a 70 ma record of suprasolidus conditions in the large hot long duration grenville orogen petrochronology of magmatic monazite and apatite from a single paragneiss leucosome derived by insitu partial melting documents the thermal evolution of the allochthonous belt of the central grenville province monazite records suprasolidus metamorphism from ca 1080 to 1020 ma under high temperature up to 850 degrees c apatite from the same leucosome yields an age of 960 ma consistent with cooling of this crustal segment down to subsolidus conditions of ca 450 degrees c a pegmatitic granite dyke with lobate contacts previously dated at ca 1005 ma turlin et al 2017 is interpreted to intrude the paragneisses at a temperature of ca 650 degrees c close to the wet solidus these data document slow cooling at a rate of 2 6 degrees c ma for the middle crust of this part of the grenville hinterland marked by protracted suprasolidus conditions for at least 70 ma it supports the definition of the grenville orogen as a large hot long duration orogen

neoarchean crustal growth by accretionary processes evidence from combined zircon titanite u pb isotope studies on granitoid rocks around the hutti greenstone belt eastern dharwar craton india abstract the neoarchean hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides combined u pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution the golapalli and yelagatti granodiorites occurring to the north of the hutti greenstone belt were emplaced at 2569 17 ma the yelagatti granodiorite yielded a younger titanite age of 2530 6 ma which indicates that it was affected by a post crystallization thermal event that exceeded the titanite closure temperature the western granodiorites from kardikal have identical titanite and zircon ages of 2557 6 ma and 2559 19 ma respectively the eastern kavital granodiorites yielded titanite ages of 2547 6 ma and 2544 24 ma which are identical to the published u pb zircon shrimp ages these ages imply that the granitoid rocks surrounding the hutti greenstone belt were formed as discrete batholiths within a short span of ca 40 ma between 2570 ma and 2530 ma ago they were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the archean the first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the hutti area a metamorphic overprint occurred at ca 2300 ma ago that reset the rb sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of au in the ore lodes the eastern dharwar craton consists of quartz monzodiorite granodiorite granite suites of rocks that are younger than the greenstone belts that are older than 2650 ma reported from earlier studies the granitoid magmatism took place between 2650 and 2510 ma ago indicating accretionary growth of the eastern dharwar craton

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

textural geochronological and chemical constraints from polygenetic titanite and monogenetic apatite from a mid crustal shear zone an integrated epma sims and tims study abstract titanites extracted from a 520 9 ma syn orogenic syenite at otjimbingwe damara orogen namibia show shape colour and u pb systematics that vary with position in the syenite relative to a major shear zone most titanite fractions from syenite cropping out outside the shear zone are characterized by a medium brown anhedral core overgrown by a pale brown subhedral rim empa and sims analyses show that the cores are enriched in al fe and sometimes na mg and mn but also in ree nb and zr relative to the rim tims u pb ages from these titanites gave relatively imprecise 207pb 206pb ages between 680 21 ma and 695 20 ma as well as 570 8 ma to 604 9 ma mainly due to their low 206pb 204pb ratios combined th u pb relationships suggest that these titanites contain a component similar to ca 715 750 ma old titanite populations observed so far only in neoproterozoic syenites from the northern part of the orogen titanites from syenites within the shear zone are dark brown mostly euhedral to subhedral and display only occasionally a pale brown rim these titanites show decreasing concentrations of na mg al and fe and increasing concentrations of mn from core to rim interpreted to reflect growth zonation abundances of mree hree y nb and zr are higher in the rim than in the core 207pb 206pb ages obtained on these titanites range from 470 2 ma to 538 3 ma for most samples only the samples with the highest 206pb 204pb ratios between 350 and 230 have u pb ages similar to the previously published u pb zircon age of 520 9 ma from the same complex other titanite fractions with lower 206pb 204pb ratios have slightly older ages than ca 520 ma indicating some inheritance inherited titanite survived multiple metamorphic episodes in the damara orogen between ca 750 ma and 470 ma and the igneous event that lead to the intrusion of the otjimbingwe syenite at 520 ma these data imply a high closure temperature of u pb in titanite of 650 700 c consistent with previous studies coeval apatite has no apparent inheritance low 206pb 204pb ratios between 55 and 170 and u pb ages similar to or only slightly younger than the u pb zircon or titanite age obtained on the titanite fractions with the highest 206pb 204pb ratios assuming a closure temperature for u pb of 900 c for zircon 700 c for titanite and 450 c for apatite initial fast cooling of the complex can be inferred our data demonstrate that titanite can contain multiple age domains preserving a chronological and chemical record of previous igneous events

can fission track annealing in apatite be described by first order kinetics abstract it is becoming commonplace to suggest that fission track annealing in apatite can be described by first order kinetics that is the decrease in track density and or length with time during isothermal annealing can be described by a single exponential equation a review of the literature reveals that there is no convincing evidence for such an assertion but more than ample evidence to the contrary simple graphical analysis of available annealing data from the literature clearly shows that the single exponential fails to adequately describe the data more rigorous statistical model fitting is not attempted due to the generally inadequate specification of annealing data in the literature most published annealing data suffer from flaws in experimental design that render them of limited value in defining viable kinetic models for the annealing process consideration of the processes taking place during annealing suggests that there is no good reason why first order kinetics would be expected to describe fission track annealing and more complex models should be considered in order to provide reliable data capable of yielding practically useful kinetic models laboratory annealing experiments should be designed to meet a number of strict criteria in identifying viable models the need is for those which adequately describe the data rather than for physically meaningful models although models should preferably be kept as simple as possible

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

direct ion microprobe u pb dating of fossil tooth of a permian shark abstract we report here direct ion microprobe dating of fossil tooth dentine of a permian fresh water shark orthacanthus senckenbergianus using the shrimp instrument recently installed at hiroshima university fifteen spots on the small sample approximately 2 mm 1 mm indicate a 238 u 206 pb isochron age of 266 18 ma and a tera wasserburg concordia intercept age of 266 24 ma in a three dimensional 238 u 206 pb 207 pb 206 pb 204 pb 206 pb diagram these permian ages are consistent with a 235 u 207 pb age of 453 170 ma and a 232 th 208 pb age of 235 310 ma suggesting indistinguishable depositional and early diagenetic ages of the fossil in its sedimentary sequences the success of the method depends on the chemical fractionation of uranium from lead in a specimen a few hundred microns in size and the consequent variations in lead isotopic compositions due to radioactive decay

