paleo to mesoarchean ttg accretion and continental growth in the western dharwar craton southern india constraints from shrimp u pb zircon geochronology whole rock geochemistry and nd sr isotopes a multidisciplinary study involving field petrographic shrimp u pb zircon titanite ages whole rock geochemical and nd sr isotope data is presented for the peninsular gneisses and associated plutons forming core of the dharwar craton two major periods 3350 3280 ma and 3230 3200 ma of crustal growth through ttg accretion sub contemporaneous with greenstone volcanism are documented elemental and nd sr isotope data suggest that the ttgs originated by low and high pressure melting of heterogeneous mafic sources thickened island arc or oceanic plateau crust at different depths among the early accreted ttgs magmatic protoliths of low al gneisses formed by low pressure 10 12 kbar melting of a depleted mafic source at shallow levels island arc type crust with plagioclase amphibole ilmenite in the residue whilst high al gneisses were probably produced by high pressure melting 14 18 kbar of a chondritic or a less depleted mafic source with garnet amphibole pyroxene ilmenite in the residue base of island arc crust or thickened oceanic plateau the 3230 3200 ma trondhjemitic plutons marking the second accretion event derived by higher pressure 14 16 kbar melting of a mafic source island arc type crust with garnet plagiocalse hornblende ilmenite with a minor involvement of previously accreted ttgs this plutonism was coeval with a major phase of crustal scale diapiric overturn that had a major imprint on crustal reworking a later magmatic episode at ca 3100 ma produced granitic plutons derived from crustal melting and leucocratic gneisses derived from a depleted mafic source in the lower crust documentation of depleted shallow and deep mantle reservoirs during 3350 3280 ma magmatism implies massive differentiation of the mantle during an earlier episode of crust formation around 3800 600 ma as substantiated by nd model ages and published pb isotope data as well as u pb detrital zircon ages with in situ hf isotope data

emergence of modern continental crust about 3 billion years ago the continental crust provides a record of earth s evolution analysis of the geochemical signature of continental crust formed since the hadean points to the initiation of plate tectonics about 3 billion years ago

an exotic mesoarchean microcontinent the coorg block southern india abstract sandwiched between the dharwar craton in the north and the neoarchean proterozoic crustal blocks to the south the coorg block in southern india is composed dominantly of a suite of arc magmatic rocks including charnockites ttg tonalite trondhjemite granodiorite related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block here we present the petrology geochemistry zircon u pb geochronology and lu hf isotopes of all the major lithologies from this block computation of metamorphic p t conditions from mineral chemical data shows consistent granulite facies p t conditions of 820 870 c and up to 6 kbar our geochemical data from major trace and ree on representative samples of the dominant rock types from the coorg block corroborate an arc related signature with magma generation in a convergent margin setting the zircon data yield weighted mean 207pb 206pb ages of 3153 4 9 to 3184 0 5 5 ma for syenogranites 3170 3 6 8 ma for biotite granite 3275 5 1 ma for trondhjemite 3133 12 to 3163 8 6 9 ma for charnockites 3156 10 to 3158 3 8 2 for mafic enclaves 3161 16 ma for diorite and 3173 16 ma for felsic volcanic tuff an upper intercept age of 3363 59 ma and a lower intercept age of 2896 130 ma on zircons from a charnockite as well as an evaluation of the th u values of the zircon domains against respective 207pb 206pb ages suggest that the mesoarchean magma emplacement which probably ranged from 3 3 to 3 1 ga was immediately followed by metamorphism at ca 3 0 to 2 9 ga the ages of magmatic zircons from the charnockites and their mafic granulite enclaves as well as those from the volcanic tuff and biotite granite are all remarkably consistent and concordant marking ca 3 1 ga as the peak of subduction related crust building in this block within the tectonic milieu of an active convergent margin the majority of zircons from the coorg rocks show hf isotope features typical of crystallization from magmas derived from juvenile sources their hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga the crustal blocks in the southern granulite terrane in india preserve strong imprints of major tectonothermal events at 2 5 ga 2 0 ga 0 8 ga and 0 55 ga associated with various subduction accretion collision or rifting events however the coorg block is exceptional with our data suggesting that none of the above events affected this block importantly there is also no record in the coorg block for the 2 5 ga pervasive regional metamorphism that affected all the other blocks in this region the geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly neoarchean collage in the northern domain of the southern granulite terrane of india the mesoarchean arc related rocks in the coorg block suggest that the magma factories and their tectonic architecture in the early earth were not markedly different from those associated with the modern style plate tectonics

