apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

u pb and th pb dating of apatite by la icpms abstract apatite is a common u and th bearing accessory mineral in igneous and metamorphic rocks and a minor but widespread detrital component in clastic sedimentary rocks u pb and th pb dating of apatite has potential application in sedimentary provenance studies as it likely represents first cycle detritus compared to the polycyclic behavior of zircon however low u th and radiogenic pb concentrations elevated common pb and the lack of a u th pb apatite standard remain significant challenges in dating apatite by la icpms and consequently in developing the chronometer as a provenance tool this study has determined u pb and th pb ages for seven well known apatite occurrences durango emerald lake kovdor mineville mud tank otter lake and slyudyanka by la icpms analytical procedures involved rastering a 10 m spot over a 40 40 m square to a depth of 10 m using a geolas 193 nm arf excimer laser coupled to a thermo elementxr single collector icpms these raster conditions minimized laser induced inter element fractionation which was corrected for using the back calculated intercept of the time resolved signal a tl u bi np tracer solution was aspirated with the sample into the plasma to correct for instrument mass bias external standards plesovice and 91500 zircon nist srm 610 and 612 silicate glasses and stdp5 phosphate glass along with kovdor apatite were analyzed to monitor u pb th pb u th and pb pb ratios common pb correction employed the 207pb method and also a 208pb correction method for samples with low th u the 207pb and 208pb corrections employed either the initial pb isotopic composition or the stacey and kramers model and propagated conservative uncertainties in the initial pb isotopic composition common pb correction using the stacey and kramers 1975 model employed an initial pb isotopic composition calculated from either the estimated u pb age of the sample or an iterative approach the age difference between these two methods is typically less than 2 suggesting that the iterative approach works well for samples where there are no constraints on the initial pb composition such as a detrital sample no 204pb correction was undertaken because of low 204pb counts on single collector instruments and 204pb interference by 204hg in the argon gas supply age calculations employed between 11 and 33 analyses per sample and used a weighted average of the common pb corrected ages a tera wasserburg concordia intercept age and a tera wasserburg concordia intercept age anchored through common pb the samples in general yield ages consistent at the 2 level with independent estimates of the u pb apatite age which demonstrates the suitability of the analytical protocol employed weighted mean age uncertainties are as low as 1 2 for u and or th rich palaeozoic neoproterozoic samples the uncertainty on the youngest sample the cenozoic 31 44 ma durango apatite ranges from 3 7 7 6 according to the common pb correction method employed the accurate and relatively precise common pb corrected ages demonstrate the u pb and th pb apatite chronometers are suitable as sedimentary provenance tools the kovdor carbonatite apatite is recommended as a potential u pb and th pb apatite standard as it yields precise and reproducible 207pb corrected 232th 208pb and common pb anchored tera wasserburg concordia intercept ages

late quaternary climate and hydrology of tropical south america inferred from an isotopic and chemical model of lake titicaca bolivia and peru abstract a simple mass balance model provides insight into the hydrologic isotopic and chemical responses of lake titicaca to past climatic changes latest pleistocene climate of the altiplano is assumed to have been 20 wetter and 5 c colder than today based on previous modeling our simulation of lacustrine change since 15 000 cal yr b p is forced by these modeled climate changes the latest pleistocene lake titicaca was deep fresh and overflowing the latest pleistocene riverine discharge from the lake was about 8 times greater than the modern average sufficient to allow the expansion of the great paleolake tauca on the central altiplano the lake 18o value averaged about 13 smow the modern value is about 4 2 the early holocene decrease in precipitation caused lake titicaca to fall below its outlet and contributed to a rapid desiccation of paleolake tauca continued evaporation caused the 100 m drop in lake level but only a slight 1 2 increase relative to modern in 18o of early holocene lake waters this holocene lowstand level of nearly 100 m was most likely produced by a precipitation decrease relative to modern of about 40 the lake was saline as recently as 2000 cal yr b p the timing of these hydrologic changes is in general agreement with calculated changes of insolation forcing of the south american summer monsoon

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

in situ la icp ms u pb dating of metavolcanics of norrbotten sweden records of extended geological histories in complex titanite grains in situ u pb dating of a variety of mineral phases is an important goal in petrology this study reports data chiefly from titanite but also from rutile and apatite obtained using the laser ablation la icp ms methodology on polished thick sections in order to retain as much petrologic information as possible and allowing trace element analyses from adjacent areas to the u pb analyses the samples analysed come from svecofennian intermediate to acid volcanic rocks of the porphyry group within the major iron ore province of norrbotten northern sweden where titanite is a common phase associated with the mineralisation using a laser beam of 30 45 m in diameter accurate and relatively precise 2 3 2 on 206pb 238u ratios data can be obtained on titanite similar precision on pb u ratios can be obtained in rutile but accuracy cannot be assessed because of the lack of well characterised standards however the data are a priori accurate as they are geologically reasonable apatite shows reverse discordance which could be explained by a number of scenarios however the 207pb 206pb ages are fairly precise 1 3 2 and the calculated age is geologically reasonable suggesting that the data may be accurate the titanite grains studied show complex internal structures rare earth element ree analysis by la icp ms demonstrates that the core and rim zones are distinct and supports a model for two stage evolution in two out of three samples with the rims in these samples being enriched in total ree and particularly in lree in one sample and displaying a positive y anomaly in contrast to the cores in situ u pb analysis reveals that core zones from all samples retain distinct older ages of c 2050 ma whereas the reworked rims have either distinct younger or strongly reset ages the older cores are interpreted as representing the first alteration metamorphism of the volcanic pile titanite rims record a u pb age of 1870 24 ma 2 at luossavaara and 1826 15 ma 2 at the smaller fe oxide apatite body at gruvberget in both cases the ages are consistent with previous age determinations from the fe oxide apatite deposits a sample from malmberget records a slightly different scenario where u pb ages spread along concordia from c 1920 to 1708 ma we attribute this pattern to metamorphic reworking of primary metasomatic titanite rutile ages from inclusions within the core of gruvberget titanite essentially mimic the ages obtained from the surrounding titanite and suggest that they have undergone the same history apatite inclusions within titanite at malmberget record a younger age of 1584 12 ma 2 which we attribute to later events known to have affected the area these data indicate that the porphyry group volcanics must be at least c 2050 ma in age in contrast to previous age determinations from zircon which suggested a deposition age of c 1960 to 1880 ma the large grain size of the titanites studied coupled with the ability for the first time to obtain in situ u pb ages from small areas has allowed probing of this hitherto unrecognised event smaller zircon grain size coupled with extremely saline hot pressurised high p t metasomatic fluids relating to regional magnetite apatite mineralisation in the period 1900 to 1800 ma may have resulted in resetting of zircon u pb systematics 2007 elsevier b v all rights reserved

