identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

post granulite facies monazite growth and rejuvenation during permian to lower jurassic thermal and fluid events in the ivrea zone southern alps u pb analyses of single monazite grains from two granulite facies metapelites in the ivrea zone southern alps reveal the presence in both samples of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted one group of monazite data defines a subconcordant discordia line with an upper intercept age of 293 4 5 8 ma and a lower intercept age of 210 14 ma the upper intercept is interpreted as the real cooling age of the monazites the lower intercept is interpreted as an episode of fluid driven pb loss indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 12 ma another group of monazite data lies above the concordia the presence of excess 206pb indicates that these crystals have grown below the monazite blocking temperature thus after the granulite facies metamorphism the age of growth of the new monazite crystals is approached by their 207pb 235u ages that range between 273 and 244 ma the two groups of post cooling age post 293 4 5 8 ma monazite data correspond to two distinct late and post variscan geotectonic regimes that affected the southern alps 1 permian transtension with decompression and anatectic melting 2 upper triassic to lower jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism

late cadomian crustal tilting and cambrian transtension in the tepla barrandian unit bohemian massif central european variscides the cadomian basement of the tepla barrandian unit is characterized by a classic barrovian type metamorphism the degree of which increases considerably towards the west reaching amphibolite facies conditions in the domaz lice crystalline complex dcc the number and volume of plutons also increases towards the west the emplacement ages of the te s ovice granite and the mrac nice trondhjemite have been determined at 521 7 2 ma and 523 4 5 ma respectively applying conventional u pb analyses of zircons pervasive high temperature prolate fabrics and north northwest dipping dextral oblique slip shear zones within the mrac nice trondhjemite suggest a synkinematic melt emplacement within a lower cambrian transtensional setting transtension is probably related to early stage rifting that introduced the separation of the tepla barrandian unit as part of armorica from gondwana structural and petrological data of the country rocks show that the barrovian type metamorphism and two deformation stages d1 with unknown kinematics and d2 top to the north shearing are older than the melt emplacement and thus can be attributed to the cadomian orogeny the intrusion depth of both plutons is nearly the same ca 7 km although the degree of barrovian type metamorphism differs significantly within the country rocks this suggests late cadomian eastward tilting of the metamorphic isograd planes the weak post plutonic lower greenschist to subgreenschist facies folding and thrusting result from variscan northwest southeast compression

conventional and in situ geochronology of the tepla crystalline unit bohemian massif implications for the processes involving monazite formation the tepla crystalline unit tcu western bohemian massif proves highly suitable for studying the effects of differential metamorphic reworking on the u th pb systematics in monazite as the overprint of variscan regional metamorphism onto high grade cadomian paragneisses intensifies progressively towards the northwest although variably hampered by scarcity small size and low uranium contents of monazite isotope dilution thermal ionisation mass spectrometry of monazite from paragneisses from the garnet staurolite and kyanite zones of the tcu gives a narrow 206pb 238u age range from 387 to 382 ma for variscan peak metamorphism these data are supported by 382 373 ma monazite ages derived from electron microprobe analyses inheritance of older components in grains from the central tcu imply major resetting of pre variscan monazite around 380 ma possibly due to widespread garnet growth during variscan metamorphism which led to the consumption of pre variscan high y monazite and subsequent growth of new low y monazite concordant 498 494 ma monazite ages in a migmatitic paragneiss close to the adjacent marianske lazn complex mlc grew in response to metagabbro emplacement in the mlc from 503 to 496 ma and not during either cadomian or variscan regional metamorphism backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca 540 to 380 ma combined evidence presented here suggests that instead of pb loss by volume diffusion the apparent resetting of the u th pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution reprecipitation