quantifying crustal thickness variations in evolving orogens correlation between arc basalt composition and moho depth a compilation of geochemical data from 50 active volcanoes where seismically defined moho depth measurements are available representing most volcanic arcs around the world indicates a relation exists between crustal thickness moho depth and the trace element composition of basalts the best correlation exists for maximum light heavy rare earth element ree ratios and we use maximum ce y r2 0 90 to show the exponential function with increasing depth to 50 km application to new zealand part of the eastern gondwanan margin through much of the phanerozoic shows that higher ce y ratios in arc basalts correspond to two periods of recognized crustal thickening plus an additional event at ca 240 ma which links new zealand to the gondwanide orogeny conversely low ce y ratios in basalts correlate with periods of recognized extension in eastern gondwana very high ce y ratios in ca 120 ma rocks imply an unusually thickened 50 km arc crust consistent with independent geological estimates of crustal thickness at that time the correlation shows that crustal thickness variations during orogenesis can be conservatively estimated to within 3 km

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

geochemical and isotopic study to determine sources and processes affecting nitrate and sulphate in groundwater influenced by intensive human activity carbonate aquifer gliwice southern poland abstract a multi species multi stable isotope approach 15nno3 18ono3 34sso4 18oso4 18oh2o and 2hh2o was used together with environmental tracers ar ne cfc 11 and cfc 12 and geochemical modelling to characterize sources and processes controlling concentrations of no3 and so42 in groundwater of the carbonate aquifer gliwice southern poland the study area represents a strongly transformed environment with a range of human activities i a agriculture urbanization and industry the 15nno3 and 18ono3 indicated that most samples contained no3 of mixed sources artificial fertilizers municipal and industrial sewage while very good correlation between no3 and cfc 12 suggested that nitrate originated primarily from residential and industrial sewage conversely isotopic composition of sulphate in groundwater suggested agriculture as well as oxidation of sulphides as dominant the conclusion was supported by the comparison of cfcs and sulphate concentrations which revealed no relevant correlation geochemical modelling confirmed the presence of sulphate reduction in areas where isotopic analyses were not possible due to undetectable sulphate thus the integrated application of stable isotopes environmental tracers groundwater chemistry and geochemical modelling shows a complex origin of groundwater pollution in the study area as well as variety of geochemical processes controlling chemistry of groundwater in a triple porosity aquifer influenced by different types of human activity

sulfur and carbon isotopes in archean banded iron formations implications for sulfur sources abstract sulfur isotope ratios of pyrite from two late archean banded iron formations were used to constrain possible sources of sulfur the iron formations studied were the temagami iron formation which hosts the sherman mine near temagami and the boston iron formation including the adams mine south of kirkland lake both in ontario in both cases algomantype oxide facies magnetite chert ores predominate the sulfur studies were supplemented by limited carbon isotopic data for the sherman mine iron formation in the sherman mine occurs on both limbs of a regional synformal structure each iron formation unit caps a thick calc alkaline andesite dominated volcanic sequence with one band hosted in sedimentary rocks of turbidite origin the adams mine includes four major iron formation bands interbedded with mafic and ultramafic volcanic rocks and greywacke of turbidite origin the sulfur isotope statistics for averages of pyrites from these three units are sherman mine volcanic hosted suite 34 s 1 7 4 6 n 14 sherman mine sediment hosted suite 34 s 1 4 1 6 n 43 and adams mine 34 s 1 8 2 9 n 17 these averages differ by from juvenile sulfur and together with their limited dispersions suggest a predominantly unfractionated source of sulfur it is argued that this sulfur has been derived from the associated volcanic piles most of the pyrite occurs as disseminated grains in oxide facies iron formation and interbedded clastic sediments a distinctive nodular pyrite in the sherman mine sediment hosted suite occurs in cherty graphitic sulfide facies iron formation while the sulfur isotopic properties of this subset 34 s 0 0 0 8 are not suggestive of biogenic sulfur the geological setting and carbon isotopic compositions of associated graphite 13 c 34 imply that anaerobic bacterial processes were active in this reducing environment the apparent lack of biogenic sulfide is rationalized by models suggesting that archean marine basins differed from phanerozoic analogues in various parameters which influence sulfur isotopic characteristics it is argued that biogenic sulfide is probably a significant component of the sulfur in sherman mine nodular pyrite

