on the possibility of determining rates of removal of uranium from crystalline igneous rocks using u series disequilibria 2 applicability of a u leach model to mineral separates abstract preliminary u series data are presented on the disequilibria of mineral separates of surface samples from the eye dashwa lakes granite canada most analyses of the ferromagnesian quartz plus feldspar and zircon plus sphene grains show that leaching of u has taken place such that 234 u 239 u 1 and 230 th 234 u 1 the simple leach model presented in a companion paper was used to explain these characteristic ratios it is suggested that the leach model may be used on zircon and sphere grains to estimate u leach rates and this is relevant to the suggestion of using sphene like hosts as the primary barrier in the safe disposal of nuclear waste a full analysis of u mobility within a given rock volume was not possible due to our limited data base and to our inability to monitor labile u directly

determination of recent 238u 234u and 230th mobility in granitic rocks application of a natural analogue to the high level waste repository environment abstract the isotope activity ratios 234u 238u and 230th 234u have been investigated in drill core samples up to 1300 m depth in three granitoids from australia to identify the nature of possible recent 1 0 which has generated excess 230th these enriched ground waters do not appear to have penetrated below 530 m depth and 230th excess indicates bulk loss of u the observed disequilibria indicate that significant radionuclide mobilisation has occurred in these rocks during the recent past

gravity field determination and characteristics retrospective and prospective gravimetry has had a long history using pendulums torsion balances and static spring gravimeters relative accuracy adequate for many geophysical problems was already attained by 1900 but it took another half century to build readily portable gravimeters calibration and datum definition remained problems until the 1970s when free fall absolute gravimeters were developed that now have a precision of 10 3 mgal the problems of geographic inaccessibility and field party costs notably in areas of greatest tectonic interest are now being overcome by airborne gravimetry that has already achieved accuracies of 1 3 mgal with resolutions of 10 to 20 km satellite techniques are the best way to determine the long wavelength variations of the gravity field the resolution of the models has steadily improved with the number of satellites and the precision of the observations the best current model includes tracking data from more than 30 satellites satellite altimetry and surface gravimetry and has a resolution of about 290 km harmonic degree 70 with the most recent improvements coming from doppler orbitography and radiopositioning integrated by satellite doris tracking of the spot 2 satellite and satellite laser ranging slr doris and global positioning system gps tracking of the topex poseidon satellite meanwhile radar altimetry has become the dominant technique to infer the marine geoid with a resolution of tens of kilometers or shorter similarly the gravity fields of the moon venus and mars have been determined to harmonic degrees 70 75 and 50 respectively although tracking limitations result in variations of spatial resolution modeling earth s gravity field from the abundance of precise data has become an increasingly complex task with which the development of computer capacity has kept pace contemporary solutions now entail about 10 000 parameters half of them for effects other than the fixed gravity field of earth temporal variations arising from tides have long been well modeled and nontidal changes are now being identified the improvement in gravitational models engendered corresponding advances in geophysical interpretation isostatic models were refined and expanded to account for regional thermal and tectonic histories interpretation of the long wavelength gravity field determined by satellite techniques has been mainly in terms of plate tectonics as a manifestation of mantle convection gravity has been significant in inferring that there must be a large increase in viscosity with depth most strongly from the apparent slow sinking of subducted slabs the prospects for increasing accuracy and resolution in the determination of earth s gravity field rest primarily with the development of new measurement systems airborne gravimetry is taking promising new steps using gps but significant global model improvement awaits a dedicated satellite gravimetry system and future satellite altimeter missions will do more for ocean dynamics studies than geoid improvement advances in interpretation will occur through the development of other data such as seismic tomography and larger scale computer modeling of tectonics and convection

applications of uthpb isotope systematics to the problems of radioactive waste disposal abstract concentrations of u th and pb and the isotopic composition of pb for whole rock samples of granitoids show 1 that open system behavior is nearly universal in the surface and near surface environment and 2 that elemental mobility is possible to depths of several hundred meters several identified or at least postulated factors that control u and or pb mobility include 1 the mineralogical sites for u and its daughter products 2 access of groundwater to these sites 3 the volume of circulating water and 4 the chemistry of the groundwater studies of granitic samples from peralkaline complexes in the arabian shield have shown that most samples lost less than 20 of their u during recent exposure to the near surface environment most of the u in these samples appears to be firmly bound in zircons in contrast most surface and shallow drill core samples of the granite of lankin dome granite mountains wyoming have lost 70 of their u most of the u in these samples is weakly bound in biotite and epidote family minerals the granite recovered during the illinois deep drill hole project stephenson county illinois is mineralogically similar to the granite of lankin dome but this granite lost radiogenic pb rather than u probably as a result of exposure to groundwater that had a markedly different chemistry from that in the granite mountains studies of the sherman granite wyoming and the gotemar granite southeastern sweden have shown that u and or pb mobility is greatest in and near fractured rock the greater mobility is interpreted to be the result of both a larger water rock ratio in the fractured rock and exposure to water over an increased surface area and consequently a greater number of uranium sites several types of geochemical and mineralogic data can be used to identify rock water interaction in granites however if rock samples have favorable radiogenic to common pb ratios both the amount and approximate timing of u or pb mobility can be obtained through the use of isotopic studies such information can be extremely important in the search for favorable hosts for containment of radioactive waste rocks such as the gotemar granite have undergone considerable rock water interaction most of which occurred 400 myr ago and little in recent times thus a search for zones that have experienced only a little interaction with water may provide a misleading prediction as to the ability of such zones to shield radioactive wastes from the modern biosphere from an isotopic point of view an ideal candidate for evaluation as a host rock for radioactive wastes would have the following characteristics 1 a high ratio 2 of radiogenic to common pb in order to optimize precision of the results 2 a simple two stage geologic history so that results could be interpreted without multiple working hypotheses and 3 an originally high percentage 50 of labile u so that the results would be highly sensitive to even small amount of rock water interaction these characteristics should produce rocks with marked radioactive disequilibrium in surface samples the disequilibrium should grade to radioactive equilibrium with increasing depth until zones in which water has not circulated are found extensive regions of such zones must exist because u th pb systematics of most analyzed granitoids demonstrate closed system behavior for almost all of their history except for their recent history in the near surface environment

distribution of uranium thorium and rare earth elements in the eye dashwa lakes pluton a study of some analogue elements abstract the distribution of u th and rare earth elements ree s in the eye dashwa lakes pluton near atikokan ontario is examined materials analysed include a unaltered granite b altered granite c highly altered granite adjacent to open fractures and d fracture filling minerals distinct distribution patterns are noted for u and some ree s among the four types of samples th shows higher concentrations in materials c and d compared with unaltered and altered granite the highly altered granite and some fracture fillings have greater concentrations of u th and ree s the large concentrations of u th and ree s in these materials are due to both sorption on selective materials and precipitation as mineral phases containing these elements the occurrence of uraninite bastnaesite and thorogummite coatings on fracture fillings are examples of this precipitation

joint analysis of p wave velocity and resistivity for morphology identification and quantification of gas hydrate abstract a knowledge of hydrate morphology is essential for assessing its resource potential understanding its formation and determining optimum strategies for exploitation previous studies have suggested two general morphologies of gas hydrates distributions in sediment settings pore and fracture filling however because of a lack of effective morphology identification methods it is currently difficult to quantitatively model gas hydrates here we propose a joint analysis of p wave velocity and resistivity to identify hydrate morphology and estimate hydrate saturation in a continuous depth profile first we perform numerical modeling to investigate the effects of hydrate morphology on the p wave velocity and resistivity properties of gas hydrate bearing sediments ghbs the results demonstrate that in the case of identical hydrate concentration fracture filling ghbs typically exhibit higher resistivity but lower p wave velocity than those of pore filling ghbs consequently the cross plot between these two properties are strikingly different for the two types of ghbs by comparing the cross plot of field measurements to the theoretical cross plot we hypothesize that hydrate morphology can be identified and hydrate saturations estimated we test and verify the hypothesis using the velocity and resistivity log data at different sites in china s second gas hydrate expedition in 2013 in the south china sea where the two morphologies were confirmed via core samples cross plots of field measurements agree closely with the theoretical results and gas hydrate morphologies are successfully identified the estimated hydrate saturations are comparable to measurements of pore water freshening in the majority of cases

introduction to the illinois deep hole project commonwealth edison recently drilled 3 engineering test holes in n w illinois the 3 holes uph 1 2 and 3 were 0 6 1 6 and 1 6 km deep respectively and the latter 2 penetrated nearly one kilometer of precambrian granitic basement in 1980 continuous core from all 3 holes and access to uph 3 were made available for scientific investigations through the illinois deep hole project the principal study of these holes was an integrated investigation of surface and in hole experiments plus detailed petrologic geochemical and physical property studies of the core samples 50 references

