lewisian crustal history from in situ sims mineral chronometry and related metamorphic textures abstract a combination of detailed petrography and in situ sims 207 pb 206 pb micro chronometry of monazite has been carried out on samples from the lewisian high grade metamorphic terrain northwest scotland two episodes of granulite facies metamorphism have been recognized the thermal maxima of which are estimated at or before 2760 12 ma 2 and at 2526 8 ma respectively pegmatites at scourie crystallised at 2480 ma the amphibolite facies metamorphism of the main laxfordian event is estimated at 1752 12 ma and the formation of kyanite and staurolite is associated with this event the oldest ages of 2760 ma have been obtained from micro inclusions of monazite in garnet it appears that garnet which has a high closure temperature for the u pb system plays an important role in the behaviour of the monazite u pb system it serves to shield monazite from reaction with the other phases and to armour monazine from diffusive pb loss or from resetting

the late archean history of the wyoming province as recorded by granitic magmatism in the wind river range wyoming abstract the wyoming province a small ca 500 000 km 2 archean craton is the most southwestern of the archean provinces in north america it is composed primarily of late archean potassium rich granitic rocks in contrast to many other archean provinces rocks of tonalite trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2 8 ga field petrologic geochemical and isotopic study of the late archean granites exposed in the wind river range have allowed us to identify at least four periods of potassic calc alkalic magmatism at 2 8 2 67 2 63 and 2 55 ga granitic rocks of these ages appear to be widespread across the wyoming province the oldest calc alkalic granites of the wind river range emplaced at ca 2 8 ga appear to be derived predominantly from pre existing crust however nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses during at least two other periods of plutonism at 2 67 and 2 63 ga generation of the wind river range batholiths involved the incorporation of substantial amounts of isotopically juvenile material either from depleted mantle or young continental crust the information presented below as well as data available from elsewhere in the wyoming province is interpreted to suggest that the wyoming province unlike other archean cratons is not composed of a tectonic amalgamation of smaller exotic terranes although the wyoming province did experience crustal addition in archean time it was not by lateral accretion but by incorporation of mantle derived melts into large granitic batholiths

evidence for proterozoic mylonitization in the northwestern wyoming province the 3 km thick madison mylonite zone encompasses the contact between the cherry creek metamorphic suite and the pre cherry creek metamorphic complex in the southern madison range of southwest montana in the shear zone greenschist and epidote amphibolite facies assemblages overprint earlier amphibolite and granulite facies assemblages large scale metasomatic conversion of granitic gneiss to mylonite gneiss increased siderophile fe mn mg p and ca elements and decreased k al and si in the mylonite gneiss suggesting extensive fluid flux in the madison mylonite zone southeast dipping foliations adjacent to the madison mylonite zone are rotated about their northeastern strike through the vertical to northwest dipping orientations indicating southeast directed thrusting of the pre cherry creek metamorphic complex over the cherry creek metamorphic suite complete stratigraphic continuity of cherry creek units within the mylonite zone shows the predominance of ductile shearing ductile thrusting is also indicated by asymmetric fabrics stretching lineations finite strain and metamorphic gradients within the shear zone if the foliations were passively rotated during simple shear a minimum of 10 km of distributed ductile slip occurred in the madison mylonite zone 40ar 39ar thermochronology of hornblende and muscovite adjacent to the madison mylonite zone records a late archean 2 5 ga cooling event within the shear zone a discordant 40ar 39ar muscovite spectrum is consistent with initial cooling to argon closure at about 1 9 ga with subsequent disturbance at about 1 6 ga hornblendes from the madison mylonite zone show highly discordant argon loss spectra with temperature steps starting at 1 8 ga and ending at 2 5 ga these argon age spectra indicate early proterozoic heating and argon loss in the madison mylonite zone resetting and subsequent retrogressing of muscovite but only partial resetting of hornblende due to its higher argon closure temperature this study strongly suggests early proterozoic 1 80 1 90 ga and possibly later thrusting in the madison mylonite zone which may be correlative with retrograde metamorphism and early proterozoic isotopic resetting northwest of the madison range the madison mylonite zone is probably a foreland thrust zone on the margin of a major compressional orogen of early proterozoic age that reworked the archean basement of the northwestern wyoming province

