petrology geochemistry and u pb geochronology of the betic ophiolites inferences for pangaea break up and birth of the westernmost tethys ocean abstract the betic ophiolitic unit part of the mulhacen complex in the betic cordillera se spain is composed of several tectonic slices of eclogitized mafic and ultramafic rocks together with oceanic metasediments a comprehensive review of the petrological characteristics of the betic ophiolites shows notable analogies with other ophiolites from the alps apennines system indicating that they represent relics of oceanic lithosphere created at the westernmost end of the western tethys u pb dating by shrimp sensitive high resolution ion microprobe on zircon grains carefully investigated by cathodoluminescence images has been carried out on an eclogitized metagabbro lugros outcrop and a metadolerite dyke intruded in ultramafic rocks almirez outcrop giving consistent magmatic ages of around 185 3 ma this age marks the beginning of the betic oceanic magmatism transitional to enriched morb that followed pangaea break up and the earliest oceanic accretion of the western tethys ocean subsequently ultraslow mid ocean ridge spreading developed from the betic oceanic basin northeastward to the ligurian and alpine tethys domains mainly around 165 to 140 ma

mean meridional currents in the central and eastern equatorial atlantic ship based acoustic doppler current profiler adcp velocity measurements collected by several major field programs in the tropical atlantic are averaged and combined with estimates of the mean near surface velocity derived from drifters and argo float surface drifts adcp d to describe the mean cross equatorial and vertical structure of the meridional currents along 23 w and 10 w data from moored adcps and fixed depth current meters a satellite derived velocity product and a global ocean reanalysis were additionally used to evaluate the mean adcp d meridional velocity the dominant circulation features in the long term mean adcp d meridional velocity in the upper 100 m are the tropical cells tcs located approximately between 5 s and 5 n with near surface poleward flow and subsurface equatorward flow that is stronger and shallower in the northern cell compared to the southern cell the thickness of the surface limb of the tcs decreases and the northern cell is found to shift further south of the equator from the central to eastern tropical atlantic analysis of two season means estimated from the ship based adcp near surface drift and moored velocity data as well as the simulated fields indicates that the maximum poleward velocity in the surface limb of the tcs intensifies during december may along 23 w largely due to seasonal compensation between the geostrophic and ageostrophic or wind driven components of the meridional velocity whereas the maximum equatorward flow in the subsurface limb of the northern cell intensifies during june november along both 23 w and 10 w due to the seasonality of the geostrophic meridional velocity

history of the west african neoproterozoic ocean key to the geotectonic history of circum atlantic peri gondwana adrar souttouf massif moroccan sahara abstract bordered by the archaean basement of the reguibat shield to the e and s the adrar souttouf massif is located in the southern regions of the moroccan sahara and represents the northern part of the mauritanide belt the central areas of this massif comprise the dayet lawda and sebkha matallah units that are mainly composed of mafic rocks ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology u th pb and lu hf isotope composition additionally 155 apatite grains from six samples were also dated by the u th pb method mostly well rounded zircon grains commonly showed bright cathodoluminescent cl overgrowth domains and leaching zones indicating metamorphic overprint around unaffected oscillatory zoned core areas all samples but one yielded two significant zircon age populations 605 ma and 634 ma with some inherited grains in the range of 650 740 ma and very scarcely up to 1190 ma for six samples the hf isotope composition suggests a major contribution of neoproterozoic juvenile magmas from depleted mantle source the zircon hf data for the remaining samples points to a predominant recycling of older archaean to palaeoproterozoic crust based on the geochemical composition of the rocks an island arc setting is assumed at the periphery of the west african craton close to the cryogenian ediacaran boundary subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 ma a minor variscan overprint could be demonstrated for only one sample this case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the west african craton for palaeogeographic and geodynamic reconstructions including the peri gondwanan terranes during the late neoproterozoic

the rate of oceanic detachment faulting at atlantis bank sw indian ridge abstract the rates of slip on oceanic detachment faults and how those rates compare to sea floor spreading rates constitute fundamental data required to constrain how oceanic core complexes form and their role during crustal accretion we combine sea surface magnetic data with the magnetic polarity of shallow core samples and pb u shrimp ages of igneous zircon to determine the time averaged half spreading rate during oceanic detachment faulting at atlantis bank 100 km south of the ultraslow spreading southwest indian ridge swir the pb u zircon ages correlate well with the magnetic ages and so highlight that magmatic accretion and faulting were coeval for over 2 myr creating and exposing a 1 5 km thick layer of gabbro for 35 km parallel to spreading we use bivariate linear regression of distance age data and forward modeling of magnetic anomaly data to calculate a half spreading rate during detachment faulting of 14 1 1 8 1 5 km myr 95 confidence limits when integrated with regional constraints on spreading history we note that detachment faulting coincided with a short lived regional increase in the full spreading rate along the swir and for the ridge segment containing atlantis bank spreading was highly asymmetric with 80 of plate motion accommodated by detachment faulting consequently the detachment fault effectively formed the plate boundary at the surface in this spreading segment highly asymmetric spreading was confined to the spreading segment containing atlantis bank and to the duration of detachment faulting so the ridge segment containing atlantis bank migrated northward relative to its symmetrically spreading eastern neighbour such that the intervening non transform discontinuity shortened we suggest that the highly asymmetric spreading may be a characteristic feature of oceanic detachment faulting an inference supported by more poorly constrained half spreading rates determined at several other oceanic core complexes

seasonal response of the sea surface to the wind in the equatorial atlantic from february 1983 through september 1984 18 inverted echo sounder deployments yielded time series from 10 locations as part of the seasonal response of the equatorial atlantic sequal program using historical and in situ data the sounders travel time data are interpreted as dynamic height 0 relative to 500 dbar and then related to representations of the synoptic wind field the seasonal variations of the zonal pressure gradient along the equator and the height of the dynamic ridge along 3 n relative to the equator and 9 n are described the pressure gradient between 10 w and 34 w has a minimum each april and a maximum each july its value ranges between 1 4 not significantly different from zero and 8 3 10 5 dyn g 1 during the observational period the zonal pressure gradient is correlated with zonal wind stress from an in situ wind sensor at midfetch and a 10 day lag time is indicated the north equatorial countercurrent trough attains a maximum in the late boreal summer and fall and a minimum in the spring it ranges from 39 to 8 dyn cm deep at 38 5 w and from 18 to 1 dyn cm at 28 w a two layer model calculation using monthly mean winds derived by servain et al 1986 for the appropriate time period generally reproduces the observed seasonal variation along the two meridians and the large zonal variation between them tracking the individual terms month by month illustrates the rate of the sverdrup adjustment to the local wind stress with surface elevation rising as fast as 10 cm per month

forced oscillations on the equatorial atlantic basin during the seasonal response of the equatorial atlantic program 1983 1984 a subset of data from an array of eight inverted echo sounders and a meteorological station deployed and maintained for 15 months in the equatorial atlantic as part of the seasonal response of the equatorial atlantic program are analyzed to determine the variability of forced oscillations of the basin the analysis is done to study both low and high frequency variability at low frequencies the equatorial basin responds as a whole to the seasonally varying winds the dominant signal is the annual cycle of the equatorial system as a composite of the response to the strong seasonal wind curl that is maximum in the western basin and the part of the zonal transfer to mass associated with the seasonal equatorial upwelling and downwelling system three bands of energy appear as dominant in the combined spectrum of the ocean and the atmosphere at higher frequencies 20 30 days 13 16 days and the inertial gravity wave band each one of the bands is analyzed and discussed instability waves are detected on the echo sounder records the waves have periods centered at 30 2 days and propagate westward with a wavelength of 1000 km and a phase speed of 0 35 m s the most energetic oscillation in the analyzed wind records is at periods centered around 14 days the response of the ocean to this oscillation has a meridional distribution with maximum amplitude at 3 n the most significant oceanic oscillations in the inertial gravity band are centered at 5 2 and 3 5 days from the study of their meridional structure it is concluded that those oscillations correspond to forced inertial gravity waves with meridional structures corresponding to the meridional numbers n 1 and n 3 respectively

structure and chemistry of zircon and zircon group minerals zircon zrsio4 is a common accessory mineral in nature occurring in a wide variety of sedimentary igneous and metamorphic rocks known to incorporate an assortment of minor and trace elements zircon has the ability to retain substantial chemical and isotopic information leading to its use in a wide range of geochemical investigations including studies on the evolution of earth s crust and mantle e g hanchar et al 1994 bowring 1995 vervoort et al 1996 hoskin and schaltegger this volume valley this volume as well as age dating e g gibson and ireland 1995 bowring et al 1998 solar et al 1998 bowring and schmitz this volume ireland and williams this volume parrish et al this volume the physical and chemical durability of zircon is a major factor in it being the mineral by which many of earth s oldest known rocks have been dated bowring et al 1989 maas et al 1992 buick et al 1995 bowring and williams 1999 wilde et al 2001 and is also an important factor in zircon being proposed as a candidate waste form for the geologic disposal of excess plutonium from dismantled nuclear weapons ewing and lutze 1997 ewing 1999 burakov et al 2002 burakov et al 2003 ewing et al this volume the chemical and physical properties of zircon and its ability to incorporate and retain trace elements are largely determined by its crystal structure the zircon structure is adopted by numerous minerals and synthetic compounds with the general formula at o4 in which high field strength t site cations occupy isolated tetrahedra and a site cations occupy larger eight coordinated structural sites zircon type compounds share many physical properties as well as displaying variable degrees of solid solution among end members

