oxygen isotopic composition and u pb discordance in zircon we have investigated u pb discordance and oxygen isotopic composition of zircon using high spatial resolution 18o measurement by ion microprobe 18o in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance our results suggest that three characteristics of zircon are interrelated 1 u pb systematics and concomitant age discordance 2 18o and the water rock interactions implied therein and 3 zircon texture as revealed by cathodoluminescence and bse imaging a key observation is that u pb disturbed zircons are often also variably depleted in 18o but the relationship between discordance and 18o is not systematic 18o values of discordant zircons are generally lighter but irregular in their distribution textural differences between zircon grains can be correlated with both u pb discordance and 18o discordant grains exhibit either a recrystallized fractured or strongly zoned cl texture and are characteristic of 18o depletion we interpret this to be a result of metamictization leading to destruction of the zircon lattice and an increased susceptibility to lead loss conversely grains that are concordant have less expressed zoning and a smoother cl texture and are enriched in 18o from this it is apparent that various stages of water rock interaction as evidenced by systematic variations in 18o leave their imprint on both the texture and u pb systematics of zircon

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

the accumulation of non formula elements in zircons during weathering ancient zircons from the jack hills western australia abstract in this contribution we describe the influx of non formula elements fe ca al y u and th into fractures and selected zone lamellae in zircons from jack hills during recent weathering and discuss the effects of this on overlapping sims u th pb and oxygen isotope analyses previous research has recognised the importance of fractures in the generation of anomalous u th pb and oxygen isotope systems in this report we show that besides fractures specific zones in euhedrally zoned zircon can act as pathways for the influx of weathering solutions and contain a similar range of trace element materials as do the fractures whereas zero age discordant u pb systems of jack hills zircons have been explained by many authors in terms of pb loss present results confirm conclusions of our previous study that the main discordance mechanism of jack hills zircons is u th gain due to overlap of sims analyses with mineralized fractures and zone lamellae with excess weathering fluid deposited u and th we explain the anomalously light and heavy oxygen isotopes and significant oh in sims analyses that overlap fractures and mineralized zones as due to the presence in the fractures of ca fe al oxides and hydroxides with complexly fractionated oxygen isotopic systems there is a suggestion in some of the elemental maps that there has been minor dispersion of trace elements away from fractures but sims u th pb and oxygen isotope analyses on parts of the zircon away from fractures and mineralized zones show no evidence of interaction with weathering fluid indicating that penetration of weathering fluids into the body of the zircon at the location of the sims spots has not occurred results of this study have implications for other sims u th pb and oxygen isotope studies of zircons from rocks that have been subjected to weathering and also for early tims u pb measurements of bulk zircon samples that show zero ma u pb discordance

nano and micro geochronology in hadean and archean zircons by atom probe tomography and sims new tools for old minerals atom probe tomography apt and secondary ion mass spectrometry sims provide complementary in situ element and isotope data in minerals such as zircon sims measures isotope ratios and trace elements from 1 20 m spots with excellent accuracy and precision apt identifies mass charge and three dimensional position of individual atoms 0 3 nm in 100 nm scale samples volumes up to one million times smaller than sims apt data provide unique information for understanding element and isotope distribution crystallization and thermal history and mechanisms of mineral reaction and exchange this atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage and can test for intracrystalline element mobility nano geochronology is one application of apt in which pb isotope ratios from sub micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating based on sem imaging and sims analysis 11 needle shaped specimens 100 nm in diameter were sampled from one archean and two hadean zircons by focused ion beam milling and analyzed with apt the three dimensional distribution of pb and nominally incompatible elements y rees differs at the atomic scale in each zircon zircon jh4 0 4 007 ga jack hills western australia is homogeneous in pb y and rees in contrast pb and y and rees are clustered in sub equant 10 nm diameter domains spaced 10 40 nm apart in zircons arg2 5 2 542 ga grouse creek mountains utah and jh4 4 4 374 ga jack hills most clusters are flattened parallel to 100 or 010 u and th are not collocated with pb in clusters and appear to be homogeneously distributed in all three zircons the analyzed domains experienced 4 to 8 10 15 decay events mg due to u and th decay and yet all zircons yield u pb ages by sims that are better than 97 concordant consistent with annealing of most radiation damage the 207 pb 206 pb ratios for the 100 nm scale specimens measured by apt average 0 17 for arg2 5 0 42 for the jh4 0 and 0 52 for jh4 4 these ratios are less precise 10 18 2 due to the ultra small sample size but in excellent agreement with values measured by sims 0 1684 0 4269 and 0 5472 respectively and the crystallization ages of the zircons thus pb in these clusters is radiogenic but unsupported meaning that the pb is not spatially associated with its parent isotopes of u and th for the domain outside of clusters in jh4 4 the 207 pb

