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

the tectonic significance of u th pb ages of monazite inclusions in garnet from the himalaya of central nepal abstract textural observations chemical zoning and 196 in situ la icp ms 232 th 208 pb and 238 u 206 pb dates of monazite inclusions in greater himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite cenozoic prograde and retrograde monazite are commonly intergrown and both of these components are frequently intergrown with lower paleozoic monazite because zones in most inclusions are too small to date and because some inclusions show little zoning of y and th chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of th pb dates comparison of 238 u 206 pb and 232 th 208 pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions most cenozoic monazite crystallized between 42 29 and 22 12 ma and these age ranges may record prograde and retrograde metamorphism of greater himalayan rocks respectively dates younger than 12 ma are discordant without exception the

downward continuation of heat flow density data and thermal regime in eastern canada abstract heat flow density hfd data from eastern canada were analyzed by downward continuation the data cover parts of the superior and grenville provinces of the canadian shield and parts of the appalachian orogen the downward continuation eliminates the effect of shallow crustal radiogenic heat production and better reveals differences in the thermal regime of deeper origin the analysis shows that the deep heat flow beneath the superior and grenville provinces is low and the temperature in the lower crust is also low the thermal regime of the crust changes markedly across a boundary that follows the st lawrence river surface heat flow and heat production are markedly higher in the appalachian province with much higher temperature and heat flow the thermal regime in the appalachian lower crust is also distinctly different from that of the shield temperature and heat flow increase eastward toward the atlantic margin

near surface thermal prospecting review of processing and interpretation temperature measurements at shallow depths up to 3 m contain useful information about features of the geological structures in the areas under investigation however the noise caused by seasonal temperature variations and terrain relief effects may significantly distort the observed temperature field therefore procedures are developed for the calculation and removal of these noise sources a seasonal variations are first eliminated by a procedure using repeated observations b terrain relief corrections are calculated by a correlation technique which facilitates the identification of anomalies associated with concealed geological features essential similarities between thermal and magnetic prospecting make it possible to apply to thermal prospecting modifications of the rapid methods of characteristic points and tangents developed for magnetic prospecting these methods are applicable to conditions of inclined relief arbitrary magnetization polarization and an unknown level of the normal field the methods can be used to locate disturbing bodies by their associated temperature anomalies interpretation is made possible by approximating bodies by a dipping thin sheet or a horizontal circular cylinder the interpretation results obtained both on models and polymetallic greater caucasus and oil and gas middle kura depression deposits testify to the accuracy and reliability of these methods these methods were also used successfully for interpretation of temperature anomaly over underground cavity in cracov poland

disturbance versus preservation of u th pb ages in monazite during fluid rock interaction textural chemical and isotopic in situ study in microgranites velay dome france monazite is extensively used to date crustal processes and is usually considered to be resistant to diffusive pb loss nevertheless fluid assisted recrystallisation is known to be capable of resetting the monazite chronometer this study focuses on chemical and isotopic disturbances in monazite grains from two microgranite intrusions in the french central massif charron and montasset petrologic data and oxygen isotopes suggest that both intrusions have interacted with alkali bearing hydrothermal magmatic fluids in the charron intrusion regardless of their textural location monazite grains are sub euhedral and cover a large domain of compositions u pb chronometers yield a lower intercept age of 297 4 ma an inherited component at 320 ma is responsible for the scattering of the u th pb ages the montasset intrusion was later affected by an additional f rich crustal fluid with crystallisation of ca ree fluorocarbonates fluorite calcite and chloritisation pristine monazite is chemically homogeneous and displays 208pb 232th and 206pb 238u concordant ages at 307 2 ma by contrast groundmass monazite shows dissolution recrystallisation features associated with apatite and thorite precipitation th silicate and strong chemical reequilibration 208pb 232th ages are disturbed and range between 270 and 690 ma showing that the th pb ratio is highly fractionated during the interaction with fluids apparent u pb ages are older due to common pb incorporation yielding a lower intercept age at 312 10 ma the age of the pristine monazite these results show that f rich fluids are responsible for th mobility and incorporation of excess pb which thus strongly disturbed the u th pb chronometers in the monazite

a new geochronometer using lanthanum 138 we have applied a new geochronometer based on the decay of 138la to 138ba to a sample of amitsoq gneiss from west greenland using a value for the electron capture decay constant ec of 4 44 10 12 yr 1 the resulting age 2 408 24 myr is in good agreement with that from sm nd systematics 2 410 54 myr application of this geological clock is limited to ree rare earth element rich minerals but its merit is that for archaean rocks the age can be evaluated from one specimen on the assumption that the initial ba isotope ratios are effectively constant

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

evaluation of the thermal regime of the valles caldera new mexico u s a by downward continuation of temperature data abstract downward continuation of temperature data from 73 wells extending to depths of 250 ft 76 m provides constraints on the thermal regime of the valles caldera surface temperature gradient data and bottom hole temperatures were used as constraints in the downward continuation three factors were found to control the shallow thermal regime 1 1 heat associated with the main geothermal source 2 2 local topography and 3 3 west southwest groundwater flow although the well density is relatively high comparison with the topography shows that the wells are not randomly distributed and tend to be clustered in valleys many details in the thermal regime appear to be related to groundwater drainage in these valleys temperature gradients and temperatures generally increase in the same direction as the regional drainage of the caldera suggesting a long wavelength shallow component to this regional gradient trend inversion of gradient and temperature data show additional deep heat input in the west southwest sector of the caldera which appears to be spatially associated to the youngest volcanism a previously reported northeast displacement of the main heat source from the surface anomaly has not been confirmed

u pb shrimp geochronology of th poor hydrothermal monazite an example from the llallagua tin porphyry deposit bolivia in situ u pb shrimp analysis of hydrothermal monazite virtually free of th and poor in u 0 2 ppm th 40 103 ppm u from the world class llallagua tin porphyry deposit in bolivia defines a mineralization age of 23 4 2 2 ma mswd 0 48 confirming earlier k ar sericite alteration age data these ages are however in contrast with a weighted mean single crystal 207pb 206pb evaporation age of 39 3 6 0 ma and a related pb pb inverse isochron age of 42 4 4 0 ma mswd 0 66 on zircon from a post porphyry dike as well as with an earlier single crystal sm nd apatite isochron age our data points to a significant time gap between emplacement of the ore hosting porphyry intrusion magmatism and its hydrothermal overprint tin mineralization suggesting long lived magmatic hydrothermal activity in this part of the andean back arc crust the decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization our study demonstrates the usefulness of the application of the u pb shrimp method to direct age determination of ore mineralization using th poor hydrothermal monazite even when dealing with geological young events the common assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted

detrital and newly formed metamorphic monazite in amphibolite facies metapelites from the motajica massif bosnia abstract the motajica massif in bosnia situated in the sava zone between the tisza unit and the dinarides hosts a late cretaceous flysch series which is partly preserved in an anchimetamorphic state partly affected by amphibolite facies overprint samples from both parts contain abundant accessory monazite which was investigated by means of the electron microprobe with reference to age and composition in the amphibolite facies samples new metamorphic monazite has formed chemical th u pb monazite ages indicate that the regional metamorphic event happened in the late cretaceous or the early palaeogene in addition inherited detrital monazite of permian age has been identified based on various observations it is argued that the detrital monazite was not the direct precursor to the metamorphic monazite the latter may rather have grown from other more reactive ree carrier minerals either detrital ones e g detrital allanite or such that formed during diagenesis and low grade metamorphism a later greenschist facies event that occurred in connection with a ca 27 ma old granodiorite intrusion led in some samples to the formation of late fluorapatite allanite coronas around both detrital and newly formed monazites the anchimetamorphic samples contain exclusively detrital monazite these are partly of permian as well as jurassic to early cretaceous age based on their compositional and textural features the jurassic to early cretaceous monazite relics are considered to be mainly derived from metamorphic rocks the permian monazite displays high yttrium content up to ca 3 5 wt y 2 o 3 indicating a high formation temperature 650 c and therefore probably a magmatic origin apart from almost ubiquitous permian high y monazite the motajica flysch also contains large permian xenotime with magmatic zoning patterns fluorapatite with parasitic monazite inclusions formerly ree rich fluorapatite and zircon with a type granite morphology this accessory mineral spectrum implies that among others rift related ree rich permian a type granites or rhyolites should be considered as a source for the sava zone flysch

alleghanian tectono thermal evolution of the dextral transcurrent hylas zone virginia piedmont u s a abstract the hylas zone is the northernmost segment of the eastern piedmont fault system and lies in the grenville goochland terrane virginia like the other major segments the nutbush creek zone the hollister zone and the modoc irmo zone the hylas zone experienced late paleozoic dextral transcurrent shearing early deformation in the zone was ductile and produced type i and type ii s c mylonites under amphibolite grade metamorphic conditions later deformation was at the brittle ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons the feldspar exhibits a strain dependent sequence of microstructures including kink bands antithetic extensional microfaults pull apart bends in the microfaults by development of transverse fractures and microboudinage correlation of mineral mechanical response with a thermal decay curve based on isotopic mineral ages yields a temporally constrained deformation history ductile dextral shearing occurred subsequent to the intrusion of the 330 ma petersburg granite and passed into the brittle ductile transition by approximately 260 ma dextral faulting terminated before 240 ma the appalachian dextral transcurrent faulting event therefore continued through permian in some areas

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

ion microprobe 232th 208pb ages from high common pb monazite morefield mine amelia county virginia implications for alleghanian tectonics monazite ce th po4 from a pegmatite in the morefield mine of the eastern piedmont of central virginia has unusually high and variable amounts of common pb leading to problematic interpretations of its u th pb ages and how the monazite relates to nearby granite intrusions and faults to address these issues we analyze a single large monazite grain from the pegmatite using electron microprobe analysis empa n 64 laser ablation inductively coupled plasma mass spectrometry la icp ms n 58 and secondary ion mass spectrometry sims n 59 the monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization although the compositions of these regions fit the ideal stoichiometry of monazite they have lower si th u and y and higher p rare earth element ree and ca concentrations compared to visibly unaltered portions of the grain la icp ms and sims analyses demonstrate that common pb as proxied by 204pb is enriched in proximity to microcrack regions and correlates with 137ba sims 232th 208pb analysis from grain regions with lowest contents of common pb 208pb comprises 99 of pb isotopes yields two sets of ages 263 5 3 0 ma 1 mswd 1 7 n 11 and 234 1 3 3 ma 1 mswd 0 4 n 13 regionally the ages are similar to the youngest appalachian pegmatite bodies emplaced during the terminal alleghanian laurentia africa collision however the monazite ages are younger than locally surrounding intrusions the closest intrusive in distance 30 km and age is the petersburg granite 296 33 0 11 ma zircon 238u 206pb the emplacement of which coincided with activity along the hylas fault because the fault experienced a brittle ductile transition in the late permian 262 ma coeval with the older 232th 208pb monazite age group we postulate that the hylas fault may have operated as a fluid migration system between the petersburg granite and morefield mine pegmatite the younger monazite age coincides with triassic normal and or sinistral faulting linked to the development and deformation of local rift basins

ages of regional metamorphism and ductile deformation in the central and southern appalachians abstract a summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation the ca 480 435 ma taconic event affected most of the piedmont and blue ridge provinces the ca 380 340 ma acadian event was milder and was centered in the west central piedmont and eastern blue ridge the 330 230 ma alleghanian event exhibits an abrupt ductile deformation front in the eastern piedmont and extends an unknown distance eastward below the atlantic coastal plain

dating rock deformation with monazite the impact of dissolution precipitation creep abstract the u th pb system of monazite behaves differently dependent on the deformation mechanism dissolution precipitation creep dpc or dislocation creep activated in the hosting metamorphic rocks this can be exploited to use monazite for dating deformation as is shown in rocks subsequently deformed by dislocation creep and dpc in rock layers intensely deformed by dpc mineral reactions particularly the dissolution of feldspar and apatite increased the alkali content and reactivity of the fluid this in turn led to dissolution of old predeformative monazite grains new synmetamorphic monazite grains formed as the result of inter grain transport of material over distances within the grain scale this process efficiently led to complete resetting of the monazite u th pb system even at temperatures prevailing during greenschist facies conditions the chemical composition of the new monazite records the dissolution of the old feldspar by a less pronounced negative eu anomaly compared to old monazite the shape of the monazite grains that precipitated during creep indicates the sense of shear in the shear zone thus linking the obtained ages directly to the map scale tectonic transport in rock layers predominantly deformed by dislocation creep old monazite grains survived intense mylonitization and high strain and show a core rim structure the cores are patchy reflecting intra grain coupled dissolution reprecipitation replacement processes a wide range in apparent geologically inaccurate th pb and u pb ages among the patchy zones is the result of incomplete removal of in situ grown radiogenic pb from the patchy domains depletion of th and u and the redistribution of th and u among the domains exclusively in the rims of the old monazite the chemical composition correlates to that of the syndeformative monazites and the u th pb system reflects the subsequent dpc accordingly rocks pervasively deformed by dpc should be preferably used to obtain monazite most suitable for precise dating of creep episodes linked to shear deformation and for determination of deformation rates

