the mont saint hilaire plutonic complex occurrence of excess 40ar and short intrusion history mont saint hilaire is an approximately 10 km2 alkaline plutonic complex in the monteregian petrographic province of quebec the complex consists of an older western half of alkali gabbros and an eastern portion of nepheline syenites and magmatic breccias the intrusives were emplaced at depths of no more than a few kilometres into paleozoic sedimentary rocks through the underlying grenville basement 40ar 39ar analyses of amphibole yield complex age spectra in which the apparent ages decrease with increasing heating temperature conventional k ar analyses of amphibole pyroxene and feldspar yield anomalously old dates these data indicate the presence of small yet significant amounts up to 8 10 11 mol g of excess 40ar the quantities of excess argon are variable between rocks and minerals from the same rock in contrast biotites yield concordant plateaus and consistent ages ranging only from 124 1 to 124 6 ma these data define the age of the complex 124 4 1 2 ma and imply a short time span of

automated velocity model building with wavepath tomography wavefield continuation migration generally is recognized as superior to kirchhoff methods in complex velocity models such as below rugose salt bodies it accounts for multipathing sharp velocity contrasts and the limited bandwidth of seismic wave propagation wavepath tomography builds the velocity model in a way that is consistent with the wavefield migration operator traveltime residuals are back projected along a wavepath instead of rays the actual wavefield continuation operator is used to represent the wave propagation between surface source receiver pairs and subsurface reflection points a wavepath is obtained by multiplying impulse responses from a surface location and a reflection point the inversion matrix is kept to a manageable size by restricting the wavepath to the first fresnel zone the considerable expense of computing a single wavepath kernel in comparison to ray tomography is partially offset by the smaller number of back projections necessary to sample the velocity model adequately we have used wavepath tomography to build subsalt velocity models using 2d synthetic data wavepath tomography was implemented in 3d

excess argon in amphiboles from fluid interaction and short intrusion interval at the epizonal marangudzi complex zimbabwe the epizonal marangudzi complex of the karoo associated nuanetsi igneous province in southern zimbabwe was emplaced into precambrian gneisses and consists of a gabbro mass intruded in turn by ring dikes of quartz and nepheline syenites amphibole and biotite from various lithologies have been examined using 40ar 39ar techniques in order to investigate the temporal distribution of intrusions and the possible occurrence of excess ar with one important exception samples give internally concordant incremental release spectra average 40ar 39ar dates range from 177 6 to 181 3 ma three amphiboles give dates which are significantly older than the other samples and contain small yet discernible quantities of excess 40ar excess ar is restricted to amphiboles displaying substantial fe mg zoning which is interpreted as reflecting late magmatic or early sub solidus reaction with fluid phases argon was apparently acquired from the fluid during alteration and ultimately could have been magmatic or could have come from degassing of the country rocks the remaining samples both amphibole and biotite show only a restricted range 1 m y in apparent age and are essentially analytically indistinguishable implying rapid cooling of the complex the average for these analyses 178 1 ma reflects the mean age of emplacement of cogenetic intrusions within a short interval 1 m y in duration this revised age of the marangudzi complex fits well within the temporal framework of karoo province magmatism in the region

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

heightened hurricane surge risk in northwest florida revealed from climatological hydrodynamic modeling and paleorecord reconstruction historical tropical cyclone tc and storm surge records are often too limited to quantify the risk to local populations paleohurricane sediment records uncover long term tc activity but interpreting these records can be difficult and can introduce significant uncertainties here we compare and combine climatological hydrodynamic modeling including a method to account for storm size uncertainty historical observations and paleohurricane records to investigate local surge risk using apalachee bay in northwest florida as an example the modeling reveals relatively high risk with 100 year 500 year and worst case surges estimated to be about 6 3 m 8 3 m and 11 3 m respectively at bald point a paleorecord site and about 7 4 m 9 7 m and 13 3 m respectively at st marks the head of the bay supporting the inference from paleorecords that apalachee bay has frequently suffered severe inundation for thousands of years both the synthetic database and paleorecords contain a much higher frequency of extreme events than the historical record the mean return period of surges greater than 5 m is about 40 years based on synthetic modeling and paleoreconstruction whereas it is about 400 years based on historical storm analysis apalachee bay surge risk is determined by storms of broad characteristics varies spatially over the area and is affected by coastally trapped kelvin waves all of which are important features to consider when accessing the risk and interpreting paleohurricane records in particular neglecting size uncertainty may induce great underestimation in surge risk as the size distribution is positively skewed while the most extreme surges were generated by the uppermost storm intensities medium intensity storms categories 1 3 can produce large to extreme surges due to their larger inner core sizes for apalachee bay the storms that induced localized barrier breaching and limited sediment transport overwash regime surge between 3 and 5 m are most likely to be category 2 or 3 storms and the storms that inundated the entire barrier and deposited significantly more coarse materials inundation regime surge 5 m are most likely to be category 3 or 4 storms

