apatite feldspar u pb thermochronometer a reliable mid range 450 c diffusion controlled system the u pb systematics of apatite are commonly dominated by simple volume diffusion without secondary growth and recrystallization that can affect u pb systematics of sphene monazite and zircon although apatite is a relatively low u mineral both u and pb are lattice bound and thus the u pb systematics are not controlled by micro inclusions as appears to be the case for other low u minerals such as garnet and staurolite closure temperature estimates from both experimental and empirical studies range from 425 c to as high as 500 c for typical diffusion radii 25 m and cooling rates 2 100 c ma respectively this range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks in addition the closure temperature for pb diffusion in apatite fills a gap between that for pb diffusion in sphene 650 c and diffusion of ar in hornblende 500 c muscovite 350 c and biotite 300 325 c apatite is a common accessory phase in many rock types and u pb apatite dating can be a reliable and powerful thermochronometer as it combines mid range cooling ages with the strength of the u pb system in testing for isotopic closure precisions of u pb apatite dates can range from 1 2 to as low as 0 3 for proterozoic or archean dates the main limitation to accuracy of u pb apatite dates is a moderate to low ratio of u to pb and corresponding dependence of concordia coordinates on the choice of initial pb isotopic composition for proterozoic aged samples with measured 206pb 204pb of 100 the range of geologically reasonable initial pb isotopic compositions can shift the calculated u pb age by 60 ma fortunately even though the diffusion rate of pb in alkali feldspar is less than in apatite it appears that the isotopic composition of pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings examples are given from an archean terrane in se wyoming that was unroofed and cooled during proterozoic uplift and from proterozoic terranes in sw us that have contrasting thermal histories

u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

thermo rheological shear heating model for leucogranite generation metamorphism and deformation during the proterozoic trans hudson orogeny black hills south dakota abstract this paper evaluates thermotectonic models for metamorphism and leucogranite generation during the proterozoic trans hudson orogeny as recorded in rocks exposed in the black hills sd intrusion of the harney peak granite and associated pegmatites at 1715 ma occurred at the waning stages of regional deformation and staurolite grade regional metamorphism published consortium for continental reflection profiling cocorp results indicate that proterozoic sedimentary rocks were thrust over the archean wyoming province during the trans hudson collision isotopic compositions of the harney peak granite suggest that the exposed proterozoic and archean metasedimentary rocks in the black hills represent source rocks of the granites numerical simulations of the regional metamorphism and harney peak granite generation assuming crustal thickening by thrusting coupled with erosion show the following 1 doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust 2 a 35 km drop off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level it is however insufficient to produce staurolite and melting can occur only in the deepest parts of the crust 3 temperatures in crust with stable 70 km thickness for 40 ma and 35 km drop off length for heat production could become sufficient to produce staurolite at the current erosion level and subsequent rapid denudation of the crust could potentially trigger decompression melting of lower crustal rocks although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation it is inconsistent with melting of upper crustal proterozoic source rocks that is indicated by isotopic compositions of the granites with lack of evidence for rapid denudation of the trans hudson orogen and with confinement of the leucogranites to the deformed proterozoic metapelitic rocks production of the harney peak granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the wyoming province and overthrusted sedimentary rocks we suggest that with reasonable rheologic properties of metapelites and rates of plate convergence shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust modeling rheology of the lithologically stratified thickened crust with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny which explains regional folding of the schists and predicts ductile shear zones in the granite source region because of the lithologic stratification the granitic basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists a weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens

state of the art of modelling two phase flow in fractured rock two phase flow in fractured rock is an important phenomenon for a range of practical problems not the least of which is non aqueous phase liquid napl contamination of groundwater although multiphase systems have long been studied in the petroleum field in the hydrogeological field progress has only just reaches the point where models are being developed scale effect is one of the main issues of concern although models presented in this paper have the potential to provide useful predictions they can only be used to investigate a variety of possible scenarios with parameters being specified in the form of distribution of values the calibration and validation of all but the simplest of these models poses a formidable task with great demands on hydrogeologists and geophysicists to provide adequate data

field determination of the three dimensional hydraulic conductivity tensor of anisotropic media 2 methodology and application to fractured rocks the analytical solutions developed in the first paper can be used to interpret the results of cross hole tests conducted in anisotropic porous or fractured media in the particular case where the injection and monitoring intervals are short relative to the distance between them the test results can be analyzed graphically from the transient variation of hydraulic head in a given monitoring interval one can determine the directional hydraulic diffusivity kd e ss and the quantity d ss by curve matching here kd e is directional hydraulic conductivity parallel to the unit vector e pointing from the injection to the monitoring interval ss is specific storage and d is the determinant of the hydraulic conductivity tensor k the principal values and directions of k together with ss can then be evaluated by fitting an ellipsoid to the square roots of the directional diffusivities ideally six directional measurements are required in practice a larger number of measurements is often necessary to enable fitting an ellipsoid to the data by least squares if the computed kd e ss values fluctuate so severely that a meaningful least squares fit is not possible one has a direct indication that the subsurface does not behave as a uniform anisotropic medium on the scale of the test test results from a granitic rock near oracle in southern arizona are presented to illustrate how the method works for fractured rocks at the site the oracle granite is shown to respond as a near uniform anisotropic medium the hydraulic conductivity of which is strongly controlled by the orientations of major fracture sets the cross hole test results are shown to be consistent with the results of more than 100 single hole packer tests conducted at the site

interpretation of field tracer tests of a single fracture using a transient solute storage model the results and interpretation of five induced gradient tracer tests performed at five different average interborehole fluid velocities in a single fracture in monzonitic gneiss are described the experiments were conducted using radioactive 82br and a fluorescent dye as conservative tracers where the tracers were pulse injected into radial convergent and injection withdrawal flow fields the flow fields were established between straddle packers isolating the fracture in three boreholes over distances of 12 7 29 8 m the tracer breakthrough curves were determined from samples of the withdrawn groundwater and were interpreted using residence time distribution rtd theory and two deterministic simulation models the rtd curves of the tracer experiments were interpreted by fitting to the field data a simple advection dispersion model and an advection dispersion model with transient solute storage in immobile fluid zones both models consider the different flow field geometries associated with injection withdrawal and radial convergent tests comparison of the fits obtained by the simulation models suggest that the initial period of solute transport in single fractures is advection dominated and with increasing tracer residence time or decreasing fluid velocity transport progresses toward more fickian like behavior during the advective dominated period the transient solute storage model is shown to adequately describe the asymmetries and long tails characteristic of the fracture rtds interpretation of the tracer experiments using both simulation models further suggests that induced gradient tracer experiments are likely to underestimate the dispersive characteristics of single fractures under natural flow conditions

paleo and neoproterozoic magmatic and tectonometamorphic evolution of the isla cristalina de rivera nico perez terrane uruguay the isla cristalina de rivera crystalline complex in northeastern uruguay underwent a multistage magmatic and metamorphic evolution based on shrimp u pb zircon th u pb monazite chime epma method and k ar age data from key units several events can be recognized 1 multistage magmatism at 2 171 2 114 ma recorded on zircon of the granulitic orthogneisses and their 2 093 2 077 ma overgrowths 2 a distinct amphibolite facies metamorphism at 1 980 ma recorded by monazite 3 greenschist facies reworking and shearing at ca 606 ma monazite and k ar on muscovite along the rivera shear zone and finally 4 intrusion of the post tectonic sobresaliente and las flores granites at around 585 ma lithological similarities geographic proximity and coeval magmatic and metamorphic events indicate a similar tectonometamorphic evolution for the isla cristalina de rivera the valentines block in uruguay and the santa maria chico granulitic complex in southern brazil since at least 2 1 ga

lead isotope and trace element geochemistry of paleoproterozoic metasedimentary rocks in the lead and rochford basins black hills south dakota usa implications for genetic models mineralization ages and sources of leads in the homestake gold deposit abstract mixed silicate carbonate sulfide facies banded iron formations with elevated detrital components were deposited in the northern black hills during middle paleoproterozoic time between 2 012 and 1 974 ga homestake iron formation and equivalents and at 1 887 ga rochford iron formation these iron formations deposited atop shales and basalts in intracontinental rifted basins separated in time by 80 130 myr are the hosts of extensive late paleoproterozoic gold mineralization gold sulfide ore bodies occur within dilated segments of late stage ductile brittle shears that formed after isoclinal folding related to wyoming superior continental collision lead stepwise leaching pbsl data for monazite bearing garnet separated from a sample of homestake iron formation has yielded an isochron age of 1746 10 ma 2 mswd 0 42 which represents a maximum age for both the isoclinal folding and subsequent gold mineralization monazite bearing garnet from a nearby harney peak type granitic pegmatite believed to be coeval with latest stage semibrittle shears and quartz veins that crosscut homestake ore bodies has yielded a pbsl isochron age of 1713 10 ma 2 mswd 4 1 which probably represents a minimum age for the gold event pb isotopes of sulfides from replacement ore and shear ore within the homestake iron formation cannot distinguish between these distinct ore types however both pb isotopes and rey rare earth and yttrium abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation thus revealing an epigenetic sensu stricto component to the gold mineralization the pb isotope characteristics of the ore further reflect an admixed component with low 232th 204pb and variable th u ratios in addition to compositions directly derived from bif typical leads this added component corresponds to pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite the apparent pb isotopic match between ore and granite is further consistent with the pbsl data for shear ore type arsenopyrite which define an isochron age of 1719 45 38 ma 2 mswd 0 2 and by a striking overlap of the pb isotopic data fields defined by the homestake galenas and pegmatitic feldspars the precambrian ore leads are therefore best explained as mixtures of magmatic hydrothermal and host rock mobilized leads the lead area also contains significant gold bearing deposits of tertiary age which are hosted in both paleoproterozoic and phanerozoic rocks tertiary mineralization was temporally and spatially related to coeval plutons emplaced during laramide uplift of the black hills and represents the final mineralization event recognized in the homestake mine area like the precambrian galena leads the leads of tertiary galena hosted by middle paleoproterozoic rocks including the homestake iron formation define a trend best explained by mixtures of pb derived from magmatic fluids exsolved from tertiary intrusives and pb leached from the same country rocks that contaminated the granite derived hydrothermal fluids ascending through them during the much earlier shearing and mineralization event inferred at 1730 1715 ma

concentric slow cooling of a low p high t terrane evidence from 1600 1300 ma mica dates in the 1780 1700 ma black hills orogen south dakota u s a the crystalline core of the southern black hills south dakota exposes an extensive low p high t aureole of garnet to second sillimanite zone schists centered on the plutonic core of the 1715 ma harney peak granite hpg this paper demonstrates regional patterns of apparent ages observed for 52 40 ar 39 ar dates of muscovite and biotite in diverse rocks from across the 1000 km 2 metamorphic aureole and its plutonic center about 20 biotite dates sampled mostly near faults are influenced by excess 40 ar and obscure the regional trends the remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite with previously unrecognized elliptical age zones centered on the main hpg pluton and its outliers the regional pattern of 40 ar 39 ar cooling ages indicates non uniform slow cooling of the mid crust between 1600 1250 ma this scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite rb sr and apatite u pb which range from 1690 to 1550 ma and from 1700 to 1500 ma respectively to explain these results it is likely that ambient pre granite temperatures of the country rocks were 350 c at the 10 14 km depth of granite emplacement as previously proposed and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years alternatively or in addition the hpg and inner aureole were not uplifted until 1480 1330 ma whereupon they finally cooled through 300 350 c

geometric and seismic interpretation of the perdido fold belt northwestern deep water gulf of mexico we interpret the perdido fold belt to be composed of detachment folds flanked by megascopic scale kink bands the two previous principal fold models for the perdido fold belt incorporated components of large scale faulting an imbricate fault bend fold model and a high angle reverse faulted detachment fold model we interpret the perdido fold belt to be fold dominated the belt consists of detachment folds overlying a ductile decollement layer the detachment fold crests are bounded by kink bands which are narrow zones of angularly folded strata the low reflectivity low signal to noise zones seen in seismic profiles across the perdido fold belt are poorly imaged sections of the folds that result from steeply dipping bedding in the kink bands the substantial width of these low reflectivity zones their subparallel edges in cross section and their orientation at a high angle to layering as well as their organized spacing and conjugate geometry support a kink band interpretation propagating reverse faults are unlikely in the folds because of the absence of backlimbs overlying the locations of hypothetical faults improved seismic imaging efforts have illuminated steeply dipping bedding that is not cut by faulting in many of the low reflectivity zones in the perdido fold belt

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

analysis of hydrodynamic dispersion in discrete fracture networks using the method of moments recent field investigations in crystalline rock give strong indications that water flows in largely isolated channels in fissured rock the channels are located in fracture planes and may or may not connect to other channels in the same plane or at fracture intersections in the present paper hydrodynamic dispersion in such fracture networks is investigated using the concepts of residence time distribution theory it is shown that from a set of simple relations at least the first four statistical moments for the system response are easily obtained even for complicated networks in particular an equivalent dispersion coefficient may be calculated from the second central moment variance the effect of channeling generally decreases in systems with many mixing steps a criterion based on the coefficients of skewness and kurtosis is derived for when the response of the system is in accordance with the diffusion dispersion model examples show that this limit may not be obtained under realistic repository conditions in fissured rock

step leach pb pb dating of inclusion bearing garnet and staurolite with implications for early proterozoic tectonism in the black hills collisional orogen south dakota united states we report 207 pb 206 pb stepwise leaching pbsl ages of 1762 15 ma 1759 8 ma and 1760 7 ma for almandine garnet staurolite and garnet staurolite combined in an early proterozoic metapelite from the black hills collisional orogen south dakota pbsl derived 208 pb 206 pb th u trends reveal that staurolite contains inclusions of both cogenetic monazite ca 1760 7 ma and older detrital zircon 2040 ma whereas coexisting garnet contains only the zircon these results and petrologic data indicate prograde sequential growth of garnet monazite and staurolite during tectonic burial at temperatures between 400 c and 550 c i e well below pb closure temperatures we interpret the 1760 7 ma date as a maximum age for wyoming superior continental collision in the black hills which apparently postdated the 1800 1900 ma trans hudson orogeny in canada but was nearly synchronous with north directed accretion of the central plains orogen in southeastern wyoming

an overview of the petroleum systems of the northern deep water gulf of mexico the northern deep watergulf ofmexico is one of themost active deep water petroleum provinces in the world this paper introduces the regional geologic setting for the northern deep water gulf of mexico and briefly discusses the importance of technology in developing the area s resources exploration has focused on four major geologic provinces basins subsalt fold belt and abyssal plain these provinces formed from the complex interactions between mesozoic cenozoic sedimentation and tectonics improved understanding of the geology of these provinces has largely been accomplished by improvements in seismic acquisition and processing in addition advances in drilling technology have permitted drilling and field development in increasingly greater water depths the 226 oil and gas fields and discoveries in the northern deep water gulf of mexico are summarized in terms of their exploration and development history producing facility ages of reservoirs upper jurassic upper paleocene lower eocene oligocene lower miocene upper pleistocene and trap type structural combined structural stratigraphic and stratigraphic in addition the interpreted regional distribution of upper jurassic and possible lower cretaceous source source rocks is shown in part based on the 26wells that have penetrated these source rocks the eight papers in this special issue review the geology of the mississippi canyon and northern atwater valley protraction areas the first five papers review the subregional structural setting and the evolution of its tectonics and petroleumsystems the final three papers summarize the geologic evolution of two economically important intraslope basins thunderhorse andmensa in terms of their stratigraphy structural evolution and petroleumsystems these two basins contain two of the larger oil and gas fields respectively in the northern deep water gulf of mexico copyright 2017 the american association of petroleum geologists all rights reserved

the relationship of the degree of interconnection to permeability in fracture networks the problem of determining the permeability of a rock mass containing a system of finite fractures is highly dependent on both the degree of interconnection between fractures and the heterogeneity of individual fracture characteristics this paper examines how degree of interconnection affects both magnitude and nature of the fracture permeability the interconnection between given fracture sets is a complex function of 1 fracture density i e the number of fractures per unit volume and 2 the fracture extent or size unfortunately neither the density nor the extent of fractures is easily determined from borehole data however fracture frequency can be directly measured in a borehole because it is simply the number of fractures intersected per unit length of the borehole the frequency is a measure of the product of fracture density and size because the probability of a fracture intersecting a borehole is proportional to this product the effect of the degree of interconnection was investigated by numerically simulating flow in fracture networks where fracture size and density varied inversely while the product of these two parameters was held fixed directional permeabilities of a number of such networks were determined and the hydraulic behavior of each fracture system was compared to that of an ideal porous medium the permeability of the rock matrix between the fractures was assumed to be low enough to be negligible the results show that as fracture length increases the degree of interconnection increases thus for a given fracture frequency as measured in a borehole the permeability of the system increases as fracture length is increased and density is proportionally decreased also fracture systems with shorter but more dense fractures behave less like porous media than do systems with longer but less dense fractures knowledge of fracture frequency and orientation alone is inadequate when determining permeability or deciding the important question of whether a given system behaves like a porous medium for many cases where these parameters can be measured in the borehole it is critical to know fracture length in order to predict the degree of fracture network interconnection however for certain cases where fractures are larger than some critical size it may not be necessary to know fracture lengths exactly

neoproterozoic to early palaeozoic events in the sierra de san luis implications for the famatinian geodynamics in the eastern sierras pampeanas argentina the application of the sensitive high resolution ion microprobe shrimp u pb dating technique to zircon and monazites of different rock types of the sierra de san luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains additional constraints on the famatinian metamorphic episode are provided by pb pb stepwise leaching experiments on one staurolite and two garnet separates the results indicate that the sedimentary precursors of the conlara metamorphic complex have a maximum age of c 590 ma whereas the pringles metamorphic complex metasediments appear to be sourced from the pampean orogen in the early cambrian folded xenoliths within the c 496 ma el penon pluton suggest that the host conlara metamorphic complex underwent a pampean compression from a 208 pb 232 th monazite age of 478 ma for a migmatite from the nogoli metamorphic complex the structural evolution of this basement complex appears to be entirely post pampean onset of the famatinian high grade metamorphism between c 500 ma and c 450 ma follows a period of crustal extension on the western outboard of gondwana and might not be related directly to a mid ordovician accretion of the cuyania terrane

alpha recoil in u pb geochronology effective sample size matters displacement of the daughter isotope by recoil results in an open system on the nanoscale for a heterogeneous distribution of u and th this redistribution of intermediate and stable daughter isotopes results in subvolumes with a deficit of pb and others with an excess of pb whether such heterogeneities affect the analyzed u pb system depends on 1 the volume of the analyzed sample 2 the degree and scale of heterogeneity in the u and th distribution and 3 the analytical procedure spatial separation of parent and daughter through recoil affects the u pb systematics of leached samples where leaching gives access to domains less than 1 m wide anomalous data patterns originating from recoil induced parent to daughter fractionation are more important if there are strong heterogeneities in the u and th distribution whereby pb excess appears more pronounced than pb deficit fractionation of parent and daughter elements through selective dissolution of u ree rich growth zones in zircon and u inclusions in columbite as well as the presence of u th rich micro inclusions in silicates dated using a step leaching scheme may result in anomalous 207pbrad 206pbrad scattered 206pbrad 238u and 207pbrad 235u and reverse discordance the accumulated structural damage controls the leaching and dissolution behavior but may also influence the non stoichiometric element mobilization during sputtering or ablation in the analysis of u rich samples by shrimp and la mc icp ms