improving the resolution of single grain u pb dating by use of zircon extracted from feldspar application to the variscan magmatic cycle in the central alps abstract the usual discordant u pb zircon age patterns for intrusives in orogenic belts impede and exact determination of the time of emplacement they are the product of multiple overprinting and of the presence of inherited zircon dating of monophase zircons crystallized during a single event may yield higher age resolution and results that are easier to interpret during magma crystallization zircon is generated over a range of ptx conditions whereas rock forming minerals grow under more limited conditions and their relative ages can be determined petrographically zircons formed during earlier stages of magma crystallization and included in such mineral constituents are shielded from subsequent reactions outside the carrier mineral following partial melting of the protolith or xenolith assimilation newly crystallizing rock forming minerals may include zircon xenocrysts as individual grains using this insight and to facilitate the analysis of monophase zircons included in petrogenetically defined constituent minerals we have introduced additional steps in the mineral separation procedure we found the effort worthwhile in that the magmatic zircons extracted from plagioclase yielded the most concordant intrusive ages in contrast to zircons from microcline or from the interstices also inherited zircons occurring as individual grains in magmatic feldspar appear to have preserved some rudimentary morphology and a less disturbed radiogenic pb composition which allows us to distinguish provenance from discrete protoliths we were thus able to resolve small but structurally supported age differences between two groups of variscan intrusives in the central swiss alps we obtained 299 4 1 2 ma for the crystallization age of the granite gneiss in the gotthard massif with inherited zircons that suggest generation from late proterozoic 735 ma and mid proterozoic 1760 ma protoliths the massive leucocratic granites on the margin of the granite gneiss yielded an intrusive age of 294 3 1 1 ma with an inherited grain implying a source of caledonian vintage

monazite u pb and th pb geochronology by ion microprobe with an application to in situ dating of an archean metasedimentary rock abstract monazite grains from nine different precambrian rock samples and having concordant isotope dilution u pb and th pb ages were used to develop and evaluate techniques for u pb th pb and pb pb isotopic dating using an ion microprobe operating at high mass resolution r 5500 1 an unidentified isobar at 203 960 amu complicated the determination of 204 pb in the monazites with higher th but it appeared to have been eliminated when using a slight energy offset the 207 pb 206 pb ratios of individual spots were determined in 10 min analyses to 0 3 0 5 1 of the true values using a 20 m wide o 2 primary beam whereas the uncertainty in the mean of several spots on a single generation of monazite was typically 0 1 0 4 2 instrumental bias in the measured pb uo and pb tho values of individual analyses was monitored using linear relationships between 206 pb uo vs uo 2 uo and 208 pb tho vs uo 2 uo the bias appears to vary somewhat with monazite th content requiring the use of compositionally matched standards individual spot measurements of the 206 pb 238 u and 208 pb 232 th ratios in unknown monazites of low to moderate th can be determined to 2 1 the dating technique was applied to in situ analysis of small monazite inclusions hosted in an amphibolite grade metapelite of archean origin both the contextual setting and the morphology of the monazite grains were found to be important factors in interpreting the timing of growth of the host metamorphic mineral e g garnet staurolite plagioclase euhedral monazite grains armoured in low cao garnet and matrix plagioclase yielded an age of 2548 17 ma interpreted as dating an early episode of low pressure metamorphism partially resorbed monazite within a high cao garnet rim yielded ages of ca 2500 ma but are interpreted as inherited from the first event rather than syn genetic with respect to the growth of this garnet an age of 1755 30 ma was obtained for anhedral monazite associated with late chlorite filled fractures apparently reflecting post tectonic retrogression