a basin to channel scale unstructured grid hurricane storm surge model applied to southern louisiana abstract southern louisiana is characterized by low lying topography and an extensive network of sounds bays marshes lakes rivers and inlets that permit widespread inundation during hurricanes a basin to channel scale implementation of the advanced circulation adcirc unstructured grid hydrodynamic model has been developed that accurately simulates hurricane storm surge tides and river flow in this complex region this is accomplished by defining a domain and computational resolution appropriate for the relevant processes specifying realistic boundary conditions and implementing accurate robust and highly parallel unstructured grid numerical algorithms the model domain incorporates the western north atlantic the gulf of mexico and the caribbean sea so that interactions between basins and the shelf are explicitly modeled and the boundary condition specification of tidal and hurricane processes can be readily defined at the deep water open boundary the unstructured grid enables highly refi

numerical study of coastal hydrodynamics using a coupled model for hudhud cyclone in the bay of bengal abstract the past decade has witnessed an increased intensity of cyclones in the bay of bengal region with higher winds spread over a larger area the associated risk and coastal vulnerability have increased with wider destructive potential from high waves storm surges and associated coastal inundation the very severe cyclones that made landfall over the bay of bengal in the past decade had strong winds in their outer cores unlike those cyclones that made landfall in previous years the original parametric wind formulation performs well for more compact cyclones but at a radial distance far away from the cyclone centre the winds are under estimated hence there is a need to revisit and modify this formula for practical applications and this study attempts to provide a better representation of the overall radial distance in the wind field envelope the study postulates a 3 5 power law fitted to the original wind formulae which provides a reasonably good estimate for the surface wind field the recent very severe cyclones that developed over the bay of bengal provided an excellent test bed to verify this hypothesis which is supported by validation from six in situ buoys the modified wind formula used with a coupled hydrodynamic model adcirc swan simulated the storm surge and wave characteristics associated with a recent very severe cyclonic storm hudhud that made landfall in andhra located on the east coast of india in 2014 the study also investigated the dependence of coastal geomorphic features and beach slope on the variability of wave induced setup computed significant wave height and storm surge show an excellent match with wave rider buoy and tide gauge observations

pliensbachian environmental perturbations and their potential link with volcanic activity swiss and british geochemical records abstract the mechanisms leading to environmental and climate change prior to and anticipating the well studied toarcian oceanic anoxic event are not completely understood specifically the role of the continuing break up of pangea associated palaeogeographic rearrangements and the influence of volcanic activity are less well known therefore we studied the pliensbachian interval in the open marine hemipelagic section of the breggia gorge in southern switzerland to elucidate the potential impact of these major changes on the environment and climate we identified carbon isotope excursions towards lower values in whole rock carbonates dated from the sinemurian pliensbachian boundary davoei margaritatus and spinatum zones and possibly from the pliensbachian toarcian boundary using the whole rock oxygen isotope record and phosphorus content we associated these events with phases of increased runoff possibly related to climate warming we also established the first mercury hg record for the entire pliensbachian and in order to verify its validity we analysed an additional section located along the dorset coast in southern england the presence of hg spikes common in both sections may indicate the influence of episodes of increased volcanic activity the hg spike during the earliest pliensbachian may coincide with a late phase of the central atlantic magmatic province and or increased rifting linked with the opening of the hispanic corridor further significant maxima in hg contents are observed within the davoei and margaritatus zones and both phases may correspond to early volcanic activity of the karoo ferrar large igneous province further analyses are however needed to corroborate this interpretation and evaluate the importance of organic matter scavenging and detrital input in comparison to volcanic activity