mesoarchean convergent margin processes and crustal evolution petrologic geochemical and zircon u pb and lu hf data from the mercara suture zone southern india abstract the mercara shear zone is sandwiched between the western dharwar craton and the coorg block in the southern granulite terrain of india and is marked by steep gravity gradients interpreted to suggest the presence of underplated high density material in the lower crust here we present geological petrological and geochemical data together with zircon u pb ages and lu hf isotopes from a suite of metaigneous ttg related gneisses charnockite metagabbro mafic granulite and metasedimentary quartz mica schist khondalite garnet biotite gneiss kyanite sillimanite bearing metapelite rocks from this zone geochemical data on the magmatic suite suggests formation through subduction related arc magmatism whereas the metasediments represent volcano sedimentary trench sequences phase equilibrium modeling of mafic granulites from the mercara shear zone suggests p t range of 10 12 kbar at 700 c to 900 c the zircon data yield weighted mean 207pb 206pb ages of 3229 80 ma for metagabbro 3168 25 ma for the charnockite and 3181 20 ma for the mafic granulite ages ranging from 3248 28 ma to 3506 26 ma were obtained from zircons in the kyanite sillimanite bearing metapelite 3335 44 ma from khondalite 3135 14 ma from garnet biotite gneiss 3145 17 ma to 3292 57 ma from quartz mica schist and 3153 15 ma to 3252 36 from ttg gneiss the tightly defined ages of 3 1 to 3 2 ga from igneous zircons in the magmatic suite suggest prominent mesoarchean convergent margin magmatism the timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca 3 0 ga which might mark the collisional event between the western dharwar craton and the coorg block hf isotope features suggest magma derivation mostly from juvenile sources and the lu hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga our study defines the mercara shear zone as a terrane boundary and possible mesoarchean suture along which the coorg block was accreted to the western dharwar craton the accretion of these continental fragments might have coincided with the birth of the oldest supercontinent ur

oldest rocks from peninsular india evidence for hadean to neoarchean crustal evolution abstract the evolution of continental crust during hadean and archean and related geodynamic processes provides important clues to understand the early earth history here we report evidence for hadean and eoarchean crust from the fringe of the coorg block one of the oldest crustal blocks in peninsular india we present geological petrological and geochemical data together with zircon u pb ages and lu hf isotopes from a suite of metaigneous granitoids diorite charnockite metavolcanics and metasedimentary quartz mica schist calcareous schist ferruginous quartzite and bif rocks petrological and geochemical studies indicate that the igneous suite formed from subduction related arc magmatism and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components mineral thermometry suggests metamorphism under temperatures of 710 730 c and pressures up to 8 kbar magmatic zircons in the metaigneous suite show oscillatory zoning and high th u contents up to 3 72 and record multiple pulses of magmatism at ca 3 5 3 2 2 7 and 2 5 2 4 ga the metasedimentary rocks accreted along the margins of the coorg block show multiple zircon population with mean 207pb 206pb ages at 3 4 3 2 3 1 2 9 2 7 2 6 2 5 2 2 2 0 and 1 3 ga and overprinted by younger thermal event at ca 0 8 0 7 ga zircons in the 3 5 ga dioritic gneiss show positive ehf t values ranging from 0 0 to 4 2 and hf crustal model ages tdmc of 3517 to 3658 ma suggesting that the parent magma was derived from eoarchean juvenile sources the zircons in the 3 2 ga charnockite display ehf t values in the range of 3 0 to 2 9 and hf crustal model ages tdmc of 3345 to 3699 ma the neoarchean metagranites diorites and felsic tuff show both positive and negative ehf t values and a range of tdmc values from 2904 to 3609 ma suggesting magma derivation from meso to eoarchean juvenile and reworked components the tdmc values of the dominant zircon population in the metasedimentary suite range from 3126 to 3786 ma with the oldest value 4031 ma recorded by zircon grain in a ferruginous quartzite the ehf t values of detrital zircons also show both positive and negative values with a dominant crustal source our zircon u pb and lu hf data suggest vestiges of neohadean primordial continental crust in peninsular india with episodic crustal growth during eoarchean mesoarchean and neoarchean building the continental nuclei the results contribute to the understanding of crustal evolution in the early history of the earth