instability waves observed on the equator in the atlantic ocean during 1983 a packet of surface confined westward propagating waves apparently generated by barotropic instability of the surface currents was observed in the equatorial atlantic during 1983 initial wavenumber analyses and energetics calculations are presented local working by the horizontal reynolds stress was large enough and at the correct time to account for the wave packet s existence the amplitude of the waves was comparable to the mean currents implying that the waves are of primary importance in the momentum balance of the seasonally varying equatorial surface currents

winds at st peter and st paul rocks during the first sequal year wind observations were obtained during the first nine months of the sequal field program from a station maintained at st peter and st paul spp rocks winds are very steady from may to november and very unsteady during april when the intertropical convergence zone itcz crosses the site wind stress components are computed and comparison with 1979 data shows that the onset of the winds occurred one month earlier in 1983 but is of similar amplitude the spectral analysis of the wind shows a significant increase in the variance for periods between 3 5 days and 10 15 days

observation of the marine atmospheric boundary layer in the gulf of guinea during the 2006 boreal spring in situ satellite and model analyses data in april july 2006 are used to investigate the links between sea surface temperature sst and the marine atmospheric boundary layer mabl in the gulf of guinea the study region between 10 w and 6 e is divided into three areas with different characteristics the north area 4 5 n to 1 n with the wettest atmosphere and the warmest sst the upwelling area 1 n to 4 s with the strongest sst decrease and the south area 4 s to 8 s with a drier atmosphere and a more slowly decreasing sst than in the upwelling area the key zone of the air sea interactions in this region seems to be the sst front between north and upwelling areas on the one hand the study of the mabl on either side of the front shows a well mixed layer between the surface and about 500 m high sensitive to surface variations which gets shallower deeper when the sst decreases increases the mabl height about 1500 m follows the same variations but is not exactly collocated with the sst variations on the other hand the observation of the mabl across the sst front shows a strengthening of southeasterlies in the south area coinciding with a strong sst decrease in the upwelling area in the latter the wind weakens above the colder sst besides in the north area the wind strengthens above the warmer sst however the wind acceleration spans from the equator to 2 n in april and as far as 4 n in june a convergence zone is observed in the vicinity of 2 n in april suggesting a convection activity there favoured by the sst front copyright 2011 royal meteorological society

impact shocked zircons discovery of shock induced textures reflecting increasing degrees of shock metamorphism abstract textural effects specifically characteristic of shock metamorphism in zircons from impact environments have not been reported previously however planar deformation features pdf due to shock metamorphism are well documented in quartz and other mineral grains from these same environments an etching technique was developed that allows sem visualization of pdf and other probable shock induced textural features such as granular polycrystalline texture in zircons from a variety of impact shock environments these textural features in shocked zircons from k t boundary distal ejecta form a series related to increasing degrees of shock that should correlate with proportionate resetting of the u pb isotopic system

validation of an equatorial atlantic simulation model using inverted echo sounder data changes in the vertically averaged temperature of the water column observed over a 12 month period at eight locations in the tropical atlantic with inverted echo sounders are described in terms of empirical orthogonal functions these modes are compared with those from a similar analysis for data from a general circulation model of the tropical atlantic the method proves to be useful for the validation of models

variability in horizontal current velocities in the central and eastern equatorial atlantic in 2002 1 near surface current velocity data from the pilot research moored array in the tropical atlantic at 23 w in the year 2002 were re examined and compared to simultaneous unpublished velocity data at 10 w near the surface strong 7 day period fluctuations were observed in the zonal velocity component and in temperature records at 23 w the large temperature fluctuations near the thermocline depth suggest a vertical velocity component at the same 7 day period the 7 day period signals occurred in june july simultaneously with the seasonal upwelling the meridional velocity component presented similar spectral contents at 23 w and at 10 w two distinguishable quasiperiodic signals were observed in the meridional component at both locations the first signal occurred in the spring had a periodicity of about 14 days was subsurface intensified and had the characteristics of a mixed rossby gravity wave the second signal occurred three times in the year once in winter 2001 2002 another in summer and another during fall 2002 had a periodicity of about 20 days was strongest close to the surface and was the result of the passage of tropical instability waves tiws satellite sea surface temperature sst data show the same periodicities as the meridional velocity records and present a spatial structure in accordance with the tiw sst signature the summer of 2002 showed different tiw sst patterns depending upon location suggesting the existence of different types of tiws overall variability in the current meter data for the year 2002 was mainly due to current instabilities except around boreal spring where intraseasonal variability in surface winds was strongest

equatorial kelvin waves in the atlantic long term inverted echo sounder records along and north of the equator in the atlantic are analyzed to show the dominance of equatorially trapped and eastward propagating disturbances with first baroclinic mode kelvin wave characteristics in the frequency band of 10 1 to 40 1 cycles day 1 propagation speed is estimated as 2 35 0 22 m s 1 along the equator from 34 w to 1 w coherence is present to 3 n but absent at 6 n from mooring data and 9 n coherence is shown with the zonal wind stress observed in midbasin and it is suggested that the origin of the disturbance is not far from 40 w

the izera metabasites west sudetes poland geologic and isotopic u pb zircon evidence of devonian extension in the saxothuringian terrane abstract in the lausitz izera massif of the saxothuringian terrane part of the central european variscides 515 480 ma metagranites have been intruded by a swarm of basic veins that have within plate alkali basalt geochemistry or rarely normal mid ocean ridge basalt n morb geochemistry based on their spatial orientation distribution in the region and structural relationships to the host metagranites these basic veins could be grouped into three types that displayed six varieties of host vein contacts zircons extracted from eight of the basic veins revealed that there were two populations clear zircons and brown zircons each type were subjected to both single grain conventional u pb analysis and to sensitive high resolution ion microprobe shrimp u pb analysis the clear grains yielded ages 480 ma clustering at 500 ma the brown grains yielded ages between 390 and 365 ma clustering at 370 ma the colourless zircons possess an oscillatory zoned structure and rare earth element ree distribution pattern that suggests they come from the host granites and from deeper neoproterozoic granitoid basement however the basic host rocks show almost no obvious contamination by felsic material the turbid brown grains which are dark under cathodoluminescence also seem to be mainly magmatic because of their internal structures high th u ratios and ree pattern when the zircon results are integrated into the regional geology an 30 m y period of episodic intra plate predominantly alkaline basic magmatism at the passive margin of the saxothuringian terrane is suggested with maximum activity at 370 ma during an extensional regime combining the factors of the relatively high thermal gradient during early metamorphism of the metabasites 450 c 2 kbar the geology of saxothuringia and the regional structural data we propose a plume model of hot fingers type whereby the basic magmatism of the izera region was due to repeated devonian intrusions derived from garnet bearing enriched oib type and less commonly depleted asthenosphere sources the waning stages of this activity were coeval with the early deformation of the izera granite host basic vein system and matched by rifting and extensional widening of the saxothuringian basin in its izera kaczawa segment

in situ wind measurements and the ocean response in the equatorial atlantic during the programme francais ocean et climat dans l atlantique equatorial and seasonal response of the atlantic ocean experiment in situ wind measurements collected as part of the programme francais ocean et climat dans l atlantique equatorial focal seasonal response of the equatorial atlantic sequal experiment 1983 1984 in the western and eastern parts of the equatorial atlantic basin are described they were obtained from meteorological stations placed saint peter peter and saint paul rocks spp 1 n 29 w and at the top of a surface buoy moored in the gulf of guinea 0 n 4 w from the wind observations the wind stress was inferred and results are compared with climatology the wind stress time series show the abrupt increase of the winds during the spring that at spp reaches a value as high as 0 35 dyn cm2 in 2 weeks for both observed years the 11 day running mean time series shows that the onset of the zonal component of the wind stress occurs at spp on april 10 1983 and on may 17 1984 and in the gulf of guinea gg on april 5 1983 and april 10 1984 the monthly mean observations show an interannual variability both in the time of the onset and in the strength of the trade winds at spp and gg the total wind stress increases 1 month earlier than climatology in 1983 but at the same time as climatology in 1984 at spp the zonal component of the wind stress also intensifies 1 month earlier than climatology in 1983 but 1 month later in 1984 the equatorial temperature records at 28 w and 4 w show that the depth of the 20 c isotherm on a seasonal time scale decreases during the relaxation period of the trade winds boreal winter in 1983 1984 this occurred in december 1983 at 28 w and in march 1984 at 4 w after the onset of the local trade winds the thermocline continues to move upward during 1 month at 28 w and during 3 months at 4 w thereafter the thermocline deepens at both locations at the surface the temperature decreases when the trade winds intensify and remains low as long as the trade winds are blowing the seasonal variations of the temperature both at the surface and below the surface at 28 w and 4 w are interpreted in the light of the results of a nonlinear multilevel model in the cases of a sudden increase and a sudden relaxation of the trade winds