advanced techniques in site characterization and mining hazard detection for the underground coal industry abstract today s highly productive coal mining systems represent large capital investments careful geotechnical planning and design are essential to protect these systems and to help ensure their overall success too often however the ability of the mine operator to adequately characterize the geotechnical environment and keep up with changing geologic conditions lags the advance rate of the mining equipment seismic tomographic imaging based on the same principles as medical computer aided tomography cat scans offers the ability to identify changing geologic conditions loading patterns and the location of nearby abandoned mine workings although seismic tomography has been used for many years in the oil industry until recently its practicality in the mining industry has been limited because the processes involved interfered with mining productivity and were labor intensive in terms of data collection and interpretation new advancements in signal processing have greatly enhanced the speed resolution and range of applications of tomographic imaging in underground coal mine settings many tomography applications in the mining industry use seismic velocity and or attenuation tomography within a volume enclosed by the seismic source and receiver array although results obtained using this geometry work well in many situations recent developments in reflection tomography show promise in eliminating the requirement that the target volume be surrounded by sources receivers for example in seam seismic reflection tomography may now be used to image structures and or old workings from one general location in the mine e g face areas of mains and panel developments well ahead of planned developments largely eliminating the need to probe hole drill on regular intervals these new developments have greatly expanded the ability of the mine operator to cost effectively characterize previously inaccessible areas of the property this paper gives an overview of the theories and processes involved in seismic tomography applicable to coal mine settings recent examples are presented of tomographic imaging applied to a variety of coal mine ground control problems and old works detection in the united states and europe the applicability of both seismic refraction and seismic reflection tomography is discussed

3d common reflection point based seismic migration velocity analysis three dimensional prestack depth migration and depth residual picking in common image gathers cigs are the most time consuming parts of 3d migration velocity analysis most migration based velocity analysis algorithms need spatial coordinates of reflection points and cig depth residuals at different offsets or angles to provide updated velocity information we propose a new algorithm that can analyze 3d velocity quickly and accurately spatial coordinates and orientations of reflection points are provided by a 3d prestack parsimonious depth migration the migration involves only the time samples picked from the salient reflection events on one 3d common offset volume ray tracing from the reflection points to the surface provides a common reflection point crp gather for each reflection point predicted nonhyperbolic moveouts for local velocity perturbations based on maximizing the stacked amplitude give the estimated velocity updates for each crp gather then the velocity update for each voxel in the velocity model is obtained by averaging over all predicted velocity updates for that voxel prior model constraints may be used to stabilize velocity updating compared with other migration velocity analyses the traveltime picking is limited to only one common offset volume and needs to be done only once there is no need for intensive 3d prestack depth migration hence the computation time is orders of magnitude less than other migration based velocity analyses a 3d synthetic data test shows the algorithm works effectively and efficiently

40ar39ar dating of glauconites measured 39ar recoil loss from well crystallized specimens abstract nine glauconite samples with relatively high k concentrations and which appear to be well crystallized using normal x ray diffraction techniques have been studied using the 40 ar 39 ar method the glauconite 40 ar 39 ar apparent ages exceed their k ar rb sr and in most cases stratigraphic ages by substantial amounts 40 ar 39 ar release spectra sometimes yield plateaus but these apparent ages have no geological significance the results indicate that 39ar is lost by recoil from mineral grains during neutron irradiation consistent with previously reported observations the amount of 39ar loss was measured by isotope dilution for four samples and varied from 29 to 17 in contrast radiogenic 40ar is quantitatively retained during irradiation the very fine blades which make up glauconite grains yield the mineral susceptible to large amounts of 39ar loss and unsuitable for 40 ar 39 ar dating