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

dating low grade metamorphic events by shrimp u pb analysis of monazite in shales although the u pb isotopic system is widely used to date magmatism and medium to high grade metamorphism dating low grade metamorphic events has been hampered by a scarcity of suitable minerals we show that monazite forms in shales during low grade metamorphism and can be dated by in situ u pb analysis providing precise ages for low temperature thermal events in proterozoic shales from the pine creek inlier of northern australia monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion ca 1830 ma in the pilbara craton of northwestern australia monazite growth 2192 5 m y ago coincided with basaltic volcanism and rifting an event apparently responsible for hydrothermal iron oxide mineralization in a nearby giant iron ore deposit the mineralizing environment was highly oxic and bathed by meteoric fluids thus our results imply the existence of an oxygen rich atmosphere by 2200 ma given the abundance of shales in the stratigraphic record and the likely widespread distribution of monazite in such rocks in situ u pb dating of monazite may become an important chronometer for low grade metamorphic events

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

constriction due to subduction evidence for slab pull in the marianske lazně complex central european variscides pervasive l and l s tectonites of the marianske lazn eclogites include pencil fabrics constrictional folds and mullions that result from the first deformation increment observed petrological and microstructural data suggest that these structures formed by constrictional flow at deep subduction levels after or even during the eclogite facies stage and before the subsequent high pressure granulite facies overprint constriction at mantle depths is explained by taking into account the geometry and seismic properties of recent subduction zones here constrictional strain and associated l s tectonites should be expected below the rheologycontrolled slab bend at about 40 km depth where dip parallel extension and a reduced viscosity of the downgoing slab are compatible with a coaxial flow regime

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

different age response of zircon and monazite during the tectono metamorphic evolution of a high grade paragneiss from the ruhla crystalline complex central germany results of tims sims and sem analyses show that zircon and monazite in a high grade paragneiss of the ruhla crystalline complex central germany were formed and or altered during different stages of a tectono metamorphic history between early devonian and permian times detrital zircon cores of 460 ma place an older limit on the age of anatexis and show that the paragneiss sequence contains rocks at least as young as early cambrian metamorphic zircon growth commenced at 365 ma peaking at 360 355 ma at the same time that granite dykes were emplaced in contrast monazite in the paragneiss preserves little record of the metamorphic peak most monazite grains grew or were recrystallised in the lower carboniferous at 339 ma contemporaneous with the emplacement of voluminous diorite and granite bodies these intrusions and related tectonics caused some of the high u zircon overgrowths to undergo moderate to severe pb loss a second pb loss event between 300 and 280 ma can be related to late carboniferous early permian large scale block faulting

monazite xenotime miscibility gap thermometry i an empirical calibration rare earth element ree and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the variscan fold belt in ne bavaria germany the metapelites include a continuous prograde mainly low p 3 5 kbar metamorphic profile from greenschist c 400 c to lower granulite facies conditions c 700 c the lree la sm are incorporated preferentially in monoclinic monazite reo9 polyhedron whereas the hree plus y are concentrated in tetragonal xenotime reo8 polyhedron the major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade monazite consists mainly of la ce and nd la0 20 0 23 ce0 41 0 45 nd0 15 0 18 po4 all other elements are below 6 mol likewise xenotime consists mainly of ypo4 with some dy and gd solid solutions y0 76 0 80 dy0 05 0 07 gd0 04 0 06 in contrast the minor hree concentrations in monazite increase strongly with increasing metamorphic grade y dy and gd increase by a factor of 3 5 from greenschist to granulite facies rocks monazite crystals often show zonation with cores low in hree and rims high in hree that is interpreted as growth zonation attained during prograde metamorphism similarly sm and nd in xenotimes increase by a factor of 3 4 with increasing metamorphic grade prograde zonation in single crystals of xenotime was not observed the xhree y in monazite and xlree in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c 400 c to 700 c the empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry due to its contents of u and th monazite has often been used for u pb age determination the combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature time paths