hydrochemical evidence of the depth of penetration of anthropogenic recharge in sandstone aquifers underlying two mature cities in the uk pollution of urban groundwater is routinely reported but the profile of contamination with depth in urban aquifers is rarely resolved this limits understanding of the depth of penetration of urban recharge and contaminants and use of urban groundwater penetration of anthropogenic solutes major ions trace metals in permo triassic sandstone aquifers underlying two mature conurbations in the uk was investigated through depth specific groundwater sampling of dedicated multilevel piezometers identification of solute origin and biogeochemical processes e g denitrification mineral dissolution was aided by use of stable isotope ratios 34 s 32 s 18 o 16 o 15 n 14 n 13 c 12 c and chemical speciation modelling phreeqc depth profiles of aquifer hydrochemistry reveal penetration of anthropogenic solutes to depths of between 30 and 47 m below ground in the unconfined sandstone and confirm the contributions of faecal and industrial effluents to urban recharge they also highlight the complexity of solute loading and difficulty resolving solute origin from the range of potential sources in urban groundwater faecally derived no3 is the most pervasive contaminant exceeding drinkingwater quality guidelines and is associated with elevated concentrations of b and so4 elevated concentrations of li b cr and co are observed at depth in groundwater contaminated by long term industrial land use metalworking observed penetration of anthropogenic solutes in the unconfined sandstone is consistent with post development recharge of urban groundwater residence times 230 a indicated by flow modelling and suggests tentatively that urban abstraction to depths of up to 50 m below ground in the unconfined permo triassic sandstone is required to scavenge contaminated groundwater

using isotope fractionation of nitrate nitrogen and nitrate oxygen for evaluation of microbial denitrification in a sandy aquifer the nitrate input into the groundwater from forest and agricultural land in the fuhrberger feld catchment area is virtually completely degraded by microbial denitrification in the aquifer five multilevel groundwater sampling wells show characteristic depth functions of nitrate proving the denitrification process a series of 10 samples of arable land groundwater was taken from different depths at these multilevel wells and isotope ratios of 15n14n and 18o16o in the nitrate were measured the sampling was based on the two dimensional vertical plane groundwater flownet and on the knowledge of the landuse upgradient of the multilevel wells high nitrate concentrations are associated with low 15n and low 18o values and vice versa the 15n 18o ratio indicates a linear increase with a slope of 1 2 1 the microbial dentrification accords with a rayleigh process with calculated enrichment factors of e 15 9 for 15n and e 8 0 for 18o three samples taken from groundwater recharged under coniferous forest show deviating characteristics low nitrate concentrations with low 15n values 1 4 but very high 18o values 40 50 using the previously quantified enrichment factors for 15n and 18o an attempt was made to ascertain the significance of denitrification in specific depth sections at two other multilevel wells 7 samples in two cases quantitative assessments on the degree of nitrate concentration decrease owing to denitrification were possible however in other cases the specific nitrate isotope fractionation was masked by the variability of nitrate concentration and of nitrate isotope ratios in the corresponding groundwater recharge preventing definite interpretations

entrainment threshold of natural grains in liquids determined empirically from dimensionless settling velocities and other measures of grain size abstract grain size plays a crucial role in sediment transport but different methods of size analysis do not yield comparable results particle diameters obtained by settling tube for example cannot be employed in equations based on sieve sizes although the majority of experiments on the initiation of sediment motion has been based on sieve data settling tube analysis is probably superior as both the settling rate and the entrainment threshold of grains are determined by the same factors of size shape and density in this paper empirical equations are provided which allow the determination of a non dimensional critical shear stress directly from the equivalent sedimentation diameter the equations apply to the entrainment of well sorted grains with a high sphericity on flat sediment beds and are valid for particles and liquids of different densities

continental emergence and growth on a cooling earth isostasy considerations are connected to a 1 d model of mantle diverentiation due to pressure release partial melting to obtain a model for the evolution of the relative sea level with respect to the continent during the earth secular cooling in this context a new mechanism is derived for the selective exhumation of exposed ancient cratons the model results in a quantitative scenario for sea level fall due to the changing thicknesses of the oceanic basaltic crust and its harzburgite residual layer as a function of falling mantle temperature it is also shown that the buoyancy of the harzburgite root of a stabilized continental craton has an important evect on sea level and on the isostatic readjustment and exhumation of exposed continental surface during the earth s secular cooling the model does not depend on the usual assumption of constant continental freeboard and crustal thickness and its application is not restricted to the post archaean it predicts large scale continental emergence near the end of the archaean and the early proterozoic this provides an explanation for reported late archaean emergence and the subsequent formation of late archaean cratonic platforms and early proterozoic sedimentary basins for a period of secular cooling of 3 8 ga corresponding to the length of the geological record the model predicts a fall of the ocean floor of some 4 km or more for a constant ocean depth this implies a sea level fall of the same magnitude a formula is derived that allows for an increasing ocean depth due to either the changing ratio of continental with respect to oceanic area or to a possible increase of the oceanic volume during the geological history increasing ocean depth results in a later emergence of submarine ancient geological formations compared to the case when ocean depth is constant selective exhumation is studied for the case of constant ocean depth it is shown that for this case early exposed continental crust can be exhumed to a lower crustal depth which explains the relative vertical displacement of low grade with respect to high grade terrain increasing ocean depth is not expected to result in diminished exhumation 2000 elsevier science b v all rights reserved