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

thermal and exhumation history of sakhalin island russia constrained by apatite u pb and fission track thermochronology abstract sakhalin island represents a key locality to study the tectonic evolution of the western pacific the island is located at the amur okhotsk plate margin and records a complex thermotectonic history apatite double dating u pb and fission track and thermal history modelling were applied to three late eocene granitoid massifs within central and southern sakhalin the aniva okhotsk and langeri complexes apatite u pb results yield consistent late eocene 40 37 ma ages suggesting rapid post magmatic cooling apatite fission track results reveal bimodal age distributions with late eocene early oligocene 38 33 ma and early miocene 20 17 ma age populations that can be correlated with variations in uranium and chlorine concentrations thermal history modelling translates the aft age bimodality into two phase cooling histories the timing of the early cooling phase 38 33 ma corresponds with the apatite u pb ages indicating rapid cooling to at least 100 c during the late oligocene the second cooling phase at 20 17 ma cooled the samples to near surface temperatures both cooling phases correspond with regional unconformities and subsequent accelerations in sedimentation rate suggesting that cooling was a response to rapid exhumation in addition our data suggests that the studied terranes record differential exhumation with respect to the structural architecture the miocene exhumation pulse is coeval with the timing of transpressional fault displacement and the subsequent opening of the kuril basin

multi mineral geochronology insights into crustal behaviour during exhumation of an orogenic root under rare conditions reworked cratons and their margins preserve the orogenic roots of ancient mountain building events however based on the preservation of high temperature 800 c middle and lower crustal metamorphic assemblages present day exposure of these terrains is not simply a result of protracted denudation but also must reflect a multifaceted exhumational history in situ analysis within thin section preserves the textural setting of target minerals that can be used as thermochronometers such as u pb of zircon monazite titanite and apatite and sm nd of apatite in situ analyses of these chronometers has the potential to provide critical timing constraints on exhumation processes related to decompression melting and cooling across large metamorphic terrains the repulse bay block of the rae craton preserves a large composite amphibolite granulite area 50 000 km2 of archean orthogneiss migmatite and slivers of proterozoic metasediments that underwent high grade metamorphism partial melting ductile flow and finally exhumation during the paleoproterozoic trans hudson orogeny the granulite domain preserves dry granitoid assemblages whereas the amphibolite domain is dominated by hydrated migmatites and orthogneiss metasediments occur in both domains and preserve mineral assemblages that are consistent with having undergone tectonometamorphic conditions of 9 kbar 800 c during burial u pb thermochronometers document identical cooling histories of the granulite and amphibolite domains through the u pb closure temperatures of titanite 650 c and apatite 450 c this suggests that melt loss from the underlying granulite domain and melt gain to the amphibolite domain prior to cooling through 650 c are a controlling factor of the metamorphic assemblages across the composite granulite amphibolite terrains such as the repulse bay block rather than significant differences in burial history cooling history and or reorganization of the crust

geochemistry and origin of albite gneisses northeastern adirondack mountains new york albite gneisses containing up to 8 7 percent na2o and as little as 0 1 k2o comprise a significant part of the proterozoic lyon mountain gneiss in the ausable forks quadrangle of the northeastern adirondacks new york state two distinct types of albite gneisses are present one is a trondhjemitic leucogneiss lag consisting principally of albite ab95 ab98 and quartz with minor magnetite and locally minor amounts of amphibole or acmiterich pyroxene lag probably originated by metamorphism of a rhyolitie or rhyodacitic ash flow tuff with a type geochemical affinities following post depositional analcitization in a saline or saline alkaline environment the other type is a mafic albite gneiss mag containing albite and pyroxene along with 0 45 percent quartz minor amphibole and titanite mag locally displays pinstripe banding and contains albite ab98 megacrysts up to 5 cm across its precursor may have been a sediment composed of diagenetic analcite or albite dolomite and quartz both types of albite gneiss are interlayered with granitic gneisses lmg of variable composition derived from less altered tuffs a potassium rich up to 9 7 k2o microcline gneiss facies may have had a protolith rich in diagenetic k feldspar we propose that the albite gneisses and associated granitic gneisses are the granulite facies metamorphic equivalent of a bimodal dominantly felsic volcanic suite with minor intercalated sediments probably including evaporites the volcanics were erupted in an anorogenic setting such as an incipient or failed intracontinental rift deposition took place in a closed basin playa lake environment where diagenetic alteration resulted in redistribution of the alkalis and strong oxidation