geochemical and isotopic constraints on the evolution of late paleozoic dyke swarms in west junggar xinjiang china abstract there are two sets nw and ne trending of basic to intermediate dykes emplaced into both the carboniferous volcanic sedimentary strata and undeformed plutons in the west junggar northern xinjiang china the early to middle permian nw trending dykes are mainly diorite and diorite porphyrite characterized by moderate mg 55 66 1 belonging to calc alkaline series the late permian ne trending dykes are diabase and diabase porphyritic belonging to tholeiitic series with low mg 41 4 58 3 both sets of dykes exhibit enriched liles depleted nb ta hrees and highly depleted isotopic signatures end 5 1 8 1 isr 0 70379 0 70675 trace element and isotopic data indicate that the nw trending dykes were generated by partial melting of a recently metasomatized subducted cold lithospheric mantle sclm coupled with variable assimilation of juvenile crustal materials whereas the ne trending dykes were derived from the partial melting of a similar sclm with lower degree of metasomatism than the nw trending dykes which were subsequently subjected to magma modification by the fractional crystallization of olivine pyroxene and or plagioclase the arc signatures exhibited by the trace element patterns of both sets of dykes in north xinjiang imply a recently metasomated mantle in this region formed during subduction prior to late paleozoic the mafic dyke swarms in this region are the products of phanerozoic continental growth in north xinjiang indicating widespread lithospheric extension

zinc copper and lead in mid ocean ridge basalts and the source rock control on zn pb in ocean ridge hydrothermal deposits the contents of zn cu and pb in mid ocean ridge basalts morb and the morb source rock control on zn pb in ocean ridge hydrothermal deposits are examined the values of zn cu and pb for submarine mid ocean ridge basalts morb are respectively in ppm average morb 75 75 and 0 7 west valley juan de fuca ridge jfr 87 64 and 0 5 southern jfr 120 and 0 5 and 21 n east pacific rise epr 73 78 and 0 5 values of zn pb range from about 100 240 and cu pb from 100 156 in this study zn is found to correlate positively with tio2 feo mean square of weighted deviates mswd of 1 6 for jfr basalt and inversely with mg number mswd of 3 5 therefore contrary to statements in the literature that zn should be compatible in morb zn is a mildly incompatible element and must be enriched in the glass phase relative to olivine as zn does not fit into the other major phenocryst phase plagioclase in the source of morb zn likely is most enriched in oxides spinel magnetite and titanomagnetite copper generally does not correlate well with other elements in most morb data examined when differentiation is dominated by olivine cu has a tendency to behave incompatibly e g at mg numbers 70 but overall cu shows some tendency towards being a compatible element particularly along the mid atlantic ridge a behavior presumably due to separation of sulfides in which cu but not zn is markedly enriched copper thus may be in dispersed sulfides in the source of morb ocean ridges provide important data on source rock controls for sulfide deposits because in sediment starved ridges much is known about the possible source rocks and mineralization is presently occurring in contrast to zn pb 5 in continental hot cl rich brines zn pb in the hottest sediment starved ridge black smoker hydrothermal fluids at 21 n epr is about 110 similar to local morb 145 but cu pb is closer to 30 possibly due to subsurface deposition of cu at the jfr the best value of zn pb in the hydrothermal fluids is about 175 again similar to local morb 240 but cu is very low in the fluids that are at temperatures less than 300 c the large morb like zn pb in the hottest black smoker fluids suggests a source rock control for the metals that prohibits significant galena in the black smoker deposits of sediment starved ridges in contrast exhalative deposits on sediment swamped ridges have significant galena its presence is suggestive of pb derivation from sediments an origin supported by pb isotope studies of lehuray and colleagues in 1988

the strontium and oxygen isotopic record of hydrothermal alteration of syenites from the abu khruq complex egypt hydrothermal convection initiated by emplacement of the gabbro syenite complex of abu khruq into the egyptian basement 89 ma ago systematically altered the trace element and isotopic compositions of the syenites the scale of sr transport in migrating solutions was far larger than the scale of sr isotopic equilibration within rocks as a result sr exchange was heterogeneous in the syenites an effect which can be observed on three different scales within grains of a single mineral species heterogeneities are related to grain boundaries and microfractures through which fluids migrated among minerals within rock samples heterogeneities are related to differences in susceptibility to sr alteration among samples within a single unit heterogeneous alteration is apparently related to differences in permeability close to fracture zones during the early stages of alteration radiogenic sr derived from the country rocks was added to the syenites causing small net changes in concentration 5 ppm ave some rb sr mineral isochrons from single rock samples yield the emplacement age because isotopic equilibration of this added sr sometimes occurred within rock specimens however regressions of the whole rock rb sr data yield apparent ages that are about 10 ma too old later stage alteration involved larger changes in whole rock sr concentration 45 ppm ave but had little further effect on the isotopic relationships because the sr was derived from cogenetic gabbros rather than the country rocks alterations of rb sr and sr isotopic compositions are not well correlated with changes in 18o 16o ratio because mineralogy played an important role in decoupling trace element and oxygen isotopic alteration in general the absence of such correlations for whole rock data is not diagnostic of rocks with unaltered trace element and isotopic compositions mineral scale sr isotopic heterogeneities associated with grain boundaries and microfractures may be the most unambiguous evidence of trace element mobility

oxygen isotope study of precambrian granites from the illinois deep hole project seventeen whole rock samples and 27 mineral separates from the precambrian granites in drill cores uph 1 2 and 3 have been analyzed the 180 values for quartz remain exceedingly uniform 7 6 8 6 throughout the entire length of the three cores and are indicative of their primary igneous values the 180 for the feldspars are also uniform for samples located 70 m or deeper below the unconformity 6 0 6 9 however in the upper 70 m of the granite body a gradual upward increase of 180 in the feldspars is observed to 8 8 similarly 18 quartz feldspar show typical plutonic values 1 4 2 1 in the main part of the precambrian sections except in the upper 70 m where q f become progressively smaller upward until a reversed fractionation negative q f is observed in samples several meters below the unconformity the increase of 180 in the feldspars is interpreted as a result of hydrothermal alteration of the granites at low temperatures probably between 110 260 c the source of the hydrothermal fluid responsible for the alteration most likely derived from the formation waters in the upper cambrian ordovician sedimentary rocks overlying the unconformity the alteration occurred at least 135 200 m y ago when there was also a lead mobilization event the relative low primary 18o 16o ratios in the granites suggest that the granites may have derived from partial melting of low 180 metamorphic rocks in the lower crust

the erosion response to quaternary climate change quantified using uranium isotopes and in situ produced cosmogenic nuclides abstract studying how catchment erosion has responded to past climate change can help us better understand not only how landscape evolution operates but also predict the consequences of future climate change on soil resource availability recent years have seen the development of tools that allow a quantitative assessment of past changes in catchment erosion this work reviews the principles of the application of in situ produced cosmogenic nuclides and uranium isotopes to quantifying past erosion rates results highlight the role of periglacial processes and mass wasting in dictating how catchment erosion responds to climatic variability at the 10 kyr scale at the million year scale it is more difficult to untangle the role of climate and tectonics a strong coupling exists at the 10 kyr to 100 kyr scales between climatic cycles and the transfer time of regolith from source to sink this coupling reflects changes in sediment source that are either set by changes in vegetation cover at the catchment scale or by the storage of sediments on continental shelves at a larger scale although further analytical developments are required for these tools to reach their full potential existing works suggest that in the near future they will provide unprecedented quantitative insights on how soil and fluvial systems adapt to external perturbations climatic tectonic and or anthropic

direct radiometric age determination of carbonate diagenesis using u pb in secondary calcite abstract carbonate rocks and corals from the middle devonian lucas formation ontario have 238 u 204 pb values ranging from 132 to 559 and give concordant apparent 206 pb 238 u and 207 pb 235 u ages in approximate agreement with the primary age of sedimentation a carbonate rock from one part of the section gives an anomalously young age and contains patches of secondary calcite spar this spar is predominantly pale purple under cathodoluminescence cl and contains 250 600 ppb u and 15 22 ppb pb the values range from 2 10 3 10 4 making the pb very radiogenic apparent 206 pb 238 u ages range from 208 to 249 ma and 232 th 238 u values are from 0 005 to 0 040 the younger ages are correlated with an increase in orange cl and higher apparent values the age scatter is attributed to mixing of a subordinate calcite spar which infiltrated the older spar the young calcite is bright orange under cl has 7200 ppb u of 6 2 10 4 a of 0 23 and is dated at 45 ma the age of the older spar has been determined by a sequential dissolution technique the resulting u pb data can be plotted as u pb age spectra and yield lplateau dates ranging from 231 5 1 4 to 244 9 1 6 ma and an integrated 206 pb 238 u age of 238 2 1 1 ma also the sequential dissolution increases the values progressively to a5 10 4 making these ages independent of the initial pb correction the occurrence of two young ages of diagenetic calcite in these strata is consistent with the idea of recurrent fluid activity along fracture zones in subsurface sediments in the michigan basin this study shows that it is now possible to date secondary carbonate diagenesis by u pb and by inference to determine precisely the timing of activity on ancient fracture zones in carbonate rocks

considerations for u series dating of sediments insights from the flinders ranges south australia abstract uranium isotope ratios have been determined for the fine grained detrital fraction of pleistocene wilkawillina valley fill sediments four local proterozoic bedrock samples and fine grained aeolian material from a sand dune deposit of the flinders ranges south australia the aim was to quantify the comminution age i e the time elapsed since physical weathering of the bedrock and residence time of the valley fill sediments and to place tighter constraints on input parameters for the comminution age calculation despite using two independent approaches for determination of the recoil lost fraction of 234 u from the sediment weighted geometric and surface area estimates samples fail to produce realistic comminution ages and hence residence times the issues involved in the ability to determine sediment comminution ages are discussed the 234 u 238 u activity ratio of the local bedrock is not in secular equilibrium despite the bedrock being much older than 1 ma i e the timeframe for 234 u and 238 u to reach secular equilibrium in a closed system using the average flinders ranges bedrock 234 u 238 u ratio instead of an assumed 234 u 238 u activity ratio of unity for the source would significantly reduce calculated residence times this result warrants concern for future studies using the comminution approach for which a secular equilibrium source 234 u 238 u activity ratio is assumed significant input of aeolian material may modify the measured 234 u 238 u activity ratios such input may be more tightly constrained in future studies using rare earth element and radiogenic isotopic data future comminution studies would benefit from further consideration of the importance of 1 leaching lost 234 u from source rock and bulk sediment samples 2 wind deposition of fine grained material and 3 the appropriateness and robustness of sample pre treatment procedures

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

detection of gas hydrates in faults using azimuthal seismic velocity analysis vestnesa ridge w svalbard margin accepted for publication in journal of geophysical research solid earth copyright 2020 american geophysical union further reproduction or electronic distribution is not permitted

u th pb ree systematics of organic rich shales from the ca 2 15 ga sengoma argillite formation botswana evidence for oxidative continental weathering during the great oxidation event article i nfo article history accepted 23 october 2008 theredox sensitive geochemicalbehaviorof uraniumpermits the useof th uratiosasa geochemicalproxy for the oxidation state of the atmosphere during deposition due to the effects of post depositional uranium mobilityonth uratiosduringeventsinvolvingoxygenated fluids directmeasurementsofth uratiosareoften misleading even for drill core samples because both of these elements radioactively decay and produce lead isotopes the pb isotope composition may reflect the depositional th u ratio although the th u ratios induced bychangesshortlyafterdeposition maynotbe distinguished fromthe true depositional th uratios in order to effectively evaluate the time integrated th u ratio a values for the initial depositional pb isotope composition must be determined or accepted from the models for the whole earth while the timing for the rise of atmospheric oxygen is reasonably well constrained now its effect on continental weathering and ocean redox state remains poorly constrained and debated the ca 2 15 ga sengoma argillite formation of botswana contains organic rich shales deposited during the great oxidation event the slope of the

circulation from a joint gravity field solution determination of the general ocean with the development of satellite altimetry it is possible to infer the geostrophic velocity of the surface ocean currents if the geoid and the position of the satellite are known accurately errors in current geoid models and orbit computations both due primarily to errors in the earth s gravity field model have limited the use of altimeter data for this purpose the objective of this investigation is to demonstrate that altimeter data can be used in a joint solution to simultaneously estimate the quasi stationary sea surface topography and the model for the gravity field satellite tracking data from twelve satellites were used along with seasat altimeter data for the solution the estimated model of compares well at long wavelengths with the hydrographic model of covariance analysis indicates that the geoid is separable from up to degree 9 at which point geoid error is comparable to the signal of

temporal and spatial evolution of a gas hydrate bearing accretionary ridge on the oregon continental margin a seismic reflection survey on the oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin the seismic signature of gas hydrate a bottom simulating reflector bsr with negative polarity that locally cuts across stratigraphic horizons is especially well developed beneath hydrate ridge this anomalously shallow accretionary ridge was drilled during ocean drilling program leg 146 to study fluid venting in this paper we focus on the seismic data from the southern part of hydrate ridge where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system apparent disruptions of the bsr beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping a double bsr beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding on the basis of these and other observations we propose a qualitative model for the evolution of a hydrate bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor permitting construction of large ridges that are then eaten away by slumps along their margins the north to south variation in sea floor venting and subsurface seismic structure along hydrate ridge may reflect different stages in the temporal evolution of one of these ridges

the chemical and isotopic record of rock water interaction in the sherman granite wyoming and colorado chemical isotopic radiographic and rock leaching data are combined to describe the effects of rock water interactions in core samples of petrographically fresh 1 43 b y old sherman granite the data serve to identify sensitive indicators of incipient alteration and to estimate the degree pathways and timing of element mobilization unfractured core samples of sherman granite are remarkably fresh by most chemical or isotopic criteria but incipient alteration is indicated by the abundance and distribution of uranium and the degree of radioactive equilibration of uranium with its decay products uranium abundances which are out of equilibrium with lead decay products indicate remobilization of a portion 3 to 60 percent of original uranium in late phanerozoic time association of uranium with minor but pervasive secondary alteration products also indicates some remobilization the amount of apparent uranium mobility in unfractured sherman granite 3 to 60 percent is small compared to the results of similar studies of archean granites from nearby localities chemical and isotopic data evaluated as a function of core sample depth suggest a uranium migrational pathway involving near surface leaching and reconcentration at depth movement of solutions through the upper 200 ft 60 m of sherman granite is fracture controlled and brecciated granite shows more obvious petrographic chemical and isotopic evidence of alteration and multi element redistribution laboratory experiments using freshly crushed sherman granite confirm that uranium is leached in preference to elements such as si mg ca and k and that leachable uranium is situated close to the solid liquid interface perhaps as uranium along grain boundaries in crystal defects or on cleavage traces of minerals that exclude uranium from their structure

gas hydrate concentration and characteristics within hydrate ridge inferred from multicomponent seismic reflection data 1 a seismic experiment composed of streamer and ocean bottom seismometer obs surveys was conducted in the summer of 2002 at southern hydrate ridge offshore oregon to map the gas hydrate distribution within the hydrate stability zone gas hydrate concentrations within the reservoir can be estimated with p wave velocity vp however we can further constrain gas hydrate concentrations using s wave velocity vs and use vs through its relationship to vp vp vs to reveal additional details such as gas hydrate form within the matrix i e hydrate cements the grains becomes part of the matrix frame or floats in pore space both vp and vs can be derived simultaneously by inverting multicomponent seismic data in this study we use obs data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments once vp and vs are estimated they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration we model vp using an equation based on a modification of wood s equation that incorporates an appropriate rock physics model and vs using an empirical relation the gas hydrate concentration is estimated to be up to 7 of the rock volume or 12 of the pore space however vp and vs do not always fit the model simultaneously vp can vary substantially more than vs thus we conclude that a model in which higher concentrations of hydrate do not affect shear stiffness is more appropriate results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area

element mobility studies of two drill cores from the gotemar granite krakemala test site southeast sweden abstract a pilot study was carried out on two relatively deep drill cores 600 m from the gotemar granite massif in s e sweden this granite is typical of the 1400 ma anorogenic granites of the northern hemisphere samples from representative unfractured parts of the cores together with four samples taken along a profile tangential to a fracture plane at 280 m depth were investigated chemically mineralogically and isotopically the results show that after crystallisation subtle and pervasive open system modifications of the trace element chemistry of the granite took place whereas the major element chemistry and minera logical data emphasised the relative homogeneity of the gotemar granite samples investigated trace elements such as u rb and pb revealed irregular distributions which are probably the result of large scale hydrothermal alteration processes this conclusion is supported by isotopic studies which indicate that whole rock samples were open to a gain or loss of pb and possibly u at 420 171 ma ago in addition isotopic data for u pb and u ra are consistent with a recent minor loss of u the pervasive alteration and the more recent mobilisation of u are evident to a depth of at least 600 m the effects are most prevalent along major fracture zones and within the upper 250 300 m of one drill hole where a high frequency of crush zones has been noted higher fe oxidation ratios higher rb contents lower u contents and correspondingly higher th u ratios all characterise this zone

provenance ages and alteration histories of shales from the middle archean buhwa greenstone belt zimbabwe nd and pb isotopic evidence abstract shales of the ca 3 0 ga buhwa greenstone belt zimbabwe were derived from a compositionally diverse provenance whose ages determined by ion probe analyses of detrital zircons in interbedded sandstones range from 3 8 to 3 1 ga geochemical data for the shales were previously interpreted to indicate that sediments had been derived from an intensely weathered source ree concentrations in the shales were interpreted to suggest that the provenance was compositionally mixed with components of felsic tonalite and alkalic granitoid and mafic rocks sm nd and nd isotopic compositions of these rocks can be used to model initial nd isotopic ratios at the time of sedimentation endsed as well as model crustal formation ages tdm the former at the age of sedimentation range from 0 6 to 10 8 consistent with a range of provenance ages the latter range from 4 46 ga to 2 99 ga the oldest crustal formation ages up to 0 7 ga older than known detrital components are interpreted here to indicate that the sm nd system of the sediments experienced open system behavior the implied alteration would have included an increase in sm nd by about 20 25 percent probably in the form of preferential loss of nd with respect to sm the pb isotopic compositions of whole rock samples are quite radiogenic with a range of 206pb 204pb from 25 5 to 154 an array of ten samples lies scattered about a line with a 207pb 204pb 206pb 204pb slope age of about 2 73 ga five individual samples were sequentially leached to further test the timing and characteristics of this u th pb alteration event these arrays of a whole rock three leach steps and a residue also form linear pb pb arrays one is more scattered with ages ranging from 2260 360 ma to 2824 170 ma suggesting that all samples experienced a latest archean to earliest proterozoic enrichment in u pb this age range also may be the approximate age of sm nd enrichment for the shales all samples both whole rocks and leached samples lie grouped on a 208pb 204pb 206pb 204pb diagram around a line with 232th 238u 3 5 2 9 to 3 9 because of the lack of large differences in the th u of the samples through large ranges of u pb we interpret this consistency in th u to mean that the shales of the buhwa belt experienced pb loss rather than u and th gain circumstances that may be responsible for pb loss in a sedimentary basin include loss of saline fluids during basin dewatering such an event would likely have been related to folding associated with the thrusting and magmatic intrusion of the adjacent limpopo belt suggesting that uplift dewatering and geochemical and isotopic alteration can be genetically related

seismic anisotropy in gas hydrate and gas bearing sediments on the blake ridge from a walkaway vertical seismic profile 1 we present results from an analysis of anisotropy in marine sediments using walkaway vertical seismic profiles from the blake ridge offshore south carolina we encountered transverse isotropy ti with a vertical symmetry axis in a gas hydrate bearing unit of clay and claystone with thomsen parameters e 0 05 0 02 and 0 04 0 06 ti increased to e 0 16 0 04 and 0 19 0 12 in the underlying gas zone rock physics modeling suggests that the observed ti is caused by a partial alignment of clay particles rather than high velocity gas hydrate veins similarly the increase of ti in the gas zone is not caused by thin low velocity gas layers but rather we speculate by the sharp contrast between seismic properties of an anisotropic sediment frame and elongated gas bearing pore voids our results underscore the significance of anisotropy for integrating near vertical and wide angle seismic data

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

alleghenian age of the upper mississippi valley zinc lead deposit determined by rb sr dating of sphalerite mississippi valley type mvt ore deposits in north america are generally thought to have formed from warm brines which migrated from sedimentary basins to sites of deposition in sedimentary strata of the neighbouring craton long after unification of the sediments1 3 when and how these migrations occurred is not well understood but recent geological evidence suggests that continental scale brine migration can be triggered by tectonic collisions4 6 indirect evidence7 9has suggested an association of north american mvt mineralization with the late palaeozoic alleghenian ouachita orogeny recently however nakai et al 10obtained a rb sr age of 377 29myr for sphalerites from the east tennessee mvt district which they linked to the middle palaeozoic acadian orogeny here we report rb sr dates for sphalerites from the mvt type section the upper mississippi valley umv zinc lead district rb sr data from two distinct sphalerite bands from the west hayden ore body yield two independent but indistinguishable isochrons of permian age 269 6 myr and 270 4 myr consistent with a brine migration episode initiated by the alleghenian ouachita orogeny in conjunction with the previous data indicating a link to the acadian orogeny10 11 our results indicate that north american mvt deposits formed in at least two distinct tectonic episodes

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

u th pb systematics in hydrothermally altered granites from the granite mountains wyoming abstract u th pb systematics were investigated in 15 samples representing two types of deuterically altered archean granite albitized and silicified epidotized granite from the granite mountains wyoming the loss of k feldspar during both types of deuteric alteration was accompanied by an extreme reduction of pb content from roughly 40 ppm to less than 12 ppm in the most altered samples nine of the 15 samples yield anomalously young whole rock pb pb and th pb ages compared to concordia ages for zircons and to whole rock pb pb and th pb ages for samples of unaltered granite the young ages are interpreted to be the result of radiogenic pb loss during a middle proterozoic metamorphism that disturbed several isotopic systems in the unaltered granite the loss of radiogenic pb from the whole rock systems of the deuterically altered granites is most likely due to the absence of microcline in many granitic rocks potassium feldspar tends to act as a receptor for pb that has been mobilized and this effect may account for the closed system behavior of pb in whole rock samples of the unaltered granite it may also account for the apparent gain of radiogenic pb during the proterozoic by one sample which was collected at the edge of an alteration zone the deuterically altered granites may have also lost u during the proterozoic but except for two samples the dominant u loss occurred relatively recently probably during tertiary uplift and erosion the low common pb contents and rather high u and th contents of the hydrothermally altered granites might seem to indicate that these rocks are well suited for geochronologic studies in the u th pb system because such rocks should have a high percentage of radiogenic pb unfortunately open system behavior in response to post crystallization metamorphism shows that apparent ages for these types of rocks must be interpreted with caution

evaluation of pleistocene groundwater flow through fractured tuffs using a u series disequilibrium approach pahute mesa nevada usa abstract groundwater flow through fractured felsic tuffs and lavas at the nevada national security site represents the most likely mechanism for transport of radionuclides away from underground nuclear tests at pahute mesa to help evaluate fracture flow and matrix water exchange we have determined u series isotopic compositions on more than 40 drill core samples from 5 boreholes that represent discrete fracture surfaces breccia zones and interiors of unfractured core the u series approach relies on the disruption of radioactive secular equilibrium between isotopes in the uranium series decay chain due to preferential mobilization of 234 u relative to 238 u and u relative to th samples from discrete fractures were obtained by milling fracture surfaces containing thin secondary mineral coatings of clays silica fe mn oxyhydroxides and zeolite intact core interiors and breccia fragments were sampled in bulk in addition profiles of rock matrix extending 15 to 44 mm away from several fractures that show evidence of recent flow were analyzed to investigate the extent of fracture matrix water exchange samples of rock matrix have 234 u 238 u and 230 th 238 u activity ratios ar closest to radioactive secular equilibrium indicating only small amounts of groundwater penetrated unfractured matrix greater u mobility was observed in welded tuff matrix with elevated porosity and in zeolitized bedded tuff samples of brecciated core were also in secular equilibrium implying a lack of long range hydraulic connectivity in these cases samples of discrete fracture surfaces typically but not always were in radioactive disequilibrium many fractures had isotopic compositions plotting near the 230 th 234 u 1 1 line indicating a steady state balance between u input and removal along with radioactive decay numerical simulations of u series isotope evolution indicate that 0 5 to 1 million years are required to reach steady state compositions once attained disequilibrium 234 u 238 u and 230 th 238 u ar values can be maintained indefinitely as long as hydrological and geochemical processes remain stable therefore many pahute mesa fractures represent stable hydrologic pathways over million year timescales a smaller number of samples have non steady state compositions indicating transient conditions in the last several hundred thousand years in these cases u mobility is dominated by overall gains rather than losses of u

infilling history of a neoproterozoic intracratonic basin nd isotope provenance studies of the uinta mountain group western united states abstract neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the neoproterozoic uinta mountain group umg in north eastern utah and northwestern colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the proterozoic geologic history of the southern margin of the archean wyoming province in the east lobe of the uinta mountains coarse grained components of the jesse ewing canyon formation the oldest recognized formation of the umg have measured nd values of 23 to 28 and were most probably derived from archean basement of the wyoming province to the north fine grained sediment in the southern portions of the jesse ewing canyon formation and shales and quartz arenites from overlying undifferentiated umg all contain at least a component of detritus derived from proterozoic crust in the west lobe the oldest members of the umg the red castle lake gilbert peak and moosehorn lake formations and arkosic arenite comprising the northern portions of the overlying mount watson formation were also derived exclusively from archean sources however as in the east lobe the more mature sandstones that dominate the southern sections of the mount watson formation have isotopic compositions that require a significant proterozoic crust derived detrital component our data support a model in which sedimentary rocks with an archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the umg was deposited a source for these sediments within the archean wyoming province is consistent with the high light rare earth element lree and th abundances of finegrained low nd sedimentary rocks given that late archean granitic rocks within the wyoming province have similar trace element characteristics sediments with proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis the proterozoic detrital component in these sediments originated either through direct erosion of the colorado province in the neoproterozoic or through erosion of mixed archean and proterozoic crust located to the east of the umg basin that originally formed during the paleoproterozoic accretion of the colorado province to the southern margin of the wyoming craton

on the possibility of determining rates of removal of uranium from crystalline igneous rocks using u series disequilibria 1 a u leach model and its applicability to whole rock data the removal of u from granitic rocks is characterised by234u 238u 1 and 230th 234u 1 it is notable from weathered eye dashwa lakes canada granite samples and from the published data that230th 234u ratios seldom exceed a limiting value of about 2 a u leach model is discussed in detail which may account for these observations the model assumes that 230th is practically immobile and that the removal of u can then be measured against the decay rate of the unsupported230th a property of the model is that the isotope ratios reach constant values reflecting a kind of secular equilibrium and this implies that u is seldom removed faster than the decay of230th if the limited230th 234u ratios from the data correspond to secular equilibrium ratios then this allows simple calculation of u leach rates from any given subsurface sample a test for the model using235u 231pa ratios is suggested a comparison of leach rates derivable from u th pb systematics of colorado and wyoming granite samples with the corresponding estimates from the model shows that the latter are high by a factor of 10 50 data from rapidly weathered surface rinds suggests that the presence of labile u restricts the application of the model to subsurface low weathered zones

global gravity field recovery from the aristoteles satellite mission one of the primary objectives of the future aristoteles satellite mission is to map earth s gravity field with high resolution and accuracy in order to achieve this objective the aristoteles satellite will be equipped with a gravity gradiometer and a global positioning system gps receiver global gravity field error analyses have been performed for several combinations of gradiometer and gps observations these analyses indicated that the bandwidth limitation of the gradiometer prevents a stable high accuracy high resolution gravity solution if no additional information is available however with the addition of high accuracy gps observations a stable gravity field solution can be obtained a combination of the measurements acquired by the high quality gps receiver and the bandwidth limited gradiometer on board aristoteles will yield a global gravity field model with a resolution of less than 100 km and with an accuracy of better than 5 mgal for gravity anomalies and 10 cm for geoid undulations

isotopic composition of uranium and thorium in crystalline rocks the 238u 234u 230th 232th system has been investigated in 84 silicic crystalline rocks obtained from drill cores surface and near surface samples in california wyoming colorado and illinois results of these analyses displayed on ternary diagrams with apexes for 238u 234u and 230th indicate five predominant geochemical processes that affected uranium in the rock 1 bulk uranium leaching where 238u and 234u were removed with little or no fractionation 2 preferential 234u leaching by alpha recoil displacement 234u recoil loss with lesser 238u loss 3 234u recoil loss with little or no 238u loss 4 uranium assimilation where both 238u and 234u were added with present day 234u 238u activity ratios varying from 0 8 to 1 2 and 5 addition of 234u and 230th by daughter emplacement processes 234u 230th recoil gain evidence for the existence of 234u and 230th recoil gain in rocks is the most important finding of this investigation radioactive disequilibrium occurs in the majority of rocks analyzed where 234u recoil loss is the predominant process associated with incipient weathering u assimilation and 234u 230th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks relatively unfractured and petrographically fresh rocks from the uph 3 drill hole in northern illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study and they demonstrate that equilibrium during the last 0 5 m y can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock the 238u 234u 230th system can be a sensitive indicator of geologically recent u mobility and rock water interaction both in petrographically fresh core samples and in sealed fracture zones

detection of seismic anisotropy using ocean bottom seismometers a case study from the northern headwall of the storegga slide summary azimuthal seismic anisotropy has been identified from the analysis of s waves generated by p to s mode conversion in the pleistocene sediments that form the northern headwall of the storegga slide which were investigated with a seismic experiment employing a seabed array of ocean bottom seismometers and a grid of airgun shots the principal technique used to detect the anisotropy was azimuthal stacking of the radial and transverse horizontal geophone components after the application of moveout to show the variations in amplitude phase and cumulative traveltime effects of s waves ultimately providing information that identified the fast and slow s wave polarization orientations particle motion analysis was used to corroborate the results and provide further information on the magnitudes of cumulative s wave splitting a 2 d ray traced inversion of the traveltimes of pre critical p and ps arrivals provided a velocity model from which the variation with depth of vp vs and anisotropy could be compared with lithological and stratigraphic data from a borehole at the centre of the obs array increased anisotropic response was observed to be coincident with high velocity units which have high mica but low water content and are interpreted to be of glacial origin the analysis of azimuthal seismic anisotropy shows clear evidence for horizontal transverse isotropy or an orthorhombic symmetry the distribution in orientations of the fast plane of symmetry is broadly bimodal e w and ne sw across the obs array the e w group showed correlation with the headwalls of old buried slides and other faults visible within coherency attributes calculated from an accompanying 3 d seismic data set and with the strike of some of the headwalls of slides shown in multibeam bathymetry however the pattern of headwall fractures shown in the bathymetry is complicated and reticulate and the ne sw orientation is also well represented we infer that the cause of the anisotropy is the presence of vertical to sub vertical fluid filled fractures and micro cracks partially held open by high pore fluid pressure the fracture orientations are controlled primarily by the present day gravitationally induced down slope stress which is mediated by the heterogeneous nature of sub surface causing local changes in the orientation of the principal stresses at the margins of incipient or failed slides the fractures if connected are likely to increase vertical permeability within the sediment column significantly and influence the distribution of gas hydrate within the strata

anisotropic amplitude variation of the bottom simulating reflector beneath fracture filled gas hydrate deposit 1 for the first time we report the amplitude variation with angle ava pattern of bottom simulating reflectors bsrs beneath fracture filled gas hydrate deposits when the effective medium is anisotropic the common depth point cdp gathers of two mutually perpendicular multichannel seismic profiles located in the vicinity of site nghp 01 10 are appropriately processed such that they are fit for ava analysis ava analysis of the bsr shows normal incidence reflection coefficients of 0 04 to 0 11 with positive gradients of 0 04 to 0 31 indicating class iv pattern the acoustic properties from isotropic rock physics model predict class iii ava pattern which cannot explain the observed class iv ava pattern in krishna godavari basin due to the anisotropic nature of fracture filled gas hydrate deposits we modeled the observed class iv ava of the bsr by assuming that the gas hydrate bearing sediment can be represented by horizontally transversely isotropic hti medium after accounting for anisotropic wave propagation effects on bsr amplitudes the effective medium properties are estimated using backus averaging technique and the ava pattern of bsrs is modeled using the properties of overlying hti and underlying isotropy hti media with or without free gas anisotropic ava analysis of the bsr from the inline seismic profile shows 5 30 gas hydrate concentration equivalent to fracture density and the azimuth of fracture system fracture orientation with respect to the seismic profile is close to 45 free gas below the base of gas hydrate stability zone is interpreted in the vicinity of fault system f1

sediment residence times constrained by uranium series isotopes a critical appraisal of the comminution approach abstract quantifying the rates of landscape evolution in response to climate change is inhibited by the difficulty of dating the formation of continental detrital sediments we present uranium isotope data for cooper creek palaeochannel sediments from the lake eyre basin in semi arid south australia in order to attempt to determine the formation ages and hence residence times of the sediments to calculate the amount of recoil loss of 234u a key input parameter used in the comminution approach we use two suggested methods weighted geometric and surface area measurement with an incorporated fractal correction and typical assumed input parameter values found in the literature the calculated recoil loss factors and comminution ages are highly dependent on the method of recoil loss factor determination used and the chosen assumptions to appraise the ramifications of the assumptions inherent in the comminution age approach and determine individual and combined comminution age uncertainties associated to each variable monte carlo simulations were conducted for a synthetic sediment sample using a reasonable associated uncertainty for each input factor and including variations in the source rock and measured 234u 238u ratios the total combined uncertainty on comminution age in our simulation for both methods of recoil loss factor estimation can amount to 220 280 ka the modelling shows that small changes in assumed input values translate into large effects on absolute comminution age to improve the accuracy of the technique and provide meaningful absolute comminution ages much tighter constraints are required on the assumptions for input factors such as the fraction of recoil lost 234th and the initial 234u 238u ratio of the source material in order to be able to directly compare calculated comminution ages produced by different research groups the standardisation of pre treatment procedures recoil loss factor estimation and assumed input parameter values is required we suggest a set of input parameter values for such a purpose additional considerations for calculating comminution ages of sediments deposited within large semi arid drainage basins are discussed

secondary k feldspar at the precambrian paleozoic unconformity southwestern ontario the precambrian paleozoic boundary in the subsurface of southwestern ontario commonly is characterized by secondary k feldspar in the weathered and altered precambrian granitoid gneisses at the unconformity secondary k feldspar has replaced preexisting minerals and also occurs as discrete crystals of adularia overgrowths on altered minerals and microcrystalline veinlets the k feldspar is chemically pure or99 100 and has high 18o values 18 9 to 21 4 vienna standard mean ocean water features that indicate crystallization at low temperatures secondary k feldspar also occurs in cambro ordovician clastic and carbonate rocks that immediately overlie the precambrian basement k ar 453 9 to 412 8 ma and rb sr 440 50 ma dates obtained for secondary k feldspar from the precambrian host rocks suggest that its crystallization is unrelated to precambrian weathering or early diagenesis of the immediately overlying cambro ordovician strata estimated crystallization temperatures for the seconda

dynamic sea surface topography gravity and improved orbit accuracies from the direct evaluation of seasat altimeter data a gravitational model incorporating seasat altimetry surface gravimetry and satellite tracking data has been determined in terms of global spherical harmonics complete to degree and order 50 this model pgs 3337 uses altimeter data as a dynamic observation of the satellite s height above the sea surface a solution for the ocean s dynamic topography is recovered simultaneously with the orbit parameters gravity and ocean tidal terms the recovered dynamic topography reveals the global long wavelength circulation of the oceans with a resolution of 2 000 km and is very similar to the mean upper ocean dynamic height derived from historical ship observations the pgs 3337 geoid has an uncertainty of 60 cm rms globally but only 25 cm rms over the ocean because of the altimeter measurements seasat orbits determined in this solution have an estimated accuracy for the radial position of 20 cm rms the difference between the altimeter observed sea height and the geoid plus dynamic topography model is 30 cm rms contained in these residuals are the sea height variability as well as errors from the geoid orbits tidal models and altimeter range measurements this performance level is 2 to 3 times better than that achieved with previous goddard gravitational models

geodetic and oceanographic results from the analysis of 1 year of geosat data the first year of the geosat exact repeat mission has been analyzed to obtain improved orbits gravity model and representation of sea surface topography the starting point for this study were the original geophysical data records merged with orbits by haines et al 1990 computed with the gem t1 gravity model a radial orbit error model following the development of engelis 1987a was modified to account for resonant effects above degree 36 twenty four 17 day arcs were analyzed separate 17 day and a single 1 year solution for a gravity model and a spherical harmonic representation of the sea surface topography to degree 10 were found the root mean square crossover discrepancy with the a priori i e gem t1 model was 1 1 m which was reduced to 0 22 m with the 1 year solution comparison of overlaps in the arcs showed discrepancies of 1 3 m with the a priori field which were reduced to 0 07 m with the 1 year solution the determination of the degree 1 0 term in the sea surface topography was not possible in this analysis because of the high correlation of this term with an orbit error term in addition the 1 1 terms showed serious incompatibility with that implied by historical oceanographic information the final solution used here incorporated the degree 1 coefficients from oceanographic data the problem with the 1 1 coefficients may imply a small center of mass offset in the tracking station positions used in the orbit calculations or may result from the poor distribution of the available tracking stations error analysis indicated a 10 cm sea surface topography commission error to degree 10 the geoid commission error to degree 36 was 9 cm in the ocean areas the new potential coefficient model was compared with terrestrial data in a manner to that done by engelis and knudsen 1989 to show the improvement of the new model

p wave and s wave velocity structure of northern cascadia margin gas hydrates summary single channel seismic and wide angle reflection data collected in september 2005 were analysed along a 2 d profile of 10 ocean bottom seismometers obss on the continental slope region off vancouver island near odp site 889 and iodp site u1327 the objectives were to determine the shallow p wave and s wave velocity structure associated with marine gas hydrates and to estimate the hydrate concentration and distribution in the sediment pore space combined traveltime inversion of single channel and obs data produced a p wave velocity model down to the depth of the bottom simulating reflector bsr at 230 5 m below the seafloor mbsf mean velocities which increased from 1 50 km s 1 at the seafloor to 1 88 km s 1 at the bsr are in good agreement with the sonic log data from sites 889 and u1327 the increase in p wave velocity of the hydrate bearing sediments relative to a background no hydrate velocity was utilized to estimate the hydrate concentration by using effective medium theory an average concentration of 13 per cent in the interval from 120 230 mbsf was estimated from the p wave velocity model lateral continuity of the model data confirms that these average hydrate concentrations are also found around the drillsites out to distances of a few kilometres forward modelling of s waves was carried out using the data from the obs horizontal components above the bsr s wave velocities are higher than a background velocity profile based on a rock physics model and on global averages for unconsolidated sediments this increase in velocity suggests that the hydrate is distributed as part of the load bearing matrix to increase the rigidity of the sediment

age and significance of precambrian basement samples from northern illinois and adjacent states geochronologic results are reported for 12 localities where deep drilling has penetrated precambrian basement in iowa illinois indiana michigan and kentucky for the northern illinois deep drill holes uph 1 and uph 3 a zircon u pb age of 1463 10 m y was obtained rb sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event u pb ages for other localities fall in the interval 1450 1510 m y and indicate that this was a major period of igneous activity throughout the southern great lakes region rb sr results indicate that alteration was ubiquitous and that most ages in the range 1100 1400 m y probably represent disturbed systems the results obtained so far tend to support interpretations in which kentucky indiana and illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450 1500 m y old older proterozoic rocks may be absent or more deeply buried

neural network analysis of crosshole tomographic images the seismic signature of gas hydrate bearing sediments in the mackenzie delta nw canada 1 crosshole seismic experiments were conducted to study the in situ properties of gas hydrate bearing sediments ghbs in the mackenzie delta nw canada seismic tomography provided images of p velocity anisotropy and attenuation self organizing maps som are powerful neural network techniques to classify and interpret multi attribute data sets the coincident tomographic images are translated to a set of data vectors in order to train a kohonen layer the total gradient of the model vectors is determined for the trained som and a watershed segmentation algorithm is used to visualize and map the lithological clusters with well defined seismic signatures application to the mallik data reveals four major litho types 1 ghbs 2 sands 3 shale coal interlayering and 4 silt the signature of seismic p wave characteristics distinguished for the ghbs high velocities strong anisotropy and attenuation is new and can be used for new exploration strategies to map and quantify gas hydrates

the migration of radionuclides in granite a review based on natural analogues abstract the characterization of a geological formation for nuclear waste disposal should be carried out on a site by site basis however the study of analogous granitic formations all around the world gives a few general guidelines for a better understanding of elemental migration processes in this review focused on mineralogical petrological and geochemical phenomena it is shown that fractures should play a dominant role in transport through granites moreover mineralogical transformations as well as their textural and petrophysical consequences e g permeability changes should be reliably predicted finally this review confirms that water rock interactions do mobilize many elements including analogues of actinides u th ree however most alteration minerals of granite e g clay minerals carbonates exhibit a high capability for efficiently trapping these heavy elements it is concluded that natural analogues are very useful for specifying the appropriate approach for the characterization of a geological site they have also proved essential for developing and validating geochemical models of mass transfers around a repository in granite

226ra 238u and 228th 228ra disequilibrium as weathering indices in beach sand sediments associated with granitoids from cyclades greece abstract radioactive secular disequilibrium measured as the absolute difference from 1 of the 226ra 238u and 228ra 228th ratios have been used to determine weathering extent twenty eight samples from beach sands of cyclades islands all of them being the weathering products of the local granitic plutons are studied for their secular disequilibrium in both 238u and 232th series 226ra 238u and 228th 228ra respectively the results are compared to weathering indices already in use worldwide and suggest a significant linear correlation of u series disequilibrium with wip alkaline and alkaline earth element leaching and pwi silica leaching for the beach sands studied and significant non linear correlations with cia chemical index of alteration and pia plagioclase index of alteration considering only the heavy mineral enriched samples significant correlations exist between u series disequilibrium and cia ciw chemical index of weathering and pia therefore with an appropriate choice of the weathering index and samples both u series and weathering index can recover the extent of chemical weathering on the other hand the use of th series disequilibrium as a weathering index is limited due to 228th and 228ra geochemistry and especially due to their short radionuclide half lives u series disequilibrium can therefore be used as a relatively sensitive weathering index in sediments consistent with cia ciw and pia indices however the mineralogical composition must be considered more specifically the mineral content of the sediments should involve considerable amounts of sensitive to weathering minerals e g fe mg minerals as well as feldspars

the possible bearing of the granite of the uph deep drill holes northern illinois on the origin of mississippi valley ore deposits the pb pb th pb and u pb systems of whole rock drill core samples of the granite of uph record at least one postcrystallization event treatment of the data on a concordia plot indicate the timing of this disturbance was probably 260 35 m y ago and probably no older than 400 m y ago the upper intercept age on concordia is 1416 20 m y and the pb pb isochron age for the four least disturbed rocks plus the four potassium feldspars is 1451 19 m y these ages are in close agreement with rb sr whole rock and u pb zircon ages determined by others and should be the age of granite formation considerable pb loss is indicated for many of the shallower samples that have a more red rather than pink color probably to a factor of two to three or more as a result of the disturbance although the granite remains pb rich today 40 80 ppm in some other studies of disturbed u th pb systems the radiogenic lead has been taken up by potassium feldspars however the pb for most samples seems to have been lost from the system for the granite of uph one sample with an unusually high pb content of about 195 ppm has the most radiogenic pb of any sample but normal values of th u this sample has obviously gained radiogenic pb by more than a factor of four and confirms that pb was indeed mobilized in parts of the system within the limits of the uncertainties equal amounts of pb were lost or gained in both the u pb and th pb systems which is compatible with mobilization being restricted to lead there is some tendency however for the more affected samples to have lower th u values and higher u contents a complex model in which u is gained pb lost and 208pb lost preferentially to 207pb and 206pb cannot be ruled out u and th have somewhat different crystallographic locations and decay energies so that 208pb might be in more accessible sites than the other pb isotopes fission track mapping of the samples indicates most of the u today is located in submicroscopic sites along edges and cleavages of biotite and iron oxides so u is readily available for remobilization lead extraction with only minor u or th removal may have been accomplished by chloride rich brines lead isotope data on the granite of uph indicates that some of the pb in the ore in the nearby upper mississippi valley lead zinc district may have been derived from these granites an even better match is made with the ores of southeastern missouri although a granite like that of uph would only be one end member most likely the radiogenic end member of a mixed source curiously similar aged and heavy metal rich granites from southeastern missouri are not a good match for theckn nearby ores basement rocks such as the granite of uph must be reconsidered as an important contributor to the ores of the mississippi valley but it is unlikely that they constitute the sole source

geochemical baselines for the radioelements k u and th in the campania region italy a comparison of stream sediment geochemistry and gamma ray surveys abstract this paper presents new data on the baseline concentrations of u th and k in 2389 stream sediments over the whole campania region these data based on systematic sampling and analysis are compared with those obtained by gamma ray spectrometry surveys variations in the u th and k concentration in the surficial environment of the campania region appear to be related to bedrock lithology generally high u th and k values in stream sediments correspond well with the occurrence of volcanic rocks in the central western part of the region whereas low values are found in areas characterized by silico clastic and carbonate deposits occurring mostly in the southern and eastern part of the region gamma ray spectrometry maps show a similar pattern although the distribution of the highest radioactivity levels define more restricted areas than the ones resulting from mapping stream sediment geochemistry particularly high 40k radioactivity levels delimit all the known eruptive centers roccamonfina campi flegrei and somma vesuvius including the fissural sources of campania ignimbrites much better than u and th radioactivity one of the concerns for human health in the campania region is the total gamma radiation and rn potential related mostly to alkaline volcanics of the neapolitan volcanological province in particular geothermal activity occurring in all the campanian volcanic areas represents a potential hazard for rn gas

pb isotope evidence on the origin of the west shropsh orefield england pb isotope data on ore galenas from the west shropshire orefield show a significant spread of 20 pb 204 pb values with minor variation in 207pb 204 pb and 208pb 204 pb ratios these indicate that lead was derived from multiple sources some of them uranium enriched and incompletely mixed prior to ore deposition four possible mineralizing agents are considered circulating sea water metamorphic waters basinal brines and convecting formation waters pb isotope data exclude a circulating sea water origin for the mineralization and best support a convecting formation water mineralizing agent a model involving a single fluid tapping multiple lead sources is proposed to explain the observed pb isotope variation

dating of archean basement in northeastern wyoming and southern montana rb sr whole rock and u pb zircon ages of granite and gneiss cores from three deep drill holes extend known occurrences of archean rocks in the subsurface of northeastern wyoming and southern montana rb sr and k ar mineral ages are discordant and reflect early or middle proterozoic disturbance highly altered rocks occur in a thin zone immediately below the sub cambrian unconformity samples from a few metres deeper in the basement are much fresher but show the effects of this alteration in filled fractures and thin adjacent alteration haloes whole rock rb sr systems have retained a fair degree of integrity in spite of increased susceptibility to modification because of the disturbed mineral systems interaction of the rocks with water a few metres below the sub cambrian unconformity probably occurred for only a relatively short time fractures filled rapidly with secondary minerals such as chlorite anhydrite and carbonate to maintain a relatively impermeable crystalline basement in which the silicates and their contained isotopic systems were preserved ages of cores from two wells extend the archean basement to the nacp anomaly a possible suture related to early proterozoic tectonism these and published ages limit a possible ensimatic mobile belt to a zone 300 km wide beneath the williston basin however the possibility remains that archean crust was continuous between the wyoming age province and the superior province

u th pb systematics of morphologically characterized zircon and allanite a high resolution isotopic study of the alpine rensen pluton northern italy abstract the rensen pluton is a typical member of the tertiary calc alkaline plutonic suite structurally connected to the periadriatic lineament it intrudes the austroalpine polymetamorphic basement of the eastern alps southwest of the tauern window northern italy zircons and allanites from tonalitic quartz dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain by grain microanalytical techniques high resolution isotope analyses show that morphology and u pb systematics of zircon are highly correlated with magma evolution zircons from the main tonalitic part of the intrusion display a gradual increase of u content from early stage s type morphology to late stage g1 type morphology most of the zircon crystals include premagmatic seeds derived from old crustal sources unevolved s type zircons contain inherited components of late proterozoic early paleozoic apparent age whereas late stage g1 type zircons show inheritance of early proterozoic apparent age both types of inherited components are present in zircons from a fine grained tonalitic domain located at the center of the pluton which shows restitic structures the crustal affinity of this rock is corroborated by its relatively low e nd value 6 0 as compared to values obtained for the main tonalite 3 9 and the quartz diorite 3 5 the accuracy of cainozoic zircon ages is affected by initial radioactive disequilibrium in the u pb decay chains particularly uncertain is the fractionation behavior of protactinium which could be enriched in zircon due to the high stability of the pa 5 oxidation state because the th pb system is devoid of long lived intermediate radioactive nuclides the best approximation to the emplacement age of the rensen pluton is given by allanite th pb ages of 31 70 0 25ma 95 c l for the main tonalite and by a slightly younger age of 31 09 0 32ma for quartz diorite

widespread buried precambrian layered sequences in the u s mid continent evidence for large proterozoic depositional basins large regions of the north american mid continent are underlain by precambrian layered rocks buried beneath phanerozoic sedimentary strata south of the wichita mountains published seismic reflection profiles show a precambrian layered assemblage extending for at least 40 km in both the north south and east west directions and industry data show that it may continue 150 km to the southeast seismic reflection data in the illinois region show a precambrian layered assemblage extending 320 km in an east west direction and 200 km in a north south direction in both cases the layered rocks are as much as 12 km thick apparent sequence boundaries onlap downlap within these assemblages suggest they are parts of large depositional basins with diffractions and dipping strat due to faulting the layered sequences correlate with regions of relatively long wavelength and low amplitude magnetic anomalies the extent of this magnetic signature suggests that about 200 000 km 2 of illinois indiana and western ohio about 50 000 km 2 of southernmost oklahoma and north central texas and about 32 000 km 2 of southern missouri and northern arkansas may be underlain by similar precambrian strata drill holes indicate that the top of the mid continent precambrian basement is composed largely of silicic igneous rocks such material may comprise a large part of the layered sequences alternatively these igneous rocks could be intermixed with or underlain by nonvolcanic meta sedimentary strata the strong reflectivity of some layers suggest that minor mafic flows and or sills may also be present analysis of u pb and nd sm isotopes within the granites and rhyolites imply that the layered sequences postdate crustal formation at 1 7 2 0 ga and predate or are contemporaneous with the 1 3 1 5 ga crystallization ages of the granites and rhyolites though these layered rocks have a spatial association with igneous rocks and thus have likely been metamorphosed the possibility tha they contain precambrian hydrocarbons that escaped heating is as yet untested

representative uranium deposits and occurrences exclusive of sandstone impregnations and quartz conglomerates abstract all the known non sandstone and non quartz conglomerate types of uranium deposits and occurrences were reviewed to focus on geologic environments from which unknown large economic types might be expected one or more representative examples of each type was selected and its major occurrence characteristics identified following a new limited genetic classification potential significance was judged by capacity of the mineralization process to produce large deposits rather than by the size and grade of known examples the precedent for this approach is set by uranium exploration history exploration occurred in spurts stimulated by demand beyond the capability of known deposits and distinguished by recognition of new types of larger richer deposits which could supply the demand stagnation in the 1960 s and 1970 s of the discovery rate for conventional sandstone impregnations and quartz pebble conglomerates which contributed the bulk of uranium production illustrated the need for another innovative spurt the study encompassed the then unconventional types of uranium deposits however during the investigation exploration discovered so many large rich unconventional deposits that the connotation conventional on the production basis embraced them this included the rossing late magmatic replacement dissemination and a number of impregnation replacement masses related to proterozoic unconformities in canada and australia these types were included in the original project and are retained in this discussion but their inclusion reduces the significance precision and impact originally intended by designating the entire range of considered deposit types as unconventional accordingly the designation conventional unconventional has been removed or subdued however the principle that surprising discoveries introduce new economic types of deposits which were subtly evidenced in known occurrences and shift the designation of conventional was emphatically confirmed by both this project and concurrent exploration recognition of this principle lends potential significance to the remaining unconventional deposits and occurrences discussed their occurrence parameters may contain clues to yet other unknown types of large rich deposits however little effort was made in this paper to identify specific potentially economic exploration targets

copper and sulfur isotope ratios in paleozoic hosted mississippi valley type mineralization in wisconsin usa abstract we provide the first cu isotope data for chalcopyrite in paleozoic carbonate hosted mississippi valley type mvt mineralization along with corresponding sulfur isotopes for eastern wisconsin usa chalcopyrite in this region occurs mainly as disseminated crystals and is associated with regionally extensive dolomite hosted subeconomic mvt mineralization on the western edge of the ancestral michigan basin a narrow range in 65cu values 0 0 5 for chalcopyrite is observed along a large region 125 km of the western margin of the basin that is consistent with a homogeneous cu source in the crust with a 65cu composition near 0 along with limited redox modification during regional flow or hydrothermal precipitation processes cu and s isotope values in conjunction with existing lead strontium and sulfur isotope data from other sulfide minerals in the region are consistent with a precambrian source similar to parts of the midcontinent rift system but distinct from some other precambrian deposits in the region

the distribution of uranium and thorium in the cape columbine granite from the southwestern cape province south africa abstract the coastal outcrops of the cape columbine granite were mapped radiometrically on a 10 m grid by gamma spectrometry numerous radioelement enriched anomalies were found each a few square metres in outcrop some of the anomalies but by no means all are associated with small bodies of dequartzified granite episyenite chloritized or fluoritized granite evidently testifying to localized hydrothermal activity of considerable diversity the uranium and thorium in the relatively unaltered main granite are present in a variety of host minerals a rock forming minerals b accessory minerals including u and th minerals c secondary minerals associated with hydrothermal activity and d secondary minerals associated with weathering thin coatings of cryptocrystalline material in micro fractures within rock forming minerals may belong to either of the last mentioned two groups variations in the relatively unaltered granite are described to both primary differences in the amount of accessory minerals and the percentage of uranium hosted by secondary minerals the uranium is present in order of decreasing abundance in xenotime thorite along grain boundaries and within biotite and zircon while the thorium occurs in monazite thorite xenotime apatite and epidote along grain boundaries and within zircon and biotite the different types of alteration bodies are associated with wide variations in whole rock radio element distributions quantitative mineralogical study of one occurrence of fluoritized granite has made it possible to calculate the relative importance of local uranium host minerals zircon 30 of the uranium xenotime 13 thorite 13 along grain boundaries 9 and in biotite 9 and other rock forming minerals 4 the thorium is present in zircon 28 of the thorium monazite 20 thorite 14 biotite 9 xenotime 2 and along grain boundaries 2 the uranium was apparently liberated from primary minerals during alteration and trapped in secondary minerals causing a major radio element redistribution the redistributed uranium and thorium are more readily leachable during weathering resulting in some variation in surface samples and outcrops the demonstrated dekametre scale anomalies are nevertheless considered to be a pristine feature of the rocks

