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

u pb monazite ages in amphibolite to granulite facies orthogneiss reflect hydrous mineral breakdown reactions sveconorwegian province of sw norway in the rogaland vest agder terrain of the sveconorwegian province of sw norway two main sveconorwegian metamorphic phases are reported a phase of regional metamorphism linked to orogenic thickening m1 and a phase of low pressure thermal metamorphism associated with the intrusion of the 931 2 ma anorthosite charnockite rogaland igneous complex m2 phase m1 reached granulite facies to the west of the terrane and m2 culminated locally at 800 850 c with the formation of dry osumilite bearing mineral associations monazite and titanite u pb geochronology was conducted on 17 amphibolite to granulite facies orthogneiss samples mainly from a suite of 1050 2 8 ma calc alkaline augen gneisses the feda suite in these rocks prograde negatively discordant monazite crystallized during breakdown of allanite and titanite in upper amphibolite facies at 1012 1006 ma in the feda suite and other charnockitic gneisses concordant to slightly discordant monazite at 1024 997 ma probably reflects breakdown of biotite during granulite facies m1 metamorphism a spread of monazite ages down to 970 ma in biotite hornblende samples possibly corresponds to the waning stage of this first event in the feda suite a well defined monazite growth episode at 930 925 ma in the amphibolite facies domain corresponds to major clinopyroxene formation at the expense of hornblende during m2 growth or resetting of monazite was extremely limited during this phase in the granulite facies domain up to the direct vicinity of the anorthosite complex the m2 event was shortly followed by cooling through ca 610 c as indicated by tightly grouped u pb ages of accessory titanite and titanite relict inclusions at 918 2 ma over the entire region a last generation of u poor monazite formed during regional cooling below 610 c in hornblende rich samples at 912 904 ma this study suggests 1 that monazite formed during the prograde path of high grade metamorphism may be preserved 2 that monazite ages reflect primary or secondary growth of monazite linked to metamorphic reactions involving redistribution of rees and th and or fluid mobilisation 3 that the u pb system in monazite is not affected by thermal events up to 800 850 c provided that conditions were dry during metamorphism

deformed monazite yields high temperature tectonic ages the deformation of monazite in the polymetamorphic sandmata granulite complex in india has been investigated by electron backscattered diffraction and sensitive high resolution ion microprobe shrimp quantitative microstructural analyses document the development of deformation twins in 100 001 and graphic 1 orientations low angle 10 boundary development associated with dislocation creep and the development of new grains due to dynamic recrystallization these data represent the first quantitative evidence of crystal plastic deformation of natural monazite shrimp u th pb analysis shows that the host monazite preserves discordant ages as old as 1666 28 ma along a trend from ca 1720 ma to ca 1000 ma with increasingly discordant ages recorded in zones of higher lattice distortion domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 14 ma this age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith and is consistent with partial resetting and pb loss from domains deforming by dislocation creep the complex but systematic relationship between microstructure and age data in monazite provide the first direct evidence of pb isotope resetting during deformation the approach illustrates a new methodology for the dating of deformation events in high grade metamorphic rocks which are typically difficult to constrain 1 embed inline graphic 1 gif

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

the role of biota in the initiation and growth of islands on the floodplain of the okavango alluvial fan botswana a group of islands of varying size on the floodplain of the okavango alluvial fan were studied to establish the processes which lead to the initiation and growth of islands it was found that islands are initiated by the mound building activities of the termite macrotermes michaelseni these termites import fine grained materials to use as a mortar for the construction of epigeal mounds their activities create a topographic feature raised above the level of seasonal flooding and also change the physical properties and nutrient status of the mound soil shrubs and trees are able to colonize these mounds which results in increased transpiration as a result precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds resulting in vertical and especially lateral growth causing island expansion 1998 john wiley sons ltd