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

u pb zircon dating of granulite metamorphism in the sludyanskiy complex eastern siberia abstract conventional multigrain and single grain zircon u pb ages were obtained for syn and post metamorphic granitoids from the sludyanskiy complex in the south western baikal region regarded as a classical precambrian terrain in the eastern siberia single grains of magmatic zircon from the syn metamorphic hypersthene bearing biotite trondhjemite and two pyroxene trondhjemite have yielded concordant u pb ages of 488 5 0 6 ma and 488 0 5 ma concordant u pb data for zircons of metamorphic origin in the syn metamorphic two pyroxene trondhjemite yield an age for the peak of the granulite facies metamorphism in the sludyanskiy complex as 477 6 2 ma the pattern of multigrain magmatic zircon samples from syn metamorphic granitoids reflects significant pb loss during the metamorphic episode this came to an end before 471 1 2 ma as demonstrated by concordant zircon data for the intrusion of post metamorphic amphibole two pyroxene quartz syenite our zircon data establish that the granulite facies reworking of the sludyanskiy complex took place during the palaeozoic and not during the paleoproterozoic as was previously suggested the age span between the intrusions of syn metamorphic and post metamorphic granitoids constrains the duration of the metamorphic episode to about 20 ma

la mc icpms and shrimp u pb dating of complex zircons from quaternary tephras from the french massif central magma residence time and geochemical implications abstract analyses of zircon grains from the queureuilh quaternary tephras pumice provide new information about their pre eruptive history u pb dating was performed in situ using two methods shrimp and la mc icpms equipped with a multi ion counting system both methods provided reliable 207pb 206pb and 206pb 238u ratios as well as u and th abundances required for u pb concordia intercept age determination after initial 230th disequilibrium correction the new la mc icpms method was validated by dating a reference zircon 61 308b and zircons from a phonolitic lava dated independently with the two techniques a time resolution of about 20 kyr for 1 ma zircon crystals was achieved for both methods the clear euhedral zircon population from queureuilh tephras is quite complex from several points of view 1 some grains are reddish or yellowish while others are colorless 2 the u and th composition changes by more than an order of magnitude and th u is generally high 1 2 3 there are three discrete ages recorded at 2 35 0 04 1 017 0 008 and 0 640 0 010 ma from the previously determined 40ar 39ar age at 0 571 0 060 ma duffell h 1999 contribution geochronologique a la stratigraphie volcanique du massif des monts dore par la methode 40ar 39ar d e a univ clermont ferrand 56 p the discontinuous zircon age populations the color of the grains and their composition we favor the following model as explanation the oldest less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent a primitive magma chamber perhaps deep in crustal level was formed at 1 0 ma the related magma previously characterized by high th u ratio 2 2 1 1 underwent rejuvenation during ascent to a new chamber at shallow depth and or during injection of more mafic magmas during this stage at 0 64 ma the colorless zircon grains of lower th u ratio 1 3 0 5 crystallized this last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40ar 39ar method however if the primitive magma is considered the magma residence time as a whole from this first stage reached 446 kyr in the light of the complex history of such magmas which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions the use of zr concentration in geochemical modeling of whole rock compositional data can be problematic

improving constraints on apatite provenance nd measurement on fission track dated grains detrital thermochronology is commonly used to locate source and improve the understanding of a region s long term development provenance detrital age data typically comprise a number of distinct age modes representing contributions from a number of different sources for methods such as apatite fission track ft this can present a challenge in that direct assignment of age modes to particular lithological or tectono stratigraphic units is rarely possible particularly when ancient orogenic sediments are examined a new approach described here is based on measuring nd isotopic data on single apatites by laser ablation icpms where the 143nd 144nd ratios are diagnostic of grain source by combining nd isotopic measurement with apatite ft analysis it is possible to link detrital apatite ft ages to specific rock unit sources the methodology and wider benefits of this new approach are discussed using examples from the himalaya and andaman islands geological society of london 2009

salmon origin in california s sacramento san joaquin river system as determined by otolith strontium isotopic composition geochemical methods for distinguishing salmon of different runs would improve management practices designed to mitigate for declines in salmon populations in california9s sacramento san joaquin river system strontium isotopic measurements show a strong relationship between the 87 sr 86 sr ratio in hatchery water and the 87 sr 86 sr ratio in the otoliths aragonitic ear bones of juvenile chinook salmon oncorhynchus tshawytscha raised in those waters as a result of differences in basin geology from north to south along the western slope of the sierra nevada important salmon spawning rivers within the sacramento san joaquin river system have distinct 87 sr 86 sr ratios of the 10 rivers in this study those in the sacramento river drainage have lower 87 sr 86 sr ratios 0 7039 0 7063 than those in the san joaquin river basin 0 7068 0 7092 with the exception of the american river which has the highest 87 sr 86 sr ratios in this study average 0 7100 the combination of distinct river 87 sr 86 sr ratios and the relationship between water and otolith sr isotope ratios indicates that this geochemical method can be used to identify the origin and potentially the migration history of juvenile out migrating salmon in the sacramento san joaquin system

apatite triple dating and white mica 40ar 39ar thermochronology of syntectonic detritus in the central andes a multiphase tectonothermal history we applied apatite u pb fission track and u th he triple dating and white mica 40 ar 39 ar thermochronology to syntectonic sedimentary rocks from the central andean puna plateau in order to determine the source area geochronology and source sedimentary basin thermal histories and ultimately the timing of multiple tectonothermal events in the central andes apatite triple dating of samples from the eocene geste formation in the salar de pastos grandes basin shows late precambrian devonian apatite u pb crystallization ages eocene apatite fission track aft and eocene miocene u th he ca 8 47 ma cooling ages double dating of cobbles from equivalent strata in the arizaro basin documents early eocene 46 2 3 9 ma and cretaceous 107 6 7 6 109 5 7 7 ma aft and eocene oligocene ca 55 30 ma u th he ages thermal modeling suggests relatively rapid cooling between ca 80 and 50 ma and reheating and subsequent diachronous basin exhumation between ca 30 ma and 5 ma the 40 ar 39 ar white mica ages from the same samples in the salar de pastos grandes area are mainly 400 350 ma younger than apatite u pb ages suggesting source terrane cooling and exhumation during the devonian early carboniferous together these data reveal multiple phases of mountain building in the paleozoic and cenozoic basin burial temperatures within the plateau were limited to

