a combined in situ oxygen silicon isotopic and fluid inclusion study of a chert sample from onverwacht group 3 35ga south africa new constraints on fluid circulation abstract cherts have been extensively used as a proxy of precambrian environmental conditions of the earth cherts from the onverwacht group 3 5 3 3 ga south africa are considered as well preserved and have been used as a paleo oceanic thermometer especially by using the oxygen isotope composition of the chert nevertheless there are several lines of evidence size and number of the quartz veins that fluids circulated through these cherts after their deposition these cherts are composed mainly of microquartz with large quartz veins 100 m and iron carbonates as ankerites a combined study of oxygen silicon and fluid inclusions study have been performed on one chert sample from the onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions in situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector cameca 1270 ion probe crpg cnrs nancy moreover fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation three types of primary fluid inclusions carbonic lc aqueous carbonic fluids lc w and aqueous fluid lw have been observed the temperatures of this fluid circulation were determined by microthermometry and using quartz carbonate pair thermometry at between 200 c and 300 c fluid pressures range from 100 mpa to 30 mpa which correspond to depths ranging from 4 to 1 km this evolution could be related to an exhumation associated with a dilution and cooling process this fluid circulation could be associated to the gold bearing fluid responsible of the sheba fairview ore system dated at 3074 6 ma the results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz the 18 o value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold bearing fluid the silicon isotope composition of the microquartz allows determination of the initial 30 si of the microquartz precursor this precursor has been precipitated from a hydrothermal fluid hence a combined 18 o 30 si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined

stable isotope and rare earth element evidence for recent ironstone pods within the archean barberton greenstone belt south africa there is considerable debate about the mode and age of formation of large up to 200 m long hematite and goethite ironstone bodies within the 3 2 to 3 5 ga barberton greenstone belt we examined oxygen and hydrogen isotopes and rare earth element ree concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea floor vents in the archean ocean or from recent surface and shallow subsurface spring systems goethite d 18 o values range from 0 7 to 1 0 and dd from 125 to 146 which is consistent with formation from modern meteoric waters at 20 to 25 c hematite d 18 o values range from 0 7 to 2 0 which is consistent with formation at low to moderate temperatures 40 55 c from modern meteoric water ree in the goethite and hematite are derived from the weathering of local sideritic ironstones silicified ultramafic rocks sideritic black cherts and local felsic volcanic rocks falling along a mixing line between the eu eu and shale normalized hreeavg lreeavg values for the associated silicified ultramafic rocks and felsic volcanic rocks contrasting positive ce ce of 1 3 to 3 5 in hematite and negative ce ce of 0 2 to 0 9 in goethite provides evidence of oxidative scavenging of ce on hematite surfaces during mineral precipitation these isotopic and ree data taken together suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and or subsurface warm springs

sedimentary geology of the palaeoarchaean buck ridge south africa and kittys gap western australia volcano sedimentary complexes abstract the two palaeoarchaean volcano sedimentary complexes of the buck ridge barberton greenstone belt south africa and kittys gap coppin gap greenstone belt east pilbara australia have a similar geological setting and age 3 45 ga the predominantly volcaniclastic sediments are concentrated at the top of these complexes and experienced thorough very early diagenetic silicification in many places the silicification process has led to excellent preservation of the primary sedimentary structures elsewhere it has resulted in their obliteration or replacement by diagenetic structures the buck ridge chert forms a regressive transgressive succession deposited around base level with lacustrine and littoral marine facies deposition of the kittys gap chert was also close to base level almost exclusively subaqueous with tidal influence and a regressive sequential trend in both volcano sedimentary complexes these low energy sediments are juxtaposed with high energy breccia pods and layers with often a high fe oxide content the breccias are interpreted as being the result of explosive hydrothermal activity sedimentation was strongly controlled by normal faulting

