past present and future of thermochronology in one form or another geochronologists have been practicing thermochronology 1 the use of radioisotopic dating to constrain thermal histories of rocks and minerals for over 40 years building from lessons learned over these four decades thermochronology continues to evolve due to technical developments increasingly sophisticated theoretical models and an expanding range of applications in geologic and planetary science most recently interest in earth surface processes and interactions between tectonics erosion and climate has drawn attention to techniques that can address the timing and rates of processes operating at temperatures below about 300 c the purpose of this rimg volume is to assess the current state of thermochronology as of circa 2005 which is coincidentally the 100th anniversary of the first radioisotopic date rutherford 1905 1906 excellent review papers and books on specific topics within this field have been published but no single volume has yet provided a comprehensive review of current practices basic theory and illustrative examples the motivation for this volume stems from these considerations knowing that in a fast developing field a book like this can quickly become dated we tried to include sufficient review of fundamentals and the literature to offer students and new users a useful introduction to thermochronology that may have some staying power in this chapter we first review the salient points of thermochronology s history before assessing our current capabilities and challenges and then taking the risk of suggesting where the field is headed we do not provide a comprehensive history of the method that does full justice to the work of the large and growing cohort of thermochronologists in this short space we instead opted to give our perspectives on where the intellectual and technological roots of the discipline lie which run deeper and go back farther than is sometimes appreciated

solubility and trapping of helium in apatite abstract a fundamental but unquantified assumption in u th he dating of apatite is that grains do not incorporate extraneous helium by solution or other processes but large age dispersion seen in some samples suggests that this assumption might be violated our laboratory experiments show that helium solubility in apatite is quite low and unlikely to lead to age dispersion in most samples however in some samples highly variable and sometimes large helium uptake suggests that apatite grains can trap helium in microvoids that could be derived from fluid inclusions or other microstructures a conclusion supported by crushing and step heating experiments the presence of such microvoids raises the possibility that closure and age systematics could be complicated either by trapping of internally generated radiogenic helium and or alteration of helium diffusion kinetics by impeding diffusion

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

provenance shallow to deep diagenesis and chemical mass balance in supermature arenites and pelites ordovician simpson group oklahoma and kansas u s a abstract this study considering original detrital compositions and diagenetic alterations relates sandstone and shale mineral and chemical composition to provenance and mass balance between the sedimentary assemblage and its provenance mature quartz rich sandstones and siltstones with clay rich shales in cores at depths of 60 m 5 2 km and burial temperatures of 150 c from over 50 wells were utilized in this work these provide the material for analysis of compositions and alterations in coastal and shallow marine clastic sedimentary rocks of the middle ordovician simpson group in oklahoma and kansas u s a mineral and geochemical data were obtained by optical petrography x ray diffraction scanning electron microscopy electron microprobe and whole rock x ray fluorescence and icp ms analyses results of the study show significant progressive diagenetic changes with increased burial depth and temperature mineralogically and chemically less altered rocks are comparable to modern quartz rich sands and kaolinitic muds in siliciclastic sediments developed during humid tropical weathering in crystalline basement shield provenance in africa and south america by analogy to modern tropical cases muds composed of major kaolinite with minor smectite and illite at deposition were diagenetically transformed to illite rich rocks by possible microbial action at surface to shallow 20 in sandstone buried 3 km during deep burial high temperature 110 c calcite cement from deep basin sources was precipitated in sandstones geochemical mass balance estimates physical and chemical evidence suggest quartz cement was sourced from within clastic rocks of the simpson group with minor external contribution of sio2 na is in low abundance due to loss in solution during weathering in the provenance rare earth elements ree have typical abundances and distributions in shales with enrichment in light ree littoral environment siltstones and matrix rich sandstones display enrichment of heavy ree in phosphatic brachiopod shell fragments

spatially correlated anomalous 40ar 39ar age variations in biotites about a lithologic contact near simplon pass switzerland a mechanistic explanation for excess ar forty four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40ar 39ar and demonstrate the importance of bulk ar diffusivity and system geometry factors not usually considered in the interpretation and collection of 40ar 39ar age data the resulting 40ar 39ar apparent ages range from 11 30 0 05 ma to 17 90 0 10 ma the ages and excess argon contents are spatially and lithologically correlated the pelite samples all yield ages clustering around 12 ma the age expected for cooling through biotite closure 360 c in this region of the alps ages in the amphibolite biotites are older showing a smooth trend between 15 ma at the contact with the pelite to 18 ma 34 cm from the contact this data shows that characterization of the ar closure age for biotite in a given system should not rest on a single sample as otherwise irresolvable differences in age between samples within the same outcrop can exist a generalized mechanistic model for excess argon is presented the presence or absence of excess ar depends on an intrinsic system parameter t the transmissive timescale which is the characteristic time for 40ar to escape through the local intergranular transporting medium itm to some sink for argon to prevent buildup of geochronologically significant excess 40ar t must be very short relative to the true closure age of the mineral a fortran code including radiogenic ar production diffusive loss of ar from biotite and bulk ar diffusion through the itm has been developed application of numerical modeling suggests that the time averaged effective bulk diffusivity deffar in the biotite amphibolite rock during early retrograde cooling is 2 2 1 0 10 8 m2 yr assuming steady state conditions the first such measurement available numerical modeling also provides information about the transmissivity and geologic history specific to the field site including a drop in deffar at 15 5 1 0 ma the timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack

understanding phengite argon closure using single grain fusion age distributions in the cycladic blueschist unit on syros greece abstract the preservation of 40ar 39ar ages of high pressure hp metamorphic white mica reflects an interplay of processes that mobilise 40ar either through mica recrystallisation or by diffusive 40ar loss the applicability of resulting ages for dating tectonic processes is critically dependent on whether either of these processes can be proven to be efficient and exclusively active in removing 40ar from mica if not preservation of an inherited or mixed age signal in a sample must be considered for interpretation the cycladic blueschist unit on syros has become a new focal area in the discussion of the geological significance of argon age results from multi grain step heating experiments while some argue that age results can directly be linked to deformation or metamorphic growth events others interpret age results to reflect the interplay of protracted recrystallisation and partial resetting preserving a mixed age signal here we demonstrate the potential of a new approach of multiple single grain fusion dating using the distribution of ages at the sample section and regional scale we show that in northern syros mica ages display systematic trends that can be understood as the result of three competing processes 1 crystallisation along the prograde to peak metamorphic path 2 a southward trend of increasing 40ar loss by diffusion and 3 localised and rock type dependent deformation or metamorphic reactions leading to an observed age spread typically limited to 10 myr at the section scale none of the sections yielded the anomalously old age results that would be diagnostic for significant excess 40ar the recorded trends in ages for each of the studied sections reflect a range of p t conditions and duration of metamorphism diffusion modelling shows that in a typical subduction metamorphic loop subtle variations in p t t history can explain that age contrasts occur on a regional scale but are limited on the outcrop scale our new approach provides a comprehensive inventory of the range of ages present in different rocks and at different scales which results in a more refined understanding of argon retention and isotopic closure of phengite and the geological significance of the ages we verify the added value of our new approach by comparison with multi grain step heating experiments on selected samples from the same sections

