larger degree by secondary processes during early diagenesis which also affected the mo isotopic composition of the samples on a local scale our results indicate that mo concentrations and isotopic compositions in ancient stromatolitic carbonates were subject to redox changes within microbial mats and within the soft sediment during early diagenesis and later lithification and as such cannot be used to quantitatively reconstruct the amount of free atmospheric oxygen or its fluctuations through earth s history nevertheless we interpret our heavy mo isotopic signatures from carbonates as a minimum value for neoarchean seawater and reinforce the assumption that free atmospheric oxygen built up a heavy oceanic mo reservoir at that time

discriminating stromatolite formation modes using rare earth element geochemistry trapping and binding versus in situ precipitation of stromatolites from the neoproterozoic bitter springs formation northern territory australia stromatolites consist primarily of trapped and bound ambient sediment and or authigenic mineral precipitates but discrimination of the two constituents is difficult where stromatolites have a fine texture we used laser ablation inductively coupled plasma mass spectrometry to measure trace element rare earth element ree y and th concentrations in both stromatolites domical and branched and closely associated particulate carbonate sediment in interspaces spaces between columns or branches from bioherms within the neoproterozoic bitter springs formation central australia our high resolution sampling allows discrimination of shale normalised ree patterns between carbonate in stromatolites and immediately adjacent fine grained ambient particulate carbonate sediment from interspaces whereas all samples show similar negative la and ce anomalies positive gd anomalies and chondritic y ho ratios the stromatolites and non stromatolite sediment are distinguishable on the basis of consistently elevated light rees lrees in the stromatolitic laminae and relatively depleted lrees in the particulate sediment samples additionally concentrations of the lithophile element th are higher in ambient sediment samples than in stromatolites consistent with accumulation of some fine siliciclastic detrital material in the ambient sediment but a near absence in the stromatolites these findings are consistent with the stromatolites consisting dominantly of in situ carbonate precipitates rather than trapped and bound ambient sediment hence high resolution trace element ree y th geochemistry can discriminate fine grained carbonates in these stromatolites from coeval non stromatolitic carbonate sediment and demonstrates that the sampled stromatolites formed primarily from in situ precipitation presumably within microbial mats biofilms rather than by trapping and binding of ambient sediment identification of the source of fine carbonate in stromatolites is significant because if it is not too heavily contaminated by trapped ambient sediment it may contain geochemical biosignatures and or direct evidence of the local water chemistry in which the precipitates formed

metal cycling in mesoproterozoic microbial habitats insights from trace elements and stable cd isotopes in stromatolites abstract reconstructing the environmental conditions that supported early life on earth relies on well preserved geochemical archives in the rock record however many geochemical tracers either lack specificity or they are affected by post depositional alteration we present a data set of major and trace element abundances and cd isotope compositions of dome shaped and conophyton type stromatolites of the late mesoproterozoic paranoa group brazil showing distinct values with unprecedented resolution at the lamina scale the studied stromatolites consist of dolomite with a high purity and a negligible content of immobile elements e g two endmember compositions can be distinguished by multiple proxy analysis reflecting the contrasting depositional environments of the two types of stromatolites shale normalized rare earth elements including yttrium reysn patterns of domal stromatolites show a light reysn lrey enrichment ybsn prsn in contrast reysn patterns of the conophyta are parallel those of modern seawater with an lreysn depletion relative to hreysn ybsn prsn 2 1 to 3 9 positive gdsn anomalies 1 1 to 1 4 and strong super chondritic y ho ratios 37 9 to 46 2 suggesting a microbial habitat that was dominated by seawater cd isotopes correlate negatively with cd and u but positively with mn and ce concentrations reflecting authigenic carbonate formation at different depths within a redox gradient of the ancient microbial mat e112 110cddol values increase from 3 52 at the mat surface to 3 46 in the interior of the mat due to the effect of kinetic fractionation during cd uptake e g by adsorption onto organic matter or by precipitation of sulfides in addition to incorporation into carbonate minerals hence our multi proxy approach including cd isotopes bears a high potential to shed light on environmental conditions in ancient microbial habitats and the activity of microbial life on early earth