significance of in situ sims chronometry of zoned monazite from the lewisian granulites northwest scotland abstract in situ studies of lewisian metamorphic monazites show them to be strongly zones in th content th u ratio and 207 pb 206 pb age traverses across compositional zones of individual monazites by sims and empa show that there are no resolvable shifts between 232 th 238 u and 208 pb 206 pb ratios and between th and pb contents in any of the profiles obtained furthermore the difference in 208 pb 232 th ages between two zones of monazites in the same thin section is indistinguishable from the difference in 207 pb 206 pb ages determined by sims these observations together with simple models suggest that transport of pb relative to th and u did not occur significantly during younger e g the laxfordian events in the lewisian and the ages preserved in these zoned monazites are close to concordant moreover monazites having different compositions yield indistinguishable 207 pb 206 pb ages which suggests that the closure temperature of the u pb system in these monazites is not significantly dependent upon their composition consequently the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the lewisian the results are consistent with the recent interpretations of the lewisian thermal history which suggest two high grade metamorphic events separated by 200 ma

resolving cambrian carboniferous permian and alpine monazite generations in the polymetamorphic basement of eastern crete greece by means of the electron microprobe the electron microprobe based investigation of accessory monazites in polished thin sections is a helpful tool in resolving the geochronology of a polymetamorphic basement the method was applied to variably altered gneisses and micaschists from the retrogressed originally amphibolite facies basement in eastern crete greece the presented data indicate that most monazite formed or recrystallized in response to high fluid activity during alpine low temperature metamorphism this low temperature monazite is characterized by distinctly low yttrium uranium and thorium contents however older grains were able to survive in less retrogressed samples and have been traced with the electron microprobe using microstructural and compositional criteria in situ chemical th u pb dating of these pre alpine monazites provides evidence for an igneous event in the cambrian and two different metamorphic events in the carboniferous and permian

middle precambrian provenance of jurassic sandstone in the mino terrane central japan th u total pb evidence from an electron microprobe monazite study precise electron microprobe analyses have been made on detrital monazites in jurassic sandstones of the mino terrane central japan and their th u pb relations are examined in terms of pbo and tho2 tho2 tho2 plus the equivalent of uo2 the pbo tho2 ratios of most detrital monazites 403 of 581 grains cluster in the range from 0 0763 to 0 0618 other grains show the ratio of about 0 0541 46 grains 0 0364 29 grains 0 0170 24 grains and 0 0116 79 grains the pbo tho2 ratio of 0 0763 0 0618 is nearly identical with that of monazites in a middle precambrian 1740 ma gneiss clast from the kamiaso conglomerate of the mino terrane this strongly suggests that the detrital monazites were derived from gneisses and or granitoids of middle precambrian age the ratio of 0 0116 is nearly the same as that of monazites in a permian 274 ma paragneiss of the hida terrane north to the mino terrane the pbo tho2 monazite data coupled with palaeocurrent analyses and petrological characteristics of sandstones show that middle precambrian high grade metamorphic rocks and granitoids were widespread both in the northern and southern source regions of the e w trending jurassic basin of the mino terrane the pbo tho2 ratio of detrial monazites is a useful new measure for the provenance analysis of clastic rocks

