ree th and u behaviour during hydrothermal and supergene processes in a granitic environment the two mica granites which host the two main uranium deposits from the northwestern massif central france margnac fanay and le bernardan deposits were hydrothermally altered both before and during the deposition of the primary u mineralization both altered rocks and mineralizations were then subjected to supergene processes we have analyzed the rare earth elements ree th and u in fresh and altered mineralized and barren whole rocks and in the pitchblendes to study the behaviour of these elements during hydrothermal and supergene processes in a granitic environment when fluids are aqueous with a low salt content mainly chloride then irrespective of temperature 400 450 c for feldspar episyenitization 350 c for mica episyenitization and 100 150 c for the quartz coffinite montmorillonite adularia stage the ree patterns and parameters the hree lree fractionation the th concentration and the th ce ratio are not modified this indicates that monazite which controls these elements in the two mica granites remains stable under such conditions in contrast when ligands are present in the solutions co2 3 or hco 3 with the pitchblende stage at 350 c hpo2 4 or other phosphorous anions during supergene events at low temperatures ree are either leached or concentrated depending on the prevailing redox conditions and there is hree lree fractionation the ree content of the newly formed minerals pitchblende appears to be mainly controlled in such deposits by the fluid composition

transport and selective uptake of radium into natural clay minerals abstract understanding of the environmental behavior of ra is important from the viewpoint of the long termed repository safety of radioactive waste but investigation of ra behavior in natural environment is difficult to detect we found isotopic evidence of ra transportation and its selective uptake into clay minerals from pb isotopic analyses illite grains found in calcite veins included in sandstone near the oklo uranium deposit republic of gabon show extremely low 207 pb 206 pb 0 0158 isotopic ratios although the pb isotopic ratios of calcite and quartz coexisting with illite indicate the formation age of each component those of illite do not in addition illite grains having low 207 pb 206 pb isotopic ratios contain a strongly large amount of ba 1230 to 6010 ppm in contrast with low contents of ba in calcite and quartz 207 pb 206 pb isotopic data suggest an excess of 206 pb due to selective adsorption of 226 ra and also ba into illite grains this is a very rare example to show evidence of the selective adsorption behavior of ra from the isotopic excesses of 206 pb although the adsorption ability of ra itself in nature was largely reported

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

meandering and transport variations of the florida current abstract current meter observations were collected from a three dimensional array moored in the florida straits between december 1983 and june 1984 as part of the subtropical atlantic climate studies stacs program approximately one fourth of the total subinertial velocity and temperature variance contained in these records is associated with meandering of the florida current on time scales ranging from several days to a few weeks there approach to be no strong correlation between the occurrence of meanders and variations in florida current volume transport or local wind forcing utilizing frequency domain empirical mode analysis we find the most coherent energetic meandering signals within two limited frequency bands centered near periods of 12 days and 5 days these meanders propagate downstream northward with phase speeds and wavelengths of approximately 28 km d 1 340 km and 36 km d 1 170 km respectively periodic waveforms composed from these modes indicate an asymmetric meander pattern wi

wellen radar wera a new ground wave hf radar for ocean remote sensing hf radars can be used to measure surface currents and wave spectra the coastal radar codar used by the university of hamburg was designed for current mapping only it has been operated for 15 field experiments during the past 15 years recently a new hf radar called wellen radar wera has been developed at the university of hamburg one main advantage of the system is the possibility of connecting different configurations of receive antennas when operated with a linear array information on the sea state can be obtained via second order spectral bands a further advantage is the flexibility in range resolution between 0 3 and 1 2 km instead of the fixed resolution of about 2 km of codar this is achieved by transmitting frequency modulated continuous wave fmcw chirps instead of continuous wave cw pulses in addition this technique avoids the blind range of about 3 km in front of the codar the technical design of wera is described and first experimental results are presented

deformed monazite yields high temperature tectonic ages the deformation of monazite in the polymetamorphic sandmata granulite complex in india has been investigated by electron backscattered diffraction and sensitive high resolution ion microprobe shrimp quantitative microstructural analyses document the development of deformation twins in 100 001 and graphic 1 orientations low angle 10 boundary development associated with dislocation creep and the development of new grains due to dynamic recrystallization these data represent the first quantitative evidence of crystal plastic deformation of natural monazite shrimp u th pb analysis shows that the host monazite preserves discordant ages as old as 1666 28 ma along a trend from ca 1720 ma to ca 1000 ma with increasingly discordant ages recorded in zones of higher lattice distortion domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 14 ma this age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith and is consistent with partial resetting and pb loss from domains deforming by dislocation creep the complex but systematic relationship between microstructure and age data in monazite provide the first direct evidence of pb isotope resetting during deformation the approach illustrates a new methodology for the dating of deformation events in high grade metamorphic rocks which are typically difficult to constrain 1 embed inline graphic 1 gif

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

formation and spreading of subtropical mode water in the north pacific a time averaged description of the formation and spreading of north pacific subtropical mode water npstmw is presented the formation of npstmw is studied using transpac expendable bathythermograph data the data are transformed into a stream coordinate system and averaged the coordinate system uses the position of the kuroshio front as its origin maps are presented of the temperature structure and mixed layer distribution in winter relative to the kuroshio front the formation of npstmw occurs in two separate stages cooling and thickening as water flows at the surface in the kuroshio in winter the water is cooled by the intense ocean atmosphere heat flux taking place in the area when water leaves the kuroshio to the south the thermocline underneath becomes deeper convection is able to penetrate deep into the water column and the heat flux acts more to thicken the surface mixed layer rather than reduce the temperature the area south of the kuroshio where relatively homogeneous waters are found is the source region for npstmw at the end of the winter the thick mixed layers are assumed to be covered over by a seasonal thermocline and become npstmw the levitus 1982 data are used to study the way in which npstmw is spread away from its source region by the subtropical circulation the distribution of potential vorticity pv on four npstmw isopycnals is studied with low pv used as a tracer for npstmw the pv reveals a pattern of differential spreading whereby npstmw formed in the western part of the source region is advected to the west after its formation npstmw formed farther east is advected more to the south or east after formation differential spreading allows npstmw to be found in a large part of the northwestern subtropical gyre well away from its source region the distribution of npstmw described by masuzawa 1969 with npstmw getting colder from west to east is explained as a combination of the distribution of npstmw at its source and the way it is spread after it is formed