deducing the distribution of terminal electron accepting processes in hydrologically diverse groundwater systems the distribution of microbially mediated terminal electron accepting processes teaps was investigated in four hydrologically diverse groundwater systems by considering patterns of electron acceptor nitrate sulfate consumption intermediate product hydrogen h sub 2 concentrations and final product ferrous ion sulfide and methane production in each hydrologic system a determination of predominant teaps could be arrived at but the level of confidence appropriate for each determination differed in a portion of the lacustrine aquifer of the san joaquin valley for example all three indicators sulfate concentrations decreasing h sub 2 concentrations in the 1 2 nmol range and sulfide concentrations increasing along flow paths identified sulfate reduction as the predominant teap leading to a high degree of confidence in the determination in portions of the floridan aquifer and a petroleum hydrocarbon contaminated aquifer sulfate reduction and methanogenesis are indicated by production of sulfide and methane and hydrogen concentrations in the 1 4 nmol and 5 14 nmol range respectively however because electron acceptor consumption could not be documented in these systems less confidence is warranted in the teap determination in the black creek aquifer no pattern of sulfate consumption and sulfide production were observed but h sub 2 concentrations indicated sulfate reduction as the predominant teap in this case more where just a single line of evidence is available the least confidence in the teap diagnosis is justified because this methodology is based on measurable water chemistry parameters and upon the physiology of microbial electron transfer processes it provides a better description of predominant redox processes in groundwater systems than more traditional eh based methods less

sulfur and oxygen isotopes in italian marine sulfates of permian and triassic ages evaporitic sulfates of permian and triassic ages from italian occurrences were analyzed for sulfur and oxygen isotopes in order to supplement the corresponding isotope age curves and to use isotopic data to trace the origin of the sulfate in hydrologic systems the sulfur isotopic composition age relationships obtained in this study generally agree with those found throughout the world in two cases i e in samples from the carniola di bovegno formation bergamo and brescia alps and from the lower part of the gracilis formation recoaro vicenza sulfur isotopes suggest different ages for the rocks than the formerly ascribed ones oxygen isotopic compositions show the same range from 10 5 to 18 5 without any consistent relationship with sulfur these ranges of values are almost identical to those published previously for sulfates of permian and triassic ages from other locations

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

15n isotope biogeochemistry and natural denitrification process in groundwater application to the chalk aquifer of northern france the use of 15n natural isotope tracing in an aquifer contained within chalk rocks in northern france indicates that under certain hydrogeological conditions major denitrification occurs at the boundary where the aquifer becomes confined the nitrate concentrations decrease in the direction of groundwater flow accompanied by an exponential increase in 15n expressed in 15n of the residual nitrate this is characteristic of kinetic isotope effects which accompany the reduction of the nitrate ion during denitrification hydrogeochemical and bacteriological observations confirm this process natural isotope tracing also permits this process to be distinguished from local dilution with nitrate free surface water which would entail a major drop in nitrate values without 15n isotopic enrichment a model is proposed to explain the relatively small observed magnitude of the isotopic fractionation effect

controls on δ 34 s and δ 18 o of dissolved sulfate in aquifers of the murray basin australia and their use as indicators of flow processes the usefulness of stable isotopes of dissolved so4 34s and 18o to study recharge processes and to identify areas of significant inter aquifer mixing was evaluated in a large semi arid groundwater basin in south eastern australia the murray basin the distinct isotopic signatures in the oxidizing unconfined murray group aquifer and the deeper reducing renmark group confined aquifer may be more sensitive than conventional chemical tracers in establishing aquifer connections 34s values in the unconfined murray group aquifer in the south and central part of the study area decrease along the hydraulic gradient from 20 8 to 0 3 the concomitant increasing so4 cl ratios as well as relatively low 18oso4 values suggest that vertical input of biogenically derived so4 via diffuse recharge is the predominant source of dissolved so4 to the aquifer further along the hydraulic gradient towards the discharge area near the river murray 34s values in the unconfined murray group aquifer increase and so4 cl ratios decrease due to upward leakage of waters from the confined renmark group aquifer which has a distinctly low so4 cl and high 34s 14 9 56 4 relatively positive 34s and 18oso4 values and low so4 cl in the renmark group aquifer is typical of so4 removal by bacterial reduction the s isotope fractionation between so4 and hs of 24 estimated for the confined aquifer is similar to the experimentally determined chemical fractionation factor for the reduction process but much lower than the equilibrium fractionation 70 even though the confined groundwater residence time is 300 ka years mapping the spatial distribution of 34s and so4 cl of the unconfined murray group aquifer provides an indicative tool for identifying the approximate extent of mixing however the poorly defined end member isotopic signatures precludes quantitative estimates of mixing fractions

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

multi isotopic study 15n 34s 18o 13c to identify processes affecting nitrate and sulfate in response to local and regional groundwater mixing in a large scale flow system the integrated use of hydrogeologic and multi isotopic approaches 15n 18ono3 34s 18oso4 and 13chco3 was applied in the selva basin area ne spain to characterize no3 and so42 sources and to evaluate which geochemical processes affect no3 in groundwater the studied basin is within a basin and range physiographic province where natural hydrodynamics have been modified and different scale flow systems converge as a consequence of recent groundwater development and exploitation rates as a result groundwaters related to the local recharge flow system affected by anthropogenic activities and to the generally deeper regional flow system recharged from the surrounding ranges undergo mixing processes the 15n 18ono3 and 34s indicated that the predominant sources of contamination in the basin are pig manure and synthetic fertilizers hydrochemical data along with 15n 18ono3 34s 18oso4 and 13chco3 of some wells confirmed mixing between regional and local flow systems apart from dilution processes that can contribute to the decrease of no3 concentrations the positive correlation between 15n and 18ono3 agreed with the occurrence of denitrification processes the 34s and 18oso4 indicated that pyrite oxidation is not linked to denitrification and 13chco3 did not clearly point to a role of organic matter as an electron donor therefore it is proposed that the mixing processes between deeper regional and local surface groundwater allow denitrification to occur due to the reducing conditions of the regional groundwater thus isotopic data add useful complementary information to hydrochemical studies especially in those areas where hydrochemical data is not conclusive

application of multi isotope data o d c and s to quantify redox processes in urban groundwater abstract the different recharge sources and their mixing ratios were investigated in water samples from an urban aquifer two distinct zones from barcelona city were compared 1 poble sec and 2 besos river delta which are located in a densely populated area with a strong industrial impact in this study 106 water samples were collected from july 2007 to may 2010 for hydrochemical and isotopic characterisation the application of environmental isotopes coupled with hydrochemistry provided the necessary information to isotopically quantify groundwater recharge sources and evaluate the occurrence of redox processes in the besos river delta a decrease in dissolved so 4 2 concentration and an increase in 34sso4 and 18oso4 values were observed in groundwater samples both indicating so 4 2 reduction moreover other chemical indicators supported a reducing environment such as low or null levels of dissolved o2 and no 3 the presence of nh 4 and an increase in dissolved fe and as the reducing conditions were probably induced by the organic c dissolved in water infiltrating from the river besos in poble sec the relationship between 34sso4 and 18oso4 showed a strong influence of sewage water infiltration into the aquifer however the aquifer is oxic and there is no influence of so 4 2 reduction

neoarchean lithospheric strengthening and the coupling of earth s geochemical reservoirs this paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence we show that until the neoarchean continents were unable to sustain topography 2500 m during the neoarchean the continental lithosphere evolved through a rheological threshold allowing for the development of signifi cant topography the consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the earth s geochemical reservoirs the neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values we propose that during the neoarchean the exogenic envelopes which were until then coupled to the mantle through hydrothermal processes and volcanism also became coupled to the continental crust through reliefgenerating tectonics processes and erosion hence changing the balance between mantle versus crustal interaction with the exogenic earth

crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks new compilations of global o isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in 18o values in both systems since the late archean this is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks each of these factors can arguably be achieved by increased crustal thickening in the late archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks despite the suggested change in tectonic regimes in the late archean stochastic modelling in this study demonstrates that 18o data do not provide evidence for a secular decrease in the proportion of mantle derived magmas in granitoid rocks instead best fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term 100 200 myr tectonic cycles preserved in the continental crust hence major step changes in global tectonic regimes in the post hadean such as the initiation of subduction in the mid to late archean are not supported by global zircon o isotope datasets and instead minor progressive changes are indicated for earth s tectonic regimes

a case for late archaean continental emergence from thermal evolution models and hypsometry abstract the secular cooling of the earth s mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans in the oceanic bathymetry and in the area of emerged continental crust the evolution of these variables is of fundamental importance to the geochemical coupling of mantle continental crust atmosphere and ocean to explore this further we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature continental area and hypsometry in this paper we investigate the continental freeboard predicted using different models for the cooling of the earth we show that constancy of the continental freeboard 200 m is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110 210 c hotter than present such numbers imply either a very limited cooling of the planet or most likely a change in continental freeboard since the archaean during the archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere thus limiting crustal thickening due to mountain building processes and the maximum elevation in the earth s topography rey p f coltice n neoarchean strengthening of the lithosphere and the coupling of the earth s geochemical reservoirs geology 36 635 638 2008 taking this into account we show that the continents were mostly flooded until the end of the archaean and that only 2 3 of the earth s area consisted of emerged continental crust by around 2 5 ga these results are consistent with widespread archaean submarine continental flood basalts and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from 2 5 ga the progressive emergence of the continents as shown by our models from the late archaean onward had major implications for the earth s environment particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the earth s deep and surface reservoirs

the lower crust of the dharwar craton southern india patchwork of archean granulitic domains u pb geochronological major and trace element and isotope geochemical data identify crustal domains of contrasted petrological origins and histories in the lower crust of the dharwar craton all affected by granulite facies metamorphism and deformation in the neoarchean four lower crustal domains have been identified and investigated across the neoarchean amphibolite to granulite facies transition zone 1 in the southern part of the western dharwar craton wdc high grade metamorphism affects rocks formed before 3 0 ga high grade metamorphism is recorded at ca 2 51 ga followed by possible complex cooling and resetting until 2 2 ga 2 the br hills mm hills domain represents the lower crust of the central dharwar province cdp which is here defined as a transitional crust between the old 3 ga wdc and the eastern dharwar province edp not older than 2 7 ga this granulitic domain contains the oldest rocks of the dharwar craton with ttg suites formed at ca 3 4 ga and ca 2 6 ga suites of probable crustal recycling origin both suites are involved in 2513 5ma granulite facies metamorphism 3 the nilgiri granulitic domain is tectonically juxtaposed with the deepest level of the wdc as a consequence of neoarchean dextral slip along the moyar shear zone the massif consists of rocks of orogenic affinities formed between 2 70 and 2 56 ga and overprinted by granulite facies metamorphism at 2518 10 ma 4 the krishnagiri domain exposes the deepest levels of the eastern dharwar craton it is composed of ttg and calc alkaline igneous protoliths formed between 2 55 and 2 53 ga migmatization in the upper part of the domain took place at 2507 6ma and granulite facies overprint in the deepest parts of the domain is constrained at 2516 5 ma the krishnagiri domain was magmatically accreted to the lower crust of both the cdp and edp by interaction with mafic magmatism inducing reworking of the 2 7 2 6 ga or older overlying crust the nilgiri granulites share several temporal and petrological properties with the madras granulites that make the easternmost outcrop of the dharwar craton we tentatively suggest that the nilgiri and madras granulites evolved as parts of a common madras domain in the lower crust of the edp before activation of the moyar shear zone in this sense the nilgiri massif may be considered as an allochtonous unit in a context of neoarchean ultra hot orogeny in south india

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

isotopic composition of sulfate as a tracer of natural and anthropogenic influences on groundwater geochemistry in an urban sandstone aquifer birmingham uk abstract development and management of urban groundwater resources is limited in practice by a lack of knowledge of the nature distribution and sources of groundwater contamination in an urban aquifer that may have had complex land use and abstraction history sulfate is associated with many of the solute sources to urban groundwater and in this study the use of dual isotope 18o and 34s fingerprinting of so 4 2 in groundwater to assess the contribution of different sources to urban groundwater is investigated groundwater 70 locations and surface water samples from the city of birmingham on the triassic sherwood sandstone aquifer have been analyzed for inorganic chemical and so 4 2 isotopic composition isotopic compositions of so 4 2 associated with various solute sources have also been determined sulfate derived from pyrite oxidation during recharge through quaternary drift deposits is characteristically depleted in 18o and 34s compared to other sources and is ubiquitous in unconfined zone groundwater though the contribution from this source has increased markedly following draw down induced by abstraction from the aquifer in unpolluted groundwaters other so 4 2 sources are dissolution of evaporite minerals confined zone and rainfall unconfined zone unfortunately so 4 2 isotopic composition cannot distinguish between made ground i e artificial man made ground usually demolition waste and sewage sources of so 4 2 which constitute the major contributions to so 4 2 in polluted urban groundwater at most sites contributions of so 4 2 from spilt industrial acids do have a distinctive isotopic composition seen at many metal working and former metal working sites estimates of solute contributions derived from these sources derived in this way provide a useful check on the calibration of aquifer and pollutant flux recharge models for urban groundwater

oxygen isotope perspective on crustal evolution on early earth a record of precambrian shales with emphasis on paleoproterozoic glaciations and great oxygenation event abstract we present stable isotope and chemical data for 206 precambrian bulk shale and tillite samples that were collected mostly from drillholes on all continents and span the age range from 0 5 to 3 5 ga with a dense coverage for 2 5 2 2 ga time interval when earth experienced four snowball earth glaciations and the irreversible rise in atmospheric o2 we observe significant downward shift of several and a smaller range of 18 o values 7 to 9 in shales that are associated with the paleoproterozoic and potentially neoproterozoic glaciations the paleoproterozoic samples consist of more than 50 mica minerals and have equal or higher chemical index of alteration than overlying and underlying formations and thus underwent equal or greater degrees of chemical weathering their pervasively low 18 o and d down to 85 values provide strong evidence of alteration and diagenesis in contact with ultra low 18 o glacial meltwaters in lacustrine deltaic or periglacial lake sikussak type environments associated with the paleoproterozoic glaciations the d silicate values for the rest of precambrian shales range from 75 to 50 and are comparable to those for phanerozoic and archean shales likewise these samples have similar ranges in 13 c org values 23 to 33 pdb and corg content 0 0 to 10 wt to phanerozoic shales precambrian shales have a large range of 18 o values comparable to that of the phanerozoic shales in each age group and formation suggesting similar variability in the provenance and intensity of chemical weathering except for the earliest 3 3 3 5 ga archean shales which have consistently lower 18 o values moreover paleoproterozoic shales that bracket in age the great oxidation event goe overlap in 18 o values absence of a step wise increase in 18 o and d values suggests that despite the first order change in the composition of the atmosphere weathering cycle was not dramatically affected by the goe at 2 4 2 3 ga shales do not show comparable 18 o rise in the early phanerozoic as is observed in the coeval 18 o trends for cherts and carbonates there is however a sharp increase in the average 18 o value from the early archean to the late archean followed by a progressively decelerating increase into the phanerozoic this decelerating increase with time likely reflects declining contribution of mantle extracted normal 18 o crust and lends support to crustal maturation and increasing 18o sequestration into the crust and recycling of high 18 o and 87sr 86sr sedimentary rocks this secular increase in the 18 o composition of the continental crust could have also had a mild effect on seawater 18 o composition

regimes of subduction and lithospheric dynamics in the precambrian 3d thermomechanical modelling abstract comparing the early earth to the present day geological geochemical evidence points towards higher mantle potential temperature and a different type of tectonics in order to investigate possible changes in precambrian tectonic styles we conduct 3d high resolution petrological thermomechanical numerical modelling experiments for oceanic plate subduction under an active continental margin at a wide range of mantle potential temperature t p t p 0 250 k compared to present day conditions at present day mantle temperatures t p 0 k results of numerical experiments correspond to modern style subduction whereas at higher temperature conditions important systematic changes in the styles of both lithospheric deformation and mantle convection occur for t p 50 100 k a regime of dripping subduction emerges which is still very similar to present day subduction but is characterised by frequent dripping from the slab tip and a loss of coherence of the slab which suggests a close relationship between dripping subduction and episodic subduction at further increasing t p 150 200 k dripping subduction is observed together with unstable dripping lithosphere which corresponds to a transitional regime for t p 250 k presumably equivalent to early archean the dominating tectonic style is characterised by small scale mantle convection unstable dripping lithosphere thick basaltic crust and small plates even though the initial setup is still defined by present day subduction this final regime shows many characteristics of plume lid tectonics transition between the two end members plume lid tectonics and plate tectonics happens gradually and at intermediate temperatures elements of both tectonic regimes are present we conclude therefore that most likely no abrupt geodynamic regime transition point can be specified in the earth s history and its global geodynamic regime gradually evolved over time from plume lid tectonics into modern style plate tectonics