th diffusion in monazite abstract diffusion of th has been characterized in synthetic monazite under dry conditions the synthetic monazites either pure cepo4 ndpo4 or a mixed lree phosphate containing ce nd and sm were grown via a na2co3 moo3 flux method the sources of diffusant for the experiments were either synthesized thsio4 or cath po4 2 powders mixed with cepo4 experiments were performed by placing source and monazite in pt capsules and annealing capsules in 1 atm furnaces for times ranging from 3 weeks to a few hours at temperatures from 1350 to 1550 c the th distributions in the monazite were profiled by rutherford backscattering spectrometry rbs the following arrhenius relation was obtained for th diffusion in monazite d 6 7 10 1 exp 740 63 kj mol 1 r t m 2 s 1 the diffusivity of th was found to be similar for monazites containing a single ree ce or nd and the mixed lree phosphates th diffusion was also similar for experiments run using the th silicate and ca th phosphate sources suggesting that the substitutional mechanism for th in monazite i e th 4 si 4 ree 3 p 5 for the thsio4 source and th 4 ca 2 2ree 3 for the cath po4 2 source does not significantly affect th diffusivities and that th is likely the rate limiting species in diffusion th diffusion in monazite is about 3 orders of magnitude slower than pb diffusion cherniak d j watson e b grove m harrison t m 2004a pb diffusion in monazite a combined rbs sims study geochimica et cosmochimica acta 68 829 840 this contrasts with findings of gardes et al gardes e jaoul o montel j m seydoux guillaume a m wirth r 2006 pb diffusion in monazite an experimental study of pb2 th4 2nd3 interdiffusion geochimica et cosmochimica acta 70 2325 2336 who determined that diffusivities measured in pb th ree exchange which would under most circumstances be rate limited by the slowest diffusing species are comparable to those measured by cherniak et al cherniak d j watson e b grove m harrison t m 2004a pb diffusion in monazite a combined rbs sims study geochimica et cosmochimica acta 68 829 840 for pb th diffusion in zircon cherniak d j hanchar j m watson e b 1997 diffusion of tetravalent cations in zircon contributions to mineralogy and petrology 127 383 390 is about an order of magnitude slower than in monazite but with similar activation energy for diffusion the lower diffusivities in zircon may be a consequence of the larger disparity in size between th and the zr site in zircon as compared with th and the ree site in monazite nonetheless th is essentially immobile in monazite with respect to exchange by volume diffusion under most geologic conditions these findings may also have implications for containment of high level actinide based nuclear waste in monazite ceramic waste forms but it should be noted that mechanisms other than diffusive exchange e g recrystallization dissolution reprecipitation may be important to consider along with diffusion when evaluating the retentivity of monazite for th and other actinide elements

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

nano and micro geochronology in hadean and archean zircons by atom probe tomography and sims new tools for old minerals atom probe tomography apt and secondary ion mass spectrometry sims provide complementary in situ element and isotope data in minerals such as zircon sims measures isotope ratios and trace elements from 1 20 m spots with excellent accuracy and precision apt identifies mass charge and three dimensional position of individual atoms 0 3 nm in 100 nm scale samples volumes up to one million times smaller than sims apt data provide unique information for understanding element and isotope distribution crystallization and thermal history and mechanisms of mineral reaction and exchange this atomistic view enables evaluation of the fidelity of geochemical data for zircon because it provides new understanding of radiation damage and can test for intracrystalline element mobility nano geochronology is one application of apt in which pb isotope ratios from sub micrometer domains of zircon provide model ages of crystallization and identify later magmatic and metamorphic reheating based on sem imaging and sims analysis 11 needle shaped specimens 100 nm in diameter were sampled from one archean and two hadean zircons by focused ion beam milling and analyzed with apt the three dimensional distribution of pb and nominally incompatible elements y rees differs at the atomic scale in each zircon zircon jh4 0 4 007 ga jack hills western australia is homogeneous in pb y and rees in contrast pb and y and rees are clustered in sub equant 10 nm diameter domains spaced 10 40 nm apart in zircons arg2 5 2 542 ga grouse creek mountains utah and jh4 4 4 374 ga jack hills most clusters are flattened parallel to 100 or 010 u and th are not collocated with pb in clusters and appear to be homogeneously distributed in all three zircons the analyzed domains experienced 4 to 8 10 15 decay events mg due to u and th decay and yet all zircons yield u pb ages by sims that are better than 97 concordant consistent with annealing of most radiation damage the 207 pb 206 pb ratios for the 100 nm scale specimens measured by apt average 0 17 for arg2 5 0 42 for the jh4 0 and 0 52 for jh4 4 these ratios are less precise 10 18 2 due to the ultra small sample size but in excellent agreement with values measured by sims 0 1684 0 4269 and 0 5472 respectively and the crystallization ages of the zircons thus pb in these clusters is radiogenic but unsupported meaning that the pb is not spatially associated with its parent isotopes of u and th for the domain outside of clusters in jh4 4 the 207 pb

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

experimental determination of ree distributions between monazite and xenotime potential for temperature calibrated geochronology solid solutions of the ree y orthophosphates monazite and xenotime were synthesized hydrothermally between 300 c and 800 c at 2 and 5 kbar using ree y oxides and h3po4 as starting materials the ree bulk composition was similar to those found in metapelitic rocks all experiments produced two solid phases monoclinic monazite and tetragonal xenotime in volumetric ratios similar to relative abundances in metapelites ree concentrations of coexisting monazite xenotime pairs were determined by emp analysis and ree distribution coefficients dree xreemonazite xreexenotime were derived for the entire ree y suite values of dree depend strongly on ionic radii of the rees and on temperature the log dree vs ionic radius curve shows a nearly linear decrease from la to lu and comprises almost five orders of magnitude from dla 102 5 to dlu 10 2 between 450 and 800 c most dree values vary continuously by a factor of about 2 to 3 dnd decreases from 48 to 16 and dsm from 9 to 5 dtb increases from about 0 2 to about 0 4 ddy from 0 1 to 0 2 and dy from 0 06 to 0 12 along this temperature range experimentally determined values of dree are in close agreement with those derived from natural monazite xenotime pairs equilibrated between 400 and 700 c ree partitioning between monazite and xenotime is appropriate for geothermometry the most useful application is that equilibrium and disequilibrium partitioning of ree in metamorphic rocks are easily recognized in the case of equilibrium distributions u pb chronometry of single monazites or xenotimes provides a direct link to their growth conditions and information on temperature time paths equilibrium distributions of sm and nd expressed in molar fractions result in kd xsmmonazitexndxenotime xsmxenotimexndmonazite 0 2 to 0 3 from 600 to 800 c and sm nd chronometry should provide reasonable two mineral isochrons for such monazite xenotime pairs

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

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

first delimitation of areas affected by ground deformations in the guadalfeo river valley and granada metropolitan area spain using the dinsar technique abstract a differential interferometric sar dinsar analysis of terrain displacements in the province of granada southern spain is presented here for two different study zones the first zone concerns the guadalfeo river basin where abundant landslides and unstable slopes were previously identified and inventoried on a gis application however no instrumental quantification of landslide activity was available considering morphological criteria and field observations these deep seated landslides were considered to be dormant or moving extremely slowly the second study zone corresponds to the granada metropolitan area where no previous information was available on any vertical movement the analysis was based on ers1 and ers2 images covering the period from 1993 to 2000 by using dinsar we made an initial qualitative assessment of movements on landsliding slopes in the guadalfeo river basin and also vertical land movements in the alluvial quaternary sediments to the west and south of the granada metropolitan area the terrain instability assessment resulting from this research shows maximum annual velocities along the sar line of sight los of 6 mm yr for the tablones landslide and up to 13 mm yr for the albunuelas village where progressive differential settlement processes were established as the origin of the movement in the case of the granada metropolitan area two sites were detected with subsidence affecting two villages santa fe located in the western part of the area with an estimated average velocity of 8 mm yr and otura in the south of the area with maximum velocities of 12 mm yr a description of geological features of the different sites showing los ground displacement is presented with a discussion of the application of the rates to landslide activity assessment found local ground conditions or exploitation of subterranean water in urban areas

