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

the incorporation of pb into zircon abstract the incorporation of pb into zircons grown from pb rich solutions was evaluated using three different approaches 1 high temperature growth of large crystals from pb silicate melts 2 hydrothermal overplating of thin epitaxial layers on substrates of natural zircon and 3 growth of small homogeneously nucleated crystals from aqueous fluids the melt grown zircons 50 400 m were crystallized from pbo sio2 zro2 p2o5 liquid at atmospheric pressure by cooling from 1430 to 1350 c in the p2o5 free system despite 66 wt pbo in the melt these zircons contain 3 atom pb with apparent zircon fluid partition coefficients of 4 2 and 2 6 respectively for pb4 and pb2 in contrast to the case of melt grown zircons available p is excluded from the aqueous epitaxial zircon suggesting that charge balance is accomplished by h instead small 2 5 m zircons grown by cooling aqueous solutions pbo sio2 zro2 p2o5 from 800 c or 900 c contain 0 25 0 5 atom pb 2 4 wt pbo yielding apparent dpb values of 0 2 0 3 available p5 is incorporated in a 2 1 ratio with pb2 suggesting a specific charge balance mechanism 2p5 pb2 2si4 zr4 however pb enters the zircon even when p is unavailable so h may again play a charge balancing role because of the rapid polythermal modes of zircon growth and the high pb content of the experimental systems the apparent partition coefficients should not be viewed as equilibrium values but as qualitative indicators of pb compatibility under various growth circumstances the overall results are consistent with the low but variable levels of non radiogenic common pb in natural zircons the increased compatibility of pb in fluid grown low temperature zircons suggests a possible fingerprint for zircons from hydrothermal and wet metamorphic rocks i e high concentrations of common pb

paleomagnetic and 40ar 39ar geochronologic data bearing on the structural evolution of the silver peak extensional complex west central nevada the silver peak extensional complex located in the silver peak range of west central nevada is a displacement transfer system linking the furnace creek fish lake valley fault system and transcurrent faults of the central walker lane late neogene northwest directed motion of an upper plate composed of lower paleozoic sedimentary rocks and late tertiary volcanic and volcaniclastic strata exhumed a lower plate assemblage of metamorphic tectonites with proterozoic and mesozoic protoliths paleomagnetic investigation of miocene pliocene pyroclastic and sedimentary rocks of the upper plate and miocene mafic dikes in the lower plate reveals modest horizontal axis tilting northwest side up and vertical axis rotation clockwise within the extensional complex eight to ten samples from each of 123 sites were demagnetized 95 sites yielded interpretable results dual polarity results from one population of mafic dikes in the lower plate assemblage indicate moderate northwest side up tilting declination d 329 inclination i 37 95 4 3 number n 30 sites in situ 95 the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction some dikes yield exclusively normal polarity results that are interpreted to indicate modest clockwise vertical axis rotation d 021 i 57 95 4 3 n 19 sites in situ concurrent with uplift of the lower plate rocks and nine sites yield magnetization directions that are north directed with positive inclinations of moderate steepness similar to an expected miocene field late miocene pyroclastic rocks in the upper plate yield normal polarity magnetizations suggestive of moderate clockwise vertical axis rotation d 032 i 53 95 8 8 n 10 sites the apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field because the overall dispersion of the vgp virtual geomagnetic pole positions is high for the latitude of the site location middle miocene sedimentary rocks probably were remagnetized shortly after deposition of eight 40ar 39ar determinations from mafic dikes in the lower plate five groundmass concentrates yield saddle shaped age spectra and one separate provided a plateau date of low confidence isochron analysis reveals that all six groundmass concentrates contain excess ar if rapid cooling and ar retention below 250 c are assumed the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10 5 ma the 40ar 39ar age spectrum data are consistent with existing fission track cooling and k ar isotopic age information from lower plate granitic rocks and indicate rapid cooling of the lower plate assemblage from well above 300 c to 100 c between 13 and 5 ma rapid cooling may explain the overall distribution of paleomagnetic results from lower plate intrusions such that the earliest acquired magnetizations reflect both northwest side up tilt and clockwise rotation and the younger magnetizations reflect northwest side up tilt overall the paleomagnetic data from the silver peak extensional complex are interpreted to suggest that vertical axis rotation of crustal scale blocks associated with displacement transfer in the central walker lane may play an integral part in accommodating strain within a continental displacement transfer system

u pb shrimp and 40ar 39ar geochronological constraints on the evolution of the xingxingxia shear zone nw china a triassic segment of the altyn tagh fault system the development of structures and their age along the segment of the altyn tagh fault system and the eastward extension of the tianshan orogenic belt remain speculative recent investigations on the structural framework granitic intrusions and metamorphic rocks in the eastern tianshan and adjacent areas show that the ne striking xingxingxia sinistral ductile shear zone nw china is subparallel to the altyn tagh fault zone and is superposed on the eastern tianshan orogenic belt u pb zircon sensitive high resolution ion microprobe shrimp dating and muscovite biotite and k feldspar 40 ar 39 ar thermochronology indicate that sinistral shear along the xingxingxia shear zone initiated at 240 235 ma broadly at the same time as initial formation of the altyn tagh fault zone but later than initiation of dextral strike slip motion along ew trending eastern tianshan orogenic belt at 270 245 ma formation of the xingxingxia ductile shear zone was associated with gondwanaland convergence along the southern margin of the eurasian continent during the late permian early triassic