present trends and the future of zircon in geochronology laser ablation icpms in situ u th pb geochronology was born some two decades ago with the introduction and development of high resolution secondary ion mass spectrometry sims or shrimp sensitive high mass resolution ion microprobe compston et al 1984 williams 1998 compston 1999 davis et al this volume ireland and williams this volume this technique clearly demonstrated the existence of age heterogeneities within the single crystals of zircon and other accessory phases and therefore the need for high spatial resolution tens to hundreds of cubic micrometers geochronological data in situ dating by ion probe is capable of achieving an analytical precision that is only an order of magnitude worse than the conventional isotope dilution thermal ionization mass spectrometry id tims dating technique it has the advantage however of more readily identifying concordant portions of grains does not require chemical treatment of samples prior to the analysis is essentially nondestructive and can achieve greater sample throughputs a major obstacle to the wider use of ion probe dating has always been the high cost of instrumentation and hence relative scarcity of suitably equipped geologic laboratories laser ablation inductively coupled plasma mass spectrometry la icpms emerged in 1985 and rapidly became an important analytical tool for trace element determinations in geological samples jackson et al 1992 it was soon realized that the large variations in radiogenic pb and pb u isotopic ratios found in nature could be resolved by icpms techniques and when coupled to a laser icpms could be used as a dating tool similar to the ion probe the pioneering work of feng et al 1993 fryer et al 1993 hirata and nesbitt 1995 and jackson et al 1996 illustrated the potential usefulness of laser sampling for in situ dating by icpms particularly well however these studies and others that followed also revealed the major

magmatic zoning in apatite a monitor of porosity and permeability change in granites apatites from the shap granite northern england are strongly zoned reflecting multiple generations of growth and dissolution such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using la icp ms analyses the zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries the main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning followed by the growth of a thin rim of relatively uniform composition the chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets

phosphate lu hf geochronology abstract igneous metamorphic and biogenic apatite contains lu and hf in proportions that make this mineral suitable for dating by the 176lu 176hf method we present a new method for separation of lu and hf from apatite that involves a single extraction column step for hf and a second exchange column step for lu this procedure allows rapid sample processing prior to analysis by mc icp ms results from the igneous gardiner skaergaard and khibina intrusions indicate that the closure temperature for the lu hf isotopic system in apatite is relatively high the lu hf isochron age of 60 ma for the skaergaard intrusion east greenland is older than the generally accepted emplacement age but does not appear to be a product of mixing a centimeter sized apatite from the otter lake area grenvillian province yields high and variable 176lu 177hf ratios showing that metamorphic apatite may in some cases allow for mono mineral dating the most important application of the method is undoubtedly its potential use for dating sedimentary formations although we find biogenic apatite in porous matrix highly susceptible to post depositional resetting of the lu hf isotopic system the preservation of a near depositional age for a shark tooth from the impermeable london clay and recent work on crystalline authigenic phosphorites earth planet sci lett 201 2002 203 show the lu hf system to be a promising tool for direct dating of sedimentary formations

composition and structural control of crustal domains in the central andes 1 present day ratios of pb isotopes 324 published samples 435 new and nd sr isotopes 150 published 180 new on proterozoic to holocene igneous metamorphic and sedimentary rocks define at high spatial resolution distinct isotopic domains of the crust in the central andes these domains correlate with the internal compositional structure of the crust as revealed by a three dimensional density model pb nd isotopic boundaries thus correspond to variations in crustal compositional structure and reflect proterozoic mafic dominated and paleozoic felsic dominated crustal lithologies age and composition mafic versus felsic of these domains have controlled the rheology of the andean crust have influenced crustal deformation patterns and correlate with the central andean plateau segmentation

u pb dating of perovskite by la icp ms an example from the oka carbonatite quebec canada abstract the potential for u pb dating of the mineral perovskite using laser ablation inductively coupled plasma mass spectrometry la icp ms has been evaluated perovskite can have relatively elevated u and th contents making it a potentially useful geochronometer kimberlite lamprophyre and carbonatite intrusions are prone to alteration and typically contain abundant xenocrysts thus standard techniques for dating the magmatic emplacement of these rocks becomes highly problematic perovskite however is commonly present as a primary magmatic mineral in these rocks and has been used to constrain the timing of kimberlite and lamprophyre emplacement the main limitations to the production of accurate la icp ms u pb ages for perovskite are the lack of suitable standards coupled with the large common pb correction required 204 hg can produce large isobaric interference on the non radiogenic 204 pb isotope and methods based on 208 pb 206 pb and u th ratios are hampered by the enhanced th content present in most perovskite crystals after careful calibration using zircon reference materials common pb corrected 206 pb 238 u ages can be determined on each individual la icp ms analysis by using the measured 207 pb 206 pb ratio which is dominated by the common pb proportion in phanerozoic rocks this method assumes that the perovskite crystals analyzed are cogenetic and have the same initial pb ratios the la icp ms method has been applied to data collected on two large single perovskite crystals from the oka carbonatite complex quebec canada the weighted mean 206 pb 238 u age of 131 7 ma n 25 mswd 0 42 is consistent with published geochronology data from the region and suggests that la icp ms dating of perovskite could be a useful low cost alternative to compliment existing techniques