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

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

high resolution 5 μm u th pb isotope dating of monazite with excimer laser ablation ela icpms abstract monazite lree po 4 a common accessory mineral in magmatic and metamorphic rocks is complementary to zircon in u th pb geochronology because the mineral can record successive growth phases it is useful for unravelling complex geological histories a high spatial resolution is required to identify contrasted age domains that may occur at the crystal scale bulk mineral techniques such as id tims applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages laser ablation la icpms has been demonstrated to be a useful technique for u th pb dating of zircons and this study tests its analytical capabilities for dating monazite a sector field high resolution icpms coupled with a 193 nm arf excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements the u th pb systematic was applied to monazite grains from several samples a lower palaeozoic lens from high grade terrains in southern madagascar neogene hydrothermal crystals from the western alps a palaeoproterozoic very high temperature granulite from central madagascar and a variscan leucogranite from spain directly on a polished thin section the major aim was to compare and or reproduce tims and emp ages of monazite from a variety of settings and ages the three independent 206 pb 238 u 207 pb 235 u and 208 pb 232 th ratios and ages were calculated isotope fractionation effects mass bias laser induced fractionation were corrected using a chemically homogeneous and u pb concordant monazite as external standard this study demonstrates that excimer laser ablation ela icpms allows u th pb dating of monazite with a high level of repeatability accuracy and precision as well as rapidity of analysis a spatial resolution almost comparable to that of emp in terms of crater width 5 m produced precise 208 pb 232 th 206 pb 238 u and 207 pb 235 u ratios for dating palaeozoic to precambrian monazites the advantages of ela icpms isotope dating are precision accuracy and the ability to detect discordance in the case of late miocene hydrothermal monazites from the alps a larger spot size of 25 m diameter is required and precise and accurate ages were obtained only for 208 pb 232 th systematics results from the variscan granite show that in situ u th pb dating of monazites with ela icpms is possible using a 5 m spot directly on thin sections so that age data can be placed in a textural context

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

dating diagenetic monazite in mudrocks constraining the oil window authigenic monazite nodules increasingly recognized in pre mesozoic mudrock successions provide a means for dating diagenesis by exploiting trace uranium within the monazite structure but dating is difficult in practice because of zonation and abundant host rock inclusions we report here a refined technique that has yielded a date of 419 14 ma 2 mswd 1 2 for a 17 point pb pb regression age from a telychian silurian late llandovery mudrock from central wales shrimp analyses of monazite nodules from the same horizon are consistent with this and additionally demonstrate concordance of the u pb and th pb systems these results open the door to providing robust dates for a phase of burial related diagenesis which in wales seems approximately coincident with and perhaps was related to hydrocarbon expulsion

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

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

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

complexity in the behavior and recrystallization of monazite during high t metamorphism and fluid infiltration abstract a detailed study of monazite grains in silica undersaturated and quartz bearing mg al metapelite from the oygarden group of islands east antarctica reveals a complex history of growth and recrystallization during two separate events in the neoproterozoic and earliest cambrian monazite grains from garnet poor and garnet rich metapelite preserve core domains that have ages corresponding to growth and or recrystallization at granulite facies p 9 1 0 gpa t 850 900 c conditions during the rayner structural episode between 930 and 890 ma high th rims 22 wt tho 2 that are in textural equilibrium with sapphirine orthopyroxene symplectites formed after garnet during late rayner decompression occur on monazite grains in garnet rich assemblages and give electron microprobe ages 883 18 ma within error of core domains 903 14 ma these high th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth in garnet poor metapelite monazite grains that have grain boundaries in textural equilibrium with the granulite facies assemblage also show alteration to higher th compositions on rims and along vein like fractures this compositional shift is accompanied by partial to complete resetting of ages to 500 ma along with minor modification of core domain ages textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution reprecipitation reaction process but in contrast to garnet bearing assemblages this alteration occurred during a fluid influx at lower temperatures during regional pan african tectonism this study highlights the susceptibility of monazite to resetting with fluid present conditions dramatically increasing the potential for recrystallization

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

timing of diagenesis and very low grade metamorphism in the eastern sector of the sierra de cameros iberian range spain a u pb shrimp study on monazite the sierra de cameros is an intracontinental orogen and represents the north western part of the iberian range in northern spain it comprises a thick sequence of syn rift continental sediments mainly sandstones and carbonates deposited during lower cretaceous times a unique characteristic of the sierra de cameros in relation to the rest of the iberian range is the presence of low grade metamorphism in certain parts of the basin an event that predates basin inversion this paper describes the presence textural relationships and geochemical aspects of authigenic and or metamorphic monazite within different lithologies from the deepest parts of the basin sensitive high resolution ion microprobe shrimp u pb dating of monazite records the presence of two age populations the first with 206 pb 238 u ages ranging from 122 to 116 ma which is considered as diagenetic in origin whilst the second is dated at 99 2 ma and postdates the metamorphic climax