pbsl dating of garnet and staurolite constraints on the paleoproterozoic crustal evolution of the eastern block north china craton abstract it is well accepted that the north china craton formed by assembly of the eastern and western blocks along the trans north china orogen with cratonization in the paleoproterozoic 1 85 ma however there is controversy about the formation of the eastern and western blocks themselves especially the timing of cratonization of the eastern block the jiao liao ji orogenic belt is the most important paleoproterozoic orogenic belt in the eastern block and consists mainly of the north and south liaohe groups and the liaoji granitoids the timing of metamorphism of the liaohe group is the key to understanding the evolution of the eastern block of the north china craton because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts dating of minerals such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure temperature deformation history recorded by the same mineral in this paper we present results of precise 207pb 206pb stepwise leaching pbsl dating of garnet and staurolite from the metasedimentary rocks of the south liaohe group the primitive pb and radiogenic pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector mc icp ms the garnet and staurolite record two groups of 207pb 206pb ages at 1 93 1 91 ga and 1 86 ga respectively these constrain the time of peak metamorphism of the liaohe group during collision between the longgang block in the north and the liaonan nangrim block in the south and a subsequent post collisional event respectively our data for the south liaohe group and previously published data for the liaoji granite combined with key lithological structural and metamorphic information indicate that the cratonization of the eastern block of the north china craton occurred at 1 93 1 91 ga

geochronology of the hout river shear zone and the metamorphism in the southern marginal zone of the limpopo belt southern africa abstract in this paper monazite u pb and zircon evaporation dates stepleaching pb pb results on garnet staurolite and kyanite and hornblende ar ar data are presented which constrain the timing of granulite facies metamorphism in the southern marginal zone of the limpopo belt and its thrusting onto the kaapvaal craton the southern marginal zone of the limpopo belt is considered to be a lower crustal equivalent of the northern kaapvaal craton granulite exhumation is associated with southward thrusting along the hout river shear zone which is a set of thrust and strike slip shear zones zircon ages for the matok intrusive complex which was emplaced within the zone during this thrusting charno enderbites 2671 4 ma granodiorites between 2667 and 2664 ma have previously been interpreted as evidence for rapid exhumation of the southern marginal zone within only 7 ma we have obtained a u pb date of 2691 7 ma for monazite from the bandelierkop quarry in the southern marginal zone interpreted as the age of high grade metamorphism a single zircon evaporation pb pb date of 2643 1 ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation anatexis of metapelitic xenoliths within the matok intrusive complex was dated at 2663 4 ma by u pb on monazite indistinguishable from existing zircon ages for this complex pb pb step leaching dates obtained on synkinematically grown garnet 2691 20 ma staurolite 2712 37 ma and kyanite 2672 51 ma from the khavagari hills in the giyani greenstone belt in the immediate footwall of the hout river shear zone indicate that early thrusting was contemporaneous with peak metamorphism in the southern marginal zone ar ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations probably reflecting repeated or continuous tectonic activity of the hout river shear zone up to about 2600 ma

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

the effect of allochthonous salt on the petroleum systems of northern green canyon and ewing bank offshore louisiana northern gulf of mexico the northern green canyon ewing bank region northern gulf of mexico basin contains the oxfordian neogene tithonian neogene albian neogene turonian neogene and eocene neogene petroleum systems the systems encompass 42 fields or discoveries in the study area and include four subsalt discoveries essential elements of the systems include source shales of oxfordian tithonian albian turonian and eocene age neogene siliciclastic turbidite reservoirs allochthonous salt and overburden strata ranging in age from jurassic to quaternary the petroleum systems of the area are significantly affected by the evolution of allochthonous salt the high thermal conductivity of salt retards the thermal maturation of subsalt petroleum source rocks and causes late generation and migration from them most traps were formed during the pliocene pleistocene and the generation migration accumulation of petroleum ranges from early miocene to the holocene the critical moment of peak oil generation for each source varies spatially and temporally as a function of the overlying sediments and allochthonous salt evolution the impermeability of salt prevents vertical petroleum migration and causes migration pathways to be deflected laterally up the dip of base salt where salt welds form petroleum migration is unimpeded and continues vertically by integrating predictions of potential source rocks structural restorations thermal maturation modeling regional salt maps and petroleum systems logic we can determine petroleum migration pathways and zones of concentration all 42 fields or discoveries within the study area are associated with predicted zones of paleosubsalt petroleum concentration present day salt geometries do not delineate many of these zones because of salt weld formation during the pleistocene this generation migration and accumulation technique enables geoscientists to focus their exploration efforts toward areas with a greater probability of success

tracer movement in a single fissure in granitic rock some experimental results and their interpretation radionuclide migration was studied in a natural fissure in a granite core the fissure was oriented parallel to the axis in a cylindrical core 30 cm long and 20 cm in diameter the traced solution was injected at one end of the core and collected at the other breakthrough curves were obtained for the nonsorbing tracers tritiated water and a large molecular weight lignosulphonate molecule and for the sorbing tracers cesium and strontium from the breakthrough curves for the nonsorbing tracers it could be concluded that channeling occurs in the single fissure a dispersion model based on channeling is presented the results from the sorbing tracers indicate that there is substantial diffusion into and sorption in the rock matrix sorption on the surface of the fissure also accounts for a part of the retardation effect of the sorbing species a model which includes the mechanisms of channeling surface sorption matrix diffusion and matrix sorption is presented the experimental breakthrough curves can be fitted fairly well by this model by use of independently obtained data on diffusivities and matrix sorption

the effect of allanite inclusions on u pb step leaching ages and sm nd isotope systematics of garnet from the ogcheon metamorphic belt south korea abstract the ogcheon metamorphic belt consists primarily of metasedimentary and metavolcanic rocks that have experienced polyphase tectonometamorphism since the neoproterozoic peak metamorphism reaching up to lower amphibolite facies produced ubiquitous garnet porphyroblasts in pelitic and mafic schists to determine the timing of their formation step leaching experiments were undertaken for five garnet fractions separated from pelitic and quartz hornblende garnet schists the u pb ages from three samples are identical within 2 errors ranging from 291 41 ma to 276 29 ma the quasi linearity of leachates in 238u 206pb and 208pb 206pb diagrams suggests that u and pb are released from a single mineral phase and that minor chemical fractionation between u and pb may have occurred during the leaching experiment deviations of residues and bulk garnet fractions from the linear trend are attributed to partial dissolution of refractory inclusions of detrital zircon th u ratios of leachates are in the range of 3 4 12 much higher than those of pure garnet and suggest the contribution of allanite negative relationships in the sm nd isochron diagram and similar 147sm 144nd ratios between whole rock and garnet corroborate the influence of light rare earth element lree rich allanite on the sm nd isotopic system simple mass balance calculations indicate that only a trace amount 0 35 modal of allanite inclusions should govern the u th pb systematics of garnet petrographic evidence together with the consistency in u pb ages suggests that allanite is a product of prograde metamorphism thus peak metamorphism responsible for the growth of allanite bearing garnet porphyroblasts in the ogcheon metamorphic belt is best estimated to be early permian

geochronological constraints on tourmaline formation in the western fold belt of the mount isa inlier australia evidence for large scale metamorphism at 1 57 ga boron metasomatism during two deformational events resulted in tourmaline formation in parts of late palaeoproterozoic eastern creek volcanics of the western fold belt of the mount isa inlier despite low u pb ratios 1 as measured by laser ablation icp ms when the step leaching process was applied to the tourmaline it process extracted pb fractions with 206 pb 204 pb up to 76 illustrating the method s ability to efficiently separate radiogenic and common pb that are hosted in different crystal lattice sites tourmaline from two syn d2 metamorphic hydrothermal veins at anderson s lode and eldorado yield similar pb step leaching ages of 1573 12 and 1577 48 ma respectively tourmaline from a post d2 but pre d3a vein at king was dated at 1528 51 ma although there is evidence to suggest some contribution from u rich inclusions i e zircon and monazite to some leach steps the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events this is supported by estimates of pb closure temperatures in tourmaline derived from ionic porosity models the 1575 ma ages for d2 also reflect the main metamorphic event in the western fold belt but are 40 myr older than accepted age estimates for this episode the revised timing for peak metamorphism tectonism in the western fold belt corresponds with the age of metamorphism in the eastern fold belt suggesting broadly coeval tectonic histories across the entire inlier from at least 1600 ma

a field example of measuring hydrodynamic dispersion in a single fracture a field example of measuring the dispersive properties of a single fracture in fractured plutonic rock is presented the experimental technique involves injecting a slug of conservative tracer into a steady groundwater flow field established between a pumping and recharging borehole and monitoring the tracer breakthrough by sampling the withdrawal water directly the breakthrough curves from two experiments were analyzed with a model which describes the flow field geometry either analytically or numerically and solves for hydrodynamic dispersion analytically a longitudinal dispersivity of 1 40 m was estimated by fitting the model to each set of field data the magnitude of the dispersion was determined to be independent of dispersive effects created by flow through the borehole instrumentation and thought to be purely hydrodynamic in nature

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

on two phase relative permeability and capillary pressure of rough walled rock fractures this paper presents a conceptual and numerical model of multiphase flow in fractures the void space of real rough walled rock fractures is conceptualized as a two dimensional heterogeneous porous medium characterized by aperture as a function of position in the fracture plane portions of a fracture are occupied by wetting and nonwetting phase respectively according to local capillary pressure and accessibility criteria phase occupancy and permeability are derived by assuming a parallel plate approximation for suitably small subregions in the fracture plane for log normal aperture distributions a simple approximation to fracture capillary pressure is obtained in closed form it is found to resemble the typical shape of leverett s j function wetting and non wetting phase relative permeabilities are calculated by numerically simulating single phase flows separately in the wetted and non wetted pore spaces illustrative examples indicate that relative permeabilities depend sensitively on the nature and range of spatial correlation between apertures it is also observed that interference between fluid phases flowing in a fracture tends to be strong with the sum of wetting and nonwetting phase relative permeabilities being considerably less than 1 at intermediate saturations

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

structural geometry and evolution of the ku zaap and maloob structures campeche bay mexico the ku zaap and maloob structures consist of an arcuate series of fold thrust structures which change orientation from approximately east west to north northwest south southeast along a structural salient in the offshore campeche bay mexico the structures are related to a series of thrust faults which merge into the maloob thrust the ku zaap and maloob fields located on these structures together constitute an important complex of fields with proven remaining reserves of approximately 1850 mmboe the structures formed during three main periods of deformation 1 a jurassic cretaceous extensional episode 2 a miocene compressive phase during which the main trap forming structures were formed and 3 a pliocene to holocene extensional episode resulting in several listric growth faults three dimensional 3 d models of the main reservoir unit upper cretaceous breccia and other prospective reservoir units eocene and kimmeridgian constructed by integrating 3 d seismic interpretation well data and a series of balanced structural cross sections provide an understanding of the geometry and evolution of the structures the zaap maloob and sub maloob structures are interpreted as a series of faulted detachment folds detaching within salt and other jurassic ductile units the front limbs of these structures are typically cut by thrust faults with small displacements terminating in the lower tertiary section the ku and pakal structures also originated as a detachment fold but was faulted relatively early in its deformational history the frontal zone of this structure is marked by a closely spaced thrust system consisting of the ku pakal 1 and maloob thrusts the 3 d models provide an accurate picture of the structural geometry of the principal reservoir units which will be useful for future production from the fields and for exploration in adjacent areas in the basin

petrologic and geochemical links between the post collisional proterozoic harney peak leucogranite south dakota usa and its source rocks abstract the proterozoic terrane of the black hills south dakota includes the composite harney peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes available dates indicate that granite generation post dated regional metamorphism and deformation that have been attributed to collision of the wyoming and superior cratons at 1760 ma previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites in this study whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks these were then used to model observed trace element compositions of the granites using published mineral melt distribution coefficients model trace element melt compositions using pelitic and graywacke protoliths yield similar results the models reproduce well the observed depletion of transition metals and ba in the granites relative to metasedimentary protoliths the depletion is due mainly to high proportion of biotite with variable amounts of k feldspar in the model residue sr is also depleted in the granites compared to source rocks but to a lesser relative extent than ba this is because of the low biotite melt distribution coefficient for sr and because high proportion of plagioclase in the residue is compensated by high sr concentrations in protoliths rubidium cs and ta behaved as slightly compatible to incompatible elements and therefore were not strongly fractionated during melting of the considered elements only b appears to have been highly incompatible relative to residue during melting the protoliths had sufficient b to allow tourmaline crystallization in those parts of the harney peak granite in which ti concentration was sufficiently low not to enhance crystallization of biotite the reproducibility of observed trace element concentrations in the harney peak granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites this argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post collisional harney peak and other similar peraluminous granite suites

a large scale flow and tracer experiment in granite 2 results and interpretation water and tracer flow has been monitored in a specially excavated drift in the stripa mine several new experimental techniques and equipment were developed and used the whole ceiling and the upper part of the walls were covered with 375 individual plastic sheets where the water flow into the drift could be collected eleven different tracers were injected at distances between 11 and 50 m from the ceiling of the drift the flow rate and tracer monitoring was kept up for more than 2 years the tracer breakthrough curves and flow rate distributions were used to study the flow paths velocities hydraulic conductivities dispersivities and channeling effects in the rock in a companion paper the experimental design and performed experiments are described the present paper describes the interpretation of flow and tracer movement in the rock outside the drift the tracer movement was measured by the more than 160 individual tracer curves the tracer experiments have permitted the flow porosity and dispersion to be studied the possible effects of channeling and the diffusion of tracers into stagnant waters in the rock matrix and in stagnant waters in the fractures have also been addressed

b and li in proterozoic metapelites from the black hills u s a implications for the origin of leucogranitic magmas the abundance and distribution of b and li in metasedimentary rocks and in the harney peak granite hpg in the black hills south dakota u s a elucidate the behavior of these two elements during 1 regional metamorphism that began at 1760 ma 2 subsequent contact metamorphism associated with emplacement of the hpg and associated pegmatites at 1715 ma and 3 anatexis leading to production of the granite there is no clear evidence for changes in b and li concentrations with progressive regional metamorphism of the metapelites from chlorite biotite grade up to staurolite grade rocks outside the pegmatite aureole that surround the hpg have average b contents of 80 ppm there is no correlation of b with other elements in the pelitic schists which indicates that none of the major phases is the dominant host for b boron content is mainly controlled by small amounts of randomly distributed tourmaline that was identified in alpha track maps of thin sections in the aureole of the granite b concentrations are depleted in many samples the depletion is attributed to consumption of tourmaline during interaction of the rocks with alkalic fluids the average li contents in metapelites far away from the hpg are also 80 ppm near the hpg li concentrations reach roughly 190 ppm indicating significant metasomatism by fluids that emanated from the granite and pegmatites the concentrations of b and li in the low grade metapelites are sufficient to explain their abundances in the hpg if the granite formed by partial melting of the metapelites and if there was total breakdown of tourmaline stabilization of tourmaline in leucogranites is not necessarily related to sources enriched in b but may be related to its competition for fe and mg with biotite

leucogranites in the black hills of south dakota the consequence of shear heating during continental collision leucogranites are common features of convergent orogens however the heat necessary for producing leucogranite magma from pelitic sources in convergent orogens is a major unresolved issue we suggest that the generation of the harney peak granite black hills south dakota during the proterozoic trans hudson orogeny is best explained by shear heating during thrusting of a proterozoic sedimentary sequence over the archean wyoming basement our thermotectonic model successfully reproduces pressure temperature time constraints on early regional metamorphism of schists and subsequent generation of the late orogenic harney peak granite the model is consistent with isotopic data for the granite which require both proterozoic and archean source rocks and its emplacement as small batches of melt the similarity of geologic relationships in the black hills to those of other collisional orogens suggests that shear heating may be a viable mechanism for leucogranite generation in thickened crusts

assessing the validity of the channel model of fracture aperture under field conditions in recent investigations of fluid and solute movement in discrete fractures spatial heterogeneity of the fracture aperture has been conceptualized as a series of noninterconnecting constant aperture flow paths or channels two methods of estimating the distribution of the aperture sizes are presented using information from a single hole pumping test and a radially converging tracer test the first method uses the transmissivity of the fracture and mean arrival time of the tracer while the second method uses the mean and variance of the solute arrival time if the fracture can be conceptualized as a series of nonintersecting flow paths of constant aperture the two methods should yield identical estimates for the parameters of the distribution the validity of the channel model can be assessed by comparing the parameters of the distribution that are estimated by the two methods this technique was demonstrated using hydraulic and tracer tests conducted in a discrete areally extensive horizontal fracture in the silurian dolomite in the northeastern illinois a lognormal and a truncated gamma distribution were used to describe the distribution of constant aperture flow paths the lognormal distribution could not reproduce the abrupt rising limb and maximum rate of mass arrival that characterized the tracer tests the gamma distribution more accurately predicted the shape of the breakthrough curves however the two methods of estimating the parameters of the distribution provided significantly different estimates of the variances of the aperture sizes the mean aperture as estimated by the two methods was similar the difference in the variance of the aperture as estimated by the two methods indicates that alternative conceptual models of aperture heterogeneity are required to more accurately describe both fluid and solute movement in this field situation

constraints on the genesis of gold mineralization at the homestake gold deposit black hills south dakota from rhenium osmium sulfide geochronology the homestake gold deposit located in the black hills south dakota usa is one of the largest known hydrothermal gold deposits globally with total mining production exceeding 40 moz au rhenium osmium geochronology of ore associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern black hills arsenopyrite yields a rhenium osmium re os age of 1 736 8 ma mean squared weighted deviation 1 6 consistent with existing age constraints for gold mineralization whereas re os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age this is taken to indicate that the re os arsenopyrite chronometer is robust to at least 400 c whereas the re os pyrrhotite chronometer is likely disturbed by temperatures of 300 350 c the re os arsenopyrite age and initial os ratio 0 28 0 15 are interpreted to indicate that gold was introduced at ca 1 730 ma coincident with the onset of exhumation of crustal blocks and possibly the earliest intrusive phases of harney peak granite magmatism new in situ u pb monazite analyses from an aplite dike in the east central black hills indicate that granite magmatism was a protracted event persisting until at least ca 1 690 ma

time constraints on the tectonic evolution of the eastern sierras pampeanas central argentina the application of the shrimp u pb dating technique to zircon and monazite of different rock types of the sierras de cordoba provides an important insight into the metamorphic history of the basement domains additional constraints on the pampean metamorphic episode were gained by pb pb stepwise leaching pbsl experiments on two titanite and garnet separates results indicate that the metamorphic history recorded by crd free gneisses m2 started in the latest neoproterozoic earliest cambrian 553 and 543 ma followed by the m4 metamorphism at 530 ma that is documented in the diatexites zircon ages of 492 ma in the san carlos massif correlate partly with rather low th u ratios 0 1 suggesting their growth by metamorphic fluids this age is even younger than the pbsl titanite ages of 506 ma it is suggested that the fluid alteration relates to the beginning of the famatinien metamorphic cycle in the neighbouring sierra de san luis and has not affected the titanite ages the ptt evolution can be correlated with the plate tectonic processes responsible for the formation of the pampean orogene i e the accretion of the pampean basement to the rio de la plata craton m2 and the later collision of the western pampean basement with the pampean basement