u pb and sm nd ages for the stillwater complex and its associated sills and dikes beartooth mountains montana identification of a parent magma seven analyses from two samples of the lower banded series of the stillwater complex montana yielded a u pb zircon baddeleyite age of 2 705 plus minus 4 ma a more precise age of 2 704 plus minus 1 ma is obtained if the authors regress only five of seven analyses in either case the age is in very good agreement with the previously reported sm nd mineral isochron age of 2 701 plus minus 8 ma for a gabbro from the lower banded series noncumulate mafic sills and dikes that immediately underlie the stillwater complex are separated into five compositional groups u pb zircon ages for a group 1 gabbronorite 2 711 plus minus 1 ma and a group 3 mafic norite 2 703 plus minus 10 ma are comparable to previously reported u pb zircon ages for a group 1 diabase 2 713 plus minus 3 ma and a group 4 high ti norite 2 712 plus minus 3 ma although group 2 high mg gabbronorite and group 6 olivine gabbro have not yet yielded significant amounts of zircon baddeleyite or sphene mineral separates from group 2 yielded a four point pb pb internal isochron age of 2 704 plus minus 25 ma and separates from group 6 yielded a sm nd internal isochron age of 2 706 plus minus 64 ma indicating that these groupsmore are at least contemporaneous with intrusion of the stillwater complex these results indicate that groups 1 and 4 of the associated sills and dikes are perhaps slightly older than the complex and that groups 2 3 and 6 may be coeval with it supporting the idea that these three groups are the best candidates for a stillwater parent magma a conclusion previously presented on the basis of detailed mineralogic and geochemical evidence less

quartz shielding of sub 10 μm zircons from radiation damage enhanced pb loss an example from a metamorphosed mafic dike northwestern wyoming craton abstract the coupling of two new approaches enabled acquisition of in situ u pb data from zircons as small as 5 m in a metamorphosed mafic dike from the northern madison range montana despite negligible zircon yield by mineral separation automated mineralogy rapidly identified 375 sub 20 m zircons in a single thin section subsequently zircon crystals were dated using modifications to the conventional sims technique to preferentially collect secondary ions emitted from a domain a few microns in size within the 20 m diameter analysis pit this approach allowed analysis of zircons too small to be dated by standard sims and tims methods u pb data define a discordia array with upper and lower intercepts of 1753 9 ma and 63 8 ma respectively 1 error mswd 1 5 we interpret the upper intercept to reflect zircon growth during high temperature and high pressure metamorphism 800 c 1 2 gpa based on textural relationships between the dated zircons and the peak metamorphic assemblage the lower intercept is attributable to the thermal pulse associated with emplacement of the nearby ca 75 ma tobacco root batholith percent discordance is linked with both textural setting and u concentration zircons located along grain boundaries or within fractured host grains display a positive correlation between u 1049 2817 ppm and percent discordance 12 82 that is consistent with radiation damage enhanced pb loss in contrast three zircons housed completely within unfractured quartz yield the most concordant analyses of the dataset 7 discordant despite u concentrations comparable to highly discordant matrix grains this relationship suggests that the included zircons were shielded from pb loss by the encapsulating quartz crystals the results imply that targeting zircons located completely within unfractured host phases may aid in isolating earlier portions of geologic histories the primary geological implication of this dataset is to increase the documented extent of paleoproterozoic high grade metamorphism in the northwestern wyoming craton

a crystal chemical basis for pb retention and fission track annealing systematics in u bearing minerals with implications for geochronology this study develops an empirical crystal chemical framework for systematizing the kinetics of pb loss and fission track annealing in u bearing minerals ionic porosity z the fraction of a mineral s unit cell volume not occupied by ions potentially accounts for kinetic behavior by monitoring mean metal oxygen bond length strength various tests of a general kinetics porosity relationship are presented based upon diverse mineral data including 1 pb diffusion parameters 2 measured closure temperatures tc for fission track annealing and 3 retentivities of both pb and fission tracks from apparent age data every kinetic parameter including tc and mineral age for both the u pb and fission track systems is inversely correlated with z within the sub assemblage zircon z 29 titanite 34 and apatite 38 assuming a diffusional closure model pb isotopic transport phenomena are described by a tc z scale calibrated with field based tc data for titanite 680 20 c and apatite 500 c extrapolation of this scale yields tc estimates for the following minerals staurolite tc 1060 c z 25 garnet 1010 c 26 5 zircon 900 c monazite xenotime and epidote 750 c 32 and ca clinopyroxene 670 30 c 34 1 depending on composition these empirical results imply that a u pb pb date for staurolite or garnet records the time of mineral growth not post growth isotopic closure as also concluded in recent field studies because z systematizes fission track annealing this recrystallization process like volume diffusion must also be rate limited by the strength of chemical bonds the extent to which other recrystallization processes are likewise rate limited is important to u pb geochronology because they potentially compete with diffusion as mechanisms for pb isotopic resetting in nature