quartz veins record vertical flow at a graben edge fulmar oil field central north sea the fulmar oil field lies at the faulted western margin of the central graben in the central north sea chalcedony and quartz and later dolomite occur as diagenetic cements in veins within the upper jurassic fulmar formation quartz in one particular vein contains primary aqueous fluid inclusions with homogenization temperatures ranging from 85 degrees to 140 degrees c with a mean of 117 degrees c the measured d18o of the authigenic quartz ranges from 25 1 to 27 1o oo smow the d18o of the water from which most of the quartz precipitated is calculated to have been around 8o oo which is significantly more positive than the measured present day formation water d18o of 4o oo quartz and chalcedony cements in the vein are postdated texturally by dolomite that has a d18o of 23 5o oo smow high salinities are recorded in the fluid inclusions in the quartz the majority ranging from 15 to 19 wt nacl equivalent also the quartz contains inclusions of anhydrite high salinity aqueous inclusions combined with anhydrite inclusions in the quartz and the 18o enriched calculated water composition suggest that the quartz precipitated from an evaporite influenced fluid the same fluid contributed mg for dolomite precipitation more deeply buried sections including the permian zechstein evaporites underlying the fulmar field are the most obvious source of an 18o enriched saline fluid mineralization of the vein resulted from cross formational flow of warm fluids from depth through the zechstein and into the jurassic succession over vertical distances of 600 2000 m fluids most probably migrated up faults and fractures during episodes of salt movement or overpressure release

morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy in pelitic schists composed mainly of quartz and albite grains the morphology of intergranular pores which were filled with water was studied by transmission electron microscopy tem although some pores are defined by crystallographic planes f face most of their form has an ideal shape determined by interface tensions between grains and fluid high resolution tem observations demonstrate that pore free regions at grain boundaries are tight even at the nanometer scale showing that the wetting angle is larger than 0 in this rock the pore distribution in two grain junctions can be compared to a necklace microstructure developed by instability of a fluid film along the boundary induced by microcracking wetting angles for pores located at grain edges of quartz and albite decrease in the order albite albite quartz quartz and quartz albite the quartz quartz wetting angle in a calcite free sample is smaller than that in a calcite containing sample this angle also changes due to grain misorientation our results confirm that solid solid and solid fluid interfacial energies control the geometry of intergranular fluid in natural rocks

quantification of the factors controlling the presence of excess 40 ar or 4 he abstract a quantitative physical model is presented which includes the factors that control the presence or absence of internally derived excess 40ar or excess 4he in geological systems in particular the model incorporates the transport and partitioning properties of the rock surrounding the mineral of thermochronologic interest and illuminates the related effects on the amount of excess 40ar or 4he preserved in the system modeling of a simplified 1 d rock column bounded by an external sink for 40ar or 4he shows that a steady state excess 40ar or 4he profile develops the magnitude of which is determined by a system parameter called the transmissive timescale t the characteristic time required to reach this steady state depends upon t and the total local sink capacity tlsc wherein the important role of local matrix mineral and fluid phases is incorporated together these two system parameters t and tlsc determine the evolution of excess 40ar or 4he buildup within a system above the closure temperatures of all minerals involved an analytical expression for the 1 d system describing the evolution of excess 40ar or by analogy 4he in a particular potassium bearing or u th bearing mineral located at a distance l from an external sink has been derived empirically from model results 40 ar age equivalent l t t ar 2 1 exp 5 2 t t ar 1 tlsc ar local matrix minerals perhaps most notably quartz may act as important sinks for 40ar except in the most fluid rich systems where fluids dominate and thus are fundamental in controlling and limiting thermochronologically problematic excess 40ar in neighboring potassium bearing minerals in general the model provides a rigorous means of predicting excess noble gas content residence release and transport within the compositionally variable and thermally evolving crust

a microstructural and argon laserprobe study of shear zone development at the western margin of the nanga parbat haramosh massif western himalaya a sample of banded amphibolite from the western margin of the nanga parbat haramosh massif as sassi has been studied using microstructural and 40ar 39ar laserprobe techniques to investigate the relationship between deformation and argon isotope variations in a natural system amphibolite grade deformation occurred during south directed overthrusting of the kohistan arc over india along the main mantle thrust and was overprinted by extensional reactivation of the earlier fabric and the formation of biotite rich shear zones subsequent deformation along discrete fine grained fault zones was characterised by the formation of scapolite chlorite and k feldspar early plastic deformation and later cataclasis different minerals developed during this history show a wide range in apparent 40ar 39ar ages biotite chlorite and scapolite exhibit much lower concentrations of excess argon indicating their equilibration in a fluid relatively poor in excess argon a true age of ca 8 ma from biotite represents a minimum age for deformation associated with formation of the nanga parbat syntaxis and also precludes pliocene metamorphism in this area of the syntaxis both high and low closure temperature minerals amphiboles and feldspars record apparent ages which are associated with the incorporation of excess argon within the mineral lattice although differential thermal resetting of minerals at different closure temperatures is important variations in the inherited 40ar 36ar ratio throughout the sample is dominated by deformation and fluid infiltration consequently it appears that within deforming metamorphic rocks areas with significantly different argon isotope compositions may be present and need not be homogenised by diffusion

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

recrystallization in quartz overgrowths quartz overgrowths are among the most volumetrically significant diagenetic phases in sandstones many combined petrographic and geochemical studies of quartz overgrowths rely heavily on the long held underlying assumption of their stability this study of sem cathodoluminescence sem cl imaging of quartz overgrowths from the jurassic khatatba formation of egypt calls this assumption into question these overgrowths preserve spectacular sem cl textures that can be interpreted as recrystallization fabrics there are three major growth zones in the overgrowths q1 to q3 in the first q1 fine sem cl zoning is preserved some with relict fibrous textures and smooth or bulging growth surfaces these textures could indicate initial precipitation as opaline silica chalcedony microquartz or some phase other than coarse quartz overgrowth in q1 there are discordant areas locally showing smooth and scalloped boundaries and filled with quartz with the sem cl character of later growth zones such textures suggest that earlier zones were crosscut by later phases of quartz the smooth and scalloped boundaries and their position in the overall framework of cements and grains are in places inconsistent with simple fracturing but are consistent with dissolution of earlier growth zones followed by filling with later quartz these features suggest that earlier growth zones were unstable and were replaced by coarsely crystalline quartz overgrowth such observations and other empirical and chemical considerations suggest that in some cases quartz overgrowths may result from recrystallization of less stable quartz and silica phases thus any study of sandstone diagenesis focused on quartz overgrowths should take this possibility into account

field measurement of high temperature bulk reaction rates i theory and technique knowledge of metamorphic reaction rates is crucial to accurately interpret rock and mineral chemistry the local equilibrium assumption used in geochronology geothermobarometry and material flux estimates requires that local reaction rates among system phases are fast relative to local rates of p t x change natural metamorphic reaction rates are essentially unknown and difficulties and disagreements exist regarding extrapolations of existing laboratory data to natural conditions growing recognition of natural effects that could be attributed to slower reaction rates justifies the need for a field based quantification of reaction rates to assess the accuracy of lab based predictions we describe in detail the theory and methodology of a technique for extracting bulk reaction rates directly from isotopic data derived from natural samples baxter and depaolo 2000 reaction rates measured using this technique may be judiciously applied to isotopic exchange or net reaction kinetics in other natural systems the technique requires collection of whole rock and garnet 87sr 86sr data along a sampling traverse normal to a lithologic contact where there was prior to metamorphism a sharp isotopic discontinuity garnet data provide information on syn metamorphic conditions and the time interval for the exchange process forward modeling of the reactive transport process using numerical methods and the equations for diffusive reactive transport allow determination of the reaction rate and bulk sr diffusivity that provides the best fit to the data measurement of reaction rates is best constrained if an isotopic step is preserved at the contact the size of which is directly proportional to the bulk reaction rate this contribution is intended to serve as a template for future use and development of this technique to acquire natural reaction rate data from metamorphic systems

