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

deposit from a giant wave on the island of lanai hawaii limestone bearing gravel the newly named hulopoe gravel blankets the coastal slopes on lanai the deposit which reaches a maximum altitude of 326 meters formerly was believed to have been deposited along several different ancient marine strandlines but dated submerged coral reefs and tide gauge measurements indicate that the southeastern hawaiian islands sink so fast that former worldwide high stands of the sea now lie beneath local sea level evidence indicates that the hulopoe gravel and similar deposits on nearby islands were deposited during the pleistocene by a giant wave generated by a submarine landslide on a sea scarp south of lanai

geology of new providence island bahamas contrary to the popular notion that the bahama islands are built of eolianite deposits at least new providence island consists principally of elevated marine sand flat and protected lagoon deposits narrow eolianite ridges separate such deposits from reef tract deposits capped by prograding beach deposits on the northern bank margin side of the island all exposed depositional phases are quaternary the most extensively exposed deposits we correlate with the 125 000 yr high sea level recognized world wide elevations of keystone vugs in beach deposits of that depositional phase indicate paleo mean sea levels of as high as 10 m deposits of an earlier depositional phase suggest that there was then no island but only a barrier sand shoal and reef tract holocene additions to the island9s area have been minor and in the form of prograding beach deposits classical superpositional stratigraphy has limited value in the elucidation of new providence geology due to the nature of the deposits which are partially overlapping thin facies sheets and lenses therefore we have also used a morphostratigraphic approach and as paleontological markers species of cerion a very rapidly evolving genus of land snail limited radiometric dating is reported islands on bahama type banks of earlier geologic periods were probably built almost entirely of beach deposits rather than the eolianites and elevated marine deposits of which new providence is built

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

boulder deposits from large waves during the last interglaciation on north eleuthera island bahamas abstract seven boulders measuring 100 to 1000 m 3 are scattered along the coastal ridge of north eleuthera some are situated on ridge crests up to 20 m above present sea level the boulders were probably deposited during oxygen isotope substage 5e or 5d as shown by their stratigraphic setting and by amino acid racemization ratios d alloisoleucine l isoleucine ratios were determined for land snails and oolite of both marine and eolian origin was associated with the boulders like the boulders the probable source rocks exposed in the adjacent cliffs are composed of marine and eolian limestone of oolitic and peloidal composition the source beds are correlated with stage 9 or 11 the largest boulder is about 10 times the size of the largest holocene ones moved by waves in the area tsunamis are a reasonable possibility as a transporting mechanism of the pleistocene boulders however if deposited instead by storms during the last interglaciation the storms were of much greater intensity than those occurring in the region during the late holocene

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

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

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

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

scalloped bank margins beginning of the end for carbonate platforms bathymetric seismic reflection and satellite data have revealed the common existence of a new type of carbonate bank margin scalloped margin throughout the gulf of mexico and the caribbean region these large up to tens of kilometres across irregular convexbankward embayments are interpreted as predominantly erosional features produced by large scale failure of platform margins in tectonically active areas they may be triggered by earthquake shocks whereas in stable regions they may more likely be the result of undercutting by dissolution and or deep water currents scalloped margins are common in the south eastern bahamas this suggests that these relatively small isolated bahama banks may be erosional remnants of once larger platforms the processes that create scalloped margins may result in erosional deep water carbonate basins and potentially the demise and disintegration of carbonate platforms