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

radiation damage in zircon and monazite monazite and zircon respond differently to ion irradiation and to thermal and irradiation enhanced annealing monazite cannot be amorphized by 800 kev kr ions at temperatures greater than 175 c whereas zircon can be amorphized at temperatures up to 740 c the damage process i e elastic interactions leading to amorphization in radioactive minerals metamictization is basically the same as for the ion beam irradiated samples with the exception of the dose rate which is much lower in the case of natural samples the crystalline to metamict transition in natural samples with different degrees of damage from almost fully crystalline to completely metamict is compared to the sequence of microstructures observed for ion beam irradiated monazite and zircon the damage accumulation process representing the competing effects of radiation induced structural disorder and subsequent annealing mechanisms irradiation enhanced and thermal occurs at much higher temperatures for zircon than for monazite the amorphization dose expressed as displacements per atom is considerably higher in the natural samples and the atomic scale process leading to metamictization appears to develop differently ion beam induced amorphization data were used to calculate the decay event dose required for amorphization in terms of a critical radionuclide concentration i e the concentration above which a sample of a given age will become metamict at a specific temperature this equation was applied to estimate the reliability of u pb ages to provide a qualitative estimate of the thermal history of high u natural zircons and to predict whether actinide bearing zircon or monazite nuclear waste forms will become amorphous metamict over long timescales

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

nanoscale distribution of pb in monazite revealed by atom probe microscopy abstract the widespread use of monazite lreepo4 in u pb geochronology is underpinned by the assumption that it incorporates negligible amounts of pb during initial growth and that radiogenic pb remains immobile after formation we have investigated the nanoscale distribution of pb in monazite from granulite facies rocks of the sandmata metamorphic complex rajasthan india by atom probe microscopy to further understand the utility of monazite as a geochronometer the studied monazite contains distinct 10 nm clusters enriched in ca and with a bulk composition consistent with them being apatite ca5 po4 3 oh that are also enriched in si and pb relative to the monazite host the 208pb 232th ratios of the clusters ranged from 1 1 0 1 to 1 4 0 2 2 indicating that the clusters hold unsupported pb the 208pb 232th ratios of the whole specimen including clusters and the matrix alone are similar

u th pb dating of collision in the external alpine domains urseren zone switzerland using low temperature allanite and monazite article i nfo dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low temperature metamorphic con ditions in the external domains of the central alps urseren zone the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two ree minerals to date successive metamorphic stages within a collisional cycle the studied outcrop locality of garvera exposes mesozoic metasediments that were metamorphosed under greenschist facies conditions t b 450 c under these conditions ree mineral grains or domains are typically small 5 50 m and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins allanite grains n200 m based on the texture and mineral assemblages ree mineral growth conditions were attributed to be detrital mnz1 syn kinematic mnz2 and aln1 post kinematic at the thermal peak aln2 and retrograde during late stage deformation mnz3 to constrain temporally the successive crystallisation conditions ree minerals have been dated using shrimp ion microprobe u th pb analyses show that the alpine ree minerals have a high th u and while most of the 206 pb is non radiogenic f206 that can reach up to 99 for allanite th pb ages can be obtained using 206 pbc corrected th pb isochron the age of different generations of allanite and monazite constrains the timing of successive collisional stages for the prograde accretion at 22 5 1 5 ma mnz1 and aln1 thermal peak at 19 3 2 0 ma aln2 and late stage hydrothermal veining at13 6 1 4 ma mnz3 these novel geochronological data confirmtheregional diachroneity throughthe cen tral alps with younger ages burial thermal peak and exhumation toward the external domains the similarity betweenmetamorphicagesofsamplestakenintheurserenzoneandthenearbycrystallinemassifssuggeststhat themetasedimentarycoveroftheurserenzoneremainedcloselyjuxtaposedtotheexternalcrystallinebasement during the alpine collision cycle the monazite age of the late stage vein testifies for active tectonic deformation at 13 6 1 5 ma which likely played a crucial control on the exhumation of the external massifs in the middle miocene