martian high latitude permafrost depth and surface cover thermal inertia distributions 1 martian high latitudes have thermal properties consistent with an extensive high thermal inertia permafrost layer near the surface surface cover thermal inertias and permafrost depths at martian high latitudes 50 80 n s are derived from thermal emission spectrometer tes data and compared with previously published water ice depths determined from the mars odyssey neutron spectrometer mons the depth to the permafrost layer is correlated with surface cover thermal inertia albedo and latitude in general agreement with predicted trends of water ice stability comparison of permafrost depths with water ice rich layer depths derived from mons data displays good qualitative agreement although a divergence is present at greater burial depths this disparity may be due to the presence of hydrated minerals at shallow depths or a lower than expected permafrost thermal inertia corresponding with low water ice concentrations at greater depths surface cover thermal inertias are greater in the northern high latitudes than in the south and differences between these and previous results will have significant effects on the predicted depth of martian water ice stability several regions in the northern hemisphere display high surface cover thermal inertia associated with possible receding water ice deposits that are shallow enough to influence diurnal surface temperatures significant lateral and vertical heterogeneity in water ice distributions are present and the martian regolith is likely more complicated than can be described by simple two layered models and a single mode of water ice emplacement

the oldest zircons of africa their u pb hf o isotope and trace element systematics and implications for hadean to archean crust mantle evolution abstract more than 450 detrital zircon grains from two limpopo belt quartzite samples were investigated by a combination of scanning electron imaging u pb dating 18 o lu hf isotope and trace element analyses in order to get robust information about the early earth s crust mantle evolution the detrital zircon grains have crystallization ages between 3 95 ga and 3 18 ga show hf t between 1 to 15 1 unit and 18 o vsmow mostly between 5 5 and 8 1 0 2 pristine zircon domains reveal ti in zircon temperatures between 700 and 865 c and th u of 0 3 2 3 trace elements point to zircon formation in predominately granitoid rocks metamorphic zircon rims have ages 2 65 ga hf 2 65ga 15 18 o 7 0 8 1 and th u mostly hf t age array i which starts from a chondritic uniform reservoir chur at about 4 5 ga and requires 176 lu 177 hf 0 020 indicative for mafic crust most zircon grains however plot on or above an hf t age array ii which runs parallel to array i 176 lu 177 hf 0 021 and starts from chur at 4 01 ga oxygen isotope compositions of 18 o 5 5 indicate that the magmatic host rocks of the zircons have been formed either by melting of altered mafic crust which interacted with cold water prior to granitoid formation and or that ancient sedimentary and or magmatic rocks were involved in the melting process the new u pb hf 18 o datasets together with compiled data from worldwide sources indicate a significant gap of about 5 epsilon units between arrays i and ii furthermore they illustrate that many hadean zircon analyses plot well below array i and some above chur these findings support an interpretation that the hadean earth was covered by a long lived mafic protocrust perhaps forming a partially open stagnant lid this protocrust was affected by internal reworking but also injected and overlain by ultra mafic rocks derived from chondritic and highly depleted mantle sources at

isotopic insight into the proterozoic crustal evolution of the rudall province western australia abstract the proterozoic assembly of australia involved the convergence of three main archean cratonic entities the north west and south australian cratons and is recorded in the proterozoic orogenic belts surrounding these continental nuclei the rudall province of northern western australia is the sole exposure of a paleo to mesoproterozoic orogen lying between the north and west australian cratons and may record the effects of their amalgamation we present new zircon o u pb and lu hf isotope data from magmatic rocks across the rudall province to which we add existing isotope data to yield a crustal evolution overview hf evolution trends for the ca 1804 1762 ma kalkan supersuite the ca 1589 1549 ma krackatinny supersuite and the ca 1310 1286 ma camel suite indicate a significant input of archean east pilbara basement material albeit as a mix with more juvenile material including a possible ca 1900 ma component zircon 18o data suggest a contribution from supracrustal material into the magmatic source of the kalkan supersuite which may have been emplaced in an extensional setting in contrast the krackatinny supersuite and camel suite have mantle like 18o which may reflect partial melting of deeper archean sources geochemical data for the krackatinny supersuite shows geochemical trends implying that melting of thickened mafic crust progressed from deeper to shallower levels possibly in a rift setting camel suite k rich leucogranites may also have been emplaced in an extensional setting towards the end of high p metamorphism all terranes of the rudall province are para autochthonous with respect to the pilbara craton with no requirement for arc related magmatism we outline two potential scenarios for the paleo to mesoproterozoic geodynamic evolution of the rudall province an early cratonic amalgamation between the west and north australian cratons ca 1680 ma followed by mesoproterozoic intraplate events or a later assembly ca 1377 1275 ma we lean towards this later amalgamation scenario

zircon hf and o isotope constraints on the evolution of the paleoproterozoic baoule mossi domain of the southern west african craton abstract the use of zircon isotopic systems such as lu hf and o has proven to be an invaluable tool for unraveling the evolution of the early earth in order to achieve a comprehensive understanding of the evolution of the southern west african craton and in particular the paleoproterozoic baoule mossi domain zircon grains from 75 felsic intrusions were analyzed for hf isotopes 22 of those samples were analyzed for o isotopes covering an area that covers more than 1000 km2 from southwest burkina faso across southern mali and into eastern guinea the results of the study reveal that the baoule mossi domain comprises two magmatic regions that display broadly juvenile isotopic character but with significant localized evidence of crustal contamination the hf model age data show that the crustal contaminant is potentially as old as ca 2800 ma and indicate that the two domains may have evolved slightly differently in fact the westernmost region presents a slightly older or less radiogenic hf isotope signature when compared to the easternmost region furthermore the o isotopic data support the notion that the felsic magmas were subject to supra crustal contamination as 18o values range between 6 and 12 the data presented in this study provide robust evidence that the baoule mossi domain of the southern west african craton evolved by accretionary processes between ca 2250 2050 ma

non uniform seasonal defrosting of subpolar dune field on mars abstract images of the sub polar regions of mars show existence of various seasonal depending features including mosaics of dark patches called dalmatian spots we interpret formation of dark patches on dune fields by means of the non uniform sublimation of winter co 2 ice deposit

extending the krogh legacy development of the ca tims method for zircon u pb geochronologythis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh tom krogh was without a doubt the most influential u pb geochronologist of the modern era his career was marked by a continuing series of breakthroughs both revolutionary and evolutionary his legacy is such that every lab around the world uses the methods he developed after some fascinating insights into chemical leaching effects in zircon tom largely dropped this approach in favor of his enormously successful air abrasion technique coupled with careful sample selection at the individual grain level here i describe continued experiments with leaching and step wise dissolution techniques and how a series of alternating steps forward and setbacks eventually led to a new chemical abrasion or ca tims method for selectively removing those domains within zircon that have lost pb continuing to strive for improvements in analytical technique is perhaps the best way to honor tom s many contributions and extend his legacy

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

age and provenance constraints on seismically determined crustal layers beneath the paleozoic southern central asian orogen inner mongolia china abstract we present 110 ages and 51 in situ 18o values for zircon xenocrysts from a post 99 ma intraplate basaltic rock suite hosted in a subduction accretion complex of the southern central asian orogenic belt in order to constrain a seismic profile across the paleozoic southern orogen of inner mongolia and the northern margin of the north china craton two zircon populations are recognized namely a phanerozoic group of 70 zircons comprising granitoid derived ca 431 99 ma n 31 peak at 256 ma meta granitoid derived ca 449 113 ma n 24 peak at 251 ma and gabbro derived 436 242 ma n 15 peaks at 264 and 244 ma grains each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation the precambrian population 2605 741 ma n 40 exhibits a prominent age peak at 2520 ma granulite facies metamorphism and four small peaks at ca 1900 1600 and 800 ma our new data together with literature zircon ages significantly constrain models of three seismically determined deep crustal layers beneath the fossil subduction zone forearc along the active northern margin of the north china craton namely 1 an upper arc crust of early to mid paleozoic age intruded by a major permian triassic composite granitoid gabbroic pluton 8 20 km depth 2 a middle crust predominantly consisting of mid meso to neoproterozoic felsic and mafic gneisses and 3 a lower crust composed predominantly of late archean granulite facies rocks we conclude that the paleozoic orogenic crust is limited to the upper crustal level and the middle to lower crust has a north china craton affinity furthermore integrating our data with surface geological petrological and geochronological constraints we present a new conceptual model of orogenic uplift lithospheric delamination and crustal underthrusting for this key ocean continent convergent margin

hydrous species in zircon a survey of hydrous species in zircon from a number of localities with a range of occurrences and structural states was carried out using infrared spectroscopy and electron microprobe analysis stepwise heating experiments on a crystalline and a nearly metamict zircon were also performed the region of ir spectra of oh bearing crystalline zircon from 4000 to 2500 cm 1 exhibits anisotropic oh bands resulting from at least three oh sites one at vacant tetrahedra and two that may be at si occupied tetrahedra one site associated with si occupied tetrahedra is the most important producing ir absorption bands at 3420cm 1 e c and 3385cm 1 e c oh maybe introduced during crystal growth by a coupled substitution of the form m 3 h 4 zr 4 the sites at vacant tetrahedra account for considerably less oh producing relatively weak ir absorption bands at 3510 cm 1 e c e c oh may be introduced during crystal growth by the hydrogrossular substitution of the form 4 oh sio 4 4 an additional oh band at 3270 cm 1 appears to be associated with an occupied tetrahedron heating experiments produced two more oh environments and indicate that h can migrate among the sites at high temperatures some crystalline samples are entirely free of oh ir spectra of metamict zircon exhibit broad asymmetric isotropic oh bands similar to h 2o bands but the absence of the h 2o bending mode band indicates that oh is the dominant hydrous species present ir spectra of partially metamict zircon samples retain some anisotropy the breadth of their oh bands is a function of degree of metamictizatron the maximum oh observed in any metamict or partially metamict zircon sample was approximately 0 1wt expressed as h 2o sucho h is probably secondary the maximum primary oh observed is 0 05 wt partially metamict zircon from a hydrothermal vug crystalline crustal zircon contains more than an order of magnitude less oh the maximum oh content observed in crystalline zircon 0 01 wt i s in a mantle zircon sample from a kimberlite suggesting high activity of h 2o in the mantle additional oh is present in some metamict and partially metamict zircon samples in the form of inclusions of hydrous silicates such as kaolinite fluid inclusions containing h 2o occur in some samples this may be a partial explanation of the high h 2o contents reported for some radiation damaged zircon based on loss on heating measurements

transport of uranium thorium and lead in metamict zircon under low temperature hydrothermal conditions abstract understanding the stability of radiation damaged zircon under low temperature hydrothermal conditions is crucial to the application of zircon for u pb geochronology and as a host phase for the disposal of plutonium waste we report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 c with a 2 m alcl 3 and a 1 m hcl cacl 2 solution as hydrothermal fluids for 1340 h cathodoluminescence cl and backscattered electron bse images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon these zones have experienced severe loss of si u th and pb and gain of al or ca and a water species as revealed by electron microprobe sensitive high resolution ion microprobe shrimp analyses and infrared spectroscopy micro raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered whereas recrystallization of the embedding amorphous phase was not observed no detectable structural or chemical changes were detected inside the unaltered areas intensive fracturing which was most intense in the hcl cacl 2 experiment occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions we explain the formation of deep penetrating alteration zones by a percolation type diffusion model which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible the idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon the model was used to estimate for the first time diffusion coefficients for u th and pb diffusion in amorphous zircon at 175 c by assuming that volume diffusion inside the amorphous domains is the loss rate limiting process these estimates show that volume diffusion in metamict zircon can cause significant loss of u th and especially loss of radiogenic pb over geological time scales even at temperatures as low as 175 c our results show that recent pb loss discordias can be generated 1 by predominate pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant and 2 by leaching of pb and u and th from metamict zircon through an external fluid

strengths and limitations of zircon lu hf and o isotopes in modelling crustal growth abstract the robust nature of the mineral zircon combined with our analytical ability to readily acquire in situ uranium lead u pb lutetium hafnium lu hf and oxygen o isotopic data has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through earth history in such studies there has been a strong focus on developing methods to determine the timing and or proportion of juvenile magmatic addition to the continental crust one widespread approach to determine the timing of crustal growth has been the construction or fitting of reworking arrays to regional hf isotopic datasets simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on going dilution and refertilisation of ancient crust consistent with hot zone models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism a new compilation of magmatic rock zircon lu hf and o isotope data is used to demonstrate that the use of mantle like o isotope data as a screening tool for meaningful hf model ages is also unlikely to be reliable with independently constrained data indicating that as few as 14 of hf model ages provide a meaningful indicator of the timing of crustal growth the limitations of hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts in an effort to address some of the limitations within existing models we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions

interaction of weathering solutions with oxygen and u pb isotopic systems of radiation damaged zircon from an archean granite darling range batholith western australia zircons from archean granites from the darling range batholith in the yilgarn craton of western australia have been shown to have complexly discordant u pb systems with a strong component of zero age disturbance the only geological event that has affected the granites in recent times is the pervasive regional weathering our aim in this study was to investigate the effects of weathering on the u pb and oxygen systems of the zircons and to this end we report secondary ion mass spectrometry sims results of oh oxygen and u pb isotope systems of six typical zircons from a sample of the granite these results confirmed the presence of oh in highly radiation damaged parts of the zircons demonstrating fluid interaction within grains the presence of oh was accompanied by significant changes in the 18o 16o ratios the data suggest trends where 18o values in individual grains both increase and decrease with increasing oh sims measurements showed the u pb systems are variably and unsystematically discordant in radiation damaged parts of the zircons particularly those with elevated oh contents the complex u pb systems are interpreted in terms of multiple disturbance events between 450 and 0 ma involving low temperature fluid induced movement of radiogenic pb decoupled from parent u and th within the radiation damaged zircons together with some pb loss

mapping temporal and spatial patterns of zircon u pb disturbance a yilgarn craton case study abstract the zircon u pb system is a robust geochronometer however disturbance to this system can be widely diagnosed on u pb concordia diagrams where it is typically interpreted as a consequence of radiogenic pb loss in many cases removal of radiogenic pb is not complete and discordia regressions on concordia diagrams may then potentially track both the original age of crystallization of the rock and the timing of isotopic disturbance however u pb disturbance resulting in normal discordance can also be driven by u gain in this contribution we expand on a novel approach to determine the most pervasive time of u pb disturbance in a sample we introduce the concordant discordant comparison cdc test which evaluates the similarity between a sample s concordant age structure and a modelled age structure from discordant analyses over a wide range of potential disturbance times the closest similarity in concordant and discordant age populations as derived from a specified time of elemental mobility is interpreted as the best estimate for the time of u pb disturbance the cdc test is appropriate for magmatic as well as detrital samples and yields dates that can be spatially interpolated to produce regional maps that define domains of u pb disturbance we highlight the application of the cdc test using a case study from the eastern goldfields superterrane of the yilgarn craton western australia u pb sims analysis of 95 zircon samples from across the eastern goldfields superterrane reveals two dominant times of u pb disturbance recent and at 600 ma the 600 ma time of u pb disturbance shows a statistically significant spatial relationship to a buried set of undated dykes the cosmo newbery dyke suite which are apparent in geophysical images thermal modelling indicates dyke temperatures alone are insufficient to cause widespread pb loss in host rock zircon we therefore suggest dyke emplacement caused u pb disturbance through reactive fluids leaching radiogenic pb additionally we use the difference between the measured and calculated zircon th u ratio to distinguish between pb loss and elemental gain processes as the calculated th u ratio derived from 208 pb 206 pb will only match measured th u where no later addition or loss of th or u has occurred cdc modelling of discordant u pb zircon analyses may provide a means to recognise the distal footprint of otherwise difficult to date tectonothermal events and extract useful information from often discarded analyses

hf isotope evidence for effective impact melt homogenisation at the sudbury impact crater ontario canada abstract we report on the first zircon hafnium oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets the differentiated melt sheet at the 1 85 ga originally ca 200 km in diameter sudbury impact crater ontario canada yields a tight range of uniform zircon hf isotope compositions e hf 1850 of ca 9 to 12 this is consistent with its well established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised we propose that the heterogeneity in other isotopic systems such as pb in early emplaced impact melt at sudbury is associated with volatility related depletion during the impact cratering process this depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post impact assimilation of country rock whereas the systems of more refractory elements preserve initial homogeneities zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks however they display a marked offset approximately one third up the melt sheet stratigraphy that is interpreted to be a result of post impact assimilation of 18 o enirched rocks into the base of the cooling impact melt given that impact cratering was a more dominant process in the early history of the inner solar system than it is today and the possibility that impact melt sheets were sources of ex situ hadean zircon grains these findings may have significance for the interpretation of the early zircon hf record we speculate that apparent e hf time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust we also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself

structural relationships along a neoarchean arc continent collision zone north china craton the archean north china craton is composed of the western block eastern block and the intervening central orogenic belt a 4 10 km wide and 85 km long tectonic melange belt informally called the zanhuang tectonic melange is documented in the zanhuang massif of the central orogenic belt separating the eastern block from an archean arc terrane in the central orogenic belt the melange belt contains a structurally complex tectonic mixture of metapelites metapsammites marbles and quartzites mixed with exotic tectonic blocks of volcanic mafic and ultramafic rocks metabasalts that locally include relict pillow structures and tonalite trondhjemite granodiorite ttg gneisses the zanhuang tectonic melange marks the suture of an arc continent collisional zone between the western zanhuang massif in the central orogenic belt and the eastern block of the north china craton and it is one of the best preserved archean tectonic melanges in the world here using zircon u pb dating of various types of blocks from the zanhuang melange we show that the formation and associated deformation of the zanhuang melange occurred in the neoarchean ca 2 5 ga high precision 1 20 1 200 lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the melange and regional structural relationships along the arc continent collisional zone a synthesis of studies on the tectonic evolution of the north china craton coupled with our new fabric and kinematic analysis of the zanhuang melange further constrains the initial amalgamation timing and geometry of the arc continent collision between the fuping arc terrane in the central orogenic belt and the eastern block with a northwest dipping subduction polarity the asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the zanhuang melange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the eastern block by 2 5 ga lithostructural mapping shows that the classic melange and fold and thrust structures along the neoarchean arc continent collisional zone are broadly similar to phanerozoic collisional belts

the origin of high δ18o zircons marbles megacrysts and metamorphism the oxygen isotope ratios 18o of most igneous zircons range from 5 to 8 with 99 of published values from 1345 rocks below 10 metamorphic zircons from quartzite metapelite metabasite and eclogite record 18o values from 5 to 17 with 99 below 15 however zircons with anomalously high 18o up to 23 have been reported in detrital suites source rocks for these unusual zircons have not been identified we report data for zircons from sri lanka and myanmar that constrain a metamorphic petrogenesis for anomalously high 18o in zircon a suite of 28 large detrital zircon megacrysts from mogok myanmar analyzed by laser fluorination yields 18o from 9 4 to 25 5 the u pb standard cz3 a large detrital zircon megacryst from sri lanka yields 18o 15 4 0 1 2 se by ion microprobe a euhedral unzoned zircon in a thin section of sri lanka granulite facies calcite marble yields 18o 19 4 by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble small oxygen isotope fractionations between zircon and most minerals require a high 18o source for the high 18o zircons predicted equilibrium values of 18o calcite zircon 2 3 from 800 to 600 c show that metamorphic zircon crystallizing in a high 18o marble will have high 18o the high 18o zircons 15 from both sri lanka and mogok overlap the values of primary marine carbonates and marbles are known detrital gemstone sources in both localities the high 18o zircons are thus metamorphic the 15 25 zircon values are consistent with a marble origin in a rock dominated system i e low fluid external rock the lower 18o zircon values 9 15 are consistent with an origin in an external fluid dominated system such as skarn derived from marble although many non metasomatized marbles also fall in this range of 18o high 18o 15 and the absence of zoning can thus be used as a tracer to identify a marble source for high 18o detrital zircons this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks metamorphic zircon megacrysts have not been reported previously and appear to be associated with high grade marble identification of high 18o zircons can also aid geochronology studies that seek to date high grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble

neoarchean seafloor hydrothermal metamorphism of basalts in the zanhuang ophiolitic melange north china craton abstract epidosites are suggested to form in up flow zones located at the base of ore forming oceanic hydrothermal systems and are commonly preserved in ophiolitic sequences however occurrences of epidosites in archean tectonic melanges are rarely documented in this study we report the presence of late neoarchean epidosites in the zanhuang ophiolitic melange in the north china craton using meter scale litho structural and petrologic analysis zircon u pb dating major and trace element and oxygen and strontium isotopic geochemistry field studies show that the epidosites are mainly preserved as the cores of meta pillow lavas and are distributed as blocks in different matrices within the tectonic melange the pillow rims are used as protolith to estimate the element mobility during the alteration process the epidosites display strong depletion in na2o 0 01 0 27 wt mgo 0 43 3 01 wt depletion in large ion lithophile elements lile e g rb ba cs u and transition metals e g co sc cu ni zn and show enrichment of cao 13 35 21 0 wt sr and pb the high field strength elements hfse e g zr hf ta th of epidosites are varied but rare earth elements ree are unchanged the amphibolitic pillow rims are characterized by enrichment of lile flat heavy rare earth elements hree patterns and depletion in hfse with negative ti anomalies and positive pb and ba anomalies oxygen and strontium isotopic ratios of epidosites are 5 0 8 6 and 0 70284 0 70551 respectively both of which are greater than those of fresh basalts the major and trace elements oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence magmatic zircons from two samples of epidosites yield 207pb 206pb ages of 2512 13 ma mswd 0 51 and 2542 13 ma mswd 1 4 confirming that the epidosites were generated at ca 2 51 2 54 ga based on geochemical data and field observations we suggest that the protolith of the epidosites formed in an arc related tectonic setting during the late neoarchean this study shows that hydrothermal activity operating in the late neoarchean was similar to that of the modern ocean floor

evolution of the darling range batholith yilgarn craton western australia a shrimp zircon study ma and slow cooling indicated by marginal recrystallization and primary and secondary internal structures observed in zircons from continued pb loss from the zircons until 2628 2616 ma granites from the late archaean darling range batholith record stages in the evolutionary history of the granites and provide a basis for a shrimp u pb study on the timing of granite evolution many granite zircons contain cores a few low u cores retain concordant to slightly discordant u pb ages of 2 8 ga some show discordant intermediate ages but most have nearly concordant