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

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

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

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

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

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

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

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

precise chronology of the last interglacial period 234u 230th data from fossil coral reefs in the bahamas a detailed study of 238 u 234 u 230 th ages was made for different coral species from two bahamian reefs to determine the time scale of the sea level high during the last interglacial period using recently developed mass spectrometric techniques for the measurement of u and th isotopes thirty seven coral samples were analyzed in replicate typical errors at 125 ky are 1 5 ky 2 and 3 ky time intervals appear to be well resolved this high precision permits detailed chronologic study with time resolution adequate to define stages of the reef s history these data demonstrate that the time between transgression and regression of the ocean in the last interglacial was about 12 ky after which sea level fell rapidly at more than 2 m ky the high sea level stand began possibly by 132 ky and certainly by 129 ky ago when sea level reached 6 m above present mean low sea level high sea level was sustained until 120 ky and then fell rapidly this time period covers the milankovitch insolation peak at 65 n but is not sharply defined and the sea level high lasted from about 4 ky before the insolation peak to 8 ky after it we find no evidence for a double peak of sea level rise in the last interglacial episode that is high stands at both 125 and 142 ky there appears to be some discrepancy between this precise chronology for a coral reef on a tectonically stable platform and the chronology assigned to deep sea sediments the present results show that the duration of the last interglacial led to a 12 ky period of high sea level stand which is much longer than the insolation peak the study area consists of two sizeable bank barrier coral reefs of sangamon age that crop out along the coast northwest of cockburn town on san salvador island and at devil s point on great inagua island the initial stage of reef development is not revealed at either site but the main parts of both reefs are well preserved at cockburn town the beginning of the reef was marked by colonization of acropora palmata on hardground areas and possibly other corals at 132 ky ago patch reefs at the southeastern flank of the developing reef were growing by 129 to 126 ky ago by 126 to 123 ky ago the crest of the reef was formed by the frame building coral a palmata and associated patch reefs flourished along its flanks a palmata coral heads grew upward to a mean low sea level 6 m above the present one the reef began to decline as sea level dropped at about 123 to 120 ky ago near two measured profiles at devil s point the top of a rubblestone dominated by a cervicornis and patch reef corals preserved in growth position have been beveled off and represent an ancient wave cut surface 230 th ages on corals below and above this surface indicate that it was cut at about 125 1 ky ago

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

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

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

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

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

catastrophic wave erosion on the southeastern coast of australia impact of the lanai tsunamis ca 105 ka sand barriers along the coast of southern new south wales dating from the last interglacial have been almost completely destroyed most probably by a catastrophic tsunami evidence for catastrophic wave erosion can also be traced to heights of at least 15 m above present sea level on coastal abrasion ramps these erosional features lie above the range of effective erosion by contemporary storm waves and cannot be attributed to either eustatic fluctuations or local uplift chronological evidence for the timing of the destruction of the last interglacial barriers suggests that tsunamis generated by the submarine slide off lanai in the hawaiian islands 105 ka traveled across the pacific and eroded this coast

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

sedimentation et fracturation de haute energie en milieu recifal tsunamis ouragans et cyclones et leurs effets sur la sedimentologie et la geomorphologie d un atoll motu et hoa a rangiroa tuamotu pacifique se abstract a longitudinal festoon fracture of the reef flat widened by slow biological and mechanical erosion transverse along the grooves divides the nw coral rim of rangiroa tuamotu se pacific into colossal blocks 15 10 5 m 1500 t the elaboration of these blocks can be attributed to the dislocation of the top of the reef flat by catastrophic events tsunamis or hurricanes typhoons in the sw and s pacific the shifting of these blocks mega boulders would necessitate a considerable amount of energy and at present we cannot decide which one of these phenomena was responsible various examples observed in the pacific are cited to illustrate abrupt changes in the sea level these could be attributed either to meteorological events hurricanes cyclones or typhoons or to seismic events of very high energy ground sea or tsunamis the reader is reminded of the different characteristics that these phenomena produce this study is mainly concerned with tsunamis especially the frequency of their occurrence energy and effects on the coasts of small oceanic islands hurricanes cyclones and typhoons roughly originate from the n ne and e whereas tsunamis originate from all directions except from se s and sw angle these very high energy events hurricanes and tsunamis seem then to be responsible for 1 the decreasing grain size of the coral rim sediments from the ocean to the lagoon through the range of colossal blocks smaller boulders scree gravels coarse and fine sands 2 the sedimentary accretion of the motu itself at each very high energy so called catastrophic event they are also responsible for the preservation of the two characteristic geomorphic forms of an atoll motu and hoa 3 finally the alternation motu hoa motu etc on the coral rim this alternation seems to result from the interaction between oceanic disturbances hurricanes or tsunamis and the water volume which must flow over the coral rim the transported material the reef flat and the angle of incidence between the swell and the coast during these catastrophic events