argon composition of metamorphic fluids implications for 40ar 39ar geochronology the concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe sup 40 ar sup 39 ar laser microprobe and fluid inclusion analyses on the same samples in our study fluid in inclusions formed during ductile deformation was probably derived locally from the paleozoic caledonian sequences whereas brines emplaced in cross cutting fractures were derived from proterozoic footwall rocks several kilometers from the mylonite by utilizing sup 38 ar sub cl to correct for open quotes excess sup 40 ar close quotes we estimate a 370 ma closure age for the amphibole which is distinctly younger than the 440 480 ma plateau ages indicated by step heating spectra the open quotes excess sup 40 ar close quotes responsible for the anomalous plateaus probably resulted from diffusive re equilibration of the amphibole with the fluids present during ductile deformation of the mylonite the significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region these ages indicate that the western gneiss terrain in our study area records deformation and cooling associated with siluro devonian tectonothermal metamorphism rather than an earlier caledonian tectonism as proposed in previous sup 40 ar sup 39 ar investigators 46 refs 8 figs

vertical effective stress as a control on quartz cementation in sandstones abstract temperature controlled precipitation kinetics has become the overwhelmingly dominant hypothesis for the control of quartz cementation in sandstones here we integrate quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation to suggest that the supply of silica from stress sensitive intergranular pressure dissolution at grain contacts is in fact a key control on quartz cementation in sandstones we present data from highly overpressured sandstones in which despite the current burial temperature of 190 c quartz cement occurs in low amounts 4 6 1 2 of bulk volume in situ oxygen isotope data across quartz overgrowths suggest that cementation occurred over 100 ma and a temperature range of 80 150 c during which time high fluid overpressures resulted in consistently low vertical effective stress we argue that the very low amounts of quartz cement can only be explained by the low vertical effective stress which occurred throughout the burial history and which restricted silica supply as a result of a low rate of intergranular pressure dissolution at grain contacts

40ar 39ar thermochronology of the fossil ll6 chondrite from the morokweng crater south africa studies of meteorites are based mostly on samples that fell to earth in the recent past i e a few million years at most the morokweng ll chondrite meteorite is a particularly interesting specimen as its fall is much older ca 145 ma than most other meteorites and because it is the only macro meteorite clast width intersected in drill core 25 cm found in a melt sheet of a large impact structure when applied to the morokweng meteorite 40ar 39ar thermochronology provides an opportunity to study 1 effects associated with pre impact and post impact processes and 2 collision events within a potentially distinct and as yet unsampled asteroid population a single multi grain aliquot yielded an inverse isochron age of 625 163 ma this suggests a major in space collisional event at this time we have modeled the diffusion of 40ar within the meteorite and plagioclase during and after the 145 ma impact on earth to tentatively explain why pre terrestrial impact 40ar has been preserved within the plagioclase grains the 145 ma terrestrial impact age is recorded in the low retentivity sites of the meteorite plagioclase grains that yielded a composite inverse isochron age at 141 15 ma and thus confirms that age information about major terrestrial or extraterrestrial impacts can be recorded in the k rich mineral phases of a meteorite and measured by the 40ar 39ar technique more studies on fossil meteorites need to be carried out to understand if the rough 0 6 ga age proposed here corresponds to major ll chondrite asteroid population destructions or rather to an isolated collision event

early quartz cements and evolution of paleohydraulic properties of basal sandstones in three paleoproterozoic continental basins evidence from in situ δ18o analysis of quartz cements quartz cement microstratigraphy and high precision in situ 18 o values obtained by secondary ion mass spectrometry sims from m size quartz cement zones have been used here to determine the timing of cementation and to evaluate precipitation mechanisms within the basal sandstones of three economically significant paleoproterozoic basins the athabasca and thelon basins canada and the mcarthur basin australia in these examples the earliest quartz cements have the highest 18 o values up to 33 mean 26 0 v smow indicative of low temperature precipitation at depths of 0 to 1 1 km some paragenetically early cement phases however have lighter isotopic values that suggest either precipitation from isotopically depleted water or precipitation at higher temperatures subsequent generations of quartz cements have progressively lower 18 o values mean 16 1 3 consistent with precipitation at higher temperatures these data combined with petrographic observations indicate quartz cementation began in near surface vadose and phreatic zones the next stage of cementation is characterized by syntaxial burial cement overgrowths regionally and stratigraphically well sorted sandstone facies that were initially aquifers preferentially experienced early quartz cementation which resulted in cement bridged pore throats at relatively shallow depths of 0 2 km and very early in their burial history transforming these units into diagenetic aquitards as a result these units did not experience mineralization compositionally and texturally immature braided fluvial facies did not experience early addition of quartz cement but are instead marked by minor syntaxial overgrowth cement that make upb5 of the intergranular volume formed at depths of 3 5 km and were variably replaced by illite and chlorite during burial these lithologies were still open to fluid movement at critical times in the basin when u and

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

excess argon evolution in hp lt rocks a uvlamp study of phengite and k free minerals nw turkey excess argon is a common problem in high pressure metamorphic rocks here we present 40ar 39ar data from metabasite metachert and metapelite lithologies and a range of minerals the results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high pressure metamorphism samples were derived from blueschist and low temperature eclogite facies rocks from the tavsanli zone of nw turkey samples were analysed using the 40ar 39ar ultra violet laser ablation microprobe uvlamp and yield ages ranging from 72 3 to 154 7 ma detailed intra grain profiling of phengites in situ and analyses of sodic amphibole lawsonite quartz and garnet present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite whole rock xrf data indicates a scattered relationship between bulk k2o content of the rock and the concentration of excess argon in phengite we conclude that excess argon was derived in situ in a relatively closed fluid absent system rather than introduced by fluids

production redistribution and loss of helium and argon isotopes in a thick sedimentary aquitard aquifer system molasse basin switzerland abstract in a rock water system an apparent residence time for he can be derived from the bulk he concentration in the rock and the porewater and the he production rate provided that 1 the system is at steady state and 2 the he flux from external sources is negligible this second condition is crucial and needs to be assessed by identifying the various he sources in a given hydrologic environment he and ar isotope abundances were investigated in whole rock samples and mineral separates of the alternating shale sandstone rock sequence of the permo carboniferous trough pct of the molasse basin in northern switzerland 4 he concentrations and 40 ar 36 ar ratios in groundwater in the bounding aquifers are lower than those in pct groundwaters thus eliminating a possible external source of radiogenic noble gases the elements producing radiogenic helium isotopes u th and li are mainly concentrated in the shales where they reside in biotite muscovite clay minerals and organic matter contents of these elements as well as chronological and chemical data allow us to calculate maximum closed system he isotope concentrations and the retention coefficients e g 3 he measured 3 he calculated ratios as deduced from the low retention coefficients almost all helium 3 that has been produced in these minerals since their deposition has been lost into the complementary reservoir i e the porewater the same conclusion is valid for 4 he this is also indicated by the similarity between the calculated 4 he 3 he production ratio of the shales and that observed in pct groundwater 1 3 10 7 moreover similar 4 he 3 he ratios are measured in quartz and plagioclase mineral grains from the sandstone layers by far exceeding the production ratios derived for these minerals also the measured concentrations of he isotopes especially 3 he are higher than the calculated production capacities of these minerals these observations suggest that some of the he produced in the shale and released from these rocks migrates via the porewater into the sandstone layers and penetrates further into some rock forming minerals such as quartz and plagioclase measurements of the migration rate of he atoms through quartz crystals showed that equilibrium between internal gas fluid vesicles and external porewater he concentrations is reached on a short time scale of 10 4 years at an in situ temperature of 70 c the equilibrium allows the he concentrations in the porewater across the pct sequence to be quantified using quartz crystals as detectors for the pct segment opened by the weiach borehole these quartz derived concentrations as well as those directly measured in pct groundwaters are rather high implying a long residence time for the he atoms in the pct rock porewater system well exceeding 10 7 years

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

emplacement age and thermal footprint of the diamondiferous ellendale e9 lamproite pipe western australia the diamondiferous ellendale 9 e9 pipe is a funnel shaped maar diatreme volcano consisting of inward dipping tuff sequences intruded by lamproite plugs and dykes the host rocks for the e9 pipe are permian sandstones the multiple lithological contacts exposed within the mined maar volcano provide a natural laboratory in which to study the effect of volcanic processes on u th pb he systematics zircon from the regional sandstone and e9 lamproite display a bimodal distribution of ages on u th he u pb plots the zircon u pb ages for the e9 pipe n 52 range from 440 to 2 725 ma while the cluster of u th he ages for the lamproite dyke zircon indicate that dyke emplacement occurred at 20 6 2 8 ma concordant with a maximum emplacement age of about 22 ma from phlogopite 40ar 39ar these ages indicate a xenocrystic origin for the zircon entrained in the e9 dyke the u pb ages of detrital zircon from the regional sandstone host 373 3 248 ma n 41 are indistinguishable from those of the lamproite zircon xenocrysts whereas the detrital zircon in the host sandstone yield u th he ages from 260 to 1 500 ma a thermochronology traverse across the e9 lamproite dyke reveals that the zircon u th he ages in the host sandstone have not been significantly thermally reset during dyke emplacement even at the contact the capability of the zircon u th he method to distinguish deep mantle source lithologies from upper crustal source lithologies could be used in geochemical exploration for diamonds pre screening of detrital samples using etching and helium assay methods will improve the efficiency and decrease the cost of greenfields exploration

40ar39ar dating of the quartz samples from two mineral deposits in western yunnan sw china by crushing in vacuum abstract the primary aim of this study is to investigate the possibility of dating fluid inclusions of quartz samples using the 40 ar 39 ar method by crushing in vacuo the samples are from the lushui tin tungsten deposit and the shangmanggang gold deposit in western yunnan sw china coexisting muscovite s 90ms and quartz s 90qw in a piece of tungsten ore from the former deposit were selected and the isochron age of s 90qw by crushing 38 6 1 9 ma shows excellent coincidence with that of s 90ms by stepped heating 38 6 0 8 ma the isochron age and plateau age by crushing of the quartz smg 8qz from the latter deposit indicate that the gold mineralization occurred at 144 ma these ages are respectively agreeable with the evaluated ages from the geologic facts in addition since the potassium concentration calculated from 39 ar released by crushing in the quartz sample smg 8qz was as high as 0 24 electron probe analysis was applied to investigate the occurrence of potassium and the results reveal that the majority of potassium scattered in the microcrystal quartz grains

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe 40 ar 39 ar technique ultra high pressure eclogite amphibolite grade metamorphism of the dora maira massif in the western alps is a well established and intensively studied event however the age of peak metamorphism and early cooling remains controversial the 40ar 39 ar step heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 ma which have been interpreted as the time of early cooling after the high pressure event recent u pb and sm nd results challenge this assertion indicating a much younger age for the event around 45 ma and hence a radically different timing for the tectonic evolution of the western alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39 ar dating the new technique allowed selective in situ analysis at a spatial resolution of 50 urn of quartz phengite biotite and k feldspar the results demonstrate the frequent occurrence of excess argon with high 40ar 39 ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios step heating analysis of minerals with this mixture would produce linear arrays on a40ar 36ar versus 40ar 36 ar correlation diagram leading to geologically meaningless plateau ages older than the true closure age in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 36 ar ages in many high pressure terrains

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

oxygen isotope microanalysis by secondary ion mass spectrometry suggests continuous 300 million year history of calcite cementation and dolomitization in the devonian bakken formation calcite cementation and dolomitization are key diagenetic processes in many sedimentary systems unravelling detailed histories and timescales of cementation and replacement is however often compromised by the limited spatial resolution of many analytical techniques in some cases multiple grains are co analyzed so that diagenetic histories are blurred and reaction periods are difficult to establish in this study we have used 10 micrometer resolution in situ secondary ion mass spectrometry to determine the oxygen isotope composition of 197 individual 10 50 micrometer size crystals of dolomite and calcite from six samples in a single core of upper devonian middle bakken member siltstones and sandstones the major tight oil formation of the williston basin usa this amount of data places important constraints on the range of temperatures and times that carbonate cementation and replacement occurred petrographic data show that microcrystalline calcite cement is an early phase and combined with mineralogical data suggest that much of the dolomite replaces calcite over spatial scales of less than a centimeter analyses of individual calcite crystals have a range of 5 for 18o in the group of crystals and for the group of individual dolomite crystals 10 these sub centimeter ranges are as great as those observed in previous studies of carbonate cements sampled over many meters and remind us that previous low resolution studies may have inadvertently analyzed mixed phases there is no relationship between dolomite texture and isotopic composition at this spatial scale microscale backscattered electron imagery and scanning electron microscopy cathodoluminescence zoning is seen but cannot be resolved with a 10 micrometer spot size assuming since it is an early cement that calcite precipitated from seawater 18o 1 5 it formed at ca 15 40 c mainly at the lower temperatures present day formation waters in devonian rocks in this region have oxygen isotope compositions of 7 8 vsmow using these values as a likely dolomitizing fluid we suggest that dolomitization occurred continuously between 40 and 140 c over 150 200 million years most likely in a fluid with a high mg ca ratio resulting from gypsum formation in local evaporites we suggest that this exceptionally low rate of dolomitization was controlled by the low rate of supply of mg in a very sluggish flow regime dolomitization is incomplete because of a limited supply of mg

conditions and timing of quartz cementation in brent reservoirs hild field north sea constraints from fluid inclusions and sims oxygen isotope microanalysis abstract the conditions and timing of quartz cementation in the brent sandstones in the hild field norwegian north sea were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence cl microscopy studies samples of the tarbert and the ness formations were investigated in three different wells presently lying at 3 8 4 3 km subsea and at 140 155 c quartz cement is present in all wells abundance varies from 3 to 26 and tends to increase with depth of burial two distinct separate episodes of quartz cementation referred to as q1 and q2 are readily distinguished by cl microscopy the two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses the data indicate that q1 quartz formed in the early tertiary 65 35 ma at 95 110 c and 2 3 3 0 km burial depth and that q2 mainly precipitated in the oligocene miocene 40 10 ma at 125 135 c and 3 1 3 7 km q1 quartz precipitation follows the onset of overpressuring in the reservoir while precipitation of q2 seems to slightly precede and accompany the main phase of oil filling 35 10 ma secondary ion mass spectrometry sims oxygen isotope microanalyses yield 18o values ranging from 17 to 25 averaging 23 2 for q1 and 19 5 1 5 for q2 calculated 18o values of quartz forming waters are similar for q1 and q2 ranging from 1 to 3 and are similar to the 18o value 2 of present day formation water in the reservoir quartz cementation in the brent reservoir of the hild field occurred during deep burial at high temperature from evolved 18o rich basinal water most likely derived from the viking graben two episodes of enhanced quartz precipitation are recorded around 100 c and 130 c no evidence was found for any significant contribution of 18o depleted jurassic meteoric water and or low temperature precipitation of quartz cement

oxygen isotopic indications of the mechanisms of silica transport and quartz cementation in deeply buried sandstones the oxygen isotopic compositions and inferred precipitation temperatures of quartz cements in a suite of deeply buried sandstones from the north sea gulf coast and western canada basins indicate that in almost every case the quartz precipitated from 18 o rich basinal waters 18 o rich waters are found in strictly limited volumes in sedimentary basins silica transport and quartz cementation could not therefore have occurred as a result of large scale advection of basinal or meteoric waters because this would require very large fluid fluxes quartz cementation was driven by processes operating on a local scale the isotopic data do not uniquely indicate the mechanisms of silica transport but are consistent with both diffusion and local recycling of fluid

diagenesis and reservoir quality of sandstones with ancient deep incursion of meteoric freshwater an example in the nanpu sag bohai bay basin east china abstract an example of diagenesis and reservoir quality of buried sandstones with ancient incursion of meteoric freshwater is presented in this study the interpretation is based on information including porosity and permeability petrography stable isotopic composition of authigenic minerals homogenization temperatures th of aqueous fluid inclusions afis and pore water chemistry these sandstones closely beneath or far from the regional unconformity formed during the late paleogene period are located in the thick shahejie formation in the gaoliu area of nanpu sag bohai bay basin east china early diagenetic calcite cements were leached to form intergranular secondary pores without precipitation of late diagenetic calcite cements in most sandstones feldspars were leached to form abundant intragranular secondary pores but with small amounts of concomitant secondary minerals including authigenic quartz and kaolinite the mass imbalance between the amount of leached minerals and associated secondary minerals suggests that mineral leaching reactions occurred most likely in an open geochemical system and diagenetic petrography textures suggest that advective flow dominated the transfer of solutes from leached feldspars and calcites low salinity and ion concentrations of present pore waters and extensive water rock interactions suggest significant incursion of meteoric freshwater flux in the sandstones distances of the sandstones to the regional unconformity can reach up to 1800 m while with significant uplift in the gaoliu area the burial depth of such sandstones below sea level can be less than 800 1000 m during the uplift and initial reburial stage significant uplift during the oligocene period provided substantial hydraulic drive and widely developed faults served as favorable conduits for downward penetration of meteoric freshwater from the earth s surface unconformity to these sandstone beds extensive feldspar leaching has been occurring since the uplift period coupled high th 95 115 c of afi and low 18 o smow values 17 20 within the quartz overgrowths show that quartz cementation occurred in the presence of diagenetic modified meteoric freshwater with 18 o smow values of 7 2 indicating that authigenic quartz only have been formed during the late reburial stage when meteoric fresh water penetration slowed down secondary pores in thin sections and tested porosity suggest that meteoric freshwater leaching of feldspars and calcite minerals generated approximately 7 10 enhanced secondary porosity in these sandstones meteoric freshwater leaching reactions cannot be ignored in similar sandstones that located deep beneath the unconformity with great uplift moving these sandstones above or close to sea level and with faults connecting the earth s surface with the sandstone beds

new evidence for the origin of quartz cements in hydrocarbon reservoirs revealed by oxygen isotope microanalyses quartz cements degrade hydrocarbon reservoir quality by choking off fluid flow paths therefore it is important to understand the sources and timing of quartz cementation we present in situ microanalyses of oxygen isotope ratios in quartz cements from a jurassic hydrocarbon reservoir located offshore norway showing systematic variations that reveal the isotopic history of the pore fluids that precipitated the quartz earliest quartz cements have 18o values of 20 26 subsequent cements drop in 18o value as low as 13 but then increase back to 23 with increasing cement thickness this isotopic trend was observed in multiple overgrowths with temperatures constrained by fluid inclusions and the burial history of the reservoir the isotopic composition of porewaters can be estimated from the measured isotopic ratios in quartz cement results indicate that 97 of the quartz cement precipitated from negative 18o waters originally meteoric that were gradually modified by burial diagenetic reactions as temperatures increased precipitation began in the middle to late cretaceous however the rate of quartz precipitation increased dramatically in the last 3 5 ma as the reservoir was rapidly buried the data show that quartz precipitation is a continuous process during burial and that silica is derived locally from silicate reactions occurring within the sandstone reservoir the isotopic data do not record evidence of episodic cementation from convection driven hot basinal fluids

origin of quartz cement in the lower ordovician dongjeom formation korea abstract this study deals with the origin of quartz cement developed in deeply buried mature sandstones intercalated in a thick carbonate sequence the dongjeom formation early ordovician korea is composed of three coarsening upward sequences and is divided into two facies according to depositional conditions with respect to changes in relative sea level transgressive facies and regressive facies the transgressive facies are characterized by calcite cemented allochem bearing sandstones whereas the regressive facies is characterized by non calcareous quartzose to arkosic sandstones quartz overgrowths occupy significant amounts of pore space average 16 in the regressive facies sandstones fluid inclusions reveal that quartz overgrowths precipitated between 110 and 165 c petrographical study and homogenization temperatures indicate that quartz overgrowths precipitated mostly during and after chemical compaction during the late diagenetic stage considering the 18 o values of detrital quartz grains and the volumes of quartz overgrowth the 18 o values of quartz overgrowth are estimated to range from 14 4 to 19 0 smow on the basis of mass balance silica for quartz overgrowths was principally derived from chemical compaction pressure solution of detrital quartz quartz overgrowths in the dongjeom sandstones precipitated from formation waters 2 1 to 2 5 smow that evolved from contemporary meteoric waters by diffusion

petrography fluid inclusion isotope and trace element constraints on the origin of quartz cementation and feldspar dissolution and the associated fluid evolution in arkosic sandstones feldspar dissolution quartz cementation and clay cementation are significant diagenetic processes in deep water fan feldspar rich sandstones in the shengtuo area dongying sag east china the timing and origin of these water rock interactions as well as the paleofluids in which relevant chemical reactions occurred were deduced using data from microscopy fluid inclusion microthermometry oxygen isotope microanalysis and trace element microanalysis three distinct separate episodes of quartz overgrowths referred to as q1 q2 and q3 were distinguished by cathodoluminescence microscopy the q1 quartz identified in all porous sandstones from 2500 to 3600 m 8200 to 11 800 ft was formed at approximately 100 c 115 c 212 f 239 f before oil filled the reservoirs the q2 quartz was mainly precipitated at 115 c 130 c 239 f 275 f accompanying or slightly postdating the main phase of oil filling and was identified in samples from 2800 to 3600 m 9200 to 11 800 ft the q3 quartz was only identified in sandstones buried deeper than 3500 m 11 400 ft and was likely precipitated in the quaternary when temperature exceeded 130 c 135 c 266 f 275 f secondary ion mass spectrometry oxygen isotope microanalyses yielded 18ovsmow vienna standard mean ocean water values ranging from 21 42 to 24 35 for q1 quartz from 22 03 to 24 99 for q2 quartz and from 21 72 to 22 91 for q3 quartz a mass balance calculation and quantitative petrography data of the amount of leached feldspars and associated secondary minerals suggest that the internal feldspar dissolution is likely the primary source for the authigenic clays and quartz cements in these sandstones positive 18o water values 0 5 to 4 5 vsmow of quartz forming waters indicate that quartz cementation and feldspar dissolution occurred in a geochemical system with a limited volume of diagenetically modified connate water the variations of 18o cements and trace element compositions from q1 quartz to q2 quartz in individual overgrowth suggest that hydrocarbon filling changed the chemistry of the pore fluid significantly further 18o water values of the pore water increased by approximately 2 vsmow after oil filling meteoric freshwater did not contribute to quartz cementation and simultaneous feldspar dissolution the detected co2 and hydrocarbons in fluid inclusions in the quartz cements the existence of co2 in hydrocarbon rich natural gas and the pyrobitumen in the feldspar hosted pores suggest that acids derived from organic matter kerogen in source rocks and hydrocarbons in reservoirs probably have promoted the extensive subsurface leaching reactions of feldspars in these sandstones the ongoing development of q2 and q3 quartz suggests that quartz precipitation did not cease after oil filling further diagenetic reactions likely have proceeded from water rock interactions to hydrocarbon water rock interactions

fluid channelling during ductile shearing transformation of granodiorite into aluminous schist in the tauern window eastern alps ductile shearing in the core of the tauern window austria transformed metagranodiorite into si undersaturated garnet chlorite staurolite schist at a depth of c 35 40 km during the alpine orogeny four distinct zones have been recognized extending from the wallrock into the centre of the shear zone zone i unaltered metagranodiorite with subordinate amphibolite zone ii biotite white mica garnet schist zone iii biotite phengite schist zone iv quartz absent garnet chlorite staurolite schist with garnets up to 10 cm across whole rock analyses show a dramatic decrease in sio2 from 65 wt in zone i to 35 wt in zone iv ca na and sr also decrease across the shear zone whereas al ti fe mg p cr ni zn and rb all increase towards zone iv mass balance calculations indicate that shearing was accompanied by up to 60 volume loss near the centre of the shear zone comparison of the tauern window samples with other shear zones in granitic hosts indicates that silica loss accompanied by gains in mg fe and ti is typical for volume loss shear zones but is distinctly different from the element behaviour exhibited in shear zones that are thought to represent approximately isovolumetric behaviour in the samples studied here volume loss appears to have resulted from channellized fluid flow during shearing producing time integrated fluid fluxes of 108 cm3 cm 2 in zone iv this large volume of fluid may have originated in part from dehydration of flysch carried beneath the metagranodiorites during eocene movement on the north penninic subduction zone development of an inverted thermal gradient during subduction would have allowed the fluid to scavenge large amounts of silica from the shear zone during ascent and heating

palaeo climate controlled diagenesis of the westphalian c d fluvial sandstones in the campine basin north east belgium the westphalian c and d fluvial sandstones in the campine basin north east belgium are potential reservoirs for the sequestration of co2 and interesting analogues of the hydrocarbon reservoirs in the southern north sea although these sandstones were deposited in a relatively short period of time their reservoir properties and mineralogical compositions are very different a petrographic study complemented with stable isotope analyses fluid inclusion microthermometry and x ray diffraction analyses of the clay fractions of the sandstones which were sampled from deep boreholes 1000 m in the campine basin revealed that these differences are related mainly to the climate at the time of deposition the most important eogenetic processes affecting the westphalian sandstones were the generation of a pseudomatrix by physical compaction of al silicates and lithic fragments that were strongly altered by extensive meteoric leaching kaolinitization of unstable silicates and precipitation of siderite these processes had a detrimental influence on the reservoir properties of westphalian c sandstones but their impact on the westphalian d sandstones was minimal the difference is assumed to be related to the climate at the time of deposition which changed from tropical humid in the westphalian c to semi arid arid during the late westphalian d both the westphalian c and d sandstones were affected by similar mesogenetic processes mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite k feldspar and smectitic clays illitization of kaolinite was controlled by the available quantities of co existing kaolinite and k feldspar and mainly affected the westphalian d sandstones illitization of k feldspar was controlled by the k feldspar content it had a much larger impact on the reservoir properties of the westphalian d as in these sandstones k feldspar was less affected by eogenetic alteration the illitization of smectitic clays resulted in illite quartz and ankerite cementation in both reservoirs this process had a more important impact on the westphalian c reservoir since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales the effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis mesogenetic alterations do exert a significant control on the properties of the westphalian d the vast impact of eodiagenesis on the westphalian c sandstones made them less susceptible to mesogenetic alteration the effect of telogenetic processes on the porosity and permeability of the westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the cimmerian unconformity no significant telogenetic alterations related to the variscan unconformity were observed

the thermal response of a metamorphic belt to extension constraints from laser ar data on metamorphic micas abstract metamorphic rocks may be exhumed to the surface either by isostatic rebound uplift and erosion or tectonically via thrusting or extension the latter mechanism is particularly important in the himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at 20 ma here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension the data when combined with existing constraints on the prograde evolution lead to a uniquely complete thermal history for a metamorphic belt laser argon data are reported for quartz and micas from semi pelitic metamorphic rocks from the zanskar himalayas nw india the samples derive from three traverses extending from low grade rocks near the detachment to high grade rocks in the core of the belt most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment the ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached muscovite and biotite ages from within the shear zone date deformation associated with extension at 20 25 ma the remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active rocks in the core of the orogen were still heating at this time later fast cooling of the high grade rocks in the core of the belt ensued when the extensional regime became active simple heat transfer calculations based on the distance of the high grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23 25 ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima

depositional age and triggering event of turbidites in the western kumano trough central japan during the last ca 100 years abstract for correlating a turbidite record with a detailed land record of natural disasters during the last ca 100 years the depositional age of the surface hemipelagic sediments of the kumano trough located in central japan were estimated we attempted to extract mass accumulation rate of hemipelagic fallout from hemipelagic intervals on the basis of 137 cs geochronology x radiograph dry bulk density and grain size distribution based on assumptions of the biogenic mixing effect on the 137 cs appearance horizon and hemipelagite erosion by turbidity current the reconstructed turbidite record from hemipelagic fallout mass accumulation rate is well correlated with the record of major natural disasters in the kumano area based on the correlation most of turbidite layers are flood storm origin except for a well sorted seismo turbidite induced by the tonankai earthquake that occurred the mid 20th century although the wide distribution of a flood induced turbidite layer due to the 19th century tostukawa flood which was a huge disaster in the area caused by heavy rain and dams breaching it does not show a hyperpycnite succession the flood by dams breaching may cause surge like flow angular to subangular sand sized lithic fragments in the turbidite show that sand grains were transported from the kumano river directly plant debris and microfossils are common in both seismo turbidite and flood storm induced turbidites

a new look at stable isotope thermometry interdiffusion between coexisting minerals affects all rocks and causes resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks the fast grain boundary fgb model describes the stable isotope fractionations and intracrystalline zonation which result from closed system interdiffusion eiler et al 1991 1992 this model assumes that grain boundary diffusion is much faster than volume diffusion and it accounts for exchange among all minerals in a rock previous models of closure temperature violate mass balance restrictions and will be inaccurate in most rocks modeling results are described for amphibolites and hornblende granites and gneisses biotite granites schists and gneisses pelitic and semi pelitic rocks garnet peridotites anorthosites gabbros pyroxenites and related rocks and calc silicate rocks examples of mineral pairs and specific rock types that allow accurate stable isotope thermometry include plagioclase pyroxene in pyroxene bearing anorthosites and garnet quartz in garnetiferous quartzites in contrast the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures closed system processes are capable of producing a variety of patterns of stable isotope resetting discordance mineral zonation and fractionation reversals examples include large reversals of quartz feldspar fractionations in micaceous rocks and oscillatory zonation in feldspar from some quartz rich rocks these results permit reinterpretation of many studies of stable isotope thermometry speedometry and retrograde alteration history fgb modeling of mineral zonation provides an important new guide to applying recently developed micro analytical tools to slowly cooled rocks application of the fgb model to quartzo feldspathic gneisses from the adirondack mountains new york demonstrates the usefulness of diffusion modeling in discriminating closed system diffusion controlled retrogression from open system retrogression and illustrates the possible importance of incorporating the effect of water activity on mineral diffusivity

excess 40ar transport scale and mechanism in high pressure phengites a case study from an eclogitized metabasite of the dora maira nappe western alps application of the bulk separate furnace heating laser step heating and in situ laser agemapping 40ar 39ar techniques reveals that high pressure phengites from an alpine eclogite of the dora maira basement nappe western alps were pervasively contaminated with excess 40ar to yield ages 130 ma spot fusion dating of individual grains with the 40ar 39ar laserprobe reveals concentric radial patterns with core ages as high as 320 ma showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse the single grains and population from this hand sample retain major differences in apparent ages indicating that the extent of the excess 40ar contamination varies from grain to grain attempt to model these differences in terms of grain size dependent fractional loss falls short and demonstrates that strong cm scale argon isotopic gradients locally prevailed during syn kinematic high pressure metamorphism on petrostructural grounds a model of internal isotopic buffering is outlined whereby the excess 40ar is argued to have been internally derived from within the eclogite protolith itself low intergranular ar diffusion rates due to reduced water activity buffered below unity by the phengite assemblage resulted in the local segregation and partitioning of the excess 40ar along grain boundaries during ongoing ductile deformation supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in alpine eclogites and very high pressure metasediments which shows that volatile recycling during high pressure metamorphism may occur on a very limited scale

intercrystalline stable isotope diffusion a fast grain boundary model we formulated a numerical model for stable isotope interdiffusion which predicts the temperatures recorded between two or more minerals and the intragranular distribution of stable isotopes in each mineral as functions of mineral grain sizes and shapes diffusivities modes equilibrium isotopic fractionations and the cooling rate of a rock one of the principal assumptions of the model is that grain boundaries are regions of rapid transport of stable isotopes this fast grain boundary fgb model describes interdiffusion between any number of mineral grains assuming that local equilibrium and mass balance restrictions apply on the grain boundaries throughout the volume modeled the model can be used for a rock containing any number of minerals and number of grain sizes of each mineral several grain shapes and any thermal history or domain size desired previous models describing stable isotope interdiffusion upon cooling have been based on dodson s equation or an equivalent numerical analogue the closure temperature of dodson is the average bulk temperature recorded between a mineral and an infinite reservoir by using dodson s equation these models have treated the closure temperature as an innate characteristic of a given mineral independent of the amounts and diffusion rates of other minerals such models do not accurately describe the mass balance of many stable isotope interdiffusion problems existing models for cation interdiffusion could be applied to stable isotopes with some modifications but only describe exchange between two minerals under specific conditions the results of fgb calculations differ considerably from the predictions of dodson s equation in many rock types of interest actual calculations using the fgb model indicate that closure temperature and diffusion profiles are as strongly functions of modal abundance and relative differences in diffusion coefficient as they are functions of grain size and cooling rate closure temperatures recorded between two minerals which exchanged stable isotopes by diffusion are a function of modal abundance and differences in diffusion coefficient and may differ from that predicted by dodson s equation by hundreds of degrees c either or both of two minerals may preserve detectable zonation which may in some instances be larger in the faster diffusing mineral rocks containing three or more minerals can record a large span of fractionations resulting from closed system processes alone the results of fgb diffusion modeling indicate that the effects of diffusive exchange must be evaluated before interpreting mineral fractionations concordant or discordant recorded within any rock in which diffusion could have acted over observable scales the predictions of this model are applicable to thermometry evaluation of open or closed system retrogression and determination of cooling rates or diffusion coefficients

ar diffusion and solubility measurements in plagioclases using the ultra violet laser depth profiling technique we describe the first direct measurements of ar diffusion and solubility in plagioclases using ultra violet uv laser ablation depth profiling and noble gas mass spectrometer analyses of experimentallytreated 599 1000 8c 50 200 mpaofar crystalfragmentsoflabradoriteandoli goclase labradorite 40 ar gain diffusion profiles were measured yielding an activation energy of 26 72 4 58 kcal mol 21 118 0 19 16 kj mol 21 and a frequency factor of 9 77 10 29 8 79 10 28 28 79 10 29 c m 2 s 21 95 confidence the ar solubility in labradorite was measured yielding a value of 0 2 ppb bar 21 which is similar to or lower than many rock forming minerals the labradorite diffusion parameters indicate ar closure temperatures of 211 8c for a spherical diffusion geometry and 243 8c for a planar diffusion geometry for 100 mm diameter grains with cooling rates of 10 8 cm a 21 the data indicate that labradorite is less ar retentive than k feldspar at low temperatures but more ar retentive than k feldspar at high temperatures corroborating previous work on plagioclase the relatively slow ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems supplementary material details of the uv laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at www geolsoc org uk sup18608

using argon as a temporal tracer of large scale geologic processes abstract argon ar has proven to be one of the most useful elements in understanding the timing and duration of geological processes because of its chemical and physical properties ar is a particularly sensitive indicator of temperature variations associated with large scale geological events ar mobility in minerals is commonly described using solid state diffusion theory volume diffusion which has been used extensively to model diffusion profiles and closure temperatures in minerals because 40 ar is the radiogenic daughter product of radioactive 40 k and therefore occurs in many common rock forming minerals ar has been used effectively as a temporal tracer to understand a wide variety of geological phenomena ar isotopes have been applied with great success in three broad fields of earth science the origin and evolution of the earth s atmosphere the tectonothermal history of geologic terranes and the timing and duration of large scale tectonic events from ar isotopic data the earth s atmosphere is now known to be mainly secondary in nature although evolutionary models have undergone a recent paradigm shift due to geophysical evidence moving away from the earlier notion of a chemically layered upper and lower mantle to a whole earth mantle structure that may be chemically inhomogeneous although recent models are growing in sophistication by integrating additional information and measurements reflecting the behaviour of other noble gases through time the temporal evolution of the earth s atmosphere still remains ambiguous because measured compositions of ancient atmosphere are sparse and remain difficult to obtain 40 ar 39 ar thermochronology is one of the most commonly used tools in tectonothermal investigations because of the large range of ar closure temperatures recorded by various common k bearing minerals although more recent thermochronological models such as multidiffusion domain mdd modelling remain controversial the combination of 40 ar 39 ar thermochronology with both higher and lower temperature thermochronological methods utilizing other isotopic systems makes this an extremely versatile method of understanding the temperature time evolution of geologic regions recent developments involving the numerical modelling of ar diffusion coupled with the geodynamic modelling of orogens have led to new ways of estimating the timing and duration of a complete tectonic cycle of subduction and exhumation by applying these new methods to rocks from the bergen arcs in norway studies have shown that this cycle can be so rapid that the subducted and exhumed crust can remain relatively cool never achieving thermal equilibrium with its ambient environment throughout the entire process in summary the versatility of the 40 ar 39 ar dating method combined with the effective application of various techniques e g laser spot dating and step heating and a detailed understanding of the diffusional behaviour of ar has proven to be a powerful research tool in elucidating the timing and duration of many large scale earth system processes

quartz cementation history of sandstones revealed by high resolution sims oxygen isotope analysis abstract evaluating the timing and origin of quartz cement is central to understanding how porosity is lost in sandstones during burial kinetic models of quartz cementation have been calibrated using large scale datasets but have never been tested at the microscopic level at which cement forms here we use high precision in situ oxygen isotope analyses on sandstone from the jurassic ness formation from the north sea to reveal the growth history of single quartz overgrowths to a resolution of 2 m measured 18o cement range from 28 to 20 v smow in early to late cement and are consistent with quartz cementation models that propose the bulk of quartz precipitates as a continuous process beginning at 60 70 c quantitative x ray diffraction analyses and clay mineralogical analysis of interbedded shales are inconsistent with a silica source from shale implying that silica for the cement is sourced internally to the sand these isotope data are broadly consistent with predictive conceptual quartz cementation models and provide a critical link from micron scale measurements to basin scale predictions and observations

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

evidence for excess argon during high pressure metamorphism in the dora maira massif western alps italy using an ultra violet laser ablation microprobe super 40 ar super 39 ar technique the age of ultra high pressure metamorphism and early cooling remains controversial in the alps in a new approach to the problem samples from the undeformed hercynian metagranite brossasco were studied using an ultra violet laser ablation microprobe technique for 40ar 39ar dating the results demonstrate the frequent occurence of excess argon with high 40ar 36ar ratios 1000 10000 and a strong relationship between apparent ages and metamorphic textures the highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases the best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event then mixed with an atmospheric component during rapid cooling and retrogression producing a wide range of argon concentrations and 40ar 36ar ratios in the present case some ages in the range 60 110 ma could be explained by the presence of excess argon incorporated around 40 50 ma ago similar results found in other high pressure terrains in the alps may reconcile the argon geochronometer with other systems such as rb sr u pb or sm nd this study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40ar 39ar ages in many high pressure terrains

early cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the franciscan subduction complex 1 during subduction at the franciscan trench beginning at 170 160 ma and continuing to the present marine sedimentary and lesser volcanic rocks have been underthrust accreted and metamorphosed to form the franciscan accretionary wedge the south fork mountain schist sfms forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the franciscan new u pb ages of detrital zircons and 40ar 39ar ages of metamorphic white micas indicate that the sfms accreted surprisingly late in the time span of franciscan subduction probably at ca 123 ma at least parts of the valentine spring formation and yolla bolly terrane accreted soon thereafter therefore accretion of the sfms apparently marks a fundamental transition from virtually nonaccretionary behavior to strongly accretionary behavior in the franciscan forearc about 75 of modern forearcs are nonaccretionary so recognition of an early nonaccretionary period is consistent with modern examples the franciscan mafic high grade exotic blocks and related very small slabs which yield ca 132 to 169 ma metamorphic ages are the only well documented rock record of the early 40 m y long nonaccretionary period and represent very sparse accretion during that interval the transition into an accretionary regime at ca 123 ma was essentially synchronous with the end of the late jurassic early cretaceous magmatic lull in the sierra nevada magmatic arc with the waning of early cretaceous faulting and warping within the great valley forearc basin and possibly with changes in motion vectors of the pacific and farallon plates

ion microprobe analysis of 18o 16o in authigenic and detrital quartz in the st peter sandstone michigan basin and wisconsin arch usa contrasting diagenetic histories abstract the oxygen isotopic compositions of authigenic quartz cements in sandstones provide a monitor of the temperatures compositions and origins of pore occluding fluids during diagenesis but quartz overgrowths are too fine grained to be amenable to conventional isotopic analysis we have used a cameca ims 4f ion microprobe to determine oxygen isotopic variations in authigenic and detrital quartz in four samples of the ordovician st peter sandstone from the michigan basin and wisconsin arch midwestern usa ion microprobe isotopic analyses have been successfully accomplished with an internal precision of 1 1 and a spatial resolution of 20 30 m at low mass resolution using a high voltage offset technique repeated analyses of the quartz standard demonstrate a reproducibility of close to 1 1 sd in good agreement with that expected from counting statistics the four ion probe samples were chosen as representative of thirteen samples that were mechanically and chemically disaggregated sieved and analysed as a function of sieve size by conventional bulk mineral techniques conventional and ion microprobe analyses are mutually consistent supporting the accuracy of the ion microprobe analyses within sample isotopic variations of up to 13 and micro scale isotopic variations of at least 4 over a distance of 100 m have been measured within quartz overgrowths in a sandstone from the wisconsin arch overgrowths are uniformly higher in 18o than detrital grains and gradients of up to 25 exist across a few microns 18o enriched quartz overgrowths in sandstones from the wisconsin arch show complex cl zonation and reflect one of two possible processes 1 low temperature quartz precipitation during mixing of meteoric waters with upwelling basinal fluids 2 higher temperature quartz precipitation during episodic gravity driven upwelling of warm basinal fluids of comparable isotopic composition to michigan basin fluids from the illinois basin related to evolution of mississippi valley type pb zn ore forming fluids quartz overgrowths in michigan basin sandstones which derived their silica locally by pressure solution are thought to have precipitated continuously over a range of temperatures from hot basinal fluids of restricted isotopic composition and circulation detrital quartz shows significant intea grain and intergrain isotopic variation 5 13 but no significant inter sample variation consistent with a uniform source of granitoid and metamorphic quartz in space and time

the noble gas isotope record of hydrocarbon field formation time scales abstract noble gases may be considered as the most prominent tracers of natural fluids including hydrocarbons the atmosphere is the only source of 20ne 36ar 84kr 130xe in subsurface environments and their concentrations in pore waters after recharge are known from the solubility data this allows modelling of noble gas partitioning between coexisting gas oil and water phases in the course of hydrocarbon formation migration and storage radiogenic isotopes 4he 21ne 40ar 136xe after being released from source rocks are mixed with air derived noble gases already present in the pore space concentrations of radiogenic species in the pore space of typical hydrocarbon fields are generally so high that they can hardly be accumulated in situ and thus indicate noble gas transfer from ground waters the time bearing ratios 4he 20ne 21ne 20ne 4he 40arair 40ar 40arair in hydrocarbon fields are thus proportional to the time interval between the ground water recharge and noble gases partitioning into the hydrocarbon phase s the recharge partition interval the largest available data set allows the recharge partition intervals to be constrained for a large number of hydrocarbon fields situated in different tectonic settings ancient plates young plates mobile belts these intervals increase systematically with the ages of hydrocarbon source and trap lithologies and are comparable with these ages this important feature valid in general for different hydrocarbon fields implies i local sources of radiogenic noble gas isotopes in ground waters ii relatively recent formation of hydrocarbon fields and iii their short formation time scales in some cases the duration of formation of a hydrocarbon field can be constrained for example nearly constant 21ne 20ne 40ar 40arair ratios measured in samples from the magnus oil field north sea give an accumulation time scale 10 ma it should be emphasized that the above noble gas isotope ratios give the time estimates which are independent of geological reconstructions sometimes the noble gas inventory in a hydrocarbon field and ground waters allows characterization of the source rock volume involved in formation of the field generally this volume exceeds that of the hydrocarbon field rocks by orders of magnitude

imported quartz cement in aeolian sandstone grew from water of uniform composition but has complex zonation the origins and volumes of waters which mass transport silica in sedimentary basins remain obscure many previous analytical studies have illustrated cases where quartz cements in sandstones originate from complex and variable fluids by contrast we show by using a combination of separation and analytical techniques that in lower permian sandstones of the southern north sea the cementing fluid was isotopically uniform during growth of quartz cement with a 1b o of 19 6 1 0 v smow in this relatively uniform fluid quartz cements grew and developed complex cathodoluminescence cl zonations petrographic data show that 8 10 quartz cement locally 30 was imported into this 380 km 2 180 m thick aeolian sandstone and cement distribution controlled by depositional permeabilities we infer a large scale high volume flux of evolved meteoric fluid during 2 km deep burial and show that complex cl zonation may arise from relatively subtle changes in water composition

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