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

stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western canadian shield new u pb geochronology for an extensive exposure of high pressure granulites in the east lake athabasca region of the western canadian shield is consistent with a history characterized by 2 55 ga stabilization of cratonic lithosphere 650 million years of lower crustal residence and cratonic stability and 1 9 ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling high precision single grain and fragment zircon data define distinctive discordia arrays between 2 55 and 1 9 ga u pb ion microprobe spot analyses yield a similar range of u pb dates with no obvious correlation between date and cathodoluminescence zonation we attribute the complex u pb zircon systematics to growth of the primary populations during a 2 55 ga high pressure granulite facies event 1 3 gpa 850 c recorded by the dominant mineral assemblage of the mafic granulite gneisses with subsequent zircon recrystallization and minor secondary zircon growth during a second high pressure granulite facies event 1 0 gpa 800 c at 1 9 ga the occurrence of two discrete granulite facies metamorphic events in the lower crust separated by an interval of 650 million years that included isobaric cooling for at least some of this time suggests that the rocks resided at lower crustal depths until 1 9 ga we infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions but the unusual aspect of the history in the east lake athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface we suggest that a weakened strength profile related to the 1 9 ga heating left the lithosphere susceptible to far field tectonic stresses from bounding orogens that drove the lower crustal exhumation an ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1 7 ga

mineralogical constraints on the thermal history of martian regolith breccia northwest africa 8114 abstract martian meteorite northwest africa nwa 8114 a paired stone to nwa 7034 provides an opportunity to examine the thermal history of a martian regolith and study near surface processes and ancient environmental conditions near an impact crater on mars our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains some of the pyroxene clasts show exsolution lamellae indicating a high temperature magmatic origin and slow cooling however transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re crystallised to form magnetite and a k bearing feldspathic glassy material together with relict pyroxene this breakdown event was associated with oxidation with up to 25 fe3 fe in the relict pyroxene measured using fe k xanes by comparison with previous studies this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating being held at a temperature above 700 c for at least 7 days in an oxidising regolith environment we report an approximate 40ar 39ar maximum age of 1 13 1 25 ga for an individual separated augite clast the disturbed nature of the spectra precludes precise age determination in section this clast is porous and contains iron oxide grains this shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts and has up to 25 fe3 fe we infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes after consolidation of the impact ejecta blanket high temperatures above 700 c may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut and in some cases surround the oxidised pyroxene however the veins could alternatively be the result of a hydrothermal event in the impact regolith a simple fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days low temperature hydrous alteration took place forming goethite identified via xrd xanes and ftir comparing with previous studies the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite

observations and controls on the occurrence of inherited zircon in concord type granitoids new hampshire u pb analyses of zircons separated from two concord type plutons near sunapee and dixville notch new hampshire reveal differences in the pattern and magnitude of zircon inheritance which are related to differences in melt chemistry the sunapee pluton contains only slightly more zr than required to saturate the melt at the peak temperature of 700 30 sup 0 c traces of inherited zircon in this separate are inferred to be present as small largely resorbed grains in contrast the long mountain pluton near dixville notch contains about 240 more zr than required to saturate the melt thus more than half of the zr existed as stable inherited zircon crystals during the partial fusion event consistent with the observation of substantial inheritance in all grain size fractions ion probe intra grain analyses of zircon from the long mountain pluton indicate a complex pattern of inheritance with contributions from at least two proterozoic terranes and caution against simple interpretations of upper and lower intercepts of chords containing an inherited component ion probe analyses of zircons from the sunapee pluton reveal clear evidence of u loss which results in incorrect apparent conventional u pb ages ages of crystallization for the long mountain and sunapee pluton are approx more 350 and 354 5 ma respectively a sm nd measurement for the long mountain pluton yields a depleted mantle model age of 1 5 ga consistent with the observed inheritance pattern in contrast a sm nd model age for the sunapee pluton is improbably old due to minor monazite fractionation less

moon based euv imaging of the earth s plasmasphere model simulations the euv imager on board the chang e 3 lunar lander will image the earth s plasmasphere from a lunar perspective to focus on some of the open questions in plasmaspheric researches i e global structures erosion and refilling of plasmasphere in order to achieve the understanding of the plasmaspheric dynamics in relation to these euv images in lunar perspective the he 30 4nm emission intensities and global structures of the plasmasphere viewed from the moon are investigated using a dynamic global core plasma model embedded with ts07 magnetic field model and w05 electric field model two typical storms observed by the image euv imager are systematically simulated from the perspectives of the moon it is found from the simulations that the maximum emission intensity of the plasmasphere is similar to 12 3 r which is greater than that detected from polar orbit and the global shapes and temporal evolutions of large scale plasmaspheric structures plasmapause shoulder and plume also have different patterns in moon based simulated images it is also shown that the plasmaspheric structures extracted from moon based euv images are in agreement with those from image euv images systematic simulations demonstrate that specific latitudinal distribution of the plasmaspheric structures can only be imaged at specific positions in lunar orbit it is expected that this investigation provides us with an overall understanding on moon based euv images and helps to identify the plasmaspheric structures and evolution patterns in future moon based euv imaging

u pb systematics of the unique achondrite ibitira precise age determination and petrogenetic implications abstract ibitira is an unbrecciated equilibrated vesicular basaltic achondrite that is considered to have originated on a parent body distinct from all other known meteorites we present the first combined high precision u and pb isotopic data for this unique meteorite the 238 u 235 u value of 137 777 0 013 determined for the whole rock is comparable to values determined for bulk chondrites and other basaltic achondrites this value results in corrections of 1 1 ma for pb pb dates calculated using the previously assumed invariant 238 u 235 u value of 137 88 using the determined 238 u 235 u value the 7 most radiogenic pb isotopic analyses for acid leached pyroxene rich and whole rock fractions yield an isochron pb pb age of 4556 75 0 57 ma in excellent agreement with the results of mn cr chronology which give the ages of 4557 4 2 5 ma and 4555 9 3 2 ma using the u corrected pb pb age of d orbigny as a time anchor along with the previously proposed thermal history of ibitira and our closure temperature estimates for pb diffusion the pb pb age is interpreted as the timing of the last chemical equilibration and coarse pyroxene exsolution that occurred during high temperature metamorphism the metamorphism may have been caused by burial of ibitira lava under successive lava flows and if so the pb pb age should post date the crystallization by a short time interval the pb isotopic data for acid leachates suggest partial re equilibration of pb between plagioclase and phosphate perhaps during an impact event at 4 49 ga as recorded by k ar systematics the whole rock 238 u 204 pb indicates that compared to ci chondrites ibitira is less depleted in pb than in some alkali elements despite a lower condensation temperature of pb than the alkali elements the restricted pb depletion may reflect preferential concentration of metals with high fluid melt partition coefficients including pb and zn as a result of fluid exsolution and migration within the parent magma we discuss the implications of the u pb systematics for the origin and differentiation of the parent body