relationship between the ellisville pluton and chopawamsic fault establishment of significant late ordovician faulting in the appalachian piedmont of virginia

an investigation of low stress ice rheology on the ward hunt ice shelf deformation of the ward hunt ice shelf northwest territories canada can be used to study the rheology of ice in the low stress low strain rate regime 10 12s 1 finite element simulations are separately displayed to demonstrate what the effects of ice salinity ice temperature and sea ice back pressure have on the spreading rates of this ice shelf we conclude that in general these effects tend to obscure any differences between the commonly used non newtonian flow law and the newtonian flow law recently reinterpreted from field measurements the numerical simulations indicate that there are specific locations where useful field measurements could be carried out on the ice shelf

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

zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology a case study from the lewisian terrain abstract monazite is a valuable chronometer for many geological processes and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes this contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in lewisian granulites which is of general significance for monazite geochronology in high grade metamorphic rocks detailed investigations on two lewisian granulite samples using sem epma and sims have revealed that lewisian monazites frequently show petrographic chemical and age zonation the petrographical zonation which reflects the chemical compositional variation can be characterised as follows on the basis of the bse images 1 concentric zoning 2 patchy zoning and 3 intergrowth like zoning the chondrite normalised ree distribution patterns are remarkably similar between different zones of single monazite grains this rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin some possible mechanisms are proposed for the formation of monazite zonation these include 1 intergrowth of monazite crystals with different composition 2 episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event s which may involve recrystallisation or replacement of the original crystals in short the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism accordingly interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism line scans of th u pb la ce nd sm ca and si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism the very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism the effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase and recrystallisation of the mineral itself thus great care needs to be taken for geological application of the closure temperature concept

heat flow and heat generation in the churchill province of the canadian shield and their palaeotectonic significance six new heat flow determinations are presented for proterozoic mobile belts of the churchill province of the canadian shield an area that was affected by several stages of the hudsonian orogenic sequence 1 9 1 6 ga ago with other previously published values the mean of eight determinations considered reliable and representative and corrected for the effects of pleistocene glaciation is 44 7 mw m 2 heat generation measurements have also been made values range from 0 1 1 04 w m 3 a linear relation between heat flow and heat production is apparent the heat flow axis intercept is 37 mw m 2 and the scale depth is 11 km compared with 28 mw m 2 and 13 6 km for the archaean superior province approximately 20 of the churchill heat flow appears to be derived from radioactive decay in the upper crust compared with 30 for the superior province and shields as a whole the observations imply that the heat flow heat production relation for the churchill province should be written as q qc qe a0b where qc is equivalent to the reduced heat flow for the archaean terrain b is similar for the two and qe is an additional component of heat flow in the proterozoic mobile belts of the churchill province a speculative tectonic model is presented it is suggested that rifting along two axes of an original craton which had lateral variations in near surface radiogenic element concentration followed by erosion of the radiogenic layer and subsequent reconvergence of the cratonic segments led to widespread redistribution of radioactive elements into the reactivated inter rift crustal block one result would be that crustal temperatures are higher in that part of the churchill province than in the superior

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

docking carolina mid paleozoic accretion in the southern appalachians accretion of the extensive carolina zone to laurentia is one of the most critical unresolved problems in southern appalachian tectonics synthesis of southern appalachian tectonism reveals three lines of evidence from across the orogen that indicate a late ordovician to silurian time of accretion a tectonic unconformity in native laurentian rocks an extensive magmatic pulse in the piedmont zone of the orogen and active uplift in the carolina zone are all related to this accretionary event structural geometries in the carolina zone suggest that accretion involved sinistral transpression this new model for carolina accretion is consistent with models for the accretion of northern appalachian peri gondwanan terranes collectively their timing and mode of accretion to laurentia place tectonic constraints on paleozoic global reconstructions

triassic mineralization with cretaceous overprint in the dahu au mo deposit xiaoqinling gold province constraints from shrimp monazite u th pb geochronology abstract the xiaoqinling terrane in central china is the second largest gold producing area in china previous application of age constraints has yielded ages for the gold mineralization ranging from paleoproterozoic through triassic indosinian to jurassic and cretaceous yanshanian shrimp u th pb dating of hydrothermal monazite in a mo mineralized quartz vein from the dahu au mo deposit in the northern xiaoqinling terrane identifies two mineralization episodes hydrothermal monazite intergrown with molybdenite yields an initial precipitation age of 216 5 ma which is identical to but more precise than the molybdenite re os isochron age of 218 41 ma a younger generation of monazite is distinguished by data falling on a mixing array it is inferred that the younger monazite was formed at the expense of the older ones during the yanshanian probably during a single event that occurred at 125 ma combining these results with available geochronological data and geological evidence we conclude that molybdenum mineralization accompanied by monazite deposition took place in the indosinian followed by a yanshanian tectono hydrothermal event that resulted in large scale gold mineralization in the xiaoqinling terrane our observation demonstrates that monazite is sensitive to hydrothermal overprint and thus is useful for dating fluid infiltration event

age trends in garnet hosted monazite inclusions from upper amphibolite facies schist in the northern grouse creek mountains utah we performed in situ th pb dating of monazite in upper amphibolite facies pelitic schist from the grouse creek mountains in northwest utah sixty six ages from inclusions in four garnet grains range from 37 to 72 ma and decrease with radial distance from garnet cores the age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions 25 58 ma the monazite grains are not intersected by cracks in the garnets through which dissolution reprecipitation or pb loss might occur and are generally too small 20 lm to allow for more than one age determination on any one grain processes that might explain inclusion ages that decrease with radial distance from garnet cores include 1 pb diffusion in monazite 2 dissolution and reprecipitation of monazite and 3 co crystallization of monazite and garnet after consideration of these possibilities it is concluded that the co crystallization of monazite and garnet is the most plausible with monazite neoblasts deriving rees from the breakdown of muscovite garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m y