u and th zonation in fish canyon tuff zircons implications for a zircon u th he standard the inability to accurately determine the distribution of u and th in zircon crystals analysed using u th he is a major source of error and limits the confidence in subsequent data interpretation the fish canyon tuff fct zircon standard shows u th he age reproducibility in excess of 10 we have tested the extent to which this is due to u and th zonation using a combination of cathodoluminescence and ion probe analysis of a population of fct zircons primarily we find that fct zircons exhibit extreme u and th zonation and the population has large inter crystalline variability furthermore the net cathodoluminescence intensity from the fct zircons is negatively correlated with u and th concentration allowing cathodoluminescence emission to be used as a proxy for u and th zonation this correlation was exploited to constrain the u and th zonation within the crystal population and to show that the poor age reproducibility of the fct zircons is consistent with the observed zonation

alpine tectonics in the calabrian peloritan belt southern italy new 40ar 39ar data in the aspromonte massif area this study provides new ar 40 ar 39 geochronological constraints on the age of the alpine tectonics in the aspromonte massif southern part of the calabrian peloritan belt this massif exposes the upper units of the calabride complex which originated from the european continental margin the calabride complex was incorporated in the alpine orogenic wedge and then integrated into the apennines and maghrebides fold and thrust belts throughout the calabride complex there is evidence for a two stage tectonic history which remains however rather poorly dated alpine nappe stacking is followed by extensional reworking along the former thrust contacts or along new detachment surfaces our new ages suggest that exhumation of the uppermost units which accompanied nappe stacking probably started at 45 ma and that the deepest units were almost completely exhumed at 33 ma this kinematics probably corresponds to syn orogenic extension while the end of exhumation is clearly related to the extensional tectonics dated at 28 6 ma along detachment structures our geochronological data reveal a very short lag time between accretional and extensional processes in this part of the mediterranean alpine orogenic belt the direction of extension when the units are restored to their initial position i e before the opening of the western mediterranean basins and the bending of the arc is nne ssw such a direction does not fit with the eastward slab retreat model generally put forward to explain extension in the western mediterranean in contrast we provide evidence for roughly n s middle oligocene extension in the accretionary prism not previously described in this part of the mediterranean domain

an attempt to constrain the age duration and eruptive history of the karoo flood basalt naude s nek section south africa we have carried out paleomagnetic sampling of a similar to 750 m sequence of the karoo large igneous province naude s nek pass south africa k ar dating cassignol gillot has been performed on four samples from the 650 m upper unit mean age 179 2 1 8 ma and a sample from the lower unit 184 8 2 6 ma a succession of two phases of volcanism is suggested the lower 25 flows 115 m thick have recorded a reversed polarity the next 23 flows 135 m thick are transitional and contribute a detailed record of the van zijl 1962 jurassic reversal the upper 38 flows 500 m thick have normal polarity directional groups dgs of lava flows with quasi identical remanence directions indicate eruption durations too short to have recorded geomagnetic secular variation and hence are interpreted as single eruptive events altogether 19 dgs and 10 sheet lobes yield a sequence of 29 distinct directions this could correspond to a total eruptive activity shorter than 3000 years less than one per mil of the total duration over which the section was emplaced we obtain a new paleomagnetic pole for south africa at similar to 180 ma lambda 75 2 degrees n phi 276 4 degrees e a 95 5 8 degrees n 19 which is consistent with earlier reports

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

the multi scale influence of topography on lava flow morphology predicting lava flow pathways is important for understanding effusive eruptions and for volcanic hazard assessment one particular challenge is understanding the interplay between flow pathways and substrate topography that is often rough on a variety of scales 1 m to 10 s km to study this problem we develop a lava flow modeling framework that combines spectral analysis of substrate roughness with a new lava flow model multiflow the mutliflow model includes a multiple flow direction routing algorithm in conjunction with a thresholding function that limits the extent of the flow comparison of multiflow results with recent flows on mt etna mauna loa and kilauea suggests that multiflow accurately predicts the influence of topography on lava flow paths and morphology spectral filtering of pre flow digital elevation models allow us to quantify which topographic wavelengths are most important for influencing lava flow pathways filtering constrains the height of pre existing objects that flow overtop as a measure of flow thickness and aids in identifying divergence of flow pathways from pre eruptive topography that would result from modification of the substrate by the flow low pass filter cutoffs in the range of 10 100 m significantly improve the fit between modeled flow and real flow paths suggesting a sensitivity of lava flow paths to such mesoscale topographic wavelengths the three case studies are generalized with a parameter investigation of lava flow branching on synthetic red noise topography we demonstrate that the tendency of a flow to branch or not can be predicted on the basis of spectral characteristics of the underlying surface we end by examining spectral signatures associated with volcanic and erosional processes on the island of hawai i that may aid in forecasting lava flow pathways and deconvolving competing topographic construction and erosion in volcanic landscapes on longer timescales

heterogeneous metamictization of zircon on a microscale the ability of modern raman microprobes to determine the crystallinity of zircon with a lateral resolution of about 1 m and a volume resolution of less than 5 m3 opens up new opportunities for correlating structural chemical and isotopic data on the same micro area within a zircon grain combining this information we confirm by direct measurement the inhomogeneity in the degree of crystallinity of complex zircon grains and the correlation between u th concentration isotopic discordance and degree of metamictization in micro areas of these grains micro areas with a higher degree of metamictization are more chemically heterogeneous than neighbouring more crystalline areas and contain higher concentrations of ca fe u th and y but not always hf which in some cases shows higher concentration in the less metamict areas raman observations that there are no radiation damage halos extending over distances greater than 1 1 5 m preclude alpha and beta radiation as well as fission recoil as viable causes for amorphization and confirm that alpha recoil is the most important mechanism causing zircons to disorder into the metamict state

an intercalibration study of the fish canyon sanidine and biotite 40ar 39ar standards and some comments on the age of the fish canyon tuff the age of the fish canyon tuff fct has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the k ar and u pb isotopic systems minerals from the fct notably sanidine and biotite are in widespread use as 40ar 39ar fluence monitors standards and have been employed as both primary and intercalibrated standards in attempting to constrain the true age of fct the distinction between primary vs intercalibrated ages is important as only the primary age determinations are valid an investigation of both fish canyon sanidine fcs and fish canyon biotite fct 3 was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain to grain scale electron microprobe results show that fcs is chemically homogeneous verifying previous studies although there are some very small zones of minor k depletion fct 3 biotite commonly contains numerous and small mineral inclusions 40ar 39ar experiments further show that fcs is remarkably homogeneous confirming previous observations whereas fct 3 shows much greater isotopic variability due to contamination from the mineral inclusions by setting the age of fcs equal to 27 98 0 15 ma 2 as intercalibrated to the u pb system by villeneuve et al 2000 the resultant intercalibration factor between fcs and fct 3 is 1 005 0 017 yielding an age of 28 13 0 47 ma 2 for fct 3 although the significant intragrain chemical and isotopic variability precludes its use as an effective 40ar 39ar fluence monitor the true age of the fct is problematic a survey of all primary ages for fct shows that mean k ar ages for plagioclase hornblende biotite and sanidine are consistently younger than mean u pb ages from titanite and zircon in conjunction with recently published petrological results which indicate that the fct contains material from multiple crystallization events this significant discrepancy between the k ar and u pb data sets demonstrates that the fish canyon tuff cannot serve as a golden standard between the k ar and u pb systems

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

exploring the multicollection approach for the 40ar 39ar dating technique we present an original analytical system for 40ar 39ar dating it makes use of a 180 sector multicollection mass spectrometer equipped with five faraday cups which renders peak switching unnecessary during argon isotopic analyses compared to the single collector approach commonly used for argon isotopic analyses our system presents greater stability during data acquisition faraday cup efficiencies which can be a limiting factor for the multicollection mass spectrometer were highly reproducible the analytical validity of the 40ar 39ar ages obtained using this new mass spectrometer has been preliminarily tested using geological standard minerals mmhb 1 fct san and hd b1 commonly used as neutron fluence monitors independent plateau age determinations of ethiopian samples duplicated over a 1 year interval demonstrated the reproducibility of the analyses the results of these measurements highlight the good behavior of this new instrument for 40ar 39ar step heating dating the age reproducibility of successive steps leads to analytical errors lower than 0 1 for well behaved samples this system still in its initial stage of development represents an alternative solution that is worth exploring in order to improve absolute dating by the 40ar 39ar technique

the santa rosa event 40ar 39ar and paleomagnetic results from the valles rhyolite near jaramillo creek jemez mountains new mexico the jaramillo event was originally defined by doell and dalrymple in 1966 on the basis of k ar ages from sanidine in the normally transitionally and reversely magnetized rhyolite domes named cerro del abrigo cerro santa rosa i and cerro santa rosa ii respectively that erupted following collapse of the archetypal valles caldera new mexico we have collected new paleomagnetic data from the three domes and new 40 ar 39 ar laser fusion and furnace incremental heating experiments on sanidine crystals from the cerro santa rosa i rhyolite step wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of doell and dalrymple science 152 1966 1060 1061 are valid cerro del abrigo is normally magnetized whereas the santa rosa i dome is transitional with an inclination of 363 toward the east and the santa rosa ii dome is of reversed polarity twenty five laser fusion experiments on sanidine crystals from the the cerro santa rosa i dome together with the saddle shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption our preferred age of 936 8 ka 2c for the santa rosa i rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages this age is significantly older than was inferred on the basis of earlier laser fusion 40 ar 39 ar results that suggested a trapped component characterized by a 40 ar 36 ar ratio lower than the atmospheric value of 295 5 our new age distinguishes the santa rosa i dome by 65 kyr from the termination of the jaramillo normal subchron 1001 10 ka and by 37 kyr from basaltic lavas at haleakala and tahiti which record the kamikatsura event 899 6 ka moreover this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period the cerro santa rosa i rhyolite dome once intimately linked with the end of the jaramillo event and the acceptance of plate tectonic theory now defines a highly resolved feature the santa rosa event in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments fl 2002 elsevier science b v all rights reserved

40ar 39ar stratigraphy of pyroclastic units from the canadas volcanic edifice tenerife canary islands and their bearing on the structural evolution many felsic pyroclastic units of various types are exposed in different sectors of tenerife new 40ar 39ar determinations allow them to be placed more precisely in the general volcano stratigraphic succession according to geographic distribution stratigraphic position and isotopic ages four main pyroclastic phases may be identified the first san juan de la rambla phase 2 1 ma is only known in the north of tenerife in the tigaiga massif the second adeje phase 1 8 1 5 ma is most completely developed in the southwest of the island but occasionally occurs in the other sectors the third las americas phase 1 ma is only presently known in the southern region the fourth bandas del sur phase 0 7 0 15 ma is essentially exposed in the southeast sector the results of this work emphasise the complexity of the pre 1 ma eruptive history of tenerife and underline the fact that explosive volcanic activity has taken place for at least the last 2 ma vertical collapse structures have developed as a result of pyroclastic flow activity and these may be as old as 1 6 1 8 ma therefore much older than generally considered the precise location of calderas is difficult to ascertain as a result of the repeated lateral flank collapse during the construction of the canadas volcano