nanoscale occurrence of pb in an archean zircon abstract we report for the first time a direct atomic scale characterization of pb in zircon 4 4 3 1 ga from the early archean yilgarn craton in australia using high resolution haadf stem two forms of pb have been identified pb concentrated at 3 atom as a nanoscale patch in zircon structure and pb concentrated within the amorphous domain created by fission fragment damage the first result suggests that the pb atoms directly substitute for zr4 in the zircon structure and the latter observation demonstrates that pb diffusion can occur through amorphous regions created by radiation damage although volume diffusion is typically considered to be the dominant mechanism for pb diffusion beyond the first percolation point i e when the amorphous domains overlap and form a fully interconnected network of amorphous domains there is a new pathway for the diffusion of pb that is faster than volume diffusion through crystalline zircon

single grain u th he ages from phosphates in acapulco meteorite and implications for thermal history the cooling history of the acapulco meteorite for 400 c is well established using various chronometers suggesting extremely fast cooling 1000 c ma in contrast the thermal history for low temperatures 400 c is poorly understood because of large uncertainties in the chronometers applicable to this temperature range to better constrain the cooling history for the low temperature range we applied u th he dating techniques to individual phosphate grains one whitlockite and 11 apatite grains yielded u th he ages ranging from 1272 22 1 analytical error only ma to 4584 51 ma with tight clustering at 4 55 ga the weighted mean of the five oldest ages 4538 32 ma 1 uncertainty including systematic error is suggested to be the minimum age representing primary cooling of the acapulco body passing through 120 c although it is impossible to precisely quantify the effects of energetic particle ejection from the outermost 20 m of the phosphates petrographic evidence suggests that most dated samples are fragments likely derived from the interior of larger grains thus greatly reducing this source of error indeed the five oldest samples cannot have suffered substantial ejection since the uncorrected ages are identical with the crystallization age of the acapulco meteorite the new u th he data suggest rapid cooling of acapulco down to 120 c this evidence suggests that the younger 40 ar 39 ar age 4507 9 1 ma obtained from acapulco plagioclase which should reflect cooling through 300 c is spuriously young due to systematic errors i e decay constants and or standard data in the 40 ar 39 ar method as suggested by comparison between high precision 40 ar 39 ar and u pb ages for terrestrial volcanic rocks the scattered he age distribution 4 0 ga implies very heterogeneous thermal disturbances after the primary cooling of the body

combined high precision chronometry and geochemical tracing using accessory minerals applied to the central alpine bergell intrusion central europe abstract the behaviour of nd u th and pb isotopes in granitic rocks is usually controlled by accessory minerals in this study an approach is adopted which uses the initial 143 nd 144 nd as a geochemical tracer together with high precision u th pb geochronology for accessory phases and their respective whole rock this combination of techniques is applied to zircon allanite sphene apatite and thorite from both a tonalite and a granodiorite from the tertiary alpine bergell intrusion inherited isotopic signatures have been identified in bulk zircon fractions by inverse discordance in the u pb system in the case of the granodiorite the initial nd in the zircon is 1 77 units lower than the bulk rock this result indicates that nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks all other accessories yield an initial nd equal to their respective whole rock and u th pb ages equal to or lower than the respective intrusion ages this indicates either the absence of inherited cores or that such cores have exchanged their nd and pb in the case of apatite also sr by diffusion where available experimental mineral melt equilibria and diffusion models are consistent with this behaviour u pb geochronology of carefully selected core free zircons yields a precise intrusion age of 31 88 0 09 ma for the tonalite in contrast granodioritic zircons are partially reset due to radiation damage caused by high u concentrations however an alternative means of dating young intrusions is to use 232th 208pb ages of allanite in the case of allanite the presence of a large excess of 206pb derived from 230th indicates that no total pb loss has occurred since crystallization and the nd isotopes indicate the absence of inheritance using both the allanite age and a 238u 206pb age for sphene an intrusion age of 30 13 0 17 ma is indicated for the granodiorite taken together with an allanite th pb age for the tonalite these results strongly support the high closure temperatures for the allanites studied here 700 800 c at high post magmatic cooling rates all other accessory mineral ages are reset to values similar to hornblende 39ar 40ar ages and reflect the effect of post magmatic cooling superimposed on post metamorphic cooling

petrogenesis and chronology of lunar meteorite northwest africa 4472 a kreepy regolith breccia from the moon northwest africa nwa 4472 is a polymict lunar regolith meteorite the sample is kreep rich high concentrations of potassium rare earth elements and phosphorus and comprises a heterogeneous array of lithic and mineral fragments these clasts and mineral fragments were sourced from a range of lunar rock types including the lunar high magnesian suite the high alkali suite kreep basalts mare basalts and a variety of impact crater environments the kreep rich nature of nwa 4472 indicates that the sample was ejected from regolith on the nearside of the moon in the procellarum kreep terrane and we have used lunar prospector gamma ray remote sensing data to show that the meteorite is most similar to and most likely sourced from regoliths adjacent to the imbrium impact basin u pb and pb pb age dates of nwa 4472 phosphate phases reveal that the breccia has sampled pre nectarian 4 35 ga rocks related to early episodes of kreep driven magmatism some younger phosphate u pb and pb pb age dates are likely indicative of impact resetting events at 3 9 4 ga consistent with the suggested timing of basin formation on the moon our study also shows that nwa 4472 has sampled impact melts and glass with an alkali depleted incompatible trace element rich high sc low rb th ratios low k compositional signature not related to typical apollo high k kreep or that sampled by kreepy lunar meteorite sayh al uhaymir sau 169 this provides evidence that there are numerous sources of kreep rich protoliths on the moon