exsolution of thortveitite yttrialite and xenotime during low temperature recrystallization of zircon from new caledonia and their significance for trace element incorporation in zircon recrystallized zircon grains in a phengite epidote chlorite schist from north eastern new caledonia contain as inclusions a mineral assemblage consisting of celadonite kaolinite quartz fe oxy hydroxide smectite chlorite xenotime y thortveitite yttrialite and allanite ce this assemblage formed during low temperature 100 c seafloor alteration of a plagioclase rich mafic rock and represents the first documented evidence of this alteration event in the high p belt of northeastern new caledonia the survival of this low temperature mineral paragenesis in zircon of a rock that has undergone subsequent eclogite facies metamorphism testifies to the strength and value of zircon as a container for mineral inclusions thortveitite sc2si2o7 and yttrialite y2si2o7 inclusions in the altered zircon cores represent a new occurrence for these minerals and suggest that they may be more common than is currently recognized the altered zircon cores generally have lower trace element contents than the pristine igneous zircon cores and we suggest that thortveitite yttrialite and xenotime y formed as a result of trace element expulsion from zircon during low temperature recrystallization trace element concentrations in the zircon cores indicate that trivalent cations ree y sc in zircon cannot be charge balanced by xenotime substitution alone pentavalent cation concentrations nb as are also insufficient for charge balance the presence of thortveitite sc2si2o7 and yttrialite y2si2o7 suggests that trivalent cations in zircon might be charge balanced either by monovalent anions substituting for oxygen or by small monovalent cations such as h occurring interstitially in the zircon lattice

the forced annual reversal of the atlantic north equatorial countercurrent abstract we analyze the variability of the thermal structure associated with the atlantic north equatorial countercurrent necc and its relation to the seasonally varying winds the analysis performed allows us to establish the period of time and the region where the necc reverses direction west of 25 w the depth of the thermocline in the southern side of the necc 4 to 7 n annually oscillates 180 out of phase to the oscillations on the northern side 7 to 10 n east of 25 w the thermocline rises and falls nearly in phase across the countercurrent if the flow can be uniquely presumed from the thermal structure the necc disappears during the boreal spring in the western basin the reversal of phase from north to south over one part of the ocean but no the other is shown to be mirrored in the annual variation of the curl of the stress induced at the surface by the winds in order to quantify this result the different terms of the vorticity equation are calculated from the data we conclude that t

modes of variability of the 1983 thermocline signal acoustic travel time series from an array of nine inverted echo sounders deployed along the equator and in the north equatorial countercurrent necc are decomposed into empirical orthogonal functions eof to analyze the dominant frequencies of oscillations in the thermocline field at low frequencies the first eof mode describes the seasonal signal shows the reversal of the necc and separates the behavior of the gulf of guinea from the rest of the basin the second eof mode shows rapid changes in the depth of the thermocline in short periods of time on the equator at 10 w and at 3 n 38 30 w comparison with a simulation model by philander and pacanowski shows that the theoretical time series behave in a similar way at these two locations at high frequencies the modes show oscillations in the inertia gravity wave band

recrystallization and hydrothermal growth of high u th zircon in the weondong deposit korea record of post magmatic alteration abstract in this study we investigate zircons with high u and th contents of 12 000 24 000 and 11 000 40 000 ppm respectively from leucocratic granite in the weondong region of south korea hydrothermally epitaxial growth textures are observed in addition to four distinct textures with different backscattered electron bse intensities within single zircon crystals we describe the internal textures of the zircon crystals and define the chemical characteristics of each textural domain the zircon crystals show internally recrystallized and externally crystallized textures supporting the concept of post magmatic fluid control after crystallization of the primary zircon by late magmatism four types of secondary textures were developed the type i domain shows patchy forms within the primary domain due to the structural and chemical recrystallization of self irradiated zircon by a fluid dominated diffusion reaction process the type ii domain is characterized by a pure zircon composition mineral inclusions mainly thorite and micropores due to chemical recrystallization by a coupled dissolution reprecipitation process during interaction with aqueous fluids the type iii domain is the purest zircon is interconnected with type ii and formed by a second coupled dissolution reprecipitation process the type iv domain is the hydrothermal recrystallization overgrowth texture formed by the direct crystallization process from fluorine enriched zircon saturated aqueous fluid these results indicate that the zircon crystals were formed sequentially or intermittently through multi genetic processes by post magmatic fluids furthermore we conclude that f enriched post magmatic fluid migrated large amounts of zirconium resulting in the typical overgrowth texture of zircon

evidence for the protracted construction of slow spread oceanic crust by small magmatic injections abstract gabbroic cumulates drilled south of the kane transform fault on the slow spread mid atlantic ridge preserve up to three discrete magnetization components here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust by comparing the paleomagnetic data with mineral compositions and based on thermal models of local reheating we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter scale sills some of these magmas were crystal laden and the subsequent expulsion of interstitial liquid from them produced cumulus sills these small scale magmatic injections took place over at least 210 000 years and at distances of 3 km from the ridge axis and may have formed much of the lower crust this model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow spread ridges

time scales of upper ocean temperature variability inferred from the pirata data 1997 2000 1 time series from the pirata 10 minute measurements are investigated in order to study the upper ocean thermal field time scales of variability using the serial covariance power spectra method and wavelet analysis this is one of the few studies that analyze in time and space the pirata buoy data in the region analysis of the depth of the 20 c isotherm show distinct patterns for the northwestern atlantic at 8n 38w and the southeastern atlantic at 10s 10w in the west warm surface waters are accompanied by a shallower thermocline while in the east warm surface waters are associated with its deepening consistent with houghton 1991 at higher frequencies the dominant signals in sea surface temperature are the diurnal and semi diurnal cycles spectral analysis of the depth of the 20 c isotherm show a predominance of the semi diurnal period in the eastern atlantic there are two additional dominant bands in the spectra 10 14 days 25 27 days and for the depth of the 20 c isotherm there is also a lower frequency band at 20 40 days associated with tropical instability waves at the equator the first band can be associated with atmospherically forced motions and the lower frequency band with barotropic instability waves

pb u and th diffusion in natural zircon zircon zrsio4 is one of the most widely used minerals for determining the age origin and thermal history of rocks by u th pb geochronology but the parameters describing the solid state volume diffusion rates of these elements in natural zircon which are themselves important for establishing the limits on the applicability of zircon for geochronological studies have remained poorly quantified1 this is because of the measurement difficulties associated with the low p p m concentrations and low diffusion rates of these elements in natural zircon and the chemical and physical heterogeneity present in most crystals here we present direct measurements of the uranium thorium and lead loss from a thermally treated gem quality natural zircon and show that lead diffuses much faster than uranium or thorium we find that the u th pb isotopic system in natural zircon will typically have a closure temperature greater than 900 c which explains why zircon is apparently such a robust geochrometer and is capable of remaining isotopically closed through extended periods of high grade metamorphism and partial melting of the host rock

rare earth elements in synthetic zircon part 1 synthesis and rare earth element and phosphorus doping zircon crystals were grown from a li mo flux 7 5 mol li2moo4 86 5 mol moo3 to which equal molar proportions zro2 and li2sio3 were added 3 mol each the crystals were initially grown undoped but later dy was added to the flux without any other charge compensating species with dy and p added in equal molar proportions the zircon crystals incorporated 1 37 mol 6 99 wt dy and 1 36 mol 1 33 wt p a factor of 5 3 increase in dy over the crystals grown without p intentionally added to the flux the other p ree doped zircon crystals displayed an approximately 1000 fold increase in ree and p from la through lu a result of decreasing ionic radii the incorporation of p5 allowed in a general sense the xenotime type coupled substitution of ree3 y3 p5 zr4 si4 from la to nd however p exceeds ree from sm to gd the ree are generally equal to p and from tb to lu the ree exceed p the y and p doped zircon behaved more like middle ree mree doped zircon than heavy ree hree doped zircon crystals in their ability to incorporate y and p and to maintain charge neutrality to investigate the incorporation of dy with no p added to the flux the p to lree excess and the hree to p excess in the doped zircon secondary ion mass spectrometry sims analyses were completed on selected crystals in the dy doped crystals the sims analyses revealed minor amounts of p li and mo in the crystals these elements contributed to charge balance required by the excess dy in ree and p doped zircon the sims analyses detected li and mo and the li and mo may also provide charge balance for excess ree in the hree p doped crystals

zircon dating of oceanic crustal accretion most of earth s present day crust formed at mid ocean ridges high precision uranium lead dating of zircons in gabbros from the vema fracture zone on the mid atlantic ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery combined with results from previous dating studies this finding suggests that two distinct modes of crustal accretion occur along slow spreading ridges individual samples record a zircon date range of 90 000 to 235 000 years which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks

origin of mesozoic ophiolitic melanges in the western yarlung zangbo suture zone sw tibet abstract the petrogenesis and tectonic evolution of the mesozoic ophiolitic melanges in the western section of the yarlung zangbo suture zone yzsz remain controversial in this paper we present the results of whole rock geochemical and sr nd isotope analyses zircon u pb ages and in situ lu hf isotopic data obtained from mafic rocks of the northern and southern sub belts of the western yzsz mesozoic ophiolitic melanges to help us understand these controversial issues diabases and dolerites from the northern sub belt and gabbros from the southern sub belt exhibit variable fore arc basalt fab like geochemical compositions and have zircon u pb ages of 126 4 120 3 ma in addition gabbro diabases from the northern sub belt have boninite series affinities and yield a zircon u pb age of 125 7 ma these results along with previous studies on the yzsz mesozoic ophiolitic melanges and the gangdese arc reveal that the western yzsz mesozoic ophiolites were likely generated over multiple stages in the epicontinental gangdese fore arc basin as the yarlung zangbo neo tethyan ocean subducted northward in front of the lhasa terrane the early cretaceous fab like and boninite series mafic rocks were formed by the reinitiation of subduction which was followed by a retreat of the subduction zone and the creation of the fore arc basin and strong hyperextension accompanied by asthenosphere upwelling at 130 120 ma during this process the upwelling asthenosphere underwent decompressional melting with limited penetration of slab derived fluids and gave rise to the n morb normal mid ocean ridge basalt like basaltic magmas that intruded the overlying previously generated depleted mantle as fab like gabbro diabase and dolerite sills or dykes then boninitic magmas represented by boninitic gabbro diabases were generated by remelting the extremely depleted residual mantle source which was metasomatized by a small amount of slab derived fluids following previous extractions of fab like magma

why were sea surface temperatures so different in the eastern equatorial atlantic in june 2005 and 2006 abstract a comparison of june 2005 and june 2006 sea surface temperatures in the eastern equatorial atlantic exhibits large variability in the properties of the equatorial cold tongue with far colder temperatures in 2005 than in 2006 this difference is found to result mainly from a time shift in the development of the cold tongue between the two years easterlies were observed to be stronger in the western tropical atlantic in april may 2005 than in april may 2006 and these winds favorably preconditioned oceanic subsurface conditions in the eastern atlantic however it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid may 2005 over a broad region extending from 20 w to the african coast and from 6 s to the equator this particular event underscores the ability of local intraseasonal wind stress variability in the gulf of guinea to initiate the cold tongue seaso

dissolution reprecipitation metasomatism and growth of zircon within phosphatic garnet in metapelites from western massachusetts highly restitic garnet kyanite phlogopite metapelitic schists from the goshen dome of western massachusetts contain a population of prograde monocrystalline megacrystic garnet some with significant p in substitution for si precipitates of hydroxylapatite and rutile and 5 mm 3 the unusual p content and the abundant internal precipitate suite are similar to features reported in garnet from ultrahigh pressure uhp and mantle settings suggesting a potential u hp origin for the garnet megacrysts zircon included in megacrysts is surrounded by radial fractures indicating in situ volumetric expansion or new growth cores display rare earth element ree profiles and cathodoluminescence cl zoning consistent with magmatic growth and yield only paleozoic dates 447 404 ma the embayed core rim boundary is marked by a several micrometers wide band of cl dark zircon enriched in y p u and th that is interpreted as the accumulation of redistributed xenotime component from the original zircon rim during metamorphism outside of this band the rim has elevated hf th u 1 and steep heavy ree profiles the metamorphic rims yield concordant dates from 400 to 381 ma matrix zircon grains have magmatic cores 1726 415 ma with similar core rim boundaries enriched in y p u and th metamorphic rims on matrix zircon yield slightly younger dates 393 365 ma and are compositionally heterogeneous the difference between the youngest core and oldest rim indicates a short interval ca 4 ma between deposition of detrital zircon and the onset of metamorphism in the earliest acadian the oldest zircon rim dates are found within phosphatic garnet megacrysts of possible very high pressure origin the compositional uniformity of these rims indicates equilibrium with a single source the anomalous composition suggests a combination of dissolution reprecipitation and new growth of zircon that is derived from garnet the range in both composition and dates indicates that matrix zircon rims formed in response to local changes in mineralogy and fluid melt composition and or availability new growth of zircon on these grains cannot be confirmed suggesting that dissolution reprecipitation reactions during continued metamorphism may be the dominant mechanism that formed these rims the data collectively suggest that dissolution reprecipitation may be a common mechanism for producing metamorphic rims on zircon that does not require additional zr and hf which are limited within most metamorphic settings

wind driven meridional eddy heat flux in the gulf of guinea seasonal response of the equatorial atlantic programme francais ocean et climat dans l atlantique equatorial data for 1984 are analyzed to estimate heat fluxes in the interior of the gulf of guinea between 1 n and 4 n in this region during may july the mixed layer roughly 35 m deep cools uniformly by nearly 4 c in 3 months representing a mean rate of heat loss of approximately 70 w m2 climatic heat budget calculations by hastenrath and lamb 1978 show a surface flux heat loss of 30 w m2 during this time this suggests that although surface flux makes a significant contribution to the mixed layer cooling additional divergent heat flux is required to balance the local heat budget all advective and diffusive terms are estimated to be small except for the meridional eddy heat flux v t the cospectrum of v and t has a peak near 15 days at this frequency v is coherent with the meridional wind the thermocline displacements forced by this meridional wind are nearly antisymmetric about the equator the frequency meridional length scale and asymmetry of these fluctuations suggest that a second baroclinic rossby gravity wave is being locally forced thus in the interior of the gulf of guinea the eddy flux driven by the fortnightly wind fluctuations makes a significant contribution to the cooling of the mixed layer during the seasonal upwelling this is in contrast to the central and western equatorial atlantic ocean where a meridional heat flux is produced by 25 to 30 day fluctuations generated by current instabilities

textures and high field strength elements in hydrothermal magnetite from a skarn system implications for coupled dissolution reprecipitation reactions there is increasing evidence to suggest that high field strength elements hfse could be mobile to some extent in hydrothermal fluid due to the influence of halogens e g fluorine and chlorine however in natural hydrothermal fluid systems coupled dissolution reprecipitation cdr reactions at fluid mineral interfaces that have been emphasized in the past decade may play a key role in controlling the final textures and mineral assemblages the influences of the cdr reactions in hydrothermal systems on hfse enrichment or depletion at the mineral scale are enigmatic in this study we show that enrichment of nb and zr can occur in magnetite on the mineral scale formed by hydrothermal fluids at medium to lower temperature in a skarn system four stages of mineralization and alteration of magnetite have been identified in the baishiya iron skarn deposit of the east kunlun mountains of china magnetite formed in stage 1 s1 developed obvious oscillatory zonation whereas that formed in stages 2 s2 and 3 s3 shows hydrothermal alteration and metasomatic textures and that in stage 4 s4 developed euhedral crystals with simple zoning systematic variations in the trace element compositions of different magnetite grains analyzed by empa and la icp ms suggest that the magnetite from s1 to s3 may have formed in a metasomatic process at relatively constant temperature whereas the magnetite from s4 formed by re equilibrium processes at lower temperature the magnetite from each stage can be divided into light and dark domains based on backscattered electron images the dark domains in the magnetite from s1 and s2 have higher nb ta 8 52 27 00 and zr hf 18 22 52 64 ratios and silicon contents than the light domains 0 55 5 66 and 2 54 16 43 respectively compared with other magnetite ores the ores from s1 and s2 are depleted of v and ni this depletion may be induced by increased oxygen and co crystallized sulfide however these variations are unlikely to be responsible for the enrichment of nb and zr in magnetite at equilibrium conditions conversely the dark domains of the magnetite from s1 and s2 are porous irregular and or oscillatory with quartz inclusions indicating nonequilibrium conditions these textural features could be attributed to the cdr reactions that are ubiquitous in skarn systems the increased silicon concentrations in magnetite due to the cdr reactions could affect the lattice parameters of the magnetite structure leading to an overall change in the volume of magnetite ores the reduplicative processes of volume change dissolution and porosity formation within magnetite are further imporved due to an incresed oxygen fugacity and co crystallized sulfide e g decreased temperature or increased sulfur fugacity at far from equilibrium or local equilibrium conditions resulting in oscillatory magnetite dark domains of s1 ripening of the transient porosity can trap nanoscale precipitates of columbite and zircon within pores of si magnetite and this precipitation could be attributed to the co crystallized phlogopite that would incorporate fluorine from the hydrothermal fluid and subsequently decrease the solubility of nb and zr in the skarn system this scenario highlights that nb and zr could be scavenged and enriched into in the reaction fronts porosity by controlling the reaction pathway at a local scale that does not reflect the overall fluid rock interaction history of the mineral assemblage

diffusion in zircon despite its low abundance in most rocks zircon is extraordinarily useful in interpreting crustal histories the importance of u th pb isotopic dating of zircon is well and long established davis et al this volume parrish et al this volume zircon also tends to incorporate trace elements useful as geochemical tracers such as the ree y and hf a number of characteristics of zircon encourage the preservation of internal isotopic and chemical variations often on extremely fine scale which provide valuable insight into thermal histories and past geochemical environments the relative insolubility of zircon in crustal melts and fluids as well as its general resistance to chemical and physical breakdown often result in the existence of several generations of geochemical information in a single zircon grain the fact that this information is so frequently retained as evidenced through backscattered electron or cathodoluminescence imaging that often reveal fine scale zoning down to the sub micron scale has long suggested that diffusion of most elements is quite sluggish in zircon in this chapter we present an overview of the findings to date from laboratory measurements of diffusion of cations and oxygen in zircon because of its importance as a geochronometer attempts have been made to measure diffusion especially of pb for over 30 years but only in the last decade or so have profiling techniques with adequate depth resolution been employed in these studies resulting in a plethora of new diffusion data these findings have important implications for isotopic dating interpretation of stable isotope ratios closure temperatures and formation and preservation of primary chemical composition and zoning in zircon efforts have been made for some time to quantify and characterize diffusion in zircon most notably of pb in deference to its significance in interpreting pb isotopic signatures and refining understanding of thermal histories as is evident from

