new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

evidence of a collision between the yucatan block and mexico in the miocene we present the evidence for an anomalous southwest dipping slab in southern mexico the main evidence comes from a clear receiver function image along a seismic line across the isthmus of tehuantepec and is also supported by a previous global tomographic model the slab dips at 35 is approximately 250 km in length and appears to truncate the cocos slab at about 120 km depth we hypothesize that the slab was created by subduction of oceanic lithosphere prior to the collision of the yucatan block with mexico at approximately 12 ma this scenario would explain the chiapas fold and thrust belt as the product of this collision and its age constrains the date of the event to be in the miocene

the origin and evolution of canadian shield brines evaporation or freezing of seawater new lithium isotope and geochemical evidence from the slave craton abstract new chemical and isotopic data for deep seated calcium chloride brine from the miramar con gold mine yellowknife n w t strongly suggest that the brine salinity is of marine origin diagnostic marine properties include uniformly elevated br cl ratios typical of seawater concentrated beyond halite saturation and li br ratios 0 0254 0 0325 and 6 li compositions 32 1 to 36 3 similar to seawater 32 3 the mean 6 li for all mine water samples of 35 1 may reflect minor uptake of li by secondary minerals this interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in li but depleted in 6 li 14 7 to 15 6 relative to the unaltered metabasalt 5 4 the mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes on the one hand the devonian sedimentary record in the great slave lake region in conjunction with yellowknife brine isotopic compositions 2 h and 34 s so 4 that are similar to various devonian fluids support an evaporative origin on the other hand the na cl br cl relationship in the brine strongly suggests a cryogenic mechanism regardless of the concentrative mechanism the chemical data indicate that the yellowknife parent brine was concentrated 28 to 30 fold relative to seawater the extreme depletion of mg and enrichment of ca in the brines accompanied by sr ca ratios similar to that of seawater are accounted for by dolomitization of an aragonite rich marine sediment by the brine before infiltration into the crystalline basement rocks subsequent alteration of silicate minerals in the shield added additional ca and sr to the brine as indicated by their radiogenic 87 sr 86 sr ratios up to 0 7147 based on mineral balance calculations the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different the cryogenic evolution results in some 15 mirabilite 60 hydrohalite and 18 dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36 halite 8 gypsum 17 dolomite and 30 albite the great similarity between the calcium chloride brine from yellowknife and other such canadian shield brines indicates that they may share a common marine origin

gulf of mexico tectonic history hotspot tracks crustal boundaries and early salt distribution a late jurassic mantle plume may have generated hotspot tracks on the north american plate and the yucatan penninsula tectonic block as the gulf of mexico opened ca 150 ma the tracks are identified from deep basement structural highs that have been mapped by integrating seismic refraction and gravity data they are associated with high amplitude distinctive gravity anomalies that provide the basis for a kinematic reconstruction that restores the western ends of the hotspot tracks with a 20 clockwise rotation of the yucatan block or almost one half the total rotation required to open the gulf of mexico basin the duration of track generation is estimated to have been about 8 10 m y or almost one half the total time required to open the gulf of mexico basin prior to this rotation extension of continental crust over a 10 12 m y interval was the result of approximately 22 of counterclockwise rotation and crustal thinning autochthonous salt appears to be confined to the continental flanks of the hotspot tracks confirming that salt was deposited during continental extension and not after ocean floor had begun to form a prominent gravity anomaly along the western boundary of the basin is interpreted to be produced by a marginal ridge which was created along the ocean continent transform boundary as the basin opened the eastern flank of this marginal ridge and the northernmost easternmost and southernmost terminations of the hotspot tracks are interpreted to coincide with the oceanic continental crustal boundary in the basin dale e bird consults in the oil and gas exploration industry as a potential fields specialist since 1981 his experience in the industry includes acquisition processing interpreting and marketing geophysical data including positions with aerodat world geoscience marathon oil company digicon and aero service he earned a ph d from the university of houston in 2004 and his research interests are regional geology and tectonics and the integration and interpretation of gravity and magnetic data he is a member of several local and international geological and geophysical societies kevin burke has been a professor of geology in the university of houston since 1983 having previously lived and worked in several parts of the world his main research is in tectonics the large scale evolution of planetary lithospheres his current interests include african and asian geology and the derivation of large igneous provinces from the core mantle boundary stuart a hall is a professor of geosciences specializing in the study of gravity and magnetic fields he received a b sc honors degree in physics from the university of birmingham united kingdom 1968 and a ph d in geophysics from the university of newcastle upon tyne united kingdom 1976 his current research interests are mainly focused on the use of geophysical data to investigate the development of small ocean basins and orogenic belts he is a member of the society of exploration geophysicists american geophysical union royal astronomical society and sigma xi research society john f casey is a professor of geology and chair of the department of geosciences at the university of houston specializing in structure tectonics and geochemisty he received a b a degree in geology from the university of pennsylvania in 1975 and a ph d in geology with honors from the state university of new york albany in 1980 his research has focused on the tectonics and geochemistry of midocean ridges transform faults and ophiolite bearing orogenic belts