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

mesozoic and cenozoic tectonics of the northeastern edge of the tibetan plateau evidence from modern river detrital apatite fission track age constraints abstract thermochronometry of modern river sands exploits the natural sampling of detritus in a river catchment to infer plausible regional exhumation patterns when combined with available bedrock thermochronometric information apatite fission track age distributions for six detrital samples are reported in this study to resolve the exhumation history of the upper reaches of the heihe river draining north qilian shan and tuole shan the northeastern margin of the tibetan plateau the apatite fission track aft ages include four major populations late paleozoic 300 55 293 43 ma early mesozoic 223 15 205 17 ma late jurassic cretaceous 112 6 2 84 8 5 6 ma and cenozoic 63 8 5 9 19 3 1 7 ma the ages of these populations correspond well with the deformation and cooling history of the study area the late paleozoic and early mesozoic aft age populations record cooling events resulting from continental collision between the qiantang and kunlun blocks the late jurassic cretaceous aft age populations are interpreted to record cooling events associated with exhumation of north qilian shan and tuole shan due to the collision of the lhasa block with asian continent the cenozoic aft age populations record several cooling events a cooling event is recorded corresponding to exhumation of north qilian shan and tuole shan due to late stress from the lhasa asia collision causing deformation nearly synchronous with the india asia collision another cooling event is recorded corresponding to uplift and exhumation in oligocene time due to ongoing india asia convergence north qilian shan and tuole shan were constructed as a distal effect of india asia convergence in the early miocene while approaching their present mountain river basin distributions thermochronometry applied to heihe river sediments provide convinced evidence of multi stage exhumation of most northeastern margin of the tibetan plateau in the late paleozoic cenozoic

age constraints on the hydrothermal history of the prominent hill iron oxide copper gold deposit south australia the mesoproterozoic prominent hill iron oxide copper gold deposit lies on the fault bound southern edge of the mt woods domain gawler craton south australia chalcocite bornite chalcopyrite ores occur in a hematitic breccia complex that has similarities to the olympic dam deposit but were emplaced in a shallow water clastic carbonate package overlying a thick andesite dacite pile the sequence has been overturned against the major steep east west hangingwall fault beyond which lies the clastic to potentially evaporitic blue duck metasediments immediately north of the deposit these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite calc silicate skarn pyrite chalcopyrite the hematitic breccia complex is strongly sericitised and silicified has a large sericite chlorite halo and was intruded by dykes during and after sericitisation this paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation the breccia complex contains fragments of granitoid and porphyry that are found here to be part of the gawler range volcanics hiltaba suite magmatic event at 1600 1570 ma this indicates that some breccia formation post dated granitoid intrusion monazite and apatite in fe p ree albite metasomatised granitoid paragenetically linked with magnetite skarn formation north of the hangingwall fault grew soon after granitoid intrusion although the apatite experienced u pb lree loss during later fluid mineral interaction this accounts for its calculated age of 1544 39 ma to the south of the fault within the breccia 40ar 39ar ages yield a minimum age of sericitisation cu fe ree of dykes and volcanics of 1575 ma firmly placing prominent hill ore formation as part of the gawler range volcanics hiltaba suite magmatic event within the olympic cu au province of the gawler craton

approaches to equilibrium in the distribution of trace elements among the principal minerals in a high grade metamorphic terrane x ray fluorescence instrumental neutron activation and particle induced x ray emission methods were used to determine the distribution of numerous trace elements among garnet grt ca pyroxene cpx hornblende hbl biotite bt plagioclase pl and k feldspar kf in a high grade metamorphic terrane within the grenville province of the canadian shield results are presented as distribution formulae e g sr kf 1 1 pl 16 hbl 2 2 cpx 1 0 bt 1 2 grt sc hbl 1 1 cpx 1 0 grt 7 8 bt 22 pl 2 6 kf v hbl 1 15 bt 2 07 cpx 6 0 grt 1 4 cao 1 pl kf zn bt 1 6 hbl 1 62 cpx 2 9 grt 10 pl ga bt 1 2 hbl 1 2 pl 2 5 cpx 1 3 grt where numbers are distribution ratios e g ppm sr in hbl ppm sr in cpx 2 2 examples of inter element similarities and differences are a both rb and cs are concentrated in biotite relative to k feldspar but for rb the ratio is 2 3 and for cs it is 16 b the distribution formulae for seven lanthanides are similar except for the position of garnet e g ce hbl 2 7 cpx 2 8 pl 1 1 bt 11 kf 16 grt yb grt 2 8 hbl 2 7 cpx 9 pl 1 0 bt 7 kf and c all of sr eight lanthanides zr v and cr are concentrated in hornblende relative to ca pyroxene by a factor that lies in the narrow range of 2 2 3 1 there is a larger variation departure from the mean in some distribution ratios than in others thus the mean ratios hbl cpx for each of six elements and in parentheses the percentage relative standard deviation are zn 1 62 8 6 v 2 38 12 cr 2 42 18 sr 2 7 28 ba 2 9 36 and ni 1 66 38 we suggest that variation of this kind is the result of differences from place to place in the magnitude of deformation and recrystallization which facilitated the rearrangement of atoms combined with rates of lattice and crystal boundary diffusion that are unique for the various elements thus permitting some trace elements to approach equilibrium more closely than others

