apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

characterization of magma from inclusions in zircon apatite and biotite work well feldspar less so detrital zircon grains are employed to decipher sediment provenances and crustal evolution and they provide unique evidence of hadean crust mantle differentiation processes we demonstrate that mineral inclusions in zircon provide valuable information on the conditions under which zircon crystallized zircon grains from selected plutonic rocks from dronning maud land antarctica contain inclusions of apatite biotite amphibole and pyroxenes that accurately reflect the chemical compositions of the equivalent phases in the host rock matrix and the compositions of the whole rocks high concentrations of y and low concentrations of sr in apatite inclusions in zircon are diagnostic of evolved felsic granitoid host rocks in contrast the relative abundances and compositions of plagioclase and alkali feldspar inclusions in zircon are decoupled from the composition of the whole rock and they are generally indicative of granitic melts regardless of the bulk rock this is best explained by the late crystallization of zircon relative to the bulk of the feldspars we conclude that inclusions of apatite and mafic phases in zircon constrain the potential source rocks of detrital zircon whereas feldspar inclusions do not

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

oxygen isotope analyses and pleistocene ice volumes abstract the oxygen isotope record from fossil foraminifera in deep sea sediments is commonly used as a proxy for global ice volume the linkage between 18 o and ice volume however is probably nonlinear we have developed a simple numerical model of the isotopic response of the oceans to ice volume change the major features it simulates are 1 the changing mean isotopic composition of snow as a function of ice volume colder snow temperatures forced by climate change and higher elevation accumulation areas imply more negative mean 18 o 2 the nonequilibrium isotopic composition of ice sheets the past history of an ice sheet is integrated into its mean isotopic composition which introduces a lag of isotopic ice volume i e the measured 18 o record scaled to ice volume units behind true ice volume 3 selective preservation of isotopically more negative colder higher latitude ice this geographic effect can selectively amplify or dampen the isotopic response to the ice volume signal we illustrate the response of our model to simple hypothetical ice volume transitions of ice growth and ice decay sensitivity tests are illustrated for all model parameters the results suggest that oxygen isotope records reproduce the general patterns of ice volume change fairly accurately the foraminiferal isotope record however may misrepresent the true amplitude of the ice volume signal and lag true ice volume by 1000 to 3000 yr

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

the effect of dispersion on the establishment of a paleoclimatic record from groundwater a mathematical model which can account for the dispersion of solute in underground water over time scales exceeding 104 yr is described in its simplest form the model postulates a diffusion limited exchange of material between flowing groundwater and the stationary water in the confining aquiclude leading to the spreading of the solute in the mobile water and a consequent reduction in its concentration variability the model is used to study the increasing attenuation with age of the perturbations in groundwater composition induced by paleoclimatic oscillations in the late quaternary specific reference is made to noble gas concentrations stable isotope ratios and chloride levels the extreme attenuation of stable isotope ratios in the australian great artesian basin cannot yet be adequately explained and may provide the basis of a critical evaluation of different dispersion models in large hydraulic basins

replacement of primary monazite by apatite allanite epidote coronas in an amphibolite facies granite gneiss from the eastern alps accessory monazite crystals in granites are commonly unstable during amphibolite facies regional metamorphism and typically become mantled by newly formed apatite allanite epidote coronas this distinct textural feature of altered monazite and its growth mechanism were studied in detail using backscattered electron imaging in a sample of metagranite from the tauern window in the eastern alps it appears that the outer rims of the former monazites were replaced directly by an apatite ring with tiny thorite intergrowths in connection with ca supply through metamorphic fluid around the apatite zone a proximal allanite ring and a distal epidote ring developed this concentric corona structure with the monazite core regularly preserved in the center shows that the reaction kinetics were diffusion controlled and relatively slow quantitative electron microprobe analyses suggest that the elements released from monazite breakdown p ree y th u were diluted and redistributed in the newly formed apatite allanite and epidote overgrowth rings and were unable to leave the corona this supports the common hypothesis that these trace elements are highly immobile during metamorphism furthermore microprobe data suggest that the preserved monazite cores lost little possibly none of their radiogenic lead during metamorphism thus metastable monazite grains from orthogneisses appear to be very useful for constraining u th pb protolith ages on the basis of these findings and a review of literature data it seems that monazite stability in amphibolite facies metamorphic rocks depends strongly on lithologic composition while breaking down in granitoids monazite may grow during prograde metamorphism in other rocks such as metapelites

apatite composition tracing petrogenetic processes in transhimalayan granitoids apatites crystallized from different types of igneous rocks show significant variations in the abundances of some minor and trace elements in this study electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry were used to determine the concentrations of 25 minor and trace elements in apatite separated from three principal rock types of thetranshimalayan igneous plutonic suite s type granites the i type gangdese batholith and postcollisional adakites f mn sr and rare earth elements ree in apatite vary systematically with the composition of the host magma and thus have high potential as petrogenetic tracers more specifically the f and mn contents of apatite can be used as an indicator of magma aluminosity or differentiation index combined with sr and ree data which show significant variations in apatite from different rock types these elements are useful for constructing discrimination diagrams this study also reveals that apatite has the capacity to retain geochemical information about the host magma through the course of magmatic evolution systematic variations of sr and ree in apatite with bulk rock aluminosity are the results of partition competition with pre existing and coexisting major and accessory minerals in silicate melts and thus are useful for more detailed investigations of petrogenetic processes such as fractional crystallization and magma mixing which is signaled by inconsistent eu anomalies sr abundances and ree patterns relative to bulkrock compositions

apatite chemical composition determined by electron microprobe and laser ablation inductively coupled plasma mass spectrometry as a probe into granite petrogenesis abstract major minor and trace element abundances in apatites from various i and s type igneous and sedimentary granites of the lachlan fold belt have been determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometer the results show that apatite can accommodate many minor and trace elements whose concentrations and ratios are relatively sensitive to factors controlling many of the fundamental differences between i and s type granites apatites from s type granites generally have higher f but lower cl contents than those from i type granites which is ascribed mainly to the loss of cl during the weathering processes forming the source rocks of s type granites although fractional crystallisation can cause significant enrichment in f as well high mn and fe contents in apatites from s type granites and high s and as abundances in apatites from mafic i type granites result from different oxygen fugacities and degrees of al saturation or aluminosity between metaluminous mafic i type magmas and peraluminous s type and felsic i type magmas there are systematic and distinctive differences in absolute rare earth element ree abundances ree distribution patterns and element ratios e g la y sm nd etc between apatites from different types of granite the strong eu depletion that characterises apatites from s and felsic i type granites is interpreted here to be a result of the uniqueness of crystal chemistry of apatite and high eu2 eu3 ratios in s type and felsic i type magmas which are more reduced and peraluminous than mafic i type magmas strong ree la to eu and th enrichment in apatites from mafic i type granites and marked nd depletion in apatites from most s type and felsic i type granites are caused by the precipitation and fractionation of monazite in the parental magmas of the latter rocks substitution mechanisms are responsible for high na in apatites from s type and felsic i type granites and for high si in apatites from mafic i type granites and may also have important effects on ree partitioning between apatite and melt thus apatite chemistry can be used as an excellent indicator of granite petrogenesis the results have important implications for identifying different types of granite and are potentially significant for determining the provenance of sedimentary rocks

thermochronology of isotopically zoned micas insights from the southwestern usa proterozoic orogen abstract we have used three different 40 ar 39 ar laser microprobe methods to explore the distribution of radiogenic 40 ar in 1 0 1 5 mm biotite crystals from the ca 1680 ma horse mountain monzogranite of central arizona incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near plateau ages of 1190 ma showing no indication of intracrystalline 40 ar inhomogeneity in contrast total fusion of twenty five biotite fragments 100 pm yielded apparent ages ranging from 1006 7 to 1212 0 ma detailed age mapping in the 001 plane of two crystals with the laser focused to a minimum spot size confirms that the age dispersion in the fragment data reflects the existence of 200 m y age gradients in single crystals the two mapped crystals display very different age distribution patterns that suggest radiogenic 40 ar loss through two mechanisms volume diffusion on a scale comparable to that of the grain radius and more rapid diffusion along discrete zones of high crystal defect density simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of 0 5 k m y the horse mountain results as well as previously published studies indicate that conventional incremental heating of hydrous phases can homogenize intracrystalline gradients in 40 ar thereby masking important details of the thermal history of analyzed samples in contrast detailed isotopic mapping studies offer a wealth of information and will become more powerful with continued improvement in the spatial resolution of 40 ar 39 ar laser microprobes total fusion studies of crystal fragment populations can be readily automated making them less labor intensive than mapping studies our preliminary experiment on a limited horse mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research

understanding preservation of primary signatures in apatite by comparing matrix and zircon hosted crystals from the eoarchean acasta gneiss complex canada

an apatite for progress inclusions in zircon and titanite constrain petrogenesis and provenance apatite has recently gained considerable attention as a mineral with many uses within the earth and planetary sciences apatite chemistry has recently given new insight into a wide range of geological processes and tools for example magmatism metasomatism planetary geochemistry and geochronology we expand the utility of apatite here by presenting a novel way to fingerprint magma chemistry and petrogenesis using apatite inclusions within robust titanite and zircon we present trace element data from apatite mineral inclusions shielded within magmatic zircon and titanite importantly apatite inclusion and host titanite chemistries detailed in this study allow estimation of the whole rock sr and sio2 we show how these data can be used to assess the degree of fractionation of the host magma and to calculate key trace element abundances and ratios we demonstrate that the inclusions can be linked to discrete periods in the crystallization history of the host phases thus providing insight into petrogenesis the results highlight that apatite compositions might discriminate modern granitoids younger than 2 5 ga from archean proterozoic transitional granitoid compositions sanukitoid signatures development of such a petrological tool has important potential for interpretation of provenance and a better understanding of the secular evolution of the continental crust including that of early earth

rare earth elements in apatite as a monitor of magmatic and metasomatic processes the ilimaussaq complex south greenland abstract textural and compositional variations of apatite from the plutonic ilimaussaq complex in south greenland and its surrounding country rocks track magmatic and metasomatic processes detailed back scattered electron bse imaging reveals various types of apatite textures including i growth zonation concentric oscillatory as well as sector zonation formed during magmatic differentiation ii resorption and overgrowth textures due to fluid melt induced metasomatic overprint of precursor apatite and iii replacement textures indicating the destabilization of apatite in favor of monazite in the least evolved rocks of the ilimaussaq complex apatite incorporates rare earth elements and y rey mainly via the coupled substitution ca 2 p 5 rey 3 si 4 in the more evolved peralkaline rocks and in metasomatically overprinted rocks however the coupled substitution 2 ca 2 rey 3 na becomes increasingly relevant and apatite incorporates successively more lree compared to hree as shown by increasing la gd n and gd yb n ratios similarly at the contact between the ilimaussaq rocks and the granitic country rocks the metasomatic effect of the emplacement of the ilimaussaq melts is preserved in partly resorbed precursor apatite which is overgrown by rey enriched apatite with higher la gd n and gd yb n ratios compared to apatite from granites more distant from the contact this study shows that apatite textures and compositions are suitable to track both primary magmatic and later fluid present metasomatic processes the incorporation of rey in apatite is not only dependent on the amount of rey present but also depends largely on na activity in the apatite precipitating melts and fluids

petrogenesis of the nanling mountains granites from south china constraints from systematic apatite geochemistry and whole rock geochemical and sr nd isotope compositions abstract the widespread mesozoic granitoids in south china 135 300 km2 were emplaced in three main periods triassic 16 of the total surface area of mesozoic granitoids jurassic 47 and cretaceous 37 though much study has been conducted on the most abundant jurassic nanling mountains nlm granites their rock affinities relative to the triassic darongshan drs and cretaceous fuzhou zhangzhou complex fzc granites which are typical s and i type respectively and the issue of their petrogenetic evolution is still the subject of much debate in this study we discuss the petrogenesis of nlm granites using apatite geochemistry combined with whole rock geochemical and sr nd isotope compositions sixteen apatite samples from six granite batholiths one gabbro and three syenite bodies in the nlm area were analyzed for their major and trace element abundances and compared with those collected from drs n 7 and fzc n 6 granites the apatite geochemistry reveals that na si s mn sr u th concentrations and ree distribution patterns for apatites from drs and fzc granites basically are similar to the s and i granite types of the lachlan fold belt australia whereas those from nlm granites have intermediate properties and cannot be correlated directly with these granite types according to some indications set by the apatite geochemistry e g lower u and higher eu abundances nlm apatites appear to have formed under oxidizing conditions in addition we further found that their ree distribution patterns are closely related to aluminum saturation index asi and nd isotope composition rather than sio2 content or degree of differentiation of the host rock the majority of apatites from nlm granites asi 0 97 1 08 and end t 8 8 to 11 6 display slightly right inclined apatite ree patterns distinguishable from the typical s and i type however those from few granites with asi 1 1 and end t 6 6 and gabbro and syenite are similar to fzc apatites strongly right inclined in light of sr and nd isotope compositions magmas of nlm intrusives except gabbro and syenite and few granites with end t 8 generally do not involve a mantle component instead they fit with a melt derived largely from in situ melting or anatexis of the pre mesozoic mainly caledonian granitic crust with subordinate pre yanshanian mainly indosinian granitic crust we suggest that an application using combined whole rock asi and end t values is as useful as the apatite geochemistry for recognizing possible sources for the nlm granites

the behaviour of monazite from greenschist facies phyllites to anatectic gneisses an example from the chugach metamorphic complex southern alaska monazite is a common accessory mineral in various metamorphic and magmatic rocks and is widely used for u pb geochronology however linking monazite u pb ages with the pt evolution of the rock is not always straightforward we investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses which is exposed in the eocene chugach metamorphic complex of southern alaska we investigated textures chemical compositions and u pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade with particular focus on the relationship between monazite and other ree bearing minerals such as allanite and xenotime in the greenschist facies phyllites detrital and metamorphic allanite is present whereas monazite is absent in lower amphibolites facies schists 550 650 c and 3 4 kbar small medium y monazite is wide spread mnz1 indicating monazite growth prior and or simultaneous with growth of garnet and andalusite in anatectic gneisses new low y high th monazite mnz2 crystallised from partial melts and a third high y low th monazite generation mnz3 formed during initial cooling and garnet resorption u pb shrimp analysis of the second and third monazite generations yields ages of 55 50 ma monazite became unstable and was overgrown by allanite and or allanite epidote apatite coronas within retrograde muscovite and or chlorite bearing shear zones this study documents polyphase complex monazite growth and dissolution during a single relatively short lived metamorphic cycle

paleoclimatic inferences from an isotopic investigation of groundwater in the central san juan basin new mexico groundwater from the ojo alamo and nacimiento aquifers in the central san juan basin new mexico has yielded 14c ages ranging from modern to 35 000 yr b p the pleistocene age samples are characterized by a stable isotope content about 25 lighter in d and 3 lighter in 18o than modern precipitation and groundwater we attribute this difference to a colder mean annual temperature and perhaps increased winter precipitation consideration of various factors controlling the stable isotope composition of the groundwater allows estimation of a 5 to 7 c temperature decrease during the late wisconsin accompanied by increased effective precipitation a similar estimate of the temperature change is obtained from noble gas paleothermometry these data support a model of moderately cooler late pleistocene climate in the american southwest characterized by summers with less precipitation than today but wetter winters

magmatic zoning in apatite a monitor of porosity and permeability change in granites apatites from the shap granite northern england are strongly zoned reflecting multiple generations of growth and dissolution such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using la icp ms analyses the zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries the main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning followed by the growth of a thin rim of relatively uniform composition the chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets

mobilization of radiogenic pb in zircon revealed by ion imaging implications for early earth geochronology zircon is arguably the most commonly used geochronometer but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its u pb systematics here we present the results of a multifaceted ion microprobe study of zircon grains from the napier complex east antarctica a region affected by pervasive high temperature metamorphism at 2 5 ga and from which previous zircon geochronological interpretations have been problematic both u pb spot analysis 15 m and high spatial resolution 2 m scanning ion imaging of pb isotopes have been applied in an attempt to quantify the effects of metamorphism spot analyses spread along concordia yielding 207 pb 206 pb ages from 2 5 ga to 3 9 ga with the oldest grains reversely discordant ion images of uranogenic pb reveal a surprising micrometer scale patchy distribution that is unrelated to crystal morphology or damage the 207 pb 206 pb ratios within these subdomains correspond to apparent zircon ages as old as 4 2 ga these are interpreted as artifacts of ancient redistribution of radiogenic pb a process that can generate meaningless ages and are not relicts of ancient including hadean zircon scanning ion imaging thus facilitates identification of unsupported radiogenic pb and enables testing of the validity of old ages from zircon known to have a long and complicated history

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

improvements of precision and accuracy in in situ hf isotope microanalysis of zircon using the laser ablation mc icpms technique abstract we have developed a new analytical technique for precise and accurate in situ hf isotope ratio measurements for zircons by means of laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icpms newly designed sample cell and n 2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles in order to improve the accuracy of the hf isotope ratio analysis previously reported correction methods for isobaric interferences on 176 hf by 176 yb and 176 lu were rigorously evaluated and we found that the use of independent mass bias factors for hf and yb in the isobaric interference correction resulted in more reliable hf isotopic data in order to test the reliability of the hf isotopic data from 35 m diameter ablation pits that are smaller than the typical size of most zircon grains lu hf isotopes for three zircons harvard 91500 antarctic zircon pma7 and acasta zircon ay199 were measured the present 176 hf 177 hf isotope ratio of 0 282321 46 2 s d and initial 176 hf 177 hf isotope ratio of 0 282315 47 2 s d were obtained for zircon standard 91500 the resulting present 176 hf 177 hf isotope ratio shows an excellent agreement with the previously reported data our in situ isotopic data revealed that the lu hf ratio varied significantly within the 91500 grain 176 lu 177 hf 0 000130 0 000345 suggesting the existence of slight isotopic heterogeneity in present 176 hf 177 hf approximately 15 ppm for antarctic zircon pma7 despite the smaller size of the single grain 50 100 m wide 80 300 m long and the presence of various mineral inclusions resulting analytical precision for the initial 176 hf 177 hf ratio was 128 ppm 0 281208 36 2 s d for acasta zircon ay199 despite the high yb hf 0 05 and lu hf 0 01 isotope ratios with the early archean crystallization age 3 74 ga resulting analytical precision for the initial 176 hf 177 hf ratio was 187 ppm 0 280160 52 2 s d these results demonstrate clearly that in situ hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon lu hf isotopic study

disturbances in the sm nd isotope system of the acasta gneiss complex implications for the nd isotope record of the early earth abstract the whole rock 143nd isotope record of the acasta gneiss complex agc has been used to argue for the development of both depleted mantle and enriched crustal reservoirs during the hadean the 147sm 143nd isotope compositions of rocks from agc however fall on an array with an errorchron date of 3 3 ga despite the majority of samples yielding substantially older zircon u pb ages this has led to the suggestion of a major sm nd resetting event at this time in the agc to better understand the sm nd bulk rock systematics of the agc we investigate the u pb age and sm nd isotope compositions of apatite and titanite from the agc using the laser ablation split stream lass method apatite from all samples yield broadly similar u pb dates of 1 89 to 1 83 ga in agreement with resetting during the 1 9 ga wopmay orogen the sm nd isotope compositions of the apatite however lie off the wr errorchron and show a high degree of initial nd isotope heterogeneity conversely titanite sm nd systematics are consistent with the 3 3 ga whole rock errorchron further supporting sm nd modification at 3 3 ga taken together these show that the oldest rocks in the agc underwent at least two post crystallization events at 3 3 and 1 9 ga that modified the sm nd isotope system these results also demonstrate the relative mobility of the sm nd isotope system during orogenic events which may compromise the ability to determine robust bulk rock 143nd isotope signatures

routine low damage apatite u pb dating using laser ablation multicollector icpms 1 apatite is a common u bearing accessory mineral with a u pb closure temperature of 500 c making u pb dating of apatite a potentially valuable thermochronometer however its low u concentration and tendency to incorporate common lead has limited widespread application to destructive isotope dilution methods we overcome previous limitations by using a nu plasma multicollector icpms with an attached short pulse excimer laser and by identifying two new matrix matched reference apatites to correct for elemental fractionation gem quality 485 myr old apatite from madagascar which we independently characterized by id tims analysis and 523 5 ma apatite from the mcclure mountain syenite source of the40ar 39ar reference mmhb common pb is corrected using measured 204pb isobarically corrected for hg interference and a five step iterative process using stacey and kramers common pb model we accurately reproduce ages of numerous independently characterized apatites regularly achieving precision of 2 2 by pooling as few as five 30 m spot analyses data quality in apatite with low u concentrations low 206pb 204pb values 30 and young ages 75 ma is compromised by the goal of avoiding significant grain damage such limitations can be overcome by using spot sizes 65 m or greater but at the expense of substantial grain damage for single detrital apatite grains with ages of 500 ma precision of 4 2 was achieved by pooling 2 to 3 spots per grain the accuracy of our detrital results is supported by a good age match with similar closure temperature 40ar 39ar detrital hornblende ages from the same sediment

isotopic evidence for glacial meltwater recharge to the cambrian ordovician aquifer north central united states the chemistry of water in the cambrian ordovician aquifer in six midwestern states has been studied as part of the northern midwest regional aquifer system analysis of the u s geological survey dissolved solids concentrations generally increase perpendicular to the direction of regional groundwater flow from less than 400 mg liter in southeast minnesota southwest wisconsin and northeast iowa to more than 10 000 mg liter in northwest missouri isotopic ratios of hydrogen and oxygen are significantly depleted from north to south with an areal distribution approximately parallel to the distribution of dissolved solids for example 18o in southern iowa and northern missouri is about 6 parts per thousand lighter than 18o of modern recharge water in minnesota and wisconsin covariance between 18o and d of the groundwater similar to that of modern precipitation suggests that the differences in isotopic ratios between groundwater and modern recharge water reflect meteoric signatures of water during past recharge events rather than geochemical processes such as isotopic exchange with aquifer materials the pronounced parallelism between the distribution of isotopes and dissolved solids over large areas probably reflects largescale recharge of pleistocene glacial meltwater into the aquifer system which probably had a paleoflow system with a gradient from northeast to southwest rather than from northwest to southeast

coupled zircon lu hf and u pb isotopic analyses of the oldest terrestrial crust the 4 03 ga acasta gneiss complex abstract the acasta gneiss complex of the northwest territories canada contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust forming processes on the early earth here we report the results of a combined lu hf and u pb isotopic study of zircons from predominantly felsic orthogneisses from the acasta gneiss complex that crystallized between 4 0 and 2 9 ga many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and hf isotopic domains to ensure the reliability of the analyses and of subsequent geologic interpretations we employed the laser ablation split stream lass technique to concurrently measure the lu hf and u pb isotopic systems in zircon our results confirm prior findings of precursor hadean crust 4 0 ga in the source of these rocks and the continued involvement of this reservoir until 3 6 ga we present evidence for the input of relatively more juvenile material at 3 6 ga which we suggest corresponds to a fundamental change in the source of the magmas this study extends the lower bound of the published acasta hf isotopic record from 3 6 ga to 2 9 ga and demonstrates that the 3 6 ga 2 9 ga interval is largely represented by reworking of relatively juvenile 3 6 ga crust and the diminution of the 4 0 ga crustal signal significantly there is no evidence that rocks within the acasta gneiss complex were derived from a strongly depleted mantle

crustal melting in the lower parts of island arcs an example from the bremanger granitoid complex west norwegian caledonides the 440 5 ma bremanger granitoid complex bgc west norwegian caledonides comprises tonalites granodiorites and minor granites the bgc is coeval with a nearby subduction related gabbronoritic dioritic intrusion the 443 4 ma gasoy intrusion and an ophiolite complex generated in a back arc basin the 443 3 ma solund stavfjord ophiolite complex the bgc has a restricted isotopic variation nd440ma 0 04 to 2 21 87sr 86sr440 ma 0 7045 to 0 7054 indicating a substantial mantle component but it is enriched in crustal material relative to the primitive parts of the gabbronoritic rocks of the gasoy intrusion nd440 ma 1 67 87sr 86sr440 ma 0 7048 the chondrite normalised ree patterns are fractionated la yb n 8 to 16 smooth or with negative or positive eu anomalies geochemical modelling suggests that the bgc does not represent residual melts produced by fractional crystallisation of basic magma but is compatible with partial melting of mafic source rocks with various amounts of h2o residual garnet is suggested by hree depletion indicating a minimum pressure of 9 10 kbar during melting

petrogenesis and tectonics of the acasta gneiss complex derived from integrated petrology and 142nd and 182w extinct nuclide geochemistry abstract the timing and mechanisms of continental crust formation represent major outstanding questions in the earth sciences extinct nuclide radioactive systems offer the potential to evaluate the temporal relations of a variety of differentiation processes on the early earth including crust formation here we investigate the whole rock 182w 184w and 142nd 144nd ratios and zircon 17 o values of a suite of well studied and lithologically homogeneous meta igneous rocks from the acasta gneiss complex northwest territories canada including the oldest known zircon bearing rocks on earth in the context of previously published geochemical data and petrogenetic models the new 142nd 144nd data indicate that formation of the hadean eoarchean acasta crust was ultimately derived from variable sources both in age and composition although 4 02 ga crust was extracted from a nearly bulk earth source heterogeneous 142nd signatures indicate that eoarchean rocks of the acasta gneiss complex were formed by partial melting of hydrated hadean age mafic crust at depths shallower than the garnet stability field by 3 6 ga granodioritic granitic rocks were formed by partial melting of archean hydrated mafic crust that was melted at greater depth well into the garnet stability field our 182w results indicate that the sources to the acasta gneiss complex had homogeneous high 182w on the order of 10 ppm a signature ubiquitous in other eoarchean terranes no significant deviation from the terrestrial mass fractionation line was found in the triple oxygen isotope 16o 17o 18o compositions of acasta zircons confirming homogeneous oxygen isotope compositions in earth s mantle by 4 02 ga

origin age and movement of pore water in argillaceous quaternary deposits at four sites in southwestern ontario hydrogeologic information in the form of textural properties field and laboratory determined hydraulic conductivity values vertical hydraulic gradients from piezometer nests and major ion and isotopic contents 18o 2h 3h 13c 14c was obtained from four sites located in southwestern ontario on thick deposits of clayey till and glaciolacustrine clay at each site the piezometers were installed from the water table zone situated 1 4 m below the surface to a maximum depth between 20 and 35 m below the surface values of hydraulic conductivity determined in the laboratory by consolidometer and triaxial methods are in the order of 10 8 10 7 cm s and are similar to values calculated from rates of water level rise in piezometers mean values of average vertical groundwater velocity calculated with the measured values of hydraulic conductivity porosity and hydraulic gradient using the darcy equation for a saturated non fractured porous medium range from 0 13 to 0 26 cm yr tritiated pore waters only occur within 3 6 m of ground surface thereby indicating that groundwaters below this depth recharged prior to at least 1952 18o in the pore waters exhibit a distinctive regular shift with depth from water table values of 9 to 10 smow characteristic of present day precipitation to values between 14 and 17 at depths of 20 40 m which are characteristic of much cooler waters corrected 14c ages of the groundwater are greater than 8000 yr b p thereby suggesting that the groundwaters at depth entered these deposits during or after the formation of these deposits approximately 11 000 to 14 000 yr ago 18o 2h and cl concentration profiles were simulated with a one dimensional model for transport by advection and diffusion in a saturated porous medium reasonable agreement between the model and the field profiles was obtained with values of effective diffusion coefficients of 3 0 10 6 cm2 s for 18o 2h and cl and with groundwater velocities between 0 03 to 0 05 cm yr based on these results it is concluded that the pore water in these deposits is a mixture of late pleistocene and modern waters and that the distribution of 18c 2h and cl in these deposits is influenced predominantly by molecular diffusion apparently more so than by hydraulic flow

trace element partitioning between apatite and silicate melts we present new experimental apatite melt trace element partition coefficients for a large number of trace elements cs rb ba la ce pr sm gd lu y sr zr hf nb ta u pb and th the experiments were conducted at pressures of 1 0 gpa and temperatures of 1250 c the rare earth elements la ce pr sm gd and lu y and sr are compatible in apatite whereas the larger lithophile elements cs rb and ba are strongly incompatible other trace elements such as u th and pb have partition coefficients close to unity

new zealand 20th century sea level rise resolving the vertical land motion using space geodetic and geological data 1 investigations in long term instrumental tidal records reveal that 20th century sea level along the coast of new zealand is rising at 1 46 0 10 mm yr in agreement with the regional rates from southern australia and tasmania we extend the advanced altimeter gauge approach of combining satellite altimetry and tide gauge data with constraint equations from long term adjacent tide gauge records to assess its performance in open seas and to explore the impact of vertical land motion on the observed relative sea level this approach has again proven to be a robust method with an accuracy of 0 4 mm yr while no clear sea level rise pattern can be inferred once the tide gauge apparent sea level trends are corrected for vertical land motions from gps the advanced altimeter gauge and geological vertical rates are completely consistent and reveal three temporal phases of sea level rise marked by an increase from 1 46 0 10 mm yr to 1 72 0 10 mm yr during the period 1900 1936 followed by a decrease to 1 48 0 10 mm yr during the period 1936 1956 and a substantial increase to 2 60 0 10 mm yr during the period 1956 1975 in contrast the 20th century microfossil proxy records of absolute sea level rise display twice the tide gauge sea level rise rate of 3 17 0 30 mm yr and 3 28 0 45 mm yr respectively once salt marsh records are corrected using gps and geological vertical rates differential autocompaction and transfer functions are possible factors which need further investigation

reworking of hadean crust in the acasta gneisses northwestern canada evidence from in situ lu hf isotope analysis of zircon abstract the acasta gneiss complex of northwestern canada comprises early archean orthogneisses and includes the oldest known terrestrial rocks 3 94 4 03 ga previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4 2 ga in the oldest rocks indicating that the source of early archean granitoids contained hadean 4 03 ga crust in this study we have determined the lu hf isotopic compositions of zircon grains extracted from the early archean acasta gneisses to evaluate the extent of the hadean crust contribution to the formation of these granitoids analyses of the lu hf isotopes were carried out using in situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains two ca 3 59 ga granitoids have e hf t of 2 4 2 2 and 3 2 2 5 suggesting that the source of the granitoids was extracted from the mantle as far back as 3 8 ga this is consistent with the presence of abundant zircon xenocrysts with ages up to 3 9 ga the hf isotopic composition of a 3 72 ga granitoid is significantly less radiogenic with an e hf t of 6 1 2 5 this indicates that the granitoid formed by remelting of very old crust the e hf t values for the two oldest rocks the ca 3 97 ga tonalites are 1 2 3 3 and 3 3 1 7 respectively indicating that even the oldest known granitoids contain reworked older crustal materials these results suggest that hadean crust had significantly contributed to the genesis of some of the early archean acasta granitoids

deuterium excess in an east antarctic ice core suggests higher relative humidity at the oceanic surface during the last glacial maximum a continuous deuterium excess profile d d 8 18o was obtained from the dome c ice core spanning 32 000 yr it exhibits significant variations over this period in particular d was 4 5 lower around 18 000 yr bp than over the past 7 500 yr interpreted as reflecting higher relative humidity estimated 90 over the oceanic areas providing moisture for antarctic precipitation

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

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

modelling u pb discordance in the acasta gneiss implications for fluid rock interaction in earth s oldest dated crust abstract the u pb isotopic system in zircon is the tool of choice to interrogate high temperature geological processes yet this system has potential to investigate lower temperature fluid rock interaction as well in some cases removal of radiogenic pb is incomplete potentially allowing regression of discordant u pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid driven isotopic disturbance however in rocks preserving more complex histories simple regression is not effective at resolving multiple pb loss events here we use a concordant discordant comparison cdc test to establish the times of u pb disturbance in the acasta gneiss complex agc canada agc c 4 03 to c 3 40 ga orthogneisses experienced a long and complex post crystallization history for which u pb zircon data reflects not only the heterogeneous nature of the rock but also the varying degrees and duration of crustal reworking that inevitably involved open system processes the cdc test calculates the similarity between the concordant age structure and a modelled age structure the latter inferred from discordant analyses over a wide range of potential disturbance times our analysis reveals concordant zircon components implying new growth and or recrystallization at 3992 5 3501 6 3442 5 and 3126 6 ma in addition we establish episodes of radiogenic pb loss driven by fluid rock interaction at 3150 50 ma and probably at 2875 50 ma and c 2590 ma these pb loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism granite emplacement and unroofing pb loss within the agc shows an antithetic relationship in different samples that are in close geographic proximity we suggest that zircon alteration and associated new growth effectively rendered those grains that underwent pb loss at a particular time less susceptible to alteration during the next episode of fluid interaction

palaeoproterozoic thermal events recorded in the 4 0 ga acasta gneiss canada evidence from shrimp u pb dating of apatite and zircon u pb isotopes of apatites and zircons extracted from a sample of the acasta gneisses that occurs along the western margin of the slave craton in northwest territories of canada have been measured using a shrimp ii ion microprobe eleven apatite analyses 30 m spot give 238u 204pb 206pb 204pb and 204pb 206pb 207pb 206pb isochron ages of 1905 86 ma 2 and 1936 28 ma 2 respectively analyses 20 m spot on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238u 206pb 207pb 206pb concordia plot with concordia intercepts at 4014 25 ma 2 and 1967 93 ma 2 the former agrees with 3 96 4 02 ga protolith ages obtained on the oldest components of the acasta gneisses by other workers the lower concordia intercept for the zircon data is consistent with the u pb age of apatites and may be related to a 500 c thermal event perhaps early in the palaeoproterozoic wopmay orogeny