structural characterization of an island aquifer via tidal methods 1 a set of water level time series collected along a transect through a sedimentary island aquifer was used to test the utility of various simple models of ocean tidal propagation in bounded one dimensional aquifers fourier spectra were calculated for the ocean tidal modes and compared with spectra measured in wells along the island transect other sources of fluctuation could be neglected an observed spatial bias in the well responses attenuations and lags could not be modeled by a homogeneous aquifer theory a theory involving composite heterogeneity accounted well for the spatial bias yielding estimates of aquifer transmissivities and storage coefficients indicating a fivefold difference in hydraulic diffusivity along the transect a lack of well locations toward one end of the transect reduced the statistical significance of this result with correlations between regression parameters evident at the same time a second bias was seen involving the ratio of signal amplitude and lag with penetration distance into the aquifer as observed in prior tidal studies a brief set of numerical experiments showed that horizontal layering in aquifer properties was the most probable cause of this propagation bias application of these results to the island data set supported a conceptual stratigraphic model of a highly conductive sloping stratum underlying a less conductive superficial sand layer this model is inconsistent with well logs along the island transect but is supported by additional off transect well logs it was concluded that one dimensional tidal propagation models may be useful in inverse characterization of aquifers with macroscale hydrogeological structures and that the analysis of measured propagation bias has the potential to yield extra information on aquifer properties in the vertical direction

diffusion in silicate minerals and glasses a data digest and guide to the literature the rates of diffusion of cations oxygen and hydrogen in silicates are of fundamental importance to a wide range of petrological and geochronological problems in order that the earth scientist may readily apply the available experimentally determined diffusion rates to such problems a compilation of data for diffusion in silicate minerals and glasses is presented along with a working guide for its use published data and experimental conditions are listed in tables and these are preceded by an outline of diffusion mechanisms terminology equations and units diffusion regimes factors controlling diffusion rates measurement techniques and diffusional behaviour

the akilia controversy field structural and geochronological evidence questions interpretations of 3 8 ga life in sw greenland abstract field relations structure and geochronology demonstrate that controversial rocks on akilia sw greenland cannot host evidence of earth9s oldest life mafic ultramafic gneiss that contains a layer of quartz amphibole pyroxene gneiss a few metres thick with purported biogenic graphite is not cross cut by the protoliths of 3 82 3 65 ga tonalitic gneiss the contact between these gneisses is tectonic so their relative ages are unknown multiple episodes of intrusion and deformation in the tonalitic gneiss preceded the earliest deformation fabrics and structures seen in the mafic ultramafic gneiss although previously interpreted as a volcano sedimentary stratigraphy the mafic ultramafic gneiss displays no evidence of such an origin and could be partly or wholly derived from igneous intrusions existing geochronology indicates that the mafic ultramafic and qtz am px gneisses have ages c 2 68 ga during late archaean high grade events that complicate any protolith interpretation u pb dating of apatite from a variety of rocks on akilia shows that this mineral crystallized or possibly recrystallized at c 1 75 ga and it thus provides no indication of an early archaean age for any associated graphite regardless of whether or not the latter is biogenic supplementary material ion microprobe u pb analytical methods and isotope data tables are available at http www geolsoc org uk sup18333

thermochronological insights into the structural contact between the tian shan and pamirs tajikistan gilby jepson stijn glorie dmitry konopelko jack gillespie martin danisik noreen j evans yunus mamadjanov alan s collins

multistage exhumation and juxtaposition of lower continental crust in the western canadian shield linking high resolution u pb and 40ar 39ar thermochronometry with pressure temperature deformation paths 1 new u pb titanite apatite rutile and 40ar 39ar hornblende muscovite biotite data are linked with existing pressure temperature deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive 20 000 km2 region of exhumed lower crust in the east lake athabasca region of the western canadian shield in the 200 m y between circa 1 9 ga high pressure granulite facies metamorphism and circa 1 7 ga unconformable deposition of athabasca basin sediments on the exhumed rocks our analysis reveals at least three distinct phases of unroofing at different rates specifically we distinguish 1 an early phase of extensional unroofing in the chipman domain at rates of 1 5 2 0 km m y from 1 0 gpa to 0 8 gpa associated with mafic magmatism and metamorphism 2 an episode of regional contractional uplift and associated unroofing along the legs lake shear zone at circa 1850 ma from 0 7 0 8 gpa to 0 4 0 5 gpa and 3 a final period of extensional unroofing at rates of 0 2 0 3 km m y from 0 4 to 0 5 gpa that culminated in transport of current exposures to near surface conditions the cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases the apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1 89 1 88 ga implies their juxtaposition at 0 7 0 8 gpa conditions regional east directed thrusting of the deep crustal domains as a coherent unit at 1 85 ga juxtaposed the granulites with middle crustal hearne domain rocks at 0 4 0 5 gpa conditions the detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of laurentia the temporal and spatial heterogeneity of exhumation patterns in the east lake athabasca region may be a common feature of the unroofing histories of lower crustal rocks

concurrent fractional and equilibrium crystallisation abstract this paper proposes the concept of concurrent fractional and equilibrium crystallisation cfec in a multi phase magmatic system in light of experimental results on diffusivities of elements and other species in minerals and melts a group of equations are presented to describe how the concentrations of an element or isotope change in fractionated solid equilibrated solid melt liquid and gas phases as well as in magma as a function of distribution coefficients and mass fractions during the cfec process cfec model is a generalised and unified formulation that is valid not only for pure fractional crystallisation fc and perfect equilibrium crystallisation ec singly as two of its limiting end member cases but also for the geologically more important process of concurrent fractional and equilibrium crystallisation the concept that both fractional and equilibrium crystallisation can operate concurrently in a magmatic system for a given element among different minerals and even within different sized crystal grains of the very same mineral phase is of fundamental importance in deepening our current understanding of magmatic differentiation processes cfec probably occurs more frequently in the natural world than either pure fractional or perfect equilibrium crystallisation alone as a result of the interplay of varying diffusivities of elements under diverse physicochemical conditions different residence time and growth rates of mineral phases in magmas and varying grain sizes within each phase and among different phases the marked systematic variations in trace element concentrations in the melts of the bishop tuff have long been perplexing and difficult to reconcile with existing models of differentiation cfec which is able to better explain the scatter trends in a systematic way than fractional crystallisation is considered to be the cause

high temperature 350 c thermochronology and mechanisms of pb loss in apatite abstract natural processes driven by heat flow can be understood using quantitative reconstruction of the thermal history of accessory and common minerals that were formed or modified in these processes thermochronology assumes that isotopes are lost from minerals by thermally activated volume diffusion and forms the basis of many studies of the thermal evolution of the crust however some studies challenge this assumption and suggest that the mechanisms controlling isotope transport in minerals over geological time scales are dominated by aqueous fluid flow within mineral pathways here we test these contrasting hypotheses by inverse modelling apatite uranium lead u pb dates to produce theoretical t t solutions assuming pb was lost by volume diffusion these solutions are compared with independent geological constraints and intra grain apatite u pb dates which demonstrate that volume diffusion governed the displacement of pb this confirmation combined with an inverse modeling procedure that permits reheating and cooling paths to be distinguished between 375 and 570 c provides geologists with a unique tool for investigating the high temperature thermal evolution of accessory minerals using the u pb method

pbpb dating of apatite by a stepwise dissolution technique abstract apatites from different geologic settings have been dated by a new sequential dissolution method the procedure is based on stepwise exposure of apatite to weak intermediate acids a pegmatite sample from the kerala khondalite belt in southern india yielded an age of 509 25 ma apatites from an enderbite sample from the northern marginal zone of the limpopo belt in southern africa yielded an age of 1970 26 ma which is in agreement with the high grade metamorphism at around 2 0 ga reported from the triangle shear zone and amphibolite facies metamorphism in the transition zone apatites from an amphibolite facies gneiss from the transition zone south of the nmz yielded an age of 1878 18 ma this age is somewhat younger than a sphene age on the same rock of 1971 17 ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase the sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic pb ratios because the spread can be significantly increased and no correction for common lead is needed this study further demonstrates that apatites from magmatic and high grade metamorphic rocks yield ages very close to those of monazite zircon or garnet unless the rock subsequently interacted with a fluid phase

the cooling history of the acapulco meteorite as recorded by the 244pu and 40ar 39ar chronometers 244pu fission track densities recorded in phosphates merrillite and apatite and in orthopyroxenes adjacent to phosphates along with 40ar 39ar dating were used to retrace the cooling evolution in the low temperature regime of the acapulco meteorite over a time interval of 110 ma high resolution 40ar 39ar dating yielded a plateau age of 4514 16 ma and a precise 40ar closure temperature of 560 20 k in plagioclase this ar closure temperature is essentially indistinguishable from the 50 retention temperature of fission tracks registered in orthopyroxene 550 25 k this allows us to anchor the floating relative 244pu timescale to the absolute timescale defined by the 40ar 39ar chronometer plutonium contents of bulk phosphates inferred from fission xenon are 2x higher than those inferred from tracks in the same phosphates reflecting the low track retention temperatures of phosphates however the pu contents from tracks in orthopyroxenes adjacent to phosphates agree with those from fission xenon suggesting similar retention temperatures and times our results together with an estimated peak metamorphic temperature 1300 k and the u pbpb datum of acapulco phosphates gopel et al 1992 outline an early thermochronological history of acapulco that is more detailed than obtained by earlier attempts and spans a time of 160 ma fast cooling in the high temperature regime 1300 720 k of 100 40 k ma was followed by a drastic change of the cooling rate between 720 and 560 k 3 7 k ma down to a very slow cooling 1 7 0 5 k ma from 550 to 360 k the fast cooling at high temperatures shared also by other acapulcoite lodranites a l suggests that the parent body of acapulco was distinctly smaller than the h chondrite asteroid in contrast the very slow cooling in the low temperature regime should imply that acapulco like material became effectively insulated during a later stage of its history 6 ma after the asteroid formation if a formation time of 4565 ma is assumed for the a l body extrapolations back in time of the pu contents of phosphates give initial 244pu 238u ratios of 0 4 10 2 and 11 10 2 for apatite and merrillite respectively for the bulk phosphates an initial pu u value of 0 8 0 16 10 2 is obtained for acapulco which is in full agreement with the preferred chondritic ratio of 0 7 10 2 proposed by hagee et al 1990 trapped ar contents are surprisingly high both primordial 36ar at levels of type 4 ordinary chondrites and excess radiogenic 40ar were detected in our sample as in some acapulcoites by other authors this partial closed system behaviour could be the result of the partial melt and sintering effects that the high peak metamorphic temperatures have induced in a l meteorites in addition the spread in the early peak temperatures 1300 1450 k as reflected by the diverse degrees of partial melting observed in a l meteorites suggests heterogeneously distributed short lived heating sources 26al60fe and disfavours the homogeneous heating expected from an electromagnetic induction arising from an active pre main sequence sun the possibility that some specific collisional event s with or without disruption of the a l planetesimal could satisfactorily explain the thermal histories and diverse characteristics of these meteorites appears difficult to envision in the absence of evident shock effects in most if not all a l meteorites

nickeliferous pyrite tracks pervasive hydrothermal alteration in martian regolith breccia a study in nwa 7533 martian regolith breccia nwa 7533 and the seven paired samples is unique among martian meteorites in showing accessory pyrite up to 1 by weight pyrite is a late mineral crystallized after the final assembly of the breccia it is present in all of the lithologies i e the fine grained matrix icm clast laden impact melt rocks climr melt spherules microbasalts lithic clasts and mineral clasts all lacking magmatic sulfides due to degassing pyrite crystals show combinations of cubes truncated cubes and octahedra polycrystalline clusters can reach 200 lm in maximum dimensions regardless of their shape pyrite crystals display evidence of very weak shock metamorphism such as planar features fracture networks and disruption into subgrains the late fracture systems acted as preferential pathways for partial replacement of pyrite by iron oxyhydroxides interpreted as resulting from hot desert terrestrial alteration the distribution and shape of pyrite crystals argue for growth at moderate to low growth rate from just saturated near neutral 6 fmq 2 log units it is inferred from the maximum ni contents 4 5 wt that pyrite started crystallizing at 400 500 c during or shortly after a short duration relatively low temperature thermal event that lithified and sintered the regolith breccias 1 4 ga ago as deduced from disturbance in several isotope systematics

reverse age zonation of zircon formed by metamictisation and hydrothermal fluid leaching abstract understanding the stability of radiation damaged zircon is crucial to the application of zircon to u pb geochronology especially for zircons rich in radioactive nuclides electron microprobe and laser raman spectral analyses show that zircons from the ejinao sodalite syenite in southeast china are composed of cores without zonation and oscillatory rims the cores are rich in uranium and have experienced increased metamictisation an internal sieved structure and radial cracks were formed by radiation damage and radiation induced volume expansion infrared spectral analyses show that zircon from the ejinao sodalite syenite contains large amounts of oh and h 2 o this finding suggests that the zircon crystals experienced hydrothermal alteration the pb loss of zircons is ascribed to their interaction with a hydrothermal fluid derived from the ejinao syenite radiogenic pb 4 situated in recoil damaged sites has been significantly reduced to pb 2 by the hydrothermal fluid becoming incompatible in the crystal lattice and highly mobile in solution consequently it has been leached out from the zircon this pb loss leads to diverse 206 pb 238 u ages from 134 ma to 147 ma and severe pb loss from the inner core even produces reverse age zonations with cores approximately 5 ma younger than the rims the real crystallisation age for zircons from the ejinao sodalite syenite should be approximately 145 ma considering the entire range of analytical data obtained by both the shrimp and la icpms methods

impact history of the apollo 17 landing site revealed by u pb sims ages secondary ion mass spectrometry sims u pb ages of ca phosphates from four texturally distinct breccia samples 72255 76055 76015 76215 collected at the apollo 17 landing site were obtained in

distinct 238 u 235 u ratios and ree patterns in plutonic and volcanic angrites geochronologic implications and evidence for u isotope fractionation during magmatic processes angrites are differentiated meteorites that formed between 4 and 11 myr after solar systemformation when several short lived nuclides e g 26 al 26 mg 53 mn 53 cr 182 hf 182 w were still alive as such angrites are prime anchors to tie the relative chronology inferred from these short lived radionuclides to the absolute pb pb clock the discovery of variable u isotopic composition at the sub permil level calls for a revision of pb pb ages calculated using an assumed constant 238 u 235 u ratio i e pb pb ages published before 2009 2010 in this paper we report high precision u isotope measurement for six angrite samples nwa 4590 nwa 4801 nwa 6291 angra dos reis d orbigny and sahara 99555 using multi collector inductively coupled plasma mass spectrometry and the irmm 3636 u double spike the age corrections range from 0 17 to 1 20 myr depending on the samples after correction concordance between the revised pb pb and hf w and mn cr ages of plutonic and quenched angrites is good and the initial 53 mn 55 mn 0 ratio in the early solar system ess is recalculated as being 7 1 10 6 at the formation of the solar system the error bar incorporates uncertainty in the absolute age of calcium aluminum rich inclusions cais an uncertainty remains as to whether the al mg and pb pb systems agree in large part due to uncertainties in the pb pb age of cais a systematic difference is found in the u isotopic compositions of quenched and plutonic angrites of 0 17 a difference is also found between the rare earth element ree patterns of these two angrite subgroups the 238 u values are consistent with fractionationduring magmatic evolution of the angrite parent melt stable u isotope fractionation due to a change in the coordination environment of u during incorporation into pyroxene could be responsible for such a fractionation in this context pb pb ages derived from pyroxenes fraction should be corrected using the u isotope composition measured in the same pyroxene fraction

variable tl pb and cd concentrations and isotope compositions of enstatite and ordinary chondrites evidence for volatile element mobilization and decay of extinct 205pb new tl pb and cd concentration and tl pb isotope data are presented for enstatite as well as l and ll type ordinary chondrites with additional cd stable isotope results for the former all three chondrite suites have tl and cd contents that vary by more than 1 2 orders of magnitude but pb concentrations are more uniform as a result of terrestrial pb contamination model calculations based on pb isotope compositions indicate that for more than half of the samples more than 50 of the measured pb contents are due to addition of modern terrestrial pb in part this is responsible for the relatively young and imprecise pb pb ages determined for eh l and ll chondrites which are hence only of limited chronological utility in contrast four particularly pristine el chondrites define a precise pb pb cooling age of 4559 6 ma the enstatite chondrites ecs have highly variable e114 110cd of between about 3 and 70 due to stable isotope fractionation from thermal and shock metamorphism furthermore nearly all enstatite meteorites display e205tl values from 3 3 to 0 8 while a single anomalous sample is highly fractionated in both tl and cd isotopes the majority of the ecs thereby define a correlation of e205tl with e114 110cd which suggests that at least some of the tl isotope variability reflects stable isotope fractionation rather than radiogenic ingrowth of 205tl from 205pb decay considering l chondrites most e205tl values range between 4 and 1 while two outliers with e205tl 10 are indicative of stable isotope fractionation considering only those l chondrites which are least likely to feature pb contamination or stable tl isotope effects the results are in accord with the former presence of live 205pb on the parent body with an initial 205pb 204pb 1 5 1 4 10 4 which suggests late equilibration of the pb tl system 26 113 ma after carbonaceous chondrites ccs the ll chondrites display highly variable e205tl values from 12 5 to 14 9 also indicative of stable isotope effects however the data for three pristine ll3 ll4 chondrites display an excellent correlation between e205tl and 204pb 203tl this defines an initial 205pb 204pb of 1 4 0 3 10 4 equivalent to a 205pb 205tl cooling age of 55 12 24 ma 31 67 ma after ccs