precise and accurate in situ pb pb dating of apatite monazite and sphene by laser ablation multiple collector icp ms to evaluate in situ pb dating by laser ablation multiple collector inductively coupled plasma mass spectrometry la mc icp ms we analysed apatite sphene and monazite from paleoproterozoic metamorphic rocks from west greenland pb isotope ratios were also determined in the national institute of standards and technology nist 610 glass standard and were corrected for mass fractionation by reference to the measured thallium isotope ratio the nist 610 glass was used to monitor pb isotope mass fractionation in the low tl pb accessory minerals replicate analyses of the glass 1 to 2 min yielded ratios with an external reproducibility comparable to conventional analyses of standard reference material 981 by thermal ionisation mass spectrometry tims mineral grains were generally analysed with a 100 m laser beam although some monazite crystals were analysed at smaller spot sizes 10 and 25 m the common pb isotope ratios required for age calculations were either measured on coexisting plagioclase by la mc icp ms or could be ignored as individual crystals exhibit sufficient pb isotopic heterogeneity to perform isochron calculations on replicate analyses of single crystals mean mineral ages with the 204pb ion beam measured in the multiplier were as follows apatite 1715 23 m y sphene 1789 11 m y and monazite 1783 to 1888 m y with relative uncertainties on individual monazite ages of 0 2 but highly reproducible age determinations on single monazite crystals 1 isochron ages calculated from several mineral analyses without assumption of common pb also yield precise age determinations apatite and monazite pb ages determined by in situ pb isotope analysis are identical to those determined by conventional tims analysis of bulk mineral separates and the analytical uncertainties of these short laser analyses with no prior mechanical or chemical separation are comparable to those obtained by tims detailed examination of the sphene in situ age data does however show a small discrepancy between the la mc icp ms and tims ages 1 younger high resolution mass scans of the sphene during ablation clearly showed several small and as yet unidentified isobaric interferences that overlap with the 207pb peak at the resolution conditions for measurement of isotope ratios these might account for the age discrepancy between the la mc icp ms and tims sphene ages la mc icp ms is a rapid accurate and precise method for in situ determination of pb isotope ratios that can be used for geochronological studies in a manner similar to an ion microprobe albeit currently at a somewhat degraded spatial resolution further modifications to the la mc icp ms system such as improved sensitivity ion transmission and la methodology may lead to this type of instrument becoming the method of choice for many types of in situ pb isotope dating

time scales of assembly and thermal history of a composite felsic pluton constraints from the emerald lake area northern canadian cordillera yukon knowledge of the time scales of emplacement and thermal history during assembly of composite felsic plutons in the shallow crust are critical to deciphering the processes of crustal growth and magma chamber development detailed petrological and chemical study of the mid cretaceous composite emerald lake pluton from the northern canadian cordillera yukon territory coupled with u pb and 40 ar 39 ar geochronology indicates that this pluton was intruded as a series of magmatic pulses intrusion of these pulses produced a strong petrological zonation from augite syenite hornblende quartz syenite and monzonite to biotite granite our data further indicate that multiple phases were emplaced and cooled to below the mineral closure temperatures over a time scale on the order of the resolution of the 40 ar 39 ar technique v1 myr and that emplacement occurred at 94 3 ma simple thermal modelling and heat conduction calculations were used to further constrain the temporal relationships within the intrusion these calculations are consistent with the geochronology and show that emplacement and cooling were complete in less than 100 kyr and probably 70 5 kyr these results demonstrate that production transport and emplacement of the different phases of the emerald lake pluton occurred essentially simultaneously and that these processes must also have been closely related in time and space by analogy these results provide insights into the assembly and petrogenesis of other complex intrusions and ultimately lead to an understanding of the processes involved in crustal development 2002 elsevier science b v all rights reserved

initiation of zn pb mineralization in the pingbao pb zn skarn district south china constraints from u pb dating of grossular rich garnet abstract in this study we discuss the application of u pb dating on u rich common pb bearing grossular rich garnet and the issues of common pb correction grossular rich garnet samples were collected from the baoshan and huangshaping areas of the pingbao skarn district hunan south china the baoshan garnet comprises gr64 3 and34 0 to and99 0 gr0 2 gr grossular and andradite with minor spessartine 0 43 1 45 almandine 0 0 61 and pyrope 0 01 0 91 the huangshaping 304 garnet comprises gr41 7 and56 0 to gr70 2 and25 9 with minor spessartine 1 42 2 10 whereas the huangshaping 301 garnet comprises gr62 0 and26 6 spe11 2 to gr74 5 and20 9 pyr2 5 spe2 1 spe spessartine pyr pyrope the baoshan garnet grains are subhedral to euhedral with well developed core rim textures and contain approximately 1 10 ppm u the huangshaping 304 garnet is also euhedral with grains size ranging from several to tens of millimeters the core rim textured garnet grains are compacted together with interstitial quartz and contain about 5 10 ppm u the huangshaping 301 garnet is anhedral to tabular with interstitial quartz and magnetite and contains up to 579 ppm u in situ u pb dating of the baoshan huangshaping 304 and 301 garnet samples yielded consistent anchored y intercept ages at 162 6 2 9 ma n 60 mswd 1 5 160 2 1 5 ma n 59 mswd 1 9 and 160 9 0 3 ma n 60 mswd 4 1 respectively these ages are consistent with the new and published zircon and titanite u pb ages 162 160 ma indicating that the garnet u pb ages are reliable and the technique is robust we suggest that the non u bearing mineral correction has involved in the third party common pb ratio which is believed to be a better correction for the system common pb

drainage reorganization during breakup of pangea revealed by in situ pb isotopic analysis of detrital k feldspar pb isotopes in detrital k feldspar grains provide a powerful provenance tracer for feldspathic sandstones common pb isotopic compositions show broad hundred kilometer scale regional variation and this signature can survive weathering transport and diagenesis the feldspar pb signature can be measured rapidly using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms and careful targeting can avoid inclusions and altered regions within grains here we combine a new pb domain map for the circum north atlantic with detrital k feldspar pb isotopic data from triassic and jurassic sandstones from basins on the irish atlantic margin the pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern triassic sandstones originated from distant archean and paleoproterozoic rocks probably in green land labrador and the rockall bank to the nw implying long 500 km transport across a nascent rift system later jurassic sandstones had a composite paleo and mesoproterozoic source in more proximal sources to the north

trace element partitioning between apatite and silicate melts we present new experimental apatite melt trace element partition coefficients for a large number of trace elements cs rb ba la ce pr sm gd lu y sr zr hf nb ta u pb and th the experiments were conducted at pressures of 1 0 gpa and temperatures of 1250 c the rare earth elements la ce pr sm gd and lu y and sr are compatible in apatite whereas the larger lithophile elements cs rb and ba are strongly incompatible other trace elements such as u th and pb have partition coefficients close to unity

the kola alkaline province of the cis and finland precise rbsr ages define 380 360 ma age range for all magmatism abstract twenty one new rb sr mineral and biotite whole rock isochrons are presented for intrusive rock complexes of the palaeozoic kola alkaline province in the cis and finland samples from the ultrabasic alkaline and nepheline syenite complexes khibina lovozero afrikanda turiy mys ozernaya varaka sokli and iivaara all yield ages in the very narrow range from 361 7 to 377 3 ma i e late middle devonian to upper devonian these data represent crystallization rather than intrusion ages but are nevertheless argued as marking very closely the time of intrusion there is no indication of bimodal age distribution the emplacement of the alkaline magmas is related temporally to the subsidence of the kontozero graben of the central kola peninsula sr initial ratios of the agpaitic nepheline syenites of the giant khibina and lovozero intrusions scatter between 0 703 and 0 704 because of high rb sr ratios of the rocks these isotope ratios exclude long term pre enrichment of alkalis in the sources of the magmas prior to segregation and intrusion and thus episodic enrichment during earlier rifting events in the northern baltic shield the magma formation may be related to wrench movements in the surroundings of the major trollfjord komagelv fault which forms part of the late caledonian arctic north atlantic megashear system

la q icp ms apatite u pb geochronology using common pb in plagioclase examples from layered mafic intrusions apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks the la icp ms technique is widely applied to u pb geochronology using various minerals apatite u pb geochronology in contrast to e g zircon is compromised by variable amounts of common pb incorporated into the crystal during growth magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age if this is not the case the initial pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite two approaches are common one being the estimation of common pb from a pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate pb but not u e g feldspar most recent studies applying la icp ms to the analysis of pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers in this study we first evaluate the application of quadrupole mass spectrometry for apatite u pb geochronology combined with pb isotopic measurements in feldspar and compare the results with modeled initial pb isotopic compositions the resulting age information is accurate and precise despite using plagioclase rather than k feldspar as is normally used to define initial pb isotope compositions we apply this method to apatite bearing gabbroic rocks from layered intrusions bushveld bjerkreim sokndal hasvik and skaergaard ranging in age from ca 2 ga to ca 55 ma and generate metamorphic cooling ages generally consistent with the known geologic history of these intrusions

conflicting u th he and fission track ages in apatite enhanced he retention not anomalous annealing behaviour while initial studies showed a high degree of consistency between afta and apatite u th he dating an increasing number of studies are reporting apatite u th he ages which are older than expected on the basis of apatite fission track data from the same region we present data from a range of geological settings to document a systematic discrepancy between the two systems which becomes more pronounced in samples with older fission track ages except for samples with very low uranium contents results from a granite pebble and enclosing volcanogenic sandstone provide a well controlled test case in which independent constraints are available on the underlying thermal history these demonstrate that the progressive discrepancy between the two techniques arises not from anomalous fission track annealing behaviour as has been suggested but as a result of a change in the he retention properties of apatite this change appears to be linked to the degree of accumulated radiation damage within the crystal lattice although underlying mechanisms remain unclear we suggest detailed experiments should be performed to delineate and quantify the processes responsible until this is achieved we suggest that apatite u th he studies should also incorporate apatite fission track data as well as other low temperature indicators in order to monitor the u th he system response and to guard against the anomalous behaviour described here

precambrian olistoliths masquerading as sills from death valley california olistolith production and magmatism are processes commonly associated with extensional tectonic settings such as rift basins we present a cautionary exemplar from one such precambrian basin in which we reinterpret metabasite bodies previously documented as sills to be olistoliths we nevertheless demonstrate that on the basis of field observation alone the previous but erroneous sill interpretation is parsimonious indeed it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed we present new data from metabasites and metasedimentary strata of the kingston peak formation cryogenian and crystal spring formation mesoproterozoic of death valley usa these include field observations u pb apatite ages u pb zircon ages detrital and igneous and whole rock geochemistry these data also provide a new maximum age for the base of the pahrump group and suggest that the crystal spring diabase was more tholeiitic than previously thought similar sill olistolith misinterpretations may have occurred elsewhere potentially producing erroneous age and tectonic setting interpretations of surrounding strata this is particularly relevant in precambrian rocks where fossil age constraints are rare this is illustrated herein using a potential example from the neoproterozoic literature of the lufilian belt africa we caution others against precambrian olistoliths masquerading as sills supplementary material details of a meta igneous boulder from p12 of the silurian hills la icp ms and whole rock geochemistry methods and standards and u pb apatite and zircon isotopic data including standards and selected cathodoluminescence images are available at https doi org 10 6084 m9 figshare c 3990639

apatite fission track chronometry using laser ablation icp ms abstract comparison is made of apatite fission track ft ages measured by using conventional thermal neutron irradiation and laser ablation inductively coupled plasma mass spectrometry la icp ms to determine sample uranium content spontaneous track densities were determined on specified apatite grains from the durango fish canyon tuff and mount dromedary complex age standards neutron induced track densities and icp ms uranium values were determined for the same specified grains ages determined using the two approaches show reasonable consistency indicating the possibility of routine fission track analysis using la icp ms uncertainties and assumptions in the la icp ms approach include the laser ablation of chemically etched mineral surfaces the suitability of reference materials for calibration and the derivation of ft ages using the alternatives of selection of a f value or a revised zeta comparative approach the use of la icp ms in ft chronometry would dramatically increase the speed of analysis and hence sample throughput as well as avoiding the need for neutron irradiation and the production transport and handling of radioactive samples with the acknowledged dependency of ft annealing in apatite on composition such grain by grain multi element analysis could provide a more robust definition of the specific track annealing characteristics of each apatite crystal