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

continuation of heat flow data a method to construct isotherms in geothermal areas a continuation technique for conductive heat flow in a homogeneous isotropic medium is presented which utilizes observe surface heat flow data the technique uses equivalent point sources and is developed for transient or steady state conductive heat flow problems for a homogeneons half space with plane surface and a surface with topographic relief the technique is demonstrated by comparison with a steady state fault model and the terrain correction problem it is also compared to observed heat flow data in two geothermal areas marysville montana and east mesa imperial valley california calculated subsurface temperature distributions are compared to analytical models and the results of geophysical studies in deep drillholes in geothermal systems even in geothermal systems where convection is involved in the heat transfer the boundaries of the reservoir associated with the convective system can be treated as a boundary condition and the depth and shape of this boundary can be calculated since

new heat flow density and radiogenic heat production data in the canadian shield and the quebec appalachians new heat flow density hfd measurements were performed at 10 sites in quebec for five of the sites located in the superior province the heat flow density varies between 24 and 35 mw m2 26 and 37 mw m2 after adjustment for pleistocene climatic variations in the grenville province the values obtained range between 25 and 28 mw m2 29 and 31 mw m2 after adjustment for two nearby sites in the gaspe region appalachians the heat flow density is 47 mw m2 48 mw m2 after adjustment radiogenic heat production was also measured at the sites located in the meta volcano sedimentary sequences of the superior province the heat production is low 0 1 0 6 w m3 and it does not always correlate with the surface heat flow in the grenville province the hfd is close to slightly higher than the reduced heat flow of the superior the higher hfd in the appalachians is partly explained by the higher crustal heat production and partly by higher deep heat flow

uraniferous gorceixite in the south carolina coastal plain u s a abstract gorceixite a rare ba al phosphate containing 200 2000 ppm u occurs as thin seams or nodules in upper eocene deposits of the coastal plain province in aiken county south carolina at every locality gorceixite appears to be a post depositional feature radiochemical results for various daughter parent pairs in the 238u decay series show that uranium enrichment occurred at least 1 ma ago and there has been little uranium migration since there has been extensive recent redistribution of 226ra relative to 230th and 210po with 226ra suddenly migrating from the gorceixite into the overlying sediments within the last one hundred years the clay mineral and elemental distributions through a gorceixite outcrop suggest that gorceixite formed as a result of intensive leaching by acidic groundwater during development of a paleo soil horizon the sources for the components of gorceixite may have been leached from local sedimentary deposits derived from the barite monazite and granitic occurrences in the adjacent piedmont rocks this model provides for the predominance of gibbsite above and kaolinite below the gorceixite mechanisms for phosphate fixation and uranium enrichment depletion of the more soluble elements above the gorceixite and the thinness and limited areal extent of known occurrences

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

metasomatic thorite and uraninite inclusions in xenotime and monazite from granitic pegmatites hidra anorthosite massif southwestern norway mechanics and fluid chemistry thorite and uraninite inclusions are documented in metasomatized regions in three xenotime crystals and one monazite crystal collected from three granitic pegmatites hosted within an anorthosite intrusion located on the island of hidra southwestern norway formation of these inclusions is demonstrated to be the result of dissolution reprecipitation processes that occurred in an effectively closed chemical system the reaction was initiated by interaction between the early crystallizing orthophosphate minerals and the evolving pegmatite fluid this fluid was dominated by h 2 o but also contained f and minor amounts of cl and significant quantities of dissolved na and k this is an example of auto metasomatism and highlights a natural case where th u si enriched orthophosphate minerals have been partially altered such that the th u and si have been converted to secondary inclusions within the now th u si depleted orthophosphate via reaction with fluids rich in alkali elements

detection of inherited monazite in the manaslu leucogranite by 208pb232th ion microprobe dating crystallization age and tectonic implications abstract although leucogranites are among the least petrologically variable of all igneous rocks ironically they are among the most difficult to reliably date the high himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelationship with the most significant tectonic features of that mountain belt for a variety of reasons linked to the minimum melt composition of these leucogranites these dating studies have not been entirely successful we report results of a new ion microprobe dating method based on the decay of 232 th to 208 pb in monazite that has directly revealed the presence of inherited pb in monazite from the manalsu granite casting doubt on its previously accepted age monazite ages obtained from this leucogranite yield two distinct populations a large number of ages with a normal distribution and mean age of 22 4 0 5 ma 2 s e that we interpret to be the crystallization age and a smaller inherited fraction with an age of ca 600 ma because formation of the granite is thought to be related to slip on the main central thrust both this date and a second less precise result from a structurally similar pluton near mt everest indicate that the main central thrust was active in the interval 24 22 ma but do not constrain its initiation together with crosscutting relationships these data require that movement on the north himalayan fault occurred prior to 22 ma at both locations

some new measurements of heat flow in the superior province of the canadian shield borehole heat flow measurements are reported from six new sites in the superior province of the canadian shield values adjusted for glaciation effects but not for holocene climatic variations range from 42 to 56 mw m2 when these new values are combined with 21 previously published borehole values the mean is 42 mw m2 with a standard deviation of 11 mw m2 the data for a site on the lac du bonnet batholith suggest that the batholith has a thin veneer less than 3 km of rock of high radiogenic heat production at the surface

geodynamic evolution of the central appalachian orogen geochronology and compositional diversity of magmatism from ordovician through devonian

monazite ages and the evolution of the menderes massif western turkey the menderes massif experienced polyphase deformation but distinguishing pan african events from alpine tectono metamorphic evolution and discriminating eocene oligocene shortening from recent extension remain controversial to address this monazite in garnet bearing rocks from the massif s gordes central and cine sections were dated in thin section in situ using the th pb ion microprobe method cambro ordovician monazite inclusions in cine and central menderes massif garnets are 450 m y older than matrix grains monazites in reaction with allanite from the kuzey detachment which bounds the northern edge of the central menderes massif are 17 5 ma and 4 5 1 0 ma the pliocene result shows that dating of monazite can record the time of extension the kuzey detachment might have exhumed rocks a lateral distance of 53 km at a rapid rate of 12 mm year assuming the present 20 ramp dip pliocene monazite crystallization at 450 c and a geothermal gradient of 25 c km assuming an angle of 60 the rate decreases to 5 mm year with the detachment surface at 21 km depth in the pliocene two gordes massif monazites show a similar allanite monazite reaction relationship and are 29 6 1 1 ma and 27 9 1 0 ma suggesting that the cenozoic extension in the gordes massif and possibly the entire menderes massif might have begun in the late oligocene

la ce dating of lewisian granulites to constrain the 138la β decay half life the isotope 138la makes up 0 089 of natural lanthanum and exhibits a branched decay by to 138ce and electron capture to 138ba the la ce isotope scheme is a potentially powerful adjunct to the sm nd method for petrogenetic studies but the best determination by counting of the decay half life of la 3 02 x 1011 yr 1 is in conflict with geological half life determinations based on la ce and sm nd mineral isochrons2 4 to test these alternatives i determined a six point la ce isochron on whole rock samples of lewisian granulite facies archaean gneiss from nw scotland here i report based on a half life of 3 02 1011yr a la ce age of 2 990 400 myr 2 which falls very well within the error of a whole rock sm nd date on lewisian granulites of 2 895 100 myr 2 this half life equivalent to a la decay constant of 2 30 10 12 yr 1 is therefore recommended for geological application of the ce isotope method

constraints on the genesis of gold mineralization at the homestake gold deposit black hills south dakota from rhenium osmium sulfide geochronology the homestake gold deposit located in the black hills south dakota usa is one of the largest known hydrothermal gold deposits globally with total mining production exceeding 40 moz au rhenium osmium geochronology of ore associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern black hills arsenopyrite yields a rhenium osmium re os age of 1 736 8 ma mean squared weighted deviation 1 6 consistent with existing age constraints for gold mineralization whereas re os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age this is taken to indicate that the re os arsenopyrite chronometer is robust to at least 400 c whereas the re os pyrrhotite chronometer is likely disturbed by temperatures of 300 350 c the re os arsenopyrite age and initial os ratio 0 28 0 15 are interpreted to indicate that gold was introduced at ca 1 730 ma coincident with the onset of exhumation of crustal blocks and possibly the earliest intrusive phases of harney peak granite magmatism new in situ u pb monazite analyses from an aplite dike in the east central black hills indicate that granite magmatism was a protracted event persisting until at least ca 1 690 ma

reconnaissance fluid inclusion study of the morefield pegmatite amelia county virginia over 90 of the fluid inclusions in quartz topaz and beryl from the morefield pegmatite amelia co virginia contain mixed co 2 h 2 o fluids in order of abundance there are four inclusion types 1 three phase l l v 2 two phase h 2 o rich 3 three or four phase l v s and l l v s and 4 two phase co 2 rich co 2 rich inclusions formed by necking of inclusions containing mixed fluids topaz and quartz contain all four inclusion types and all types occur throughout the pegmatite although no h 2 o rich inclusions occur in beryl quartz from gneiss that hosts the pegmatite contains dominantly h 2 o rich inclusions most inclusions are secondary and occur along healed fractures possible primary inclusions occur only among those that contain solids all inclusions homogenize into the liquid phase a few homogenize to liquid co 2 h 2 o rich inclusions with 2 o rich and mixed fluid inclusions with 3 7 wt nacl eq and homogenization temperatures 290 degrees c contain fluids interpreted to derive primarily from the pegmatite inclusions with salinities between 3 13 wt nacl eq and intermediate homogenization temperatures are in part cooled equivalents of the pegmatite fluids but the origin of those with salinity 7 wt nacl eq is uncertain homogenization temperatures for co 2 indicate average co 2 densities near 0 65 g cm 3 for mixed fluids homogenizing near 300 degrees c and this co 2 density the trapping pressure is estimated to be 2 5 kb neither age relations nor physical proximity tie the morefield pegmatite to known granitic intrusions fluid compositions preclude a genetic relation between the pegmatite and its host gneisses

low temperature alteration of monazite fluid mediated coupled dissolution precipitation irradiation damage and disturbance of the u pb and th pb chronometers abstract low temperature alteration of monazite is documented in three centimeter sized monazite crystals from norway arendal madagascar ambato and sri lanka the three crystals have different chemical compositions especially in their u th y and pb contents and have 208 pb 232 th ages ranging from 491 to 900 ma optical microscope om scanning electron microscope sem and transmission electron microscope tem images and electron microprobe analyses epma show that all three preserve a similar patchy texture this texture is interpreted as resulting from an alteration reaction in which unaltered monazite mnz1 reacts to form a secondary th u y depleted high th u monazite mnz2 accompanied by thorite huttonite thsio 4 thorianite tho 2 and xenotime ypo 4 the proportions of which are dependent upon the initial composition of the monazite mnz1 images reveal variably intense internal fracturing with cracks filled with th rich fe rich phases monazite xenotime thermometry demonstrates that the pristine monazites mnz1 interacted with a low temperature fluid the alteration process is interpreted to follow a mechanism of fluid mediated coupled dissolution precipitation chemical dating with the electron microprobe shows no th u pb age differences between primary and secondary monazites except in the case of the ambato monazite in which altered domains yield older 750 ma apparent ages than the pristine mnz1 domains u pb and th pb isotope dating using la icp ms yields ages consistent with electron probe dates for pristine mnz1 zones however disturbance of these systems in the altered monazite domains leads to variable age results for these depending on sample in the case of sri lanka and arendal only 208 pb 232 th dates provide a reasonable estimate of the age of alteration which are constrained to be 450 and 864 ma respectively u pb systems are disturbed due to common pb contamination up to 40 and u fractionation relative to th during alteration responsible for depletion of u in altered monazites and increase of th u in contrast for the ambato monazite both the u pb and th pb systems were affected and yield inconsistent older dates for altered zones this is attributed to significant common pb contamination up to 80 which affects all pb isotopes and explains why electron probe ages are erroneous th u silicate contamination during measurement resulting from the presence of a numerous nano phases and nano fractures filled with th u silicates that are visible only under tem also contributes to the anomalously old ages for these disturbed mnz2 domains these results demonstrate the important role of radiation damage effects in particular swelling induced fracturing and the essential role of porosity and cracks which allow fluid charged with elements migration through monazite during low temperature alteration

fluid induced dissolution breakdown of monazite from tso morari complex nw himalayas evidence for immobility of trace elements primary igneous monazite from the polokongka la granite of the tso morari complex in the western himalayas has been partially replaced by a three layered corona of metamorphic fluor apatite allanite u and th bearing phases huttonite brabantite and epidote the alteration is related to high pressure amphibolite facies 10 11 kbar and 587 695 c fluid induced retrogression of the ultra high pressure granite during exhumation after india asia collision the corona textures can be explained by pseudomorphic partial replacement of the original monazite to apatite and allanite via a fluid mediated coupled dissolution reprecipitation process mass balance calculations using the volume proportions and compositions of coronal minerals show that the ree u th pb ba and p were conserved and not transported outside the alteration corona the formation of fluor apatite allanite huttonite and coffinite from monazite and the immobility of ree u and th require an influx of alkali and f bearing ca rich fluid having high ca na into the corona we are aware of only two other occurrences of such alteration textures and these have several similarities in terms of geodynamic setting and p t histories of the host rocks we suggest that there may be a common mechanism of exhumation style and source and composition of fluids during retrogression of granitoid rocks in collisional orogens and that such breakdown textures can be used to identify metagranites that have experienced high p metamorphism in continental collision zones which is otherwise difficult to constrain due to the high variance of the mineral assemblages in these rocks

postorogenic alkali feldspar granite and associated pegmatites in west avalonia the petrology of the neoproterozoic georgeville pluton antigonish highlands nova scotia the 579 8 2 2 ma 40ar 39ar muscovite georgeville pluton in mainland nova scotia is an epizonal body consisting of alkali feldspar granite and related pegmatite the pluton intrudes the ca 619

northern ancestry for the goochland terrane as a displaced fragment of laurentia the ancestral position of the goochland terrane an isolated block containing mesoproterozoic crust in the appalachian piedmont of eastern virginia during grenvillian orogenesis has direct bearing on the breakup of rodinia ages lithology geochemistry and pb and nd isotope compositions of the montpelier anorthosite and state farm gneiss of the goochland terrane allow correlation with the grenvillian roseland anorthosite and associated igneous suites of the blue ridge farther west in virginia in contrast extensive metapelites which underwent devonian metamorphism and intrusion and amphibolites of the goochland terrane lack equivalents in the blue ridge dextral slip faults which bound the goochland terrane as well as dextral faults farther north suggest large scale translation prior to late paleozoic reaccretion ages of extension related a type granitoids within the virginia blue ridge 735 680 ma suggest physical separation from compositionally similar granitoids in the goochland terrane 630 588 ma reading prong 602 ma and manhattan prong 563 ma pre iapetan restoration of the goochland terrane northeastward of the blue ridge and outboard of the reading prong accounts for its affinities to the blue ridge unique anorthosites manhattan prong abundant amphibolites and reading prong ca 600 ma magmatism thus similar to the translation of madagascar out of africa the goochland terrane was rifted from laurentia marooned in oceanic crust and then dextrally translated southward 500 km prior to late paleozoic reaccretion documentation of large scale dextral translation of eastern laurentia relative to its fragments and other cratons during the neoproterozoic and paleozoic may assist in locating far traveled pieces of the grenville orogen for reconstruction of rodinia

the relationship between ree y nb th minerals and the evolution of an a type granite wentworth pluton nova scotia the wentworth pluton in the eastern cobequid highlands consists principally of metaluminous to peralkaline a type granite 362 ma a large part of which was remelted by a major gabbro intrusion 357 ma magmatic minerals like allanite ce chevkinite ce zircon and hingganite y and post magmatic mineral phases such as ree bearing epidote samarskite y aeschynite y fersmite thorite like phases and hydroxylbastnasite ce were identified the presence of fluorine in the parental magma indicated by whole rock geochemical data and the presence of fluorite increased the solubility of monazite and xenotime and thus facilitated retainment of rare metals in the magmatic system fractionation of allanite ce and chevkinite ce led to a melt enriched in heavy rare earth elements hree from which hingganite y crystallized during late magmatic stages the remelting of the early granite led to fluorine and sulfur release in volatile phases which circulated with hydrothermal fluids thus mobilizing the ree and other rare metals reduction of fluorine activity during the late to post solidus crystallization resulted in the precipitation of hree and rare metals in samarskite y thereby enriching the residual hydrothermal fluids in light rare earth elements lree post magmatic lree minerals such as hydroxylbastnasite ce either replaced earlier minerals or precipitated from these hydrothermal fluids carbonate fluids involved in a late regional hydrothermal circulation event along the cobequid chedabucto fault 320 315 ma promoted ti mobility and the formation of tio 2 minerals and probably of aeschynite y this mineralogical diversity in addition to the complex geological history of the pluton provides a unique opportunity to correlate the formation of individual rare metal minerals to different stages of pluton evolution and thus provide an insight to the formation conditions of these minerals

x ray diffraction study of brabantite monazite solid solutions abstract we synthesized compounds with stoichiometry of natural brabantite m2 th po4 2 with m2 ca cd sr pb ba at 1 bar 1200 c and 2 5 kbar 700 c those compounds were studied by powder x ray diffraction and electron microprobe for ca sr and pb we obtained always crystals with the monazite structure for cd the monazite structure is obtained at 1 bar but not at 2 5 kbar for ba we obtained the monazite structure only at 2 5 kbar the unit cell parameters of the compounds with monazite structure vary regularly with the size of the m2 ion we studied also the solid solution lapo4 m2 th po4 2 at 1200 c 1 bar the solid solutions for cd ca and sr are continuous and the unit cell parameters vary linearly with the degree of substitution for pb the solid solution is continuous but the unit cell parameters do not vary linearly for ba there is a miscibility gap with a maximum of about 50 mol of bath po4 2 in lapo4 these results have several consequences for u th pb geochronology of monazite and for utilization of monazite as a nuclear waste ceramic

crystal chemistry of the monazite and xenotime structures monazite and xenotime the re p04 dimorphs are the most ubiquitous rare earth re minerals yet accurate structure studies of the natural phases have not been reported here we report the results of high precision structure studies of both the natural phases and the synthetic re p04 phases for all individual stable rare earth elements monazite is monoclinic p2 n and xenotime is isostructural with zircon space group 14 amd both atomic arrangements are based on 001 chains of intervening phosphate tetrahedra and re polyhedra with a reogpolyhedron in xenotime that accommodates the heavy lanthanides tb lu in the synthetic phases and a re09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements la gd as the structure transforms from xenotime to monazite the crystallographic properties are comparable along the 00i chains with structural adjustments to the different sizes of re atoms occurring principally in 00i there are distinct similarities between the structures that are evident when their atomic arrangements are projected down 001 in that projection the chains exist in 100 planes with two planes per unit cell in monazite the planes are offset by 2 2 a along 010 relative to those in xenotime in order to accommodate the larger light re atoms the shift of the planes in monazite allows the re atom in that phase to bond to an additional 02 atom to complete the re09 polyhedron