a paleotemperature record derived from dissolved noble gases in groundwater of the aquia aquifer maryland usa low 14c activities in groundwater of the confined part of the aquia aquifer in southeastern maryland suggest that most of this water infiltrated at least 30 000 years ago however radiocarbon contents of the dissolved inorganic carbon seem to be affected by isotopic exchange possibly with secondary calcite deposits in the formation leading to overestimated 14c ages whereas the geochemistry of the aquia aquifer complicates the application of the widely used 14c dating method the accumulation of radiogenic he seems to provide a viable alternative for establishing a chronology the quasi linear increase of he concentrations with flow distance observed in the aquia aquifer can be explained entirely by accumulation of in situ produced radiogenic he u and th concentrations in aquia sand were measured in order to determine the accumulation rate of 4he with sufficient confidence to establish a he time scale concentrations of dissolved atmospheric noble gases were used to derive mean annual ground temperatures at the time of infiltration these noble gas temperatures ngts clearly show the presence of water that infiltrated under much cooler conditions than at present ngts are correlated with chloride concentrations corroborating the hypothesis that chloride variations in this aquifer constitute a climate signal in contrast the stable isotope ratios 180 and d do not provide a clear record of past climatic changes in the aquia aquifer and the correlation between ngts and stable isotope ratios is weak the ngt record suggests that mean annual temperatures in this midlatitude coastal site during the last glacial maximum lgm were 9 0 0 6 c colder than during the holocene this difference is slightly lower than estimates derived from pollen data for this region but considerably larger than the rather uniform cooling of about 5 c indicated by noble gas studies in more southern locations of north america the larger cooling is ascribed to the influence of the laurentide ice sheet which at its maximum extension came as close as 250 km to our study site

oceanic crustal thickness versus spreading rate a compilation of oceanic crustal thickness from seismic observations collected over the past two decades shows that the average crustal thickness away from plateaus is 6 km no systematic increase of crustal thickness with spreading rate is observed instead the data show large variations in crustal thickness at slow spreading ridges 3 8 km for half rates 30 mm yr the large variations at slow ridges and small variations at fast ridges are consistent with the results inferred from recent gravity studies of mid ocean ridges both data sets support the speculation of a transition from a 3 d structure of crustal accretion at slow ridges to a 2 d accretion pattern at fast ridges

continuous exhumation of mantle derived rocks at the southwest indian ridge for 11 million years the global mid ocean ridge system where tectonic plates diverge is traditionally thought of as the largest single volcanic feature on the earth yet wide expanses of smooth sea floor in the easternmost part of the southwest indian ridge in the indian ocean lacks the hummocky morphology that is typical for submarine volcanism at other slow spreading ridges the sea floor can extend by faulting the existing lithosphere along only one side of the ridge axis however the smooth sea floor in the easternmost southwest indian ridge also lacks the corrugated texture created by such faulting instead the sea floor is smooth on both sides of the ridge axis and is thought to be composed of altered mantle derived rocks here we use side scan sonar to image the sea floor and dredge samples to analyse the composition of two sections of the southwest indian ridge between 62 050 e and 64 400 e where the sea floor formed over the past 11 million years we show that the smooth floor is almost entirely composed of seawater altered mantle derived rocks that were brought to the surface by large detachment faults on both sides of the ridge axis faulting accommodates almost 100 of plate divergence and the detachment faults have repeatedly flipped polarity we suggest that this tectonic process could also explain the exhumation of mantle derived rocks at the magma poor margins of rifted continents

groundwater chemistry and water rock interactions at stripa groundwaters from near surface to a depth of 1232 m in the stripa granite have been sampled and analyzed for major and trace constituents the groundwater composition consists of two general types a typical recharge water of ca hco3 type 700 m depth of high ph 8 10 that reaches a maximum of 1250 mg l in total dissolved solids tds intermediate depths show mixtures of the two types that are highly fracture dependent rather than depth dependent any borehole can vary significantly and erratically in tds for either a horizontal or vertical direction the general transition from ca hco3 type to na ca cl type correlates with the depth profile for hydraulic conductivity that drops from 10 8 m s to 10 11 m s or lower thermomechanical stress from heater experiments clearly shows an effect on the groundwater composition that could be caused by changing flow paths leakage of fluid inclusions or both dissolution and precipitation of calcite fluorite and barite aluminosilicate hydrolysis and addition of a saline source possibly fluid inclusion leakage play the major roles in defining the groundwater composition the low permeability of the stripa granite has produced a groundwater composition that appears intermediate between the dilute shallow groundwaters typical of recharge in a crystalline rock terrain and the saline waters and brines typical of cratonic shield areas at depth

saline minewaters of the keweenaw peninsula northern michigan their nature origin and relation to similar deep waters in precambrian crystalline rocks of the canadian shield

fluid inclusion evidence of basinal brines in archean basement thunder bay pb zn ba district ontario canada fluid inclusions from three quartz galena sphalerite barite calcite veins in the thunder bay district of western ontario contain liquid vapor halite and homogenize by vapor disappearance or halite dissolution at temperatures of 90 200 c cyclically frozen liquid vapor type i inclusions undergo four melting events upon gradual warming initial melting at 55 to 46 c ice disappearance at 30 2 to 25 4 c inversion of hydrohalite to halite at 8 0 to 0 7 c and halite melting at 14 0 to 56 3 c liquid vapor halite type ii inclusions behave similarly but have higher t m ice 27 2 to 21 7 c and t m halite 105 203 c scanning electron microscopy and energy dispersive analysis of fluid inclusion derived decrepitates indicate that the solutes consist of nacl cacl 2 kcl and are consistent with the low temperature phase observations in that they define two distinct populations based on cacl 2 cacl 2 nacl ratios the temperatures and compositional trends defined by the inclusion results are similar to those documented for basinal brines and from fluid inclusions in mississippi valley type ore deposits the thunder bay veins cross the basal unconformity of the middle proterozoic sibley basin and extend into archean basement granites such that the fluid inclusions results provide direct evidence that basinal waters infiltrated basement rock in western ontario the inclusion fluids and associated mineralization are thought to result either from dewatering of the sibley basin during keweenaw age rifting or from the introduction of exotic paleozoic basinal waters when the michigan basin extended over the region