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

domainal variations of u pb monazite ages and rb sr whole rock dates in polymetamorphic paragneisses ktb drill core germany influence of strain and deformation mechanisms on isotope systems polyphase metamorphic paragneisses from the drill core of the continental deep drilling project ktb nw bohemian massif are characterized by peak pressures of about 8 kbar medium p metamorphism followed by strain accumulation at t 650 c initially by dislocation creep and subsequently by diffusion creep u pb monazite ages and rb sr whole rock data vary in the dm scale indicating ordovician and mid devonian metamorphic events such age variations are closely interconnected with dm scale domainal variations of microfabrics that indicate different predominant deformation mechanisms u pb monazite age variations dependent on microfabric domains exceed grain size dependent age variations in mylonitic domains recording high magnitudes of plastic strain dislocation creep and minor static annealing monazite yields concordant and near concordant lower ordovician u pb ages and the rb sr whole rock system shows isotopic disequilibrium at an mm scale in mineral growth mobilisate domains in which diffusive mass transfer was a major strain producing mechanism promoting diffusion creep of quartz and feldspar and in which static recrystallization annealing reduced the internal free energy of the strained mineral aggregates concordant u pb ages are mid devonian locally in such domains rb sr dates among mm3 sized whole rock slabs reflect post ordovician resetting in transitional domains the u pb ages are discordant we conclude that medium p metamorphism occurred at 484 2 ma and a second metamorphic event at 380 370 ma mid devonian caused progressive strain in the rocks dislocation creep at high rates even at high temperatures does not reset the rb sr whole rock system while diffusion creep at low rates and stresses i e low e deff ratios static annealing and the presence of intergranular fluids locally assist resetting at temperatures above 650 c diffusive pb loss did not reset ordovician u pb monazite ages and in domains of overall high imposed strain rates and stresses resetting was not assisted by dynamic recrystallization crystal plasticity however during diffusion creep at low rates pb loss by dissolution and precipitation recrystallization of monazite produces discordance and devonian concordant u pb monazite ages hence resetting of these isotope systems reflects neither changes of temperature nor directly the presence or absence of strain

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif abstract investigations of variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the tepla barrandian unit bohemian massif the study focuses on determining differential stress deformation mechanisms and geometry of cadomian metagreywackes and cambrian granitoids which show different grade of variscan barrovian type metamorphism ranging from the greenschist subgreenschist facies boundary semibrittle deformation regime to the high grade amphibolite facies the semibrittle deformation regime t ca 300 c is characterized by large scale thrusting within a folding environment and by similar intensities of cataclasis diffusive mass transfer and crystal plasticity of quartz the latter including low temperature migration recrystallization within cadomian quartz veins of the greenschist facies part dislocation creep of quartz operated at differential stresses ranging from ca 70 mpa in the lowermost greenschist facies to ca 20 mpa in the uppermost greenschist facies these values have been derived by paleopiezometry using the grain size of recrystallized quartz they are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes a marking increase in recrystallized grain size of quartz occurs within the lower greenschist facies where low temperature grain boundary migration is replaced by subgrain rotation thus causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level pressure solution of quartz and development of discrete crenulation cleavage was common in fine grained metagreywackes of the lower greenschist facies whereas the upper greenschist facies t ca 500 570 c is characterized by increasing intensity of metamorphic reactions grain coarsening and development of a schistosity the entire greenschist facies level forms a suprastructure where viscosity contrasts between mica and quartz rich layers were sufficiently high for buckle folding strain protected intercalated metagranitoids on the other hand do not show pervasive deformation fabrics the amphibolite facies part forms an infrastructure characterized by pervasive mylonitic shearing of both cambrian granitoids and metagreywackes it is suggested that the combination of the following processes has contributed to weaken the amphibolite facies level making nonlocalized flow possible 1 increased recrystallization of feldspar 2 crystal plastic softening of quartz due to high temperature grain boundary migration and activity of prism slip and 3 enhanced diffusive mass transfer in the form of metamorphic reactions the structural breaks at the semibrittle crustal level and at the greenschist amphibolite facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust

neoproterozoic to early cambrian history of an active plate margin in the tepla barrandian unit a correlation of u pb isotopic dilution tims ages bohemia czech republic abstract the tepla barrandian unit tbu of the bohemian massif shared a common geological history throughout the neoproterozoic and cambrian with the avalonian cadomian terranes the neoproterozoic evolution of an active plate margin in the tepla barrandian is similar to avalonian rocks in newfoundland whereas the cambrian transtension and related calc alkaline plutons are reminiscent of the cadomian ossa morena zone and the armorican massif in western europe the neoproterozoic evolution of the tepla barrandian unit fits well with that of the lausitz area saxothuringian unit but is significantly distinct from the history of the moravo silesian unit the oldest volcanic activity in the bohemian massif is dated at 609 17 19 ma u pb upper intercept subduction related volcanic rocks have been dated from 585 7 to 568 3 ma lower intercept rhyolite boulders which pre dates the age of sedimentation of the cadomian flysch st chovice group accretion uplift and erosion of the volcanic arc is documented by the neoproterozoic dob is conglomerate of the upper part of the flysch the intrusion age of 541 7 8 ma from the zgorzelec granodiorite is interpreted as a minimum age of the neoproterozoic sequence the neoproterozoic crust was tilted and subsequently early cambrian intrusions dated at 522 2 ma t sovice granite 524 3 ma vsepadly granodiorite 523 3 ma smr ovice tonalite 523 1 ma smr ovice gabbro and 524 0 8 ma orlovice gabbro were emplaced into transtensive shear zones

u pb zircon ages and structural development of metagranitoids of the tepla crystalline complex evidence for pervasive cambrian plutonism within the bohemian massif czech republic u pb zircon dating of three metagranitoids situated within a tilted crustal section at the northwestern border of the tepla barrandian unit tepla crystalline complex tcc yields similar cambrian ages the u pb data of zircons of the tepla orthogneiss define an upper intercept age of 513 7 6 ma the 207pb 206pb ages of 516 10 and 511 10 ma of nearly concordant zircons of the hanov orthogneiss and the lestkov granite are interpreted to be close to the formation age of the granitoid protolith similar to the cambrian granitoids of the southwestern part of the tepla barrandian unit doma lice crystalline complex dcc the middle cambrian emplacement of the tcc granitoids postdates cadomian deformation and metamorphism of the upper proterozoic country rocks but predates variscan tectonometamorphic imprints structural data as well as sedimentological criteria suggest a dextral transtensional setting during the cambrian plutonism related to the early paleozoic break up of northern gondwana due to strong variscan crustal tilting the degree of variscan tectonometamorphic overprint is strikingly different in the dated granitoids it is lowest in the weakly or undeformed lestkov granite located in the greenschist facies domain the tepla orthogneiss in the north underwent pervasive top to nw mylonitic shearing under amphibolite facies conditions there is no indication for a resetting of the u pb isotopic system of the tepla orthogneiss zircons that could be attributed to this imprint radiation damages accumulated until recent have probably caused lead loss

10 km minimum throw along the west bohemian shear zone evidence for dramatic crustal thickening and high topography in the bohemian massif european variscides the west bohemian shear zone wbsz forms a steep collapse structure along which east side down normal movements led to the juxtaposition of the relatively cold cadomian basement of the tepla barrandian unit against high grade moldanubian rocks synkinematic plutons straddle the wbsz the mut nin pluton intruded into moldanubian crust at a depth of 23 4 km as derived by using al in hornblende barometry the tepla barrandian babylon pluton intruded at 12 km depth as indicated by phengite barometry and petrogenetic considerations both emplacement depths together with mineral cooling ages result in a minimum vertical displacement of 10 km between 340 and 320 ma this large throw could be explained by over thickened crust that was weakened from below the alkaline signature of the mut nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 ma the cold tepla barrandian upper crust sank into its weak partly molten moldanubian substratum resulting in elevator style movements not only along the wbsz but also along the hoher bogen and central bohemian shear zone all these ductile normal shear zones were active simultaneously during the lower carboniferous and dip steeply towards the tepla barrandian unit that probably formed a highly elevated plateau at this time