annealing radiation damage and the recovery of cathodoluminescence the structural recovery upon heat treatment of a highly metamict actinide rich zircon u 6000 ppm has been studied in detail using a range of techniques including x ray powder diffraction raman spectroscopy shrimp ion probe electron microprobe transmission electron microscopy and cathodoluminescence analysis the structural regeneration of the amorphous starting material depends on random nucleation it starts between 800 and 950 c when amorphous zrsio4 decomposes to form crystalline zro2 and amorphous sio2 at around 1100 c well crystallised zrsio4 grows at the expense of the oxides u has been retained in the newly grown zircon whereas pb was evaporated during the heat treatment this process is in marked opposition to the reconstitution of moderately metamict minerals which experience a gradual recovery controlled by the epitaxial growth at the crystalline amorphous boundaries both of these recovery processes are not the direct inverse of metamictisation the structural regeneration was found to be connected with a significant increase in the emission of cl in all cases annealing heavily damaged zircon and moderately damaged zircon and monazite we observe that the final wellcrystallised annealing products emit more intense cl than their radiation damaged starting minerals although having almost identical elemental composition our observations are taken as evidence that the cl is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres

in situ u pb sims in sims micro baddeleyite dating of mafic rocks method with examples abstract an in situ u pb sims in sims method to date micro baddeleyite crystals as small as 3 m is presented with results from three samples that span a variety of ages and geologic settings the method complements id tims geochronology by extending the range of dateable crystals to sizes smaller than can be recovered by physical separation x ray mapping and bse imaging are used to locate target grains in thin section followed by sims analysis on a cameca ims 1270 using the field aperture in the transfer column to screen out ions from host phases internal age precisions for the method are anticipated to range from 0 1 for precambrian rocks to 3 7 for phanerozoic rocks results establish a 2689 5 ma age for mafic dikes in the wyoming craton usa a 1540 30 ma age for a subaerial lava flow from the thelon basin of northern canada and a 457 34 ma age for mafic dikes in the platform sequence of southeastern siberia the method is ideal for relatively non destructive dating of small samples such as extraterrestrial rocks and precious terrestrial samples

high precision u pb zircon geochronology of the late triassic chinle formation petrified forest national park arizona usa temporal constraints on the early evolution of dinosaurs the triassic successions of the colorado plateau preserve an important record of vertebrate evolution and climate change but correlations to a global triassic framework are hampered by a lack of geochronological control tuffaceous sandstones and siltstones were collected from the upper triassic chinle formation exposed in the petrified forest national park arizona usa within a refined stratigraphic context of 31 detailed measured sections u pb analyses by the isotope dilution thermal ionization mass spectrometry id tims method constrain maximum depositional ages for nine tuffaceous beds and provide new insights into the depositional history of the chinle fluvial system the base of the blue mesa member of the chinle formation is placed at ca 225 ma and the top of the petrified forest member is placed at 208 ma or younger bracketing an 280 m thick section that spans nearly the entire norian stage of the late triassic estimated sediment accumulation rates throughout the section reflect extensive hiatuses and or sediment removal by channel erosion the new geochronology for the chinle formation underscores the potential pitfalls of correlation of fluvial units based solely on lithostratigraphic criteria a mid norian age ca 219 213 ma for the distinctive sonsela conglomeratic sandstone bed constrains the adamanian revueltian land vertebrate faunachron boundary our new data permit a significant time overlap between the lower chinle sequence and the dinosauromorph rich ischigualasto formation of northwestern argentina near contemporaneity of the trans american deposits and their faunal similarities imply that early dinosaur evolution occurred rapidly across the americas

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

coseismic vs climatic factors in the record of relative sea level changes an example from the last interglacials in se spain abstract trying to decipher how paleoseismic activity has influenced the altitudinal disposition of interglacial deposits is one of the main challenges to be confronted before quantifying relative sea level changes between consecutive highstands at the same time identifying paleoseismic features in an area with low instrumental seismic activity can complement the seismic record thereby contributing to better seismic hazard knowledge in this regard cope basin se spain meets all the requirements necessary for these kinds of analysis firstly on account of its excellent outcrop conditions and secondly because its sea level record is undoubtedly conditioned by both factors climatically driven changes and local and regional seismotectonics the morpho sedimentary sequence presented in this paper provides evidence from three different highstands two littoral sedimentary units separated by terrestrial deposits and affected by liquefaction paleoseismic activity and a younger paleo cliff and its associated wave cut platform carved into those sedimentary units this wave cut platform now tilted and folded reaches heights ranging from 0 to 8 m which has been interpreted as a consequence of cosesimic effect the age of this wave cut platform last interglacial or holocene supports uplift rate estimations of from 0 016 to 0 15 mm yr since the last interglacial holocene active deformation observed on pleistocene holocene sea level geomarkers can not only furnish useful information about seismic activity within the major structure of the aguilas arc but also offer new insight into the cope basin s geodynamic model

isotopic dating of meso and neoproterozoic mafic magmatism in the southern tobacco root mountains southwestern montana abstract new isotopic dating of mafic dikes in the southern tobacco root mountains of southwestern montana provides evidence for two distinct episodes of subparallel proterozoic mafic magmatism separated by about 700 m y previously published geochemical data from dikes in the southern tobacco root mountains had identified three geochemical groups termed groups a b and c with apparent rb sr ages of 1455 ma group a and 1100 1120 ma groups b and c sm nd dating of a geochemical group a dike yields an age of 1448 49 ma that is interpreted to represent the emplacement age of the dikes this age is similar to published u pb dates from mafic sills that intrude the nearby mesoproterozoic belt supergroup and laramide basement cored uplifts of the archean wyoming province paleomagnetic results from groups a and b dikes are similar suggesting that the were emplaced at about the same time we suggest that the mafic magmatism recorded by the southern tobacco root mountains group a c dikes at about 1450 ma probably corresponds to continental extension that accompanied development of the mesoproterozoic belt basin u pb dating of baddeleyite from the group b dikes provides evidence for a younger magmatic event with an age of 782 4 4 9 ma 95 confidence 7 1 ma incorporating uranium decay constant errors this date is similar to isotopic dates from neoproterozoic mafic dikes and sills exposed elsewhere in uplifts of the rocky mountain foreland and that intrude the mesoproterozoic strata of the belt supergroup geochronologic data from elsewhere in the northern cordillera and canadian shield indicate that the group b dikes in the southern tobacco root mountains are part of a regional 780 ma magmatic event that extended for more than 2400 km along the western margin of laurentia the origin of the magmatic event is not clear but may be related to a mantle plume and crustal extension accompanying initial breakup of the supercontinent rodinia and development of the proto pacific ocean no evidence of ca 1100 ma mafic magmatism in this part of the northern cordillera was found