slow cooling of deep crustal granulites and pb loss in zircon abstract we attempted to obtain the magmatic crystallization age of a metamorphosed granulite facies t 800 90 c p 8 1 kbar massif type anorthosite from the ankafotia body of southwest madagascar the sample studied is a coarse grained leuconorite with good preservation of igneous texture and mineralogy although plagioclase which contains abundant rutile and zircon inclusions has been slightly recrystallized thirty three isotope dilution u pb analyses of zircons representing single grain fragments 29 analyses and multi grain fractions 4 analyses yield a spectrum of concordant ages from 631 to 549 ma a time span of more than 80 myr back scattered electron and cathodoluminescence images show that most grains are either homogeneous structureless fragments 35 or are permeated to a variable degree by anastamosing cracks occupied by relatively u and or th enriched zircon 45 a smaller percentage of grains show relict magmatic zoning 20 thin u and or th rich overgrowths occur on about 25 of grains raman spot analyses demonstrate that all fragments are highly crystalline and non metamict there are marked correlations between zircon grain size and internal features such that the oldest grains are larger and show relict magmatic zoning the youngest grains are small fragments containing high u crack networks ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature rare earth element patterns show heavy ree enrichment with negative eu anomalies and positive ce anomalies we suggest that the ca 80 myr spread in concordant u pb ages in this sample is indicative of high temperature pb loss during one or more protracted periods of granulite facies metamorphism with only minor episodic or continuous metamorphic zircon growth volume diffusion and or fracture assisted diffusion seems to be the dominant mechanism of pb loss cooling curves calculated using recently measured pb diffusion parameters conform to the age size relationship and imply very slow cooling rates 1 2 c myr or less as might be expected for a terrane in which granulite conditions were maintained for an extended period of time our results therefore suggest a note of caution for interpretation of concordant zircon ages in meta igneous rocks affected by high grade metamorphism of long duration

la q icp ms apatite u pb geochronology using common pb in plagioclase examples from layered mafic intrusions apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks the la icp ms technique is widely applied to u pb geochronology using various minerals apatite u pb geochronology in contrast to e g zircon is compromised by variable amounts of common pb incorporated into the crystal during growth magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age if this is not the case the initial pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite two approaches are common one being the estimation of common pb from a pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate pb but not u e g feldspar most recent studies applying la icp ms to the analysis of pb isotopes in feldspar utilize either multicollector or magnetic sector mass spectrometers in this study we first evaluate the application of quadrupole mass spectrometry for apatite u pb geochronology combined with pb isotopic measurements in feldspar and compare the results with modeled initial pb isotopic compositions the resulting age information is accurate and precise despite using plagioclase rather than k feldspar as is normally used to define initial pb isotope compositions we apply this method to apatite bearing gabbroic rocks from layered intrusions bushveld bjerkreim sokndal hasvik and skaergaard ranging in age from ca 2 ga to ca 55 ma and generate metamorphic cooling ages generally consistent with the known geologic history of these intrusions

isotopic peculiarities of an archaean pegmatite union mine mica south africa abstract an isotopic investigation rb sr u pb pb pb and sm nd of the archaean union mine pegmatite northern province south africa is reported the intrusion of this pegmatite devoid of zircon and monazite has been dated at 2912 2 6 ma by pb pb step leaching of magmatic garnet this age is supported by an identical two garnet apatite sm nd isochron of 2917 27 ma a rb sr k feldspar garnet albite isochron of 2023 11 ma corresponds to a later hydro thermal overprint isotope resetting during this overprint was incomplete and revealed that various parent daughter isotope pairs within minerals of the union mine pegmatite behaved in exceedingly different manners under the same physico chemical conditions sr isotope resetting was incomplete and resulted in open system behaviour of the pegmatite k feldspar almost quantitatively lost its radiogenic sr which had accumulated between the time of intrusion and the overprint sr loss from muscovite was variable and incomplete as indicated by a wide range in ages from 2088 74 ma to 2744 13 ma only insignificant portions of this sr were incorporated into adjacent albite as revealed by a micro drilled rb sr profile through a muscovite albite grain boundary the 87 sr 86 sr initial ratio is 0 8301 11 for the proterozoic isochron which is far less radiogenic than the calculated value of 2 24 for a closed system it is therefore concluded that sr was lost from the system and that the rb sr ratio of the whole rock was changed apatite on the other hand seems to have behaved as a closed system and retains a low initial sr ratio of 0 7125 u pb analyses of apatite reveal that this mineral has behaved as an almost closed system for pb diffusion as well but was open for u significant loss during the overprint this has resulted in high reverse discordance of apatite u pb analysis and in a geologically meaningless secondary pb pb isochron the data demonstrate that both preferential loss of daughter sr over parent rb and parent u over daughter pb isotopes can occur in some minerals of one and the same rock this process altered the parent daughter ratios to such an extent that in the case of apatite a secondary pb pb isochron with an erroneous age of 4119 66 ma was established it appears likely that preferential loss of daughter or parent isotopes is the reason for spurious secondary whole rock isochrons reported and reviewed by moorbath and taylor 1986 and moorbath et al 1986 such losses far exceeding predictions based on volume diffusion experiments may occur by means of fast transport diffusion which in part may depend on the micro textures of the host minerals geochemical studies of early archaean rocks require substantial back correction for radiogenic decay and the example of the union mine pegmatite shows that changes of parent daughter isotope ratios may occur long after rock formation however such changes may go unnoticed in high precision geochronology i e garnet dating in this study but will lead to erroneous correction for radiogenic decay and hence to erroneous conclusions regarding the origin of the studied samples

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

filling the north american proterozoic tectonic gap 1 60 1 59 ga deformation and orogenesis in southern wyoming usa abstract structural and thermochronologic studies in the sierra madre southern wyoming indicate that parts of the 1 78 1 75 ga cheyenne belt an archean proterozoic suture were reactivated at ca 1 6 ga new shear zones notably the brittle battle lake thrust tear fault zone also formed at this time the battle lake fault zone blfz which displaced parts of the cheyenne belt at least 30 km to the north corresponds to the surface expression of a south dipping reflector imaged by regional seismic studies continental dynamics of the rocky mountains experiment 40ar 39ar and u pb data define age patterns in the sierra madre that are delineated by discrete structures north of the 1 78 ga divide peak mylonite zone a foreland structure associated with the medicine bow orogeny 40ar 39ar biotite dates suggest cooling below 300 c by 2 0 ga south of the divide peak mylonite zone but north of the blfz hornblendes yield apparent 40ar 39ar ages of ca 1 80 1 78 ga that may record tectonic burial and cooli