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

the problem of dating high pressure metamorphism a u pb isotope and geochemical study on eclogites and related rocks of the marianske lazně complex czech republic this study presents new geochemical major and trace element nd sr isotope and u pb zircon monazite titanite and rutile data for various rock types eclogite high pressure granulite amphibolite orthogneiss leucosome of the high grade metamorphic marianske lazn complex in the western bohemian massif concordant u pb zircon analyses from the mafic to intermediate samples disclose the marianske lazn complex as a mixture of c 540 ma oceanic rocks juxtaposed at depth with lower crustal rocks of the structurally overlying tepla barrandian unit this interpretation refutes earlier models in which the marianske lazn complex was interpreted as a dismembered cambro ordovician ophiolite complex affected by variscan subduction metamorphic zircon in mafic rocks of the marianske lazn complex as well as monazite from orthogneiss and titanite in leucosome yield ages around 380 ma on the basis of detailed petrography and cathodoluminescence imaging we conclude that the c 380 ma age reflects the timing of variscan exhumation and associated decompression melting under upper amphibolite facies to granulite facies conditions thereby the data derived from metamorphic zircon of eclogites and high pressure granulite unexpectedly do not date the timing of eclogitization which could have happened just before variscan exhumation or even shortly after late cadomian protolith formation

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

u pb zircon and sr nd pb whole rock investigations from the continental deep drilling ktb among the large number of gabbroic intrusions within the profile of the continental deep drilling ktb several types can be distinguished in this study petrographic geochemical investigations conventional u pb zircon data and results of sr nd pb isotope characteristics of meta gabbros amphibolites and felsic rocks are reported the main results are u pb zircon age data reveal a magmatic cambro ordovician evolution of the erbendorf vohenstrauss zone zev the range of the ages of magmatic intrusions varies from 496 to 476 ma probably as a result of overlap of magmatic and metamorphic processes cambro ordovician magmatic ages were observed within different mafic b unit as well as in variegated units v unit of the ktb profile the silurian devonian metamorphism is dated at around 398 ma the silurian as well as the variscan metamorphic events apparently did not influence the u pb system the cambro ordovician mafic rocks from the ktb scatter on a nd evolution diagram several meta gabbros cluster around the ndt 500 value of 5 but others vary from 8 6 to 0 65 and suggest variable degrees of contamination with continental crust material a contamination of only 5 10 is required to reduce nd from 8 6 to approximately 5 according to the ree patterns and the sr nd pb data the mafic rocks could either derive from an enriched mantle source or else their protolith magma originated from a depleted mantle and was contaminated by continental crust the data for the mafic rocks of the zev are in accord with observation by other authors regarding the evolution of the late proterozoic to early palaeozoic magmatism in the bohemian massif

coronitic metagabbros of the marianske lazně complex and tepla crystalline unit inferences for the tectonometamorphic evolution of the western margin of the tepla barrandian unit bohemian massif

emplacement depths and radiometric ages of paleozoic plutons of the neukirchen kdyně massif differential uplift and exhumation of cadomian basement due to carboniferous orogenic collapse bohemian massif abstract the igneous complex of neukirchen kdyn is located in the southwestern part of the tepla barrandian unit tbu in the bohemian massif the tbu forms the most extensive surface exposure of cadomian basement in central europe cambrian plutons show significant changes in composition emplacement depth isotopic cooling ages and tectonometamorphic overprint from ne to sw in the ne the vsepadly granodiorite and the smr ovice diorite intruded at shallow crustal levels t 20 km the teufelsberg cert v kamen diorite on the other hand forms an unusual intrusion dated at 359 2 ma concordant u pb zircon age k ar dating of biotite of the teufelsberg diorite yields 342 4 ma these ages together with published cooling ages of hornblende and mica in adjacent plutons are compatible with widespread medium to high grade metamorphism and strong deformation fabrics suggesting a strong variscan impact under elevated temperatures at deeper structural levels the plutons of the neukirchen area are cut by the steeply ne dipping hoher bogen shear zone hbsz which forms the boundary with the adjacent moldanubian unit the hbsz is characterized by top to the ne normal movements which were particularly active during the lower carboniferous a geodynamic model is presented that explains the lateral gradients in cambrian pluton composition and emplacement depth by differential uplift and exhumation the latter being probably related to long lasting movements along the hbsz as a consequence of lower carboniferous orogenic collapse

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool