arar a software tool to promote the robust comparison of k ar and 40ar 39ar dates published using different decay isotopic and monitor age parameters the comparison of k ar and 40ar 39ar geochronologic data published by different laboratories is markedly hindered by the inconsistent use of the parameters necessary to convert isotopic analyses to dates this problem is particularly acute when we try to evaluate the significance of datasets obtained prior to the development of community consensus values for basic decay constants isotopic abundances and the ages of common monitor minerals unfortunately the effect of using different parameters for the same dataset can sometimes exceed the quoted analytical precision of derived dates we created the argon age recalculator or arar to help researchers account for such discrepancies in a simple efficient manner allowing for more robust comparisons among datasets and more effective compilation of existing datasets using self consistent parameter sets arar is freely available as a platform independent executable application at http group18software asu edu

intercalibration of 40ar39ar dating standards a number of standards used for k ar and 40ar39ar dating have been intercalibrated multiple splits 15 40 mg each of mmhb 1 sb 3 biotite lp 6 biotite 40 60 ga1550 biotite fish canyon tuff 3 biotite taylor creek rhyolite sanidine bern 4 muscovite bern 4 biotite and pennsylvania state university orthoclase 1a were irradiated in capsules fused and analyzed in replicate analyses most of these standards proved to be homogeneous in 40ar 39ark ratio age k ca and 36ar contents mmhb 1 is inhomogeneous in age at the 15 mg level indicating it is unsuitable for use as an interlaboratory standard using material taken straight from the bottle the orthoclase sample is also inhomogeneous at the 30 mg level quoted relative to an age of 162 9 ma for the sb 3 biotite standard total fusion ages are lp 6 biotite 40 60 128 1 ma ga1550 biotite 97 8 ma fish canyon tuff 3 biotite 27 95 ma taylor creek rhyolite sanidine 28 0 ma bern 4 muscovite 18 51 ma bern 4 biotite 17 21 ma these ages are in good agreement with the absolute k ar ages of these minerals different preparations of biotite and sanidine standards from a welded tuff such as the fish canyon tuff may not be of identical ages tests on submilligram splits reveals that sb 3 biotite ga1550 biotite fish canyon tuff 3 biotite and taylor creek rhyolite sanidine are suitable for use as monitors in laser work the sanidine specimen appears to be the most suitable for use as the primary monitor for 40ar39ar dating

three dimensional effects causing artifacts in two dimensional cross borehole electrical imaging summary cross borehole electrical resistance tomography ert experiments utilize downhole electrodes typically placed in a borehole filled with material of contrasting resistivity to the host rock the circular geometry of the boreholes is three dimensional and inversion routines are typically two dimensional because of the resistivity contrast artifacts in the form of borehole inversion effects develop in the resistivity images other 3d effects resulting in inversion artifacts are shadow effects caused by use of a 2d code to invert data from a 3d body located outside the image plane in both cases the inversion model misrepresents the spatial change in voltages as a corresponding spatial change in resistivity borehole inversion effects and shadow effects result as the forward solver attempts to resolve the discrepancy in the voltages for numerous electrode pairs into a meaningful resistivity distribution borehole inversion effects are shown to be related to the resistivity contrast between the borehole fill and the host medium and to borehole diameter borehole inversion effects do not materialize with small diameter boreholes e g 0 1 m when the fill resistivity contrast is one order of magnitude or less however a borehole fill resistivity contrast of two orders of magnitude causes artifacts in the form of sheaths near the boreholes and a conductive artifact between the boreholes larger diameter borehole e g 0 2 m induce significant borehole inversion effects with as little as one order of magnitude fill resistivity contrast the general resistivity patterns are similar for the borehole inversion effects however the artifacts are amplified as resistivity contrast and or borehole diameter increase these results are significant because borehole inversion effects may mask a target heterogeneity or an artifact may be confused for an anomaly resulting in improper actions for site characterizations or remediation strategies suggestions of best practices for experimental design to prevent or minimize borehole inversion effects include minimizing borehole diameter minimizing borehole fill resistivity contrast measuring and inputting the resistivity of the borehole fill water into the model and inverting changes in resistivity for time lapse ert data shadow effects are shown to be related directly to the distance of the target heterogeneity out of the image plane our results show that shadow effects become insignificant when the target is between 3 and 5 m outside the 10 m wide image plane because the size of the target and resistivity contrast of the target to the host rock are site specific variables the actual distance at which shadow effects become insignificant will vary from site to site to reduce misinterpretations by shadow effects reduce the dipole length of the four electrode measurements and supplement data from other well pairs and or other methods

eruptive history of the karoo lava flows and their impact on early jurassic environmental change this paper reports new paleomagnetic and geochronologic data from a 1500 m thick composite section belonging to the drakensberg group the thickest remnant of the karoo lavas in northern lesotho flow by flow analysis of paleomagnetic directions reveals twenty one magnetic directional groups dg corresponding to single eruptive events see and sixteen individual lava flows the new age determinations of lava flows range from 180 1 1 4 to 182 8 2 6 ma these data combined with previous results allow us to propose that the main part of the drakensberg group and the karoo intrusive complex dated around 181 183 ma may have been erupted over a period as short as 250 kyr and may have coincided with the two main phases of extinction in the early toarcian this scenario agrees well with the discontinuous rhythm of environmental and biotic perturbations in the late pliensbachian toarcian interval

saline tracer visualized with three dimensional electrical resistivity tomography field scale spatial moment analysis 1 cross well electrical resistivity tomography ert was used to monitor the migration of a saline tracer in a two well pumping injection experiment conducted at the massachusetts military reservation in cape cod massachusetts after injecting 2200 mg l of sodium chloride for 9 hours ert data sets were collected from four wells every 6 hours for 20 days more than 180 000 resistance measurements were collected during the tracer test each ert data set was inverted to produce a sequence of 3 d snapshot maps that track the plume in addition to the ert experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values using modified moment analysis of the electrical conductivity tomograms the mass center of mass and spatial variance of the imaged tracer plume were estimated although the tomograms provide valuable insights into field scale tracer migration behavior and aquifer heterogeneity standard tomographic inversion and application of archie s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass such underestimation is attributed to 1 reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and 2 spatial smoothing regularization from tomographic inversion the center of mass estimated from the ert inversions coincided with that given by migration of the tracer plume using 3 d advective dispersion simulation the 3 d plumes seen using ert exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well

intercalibration of the hb3gr 40ar 39ar dating standard abstract the 40 ar 39 ar dating technique is based on the knowledge of the age of neutron fluence monitors standards recent investigations have improved the accuracy and precision of the ages of most of the phanerozoic aged standards e g fish canyon tuff sanidine fcs alder creek sanidine ga1550 biotite and lp 6 biotite however no specific study has been undertaken on the older standards i e hb3gr hornblende and nl 25 hornblende generally used to date precambrian high ca k and or meteoritic rocks in this study we show that hb3gr hornblende is relatively homogenous in age composition ca k and atmospheric contamination at the single grain level the mean standard deviation of the 40 ar 39 ar k f value derived from this study is 0 49 comparable to the most homogeneous standards the intercalibration factor which allows direct comparison between standards between hb3gr and fcs is r fcs hb3gr 51 945 0 167 using an age of 28 02 ma for fcs the age of hb3gr derived from the r value is 1073 6 5 3 ma 1 internal error only and 8 8 ma including all sources of error this age is indistinguishable within uncertainty from the k ar age previously reported at 1072 11 ma turner g huneke j c podosek f a wasserburg g j 1971 40 ar 39 ar ages and cosmic ray exposure ages of apollo 14 samples earth planet sci lett 12 19 35 the r value determined in this study can also be used to intercalibrate fcs if we consider the k ar date of 1072 ma as a reference age for hb3gr we derive an age of 27 95 0 19 ma 1 internal error only for fcs which is in agreement with the previous determinations altogether this shows that hb3gr is a suitable standard for 40 ar 39 ar geochronology

electrical resistivity tomography of vadose water movement cross borehole electrical resistivity tomography ert was used to image the resistivity distribution before and during two infiltration experiments in both cases water was introduced into the vadose zone and the change in resistivity associated with the plume of wetted soil was imaged as a function of time the primary purpose of this work was to study the capabilities and limitations of ert to image underground structure and ground water movement in the vadose zone a secondary goal was to learn specifics of unsaturated flow in a complex geologic setting tomographs of electrical resistivity taken before infiltration image coarser well drained soils sands and gravels as more resistive zones whereas finer grained soils silts and clays which hold more water by capillarity are imaged as more conductive images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology in the ert images we see the effects of capillary barriers and infer differences between capillary driven flow through fine sediments and gravity driven flow through very permeable sediments images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs ert can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements

hydrogeophysical tracking of three dimensional tracer migration the concept and application of apparent petrophysical relations 1 direct estimation of groundwater solute concentrations from geophysical tomograms has been only moderately successful because 1 reconstructed tomograms are often highly uncertain and subject to inversion artifacts 2 the range of subsurface conditions represented in data sets is incomplete because of the paucity of colocated well or core data and aquifer heterogeneity and 3 geophysical methods exhibit spatially variable sensitivity we show that electrical resistivity tomography ert can be used to estimate groundwater solute concentrations if a relation between concentration and inverted resistivity is used to deal quantitatively with these issues we use numerical simulation of solute transport and electrical current flow to develop these relations which we call apparent petrophysical relations they provide the connection between concentration or local resistivity and inverted resistivity which is measured at the field scale based on ert for media containing ionic solute the apparent petrophysical relations are applied to tomograms of electrical resistivity obtained from field measurements of resistance from cross well ert to create maps of tracer concentration on the basis of synthetic and field cases we demonstrate that tracer mass and concentration estimates obtained using these apparent petrophysical relations are far better than those obtained using direct application of archie s law applied to three dimensional tomograms from ert which gives severe underestimates

refining the mediterranean messinian gap with high precision u pb zircon geochronology central and northern italy astronomically tuned cyclic sedimentary successions provide unprecedented insight into the temporal evolution of depositional systems and major geologic events however placing astronomically calibrated records into an absolute time frame with confidence requires independent and precise geochronologic constraints astronomical tuning of the precessionally modulated sedimentary cycles of the mediterranean basin deposited during the messinian salinity crisis 5 96 5 33 ma has indicated an 90 k y messinian gap corresponding to the evaporative drawdown of the mediterranean following the closure of the mediterranean atlantic gateway in the messinian deposits a volcanic ash dated by 40 ar 39 ar geochronology was used to anchor the sedimentary cycles to the insolation curve however the uncertainty of the 40 ar 39 ar date introduces a potential two cycle 40 k y uncertainty in the tuning using high precision chemical abrasion thermal ionization mass spectrometry ca tims u pb geochronology on single zircon grains from two messinian ash layers in italy we obtained dates of 5 5320 0 0046 ma and 5 5320 0 0074 ma with sub precessional resolution combined with our astronomical tuning of the messinian lower evaporites the results refine the duration of the messinian gap to at most 28 or 58 9 6 k y which correlates with either the tg12 glacial interval alone or both tg12 and tg14 glacial intervals supporting the hypothesis of a glacio eustatic contribution in fully isolating the mediterranean from the atlantic ocean our new u pb dates also allow us to infer a precessionally modulated cyclicity for the post evaporitic deposits and hence enable us to tune those successions to the insolation curve

leakage detection of water reservoirs using a mise a la masse approach abstract localizing leaks of water and fluids from storage tanks and water reservoirs with geomembranes is an important task for a variety of environmental applications and water resources applications the minimally intrusive mise a la masse method is used to detect leaks with the current injected inside the reservoir and a return current electrode located remotely we test a new approach for the inversion of the voltage data using sandbox experiments and numerical modeling a method similar to the self potential inversion method is proposed to inverse the voltages recorded around the tank or reservoir a global objective function with a data misfit term and regularization term is minimized to invert the voltages in the inversion process a depth weighting matrix is used to strengthen the depth resolution of the current source and the minimum support method is used to avoid oversmoothed results in terms of leak detection the distributions of electrical current density on the walls of reservoir indicate the position of leaks the results show that the inversion method with source compaction accurately identifies the location of single leaks for two separated leaks there is an obvious bias for the deeper hole and the bias increases with its depth for three holes the source compaction method generally identifies the location of the three leaks when their depth ranges are similar when one of the leaks becomes deeper localization of the deeper one becomes more difficult the influence of the size of the leak on the inversion results is also investigated the inversion algorithm overestimates the depth of small leaks while it slightly underestimates the depth of large leaks for a leak having the form of a crack the inversion results using the source compaction method agree with the position of the leak and its shape

a middle late triassic 40ar 39ar age for the paasselka impact structure se finland abstract 40ar 39ar dating of recrystallized feldspar glass particles separated from clast rich impact melt rocks from the approximately 10 km paasselka impact structure se finland yielded a middle to late triassic ladinian karnian pseudo plateau age of 228 7 3 0 3 4 ma 2 this new age makes paasselka the first known triassic impact structure dated by isotopic methods on the baltic shield the new paasselka impact age is within uncertainty coeval with isotopic ages recently obtained for the lake saint martin impact structure in canada indicating a new middle to late triassic impact crater population on earth the comparatively small crater size however suggests no relationship between the paasselka impact and a postulated extinction event at the middle late triassic boundary

establishing a 14 6 0 2 ma age for the nordlinger ries impact germany a prime example for concordant isotopic ages from various dating materials abstract 40ar 39ar dating of recrystallized k feldspar melt particles separated from partially molten biotite granite in impact melt rocks from the approximately 24 km nordlinger ries crater southern germany yielded a plateau age of 14 37 0 30 0 32 ma 2 this new age for the nordlinger ries is the first age obtained from 1 monomineralic melt 2 separated from an impact metamorphosed target rock clast within 3 ries melt rocks and therewith extends the extensive isotopic age data set for this long time studied impact structure the new age goes very well with the 40ar 39ar step heating and laser probe dating results achieved from mixed glass samples suevite glass and tektites and is slightly younger than the previously obtained fission track and k ar and ages of about 15 ma as well as the k ar and 40ar 39ar age data obtained in the early 1990s taking all the 40ar 39ar age data obtained from ries impact melt lithologies into account data from the literature and this study we suggest an age of 14 59 0 20 ma 2 as best value for the ries impact event

electrical characterization of non fickian transport in groundwater and hyporheic systems 1 recent work indicates that processes controlling solute mass transfer between mobile and less mobile domains in porous media may be quantified by combining electrical geophysical methods and electrically conductive tracers whereas direct geochemical measurements of solute preferentially sample the mobile domain electrical geophysical methods are sensitive to changes in bulk electrical conductivity bulk ec and therefore sample ec in both the mobile and immobile domains consequently the conductivity difference between direct geochemical samples and remotely sensed electrical geophysical measurements may provide an indication of mass transfer rates and mobile and immobile porosities in situ here we present 1 an overview of a theoretical framework for determining parameters controlling mass transfer with electrical resistivity in situ 2 a review of a case study estimating mass transfer processes in a pilot scale aquifer storage recovery test and 3 an example application of this method for estimating mass transfer in watershed settings between streams and the hyporheic corridor we demonstrate that numerical simulations of electrical resistivity studies of the stream hyporheic boundary can help constrain volumes and rates of mobile immobile mass transfer we conclude with directions for future research applying electrical geophysics to understand field scale transport in aquifer and fluvial systems subject to rate limited mass transfer

application of the mise a la masse method to detect the bottom leakage of water reservoirs abstract leakages of reservoirs are responsible for the loss of water resources and the spread of contaminants we develop a methodology to detect leaks located at the bottom side of a reservoir with the minimally invasive mise a la masse method which involves the potential electrodes located at the ground surface around the reservoir and the current source and sink placed in and out of the reservoir respectively this method allows localizing the secondary current distribution associated with leakage using the distribution of potential recorded on a set of electrodes during the mise a la masse experiment an initial model based on the distribution of root mean square values between the observation and the simulation data is first given to the inversion algorithm the tikhonov regularization which includes a weighting matrix and a minimum support function is used to strengthen the detection resolution of the leak 29 sandbox experiments show that the proposed method and inversion algorithm can localize a single leak for a leak with a crack shape the inversion algorithm detects the location of the leak with a small bias when the leak lasts a conductive zone may occur below the leak due to the increase of water content or ionic strength of the pore water the occurrence of such a conductive body could affect the localization of the leak because the conductivity distribution may not be well resolved the effect is analyzed using synthetic experiments the results show that the bias between the real leak and inversion results increases with the position and size of the undetected conductive zone two small scale field tests were conducted to test the performance of the mise a la masse method the two leaks are properly identified this study provides an efficient approach to detect the bottom leakage of reservoirs

