effects of experimental leaching on rbsr and kar isotopic systems and ree contents of diagenetic illite leaching with 1 n hcl 1 n nh4cl acetone humic acid extracts cation exchange resin and nh4 edta have negligible effects on the rb sr and k ar systems of different size fractions 0 4 and 0 8 2 m of a 1m diagenetic illite none of these treatments including the 1 n hcl treatment induced preferential removal of radiogenic 87sr and 40ar from mineral structures however the 1 n hcl treatment removed significant amounts of sr and ree as compared to the treatments by nh4 edta and cation exchange resin however much of these hcl derived elements resided in non clay minerals such as carbonate and fe oxide phases the k ar and rb sr ages of the samples were not affected by the hcl treatment any more than by the nh4 edta or the cation exchange resin because of potential loss of solid material during elutriation of differentially treated samples the hcl treatment is preferred over other methods allowing sufficient controls on the isotopic and mass balances between the leachate and the corresponding residue of an untreated sample for rb sr isotopic dating purposes of clay minerals from sedimentary and metasedimentary rocks leaching experiments give information about the ambient environment of crystallization analysis of both the leachate and residue under carefully controlled conditions is recommended

late cenozoic sedimentation of nihewan basin central north china and its tectonic significance abstract late cenozoic sediments of the nihewan basin in the northeastern part of the shanxi graben north china craton include buried late neogene rocks and exposures of the pliocene yuxian formation the pleistocene nihewan formation and xiaodukou formation with the age at least from 3 6 to 0 11 ma the sedimentary succession records the tectonic processes of the graben in the eastern nihewan basin on the basis of sedimentary facies and provenance analysis we have identified three main evolution stages during the late cenozoic the first stage was rapid expansion of the basin in the neogene which is thought to have commenced at least from 3 6 ma and lasted until 2 6 ma this stage is characterized by lacustrine facies dark colored mud and silt in depocenter of the basin with a potential source area from the northeast of the basin the following stage was a period of contraction at 2 6 1 8 ma characterized by deltaic deposits in depocenter of the basin and braided fluvial deposits in northeast margin of the basin and the deposition of cross bedded sands that were derived from erosion of archean proterozoic paleozoic and jurassic rocks from the north northwest and south of the basin the last stage was one era of gradually extinction of the basin from 1 8 to 0 11 ma followed by dried up of the basin and replaced by loess sediments were deposited in deltaic in depocenter of the basin and alluvial fan in northeast margin of the basin these coarse grained sands and pebbles were derived mainly from north and north northeast of the basin and generally from archean proterozoic and paleozoic rocks these three stages and the conversion ages 2 6 1 80 and 0 11 ma are largely consistent with three episodes of tibetan plateau rapid uplift therefore we infer that the evolution of the nihewan basin was predominantly influenced by the far field effect of the progressive uplift and northeastward expansion of the tibetan plateau

late paleoproterozoic extension and a paleostress field reconstruction of the north china craton abstract mafic dyke swarms and aulacogens are major anorogenic extensional events in the late paleoproterozoic north china craton ncc the n nnw mafic dyke swarms are widespread in the ncc whose ages span between 1 83 and 1 77 ga the similar ages and orientations of 1 8 ga dyke swarms in the ncc demonstrate that the amalgamated ncc experienced widespread extension at this time based on the width statistics of dyke swarms on ten survey lines an average crustal extension ratio of 0 35 was found for the ncc the small magnitude of overall extension suggests that the mafic dyke swarms were emplaced into the elastic fractures and indicates that the ncc had become a brittle plate prior to the emplacement of the mafic dyke swarms precisely dated mafic dyke swarms when used as paleostress indicators can be employed in the paleostress field reconstruction of precambrian cratons two dimensional finite element modeling 2 d fem of the ncc in which the various blocks were assigned densities and elastic constants shows that north south compression favors dyke intrusion along generally n nw lines and that deviations in dyke trends can be explained by the effects of boundary constraints and the physical properties of the crust the best fitting model can be considered a plausible representation of the tectonic force acting on the ncc that produces the intraplate stress field that is most consistent with the observed orientation of dyke swarms the results of modeling of the late paleoproterozoic stress field suggest a common tectonic setting for the emplacement of mafic dyke swarms in the central orogenic zone western and east blocks of the ncc the results also show that the north south tectonic forces play an important role in determining the paleostress field in the ncc the widespread extension of the ncc resulted from the north south tectonic forces which may be related to the break up of the late paleoproterozoic supercontinent the paleostress field modeling provides a possible approach to consider the supercontinent paleostress reconstruction and to reveal the mechanisms of the supercontinent break up

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

diffusion of ar in k feldspar present and absent the mechanisms of ar release from k feldspar samples in laboratory experiments and during their geological history are assessed here modern petrology clearly established that the chemical and isotopic record of minerals is normally dominated by aqueous recrystallization the laboratory critique is trickier which explains why so many conflicting approaches have been able to survive long past their expiration date current models are evaluated for self consistency especially arrhenian non linearity leads to paradoxes the models testable geological predictions suggest that temperature based downslope extrapolations often overestimate observed geological ar mobility substantially an updated interpretation is based on the unrelatedness of geological behaviour to laboratory experiments the isotopic record of k feldspar in geological samples is not a unique function of temperature as recrystallisation promoted by aqueous fluids is the predominant mechanism controlling isotope transport k feldspar should therefore be viewed as a hygrochronometer laboratory degassing proceeds from structural rearrangements and phase transitions such as are observed in situ at high temperature in na and pb feldspars these effects violate the mathematics of an inert fick s law matrix and preclude downslope extrapolation the similar upward concave non linear shapes of arrhenius trajectories of many silicates hydrous and anhydrous are likely common manifestations of structural rearrangements in silicate structures

geophysical investigations of the lake lappajarvi impact structure western finland abstract a circular negative bouguer anomaly approximately 10 mgal in amplitude and 17 km in diameter is associated with the lappajarvi impact structure the related mass deficit is 440 10 11 kg there are irregularities in the anomaly pattern due to differentially faulted bedrock blocks and lateral density contrasts between bedrock shocked bedrock breccia suevite impact melt rock and overburden the minimum amount of impact melt rock was interpreted from the positive residual bouguer anomaly between the centre and the rim in the northwestern part of the crater as being 1 5 2 0 km 3 there are a few small aeromagnetic anomalies up to 200 nt at an altitude of 40 m within the crater which is mostly characterized by a lack of magnetized material strong magnetic anomalies a few thousand nt which are abundant in the surroundings disappear in the crater area the aero electromagnetic aem data show strong anomalies with inphase quadrature ratios close to 1 indicating moderately conducting material within the crater an electromagnetic wide band sampo em profile was measured across the annular depression between the centre and the rim of the crater three layers were interpreted the thickness of the uppermost layer mostly conductive overburden is about 100 200 m and the resistivity 60 170 m in the annular depression beneath the uppermost layer there is a 50 250 m thick layer of suevites and breccias with resistivity of about 17 50 m the resistive lowest layer follows the base of breccia and suevite some separate soundings were carried out near the centre of the crater according to one of these soundings there is a conductive layer of suevite and breccia with a resistivity of 30 40 m beginning at depth of 150 m beneath the exposed impact melt rock the results of geophysical interpretations such as the annular depression between the centre and the rim of the crater the small thickness diameter ratio of the geophysically anomalous part of the crater and the calculated amount of impact melt rock indicate that the lappajarvi structure belongs to the category of complex craters rhomboidal geophysical anomaly patterns in particular aem anomalies parallel to old bedrock structures suggest that the collapse of the transient crater and or subsequent crater modification were controlled by nw se and ne sw trending pre existing fracture and fault zones