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

u pb shrimp monazite ages of the giant morro velho and cuiaba gold deposits rio das velhas greenstone belt quadrilatero ferrifero minas gerais brazil abstract u pb shrimp results of 2672 14 ma obtained on hydrothermal monazite crystals from ore samples of the giant morro velho and cuiaba archean orogenic deposits represent the first reliable and precise age of gold mineralization associated with the rio das velhas greenstone belt evolution in the quadrilatero ferrifero brazil in the basal nova lima group of the rio das velhas greenstone belt felsic volcanic and volcaniclastic rocks have been dated between 2792 11 and 2751 9 ma coeval with the intrusion of syn tectonic tonalite and granodiorite plutons and also with the metamorphic overprint of older tonalite trondhjemite granodiorite crust since cratonization and stable shelf sedimentation followed intrusion of neoarchean granites at 2612 3 2 ma it is clear that like other granite greenstone terranes in the world gold mineralization is constrained to the latest stages of greenstone evolution

epma u th pb monazite and shrimp u pb zircon geochronology of high pressure pelitic granulites in the jiaobei massif of the north china craton the jiaobei massif constitutes the southern segment of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton and contains high pressure hp mafic and pelitic granulites the hp pelitic granulites contain four distinct mineral assemblages garnet kyanite biotite muscovite rutile m1 garnet kyanite ternary feldspar biotite muscovite rutile m2 garnet sillimanite biotite perthite anti perthite m3 and biotite muscovite sillimanite plagioclase m4 of which m2 is a typical hp pelitic assemblage the origin of these hp granulites is important for understanding the tectonic evolution of the jiao liao ji belt but the timing of the hp event still remains unknown this study applied the epma u th pb monazite and shrimp u pb zircon dating techniques to determine the metamorphic ages of the hp pelitic granulites in the jiaobei massif cl images and th u ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin eighty two epma analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 ma 1814 to 1813 ma and 1706 to 1692 ma and the ages of 16 shrimp analyses on 14 zircon grains from one sample range from 1864 to 1803 ma the oldest ages of 1895 to 1882 ma were yielded from high y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite and thus interpreted as the time of the hp m2 granulite facies metamorphism the epma monazite ages of 1814 to 1813 ma and shrimp zircon ages of 1864 to 1803 ma are interpreted as the approximate ages of the post hp granulite facies metamorphism m3 most probably resulting from the exhumation of the hp granulites to the medium pressure granulite facies levels the youngest age group of 1706 to 1692 ma obtained from low y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism m4 that occurred when the hp pelitic granulites were exhumed to the upper crust the presence of paleoproterozoic hp mafic and pelitic granulites in the southern segment of the jiao liao ji belt suggests that the evolution of the jiao liao ji belt in the eastern block of the north china craton must have been involved in subduction or collision related tectonic processes

multi step tims and ca tims monazite u pb geochronology abstract multi step chemical abrasion thermal ionization mass spectrometry ca tims methods were developed for monazite using six samples that vary in composition and age amelia jefferson county burke elk mountain 554 and madagascar to evaluate whether the multi step ca tims approach reveals complexities in either age or composition that might be masked by single step analysis this study presents a side by side comparison of monazite samples dated via multi step tims not annealed and ca tims annealed and measurement of rare earth element ratios for each dissolution step the data demonstrate three important contributions first annealing reduces solubility after one partial dissolution step not annealed fractions were 3 3 4 1 times more digested than the annealed fractions the difference in solubility suggests that monazite does not fully self anneal at low temperatures second multi step tims analyses yielded high precision u pb plateau ages for the burke and amelia monazites ca tims analyses yielded high precision u pb plateau ages for madagascar and amelia monazites jefferson county elk mountain and 554 yielded more complex results and no u pb plateau ages third chemical analysis of partial dissolution steps reveals heterogeneous age and compositional data for annealed samples because not annealed samples yielded more consistent age and compositional data high temperature annealing is not recommended for monazite instead optimal tims results are provided by slow partial dissolution of monazite in weak acid

two dimensional inverse modeling of crustal thermal regime with application to east european geotraverses the modeling of crustal thermal regimes is basically the inference of subsurface distributions of heat flow density and temperature by continuing downward the near surface measurements of heat flow density temperature thermal conductivity and heat production to account for the uncertainties in the near surface data and in the subsurface distributions of thermal conductivity and heat production and to provide explicit estimates for the uncertainties in the resulting models we have employed a bayesian inverse formulation that incorporates the a priori information on model parameters in the form of a joint gaussian probability density function and that gives the inverse solution in the form of the most probable estimates and the a posteriori covariance matrix for the model parameters to illustrate the method we apply it to the modeling of thermal regimes along east european geotraverses eegt 5 and eegt 1 using both a linear 5 point finite difference and a quadrilateral isoparametric finite element discretization two features are emphasized first we introduce an a priori autocovariance function for the heat flow density at model base to help stabilize the solution second we demonstrate the improvement in solution accuracy and resolution when a priori temperature information is available and incorporated in the inversion for the geotraverses studied the finite difference and finite element formulations yield essentially identical results although there is a substantial difference in the extent of their parameterization in general the surface and basal heat flow densities are similar in shape implying a relatively uniform total crustal heat production across the geotraverses both the heat flow densities and the crustal temperatures exhibit strong lateral variations that appear to correlate with the tectonic age the heat flow densities and temperatures are low in the precambrian east european platform relatively higher in the variscan units high in the alps and very high in the pannonian basin the results support the presence of lateral variations of up to 40 m w m 2 in the moho heat flow density provided that one accepts as reasonable an a priori standard deviation of 25 for the surface heat flow density and an uncertainty of a factor of two for the heat production finally the a posteriori and the observed surface heat flow densities match well but exceptions occur in localized regions suggesting the presence of perturbing effects that cannot be accounted for in a steady state purely conductive model