remotely forced biweekly deep oscillations on the continental slope of the gulf of guinea current meter measurements on the continental slope of the gulf of guinea at 7 20 s and 1300 m depth have revealed biweekly oscillations of the currents bottom intensified and oriented along the bathymetry we develop a three dimensional primitive equation model of the gulf of guinea to study the oscillations and their forcing mechanism the high resolution 1 12 regional model reproduces remarkably well the main characteristics of the deep currents on the continental slope experiments with different forcings demonstrate that the biweekly variability at 1300 m depth is remotely forced by equatorial winds deep yanai waves generated by the wind propagate eastward along the equator upon reaching the african coast the energy propagates poleward in both directions as coastal trapped waves the selection of the dominant biweekly period is explained by the absence of equatorial waves with westward group velocities in that frequency band contrary to a previous hypothesis involving tidal forcing our interpretation is coherent with the significant interannual variability of the biweekly energy

low frequency variability in the equatorial atlantic abstract low frequency v and u component oscillations in the eastern equatorial atlantic are addressed using lime series of up to 1 5 years duration from the lower portion of the main thermocline a distinct frequency separation is observed with characteristic time scales of 1 month for v and 4 8 months for u the principal v component oscillations correspond to seasonally modulated linear equatorially trapped rossby gravity waves apparently generated at the surface in the central equatorial atlantic by shear instability between the south equatorial current and the north equatorial countercurrent their length and time scales are well defined and they agree with theoretical predictions the principal u component oscillations correspond to the deep zonal jets observed by recent profiling experiments in the indian and pacific oceans the vertical scales found here agree with the results from the other oceans however insufficient data are available anywhere to ascertain zonal scales linear theory would

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

zircon thermometry and u pb ion microprobe dating of the gabbros and associated migmatites of the variscan toledo anatectic complex central iberia in the central iberian zone there are several large thermal domes in which small bodies of ultramafic mafic and intermediate rocks appear intimately associated with crustal granites and migmatites the closest spatial association between the ultramafic mafic and intermediate rocks and migmatites is in the toledo anatectic complex where field relationships suggest that these rocks are coeval and have an age close to 340 ma this and the recent discovery in the neighbouring ossa morena zone of a large mid crustal seismic reflector interpreted as a 335 350 ma mafic sill reinforce the hypothesis that heat for crustal melting was supplied from early variscan mantle magmas emplaced in the middle crust however precise ion microprobe u pb zircon dating and ti in zircon thermometry in toledo do not support this idea whereas the mean age of four mafic bodies is 307 2 ma the migmatites are c 25 ma older the migmatites hosting ultramafic mafic and intermediate bodies have the same age and ti in zircon temperatures as migmatites far from any mafic intrusion these data reveal that ultramafic mafic and intermediate magmas are late variscan they were emplaced in already cooling anatectic zones once the extensional collapse was initiated and their thermal impact on the mid crustal variscan anatexis of central iberia was negligible

chicxulub crater source for shocked zircon crystals from the cretaceous tertiary boundary layer saskatchewan evidence from new u pb data new u pb data for five of six individual shocked zircon grains from the cretaceous tertiary k t boundary fireball layer rock creek site south central saskatchewan yield a source age of 548 6 ma the collinear data points are variably discordant in proportion to the intensity of shock metamorphism exhibited by each zircon the age and zircon shock features establish a strong temporal and genetic link with k t distal ejecta from the fireball layer at the berwind canyon site in the raton basin colorado where texturally similar zircons from a previous study gave an indistinguishable source age of 544 5 ma these ages are in agreement with independent ages obtained from a previous study for single shocked zircons from the chicxulub crater mexico and from beloc haiti the u pb zircon data unequivocally support a meteorite impact origin for the global k t boundary layer and imply that the chicxulub crater is the sole north american target source

deep intraseasonal variability in the central equatorial atlantic besides the zonal flow that dominates the seasonal and long term variability in the equatorial atlantic energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column 15 years of full depth velocity data from an equatorial mooring at 23 w are used to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the surface into the deep ocean near the surface 20 to 50 m intraseasonal variability at 23 w peaks at periods between 30 to 40 days it is associated with westward propagating tropical instability waves which undergo an annual intensification in august enhanced energy levels of equatorial intraseasonal variability are observed down to about 2000 m a frequency vertical mode decomposition shows that meridional velocity fluctuations are more energetic than the zonal ones for periods 50 days the energy peak at 30 to 40 days and vertical modes 2 to 5 excludes equatorial rossby or gravity waves and suggests yanai waves to be associated with the observed intraseasonal energy yanai waves that are considered to be generated by tropical instability waves propagate their energy from near the surface west of 23 w down and eastward to eventually reach the mooring location the distribution of intraseasonal energy depends largely on the dominant frequency and the time depth and longitude of excitation with the dominant vertical mode of the yanai waves playing only a minor role observations also indicate the presence of weaker intraseasonal variability at 23 w below 2000 m that is not associated with tropical instability waves

vertical structure of the seasonal cycle in the central equatorial atlantic ocean xbt sections from 1980 to 1988 abstract a set of temperature profiles from expendable bathythermographs collected from 1980 to april 1988 along two ship routes transecting the equatorial atlantic from 11 n to 11 s is analyzed to inter the vertical structure of the annual variability of the temperature and the currents in the upper ocean during the average seasonal cycle the vertical isotherm displacements occur earlier below 300 meters than near the surface at most locations within 4 degrees of the equator at the equator the amplitude of the displacements does not decrease with depth in the upper 500 meters this still holds down to 700 meters but there are less data at these depths the lead of the deeper isotherm displacements with respect to those in the upper thermocline implies that there is a contribution to the pressure forces from these layers that is not in phase with the contribution of the upper thermocline this also suggests that the energy source of the seasonal variability is close to the surface dynamic height and

geochronologic constraints on deformation and metasomatism along an exhumed mylonitic shear zone using apatite u pb geochemistry and microtextural analysis abstract directly dating and characterizing deformation and metasomatism in fault zones is critical to understanding fault zone processes and lithospheric evolution integrated apatite u pb geo thermochronometry geochemistry and microtextural analysis has potential to directly date and characterize deformation and fluid rock interactions in shear zones using a single mineral phase integrated apatite u pb microtextural and trace and rare earth element results from an exhumed mylonitic shear zone the main mylonitic band in the northern pyrenees constrain timing of deformation and metasomatism along the shear zone as early cretaceous synchronous with ductile deformation and exhumation of middle lower crustal gneisses in the footwall and significant mineralization and metasomatism in the brittle hanging wall samples from the hanging wall preserve magmatic apatite u pb crystallization ages that overlap with zircon and monazite u pb ages of 300 ma granitic geochemical trends and primary magmatic growth zoning footwall samples below the mmb record dynamic recrystallization of apatite associated with mylonitization and exhumation of the gneisses in the early cretaceous apatite along the mmb record partial recrystallization via dissolution reprecipitation associated with shearing and circulation of deeply sourced fluids along the shear zone evident by mixed ages 300 100 ma geochemical signatures enriched in light rare earth elements and patchy microtextures the mmb likely acted as a conduit for deeply sourced fluids that affected the brittle ductile transition strain localization and fluid pathways through the continental crust this study demonstrates that the integration of apatite u pb geo thermochronology geochemistry and microtextural analysis is a powerful approach to dating deformation and metasomatism characterizing grain scale deformation and re crystallization processes and geochemically characterizing fluid rock interactions in exhumed middle crustal shear zones