preferential dissolution of 234u and radiogenic pb from α recoil damaged lattice sites in zircon implications for thermal histories and pb isotopic fractionation in the near surface environment abstract zircons from two archean samples a 2732 ma trondhjemite and a 2728 ma dacite both from the western superior province were washed in hydrofluoric acid hf at moderate temperatures zircon residues and hf wash solutions were analyzed for u pb isotope systematics and 234u 238u ratios in an attempt to study the effect of hf washing on selective dissolution of radiogenic isotopes in areas damaged by recoil on a conventional concordia diagram the datum from one analyzed wash solution plots below concordia near the array of discordant data from magnetic unabraded zircon fractions which record a lower concordia intersection of about 230 ma data points from hf wash residues of two different zircon fractions from the trondhjemite are also discordant and collinear with near concordant data from abraded unleached fractions which record the true age of crystallization however this array has a negative lower concordia intercept of 749 ma the residue and wash from hf treatment of a single zircon fraction from the dacite are collinear with the crystallization age of the sample as defined by near concordant data from unleached fractions this line has a lower concordia intersection of 1388 ma also negative but distinct from the trondhjemite residue line the discordance of strongly washed residues is proportional to the calculated recoil dosage of the grains leach residues show negative 234u 238u anomalies while the leaches themselves show 234u excess on the order of 10 these results can be explained by the bulk dissolution of altered and metamict crystal domains during the hf wash accompanied by leaching of 234u and radiogenic pb from recoil damaged lattice sites in alteration free zircon residues studies were also carried out on zircons washed in nitric acid these show small ca 0 5 negative 234u anomalies this suggests that small amounts of radiogenic pb can be leached from damaged lattice sites by nitric acid the negative age lower concordia intercepts of pb loss lines defined by hf leach residues can be explained if early radiation damage has been annealed so that only more recently formed radiogenic pb atoms are in damaged crystal lattice sites and are susceptible to leaching since this pb fraction is relatively young the residue pb is biased toward anomalously old values a model is proposed for early burial and heating of the rocks above the annealing temperature for recoil damage followed by uplift and cooling several hundred million years after crystallization this model predicts a cooling time of 2500 20 ma for the trondhjemite and 2295 35 ma for the rhyolite comparison with metamorphic and cooling histories from ar ar studies suggest that the long term annealing temperature for low levels of recoil damage in zircon is in the range 250 300 c approximately the same as for fission tracks these results demonstrate that hf and possibly to a small extent hno3 can preferentially leach radioactive daughter elements from damaged zircon lattice sites in addition to dissolving metamict domains the threshold recoil dosage for dissolution of damaged zircon appears to be slightly in excess of 1 5 1015 decay events mg the isotopic composition of the leached pb is sensitive to the annealing history of the zircon thus in addition to supplying the magmatic crystallization age of the sample the hf leaching of zircon might be useful for constraining its later low temperature thermal history leaching from recoil damaged sites also occurs in the natural environment and this may lead to significant enhancement of 206pb 204pb in groundwater

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

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

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales

evaluating baddeleyite oxygen isotope analysis by secondary ion mass spectrometry sims abstract two baddeleyite megacrysts were evaluated as potential reference materials rms for sims oxygen isotope analysis and utilized to understand and calibrate instrumental mass fractionation imf a baddeleyite crystal s0045 from the phalaborwa carbonatite south africa has a mean 18ovsmow 4 6 0 3 range 0 75 measured using laser fluorination gas source mass spectrometry lf gms and one s0069 from the mogok metamorphic belt myanmar has 18ovsmow 22 2 0 4 range 0 89 sims standardization utilizing these inherently heterogeneous rms is possible by analyzing a number of crystal fragments and utilizing one of them lying at the median of the range metamictization lattice orientation and chemical composition do not appear to be significant sims oxygen isotope analysis of co crystalline zircon and baddeleyite from the 2 2 ga duck lake sill dls in the northwest territories canada yield predominant 18ovsmow modes of 6 0 and 3 2 respectively this difference is consistent with preserving high temperature isotopic equilibrium between zircon and baddeleyite dls baddeleyite 18o data as a whole are negatively skewed to 0 0 and interpreted to reflect low temperature open system behaviour zircon 18o are less affected but also show hints of the same influences of secondary alteration and oxygen isotope exchange