constraints on fluid evolution during metamorphism from u th pb systematics in alpine hydrothermal monazite abstract the timing and duration of hydrothermal activity during orogenesis are difficult to constrain because such systems are open and multistage using mm sized monazite crystals from two alpine clefts from the central alps switzerland we demonstrate that combined u th pb isotopic systematics of hydrothermal monazite can constrain both timing and fluid evolution during crystallization our data highlight four major characteristics of the cleft monazites i extreme th u ratios ii significant common pb in the u system up to 39 206 pb iii excess 206 pb up to 54 and iv precise and reliable th pb ages comparison of our results with literature data indicates that th u in monazite is a remarkable discriminant of geological conditions capable of distinguishing metasedimentary magmatic and hydrothermal origin hydrothermal monazite crystals are characterized by th u ratios up to 629 among the highest values ever reported extreme th u ratios in monazite enhance incorporation of 230 th a short lived intermediate product in the 238 u 206 pb decay chain generating 206 pb excess which if not accounted for results in 206 pb 238 u ages that are too old for example in the samples investigated here by as much as 300 for the two zoned monazite crystals studied in detail 208 pb 232 th ages are reliable for the different compositional domains visible in back scattered electron bse images 232 th 208 pb ages for the older domains 15 2 0 3 ma and 14 1 0 3 ma respectively are consistent with the structural relationships of the hydrothermal veins indicating early retrograde genesis the youngest rim age at 13 5 0 4 ma for the blauberg crystal marks termination of monazite precipitation the resolvable age difference is interpreted to reflect pulsed monazite growth over an extended period of hydrothermal activity in both crystals the outer rims have the highest 206 pb excess confirming a two stage crystallization two scenarios are envisaged to account for the 206 pb excess evolution in the first increasing 206 pb excess is caused by a modification of the th u fractionation between monazite and fluid due to an evolution of the fluid towards more oxidizing conditions that favor partitioning of hexavalent u into the fluid alternatively it is possible that 230 th increased with time in the fluid in this second case monazite growth would have occurred in a closed system from a fluid that was initially in disequilibrium with the 238 u 206 pb decay chain and progressively equilibrated t

early diagenetic ree phosphate minerals florencite gorceixite crandallite and xenotime in marine sandstones a major sink for oceanic phosphorus

downward continuation of heat flow data method and examples from the western united states we outline a method to determine directly from the data a subsurface temperature distribution which is compatible with the surface heat flow and takes into consideration the effect of the shallow heat sources the analysis assumes that 1 the crust is in conductive equilibrium 2 the lateral changes in thermal conductivity can be neglected and 3 the depth of the heat sources is proportional to the wavelength of the variations in surface heat flow we apply the technique to determine the crustal temperatures in two tectonically active regions of the western united states in the rio grande rift temperatures higher than 1 100 c are predicted at a depth of 20 km in the transition region between the colorado plateau and the basin and range the variability in surface heat flow is partially accounted for by shallow heat sources however the increase in heat flow toward the basin and range seems associated with deep seated i e lower crust or upper mantle variations in the thermal regime

understanding geologic processes with xenotime composition chronology and a protocol for electron probe microanalysis abstract xenotime ypo 4 is iso structural with zircon group minerals and lies at one end of a compositional solid solution with monazite as with monazite and zircon naturally occurring xenotime may accommodate significant concentrations of rare earth elements as well as th and u making it an important repository for these valuable geochemical marker elements and a potential geochronometer xenotime grows in sediments during diagenesis in low to granulite grade metamorphic rocks in migmatites and is common in peraluminous igneous rocks xenotime can be a complement or indeed an alternative to monazite and or zircon in geochemical and geochronological studies to maximise the petrographic potential of xenotime a complete understanding of its composition and relationship to rock forming assemblages is essential electron beam techniques provide micron scale resolution of compositional and textural variation and yield precise dates for individual micro volumes in multi domain grains methods for collecting trace element data by electron probe microanalysis differ from those routinely employed to collect major element concentrations careful characterisation of background curvature and background interferences around the th u and pb peaks is required to avoid large overlap correction for y l 2 3 on pb m analysis of pb is made on the m peak a broad array of geochemical and geochronological applications can take advantage of the links between xenotime composition and texture and the evolution of silicate mineral assemblages in a diversity of environments one example of metasomatised xenotime from a pegmatite in sw norway demonstrates a practical application to dating geologic processes as well as the potential limitations that arise in compositionally heterogeneous xenotime

northern ancestry for the goochland terrane as a displaced fragment of laurentia comment and reply comment the recent article by bartholomew and tollo 2004 1 concludes that the enigmatic goochland terrane is a displaced laurentian fragment in their model the goochland terrane originated in the new york promontory rifted away from laurentia during the formation of iapetus and was subsequently

versatile monazite resolving geological records and solving challenges in materials science generalizations about monazite implications for geochronologic studies with the advent of techniques that preclude mineral separation ages from specific compositional domains in monazite ce la th po 4 have provided a wealth of information regarding the timing of the geologic evolution of numerous regions however confusion can arise when single grains show large differences in age that fail to correlate to chemistry or location within the monazite generalizations that lead to incorrect age interpretations include that monazite zoning in y th and or the rare earth elements ree always identify 1 distinct tectonic events 2 environment of crystallization and 3 provenance of detrital grains increasing th contents in monazite do not always reflect 1 increasing grade in metamorphic grains 2 changes in silicate melt composition in igneous grains 3 make the mineral more susceptible to alteration nor 4 control the mineral s uptake of ree metamorphic monazites from himalayan garnet bearing rocks with coexisting allanite show no relationship between th content and ree instead chondrite normalized ree patterns of the allanite mirror those of the monazite indicating the variations are related to the reactant that formed the mineral generalizations about pb behavior in monazite remain problematic incorporation of pb into monazite has thus far been precluded by experimental studies yet common pb has been measured in many studies of natural monazite a clear understanding about controls of monazite composition and the role of the chemical and or pressure temperature p t environment of the rocks in which it forms is required to correctly interpret the meaning of the mineral s age s

downward continuation of heat flow data abstract downward continuation operators are derived and used to calculate the subsurface temperature from surface heat flow data if the distribution of the shallow heat sources were known their effect on the temperature field could be included in the downward continuation operator the distribution of heat sources with depth can be estimated from the fourier spectral content of the heat flow data but the relationship is non unique several operators have been derived with different assumptions regarding the relationship between the wavelength of the surface heat flow variations and the depth of the corresponding heat sources these downward continuation operators were applied to determine the subsurface temperature and the deep heat flow beneath the colorado plateau the calculations show that the temperature in the crust beneath the plateau is lower than in the surrounding basin and range and rio grande rift provinces however the temperature is locally higher in regions of recent tectonic or magmatic activity the same downward continuation operators can also be used to model the effect of hydrothermal convection on the temperature field an example is presented where closely spaced heat flow data from the juan de fuca ridge have been used to calculate the crustal temperature and to estimate the heat flow below the convecting layer

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets