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

early permian peak metamorphism recorded in u pb system of black slates from the ogcheon metamorphic belt south korea and its tectonic implication abstract we report pb pb whole rock and uraninite chemical th u total pb isochron method chime ages of carbonaceous black slates from the ogcheon metamorphic belt south korea the pb isotopic data of whole rock samples yield 207 pb 206 pb ages of 283 33 and 291 13 ma for two outcrops from the northeastern part of the belt the uraninite chime age is estimated at 283 26 and 281 27 ma for the northeastern and the middle part of the belt respectively all of the above ages are identical within error ranges and represent the timing of peak metamorphism after the late precambrian intraplate rifting on the other hand the 207 pb 206 pb whole rock age for the southwestern part of the belt is estimated at 194 27 ma probably representing the timing of contact thermal metamorphism associated with the intrusion of jurassic granitic plutons rb sr isotopic data of the black slates do not define any meaningful isochron the early permian metamorphic age of this study does not support any tectonic scheme in favor of major tectonometamorphism at either the silurian devonian or the triassic time instead it corroborates the probability that the two zones in the ogcheon belt the ogcheon metamorphic belt and the taebaegsan zone were separated from each other before the development of major structural framework in the former our data do not support an idea that the ogcheon belt corresponds to the continuation of the triassic collision belt in east central china

nondestructive chemical dating of young monazite using xrf 1 design of a mini probe age data for samples from the central alps and comparison to u pb tims data the analytical capabilities of an improved mini xrf instrument for trace element analysis are explored to obtain u th pb ages for single monazite grains as small as 50 m the detection limits for u th and pb with counting times of 10 min are 10 ppm counts collected for 40 min are sufficient to date single monazite grains as young as 15 ma the precision of the age data depends essentially on the counting statistics of the main x ray peak npb l 1 2 used to analyze for lead the age uncertainty is proportional to the age of the grain the total contents of th and u and the squared integration time the reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central alps for which the chemical ages are compared to mass spectrometric u pb tims ages we show that populations of analyses obtained using the new instrument on single grains as young as 25 30 ma agree within error with the tims data for monazite separates with complex age patterns such as polymetamorphic samples yielding discordant isotopic ages the statistical analysis of chemical age data obtained from populations of 26 50 grains is advantageous in that it may provide additional insight into the reasons for the discordancy chemical th u pb dating of monazite by the xrf method offers considerably improved accuracy and precision compared with chemical dating by electron microprobe which typically becomes impractical for samples less than 100 200 ma in age conversely the minimum grain size required for the new instrument 50 m is well above the spatial resolution of the electron microprobe 2 m a companion paper scherrer et al this volume describes further developments towards microanalysis by xrf with improved spatial resolution

paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

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

corrections to heat flow and energetics of the san andreas fault zone and some additional comments on the relation between fault friction and observed heat flow

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

monazite age constraints on the tectono thermal evolution of the central appalachian piedmont the central appalachian piedmont lies in the critical juncture between the northern and southern appalachians portions of the orogen with distinct middle to late paleozoic accretionary histories orogen scale compilation maps link the central and southern appalachians but until recently limited geochronological data prevented robust tectonic comparisons between high grade metamorphic rocks in different parts of the orogen we report the results of in situ u th total pb monazite geochronology that date significant deformation and metamorphism as middle silurian 425 ma through middle devonian 385 ma and demonstrate the diachronous nature of orogen development the rosemont shear zone is identified as a major tectonic boundary in southeastern pennsylvania and northern delaware separating the rifted laurentian margin from younger rock units that formed in a magmatic arc setting the laurentian margin rocks occur in a series of nappes in which the metamorphic grade decreases from the structurally highest nappe to the lowest the in situ monazite ages show that maximum temperature in the lowest nappe may have been attained some 15 million years after maximum temperature in the highest nappe we interpret this to be the result of successive nappe emplacement with the warmer overriding sheets contributing heat to lower levels combining geochronologic and thermobarometric results with the geometry of deformation results in a new picture of the tectonic development of the central appalachian piedmont that further links the evolution of the southern and northern appalachians for the laurentian margin rocks tectonism resulted from the approach and collision of peri gondwanan terranes during the silurian to early devonian in a dominantly sinistral transpressive tectonic regime this portion of the pennsylvania delaware piedmont inboard of the rosemont shear zone is contiguous with comparable rocks in the southern appalachians in contrast arc related rock units outboard of the rosemont shear zone experienced primarily thermal metamorphism in the silurian while crustal thickening and associated regional metamorphism is middle devonian in age and likely the result of the accretion of avalonia during the acadian orogeny these arc related and younger rocks probably originated to the north of their present location as part of the northern appalachians they were ultimately emplaced in a right lateral transcurrent regime sometime after the middle devonian thus it is in this portion of the central appalachian piedmont that the northern and southern appalachians are joined

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

monazite geochronology of uhp and hp metamorphism deformation and exhumation nordoyane western gneiss region norway u th pb monazite geochronology is combined with previous structural analysis and quantitative estimates of metamorphic conditions to date the thermotectonic evolution of uhp and hp plates 820 c 39 kbar 780 c 18 kbar metamorphosed during the late silurian early devonian collision between baltica and laurentia the upper plate contains a microdiamond bearing kyanite garnet graphite gneiss and associated kyanite eclogites independently indicating probable diamond forming conditions in situ dating of monazite in the microdiamond sample using the shrimp ii at the geological survey of canada yielded ages of 415 6 8 ma for those included in garnet and 398 6 ma for those in the matrix these ages compare to 408 0 5 6 and 397 5 4 4 ma determined using the electron microprobe at the university of massachusetts both methods also identified complexly zoned detrital cores up to 150 micrometers in diameter with ages of 1100 950 ma and scattered grains with ages of 900 500 ma but no ages of 1680 1650 ma equivalent to the local baltica basement were found agreement between the two techniques allowed evaluation of monazite age domains 198 analyses from the microdiamond rock and a kyanite garnet sillimanite mylonite produced from it using a combination of high resolution element imaging and trace element analysis of u th pb and y this comparison yielded three mean ages of 407 0 2 1 ma 394 8 2 3 ma and 374 6 2 7 ma combining this geochronology with previous p t estimates we propose that the uhp unit reached its maximum depth of 125 km at a maximum age of 407 ma when monazite was included in garnet and experienced 65 km of exhumation at an average rate of 10 9 mm year during top southeast thrusting that brought it into contact with the hp unit following these events both units were exhumed together at an average rate of 3 8 mm year until reaching a depth of 37 km at 395 ma where these rocks experienced extensive re equilibration and top west and left lateral shearing after 395 ma these units continued to be exhumed at an average rate of 0 8 to 1 4 mm year until 375 ma the time of last equilibration of asymmetric monazite porphyroclasts in the mylonite the exhumation histories of these units record a change in mechanism from syncollisional exhumation through late to post orogenic collapse that was a consequence of plate reorganization

analytical perils and progress in electron microprobe trace element analysis applied to geochronology background acquisition interferences and beam irradiation effects electron probe microanalysis epma of accessory minerals such as monazite xenotime and thorite for minor and trace element concentrations and geochronology requires consideration of beam irradiation effects increasing heat and charge as higher current densities and lengthy counting times are employed and requires careful detailed assessment of background intensities and interferences a carbon coat 250 a thickness is generally inadequate for prevention of absorbed current s uctuation and beam damage when using the high current densities applied for high precision e g 200 na focused beam beam irradiation effects include element mobility in monazite resulting in p loss relative to ree coating materials of higher electrical and thermal conductivity are indicated and use of gold 100 a is strongly suggested systematic compositional and therefore age variability can result simply from analytical effects requiring evaluation of all aspects of data acquisition the spectra relevant to measurement of y th pb and u are complex especially in ree bearing minerals acquisition of detailed spectral wavelength scans allows recognition of background and peak interferences as well as curvature background intensities can be extracted directly from scan data by regression x ray mapping allows delineation of domains guiding background acquisition and detailed quantitative analysis minor substitution or s uorescence of unexpected elements can compromise analyses one documented effect being the s uorescence of k in monazite adjacent to or hosted by k feldspar or micas this effect clearly evident within 10 micrometers from k feldspar can result in erroneous u concentrations leading to misinterpretation of rim ages as younger overgrowths absorption edges associated with th also become relevant to the measurement of u at high th concentrations because background intensity is sensitive to variation in average atomic number backgrounds must be acquired from each identi ed domain with particular attention being paid to th variation misapplication of background intensities can result in large age discrepancies for example application of backgrounds obtained from a high th domain in monazite to measurement of a low th domain 1 3 of the amount in the high th domain results in an overestimation of the um background intensity of 0 008 cps na and an overall increase in age of 70 m y

transmission electron microscope study of polyphase and discordant monazites site specific specimen preparation using the focused ion beam technique electron microprobe emp u th pb dating on polyphase and discordant monazites from polymetamorphic granulites of the andriamena unit north central madagascar reveals inconsistent chemical ages to explain these drastic variations transmission electron microscopy tem foils were prepared directly from thin sections by using the focused ion beam technique the most important result of the tem study is the demonstration of the presence of small 50 nm pb rich domains where large variations in emp ages occur we suggest that radiogenic pb was partially reincorporated in monazite during the recrystallization at 790 ma because the excited volume of emp is 4 m3 u th pb dating yielded various apparent older ages without geological significance in addition tem analysis of the foils revealed the presence of an 150 nm wide amorphous zone along the grain boundary of monazite and its host quartz this fe si al rich phase may have formed as a result of fluid activity at 500 ma and the phase s amorphous state may be due to the irradiation from u and th decay in the monazite this demonstrates for the first time the enormous potential of the tem investigations on site specific specimens prepared with the focused ion beam technique for the interpretation of geochronological data

in situ trace element analysis of monazite and other fine grained accessory minerals by epma abstract trace element analysis of high z accessory phases by electron probe micro analysis offers unique access to data unattainable by other analytical techniques but also presents great challenges the intimate relationships between beam conditions voltage current spatial resolution precision and spectral complexity require the careful examination of all analytical parameters and their consequences at a level commensurate with the sensitivity of the desired analysis the high ree and actinide content and resulting high z of monazite allows high direct analytical resolution below 500 nm at 10 kv depending on the realized beam diameter due to limited electron range but the high concentration of these elements will also produce significant fluorescence at a distance the use of high accelerating potential in an effort to improve counting statistics is injudicious in most cases as direct analytical resolution decreases and high overvoltage on rees and th in monazite results in efficient production of energetic ionizing radiation l series capable of fluorescing elements of interest or interfering elements at remarkable distances at least several 10 s of microns from the beam boundary fluorescence is a very significant problem in trace element analysis and in geochronologic analysis of monazite as grains are commonly complex with micro domains that differ considerably in composition especially th ree in such instances a 15 kv beam can produce fluorescence at a distance leading to errors of 10 s of ppm within 5 m of a compositional boundary for trace element analysis effects such as background curvature and minor interferences can result in very large errors 50 or more for curvature alone for concentrations at or below 100 ppm strong background curvature is observed for all makes and varieties of spectrometers in the pb m region in addition for ree and actinide bearing phases such as monazite the wavelength regions available for background determination are very limited requiring routine high precision scanning and regression modeling to determine reasonably accurate background intensities two point background interpolation methods are inappropriate for high sensitivity analysis in all cases without a direct means to verify the accuracy of low concentration analyses great attention must be paid to analytical protocol and very detailed spectral analysis the use of consistency standards to monitor changes is important but caution must be exercised in the use of secondary age standards in the case of monazite epma because systematic error can result in cancellation effects when concentrations of both parent and daughter are proportionally incorrect in all cases caution in interpretation and integration of all available information is necessary if the potential of the method is to be realized

neoarchean arc magmatism and subsequent collisional orogenesis along the eastern rae domain western churchill province implications for the early growth of laurentia abstract the athabasca granulite terrane on the eastern margin of the rae domain of the western churchill province contains a well exposed record of neoarchean magmatism deformation and metamorphism circa 2 63 2 60 ga plutonic rocks range from 58 01 to 73 71 sio2 and vary from ferroan to magnesian they form shallow linear trends on modified alkali lime index diagram plotting largely in the calc alkaline field on tectonic discrimination diagrams all samples plot in the volcanic arc field samples are lree enriched and contain relatively steep negative slopes and flat depleted hree profiles when normalized to chondrites the entire neoarchean plutonic complex independent of lithology contains strong enrichments in the liles and pb and depletions in hfses monazite u th total pb results from a migmatitic paragneiss suggest that partial melting occurred at ca 2 57 ga within the chipman subdomain monazite results from the adjacent northwestern subdomain suggest that deformation occurred at ca 2 57 ga in sims and icp ms zircon u pb geochronology corroborates these conclusions although the region is dominated by paleoproterozoic localized shear zones internal to each subdomain is evidence of a widespread tectonothermal event during the neoarchean that involved arc plutonism crustal thickening lower crustal flow at pressures exceeding 1 0 gpa and extensive partial melting of the lower continental crust we interpret this sequence of magmatic deformational and anatectic processes to reflect the transition from a continental arc to a collisional tectonic setting the recognized series of events presented herein is likely correlative to the 2 60 2 50 ga macquoid orogen which has been recognized further to the northeast and likely parallels the eastern margin of the rae domain of the western churchill province these data support an important component of modern style plate tectonics during continental magmatism and accretion in the neoarchean