geochemistry and origin of saline groundwaters in the fennoscandian shield chemical and isotopic analyses of water from drill holes and mines throughout the fennoscandian shield show that distinct layers of groundwater are present an upper layer of fresh groundwater is underlain by several sharply differentiated saline layers which may differ in salinity relative abundance of solutes and o h sr and s isotope signature saline groundwater can be classified into four major groups based on geochemistry and presumed origin brackish and saline waters from 50 200 m depth in coastal areas around the baltic sea exhibit distinct marine chemical and isotopic fingerprints modified by reactions with host rocks these waters represent relict holocene seawater inland three types of saline groundwater are observed an uppermost layer of brackish and saline water from 300 900 m depth saline water and brines from 1000 2000 m depth and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the kola peninsula u s s r electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths the salinity of all inland groundwaters is attributed predominantly to water rock interaction the main solutes are cl ca na and mg in varying proportions depending on the host rock lithology the abundance of dissolved gases increases with depth but varies from site to site the main gas components are n2 ch4 up to 87 vol and locally h2 the 13c value for methane is highly variable 25 to 46 and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of ch4 the uppermost saline water has meteoric oxygen hydrogen isotopic compositions whereas values from deeper water plot above the meteoric water line indicating considerably longer mean residence time and effective low temperature equilibration with host rocks geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin the source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict precambrian hydrothermal or metamorphic fluids

formation of saline groundwaters in the baltic region through freezing of seawater during glacial periods typical ca cl brines occur in crystalline and metamorphic rocks below freshwater horizons at various localities in sweden and finland total dissolved solids tds range in concentration between 2 and 120 gl 1 and have long been thought to derive from water rock interactions the relationships between na cl and br in these brines suggest however that they were derived from freezing of seawater during glacial periods the brines were subsequently diluted by meteoric waters and their ca mg ratio was increased through water rock interactions in the subsurface the hydrogeological model for both the formation of freeze derived marine brines and their lateral intrusion involves restricted inland marine basins in recent and subrecent polar climatic belts seawater in such basins gradually freezes in response to glaciation the solutes which concentrate in the remaining water body are residual after precipitation of a sequence of minerals which include carbonates mirabilite and hydrohalite hydraulic pressure of the growing ice sheet over the frozen seas is gradually added to the ambient hydrostatic pressure exerted on the brines this together with their increased density increases the intrusional potential of the brines as the land ice cannot exert hydraulic pressure on continental groundwater in the aquifers the balance of pressure favours deep landward intrusion of brines post glacial processes cause the subsurface dilution and replacement of the brines both by seawater and fresh waters the presence of such brines also far from present day coastal settings reflects the shifting of coastlines as a result of isostatic movements and eustatic sea level changes associated with glaciation and deglaciation

evidence of ridge propagation in the eastern gulf of mexico from integrated analysis of potential fields and seismic data abstract integrated analysis of gravity magnetic and seismic data reveals two phases of spreading in the eastern gulf of mexico gom including two distinct spreading centers suggesting a major ridge reorganization during the opening of the eastern part of the gom ridge propagation between the two spreading episodes explains the following observations 1 the drastic asymmetry in the oceanic domain of northeastern gom 2 the presence of two distinct crustal zones with dramatically different thickness and physical properties and 3 the observed seismicity within the oceanic domain that is not aligned with any known tectonic structure the initial late jurassic spreading center 160 ma resulted in thin 5 km and uniform oceanic crust with a fast compressional velocity 7 km s based on our analysis the estimated full spreading rate of this older spreading event is less than 1 cm yr the spreading regime changed in early cretaceous around 150 ma resulting in a propagation i e jump of the spreading center the new spreading episode was characterized by a change in spreading direction and increased magma supply as it produced thicker up to 9 km oceanic crust with a typical two layered structure despite the increase in magmatic material the full rate of this younger spreading event estimated from our analysis is only slightly faster 1 1 cm yr assuming that spreading ceased at 137 ma the later conclusion is consistent with the morphology of the spreading centers mapped by seismic data our analysis shows that recent deep crustal earthquakes in the middle of the gulf of mexico are aligned with the boundary between the two identified distinct oceanic zones referred here as a pseudofault

saline groundwaters in the canadian shield a first overview abstract the chemical and isotopic compositions of saline groundwaters and brines in the canadian shield are described the occurrence of such waters is very widespread not related to any specific rock type or mineralization and generally can be expected at depths below 1 km the most saline brines have salinities exceeding 200 g l 1 dissolved solids and their chemistries tend to suggest that in the past saline waters penetrated the canadian shield the original chemistry of these solutions was subsequently modified through strong rock water interactions and it appears that their final chemical compositions are independent of the original chemistries stable isotope analyses indicate that these waters are not simple evaporated seawater sedimentary basin brines hydrothermal solutions or residual ore forming fluids although any of these could be a precursor of the brines secondary isotope exchange reactions and the hydration of primary silicates accompanied by the formation of clay minerals could have modified not only the primary isotopic composition but may have also profoundly influenced the chemistries of these brines to be effective these processes would have had to occur in closed systems

dating ultra deep mine waters with noble gases and 36cl witwatersrand basin south africa abstract concentrations and isotopic ratios of dissolved noble gases 36cl d and 18o in water samples from the ultra deep gold mines 0 718 to 3 3 km below the surface in the witwatersrand basin south africa were investigated to quantify the dynamics of these ultra deep crustal fluids the mining activity has a significant impact on the concentrations of dissolved gases as the associated pressure release causes the degassing of the fissure water the observed under saturation of the atmospheric noble gases in the fissure water samples 70 98 normalized to asw at 20 c and 1013 mbar is reproduced by a model that considers diffusive degassing and solubility equilibration with a gas phase at sampling temperature corrections for degassing result in 4he concentrations as high as 1 55 10 1cm3stp4he g 1 40ar 36ar ranging between 806 and 10331 and 134xe 132xe and 136xe 132xe ratios above 0 46 and 0 44 respectively corrected 134 136 xe 132xe and 134 136 xe 4he ratios are consistent with their production ratios whereas the nucleogenic 4he 40ar and 134 136 xe 40ar ratios generally indicate that these gases are produced in an environment with an average u th k content 2 3 times above that of crustal average in two scenarios one considering only accumulation of in situ produced noble gases the other additionally crustal flux components the model ages for 14 individual water samples range from 13 to 168 ma and from 1 to 23 ma respectively the low 36cl ratios of 4 37 10 15 and comparatively high 36cl concentrations of 8 350 10 15 atoms 36cl l 1 reflect subsurface production in secular equilibrium indicating an age in excess of 1 5 ma or 5 times the half life of 36cl in combination the results suggest residence times of the fluids in fissures in this region up to 3 3 km depth are of the order of 1 100 ma we cannot exclude the possibility of mixing and that small quantities of younger water have been mixed with the very old bulk

tropical sst preconditioning of the sh polar vortex during winter 2002 the response of the southern hemisphere sh polar atmosphere to the tropical sea surface temperature sst during the 2002 winter spring season is investigated by using a general circulation model gcm the sh stratospheric winter of 2002 was particularly unusual characterized by a weaker than normal polar vortex during the whole season it also registered at the end of september the first major warming yet observed in the sh this event is unexpected in the sh and it is supposed to be induced by a preconditioning of the polar vortex starting at the beginning of the winter atmospheric gcm experiments with prescribed sst boundary conditions are performed the sensitivity of the antarctic dynamics to the tropical sst of 2002 a year characterized by an el nino event of moderate intensity is studied and the uniqueness of the 2002 tropical oceanic condition is investigated through the comparison of the simulated response of the climatic system to 2002 and 1997 tropical sst 1997 being a year with a strong el nino event model results highlight a primary role played by the tropical sst of 2002 in the development of the peculiar characteristics of the antarctic dynamics during the winter months that appears to be a necessary condition for the generation of the anomalous destabilization of the polar vortex during the following spring results for june 2002 show a strong generation of vertically propagating waves resulting from the tropical sst that through the perturbation of the westerly jet at middle latitudes produces a preconditioning of the polar vortex by affecting the wave refraction index the particular structure of the tropical sst anomalies during the winter of 2002 is thought to have influenced the subsequent preconditioning of the stratospheric vortex

influence of deep louann structure on the evolution of the northern gulf of mexico three aspects of basement structure and rift related salt distribution have especially influenced the evolution of the deep water northern gulf of mexico 1 creation of a basement high toledo bend flexure separating a chain of interior basins from the central louann salt basin 2 segmentation of the central louann salt basin by the brazos transfer fault into eastern and central domains and 3 salt provinces formed during basin opening the toledo bend flexure was reactivated as a hinge during the cenozoic uplift of the north american craton this uplift triggered gravity gliding forming fold belts in the seaward parts of the continental margin the geometry of the toledo bend flexure influenced the position of these fold belts the brazos transfer fault separates the west sector of the study area from the central and east sectors most of the salt in the deep water northern gulf of mexico lay in the central sector which sourced most of the sigsbee salt canopy the western sector was narrower and was subdivided by the east breaks basement high splitting the callovian salt basin in two as the gulf opened created a southward thinning wedge of salt at the seaward end of the northern gulf of mexico we divide this wedge into a series of provinces on the basis of the geometry of the base of the deep salt original salt thickness influenced diapir location the geometry of the sigsbee canopy the geometry and style of later compressional fold belts and petroleum systems

origin of cacl brines in devonian formations western canada sedimentary basin abstract the majority of waters in devonian formations of the western canada sedimentary basin are high salinity ca cl brines this relatively rare type of brine is of economic interest because of its association with both oil and hydrothermal ore deposits these brines are important sources of ca2 mg2 and ore metals in sedimentary basins it is possible to generate the ca cl brines by modification of residual evaporite brines the evaporation of devonian sea water produced large quantities of gypsum and halite such as the prairie formation residual brines were not retained in the evaporating system quantities of dense halite saturated brine sufficient to account for the high salinity formation waters in the basin were lost syndepositionally the cl and br content of waters within devonian formations in the basin can be explained by dilution of the residual evaporite brines brine composition was modified as a result of density driven reflux to depths of several kilometers this placed concentrated brines in contact with precambrian basement rocks at elevated temperatures 100 300 c the most important reactions between the brine and basement rocks appear to involve albitization of feldspars further modification of the brines occured as cements formed in devonian carbonates volumetrically the most important of these is dolomite in addition some devonian carbonates were dolomitized during ascent along faults mg2 was lost from hot brines as dolomite formed

a new look at the stratigraphy of the yellowknife supergroup at yellowknife n w t implications for the age of gold bearing shear zones and archean basin evolution based on recent detailed mapping a revised stratigraphic column is proposed for the rocks of the archean yellowknife supergroup in the yellowknife greenstone belt the mafic volcanic rocks of the kam formation previously thought to represent the oldest supracrustal rocks of the belt overlap remnants of an earlier volcanic sedimentary sequence here referred to as the octopus formation as its enormous thickness makes it too unwieldy to be described as a single formation the kam formation is raised to group status and subdivided into four formations it is proposed that the kam group should replace the beaulieu group in the yellowknife area the chan formation at the base of the kam group consists of multiple gabbroic intrusions that were emplaced into a carapace of pillowed flows the intrusions locally resemble sheeted mafic dyke complexes in phanerozoic ophiolites thought to represent evidence for sea floor spreading the crestaurum formation which overlies the chan formation is characterized b

the role of seawater freezing in the formation of subsurface brines abstract several mechanisms evaporation water rock interaction ultra filtration have been suggested to explain the evolution of ubiquitous ca chloride subsurface brines in the present paper the freezing of seawater in polar regions and in even wider areas during glacial periods is proposed as an additional possible path of brine formation four detailed seawater freezing experiments to 14 c resulting in a concentration factor of about 5 were carried out and na k ca mg sr cl so4 and br were analysed in the residual brines and in the ice br and sr whose behavior during the freezing of seawater is reported here for the first time show a conservative behavior throughout the studied temperature range our data and earlier literature show that the high salinities which are common in subsurface brines 300 g l may be obtained by the removal of h2o as ice in the primary glacial environment the decrease in the na cl ratio is caused by the crystallization of mirabilite na2so4 10h2o supplemented by hydrohalite nacl 2h2o sulfate is removed both in mirabilite and by bacterial reduction the brine then migrates to the subsurface heats up under the local geothermal gradient and interacts with the adjacent rocks at this stage it may be diluted by meteoric waters its mg ca ratio decreases dolomitization and chloritization the so 4 cl ratio varies according to the local gypsum anhydrite equilibrium conditions and the ca so 4 hco 3 ratio increases as a result of dolomitization or chloritization the interaction with rocks in the subsurface may affect both the original 87 sr 86 sr and the 18 o 16 o ratios of the brine although several of the processes which lead to the formation of ca chloride brines are common for both the evaporative and the freezing models the na br cl relationship in a given brine can be used to discriminate between the two modes of brine evolution several subsurface brines from the canadian shield and one brine from finland are used as examples of the seawater freezing model and an explanation is proposed for the necessary mass production of brines in glacial environments

geochemical and isotopic evidence for a genetic link between canadian shield brines dolomitization in the western canada sedimentary basin and devonian calcium chloridic seawater hypersaline brines of marine origin deep within fractured crystalline rocks of the canadian shield are characterized by calcium chloride compositions with relatively low concentrations of magnesi

deep crustal structure in the eastern gulf of mexico we use air gun data recorded by ocean bottom seismometers to constrain the velocity structure along gulf of mexico basin opening line 4 a profile extending from the northwestern florida peninsula across the florida escarpment to the central gulf of mexico moderately thinned continental crust with a moho depth of 32 33 km average sediment thickness of 6 km and an average crustal thickness of 27 km is interpreted on the northeast end of the profile offshore florida thinned and intruded continental crust is identified over a horizontal distance of 225 km where the crustal layer thins from 25 km to 6 7 km mean seismic velocities of the crust in this region increase from 6 55 km s to 6 95 km s from northeast to southwest and are evidence for increased magmatic input as rifting developed oceanic crust with an average thickness of 5 6 5 7 km is observed over a distance of 175 km on the southwest end of the profile with an extinct spreading ridge with an axial valley morphology imaged on a coincident seismic reflection profile anomalously high upper oceanic crust velocities of 6 0 6 7 km s are interpreted as massive basalt flows and could reflect increased temperatures during emplacement integrating well seismic reflection and our seismic refraction data allow us to estimate a full spreading rate of 2 2 cm yr for seafloor spreading along the profile this indicates that oceanic crust was emplaced at a slow spreading center

geochemistry of the archean kam group yellowknife greenstone belt slave province canada abstract the geochemistry and isotope systematics of archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of archean cratons the kam group is a 10 km thick pile of submarine tholeiitic mafic and subordinate felsic volcanic rocks erupted between 2712 and 2701 ma that forms the bulk of the yellowknife greenstone belt in the dominantly granite metasedimentary slave province mafic rocks range from normal mid ocean range basalt like basalts to slightly light rare earth element enriched lree enriched but nb depleted basaltic andesites and andesites whereas dacitic to rhyodacitic felsic rocks are strongly lree enriched and highly depleted in nb the 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 newcomma

iodine 129 constraints on residence times of deep marine brines in the canadian shield canadian shield brines are characterized by high concentrations of both stable iodine and 1 2 9 i the former reflecting its concentrated but diagenetically modified seawater origin and the latter its long subsurface residence time within its host shield rocks stable iodine concentrations in the yellowknife brine are as high as 18 mg l requiring the contribution of iodine from an external marine reservoir this reservoir likely comprised organic rich marine sediments in the basin where the brine was produced diagenesis of these sediments released iodine to the brine during its infiltration into the subsurface 1 2 9 i concentrations in the yellowknife brine are as high as 3 4 x 10 8 atom l and are due to the fission of 2 3 8 u in the rock matrix and subsequent diffusion of the radiogenic 1 2 9 i into the fracture fluids this concentration is close to the predicted secular equilibrium concentration for a fluid in basaltic rock with a uranium concentration of 1 mg kg after the residence time of 80 m y required for secular equilibrium has been attained this time period is interpreted as the minimum residence time of the yellowknife brine in the shield but it may be much greater possibly dating back to middle devonian time when most of the present day shield was covered by seawater similarly high stable iodine and 1 2 9 i concentrations measured in the sudbury brine support a generic ancient marine hypothesis for the origin of shield brines stable iodine and 1 2 9 i concentrations in the shallower mine waters are the result of mixing between the brine and low iodine modern meteoric recharge that is enriched in 1 2 9 i due to global fallout from nuclear fuel waste reprocessing activities

comparison of the sr isotopic signatures in brines of the canadian and fennoscandian shields abstract a synthesis of sr isotope data from shallow and deep groundwaters and brines from the fennoscandian and canadian shields is presented a salinity gradient is evident in the water with concentrations varying from approximately 1 75 g l 1 below 1500 m depth in the fennoscandian shield and from 10 up to 300 g l 1 below 650 m depth in the canadian shield strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water rock interaction in the systems in both shields the sr concentrations are enriched relative to cl defining a positive trend parallel to the seawater dilution line and indicative of sr addition through weathering processes the depth distribution for sr concentration increases strongly with increasing depth in both shields although the variation in sr isotope composition does not mirror that of sr concentrations strontium isotope compositions are presented for surface waters and groundwaters in several sites in the fennoscandian and canadian shields numerous mixing lines can be drawn reflecting water rock interaction a series of calculated lines links the surface end members surface water and shallow groundwater and the deep brines these mixing lines define a range of 87 sr 86 sr ratios for the deep brines in different selected sites all sites show a specific 87 sr 86 sr signature and the occurrence of large 87 sr 86 sr variations is site specific in both shields in canadian shield brines the sr isotope ratios clearly highlight large water rock interaction that increases the 87 sr 86 sr ratio from water that could have been of marine origin in contrast to the canadian shield groundwater does not occur in closed pockets in the fennoscandian and the well constrained 87 sr 86 sr signatures in deep brines should correspond to a large well mixed and homogeneous water reservoir whose sr isotope signature results from water rock interaction

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

exotic stable isotope compositions of saline waters and brines from the crystalline basement the overwhelming majority of groundwaters reported in literature show isotopic compositions that place them on or right of the meteoric water line mwl in a 2h vs 18o diagram a closer look to the steadily increasing data from the deeper crystalline basement reveals nevertheless that points left of the mwl are not so unusual as has been suggested during the last decades their supposed rareness seems to be due to a bias of sampling the concerned fluids develop only at greater depths and in low permeability environments which makes access to them so difficult paradoxically exotic stable isotope compositions may be the rule in a restricted depth zone in all crystalline basements with slow groundwater movements but not too high temperatures the present study takes stable isotope data on saline fluids from the deep french granitic basement as a starting point in order to situate the french data from the vienne region in a more general context a literature review has been undertaken assembling more than 1300 stable isotope analyses of waters and brines in crystalline rocks from stable cratons and mountain belts world wide the french fluids show some particular features with respect to the majority of other studies as their major ion composition close to seawater or a significant isotopic shift at relatively low salinities detailed knowledge on the hydrological and chemical history of the site allowed to develop a conceptual model of isotopic evolution by interaction of fracture fluids with fracture minerals this model considers interactions of the last fluids flushing the system marine water unaltered evaporated or diluted with previously precipitated mineral phases and the neoformation of the last generation of fracture minerals the model has been tested quantitatively for several combinations of dissolution precipitation reactions involving formation of carbonates clay minerals and fe hydroxides most of the modelled scenarios would leave the residual liquid phase depleted in 18o and either enriched or depleted in 2h and produce waters with isotopic signatures corresponding to the observed 2h vs 18o correlation it is therefore suggested that the general phenomenon of an isotopic shift in deep fissured silicate rocks which seems to be rather independent of lithology may result from a combination of different reaction mechanisms implying not solely the rock matrix but also the different generations of fracture minerals

upper crustal velocity structure beneath the central lucky strike segment from seismic refraction measurements we present a three dimensional velocity model of the upper crust around the central volcano of the lucky strike segment mid atlantic ridge the model constructed from a 3 d array of air gun shots 37 5 m spacing along line and 100 m between lines to ocean bottom seismometers fired during a 3 d seismic reflection survey shows an off axis velocity increase 1 km s a low velocity region within the median valley and a low velocity anomaly underneath the lucky strike volcano our observations indicate a porosity decrease of 1 9 corresponding to a velocity increase of 0 5 1 km s over a distance of 8 km from the ridge axis 0 7 ma and a porosity decrease of 4 11 corresponding to a velocity increase of 2 km s between a depth of 0 5 and 1 75 km below seafloor a sinusoidal variation in the traveltime residuals indicates the presence of azimuthal anisotropy with cracks aligned approximately along the ridge axis we favor an interpretation in which upper crustal porosities are created by a combination of magmatic accretion lava sheeted dike boundary and active extension faults fractures and fissures the porosity variation with depth probably depends on pore space collapse hydrothermal alteration and a change of stress accommodation the off axis porosities are possibly influenced by both hydrothermal precipitation and the aging of the crust

dating very old pore waters in impermeable rocks by noble gas isotopes the sup 4 he sup 40 ar and sup 136 xe content dissolved in the pore water of sedimentary rock samples was measured on samples from borehole cores near the repository for nuclear waste in morsleben germany due to the very low permeabilities of the rock formations conventional groundwater sampling was almost impossible hence the authors developed a new sampling method for noble gases in the pore water of freshly drilled rock cores this method provides vertical noble gas profiles in high depth resolution even in impermeable rocks by application of the new technique quantitative age information of groundwater and pore water have been derived the authors find palaeowaters from the last glaciation depleted in delta d and delta sup 18 o with a sup 4 he age of about 55 kyr the high saline pore solutions below are at least 6 mio years old this has been concluded from the profiles of radiogenic sup 4 he and sup 40 ar close to diffusion in steady state and from xenon isotopes produced by spontaneous fission of sup 238 u in the rocks a sup 4 he flux of 2 center dot 10 sup minus 7 cc stp cm sup 2 yr is derived from the profile which is due to local sup 4 he production within the investigatedmore sediments less

canadian shield brine from the con mine yellowknife nt canada noble gas evidence for an evaporated palaeozoic seawater origin mixed with glacial meltwater and holocene recharge abstract dissolved noble gas concentrations were measured in high salinity 270 g l ca na cl groundwaters from the con mine yellowknife canada in an effort to discriminate between two possible origins as either a brine generated by evaporative enrichment in a paleozoic inland sea or marine water concentrated by freezing during glacial times major ion and isotope geochemistry indicate that brines from the deepest level remain relatively undisturbed by mixing with modern water introduced by mining mixing calculations are used to quantify fractions of brine glacial meltwater and modern water from this noble gas concentrations were corrected for excess air with ne and normalized to 100 brine solution over pressuring of helium and argon in the brine provide age constraints based on the accumulation of geogenic 4he and 40ar radiogenic age calculations together with the local geological history suggest brine emplacement during early palaeozoic time likely during the devonian when evaporitic inland seas existed in this region the concentrations of the atmospherically derived noble gases in the brine fraction kr 1 4e 8 xe 8 5e 10 cc stp cc h 2 o are close to atmospheric equilibrium for brine at 25 c kr 7 3e 9 xe 8 0e 10 cc stp cc h 2 o but are far lower than would be expected for closed system concentration of seawater by freezing kr 2 8e 6 xe 4 2e 7 cc stp cc h 2 o thus despite the complicated mixing history of the brine the atmospheric and geogenic noble gases provide strong evidence for an origin as air equilibrated brine from evaporated paleozoic seawater which infiltrated via density displacement through existing fractures and faults into the canadian shield

hydrogeochemistry groundwater ages and sources of salts in a granitic batholith on the canadian shield southeastern manitoba abstract the hydrogeochemistry of the lac du bonnet granitic batholith has been determined for the region of the whiteshell research area wra in southeastern manitoba canada this work forms part of the geosciences studies performed for the canadian nuclear fuel waste management program over the period 1980 1995 by atomic energy of canada limited aecl knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the canadian shield groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay rich overburden from packer isolated borehole zones intersecting fractures or fault zones in the bedrock and from boreholes in unfractured rock in aecl s underground research laboratory url eighty six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the lac du bonnet batholith in addition 28 groundwaters from piezometers in a large wetland area near the url have been sampled and analysed to determine the influence of clay rich overburden on the bedrock hydrogeochemistry analyses have been made for major and minor ions ph eh trace metals and stable and radioactive isotopes to characterise these groundwaters and relate them to their hydrogeologic regimes shallow groundwaters in the fractured bedrock are generally dilute tds 500 m na ca cl so4 waters of increasing salinity up to 50 g l with depth are found and in some fractures the waters have evolved to a ca na cl composition isotopic data indicate that these waters are warm climate and pre glacial in origin with residence times of over 1 ma pore fluids observed to drain from the unfractured rock matrix in the url facility are almost pure ca cl in composition 90 g l salinity and have a 2h 18o composition displaced well to the left of the global meteoric water line about which all other wra groundwaters lie this information indicates that these pore fluids have undergone prolonged water rock interaction and have residence times of 101 103 ma most of the deeper fracture groundwaters and pore fluids have low br cl ratios and moderate to high 34s values of dissolved so4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith these fluids may have entered the batholith during early paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western canada the hydrogeochemical data and interpretations show that below 500 m in the wra fracture hosted groundwaters are very saline reducing and old and are therefore indicative of stagnant conditions over the period of concern for nuclear waste disposal 1 ma the intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics the pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 103 ma

geochemical evidence for efficient aquifer isolation over geological timeframes aquitards layers of rock having low permeability have been suggested as potential long term reservoirs for toxic materials such as nuclear or chemical waste but information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs here we investigate the long term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the paris basin france using helium isotopes as a geochemical tracer the deeper trias sandstone aquifer which lies above the crystalline basement accumulates radiogenic 4he and primordial 3he from large regions of the crust and mantle at rates comparable to the degassing of the whole crust1 and of mid ocean ridges2 we show that the overlying carbonate dogger aquifer which is separated from the trias aquifer by an aquitard layer consisting of a 600 m succession of shales and clays is stagnant and has been extremely well isolated from the trias over the past several million years this finding together with previous studies at the centre of the paris basin3 4 shows that diffusive mass transfer across aquitards is negligible and that cross formational flow in basins takes place preferentially in faulted areas

helium isotope fluxes and groundwater ages in the dogger aquifer paris basin the dogger aquifer of the paris basin has been shown to be first order homogeneous in its helium concentration and its helium isotopic ratio although slight differences are noted in relation to specific chemical water types the helium isotopic composition of the dogger aquifer contains a small component of mantle helium that is associated with a magmatic input although the heat accompanying this magma is small constraints on the age of the overlying albien aquifer are used to calculate the flux of helium out of the dogger this constraint allows a calculation of the residence time of waters in the dogger aquifer of 4 myr 50 which is significantly longer than estimates derived from hydrologic models and hydrologic testing it is possible that the hydrologic ages and the helium ages can be reconciled by a vertically layered dogger aquifer with gradients in he salinity and fluid flow vertical profiles of fluid composition helium concentration and permeability are required in the paris basin to resolve this question

variation with spreading rate of oceanic crustal thickness and geochemistry abstract away from the influence of fracture zones hotspots and marginal basins oceanic crust exhibits remarkably uniform crustal thickness rare earth element concentrations and bulk composition at all but the most slowly spreading ridges at full spreading rates below 15 mm a however all these observed parameters show marked and abrupt changes the seismically determined crustal thicknesses decrease sharply from their average of 7 1 km as do melt thicknesses inferred from rare earth element inversions of basalts bulk element compositions of basalts show an increase in the percentage of na2o and decreases in the feo content and cao al2o3 ratio as the full spreading rate drops below 15 mm a we present a model of the melting beneath oceanic spreading centres which reproduces the observed variations with spreading rate of crustal thickness and the main geochemical changes observed in basalts changes in the volume and composition of melt generated at very slow spreading rates are caused by conductive heat loss from the mantle welling up beneath rifts we model the observed behaviour with normal mantle potential temperatures of 1300 20 c greater crustal thicknesses are generated at the north atlantic spreading centre adjacent to the iceland mantle plume suggesting mantle temperatures about 80 c hotter than normal in this area

crustal structure of a super slow spreading centre a seismic refraction study of mohns ridge 72 n summary a series of eight high resolution seismic refraction profiles from the ultra slow spreading 16 mm yr 1 full spreading rate mohns ridge in the norwegian greenland sea has been treated with modern inversion methods the profiles were shot parallel to the ridge at an off axis distance of 0 135 km corresponding to crustal ages of 0 22 ma the resulting models are constrained by synthetic seismograms and gravity modelling the crustal thickness in all profiles is well below the global average for typical oceanic crust and shows a high variability with a mean thickness of 4 0 0 5 km this is mainly due to a very thin and variable lower crustal layer layer 3 generally the crust is thicker beneath basement highs and thinner beneath basins implying local isostatic compensation the top of the basement layer 2a consists of a zone with low p wave velocities 2 5 3 0 km s 1 the mean thickness of this layer decreases with distance from the ridge beneath it lies a layer with slightly higher velocities layer 2b its thickness shows less variability along a given profile and an overall increase with age the combined average thickness of the upper two layers remains nearly constant indicating that the boundary between layer 2a and 2b may represent an alteration front upper mantle velocities are generally slow around 7 5 km s 1 for the profile directly within the rift valley a model without a third layer incorporating a constant gradient up to upper mantle velocities and a model with a moho depth inferred from neighbouring profiles and upper mantle velocity as slow as 7 2 km s 1 fit the seismic and gravity data equally well the crustal structure is not mature below the ridge these observations support previous models suggesting the presence of low densities and velocities at about 2 km below the rift axis poisson s ratios determined from converted s wave modelling are incompatible with a layer 3 consisting of purely serpentinized peridotite however a volume fraction of 10 40 per cent serpentinite cannot be ruled out

saline ground waters and brines in the canadian shield geochemical and isotopic evidence for a residual evaporite brine component abstract saline ca na cl type groundwaters and brines sampled in deep mines over an extensive area of the canadian precambrian shield have elevated br cl ratios which may indicate that the chlorinity of these waters was derived from the infiltration of residual evaporitic brines remnants of the great marine incursions of the paleozoic era boron concentrations in these waters are generally low i e 11 b 10 b ratios of these waters are significantly greater than the average value for continental crustal rocks with the highest values 4 19 approaching that of present day seawater 4 20 moreover the boron isotopic ratios generally trend to higher values with increasing chlorinity which supports the conclusion from the br cl relationship that most of the chloride in shield brines is of marine origin rather than a product of water rock interactions if this is correct crystalline rocks must then be sufficiently permeable on a regional scale to have allowed the brine to infiltrate to depths of several kilometers where it now resides

potassium and boron co depletion in canadian shield brines evidence for diagenetic interactions between marine brines and basin sediments abstract hypersaline ca cl brines of marine origin deep within the crystalline rocks of the canadian shield are typically highly depleted in potassium and boron compared to their concentrated seawater parent brine while this depletion in both solutes is not accompanied by a major alteration in their original marine b k ratios their initial marine 11 b has been lowered to 20 30 values that are intermediate between that of present day seawater 39 and typical crustal values 3 5 together these observations eliminate 3 of 4 potential k and b removal mechanisms that may have operated at various times over the long history of shield brines 1 co precipitation in late stage evaporative salts in the marine basin where the brine was formed 2 incorporation in secondary feldspars formed either within basin sediments or the host rocks and 3 in situ adsorption by secondary clay minerals on fracture surfaces rather the 11 b values together with the consistency in the depleted but similar k br and b br ratios suggest that k and b were removed from shield brines by fixation in illitic clay minerals this likely occurred during the diagenetic illitization of smectite in argillaceous marine sediments through which the brine infiltrated prior to penetrating the underlying crystalline basement rocks

87sr 86sr of brines from the fennoscandian shield a synthesis of groundwater isotopic data from the baltic sea region high salinity ca na cl brines occur in sweden and finland the complex geological history of the fennoscandian shield is reflected by the wide range of saline waters whose chemical compositions have been explained by local water rock interaction simple evolution of relict sea water or freezing of sea water during glacial periods this study deals with the present knowledge of sr isotopes in groundwater from the fennoscandian shield to better constrain the origin of their deep saline component two different mixing trends are seen between aspo island groundwater and the baltic sea the first trend links the north sea to rivers through various dilution levels in the baltic the second trend is a simple linear relationship through aspo groundwaters agreeing with a binary mixing between a baltic end member and a highly sr concentrated fluid strontium isotope compositions are presented for precipitation in finland and sweden the baltic sea and the river waters draining the fennoscandian shield and for g