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

dissolution reprecipitation of igneous zircon in mid ocean ridge gabbro atlantis bank southwest indian ridge abstract zircons recovered from oceanic gabbro exposed on atlantis bank southwest indian ridge typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas in one rock of twenty investigated weak oscillatory zonation patterns are overprinted by secondary textural features characterized by mottled convoluted and wavy internal zonation patterns that are frequently associated with secondary micron to submicron scale micro porosity these zircons are hosted in a felsic vein that intruded an oxide gabbro both of which are cross cut by monomineralic amphibole and quartz rich veinlets zircons with weak oscillatory zonation patterns record a weighted average 206pb 238u age of 12 76 0 20 ma mswd 1 5 and have high trace element concentrations e g rees 0 4 2 2 wt y 0 6 2 8 wt p 0 4 0 9 wt and th u 0 1 0 5 these zircons are anomalously old 1 myr relative to the magnetic age for this portion of oceanic crust 11 75 ma in contrast zircons with non igneous secondary textures have a younger weighted average 206pb 238u age of 12 00 0 16 ma mswd 1 7 and have lower trace element concentrations e g rees 0 2 0 8 wt y 0 3 1 0 wt p 0 1 0 3 wt and slightly lower th u 0 1 0 3 the weighted average age of these zircons is similar to the magnetic anomaly age and other 206pb 238u ages of nearby rocks we do not observe a correlation between crystallographic misorientation internal texture or trace element chemistry we suggest that the decrease in trace element concentrations associated with the development of non igneous alteration textures is attributed to the purging of non essential structural constituent cations from the zircon crystal lattice at amphibolite facies conditions the mechanism of alteration re equilibration was likely an interface coupled dissolution reprecipitation processes that affected pre existing anomalously old zircons during shallow level magmatic construction of atlantis bank at 12 0 ma

variability of the horizontal velocity structure in the upper 1600 m of the water column on the equator at 10 w this paper presents initial results from new velocity observations in the eastern part of the equatorial atlantic ocean from a moored current meter array during the equalant program 1999 2000 a mooring array was deployed around the equator near 10 w that recorded one year of measurements at various depths horizontal velocities were obtained in the upper 60 m from an upward looking acoustic doppler current profiler adcp and at 13 deeper levels from current meters between 745 and 1525 m to analyze the quasiperiodic variability observed in these records a wavelet based technique was used quasiperiodic oscillations having periods between 5 and 100 days were separated into four bands 5 10 10 20 20 40 and 40 100 days the variability shows i a strong seasonality the first half of the series is dominated by larger periods than the second one and ii a strong dependence with depth some oscillations are present in the entire water column while others are only present at certain depths for the oscillations that are present in the entire water column the origin of the forcing can be traced to the surface while for the others the question of their origin remains open phase shifts at different depths generate vertical shears in the horizontal velocity component with relatively short vertical scales this is especially visible in long duration events 100 days of the zonal velocity component comparison with a simultaneous lowered acoustic doppler current profiler ladcp section suggests that some of these flows may be identified with equatorial deep jets a striking feature is a strong vertical shear lasting about 7 months between 745 and 1000 m these deep current meter observations would then imply a few months of duration for the jets in this region

direct dating of adirondack massif anorthosite by u pb shrimp analysis of igneous zircon implications for amcg complexes the low abundance of igneous zircon in proterozoic massif anorthosites has presented a major obstacle to the acquisition of direct absolute ages of crystallization for these important rocks indirect dating that relies on zircon ages from associated mangerite charnockite granite granitoids assumes that they have a coeval relationship with anorthosite that requires documentation shrimp sensitive high resolution ion microprobe u pb zircon dating techniques provide a powerful means for directly dating the small populations of zircons in anorthositic rocks and for resolving problems with inheritance within the adirondack mountains 10 samples of massif anorthosite have yielded more than sufficient quantities of igneous zircon to establish directly the ages of the region9s classic anorthosite occurrences e g the marcy and oregon dome massifs in addition a ferrogabbro a ferrodiorite and a coronitic olivine metagabbro all crosscutting massif anorthosite were dated the average age of this suite of 13 anorthositic samples is 1154 6 ma mswd mean square of weighted deviates 0 26 probability 0 99 in addition eight associated granitoids have been dated by shrimp techniques and complement another five previously dated by multi grain thermal ionization mass spectrometry tims methods the 13 granitoids yield an average age of 1158 5 mswd 0 89 probability 0 60 and are broadly coeval with the massif anorthosite the overlapping ages provide evidence that these rocks constitute a single composite anorthosite mangerite charnockite granite amcg suite intruded at ca 1155 ma an age corresponding to the ages of major amcg suites in the grenville province in canada e g morin and lac st jean although rocks of the adirondack amcg suite are now documented as broadly coeval it does not follow that the constituent amcg lithologies were comagmatic field relationships and mineral disequilibria in transitional zones are inconsistent with derivation from a single parental magma moreover the presence of older ca 1 2 1 3 ga inherited cores in some zircons from amcg granitoids conflicts with derivation of these rocks from magmas that formed anorthosite gabbro or ferrodiorite or jotunite in which zircons are highly soluble the slightly older ca 1158 ma average age of the mangeritic and charnockitic members of the amcg suite is consistent with an origin as early lower crustal anatectites that left behind pyroxene plagioclase restites this refractory material then reacted by assimilation fractional crystallization afc with ponded mantle derived gabbroic magmas to produce plagioclase rich crystal mushes with crustal isotopic signatures as proposed much earlier by r f emslie these magmas are considered to be parental to the adirondack anorthosite and upon ascent they were emplaced in proximity to still hot earlier mangeritic and charnockitic bodies where they underwent further fractionation the composite nature of the marcy massif documents that this process was repeated in several sequential pulses

dynamics of intraseasonal sea level and thermocline variability in the equatorial atlantic during 2002 03 abstract satellite and in situ observations in the equatorial atlantic ocean during 2002 03 show dominant spectral peaks at 40 60 days and secondary peaks at 10 40 days in sea level and thermocline within the intraseasonal period band 10 80 days a detailed investigation of the dynamics of the intraseasonal variations is carried out using an ocean general circulation model namely the hybrid coordinate ocean model hycom two parallel experiments are performed in the tropical atlantic ocean basin for the period 2000 03 one is forced by daily scatterometer winds from the quick scatterometer quikscat satellite together with other forcing fields and the other is forced by the low passed 80 day version of the above fields to help in understanding the role played by the wind driven equatorial waves a linear continuously stratified ocean model is also used within 3 s 3 n of the equatorial region the strong 40 60 day sea surface height anomaly ssha and thermocline variability result mainly from the

the upstream spreading of bottom trapped plumes it is well known that numerical simulations of freshwater discharges produce plumes that spread in the direction opposite to that of the propagation of coastally trapped waves the upstream direction the lack of a theory explaining these motions in unforced environments deemed the numerical results suspect thus it became a common practice in numerical studies to add a downstream mean flow to arrest the development of the upstream perturbation this approach is generally unjustified and it remains a matter of interest to determine if the upstream displacement produced by models is a geophysical phenomenon or a consequence of erroneous assumptions in the model setup in this article the results of highly idealized numerical experiments are used to investigate these matters it is shown that this phenomenon is associated with the geostrophic adjustment of the discharge and that upstream motion is endemic to the baroclinic structure of bottom trapped plumes it is also shown that downstream displacements are generated by the cross shelf barotropic pressure gradient generated by the propagation of coastally trapped waves sensitivity experiments indicate that the speed of upstream propagation and the density structure of the plume are affected by bottom friction the slope of the bottom and the magnitude of the density anomaly bottom friction in particular slows down the progression of the plume and changes its density structure producing a more homogeneous downstream region and a more stratified upstream region

evolution of the southwest indian ridge from 55 45 e to 62 e changes in plate boundary geometry since 26 ma from 55 45 e to 58 45 e and from 60 30 e to 62 00 e the ultraslow spreading southwest indian ridge swir consists of magmatic spreading segments separated by oblique amagmatic spreading segments transform faults and nontransform discontinuities off axis magnetic and multibeam bathymetric data permit investigation of the evolution of this part of the swir individual magmatic segments show varying magnitudes and directions of asymmetric spreading which requires that the shape of the plate boundary has changed significantly over time in particular since 26 ma the atlantis ii transform fault grew by 90 km to reach 199 km while a 45 km long transform fault at 56 30 e shrank to become an 11 km offset nontransform discontinuity conversely an oblique amagmatic segment at the center of a first order spreading segment shows little change in orientation with time these changes are consistent with the clockwise rotation of two 450 km wide first order spreading segments between the gallieni and melville transform faults 52 60 e to become more orthogonal to spreading we suggest that suborthogonal first order spreading segments reflect a stable configuration for mid ocean ridges that maximizes upwelling rates in the asthenospheric mantle and results in a hotter and weaker ridge axis that can more easily accommodate seafloor spreading

low frequency meandering of the atlantic north equatorial countercurrent four 19 month time series of indirect measurements of dynamic height were obtained in the tropical atlantic along 28 w at 0 3 6 and 9 n with three inverted echo sounders and one current meter mooring the series were analyzed to study the time latitude variability of the north equatorial countercurrent necc the eastward flow associated with the necc was present at 28 w from 3 to 9 n during most of the observed period except in march april 1983 and april may 1984 periods that coincided with the onset of the wind at the equator the amplitude of the necc s annual cycle was maximum at 6 n and was larger in 1983 than in 1984 the analysis of the time latitude variability of differences in dynamic height shows a long period meridional shift of the necc the core of the current attains its northernmost location during august september in both years and its southernmost location during march april in 1983 and march in 1984 the location of the core is directly related to the position of the intertropical convergence zone from the time series of dynamic height obtained from the indirect measurements geostrophic velocities and transports were estimated and compared with direct observations of currents and values obtained from hydrographic casts

protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole u1309d 30 n mar sensitive high resolution ion microprobe shrimp u pb zircon ages of 24 samples from oceanic crust recovered in integrated ocean drilling program iodp hole u1309d and from the surface of atlantis massif mid atlantic ridge mar 30 n document a protracted history of accretion in the footwall to an oceanic detachment fault ages for 18 samples of evolved fe ti oxide gabbro and felsic dikes collected 40 1415 m below seafloor in u1309d yield a weighted mean of 1 20 0 03 ma mean square of weighted deviates 7 1 however the ages range from 1 08 0 07 ma and 1 28 0 05 ma indicating crustal construction occurred over a minimum of 100 200 ka the zircon ages along with petrologic observations indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka the oldest ages are observed below 600 mbsf an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault the data are most consistent with a multiple sill model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1 4 km zircon ages from broadly spaced samples collected along the southern ridge of atlantis massif yield a detachment fault slip rate of 28 7 6 7 mm a and imply significant asymmetric plate spreading up to 100 on the north american plate for at least 200 ka during core complex formation

tropical atlantic sea level variability from geosat 1985 1989 geosat altimeter data between april 1985 and september 1989 are analyzed in the tropical atlantic ocean first improvements due to the use of new corrections and orbit computations are found to be effective especially in the gulf of guinea where part of the previously missing signal is recovered then the variability of the ocean is examined using empirical orthogonal functions eofs only three eofs are needed to describe 80 of the seasonal variance the first one describes the meridional tilting of the tropical atlantic along the mean location of the intertropical convergence zone with an annual period the second describes a mass redistribution due to the equatorial upwelling peaking in june july the third function presents a clear semiannual signal looking at interannual variability the first eof reveals a mass redistribution between the equatorial region 10 n to 10 s and the northern and southern ones 10 n to 30 n 10 s to 30 s in the equatorial region the upper layer volume increases from about 1 5 to 1 5 x 1014 m3 between 1987 and 1989 occurring 1 year after the pacific el nino this phenomenon recalls the 1984 anomaly observed during the programme francais occan et climat en atlantique tropicavseasonal equatorial atlantic experiments

termination of backarc spreading zircon dating of a giant oceanic core complex the godzilla megamullion is the largest oceanic core complex occ currently known and is adjacent to the spreading center of the parece vela basin pvb an extinct backarc basin in the philippine sea the duration and termination of tectonomagmatic processes during occ formation are poorly constrained due to the weak geomagnetic anomalies in the region zircon u pb dating of gabbroic and leucocratic rocks from the godzilla megamullion reveals that fault induced spreading over the 125 km length of the occ lasted for 4 m y with continuous magmatic accretion at the spreading axis the latest magmatism constrains the cessation of pvb spreading to ca 7 9 ma or later significantly younger than a previous estimate of ca 12 ma the new ages show that backarc basin formation migrated to the present day mariana trough soon after the cessation of spreading in the pvb

perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon abstract the formation of oceanic plagiogranite has been attributed primarily to either 1 extreme fractional crystallization of a mantle melt or 2 partial melting of hydrated mafic crust with support for the latter from field evidence and recent melting experiments remelting of hydrothermally altered ocean crust could yield rocks and minerals with diverse primary magmatic 18 o values forming proximal to the magmatic center where crustal growth is occurring to constrain the magmatic 18 o of a wide range of silicic rocks in oceanic crust and evaluate their petrogenesis we characterized the 18 o of zircons in 22 plagiogranite samples tonalite and trondhjemite from 8 different ophiolites and one dacite sampled along the east pacific rise using secondary ion mass spectrometry sims the 18 o values of 202 magmatic zircons from ophiolites range from 3 9 to 5 6 n 244 spots average 4 9 0 6 2sd extending 1 below typical zircon in equilibrium with mantle and from gabbroic massifs along slow spreading mid ocean ridges 4 7 5 9 east pacific rise dacite zircons range from 4 6 to 5 0 n 12 spots plagiogranite from the dike gabbro transition zone of the northern oman ophiolite yield the lowest 18 o zrn with rock average values of 4 3 5 0 the low 18 o values are best explained by remelting of crust altered by hydrothermal fluids with seawater like isotopic compositions at high temperatures possibly due to vertical migration of the boundary between an active magma chamber and a vigorous high temperature hydrothermal system in the overlying crust if the partial melt was assimilated into a fractionating melt lens with morb like 18 o as envisioned for km scale plagiogranite bodies in oman up to 20 contamination by a protolith with 18 o 2 would be required previous oxygen isotope constraints from quartz in oman plagiogranite suggested melting of both high and low 18 o crust had occurred comparison of quartz zircon pairs indicates that quartz has been modified in most samples and we find no evidence for the involvement of high 18 o rocks during plagiogranite formation

zircons from syros cyclades greece recrystallization and mobilization of zircon during high pressure metamorphism zircons were studied from high pressure low temperature metamorphosed meta igneous lithologies from syros these rocks carry several zircon generations related to each other by dissolution reprecipitation processes one generation is pristine zircon that shows growth zoning relatively elevated contents of trivalent cations and high th u ratios the other end member is a skeletal zircon generation with negligible trivalent cation contents and low th u ratios 0 1 texturally between these two there is a range of zircon crystals with complex inclusion populations of y hree th phases and fluid inclusions showing variable progress of replacement recrystallization both end members yield distinct sensitive high resolution ion microprobe shrimp u pb ages the pristine generation has an age of 80 2 1 6ma from a metagabbro and 76 4 2 1ma from a meta plagiogranite dyke the skeletal low th u zircon generation yields an age of 52 4 0 8ma the older late cretaceous zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust the younger eocene age however dates a zircon recrystallization event which possibly coincides with high solubility and mobility of high field strength elements in a high pressure aqueous fluid phase intergrowth relations between zircon and peak metamorphic garnet and excellent agreement of the u pb ages with white mica ar ar ages for the same samples support the conclusion that eocene is the true age of high pressure metamorphism on syros

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

on the occurrence trace element geochemistry and crystallization history of zircon from in situ ocean lithosphere we characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite evolved gabbro and metamorphosed ultramafic host rocks collected along present day slow and ultraslow spreading mid ocean ridges mors the geochemistry of 267 zircon grains was measured by sensitive high resolution ion microprobe reverse geometry at the usgs stanford ion microprobe facility three types of zircon are recognized based on texture and geochemistry most ocean crustal zircons resemble young magmatic zircon from other crustal settings occurring as pristine colorless euhedral type 1 or subhedral to anhedral type 2 grains in these grains hf and most trace elements vary systematically with ti typically becoming enriched with falling ti in zircon temperature ti in zircon temperatures range from 1 040 to 660 c corrected for a tio2 0 7 a sio2 1 0 pressure 2 kbar intra sample variation is typically 60 150 c decreasing ti correlates with enrichment in hf to 2 wt while additional hf enrichment occurs at relatively constant temperature trends between ti and u y ree and eu eu exhibit a similar inflection which may denote the onset of eutectic crystallization the inflection is well defined by zircons from plagiogranite and implies solidus temperatures of 680 740 c a third type of zircon is defined as being porous and colored with chaotic cl zoning and occurs in 25 of rock samples studied these features along with high measured la cl s ca and fe and low sm la n ratios are suggestive of interaction with aqueous fluids non porous luminescent cl overgrowth rims on porous grains record uniform temperatures averaging 615 26 c 2sd n 7 implying zircon formation below the wet granite solidus and under water saturated conditions zircon geochemistry reflects in part source region elevated hree coupled with low u concentrations allow effective discrimination of 80 of zircon formed at modern mors from zircon in continental crust the geochemistry and textural observations reported here serve as an important database for comparison with detrital xenocrystic and metamorphosed mafic rock hosted zircon populations to evaluate provenance

cooling history of atlantis bank oceanic core complex evidence for hydrothermal activity 2 6 ma off axis 1 we report 26 u th he zircon ages from atlantis bank southwest indian ridge which constrain time scales and rates of lower crustal cooling in ultraslow spreading oceanic crust in this setting samples from the detachment fault surface indicate that denuded oceanic crust cooled rapidly 1200 c ma consistent with existing models for the cooling of oceanic crust u th he zircon ages from samples collected along n s and e w trending faults scarps record young ages inconsistent with standard cooling models for lower oceanic crust these samples have a mean u th he zircon age 2 6 ma younger than their corresponding igneous crystallization ages and record cooling through 200 c well outside the rift valley similar anomalously young ages are recorded by zircon sphene and apatite fission track data from odp hole 735b we interpret these young ages as recording an off axis thermal heating event associated with localized high temperature 300 c hydrothermal fluid flow resulting from underplated mafic magmas

cross equatorial structure and temporal modulation of intraseasonal variability at the surface of the tropical atlantic ocean intraseasonal variability 10 50 days in the equatorial atlantic ocean is analyzed from multiyear 1999 2005 satellite gridded products of sea level anomalies sla and sea surface temperature sst two regions with distinct intraseasonal variability have been identified the first one west of 10 degrees w is dominated by westward propagating anomalies with maximum values in sla along 5 degrees n and in sst along 2 degrees n they occur in boreal summer with periods of 25 50 days and are known to correspond to tropical instability waves tiws we show that tiws have also a signature though weaker south of the equator especially along 5 degrees s in sla northern and southern anomalies propagate together westward being mostly out of phase suggesting that equatorial wave dynamics is involved in tiws variability an sst signature of tiws is also observed near 2 degrees s in quadrature with sst anomalies detected in the northern hemisphere the interannual modulations of the tiw signature in sla and sst are compared and discussed the second dominant intraseasonal signal is only seen east of 10 degrees w in sst and corresponds to an equatorially trapped variability confined to the gulf of guinea with periods between 10 and 20 days this signal is present in boreal summer when an intense sst front is observed just north of the equator intraseasonal variability with comparable periods is also observed in the meridional wind stress throughout the year comparison of sst and meridional wind stress anomalies suggests that the 10 to 20 day variability in sst is forced by the wind stress but seasonally modulated by the presence of the sst front