format and philosophy for collecting compiling and reporting microprobe monazite ages microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains down to 5 m or less in structural and petrologic context this paper presents an analytical strategy presentation format and error considerations for microprobe monazite dating the strategy involves high resolution compositional mapping to delineate compositional domains within monazite crystals then for each compositional domain a series of th u and pb analyses are made and a single date and error are calculated the number of analyses in each domain is determined by the desired statistical precision of the date results from several monazite grains are typically combined and along with textural relationships are used to build an argument that the dates constrain the age of a deformation or metamorphic event the total error involves three components short term random error dominated by counting statistical uncertainty short term systematic error uncertainty in background correction conductive coating variation and calibration and long term systematic error uncertainty in standard composition mass absorption factors decay constants etc in homogeneous compositional domains short term random errors 2 of less than 10 m y can be obtained from five to ten analyses however short term systematic error mainly background estimation uncertainty would typically result in a doubling of the magnitude of random error microprobe dates are presented as a single gaussian probability distribution for each domain along with representative compositional maps it is recommended that a consistency standard be analyzed during each analytical session and the results be reported along with those from the unknown this proposed strategy and format are compatible with those of other geochronological techniques they incorporate analytical limitations associated with trace as opposed to major element microprobe analysis and will allow better comparisons to be made between labs and between different geochronological techniques

nondestructive chemical dating of young monazite using xrf 2 context sensitive microanalysis and comparison with th pb laser ablation mass spectrometric data a newly developed xrf microprobe at the institute of mineralogy and petrology university of bern switzerland has been applied for precise chemical th u pb dating of individual monazite grains separated from pb free polished petrographic thin sections the nondestructive nature of the xrf measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis empa chemical dating by xrf microprobe analysis and isotopic 208pb 232th dating by laser ablation plasma ionisation multi collector mass spectrometry la pimms analysis as an example the 2 precision achieved with the xrf microprobe for well characterised reference material monazite fc 1 tims age 54 3 1 ma xrf age 55 3 2 6 ma doubly polished to 30 m in thickness is below 5 after 90 min integration time 50 kv 30 ma at a spatial resolution of 90 m at 38 m spatial resolution the uncertainty is 35 for the same integration time the sample characteristics are 200 300 ppm of pb xrf 3 8 5 1 wt of th empa and 0 4 1 4 wt u empa combined with an electron microprobe and conventional optical microscopy the xrf microprobe is thus a competitive low cost and nondestructive alternative to more costly isotopic methods the xrf microprobe is easy to use and maintain

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

an improved u th pb age calculation for electron microprobe dating of monazite abstract determination of u th pb ages using the electron probe microanalyser epma is an inexpensive alternative method for dating monazite the method is rapid and reliable both for simple monogenenetic monazite and for complex polygenetic monazite having undergone metamorphic events involving fluid interaction and recrystallization the main limitation of the method is its rather poor precision i e 45 to 120 ma for ages ranging from 300 to 3000 ma calculated on each spot the precision is limited by pb content and by counting statistics which cannot give precision better than 2 for individual determinations on u th and pb even at high levels a procedure that uses the new th pb vs u pb diagram to improve the calculated precision on u th pb ages gives results to within 5 to 15 ma for ages ranging from 300 to 3000 ma with complex polygenetic monazite in which either the points show large scattering indicated by a large mswd or the regression line exhibits a slope very different from neighbour theoretical isochrons the procedure must be applied separately on homogeneous domains only this makes it possible to distinguish between events separated by a gap of 20 to 60 ma according to the range of ages concerned i e 300 to 3000 ma several examples are given to illustrate these systematics

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

a three dimensional method for calculating independent chemical u pb and th pb ages of accessory minerals abstract previous theoretical considerations on the chemical u th total pb dating method failed to distinguish between thorogenic and uranogenic lead however it can be shown that the data points are located on a plane in the three dimensional tho2 pbo uo2 space the calculation of the best fit plane yields a slope in the tho2 pbo and uo2 pbo coordinate projections and an initial pbo value from the two slopes th pb and u pb ages can be calculated independently the method described in this paper is applied to monazites from the hercynian g4 granite of the fichtelgebirge germany th pb and u pb ages were calculated at 323 20 ma and 304 15 ma respectively the intercept value close to zero indicates that no significant amounts of common lead are present in the monazites studied

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

microprobe monazite geochronology putting absolute time into microstructural analysis abstract high resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains but more importantly its in situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on p t d paths constrain the rates of metamorphic and deformational processes and provide new links between metamorphism and deformation microprobe geochronology is particularly applicable to three persistent microstructural microtextural problem areas 1 constraining the chronology of metamorphic assemblages 2 constraining the timing of deformational fabrics and 3 interpreting other geochronological results although some monazite generations can be directly tied to metamorphism or deformation at present the most common constraints rely on monazite inclusion relations in porphyroblasts that in turn can be tied to the deformation and or metamorphic history microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process resulting in a new level of integration of time t into p t d histories

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

electron microprobe age mapping of monazite high resolution x ray maps of th u pb and y in monazite can be used to construct age maps which reveal the continuous spatial distribution of ages in a single grain of monazite the age mapping algorithm and three examples are presented to illustrate the capabilities and applications of this mapping technique the insights it can provide into monazite geochronology in general and its limitations compared to electron microprobe emp quantitative dating and other in situ geochronologic techniques age maps offer critical information for unraveling metamorphic and tectonic histories and for interpreting results from other geochronologic techniques and they are a valuable aid for rigorously locating in situ analytical points age mapping also can be used to better understand the behavior of the u th pb system in monazite during metamorphism deformation and s uid circulation events age maps presented in this paper reveal unsuspected age heterogeneities on the micrometer scale like a now healed fracture not visible in back scattered electron bse images or young domains less than 5 m in width located inside an older core in both cases using age maps as a template for locating in situ analysis points will minimize the peril of age mixing and erroneous geological interpretations in addition to providing critical information for illustrating and interpreting the history of complex polygenetic monazite age mapping may ultimately lead to a better understanding of the processes involved with monazite growth and recrystallization and thus even more powerful applications of the monazite geochronometer the agemap program is available in a windows version and can be downloaded from the internet at the following address http www geo umass edu probe agemap

epma and pixe dating of monazite in granulites from stary gieraltow ne bohemian massif poland abstract chemical th u total pb chime dating of monazite by electron probe microanalyzer epma and proton microprobe pixe was carried out on felsic granulites from stary gieraltow poland which represent part of the orlica nieznik dome in the ne bohemian massif analyzed monazite is characterized by mosaic zoning rather than simple core to rim growth and strontium contents of up to 750ppm an isochron age of 347 13ma represents timing of amphibolite facies metamorphism in agreement with previously published estimates

smith river allochthon a southern appalachian peri gondwanan terrane emplaced directly on laurentia the smith river allochthon is in a southern appalachian belt of metaclastic rocks that has traditionally been considered to be of peri laurentian origin new th u pb monazite ages confirm that the allochthon was involved in an early cambrian tectonothermal event indicate that it was exotic with respect to adjacent laurentian rocks and indicate that it is likely of gondwanan origin the allochthon may form a new link between the appalachians and the pampean terrane of western south america in addition its position in the orogen has implications for recent models of the opening of the iapetus

denudation history of the high t p ryoke metamorphic belt southwest japan constraints from chime monazite ages of gneisses and granitoids chime chemical th u total pb isochron method monazite ages were determined for gneisses and granitoids from the eastern and western parts of the ryoke belt separated by about 500 km the monazite ages for the gneisses are concentrated between 102 and 98 ma and are interpreted as the time of monazite formation under lower amphibolite facies conditions the peak metamorphism seems to be contemporaneous with the emplacement of the geologically oldest plutons that are dated at c 95 ma in both the eastern and western parts in the eastern part plutonism continued from c 95 ma to c 68 ma at intervals of 2 10 ma whereas in the western part it ceased at c 85 ma the chime monazite ages agree well with the relative age of granitoids derived from intrusive relationships of granitoids in both parts these lines of evidence are incompatible with a current view that the plutonometamorphism in the ryoke belt becomes younger towards the east the chime monazite ages coupled with available data on the depth at which the ryoke metamorphism took place and the emplacement of individual plutons show that the western part was eroded more rapidly about 1 5 mm year 1 than the eastern part about 0 8 mm year 1 over the time span from 91 to 85 ma the denudation rates agree well with those in active orogenic belts like the alps and himalayas

yttrium zoning in garnet coupling of major and accessory phases during metamorphic reactions the concentration of yttrium in pelitic garnets as a function of metamorphic grade has been examined in relation to the distribution of xenotime ypo 4 in samples from new england and british columbia samples with xenotime present only as inclusions in garnet generally possess high y cores and concentrations that drop off discontinuously along zoning shoulders of variable width to low y outboard regions samples with matrix xenotime are restricted to the garnet zone y concentration of these garnets generally decreases smoothly from core to rim xenotime may also be present in reaction zones around garnet in xenotime bearing samples y g r t is strongly temperature dependent and ranges from 5000 ppm in the garnet zone to 150 ppm in the sillimanite zone measured yttrium zoning profiles in xenotime absent samples are reproduced with both rayleigh fractionation and diffusion models but p t histories of the samples examined favor the rayleigh model with garnet volume bulk rock yttrium and mode of y hree accessory phases controlling the profile shape high yttrium annuli in staurolite zone samples may form by garnet overgrowth of proximal matrix enriched in yttrium due to garnet consumption during discontinuous staurolite forming reactions an increase in y g r t and hree g r t in garnet from anatectic samples is related to dissolution of phosphates in vapor absent peraluminous melt with partitioning of highly compatible y and hree into garnet grown during anatexis textural analysis reveals that phosphates are absent from regions of garnet grown in equilibrium with melt a main result of this study is identification of an intimate coupling between major pelite phases and accessory phases during reaction progress this coupling is of great advantage in that it may be used to 1 calibrate sensitive geothermometers and geobarometers 2 identify particular regions of garnet grown in different garnet producing reactions over a range of grades and 3 reveal portions of pelite reaction history invisible to major elements

jurassic granitoid magmatism in the dinaride neotethys geochronological constraints from detrital minerals three independent single grain geochronometers applied to detrital minerals from central dinaride sediments constrain the timing of felsic magmatism that associated the jurassic evolution of the neotethys the lower cretaceous clastic wedge of the bosnian flysch sourced from the dinaride ophiolitic thrust complex yields magmatic monazite and zircon grains with dominant age components of 164 3 and 152 10 ma respectively a unique tephra horizon within the adriatic carbonate platform was dated at 148 11 ma by apatite fission track analysis these consistent results suggest that leucocractic melt generation in the central dinaride segment of the neotethys culminated in middle to late jurassic times coeval with and slightly post dating subophiolitic sole metamorphism growth of magmatic monazite and explosive volcanism call for supra subduction zone processes at the convergent neotethyan margin new compilation of geochronological data demonstrates that such jurassic felsic rocks are widespread in the entire dinaride hellenide orogen

the timing of metamorphism in the odenwald spessart basement mid german crystalline zone new in situ electron microprobe monazite and white mica 40ar 39ar step heating ages support the proposition that the odenwald spessart basement mid german crystalline zone consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition the monazite ages constrain tectonothermal events at 430 43 ma 349 14 ma 331 16 ma and 317 12 ma 316 4 ma and the 40ar 39ar analyses provide white mica ages of 322 3 ma and 324 3 ma granulite facies metamorphism occurred in the western odenwald at c 430 and 349 ma and amphibolite facies metamorphism affected the eastern odenwald and the central spessart basements between c 324 and 316 ma we interpret these data to indicate that the otzberg michelbach fault zone which separates the eastern odenwald spessart basement from the western odenwald basement is part of the rheic suture which marks the position of a major variscan plate boundary separating gondwana and avalonia derived crustal terranes the age of the carboniferous granulite facies event in the western odenwald overlaps with the minimum age of eclogite facies metamorphism in the adjacent eastern odenwald the granulite and eclogite facies rocks experienced contrasting pressure temperature paths but occur in close spatial proximity being separated by the rheic suture as high pressure and high temperature metamorphisms are of similar age we interpret the odenwald spessart basement as a paired metamorphic belt and propose that the adjacent high pressure and high temperature rocks were metamorphosed in the same subduction zone system juxtaposition of these rocks occurred during the final stages of the variscan orogeny along the rheic suture

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

chemical th u total pb dating by electron microprobe analysis of monazite xenotime and zircon from the archean napier complex east antarctica evidence for ultra high temperature metamorphism at 2400 ma abstract the chemical th u total pb isochron method chime of dating was carried out on monazite xenotime zircon and polycrase in six samples of ultra high temperature uht granulites and one of pegmatite from five localities in the napier complex enderby land east antarctica despite the chronological heterogeneity in many grains the chime ages overall show strong evidence for a major event near 2420 ma analyzed monazite and xenotime grains except monazite from mt riiser larsen have a narrow range of chime ages 2412 42 ma from beaver island and 2429 36 2421 18 and 2431 22 ma from reference peak these ages are indistinguishable from chime ages on zircon 2436 17 and 2420 20 ma and on central older domains of monazite 2404 54 ma from mt riiser larsen and on a zircon rim 2430 15 ma from mt cronus we conclude that the ca 2420 ma ages relate to the uht metamorphism but we do not exclude the possibility that lead loss has lowered the ages by some 50 ma this interpretation is consistent with a preponderance of ages obtained by other methods on the napier complex particularly the ca 2475 25 ma age for a generation of pegmatites coeval with the uht event and containing the uht mineral assemblage beryllian sapphirine khmaralite quartz events prior to 2500 ma and after 2420 ma have also left their imprint on the chime ages for example a 3646 ma chime age suggests that a zircon core is inherited whereas ca 2200 and 2000 ma chime ages in zoned monazite could be due to further lead loss during post uht activity the much younger chime age of 1094 67 ma for monazite from a pegmatite at zircon point casey bay confirms 1000 1100 ma lead loss ages reported for this region and could result from fluid activity in an outlier of rayner activity near the boundary between the rayner and napier complexes approximately 2500 ma ages have been reported from granulite facies but not uht rocks in the madras and nilgiri blocks in south india these blocks and the napier complex could have constituted a single structural unit by late archean time

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

late tectonic reorientation of lineaments and fabrics in the northern eastern ghats province india evaluating the role of the mahanadi shear zone abstract south of the msz that cuts across the eastern ghats province egp of india lineament trends and early granulite facies fabrics rotate from nne ssw in the south towards the wnw ese striking northerly dipping mahanadi shear zone msz in the north using field microstructural and ebsd techniques we demonstrate that d1 and d2 planar fabrics are reoriented by an e w trending northerly dipping d3 extensional shear zone that has a subordinate sinistral strike slip component d3 microstructures are pervasive with quartz and feldspar microstructures characteristic of amphibolite facies metamorphism monazite chemical ages from the d3 shear zone yield ages in the range 730 680 ma the msz representing d4 deformation in the area is a predominantly dextral strike slip deformation during d4 microstructures evolve exclusively through plastic deformation of quartz and culminates in s4 mylonitic fabric formation in high d4 strains zones under greenschist facies conditions these microstructures are restricted to the msz and localized high d4 strain zones unlike the d3 microstructures we conclude that the earlier post granulite extensional deformation d3 and not the msz was responsible for the swing in lineament and fabric trends in the northern egp

hydration and dehydration of zeolitic tuff from yucca mountain nevada naturally occurring zeolites expand and contract when hydrated or dehydrated in tuffaceous rock composed largely of such zeolites the entire rock may swell or contract significantly as the rock becomes saturated or dries out if such rock is constrained significant stresses may develop as a result of hydration or dehydration we present experimental results that substantiate this in a zeolitized non welded tuff from yucca mountain nv rock permeability governs the swelling rate since the major constituent clinoptilolite hydrates as fast as it can be exposed to water at yucca mountain where a nuclear waste repository is proposed strata of welded devitrified tuff overlie non welded zeolitic tuff should the hydration state of the units change significantly over the repository lifetime additional stresses on the same order of magnitude as now exist may develop