sem based quantitative mineralogical analysis of peridotite kimberlite and concentrate abstract quantitative automated mineralogy is the acquisition of mineralogical and textural data by scanning electron microscopy based energy dispersive analytical methods the technique is used in the metals and energy resource extraction industries to provide accurate mineral characterization information over large data sets we have employed this method to three projects related to diamonds mantle peridotite kimberlite and garnet concentrate the first example assesses the metasomatic clinopyroxene phlogopite modal mineralogy in peridotite xenoliths from premier cullinan south africa understanding mantle mineral variability can be coupled with measureable mineral properties to develop mantle geophysical and geochemical models the second example compares kimberlites from letseng satellite pipe lesotho and ngamiland botswana to assess the variability of kimberlite mineralogy kimberlite domains can be identified on a micro scale with potential to understand parameters such as hardness and grindability or to identify kimberlite clusters with discrete mineral assemblages the third example applies mineral compositional variability in garnet concentrate samples as a possible tool for evaluating exploration projects application of garnet definitions to concentrates through digitally grouping grains into discrete populations results in improved understanding of large sample populations and hence diamond prospectivity quantitative mineralogy applied to diamond related projects utilizes the principles of geometallurgy in evaluating large data sets for greater understanding of the variability of mantle materials

in situ u pb dating of micro baddeleyite by secondary ion mass spectrometry abstract we introduce a technique for u pb dating of baddeleyite using secondary ion mass spectrometry sims in situ analysis of ng mass crystals that cannot be efficiently extracted by conventional mineral separation techniques average 207 pb 206 pb ages for precambrian baddeleyite crystals are within 206 pb 238 u ratios are corrected for instrumental fractionation calibrated through linear regression in a pb u relative sensitivity vs uo 2 u calibration plot calibration is performed on separated baddeleyite crystals 100 200 m in maximum dimension mounted in random crystallographic orientation 206 pb 238 u ages for baddeleyite from duluth gabbro fc4b and kovdor are accurate within 1 2 when averaging 15 30 individual spot analyses and relative sensitivities calibrated on phalaborwa baddeleyite the relative difference of 206 pb 238 u between large crystals and micro baddeleyite from fc4b is within 1 comparison between silicate glass and baddeleyite as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in pb u sensitivity in baddeleyite due to variable crystal orientations we successfully utilized oxygen flooding and a uo 2 u based calibration to significantly reduce orientation dependent bias

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

late cretaceous remagnetization of proterozoic mafic dikes southern highland mountains southwestern montana a paleomagnetic and 40ar 39ar study paleomagnetic results from early proterozoic metabasite sills and middle proterozoic diabase dikes from the southern highland mountains of southwestern montana give well defined dual polarity magnetizations that are statistically identical to those from a small late cretaceous pluton that cuts the dikes the concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the proterozoic rocks were remagnetized probably during late cretaceous time paleomagnetic rock magnetic and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low ti magnetite combining virtual geomagnetic poles from metabasite sills diabase dikes and the late cretaceous pluton we obtain a paleomagnetic pole at 85 5 n 310 7 e k 19 9 a 95 9 1 n 14 sites that is similar to a reference pole from the 74 ma adel mountain volcanics of western montana biotite and hornblende 40 ar 39 ar isotopic dates from host basement gneiss and a hornblende from a remagnetized metabasite sill yield ages of ca 1800 ma these dates probably record cooling of the southern highland mountains following high grade metamorphism at 1 9 1 8 ga the gneiss and metabasite age spectra show virtually no evidence of disturbance indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems thus the overprint magnetization of the highland mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby late cretaceous plutons remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of late cretaceous hydrothermal fluids related to epithermal mineralization the absence of significant disturbance to the 40 ar 39 ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may go unrecognized and undated even if 40 ar 39 ar dating is applied

pre metamorphic arar ages from biotite inclusions in garnet two populations of ar ar ages in biotite inclusions occluded by garnets from an indian polymetamorphic gneiss have been measured using uv laser ablation one population yields a reproducible age of 455 my identical to biotite ages in the adjacent groundmass and is interpreted as slow cooling following high grade metamorphism during the pan african event the second population yields ages in the range 1800 2100 ma and corresponds closely with zircon ages from the same rock these results imply that the ar ar system in a small population of biotite inclusions remained closed during high grade metamorphism 750 c and 5 kb the results of this first experiment accord with results for helium diffusion in garnet and suggest a mechanism for the long term storage of heavy noble gases in the deep crust

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