from opening to subduction of an oceanic domain constrained by la icp ms u pb zircon dating variscan belt southern armorican massif france in the variscan belt of western europe the lifetime and evolution of the oceanic domain is poorly constrained by sparse outdated and unreliable multigrain id tims u pb zircon dating in this article we present a complete in situ la icp ms dataset of about 300 u pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of southern brittany comprising new dating of previously published zircon populations and newly discovered rock samples in situ dating and cathodo luminescence imaging of each zircon grain yields new absolute time constraints on the evolution of the galicia moldanubian ocean the new results confirm that the opening of this oceanic domain is well defined at about 490 ma in contrast the generally quoted 400 410 ma age for the high pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites in light of these new data we propose that the obduction of oceanic rocks occurred at about 370 380 ma while the high pressure event is recorded at 355 ma in only a few zircon grains of some eclogite samples additionally this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their u pb systems do not record that metamorphism systematically these zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith another example of an eclogite sample from the french massif central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism

experimental shock deformation in zircon a transmission electron microscopic study abstract in recent years apparently shock induced and thus impact characteristic microdeformations in the form of planar microdeformation features and so called strawberry granular texture have been observed in zircons in rocks from confirmed impact structures and from the k t boundary the nature of the planar microdeformations in this mineral is however still unknown and critical information is needed regarding the shock pressure range in which these deformation effects are produced we experimentally shock deformed two series of thin zircon zrsio 4 target plates cut perpendicular to the c axis at shock pressures of 20 40 and 60 gpa the recovered samples were characterized by optical and scanning electron microscopy in addition one sample series was studied by transmission electron microscopy tem microdeformation effects observed at 20 gpa include pervasive micro cleavage and dislocation patterns plastic deformation is indicated by a high density of straight dislocations in glide configuration the dominant glide systems are 010 micro cleavages induced by shear stresses during the compression stage occur mostly in the 100 planes the large density of dislocations at crack tips shows that plastic deformation was initiated by the micro cracking processs at 40 gpa the sample was partly transformed from the zircon z to a scheelite cawo 4 type s structure planar deformation features pdfs containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics the phase with scheelite structure initiated in the 100 planes of zircon consists of thin 0 1 to several m bands that crosscut the zircon matrix the phase transformation is displacive martensitic and can be related by 100 z 112 s and 001 z s the scheelite structure phase is densely twinned with twins in the 112 plane the 60 gpa sample consists completely of the scheelite structure phase crosscutting and displacing relationships between twins and pdfs demonstrate that pdfs are formed in the zircon structure i e before the phase transformation to the scheelite structure occurred most likely at the shock front crystallographic orientations of optically visible planar features in zircon in comparison with orientations of planar defects at the tem scale suggest that the optically visible features are more likely planar microfractures than pdfs

zircon record of fractionation hydrous partial melting and thermal gradients at different depths in oceanic crust odp site 735b south west indian ocean felsic veins plagiogranites are distributed throughout the whole oceanic crust section and offer insight into late magmatic high temperature hydrothermal processes within the oceanic crust despite constituting only 0 5 of the oceanic crust section drilled in iodp site 735b they carry a significant budget of incompatible elements which they redistribute within the crust such melts are saturated in accessory minerals such as zircon titanite and apatite and often zircon is the only remaining phase that preserves magmatic composition and records processes of felsic melt formation and evolution in this study we analysed zircon from four depths in iodp site 735b they come from the oxide gabbro depth approximately 250 m below sea floor and plagiogranite depths c 500 860 940 m below sea floor all zircons have similar ehf composition of c 15 units indicating an isotopically homogenous source for the mafic magmas forming iodp site 735b gabbro zircons from oxide gabbro are scarce and variable in composition consistent with their crystallization from melts formed by both fractionation of mafic magmas and hydrous remelting of gabbro cumulate on the other hand zircon from plagiogranite is abundant and each sample is characterized by compositional trends consistent with crystallization of zircon in an evolving melt however the trends are different between the plagiogranite at 500 m bsf and the deeper sections which are interpreted as the record of plagiogranite formation by two processes remelting of gabbro cumulate at 500 m bsf and fractionation at deeper sections zircon from both oxide gabbro and plagiogranite has 18o from 3 5 to 6 0 values of 18o are best explained by redistribution of 18o in a thermal gradient and not by remelting of hydrothermally altered crust tentatively it is suggested that fractionation could be an older episode contemporaneous with gabbro crystallization and remelting could be a younger one triggered by deformation and uplift of the crustal pile

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

fingerprinting the k t impact site and determining the time of impact by upb dating of single shocked zircons from distal ejecta abstract u pb isotopic dating of single 1 3 g zircons from k t distal ejecta from a site in the raton basin colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site data for the least shocked zircons are slightly displaced from the 544 5 ma primary age for a component of the target site white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65 5 3 0 ma such shocked and granular zircons have never been reported from any source including explosive volcanic rocks zircon is refractory and has one of the highest thermal blocking temperatures hence it can record both shock features and primary and secondary ages without modification by post crystallization processes unlike shocked quartz which can come from almost anywhere on the earth s crust shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers with u pb zircon dating it is now possible to correlate ejecta layers derived from the same target site test the single versus multiple impact hypothesis and identify the target source of impact ejecta the ages obtained in this study indicate that the manson impact site iowa which has basement rocks that are mid proterozoic in age cannot be the source of k t distal ejecta the k t distal ejecta probably originated from a single impact site because most grains have the same primary age

high frequency oscillations and their correlation with the wind forcing during the first sequal year two arrays of inverted echo sounders one along the equator and the other across the north equatorial countercurrent are analysed together with a wind record from st peter and st paul rocks the dominant high frequency feature along the equator is an energetic oscillation with a period of 5 8 days that shows strong coherence with the meridional component of the wind the sounder at 3 n 28 w on the other hand shows a high degree of coherence with the zonal component of the wind at a period of 4 5 days comparison of these measurements in 1983 with measurements made in 1979 suggest considerable non stationarity

observations of inertia gravity waves in the atlantic from inverted echo sounders during fgge abstract the study is based on observations from inverted echo sounders anchored near the equator in the western atlantic ocean it appears to be a good method of detecting inertia gravity waves with large vertical scale the spectral analysis of the records shows variable spectral density levels in the band of 2 5 days three of the four inverted echo sounder ies records show an increase in the variance centered at 3 75 days in addition two sounders deployed during the latter half of the year stronger winds also show a peak in the variance at 2 14 and 2 31 days the most likely interpretation of the spectral peak at 3 75 days is an atmospherically forced vertically propagating inertia gravity wave with a meridional structure given by the n 3 mode the corresponding equivalent depth is h 0 7 m and the vertical wavelength is 730 m the energy density estimated from the variance in the ies records is e 0 7 103 j m 2

deformation related microstructures in magmatic zircon and implications for diffusion an undeformed glomeroporphyritic andesite from the sunda arc of java indonesia contains zoned plagioclase and amphibole glomerocrysts in a fine grained groundmass and records a complex history of adcumulate formation and subsequent magmatic disaggregation a suite of xenocrystic zircon records proterozoic and archaean dates whilst a discrete population of zoned euhedral igneous zircon yields a shrimp u pb crystallisation age of 9 3 0 2 ma quantitative microstructural analysis of zircon by electron backscatter diffraction ebsd shows no deformation in the inherited xenocrysts but intragrain orientation variations of up to 30 in 80 of the young zircon population these variations are typically accommodated by both progressive crystallographic bending and discrete low angle boundaries that overprint compositional growth zoning dispersion of crystallographic orientations are dominantly by rotation about an axis parallel to the zircon c axis 001 which is coincident with the dominant orientation of misorientation axes of adjacent analysis points in ebsd maps less common misorientation axes account for minor components of crystallographic dispersion these observations are consistent with zircon deformation by dislocation creep and the formation of tilt and twist boundaries associated with the operation of 100 and 010 slip systems the restriction of deformation microstructures to large glomerocrysts and the young magmatic zircon population and the absence of deformation within the host igneous rock and inherited zircon grains indicate that zircon deformation took place within a low melt fraction 5 melt mid lower crustal cumulate prior to fragmentation during magmatic disaggregation and entrainment of xenocrystic zircons during magmatic decompression tectonic stresses within the compressional sunda arc at the time of magmatism are considered to be the probable driver for low strain deformation of the cumulate in the late stages of initial crystallisation these results provide the first evidence of crystal plastic dislocation creep in zircon associated with magmatic crystallisation and indicate that the development of crystal plastic microstructures in zircon is not restricted to high strain rocks such microstructures have previously been shown to enhance bulk diffusion of trace elements u th and ree in zircon the development of deformation microstructures and therefore multiple diffusion pathways in zircon in the magmatic environment has significant implications for the interpretation of geochemical data from igneous zircon and the trace element budgets of melts due to the potential enhancement of bulk diffusion and dissolution rates