amino acid geochronology resolution and precision in carbonate fossils changes in indigenous proteins preserved in carbonate skeletons can be used to estimate the time elapsed since death of the organism racemization the most widely used reaction in amino acid geochronology refers to the inversion of l amino acids to their d configuration for the protein amino acid l isoleucine racemization occurs about only one of the two chiral carbon atoms this reaction epimerization proceeds from an initial dl ratio near zero to an equilibrium ratio of about 1 30 the reaction rate is controlled primarily by temperature and to a lesser degree by the nature of the proteins themselves a variable related to taxonomy and the extent of protein degradation analyzing monospecific samples minimizes the taxonomic variable the resolving power of dl ratios as relative age indices is a function of the reaction rate degree of natural variability and analytical uncertainty natural variability is defined as the spread in ratios in five well preserved shells of the same age from a single stratum and is primarily related to taxonomy it can range from 20 or more in taxa of low reliability to 1 in taxa of greatest integrity 5 to 10 is to be expected analytical uncertainty within a single laboratory is relatively insignificant amounting to no more than 2 but interlaboratory comparisons suggest caution in comparing ratios determined at different laboratories or by different techniques sampling appropriate portions of a shell is essential to minimize variability in dl ratios site temperature exerts the greatest control on reaction rate hence resolving power for tropical sites 25 c age differences 1 ka can be resolved within the holocene and ca 5 to 10 ka for older samples as the rate decreases equilibrium is attained after 150 to 300 ka the reaction rate is substantially lower at mid latitude sites ca 10 c where equilibrium requires about 2 ma and resolution within the last glacial cycle is no better than 10 to 20 ka arctic sites 10 c show almost no measurable racemization within the holocene and in extreme cases even within the last 100 ka equilibrium is not attained in 10 ma the extent of amino acid racemization epimerization in carbonate fossils provides chronological information for the critical time period beyond radiocarbon dating dl ratios can be used directly as relative age indices and with suitable calibration can provide reasonable estimates of absolute age and or effective diagenetic temperature improving the accuracy of racemization based age estimates requires a better understanding of racemization kinetics in carbonate systems

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

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

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

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

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

paleosols in shallow marine carbonate sequences abstract shallow water carbonate sequences in the geological record commonly exhibit numerous exposure surfaces reflecting emersion of sediment due to progradation processes and sealevel oscillations some ancient platform sequences contain hundreds of such surfaces associated with paleosols fossil soils typically calcretes a wide range of macro and microstructures are associated with these calcretes which are reviewed profile types and stages in profile development can be defined the nature of the exposure surface and associated paleosols can be used to identify the nature of the prevailing climate although hydromorphic effects related to marine flooding of the platforms can modify the paleosols a potentially important use of paleosols is to define the nature of sequence boundaries several studies have identified sets of paleosols with varying maturities reflecting changes in rates of accommodation space creation during sealevel rises and falls

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

the geology of eleuthera island bahamas a rosetta stone of quaternary stratigraphy and sea level history abstract a 5 km stretch of coastline in north eleuthera reveals a long and detailed stratigraphy that includes all known surficial limestone units in the bahamas and supplements the record with several previously unrecognized ones eight paleosol bounded limestone parasequences comprise at least six interglacial periods the lithostratigraphy demonstrates cyclicity at several frequencies 10 5 10 4 20 40 ka and 10 3 years and displays a variety of distal to proximal shoreline facies indicative of shifting depocenters associated with changing sea levels stratigraphy petrology pedology and whole rock aminostratigraphy are used to correlate units and subunits among the 12 described sections amino acid ratios are also converted to absolute age estimates which support the lithostratigraphy the parasequences are correlated with marine oxygen isotope stages 1 to 13 or older evidence of middle pleistocene highstands are abundant in the eleutheran stratigraphy including paleo sea levels of decreasing age at 2 m 7 m 20 m associated with stages 11 and or 9 and two near present highstands during stage 7 a complex sea level history is associated with substage 5e while substage 5a is represented by near shore aggradation of coastal dune complexes in eleuthera and throughout the bahamas concordance of sea level deposits between bermuda and the bahamas reinforce their tectonic stability while the abundance of highstand evidence during the middle pleistocene contradicts suggestions that platform subsidence has obscured all evidence of these events below present sea level the high resolution late quaternary stratigraphy of eleuthera is unrivaled among geologic records from stable carbonate coastlines and thereby offers a rosetta stone for interpretation of the quaternary evolution of the bahamas and sea level history over the past 500 ka