postcollisional eastward extrusion and tectonic exhumation along the eastern tianshan orogen central asia constraints from dextral strike slip motion and 40ar 39ar geochronological evidence abstractpostcollisional extrusion and tectonic evolution in the eastern tianshan orogenic belt etob remains poorly known especially the mechanism of dextral strike slip motion and associated tectonic exhumation to better constrain this development a structural and 40ar 39ar geochronological study was carried out on a syndextral strike slip intrusion the jueluotag batholith as well as on other granitic plutons in the etob 40ar 39ar analyses of hornblende biotite k feldspar and plagioclase from quartz mica diorite granodiorite and dioritic porphyry dykes were used to construct cooling histories of the etob hornblendes have cooling ages of 277 272 ma similar to the syntectonic granitic intrusions but biotite ages are 261 254 ma along the syndextral strike slip pluton from east to west the dextral strike slip motion cuts through 268 ma dioritic porphyry dikes as well from these data we conclude that dextral strike slip motion occurred from 270 to 245 ma based on the syntectonic granitic intr

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

geochronology of the big spruce lake alkaline intrusion a zircon age of has been determined for the silica saturated syenites of the big spruce lake alkaline complex pb pb whole rock ages of and 2155 16 ma were obtained for the silica undersaturated rocks of the complex and for the complex as a whole respectively the sm nd whole rock data yield an age of 2183 75 ma for the complex as a whole by weighting the ages by the inverse square of the standard deviations an average age for the complex of 2174 20 ma is determined however it is felt the zircon age is the most reliable for the time of crystallization of the complex minor contamination by country rock sm nd model ages of 2467 2595 ma and metasomatism contribute to scattered sm nd data points and large errors in the ages obtained for the five individual units the pb pb data enable a limiting estimate of about 50 ma for the maximum time span for the intrusion of centres 1 4

thermochronologic constrains on the processes of formation and exhumation of the xinli orogenic gold deposit jiaodong peninsula eastern china abstract the xinli orogenic gold deposit with gold resources of 40 t located in the northwestern part of the giant jiaodong gold province eastern china is controlled by the sanshandao fault and is one of a few deposits hosted by the early cretaceous 129 128 ma guojialing granitoid soon after intrusion the granitoid underwent ductile shearing at 400 500 c marked by recrystallized quartz ribbons and bending of plagioclase lamellae with rapid cooling from zircon crystallization temperature of 750 800 c at 129 128 ma u pb to closure temperature of 300 50 c for biotite 40ar 39ar method at 124 1 ma the ductile deformation lasted 4 million years mineralization was associated with subsequent brittle reactivation of normal movement on the fault indicated by a se plunging downdip lineation on the fault a hydrothermal sericite 40ar 39ar age of 121 ma inferred to date the alteration associated with gold mineralization has been previously obtained for the adjoining cangshang gold deposit also controlled by the sanshandao fault at the xinli deposit the 121 5 1 3 ma and 120 5 1 2 ma 40ar 39ar plateau ages of weakly fractured igneous k feldspar record closely the time of normal faulting and cooling within the range of mineralization temperatures 350 250 c two zircon fission track zft ages of 91 4 ma and 90 3 ma constrain the time of cooling through 240 50 c unimodal distribution of apatite fission track aft lengths with a slightly negative skew and mean fission track lengths of 12 3 0 2 m indicate relative slow continuous cooling through 125 60 c at 60 6 ma the slight acceleration of cooling around 90 ma constrained by the 40ar 39ar zft and aft data and thermal modelling may have resulted from the late normal reactivation of the sanshandao fault in summary extension and normal faulting not only created the channelways for the mineralizing fluids but also gave rise to the subdued topography and post mineralization exhumation that preserved the deposit

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

on the calibration of fission track annealing models to obtain thermal histories from fission track length data the rates at which track lengths are reduced must be estimated the usual method is to use empirical equations calibrated via laboratory analogues a major concern is the extrapolation of the equations to predict temperatures on the geological time scale which is far outside the range of the calibration data using objective statistical criteria it is shown that the inclusion of geological data from age standards and deep boreholes is an important measure to ensure that model predictions of geological annealing rates are at least close to being correct it is shown that high temperature geological data should be the preferential target in the search for new field evidence a discussion on the use and limitations of geological data and their consequences is presented

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

ar ar ages from transitionally magnetized lavas on la palma canary islands and the geomagnetic instability timescale 1 a detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls of barranco de los tilos on the island of la palma canary islands reveals a complex temporally segmented record of geodynamo behavior that contains no less than three distinct geomagnetic events the matuyama brunhes m b reversal is recorded in five transitionally magnetized lava flows from the north tn section the isochrons obtained from three of the lower four m b lavas are defined by 14 incremental heating experiments that together with a previous age determination yielded a weighted mean of 798 4 6 2 ka all uncertainties 2s in addition a 780 3 10 3 ka isochron was determined for the overlying transitionally magnetized flow indicating that it was erupted during a distinctly younger portion of the transition near the base of the south ts section one finds a sequence of weakly magnetized flows associated with virtual geomagnetic pole vgp positions in the southwest indian ocean between latitudes 56 s and 65 s suggesting instability of the geomagnetic field beyond that of typical secular variation 40 ar 39 ar isochrons from three of these flows defined by 11 separate incremental heating experiments gave a weighted mean of 822 2 8 7 ka this anomalous field behavior recorded 24 11 kyr prior to the m b reversal may coincide with an event featured in several marine sediment records directly above two normal polarity flows 40 ar 39 ar isochrons of 751 9 18 1 ka and 675 0 15 7 ka are nine transitionally magnetized lavas having magnetization directions associated with low to midlatitude vgps spanning 23 60 n these flows are then capped by a single flow possessing normal polarity based on 12 incremental heating experiments 40 ar 39 ar isochrons of five of these nine lavas along with the uppermost flow gave a weighted mean age of 580 2 7 8 ka for this period of transitional to normal field behavior from these same transitional lavas quidelleur et al 1999 reported three unspiked k ar ages with a weighted mean of 602 24 ka and proposed a new event called the la palma excursion however the 40 ar 39 ar age presented here is three times more precise than the k ar age and is indistinguishable at the 95 confidence level from the 40 ar 39 ar age of a lava from the snake river plain idaho that originally defined the big lost event transitional field behavior of similar age observed in astronomically dated marine cores further establishes that the big lost event recorded at la palma was indeed global in extent rigorous temporal and geomagnetic constraints for several additional periods of geomagnetic field instability during the last several million years will comprise a geomagnetic instability timescale that can be factored confidently into models of the dynamo process index terms 1560 geomagnetism and paleomagnetism time variations secular and long term 1520 geomagnetism and paleomagnetism magnetostratigraphy 1035 geochemistry geochronology 1513 geomagnetism and paleomagnetism geomagnetic excursions keywords argonargon dating lavas excursion paleomagnetism reversal