the structure and transport of the gulf stream at 73 w abstract between september 1980 and may 1983 16 sections of temperature and velocity were obtained with the pegasus instrument along a transect crossing the gulf stream at 73 w the mean temperature and velocity fields of the upper 2000 meters were calculated the transport above 2000 meters calculated from the sections had a mean of 87 8 17 3 106 m3 s 1 a comparison with mass flux measurements obtained by previous investigators at cape fear indicated that the addition to the transport of the stream in the region was uniform with depth above 800 meters the mean inflow measured at the pegasus line was found to be uniform above 800 meters and to decrese substantially below this depth the increase in transport at the line estimated from the mean cross stream velocity field was 15 4 5 8 106 m3 s 1 per 100 km downstream distance the temporal fluctuation of the pegasus transport measurements was consistent with an annual cycle with a maximum in april the mean eddy kinetic energy at the surface

observations of the florida and yucatan currents from a caribbean cruise ship abstract the yucatan and florida currents represent the majority of the warm water return path of the global thermohaline circulation through the tropical subtropical north atlantic ocean their transports are quantified and compared by analyzing velocity data collected aboard the cruise ship explorer of the seas from 157 crossings between may 2001 and may 2006 the mean transport of the florida current at 26 n was estimated to be 30 8 3 2 sv 1 sv 106 m3 s 1 with seasonal amplitude of 2 9 sv upstream the yucatan current was estimated from 90 crossings to be 30 3 5 sv with seasonal amplitude of 2 7 sv these two currents are shown to be linked at seasonal time scales hence contrary to former results it was found that transports through the florida straits and the yucatan channel are similar with the implication that only small inflows occur through minor channels between them

eclogite facies quartz veins within metabasites of the dabie shan eastern china pressure temperature time deformation path composition of the fluid phase and fluid flow during exhumation of high pressure rocks

direct ion microprobe u pb dating of fossil tooth of a permian shark abstract we report here direct ion microprobe dating of fossil tooth dentine of a permian fresh water shark orthacanthus senckenbergianus using the shrimp instrument recently installed at hiroshima university fifteen spots on the small sample approximately 2 mm 1 mm indicate a 238 u 206 pb isochron age of 266 18 ma and a tera wasserburg concordia intercept age of 266 24 ma in a three dimensional 238 u 206 pb 207 pb 206 pb 204 pb 206 pb diagram these permian ages are consistent with a 235 u 207 pb age of 453 170 ma and a 232 th 208 pb age of 235 310 ma suggesting indistinguishable depositional and early diagenetic ages of the fossil in its sedimentary sequences the success of the method depends on the chemical fractionation of uranium from lead in a specimen a few hundred microns in size and the consequent variations in lead isotopic compositions due to radioactive decay

velocity and transport structure of the kuroshio extension at 35 n 152 e a current meter mooring maintained for 2 years at 35 n 152 e as part of the wespac array schmitz et al 1987 sampled the kuroshio extension during the last 390 days of deployment based on a method developed by hall and bryden 1985 for the gulf stream at 68 w velocity and temperature time series at five depths from 350 to 6000 m have been used to construct the representative horizontal velocity structure of the current cross stream position is quantified in terms of temperature at 350 dbar and an alongstream flow direction is defined on the basis of the current shear integrated transport of the kuroshio is 87 21 sv compared to 94 26 sv for the gulf stream at 68 w based on analogous data sets and analysis techniques the two currents are compared in terms of horizontal and vertical distributions of transport as well as relative strength of barotropic and baroclinic components the kuroshio has a larger relative barotropic component than the gulf stream while the latter has a stronger deeper thermocline expression the effects of rotating velocity measurements to a continuously changing alongstream direction are examined for both currents the major impact is to cut the apparent velocity variance in half demonstrating that much of the original variance is due to the meandering of a quasi fixed velocity profile rather than the passage of eddies results from these somewhat unconventional analysis methods are placed in broader geographical context and compare favorably with more traditional methods of determining these currents transports

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

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

transport potential vorticity and current temperature structure across northwest providence and santaren channels and the florida current off cay sal bank currents and temperatures were measured using pegasus current profilers across northwest providence and santaren channels and across the florida current off cay sal bank during four cruises from november 1990 to september 1991 on average northwest providence 1 2 sv and santaren 1 8 sv contribute about 3 sv to the total florida current transport farther north e g 27 n partitioning of transport into temperature layers shows that about one half of this transport is of 18 c water 17 c 19 5 c this can account for all of the excess 18 c water observed in previous experiments this excess is thought to be injected into the 18 c layer in its region of formation in the northwestern north atlantic ocean due to its large thickness potential vorticities in this layer in its area of formation are very low in our data lowest potential vorticities in this layer are found on the northern end of northwest providence channel and are comparable to those observed on the eastern side of the florida current at 27 n on average a low potential vorticity 18 c layer was not found in the florida current off cay sal bank

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

current variability on a narrow shelf with large ambient vorticity signals respectively and with corresponding phase speeds of 0 85 and 1 7 m s 1 the phase trend in east west direction of the 10 hour signal is consistent with unstable growing waves the 10 and 27 hour signals appear dominantly barotropic near the coast out to depths of at least 50 m but the 10 hour signal displayed a 180 phase change between surface and bottom farther away from the shelf break at 160 m depth the two dominant signals cannot be attributed to barotropic instability index terms 4219 oceanography general continental shelf processes 4520 oceanography physical eddies and mesoscale processes 4572 oceanography physical upper ocean processes keywords shelf circulation florida current vorticity flow instability surface doppler radar

temporal variation of meandering intensity and domain wide lateral oscillations of the gulf stream the path of the gulf stream exhibits two modes of variability wavelike spatial meanders associated with instability processes and large scale lateral shifts of the path presumably due to atmospheric forcing the objectives of this study are to examine the temporal variation of the intensity of spatial meandering in the stream to characterize large scale lateral oscillations in the stream s path and to study the correlation between these two dynamically distinct modes of variability the data used for this analysis are path displacements of the gulf stream between 75 and 60 w obtained from avhrr derived advanced very high resolution radiometer infrared images for the period april 1982 through december 1989 meandering intensity measured by the spatial root mean square displacement of the stream path displays a 9 month dominant periodicity which is persistent through the study period the 9 month fluctuation in meandering intensity may be related to the interaction of rossby waves with the stream interannual variation of meandering intensity is also found to be significant with meandering being much more intense during 1985 than it was in 1987 annual variation however is weak and not well defined the spatially averaged position of the stream which reflects nonmeandering large scale lateral oscillations of the stream path is dominated by an annual cycle on average the mean position is farthest north in november and farthest south in april the first empirical orthogonal function mode of the space time path displacements represents lateral oscillations that are in phase over the entire study domain interannual oscillations are also observed and are found to be weaker than the annual oscillation the eigenvalue of the first mode indicates that about 21 5 of the total space time variability of the stream path can be attributed to domain wide lateral oscillations the correlation between meandering intensity and domain wide lateral oscillations is very weak

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

hf radar comparisons with moored estimates of current speed and direction expected differences and implications the validation of estimates of ocean surface current speed and direction from high frequency hf doppler radars can be obtained through comparisons with measurements from moored near surface current meters acoustic doppler current profilers or drifters expected differences between current meter cm and hf radar estimates of ocean surface vector currents depend on numerous sources of errors and differences such as instrument and sensor limitations sampling characteristics mooring response and geophysical variability we classify these sources of errors and differences as being associated exclusively with the current meter as being associated exclusively with the hf radar or as a result of differing methodologies in which current meters and hf radars sample the spatially and temporally varying ocean surface current vector field in this latter context we consider three geophysical processes namely the stokes drift ekman drift and baroclinicity which contribute to the differences between surface and near surface vector current measurements the performance of the hf radar is evaluated on the basis of these expected differences vector currents were collected during the high resolution remote sensing experiment ii off the coast of cape hatteras north carolina in june 1993 the results of this analysis suggest that 40 60 of the observed differences between near surface cm and hf radar velocity measurements can be explained in terms of contributions from instrument noise collocation and concurrence differences and geophysical processes the rms magnitude difference ranged from 11 to 20 cm s 1 at the four mooring sites the average angular difference ranged between 15 o and 25 o of which about 10 o is attributed to the directional error of the radar current vector estimates due to the alignment of the radial beams

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

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

introducing a geochemical screen to identify geologically meaningful u pb dates in fossil teeth abstract previous studies have shown that radiometric dating of fossils can be complicated by post fossilisation alteration a key challenge to obtaining geologically meaningful fossilisation dates in bioapatite is identifying regions that have remained relatively undisturbed since the time of fossilisation in order to determine whether a geochemical indicator can be used to identify least disturbed regions we combine in situ trace element analysis and u pb geochronology of fossil teeth from the arroyo chijuillita member new mexico and the dinosaur park formation alberta the reliability of these u pb dates is supported by good agreement between the isotope dilution thermal ionization mass spectrometry 61 5 1 6 ma and laser ablation icp ms 64 2 2 4 ma dates for a targeted tooth region of fourteen teeth analyzed from these units two record dates within uncertainty of the depositional age one has a diffusion concave upward cross sectional yttrium profile the other a concave downward profile reflective of trace element loss it was found for teeth from both units that smaller yttrium concentration ranges are correlated with younger u pb dates likely the result of post fossilisation alteration a yttrium screen based on this relationship is proposed and is found to be a better indicator of post fossilisation open system behaviour in the samples analyzed than other geochemical features thought to be produced during fossilisation this screen provides a new tool for identifying fossil material least disturbed since fossilisation facilitating the use of bioapatite to date the time of diagenesis

on the seasonal variability of the currents in the straits of florida and yucatan channel 1 seasonal variability in the major passages of the caribbean are investigated using 5 years of wind and current observations explorer of the seas combined with a 3 year 1 12 simulation seasonal fluctuations in the florida and yucatan passages are both characterized by semiannual cycles with transport maxima in summer and winter and minima in spring and fall in the straits of florida seasonal variability is associated with surface intensified fluctuations of the florida current and can be largely explained by along channel winds and wind stress curl over the atlantic ocean in the yucatan channel seasonal variability is more barotropic and does not covary with wind forcing but rather is associated with fluctuations of an anticyclonic recirculation to the south of the channel upstream the seasonality in the grenada passage also covaries with seasonality in the yucatan and florida passages although the grenada passage carries only a small fraction of the total inflow to the caribbean recent in situ estimates of the mean transport in the yucatan channel differ by 7 sv 30 5 sv explorer of the seas 23 1 sv canek an assessment of sampling errors biases and interannual variability seem to account for no more than 2 sv leaving no satisfactory explanation for the difference in explorer of the seas data flows in the straits of florida and yucatan channel display similar vertical shear structure while canek data exhibit significantly lower shears in the upper 500 m which account for all the transport difference

eddy mean flow interaction in the kuroshio extension region abstractthe authors use data collected by a line of tall current meter moorings deployed across the axis of the kuroshio extension ke jet at the location of maximum time mean eddy kinetic energy to characterize the mean jet structure the eddy variability and the nature of eddy mean flow interactions observed during the kuroshio extension system study kess a picture of the 2 yr record mean jet structure is presented in both geographical and stream coordinates revealing important contrasts in jet strength width vertical structure and flanking recirculation structure eddy variability observed is discussed in the context of some of its various sources jet meandering rings waves and jet instability finally various scenarios for eddy mean flow interaction consistent with the observations are explored it is shown that the observed cross jet distributions of reynolds stresses at the kess location are consistent with wave radiation away from the jet with the sense of the eddy feedback effect on

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

protracted diagenetic alteration of ree contents in fossil bioapatites direct evidence from lu hf isotope systematics abstract fossil bones and teeth are potentially important repository for geochemical proxy data and a target for radiometric dating the concentration of many trace elements in bones and teeth increases by orders of magnitude after death and it is this diagenetic incorporation that forms the basis for several areas of geochemical study the use of bones and teeth in this context relies on two assumptions first that target metal ions are incorporated rapidly after death reflecting a known environmental signal and second that after early incorporation the bone or tooth remains as an essentially closed system resistant to later diagenetic change a wide literature has developed exploring these assumptions but relatively little direct evidence has been used to assess the long term diagenetic stability of trace elements within bones and teeth in this study we use the lu hf isotope system to show that bones and teeth of cretaceous and triassic age from both terrestrial and marine settings experience continued long term diagenetic change most likely through gradual addition of trace elements modelling suggests that diagenetic addition after initial recrystallisation may account for 50 of the total ree content in the sampled bones the extent depending on initial uptake conditions tooth enamel and enameloid may be more resistant to late diagenetic changes but dentine is probably altered to the same extent as bone these results have significant implications for the use of bones and teeth as hosts of chronological palaeoceanographic palaeoenvironmental and taphonomic information particularly in mesozoic and palaeozoic contexts

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

dating multiply overprinted sn mineralized granites examples from the erzgebirge germany granites and primary tin mineralization in the erzgebirge were dated using 1 conventional u pb dating of uraninite inclusions in mica 2 rb sr dating of inclusions in quartz that represent highly evolved melts 3 re os dating of magmatic hydrothermal molybdenite and 4 chemical th u pb dating of uraninite conventional isotope dilution and thermal ion mass spectrometry and chemical th u pb dating of uraninite in granites from the ehrenfriedersdorf mining district provide ages of 323 9 3 5 ma 2 greifenstein granite and 320 6 1 9 and 319 7 3 4 ma 2 both sauberg mine in agreement with u pb apatite ages of 323 9 2 9 and 317 3 1 6 ms 2 both sauberg mine rb sr analysis of melt inclusions from zinnwald gives highly radiogenic sr isotopic compositions that with an assumed initial sr isotopic composition permit calculation of precise ages from single inclusions the scatter of the data indicates that some quartz hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion re os dating of two molybdenite samples from altenberg provides ages of 323 9 2 5 and 317 9 2 4 ma 2 together with age data from the literature our new ages demonstrate that primary tin mineralization and the emplacement of the large sn specialized granites in the erzgebirge fall in a narrow range between 318 and 323 ma primary sn mineralization occurred within a short interval during post collisional collapse of the variscan orogen and was essentially synchronous over the entire erzgebirge in contrast to earlier claims no systematic age difference between granites of the eastern and western erzgebirge was established furthermore our data do not support a large age range for late variscan granites of the erzgebirge 330 290 ma as has been previously suggested

comment on in situ ion microprobe u pb dating and ree abundances of a carboniferous conodont by y sano and k terada

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

the surface velocity structure of the florida current in a jet coordinate frame the structure and variability of the florida current between 25 26 n are investigated using hf radar ocean current measurements to provide the most detailed view of the surface jet to date a 2 d jet coordinate analysis is performed to define lateral displacements of the jet in time meandering and associated structural variations over a 2 year period 2005 2006 in the jet coordinate frame core speed has a median value of 160 cm s 1 at the central latitude of the array 25 4 n with a standard deviation std of 35 cm s 1 the jet meanders at timescales of 3 30 days with a std of 8 km and a downstream phase speed of 80 km day 1 meandering accounts for 45 of eddy kinetic energy computed in a fixed geographical reference frame core speed width and shear undergo the same dominant 3 30 day variability plus an annual cycle that matches seasonality of alongshore wind stress jet transport at 25 5 n exhibits a different seasonality to volume transport at 27 n most likely driven by input from the northwest providence channel core speed correlates inversely with miami sea level fluctuations such that a 40 cm s 1 deceleration is associated with a 10 cm elevation in sea level although there is no correlation of sea level to jet meandering or width accurately quantifying the florida current s variability is critical to understand and forecast future changes in the climate system of the north atlantic as well as local impacts on coastal circulation and sea level variability along south florida s coastline

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

a mean synoptic view of the subantarctic front south of australia abstract a mean synoptic view of the subantarctic front saf is obtained from current meter and hydrographic data by averaging absolute and baroclinic velocity measurements in bins defined by a cross stream coordinate that moves with the current the cross stream coordinate is derived from current meter measurements of vertical shear of the horizontal velocity and is expressed in terms of specific volume anomaly at 780 dbar 780 by averaging absolute and baroclinic velocity measurements in stream coordinates the spatial smoothing that results from eulerian averaging of measurements of a meandering current is avoided the mean saf velocity profile is composed of one central peak and several smaller peaks in the central peak the 2 yr mean absolute velocity from current meters reaches 52 cm s 1 at 420 dbar the mean baroclinic velocity from six ctd sections reaches 34 cm s 1 the saf flow is coherent at all levels reaches the seafloor and is at least 220 km wide the cross stream structure of barocl

cenozoic oxidation episodes in west africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies imouraren deposit tim mersoi basin northern niger uranium u deposits can undergo oxidizing events subsequent to which the u may be redistributed or precipitated under distinct mineralogical forms occurrences of u6 minerals can be the result of either i in situ oxidation and alteration of previous u4 minerals or ii direct precipitation from u6 rich solutions samples from imouraren the largest u6 ore deposit known in the world located in the tim mersoi basin northern niger have been studied to determine the nature distribution and timing of occurrences of u and related minerals the host rock is composed of sandstones and analcime rich siliciclastic rocks of jurassic age the u ore is largely dominated by acicular hexavalent u minerals and by a very small amount of uraninite a comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed the petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite documented by textural relationships and chemical analyses using sem and electron microprobe the textural relationship between the u4 and u6 minerals is supported by the near identical ree signatures of the primary uraninite and related secondary u minerals ion microprobe u pb dating from 21 to 1 ma confirms that primary uraninites were altered to u6 minerals under supergene conditions during cenozoic oxidation events in western africa in relation with the well known stages of intense continental weathering that have affected western africa from the early cenozoic to recent times

on the accuracy of hf radar surface current measurements intercomparisons with ship based sensors high frequency hf radar systems can provide periodic two dimensional vector current estimates over an area approaching 1000 km as the use of these hf systems has gained wider acceptance a number of attempts have been made to estimate the accuracy of such systems however comparisons of hf radar current estimates with in situ sensors are difficult to interpret since hf systems measure currents averaged over an area of 1 km 2 and to a depth of only 50 cm while in situ sensors measure currents at a point and somewhat greater depths 1 to 10 m previous studies of the accuracy of hf radar technology have thus attributed the differences observed between hf radar and in situ sensors to an unknown combination of vertical shear horizontal inhomogeneity in situ instrument errors and hf radar system errors this study examines the accuracy of hf radar current measurements using data from the 1993 high resolution remote sensing experiment conducted off cape hatteras north carolina data from four shipborne in situ current meters are compared with data from an ocean surface current radar oscr a commercial current measuring radar we attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location the results suggest an upper bound on the accuracy of the oscr derived radial currents of 7 to 8 cm s the measurement of near surface currents in the coastal environ ment is an important problem

recent mobilization of u series radionuclides in the bernardan u deposit french massif central abstract several nuclides of the 238 u 234 u 230 th 226 ra 210 pb and 235 u 231 pa 227 ac decay series have been measured through gamma spectrometry in the u bearing episyenites of the bernardan mine northwest massif central france in order to study recent mobilizations in the u deposit and their time scales 230 th 238 u and 231 pa 235 u ratios show that u has been little affected during the last 350 ky especially in the u rich samples in contrast 226 ra was subjected to a general redistribution with a clear tendency for ra to be lost from the u rich samples and added to u poor samples the highest ra excesses are present in low u samples and are related to adsorption onto iron hydroxides ra appears to have been redistributed within the mineralised episyenitic bodies which seem to remain in 226 ra 238 u radioactive equilibrium a result consistent with the earlier study of leroy 1984 episyenitisation dans le gisement d uranium du bernardan marche comparaison avec des gisements similaires du nord ouest du massif central francais miner depos 19 26 35 as shown by the 226 ra and 210 pb data the ra redistribution was very active between 8 and 0 1 ky and this might be a consequence of the hydrological changes in groundwater circulation accompanying the holocene climate warming

origins of co2 in permian carbonate reservoir rocks zechstein ca2 of the nw german basin lower saxony permian carbonate reservoir rocks stassfurt carbonate of the co2 province lower saxony basin nw germany are locally characterised by high amounts of co2 leading to serious problems on exploration in order to receive information on the major co2 sources and co2 migration and accumulation processes mineralogical geochemical and stable isotope analyses c 13 o 18 were carried out on authigenic precipitations within the stassfurt carbonate rocks ca2 these rocks contain early diagenetic e g formed in soft sediment carbonate minerals calcite dolomite and anhydrite in concretions and small cracks in addition large up to 2 cm wide carbonate cemented fractures formed in solid ca2 rocks semi quantitative estimates of the diagenetic mineral content showed variations between the investigated rock cores nine out of 12 ca2 rock cores are derived from wells situated in the area of modem ch4 rich reservoirs and have distinct fracture systems developed in contrast the other three rock cores are from wells characterised by co4 rich reservoirs and lack fractures of similar thickness the assumed relationship between tectonically induced fractures and the gas content of the ca2 reservoir rocks implies co2 degassing of the ca2 rocks at approximately 102 degrees c to similar to 150 degrees c assumed delta 18o pore water variation from 0 parts per thousand to 4 parts per thousand smow ree distribution indicates at least three types of fracture and crack filling carbonates two carbonate generations formed from evolving diagenetic pore waters whereas one type reflects carbonate formation under hydrothermal influence the ages of these cementations derived from rb sr and u pb isotope analyses indicate their fort nation during late stage of or after the inversion of the lower saxony basin in late cretaceous times the stable carbon isotope composition of the authigenic carbonate minerals reflects at least two carbon source endmembers available during diagenesis whereas many early formed carbonates are related to the permian c 13 rich carbon source those carbonates which precipitated or recrystallised during early to late stage diagenesis and or were induced by local tectonic events incorporated c 12 enriched co from organic degradation processes the stable oxygen isotope values of authigenic carbonates probably still reflect low temperatures of mineral formation from permian sea water as it is assumed for non recrystallised carbonates or late stage high temperature up to similar to 150 degrees c precipitation ftoin sea water derived pore waters which may have been locally influenced by o 18 enriched fluids however most of the carbonate samples investigated are recrystallised crystal enlargement of concretionary and fracture carbonate is observed and hence these carbonates are at least partly re equilibrated original o 18 content reduced with surrounding pore waters at elevated temperature similar to the authigenic carbonate at least two sources may explain the concentration carbon isotope relationship of modem co present in the ca 2 reservoir rocks an organic derived co2 endmember source delta c 13 near 20 parts per thousand is present in relatively low concentrations whereas large co concentrations derived from an endmember source with all isotope value near 0 parts per thousand although the latter source is unknown such heavy co sources are most likely attributed to carbonate decomposition processes c 2005 elsevier b v all rights reserved

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

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

pb sr and nd isotope systematics of uranium mineralised stromatolitic dolomites from the proterozoic cuddapah supergroup south india constraints on age and provenance abstract the pb sr and nd isotopic compositions were determined on uranium mineralised and barren stromatolitic dolomite samples from the vempalle and tadpatri formations of the cuddapah supergroup in southern india subsamples from individual handspecimens were analyzed to minimize the potential of heterogeneous initial pb compositions all of the u mineralised samples gave pb pb ages close to 1800 ma but with high mswd values presumably due to local redistribution of u and pb in the samples the best pb pb age of 1756 29 ma is interpreted as the time of u mineralisation and as a minimum age for carbonate sedimentation and dolomitisation within the cuddapah supergroup the sr isotopic compositions of the samples vary with the mineralised samples being more radiogenic even the lowest 87 sr 86 sr of the barren sample exceeds the seawater 87 sr 86 sr about 1800 ma ago on the other hand the nd systematics of the samples does not appear to have been disturbed by processes subsequent to carbonate sedimentation the calculated initial nd isotopic compositions suggest a dominant continental crustal source that was at least 600 ma old at the time of deposition of the carbonate sediment

geochronologic significance of lead lines from old cratons abstract lead isotope data from geologically mixed suites of archaean gneisses often yield secondary isochrons that have statistically better fits than might be expected from the geological constraints on the range of samples used accepted as true isochrons these pb pb lines are commonly used both to determine the age of the rock suite and as evidence that all the rocks that yield data points within error of the isochron were formed at the same time from the same source using within geologically reasonable limits random data for initial lead and secondary lead growth we demonstrate that heterogeneous sample sets may yield statistically valid secondary isochrons this implies that the fit to a common secondary isochron does not necessarily mean that the samples used have the same age or an isotopically similar source numerical modelling of artificial data sets suggests that high correlation between 1 and 2 values is the single most important factor that enhances the statistical fit of the samples about a lead line and smoothes the initial scatter in contrast initial scatter will be preserved in data sets with a random relation between 1 and 2 values the 1 value estimated from the lead line will be biased towards the sample with the lowest 2 value

gulf stream structure transport and recirculation near 68 w an analysis of the structure and transport of the gulf stream is undertaken using direct current meter observations from a 13 mooring array deployed near 68 w from june 1988 to august 1990 the analysis is based on a stream coordinate approach in which velocities are rotated into a local downstream coordinate frame and averaged according to their relative cross stream location within the current the picture so obtained represents the average synoptic structure of the gulf stream rather than the eulerian averaged structure in which the current is weakened and broadened by lateral meandering of the current and adjacent recirculations many familiar features of the gulf stream are reproduced in the analysis including an asymmetric velocity profile with larger shear on the cyclonic shoreward side of the current an offshore displacement of the velocity core with depth and a subsurface velocity maximum on the offshore side of the current westward recirculations are also seen on both sides of the gulf stream maximum downstream speeds at the axis of the gulf stream reach approximately 2 0 m s at the surface and 0 7 m s at 1000 m roughly twice the corresponding eulerian averaged values the analysis also reveals a deep extension of the gulf stream at 3500 m depth with a width of 130 km and average speeds of 3 4 cm s the transport of the gulf stream in the stream coordinate frame is 113 8 sv approximately 30 larger than the eulerian averaged transport of 88 sv on the basis of these results and other recent studies the downstream transport increase of the gulf stream and the inflow structure to the gulf stream are reconsidered it is concluded that approximately 30 sv or over half of the transport increase between cape hatteras and 68 w is fed by inflow from the northern side of the gulf stream and that this inflow is concentrated near cape hatteras and 68 w where the gulf stream flows steeply across isobaths converging from the north

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

effects of low frequency current variability on near inertial submesoscale vortices from september 30 to october 14 1993 and from may 18 to june 13 1994 ocean surface currents along the inshore boundary of the florida current were measured using a land based ocean surface current radar oscr deployed along the florida keys a dual station oscr mapped the current fields at 20 min intervals with a horizontal resolution of 1 2 km over a 30 44 km domain using the high frequency hf 25 4 mhz mode an upward looking narrowband acoustic doppler current profiler adcp was moored in 150 m of water seaward of the shelf break concurrently sampling the three dimensional current vector between 15 and 130 m at 30 min intervals regression analyses between surface and subsurface currents at 15 m depth from the adcp indicated biases of 3 12 cm s 1 and slopes of o 0 8 1 root mean square differences were about 18 cm s 1 because of a 15 m depth separation between the two measurements and bulk current shears were o 10 3 s 1 in the florida current where maximum velocities exceeded 150 cm s 1 between 40 and 70 m surface tidal currents were 2 10 cm s 1 in the diurnal and semidiurnal bands and revealed fine scale variability in the tidal ellipses due to topographical variations differences in the tidal current amplitudes between the surface and 15 m ranged from 1 to 3 cm s 1 in these constituents and explained more of the current variance with depth during the second experiment a submesoscale surface feature was detected along the shelf break and progressed eastward at a rate of about 30 cm s 1 this feature was embedded in the near inertial wave band where the local inertial period was 29 hours that may have been forced by an abrupt change in the wind direction this transient feature was trapped and advected along the inshore edge of the florida current where the anticyclonic vorticity of the subinertial flow shifted the frequency of the nearinertial motions by a few percent of f these energetic near inertial currents of 15 20 cm s 1 had horizontal and vertical wavelengths of 40 km and 50 100 m respectively

nd and sr isotopic mapping of the archaean proterozoic boundary in southeastern tanzania using granites as probes for crustal growth abstract granitoids of two different generations occur across the archaean proterozoic boundary in southeastern tanzania the first generation which is confined to the archaean tanzania craton yields rb sr whole rock isochron ages of 2600 ma and low mantle like initial 87sr 86sr ratios 0 702 0 704 these coupled with positive or near zero e nd values 0 2 to 2 0 suggest that these granitoids represent juvenile mantle material which was added to the continental crust at about 2600 ma the second generation of granitoids intrudes the palaeoproterozoic usagaran belt and is characterised by rb sr whole rock isochron ages of about 1900 ma these granitoids show sm nd crustal formation ages which are 200 to 600 ma older than their emplacement ages and negative e nd values 2 2 to 6 2 mixing calculations suggest that the proterozoic granitoids may have formed by partial melting of 2000 ma usagaran mantle derived material which had incorporated between 15 and 45 of an archaean component from the adjacent tanzania craton

application of ostracoda and charophyta from the late jurassic to early cretaceous tendaguru formation at tendaguru tanzania east africa biostratigraphy palaeobiogeography and palaeoecology abstract revisitation of the famous late jurassic early cretaceous dinosaur locality of tendaguru by a german tanzanian expedition in summer 2000 resulted in a new standard section micropalaeontologic samples from this section yielded a marine and nonmarine partially mixed ostracod fauna consisting of 40 taxa and some charophyte gyrogonites and utriculi a few marine taxa provided biostratigraphic information although comparable faunas from the west indian madagascan east african faunal province are stratigraphically older middle late jurassic fully marine and strongly endemic an middle oxfordian age is suggested for the lower part of the tendaguru formation nodosoclavatoroid utriculi charophyta suggest that the jurassic cretaceous boundary may be located lower in the section than supposed previously i e in the trigonia smeei member rather than in the upper saurian member in biogeographical terms the marine ostracod fauna is relatively endemic and has most similarities with that known from eastern tanzania only a few faunal links to madagascar northwest india and somalia exist and there are no links to south africa ostracods and charophytes also provided palaeoecological data for specific horizons in the lower part of the middle saurian member a regression is documented while the nonmarine late jurassic early cretaceous ostracods of tendaguru are not applicable to biostratigraphy so far they are important because the nonmarine setting is unique in this area of the tethys

gulf stream s induced sea level rise and variability along the u s mid atlantic coast 1 recent studies indicate that the rates of sea level rise slr along the u s mid atlantic coast have accelerated in recent decades possibly due to a slowdown of the atlantic meridional overturning circulation amoc and its upper branch the gulf stream gs we analyzed the gs elevation gradient obtained from altimeter data the florida current transport obtained from cable measurements the north atlantic oscillation nao index and coastal sea level obtained from 10 tide gauge stations in the chesapeake bay and the mid atlantic coast an empirical mode decomposition hilbert huang transformation emd hht method was used to separate long term trends from oscillating modes the coastal sea level variations were found to be strongly influenced by variations in the gs on timescales ranging from a few months to decades it appears that the gs has shifted from a 6 8 year oscillation cycle to a continuous weakening trend since about 2004 and that this trend may be responsible for recent acceleration in local slr the correlation between long term changes in the coastal sea level and changes in the gs strength was extremely high r 0 85 with more than 99 99 confidence that the correlation is not zero the impact of the gs on slr rates over the past decade seems to be larger in the southern portion of the mid atlantic bight near cape hatteras and is reduced northward along the coast the study suggests that regional coastal sea level rise projections due to climate change must take into account the impact of spatial changes in ocean dynamics

remote radar measurement of shelf currents off key largo florida u s a abstract the coastal shelf of the florida keys is characterized by shallow and highly variable topography where currents are influenced by tides wind and the very energetic offshore florida current system the transient dynamics of variable scale are not easily captured with traditional fixed mooring observation platforms consequently an experiment combining moorings acoustic doppler current profiler adcp transects and a shore based high frequency hf radar system oscr was conducted to monitor the spatial and temporal variability of the flow field over the shelf and off shore deeper waters both the wind and tide driven flows in the middle to inner shelf called hawk channel and two offshore eddy events of the florida current were sampled during the experiment qualitative agreement between oscr current meter moorings and adcp transects was good in the shelf however at the inshore edge of the florida current there were significant differences the adcp captured the strong horizontal shear at the shelf edge whereas oscr smoothed the front over two kilometres due to its measurement cell size these differences in the spatial sampling resolution in the high shear region prevented direct quantitative comparisons although there were significant local variations linked to topography a major portion of the subtidal flow in the shelf was related to wind within the shelf both cross shelf and along shelf flows were highly spatially correlated with r 0 82 and r 0 86 respectively two sub mesoscale eddies that had characteristics consistent with the spin off eddies described by lee 1975 were observed to advect at velocities ranging from 0 53 ms 1 to 0 80 ms 1 along the inshore edge of the florida current the two eddies had along shelf length scales from 19 km to 47 km and cross shelf scales of 15 km and 25 km each eddy consisted of a single cyclonic vortex rotating with tangential velocities of 0 30 ms 1 and 0 50 ms 1 respectively the eddies had a strong signature at the shelf break however they did not have any observable influence on flows over the inner shelf

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

direct u pb dating of cretaceous and paleocene dinosaur bones san juan basin new mexico vertebrate fossils have been important for relative dating of terrestrial rocks for decades but direct dating of these fossils has heretofore been unsuccessful in this study we employ recent advances in laser ablation in situ u pb dating techniques to directly date two dinosaur fossils from the san juan basin of northwestern new mexico and southwestern colorado united states a cretaceous dinosaur bone collected from just below the cretaceous paleogene interface yielded a u pb date of 73 6 0 9 ma in excellent agreement with a previously determined 40 ar 39 ar date of 73 04 0 25 ma for an ash bed near this site the second dinosaur bone sample from paleocene strata just above the cretaceous paleogene interface yielded a paleocene u pb date of 64 8 0 9 ma consistent with palynologic paleomagnetic and fossil mammal biochronologic data this first successful direct dating of fossil vertebrate bone provides a new methodology with the potential to directly obtain accurate dates for any vertebrate fossil

shocked monazite chronometry integrating microstructural and in situ isotopic age data for determining precise impact ages monazite is a robust geochronometer and occurs in a wide range of rock types monazite also records shock deformation from meteorite impact but the effects of impact related microstructures on the u th pb systematics remain poorly constrained we have therefore analyzed shock deformed monazite grains from the central uplift of the vredefort impact structure south africa and impact melt from the araguainha impact structure brazil using electron backscatter diffraction electron microprobe elemental mapping and secondary ion mass spectrometry sims crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal plastic deformation features including shock twins planar deformation bands and neoblasts shock twins were documented in up to four different orientations within individual monazite grains occurring as compound and or type one twins in 001 100 left 10 bar 1 right 110 left 212 right and type two irrational twin planes with rational shear directions in 0 bar 1 bar 1 and bar 1 bar 1 0 sims u th pb analyses of the plastically deformed parent domains reveal discordant age arrays where discordance scales with increasing plastic strain the correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event neoblasts in granular monazite domains are strain free having grown during the impact events via consumption of strained parent grains neoblastic monazite from the inlandsee leucogranofels at vredefort records a 207pb 206pb age of 2010 15 ma 2 n 9 consistent with previous impact age estimates of 2020 ma neoblastic monazite from araguainha impact melt yield a concordia age of 259 5 ma 2 n 7 which is consistent with previous impact age estimates of 255 3 ma our results demonstrate that targeting discrete microstructural domains in shocked monazite as identified through orientation mapping for in situ u th pb analysis can date impact related deformation monazite is therefore one of the few high temperature geochronometers that can be used for accurate and precise dating of meteorite impacts

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

lu hf isotope systematics of fossil biogenic apatite and their effects on geochronology abstract reliable methods for direct dating of biogenic apatite from pre pleistocene fossils are currently not available and recent attempts using the lu hf decay system yielded highly inaccurate ages for both bones and teeth the geological processes accounting for this poor accuracy of lu hf chronometry are not yet understood here we explore lu hf systematics in fossil bones and teeth in detail by applying five different sample digestion techniques that are tested on bones and composites of bone and sediment our current dataset implies that dissolution methods only slightly affect the resulting lu hf ages while clear differences between the individual digestion techniques became apparent for element concentrations by analysing the insoluble leftovers from incomplete sample dissolution four main reservoirs of hf in fossil bones were identified 1 a radiogenic end member associated with apatite 2 an unradiogenic end member represented by the authigenic minerals or the embedding sediment 3 a highly unradiogenic end member that can be attributed to detrital zircon and 4 a moderately soluble phase probably a zr hf phosphate that yielded very low lu hf but a highly radiogenic hf isotope composition at the same time this zr hf phase must have been precipitated within the fossil bone sample at a late stage of burial history thereby incorporating radiogenic 176 hf released from apatite surfaces over geological timescales a second focus of our study is the effect of different sediment matrices and of crystal size on the preservation of pristine lu hf isotope compositions in bioapatite because near depositional lu hf ages of phosphate fossils have previously been reported for the london clay england and a calcareous marl from tendaguru tanzania we herein investigate specimens fossilised in carbonate matrices calcareous marl from oker germany carbonate concretions from the santana formation brazil carbonate from the eifel germany and argillaceous matrices with low permeability oil shale of messel germany posidonienschiefer of holzmaden germany materials analysed from these localities include bones teeth conodonts as well as coproliths and diagenetic minerals siderite montgomeryite and messelite near depositional lu hf ages were obtained for a bony fish sample notelops brama encapsulated in an early diagenetic carbonate concretion from the early cretaceous santana formation brazil and for conodonts from a middle devonian carbonate from the eifel germany low 176 lu 177 hf ratios in all materials from the middle eocene messel oil shale e g bones fish scales sediment siderite result in poor age precision and an age that is near depositional due to this large analytical error in agreement with previous results all other ages determined here for both bones and teeth are by far younger than respective chronostratigraphic ages a model illustrating the behaviour of lu and hf over time with respect to the fossilisation process is presented which accounts both for the formation of a late diagenetic radiogenic zr hf phase and long term open system behaviour the continuous lu hf element exchange between the fossils and the embedding sediment is probably related to the nm scale crystal size of fossil bones dentine and also of enamel that generate large surface areas facilitating sorption desorption processes and open system behaviour

timescales and mechanisms of ree and hf uptake in fossil bones abstract rare earth element ree patterns of fossil bones and teeth are widely used as proxies for provenance taphonomy and palaeoenvironment in order to investigate if fossil bones behave as closed systems over geologic time ree profiles were analysed by la icpms along cross sections of 54 bones from various well characterised and well dated settings these include terrestrial and marine diagenetic environments covering early triassic to holocene ages in general all fossil bones exhibit the highest ree concentrations at the outer rim gradually decreasing by up to four orders of magnitude toward the inner bone cortex intra bone ree concentration gradients decrease significantly from quaternary via tertiary to mesozoic specimens suggesting long term ree uptake and open system behaviour of fossil bone this view is further corroborated by 176lu 176hf dating of selected samples all yielding significantly younger ages than the known chronostratigraphic ages hence there is clear evidence for long term open system behaviour of fossil bones with respect to ree which is in marked contrast to currently accepted models suggesting that ree uptake is only early diagenetic although unexpected statistically significant four to seven point isochrons are observed for four fossil dinosaur bone samples and one upper triassic mastodonsaurus tooth with mswds ranging from 0 083 to 4 5 notably mobility of lu alone cannot account for the observed age patterns assuming constant lu uptake rates over time the radiometric ages should only be as low as half of the chronostratigraphic age however a six point isochron defined by subsamples of a single upper triassic mastodonsaurus tooth yields an age of 65 2 1 1 ma mswd 0 68 much younger than half of the stratigraphic age ca 234 ma hence hf must also undergo late diagenetic exchange likely mechanisms to account for the presence of statistically meaningful isochrons as well as for the late diagenetic exchange of both ree and hf are diffusion adsorption and dissolution reprecipitation processes

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

hf radar observations of small scale surface current variability in the straits of florida 1 a dual station high frequency wellen radar wera transmitting at 16 045 mhz was deployed along the eastern florida shelf and operated and maintained by the university of miami s rosenstiel school of marine and atmospheric science from september 2004 to june 2005 a moored acoustic doppler current profiler adcp acquired subsurface current measurements within the radar footprint along the shelf break at 86 m depth the shallowest adcp bin located at 14 m depth was used as a comparison for the wera surface measurements rms differences ranged from 0 1 to 0 3 m s 1 between the surface and 14 m depth with good agreement over most of the period regression analyses indicated slopes near unity in the north south v component and 0 5 for the east west u component velocities when utilized in tandem with the adcp subsurface measurements wera enables three dimensional snapshots of coastal oceanographic features to be resolved for example from december 2004 through february 2005 three energetic circulation patterns were observed 1 a subsurface stratified countercurrent 2 a submesoscale coherent vortex and 3 a mesoscale circulation feature i e a propagating tortugas gyre these features represent the significant current variability along the western flank of the florida current that impacts the coastal ocean