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

frutexites from microbial metazoan bioconstructions of recent and pleistocene marine caves sicily italy abstract frutexites like structures were recorded in small bioconstructions from recent and pleistocene marine caves the skeletal microbialite frameworks hosting frutexites form in cryptic environments characterized by poor light availability and reduced water circulation leading to confinement oligotrophic conditions and the development of typical cave communities biostalactites and biotic crusts develop in the more confined parts of the both recent and pleistocene caves they are composed of skeletal organisms engulfed in fine micrite sediments deposited in situ via microbial metabolic activity the frutexites composed of iron and manganese oxides are confined to microcavities or microfractures of the bioconstructions they grow from an original discontinuity surface into the surrounding consolidated micritic sediments a model based on the increase of the microporosity of micritic sediments due to the dissolution of the original material and precipitation of ferromanganesiferous compounds was proposed this process occurs during the syndepositional diagenetic stage due to the intrusion in the bioconstruction framework of acidic continental water rich in iron and manganese mesophilic fe mn autotrophic and chemoheterotrophic bacteria flourish on the microcavities and induce the precipitation of fe and mn oxides

carbonates and cherts as archives of seawater chemistry and habitability on a carbonate platform 3 35 ga ago insights from sm nd dating and trace element analysis from the strelley pool formation western australia abstract carbonates and cherts in the 3 35 billion year old strelley pool formation fm australia host stromatolites that are among the oldest remnants of life on earth however it is still not entirely clear whether these mineral phases are authigenic precipitates and whether they represent reliable geochemical archives of early earth environments here we present major trace element and nd isotope data of stromatolitic carbonates associated crystal fan carbonates and cherts in the strelley pool fm i to assess the reliability of these chemical sediments as geochemical archives of the fluids from which they precipitated ii to date the time of formation of carbonate and silica phases and iii to trace the sources of elements prevailing in microbial habitats 3 35 ga ago stromatolitic carbonates plot together with the stratigraphically underlying marble bar cherts on a sm nd regression line yielding 3253 320 ma in contrast associated crystal fan carbonates together with altered marble bar cherts yield 2718 220 ma suggesting that their sm nd isotope system was reset after deposition both types of carbonates as well as white cherts show shale normalized rare earth element and yttrium patterns reysn with the exception of redox sensitive ce and eu and heavy reysn to middle reysn depletion that are parallel to those of modern seawater indicating a predominantly seawater derived origin positive eusn anomalies 2 1 2 4 combined with heterogeneous nd3 35ga values between 3 2 and 5 8 within individual alternating stromatolite laminae further support that the dissolved fraction of seawater on the ancient carbonate platform was variably affected by both continental weathering and high temperature hydrothermal fluids contributing elements of both young mafic or older felsic rocks in conclusion trace element and nd isotope data presented here match well with the depositional environment as characterized based on lithological geochemical and stratigraphic relationships on an early continent showing at least episodic emergence above the sea level supporting microbial life on a shallow marine platform

authigenic minerals from the paola ridge southern tyrrhenian sea evidences of episodic methane seepage abstract paola ridge along the nw calabrian margin southern tyrrhenian sea is one of the few reported deep sea sites of precipitation of authigenic carbonates in the tyrrhenian sea here the changing composition of the seeping fluids and the dynamic nature of the seepage induced the precipitation of pyrite siderite and other carbonate phases the occurrence of this array of authigenic precipitates is thought to be related to fluctuation of the sulfate methane transition zone smtz concretions of authigenic minerals formed in the near sub bottom sediments of the paola ridge were investigated for their geochemical and isotopic composition these concretions were collected in an area characterized by the presence of two alleged mud volcanoes and three mud diapirs the mud diapirs are dotted by pockmarks and dissected by normal faults and are known for having been a site of fluid seepage for at least the past 40 kyrs present day venting activity occurs alongside the two alleged mud volcanoes and is dominated by co2 rich discharging fluids this discover led us to question the hypothesis of the mud volcanoes and investigate the origin of the fluids in each different domed structure of the study area in this study we used stable isotopes carbon and oxygen of carbonates coupled with rare earth element ree composition of different carbonate and non carbonate phases for tracing fluid composition and early diagenesis of authigenic precipitates the analyses on authigenic precipitates were coupled with chemical investigation of venting gas and sea water authigenic calcite aragonite concretions from surficial sediments on diapiric structures have depleted 13c isotopic composition and slightly positive 18o values by contrast siderite concretions generally found within the first 6 m of sediments on the alleged mud volcanoes yielded positive 13c and 18o values the siderite ree pattern shows consistent lree light ree fractionation mree medium ree enrichment and positive gd and la anomalies as shown by the ree distribution the 13c depleted composition and their association with chemosymbiotic fauna calcite aragonite precipitated at time of moderate to high methane flux close to the seafloor under the influence of bottom seawater authigenic siderite on the other hand formed in the subseafloor during periods of lower gas discharges under prolonged anoxic conditions within sediments in equilibrium with 13c rich dissolved inorganic carbon dic and 18o rich water likely related to methanogenesis and intermittent venting of deep sourced co2

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