the lappajarvi meteorite crater finland petrography rb sr major and trace element geochemistry of the impact melt and basement rocks abstract impact melt lithologies of the 77 m y old finnish meteorite crater lappajarvi as well as the precambrian target rocks have been studied in detail to identify and characterize different impact melt types clast poor clast rich suevitic melt and to study their chemical major and trace elements and isotopic rb sr compositions in comparison to the composition of the target rocks the rb sr system of the whole melt body including the suevitic melt is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks chemical interactions exchange of alkali elements 87sr redistribution between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt exchange reactions between clasts and melt are determined by thermal activation but the degree of shock metamorphism in the clasts plays an important role too major and trace element distributions in impact melt and basement rocks indicate that the lappajarvi melt body chemically is extremely homogeneous even volatile elements such as zn and cu were not strongly fractionated comparison of the abundances of siderophile elements in the impact melt e g 118 177 ppm cr 195 340 ppm ni 6 12 ppb ir and calculated target rock mixture 79 mica schist 11 granite pegmatite 10 amphibolite e g 85 6 ppm cr 54 8 ppm ni 0 5 ppb ir revealed the chondritic nature c or h chondritic of the meteoritic projectile less than 2 of the meteorite can be detected in the coherent melt whereas the suevitic melt is uncontaminated by the projectile

the problem of inherited 40ar in dating impact glass by the 40ar 39ar method evidence from the tswaing impact crater south africa abstract the tswaing meteorite impact crater is a 1 13 km diameter structure located in the 2 05 ga nebo granite of the bushveld complex the impact age had previously been determined by fission track dating to 220 104 ka 40ar 39ar step heating and total fusion experiments performed on single and multi grain impact glass aliquots gave apparent ages ranging from 1 0 0 3 ma to 204 6 ma these ages indicate that the radiogenic ar derived from the target rocks has not been completely degassed as a result of the impact process despite fusion of the target material results of step heating experiments imply that the 40 ar inherited trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid vapor inclusions or un melted residual clasts calculations assuming an age of 0 2 0 1 ma for tswaing fission track data reveal that the amount of inherited 40ar 40 ar inherited relative to the pre impact concentration varies from 0 015 to 4 15 the spread defined by 40 ar inherited likely reflects the various quench rates experienced by the glass most certainly due to the pre impact position of the sample relative to the center of the crater we compare the influence of 40 ar inherited on the apparent 40ar 39ar age determination of five impact structures our calculations show that the main characteristic controlling the age offset for a given proportion of 40 ar inherited is the age difference between the impact and the target rocks i e the 40ar concentration in the target rock the buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself the occurrence of 40 ar inherited is likely influenced by 1 the degree of polymerization i e silicate structure complexity of the target rock and presumably related to the diffusivity of ar in the melt and glass 2 the ar partial pressure at the grain boundary 3 the quantity of energy involved in the impact and 4 the porosity of the target rocks for glass that inevitably suffers inherited and or excess 40ar the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment

post orogenic tectonic evolution of the qinling belt central china insights from a magnetostratigraphic study of a cretaceous intra mountain basin sedimentary succession abstract the huicheng basin straddling the west and east qinling belt in central china is a key feature linking the ne tibetan plateau and the rifted basins in east china and its sedimentary sequences provide important archives to elucidate tectonic evolution of the qinling belt and possibly the eastward expansion of the tibetan plateau the age of upper red fluvial lacustrine facies sequence known as the chengxian group has been previously assigned to neogene in spite of the lack of age definitive fossils which seems problematic in order to better constrain the age of this group and to establish a chronostratigraphic framework we conducted a magnetostratigraphic study of about 420 m thick composite section and collected 738 paleomagnetic samples progressive thermal demagnetization of these samples reveals both single and two component magnetizations characteristic remanent magnetizations chrms are mostly isolated at or above 350 and show northeasterly downward directions indicating unanimously normal polarities through the composite section positive fold and tilting tests suggest that the chrms are likely primary and that this sequence was possibly deposited during the cretaceous normal superchron instead of the previously assigned neogene by correlating with other cretaceous basins developed in west and east qinling and taking into account the polyphase tectonic overprints from structural analysis we conclude that the huicheng basin was formed during the post orogenic tectonic regime in response to far field effects caused by the northward subduction of the neo tethys ocean and the subsequent india eurasia collision

chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from lappajarvi crater finland plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks karnaite were sampled from the coherent melt sheet located in the central crater area of the lappajarvi meteorite crater and analyzed by optical electron optical x ray and microprobe methods both types of feldspars display strong chemical textural and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt in a lower degree of shock both primary plagioclase and microcline transform to checkerboard textured grains that consist of 5 10 m sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz the subgrains are strongly zoned disordered labradorite to andesine and rimmed by sanidine in a high degree of shock the subgrains are larger and randomly oriented the subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place on the basis of model calculations it is concluded that the temperature of the karnaite melt at the time of the incorporation of the clastic debris was in the range of 1800 to 2100 c the equilibration temperature of the clast melt mixture is estimated accordingly to be at least in the range of 1100 to 1140 c or higher depending on the magnitude of the clast s postshock temperature this temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars the clast laden melt was at rest when the equilibrium temperature was reached suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours

astronomical calibration of the maastrichtian late cretaceous abstract recent improvements to astronomical modeling of the solar system have contributed to important refinements of the cenozoic time scale through astronomical calibration of sedimentary series we extend this astronomical calibration into the cretaceous on the base of the 405 ka orbital eccentricity variation thanks to the recovery of well preserved maastrichtian sedimentary series from odp ocean drilling program holes 1258a leg 207 equatorial atlantic 1267b leg 208 south atlantic 762c leg 122 indian ocean and dsdp deep sea drilling program hole 525a leg 74 south atlantic cyclostratigraphic analysis is performed on high resolution measurements of magnetic susceptibility on sediments cored during legs 207 and 208 and gray level variations of sediment color obtained from core photographs from legs 122 and 74 astronomical control on sedimentation is evident at every site with the presence of cycles corresponding to forcing by precession obliquity and orbital eccentricity variations identification of these cycles leads to the definition of a detailed cyclostratigraphic frame covering nearly 8 ma from the upper campanian to the cretaceous paleogene k pg boundary durations of each magnetochron from c32r 2r to c29n are inferred by cycle counting astronomical calibrations of maastrichtian sedimentary series are proposed based on the 405 ka eccentricity variation according to the most recent astronomical solution la2010a two different ages are suggested for the k pg boundary considering the uncertainty of the long term variation of the 405 ka eccentricity cycle the first proposal provides a cretaceous paleogene boundary age of 65 59 0 07 ma and the second an age of 66 0 07 ma which is coherent with the most recent radio isotopic datings magnetochron boundaries and the campanian maastrichtian boundary are dated relative to these numerical ages of the k pg boundary

the lappajarvi impact structure finland age duration of crater cooling and implications for early life abstract an in depth approach of 40 ar 39 ar dating of the 23 km lappajarvi impact structure finland was performed using carefully selected single grain aliquots of optically fresh clast poor impact melt rock and recrystallized k feldspar melt particles separated from impact metamorphosed granite pegmatite step heating analysis yielded a set of 13 statistically robust plateau ages obtained on six melt rock and seven k feldspar samples the melt rocks yielded fully concordant ages with a weighted mean of 76 37 0 46 ma dating of the k feldspar melt particles resulted in a series of younger plateau ages ranging from 75 11 0 36 to 76 11 0 35 ma the melt rock results combined with the oldest syn melt rock k feldspar age and including all sources of uncertainties yielded a weighted mean age of 76 20 0 29 ma 2 mswd 1 02 p 0 41 which is interpreted to represent the best estimate age of the lappajarvi impact the age spread of at least 1 1 0 5 ma between the crystallization of the impact melt rocks and the k feldspar melt particles warns against the use of single 40 ar 39 ar analysis to derive a meaningful impact age in turn the age offset monitors both rapid cooling of the lappajarvi melt sheet versus slow cooling and impact induced hydrothermal circulation within the crater basement most likely driven by long lasting heat flow from the hot central uplift of the impact structure this interpretation is in line with the grain size dependent argon diffusion parameters and apparent closure temperatures of 230 410 c determined for various domain sizes of k feldspars observed in this study our study shows that even in comparatively small impact craters post impact hydrothermal activity can be estimated to last between 600 ka and 1 6 ma and is technically resolved by means of the 40 ar 39 ar dating technique therefore the longevity of hydrothermal systems in medium sized impact craters might be an order of magnitude longer than previously estimated prolonged post impact heat flow in medium sized craters in analogy to lappajarvi might have played an important role in the emergence of life on early earth and possibly mars

geochronological constraints on the age of a permo triassic impact event u pb and 40ar 39ar results for the 40 km araguainha structure of central brazil abstract impact cratering has been a fundamental geological process in earth history with major ramifications for the biosphere the complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination hampering the assessment of the effects of an individual event in the geological record we demonstrate the utility of a multi chronometer approach in our study of samples from the 40 km diameter araguainha impact structure of central brazil samples of uplifted basement granite display abundant evidence of shock deformation but u pb ages of shocked zircons and the 40ar 39ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history mixed results are obtained from in situ 40ar 39ar spot analyses of shocked igneous biotites in the granite with deformation along kink bands resulting in highly localized partial resetting in these grains likewise spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material the age of the glass itself and post impact devitrification the timing of crater formation is better assessed using samples of impact generated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization granular aggregates of neocrystallized zircon form a cluster of ten u pb ages that yield a concordia age of 247 8 3 8 ma the possibility of pb loss from this population suggests that this is a minimum age for the impact event the best evidence for the age of the impact comes from the u th pb dating of neocrystallized monazite and 40ar 39ar step heating of three separate populations of post impact inclusion rich quartz grains that are derived from the infill of miarolitic cavities the 206pb 238u age of 254 5 3 2 ma 2 error and 208pb 232th age of 255 2 4 8 ma 2 error of monazite together with the inverse 18 point isochron age of 254 10 ma mswd 0 52 for the inclusion rich quartz grains yield a weighted mean age of 254 7 2 5 ma 0 99 2 error for the impact event the age of the araguainha crater overlaps with the timing of the permo triassic boundary within error but the calculated energy released by the araguainha impact is insufficient to be a direct cause of the global mass extinction however the regional effects of the araguainha impact event in the parana karoo basin may have been substantial

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

direct measurement of ar diffusion profiles in a gem quality madagascar k feldspar using the ultra violet laser ablation microprobe uvlamp abstract controversy surrounding the mechanisms and controls on argon diffusion in k feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures over a range of temperatures measurements of argon diffusion profiles in a gem quality iron rich orthoclase heated in a cold seal apparatus have been undertaken in situ using an ultra violet laser ablation microprobe uvlamp technique the results agree very closely with the previously determined bulk values for benson mines orthoclase activation energy e 43 8 1 kcal mol 1 and vacuum furnace cycle heating studies of k feldspars e 46 6 kcal mol 1 however instead of defining a single activation energy e and diffusion coefficient do the data yield two sets of parameters a low temperature 550 720 c array with an e of 47 2 2 5 kcal mol 1 198 2 10 5 kj mol 1 and a do of 0 0374 0 1123 0 0281 cm2 s 1 and a high temperature 725 1019 c array with an e of 63 8 3 4 kcal mol 1 268 0 14 3 kj mol 1 and a do of 55 0 225 5 44 2 cm2 s 1 the new results closely reproduce two sets of apparent activation energies previously measured in cycle heating studies of madagascar k feldspar 40 3 and 57 3 kcal mol 1 previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion however the uv depth profile analyses indicate simple diffusion to the grain surface and importantly diffusion radii calculated by combining the uvlamp and cycle heating data are the same as the physical grain sizes used in the experiments around 1 mm vacuum furnace stepped heating experiments on slowly cooled k feldspars have been interpreted as showing diffusion radii of around 6 m and indicate complex populations of sub grains this study indicates that madagascar k feldspar and thus probably all gem quality k feldspars act as single diffusion domains and that short circuit or pipe diffusion was not an important loss mechanism an apparent diffusion compensation relationship in the stepped heating data for madagascar k feldspar implies that similar relationships seen in other k feldspars are a result of a range of diffusion mechanisms