ion microprobe uranium thorium lead dating of shergotty phosphates we report ion microprobe u th pb dating of shergotty phosphates by means of the sensitive high resolution ion microprobe shrimp recently installed at hiroshima university japan ten analyses of whitlockite merrillite and three analyses of apatite indicate a 238u 206pb isochron age of 225 200 ma and a tera wasserburg concordia constrained linear three dimensional isochron age of 217 110 ma in the 238u 206pb 207pb 206pb204pb 206pb diagram these ages agree well with the 232th 208pb age of 189 83 ma which suggests that primary crystallization or a shock metamorphic event defined the formation age of the phosphate minerals the average of the later two ages 204 68 ma is consistent with the previously published rb sr age of 165 11 ma and u th pb age of 200 ma these show marginal agreement with the 40ar 39ar age of 254 10 ma but are significantly different from the sm nd age of 360 16 ma taking into account the closure temperature of the u pb system in apatite we suggest the time that shergotty last experienced a temperature of 900 c was 204 68 ma

cation diffusion in feldspars feldspars are among the most common minerals in the earth s crust and are found in a variety of rock types diffusion in feldspars has been studied for well over half a century with rosenqvist s 1949 measurements of ra and pb diffusion among the earliest published works major efforts in measuring diffusion in feldspars primarily of the alkali elements were undertaken in the early 1970s with the work of bailey 1971 lin and yund 1972 petrovic 1972 foland 1974 giletti et al 1974 and kasper 1975 many of these early studies used bulk exchange techniques rather than direct depth profiling or employed methods of limited depth resolution such as serial sectioning these methods while state of the art for their time impose several limitations including a range of measurement restricted to relatively fast diffusivities and the difficulty of distinguishing the effects on measured chemical or isotopic compositions that may be a result of diffusional fast paths surface reaction and or other artifacts from those contributions due solely to volume diffusion development of high resolution depth profiling techniques such as sims and rbs have permitted measurement of much smaller diffusivities for a broader range of elements and reduced some of the uncertainties characteristic of bulk exchange methods feldspar diffusion data have been previously summarized in yund 1983 and smith and brown 1988 and were included in brady s 1995 compilation of diffusion data for silicate minerals since the publication of these works there have been numerous diffusion studies of feldspars encompassing a range of compositions in alkali feldspars and across the plagioclase series these have included measurements of diffusion of major elements na k ca ba oxygen and silicon as well as na k and caal nasi interdiffusion diffusion of noble gases and diffusion of minor and trace elements including fe mg sr

crystal surface integrity and diffusion measurements on earth and planetary materials abstract characterization of diffusion behavior in minerals is key to providing quantitative constraints on the ages and thermal histories of earth and planetary materials laboratory experiments are a vital source of the needed diffusion measurements but these can pose challenges because the length scales of diffusion achievable in a laboratory time are commonly less than 1 m an effective strategy for dealing with this challenge is to conduct experiments involving inward diffusion of the element of interest from a surface source followed by quantification of the resulting diffusive uptake profile using a high resolution depth profiling technique such as rutherford backscattering spectroscopy rbs nuclear reaction analysis nra or ion microprobe sims the value of data from such experiments is crucially dependent on the assumption that diffusion in the near surface of the sample is representative of diffusion in the bulk material historical arguments suggest that the very process of preparing a polished surface for diffusion studies introduces defects in the form of dislocations and cracks in the outermost micrometer of the sample that make this region fundamentally different from the bulk crystal in terms of its diffusion properties extensive indirect evidence suggests that in fact the near surface region of carefully prepared samples is no different from the bulk crystal in terms of its diffusion properties a direct confirmation of this conclusion is nevertheless clearly important here we use transmission electron microscopy to confirm that the near surface regions of olivine quartz and feldspar crystals prepared using careful polishing protocols contain no features that could plausibly affect diffusion this finding does not preclude damage to the mineral structure from other techniques used in diffusion studies e g ion implantation but even in this case the role of possible structural damage can be objectively assessed and controlled while all evidence points to the reliability of diffusivities obtained from in diffusion experiments we do not recommend experiments of this type using a powder source as a means of obtaining diffusant solubility or partitioning information for the mineral of interest

the effect of intra crystal uranium zonation on apatite u pb thermochronology a combined id tims and la mc icp ms study abstract quantitative reconstruction of high temperature thermal histories 350 c has applications in metamorphic studies processes which exhume deep crust the evolution of active margins over hundreds of millions of years and the study of extra terrestrial rocks previous studies have demonstrated that apatite u pb data can be used to recover thermal history information at t 350 c from single crystal bulk dates we present u pb bulk grain id tims and intra grain la mc icp ms u pb data from apatites to evaluate the importance of parent isotope u distribution when extracting thermal history information from apatites that have experienced partial pb loss we find that parent u zonation can cause considerable scatter when single bulk grain dates are compared with their grain size and therefore u zonation must be taken into account when inverting u pb dates and grain sizes to seek thermal history solutions we show that accurate thermal history solutions can be obtained from apatites which have zoned concentrations when in situ variations in u concentration and u pb dates are modelled this observation validates the hypothesis that pb is lost by thermally driven volume diffusion in situ analytical techniques yield significantly more accurate thermal history solutions than bulk grain id tims analyses when investigating rocks which contain compositionally zoned apatites

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

uranium lead systematics of phosphates in lunar basaltic regolith breccia meteorite hills 01210 chronological studies of brecciated lunar meteorites have proved difficult because they are mixtures of materials from various sources and the radiometric clocks are sometimes affected by the subsequent impact events on the moon here we report the in situ u pb dating of phosphates in lunar meteorite meteorite hills met 01210 which is a regolith breccia consisting of low ti mare basalt clasts and mineral fragments with a minor anorthositic component in situ analyses of four merrillite and four apatite grains in met 01210 which are resistant to secondary events resulted in a 207pb 206pb 204pb 206pb isochron age of 3904 85 ma 95 confidence limit this phosphate formation age when considered as the crystallisation age of this low ti basalt is similar to crystallization ages of 3 8 3 9 ga for unbrecciated low ti basalt meteorites asuka 881757 and yamato 793169 this result reinforces the hypothesis that all these three meteorites originated from the same area on the moon and were launched by a single impact event consistent with the similarity of launch ages mineralogical and geochemical signatures