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

early archean 3 2 ga fe oxide rich hydrothermal discharge vents in the barberton greenstone belt south africa ironstone pods fe 2 o 3 72 5 97 2 wt interpreted to have formed by early archean sea floor related hydrothermal activity are described from the ca 3 5 3 1 ga barberton greenstone belt south africa most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy they rest on silicified ultramafic rocks and are overlain by ferruginous shales silicified sand stones other coarse clastics and barite horizons the ironstone pods grade along strike into laminated iron oxide facies banded iron formation that is inferred to represent periodic discharge of iron oxide and silica rich flocculates from the pods the massive texture of the iron stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor relict hydrothermal discharge chimney structures have been recognized honeycomb like cavities and possible fluid flow channel textures attest to primary porosity the iron stone pods are dominated by massive locally coarse grained specular hematite and goethite with lesser amounts of quartz and an x ray amorphous fe ai bearing silicate the pods are distinct from typical massive sulfide gossans in total trace element concentrations including precious and base metals indicating a different origin in contrast the ironstone pods have similarities in major oxide and trace metal concentrations to iron oxide deposits presently forming in part from low temperature hydrothermal fluids on the sea floor for example red seamount iron oxide deposit fluid inclusion studies on quartz show dominant type i primary fluid inclusions with salinities of 4 7 to 15 8 wt nacl equiv and homogenization temperatures t h of 90 to 150 c no evidence for boiling is seen type i fluid inclusions are most likely dominated by nacl cacl 2 h 2 o although their measured eutectic temperatures are consistent with the presence of fecl 2 these inclusions represent the end member hydrothermal fluid more saline 24 29 6 wt cacl 2 equiv lower temperature t h 33 109 c type ii and ila fluid inclusions represent pulses of a hydrothermal fluid of nacl cacl 2 fecl 2 or mgcl 2 h 2 o or possibly fecl 3 bearing composition mixing with ambient sea water in the case of type ii inclusions the various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine the lowest salinity measurement for a type i inclusion of 3 1 wt nacl equiv provides a constraint on the salinity of unenriched by evaporation sea water the t h data give a minimum trapping pressure of 7 bars which equates to a minimum sea water depth of 60 m the lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods quartz 18 o analyses average 17 3 0 6 1 n 5 calculated 18 o h 2 o for two samples are 1 4 and 1 2 respectively these estimates are within error of the postulated barberton early archean sea water value of 0 quartz hematite goethite mineral pairs yield temperatures inconsistent with fluid inclusion t h data suggesting isotopic disequilibrium

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

young poorly crystalline graphite in the 3 8 gyr old nuvvuagittuq banded iron formation carbonaceous minerals in 3 8 billion year old rocks from west greenland have been used as evidence for early life on earth geochemical analyses of similar carbon minerals in 3 75 to 4 2 billion year old rocks from canada show that they can be derived from subsequent alteration more than a billion years after the initial formation

growth fault structure and stratigraphic architecture of the buck ridge volcano sedimentary complex upper hooggenoeg formation barberton greenstone belt south africa abstract the 3 45 3 42 ga buck ridge volcano sedimentary complex br vsc forms the uppermost part of the hooggenoeg formation in the early archaean onverwacht group of the barberton greenstone belt south africa the complex consists mainly of massive and pillowed basalts and felsic quartz plagioclase porphyritic rocks which are capped by pervasively silicified sedimentary rocks deposition of the felsic extrusive and volcani clastic sedimentary rocks occurred in shallow water to emersion during an extensional tectonic regime the extensional regime is expressed by a number of listric normal faults that transect the entire 2 km thick complex the syndepositional i e growth fault character of the faults is indicated by variations in thickness and facies of rock units across the faults and by systematic rotation of the fault blocks broadly contemporaneous with the faulting felsic rocks intruded the complex into the highest levels the geometry of these intrusive bodies was controlled by the major growth faults extension also created space for the intrusion of approximately bedding perpendicular felsic and mafic dykes and black chert veins the new field observations and u pb zircon shrimp data have pinned the timing of events during and after deposition of the br vsc the growth fault model accounts well for previously unexplained structural observations that include both extensional and compressional features recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the barberton greenstone belt and the greenstone belts of the east pilbara in australia

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs comment and reply comment lowe and byerly s 2003 1 description of iron oxide depositing spring terraces in the barberton greenstone belt and how they relate to the iron stone pods we studied is misleading and inaccurate here we demonstrate that the archean iron deposits we studied have no relationship to the iron

rusty relics of earth history iron iii oxides isotopes and surficial environments abstract the two most common low temperature iron iii oxides on earth are goethite feooh and hematite fe2o3 the 18o values of natural goethites range from 15 5 to 3 3 whereas 18o values of low temperature hematites range from 16 7 to 4 7 plots of d against 18o for continental goethites are approximately parallel to the meteoric water line of craig h craig 1961 science 133 1702 3 this suggests that goethite water fractionation factors are systematic over a wide range of surficial environments and may indicate that isotopic equilibrium is commonly attained or closely approached several experimental or calculated mineral water oxygen isotope fractionation curves have been determined for both goethite and hematite although there is not yet a consensus on which of these curves best approximates isotopic fractionation in natural samples oxygen isotope measurements of both goethite and hematite have provided evidence of significant continental climate change on time scales that

pixe and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2 2ga ongeluk formation south africa implications for ancient seawater salinity abstract the ongeluk formation mainly consists of submarine volcanics pillow lavas and sheet flows composed of basaltic andesites that are between underlying and overlying glaciogenic deposits i e the makganyene diamictite and a dropstone layer at the base of the hotazel formation respectively the stratigraphic position of the ongeluk formation indicates that the ongeluk volcanism occurred during a period of global glaciation the ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids the hydrothermal quartz contains many primary type 1 and secondary type 2 liquid vapor fluid inclusions as well as inclusions that are randomly distributed without a trace of secondary healed cracks type 3 all these fluid inclusions types were individually analyzed with microthermometry and particle induced x ray emission pixe methods the results show that type 1 fluid inclusions are highly saline whereas type 2 fluid inclusions are relatively less saline type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of type 1 and type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types among the various fluid inclusion salinities of fluid inclusions a wide range in na ca values was identified in the high salinity fluid inclusions i e type 1 and a subset of type 3 which are thought to represent subseafloor fluids circulated by the ongeluk submarine volcanism the wide range in na ca values can be explained by a mixing process between na rich and ca rich fluids associated with albitization of the host basaltic andesites in the albitization process na is removed from the fluids and fixed by the host rocks by contrast ca is released from the host rocks into the fluids pixe analysis also showed two distinct trends i e vertical and horizontal on variation diagram of ca cl versus cu cl and positive correlations between mn cl cu cl zn cl and pb cl these data are best explained by the presence of other mixing processes between 1 a na rich ca and cu poor fluid 2 a ca rich na and cu poor fluid and 3 a na and cu rich ca poor fluid the cu mn zn and pb rich fluid likely represents a high temperature hydrothermal fluid from a deep reaction zone in the ongeluk subseafloor hydrothermal system by contrast the 1 na and k rich ca and cu poor endmember for the high salinity primary inclusions is considered to represent the composition of 2 2 ga ongeluk seawater we propose the estimation of 2 2 ga ongeluk seawater composition is 2237 mmol kg of na 200 1000 mmol kg of k

metasomatic origin of quartz pyroxene rock akilia greenland and implications for earth s earliest life a quartz pyroxene rock interpreted as a banded iron formation bif from the island of akilia southwest greenland contains 13 c depleted graphite that has been claimed as evidence for the oldest 3850 million years ago life on earth field relationships on akilia document multiple intense deformation events that have resulted in parallel transposition of early archean rocks and significant boudinage the tails of which commonly form the banding in the quartz pyroxene rock geochemical data possess distinct characteristics consistent with an ultramafic igneous not bif protolith for this lithology and the adjacent schists later metasomatic silica and iron introduction have merely resulted in a rock that superficially resembles a bif an ultramafic igneous origin invalidates claims that the carbon isotopic composition of graphite inclusions represents evidence for life at the time of crystallization