simulation of three dimensional flow of immiscible fluids within and below the unsaturated zone this paper presents a two phase flow model based on a three dimensional finite difference formulation as three dimensional simulations can require substantial computer effort a numerical technique that takes advantage of vector and parallel processing computer architecture is developed the model is posed in terms of water saturation and nonwetting fluid pressure it uses three phase capillary pressure and relative permeability relationships to permit simulation within or below the unsaturated zone a modified formulation of slice successive overtaxation an iterative matrix solution technique is introduced this technique is designed to use parallel processing capabilities of new computers the model is applied to immiscible fluid flow at two chemical waste landfills near niagara falls new york at both sites denser than water nonaqueous liquids napls are present in the groundwater regimes in relatively large quantities the model applications address several technical concerns at the two sites including the effectiveness of clay as a geologic barrier to napl migration owing to capillary pressure forces the three dimensional aspects of dense napl flow and the sensitivity of napl recovery in pumping wells due to various hydrogeologic and fluid properties the results of the applications show that 1 even under a downward hydraulic gradient natural differences in capillary pressure relationships for different lithologies can prevent downward migration of napl 2 without any lithologic capillary barrier an upward hydraulic gradient induced by a de watering system can prevent downward migration of napl 3 napl recovery at wells is sensitive to relative permeability a relationship that requires field calibration in many settings and 4 the three dimensional aspects of two phase flow and hydrogeologic heterogeneity require explicit treatment in many settings

radionuclide transport in fast channels in crystalline rock recent field investigations in crystalline rock give strong indications that water flows in largely isolated channels in fissured rock the present paper sets out to investigate radionuclide transport in such channels including diffusion into the rock matrix it is shown that the uptake from a cylindrical hole in a matrix is much more effective per unit contact area than a flat surface semi infinite solid comparison of diffusion from cylindrical and slit formed channels shows that the differences in interfacial flux are minor a slit formed channel may therefore be approximated by a cylindrical one diffusional transport in the matrix is then treated as one dimensional instead of two dimensional which gives a considerable numerical simplification a simple estimate of the penetration depth into the cylindrical region i e the location of the diffusional front is proposed by using the integrated finite difference method a number of radionuclide transport calculations are performed for the case of flow and dispersion in a cylindrical channel coupled to diffusion and sorption in the matrix

controversial pb pb and sm nd isotope results in the early archean isua west greenland oxide iron formation preservation of primary signatures versus secondary disturbances pb stepwise leaching pbsl determinations on two magnetite enriched fractions of a bif sample from the northeastern part of the isua supracrustal belt west greenland yield an isochron of 3691 22 ma mswd 0 4 in combination with previously published geochronological constraints for a minimum deposition age of 3 71 ga for volcanic sequences nutman et al 1997 comprising the studied bifs and supported by microtextural observations this demonstrates an early archean amphibolite facies metamorphic event in the supracrustal pbsl data on magnetite from slightly discordant veins within the same iron stone together with bulk pb isotope values of paragenetically late pyrite yield a 3 63 0 07 ga mswd 8 6 errorchron with significantly different pb isotopic compositions from those obtained from the main magnetite layers and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later the fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3 6 ga granitic into 3 7 ga tonalitic amitsoq gneisses enclosing the belt tremolite rich layers in the bif are characterized by sm nd ratios close to chondritic end 3800 of 1 7 and 2 1 within error of published results from adjacent basic units in the supracrustal suite in contrast the magnetite dominated layers yield unrealistically high end 3800 of 14 8 and 14 4 indicative of sm nd ratios resembling ree fractionated continental sources these high end 3800 together with radiogenic sr leached from the magnetite enriched separates is ascribed to secondary hydroxyapatite which predominantly forms as crystal overgrowths in the magnetite rich bands the timing of the hydrothermal event during which apatite was deposited within the bif remains uncertain but a tchur model age of 1 85 ga from the apatite dominated hcl leachate may point to a close genetic relationship with local proterozoic metamorphism and granite formation by crustal remelting in the isukasia area step leaching of magnetite from a similar silicate oxide facies bif from the western part of the isua supracrustal belt yield a pb pb isochron age of 2 84 0 05 ga mswd 1 43 the agreement between the pbsl age and previously published field and isotopic evidence for a major late archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2 75 2 83 ga granodioritic ikkattoq gneisses west and southwest of the western limb of the isua supracrustal belt we argue that the pbsl isochron ages date the peak of amphibolite facies metamorphic events which respectively affected the eastern and western sections of the isua supracrustal belt during the early and late archean our results give additional support to the suggestion that the scatter on published end t values from the isua supracrustal belt and adjoining gneisses can be assigned to post formational hydrothermal processes and underline the need for care in the interpretation of sm nd bulk data from polymetamorphic rocks to constrain isotopic models of early earth s evolution

discrete fracture network interpretation of field tracer migration in sparsely fractured rock field tracer migration in sparsely fractured rock is analyzed with a discrete fracture network model that has been calibrated on fracture and flow data from the stripa 3d experiment in sweden comparison between transport simulations in the flow calibrated discrete model and tracer tests of the field experiment confirms the fracture network parameters obtained from the flow analysis except for the variance of the fracture conductivities which had to be increased the discrete model reproduces the uneven spatial distribution of flow and tracers the complex dispersive behavior and channeling effects that have been observed in the field experiment as a result of flow channeling transport occurs along preferential paths whose transport properties may substantially deviate from the medium average properties for example porosity and wetted fracture surface area available to sorption may be reduced by almost 2 orders of magnitude compared to the medium average values

zircon and garnet geochronology of eclogites from the moldanubian zone of the black forest germany abstract the moldanubian zone of the black forest germany part of the internal zone of the central european variscan belt exposes high pressure metamorphic rocks as small eclogite lenses in host gneisses we report zircon u pb and pb pb ages and garnet sm nd and pb pb ages for five eclogites the less mobile trace element contents of these eclogites are similar to those of mid ocean ridge basalts but have moderate to highly fractionated concentrations of the rare earth elements enrichments in the incompatible elements and negative anomalies in the high field strength elements initial nd values range from documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfon

scale dependence of continuum models for fractured basalts numerical simulations are carried out to determine the scale above which an equivalent porous medium epm model can be used to represent flow through the dense interiors of fractured basalts steady laminar flow is assumed to occur through the interconnected network of conduits formed by the intersecting column defining fracture patterns of the basalt dense interiors the basalt rock matrix is considered to be impermeable a porous medium equivalence is established in terms of fluid darcian fluxes a polar plot for the equivalent hydraulic conductivity is used to test whether a given fractured basalt system can be approximated as an epm for interconnected colonnade network models of filled or unfilled fractures with uniform apertures and a column diameter of 1 m epm models are appropriate at a length scale of approximately 6 times the column diameter for network models of filled or unfilled fractures with a lognormal aperture b millimeters distribution mean in b 1 945 and ln b 0 896 epm models can be justified at length scales of approximately 22 and 27 times the column diameter respectively the computed equivalent k values for clay filled fractures compare favorably with the field measured data this suggests that a model comprised of an interconnected network of filled fractures may not be an unlikely representation of flow through the dense interiors at the small scales considered in this study several limitations of the study are noted

numerical investigation of steady liquid water flow in a variably saturated fracture network numerical simulation was used to study steady liquid water movement in a 5 m by 5 m vertical section containing a hypothetical fracture network under conditions of variable saturation the fracture network was assumed to be embedded within an impermeable rock matrix three variations of a network were considered the mixed network consisted of two fracture sets a subvertical set containing five 125 m average aperture fractures and a subhorizontal set containing four 25 m average aperture fractures the other two networks had identical fracture orientation and contained either all 125 m or all 25 m average aperture fractures the tough simulator was used to calculate the total steady liquid water flux through the network the flux through individual fracture segments and the pressure head at each fracture segment a unit hydraulic gradient was imposed on the network by applying fixed pressure head boundaries ranging from 0 25 to 0 0 m of water of equal value to the top and bottom saturation and permeability versus pressure head relations for the two sets of fractures were determined with the vsfrac model which assumed that aperture was variable within an individual fracture results showed that the spatial distributions of pressure head and flux within the network as well as the location of the dominant pathways depended strongly on the prescribed boundary pressure head for the mixed network both pressure head and flux tended to become more spatially uniform when the boundary pressure head approached the pressure head at which the permeability thickness products of the large and small aperture fractures are equal the crossover pressure head these results imply that for systems similar to the one considered here interpretation of actual measurements of pressure head and flux may be quite complex and that representation of variably saturated fracture networks as an equivalent continuum may be more valid for some ranges in pressure head than for others equivalent permeability as a function of pressure head was calculated for the fracture network illustrating how information collected on individual fractures may be used to estimate the flow properties of rock at larger scales

chemostratigraphy of paleoproterozoic carbonate successions of the wyoming craton tectonic forcing of biogeochemical change the archean wyoming craton is flanked on the south and east by belts of paleoproterozoic supracrustal successions whose correlation is complicated by lack of geochronologic constraints and continuous outcrop however carbonate units in these successions may be correlated by integrating carbon isotope stratigraphy with lithostratigraphy the 10 km thick paleoproterozoic snowy pass supergroup in the medicine bow mountains was deposited on the present day southern flank of the wyoming craton it contains three discrete levels of glacial diamictite correlative with those in the huronian supergroup on the southern margin of the superior craton the nash fork formation of the upper snowy pass supergroup is significantly younger than the uppermost diamictite and was deposited after the end of the paleoproterozoic glacial epoch carbonates at the base of the nash fork formation record remarkable 13 cenrichment up to 28 v pdb whereas those from overlying members of the lower nash fork formation have d 13 c values between 6 and 8 carbonates from the upper nash fork formation above the carbonaceous shale have carbon isotope values ranging between 0 and 2 5 the transition from high carbon isotope values to those near 0 in the nash fork formation is similar to that at the end of the ca 2 2 2 1 ga carbon isotope excursion in fennoscandia this chemostratigraphic trend and deposition of bifs mn rich lithologies carbonaceous shales and phosphorites at the end of the global ca 2 2 2 1 ga carbon isotope excursion are likely related to ocean overturn associated with the final breakup of the kenorland supercontinent correlative carbonates from the slaughterhouse formation in the sierra madre wy and from the whalen group in the rawhide creek area in the hartville uplift wy have highly positive carbon isotope values in contrast carbonates from other exposures of the whalen group in the hartville uplift and all carbonate units in the black hills sd have carbon isotope values close to 0 combined with existing geochronologic and stratigraphic constraints these data suggest that the slaughterhouse formation and the succession exposed in the rawhide creek area of the hartville uplift are correlative with the lower and middle nash fork formation and were deposited during the ca 2 2 2 1 ga carbon isotope excursion the estes and roberts draw formations in the black hills and carbonates from other exposures in the hartville uplift postdate the ca 2 2 2 1 ga

double porosity models for a fissured groundwater reservoir with fracture skin theories of flow to a well in a double porosity groundwater reservoir are modified to incorporate effects of a thin layer of low permeability material or fracture skin that may be present at fracture block interfaces as a result of mineral deposition or alteration the commonly used theory for flow in double porosity formations that is based upon the assumption of pseudo steady state block to fissure flow is shown to be a special case of the theory presented in this paper the latter is based on the assumption of transient block to fissure flow with fracture skin under conditions where fracture skin has a hydraulic conductivity that is less than that of the matrix rock it may be assumed to impede the interchange of fluid between the fissures and blocks resistance to flow at fracture block interfaces tends to reduce spatial variation of hydraulic head gradients within the blocks this provides theoretical justification for neglecting the divergence of flow in the blocks as required by the pseudo steady state flow model coupled boundary value problems for flow to a well discharging at a constant rate were solved in the laplace domain both slab shaped and sphere shaped blocks were considered as were effects of well bore storage and well bore skin resultsmore obtained by numerical inversion were used to construct dimensionless type curves that were applied to well test data for a pumped well and for an observation well from the fractured volcanic rock terrane of the nevada test site less

flow through fractures flow through fractures is often idealized as flow between two parallel plates plane poiseuille flow the opening or aperture between parallel plates is unambiguous and its relation to flowrate is well known however fractures in rock have uneven walls and a variable aperture a model for flow in a fracture is proposed wherein the fracture is represented by a set of parallel plate openings with different apertures the model leads to a modified poiseuille equation for flow which includes an aperture frequency distribution for the fracture any arbitrary aperture distribution can be used in order to simplify computation and demonstrate the properties of the model a log normal form of distribution is assumed even when an analytical form of the distribution is assumed two parameters rather than a single value representing aperture size are required to determine flowrate models of aperture change for a fracture undergoing compression fracture walls deforming and extension fracture walls separating are developed which constrain the additional parameter and allow calculation of flowrate as a function of mean aperture the theoretical relationships developed between mean aperture and flowrate can be used to interpret published laboratory data for single fractures

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

a large scale flow and tracer experiment in granite 1 experimental design and flow distribution this paper describes the stripa three dimensional experiment where water and tracer flow has been monitored in a specially excavated drift in the stripa mine several new experimental techniques and equipment were developed and used the experiment was performed in a specially excavated drift 100 m long at the 360 m level in granite the whole ceiling and the upper part of the walls were covered with 375 individual plastic sheets where the water flow into the drift could be collected eleven different tracers were injected at distances between 11 and 50 m from the ceiling of the drift the flow rate and tracer monitoring was kept up for more than 2 years the tracer breakthrough curves and flow rate distributions were used to study the flow paths velocities hydraulic conductivities dispersivities and channeling effects in the rock this paper describes the experimental techniques the fracture mapping the tracer and flow rate measurements and the results of the flow rate measurements the detailed observations made possible by the plastic sheeting technique have given some qualitative as well as quantitative information on flow rate distribution in fractured rock which previously has not been available these observations may be of importance for assessing the transport of dissolved species such as radionuclides through fractured rock