multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

short paper detrital mineral ages from the southern uplands using 40ar 39ar laser probe detrital mica ages in the southern uplands range from 458 to 502 ma the evidence favours a source in a basement which was part of an arc complex to the north rather than from the dalradian block which also has mica ages of this range no grenville ages have yet been recorded detrital volcanic clasts dispersed from the south have cooling ages of 560 and 530 ma indicating that they did not come from a contemporary arc but an older arc terrane of uncertain affinities

the provenance of granite boulders in conglomerates of the northern and central belts of the southern uplands of scotland rb sr whole rock isochrons from several suites of granite boulders which occur in ordovician and silurian conglomerates in the southern uplands of scotland yield ages of around 1200 ma 600 to 700 ma and 470 to 490 ma the plutonic and tectonic history recorded by these clasts corresponds more closely with that of nw newfoundland than with that of scotland the petrography of the clasts can also be matched with exposed granite plutons in newfoundland since the conglomerates were derived from the nw it is concluded that the southern uplands lay to the se of newfoundland a total of 1500 km strike slip displacement occurred between the caradoc and the early devonian consistent with oblique subduction of the iapetus ocean below the north american continental margin

resetting of rb sr whole rock ages during acadian low grade metamorphism in north wales ordovician volcanic rocks and associated sub volcanic intrusions in north wales give devonian metamorphic rb sr whole rock ages isotopic resetting is not confined to acid volcanic rocks three intrusions preserve ordovician ages the closed system resetting is not directly related to metamorphic grade but appears to correlate with the extent to which the secondary metamorphic mineral assemblage is developed rocks giving devonian ages have a well developed secondary mineralogy whereas those retaining ordovician ages are less extensively altered the metamorphic ages combine to give a lower devonian mean age and weighted 2a error of 399 f 9 ma metamorphism is therefore synchronous with acadian deformation in the north wales area

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

laser probe 40ar 39ar studies of the peace river shocked l6 chondrite abstract samples of matrix and melt glass from the shocked l6 chondrite peace river have been subjected to 40ar 39ar analysis using both stepped heating and localised outgassing by a laser probe stepped heating studies of three distinct samples of matrix gave indistinguishable and unusually well defined low temperature 40ar 39ar plateaux corresponding to an age of 450 30 ma in an attempt to investigate whether or not this plateau age is meaningful i e the time of a major collision event involving the parent body or is an artefact resulting from partial degassing during the fragmentation of the parent body which commenced the exposure of peace river to cosmic rays 40 ma ago we have used a laser probe to analyse ar from single 80 m diameter laser pits which release gas from a few micrograms of sample on this scale no areas have ages younger than the plateau age supporting the view that it is not an artefact measurements on the melt glass gave variable and meaningless ages in some cases greater than the age of the solar system due to retention of 40ar and loss of k during melting followed by rapid quenching this observation indicates the danger of attempts to date impact melt glass directly the observation that argon has been retained by the glass and lost by the matrix can be understood in terms of a range of plausible annealing histories

high velocity anomaly adjacent to the western edge of the pacific low velocity province koki idehara 1 satoru tanaka2 and nozomu takeuchi3 1department of earth and planetary science university of tokyo 7 3 1 hongo bunkyo ku tokyo 113 0033 japan e mail idehara eps s u tokyo ac jp 2institute for research on earth evolution japan agency for marine earth science and technology 2 15 natsushima cho yokosuka 237 0061 japan 3earthquake research institute university of tokyo 1 1 1 yayoi bunkyo ku tokyo 113 0032 japan

incomplete retention of radiation damage in zircon from sri lanka a suite of 18 zircon gemstones from placers in the highland southwestern complex sri lanka were subjected to a comprehensive study of their radiation damages and ages the investigation included x ray diffraction raman and pl spectroscopy electron microprobe pixe and hrtem analysis as well as u th he and shrimp u th pb age determinations zircon samples described in this study are virtually homogeneous they cover the range from slightly metamict to nearly amorphous generally concordant u th pb ages averaging 555 11 ma were obtained late ordovician zircon u th he ages scattering around 443 9 ma correspond reasonably well with previously determined biotite rb sr ages for rocks from the hswc slightly to moderately metamict zircon has retained the radiogenic he whereas only strongly radiation damaged zircon calculated total fluences exceeding 3 5 10 18 events g has experienced significant he loss when compared to unannealed zircon from other localities sri lanka zircon is about half as metamict as would correspond to complete damage accumulation over a 555 m y lasting self irradiation period suggesting significant annealing of the structural radiation damage insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon we suggest to estimate effective doses for sri lanka zircon by multiplying total fluences which were calculated using the zircon u th pb age by a correction factor of 0 55 this conversion may be applied to literature data as well because all gem zircon samples from sri lanka this work and previous studies seem to reveal the same general trends of property changes depending on the radiation damage the use of effective doses for sri lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation damaged zircon

evolution of the northern belt southern uplands implications for the southern uplands controversy conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the southern upland fault the establishment of an early caradoc age for the red chert and volcanic sequences within the northern belt of the southern uplands confounds the accretionary prism model and our data support the view that following the obduction of the ballantrae ophiolite fan complexes prograded southeastwards across a narrow fault bounded shelf into the deep basin of the northern belt the fan complexes were initiated by uplift in the hinterland caused by the intrusion of plutons into the midland valley terrane sedimentation was controlled by regional extension subsidence and eustasy through the llanvirn and caradoc basic lavas with attenuated within plate geochemical signatures were erupted during the early caradoc peak of extension and may have just post dated the initiation of southwards subduction beneath the lake district

garnet provenance studies juxtaposition of laurentian marginal terranes and timing of the grampian orogeny in scotland a study of the composition and zoning patterns of detrital garnets from ordovician greywackes from the northern belt of the southern uplands terrane of scotland reveals characteristics of the metamorphic sources very similar to the dalradian supergroup of the scottish grampian terrane the radiometric cooling and uplift ages of the dalradian metamorphic zones and the depositional ages of the southern uplands greywackes support the hypothesis for local provenance detrital metamorphic garnet identical to dalradian garnet has also been identified in the upper ordovican sandstones of the highland border complex these observations do not support proposals that the grampian midland valley and southern uplands terranes were exotic to each other in the late ordovician time these new results together with a review of published age dates clarify the late ordovician palaeogeography for this part of the laurentian margin the distances between the grampian midland valley and southern uplands terranes may have been similar to the present day it is concluded that large rivers flowed out of the uplifting mountainous grampian terrane and across the midland valley into a southern uplands trench during the late ordovician time the main orogeny i e mountain building in the grampian terrane was therefore post cambrian producing the first high mountains and resultant flysch in the caradoc

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 structure of the western part of the southern uplands of scotland the structures of two areas one in the northern belt and one in the central belt of the southern uplands are described and compared the main structural histories are the same both show a main deformation phase related to se directed thrusting with associated fold and cleavage formation both areas exhibit late minor nw directed thrusting with related refolding and crenulation cleavage development in the case of the central belt both areas also show evidence of sinistral strike slip or transpressive deformation which reactivates steepened thrust faults when these deformation histories are reviewed in the context of geophysical radiometric and biostratigraphical data from the region a forearc accretionary setting for the whole of the southern uplands appears the most likely the strike slip deformation has internally disrupted an accretionary prism but has not juxtaposed originally unrelated terranes the effects of the late silurian closure of iapetus and subsequent events are also represented in the southern uplands

hydration during uplift is recorded by reset rb sr whole rock ages the garn fadryn sub volcanic intrusion of n wales gives a u pb zircon age of 4533 2 2 ma this is the first reliable emplacement age from the welsh caradoc volcanic province proving that the rb sr whole rock age of 392 32 ma does not date intrusion but reflects resetting during early devonian low grade metamorphism resetting in this setting is chemically controlled and is a function of disturbance during water rock interaction it is not analogous to the predominantly thermally controlled resetting of k ar systems post volcanism rb sr whole rock secondary ages in this region have a mean age of 399 9 ma and record a specific event which is the time of hydration it is proposed that this event was triggered by uplift which opened fractures increased permeability facilitated formation of secondary hydrated minerals and reset the rb sr system this model for resetting should have widespread applicability to other volcanic provinces affected by low grade metamorphism

radiation enhanced fission track annealing revisited and consequences for apatite thermochronometry apatite fission track aft analyses for granitoid and metamorphic bedrock samples from the western superior province ontario the churchill rae province melville peninsula and southampton islan

detrital muscovite 40ar 39ar ages from carboniferous sandstones of the british isles provenance and implications for the uplift history of orogenic belts major progradations of clastic sediments are recorded in the sedimentary record of the famennian and the visean namurian of the united kingdom and surrounding waters we have determined 40ar 39ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 ma spanning both progradations detrital mica ages are dominated by a peak at 415 ma with minor peaks at 440 ma and 390 ma the 415 ma muscovites are derived from the unroofing of the scandian nappes during the compressional phase of the caledonian orogeny in scandinavia the 440 ma muscovites record pre scandian orogenic activity which is rarely preserved in the orogenic record thermochronological evidence suggests that episodic postorogenic uplift and exhumation events kept the scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations the near total absence of detrital muscovites with ages 415 ma suggests that the scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension the river s which supplied the sediments probably ran parallel to the strike of the major scandian thrusts along the length of the caledonian orogen in a manner analogous to the major river systems of contemporary orogenic highlands

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

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

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

detrital avalonian zircons in the laurentian southern uplands terrane scotland the silurian ordovician southern uplands terrane occupies a key position in the caledonian orogen yet its genesis is controversial marginal basin backarc and forearc tectonic regimes have all been invoked as operative at the laurentian margin of the iapetus ocean fresh andesitic detritus within turbidite sandstones has until now been assumed to provide evidence for an ordovician suprasubduction volcanic arc a central feature of most models however high precision thermal ionization mass spectrometer u pb and laser ablation data for detrital zircons from the sandstone prove neoproterozoic volcanism at 557 6 ma 2 and probably also at 613 12 ma 2 the complex crystallization history recorded by the zircons shows assimilation of 1043 7 ma 2 grenvillian basement into the andesite magma the fact that no zircons have been found having ages that overlap the caradocian depositional age of the host sedimentary rocks undermines all extant terrane models the age profile of the detrital zircons is typical of gondwana and avalonia this finding has important implications for the paleogeography of the iapetus ocean during the ordovician because the zircon data require the introduction of avalonian detritus into a sedimentary basin marginal to laurentia

alpha recoil in u pb geochronology effective sample size matters displacement of the daughter isotope by recoil results in an open system on the nanoscale for a heterogeneous distribution of u and th this redistribution of intermediate and stable daughter isotopes results in subvolumes with a deficit of pb and others with an excess of pb whether such heterogeneities affect the analyzed u pb system depends on 1 the volume of the analyzed sample 2 the degree and scale of heterogeneity in the u and th distribution and 3 the analytical procedure spatial separation of parent and daughter through recoil affects the u pb systematics of leached samples where leaching gives access to domains less than 1 m wide anomalous data patterns originating from recoil induced parent to daughter fractionation are more important if there are strong heterogeneities in the u and th distribution whereby pb excess appears more pronounced than pb deficit fractionation of parent and daughter elements through selective dissolution of u ree rich growth zones in zircon and u inclusions in columbite as well as the presence of u th rich micro inclusions in silicates dated using a step leaching scheme may result in anomalous 207pbrad 206pbrad scattered 206pbrad 238u and 207pbrad 235u and reverse discordance the accumulated structural damage controls the leaching and dissolution behavior but may also influence the non stoichiometric element mobilization during sputtering or ablation in the analysis of u rich samples by shrimp and la mc icp ms

nanoscale occurrence of pb in an archean zircon abstract we report for the first time a direct atomic scale characterization of pb in zircon 4 4 3 1 ga from the early archean yilgarn craton in australia using high resolution haadf stem two forms of pb have been identified pb concentrated at 3 atom as a nanoscale patch in zircon structure and pb concentrated within the amorphous domain created by fission fragment damage the first result suggests that the pb atoms directly substitute for zr4 in the zircon structure and the latter observation demonstrates that pb diffusion can occur through amorphous regions created by radiation damage although volume diffusion is typically considered to be the dominant mechanism for pb diffusion beyond the first percolation point i e when the amorphous domains overlap and form a fully interconnected network of amorphous domains there is a new pathway for the diffusion of pb that is faster than volume diffusion through crystalline zircon

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

40 ar 39 ar ages of detrital white mica from upper austroalpine units in the eastern alps austria evidence for cadomian and contrasting variscan sources five detrital white mica concentrates from very low grade metaclastic sequences within pre variscan basement and post variscan cover units of the upper austroalpine nappe complex eastern alps have been dated with 40ar 39ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas two samples from early palaeozoic sandstone exposed within the same alpine nappe record slightly discordant age spectra the maximum age recorded in one is 562 2 0 7 ma whereas the other yielded a 40ar 39ar plateau age of 607 3 0 3 ma these results indicate a source area affected by cadomian tectonothermal activity three detrital muscovite concentrates from post variscan late carboniferous and permian cover sequences exposed within three different alpine nappes yielded 40ar 39ar plateau ages of 359 6 1 1 ma 310 5 1 2 ma and 303 3 0 2 ma the contrasting detrital white mica ages are interpreted to reflect different source areas detrital muscovite from a post variscan carboniferous molasse type sequence and from a permian verrucano type sequence record ages which indicate late variscan e g 330 300 ma metamorphic sources by contrast detrital white mica from another permian verrucano type sequence suggests a source area affected by early variscan e g 400 360 ma metamorphism these results help clarify palinspastic relationships and tectonic correlations between pre late carboniferous metamorphic basement sequences and carboniferous to permian cover sequences

radiation damage in zircon a single zoned sri lankan zircon exhibits a range of microstructures from crystalline to nearly amorphous that are the result of radiation damage over a dose range of 2 1 10 1 1015 decay events mg 0 16 0 47 dpa the zones in the crystal were examined at a variety of length scales using optical microscopy micro raman spectroscopy electron microprobe analysis and transmission electron microscopy birefringence varies linearly with dose birefringence 4 71 10 18 g d 4 86 10 2 full width at half maximum fwhm measurements of the b1g 3 peak as determined by micro raman spectroscopy were used to estimate the extent of radiation damage in each zone the radiation dose calculated on the basis of u and th concentrations and sample age vs damage from the fwhm measurements of the b1g 3 peak relationship among zones was consistent with results for a suite of sri lankan single crystals suggesting that all of these zircon crystals have undergone a similar thermal history based on a comparison with zircon crystals that are considered to have undergone minimal annealing e g lunar zircon the sri lankan zircon crystals have accumulated less damage than is expected based on their calculated dose consistent with previous evidence for annealing to estimate the extent of annealing in sri lankan samples an equivalent damage dose i e the dose required to produce the same amount of damage in an unannealed sample was calculated for a given u and th concentration by determining the time of damage accumulation required to create an equivalent amount of damage in an unannealed zircon the dose vs damage relationship in sri lankan zircon crystals 560 ma is equivalent to that of a 375 ma unannealed zircon suggesting that approximately one third of the damage has been lost due to annealing the dose vs damage relationship is consistent with the direct impact model of damage accumulation transmission electron microscopy revealed zoning on a scale finer than could be identified optically 0 5 m and the presence of abundant zro2 nano particles at zone boundaries

the northern belt of the longford down inlier ireland and southern uplands scotland an ordovician back arc basin the northern belt of the southern uplands longford down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid to late ordovician age they extend throughout the length of the belt and are generally separated by a strike fault the northwestern metaclast petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the nw whereas the southeastern basic clast petrofacies suite contains abundant calc alkaline detritus derived from an island arc and plutonic metamorphic root zone complex to the s or se a similar source but located to the north is indicated for younger clastic sequences of similar composition in the northern part of the central belt immediately south of a major sinistral shear zone which defines the boundary between the northern and central belts it is concluded in light of other facets of regional geology and possible palaeogeographic configurations that this pattern reflects a back arc basin in the northern belt margined by a volcanic arc massif which once occupied the space now represented by the shear zone and a fore arc basin to the se central and southern belts the northern belt back arc basin is postulated to have been closed and deformed at the end ordovician and subsequently overthrust northwestward by the allochthonous thrust stack imbricated central and southern belts fore arc sequence at the end silurian this in the northern belt resulted in the development of a major recumbent structure downward facing on regional end silurian cleavage

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

basin thermal history favours an accretionary origin for the southern uplands terrane scottish caledonides there is general agreement that the siluro ordovician southern uplands suspect terrane is an imbricate thrust belt formed adjacent to the laurentian continental margin during closure of the iapetus ocean however there are conflicting models for its tectonosedimentary evolution especially for the initial depositional environment was it a convergent forearc trench with subsequent incorporation of strata into a suprasubduction accretionary prism or was it a back arc or continental margin extensional basin subsequently caught up in a foreland fold and thrust belt to address this issue the different thermal histories of the contrasting convergent and extensional sedimentary basin models have been investigated palaeogeothermal conditions in the southern uplands terrane assessed from regional metamorphic patterns clay mineral assemblages and k white mica composition rule out extension and support deposition in a low heat flow plate convergent setting there is no evidence for a change from extensional to convergent basin regimes from north to south as required in some models instead throughout the terrane there is a close temporal and spatial relationship between deposition burial metamorphism and tectonism that is elsewhere typical of accretion

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

tracing the fluid evolution of the kiruna iron oxide apatite deposits using zircon monazite and whole rock trace elements and isotopic studies the ore genesis of the paleoproterozoic iron oxide apatite deposits in the vicinity of kiruna in northern sweden is poorly understood despite a century long mining history and 2500 mt of iron ore with grades of 30 to 70 wt fe produced in the region to date zircon grains from the ore recently dated at ca 1874 ma show very different appearances compared to zircon from surrounding host rocks ca 1880 ma and related intrusions ca 1880 and ca 1874 ma particularly an inclusion rich rim in contrast zircon from the host rocks and a proximal granite intrusion exhibit typical igneous growth zoning electron microprobe results show near stoichiometric composition for zr si and hf in the host rock zircon grains the ore zircon crystals have low analytical totals with significant concentrations of ca fe y and p and infrared spectroscopy showed several weight percent of water these ore zircon grains further show fe rich inclusions zones and or veins in elemental x ray maps and light rare earth elements lree enrichment transmission electron microscopy tem shows that the lree are not due to micro or nano inclusions in the zircon but are likely hosted as lree oxides in amorphous regions of the grains based on these characteristics the rims on ore zircon grains are interpreted to be of hydrothermal origin uranium pb in monazite from the ore measured by sims suggests a secondary event influencing the area at ca 1624 ma a period of known geologic activity in fennoscandia electron microprobe x ray mapping of these monazite grains shows no zoning and relatively low u and th concentrations stark contrasts are visible between the ore depleted mantle influence and host rocks crustal influence in the whole rock lu hf and sm nd data the depleted mantle signature of the ore could be related to the kiruna greenstone group as a potential source region for the iron the sm nd isotopic composition of monazite from the ore shows a crustal influence and indicates that the younger event has not disturbed the whole rock sm nd signature of the ore the hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source

age of metamorphism in the lesser himalaya and the main central thrust zone garhwal india results of illite crystallinity 40 ar 39 ar fusion and k ar studies

unravelling dates through the ages geochronology of the scottish metamorphic complexes the paper by giletti et al 1961 is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains we consider certain critical issues from each of the main complexes of the scottish highlands studied by giletti et al to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts lewisian examples focus on the formation of archaean crust and the age of the main high grade metamorphism and the scourie dyke swarm the antiquity of moinian sedimentation its relationship to the torridonian sandstones and the timing of precambrian metamorphism have been controversial issues the timing and nature of caledonian orogenesis most clearly expressed in the dalradian complex have been the focal points for the refinement of radiometric investigation these complexes have been subject to successive developments in methodology with ever tighter constraints from rb sr and k ar mineral dating through rb sr and pb pb whole rock studies u pb dating of bulk zircon fractions and sm nd whole rock and mineral investigation up to the latest technologies of single grain zircon and ion microprobe analysis the rocks have released their secrets reluctantly and many of the questions posed in 1961 have still not been definitively answered however the hope of unambiguous solution leads towards greater efforts ever more reliable data and a clearer evolutionary picture

influence of radiation damage on titanite he diffusion kinetics titanite is a common u th bearing accessory mineral that previous study suggested has a u th he closure temperature tc of 210 175 c although it has been recognized that radiation damage influences the he retentivity of apatite and zircon this effect has not been documented in titanite we acquired 51 the analyses from 11 archean basement samples across the kaapvaal craton south africa the the dates range from 20 to 1200 ma and are negatively correlated with their 10 970 ppm span of effective uranium concentration eu raman spectra acquired for 4 of these samples display peak broadening with increasing eu 4he diffusion experiments on the same 4 samples suggest tc values that vary by 175 c estimated alpha dose versus tc for our results and published the diffusion data document tc values of 150 210 c at alpha doses 50 1016 g with a sharp decrease in he retentivity above this threshold this damage diffusivity pattern is similar to zircon but the damage level at which the reduction in retentivity occurs appears to be lower for titanite because titanite typically has substantially lower eu than zircon for the same protracted thermal history the dates are likely to record higher temperature portions of the history than zircon u th he dates the results demonstrate that the dating can access a much larger temperature range and potentially be used to decipher more detailed time temperature paths than previously thought

reply to the discussion of the reply by r l romer and u kroner on geochemical signature of ordovician mn rich sedimentary rocks on the avalonian shelf 1 1appears in the canadian journal of earth sciences 49 11 1372 1377 doi 10 1139 e2012 049 romer and kroner 2012 stated the core of the harlech dome i in part is made up of ordovician deposits and ii contains tectonic repetitions in their comment schofield et al this issue d

geographic boundary and shear wave velocity structure of the pacific anomaly near the core mantle boundary beneath western pacific we determine the geographical boundary and shear velocity structure of a very low velocity province at the base of the earth s mantle beneath western pacific we term it the pacific anomaly based on the waveform modeling and travel time analysis of scsh sh phases our seismic data are from the china national digital seismographic network the f net in japan the global seismographic network and several passcal arrays the observed scs sh differential travel time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined the seismic data also suggest that the average shearvelocity reduction inside the anomaly reaches 5 in the lowermost 300km of the mantle waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is best characterized by a shear velocity model with a velocity jump of about 2 at about 100 145km above the core mantle boundary and a thin 30 km thick basal layer with a shear wave velocity reduction of 13 stacked seismic data sampling the middle of the anomaly however show no evidence for any internal discontinuity with a velocity decrease greater than 2 in the middle of the anomaly overall the seismic data sampling the base of the pacific anomaly can be explained by a negative shear velocity gradient from 0 to 1 top to 13 bottom in the lowermost 220 km of the mantle similar to those of a very low velocity province beneath the south atlantic ocean and the indian ocean such a strong negative shear velocity gradient can be explained by partial melting of a compositional anomaly produced early in the earth s history located within a bottom thermal boundary layer our travel time data also exhibit small scale variations inside the anomaly indicating existence of internal small scale seismic heterogeneities inside the pacific anomaly

extent of the low velocity region in the lowermost mantle beneath the western pacific detected by the vietnamese broadband seismograph array 1 we present evidence showing the extent of the low velocity region in the lowermost mantle beneath the western pacific we analyzed s ss scs s and sscs ss travel times observed by the vietnamese broadband seismograph array deployed as part of the ocean hemisphere project the abrupt changes in scs s and sscs ss travel times suggest that the western geographical boundary of the low velocity region is located around 140 e and is sharp more than 4 velocity contrast within 200 km the dependency of s and ss travel time anomalies of epicentral distances suggests that the strong low velocity region is confined to within 400 km from the cmb core mantle boundary the existence of lateral heterogeneities with a 100 km scale inside the low velocity region is also suggested by the abrupt changes in s and scs waveforms

tectonic development of the southern belt of the southern uplands accretionary complex detailed structural studies in the wenlock age southern belt of the southern uplands have identified major controls on tectonic development original thickness and composition of sediment control the style of faulting thin poorly consolidated interbedded sandstone mudstone sequences form broad sheared zones while thicker well consolidated sandstone dominated sequences fail along thin bedding parallel brittle fractures with vein infill the structural evolution of the sheared zones provides an insight into melange development depth of burial within the accretionary pile governs the development of cleavage and intensity of post fl deformational phases the intensity of imbrication within accreted material probably relates to subduction rate with early wenlock sequences being highly imbricated and mid wenlock sediments preserved in internally coherent fault bounded packets sinistral strike slip movement controls the development of steeply plunging folds sub horizontal stretching directions and sideways facing bedding cleavage relationships and strongly influences fabric development in sheared zones determination of the relative importance of these factors permits a fuller understanding of the deformational history

elemental dispersion and stable isotope fractionation during reactive fluid flow and fluid immiscibility in the bufa del diente aureole ne mexico evidence from radiographies and li b sr nd and pb isotope systematics the 31 6 0 3 ma old bufa del diente alkali syenite ne mexico intruded a sequence of cretaceous limestones with intercalated sub horizontal chert layers the cherts acted as aquifers that facilitated transport of brines and pegmatitic melts within the shallow level 1 kbar contact metamorphic aureole fluid driven reactions between chert and marble wallrock and the influx of late melts and various fluids gave rise to distinct chemical and isotopic signatures within the aquifer and across the zones of infiltration and fluid driven reaction aqueous brines of magmatic origin produced thick wollastonite mantles around the chert layers wollastonite formation occurred at the expense of limestone and chert and generated co2 this co2 induced fluid unmixing into an aqueous brine and a low density co2 rich fluid which was lost to the overlying marble where it oxidized organic matter and caused 13c and 18o shifts in a zone some 5 10 cm wide after wollastonite formation the chert aquifers were locally intruded by pegmatite veins carrying alkali feldspar quartz aegirine augite eudialyte zircon and apatite aqueous fluids that exsolved during crystallization of the pegmatite veins escaped along late cross fractures and migrated along the inner and outer borders of the wollastonite margins chemical dispersion patterns of u al na k p s fe and ree across the chert to marble boundary and its metasomatic rims are shown by autoradiography and neutron induced radiography scavenging of cations at mineralogical contacts and cation transport into the marbles occurred only on the mm to cm scale isotopic data for pb and sr across a simple metachert marble boundary and for pb sr nd b and li across a metachert pegmatite marble sequence demonstrate the following 1 the pb and sr isotopic signature of early fluids was buffered by the carbonate wallrock only late fluids shielded from wallrock interaction by a wollastonite mantle variably preserved a memory of their initial magmatic signature 2 since the nd isotope signature of marble and chert is bound to calcite and clay minerals systematic shifts to unradiogenic nd in marble reflect loss of carbonate bound nd as the wollastonite margin is approached nd in the wollastonite margin is dominated by nd originally bound to clay minerals the later emplacement of the pegmatite which carried the nd isotope signature of its alkali syenite source had little effect on the nd isotopic composition of the wollastonite rim 3 although the li and b isotopic compositions reflect the alkali syenite source they are also affected by isotopic fractionation and partitioning between melt fluid and solids

complicated secondary textures in zircon record evolution of the host granitic rocks studies from western tauern window and otztal stubai crystalline complex eastern alps western austria abstract samples of metamorphosed and deformed granitic rocks were collected from two alpine complexes with well constrained metamorphic history western tauern window and otztal stubai crystalline complex zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques cathodoluminescence cl imaging and raman spectroscopy the aims were to describe and interpret complicated secondary textures and microstructures in zircon based on cross cutting relationships between secondary microstructures reconstruct the sequence of processes affecting zircon crystals link the evolution of zircon with the history of the host rocks the results indicate that zircon in the sampled granitic rocks forms growth twins and multi grain aggregates which are unusual for this mineral moreover various secondary textures have been found in the sampled zircon often cross cutting each other in a single crystal these include distorted oscillatory cl zoning with inner zones forming inward penetrating cl bright embayments which are the evidence of dry recrystallization via annealing lattice recovery cl mosaicism with no preservation of growth zoning but abundant nano and micro scale pores and mineral inclusions which are the evidence of recrystallization by coupled dissolution reprecipitation and or leaching embayed zircon boundaries filled with apatite monazite epidote and mylonitic matrix indicating mineral fluid reactions resulting in zircon dissolution and fragmentation overgrowth cl dark rims which contain nano pores and point to transport and precipitation of dissolved zircon matter we conclude that zircon in our meta granites is sensitive to metamorphism deformation events and was reactive with metamorphic fluids additionally we have found evidence of crystal plastic deformation in the form of low angle boundaries and bent grain tips which is a result of shearing and ductile deformation of the host rock we suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization prograde metamorphism peak of amphibolite facies metamorphism post peak cooling and exhumation formation of ductile shear zones and final cooling to 250 c

weathering dynamics reflected by the response of riverine uranium isotope disequilibrium to changes in denudation rate abstract the ratio between the activity of dissolved 234 u and 238 u in river water 234 u 238 u riv shows promise for tracing weathering processes it has been suggested that preferential 234 u release generates a high 234 u 238 u riv under low weathering intensity while congruent weathering as expected for high weathering intensity would release uranium with a secular equilibrium 234 u 238 u riv of 1 however weathering intensity depends on the combination of exposure age of weathering interfaces and dissolution rate complicating the use of 234 u 238 u riv as a tool in weathering studies this work attempts to resolve the weathering dynamics reflected by 234 u 238 u riv using catchment scale denudation rate d as a first order approximation for the age of weathering interfaces a global dataset n 653 including 132 new measurements from chinese catchments indicates changing responses of 234 u 238 u riv to d higher values 1 5 and larger variability of 234 u 238 u riv are observed in catchments both with high 3 mm yr and low d 234 u 238 u riv are associated with catchments of medium to high d 0 3 2 mm yr the observed pattern can be explained by a model that involves competing uranium release from active and inactive weathering interfaces in response to a shifting weathering regime from kinetically limited to supply limited 234 u 238 u riv decreases with decreasing d under a kinetically limited weathering regime due to the increasing contribution of congruent weathering relative to the preferential dissolution of 234 u from radioactively damaged sites as d decreases in contrast 234 u 238 u riv increases with decreasing d under a supply limited weathering regime because the minerals that remain under these conditions continue to release 234 u to solution by recoil while contributing little to the dissolved 238 u because of their slow dissolution rate a much deeper depth of the weathering interface is suggested for catchments with high d due to higher weathering contribution from deep fractures relative to the poorly developed surface regolith a young age of 10 kyrs is constrained by the model for complete dissolution of exposed weathering active minerals which implies a rapid shifting from kinetically limited to supply limited weathering regime and limited influence of weathering on the calculation of comminution age based on 234 u 238 u in sediments a site specific dissolution rate that is consistent with those estimated from weathering profiles is also constrained which suggests that the discrepancy between field and laboratory weathering rate may largely be controlled by the difference in chemical affinity associated with stagnant micro pores low permeability and high rock fluid ratios rather than the passivation of weathering interfaces

very low shear wave velocity at the base of the mantle under the south pacific superswell abstract s wave velocity heterogeneity at the base of the mantle beneath the south pacific superswell has been examined using skks sks and s sdiff sks differential travel times the residuals of skks sks and s sdiff sks with respect to the preliminary reference earth model prem are compared to the residuals predicted using recently published three dimensional s wave velocity models we have obtained a three dimensional model that explains the s sdiff sks residuals well and applied this model to the correction of the skks sks residuals the corrected skks sks residuals beneath the south pacific superswell remain large and positive forward modeling reveals that these residuals can be accounted for by a 5 7 low velocity region with a lateral extent of about 500 km in the lowermost 200 km of the mantle although the magnitude of the velocity reduction and the thickness of the anomalous region trade off against one another and cannot be uniquely determined the existence of a very low s wave velocity region at the base of the mantle is a robust feature this feature lies close to the ultra low velocity zone ulvz identified by p velocity data beneath the southwestern pacific and is located just below a 2 3 low s velocity body in the lower mantle at depths of 2300 2700 km beneath the south pacific superswell

high resolution 3 d s velocity structure in the d region at the western margin of the pacific llsvp evidence for small scale plumes and paleoslabs abstract although previous tomographic studies found a large low s velocity province llsvp in the lowermost mantle beneath the pacific due to a lack of resolution it remains unclear whether the llsvp consists of clusters of small scale low velocity anomalies or large scale anomalies we recently deployed a seismic array in thailand which provides a dataset with wide azimuthal coverage at the western pacific llsvp we analyze the new dataset using waveform inversion and find high velocity anomalies extending vertically to a height of 400 km above the core mantle boundary cmb beneath the philippine sea and small scale low velocity patches with a diameter of 300 km at the cmb beneath new guinea the locations of the high velocity anomalies are consistent with the past izanagi plate subduction boundary and the low velocity anomalies can be interpreted as a small scale plume cluster hence we conclude that vertical flow upwelling plumes and downwelling of slabs is dominant in the lowermost mantle beneath the western pacific region

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

long term exhumation of an aegean metamorphic core complex granitoids in the northern menderes massif western turkey the egrigoz koyunoba and alacam plutons are located in the northern portion of the menderes massif a region characterized today by large scale extension to gain a better understanding of their exhumation history we acquired zircon ages geochemical analyses and cathodoluminescence cl images to search for evidence of micro to macro scales of deformation the combination is a powerful means to decipher their tectonomagmatic history in situ ion microprobe 238u 206pb zircon ages of the granitoids range from 30 0 3 9 ma to 14 7 2 6 ma 1 and indicate the plutons crystallized over 15 m y the dated zircons show cl zoning consistent with igneous crystallization and are only located adjacent to or as inclusions in biotite grains the youngest ages are dominated by a blue color in cl whereas a majority of older zircons are yellow higher th u contents are typically shown by zircons with black or dark green colors in cl most of the samples of these granitoids indicate they are magnesian calc alkalic and peraluminous granite to granodiorites but variations exist likely reflecting heterogeneity caused by magma mixing partial melting crustal contamination and post emplacement fluid interactions as evidenced by the cl images cl images show that all samples experienced magma mixing multiple episodes of brittle deformation and fluid mediated alteration sources for the generation of northern menderes massif granitoids include simple slab induced upwelling from the subduction of the african plate along the hellenic arc adiabatic decompression as the northern edge of turkey9s anatolide tauride block delaminates and upwelling asthenosphere as the continental lithosphere thins during extension we present a model in which western turkey is an amalgamation of stacked subduction zones that transition from north to south over time northern menderes massif granitoids document their complex geologic history in their ages chemistry and textures

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

style scale and significance of sand bodies in the northern and central belts southwest southern uplands sedimentological and biostratigraphical data from the rhinns of galloway and adjacent areas in sw scotland confirm that deep water depositional systems consistently operated along and were sourced from the northwestern margin of an asymmetrical basin during the late ordovician and early silurian while pelagic facies accumulated simultaneously to the se providing a scenario analogous to many modern trench systems most of the observed sedimentological anomalies with regard to this general model can be explained within the context of the varied styles of trench filling depositional systems briefly reviewed here and the major stratigraphic and sedimentologic features can be best explained in terms of a geotectonically evolving fore arc region two main phases of development are recognized a llandeilo late ashgill during this time interval the fore arc trench region was tectonically juxtaposed against an active continental margin arc small to medium scale se prograding sand rich fans were formed within a relatively narrow trench leading to axial diversion of the fans initially to ne but later mainly to sw simultaneously a coarse volcanilithic sediment apron flanking the arc migrated gradually northeastwards probably in response to relative fault displacement of the arc and trench b llandovery during this time interval the fore arc trench region was dominated by a variety of mainly fan type depositional systems which were exclusively sourced at least until the uppermost llandovery from the northwestern margin the juxtaposition of coeval depositional systems differing significantly in sedimentological style and composition strongly suggests that the llandovery sedimentation in this region occurred in several structurally separated sub basins within the main trench system

rare earth element geochemistry of upper ordovician cherts from the southern uplands of scotland caradoc and ashgill radiolarian cherts and siliceous mudstones from the southern uplands preserve primary rare earth element ree signatures which are comparable to those of more recent deposits from continental margin settings this is incompatible with the widely held view of these rocks as open ocean deposits incorporated in an accretionary prism and reinforces the model of deposition on an extensional continental margin the ree signatures can be used as fingerprints to differentiate between some of the fault bounded formations within the area they indicate the provenance of the mud grade siliciclastic material in these distal hemipelagites and are comparable with published ree data on grey wackes in the same successions this detailed analysis of the ree patterns in lower palaeozoic cherts demonstrates the usefulness of this approach in ancient orogens

detection of ridge like structures in the pacific large low shear velocity province abstract waveform tomography is conducted for sh velocity structures of the entire mantle using approximately 3 5 times the data used for obtaining the previous model sh18ce the resultant new model sh18cex exhibits a cluster of ridge like low velocity anomalies in the western part of the pacific large low shear velocity province llsvp the location of the ridge like anomalies is in good agreement with the location of the abrupt change in the topography of the d discontinuity these results suggest that the llsvp is associated with a cluster of ridge like piles rather than a single large pile spread over the entire region the piles probably consist of intrinsically dense material however either their volume or density contrast may not be sufficiently large to develop large scale domes

improved accuracy of zircon u th he ages by rectifying parent nuclide zonation with practical methods abstract a major limitation in the accuracy of zircon u th he thermochronometry zhe is the alpha ejection correction the standard alpha ejection correction f t assumes a homogeneous u th distribution in the host crystal here two approaches are used to address parent nuclide heterogeneity often observed in natural zircons 1 a new methodology where offending zonation is mechanically removed and 2 custom alpha ejection corrections informed by a la icp ms depth profile f zac we present an empirical test of these methodologies on a suite of bedrock zircons from variscan orthogneisses of the central aegean which display marked parent nuclide zonation which has impacted the ability to obtain accurate and precise thermochronometric data during the progressive mechanical abrasion of enriched u th and he zones the change in grain diameter exhibits a parabolic relationship with respect to the apparent age where the youngest age should represent the true age of the sample the abrasion corrected weighted mean age for sample 08pa01 is 8 8 0 7 ma f t 1 a too young age possibly the result of parent nuclide enriched cores the findings also show that using f zac instead of f t may change single sample zhe aliquots up to 29 using f zac increases the weighted mean age 17 for sample 08pa01 from 9 2 0 7 ma f t to 10 8 0 9 ma f zac and 6 for sample 08pa71 from 10 9 0 8 f t ma to 11 5 0 9 ma f zac in this case the f zac correction improves the overall accuracy of the zhe data but does not impact the problem of over dispersed ages suggesting they may be separate issues the increase in zhe age brings the dataset into agreement with previous thermochronometric studies indicating that cooling of the paros footwall below 180 c may have occurred earlier than hitherto thought

the age and tectonic significance of the bute amphibolite highland border complex scotland sm nd and k ar isotopic determinations on an amphibolite from the highland border complex yield cambrian ages collisional events which might have generated the amphibolite appear incompatible with the passive margin character of laurentia at that time a possible explanation is that the part of the highland border complex containing the amphibolite and associated rock units was generated outside laurentia

laser probe40ar39ar measurements of loss profiles within individual hornblende grains from the giants range granite northern minnesota usa abstract a nd glass laser has been used as a probe mapping natural argon diffusion in partially outgassed hornblende grains at a resolution of around 50 m the hornblendes are from the giants range granite 2700 ma minnesota taken approximately 2 km from the contact with the duluth gabbro complex 1100 ma the full range of ages can be discerned within individual hornblende grains using the laser extraction technique but more importantly diffusion profiles up to 500 m in length have been detected the profiles reflect large variations in domain size within single grains diffusive argon loss was controlled not only by lattice dominated volume diffusion but a network of fast diffusion paths now defined by biotite inclusions and also indirectly by mineral reactions associated with the contact metamorphism three natural argon loss profiles from subgrains of different sizes yield good fits to theoretical volume diffusion loss curves and a mean time integrated diffusion parameter d t dt of 7 1 0 9 10 5 cm 2 this parameter constrains the possible heat loss models for the duluth gabbro

microstructure of 24 1928 ma concordant monazites implications for geochronology and nuclear waste deposits the microstructure of monazite was studied using scanning electron microscopy sem electron microprobe analysis emp x ray diffraction patterns xrd and transmission electron microscopy tem four well characterized monazites were investigated having very different concordant u pb ages 24 to 1928 ma and up to 15wt tho2 and 0 94wt uo2 the sem and emp analyses of polished single crystal fragments reveal the absence of significant chemical zoning xrd and tem investigations show that the monazites are not metamict despite their old ages very high abundances of radionuclides and hence high time integrated radiation doses except for the youngest one the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains this structure is responsible for the presence of 1 mottled diffraction contrasts on the tem and 2 a second structural phase b with very broad reflections in the xrd patterns older monazites receive higher self irradiation doses and hence they contain higher amounts of this b phase for the 1928 ma monazite xrd reveals only the broad reflections of phase b implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice it is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because selfannealing heals the defects as they are produced by alpha decay damage the only memory of irradiation induced defects is the presence of distorted domains as the diffusion rate of pb in an undisturbed monazite lattice is extremely low pb loss due to volume diffusion out of the monazite lattice is virtually impossible this is considered as one reason why almost all monazites have concordant u th pb ages moreover as long term self irradiation effects are limited in monazite we consider this phase as a good candidate for the storage of high level nuclear waste under the aspect of its high resistance to irradiation

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

turbidite geochemistry and provenance studies in the southern uplands of scotland regional lithogeochemical data from the southern uplands have been used to characterize a distinctive stratigraphy across this region a suite of 840 point counted and petrographically classified greywacke samples were used to establish chemical fingerprints for a series of greywacke dominated lithostratigraphical units these fingerprints were then used to evaluate a further 1455 greywacke samples collected throughout the southern uplands and longford down and enabled a series of strike parallel geochemical tracts to be defined four principal geochemical groups are recognized relating to cratonic and volcanic derived greywacke provenances and both carbonate rich and hydrothermally altered greywackes volcanic derived units display higher ti fe mg ca na mn cr ga ni sr v and zn values than their cratonic counterparts which with the exception of the carbonate rich hawick group display higher si k la nb rb th and zr volcaniclastic greywackes display ree patterns dissimilar to typical post archean upper crust but similar to their andesitic components whereas cratonic groups have ree patterns close to that of upper crust systematic strike parallel geochemical variation in the southern uplands is controlled by petrographical differences which directly reflect provenance with individual lithostratigraphical units derived from variable mixtures of ophiolitic calc alkaline acid igneous low grade metamorphic and carbonate rich detritus

retention of radiation damage in zircon xenocrysts from kimberlites northern yakutia abstract we have studied zircon xenocrysts from mesozoic kimberlites from the kuoika and ary mastakh fields in northern yakutia zircon xenocrysts are assumed to originate from crustal rocks our shrimp sensitive high mass resolution ion microprobe analyses yielded predominantly concordant u th pb ages up to 3570 ma paleoarchean that clearly predate kimberlite formation the general u th pb concordance observed excludes notable disturbance of the zircon xenocrysts u th pb isotope system during kimberlite ascent and emplacement in addition zircon xenocrysts were found to be significantly more radiation damaged than would correspond to damage accumulation only since the time of kimberlite formation this observation first indicates that zircon crystals were sampled by the kimberlite magma at comparably shallow depths not exceeding 10 12 km if in contrast zircon crystals originated from deeper levels of the earth s crust they would have been exposed to temperatures of 250 300 c or more this in turn would have caused long term thermal annealing of the radiation damage which was however not observed in our study second our observation contradicts the hypothesis that high temperatures experienced by zircon xenocrysts during kimberlite ascent will cause notable structural reconstitution by short term thermal annealing consequently zircon crystals cannot have spent more than a few hours at temperatures exceeding ca 700 800 c or more than a few days at temperatures exceeding ca 500 600 c this in turn suggests that i temperatures of the ascending kimberlite magmas were rather moderate and ii kimberlite ascent is a comparably short process followed by rapid cooling

nd isotope study of provenance patterns across the british sector of the iapetus suture the southern uplands greywacke succession scotland accumulated at the laurentian margin of the iapetus ocean it was sequentially incorporated into an imbricate accretionary thrust complex until closure of the ocean thereafter the thrust belt propagated across the suture zone as a foreland thrust belt directed towards the hinterland of avalonia a foreland basin migrating ahead of the thrust belt was the depositional site for the southernmost southern uplands units and the windermere supergroup english lake district a nd isotope study has shown that juvenile ophiolitic detritus was introduced into the oldest mid ordovician southern uplands greywackes before two distinct provenance areas evolved one supply ing juvenile andesitic detritus in addition to a quartzo feldspathic component the other proterozoic and exclusively quartzo feldspathic bimodal composition continued into the early silurian but was overlapped from late in the ordovician by greywackes with intermediate nd isotope composition this was not a sim ple mixing effect since the andesitic component is not represented and the necessary juvenile component comes from granodioritic and felsitic lithologies intermediate end values are then a consistent feature through the silurian both in the younger strata of the southern uplands and in the earliest foreland basin tur bidites of the windermere supergroup the transition suggests cessation of volcanicity and erosion of deeper levels of the provenance terrane s possibly linked to the evolution of the basin system from active margin accretion related to a foreland setting to the north of the southern uplands terrane beyond the southern upland fault a caradoc to wenlock turbidite sequence occupies inliers within the midland valley the older greywackes contain abundant juvenile ophiolite and plutonic detritus in addition to a quartzo feldspathic metamorphic component there are similarities with the most northerly part of the southern uplands from the late ordovician end values systematically decline so that early llandovery midland valley greywackes are exclusively quartzo feldspathic derived from an ancient source indistinguishable in isotopic terms from that periodically supplying the southern uplands in general the llandovery midland valley provenance was significantly more mature than that contemporaneously supplying the southern uplands thereafter the midland valley latest llandovery and early wenlock greywackes contain a higher proportion of a juvenile component and by the early wenlock greywackes from the midland valley southern uplands and lake district terranes are similar in terms of end a common provenance seems likely and suggests that by the mid silurian all three terrenes were in close proximity

dry annealing of metamict zircon a differential scanning calorimetry study we report the results of a differential scanning calorimeter dsc study of the annealing of a metamict sri lankan zircon raman measurements on most chips of the powdered zircon starting material sri lankan zircon wz19 showed no evidence of a crystalline structure whereas a few chips retained residual raman bands typical of highly radiation damaged zircon dsc runs on aliquots of the powdered sample were heated to 850 and 1000 c at rates of 2 and 10 c min and to 1500 c at a rate of 10 c min raman spectroscopy was used to investigate the crystallinity of grains at selected temperature stages exothermal peaks were observed at about 910 and 1260 c during the dsc run to 1500 c the 910 c peak was demonstrated by raman spectroscopy to mark the crystallization of tetragonal zirconia and the exothermic peak at about 1260 c was demonstrated to represent the reaction of zirconia and amorphous silica to form crystalline zircon the degree of crystallinity of these grains was almost identical to that of highly crystalline zircons from recent gem gravels from new south wales a small number of experimental chips from dsc analyses under 1000 c were found to have zircon raman bands that indicated they had undergone partial annealing the present experimental results suggest that reconstitution of amorphous zircon to the crystalline state by dry annealing will rarely occur in terrestrial geological settings even under extreme metamorphic conditions

isotopically heterogeneous initial pb and continuous 222 rn loss in fossils the u pb systematics of brachiosaurus brancai abstract diagenetic incorporation of uranium into bone material establishes a u pb system that provides a minimum age for the fossil loss of rn and ra from the 238u decay series recent mobility of u and pb and incorporation of radiogenic pb from an old basement which was available during diagenesis disturbed the u pb system all data fall in a 206pb 238u 207pb 235u concordia diagram to the right of the concordia while samples that do not experience redistribution of u pb 222rn and 226ra should define a linear array that intersects the concordia curve at the age of diagenesis and at the age of the source of inherited radiogenic pb loss of 222rn and 226ra has led to displacement of the present data set to lower 206pb 238u and therefore to younger intercept ages and to older 207pb 206pb ages the 207pb 204pb 208pb 204pb systematics demonstrates that 1 three out of 19 samples experienced significant postdiagenetic fractionation of u and pb and that 2 the diagenetic pb addition was isotopically heterogeneous the 235u 207pb 206pb 207pb and 206pb 238u 207pb 235u systematics combined demonstrate that 3 the samples experienced 5 to 18 222rn loss for a 1 9 ga old source of the diagenetically introduced pb and correspondingly more for a younger pb source and that 4 there is no unique solution for the age of the source of diagenetic pb since pb from a younger source appears in the 235u 207pb 206pb 207pb diagram in the same way as higher 222rn loss no precise age can be determined from the data although the most consistent solution falls in the range of 140 to 150 ma addition of radiogenic pb to the fossil bone is obvious only through its anomalous highly radiogenic character and the isotopic heterogeneity in 208pb 204pb no th i e no in situ 208pb growth the effect of unrecognized diagenetic additions of radiogenic pb is an overestimation of the age whereas unrecognized 222rn loss results in a too young discordant age since fossil material formed at times for which the discordia only shows a small curvature anomalies originating from diagenetic pb additions and 222rn loss may not always be recognized

mgsio3 postperovskite at d conditions the postperovskite transition in mgsio3 at conditions similar to those expected at the d discontinuity of earth s lower mantle offers a paradigm for interpreting the properties of this region despite consistent experimental and theoretical predictions of this phase transformation the complexity of the d region raises questions about its geophysical significance here we report the thermoelastic properties of cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle these are i the jumps in shear and longitudinal velocities similar to those observed in certain places of the d discontinuity and ii the anticorrelation between shear and bulk velocity anomalies as detected within the d region in addition the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle

raman micro spectroscopy on diamond graphite and other carbon polymorphs from the ultrahigh pressure metamorphic kimi complex of the rhodope metamorphic province ne greece raman micro spectroscopy was applied on carbon inclusions in garnet porphyroblasts from kyanite biotite garnet schists of the rhodope metamorphic province rmp ne greece diamond and cuboids of poorly to highly ordered graphite were identified either as single phase inclusions or as polyphase inclusions along with co2 and or carbonates calcite magnesian calcite questionable raman bands that may be assigned to other c phases nanodiamond lonsdaleite a different c polymorph have been observed the presence of diamond confirms beyond any doubt the ultrahigh pressure uhp metamorphism reported by mposkos and kostopoulos 1 e mposkos d kostopoulos diamond former coesite and supersilisic garnet in metasedimentary rocks from the greek rhodope a new ultrahigh pressure metamorphic province established earth planet sci lett 192 2001 497 506 in the rmp cuboid graphite showing variable degree of disordering most probably formed after diamond the possible involvement of co2 and or c o h fluids in the formation of diamond is discussed

uplift patterns and orogenic evolution in the scottish dalradian rb sr and k ar mineral ages from the dalradian rocks of angus and perthshire indicate considerable diachronism of cooling following the caledonian regional metamorphism the oldest mineral ages 515 ma are similar to the independent estimates of peak metamorphic age and hence indicate very rapid cooling they occur in the highest grade samples of the angus region and are associated with localized uplift regional rapid uplift events at 460 440 ma and 410 390 ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the scottish highlands thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the p t paths they suggest that the major diachronous metamorphism and granite production in the scottish highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources localized uplift may exercise considerable control on mineral facies variation within orogenic belts the local uplift in angus introduces hot high pressure tarfside nappe rocks into higher level tay nappe rocks thus the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

shock metamorphosed zircon in terrestrial impact craters to ascertain the progressive stages of shock metamorphism of zircon samples from three well studied impact craters were analyzed by optical microscopy scanning electron microscopy sem and raman spectroscopy in thin section and grain separates these samples are comprised of well preserved rapidly quenched impactites from the ries crater germany strongly annealed impactites from the popigai crater siberia and altered variably quenched impactites from the chicxulub crater mexico the natural samples were compared with samples of experimentally shock metamorphosed zircon below 20 gpa zircon exhibits no distinct shock features above 20 gpa optically resolvable planar microstructures occur together with the high pressure polymorph reidite which was only retained in the ries samples decomposition of zircon to zro2 only occurs in shock stage iv melt fragments that were rapidly quenched this is not only a result of post shock temperatures in excess of 1700 c but could also be shock pressure induced which is indicated by possible relics of a high pressure polymorph of zro2 however zro2 was found to revert to zircon with a granular texture during devitrification of impact melts other granular textures represent recrystallized amorphous zrsio4 and reidite that reverted to zircon this requires annealing temperatures 1100 c a systematic study of zircons from a continuous impactite sequence of the chicxulub impact structure yields implications for the post shock temperature history of suevite like rocks until cooling below 600 c

the varzaghan ahar iran earthquake doublet mw 6 4 6 2 implications for the geodynamics of northwest iran on 2012 august 11 a pair of large damaging earthquakes struck the varzaghan ahar region in northwest iran in a region where there was no major mapped fault or any well documented historical seismicity to investigate the active tectonics of the source region we applied a combination of seismological methods local aftershock network calibrated multiple event relocation and focal mechanism studies field observations structural geology and geomorphological and inversions for the regional stress field the epicentral region is north of the north tabriz fault the first main shock is characterized by right lateral strike slip motion on an almost e w fault plane of about 23 km length extending from the surface to a depth of about 14 km the second main shock occurred on an ene striking fault that dips at 60 70a degrees to the nw independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with sigma 1 oriented n132e for the region northeast of the north tabriz fault the presence of rigid lithosphere of the south caspian basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation at a global scale our results indicate that despite the basic assumption of rigid blocks in geodetic plate modelling internal deformation of block like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate

age and evolution of the lower crust beneath the western churchill province u pb zircon geochronology of kimberlite hosted granulite xenoliths nunavut canada abstract zircon from granulite facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within archean cratons we present the first such records from beneath two archean crustal blocks that were affected by multiple episodes of regional scale proterozoic metamorphism and are located within the western churchill province nunavut canada zircon u pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption as confirmed with raman spectroscopy zircon was recovered from four metabasites grt cpx plg from the rankin inlet area of the chesterfield block and five metabasites grt cpx plg hbl opx and three metatonalite anorthosites plg cpx plg hbl opx from the repulse bay area of the rae craton sem cl imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization the latter are related to local high temperature recrystallization and micro fracturing and are spatially associated with zones of apparent pb loss and or intracrystalline pb variability evidence for the formation of paleoarchean mesoarchean to neoarchean crust is preserved in oscillatory zoned igneous cores having 207pb 206pb ages of ca 3 5 ga and ca 3 0 2 6 ga two periods of paleoproterozoic metamorphism and zircon growth followed with the oldest episode at ca 2 0 1 9 ga in the chesterfield block metamorphic zircon rims from all twelve xenoliths have an age range of ca 1 75 to 1 70 ga the trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores with the rims characterized by lower hree contents and low lu gd n suggesting that garnet crystallization occurred syn to pre rim growth at ca 1 75 ga the archean and paleoproterozoic ages correspond broadly to the upper crustal histories of the western churchill province however the prevalence of ca 1 75 1 70 ga metamorphic rims indicates regional scale metamorphic reworking of the lower crust across the region during a period of intracratonic extension and associated magmatism regional magmatic underplating and or remobilization of the lithosphere at ca 1 75 1 70 ga to an extent not widely appreciated represents a terminal stage in western churchill cratonization including stabilization of reworked archean crust relative to underlying diamondiferous mantle

dating terrestrial impact structures u pb depth profiles and u th he ages of zircon we investigate the presence of epitaxial overgrowth rims and reset zircon complete loss of radiogenic lead pb from terrestrial impactites to constrain the occurrence of such phenomenon in impact environments and their possible use in dating impact events we also explore u th he dating of zircon to evaluate this geochronometer in accurately identifying impact ages particularly when no dateable melt sheet exists our results show that u th he ages of zircon from the brecciated and presumably shocked target can accurately date an impact event and provides another tool to determine impact ages when no melt sheet exists an alternative to problematic interpretations of commonly used apparent 40ar 39ar plateau ages no evidence of epitaxial overgrowth rims and or reset zircon was observed suggesting that zircons within shocked impactites have remarkably slow pb diffusion and possibly explaining the relatively few reset grains reported in terrestrial impactites

metamorphic zirconology of continental subduction zones abstract zircon is widely used to date geological events and trace geochemical sources in high pressure hp to ultrahigh pressure uhp metamorphic rocks of continental subduction zones however protolith zircons may be modified by three different types of metamorphic recrystallization via mechanisms of solid state transformation metasomatic alteration and dissolution reprecipitation new zircon growth may be induced by dehydration reactions below the wet solidus of crustal rocks metamorphic zircon or peritectic reactions above the wet solidus peritectic zircon as a consequence there are different origins of zircon domains in high grade metamorphic rocks from collisional orogens thus determining the nature of individual zircon domains is substantial to correct interpretation of their origin in studies of isotopic geochronology and geochemical tracing we advocate an integrated study of zircon mineragraphy internal structure and external morphology u pb ages mineral inclusions trace elements and lu hf and o isotope compositions only in this way we are in a position to advance the simple zircon applications to metamorphic zirconology enabling discrimination between the different origins of zircon and providing constraints on the property of fluid activity at subduction zone conditions the metamorphic recrystallization of protolith zircons and the new growth of metamorphic and peritectic zircons are prominent in hp to uhp metamorphic rocks of collisional orogens these different types of recrystallized and grown zircons can be distinguished by their differences in element and isotope compositions while the protolith nature of metamorphosed rocks dictates water availability the p t conditions of subduction zones dictate the property of subduction zone fluids the fluids of different properties may be produced at different positions of subducting and exhuming crustal slices and they may physically and chemically mix with each other in continental subduction channels such fluids can act as an important agent not only for the physical transport of protolith zircons but also for the chemical transport of element zr and other fluid mobile incompatible trace elements from the subducted crust to the mantle wedge therefore the discrimination between the different types of zircons provides a powerful means to decipher the role of fluids in subduction zone processes