fluid chemistry of archean seafloor hydrothermal vents implications for the composition of circa 3 2 ga seawater seafloor hydrothermal vents of mid archean age ca 3230 ma have been identified and mapped in the barberton greenstone belt south africa and are known as the ironstone pods fluid inclusion homogenization temperature data when combined with gas chromatographic data provide a minimum calculated water depth for the pods of 982 m ironstone pod hydrothermal fluid endmember concentrations mg 0 of various dissolved components derived from bulk fluid inclusion crush leach experiments include cl 730 mmol l br 2 59 i 0 058 na 822 nh4 11 4 k 21 5 ca 42 6 and sr 0 15 this hydrothermal fluid also contains up to 1 07 mol co2 0 03 mol n2 0 02 mol ch4 262 ppm cos and minor amounts of c2 c4 hydrocarbons hydrothermal endmember ca sr and nh4 in particular and to a lesser degree k i and co2 commonly plot on or very close to modern vent fluid trends by contrast endmember na and br concentrations are distinct higher from modern vent fluids high i and nh4 concentrations are consistent with contributions from sediments and or organic matter calculated 18oh2o values for the pod hydrothermal endmember fluid define a narrow range from 0 9 to 1 6 very similar to that of modern vent fluid values 0 4 2 1 a best estimate for the ironstone pod seawater endmember composition is cl 920 mmol l br 2 25 so4 2 3 i 0 037 na 789 nh4 5 1 k 18 9 mg 50 9 ca 232 and sr 4 52 barberton seawater components are commonly within an order of magnitude of modern seawater values with the exception of significantly higher 1 nh4 ca and sr in the inclusions sulfate concentrations are minimum estimates for barberton seawater fluid inclusion samples containing the greatest amount of seawater component have higher n2 up to 0 1 mol and low co2 when compared to samples dominated by the hydrothermal endmember fluid barberton ambient seawater is considered to have been an evaporative brine of nacl cacl2 composition during the time of pod deposition ironstone pod fluid inclusion seawater endmember br cl and i cl values of 2 45 10 3 and 40 2 10 6 respectively are within error of bulk earth 2 38 10 3 and 190 10 6 and are consistent with the chemistry of 3 23 ga barberton seawater being buffered by the mantle

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

protracted fluid rock interaction in the mesoarchaean and implication for gold mineralization example from the warrawoona syncline pilbara western australia abstract oxygen isotopic and geochemical analyses on whole rock and quartz veins are combined with structural observations in order to constrain the fluid circulation history within the mesoarchaean warrawoona syncline of the north pilbara craton western australia the plumbing system which is the focus of this study is localized in the so called fielding s find shear zone ffsz a km scale shear zone formed during the burial of greenstones and coeval exhumation of granitic complexes this shear zone runs parallel or close to the axial plane of the syncline it involves a prominent quartz vein network and is lined with strongly hydrothermally altered mafic felsic and sedimentary rocks towards the ffsz felsic and mafic volcanic rocks become intensely silicified with an increase in bulk rock 18 o values from 10 8 to 25 1 for altered felsic volcanics and from 7 1 to 18 3 for altered mafic volcanics geochemical modelling ascribes the silicification to a dissolution precipitation process ree and most other trace elements are strongly depleted in the silicified units with the exceptions of elements such as v cr ni and co which are enriched throughout the warrawoona syncline vein quartz 18 o data are within a small range of 13 2 2 significantly lower than their silicified host rocks these data are interpreted as the result of two main paleo fluid circulation stages intense silicification and 18 o enrichment represent alteration driven by low temperature hydrothermal convection probably involving archaean seawater in contrast the quartz veins network is related to the infiltration of metamorphic and or magmatic fluids during a later deformation episode these quartz veins represent the event responsible for the bulk of economic lode gold formation in the area the protracted fluid rock interaction history in the warrawoona syncline may have played a major role in setting the stage for the late mineralizing event the early hydrothermal circulation could have formed an efficient plumbing system characterized by high permeability low reactivity and possibly au enrichment upgrading the au endowment to the late hydrothermal fluids