effects of fluid rock interaction on 40ar 39ar geochronology in high pressure rocks sesia lanzo zone western alps abstract in situ uv laser spot 40ar 39ar analyses of distinct phengite types in eclogite facies rocks from the sesia lanzo zone western alps italy were combined with sims boron isotope analyses as well as boron b and lithium li concentration data to link geochronological information with constraints on fluid rock interaction in weakly deformed samples apparent 40ar 39ar ages of phengite cores span a range of 20 ma but inverse isochrons define two distinct main high pressure hp phengite core crystallization periods of 88 82 and 77 74 ma respectively the younger cores have on average lower b contents 36 g g than the older ones 43 48 g g suggesting that loss of b and resetting of the ar isotopic system were related phengite cores have variable 11b values 18 to 10 indicating the lack of km scale b homogenization during hp crystallization overprinted phengite rims in the weakly deformed samples generally yield younger apparent 40ar 39ar ages than the respective cores they also show variable effects of heterogeneous excess 40ar incorporation and ar loss one acceptable inverse isochron age of 77 1 1 1 ma for rims surrounding older cores 82 6 0 6 ma overlaps with the second period of core crystallization compared to the phengite cores all rims have lower b and li abundances but similar 11b values 15 to 9 reflecting internal redistribution of b and li and internal fluid buffering of the b isotopic composition during rim growth the combined observation of younger 40ar 39ar ages and boron loss yielding comparable values of both parameters only in cores and rims of different samples is best explained by a selective metasomatic overprint in low permeability samples this overprint caused recrystallization of phengite rims whereas higher permeability in other samples led to complete recrystallization of phengite grains strongly deformed samples from a several km long blueschist facies shear zone contain mylonitic phengite that forms a tightly clustered group of relatively young apparent 40ar 39ar ages 64 7 68 8 ma yielding an inverse isochron age of 65 0 3 0 ma almost complete b and li removal in mylonitic phengite is due to leaching into a fluid the b isotopic composition is significantly heavier than in phengites from the weakly deformed samples indicating an external control by a high 11b fluid 11b 7 4 we interpret this result as reflecting phengite recrystallization related to deformation and associated fluid flow in the shear zone this event also caused partial resetting of the ar isotope system and further b loss in more permeable rocks of the adjacent unit we conclude that geochemical evidence for pervasive or limited fluid flow is crucial for the interpretation of 40ar 39ar data in partially metasomatized rocks

annama h chondrite mineralogy physical properties cosmic ray exposure and parent body history the fall of the annama meteorite occurred early morning local time on april 19 2014 on the kola peninsula russia based on mineralogy and physical properties annama is a typical h chondrite it has a high ar ar age of 4 4 ga its cosmic ray exposure history is atypical as it is not part of the large group of h chondrites with a prominent 7 8 ma peak in the exposure age histograms instead its exposure age is within uncertainty of a smaller peak at 30 4 ma the results from short lived radionuclides are compatible with an atmospheric pre entry radius of 30 40 cm however based on noble gas and cosmogenic radionuclide data annama must have been part of a larger body radius 65 cm for a large part of its cosmic ray exposure history the 10be concentration indicates a recent 3 5 ma breakup which may be responsible for the annama parent body size reduction to 30 35 cm pre entry radius

ar ar ages and thermal histories of enstatite meteorites abstract compared with ordinary chondrites there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies in this study we report 39ar 40ar dating results for five el chondrites khairpur pillistfer hvittis blithfield and forrest five eh chondrites parsa saint marks indarch bethune and reckling peak 80259 three igneous textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies zaklodzie queen alexandra range 97348 and queen alexandra range 97289 and three aubrites norton county bishopville and cumberland falls several ar ar age spectra show unusual 39ar recoil effects possibly the result of some of the k residing in unusual sulfide minerals such as djerfisherite and rodderite and other age spectra show 40ar diffusion loss few additional ar ar ages for enstatite meteorites are available in the literature when all available ar ar data on enstatite meteorites are considered preferred ages of nine chondrites and one aubrite show a range of 4 50 4 54 ga whereas five other meteorites show only lower age limits over 4 35 4 46 ga ar ar ages of several enstatite chondrites are as old or older as the oldest ar ar ages of ordinary chondrites which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies or were metamorphosed to lower temperatures compared to other chondrite types many enstatite meteorites are brecciated and or shocked and some of the younger ar ar ages may record these impact events although impact heating of ordinary chondrites within the last 1 ga is relatively common for ordinary chondrites only bethune gives any significant evidence for such a young event

direct current and self potential monitoring of an evolving plume in partially saturated fractured rock abstract hole surface and hole hole mise a la masse malm measurements and surface self potential sp measurements were taken over a 173 day period during an 8 well radial injection tracer experiment in partially saturated fractured columbia river basalt a potassium chloride tracer stream was injected into the fractured basalt and was energized directly for the malm experiments the constant concentration tracer solution was injected for 76 days above a perched water table at an average rate of 10 07 l day tracer injection was followed by a 34 day period of tap water injection and a 63 day drainage period the associated changes in potential distributions and sp s over time indicate that an asymmetrical vadose zone plume evolved in the fractured basalt hole surface and sp measurements were used to track the lateral evolution of the tracer plume however there was some noise and variability in the sp data the hole hole measurements helped delineate the depth of preferential tracer migration in the fractured basalt

electrode arrays for measuring groundwater flow direction and velocity the movement of 2000 liters of salt water after injection into groundwater within gravels a few meters below the ground surface at three injection sites was traced by six different resistivity monitoring arrays the resistivity rectangle schlumberger sounding wenner sounding wenner fixed spacing mise a la asse and downhole electrode array five of the arrays indicated groundwater flow direction and seepage velocity as evidence indicates similar geological and hydrogeological conditions exist at the injection sites therefore comparisons between the sensitivity of the five arrays can be made and are as follows resistivity rectangle maximum decrease of 60 percent in derived potential differences schlumberger sounding maximum decrease of 28 percent in measured apparent resistivity wenner sounding maximum decrease of 20 percent in measured apparent resistivity wenner fixedspacing maximum decrease of 22 percent in apparent resistivity downhole electrode maximum decrease of 38 percent in measured r

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

40ar 39ar step heating ages of north american tektites and of impact melt rock samples from the chesapeake bay impact structure abstract this study presents 40ar 39ar step heating ages of four north american tektites three bediasites and one georgiaite and of two groundmass samples extracted at different depths from clast rich impact melt rocks cb w61 and cb w84 recovered by the usgs icdp eyreville b drill core about 9 km from the centre of the chesapeake bay impact structure radiometric age determination on both north american tektites and impact melt rocks from within chesapeake bay crater offers the first possibility to confirm the origin of these tektites for this aim argon isotopic data from 13 samples aliquots of tektite rims cores and bulk and 4 samples aliquots from two impact melt rocks were obtained over 15 to 26 step heating extractions age spectra of all tektite samples show plateaux comprising 62 98 of the 39ar release over consecutive intermediate and high temperature heating steps few low temperature extractions indicate excess 40ar inverse isochron 40ar 36ar intercepts of tektite samples are indistinguishable from air 295 5 however impact melt rock spectra presented complex ar release affecting primarily the low temperature heating steps inverse isochrones indicate excess argon from which the 40ar 36ar intercept was used to correct the age calculation cb w61 and cb w61 2 40ar 36ar intercepts are 354 5 2 5 and 327 2 6 3 respectively and those for cb w84 and cb w84 2 are 332 0 7 3 and 329 6 5 6 respectively the inverse isochron weighted mean age according to currently suggested k decay constants revisions by schwarz et al 2011 and renne et al 2011 for all four tektites is 34 86 0 25 ma mswd 0 96 p 0 41 n 4 and for the two impact melt rocks is 37 16 3 65 ma mswd 0 83 p 0 36 the combined tektite and impact melt rocks isochron mean age of 34 86 0 23 0 32 ma mswd 0 87 p 0 43 is slightly though not significantly higher than the plateau mean age of 34 55 0 27 0 36 ma mswd 0 66 p 0 62 placing this age in the global stratotype section and point gssp marine section exposed at massignano italy it falls below the eocene oligocene e o boundary overlapping with the 10 28 m ir anomaly these results agree within errors with previously reported ages of 35 20 0 54 ma especially those derived from k ar and ar ar total fusion analysis an age of 34 86 0 32 ma sets the chesapeake bay impact event close to the youngest of the three ir anomalies at 35 0 ma in the case the impactor was ir rich e g a chondrite primitive achondrite stony iron or iron meteorite the concordance with the e o boundary at 33 9 ma seems only marginally possible and only if the ir contribution from the ejecta were potentially due either to its small amount becoming diluted in the geologic record or the impactor being ir poor e g of differentiated achondritic composition this study also brings to front the need to re establish the stratigraphic and palaeo magnetic correlations across the globe for the ir anomalies and the magneto stratigraphy during the mid to late eocene and early oligocene and the need to re evaluate the markers for the eocene oligocene boundary

40ar 39ar step heating of impact glasses from the nordlinger ries impact crater implications on excess argon in impact melts and tektites seven impact melts from various places in the nordlinger ries were dated by 40ar 39ar step heating the aim of these measurements was to increase the age data base for ries impact glasses directly from the ries crater because there is only one ar ar step heating spectrum available in the literature almost all samples display saddle shaped age spectra indicating the presence of excess argon in most ries glass samples most probably inherited argon from incompletely degassed melt and possibly also excess argon incorporated during cooling from adjacent phases in contrast moldavites usually contain no inherited argon probably due to their different formation process implying solidification during ballistic transport the plateau age of the only flat spectrum is 14 60 0 16 0 20 ma 2 while the total age of this sample is 14 86 0 20 0 22 ma isochron age 14 72 0 18 0 22 ma 2 proofing the chronological relationship of the ries impact and moldavites the total ages of the other samples range between 15 77 0 52 and 20 4 1 0 ma 2 implying approximately 2 40 excess 40ar compared to the nominal age of the ries crater in respective samples thus the age of 14 60 0 16 0 20 2 14 75 0 16 0 20 ma 2 calculated using the most recent suggestions for the k decay constants can be considered as reliable and is within uncertainties indistinguishable from the most recent compilation for the age of the moldavite tektites

a rhaetian 40 ar 39 ar age for the rochechouart impact structure france and implications for the latest triassic sedimentary record ar 39 ar dating of potassium feldspar primary spherulitic blocky and secondary idiomorphic k feldspar separated from impact metamorphosed gneiss found near videix in the western central part of the rochechouart impact structure nw massif central france yielded a rhaetian combined age of 201 2 ma 2r indistinguishable within uncertainty from the age of the triassic jurassic boundary ballen quartz intergrown with the primary k feldspar indicates post shock temperatures exceeding approximately 1000 c that affected the precursor gneiss geochemically both feldspar types represent essentially pure potassium end members apart from the approximately 15 km diameter impact deposit area the youngest crystallization age known for basement rocks in this part of the massif central is approximately 300 ma no endogenic magmatic thermal events are known to have occurred later in this region the k feldspar recrystallized from local feldspar melts and superimposed post shock hydrothermal crystallization probably within some thousands of years after the impact it is therefore suggested that the 40 ar 39 ar age for the videix gneiss as a potassic impact metasomatite dates the rochechouart impact in consistence with evidence for k metasomatism in the rochechouart impactites the new age value is distinctly younger than the previously obtained karnian norian age for rochechouart and thus contradicts the late triassic multiple impact theory postulated some years ago in agreement with the paleogeographic conditions in the western tethys domain around the triassic jurassic boundary the near coastal to shallow marine rochechouart impact is compatible with the formation of seismites and tsunami deposits in the latest triassic of the british isles and possible related deposits in other parts of europe

40ar 39ar laser dating of tektites from the cheb basin czech republic evidence for coevality with moldavites and influence of the dating standard on the age of the ries impact abstract moldavites central european tektites are genetically related to the impact event that produced the 24 km diameter ries crater in germany representing one of the youngest large impact structures on earth although several geochronological studies have been completed there is still no agreement among 40ar 39ar ages on both moldavites and glasses from ries suevites even recently published data yielded within sample mean ages with a nominal spread of more than 0 6 ma 14 24 14 88 ma this age spread which significantly exceeds current internal errors must be in part ascribed to geological and or analytical causes this study reports the results of a detailed geochronological investigation of moldavites from the cheb area czech republic which have never been dated before and for comparison of two samples from type localities one in southern bohemia and the other in western moravia we used 40ar 39ar laser step heating and total fusion techniques in conjunction with microscale petrographic and chemical characterization in addition with the purpose of ascertaining the influence of the dating standards on the age of the ries impact and making data from this study and literature consistent with the now widely used fish canyon sanidine fcs standard we performed a direct calibration of multi grain splits of the fish canyon biotite fct 3 with fcs the intercalibration factors r fcs fct 3 determined for eight stack positions in one of the three performed irradiations were indistinguishable within errors and gave an arithmetic mean and a standard deviation of 1 0086 0 0031 2 in agreement with previous works suggesting that biotite from the fish canyon tuff is somewhat older 0 8 than the coexisting sanidine laser total fusion analysis of milligram to sub milligram splits of five tektite samples from the cheb area yielded mostly concordant intrasample 40ar 39ar ages and within sample weighted mean ages of 14 66 0 08 14 75 0 12 ma 2 internal errors ages relative to fcs that overlap within errors these ages match those obtained for samples from western moravia 14 66 0 08 ma and southern bohemia 14 68 0 11 ma supporting the genetic link between cheb basin tektites and moldavites and consequently between cheb basin tektites and the ries impact in contrast to samples from the cheb area and moravia 40ar 39ar ages from total fusion experiments on the bohemian specimen ranged widely from 14 6 to 17 0 ma older apparent ages however were systematically obtained from fragments characterized by visible surface alteration laser step heating experiments although displaying slightly disturbed age profiles were in line with total fusion analyses and yielded well defined plateau ages of 14 64 0 11 14 71 0 11 ma 2 internal errors ages relative to fcs a thorough comparison of our and previous 40ar 39ar ages on both moldavites and ries suevite glasses recalculated relative to the 40ar 40k ratio recently determined for fcs using intercalibration factors available in or derivable from the literature reveals some inconsistencies which may be ascribed to either geological or analytical causes based on our data decay constants in current use in geochronology and ages calculated relative to fcs we infer that the age of moldavites is 14 68 0 11 ma 2 neglecting uncertainties in the 40k decay constants

39ar 40ar ages of eucrites and thermal history of asteroid 4 vesta eucrite meteorites are igneous rocks that derived from a large asteroid probably 4 vesta past studies have shown that after most eucrites formed they underwent metamorphism in temperatures up to 800 c much later many were brecciated and heated by large impacts into the parent body surface the less common basaltic unbrecciated eucrites also formed near the surface but presumably escaped later brecciation while the cumulate eucrites formed at depths where metamorphism may havepersisted for a considerable period to further understand the complex hed parent body thermal history we determined new 39ar 40ar ages for 9 eucrites classified as basaltic but unbrecciated 6 eucrites classified as cumulate and several basaltic brecciated eucrites precise ar ar ages of 2 cumulate eucrites moama and eet 87520 and 4 unbrecciated eucrites give a tight cluster at 4 48 0 02 gyr not including any uncertainties in the flux monitor age ar ar ages of 6 additional unbrecciated eucrites are consistent with this age within theirrelatively larger age uncertainties by contrast available literature data on pb pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4 4 4 515 gyr and 147sm 143nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4 41 4 55 gyr similar ar ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites as was previously proposed we suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the k ar and some other chronometers to remain as open diffusion systems from the strong clustering of ar ar ages at 4 48 gyr we propose that these meteorites were excavated from depth in a single large impact event 4 48 gyr ago which quickly cooled the samples and started the k ar chronometer a large 460 km crater postulated to exist on vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with vesta some pb pb and sm nd ages of cumulate and unbrecciated eucrites are consistent with the ar ar age of 4 48 gyr and the few older pb pb and sm nd ages may reflect an isotopic closure before the large cratering event one cumulate eucrite gives an ar ar age of 4 25 gyr 3 additional cumulate eucrites give ar ar ages of 3 4 3 7 gyr and 2 unbrecciated eucrites give ar ar ages of 3 55 gyr we attribute these younger ages to a later impact heating furthermore the ar ar impact reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3 5 4 1 gyr among these piplia kalan the first eucrite to show evidence for extinct 26al was strongly impact heated 3 5 gyr ago when these data are combined with eucrite ar ar ages in the literature they confirm that several large impact heating events occurred on vesta between 4 1 3 4 gyr ago the onset of major impact heating may have occurred at similar times for both vesta and the moon but impact heating appears to have persisted for a somewhat later time on vesta

40ar 39ar and cosmic ray exposure ages of nakhlites nakhla lafayette governador valadares and chassigny we present 40ar 39ar dating results of handpicked mineral separates and wholerock samples of nakhla lafayette and chassigny our data on nakhla and lafayette and recently reported ages for some nakhlites and chassigny misawa et al 2006 park et al 2009 point to formation ages of approximately 1 4 ga rather than 1 3 ga that is consistent with previous suggestions of close in time formation of nakhlites and chassigny in lafayette mesostasis we detected a secondary degassing event at approximately 1 1 ga which is not related to iddingsite formation it may have been caused by a medium grade thermal event resetting the mesostasis age but not influencing the k ar system of magmatic inclusions and the original igneous texture of this rock cosmic ray exposure ages for these meteorites and for governador valadares were calculated from bulk rock concentrations of cosmogenic nuclides 3he 21ne and 38ar individual results are similar to literature data the considerable scatter of t3 t21 and t38 ages is due to systematic uncertainties related to bulk rock and target element chemistry production rates and shielding effects this hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and chassigny from a confined martian surface terrain eugster 2003 garrison and bogard 2005 cosmic ray exposure ages from stepwise release age spectra using 38ar and neutron induced 37ar from ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates although not from shielding conditions

early miocene arc volcanism in the mexico city basin inception of the trans mexican volcanic belt abstract in central mexico remnants of the early volcanic activity in the trans mexican volcanic belt tmvb are exposed at sierra de guadalupe in the northern part of the mexico city basin and to the south in the tepoztlan malinalco tenancingo and chiltepec areas a few published studies indicate a general early to middle miocene age for some of these volcanic centers we present new geologic geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character shedding light on the geodynamic evolution during this period the sierra de guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase ortho and clinopyroxene plus amphibole phenocrysts as well as dacitic lava flows and domes with plagioclase orthopyroxene amphibole and or biotite phenocrysts pyroclastic and epiclastic lahars deposits are also observed in the northern slopes of this volcanic complex the chemical composition of the sierra de guadalupe rocks is calc alkaline andesitic to dacitic new 40ar 39ar and u pb zircon ages demonstrate that volcanic activity at sierra de guadalupe lasted ca 5 0 ma from ca 20 1 ma to 14 8 ma remnants of volcanic centers found in the tepoztlan malinalco tenancingo and chiltepec areas 80 km to the south show similar compositions and eruptive styles with 40ar 39ar ages ranging between 21 and 16 ma rocks with similar ages and lithologies were also described in deep wells drilled in the mexico city basin as a whole the age distribution shows that early to middle miocene volcanism making up the initial activity of the tmvb migrated toward the north from tepoztlan 310 km from the trench up to pachuca 445 km from the trench and is likely associated with a shallowing the subducting cocos slab underneath this region our results confirm that magmatism in the tmvb began in the early miocene by dehydration of the cocos slab as attested by high ba la and th yb ratios

sahel droughts and enso dynamics correlations between summer sahel rainfall and southern oscillation index has increased during the last thirty years at high frequency time scale periods lower than 8 years an intertropical atlantic zonal divergent circulation anomaly is forced by the difference of sea surface temperature sst anomalies between the eastern equatorial parts of pacific and atlantic this zonal connection worked well during most of the e1 nino southern oscillation enso events occurring after 1970 positive negative sst anomalies in the eastern pacific atlantic led to rainfall deficits over the whole west africa at low frequency time scale periods greater than 8 years positive sst anomalies in the indian ocean and in equatorial pacific existing after 1970 have been associated with decreasing rainfall intensity over west africa through another zonal divergent circulation these different time scales remote sst forcings are combined to provide a global zonal divergent circulation anomaly pattern which could explain the strong association between sahel drought and enso dynamics after 1970

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

wa1ms a 2 61 ga muscovite standard for 40ar 39ar dating abstract the 40ar 39ar dating technique requires the use of neutron fluence monitors standards to allow calculation of the age of a sample precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40ar 39ar dates few fully intercalibrated 40ar 39ar standards with a sufficient total fusion grain to grain reproducibility are currently in use in the argon community for precambrian samples only hb3gr hornblende 1 08 ga yields sufficient grain to grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements here we present chemical and intercalibration results for a new 2 61 ga standard wa1ms is a muscovite extracted from an archaean shear zone in the lake johnston greenstone belt western australia in situ trace element analysis by ela icpms revealed consistent k contents subtle zonation and intra grain and grain to grain heterogeneities in rb sr ti and fe but a lack of mineral inclusions wa1ms has been investigated over 3 irradiations ranging from 25 to 40 h in two reactors with several disc positions and three grains sizes and has been calibrated against fcs and ga1550 and hb3gr overall we carried out 48 total fusion and 4 step heating experiments of wa1ms crystals flat age spectra and average f value 40ar 39ark relative standard deviations ranging from of 0 43 to 0 60 p 0 15 0 83 for 47 48 analyses demonstrate the reproducibility of wa1ms and its suitability as a reliable 40ar 39ar standard we calculated r wa1ms fcs 205 59 0 25 r wa1ms ga1550 57 25 0 06 and r wa1ms hb3gr 3 9713 0 014 all with p 0 14 allowing direct comparison between wa1ms and any standards in used in the community provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards the recently revised decay constant values and standard ages proposed by renne et al 2011 yield a weighted mean age of 2614 2 1 5 ma 0 055 p 0 8 1 for wa1ms against the three standards when calibrated solely against fcs in order to minimize correlated errors between fcs vs ga1550 and hb3gr we calculate a preferred age of 2613 0 2 4 ma 0 09 which is recommended when using this standard additionally this corresponds to a 40ar 40k value of 0 3375 0 0057 10 1 for wa1ms which is independent of the value of the decay constant used for calculation and allows recalculation of the age of wa1ms for any preferred set of decay constants and standard ages using the constants proposed by steiger and jager 1977 directly calculated from the three r values yields an age of 2598 2 3 5 ma corresponding to an identical although less precise 40ar 40k value of 0 3375 0 0086 10 1 benefits of this standard include 1 an increased relative precision per crystal analyzed relative to other standards for proterozoic and archaean samples 2 no post irradiation waiting time due to the absence of ca no need to wait for 37arca decay and 3 loading of standard grains that are optically different from the unknown e g pyroxene or hornblende to avoid standard unknown mixing wa1ms is freely available to the 40ar 39ar community

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

intercalibration of 40ar 39ar age standards nl 25 hb3gr hornblende ga1550 sb 3 hd b1 biotite and bmus 2 muscovite abstract 40 ar 39 ar dating requires precise and accurate age data of standard fluence monitors besides primary calibration of k and 40 ar concentrations i e the ages are calculated with precise k ar dating intercalibration of the different monitors in use provides important cross checks of the validity of standard ages and implicitly reliability and uncertainty of chronological studies that rely on these standards here we report intercalibration of 5 primary international 40 ar 39 ar standards and one internal laboratory monitor a major goal of this study was to check the ages of the nl 25 hornblende standard and of the muscovite laboratory standard bmus 2 against the international standards hd b1 ga1550 sb 3 and hb3gr in replicate measurements of three irradiation series 40 ar 39 ar ages for the fluence monitors were found which agree within 1 error limits with the reported age values of 2660 9 ma for nl 25 hb3gr 1073 6 4 6 ma bmus 2 328 5 1 1 ma sb 3 162 9 0 8 ga1550 98 79 0 54 and 24 21 0 32 for hd b1 for the hornblende nl 25 fluence monitor a new slightly different 40 ar 39 ar age of 2657 4 ma is suggested for the age of the muscovite bmus 2 laboratory standard we recommend 328 5 1 1 ma and for biotite hd b1 a more precise value of 24 18 0 09 ma

imaging and characterisation of subsurface solute transport using electrical resistivity tomography ert and equivalent transport models abstract we assess the usefulness of electrical resistivity tomography ert in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers a field tracer experiment was conducted at the krauthausen test site germany the spatial and temporal evolution of the injected nabr solute plume was monitored in a 2d ert image plane located downstream of the injection well for 90 days since ert maps changes in bulk electrical conductivity the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation the concentration maps are then analysed using an equivalent convection dispersion model cdm which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity as demonstrated by associated synthetic model studies ert resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm since for heterogeneous media local concentrations within the plume deviate from those predicted by the equivalent cdm we also interpret the ert derived pixel breakthrough curves in terms of an equivalent stream tube model stm the stm represents transport in the aquifer by a set of 1d convection dispersion processes allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified we believe that the observed tracer plume is satisfactorily described by the equivalent cdm probably because the tracer plume was small relative to the heterogeneity scale of the aquifer even though application of the stm revealed some deviation from the ideal homogeneous case the equivalent dispersivity in the stm matches the longitudinal dispersivity of the cdm closely consistent with predominantly homogeneous mixing however the stm analysis illustrates how ert results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers

40ar 39ar age determinations of lunar basalt meteorites asuka 881757 yamato 793169 miller range 05035 la paz icefield 02205 northwest africa 479 and basaltic breccia elephant moraine 96008 40ar 39ar data are presented for the unbrecciated lunar basaltic meteorites asuka a 881757 yamato y 793169 miller range mil 05035 lapaz icefield lap 02205 northwest africa nwa 479 paired with nwa 032 and basaltic fragmental breccia elephant moraine eet 96008 stepped heating 40ar 39ar analyses of several bulk fragments of related meteorites a 881757 y 793169 and mil 05035 give crystallization ages of 3 763 0 046 ga 3 811 0 098 ga and 3 845 0 014 ga which are comparable with previous age determinations by sm nd u pb th pb pb pb and rb sr methods these three meteorites differ in the degree of secondary 40ar loss with y 793169 showing relatively high ar loss probably during an impact event 200 ma ago lower ar loss in mil 05035 and no loss in a 881757 bulk and impact melt glass bearing samples of lap 02205 gave similar ages 2 985 0 016 ga and 2 874 0 056 ga and are consistent with ages previously determined using other isotope pairs the basaltic portion of eet 96008 gives an age of 2 650 0 086 ga which is considered to be the crystallization age of the basalt in this meteorite the ar release for fragmental basaltic breccia eet 96008 shows evidence of an impact event at 631 20 ma the crystallization age of 2 721 0 040 ga determined for nwa 479 is indistinguishable from the weighted mean age obtained from three samples of nwa 032 supporting the proposal that these meteorites are paired the similarity of 40ar 39ar ages with ages determined by other isotopic systems for multiple meteorites suggests that the k ar isotopic system is robust for meteorites that have experienced a significant shock event and not a prolonged heating regime

new 40ar 39ar dating of the clearwater lake impact structures quebec canada not the binary asteroid impact it seems abstract the two clearwater lake impact structures quebec canada are generally interpreted as a crater doublet formed by the impact of a binary asteroid here arguments are presented that raise important questions about the proposed double impact scenario new 40 ar 39 ar dating of two virtually fresh impact melt rock samples from the 36 km west clearwater lake impact structure yielded two statistically robust early permian plateau ages with a weighted mean of 286 2 2 2 2 6 ma 2 mswd 0 33 p 0 57 in contrast 40 ar 39 ar results for two chloritized melt rocks from the 26 km east clearwater lake impact structure produced disturbed age spectra suggestive of a distinct extraneous argon component although individually weakly robust age spectra corrected for the trapped argon component and inverse isochron plots for the east clearwater melt rocks consistently yielded apparent ages around 460 470 ma no permian signal was found in either of these melt aliquots our new 40 ar 39 ar results reproduce earlier 40 ar 39 ar plateau ages 283 ma and 465 ma respectively for the two impact structures by bottomley et al 1990 and are in conflict with a previous statistically non robust rb sr age of 287 293 26 ma for east clearwater the combined cluster of apparent ages of 460 470 ma derived from four different samples across the impact melt sheet is very unlikely to represent a false age effect due to the incorporation of extraneous argon into the melt instead it strongly favors a middle ordovician age for the east clearwater impact and impact induced hydrothermal chloritization moreover the clearwater impact structures are characterized by different natural remanent magnetizations testifying to separate geologic histories an effect unexpected in the case of a permian double impact whereas the west clearwater impact affected ordovician carbonates incorporated into the impact breccia drill core reports from the 1960s concluded that clasts of ordovician sedimentary rocks are seemingly absent in the impact breccia lens of the east clearwater lake impact structure which is overlain by 100 m of post impact sandstones shales and carbonates no resolvable impactor contamination has so far been detected in the west clearwater impact melt rocks whereas east clearwater carries a distinct ordinary possibly l chondritic impactor signature in its melt rocks east clearwater lake might thus represent one among a long list of ordovician impact structures in north america and northern europe that were presumably formed in response to the l chondrite asteroid breakup event 470 ma ago paleogeographic reconstructions show that the ordovician east clearwater impact probably occurred in a near coastal to shallow marine setting while the permian west clearwater impact hit continental pangaea along with the new 40 ar 39 ar data the paleomagnetic sedimentologic and geochemical findings suggest that the close spatial arrangement of the two clearwater lakes is most likely pure coincidence the two impact structures seem to represent a false doublet struck by impacts separated by 180 million years in time the new results for the clearwater lake impact structures have major implications for the reliable identification of doublet impact craters and the rate of binary asteroid impacts on earth and on other planetary bodies in the inner solar system

40ar 39ar age of plagioclase from acapulco meteorite and the problem of systematic errors in cosmochronology acapulcoites consistently yield some of the oldest 40ar 39ar ages reported for any meteorites all indistinguishable from the age of 4510 22 ma previously determined for whole rock samples of acapulco these ages are significantly younger at face value than the 4557 2 ma pb pb age reported for phosphates which dates cooling through 550 c metallographic evidence for rapid cooling between 650 and 350 c retention of 129xe in apatite and the absence of shock features all suggest that the 40ar 39ar ages might be expected to be more consistent with the pb pb age new 40ar 39ar data from plagioclase determined to be the sole k rich phase in acapulco yield a high precision isochron age of 4507 18 ma 2 based on currently accepted decay constants and standards but neglecting uncertainties in these quantities these data allow unambiguous association of the 40ar 39ar age with a closure temperature near 300 c and preclude the possibility of artifacts due to 39ar recoil low temperature alteration products or inherited pre solar grains the 40ar 39ar plagioclase age recalculated with decay constants believed to be more accurate than those conventionally used is 4554 ma weakening the basis of a previously inferred dramatic decrease in cooling rate shortly after closure of the pb pb system in phosphate systematic errors in 40ar 39ar dating associated chiefly with 40k decay constants and standards must be redressed before the method can be applied to high resolution inference of early solar system evolution similar limitations affect ages based on other long lived radionuclides such as 147sm

a tracer test in a shallow heterogeneous aquifer monitored via time lapse surface electrical resistivity tomography we illustrate a case study of a saline tracer test in a shallow highly heterogeneous aquifer monitored by means of surface time lapse ert the test was aimed at identifying the system s hydraulic properties some of the expected limitations of the method particularly caused by the strong decrease in ert resolution with depth and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location the vast heterogeneity of the system ranging in lithology from clay to gravel at a scale of meters to tens of meters reflects itself in the tracer migration and distribution over time the tracer is trapped in the low permeability regions and from these it is slowly released over time high resolution surface ert proves effective at picturing this system behavior over time the extreme heterogeneity is also a challenge in the attempt to translate bulk electrical

potential of electrical resistivity tomography to infer aquifer transport characteristics from tracer studies a synthetic case study 1 with time lapse electrical resistivity tomography ert transport processes in the subsurface can be imaged and monitored the information content of obtained spatiotemporal data sets opens new ways to characterize subsurface spatial variability difficulties regarding a quantitative interpretation of the imaged transport may arise from the fact that data inversion used in ert is generally underdetermined and that ert data acquisition is often limited to two dimensional 2 d image planes to address this problem we conducted a numerical tracer experiment in a synthetic heterogeneous aquifer with preset variability and spatial correlation of the hydraulic conductivity and monitored the tracer breakthrough in a 2 d image plane perpendicular to the mean flow direction using time lapse ert the tracer breakthrough patterns in the image plane were interpreted using equivalent transport models an equivalent convection dispersion equation to characterize the spatially averaged breakthrough and a stream tube model to characterize local breakthrough curves equivalent parameters derived from simulated and from ert inverted concentrations showed a good agreement which demonstrates the potential of ert to characterize subsurface transport using first order approximate solutions of stochastic flow and transport equations equivalent model parameters and their spatial variability were interpreted in terms of the local scale dispersion and the spatial variability of the hydraulic conductivity the spatial correlations of the stream tube velocity and of the hydraulic conductivity were found to be closely related consequently the hydraulic conductivity spatial correlation may be inferred from stream tube velocity distributions which can be observed with sufficiently high spatial resolution using ert