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

eruption age of a pleistocene basalt from 40 ar 39 ar analysis of partially degassed xenoliths we have applied 40 ar 39 ar dating to potassium rich granitic xenoliths and host basalt from the pleistocene big pine volcanic field california these xenoliths had been partially degassed upon their inclusion in the basaltic lava argon released from the xenoliths at extraction temperatures below 900 c yielded plateau ages indistinguishable from the total k ar age of the basalt the best estimate of the age of eruption was 1 18 0 05 2 m y 40 ar extracted at higher temperatures included radiogenic argon not degassed from the late cretaceous xenoliths 1 18 m y ago causing an increase in the apparent age for the high temperature fractions the agreement of the low temperature xenolith plateau ages and the basalt k ar ages demonstrates that 40 ar 39 ar analysis of xenoliths may be used to measure the age of eruption of very young lavas this is significant because in many instances ages cannot be reliably determined by analysis of the lavas themselves

the influence of domain size on the response characteristics of a hurricane storm surge model the influence of domain size on boundary condition specification and on computed storm surge response is investigated storm surge response along the florida shelf in the gulf of mexico due to hurricane kate is examined over three domains using two different open ocean boundary forcing functions a still water or zero elevation condition and an inverted barometer condition which accounts for the atmospheric pressure component of the meteorological forcing the first domain is relatively small and is situated primarily on the continental shelf in the region of intense storm surge generation a second domain includes the entire gulf of mexico basin the final domain covers the gulf of mexico contiguous basins and extends out into the deep atlantic ocean the computed storm surge response indicates that the small domain is inadequate since cross shelf boundaries are in regions of significant storm surge generation where surge and therefore boundary conditions are not known a priori also the behavior of resonant modes that are physically excited within the gulf of mexico due to the passage of the hurricane is unknown at the boundaries of this small domain the domain that includes the entire gulf of mexico captures the primary storm surge well but may not correctly model resonant modes in general these resonant modes are difficult to accurately set up by boundary condition specification since they may be dependent on interactions between the gulf and contiguous basins the primary storm surge response as well as resonant modes excited by the storm are best represented using a domain which encompasses the western north atlantic ocean the caribbean sea and the gulf of mexico this domain with deep atlantic ocean boundaries facilitates simple boundary condition specification and minimizes the influence of boundary conditions on storm surge generation in coastal regions basin resonant modes and basin to basin interactions are also captured

cooling pattern and mineralization history of the saint sylvestre and western marche leucogranite pluton french massif central i 40ar 39ar isotopic constraints abstract the saint sylvestre western marche leucogranite complex lc is a composite pluton emplaced 324 m y ago in the variscan continental crust of the northwestern limousin french massif central petrographic structural and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for ar retention in muscovite part i and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40 50 m y part ii a suite of thirty seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 ar 39 ar technique muscovite is characterized by well defined plateau ages ranging between 314 and 301 ma while biotite gives less well behaved age spectra with apparent ages varying between 320 and 300 ma the muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton field petrographic gravimetric and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional post cooling fault network cutting through the laccolith when restored to their approximate pre faulting geometry the age variations are shown to correlate positively with sample elevation with a top to base difference as large as 10 14 m y over a mean intrusion thickness of 3 km this pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened variscan crust during the westphalian and is confirmed by two dimensional thermal modelling part ii the data show that 40 ar 39 ar mica ages can postdate the time of crystallization of the host pluton by up to 23 ma depending on the level sampled at the surface they warn against interpreting k ar mica ages from similar low relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure cooling mechanism and post cooling faulting of the pluton in turn provided that the pre faulting geometry of the intrusion is properly assessed the vertical cooling pattern can reveal fine scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens

model simulations of the gulf of maine response to storm forcing 1 the storm response of the gulf of maine georges bank region was investigated using the dartmouth three dimensional nonlinear finite element coastal ocean circulation model called quoddy with uniform density and quadratic bottom stress a suite of model hindcast experiments were conducted for the 4 8 february 1987 period that featured a strong nor easter the model was forced at the open ocean boundaries by the m2 semidiurnal tidal sea level and in the interior by realistic surface wind stresses and in some cases atmospheric pressure the reference model run with just tidal and wind stress forcing and a time space constant bottom drag coefficient cd of 0 005 captured 65 of the observed detided low pass filtered or subtidal sea level variance of a representative array of coastal and bottom pressure stations in the gulf however there remained large unexplained model observed sea level differences particularly during the storm model sensitivity testing to a realistic range of constant cd values did not improve model results significantly however results of a model run in which atmospheric pressure forcing derived through linear interpolations of regional measurements was added to wind stress forcing captured 82 of the observed subtidal sea level variance

comparative study of 222rn 40ar 39ar and 37ar leakage from rocks and minerals implications for the role of nanopores in gas transport through natural silicates we have measured the leakage of radiogenic 222rn and 40ar and reactor produced 39ar and 37ar in samples of granites plagioclase orthoclase cleavelandite and hornblende to assess the role of nanopores in gas transport through them the 39ar and 37ar were produced by fast neutron irradiation of the samples in vacuum sealed ampoules the leakage of all three ar isotopes was barely measurable in contrast the leakage of 222rn ranged between 1 45 18 1 orders of magnitude higher than that of the ar isotopes the extremely low ar isotope leakage suggests that the samples studied are not permeated by extensive nanopore networks which intersect grain surfaces the relatively high 222rn loss from these samples therefore has to be attributed to preferential enrichment of 226ra 238u on grain surfaces or to the existence of u th accessory minerals in or around grain boundaries from which substantial quantities of 222rn escape

high resolution 40 ar 39 ar chronology of multiple intrusion igneous complexes the mount brome complex of the monteregian province of southern quebec canada consits of several major intrusions ranging compositionally from gabbro to syenite the relative ages of these intrusives have been investigated with high resolution 40ar 39ar analyses including a specially designed irradiation configuration to cancel the effects of fluence gradients small yet distinct apparent age differences are observed while a number of analytical and geological factors could be proposed to explain the small variations evaluation of these suggests the age differences reflect those in emplacement times the gabbro and nepheline diorite were emplaced within a short span 123 1 ma ago generally more evolved lithologies biotite monzodiorite pulaskite nordmarkite appear to have been emplaced within a restricted interval 1 4 0 3 ma later whole rock rb sr systematics do not give acceptable isochrons because of significant scatter interpreted to reflect initial 87sr 86sr heterogeneities resulting from crustal contamination considering the variations in initial ratio the rb sr data are consistent with the 40ar 39ar age

age and time span of emplacement of the pliny range complex northern new hampshire the pliny complex in northern new hampshire a part of the white mountain magma series is composed of eight distinct intrusions previous work has indicated chemical variations that cannot be explained by fractional crystallization and has led to the supposition that the diverse rock suite may have formed in episodes at different times the 40ar 39ar method was applied to biotite and amphibole separates from seven units in order to evaluate the time span of emplacement and to accurately establish the age the analyses reveal internally and externally concordant ages for both amphibole and biotite in all of the units the ages of individual units cannot be resolved from the mean age indicating a time span of emplacement of 1 m y or less the intrusives were emplaced in rapid succession and cooled quickly 183 2 1 8 m y ago chemical and isotopic variations among the rocks point to a complex origin involving extensive interaction of the primary magma with older crust all of which apparently required only a short period of time

40ar 39ar ages for plutons of the monteregian hills quebec evidence for a single episode of cretaceous magmatism 40ar 39ar data for biotite and amphibole from six plutonic complexes of the monteregian petrographic province in quebec canada are presented generally uncomplicated and concordant incremental heating spectra are observed although there is evidence for minor excess 40ar at several localities biotites and amphiboles are concordant consistent with rapid cooling of the high level complexes ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic the 40ar 39ar ages of six monteregian complexes st hilaire rougemont johnson shefford brome megantic all fall within the very restricted range of 124 1 m y it is suggested that all of the monteregian plutons with the exception of oka were formed within a short interval 1 2 m y during the cretaceous consistent with paleomagmetic results the new ages along with geochemical characteristics support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in new england and the chain of new england seamounts

tomographic velocity estimation with plane wave synthesis in areas with structurally complex geology tomographic velocity analysis is often required to estimate velocities in this paper i describe an algorithm for tomographic velocity estimation that uses plane wave synthesis imaging as a prestack migration the classical iterative two step process measures the traveltime errors with the current velocity model and then update the velocity model is performed as follows the events are picked in the image space after prestack migration with surface oriented plane wave synthesis imaging the traveltime deviations are measured through residual moveout rmo velocity analysis in common surface location csl gathers obtained by reflector oriented plane wave synthesis imaging and the velocity update is calculated by inverting the traveltime deviations through a conjugate gradient the results from synthetic data indicate that the tomographic method successfully estimates interval velocity models that lead to depth migrated images with no residual moveout

40ar 39ar ages of the sill complex of the karoo large igneous province implications for the pliensbachian toarcian climate change 1 reliable geochronological results gathered so far n 76 have considerably constrained the timing of the emplacement of the karoo large igneous province lip yet strikingly missing from this dating effort is the huge southern sill complex cropping out in the 0 6 106 km2 main karoo sedimentary basin we present 16 new 40ar 39ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a n s trend in the eastern part of the basin the results show a large range of plateau and miniplateau ages 176 2 1 3 to 183 8 2 4 ma with most dates suggesting a 3 ma 181 184 ma duration for the main sill events the available age database allows correlation of the karoo lip emplacement with the pliensbachian toarcian second order biotic extinction the global warming and the toarcian anoxic event provided that adequate calibration between the 40k and 238u decay constant is made the mass extinction and the isotopic excursions recorded at the base of the toarcian appear to be synchronous with both the increase of magma emission of the karoo lip and the emplacement of the sills the co2 and so2 derived from both volcanic emissions as well as carbon rich sedimentary layers intruded by sills might be the main culprits of the pliensbachian toarcian climate perturbations we propose that the relatively low eruption rate of the karoo lip is one of the main reasons explaining why its impact on the biosphere is relatively low contrary to e g the camp triassic jurassic and siberia permo triassic lips

a high resolution coupled riverine flow tide wind wind wave and storm surge model for southern louisiana and mississippi part i model development and validation abstract a coupled system of wind wind wave and coastal circulation models has been implemented for southern louisiana and mississippi to simulate riverine flows tides wind waves and hurricane storm surge in the region the system combines the noaa hurricane research division wind analysis system h wind and the interactive objective kinematic analysis ioka kinematic wind analyses the wave model wam offshore and steady state irregular wave stwave nearshore wind wave models and the advanced circulation adcirc basin to channel scale unstructured grid circulation model the system emphasizes a high resolution down to 50 m representation of the geometry bathymetry and topography nonlinear coupling of all processes including wind wave radiation stress induced set up and objective specification of frictional parameters based on land cover databases and commonly used parameters riverine flows and tides are validated for no storm conditions while winds wind waves hydrographs and high wa

coastal flooding from wave overtopping and sea level rise adaptation in the northeastern usa abstract in the northeastern united states flooding arising from wave overtopping poses a constant threat to coastal communities during storm events the purpose of this study is to construct a novel integrated atmosphere ocean coast and overtopping drainage modeling framework based on the coupled tide surge and wave model swan adcirc to assess risk and facilitate coastal adaptation and resilience to flooding in a changing climate in this region the integrated modeling system was validated against the field observations of water level wave height and period during the january 2015 north american blizzard the water level collected by a sensor in the avenues basin behind the seawall in scituate massachusetts were combined with the basin relationship between basin area and water level given by the usgs lidar data to obtain the field measurements of wave overtopping water volume in order to verify the model predictions at the storm peak the significant wave height was increased by 0 7 m at the coast by tide and surge the wave setup along the coast varied from 0 1 m to 0 25 m depending on the coastline geometry the interaction between tide surge and waves increased the wave overtopping rate by five folds mainly due to the increased wave height at the toe of the seawall the wave overtopping discharge would approximately double in an intermediate sea level rise scenario of 0 36 m by 2050 for a storm like the january 2015 north american blizzard the wave overtopping discharge would increase by 1 5 times if the seawall crest elevation was raised by the same amount as sea level rise as an adaptation strategy an increase of 0 9 m in the seawall crest elevation instead of 0 6 m currently planned by the town is required to bring the wave overtopping discharge to the current level under a 0 36 m sea level rise scenario this result is primarily due to larger waves arriving at the seawall without breaking in the presence of larger water depth

storm tide simulation in the chesapeake bay using an unstructured grid model abstract hurricane isabel made landfall near drum inlet north carolina on september 18 2003 utc 17 00 although it was classified as only a category 2 storm saffir simpson scale hurricane isabel had a significant impact on the chesapeake bay with a 1 5 2 0 m storm surge above mean sea level and was dubbed the 100 year storm a high resolution unstructured grid model untrim was applied to simulate storm tide in the chesapeake bay the application of an unstructured grid in the bay offers the greatest flexibilities in representing complex estuarine geometry near the coast and encompassing a large modeling domain necessary for storm surge simulation the resulting mesh has a total of 239 541 surface elements the model was forced by 9 tidal harmonic constituents at the open boundary and a wind field generated by a parametric wind model a hindcast simulation of hurricane isabel captures both peak storm tide and surge evolution in various sites of the bay model diagnostic studies indicate that the high surge occurring in the upper bay regions was mainly caused by the forced southerly wind whereas the offshore surge and both the northeasterly and southeasterly winds influenced the lower bay region more significantly

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

u s ioos coastal and ocean modeling testbed evaluation of tide wave and hurricane surge response sensitivities to mesh resolution and friction in the gulf of mexico 1 this paper investigates model response sensitivities to mesh resolution topographical details bottom friction formulations the interaction of wind waves and circulation and nonlinear advection on tidal and hurricane surge and wave processes at the basin shelf wetland and coastal channel scales within the gulf of mexico tides in the gulf of mexico are modestly energetic processes whereas hurricane surge and waves are highly energetic the unstructured mesh coupled wind wave and circulation modeling system swan adcirc is implemented to generate modeled tidal harmonic constituents and hurricane waves and surge for a hurricane ike 2008 hindcast in the open ocean mesh resolution requirements are less stringent in achieving accurate tidal signals or matching hurricane surge and wave responses however coarser resolution or the absence of intertidal zones decreases accuracy along protected nearshore and inland coastal areas due to improper conveyance and or lateral attenuation bottom friction formulations are shown to have little impact on tidal signal accuracy but hurricane surge is much more sensitive especially in shelf waters where development of a strong shore parallel current is essential to the development of ike s geostrophic setup the spatial and temporal contributions of wave radiation stress gradients and nonlinear advection were charted for ike nonlinear advection improves model performance by capturing an additional 10 20 cm of geostrophic setup and increasing resonant cross shelf waves by 30 40 cm wave radiation stress gradients improve performance at coastal stations by adding an extra 20 40 cm to water levels

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

biased isochron ages resulting from subsolidus isotope exchange a theoretical model and results abstract igneous rocks formed in the upper crust often undergo hydrothermal alteration at their time of emplacement sr may be mobile in hydrothermal solutions and sr isotopic heterogeneities can form as a result of isotopic exchange between the rock and solutions we develop a theoretical model of hydrothermal exchange which shows that data from altered samples cause two types of geological error on isochron diagrams 1 random variations which affect the fit of the regression and 2 systematic variations which cause shifts in the slope and intercept of the line inaccurate ages and initial ratios are expected even when the exchange process is purely stochastic i e when the amount of exchange varies from sample to sample and is independent of the original sr concentrations and rb sr ratios of the samples furthermore systematic errors are decoupled from random errors thus the fit of a regression is unrelated to the accuracy of age estimates we show that extensive alteration of the rb sr chemistry is not required for significant systematic errors to occur a method for selecting samples most likely to yield accurate ages even if altered is presented it depends only on chemical not isotopic criteria and is an alternative to a posteriori rejection of outliers

determination of velocity in estuary for bridge scour computations determination of the appropriate design for protection against bridge pier scour depends significantly upon the design velocities at the project site in estuaries with large influences by tides or storm surges the velocity is controlled by the dynamic change in water level these changes at the ocean boundary together with the upland runoff drive the hydrodynamics of the system because the ocean water surface is a function of the characteristics of storm surges the velocity is likewise similarly affected thus in order to determine the velocity at the proposed project site a simulation of the hydrodynamics is required using the ocean storm surge as the driving boundary condition a problem arises however in defining an appropriate method to correctly describe the ocean boundary this paper describes three methods for determining the boundary condition based upon historical and stochastic representations of tropical storms hurricanes at the ocean boundary of a particular estuary each of the three methods is applied to the indian river estuary in delaware for a simple and conservative approach the single design hydrograph method yields good results for the indian river example