impact induced hydrothermal activity within the haughton impact structure arctic canada generation of a transient warm wet oasis field studies and analytical scanning electron microscopy indicate that a hydrothermal system was created by the interaction of water with hot impact generated rocks following formation of the 24 km diameter 23 ma haughton impact structure hydrothermal alteration is recognized in two settings within polymict impact breccias overlying the central portion of the structure and within localized pipes in impact generated concentric fault systems the intra breccia alteration comprises three varieties of cavity and fracture filling a sulfide with carbonate b sulfate and c carbonate these are accompanied by subordinate celestite barite fluorite quartz and marcasite selenite is also developed particularly in the lower levels of the impact breccia sheet the fault related hydrothermal alteration occurs in 1 7 m diameter subvertical pipes that are exposed for lengths of up 20 m the pipes are defined by a monomict quartz carbonate breccia showing pronounced fe hydroxide alteration associated sulfides include marcasite pyrite and chalcopyrite we propose three distinct stages in the evolution of the hydrothermal system 1 early stage 200 c with the precipitation of quartz vapor phase dominated 2 main stage 200 100 c with the development of a two phase vapor plus liquid zone leading to calcite celestite barite marcasite and fluorite precipitation and 3 late stage 100 c with selenite and fibroferrite development through liquid phase dominated precipitation we estimate that it took several tens of thousands of years to cool below 50 c following impact during this time haughton supported a 14 km diameter crater lake and subsurface water system providing a warmer wetter niche relative to the surrounding terrain the results reveal how understanding the internal structure of impact craters is necessary in order to determine their plumbing and cooling systems

argon diffusion in plagioclase and implications for thermochronometry a case study from the bushveld complex south africa plagioclase is not only the most abundant mineral in the earth s crust but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material applying the k ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings nonetheless plagioclase has rarely been used for thermochronometry largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low k concentration here we describe an analytical technique that generates high precision 40 ar 39 ar data and quantifies ar diffusion kinetics of low k minerals we present results of five diffusion experiments conducted on single crystals of plagioclase from the bushveld complex south africa the observed diffusion kinetics yield internally consistent thermochronological constraints indicating that plagioclase is a reliable thermochronometer individual grains have activation energies of 155 178 kj mol and ln d0 a 2 varies between 3 5 and 6 5 these diffusion parameters correspond to closure temperatures of 225 300 c for a 10 c ma cooling rate age spectra generally conform to single domain diffusive loss profiles suggesting that grain scale diffusion dominates argon transport in this fairly simple plagioclase conjointly examining several single grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the bushveld complex cooled rapidly and monotonically from magmatic temperature to 300 c over 3 ma followed by protracted cooling to ambient crustal temperatures of 150 200 c over 600 ma 2009 elsevier ltd all rights reserved

comment on climate forcing by the volcanic eruption of mount pinatubo by david h douglass and robert s knox 1 douglass and knox 2005 hereinafter referred to as dk present a confusing and erroneous description of climate feedbacks and the climate response to the 1991 mt pinatubo eruption their conclusions of a negative climate feedback and small climate sensitivity to volcanic forcing are not supported by their arguments or the observational evidence as pointed out by wigley et al 2005a this is the consequence of assuming a one box representation for the climate system and ignoring energy exchange with the deep ocean 2 in the description of their analysis dk make a fundamental mistake in describing the problem they claim to use standard linear response theory but they confuse the response with the forcing they say the lw longwave effect is by definition the forcing function df for the climate represented by the measured surface temperature anomaly dt lw radiation changes however are produced by both the presence of a forcing agent in this case stratospheric aerosols and the response of the climate system it is the instantaneous net radiation change with no response that is the forcing the temperature anomaly is the response to forcing and not the forcing itself and the lw changes reflect both the true forcing and the effects of the temperature response 3 in spite of their statement to the contrary dk apparently do use the correct forcing as characterized by their equation 5 and illustrated in their figure 2 confusion arises because the forcing they use is a scaled version of the estimated aerosol optical depth changes which represents the true forcing yet they continually refer to the scaled optical depth as the lw changes that the two items are different is clear from dk s figure 1 4 the forcing of climate change can be defined as the change in the net radiation at the top of the atmosphere the tropopause or the surface without any response of the climate system or allowing for the stratosphere to equilibrate stenchikov and robock 1995 houghton et al 1996 and hansen et al 2005 discuss the standard definitions of radiative forcing and considerations for forcing from aerosols which are not uniformly mixed in the atmosphere for our purposes it is sufficient to consider the forcing at the top of the atmosphere allowing for no equilibration stenchikov and robock 1995 stenchikov et al 1998 this forcing can be defined as

ion microprobe uranium lead dating of zircons from the lappajarvi impact crater western finland the lake lappajarvi impact crater lies in paleoproterozoic svecofennian metasedimentary rocks on the western side of the central finland granitoid complex 1 9 ga two conflicting ages have been reported for the meteorite impact an age of 77 3 0 4 ma on the basis of ar ar whole rock data from impact melt samples and a paleomagnetic age of 195 ma during studies on impact crater indicator minerals at lappajarvi zircons with an atypical appearance were found in suevite boulders these zircons seemed to have been affected by impact shock metamorphism and it was considered that they would be good candidates for ion microprobe u pb dating allowing a new and independent age estimate for the impact event at lappajarvi four spot analyses on two black coated zircons plotted close to the upper intercept end of the concordia curve giving an approximate age of 1 8 ga for the source rock seventeen analyses were done on three dull zircon grains showing patchy impact related partial recrystallization most of these data fell fairly well on a single discordia line with intercept ages of 73 3 2 5 3 ma and 1854 2 5 1 ma however five of the data spots near the lower intercept end fell on the younger side of the line this was interpreted to indicate post impact loss of lead importantly the new ion microprobe u pb age of 73 3 f 5 3 ma is in a very good agreement with the previously reported ar ar age

systematic variations of argon diffusion in feldspars and implications for thermochronometry abstract coupled information about the time dependent production and temperature dependent diffusion of radiogenic argon in feldspars can be used to constrain the thermal evolution attending a host of earth and planetary processes to better assess the accuracy of thermal models an understanding of the mechanisms and pathways by which argon diffuses in feldspars is desirable here we present step heating ar diffusion experiments conducted on feldspars with diverse compositions structural states and microstructural characteristics the experiments reveal systematic variations in diffusive behavior that appear closely related to these variables with apparent closure temperatures for 0 1 1 mm grains of 200 400 c assuming a 10 c ma cooling rate given such variability there is no broadly applicable set of diffusion parameters that can be utilized in feldspar thermal modeling sample specific data are required diffusion experiments conducted on oriented cleavage flakes do not reveal directionally dependent diffusive anisotropy to within the resolution limits of our approach approximately a factor of 2 additional experiments aimed at constraining the physical significance of the diffusion domain are presented and indicate that unaltered feldspar crystals with or without coherent exsolution lamellae diffuse at the grain scale whereas feldspars containing hydrothermal alteration and or incoherent sub grain intergrowths do not arrhenius plots for argon diffusion in plagioclase and alkali feldspars appear to reflect a confluence of intrinsic diffusion kinetics and structural transitions that occur during incremental heating experiments these structural transitions along with sub grain domain size variations cause deviations from linearity i e upward and downward curvature on arrhenius plots an atomistic model for arrhenius behavior is proposed that incorporates the variable lattice deformations of different feldspars in response to heating and compression the resulting implications for accurately extrapolating laboratory derived diffusion parameters to natural settings and over geologic time are discussed we find that considerable inaccuracies may exist in published thermal histories obtained using multiple diffusion domain mdd models fit to arrhenius plots for exsolved alkali feldspar where the inferred ar partial retention zones may be spuriously hot