tectonic setting and provenance of the neoproterozoic uinta mountain and big cottonwood groups northern utah constraints from geochemistry nd isotopes and detrital modes abstract the neoproterozoic uinta mountain group was deposited in an east trending intracratonic rift bounded on the north by an active fault system and opening into a shallow sea on the west where the big cottonwood group was deposited in an estuary although this rift may have been associated with the early stages in the breakup of rodinia it was not an aulacogen geochemical nd isotope and detrital mode studies indicate that uinta mountain group sediments were derived from mixed archean and paleoproterozoic sources with the former dominating big cottonwood group sediments appear to have been derived predominantly from paleoproterozoic sources the archean sediment source is the wyoming craton and source rocks comprised dominantly granites enriched in th u y zr hf and ree the relative abundance of enriched granite implied by sedimentary rocks of the uinta mountain group indicates that the wyoming craton is anomalous compared to other archean cratons cia values and a cn k relationships in shales of the uinta mountain and big cottonwood groups indicate high degrees of weathering of sources probably in subtropical to tropical climates supporting a near equatorial location for southwestern laurentia at about 800 ma differences in the nd isotopic composition between the big cottonwood group and neoproterozoic sedimentary rocks in western utah and northeast nevada suggest a northwest striking uplift in northwest utah possibly ancestral to the paleozoic toole uinta arch

a marine electromagnetic survey to detect gas hydrate at hydrate ridge oregon gas hydrates are a potential energy resource and hazard for drilling and infrastructure yet estimates of global volume vary by over three orders of magnitude hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations a marine electromagnetic survey was carried out at hydrate ridge oregon usa with the aim of testing the use of controlled source electromagnetic csem and magnetotelluric mt methods to map gas hydrate and free gas below the gas hydrate stability zone a 2 d csem inversion supports the scenario deduced from previous seismic and drilling results which indicate two mechanisms of hydrate emplacement a transport dominated and reaction dominated regime a prominent resistive region of 2 5 4 m at a depth of about 130 mbsf near the seismic bottom simulating reflector bsr suggests that 27 to 46 per cent of the bulk volume is filled with hydrate depending on whether archie s law or the hashin strikman bounds are used this is representative of a reaction dominated regime for hydrate emplacement and where a significant low velocity zone exists based on a seismic tomography inversion suggests large quantities of free gas below the bsr electrical resistivity logging while drilling lwd data show general agreement with the csem inversion model except for a csem derived resistive region at seismic horizon a known to transport free gas into the gas hydrate stability zone inversion of mt data collected simultaneously during the csem survey provides a complimentary low resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model

seismic velocities for hydrate bearing sediments using weighted equation a weighted equation based on the three phase time average and wood equations is applied to derive a relationship between the compressional wave p wave velocity and the amount of hydrates filling the pore space the proposed theory predicts accurate p wave velocities of marine sediments in the porosity range of 40 80 and provides a practical means of estimating the amount of in situ hydrate using seismic velocity the shear s wave velocity is derived under the assumption that the p to s wave velocity ratio of the hydrated sediments is proportional to the weighted average of the p to s wave velocity ratios of the constituent components of the sediment in the case that all constituent components are known a weighted equation using multiphase time average and wood equations is possible however this study showed that a three phase equation with modified matrix velocity compensated for the clay content is sufficient to accurately predict the compressional wave velocities for the marine sediments this theory was applied to the laboratory measurements of the p and s wave velocities in permafrost samples to infer the amount of ice in the unconsolidated sediment the results are comparable to the results obtained by repeatedly applying the two phase wave scattering theory the theory predicts that the poisson s ratio of the hydrated sediments decreases as the hydrate concentration increases and the porosity decreases in consequence the amplitude versus offset avo data for the bottom simulating reflections may reveal positive negative or no avo anomalies depending on the concentration of hydrates in the sediments

fracture networks and hydrate distribution at hydrate ridge oregon abstract analysis of resistivity at the bit rab images acquired during ocean drilling program odp leg 204 to southern hydrate ridge oregon reveals changes in the distribution and orientation of resistive fractures between the different hydrologic provinces sampled fractures on the ridge crest at sites 1247 1248 and 1250 are typically high angle 60 with isotropically distributed strikes while those on the ridge flanks at sites 1244 1246 have lower dips 20 60 and strike parallel to the trend of the ridge the eastern slope basin site 1251 lacks significant fracturing these differences indicate that the minimum principal stress 3 direction rotates to horizontal at the ridge crest as a result of topographically driven extension and gravitational collapse of the ridge chaotic fracturing on the ridge crest results from the interaction of the stress state and free gas migration which is confined to high permeability sediments and fracture zones by capillary pressure constraints in contrast on the ridge flanks well ordered fractures that agree with stress orientations determined from borehole breakouts result from an obliquely dipping 3 and the predominance of aqueous rather than gaseous flow

seismic anisotropy at hydrate ridge 1 p wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments making it an excellent means to investigate gas hydrate systems however seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy the presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis however anisotropy can help reveal the distribution of hydrates here we report on the evidence of seismic anisotropy at hydrate ridge along the cascadia convergent margin we find that the south summit is anisotropic while the basin side east of south summit is isotropic anisotropy is likely caused by the hydrate veins we interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates

sediment residence times in catchments draining to the gulf of carpentaria northern australia inferred by uranium comminution dating abstract uranium u isotopes are useful for constraining the timescales of weathering and erosion processes the 234u 238u activity ratio parentheses denote activity ratio of fine grained detrital minerals is proposed to record the time elapsed since mineral grains were reduced to

seismic velocity increase and deep sea gas hydrate concentration above a bottom simulating reflector on the northern cascadia continental slope the amount of gas hydrates in the accretionary wedge sediments of the northern cascadia subduction zone off vancouver island has been estimated from multichannel seismic mcs and ocean drilling program odp data derailed semblance velocity analyses and full waveform inversion of mcs data combined with previously published odp sites 889 890 sonic log and vertical seismic profile vsp data show that sediment velocities increase downward more rapidly than the no hydrate no gas reference profile from about 1500 m s near the seafloor to a maximum of 1900 m s just above the bottom simulating reflector bsr at a subbottom depth of 224 m immediately below the bsr the mcs velocities drop to 1700 m s a low velocity of 1500 m s from the vsp data probably represents a thin layer 20 m containing free gas the difference between the reference and observed velocities is used to estimate hydrate concentration which reaches a maximum of 20 30 of the pore space above the bsr a simple interpretation of the drill core chlorinity dilution data at the odp site yields a similar hydrate concentration of 35 the estimated hydrate concentration with subbottom depth represents about 7 m 3 of hydrate per square meter of seafloor the total methane gas at stp including hydrate above the bsr and the small amount of free gas below is about 800 m 3 per square meter of seafloor or 200 tcf trillion cubic feet on the vancouver island continental slope application of the same method in the blake bahama region of the eastern u s margin indicates that the velocity enhancement and inferred hydrate concentration are very similar

use of u isotopes in exploring groundwater flow and inter aquifer leakage in the south western margin of the great artesian basin and arckaringa basin central australia abstract the distribution of uranium isotopes 238u and 234u in groundwaters of the south western margin of the great artesian basin gab australia and underlying arckaringa basin were examined using groundwater samples and a sequential extraction of aquifer sediments rock weathering the geochemical environment and recoil of daughter products control the 238u and 234u isotope distributions giving rise to large spatial variations generally the shallowest aquifer j aquifer contains groundwater with higher 238u activity concentrations and 234u 238u activity ratios close to secular equilibrium however the source input of uranium is spatially variable as intermittent recharge from ephemeral rivers passes through rocks that have already undergone extensive weathering and contain low 238u activity concentrations other locations in the j aquifer that receive little or no recharge contain higher 238u activity concentrations because uranium from localised uranium rich rocks have been leached into solution and the geochemical environment allows the uranium to be kept in solution the geochemical conditions of the deeper aquifers generally result in lower 238u activity concentrations in the groundwater accompanied by higher 234u 238u activity ratios the sequential extraction of aquifer sediments showed that recoil of 234u from the solid mineral phases into the groundwater rather than dissolution of or exchange with the groundwater accessible minerals in the aquifer caused enrichment of groundwater 234u 238u activity ratios in the boorthanna formation decay of 238u in uranium rich coatings on j aquifer sediments caused resistant phase 234u 238u activity ratio enrichment the groundwater 234u 238u activity ratio is dependent on groundwater residence time or flow rate depending on the flow path trajectory thus uranium isotope variations confirmed earlier groundwater flow interpretations based on other tracers however spatial heterogeneity and the lack of clear regional correlations made it difficult to identify recharge and inter aquifer leakage