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

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

reply to the comment on 40ar 39ar dating of the tephra layer in the pliocene seneze maar lacustrine sequence french massif central constraint on the age of the reunion matuyama transition and implications on paleoenvironmental archives

potassium argon geochronology of a basalt andesite dacite arc system the mount adams volcanic field cascade range of southern washington high precision k ar dating and detailed mapping have established an eruptive chronology for a cascade stratovolcano and its surrounding array of coeval basaltic centers mount adams is a 200 km 3 andesitic cone that stands at the center of a coeval 1250 km 2 quaternary volcanic field that contains 60 discrete vents k ar ages were measured for 74 samples from 63 of the 136 volcanic units defined in the course of the mapping within analytical error there are no discrepancies between k ar ages and stratigraphic sequence major activity began in the area ca 940 ka and inception of the central stratovolcano took place at ca 520 ka a plot of cumulative volume erupted versus time shows that between 940 and 520 ka the eruptive rate was 3 k y and 80 of the products were basaltic andesites are volumetrically dominant and were emplaced in three main cone building episodes centered at 500 450 and 30 ka at eruptive rates of 1 6 5 km 3 k y at a lower rate of 0 05 0 1 km 3 k y the magmatic system remained almost continuously active between the main pulses although breaks in activity as long as 30 k y are permitted by the k ar data andesitic dacitic activity in the focal region and dominantly basaltic activity on the periphery have coexisted for 520 k y and their products are interstratified the last main episode of cone construction occurred ca 40 10 ka the oldest andesite identifiably derived from within the present day edifice having an age of 33 14 ka andesites forming the south summit rim and the true summit have ages of 13 8 ka and 15 8 ka respectively the time volume composition data bear upon several fundamental questions concerning the long term behavior of arc volcanoes stratovolcanoes commonly grow in spurts but can stay active between the widely spaced episodes of peak productivity large stratocone systems may remain active for half a million years subdivision of complex stratovolcanoes into eruptive or constructional stages without detailed mapping geochronology and compositional data should be treated skeptically discussion of volumetric eruptive rates can be misleading without an adequate time scale stratovolcanoes need never develop large upper crustal magma chambers basalt erupts peripherally but its ascent is suppressed beneath stratovolcanoes by deep crustal domains of magma storage and hybridization that form where concentrated injection of basalt amplifies crustal melting

40ar 39ar dating of cataclastic k feldspar a new approach for establishing the chronology of brittle deformation abstract constraining the timing of brittle deformation such as in high angle normal or strike slip faulting is technically challenging in this study we present a new approach of age determinations from cataclastic k feldspars of brittle faults developed in rocks of the tan lu fault zone and from louzidian and liaonan metamorphic core complexes in ne china the attempt is based on field investigations microstructural observations back scattered electron bse imaging electron microprobe analyses x ray diffraction scanning electron microscope sem results and 40ar 39ar step heating of k feldspar separated from brittle fault zones of cataclastic mylonite granite and gneiss sem and bse images of individual k feldspar crystals reveal distinct grain deformation features including grain scale faults fractures and microfractures that occur parallel or perpendicular to each other the 40ar 39ar dating results provide well defined plateau spectra for the cataclastic k feldspar grains in the tan lu fault zone 74 5 1 3 ma and of brecciated gneisses and mylonites that formed at depth in the louzidian and liaonan metamorphic core complexes 120 129 ma and 116 2 ma these data suggest complete resetting of the ar isotope system of the cataclastic k feldspars allowing to constrain the timing of their deformation

high precision 40ar 39ar dating of oligocene rhyolites from the mogollon datil volcanic field using a continuous laser system replicate 40ar 39ar analyses of sanidine from seven oligocene 28 2 ma rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0 45 1s on samples smaller than 0 5 mg this is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller the system has the potential of resolving differences in apparent age of as little as 0 2 for high k ca samples as young as a few ma the analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0 1 m y or less

the 40ar 39ar age constraints on the duration of resurgence at the valles caldera new mexico 1 the 40ar 39ar ages for rocks from the valles caldera new mexico imply that resurgent uplift of the caldera floor occurred within 27 27 ka of caldera collapse the structural resurgent dome of the valles caldera was uplifted approximately 1000 m above the surrounding caldera floor during resurgence the upper bandelier tuff which yields an age of 1 256 0 010 ma 2 ages relative to fish canyon tuff sanidine at 28 02 ma is the lower constraint on the timing of resurgence the oldest postcollapse ring fracture dome cerro del medio is the upper time constraint and yields an age of 1 229 0 017 ma for one of its oldest flow lobes therefore resurgent uplift probably occurred within 54 ka at a minimum rate of 1 9 cm yr sanidine phenocrysts from rhyolites of the postcollapse intracaldera deer canyon and redondo creek members yield 40ar 39ar single crystal laser fusion ages ranging from 1 229 0 013 ma to 1 283 0 017 ma deer canyon member n 8 and 1 208 0 017 ma to 1 239 0 017 ma redondo creek member n 4 with one exception from each unit these ages are statistically indistinguishable from the upper bandelier tuff indicating that eruption of these postcollapse rhyolites probably commenced shortly after caldera formation melt inclusion hosted excess argon was not found to have a measurable effect on the age of sanidine from the upper bandelier tuff or the deer canyon member

u th ne chronometry abstract production of 21ne from the reaction 18o n 21ne in u and th rich minerals such as apatite zircon monazite and titanite can potentially be used for chronometry based on a review of the available cross section data we reevaluated the production of 21ne from this reaction using a thin target approach and compared the results against thick target situations the 21ne 4he production ratio in these minerals is about 4 10 8 but varies with oxygen content and th u ratio 21ne has a stopping range of about 1 m compared to about 20 m for particles thus the 21ne 4he production ratio also depends on crystal size when the crystals are small enough that ejection is important using a monte carlo model we computed the effects of ejection on the 21ne 4he ratio for various geometries and grain sizes we also present measurements of the 21ne 4he ratio on few mg aliquots of well dated volcanic apatites and zircons for which the duration of retention of 21ne and 4he is the same values of 4 4 1 1 10 8 and 3 5 1 2 10 8 for apatite and zircon respectively are in agreement with the theoretical values of 4 16 0 14 10 8 3 54 0 13 10 8 based on our production rate estimates the durango apatite and fish canyon tuff zircon give ne ages of 34 2 8 6 and 28 12 ma respectively which are in agreement with independently known ages our results demonstrate that the 21ne production is well understood and can be used to measure the neon retention time for slowly cooled minerals the 4he and 21ne content of zircons from the deeply exhumed crustal section in gold butte nevada crystallization age of 1 4 ga imply u th ne ages of 963 164 and 777 122 ma far older than their he ages of 16 7 1 3 and 19 1 1 5 ma respectively to explain the age difference a neon closure temperature for zircon between the values given by the thermochronometers u pb on apatite and ar ar on muscovite is derived probably 400 50 c

transitional field clusters from uppermost oligocene volcanic rocks in the central walker lane western nevada abstract three sections of the candelaria hills volcanic sequence west central nevada appear to have recorded parts of two transitional field records or reversal excursions paleomagnetic data and 40 ar 39 ar laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25 7 ma and about 23 8 ma fifteen sites yield northeast declination moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination moderate negative inclination directions both populations of site mean directions representing a total of 12 independent eruptive units are highly discordant to a time averaged late tertiary field direction and neither can be explained by a geologically reasonable magnitude of vertical axis rotation virtual paleomagnetic poles vgps estimated from the directional data lie at low to intermediate latitudes 29 of the 31 flows at intermediate latitudes

advantages and challenges of automated apatite fission track counting abstract fission track thermochronometry data are often a core element of modern tectonic and denudation studies soon after the development of the fission track method interest emerged for the development of automated counting procedures to replace the time consuming labor of counting fission tracks under the microscope automated track counting became feasible in recent years with increasing improvements in computer software and hardware and the development of microscopes and stage systems one such example examined in this study is the commercial automated fission track counting procedure from autoscan systems pty and the university of melbourne thermochronology group we conducted experiments that are designed to reliably and consistently test the ability of this fully automated counting system to recognize fission tracks in apatite and a muscovite external detector fission tracks were analyzed in a series of samples with a step wise increase in sample complexity in addition to the automated counting procedure the same samples were also analyzed using conventional counting procedures we found for all samples that the fully automated fission track counting procedure using the autoscan system yields a larger scatter in the measured fission track densities compared to conventional manual track counting this scatter typically resulted from the false identification of tracks regardless of the image filtering procedure used or sample coating large differences between track densities analyzed with the automated counting persisted between different grains analyzed in one sample as well as between different samples as a result of these differences a manual correction of the fully automated fission track counts is necessary for each individual surface area and grain counted this manual correction procedure significantly increases the time required to analyze a sample with the automated counting procedure compared to the conventional approach

rapid formation of the small isles tertiary centre constrained by precise 40ar 39ar and u pb ages the relative chronology of magmatic and tectonic events is key to an understanding of the influence of the iceland plume on the north atlantic in particular the location and duration of magmatism is of fundamental importance initial widespread flood basalt formation occurred in baffin island greenland and britain before complete plate break up at 56 ma after which time magmatism became concentrated in the active rift zone historically the british tertiary igneous province btip has been instrumental in advancing many concepts of igneous petrology however the absolute age and duration of the province remains unresolved here we present new internally consistent 40ar 39ar ages that help to constrain the volcanic activity in the small isles centre to within 2 my this short duration has implications for the onset of magmatism in the larger north atlantic province the rapid unroofing of the rum volcano and more significantly some of the evidence used to propose pulsing of the iceland plume

resolving high precision u pb ages from tertiary plutons with complex zircon systematics success in establishing high precision crystallization ages with the u pb zircon method is highly dependent on the ability to isolate or remove the effects of inherited xenocrystic components or secondary lead loss or the combined effects of both current conventional single crystal and ion probe shrimp methods are successfully applied to paleozoic and older samples but typically suffer from imprecision when applied to younger mesozoic cenozoic samples due to generally reduced radiogenic lead concentrations here we apply a series of intense partial dissolution steps to multigrain zircon fractions from a series of paleocene tonalitic to granodioritic intrusive rocks from southeastern alaska the zircon systematics are complicated by both minor inheritance and postcrystallization lead loss physical and isotopic evidence demonstrate that the partial dissolution steps preferentially remove outer layers susceptible to lead loss as well as core regions containing inherited components the final residues are often hollow shells of low u primary igneous zircon that yield highly precise reproducible and concordant ages the resulting age determinations commonly statistically differ at the 95 confidence level from ages based on apparently concordant but less precise conventional isotope dilution analyses the uncertainties of which masked minor subtle isotopic complexities this observation strongly cautions against basing age assignments of samples yielding slightly discordant data on 1 single concordant analyses accompanied by an array of discordant data or 2 the mean of several 206pb 238u or 207pb 206pb ages the step wise dissolution technique allows age determinations on young relatively low u and pb zircons at a resolution not currently possible with techniques such as single grain conventional or spot ion probe analyses widespread application of the technique will likely prove instrumental in resolving detailed magmatic histories of igneous complexes where zircon systematics are typically complex

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

twenty million years of continuous deformation along the karakorum fault western tibet a thermochronological analysis the role of the karakorum fault zone kfz is debated south of 33 n ongoing dextral oblique slip along the sw edge of the gar basin exhumes metamorphic and magmatic rocks of the ayilari range minerals have recorded a continuum of deformation from temperatures 600 400 c down to 20 ma of deformation along the fault greenschist facies deformation superimposed upon the medium to high grade deformation marks a kinematic change from pure dextral to dextral normal motion associated with the onset of rapid cooling at the regional scale the coexistence of transtension in the gar basin with transpression documented along the pangong range farther north suggests another example of the zipper tectonics model developed along the red river fault the kinematic shift induced the rise of the ayilari range starting at 16 12 ma and the incision of major river courses the indus river might have become captive of the relief at this time the river s 120 km of apparent offset implies dextral motion at a long term rate of ca 8 5 1 5 mm yr

volcanism in the earliest stage of back arc rifting in the izu bonin arc revealed by laser heating 40ar 39ar dating abstract the back arc region of the izu bonin arc has complex bathymetric and structural features which due to repeated back arc rifting and resumption of arc volcanism have prevented us from understanding the volcano tectonic history of the arc after 15 ma the laser heating 40ar 39ar dating technique combined with high density sampling of volcanic rocks from the back arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back arc rifting based on the new 40ar 39ar dating results 1 back arc rifting initiated at around 2 8 ma in the middle part of the izu bonin arc 30 30 n 32 30 n volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north south trending fissures and or lines of vents 2 following this earliest stage of volcanism at ca 2 5 ma compositionally bimodal volcanism occurred and formed small cones in the wide area this volcanism and rifting continued until about 1 ma in the region west of the currently active rift zone 3 after 1 ma active volcanism ceased in the area west of the currently active rift zone and volcanism and rifting were confined to the currently active rift zone the volcano tectonic history of the back arc region of the izu bonin arc is an example of the earliest stage of back arc rifting in the oceanic island arc age data on volcanics clearly indicate that volcanism changed its mode of activity composition and locus along with a progress of rifting

a new approach for electron microprobe zircon fission track thermochronology abstract fission track thermochronology ftt has been applied for decades to quantify rates and timing of processes in the shallow crust the most widely used approach is the external detector method edm in this conventional approach an age is obtained by counting both the fossil 238u spontaneous fission and the induced 235u induced fission tracks using an optical microscope the induced tracks are obtained through the irradiation of the sample with thermal neutrons in the nuclear reactor which causes fission of 235u based on the studies carried out by gombosi et al 2014 we present an alternative method of dating zircons using electron probe microprobe analysis epma to measure uranium concentration u the electron microprobe analysis fission track epma ft method was applied to three samples of rapidly cooled zircons the fish canyon tuff pocos de caldas syenite and serra geral zircons the analyses were performed using two approaches 1 using the age equation described in gombosi et al 2014 and 2 using a new age equation calibration developed for this work the results using the gombosi et al 2014 age equation were 26 7 4 1 ma 80 6 12 8 ma and 130 9 20 1 ma respectively and the results using the age equation from this work were 27 8 1 9 ma 83 8 7 7 ma and 136 12 ma respectively the uncertainty of the age is affected mainly by 238u concentration and s the spontaneous fission track density determinations other factors can affect the uncertainty of the age but their contributions are smaller for all samples the yield ages found by the two methods are consistent and overlap within two standard deviations of published reference ages determined from other radiometric techniques i e k ar 40ar 39ar and or u pb and traditional ftt by the edm

kinematics and timing of continental block deformation from margins to interiors during the deformation of continental blocks the magnitude of tectonic stress generally decreases with increasing distance from the margin of the block however the timing and kinematics of stress transmission from the margins to the interiors of continents are poorly resolved even though this information is critical to our understanding of the dynamics of continental deformation here we present a case study of mesozoic deformation of the north china craton ncc field investigations of mesozoic thrust faults and folds granitic intrusions and dykes combined with zircon shrimp and la icp ms dating and muscovite 40ar 39ar plateau ages reveal the age of the ne sw trending tectonic belts as 180 155 ma where the deformation of the craton margin occurred 10 20 ma earlier than that of the craton s interior although the kinematics of deformation are similar for the interior and the margin of the ncc strain decreases with increasing distance from the margin notably the bulk of the strain in the cratonic interior was focused in zones of pre existing weakness overall we determined that the ncc deformed under conditions of uniaxial compression a conclusion that is compatible with simple rheological models and that the stress magnitude attained in the cratonic interior was much less than that along its margin

perspectives on 40ar 39ar dating isotopic dating of geological samples using the k ar method and its variant the ar ar dating technique provides ages that in favourable circumstances are precise and accurate to within 1 limiting factors include the accuracy of the decay constants for k and the age of neutron fluence monitor minerals for rapidly cooled igneous rocks a k ar or ar ar age normally will give a good estimate of the age of eruption but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age as ar ar ratios can now be determined often to better than 0 1 the main limitation on accuracy relates to how well the k decay constants are known better determination of the and decays of k the basis for the decay constants is suggested at the present time rather than adoption of new decay constants linked to another decay scheme until improved independently measured decay constants for k become available there may be some circumstances where the newly proposed decay constants need to be used especially when comparing ages on volcanic igneous rocks with those measured in another system

comment on precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard by m a lanphere and h baadsgaard

independent fission track ages ϕ ages of proposed and accepted apatite age standards and a comparison of ϕ z ζ and ζ0 ages implications for method calibration abstract the results of 94 determinations of the independent fission track ages ages of proposed and accepted apatite age standards show that the ages of the accepted standards are consistent with their reference ages but that those of the proposed age standards are not the fct and dur data have been used to calculate z and 0 calibration factors the 0 values agree with independent measurements and with calculated values the z and 0 ages t t z t and t 0 of 71 samples dated in the course of geological investigations are on the whole consistent although systematic deviations from the mean t m exceed the statistical uncertainties these are however 2 and traceable to statistical variation including counting statistics and an extra poisson component resulting from somewhat inconsistent track identification criteria the overall agreement between t z t and t 0 implies that measurements of s i d and are reproducible but says nothing about their intrinsic accuracies the fact that t also agrees with t z t and t 0 t t m 1 000 0 001 is inconsistent with the 10 length reduction of confined spontaneous tracks in the standards the same applies to the t t ref ratio mean 1 009 0 010 it is concluded that the agreement between t and t ref and between t and t z t and t 0 is an indication of the fact that the calculated ages are indeed accurate this implies that the 10 reduction of the spontaneous track lengths in fct and dur has no measurable effect on their fission track ages to within the

ela icp ms u pb zircon geochronology of regional volcanism hosting the bajo de la alumbrera cu au deposit implications for porphyry related mineralization ela icp ms u pb zircon geochronology has been used to show that the porphyritic intrusions related to the formation of the bajo de la alumbrera porphyry cu au deposit nw argentina are cogenetic with stratigraphically well constrained volcanic and volcaniclastic rocks of the late miocene farallon negro volcanic complex zircon geochronology for intrusions in this deposit and the host volcanic sequence show that multiple mineralized porphyries were emplaced in a volcanic complex that developed over 1 5 million years volcanism occurred in a multi vent volcanic complex in a siliciclastic intermontane basin the complex evolved from early mafic intermediate effusive phases to a later silicic explosive phase associated with mafic intrusions zircons from the basal mafic intermediate lavas have ages that range from 8 46 0 14 to 7 94 0 27 ma regionally extensive silicic explosive volcanism occurred at 8 0 ma 8 05 0 13 and 7 96 0 11 ma which is co temporal with intrusion of the earliest mineralized porphyries at bajo de la alumbrera 8 02 0 14 and 7 98 0 14 ma regional uplift and erosion followed during which the magmatic hydrothermal system was probably unroofed shortly thereafter dacitic lava domes were extruded 7 95 0 17 ma and rhyolitic diatremes 7 79 0 13 ma deposited thick tuff blankets across the region emplacement of large intermediate composition stocks occurred at 7 37 0 22 ma shortly before renewed magmatism occurred at bajo de la alumbrera 7 10 0 07 ma the latest porphyry intrusive event is temporally associated with new ore bearing magmatic hydrothermal fluids other dacitic intrusions are associated with subeconomic deposits that formed synchronously with the mineralized porphyries at bajo de la alumbrera however their emplacement continued from 7 10 0 06 to 6 93 0 07 ma after the final intrusion at bajo de al alumbrera regional volcanism had ceased by 6 8 ma 6 92 0 07 ma

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

comparison of conventional k ar and 40ar 39ar dating of young mafic volcanic rocks abstract k ar and 40ar 39ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the snake river plain idaho mount adams washington and crater lake oregon the k ar ages were calculated from ar measurements made by isotope dilution and k2o measurements by flame photometry the 40ar 39ar ages are incremental heating experiments using a low blank resistance heated furnace the results indicate that high quality ages can be measured on young mafic volcanic rocks using either the k ar or the 40ar 39ar technique the precision of an 40ar 39ar plateau age generally is better than the precision of a k ar age because the plateau age is calculated by pooling the ages of several gas increments the precision of a plateau age generally is better than the precision of an isotope correlation isochron age for the same sample for one sample the intercept of the isochron yielded an 40ar 36ar value significantly different from the atmospheric value of 295 5 recalculation of increment ages using the isochron intercept for the composition of nonradiogenic ar in the sample resulted in much better agreement of ages for this sample the results of this study also indicate that given suitable material and modern equipment precise k ar and 40ar 39ar ages can be measured on volcanic rocks as young as the latest pleistocene and perhaps even the holocene