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

a late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

palaeomagnetism of the lappajarvi impact structure western finland abstract palaeomagnetic and rock magnetic results are presented for rocks from the lappajarvi meteorite impact structure western finland oriented samples were collected from the melt rocks in the central island of the crater and of the palaeoproterozoic svecofennian target rocks at the crater rim the natural remanent magnitization nrm of the melt rocks is generally weak and metastable during demagnetization the characteristic nrm component carried by pyrrhotite yields a palaeomagnetic pole latitude 55 4 n longitude 152 6 e dp 6 2 dm 8 7 dg 6 sites suggesting an age of 195 ma for the impact the results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations the palaeomagnetic age differs from the 40ar 3939ar age of 77 ma for the impact assuming that the 40ar 39ar age is correct the discrepancy between palaeomagnetic and radiometric ages is best explained by post impact and post cooling tilting of the melt layer the tilting 15 may be due to isostatic rebounds along joints in the melt impact remanence has also been isolated in a few target rocks at the rim this poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1 9 ga old svecofennian direction this interpretation is supported by the observation that the nrm intensity is enhanced in specimens having the impact overprint the palaeomagnetic results are consistent with earlier interpretations based on geophysical morphological mineralogical and geochemical data that the lappajarvi structure is a complex meteorite impact site less than 200 ma old

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

comment on the joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by paul r renne et al 2010

triassic diorites and granitoids in the foping area constraints on the conversion from subduction to collision in the qinling orogen china abstract the western qinling is characterized by triassic granitic plutons which are keys to constrain the timing of subduction and collisional processes between the north china and south china blocks this paper presents a set of new la icpms u pb zircon geochronology major and trace element geochemistry for several representative diorite and granitoid plutons in the foping area the diorites are characterized by high mgo al2o3 y and yb contents but low sio2 sr y and la yb ratios as well as depletion of nb ta p and ti suggesting derived from a subduction related setting la icpms zircon u pb age of 216 9 1 5 ma mswd 0 69 from the diorite constrains the time of the subduction event the medium and fine grained monzogranites as well as the biotite granites exhibit relative high sio2 mgo and rb low y and yb contents and low sr y and la yb ratios as well as various depletions of nb ta p and ti these features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn collisional setting the medium grained monzogranite and biotite granite yield la icpms zircon u pb ages of 207 3 2 1 ma mswd 1 8 and 201 6 1 2 ma mswd 1 04 respectively proposing the syn collision during ca 207 201 ma geochemical and geological evidences suggest that the granodiorites intruding in the syn collisional monzogranites were related to the post orogenic collapse after the collision between the north china and south china blocks the la icpms u pb zircon age of 189 3 2 9 ma mswd 5 6 for the granodiorite constrains the time of the post orogenic collapse our geochronological and geochemical data provide important constraints on the triassic evolution from subduction to collision then up to post collisional collapse along the qinling orogen

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

the onset of the lunar cataclysm as recorded in its ancient crater populations abstract the earliest bombardment history of the moon potentially provides powerful constraints for solar system evolution models a major uncertainty however is how much of this history is actually recorded in lunar craters for example some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes others believe that most lunar craters and large basins were formed in a narrow time interval between 3 8 and 4 0 ga the so called lunar cataclysm in the light of recent improvements in our understanding of early solar system evolution it is possible that the contributions from both scenarios could be represented in the lunar crater record if so when did the declining bombardment end and the lunar cataclysm begin here we show using new counts of 15 150 km diameter craters on the most ancient lunar terrains that the craters found on or near nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various pre nectarian era terrains this dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits which may have strongly modified the source of lunar impactors this work also suggests that the lunar cataclysm may have started near the formation time of nectaris basin this possibility implies that south pole aitken basin spa the largest lunar basin and one of the oldest by superposition was not created during the cataclysm this view is strengthened by our interpretation that a substantial fraction of ancient craters on spa were made by low velocity impactors finally we believe these results shed new light on the impact history of the primordial earth

reply to comment by a robock on climate forcing by the volcanic eruption of mount pinatubo 1 in our pinatubo paper douglass and knox 2005a hereinafter referred to as dk we concluded that there was negative climate feedback and a short climate response time robock 2005 hereinafter referred to as r claims that the outgoing long wave radiation lw was incorrectly described as the forcing and that interchange of energy with the thermocline was unjustifiably neglected in determining our results although we made certain incorrect statements about the lw radiation these were not part of our determination of the parameters as to energy flow to the thermocline as we argue elsewhere douglass and knox 2005b hereinafter referred to as dk2 this flow is estimated be small and to affect our lifetime and sensitivities by less than 15 not by a factor of three as claimed

diffusion in solid earth systems recent years have seen a rapid expansion in the acquisition and use of information on diffusive transport in phases relevant to the solid earth crystals melts and fluids although far from complete the data base on diffusion coefficients is now sufficiently large that broad constraints can be placed upon the length and time scales of many natural transport phenomena in which diffusion plays a role conversely observations of diffusion progress in specific natural samples can be used to extract time temperature information for a variety of geologic and geochemical processes ranging from sediment burial and crustal erosion to fluidmediated reactions and biosignature retention despite this undeniable progress several major challenges remain that largely define the frontiers of research in solid earth diffusion perhaps foremost among these is the need to address and understand the multi scale multi path aspects of diffusion in many systems a complication that is not limited to polyphase materials individual mineral grains can exhibit clear indications of multi path behavior even when visible evidence of such paths is lacking many other diffusion frontiers are linked in one way or another to this multi scale issue they include diffusion of molecular h2o and the effect of h species on diffusion in minerals and rocks diffusive fractionation of multiple isotopes of a single element diffusion at the extreme conditions of the deep earth reconciliation of observations from natural samples and laboratory studies and development of theoretical approaches to predict

link between oceanic heat transport thermohaline circulation and the intertropical convergence zone in the early pliocene atlantic planktonic foraminiferal oxygen isotope records from the western and eastern tropical pacific and atlantic oceans suggest a southward shift in the intertropical convergence zone toward its modern location between 4 4 and 4 3 ma a concomitant shift in the carbon isotope compositions of atlantic benthic foraminifera provides strong evidence for an increased thermohaline overturn at this time we suggest that the southward shift of the intertropical convergence zone and associated change in trade wind circulation altered equatorial surface hydrography increased the advection of warmer and more saline surface waters into the subtropical and north atlantic and contributed to thermohaline overturn