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

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

aluminous lateritic soils eleuthera bahamas a modern analog to carbonate paleosols abstract aluminous lateritic soils from governor s harbour eleuthera bahamas are described these soils are forming on eolian ridges under a humid tropical savanna climate the parent material is carbonate eolianite plus airborne dust from north africa the soils are thin discontinuous and concentrated in shallow solution pits they are stony have a crumb texture and are underlain by pedogenically altered limestones the solum has a low sio2 al2o3 ratio and is composed of boehmite hydroxy interlayered clay hic al hematite al goethite calcite and minor quartz the presence of carbonates in the solum buffers the ph and inhibits gibbsite precipitation resulting in a high availability of aluminum for incorporation in hic and iron minerals the moisture egime of these soils is characterized by frequent wetting and drying due to the high porosity and permeability of the underlying carbonates and the intermittent nature of the rainfall boehmite is the stable al phase within cryptovoids where the h2o activities are less than one the dominant pedogenic features in the altered limestones are voids produced by solution of limestone around plant roots these voids are subsequently infilled with overlying solum material and cemented with equigranular calcite alteration rinds containing micritized carbonate grains and laminations of micrite and iron rich clay develop around the voids features observed in aluminous lateritic soils are compared to other carbonate soils and paleosols preserved within carbonate sequences

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

aqueous alteration of the martian meteorite northwest africa 817 probing fluid rock interaction at the nakhlite launch site the nakhlite meteorites characteristically contain iddingsite a hydrous iron magnesium silicate that formed by aqueous alteration on mars iddingsite is most abundant in northwest africa nwa 817 and alteration products in this meteorite also have the lowest deuterium hydrogen ratio of any nakhlite taken together these distinctive properties could be interpreted to show that nwa 817 was altered under different physico chemical conditions than the other nakhlites and by liquid water from a separate reservoir here this interpretation is tested through a petrographic mineralogical chemical and isotopic study of nwa 817 we find that its iddingsite occurs as olivine hosted veins of nanocrystalline smectite and fe oxyhydroxide strong similarities in the mineralogy of iddingsite between nwa 817 and other nakhlites suggest that these meteorites were altered under comparable physico chemical conditions with the fe rich composition of nwa 817 olivine grains rendering them especially susceptible to aqueous alteration analyses of nwa 817 bulk samples by stepwise pyrolysis confirm that its iddingsite has unusually low deuterium hydrogen ratios but owing to terrestrial weathering of this meteorite the hydrogen isotopic data cannot be used with confidence to infer the origin of martian aqueous solutions nwa 817 was most probably altered along with the other nakhlites over a short time period and in a common aqueous system one interpretation of a correlation between the eruption ages of three of the nakhlites and the chemical composition of their iddingsite is that water originated from close to the surface of mars and flowed through the nakhlite lava pile under the influence of gravity

rapid sea level changes at the close of the last interglacial substage 5e recorded in bahamian island geology rapid and abrupt relative sea level changes within the last interglacial substage 5e are recorded in the island geology of the tectonically stable bahamas from 132 to 118 ka reef growth reached a maximum elevation near 2 m as indicated by fossil reef elevation across the platform whereas bioeroded notches are incised in coastal cliffs as high as 6 m the end of the interval is characterized by voluminous eolianites exhibiting palm tree and frond impressions it is inferred that sea level for most of the interval remained near 2 m restraining reef growth and that the notch at 6 m represents a rapid and brief excursion just before the close of the substage the subsequent fall must have been rapid in order to leave the notch profile intact and mobilize windward lagoon ooids into dunes before cementation could anchor them in order to explain the rapid rise to 6 m glacial surging is invoked the subsequent fall also rapid may be a consequence of the surge flooding high latitudes and providing enough moisture to initiate reglaciation and drawdown

relation of bank margin fractures to sea level change exuma islands bahamas data from carbonate bank margin fractures in the southern exuma islands bahamas in the vicinity of lee stocking island show that these fractures have a preferred bank margin parallel orientation fractures that showed evidence of pedogenic alteration to micrite calcretization had an even more pronounced orientation preference calcretization is associated with a well defined paleosol believed to have formed within the last glacial period and the transition to the present interglacial 20 13 ka horizontal laminations within the paleosol continue as vertical laminations within some fracture traces analysis of thin sections revealed root tubules and fabrics characteristic of pedogenic alteration within the calcretized parts of fracture zones the association of bank margin parallel fractures with the last interglacial suggests that fracture development is related to a eustatic process i suggest that fractures formed in response to undercutting of the bank margin during the last sea level low a fracture controlled submarine cave extending to 100 m below present sea level provides additional evidence supporting a eustatic link to fracture development

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

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