assessment of local hydraulic properties from electrical resistivity tomography monitoring of a three dimensional synthetic tracer test experiment 1 in recent years geophysical methods have become increasingly popular for hydrological applications time lapse electrical resistivity tomography ert represents a potentially powerful tool for subsurface solute transport characterization since a full picture of the spatiotemporal evolution of the process can be obtained however the quantitative interpretation of tracer tests is difficult because of the uncertainty related to the geoelectrical inversion the constitutive models linking geophysical and hydrological quantities and the a priori unknown heterogeneous properties of natural formations here an approach based on the lagrangian formulation of transport and the ensemble kalman filter enkf data assimilation technique is applied to assess the spatial distribution of hydraulic conductivity k by incorporating time lapse cross hole ert data electrical data consist of three dimensional cross hole ert images generated for a synthetic tracer test in a heterogeneous aquifer under the assumption that the solute spreads as a passive tracer for high peclet numbers the spatial moments of the evolving plume are dominated by the spatial distribution of the hydraulic conductivity the assimilation of the electrical conductivity 4d images allows updating of the hydrological state as well as the spatial distribution of k thus delineation of the tracer plume and estimation of the local aquifer heterogeneity can be achieved at the same time by means of this interpretation of time lapse electrical images from tracer tests we assess the impact on the performance of the hydrological inversion of i the uncertainty inherently affecting ert inversions in terms of tracer concentration and ii the choice of the prior statistics of k our findings show that realistic ert images can be integrated into a hydrological model even within an uncoupled inverse modeling framework the reconstruction of the hydraulic conductivity spatial distribution is satisfactory in the portion of the domain directly covered by the passage of the tracer aside from the issues commonly affecting inverse models the proposed approach is subject to the problem of the filter inbreeding and the retrieval performance is sensitive to the choice of k prior geostatistical parameters

time lapse mise a la masse measurements and modelling for tracer test monitoring in a shallow aquifer abstract the main goal of this study is to evaluate the reliability of the mise a la masse malm technique associated with saline tracer tests for the characterization of groundwater flow direction and velocity the experimental site is located in the upper part of the alento river alluvial plain campania region southern italy in this paper we present the hydrogeological setting the experimental setup and the relevant field results subsequently we compare those data against the simulated results obtained with a 3d resistivity model of the test area coupled with a model describing the advection dispersion equation for continuous tracer injection in particular we calculate a series of 3d forward solutions starting from a reference model all derived from electrical tomography results but taking into consideration different values of mean flow velocity and directions each electrical resistivity 3d model is used to produce synthetic voltage maps for malm surveys finally the synthetic malm voltage maps are compared with the ones measured in the field in order to assess the information content of the malm dataset with respect to the groundwater field characteristics the results demonstrate that the information content of the malm data is sufficient to define important characteristics of the aquifer geometry and properties this work shows how a combination of three dimensional time lapse modeling of flow tracer transport and electrical current can substantially contribute towards a quantitative interpretation of malm measurements during a saline tracer test this approach can thus revive the use of malm as a practical low cost field technique for tracer test monitoring and aquifer hydrodynamic characterization

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

new 40ar 39ar and k ar ages of the viluy traps eastern siberia further evidence for a relationship with the frasnian famennian mass extinction abstract we present here new ages combining both k ar and 40 ar 39 ar techniques reinforcing the possible link between the viluy traps eastern siberia with the late devonian mass extinctions the viluy traps also referred to as the yakutsk large igneous province are associated with the breakup of the eastern margin of the siberian platform as a triple junction rift system with volcanic outcropping along the lena markha and viluy rivers and an initial volume of volcanic products of about one million cubic kilometers k ar ages obtained on seven samples from the viluy rift display values ranging from 366 5 to 381 5 ma with a mean age of 374 5 ma our new 40 ar 39 ar results display well defined step heating plateau ages ranging from 362 3 3 4 to 379 7 3 5 ma although most k ar and 40 ar 39 ar ages agree within about 1 uncertainty which reinforces the validity of the k ar ages obtained with the cassignol and gillot unspiked technique even in the paleozoic a slightly clearer scenario appears when only the 40 ar 39 ar ages are considered effectively they cluster in two different groups with weighted mean ages of 364 4 1 7 and 376 7 1 7 ma these ages support a multi phases emplacement of the viluy traps as is the case for many continental flood basalts because the age of the first volcanic phase is undistinguishable from recent determinations of the age of the frasnian famennian boundary our results further strengthen the hypothesis of a causal relationship between the two events the second phase of emplacement of the viluy traps occurred at the end of the devonian apparently slightly before the devonian carboniferous boundary finally except for the ordovician silurian extinction which remains to be associated with a volcanic province all major phanerozoic mass extinctions have been correlated with the emplacement of a cfb thereby reinforcing the possible causal link between these events

structure and evolution of the rockeskyllerkopf volcanic complex west eifel volcanic field germany the rockeskyllerkopf volcanic complex rvc comprises three overlapping monogenetic volcanic centers southeast lammersdorf sel mauseberg m and rockeskyllerkopf rkk each volcanic center comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits that partially to completely cover the outer crater wall the sel center is a phreatomagmatic tuff ring composed of lithic rich tephra deposited by pyroclastic falls and surges the second center mauseberg with its crater to the northwest of the sel center is predominantly magmatic topographic and outcrop patterns suggest that this center may have formed a series of overlapping scoria cones along a n s trending fissure the youngest center rkk which lies on a poorly developed palaeosol within the earlier mauseberg deposits comprises a well developed proximal crater wall sequence this sequence of magmatic likely strombolian fall and grain avalanche deposits passes upward into a crater fill sequence that comprises variably welded bombs the final eruptions in the center were massive lava flows that were ponded within the rkk crater ar ar age dating of reequilibrated fragments of phlogopite megacrysts in the sel lavas indicates volcanic activity began at 474 39 ka literature k ar dates for the youngest lava flows in the rkk center give ages of 360 60 to 470 ka our interpretation of the age data and the presence of the poorly developed palaeosol between the mauseberg and rkk centers indicates that volcanism in the rvc began around 470 ka with the eruption of the sel and mauseberg centers followed a few thousand years later by the eruption of the rkk center

tracking the growth of the himalayan fold and thrust belt from lower miocene foreland basin strata dumri formation western nepal u s national science foundation nsf tectonics ear 1140068 conoco fellowship scholarship 2016 chevron texaco geology fellowship 2016 university of arizona galileo circle scholarship 2017 u s national science foundation nsf nsf ear 1338583 ear1649254

foundering triggered by the collision of india and asia captured in xenoliths xenoliths that erupted in the se pamir of tajikistan from 1000 to 1050 c and 90 km depth illuminate what happens when crust founders into the mantle 40ar 39ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic dunkeldik pipe and dike field indicates eruption at 11 2 0 2 ma u pb and trace element laser ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded the jurassic cretaceous andean style magmatic arc and its proterozoic mesozoic host rocks along the southern margin of asia recrystallization of these zircons was extensive yielding a range of dates down to 11 ma the zircons show distinct changes in eu anomaly lu gd ratio and ti concentrations compatible with garnet growth and minimal heating at 22 20 ma and then 200 300 c of heating 25 km of burial and alkali carbonate melt injection at 14 11 ma these changes are interpreted to coincide with i heat input due to indian slab breakoff at 22 20 ma and ii rapid thickening and foundering of the pamir lithosphere at 14 11 ma prior to and synchronous with collision between deep indian and asian lithospheres beneath the pamir

ar ar and i xe ages and the thermal history of iab meteorites studies of several samples of the large caddo county iab iron meteorite reveal andesitic material enriched in si na al and ca which is essentially unique among meteorites this material is believed to have formed from a chondritic source by partial melting and to have further segregated by grain coarsening such an origin implies extended metamorphism of the iab parent body new 39ar 40ar ages for silicate from three different caddo samples are consistent with a common age of 4 50 4 51 gyr less well defined ar ar degassing ages for inclusions from two other iabs eet elephant moraine 83333 and udei station are 4 32 gyr whereas the age for campo del cielo varies considerably over about 3 23 4 56 gyr new 129i 129xe ages for caddo county and eet 83333 are 4557 9 0 1 myr and 4557 4560 myr respectively relative to an age of 4562 3 myr for shallowater considering all reported ar ar degassing ages for iabs and related winonaites the range is 4 32 4 53 gyr but several iabs give similar ar ages of 4 50 4 52 gyr we interpret these older ar ages to represent cooling after the time of last significant metamorphism on the parent body and the younger ages to represent later 40ar diffusion loss the older ar ar ages for iabs are similar to sm nd and rb sr isochron ages reported in the literature for caddo county considering the possibility that iab parent body formation was followed by impact disruption reassembly and metamorphism e g benedix et al 2000 the ar ar ages and iab cooling rates deduced from ni concentration profiles in iab metal herpfer et al 1994 are consistent if the time of the postassembly metamorphism was as late as about 4 53 gyr ago however i xe ages reported for some iabs define much older ages of about 4558 4566 myr which cannot easily be reconciled with the much younger ar ar and sm nd ages an explanation for the difference in radiometric ages of iabs may reside in combinations of the following a i xe ages have very high closure temperatures and were not reset during metamorphism about 4 53 gyr ago b a bias exists in the 40k decay constants which makes these ar ar ages approximately 30 myr too young c the reported sm nd and rb sr ages for caddo are in error by amounts equal to or exceeding their reported 2 sigma uncertainties and d about 30 myr after the initial heating that produced differentiation of caddo silicate and mixing of silicate and metal a mild metamorphism of the iab parent body reset the ar ar ages

the temporal evolution of back arc magmas from the auca mahuida shield volcano payenia volcanic province argentina in order to better constrain the temporal volcanic activity of the back arc context in payenia volcanic province pvp argentina we present new k ar dating petrographic data major and trace elements from 23 samples collected on the auca mahuida shield volcano our new data coupled with published data show that this volcano was built from about 1 8 to 1 0 ma during five volcanic phases and that auca mahuida magmas were extracted from at least two slightly different oib type mantle sources with a low partial melting rate the first one containing more garnet was located deeper in the mantle while the second contains more spinel and was thus shallower the high mgo basalts or primitive basalts and the low mgo basalts or evolved basalts produced from the deeper and shallower lherzolite mantle sources respectively are found within each volcanic phase suggesting that both magmatic reservoirs were sampled during the 1 myr lifetime of the auca mahuida volcano however a slight increase of the proportion of low mgo basalts as well as of magmas sampled from the shallowest source can be observed through time similar overall petrological characteristics found in the pleistocene holocene basaltic rocks from los volcanes and auca mahuida volcano suggest that they originated from the same magmatic source consequently it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the pvp finally our data further support the hypothesis that the injection of hot asthenosphere with an oib mantle source signature which was triggered by the steepening of the nazca subducting plate induced the production of a large volume of lavas within the pvp since 2 ma

three dimensional electrical conductivity and induced polarization tomography of a rock glacier three dimensional electrical resistivity and induced polarization data were collected on an unstable alpine rock glacier in val thorens vanoise massif france in addition to these field data we also performed induced polarization measurements during freeze and thaw using a soil sample and the poorly mineralized water both from this site in the tomograms the electrical conductivity and the normalized chargeability show very distinctly the presence of the rock ice mixture the chargeability itself is however quite constant over the entire investigated area with the exception of a small area associated with the presence of carboniferous rocks rich in graphite the background chargeability is close to the dimensionless number r 8x10 2 which is consistent with the prediction of the dynamic stern layer model for the polarization of porous media the theory implies that this dimensionless number r is independent on saturation and temperature in agreement with field observations a main implication of this observation is that the classical archie s law cannot be applied to describe the electrical conductivity in this type of environments with poorly mineralized pore water surface conductivity dominates the measured conductivity of the materials implying in turn that the electrical conductivity is related to both the water content and cation exchange capacity of the material we propose new equations for both the electric conductivity and the normalized chargeability in this type of environments plain language summary induced polarization is a geophysical method looking at imaging the 3d distribution of properties related to the ability of rocks to store reversibly electrical charges under the application of a primary electrical field this method has never been applied to rock glaciers a field application and a laboratory experiment demonstrate the usefulness of this method to characterize alpine rock glaciers and point up some shortcomings in the interpretation of electrical conductivity tomography done by previous authors in this type of environments

the bombardment history of the moon as recorded by 40 ar 39 ar chronology new petrography and 40ar 39ar ages have been obtained for 1 3 mm sized rock fragments from apollo 16 station 13 soil 63503 north ray crater ejecta and chips from three rocks collected by apollo 16 and apollo 17 missions selection of these samples was aimed at the old 40ar 39ar ages to understand the early history of the lunar magnetic field and impact flux fifteen samples were studied including crustal material polymict feldspathic fragmental breccias and impact melts the impact ages obtained range between approximately 3 3 and 4 3 billion years ga polymict fragmental breccia 63503 1 exhibits the lowest signs of recrystallization observed and a probable old relic age of 4 547 0 027 the plateau age of 4 293 0 044 ga obtained for impact melt rock 63503 13 represents the oldest known age for such a lithology possibly this age represents the minimum age for the south pole aitken spa basin in agreement with literature data these results show that impact ages 3 9 ga are found in lunar rocks especially within soil 63503 impact exhumation of deep seated warm crustal material onto the lunar surface is considered to explain the common 4 2 ga ages obtained for weakly shocked samples from soil 63503 and apollo 17 this would directly imply that one or more basin forming events occurred at that time some rock fragments showing none to limited petrologic features indicate thermal annealing these rocks may have lost ar while resident within the hot ejecta of a large basin concurrent with previous studies these results lead us to advocate for a complex impact flux in the inner solar system during the initial approximately 1 3 ga

a supervolcano and its sidekicks a 100 ka eruptive chronology of the fish canyon tuff and associated units of the la garita magmatic system

late cretaceous blueschist facies metamorphism in southern thrace turkey and its geodynamic implications a blueschist facies tectonic sliver 9 km long and 1 km wide crops out within the miocene clastic rocks bounded by the strands of the north anatolian fault zone in southern thrace nw turkey two types of blueschist facies rock assemblages occur in the sliver i a serpentinite body with numerous dykes of incipient blueschist facies metadiabase ii a well foliated and thoroughly recrystallized rock assemblage consisting of blueschist marble and metachert both are partially enveloped by an upper eocene wildflysch which includes olistoliths of serpentinite metadiabase upper cretaceous and palaeogene pelagic limestone upper eocene reefal limestone radiolarian chert quartzite and minor greenschist field relations in combination with the bore core data suggest that the tectonic sliver forms a positive flower structure within the miocene clastic rocks in a transpressional strike slip setting and represents an uplifted part of the pre eocene basement the blueschists are represented by lawsonite glaucophane bearing assemblages equilibrated at 270 310 c and 0 8 gpa the metadiabase dykes in the serpentinite on the other hand are represented by pumpellyite glaucophane lawsonite assemblages that most probably equilibrated below 290 c and at 0 75 gpa one metadiabase olistolith in the upper eocene flysch sequence contains the mineral assemblage epidote pumpellyite glaucophane recording p t conditions of 290 350 c and 0 65 0 78 gpa indicative of slightly lower depths and different thermal setting timing of the blueschist facies metamorphism is constrained to c 86 ma coniacian santonian by rb sr phengite whole rock and incremental 40 ar 39 ar phengite dating on blueschists the activity of the strike slip fault post dates the blueschist facies metamorphism and exhumation and is only responsible for the present outcrop pattern and post miocene exhumation 2 km the high p t metamorphic rocks of southern thrace and the biga peninsula are located to the southeast of the circum rhodope belt and indicate late cretaceous subduction and accretion under the northern continent i e the rhodope massif enveloped by the circum rhodope belt the late cretaceous is therefore a time of continued accretionary growth of this continental domain