stable isotope composition of smectite in suevites at the ries crater germany implications for hydrous alteration of impactites abstract the 24 km diameter ries crater germany exhibits well preserved crater filling and surficial melt rich breccia deposits that are believed to have been altered by post impact hydrothermal fluids the alteration mineralogy of the crater filling breccias is characterized by clay smectite chlorite and a zeolite assemblage and secondary clay phases smectite minor halloysite in surficial melt bearing breccia deposits using stable isotope analysis of secondary smectitic clay fractions evidence of significant hydrous alteration of impactites at large water rock ratios was found the estimated fluid temperatures using data derived by 18 o and d fractionation suggest smectite precipitation in surficial breccias in equilibrium with meteoric fluids at temperatures 16 5 c in agreement with the long term variation of modern precipitation in the area the stable isotope composition of smectite in crater fill breccia however suggests a trend of monotonously increasing temperatures from 43 to 112 c with increasing depth through the breccia sequence this demonstrates a different origin of alteration and temperature distribution for the surficial and crater filling melt bearing impact breccias in the ries crater our results suggest that the inverted structure of hydrothermal systems observed in some terrestrial impact craters including the ries crater could indicate the initial configuration of a thermal anomaly in the crater filling sequence but which is replaced with a normal hydrothermal convection in crater proper during the course of post impact cooling

ar ar dating of authigenic k feldspar quantitative modelling of radiogenic argon loss through subgrain boundary networks we have analysed two distinct generations of authigenic k feldspar in fucoid bed sandstones from an t sron and skiag bridge nw highlands scotland which have experienced post growth heating to levels in excess of the predicted ar closure temperature authigenic k feldspars show microtextural similarities to patch perthites that is subgrains separated by dislocation rich boundary networks that potentially act as fast diffusion pathways for radiogenic argon the two generations of authigenic k feldspar in the fucoid bed sandstones can be distinguished by different microtextural zones bulk mineral compositions fluid inclusion populations and inferred temperatures and chemistries of parent fluids ar ar age data obtained using high resolution ultraviolet laser ablation show that the first cementing generation is ordovician and the second cementing generation is silurian modelling of ar diffusion using subgrain size as the effective diffusion dimension and a simplified tectono thermal thrust model assuming transient heating of the fucoid beds is inconsistent with observed data removal of heat from the thrust zone through rapid flushing of heated fluids rather than transient heating can be invoked to explain the observed ar ar ages for both generations of cement alternatively ar diffusion modelling using overgrowth thickness as the effective diffusion dimension instead of subgrain size also yields models that are consistent with both the fucoid bed palaeothermal maxima and determined ar ar age ages for the two generations of k feldspar cement based on this alternate explanation we propose a theoretical microtextural model that highlights fundamental differences between the microtextures of deuterically formed patch perthites and authigenic k feldspars explaining the apparent robustness of authigenic k feldspar with respect to ar retention

impact melt sheet formation on mars and its implication for hydrothermal systems and exobiology abstract hydrothermal deposits are a prime exobiological research target on mars and impact craters are potentially a significant source for these deposits key factors in the longevity of terrestrial impact crater hydrothermal systems are the volume of shock melt produced and the degree to which this melt is quenched by mixing with un melted clasts mixing occurs when the shock melt is smeared along the crater floor and the degree of mixing is inversely proportional to the thickness of this smeared layer to investigate the potential development of thick melt sheets on mars we apply an analytical impact model to asteroid and long period lp comet impacts that produces martian craters with final diameters of 30 640 km we combine our model shock melt volumes with estimates of the surface area of the transient cavity to estimate initial shock melt thickness which we use as a proxy measure of the amount of mixing between shock melt and un melted clasts we calculate this shock melt thickness parameter for a range terrestrial craters with and without thick melt sheets from which we infer the melt sheet characteristics for the model mars craters the results demonstrate that most asteroid impacts on mars do not produce thick clast free melt sheets only asteroid impact craters 580 km in diameter on mars produce shock melt thicknesses comparable to earth s 200 km diameter sudbury crater and martian craters 65 km in diameter produce shock melt thicknesses comparable to terrestrial craters that lack melts sheets entirely lp comet impacts on mars are more likely to produce thick melt sheets lp comet impact craters 280 km in diameter on mars produce shock melt thicknesses comparable to that of the sudbury crater given that lp comet impacts on mars are rare compared to asteroid impacts we conclude that large long lived impact hydrothermal systems are likewise rare on mars comparisons of the mars asteroid impact rate with the potential longevity of impact hydrothermal systems indicate that such systems were active 1 of the time in the last 3 5 ga which raises questions about the exobiological importance of impact crater hydrothermal systems on mars after the noachian

hornblende alteration and fluid inclusions in kardla impact crater estonia evidence for impact induced hydrothermal activity the well preserved krdla impact crater on hiiumaa island estonia is a 4 km diameter structure formed in a shallow ordovician sea about 455 ma ago into a target composed of thin 150 m unconsolidated sedimentary layer above a crystalline basement composed of migmatite granites amphibolites and gneisses the fractured and crushed amphibolites in the crater area are strongly altered and replaced with secondary chloritic minerals the most intensive chloritization is found in permeable breccias and heavily shattered basement around and above the central uplift alteration is believed to have resulted from convective flow of hydrothermal fluids through the central areas of the crater chloritic mineral associations suggest formation temperatures of 100 300 c in agreement with the most frequent quartz fluid inclusion homogenization temperatures of 150 300 c in allochthonous breccia the rather low salinity of fluids in krdla crater 13 wt nacleq suggests that the hydrothermal system was recharged either by infiltration of meteoric waters from the cater rim walls raised above sea level after the impact or by invasion of sea water through the disturbed sedimentary cover and fractured crystalline basement the well developed hydrothermal system in krdla crater shows that the thermal history of the shock heated and uplifted rocks in the central crater area rather than cooling of impact melt or suevite sheets controlled the distribution and intensity of the impact induced hydrothermal processes

40ar 39ar age of the lonar crater and consequence for the geochronology of planetary impacts asteroid impacts play an important role in the evolution of planetary surfaces in the inner solar system the large majority of impacts occur on bodies e g asteroids the moon mars covered by primitive igneous rocks however most of the impacts recorded on earth occur on different rock types and are poor proxies for planetary impacts the lonar crater is a 1 88 km diameter quaternary age crater fig 1 located on the ca 66 ma deccan basaltic traps in maharashtra india and is one of the very few craters on earth emplaced directly on basaltic lava flows we carried out 12 40 ar 39 ar step heating experiments on 4 melt rock samples in order to 1 obtain a precise age for the lonar crater 2 study the response of isotopic chronometers during impacts on mafic target rocks and 3 better understand the dating of extraterrestrial impact craters we obtained 10 plateau and 9 inverse isochron ages on various aliquots combination of selected data into a global inverse isochron yielded an age of 570 47 ka mswd 1 1 p 0 24 in comparison previous nonisotopic investigations on rocks thought to be affected by secondary processes yielded a range of much younger ages ca 12 62 ka the measured 40 ar 36 ar trapped values offer a direct comparison with the atmospheric benchmark value and allow us to test the inherited 40 ar degassing capacity of basaltic impact melt rocks the 40 ar 36 ar ratio of 296 5 1 7 is indistinguishable from the atmospheric composition and suggests that inherited 40 ar is absent from the melt rock this result substantiates diffusion models that predict a near complete degassing of low viscosity melt e g basalts during impact and demonstrates for the first time that inherited 40 ar is less problematic for 40 ar 39 ar dating of impact events in basaltic igneous rocks compared to si rich rocks these results provide direct evidence that basaltic melt rocks are excellent candidates for recording the timing of planetary impact events and as far as dating is concerned should be the preferred targets of sample recovery by future missions

geochronology geochemistry and sr nd hf isotopes of mafic dikes in the huicheng basin constraints on intracontinental extension of the qinling orogen abstract the diabase dikes intruded the lower cretaceous sandstone in the huicheng basin are key to understanding the mesozoic tectonic evolution of the qinling orogen la icp ms zircon u pb dating yields a crystallization age of 107 1 ma for them the dikes have low contents of sio2 42 46 50 16 wt mgo 3 47 5 59 wt with low mg of 49 59 and tio2 1 35 1 63 wt high tfe2o3 8 15 9 36 wt al2o3 14 75 17 23 wt and k2o 0 87 3 61 wt their ni and cr contents are in range of 16 8 111 and 45 7 315 ppm respectively they are significantly enriched in light rare earth elements and large ion lithophile elements e g cs pb and ba and depleted in rb k p and ti the dikes show relatively high whole rock initial 87sr 86sr ratios 0 7071 0 7079 and negative end t values 1 5 to 4 8 with single stage nd model ages of 941 1186 ma in addition they have low zircon ehf t values 8 6 to 3 5 with single stage hf model ages of 674 1117 ma both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution they were derived from partial melting of an enriched lithospheric mantle source together with regional geological data these results suggest that the qinling orogen experienced an intracontinental extension during the late early cretaceous

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

detrital zircon u th he thermochronometry of the mesozoic daba shan foreland basin central china evidence for timing of post orogenic denudation abstract the timing of the transition of a peripheral foreland basin from underfilled to overfilled provides important insights into the post orogenic dynamics of its coupled orogenic wedge such information is lacking in the mesozoic foreland basins surrounding the qinling orogen central china such as the daba shan foreland basin dsfb the northeastern part of the sichuan basin in an effort to constrain the timing of maximum burial and onset of denudation we report detrital zircon u th he zhe data from four deep boreholes in the dsfb results show that zhe ages from the oldest foreland basin formations upper triassic lower middle jurassic are mostly younger than their depositional age whereas non reset zhe ages characterize younger strata this suggests that the oldest strata attained maximum temperatures between 130 c and 200 c zhe partial retention zone which is consistent with previous independent paleotemperatures organic matter reflectance and illite crystallinity reported in two of the boreholes and another nearby the fully reset ages apparently show the youngest peaks at 80 ma and 100 ma which are interpreted as the minimum constraints on the timing of maximum basinal burial and subsequent denudation considering that the regional deposition has lasted to aptian time it is proposed that the onset of the basin exhumation in the dsfb started at aptian albian time 100 125 ma heralding the termination of regional tectonic shortening in the hinterland southern qinling

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

the significance of the contemporaneous logoisk impact structure belarus and afro arabian flood volcanism abstract the relationships between meteorite impact craters flood basalt volcanism and sudden environmental perturbation on earth have been debated extensively we present only the second example of contemporaneous meteorite impact and flood basalt volcanism the 17 km diameter logoisk impact structure belarus and afro arabian flood volcanism the new precise 40 ar 39 ar age of 29 71 0 48 ma for logoisk is coincident with the acme of flood volcanism we argue that at least one crater the size of logoisk is likely to form during the emplacement of a flood volcanic province and that contemporaneity of impacts and flood volcanism was the norm the absence of a biotic effect from the combined logoisk afro arabian volcanism we attribute to the magnitude of these combined events which are both significantly smaller than the other known example of the chicxulub impact event and the deccan traps flood volcanism supplementary material 40 ar 39 ar data are available at http www geolsoc org uk sup18329

mesozoic deformations of the dabashan in the southern qinling orogen central china abstract the dabashan a curved tectonic belt in the southern qinling orogen was formed as a result of the collision of the qinling micro plate and the yangtze plate in the late triassic this study shows that the dabashan experienced multistage structural deformations two main deformational episodes are recognized 1 foreland folding and thrusting due to the late triassic collision and 2 structural superposition during the late jurassic to early cretaceous the northern dabashan is typified by imbricate thrust sheets whereas the southern dabashan is characterized by occurrence of widespread superimposed folds the early fold axis is orientated almost due north or ne sw the folding propagated southward and southwestward into the sichuan basin from the chengkou fangxian fault which separates the southern from the northern dabashan belts it is concluded that the southern and northern dabashan were two distinct domains at least in early mesozoic this interpretation is important for understanding the tectonic evolution of the qinling orogen and the collisional process of the north and south china plates

mesozoic intracontinental orogeny in the qinling mountains central china abstract the qinling orogenic belt has been well documented that it was formed by multiple steps of convergence and subsequent collision between the north china and south china blocks during paleozoic and late triassic times following the collision in late triassic times the whole range evolved into an intracontinental tectonic process the geological geophysical and geochronological data suggest that the intracontinental tectonic evolutionary history of the qinling orogenic belt allow deduce three stages including strike slip faulting during early jurrassic n s compressional deformation during late jurassic to early cretaceous and orogenic collapse during late cretaceous to paleogene the strike slip faulting and the infills in early jurassic along some major boundary faults show flower structures and pull apart basins related to the continued compression after late triassic collision between the south qinling belt and the south china block along the mianlue suture late jurassic to early cretaceous large scale of n s compression and overthrusting progressed outwards from inner of qinling orogen to the north china block and south china block due to the renewed southward intracontinental subduction of the north china block beneath the qinling orogenic belt and continuously northward subduction of the south china block respectively after the late jurassic early cretaceous compression and denudation the qinling orogenic belt evolved into late cretaceous to paleogene orogen collapse and depression and formed many large fault basins along the major faults

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

excess argon in amphiboles linked to greenschist facies alteration in the kamila amphibolite belt kohistan island arc system northern pakistan insights from 40 ar 39 ar step heating and acid leaching experiments a mineralogical and 4oar 39ar study of 13 amphibole samples in the kamila amphibolite belt and kamila shear zone in northern pakistan has found a correlation between the degree of greenschist facies alteration and quantity of excess 40ar additionally there is a north south divide with amphibole samples from the northern region showing larger degrees of gree schist facies alteration brittle deformation and excess 40ar incorporation compared to the predominantly plastically deformed less altered amphibole samples from the kamila shear zone in the south acid leaching of two amphiboles from the kamila amphibolite belt indicates that a large proportion of the excess 40ar is correlated with later greenschist facies alteration hases and can be easily removed by acid etching thus revealing acceptable regional 40ar 39ar plateau ages

carboniferous and triassic eclogites in the western dabie mountains east central china evidence for protracted convergence of the north and south china blocks shrimp u pb dating and laser ablation icp ms trace element analyses of zircon from four eclogite samples from the north western dabie mountains central china provide evidence for two eclogite facies metamorphic events three samples from the huwan shear zone yield indistinguishable late carboniferous metamorphic ages of 312 5 307 4 and 311 17 ma with a mean age of 309 3 ma one sample from the hong an group 1 km south of the shear zone yields a late triassic age of 232 10 ma similar to the age of ultra high pressure uhp metamorphism in the east qinling dabie orogenic belt ree and other trace element compositions of the zircon from two of the huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase namely in the eclogite facies an interpretation supported by the presence of garnet omphacite and phengite inclusions zircon also grew during later retrogression zircon cores from the huwan shear zone have ordovician to devonian 440 350 ma ages flat to steep heavy ree patterns negative eu anomalies and in some cases plagioclase inclusions indicative of derivation from north china block igneous and low pressure metamorphic source rocks cores from hong an group zircon are neoproterozoic 780 610 ma consistent with derivation from the south china block in the western dabie mountains the first stage of the collision between the north and south china blocks took place in the carboniferous along a suture north of the huwan shear zone the major triassic continent continent collision occurred along a suture at the southern boundary of the shear zone the first collision produced local eclogite facies metamorphism in the huwan shear zone the second produced widespread eclogite facies metamorphism throughout the dabie mountains sulu terrane and a lower grade overprint in the shear zone

40ar 39ar geochronology of jarosite the effectiveness of hf in removing silicate contaminants abstract supergene and hypogene jarosite have been dated successfully but accurate dating of weathering derived jarosite and its application to landscape evolution studies has been limited because of difficulties in obtaining high purity mineral separates hydrofluoric acid hf can remove potassium bearing silicates from supergene samples but its effects on jarosite crystal chemistry and age dating are unquantified three experiments were conducted to determine whether hf treatment removes silicates without altering the potassium or argon composition of the jarosite prior to dating using the 40 ar 39 ar method in the first two experiments pure hypogene jarosite from pena blanca mexico pb 9 42 0 22 ma and a mixture of 85 pb 15 fish canyon sanidine fc 2 28 02 ma were crushed and treated with 25 hf for 0 30 240 and 480 min jarosite partially dissolves during hf treatment with grains becoming increasingly pitted and rounded with time in acid but pb s potassium concentration and apparent age remained constant regardless of treatment time fc 2 was absent from all treated samples suggesting that 30 min in hf is sufficient to remove mechanically mixed sanidine crystals from jarosite methods developed during experiments conducted on pb jarosite were applied to fine grained supergene jarosite from the red river valley rrv in northern new mexico rrv jarosite is mixed with oligocene 24 86 ma potassium bearing silicates that make up 50 or more of each sample four rrv jarosite samples that yielded age spectra with clear signs of contamination by older phases were treated with hf for 30 min and re dated despite treatment back scattered electron images show that silicates including sanidine and illite continued to comprise approximately 30 of hf treated rrv samples and the age spectra produced during laser step heating continued to show signs of contamination in higher wattage steps however the integrated ages of these samples were consistently less than 1 ma at least 7 ma younger than the expected integrated age if the 30 70 mixture of rrv silicates and jarosite degassed completely the young ages suggest that young jarosite controls the apparent age of these samples and that these ages can be interpreted as the maximum timing of supergene jarosite formation

cratering history and lunar chronology the moon is exceptional and important because it is the only planetary body besides the earth for which we have both a detailed stratigraphic history and datable rock samples that can be related to specific geomorphologic units fig 5 1 the moon has preserved much of its magmatic and impact record of at least the last 4 billion years while its endogenic history is of great interest for the fundamentals of planetary interiors and surfaces the moon has become a calibration plate for the cratering record of the earth moon system and by extrapolation of the entire inner solar system if one assumes a heliocentric origin for impactor populations these populations range from asteroids through long and short period comets to interplanetary dust and cover a size range from hundreds of kilometers to micrometers figure 5 1 telescopic view of the nearside of the earth s moon with landing sites of the apollo and luna missions this chapter reviews the presently available data sets in support of this paradigmatic assumption as follows 1 the phenomenology of lunar impact craters 2 the terrestrial record of the impact cratering process and the interpretation of terrestrial impactites as far as this ground truth is relevant for the interpretation of lunar impact craters and datable lunar impact breccias and melt rocks 3 the theory and numerical simulation of the cratering process and the characteristics of the earth moon crossing population of impactors asteroids and comets 4 the principles of relative age dating of lunar surface units and the general lunar stratigraphy 5 the stratigraphic significance and ages of lunar samples impactites and basalts and based on this data set the absolute ages of lunar surface units 6 the cratering rate of the moon as a function of time and 7 the time calibration of this cratering rate based on the

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

structural records of the late cretaceous cenozoic extension in eastern china and the kinematics of the southern tan lu and qinling fault zone anhui and shaanxi provinces pr china abstract our study of the late cretaceous cenozoic extension in eastern china is a kinematic analysis of faults 1500 striated fault data 175 stress solutions at 90 sites that has permitted us to separate stress fields belonging to six tectonic events a wnw ese extension 1 has affected eastern china from the campanian to the late palaeocene early ypresian under this extension the qinling qlfz and southern tan lu stlfz fault zones have been dextral transtensional fault zones a first transpressional event 2 nne ssw shortening has occurred during the late ypresian lower lutetian the qlfz and the stlfz have been dextral transpressional fault zones a ne sw extension 3 has affected the weihe graben during the palaeogene after a second transpressional event 4 the ne sw extension 5 has been rejuvenated during the late miocene and the qlfz has been a normal fault zone again and the stlfz a sinistral transtensional fault zone synrift subsidence has occurred in the weihe graben the second transpressional event 4 wnw ese shortening has occurred during the late oligocene early miocene during a period of uplifting and exhumation at that time the stlfz and the qlfz have become sinistral transpressional fault zones a period of extension 6 has taken place during the late pliocene quaternary the extensional direction has trended nnw sse during this event the qlfz has been a sinistral transtensional fault zone and the stlfz a dextral transtensional fault zone these results are compared with those previously published in order to evaluate the regional significance of these tectonic events

paleozoic metamorphism in the qinling orogen tongbai mountains central china the collision of the sino korean and yangtze blocks to form a significant part of china is recorded in the qinling tongbai and dabie mountains radiometric ages of the ultrahigh pressure metamorphic rocks in the south qinling orogenic belt suggest that subduction and collision took place during the triassic period our new 40 ar 39 ar geochronology of units in the north qinling orogenic belt confirms that high grade metamorphism and deformation took place also during the silurian devonian and carboniferous periods these results imply that the amalgamation of eastern china was a multistage process extending over at least 200 m y