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

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

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

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

diffusion and fluid interaction in itrongay pegmatite madagascar evidence from in situ 40ar 39ar dating of gem quality alkali feldspar and u pb dating of protogenetic apatite inclusions alkali feldspar 40ar 39ar and apatite upb geochronological studies have typically invoked two mechanisms to account for apparent loss of radiogenic 40ar and 206 208pb some studies have suggested that the radiogenic isotopes were lost by volume diffusion and used these dates to constrain temporal variations of rock temperatures others have argued that the radiogenic isotopes were lost due to interaction with fluids and related these dates to chemical alteration these two end member interpretations have fundamentally different implications for tectonic models derived from geochronological data and therefore it is important to reliably identify the principal mechanism for loss of radiogenic isotopes here we revisit the mechanisms of 40ar loss in the famous gem quality alkali feldspar from the itrongay pegmatite in madagascar previous studies have suggested that volume diffusion is the dominant mechanism of 40ar loss providing key evidence to support the use of 40ar 39ar dating of alkali feldspar for thermochronology we attempted to verify these results by obtaining time temperature paths from petrologically characterised cogenetic feldspar and apatite from the itrongay pegmatite and comparing them with each other however our results suggest that only a minor component in the variability of 40ar 39ar dates of itrongay feldspar is related to the diffusive loss of 40ar and that this loss was not compatible with the majority of previously proposed models which hinders quantitative interpretations the crystal studied here grew in five episodes related to the influx initially of co existing dense sio2 rich solution and co2 dominated fluid the first and supposedly the following two episodes and subsequently of h2o rich fluid supposedly the final two episodes much greater component in the variability of the acquired 40ar 39ar dates is interpreted to reflect the differences in the ages of these growth episodes which we estimate to span from 477 ma to 176 ma the first four episodes apatite inclusions in this crystal are interpreted to be xenocrysts derived from the country rocks of the itrongay pegmatite these yield older upb dates than the estimated age of their host feldspar and have apparently experienced diffusive loss of 206 207pb prior to entrapment our 40ar 39ar results indicate that there is a lack of unambiguous evidence for diffusive loss of 40ar from alkali feldspar that can be readily interpreted for thermochronological purposes however in situ 40ar 39ar dating of alkali feldspar appears to be a promising tool for tracking fluid flow events in the earth s crust whose applicability is not restricted to sedimentary rocks our upb results corroborate previous suggestions that upb dating of apatite can be used for thermochronology

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

apatite in igneous systems apatite is a minor but ubiquitous mineral in most igneous rocks although the modal proportion of apatite in common rocks is generally low it can reach high concentrations in enclaves cumulates and other rocks of low abundance i e rocks that constitute a small volume of the crust and mantle e g nelsonites the presence of apatite in most rocks is due not only to its low solubility in naturally occurring melts and aqueous solutions but also to the limited ability of common rock forming minerals to accept the amount of phosphorus that occurs in most rocks into their structure in this paper we will discuss some aspects of the occurrence texture composition physical chemistry and petrogenetic significance of apatite in felsic rocks i e andesite to rhyolite and their plutonic analogs in mafic i e basalts and related rocks and plutonic analogs and ultramafic rocks of the earth s crust and mantle fluorapatite is by far the most common member of apatite family found in igneous rocks however most natural fluorapatite contains some chlorine and hydroxyl as well and these constituents can attain high concentrations in some cases the other halogens bromine and iodine also occur in apatite but their concentrations are much lower than chlorine and fluorine many cations commonly substitute for calcium and phosphorus in apatite however they rarely reach concentrations that warrant the definition of a separate mineral species apatite can be described by the general formula a5 xo4 3z following sommerauer and katz lehnert 1985 the a site accommodates large cations e g ca2 sr2 pb2 ba2 mg2 mn2 fe2 ree3 eu2 cd2 na pan and fleet ch 2 in this volume and comprises two sites that exhibit vii fold

the tectonics and paleo drainage of the easternmost himalaya arunachal pradesh india recorded in the siwalik rocks of the foreland basin the siwalik sedimentary rocks of the himalayan foreland basin preserve a record of himalayan orogenesis paleo drainage evolution and erosion this study focuses on the still poorly studied easternmost himalaya siwalik record located directly downstream of the namche barwa syntaxis we use luminescence palaeomagnetism magnetostratigraphy and apatite fission track dating to constrain the depositional ages of three siwalik sequences the sibo outcrop upper siwalik sediments at ca 200 800 ka the remi section middle and upper siwalik rocks at 0 8 8 8 2 4 ma and the siang section middle siwalik rocks at 9 3 1 5 to 13 5 1 5 ma cretaceous paleogene detrital zircon and apatite u pb ages characteristic of the transhimalayan gangdese batholiths that crop out northwest of the syntaxis are present throughout the sibo remi and siang successions confirming the existence of a yarlung brahmaputra connection since at least the late miocene a ca 500 ma zircon population increases up section most strikingly sometime between 3 6 to 6 6 ma at the expense of transhimalayan grains we consider the ca 500 ma population to be derived from the tethyan or greater himalaya and we interpret the up section increase to reflect progressive exhumation of the namche barwa syntaxis early cretaceous zircon and apatite u pb ages are rare in the sibo remi and siang successions but abundant in modern siang river sediments zircons of this age range are characteristic of the transhimalayan bomi chayu batholiths which crop out east of the syntaxis and are eroded by the parlung river a modern tributary of the siang river we interpret the difference in relative abundance of early cretaceous zircons between the modern and ancient sediments to reflect capture of the parlung by the siang after 800 ka