plateaux ancient and modern geochemical and sedimentological perspectives on archaean oceanic magmatism unequivocal examples of oceanic plateaux older than about 2 1 ga have yet to be identified from the geological record the most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins and their subsequent juxtaposition in accretionary complexes with arc volcanic sedimentary and plutonic rocks a review of cretaceous plateaux in the eastern indian ocean suggests that certain geochemical isotopic criteria e g positive nb anomalies relative to primitive mantle employed by petrologists to identify modern oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basalts the extreme chemical and isotopic heterogeneity of indian ocean plateaux together with large variations in area and volume suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions the other extreme represented by iceland and ontong java is characterised by relatively homogeneous melt compositions reflecting steady state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas crustal thickness and compositional estimates for archaean oceanic plateaux using the mckenzie bickle method give a mean thickness of 43 km and mean mgo content of 19 wt archaean plateaux were thus compositionally distinct from surrounding ocean floor basaltic komatiite and notably more magnesian than plateaux such as iceland mean for tertiary basalts 6 3 wt mgo and ontong java mean for core samples 6 9 wt mgo by analogy with cretaceous tertiary ultramafic complexes in western colombia the deeper portions of these plateaux may have consisted of noritic rocks underlain by iherzolite pyroxenite gabbronorite and dunite the general absence of such rocks in archaean terrains suggests that these portions of plateaux are only rarely preserved the search for remnants of ancient oceanic plateaux has identified examples of archaean submarine lavas in the canadian shield 2 7 ga southern abitibi belt superior province and kaapvaal craton of southern africa 3 5 3 3 ga barberton and pietersberg belts chemical and isotopic data suggest that komatiite tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source but do not distinguish adequately the tectonic environment rifted continental margin ocean floor oceanic plateau in which these rocks were erupted however the uplift histories of these putative plateau fragments as recorded in sediments bounding and intercalated with the komatiite basalt sequences appear to be broadly comparable to those of the best documented phanerozoic oceanic plateaux although further stratigraphical investigations of presumed plateaux ancient and modern are required to test this hypothesis we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the mid archaean 3 5 ga

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

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

early archaean spherule beds in the barberton mountain land south africa no evidence for impact origin abstract detailed petrographical and geochemical studies are reported for several well documented occurrences of spherule rich layers in the barberton mountain land south africa which were previously inferred by lowe and co workers to be the result of early archaean meteorite impacts in contrast we made the following observations the textures of most spherules are not necessarily the result of impact but could instead be the product of radial or intersertal growth of crystals during secondary mineral formation there is no difference in the content of siderophile elements between spherule layers and layers devoid of spherules the siderophile element abundances are high where sulphide minerals e g pyrite gersdorffite and chalcopyrite and or chromite are present independent of the presence or absence of spherules high contents of e g ir up to 2700 ppb ni 0 96 wt and cr 1 6 wt were found in various samples abundances of these elements in chondritic meteorites are approximately 600 ppb 1 4 wt and 0 35 wt respectively resulting in respective meteoritic components in these samples of 450 70 and 460 we do not accept these high concentrations as primary meteoritic signatures impact melt rocks have typically 1 of a meteoritic component furthermore spherules of any kind are very rare in known impact deposits including the k t boundary if present at all they are not usually associated with any significant ir or pge anomaly the high abundances of the siderophile elements in some barberton samples and their enrichment in secondary minerals indicate that these elements have been remobilized and reconcentrated the pge interelement ratios have changed during remobilization as well thus the pge abundance patterns and ratios are not primary and cannot be used as an argument in favour of an impact origin either nickel rich cr spinels which are found in some of the barberton spherule samples have low fe 3 fe total ratios these ratios are incompatible with an extraterrestrial or impact origin because such impact derived spinels including those found at the k t boundary are highly oxidized and also have different chemical compositions there is also no indication of any evidence of shock metamorphism associated with the barberton spherule layers which is the commonly accepted definitive criterion for recognition of an impact origin this is unusual because such evidence is preserved even in heavily altered samples from deeply eroded archaean impact structures we conclude that while these spherule layers are clearly unusual and deserve further attention there is no convincing evidence for an origin by impact we suggest that they were formed by volcanic processes followed by extensive hydrothermal alteration

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

ironstone pods in the archean barberton greenstone belt south africa earth s oldest seafloor hydrothermal vents reinterpreted as quaternary subaerial springs bodies of iron oxide to 200 m across in the 3 5 3 2 ga barberton greenstone belt south africa have been interpreted as deposits of archean seafloor hydrothermal vents and have provided what are arguably key observations about surface environments on early earth these bodies termed ironstone pods have yielded what are putatively the oldest known complex organic compounds and have been used to estimate archean surface temperature ocean depth and volume and seawater composition and to deduce relationships between hydrothermal activity and seafloor sedimentation however the pods are composed largely of goethite a thermally unstable hydrated iron oxide mineral show abundant still open primary porosity are undeformed in a terrane of highly deformed rocks and show bedding and cavity filling dripstone indicating formation with their present orientation they are here interpreted to have formed as spring and shallow subsurface deposits of young quaternary groundwater and or low temperature hydrothermal systems iron was derived by the dissolution of siderite in surrounding archean sedimentary rocks along pod margins goethite locally replaces archean chert and silicified ultramafic rock however coarse vein and cavity fill quartz was less susceptible to replacement and remains as disrupted masses and rare remnant veins in the ironstone this quartz is of archean age but contains no more information about the archean surface environment than other late hydrothermal veins throughout the barberton greenstone belt rocks the presence of a well preserved modern iron oxide spring terrace confirms that these are deposits of young subaerial springs and contain no record of archean life or environments they do however provide a unique view of a remarkable young iron depositing hydrologic system

early archean fossil bacteria and biofilms in hydrothermally influenced sediments from the barberton greenstone belt south africa abstract sem imaging of hf etched 3 3 3 5 ga cherts from the onverwacht group south africa reveals small spherical 1 m diameter and rod shaped structures 2 3 8 m in length which are interpreted as probable fossil coccoid and bacillar bacteria prokaryotes respectively preserved by mineral replacement other possibly biogenic structures include smaller rod shaped bacteriomorphs 900 m textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature 220 c hydrothermal solutions

early archaean hydrothermal fluids a study of inclusions from the 3 4 ga buck ridge chert barberton greenstone belt south africa abstract fluid inclusions have been studied from cavities and breccias in one of the oldest well preserved relatively low grade metamorphosed sedimentary sequences on earth the fluid inclusion bearing samples are from the uppermost part of the early archaean 3 4 ga buck ridge volcano sedimentary complex in the barberton greenstone belt south africa which was affected by hydrothermal alteration during and immediately after its deposition in banded silicified sandy sediments this alteration is characterised among other features by quartz cemented hydrothermal breccias layer bound stockworks of cm size quartz veins and quartz filled cavities below stacked pebbles quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two phase aqueous or mixed h 2 o co 2 fluid the aqueous inclusions are divided into three different groups based on their microthermometric characteristics type h a relatively high salinity type m a intermediate salinity and type l a relatively low salinity optical and microthermometric characteristics of the fourth type of inclusions the mixed h 2 o co 2 fluid type c indicate that it was homogeneous at the time of trapping with only some minor changes occurring after trapping type h a and type c fluids appear as immiscible fluids in their host crystal most probably resulting from unmixing of the hydrothermal fluid phase relations in the co 2 h 2 o system show that the unmixing must have occurred at a relatively low pressure of 100 bars this conclusion is in line with the geological context of the samples which indicates a shallow environment of deposition for the sediments almost contemporaneous hydrothermal activity and a very low metamorphic grade however it is contrary to the first impression given by the homogeneous character of type c inclusions which suggests much higher p t conditions at the time of trapping the aqueous component of type c fluids may have progressively mixed with high salinity aqueous fluids type h a resulting in a dilution trend from h a to m a to l a fluids the hydrothermal fluids from the buck ridge chert in the barberton greenstone belt are strikingly similar to those encountered in present day systems they show minor differences with older ca 3 8 ga fluids preserved in hydrothermally altered rocks from the isua greenstone belt in greenland

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability