deducing the age of the dense venus atmosphere abstract we show how crater size density counts may be used to help constrain the history of the venus atmosphere based on the predictions of simple but reasonable models for crater production surface erosion and the effects of atmospheric drag and breakup on incident meteors in the venus atmosphere if the atmosphere is old we may also be able to determine the importance of breakup as a mechanism for destroying incident meteors in a dense fluid in particular if the atmosphere is young the old uneroded surfaces will have crater densities upward of 10 4 km 2 and a ratio of small 4 km craters to large 128 km craters near 103 if the atmosphere is old and the breakup mechanism is dominant absolute crater densities on venus surfaces will be diminished by several orders of magnitude relative to the young atmosphere case if atmospheric drag is dominant and the atmosphere is old the absolute crater density will be lowered by perhaps an order of magnitude relative to the young atmosphere case and the ratio of small to large craters will be reduced to a value near 101 5 according to the models the comparison of crater populations on young as well as old surfaces on venus can help in distinguishing the young and old atmosphere scenarios especially since the situation may be complicated by currently undetermined erosional and tectonic processes once a large fraction of venus surface has been imaged at kilometer resolution as the voir project promises to do it could be possible to make an early determination of the age of the venus atmosphere

the geology of the terrestrial planets during the last four years our knowledge of the geology of the terrestrial planets has advanced rapidly the advances are particularly noticeable for venus and mars improved understanding of venus has come largely from the pioneer venus mission colin 1980 which has given us our first global view of the planet albeit at fairly low resolution the period was also one of almost continuous data gathering for mars as the viking orbiters and landers emplaced at the planet in 1976 continued to function

the origin of hematite in high grade iron ores based on infrared microscopy and fluid inclusion studies the example of the conceicao mine quadrilatero ferrifero brazil petrographic and textural analysis combined with fluid inclusion studies by infrared microscopy of high grade 65 fe hematite ore samples from the conceicao deposit in the northeastern part of the quadrilatero ferrifero brazil indicate a complex process of oxidation and mineralization during two orogenic events each developed under different conditions and involving distinct fluids the earliest mineralization formed massive magnetite rich orebodies under relatively reducing conditions in the early stages of the transamazonian orogeny magnetite was oxidized martitized with the development of porous hematite crystals hematite i possibly during this stage new hematite crystals were also formed from low temperature low to medium salinity fluids as indicated by two phase fluid inclusions the origin of these fluids is still uncertain but tentatively interpreted as being modified surface water the fluids were transported along normal faults and fractures during post tectonic collapse following the transamazonian orogeny 2 1 2 0 ga and creation of the dome and keel structural pattern of the quadrilatero ferrifero these solutions were also likely responsible for the initial oxidation of the iron formations and the development of hematite i subsequent uplifted hot basement rocks or post tectonic plutons were probable heat sources for the regional metamorphism and development of a granoblastic fabric of hematite ii grains in the iron formations and high grade orebodies however the ore was only partially recrystallized as several relics of the early magnetite martite and hematite are still preserved in the granular hematite ii crystals during the brasiliano pan african orogeny 0 8 0 6 ga high salinity fluids with temperatures varying from 120 to a maximum of approximately 350 c penetrated the iron formations along shear zones crystallizing initially tabular and thereafter platy hematite crystals hematite iii and specularite forming schistose orebodies quartz veins that cut across the ore and envelop specularite plates and ore fragments formed from late stage high temperature and low salinity fluids containing co2 these later fluids did not alter the ore each of these stages of mineralization produced orebodies with distinct features recurrent hydrothermal mineralization is thought to have been responsible for the development of giant high grade iron ore deposits in structurally favorable sites fold hinges with enhanced permeability and deep faults able to conduct the fluids to the surface repeatedly over time should be important targets for exploration of new resources

methane fluxes and carbonate deposits at a cold seep area of the central nile deep sea fan eastern mediterranean sea abstract high acoustic seafloor backscatter signals characterize hundreds of patches of methane derived authigenic carbonates and chemosynthetic communities associated with hydrocarbon seepage on the nile deep sea fan ndsf in the eastern mediterranean sea during a high resolution ship based multibeam survey covering a 225 km2 large seafloor area in the central province of the ndsf we identified 163 high backscatter patches at water depths between 1500 and 1800 m and investigated the source composition turnover flux and fate of emitted hydrocarbons systematic parasound single beam echosounder surveys of the water column showed hydroacoustic anomalies flares indicative of gas bubble streams above 8 of the high backscatter patches in echosounder records flares disappeared in the water column close to the upper limit of the gas hydrate stability zone located at about 1350 m water depth due to decomposition of gas hydrate skins and subsequent gas dissolution visual inspection of three high backscatter patches demonstrated that sediment cementation has led to the formation of continuous flat pavements of authigenic carbonates typically 100 to 300 m in diameter volume estimates considering results from high resolution autonomous underwater vehicle auv based multibeam mapping were used to calculate the amount of carbonate bound carbon stored in these slabs additionally the flux of methane bubbles emitted at one high backscatter patch was estimated 0 23 to 2 3 106 mol a 1 by combined auv flare mapping with visual observations by remotely operated vehicle rov another high backscatter patch characterized by single carbonate pieces which were widely distributed and interspaced with sediments inhabited by thiotrophic chemosynthetic organisms was investigated using in situ measurements with a benthic chamber and ex situ sediment core incubation and allowed for estimates of the methane consumption 0 1 to 1 106 mol a 1 and dissolved methane flux 2 to 48 106 mol a 1 our comparison of dissolved and gaseous methane fluxes as well as methane derived carbonate reservoirs demonstrates the need for quantitative assessment of these different methane escape routes and their interaction with the geo bio and hydrosphere at cold seeps

a kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at hydrate ridge cascadia margin oregon we develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at hydrate ridge on the cascadia margin of oregon that predicts how pore water chlorinity temperature and crystallized hydrate evolve after the onset of steady venting predictions are compared to observations at ocean drilling program site 1249 in the preferred model calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20m below seafloor bsf under the southern summit of hydrate ridge in similar to 650years and the vertical water flux must be less than 50kg m 2 yr hydrate accumulates more slowly between 20m bsf and the base of the hydrate stability zone where there is no free gas accumulating to observed levels of a few volume percent of hydrate in 10 5 to 10 6 years depending on the water flux that is assumed through this zone this dichotomy means that the presently observed gas venting must have been diverted to this area similar to 650years ago or be episodic and infrequent if the gas venting for the last 650years has been as observed today the latent heat of hydrate precipitation in the upper 20m of sediments would have caused the temperature to increase similar to 0 8 degrees c at similar to 20m bsf and similar to 0 2 degrees c at similar to 100m bsf this would have caused a similar to 5m rise in the elevation of the base of hydrate stability zone and decreased the rate of hydrate crystallization from 1 5kg ch4 m 2 yr 650years ago to 0 7kg ch4 m 2 yr today

multiple metal sources in the glaciomarine facies of the neoproterozoic jacadigo iron formation in the santa cruz deposit corumba brazil abstract the rapitan type banded iron formation bif in the banda alta formation fm of the neoproterozoic jacadigo group in brazil was deposited in a redox stratified marine sub basin which was strongly influenced by glacial advance retraction cycles with temporary influx of continental freshwater and upwelling metal enriched seawater from deeper anoxic parts these new finding are based on new stratigraphic whole rock geochemical and stable fe and c isotope data from the santa cruz hematite deposit near corumba mato grosso do sul where a stratigraphy of lower and upper dolomite rich and intermediate chert rich bif facies with up to three intercalated diamictites is revealed the ca mg fe mn carbonate chert hematite and chert hematite bif 30 45 wt fe of the lower dolomite rich facies shows chemical signatures consistent with well oxidized seawater i e low pr yb paas strong negative cepaas and positive gdpaas and ypaas anomalies as well as negative 13c carbonate typical for neoproterozoic glaciogenic carbonates sedimentation in a rather shallow water depth during relatively warm interglacial periods was likely influenced by abundant freshwater from fluviatile runoff and melting icebergs in such conditions abundant microbial activity accommodated co2 sequestration in carbonates as spheroids and mats and fractionated 57fe 2 6 and 1 2 in primary fe hydroxides in contrast the intermediate chert rich facies characterized by chert hematite bif 35 55 wt fe and isolated hematite chert and hematite mud recorded trace element signatures of non oxidized absence of ce anomalies and variable ypaas anomalies more metal enriched fe mn si ni zn pb u seawater thus deposition was within and below the shallow level redoxcline during ice cover colder water and isolation from sunlight reduced microbial activity and thus an almost non fractionated 57fe 0 7 to 0 0 fluid signature reveals that low temperature based on almost absence of eu anomaly in bif hydrothermal mor vents or sub seafloor alteration of mafic or felsic rocks or shales fertilized seawater with metals all results lead to the model that the jacadigo group formed during one major marine transgression regression cycle and bif facies in the banda alta fm were a response to first and second order periodic variations of the depth of the redoxcline induced by the juxtaposition of glacial advance retraction cycles active graben tectonics and glacial isostatic adjustment or eustatic water level changes the chert rich bif facies marked the maximum of transgression age and tectonic setting of the jacadigo basin remains contentious it may represent a continental back arc basin of the brasiliano collision zone 590 ma in which submarine alteration was possibly related to low temperature hydrothermal fluids moving through the active graben system as an alternative to basin wide benthic pore water flux this would place the jacadigo group to the suite of ediacarian gaskier glaciations although a relationship to the marinoan glaciation 660 635 ma mainly based on the published sedimentation age bracket of 706 587 ma is also possible

pull aparts shear fractures and pressure solution vein arrays are often composed of pull aparts which are linked by shear fractures good examples of which occur in the lower jurassic limestones of somerset southwestern england such pull apart arrays have displacement distance characteristics which are similar to fault zones with maximum displacement indicated by the largest pull apart widths near the centre of the array and with displacement decreasing towards the tips pull apart arrays usually die out into enechelon or pinnate veins evidence for pressure solution along the shear fractures which connect pull aparts include their dark and braided nature their obliquity to the displacement direction the high dihedral angles often 90 between conjugate shear fractures and the dissolution of earlier structures a range of geometries occurs with varying relative amounts of veins and pressure solution being related to varying amounts of transtension or transpression there is a general trend for an increase in the angle between vein segments and the shear fractures as contraction increases there is therefore a trend for increased pressure solution on the shear fractures in more contractional arrays the concentration of insoluble material along shear fractures has important implications for the mechanics and sealing of faults

transience and persistence of natural hydrocarbon seepage in mississippi canyon gulf of mexico analysis of the magnitude of oil discharged from natural hydrocarbon seeps can improve understanding of the carbon cycle and the gulf of mexico gom ecosystem with use of a large archive of remote sensing data in combination with geophysical and multibeam data we identified mapped and characterized natural hydrocarbon seeps in the macondo prospect region near the wreck site of the drill rig deepwater horizon dwh satellite image processing and the cluster analysis revealed locations of previously undetected seep zones including duplicate detections a total of 562 individual gas plumes were also observed in multibeam surveys in total sar imagery confirmed 52 oil producing seep zones in the study area in almost all cases gas plumes were associated with oil producing seep zones the cluster of seeps in the vicinity of lease block mc302 appeared to host the most persistent and prolific oil vents oil slicks and gas plumes observed over the dwh site were consistent with discharges of residual oil from the wreckage in contrast with highly persistent oil seeps observed in the green canyon and garden banks lease areas the seeps in the vicinity of macondo prospect were intermittent the difference in the number of seeps and the quantity of surface oil detected in green canyon was almost two orders of magnitude greater than in mississippi canyon

measurement of marine hydrocarbon seep flow through fractured rock and unconsolidated sediment abstract three turbine seep tents measured gas flux variations including an ejection at one tent several conceptual models of hydrocarbon migration were developed from the relationship between fluxes at the tents 1 a resistor capacitor network to model oil and gas flux in the seep system including an oil regulator model 2 a fracture oil deposition model where oil deposited on fracture walls progressively clogs and then blocks fractures also measured were bubble size distributions for minor and major vents that were narrow and broad respectively and varied with time due to oil flux very oily bubbles with different trajectories were observed

deep ocean field test of methane hydrate formation from a remotely operated vehicle we have observed the process of formation of clathrate hydrates of methane in experiments conducted on the remotely operated vehicle rov ventana in the deep waters of monterey bay a tank of methane gas acrylic tubes containing seawater and seawater plus various types of sediment were carried down on ventana to a depth of 910 m where methane gas was injected at the base of the acrylic tubes by bubble stream prior calculations had shown that the local hydrographic conditions gave an upper limit of 525 m for the p t boundary defining methane hydrate formation or dissociation at this site and thus our experiment took place well within the stability range for this reaction to occur hydrate formation in free seawater occurred within minutes as a buoyant mass of translucent hydrate formed at the gas water interface in a coarse sand matrix the filling of the pore spaces with hydrate turned the sand column into a solidified block which gas pressure soon lifted and ruptured in a fine grained black mud the gas flow carved out flow channels the walls of which became coated and then filled with hydrate in larger discrete masses our experiment shows that hydrate formation is rapid in natural seawater that sediment type strongly influences the patterns of hydrate formation and that the use of rov technologies permits the synthesis of large amounts of hydrate material in natural systems under a variety of conditions so that fundamental research on the stability and growth of these substances is possible

considerable methane fluxes to the atmosphere from hydrocarbon seeps in the gulf of mexico the flux of methane a greenhouse gas from submarine hydrocarbon seeps to the atmosphere is not well quantified direct measurements of methane concentrations and isotopic depth profiles in deepwater hydrocarbon plumes indicate that a significant amount of methane from deep ocean sources could reach the surface ocean the fluxes of the greenhouse gas methane from many individual sources to the atmosphere are not well constrained1 marine geological sources may be significant2 but they are poorly quantified and are not included in the intergovernmental panel on climate change budget1 previous results based on traditional indirect sampling techniques and modelling suggested bubble plumes emitted from marine seeps at depths greater than 200 m do not reach the surface mixed layer because of bubble dissolution and methane oxidation3 4 5 here we report methane concentration and isotope depth profiles from direct submersible sampling of deepwater 550 600 m hydrocarbon plumes in the gulf of mexico we show that bubble size upwelling flows and the presence of surfactants inhibit bubble dissolution and that methane oxidation is negligible consequently methane concentrations in surface waters are up to 1 000 times saturation with respect to atmospheric equilibrium we estimate that diffusive atmospheric methane fluxes from individual plumes are one to three orders of magnitude greater than estimates from shallow water seeps6 7 8 greatly expanding the depth range from which methane seep emissions should be considered significant given the widespread occurrence of deepwater seeps we suggest that current estimates of the global oceanic methane flux to the atmosphere1 may be too low

structural control and hydrothermal evolution model of unusual high grade metasandstone hosted iron deposits mesoproterozoic eastern chapada diamantina brazil abstract the ibicoara high grade hypogene hematite deposits are located in the sao francisco craton in the chapada diamantina fold thrust belt iron mineralization host rocks are quartz and arkose sandstones from the tombador formation which present planar stratification regionally three deformational phases are identified the first d1 phase developed kink with subordinate chevron folds shear zones related to flexural slip with s c structures stretch lines lx1 slickenlines and slickensides the second phase d2 is represented by dextral and sinistral reverse high angle shear zones with nnw sse and nw se orientation trends respectively high angle tension fractures with an ene wsw orientation during the third deformation phase d3 dextral and sinistral shear zones with ene wsw and ne sw orientations are developed the richest hematite domains are massive having fe2o3total contents to 98 these irregular massive domains are distributed along the primary sedimentary foliation plane parallel stratification and tangential cross bedding of d2 shear zones or tension fractures generated during d2 the irregular massive hematite domains grade laterally into platy and stringer domains in addition there are vein stockwork and breccia mineralization styles that crosscut irregular massive domains or host rock chemical analyses of hematite in irregular massive domains show that fe2o3 correlates negatively with sio2 zr th sr nb hf and ba on the other hand it correlates positively with zn v u cu co and with the sum of rare earth elements ree when normalized to the post archean australian shale paas the distribution of ree generally show negative eu anomalies and a weak enrichment of light rare earth elements lree compared to the heavy rare earth elements hree the paragenetic succession shows the existence of three groups of hematite groups 1 and 2 are mainly related to the irregular massive domains controlled by rock stratification and the s0 s2 foliation in shear zones the distinction between them is given by the finer grain size of the first generation of hematite and the presence of breccia domain in the second group the third group is related to massive vein domains for each group the hematite grain size increases successively and there is a relative increase in the hree in relation to the lree furthermore similarities between the chemical signatures of the first groups 1 and 2 hematite generation with the signatures of the host rock reveal an important influence of the chemical composition of the original rock this is not the case for the late hematite of massive domains as well as vein style hematite suggesting high fluid rock conditions the hematite hydrothermal alteration at ibicoara has structural control with dilational structures associated with d2 shear zones the host lithotype was also a contributing factor due to its porosity and permeability and the presence of sedimentary structures that favored flow of the hydrothermal fluid from shear zones replacement and direct precipitation took place and are responsible for the transfer of solutions and formation of hematite rich areas the hydrothermal fluid probably leached components from the espinhaco supergroup metasedimentary units and dikes that are regionally present and carried these during the deformational phases that inverted the paramirim aulacogen the espinhaco supergroup is characterized by a sequence of siliciclastic rocks predominantly metasandstones and felsic metavolcanic rocks sterile in iron which were deposited between the statherian and stenian periods in the chapada diamantina area the espinhaco supergroup is comprised from base to top by the serra da gameleira formation and the rio dos remedios paraguacu and chapada diamantina groups the chapada diamantina group contains the tombador formation that hosts the high grade iron deposits investigated in the present study in light of the tectonic context and existence of shear related gold and baryte hydrothermal deposits nucleated during the paramirim aulacogen inversion the chapada diamantina thrust and fold belt is considered a potential regional exploration target

detecting surface oil slick related to gas hydrate petroleum on the ocean bed of south china sea by envi asar radar data abstract for exploring the distribution of oil slick related to gas hydrate or petroleum in the northern part of south china sea and evaluating the potential of envi asar radar data for detecting oil slick this paper tries to detect the oil slick on ocean surface of some potential areas in south china sea and map oil slick from anomaly area in radar image seven surface oil slicks in qiongdongnan basin and four potential targets of oil slicks in taixi dongsha basin were detected and extracted by using eniv asar data the anomalies of the radar image representing oil slick are validated by in site sampling and laboratory analyzing the gas chromatograph gc results of the extracted oil from the water samples gathered from the sites of detected oil slick demonstrate a typical composition of oil rather than that of ocean microbial origins and total organic carbon toc concentrations in two designed vertical profiles keep steady in different depths showing that the organic contents in water column are steady from the deep area to ocean surface these results suggest the detected oil slick is a thin floating oil rather than a thick spill influencing factors to the results of oil slick detection including wind speed current small islands and capes as well as deep sea petroleum reservoirs are discussed the results in this study exhibit that extracting the anomaly signals related to oil slick related to oil gas seeps using image processing techniques from envi asar data is helpful to predict anomaly areas as potential targets for further exploration in this area

time series video analysis of bubble release processes at natural hydrocarbon seeps in the northern gulf of mexico abstract this research quantifies the rate and volume of oil and gas released from two natural seep sites in the gulf of mexico lease blocks gc600 1200 m depth and mc118 850 m depth our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate image processing techniques determined bubble type oily gaseous and mixed oily and gaseous size distribution release rate and temporal variations observation intervals ranged from 3 h to 26 d a semi automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data this method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set ups two vents at gc600 birthday candles 1 and birthday candles 2 were analyzed they released oily bubbles with an average diameter of 5 0 mm at a rate of 4 7 bubbles s 1 and 1 3 bubbles s 1 respectively approximately 1 km away within the gc600 seep site two more vents mega plume 1 and mega plume 2 were analyzed these vents released a mixture of oily and gaseous bubbles with an average diameter of 3 9 mm at a rate of 49 bubbles s 1 and 81 bubbles s 1 respectively the fifth vent at mc118 rudyville released gaseous bubbles with an average diameter of 3 0 mm at a rate of 127 bubbles s 1 pressure records at mega plume and rudyville showed a diurnal tidal cycle 24 5 h rudyville was the only vent that demonstrated any positive correlation 0 60 to the 24 5 h diurnal tidal cycle however these observations were not conclusive regarding tidal effects on bubble release

rifting events in the southern sector of the paramirim aulacogen ne brazil new geochronological data and correlations for the sao francisco congo paleocontinent abstract the evolutionary history of the paramirim aulacogen is characterized by the superposition of rifts that developed from 1770 to 675 ma in the northern serra do espinhaco fold thrust belt two units of this aulacogen were analyzed in the present study the algodao formation espinhaco supergroup and the santo onofre group sao francisco supergroup in the study area the algodao formation consists of alternating of quartzose to feldspathic metasandstones sericitic metasandstones oligomictic metaconglomerates metasandstones and quartzites laminated metapelites metarhythmites and metasiltites and quartzites in turn the santo onofre group the serra da garapa formation was subdivided into two lithofacies associations the first is composed predominantly by quartzites and hematite rich graphite rich and or manganese rich metapelites and phyllites the second lithofacies association comprises staurolite garnet quartz biotite aluminous schist and quartzites in the boqueirao formation predominates metasubarkoses and lithic metasandstones and metasiltites besides oligomictic metamicroconglomerates u pb geochronological studies la icpms were performed on detrital zircons from these units ages ranging were 2870 17 1958 18 ma and 3177 42 894 38 ma respectively the main primary sources were related to rocks from the basement of the western bahia orogeny of siderian orosirian age as well as volcanites and plutonites both mafic and felsic with ages between ca 1 7 and 0 8 ga related to the evolution of the precursor basins of the aracuai west congo orogen as secondary sources results suggest metavolcanosedimentary sequences and greenstone belts of archean and paleoproterozoic ages as well as rocks from the espinhaco supergroup and macaubas group the lithofacies of serra da garapa formation as well as the ages found in detrital zircons suggest that this formation is chronocorrelate with rio peixe bravo formation of the macaubas group together these formations and matao and duas barras formation represent the pre glacial unit of the santo onofre macaubas basin initiated in tonian some characteristics suggest that the boqueirao formation may be correlated to some of the glacial units of this basin this work suggests that the formations fazendinha and serra da vereda are excluded from the santo onofre group

experimental formation of massive hydrate deposits from accumulation of ch4 gas bubbles within synthetic and natural sediments in order for methane to be economically produced from the seafloor prediction and detection of massive hydrate deposits will be necessary in many cases hydrate samples recovered from seafloor sediments appear as veins or nodules suggesting that there are strong geologic controls on where hydrate is likely to accumulate experiments have been conducted examining massive hydrate accumulation from methane gas bubbles within natural and synthetic sediments in a large volume pressure vessel through temperature and pressure data as well as visual observations observations of hydrate growth suggest that accumulation of gas bubbles within void spaces and at sediment interfaces likely results in the formation of massive hydrate deposits methane hydrate was first observed as a thin film forming at the gas water interface of methane bubbles trapped within sediment void spaces as bubbles accumulated massive hydrate growth occurred these experiments suggest that in systems containing free methane gas bubble pathways and accumulation points likely control the location and habit of massive hydrate deposits

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

eruptions at lone star geyser yellowstone national park usa 1 energetics and eruption dynamics 1 geysers provide a natural laboratory to study multiphase eruptive processes we present results from a 4 day experiment at lone star geyser in yellowstone national park usa we simultaneously measured water discharge acoustic emissions infrared intensity and visible and infrared video to quantify the energetics and dynamics of eruptions occurring approximately every 3 h we define four phases in the eruption cycle 1 a 28 3 min phase with liquid and steam fountaining with maximum jet velocities of 16 28 m s 1 steam mass fraction of less than 0 01 intermittently choked flow and flow oscillations with periods increasing from 20 to 40 s are coincident with a decrease in jet velocity and an increase of steam fraction 2 a 26 8 min posteruption relaxation phase with no discharge from the vent infrared ir and acoustic power oscillations gliding between 30 and 40 s 3 a 59 13 min recharge period during which the geyser is quiescent and progressively refills and 4 a 69 14 min preplay period characterized by a series of 5 10 min long pulses of steam small volumes of liquid water discharge and 50

petrographic and geochemical evidence for hydrothermal evolution of the north deposit mt tom price western australia high grade iron mineralisation 65 fe in the north deposit occurs as an e w trending synclinal sheet within banded iron formation bif of the early proterozoic dales gorge member and consists of martite microplaty hematite ore three hypogene alteration zones between unmineralised bif and high grade iron ore are observed 1 distal magnetite siderite iron silicate 2 intermediate hematite ankerite magnetite and 3 proximal martite microplaty hematite apatite alteration zones fluid inclusions trapped in ankerite within ankerite hematite veins in the hematite ankerite magnetite alteration zone revealed mostly h2o cacl2 pseudosecondary and secondary inclusions with salinities of 23 9 1 5 1 n 38 and 24 4 1 5 1 n 66 eq wt cacl2 respectively pseudosecondary inclusions homogenised at 253 59 9 c 1 n 34 and secondary inclusions at 117 10 0 c 1 n 66 the decrepitation of pseudosecondary inclusions above 350 c suggests that their trapping temperatures are likely to be higher i e 400 c hypogene siderite and ankerite from magnetite siderite iron silicate and hematite ankerite magnetite alteration zones have similar oxygen isotope compositions but increasingly enriched carbon isotopes from magnetite siderite iron silicate alteration 8 8 0 7 1 n 17 to hematite ankerite magnetite alteration zones 4 9 2 2 1 n 17 when compared to the dolomite in the wittenoom formation 0 9 0 7 1 n 15 that underlies the deposit a two stage hydrothermal supergene model is proposed for the formation of the north deposit early 1a hypogene alteration involved the upward movement of hydrothermal cacl2 rich brines 150 250 c likely from the carbonate rich wittenoom formation 13c signature of 0 9 0 7 1 n 15 within large scale folds of the dales gorge member fluid rock reactions transformed unmineralised bif to magnetite siderite iron silicate bif with subsequent desilicification of the chert bands stage 1b hypogene alteration is characterised by an increase in temperature possibly to 400 c depleted 13c signature of 4 9 2 2 1 n 17 and the formation of hematite ankerite magnetite alteration and finally the crystallisation of microplaty hematite late stage 1c hypogene alteration involved the interaction of low temperature 120 c basinal brines with the hematite ankerite magnetite hydrothermal assemblage leaving a porous martite microplaty hematite apatite mineral assemblage stage 2 supergene enrichment in the tertiary resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay

remote sensing evaluation of geophysical anomaly sites in the outer continental slope northern gulf of mexico abstract synthetic aperture radar sar images obtained from satellites are a reliable tool for localizing natural hydrocarbon seeps for this study we used the texture classifier neural network algorithm tcnna to interpret sar data from the radarsat satellite and a geostatistical clustering analysis to compare seeps detected in 579 sar images covering the northern gulf of mexico gom geostatistical analysis results indicate that in a typical active seep formation oil vents would be found within a seep formation 2 5 km in diameter repeated observations of slicks at a given seep formation indicate that advection of rising oil in the water column causes an offset from the vent depending on water depth at 500 m the radial offset is up to 1400 m at 2000 m it is up to 3270 m observations with submersibles showed that in 100 of the cases the calculated seep formations that are matched with active oil seeps correspond to anomalies interpreted from surface amplitude maps and migration pathways in the seismic data however episodically larger releases from persistent seeps occurred and also some other seep formations showed intermittent releases our analysis indicates that active oil seeps detected with sar represent a subset of the total array of geophysical features generated by hydrocarbon migration on the northern continental slope of the gulf of mexico

replacement origin for hematite in 2 5 ga banded iron formation evidence for postdepositional oxidation of iron bearing minerals banded iron formations bifs are central to interpretations about the composition of the precambrian ocean atmosphere and biosphere hematite is an important component of many bifs and its presence has been used as evidence for the former presence of hydrous ferric oxyhydroxides that formed from the oxidation of dissolved ferrous iron in seawater however textural evidence for the origin of hematite is equivocal new petrographic results show that hematite in unmineralized bif from the ca 2 5 ga dales gorge member of the brockman iron formation hamersley group western australia including morphologies previously interpreted to represent ferric oxyhydroxide precipitates formed via fluid mediated replacement of iron silicates and iron carbonates along sedimentary layering the lateral transition from stilpnomelane and siderite rich laminae to hematite dominated laminae is interpreted to reflect progressive stages of in situ alteration of reduced mineral assemblages by oxygen bearing fluids rather than changes in the chemistry of the water column during deposition although morphologies previously ascribed to primary hematite are present they are related to mineral replacement reactions raising doubts about the petrographic criteria used to identify original hematite hematite replacement in unmineralized bif postdated deposition and possibly metamorphism and predated modern weathering from a regional perspective it appears to be a distal signature of the processes that were responsible for iron ore mineralization which involved the deep infiltration of oxygen bearing meteoric fluids the mineral replacement reactions recorded in the dales gorge member are unlikely to be unique and probably occurred in bifs elsewhere at some point in their history the observation that at least some of the hematite in unmineralized bif did not form directly from ferric oxyhydroxides implies that hematite is not a reliable proxy for the composition of the precursor sediment or the redox chemistry of the ocean the oxidation of ferrous rich phases after deposition suggests that the precursor sediments of bif originally had a more reduced bulk composition this raises the possibility that in an ocean with negligible molecular oxygen and elevated si and fe the growth of iron rich clay minerals was favored over hematite

the anatomy of shallow crustal transpressional structures insights from the archaean carajas fault zone amazon brazil abstract the carajas fault zone cfz is the most prominent structure in the e w trending carajas strike slip system casss an ancient upper crustal fault network of probable late archaean age that cuts across the precambrian amazonian craton in brazil the subvertical faults reactivated pre existing high grade basement shear zone fabrics itacaiunas shear zone and display a complex long lived history of movement 1 ga dominated by oblique and strike slip displacements a postulated early regional phase of dextral movement downfaulted units of low grade to unmetamorphosed basement rocks into dilational bends and offsets in the casss subsequent sinistral transpression ca 2 6 ga led to faulting and folding during partial inversion of the cover rocks in these dilational sites with much of the associated deformation focused in the immediate vicinity

iron oxide mineralization at the contact zone between phyllite and itabirite of the pau branco deposit quadrilatero ferrifero brazil implications for fluid rock interaction during iron ore formation this study examines the whole rock geochemistry and laser ablation inductively coupled plasma mass spectrometry la icp ms iron oxide chemistry of itabirite and related iron ore as well as underlying phyllitic wall rock of the pau branco iron ore deposit quadrilatero ferrifero brazil the phyllite and iron ore of pau branco extend from hypogene deep seated into supergene weathered zones the phyllitic sequence that underlies the itabirite hosted iron ore along a sheared contact includes from the bottom to top carbonaceous and carbonate chlorite and fe rich zones which likely reflect distal to proximal alteration halos the fe rich phyllite and iron ore comprise a complex iron oxide mineralogy 1 magnetite martite is hosted in itabirite and related medium grade iron ore and present as disseminated minerals in phyllite 2 microplaty hematite replaces chlorite and carbonate in the phyllite 3 granoblastic hematite is a recrystallization precipitation product and the dominant iron oxide of the hard iron ore 4 schistose specular hematite overgrows earlier iron oxides and 5 goethite replaces earlier hematite and amphibole within the weathering horizon whole rock geochemistry and related mass balance calculations reveal that unlike the itabirite hosted iron ore interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements the iron enrichment of the phyllite is caused by addition of fe 2 o 3 the fe 2 o 3 is likely sourced from the overlying itabirite iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite the rare earth elements ree abundances of the different ore and phyllite zones reveal seawater like ree signatures and y ho ratios of the itabirite hypogene and supergene iron ore shale atypical ree patterns in the unaltered phyllite and eu fractionation trends this suggests a distinct input of hydrothermal fluids during the fe enrichment of the phyllite locally restricted ce fractionation is attributed to distinct e g mn rich lithostratigraphic intercalations within the itabirite laser ablation icp ms mineral chemistry of phyllite hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral positive ce anomalies are restricted to phyllite and itabirite hosted martite only and therefore may reflect the highly oxidative conditions during martitization at pau branco the fluid rock interaction affected not only the itabirite but also the underlying phyllite resulting in distinct alteration halos that extend beyond the itabirite toward the footwall the significant chemical influence and importance of country rock lithologies as a possible elemental source for example must be taken into account when interpreting whole rock geochemical data and investigating an itabirite banded iron formation hosted iron ore system

u pb rb sr and sm nd chronology of granitic basement hydrothermal albitites and uranium mineralization lagoa real south bahia brazil the granites orthogneisses hydrothermal albitities and rocks which have suffered uranium mineralization from the lagoa real district south state of bahia brazil have been investigated by u pb rb sr and sm nd techniques u pb values on zircons from the granitic protolith give an age of 1725 ma u pb on u mineralization dates the primary mineralization at 1395 ma and indicates a reworking at 480 ma which may represent the age of the thrusting of the lagoa real complex over espinhaco metasediments during the brazilian orogeny these two dates are given by rb sr on albitites but from sparse information and are not supported by unequivocal arguments the uranium deposition and sodium metasomatism however cannot be linked either with the thermal history of the granite or with brazilian thrusting sm nd gives scattered results which are suggestive of autochtonous reworkings of rees these results lead to the following suggestions 1 magmatic activity of subalkaline affinity existed within the sao francisco craton at about 1 7 ga 2 such subalkaline plutons are likely sources for u mineralization 3 at about 1 4 ga an unknown event caused hydrothermal activity leading to u deposition at lagoa real 4 brazilian overthrusting at about 480 ma did not play any genetic role in the hydrothermal activity

development of different types of pull apart microstructures in mylonites an experimental investigation abstract intragranular fracturing of feldspar porphyroclasts in granite mylonites of a ductile shear zone has led to development of diverse types of pull apart and bookshelf structures porphyroclasts with centrally located fractures show pull aparts with parallel walls type 1 whereas those with off centered fractures display pull aparts with both parallel type 1 and non parallel type 2 geometry analog model experiments performed by embedding segmented rigid elliptical objects within a viscous matrix indicate that the orientation and location of the fracture and the aspect ratio of the porphyroclast are the principal parameters in controlling the development of bookshelf structures and pull aparts in the case of centrally located fractures porphyroclasts of moderate aspect ratios develop either type 1 pull apart or bookshelf depending upon the fracture orientations whereas those of large aspect ratios 3 form only type 1 pull aparts irrespective of the fracture orientation off centered fracturing of porphyroclasts gives rise to fragments of unequal size which rotate independently at equal or contrasting velocities forming type 1 or type 2 v pull aparts respectively in the latter case depending upon the orientation of fracture the smaller fragment rotates faster or slower than the larger fragment showing relative tilt synthetic type 2a or antithetic type 2b to the bulk shear sense respectively type 2b geometry generally develops when the fracture angle with respect to the short axis of porphyroclasts is low antithetic to the shear direction and the long axis of the porphyroclast is at a high angle to the shear plane

a quantitative assessment of methane cycling in hikurangi margin sediments new zealand using geophysical imaging and biogeochemical modeling takahe seep located on the opouawe bank hikurangi margin is characterized by a well defined subsurface seismic chimney structure ca 80 500 m2 in area sub seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure reaction transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas the dissolution of gas up to 3761 mmol m 2 yr 1 dwarfed the rate of methanogenesis within the simulated sediment column 2 6 mmol m 2 yr 1 dissolved methane is mainly consumed by anaerobic oxidation of methane aom at the base of the sulfate reduction zone and trapped by methane hydrate formation below it with maximum rates in the central part of the chimney 946 and 2420 mmol m 2 yr 1 respectively a seep wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of aom rates gas hydrate formation and benthic dissolved methane fluxes of 3 68 104 mol yr 1 73 85 104 mol yr 1and 1 19 104 mol yr 1 respectively a much larger flux of methane probably escapes in gaseous form through focused bubble vents the approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales

natural marine seepage blowout contribution to atmospheric methane the release of methane sequestered within deep sea methane hydrates is postulated as a mechanism for abrupt climate change however whether emitted seabed methane reaches the atmosphere is debatable we observed methane emissions for a blowout from a shallow 22 m hydrocarbon seep the emission from the blowout was determined from atmospheric plume measurements simulations suggest a 1 1 gas loss to dissolution compared to 10 loss for a typical low flux bubble plume transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge this mechanism could allow methane suddenly released from deeper 250 m waters to contribute significantly to atmospheric methane budgets copyright 2006 by the american geophysical union

natural oil slicks in the gulf of mexico visible from space natural oil seepage in the gulf of mexico causes persistent surface slicks that are visible from space in predictable locations a photograph of the sun glint pattern offshore from louisiana taken from the space shuttle atlantis on may 5 1989 shows at least 124 slicks in an area of about 15 000 km2 a thematic mapper tm image collected by the landsat orbiter on july 31 1991 shows at least 66 slicks in a cloud free area of 8200 km2 that overlaps the area of the photograph samples and descriptions made from a surface ship from aircraft and from a submarine confirmed the presence of crude oil in floating slicks the imagery data show surface slicks near eight locations where chemosynthetic communities dependent upon seeping hydrocarbons are known to occur on the seafloor additionally a large surface slick above the location of an active mud volcano was evident in the tm image in one location the combined set of observations confirmed the presence of a flourishing chemosynthetic community active seafloor oil and gas seepage crude oil on the sea surface and slick features that were visible in both images we derived an analytical expression for the formation of floating slicks based on a parameterization of seafloor flow rate downstream movement on the surface half life of floating oil and threshold thickness for detection applying this equation to the lengths of observed slicks suggested that the slicks in the atlantis photograph and in the tm image represent seepage rates of 2 2 30 m3 1000 km 2 d 1 and 1 4 18 m3 1000 km 2 d 1 respectively generalizing to an annual rate suggests that total natural seepage in this region is of the order of at least 20 000 m3 yr 1 120 000 barrels yr 1

natural flux of greenhouse methane from the timor sea to the atmosphere 1 methane gas bubbles from the cornea seep were sampled at the sea surface in the timor sea continental shelf area in june 2005 total bubble gas flux was 0 076 to 0 76 l m 2 h 1 during the 6 h d 1 periods of low neap tides in june 2005 this bubble gas contained an average of 26 mmol ch4 l 1 and about 0 16 and 0 006 mmol l 1 of ethane and propane we estimate the daily flux from the sea surface to the atmosphere to be 0 012 to 0 12 mol ch4 m 2 d 1 or 0 13 to 1 3 t ch4 d 1 from an area of about 0 7 km2 this methane flux came from a 500 1400 m carbonate pavement dome on the seafloor at 84 m water depth the seep hard ground was swath mapped and 3 5 khz subbottom profile data indicate that the seep dome was strongly reflective with poor penetration into the subsurface consistent with the presence of a carbonate hard ground carbon and deuterium isotope ratios 13c 41 to 42 d 157 to 158 of the seep bubble gas indicate that this methane had a thermogenic origin and was in the same isotopic range as gas within the late cretaceous cornea oil and gas field we could not detect inputs of fluids containing nutrients or short lived radium isotopes at this site commonly associated with other cold seeps tens to a hundred of kilometers seaward from the cornea seep site water column dissolved methane concentrations in this sector of the timor sea shelf and slope were 100 500 times supersaturated with respect to the atmosphere and thus the water column is expected to be degassing additional methane to the atmosphere perhaps there are thousands of other methane seeps of similar magnitude to the cornea seep on this shelf and slope to account for all the excess dissolved methane 86 000 t measured in the water column these measured and calculated fluxes provide evidence for the hypothesis that shallow sea seeps may be a significant source of atmospheric methane in contrast to deep sea vents where most of the methane is dissolved and oxidized in seawater and does not reach the atmosphere

distinct activity phases during the recent geologic history of a gulf of mexico mud volcano abstract laser line scan imaging and chirp sub bottom profiling were used to detail the morphology of a submarine mud volcano and brine filled crater at 652 m water depth in the northern gulf of mexico the mud volcano has a relief of 6 m and a basal diameter of about 80 m the feature comprises a central brine filled crater 253 m 2 surrounded by a continuous bed of methanotrophic mussels bathymodiolus childressi covering 434 m 2 and a patchy bed covering an additional 214 m 2 of the periphery the brine pool was mostly 28 m and 12 m deep respectively at the northern end of the pool which emitted continual streams of small clear bubbles sub bottom profiles indicated three distinct strata beneath the present surface of the mud volcano integration of 17 profiles shows that the mud volcano has been built in at least three successive stages the lowest stage deposited 35 400 m 3 while the middle and upper stages deposited 7700 and 20 400 m 3 respectively piston cores were taken at the northern edge of the mussel bed and a site 100 m southwest of the pool mussel and lucinid shells were recovered from the closer core lucinid shells from the distant core a mussel shell from 3 4 m sub bottom had a 14 c age of 16 2 ka mixture of modern carbon with carbon dead reservoir material would produce actual ages 2 ka less than the radiocarbon ages

3d visualization of sheath folds in ancient roman marble wall coverings from ephesos turkey abstract archaeological excavations and restoration of a palatial roman housing complex in ephesos turkey yielded 40 wall decorating plates of folded mylonitic marble cipollino verde derived from the internal hellenides near karystos greece cipollino verde was commonly used for decoration purposes in roman buildings the plates were serial sectioned from a single quarried block of 1 25 m3 and provided a research opportunity for detailed reconstruction of the 3d geometry of meterscale folds in mylonitized marble a gocad model is used to visualize the internal fold structures of the marble comprising curtain folds and multilayered sheath folds the sheath folds are unusual in that they have their intermediate axis normal to the parent layering this agrees with regional tectonic studies which suggest that cipollino verde structures formed by local constrictional non coaxial flow sheath fold cross section geometry exposed on the surface of a plate or outcrop is found to be independent of the intersection angle of the fold structure with the studied plane consequently a single surface cannot be used as an indicator of the three dimensional geometry of transected sheath folds

limits on modes of lithospheric heat transport on venus from impact crater density we present a formalism by which the size frequency distribution of impact craters on venus may be used to estimate upper bounds on the mean global rates of volcanic resurfacing and lithospheric recycling on that planet over the past several hundred million years the principal assumptions are that an upper bound on the rate of crater production on venus may be estimated and that craters are volcanically obliterated only when flows completely cover their rims the impact crater density reported from venera 15 16 observations if valid for the entire venus surface then indicates a mean volcanic flux no greater than 2 km y corresponding to a maximum average rate of resurfacing of about 4 km b y the fraction of global heat loss due to such a rate of volcanic resurfacing is negligible for the lowest estimated crater retention age of the surface imaged by venera 15 16 if globally representative the rate of lithospheric recycling on venus does not exceed 1 5 km y corresponding to 25 of the global heat loss more likely estimates of the mean surface age limit the contribution to global heat loss of any lithospheric recycling to be less than 10 these results support the hypothesis that simple conduction dominates heat transport at lithospheric levels in the venus interior

on venus impact basins viscous relaxation of topographic relief the question of whether impact basins dating from the time of heavy bombardment of the inner solar system are preserved on the surface of venus is of paramount importance for an understanding of the geological processes that have shaped that planet two types of features have been suggested as candidates for impact basins on venus 1 large quasi circular topographic depressions and 2 large quasi circular regions of low radar backscatter surrounded by terrain of greater radar backscatter and generally displaying little topographic relief as a guide to the expected character of an impact basin on venus we have calculated the viscous relaxation with time of the topographic relief of such a basin under the assumption that the surface temperature of the planet has been at or near its presently high value since the time of basin formation the viscous relaxation model includes the effects of a decrease in effective viscosity with depth and the partial to complete isostatic compensation of initial topography we conclude that an impact basin several hundred kilometers or greater in diameter and 3 b y old or greater in age should have negligible topographic relief at present this conclusion does not exclude the hypothesis that quasi circular features of low radar backscatter but little topographic relief may be relaxed remnants of ancient impact basins large quasi circular depressions on venus such as atalanta planitia however cannot exceed a few hundred million years in age otherwise their initial topographic relief would be excessive such large depressions are thus likely to be geologically young and to have resulted from processes other than impact an additional process must therefore have operated on venus to create large roughly circular areas of lowland terrain one possibility is that these topographic features are analogs on venus to terrestrial platform basins produced by lithospheric extension and thermal subsidence

impact craters of venus a continuation of the analysis of data from the venera 15 and 16 spacecraft additional studies of venusian impact craters have been made based on an analysis of venera 15 16 radar imagery and altimetry the crater population on venus has been subdivided into groups representing different morphological classes and types the craters display the size dependent variations in morphology which are well known from other planets assuming a crater production rate based on estimates by hartmann et al 1981 their areal density indicates an age for the total population of approximately 1 b y alternative estimates based on the assumption that the recent cratering rates on the earth and venus were similar would give a significantly lower age the areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b y in age a suggested model of the effect of the atmosphere on the crater diameter frequency curve indicates that cometary impacts played only a minor role in forming the contemporary venus environment a comparative study of craters in the area where the arecibo earth based radar and venera 15 16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle arecibo images correspond to the flat bottoms of the craters seen in the venera images

geochemical evolution of the banded iron formation hosted high grade iron ore system in the koolyanobbing greenstone belt western australia the banded iron formation bif hosted iron ore deposits in the lower greenstone succession of the koolyanobbing greenstone belt 50 km north of southern cross in western australia are a 200 mt high grade fe 58 pre mining resource and represents one of the most important iron ore districts in the yilgarn craton four hypogene alteration ore forming stages and one supergene upgrading event took place 1 during ore stage 1 lree depleted transition metal enriched mg fe ca carbonates replaced quartz in bifs the deposit scale alteration was most likely induced by devolatilization of sea floor altered ca si depleted mafic rocks in the vicinity of the bif during early regional syn d 1 very low to low grade metamorphism and was most strongly developed on reactivated bif basalt contacts 2 ore stage 2 involved the formation of patchy magnetite ore by a syn d 2 to d 4 dissolution of early carbonate enrichment of fe 2o 3total in magnetite iron ore was by a factor of 2 to 2 4 and compatible trace elements in magnetite such as ga v and al were immobile a subdeposit scale ferroan talc footprint proximal to magnetite iron ore in the largest deposit k deposit was associated with ore stage 2 and resulted from dissolution of magnesite due to reaction with silica in the bif under greenschist facies conditions and potentially high fluid rock ratio 3 magnetite growth during ore stage 3 forming granular magnetite martite ore is related to a subsequent hydrothermal event occurring locally throughout the belt especially in d 2b faults 4 ore stage 4 was associated with fe ca p l ree y enriched hydrothermal fluids possibly from a magmatic source such as the postmetamorphic lake seabrook granite that crops out about 10 km west of the koolyanobbing deposits and at the southern margin of the greenstone belt these ca enriched fluids interacted with distal metamorphosed mafic rock and influenced the bif ore system in a small number of deposits they were channelled through regional d 4 faults and caused specularitedolomite quartz alteration resulting in fe grades of up to 68 5 supergene upgrade ore stage 5 by further gangue leaching in the weathering zone was most effective in carbonate altered bifs and magnetite ore this process together with supergene martitization and goethite replacement of magnetite led to the formation of high grade locally at k deposit high p goethite martite ore at koolyanobbing the two geochemically distinct stages of archean carbonate alteration clearly controlled the formation of hypogene magnetite specularite martite rich ore and recent supergene modification including the further upgrade of fe ore 2012 by economic geology

planetary radar astronomy planetary radar research reported during 1979 1982 was highlighted by radar observations of venus by the pioneer venus spacecraft earth based high resolution imaging of venus and the moon construction of accurate topographic profiles for martian tropical regions observational and theoretical advances in relating the radar signatures of mars and the moon to surface characteristics and the initial radar detections of echoes from a comet and a mainbelt asteroid this review begins with a brief synopsis of radar definitions and technical terminology and then summarizes recent results for individual targets readers interested in radar astronomy prior to the current quadrennium are directed to excellent reviews by jurgens 1982a pettengill 1978 evans 1969 muhleman 1966 and pettengill and shapiro 1965 detailed descriptions of planetary radar techniques include pettengill 1970 and evans and hagfors 1968

genesis of high grade hematite orebodies of the hamersley province western australia a reply sir we are very thankful for the constructive comments provided by kneeshaw and kepert unfortunately the competitive ownership structure of the iron ore mining industry in the hamersley province limits our personal knowledge of the mount whaleback deposit to occasional mine visits and to the available literature published and unpublished we do however have access to the data on the smaller giles mini and lido deposits 50 and 10 km west of mount whaleback respectively and to extensive exploration mapping in the ophthalmia ranges to the north direct proof for rotation of the orebody as it is preserved at paraburdoo in the approximately 50 dip of the overlying proterozoic hematite conglomerates of the mount mcgrath formation is not preserved at mount whaleback however we believe that there is sufficient information available in the literature to think that a rotational model for a large part of the deposit is perfectly plausible first and above all as stated in taylor et al 2001 f2 folds are more upright with axial planes dipping about 50 s in the southern and western part of the deposit away from the whaleback fault e g as shown on section no 315120e fig 4 in powell et al 1999 interestingly the ore controlling faults are also more upright in this part of the deposit which suggests an intimate link between the dip of these fault planes and dip of the axial planes of f2 folds secondly

the mechanics of intermittent methane venting at south hydrate ridge inferred from 4d seismic surveying sea floor methane vents and seeps direct methane generated by microbial and thermal decompositions of organic matter in sediment into the oceans and atmosphere methane vents contribute to ocean acidification global warming and providing a long term e g 500 4000 years powell et al 1998 life sustaining role for unique chemosynthetic biological communities however the role methane vents play in both climate change and chemosynthetic life remains controversial primarily because we do not understand long term methane flux and the mechanisms that control it milkov et al 2004 shakhova et al 2010 van dover 2000 vents are inherently dynamic and flux varies greatly in magnitude and even flow direction over short time periods hours to days often tidally driven boles et al 2001 tryon et al 1999 but it remains unclear if flux changes at vents occur on the order of the life cycle of various species within chemosynthetic communities months years to decades leifer et al 2004 torres et al 2001 and thus impacts their sustainability here using repeat high resolution 3d seismic surveys acquired in 2000 and 2008 we demonstrate in 4d that hydrate ridge a vent off the oregon coast has undergone significant reduction of methane flow and complete interruption in just the past few years in the subsurface below a frozen methane hydrate layer free gas appears to be migrating toward the vent but currently there is accumulating gas that is unable to reach the seafloor through the gas hydrate layer at the same time abundant authigenic carbonates show that the system has been active for several thousands of years thus it is likely that activity has been intermittent because gas hydrates clog the vertical flow pathways feeding the seafloor vent back pressure building in the subsurface will ultimately trigger hydrofracturing that will revive fluid flow to the seafloor the nature of this mechanism implies regular recurring flow interruptions and methane flux changes that threaten the viability of chemosynthetic life but simultaneously and enigmatically sustains it

the bif hosted high grade iron ore deposits in the archean koolyanobbing greenstone belt western australia structural control on synorogenic and weathering related magnetite hematite and goethite rich iron ore the koolyanobbing banded iron formation bif hosted iron ore deposits total premining resources 150 million metric tons mt indicated reserves 32 mt are located in the mesoarchean lower succession bif of the koolyanobbing greenstone belt younami terrane yilgarn craton in western australia in the koolyanobbing greenstone belt a deformation sequence that broadly correlates with the proposed deformation history of most greenstones belts within the southern cross domain includes d 1 structures mainly small scale f 1a and f 1b folds formed in a north south to northwest southeast compressional regime a ductile to brittle deformation sequence d 2 to d 4 generated during east west compression and a late stage brittle segmentation of bif and reactivation of faults attributed to d 5 the formation of the seven known medium 45 58 wt fe to high grade 58 68 wt fe magnetite martite specularite and goethite bearing orebodies can be subdivided into four archean stages and one weathering related upgrade from the permian and or mesozoic to recent times the archean ore forming stages comprise 1 early fe mg ca metasomatism causing local ferroan carbonate and ferroan talc alteration of the metamorphosed quartz magnetite bif protolith 2 sequential syn d 2a coaxial to syn d 4 transpressional tight folding driven removal of carbonate quartz and minor ferroan talc by solution and mechanical transfer producing residual enrichment of medium to high grade magnetite ore 3 magnetite mineralization in syn d 2b and syn d 4 breccias and fractures forming medium grade ore zones or overprint magnetite in bif and first stage magnetite ore and 4 mineralization of hydrothermal specularite and locally associated ferroan dolomite quartz alteration and local oxidation of magnetite in and near brittle d 4 faults fractures and reactivated f 1 and f 2a fold cores modern weathering related leaching of carbonate and minor quartz pseudomorphic goethite replacement of existing iron oxides and gangue and coeval or subsequent to oxidation in the vadose zone formed goethite martite ore with local relics of specularite or magnetite and or kenomagnetite the intensity and localization of this supergene modification is in most deposits at koolyanobbing controlled by existing hypogene magnetite specularite rich medium to high grade ore zones and or carbonate altered bif at depth the existence of high grade ore below the weathering horizons suggests the possibility of further concealed magnetite and or specularite rich orebodies within the deposits and region

quantification of gas bubble emissions from submarine hydrocarbon seeps at the makran continental margin offshore pakistan 1 evidence for twelve sites with gas bubble emissions causing hydroacoustic anomalies in 18 khz echosounder records flares was obtained at the convergent makran continental margin the hydroacoustic anomalies originating from hydrocarbon seeps at water depths between 575 and 2870 m disappeared after rising up to 2000 m in the water column dives with the remotely operated vehicle quest 4000 m revealed that several individual bubble vents contributed to one hydroacoustic anomaly analyzed gas samples suggest that bubbles were mainly composed of methane of microbial origin bubble size distributions and rise velocities were determined and the volume flux was estimated by counting the emitted bubbles and using their average volume we found that a low volume flux flare 1 at 575 mbsl 90 ml min caused a weak hydroacoustic signal in echograms whereas high volume fluxes flare 2 at 1027 mbsl 1590 ml min flare 5 c at 2870 mbsl 760 ml min caused strong anomalies the total methane bubble flux in the study area was estimated by multiplying the average methane flux causing a strong hydroacoustic anomaly in the echosounder record with the total number of equivalent anomalies an order of magnitude estimate further considers the temporal variability of some of the flares assuming a constant flux over time and allows a large range of uncertainty inherent to the method our results on the fate of bubbles and the order of magnitude estimate suggest that all of the 40 32 106 mol methane emitted per year within the gas hydrate stability zone remain in the deep ocean

geochemical constraints on the origin of the pore fluids and gas hydrate distribution at atwater valley and keathley canyon northern gulf of mexico abstract pore fluids from atwater valley at 13 14 and keathley canyon kc 151 in the northern gulf of mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values as well as hydrocarbon geochemistry suggesting that these widely separated localities share common deep subsurface fluid origins seafloor mounds with focused fluid migration pathways and inferred near seafloor gas hydrates characterize the at 13 14 region whereas the kc 151 region has a bottom simulating reflector bsr at 310 mbsf which is rather uncommon in the gulf of mexico gom at these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission the geochemical results together with the pressure core data suggest that at the at region methane hydrate mostly occurs in near surface sediments at mounds consistent with focused migration pathways in the kc region methane hydrate mostly occurs deeper in the section in highly fractured silty clayey sediments from 220 to 300 mbsf the pore fluids at the at mounds and kc 151 are characterized by higher than seawater salinity the more saline pore fluids at the at mound and at kc151 sites located 350 km apart are almost chemically indistinct ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution rather this fluid is a subsurface brine derived from jurassic or cenozoic evaporite formation modified by fluid sediment reactions and migrated to the two sites analyzed despite porewater salinities elevated above that of seawater the sediment temperatures are within the range of methane hydrate stability for each of the sites based on cl dilutions the maximum gas hydrate pore volume occupancy at the at mound sites would be 9 at kc cl concentrations in pressure cores imply that in situ hydrate is unevenly distributed with pore volume occupancy of 1 12 significant variations in sulfate gradients were observed with the sulfate to methane transition zone smtz at or near the seafloor at the at mound sites at at 13 2 the well defined smtz is at 8 mbsf and at kc 151 3 it is at 9 mbsf there is no coincidence between the steepness of the sulfate gradients and the presence or depth of a bsr suggesting that the smtz interfaces are measuring different aspects of the subsurface methane hydrology at both at and kc the 13 c dic values clearly indicate that anaerobic oxidation of methane aom is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed the most negative 13 c dic values obtained are 46 3 and 49 6 at the smtzs at at 13 2 and kc 151 3 respectively

the acoustic signature of marine seep bubbles observations of seabed bubbles 62m depth at a natural marine hydrocarbon seep by passive acoustic and optical approaches are compared the acoustic and optical methods observed a bimodal distribution with peaks at 1500 and 1750hz and 2200 and 2800 m radius respectively radii were 20 lower than predicted by the minnaert formula frequency shifts were observed for bubbles emitted within a few milliseconds and were attributed to coupling between nearby bubbles surfactants also may have played a role

marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal abstract detection of chemically identifiable hydrocarbon oil and gas seepage is an important tool in offshore exploration generated and or trapped oil and gas can leak to near surface sediments and ocean in detectable concentrations the seepage surface expression is controlled by hydrocarbon generation regional pressure regimes and migration pathways the migrated hydrocarbon concentration and composition varies due to seepage rates and near surface physical and biological processes that can alter the signal a seepage continuum exists from large volume oil and gas seeps known as macro seepage to lower volume seeps known as micro seepage only detectible with special sampling and analysis understanding the impact of migrated hydrocarbon concentration relative to in situ organic material and locally derived microbial hydrocarbons is critical this paper describes factors that control the near surface geochemical signal as well as the challenges in deciphering the migrated hydrocarbon signature to evaluate subsurface hydrocarbon generation and entrapment

high grade iron ore at windarling yilgarn craton a product of syn orogenic deformation hypogene hydrothermal alteration and supergene modification in an archean bif basalt lithostratigraphy banded iron formation bif hosted iron ore deposits in the windarling range are located in the lower greenstone succession of the marda diemals greenstone belt southern cross domain yilgarn craton and constitute a total hematite martite goethite ore resource of minimum 52 mt at 60 wt fe 0 07 p banded iron formation is interlayered with high mg basalts at windarling and precipitated during episodes of volcanic quiescence trace element content and the rare earth element ree ratios y ho 42 to 45 sm yb 1 5 together with positive la and gd anomalies in least altered hematite magnetite metachert bif indicate the precipitation from archean seawater that was fertilised by hydrothermal vent fluids with a basaltic hree y signature hypogene iron ore in sub greenschist facies metamorphosed bif formed during three distinct stages ore stage 1 was a syn to post metamorphic syn d1 fe ca mg ni co p ree metasomatism that produced local ni ree rich fe dolomite magnetite alteration in bif hydrothermal alteration was induced by hot fluid flow controlled by brittle ductile reactivation of bif basalt margins and crosscutting d1 faults the ni co rich content of dolomite and a shift in ree ratios in carbonate altered bif towards archean mafic rock signature y ho to 31 to 40 sm yb to 1 to 2 and gd gd to 1 2 to 1 4 suggest that high mg basalts in the windarling range were the primary source of introduced metals during ore stage 2 a syn deformational and likely acidic and oxidised fluid flow along bif basalt margins and within d1 faults leached carbonate and precipitated lepidoblastic and anhedral granoblastic hematite high grade magnetite hematite ore is formed during this stage ore stage 3 hydrothermal specular hematite spch fe dolomite quartz alteration was controlled by a late orogenic brittle compressional transpressional stage d4 the regional scale shear zone related d3 is not preserved in windarling this minor event remobilised iron oxides carbonate and quartz to form veins and breccia but did not generate significant volumes of iron ore ore stage 4 involved mesozoic to recent supergene oxidation and hydration in a weathering environment reaching down to depths of 100 to maximum 200 m below surface supergene ore formation involved goethite replacement of dolomite and quartz as well as martitisation important ground preparation for supergene modification and upgrade were mainly the formation of steep d1 to d4 structures steep bif basalt margins and particularly the syn d1 to syn d2 carbonate alteration of bif that is most susceptible to supergene dissolution the windarling deposits are structurally controlled supergene modified hydrothermal iron ore systems that share comparable physical chemical and ore forming characteristics to other iron ore deposits in the yilgarn craton e g koolyanobbing beebyn in the weld range mt gibson however the remarkable variety in pre syn and post deformational ore textures relative to d1 and d2 has not been described elsewhere in the yilgarn and are similar to the ore deposits in high strain zones such as of brazil quadrilatero ferrifero or iron quadrangle and nigeria the overall similarity of alteration stages i e the sequence of hydrothermal carbonate introduction and hypogene leaching with other greenstone belt hosted iron ore deposits supports the interpretation that syn orogenic bif alteration and upgrade was crucial in the formation of hypogene supergene iron ore deposits in the yilgarn craton and possibly in other archean paleoproterozoic greenstone belt settings worldwide

thermal and visual time series at a seafloor gas hydrate deposit on the gulf of mexico slope ambient temperature is a critical factor determining the stability of gas hydrate deposits on continental margins to study this process directly under varying conditions a monitoring array comprising a time lapse camera and in situ temperature probes was deployed at a hydrocarbon seep known as bush hill where gas hydrates deposits are exposed at the seafloor in a water depth of 570 m for intervals of 91 days and 30 days the digital camera recorded several daily images of a prominent gas hydrate mound consisting of structure ii gas hydrate the temperature probes were constructed with one autonomous thermistor at each end of a 50 cm pvc wand and recorded temperatures with precision of better than 0 1 8c at 30 min intervals over 327 days one probe was implanted with a tight seal into a drill hole about 7 cm deep in the top of the gas hydrate mound the second was inserted about 50 cm deep into the adjacent sediments for each probe the top thermistor recorded the ambient water temperature while the bottom thermistor synoptically recorded the internal temperature of the hydrate or sediment the bottom water temperatures ranged from 6 64 to 9 73 8c with a mean of 7 90 8c and standard deviation of 0 437 photographic results showed no dramatic changes in the size shape or gas venting from the mound by comparing the temperatures recorded at the tips of the probes with the synoptic water temperature we estimate that thermal diffusivity for the gas hydrate deposit was 7 14d 10 8 to 8 33d 10 8 m 2 s 1 and was 1 77d 10 7 to 3 01d 10 7 m 2 s 1 for the sediments at the sites where temperatures were recorded the diffusivity measured in gas hydrate was lower than that measured under laboratory conditions stability of gas hydrate in this setting is not likely to be affected by short term changes in bottom water temperature within the range observed d 2005 elsevier b v all rights reserved

nonredox transformations of magnetite hematite in hydrothermal systems the transformation of magnetite to hematite or hematite to magnetite in nature has generally been considered a redox reaction and linked to a specific redox state of fluid however a nonredox reaction fe2o3 hm fe2 h2o fe3o4 mt 2h may have been the principal mechanism for the transformations of iron oxides in nature especially in hydrothermal environments for example the transformation of goethite and or hematite primary precipitates to magnetite in banded iron formations bifs probably occurred through nonredox reactions with fe2 bearing hydrothermal fluids during the accumulation of a bif sequence rather than through redox reactions involving organic matter during and or after the bif deposition the proposed mechanisms for the transformation of magnetite to hematite provides new exploration strategies for hematite rich secondary ores extending the target for orebodies to much deeper zones below the paleosurface another important implication of the proposed mechanism is that the presence or absence of magnetite and or hematite in geologic formations may or may not provide meaningful information on the redox state of fluid

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

observations of bubbles in natural seep flares at mc 118 and gc 600 using in situ quantitative imaging this paper reports the results of quantitative imaging using a stereoscopic high speed camera system at two natural gas seep sites in the northern gulf of mexico during the gulf integrated spill research g07 cruise in july 2014 the cruise was conducted on the e v nautilus using the rov hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts the seeps originated between 890 and 1190 m depth in mississippi canyon block 118 and green canyon block 600 the imaging system provided qualitative assessment of bubble behavior e g breakup and coalescence and verified the formation of clathrate hydrate skins on all bubbles above 1 3 m altitude quantitative image analysis yielded the bubble size distributions rise velocity total gas flux and void fraction with most measurements conducted from the seafloor to an altitude of 200 m bubble size distributions fit well to lognormal distributions with median bubble sizes between 3 and 4 5 mm measurements of rise velocity fluctuated between two ranges fast rising bubbles following helical type trajectories and bubbles rising about 40 slower following a zig zag pattern rise speed was uncorrelated with hydrate formation and bubbles following both speeds were observed at both sites ship mounted multibeam sonar provided the flare rise heights which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions the evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles

hydrocarbon migration pathway and methane budget for a gulf of mexico natural seep site green canyon 600 abstract hydrocarbon seeps occur worldwide along continental margins and act as conduits for fluid discharge from the lithosphere to the overlying hydrosphere atmosphere the dynamics and rates of hydrocarbon release at cold seeps remain poorly constrained seepage enables a variety of processes that alter the seafloor morphology and affect the geochemistry of substrata supporting diverse and important chemosynthetic communities here we merge complementary geochemical oil fingerprinting geophysical deep seismic subbottom backscatter multibeam and video imaging video time lapse camera dsv alvin video rov video auv photo surveys data sets to constrain the pathways and magnitudes of methane fluxes from the source reservoir to the seafloor at a well studied prolific seep site green canyon gc 600 in the northern gulf of mexico oil samples from the reservoir an active vent and the sea surface showed compositional similarities consistent with the plumbing system structure identified in seismic data spatial distribution of seep indicators such as bacterial mats microbial communities methane derived carbonates and hydrate outcrops were then used to quantify the total magnitude of methane potentially sequestered in the study domain using a systems approach we combined published values for methane fluxes with data we collected across various scales and resolutions to compile a methane budget for gc600 the system considered for the methane budget is defined as the movement of methane from the salt ridge to the seafloor water interface for each seep domain birthday candles and mega plume total estimated input of methane was 2 8 10 8 2 2 10 9 mol yr in the birthday candles domain and 2 7 10 8 2 3 10 9 in the mega plume domain the combined total output of the system ranged from 3 2 10 5 8 2 10 5 and 3 2 10 6 5 2 10 6 mol yr respectively for birthday candles and mega plume domains leaving a potential surplus input minus output of 2 6 10 8 2 3 10 9 mol yr processes that could balance this budget include accumulation of gas hydrate and sediment free gas and the underestimated potential of biological sinks such as methane oxidation

theoretical simulation of the evolution of methane hydrates in the case of northern south china sea since the last glacial maximum the distribution and inventory of gas hydrates in a region is determined by the sediment characteristics methane supply and evolution of the reservoir in recent decades the geo environmental constraints and sources of methane have been intensively investigated and numerous experimental and numerical simulation tools have been developed to evaluate the inventory of hydrates however information regarding the evolution of hydrate reservoirs remains limited this study developed a simulator to theoretically model the evolution of specific reservoirs since the last glacial maximum lgm the lgm was a recent cold epoch that occurred approximately 18 000 years ago since the lgm the earth s climate system has experienced a continuous increase of surface temperature and rising sea level given a sufficient supply of methane and a transport system hydrates may form in marine sediments if sea level rises or melt if the temperature increases a one dimensional simulator that represents the sediment was developed and uses the current hydrate profiles as the initial conditions and reliable paleoenvironmental data obtained in sites located in the northern shelf of the south china sea scs as boundary conditions two types of hydrate profiles were reversely simulated till the lgm 1 a gaussian profile which was observed in the shenhu area and 2 a trapezoidal profile which was observed in the dongsha area scs the evolution and past quantities of local hydrate reservoirs were estimated the model results demonstrated that shallow 500 700 m below the seafloor or mbsf moderate saturation 50 pore volume or v v hydrate deposits will form if they are subjected to recent climate changes the inventory of hydrates in the nscs increased by only 0 7 over the past 18 000 years under various scenarios a sensitivity analysis was performed to identify the most pertinent parameters that control the formation and dissociation of hydrates including the grain size temperature gradient and deposition depth the distribution and depth of the reservoir were determined to be the most critical factors in the evolution of the studied hydrates

methane under saturated fluids in deep sea sediments implications for gas hydrate stability and rates of dissolution abstract deep sea sediments contain earth s largest reservoir of methane ch 4 3000 gtons c trapped within ice like crystals known as gas hydrates understanding the controls on gas hydrate stability is critical because methane released from hydrate destabilization is hypothesized to be a powerful agent of past and potentially future climate change hydrates are stable under high pressure low temperature moderate salinity and saturated gas conditions yet the degree of gas saturation is rarely known in nature because in situ dissolved pore water ch 4 concentrations are rarely measured here we report measurements of these concentrations in sediments immediately surrounding deep sea gas hydrate deposits and show that pore fluids are greatly under saturated with respect to expected values for equilibrium with methane gas hydrate this indicates that the hydrates are dissolving even though they are found within the appropriate pressure and temperature stability field however dissolution rates calculated from the in situ ch 4 data are significantly less than dissolution rates predicted for methane under saturated pore water in direct contact with pure methane gas hydrate if equilibrium ch 4 concentrations exist immediately adjacent to the hydrate surface diffusion retarding factors found naturally in ocean sediments such as oil coatings or biofilms appear to enhance stability in outcropping hydrate deposits the in situ seafloor evidence provided herein leads us to hypothesize that the stability of the worldwide hydrate deposits may be much greater than predicted from diffusion kinetics because biological microbial excretion of slime or surfactants and or physical processes oil coatings effectively armor and stabilize exposed hydrate surfaces substantially retarding their dissolution

temporal variation of methane flares in the ocean above hydrate ridge oregon abstract while bubble plumes have been acoustically imaged in the water column above marine gas hydrate deposits in many studies little is known about the temporal variation in plume intensity in july 2008 we conducted surveys using 3 5 and 12 khz echosounders and a 75 khz acoustic doppler current profiler adcp over the northern and southern summits of hydrate ridge on the cascadia continental margin our study included multiple surveys at both sites including a survey of the northern summit that was repeated 16 times in 19 h seafloor depth at the northern summit is 600 m well within the hydrate stability zone hsz which was below 510 m during our survey based on ctd data three distinct flares a term used to denote the acoustic signature of bubble plumes were detected at northern hydrate ridge nhr and one was detected at southern hydrate ridge shr coincident with where flares were observed a decade ago indicating that the supply of gas is stable on this time scale high resolution bathymetric surveys of nhr and shr acquired with an autonomous underwater vehicle auv flown 50 m above the seafloor indicate that flare locations are correlated with a distinctive pattern of short wavelength seafloor roughness supporting the inference of long term stability in the location of bubble expulsion sites as in previous studies at hydrate ridge flares were not detected with the 3 5 khz echosounder but were clearly imaged at 12 khz by reprocessing routine shipboard adcp data we show that they are also observed in at 75 khz indicating that a wide range of bubble sizes is present the intensity of the flares varied strongly with time two primary sources for flares were observed one located on the regional topographic high showed continuous activity with two times periods of particularly strong flares that are not correlated with tidal height the other located on a local topographic high shows a pulse of increased backscatter that occurred on a falling tide while the time period of observation is not enough to constrain the effect of tidal changes in seafloor pressure on venting the data suggest that tides are not the dominant factor controlling release of bubbles from the seafloor the data support previously suggested models in which temporary sealing of vents by gas hydrate formation and breaking of these barriers as gas pressure builds up is responsible for burp like pulses of gas expulsion we also report the first observations of flares originating within the hsz that extend well above the hsz with little loss of backscatter intensity these represent a new source of methane injection into the upper ocean and possibly the atmosphere flare extension above the hsz may be due to coating of some bubbles by oil or biofilms or to inclusion of particulate matter possibly including floating pieces of hydrate in the plumes although this study provides tantalizing new information on both short term variability in gas expulsion rate and long term stability of vent sites longer well calibrated observations that integrate bubble flux over entire vent fields as a function of time are needed to develop accurate models for the flux of methane into the ocean and atmosphere from seafloor methane vents

the evolution of the geothermal gradient from lambert graben and mahanadi basin a contribution to the indo antarctic rift debate abstract the east antarctic lambert graben and the indian mahanadi basin are considered to represent segments of an intra gondwanan rift structure that was active at least since the paleozoic fission track analyses of apatites from a comprehensive data set across the shoulders of both grabens were used to compare their low temperature history and to estimate the paleo geothermal gradients before the onset of the last denudation rifting stage late jurassic the paleo geothermal gradients of both juxtaposed gondwana margins similarly increase from the basement towards the respective rift shoulder from 15 20 ckm 1 to 25 30 ckm 1 this trend of increasing paleo geothermal gradients together with a denudation episode commencing in the early cretaceous and coeval igneous activity indicates a common rifting stage accompanying the breakup of gondwana in the india antarctica sector

structural and hydrothermal evolution of the shear zone related iron oxide enrichment in metavolcano sedimentary rocks of the intracontinental aracuai orogen the case of the espirito santo iron deposit abstract the rio sao joao metavolcano sedimentary sequence is located in the eastern border of the northern espinhaco fold thrust belt in the intracontinental aracuai west congo orogen mafic schist quartz amphibole and carbonate itabirites as well as a carbonate silicate rock occur in the study area these contain mineral assemblages compatible with the amphibolite facies metamorphic conditions magnetite and hematite rich itabirites form a series of iron deposits in the region near igapora caetite bahia state brazil the espirito santo iron deposit has compact hematite layers truncating quartz itabirite with magnetite varying from 10 to 55 vol of the rock volume siliciclastic and mafic rocks also host iron rich domains variably enriched in magnetite and hematite the deposit is hosted in compressional duplexes with structural top towards the sw these structures have an sn 1 sn foliation represented by compositional banding and parallel schistosity that is observed at all scales other common features are s c c boudins pinch and swell structures as well as sheath syn tectonic folds and a mineral stretching lineation lxn hydrothermal alteration was contemporaneous to the development of duplex and the sn 1 sn foliation a distal alteration zone is dominated by a potassic muscovite and or biotite chlorite assemblage and the intermediate zone by carbonate development the proximal alteration zone constitutes the iron oxide magnetite and hematite domains sulfide minerals as well as quartz veins silicification in association with other phases also formed hydrothermal magnetite is associated with c structures which grows either including silicates and skeletal carbonates or forming corrosion borders in these minerals lamellar hematite forms polycrystalline aggregates characterizing the s c structure in the sn 1 sn foliation as well as the axial planar foliation sn in intrafolial folds and the mineral stretching line lxn data obtained by the la icpms technique show that in general in quartz rich and amphibole itabirites hydrothermal magnetite is richer in ree than an early stage magnetite hydrothermal magnetite ree pattern is close to that of the country rock of the mineralization the formation of hematite and magnetite rich domains is related to the percolation of hydrothermal fluids that leached all components of the original silicates quartz and the carbonates of all host rocks and precipitated new hydrothermal phases taking advantage of the ediacaran shear structures

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

dynamics of the gas flux from shallow gas hydrate deposits interaction between oily hydrate bubbles and the oceanic environment decomposition of methane hydrates on the continental margins is a potentially significant source of atmospheric methane but the input depends upon the poorly understood fate of the hydrocarbon bubbles rising from the sea floor during a field trip to the gulf of mexico three different seepages were imaged and analyzed three different imaging techniques were tried side front and back illumination of which back illumination produced the best results the images were analyzed and the size dependent bubble distribution mass flux and rise speeds determined the total observed gas flux was 62 3 10 3 mol s 1 primarily methane of which a single vent produced seven times the next largest vent of this major vent 50 of the bubble mass was contained in the largest bubbles r 5500 m the vertical velocities demonstrated that these bubbles were heavily contaminated with oil which was also corroborated by bubble shape and oscillation observations

structural control hydrothermal alteration zonation and fluid chemistry of the concealed high grade 4ee iron orebody at the paraburdoo 4e deposit hamersley province western australia high grade iron ore of the 4ee orebody of the 4e deposit 200 mt at 63 5 wt fe occurs as a southerly dipping sheet within banded iron formation bif of the paleoproterozoic dales gorge and joffre members of the brockman iron formation structural reconstruction of the 4e deposit shows that reactivation of the 18e fault and development of the nw striking steeply sw dipping 4e and 4ee normal faults resulted in preservation of the 4ee orebody below the 4e deposit and 400 m below the modern topographic surface three hypogene alteration zones between low grade bif and high grade iron ore are observed 1 distal magnetite quartz dolomite stilpnomelane hematite pyrite 2 intermediate magnetite dolomite hematitechlorite quartz stilpnomelane and 3 proximal hematite dolomite chlorite pyrite magnetite hydrothermal alteration is temporally and spatially constrained by nw trending dolerite dikes that intruded the 4e and 4ee faults prior to hypogene alteration six vein types v1 v6 are recognized at the 4e deposit the veins both cut and parallel the primary bif layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low grade bif to high grade iron ore our integrated structural hydrothermal alteration and fluid flow model proposes that during early stage 1a hypogene fluid flow in the 4e orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the paraburdoo range heated basinal brines were focused by the nw striking steeply sw dipping 4e and 4ee normal faults and reacted with bif of the dales gorge and joffre members the warm to hot 160 255 c ca rich 26 6 31 9 equiv wt cacl2 basinal brine interacted with magnetite chert layers transforming them into magnetite quartz dolomite stilpnomelane hematite pyrite bif the iron rich brine up to 2 8 wt fe likely originated from evaporated seawater that had lost mg and na and gained li and ca through fluid rock reactions with volcaniclastic rocks and carbonate successions within the wittenoom formation the first incursion of deeply circulating low salinity 5 8 9 5 wt nacl equiv heated 106 201 c modified meteoric water is recorded in late stage 1a minerals this modified meteoric water had lost some of its na through wall rock interaction with plagioclase possibly by interaction with dolerite of the weeli wooli formation that directly overlies the joffre and dales gorge members stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite quartz dolomite stilpnomelane hematite pyrite bif and produced both the intermediate magnetite dolomite hematite chlorite pyrite and the proximal hematite dolomite magnetite stilpnomelane alteration assemblages microplaty 10 80 m platy 100 250 m and anhedral hematite increasingly replace magnetite in the intermediate alteration zone forming the proximal alteration zones that consist of microplaty platy anhedral hematite and magnetite the intermediate and proximal alteration zones represent the mixing of a hot 250 400 c high salinity ca rich 30 40 wt cacl2 equiv sr rich basinal brine with low temperature and low salinity 5 wt nacl equiv modified meteoric water that was heated 100 200 c during its descent into the upper crust heterogeneous mixing of the two end member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures up to 200 c and salinities up to 25 wt nacl equiv stage 1c of the hypogene hydrothermal fluid is characterized by low temperature 110 c low salinity 5 wt nacl meteoric water that interacted with the proximal hematite dolomite magnetite stilpnomelane altered bif leaving a porous hematite apatite high grade ore supergene alteration affected the orebody since the cretaceous and produced a hematite goethite alteration assemblage resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering discovery of the concealed 4ee orebody of the 4e deposit demonstrates that structural geology plays a critical role in the exploration for high grade iron orebodies structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist

structural controls on seepage of thermogenic and microbial methane since the last glacial maximum in the harstad basin southwest barents sea the harstad basin is a structural block on the continental shelf of sw barents sea where gas hydrates likely occurred below the grounded ice sheet during the last glaciation and it hosts active gas seepage at numerous seafloor sites we present an integrated study of fluid flow systems in the harstad basin by combining seismic profile interpretations and gas flare mapping data with the geochemical results obtained on seafloor seeping gas and methane derived carbonate crusts more than 190 acoustic gas flares were registered in water column many of them in association with pockmarks and carbonate crust fields however weak or absent seepage observed during remotely operated underwater vehicle transects across many pockmarks and crust fields suggests that seepage activity may have decreased since the last deglaciation in the western harstad basin seeps of microbial methane occur mainly above tertiary formations that are pinching out below the glacial sediments high amplitude seismic anomalies suggest the presence of gas pockets at the base of the glacial sediments and within tertiary deposits in contrast gas seeping in the eastern harstad basin originates from a biodegraded thermogenic source tentatively connected to the deeply faulted mesozoic rocks occurring below glacial sediments this spatial variability in fluid sources is also recorded in the carbon isotope data of seafloor carbonate crusts with 13 c values typically between 55 and 42 and 40 and 20 vpdb for carbonate crusts associated with microbial and thermogenic fluids respectively u th chronology combined with the stable isotope data suggests that this discrepancy in fluid sources over a distance of about 20 km has been stable since the last glaciation and highlights the significance of regional underlying geology in mediating fluid supply to the seafloor

nature and evolution of fluids associated with specularite bearing fe and au pge jacutinga mineralization during the brasiliano orogeny in the eastern sao francisco craton minas gerais brazil abstract hydrothermal fluids related to the development of the aracuai orogen during the brasiliano cycle have affected paleoproterozoic banded iron formation bearing sequences along the eastern border of the sao francisco craton controversy surround the origin of specularite in high grade schistose iron ore bodies in the serpentina range serpentina type ore versus specularite rich s free au pge quartz veins jacutinga type that occur at the eastern border of the quadrilatero ferrifero mining district chemical and thermobarometric characteristics of fluid inclusions from serpentina type quartz specularite veins show that metamorphic aqueous saline fluids of low to moderate temperature i e salinity between 6 3 and 13 7 wt nacl eqv and homogenization temperatures from 91 to 228 c were responsible for silica leaching and fe enrichment along shear zones in the statherian serra da serpentina group these serpentina type fluids present a distinct pattern with na ca k the fluids are depleted in li na ba ca k cu and mg and enriched in as and sr in comparison to the jacutinga type fluids in contrast other two distinct fluids and chemical processes were involved to precipitate fe oxides with au pge in jacutinga type veins the first type is an aqueous carbonic fluid from the catas altas and itabira deposits these jacutinga type fluids present a pattern with na k ca mg and k li ba sr the fluids are interpreted to be of brasiliano age and are typically associated with anatectic pegmatites from santa maria de itabira the high salinity of these brines 23 2 25 3 wt nacl eqv from the itabira ore likely originated from their percolation through evaporitic layers from salt concentration during deformation or from fluid rock interactions mixing between high temperature aqueous carbonic pegmatite forming fluids and low temperature aqueous metamorphic brines could have induced an abrupt change in the physical chemical conditions of the system which led to au and pge precipitation in the itabira jacutinga type deposit a second jacutinga type fluid is observed at the gongo soco and barao de cocais deposits this fluid is interpreted as an aqueous saline metamorphic fluid with the pattern na ca k mg low to moderate salinity 4 3 16 4 wt nacl eqv and temperatures of 77 to 305 c it is relatively enriched in b depleted in li ba na cu zn and k and follows the trend k li ba in both types of fluids responsible for jacutinga type mineralization gold was transported as chlorine complexes under acidic and oxidizing conditions the metal precipitation may have occurred due to a rapid depressurization during failure of shear zones operating in the upper levels of the crust or variations in the ph and redox conditions during the interaction with the host dolomitic itabirite

drivers of focused fluid flow and methane seepage at south hydrate ridge offshore oregon usa methane seepage at south hydrate ridge offshore oregon united states one of the best studied examples of gas venting through gas hydrates is the seafloor expression of a vigorous fluid flow system at depth the seeps host chemosynthetic ecosystems and release significant amounts of carbon into the ocean with new three dimensional seismic data we image strata and structures beneath the ridge in unprecedented detail to determine the geological processes controlling the style of focused fluid flow numerical fluid flow simulations reveal the influence of free gas within a stratigraphic unit known as horizon a beneath the base of gas hydrate stability bghs free gas within horizon a increases the total mobility of the composite water gas fluid resulting in high fluid flux that accumulates at the intersection between horizon a and the bghs this intersection controls the development of fluid overpressure at the bghs and together with a well defined network of faults reveals the link between the gas hydrate system at depth and methane seepage at the surface

the source and fate of massive carbon input during the latest paleocene thermal maximum lithologic faunal seismic and isotopic evidence from the blake nose subtropical western north atlantic links a massive release of biogenic methane 55 5 million years ago to a warming of deep ocean and high latitude surface waters a large perturbation in the combined ocean atmosphere carbon cycle the largest of the past 90 million years a mass extinction event in benthic faunas and a radiation of mammalian orders the deposition of a mud clast interval and seismic evidence for slope disturbance are associated with intermediate water warming massive carbon input to the global exogenic carbon cycle pelagic carbonate dissolution a decrease in dissolved oxygen and a benthic foraminiferal extinction event these events provide evidence to confirm the gas hydrate dissociation hypothesis and identify the blake nose as a site of methane release

neoproterozoic glacial dynamics revealed by provenance of diamictites of the bebedouro formation sao francisco craton central eastern brazil one of the main questions on neoproterozoic geology regards the extent and dynamics of the glacial systems that are recorded in all continents we present evidence for short transport distances and localized sediment sources for the bebedouro formation which records neoproterozoic glaciomarine sedimentation in the central eastern sao francisco craton sfc brazil new data are presented on clast composition based on point counting in thin section and shrimp dating of pebbles and detrital zircon cluster analysis of clast compositional data revealed a pronounced spatial variability of clast composition on diamictite indicating the presence of individual glaciers or ice streams feeding the basin detrital zircon ages reveal distinct populations of archean and palaeoproterozoic age the youngest detrital zircon dated at 874 9 ma constrains the maximum depositional age of these diamictites we interpret the provenance of the glacial diamictites to be restricted to sources inside the sfc suggesting deposition in an environment similar to ice streams from modern high latitude glaciers

natural oil seepage at kobuleti ridge eastern black sea abstract analysis of advanced synthetic aperture radar satellite images in combination with water column and seafloor investigations documented natural oil seepage from pechori mound and colkheti seep in 1000 1200 m water depth in the eastern black sea offshore georgia hydroacoustic imaging of the water column using multibeam echosounder evidenced numerous gas emissions from both structures gas bubbles rose as high as 45 m below sea surface it is proposed that oil coatings around gas bubbles hamper their dissolution allowing them to reach the sea surface where widespread oil slicks are formed slow rise velocities 14 1 cm s 1 of bubbles with radii of 2 6 0 3 mm n 101 derived from video data obtained with the remotely operated vehicle marum quest4000 corroborate the assumption that bubbles are oil coated high resolution seafloor mapping with the autonomous underwater vehicle marum seal5000 at colkheti seep revealed a crater strewn morphology whose formation is explained by frequent rafting of shallow gas hydrate deposits satellite imaging of oil slicks on the sea surface above both sites indicates that oil seepage is rather persistent since 2003 an order of magnitude estimation of minimum oil seepage rates suggests discharge rates of 40 liters per hour from both sites the data presented are the first comprehensive description of oil seepage in the black sea

bacterial methane oxidation in sea floor gas hydrate significance to life in extreme environments samples of thermogenic hydrocarbon gases from vents and gas hydrate mounds within a sea floor chemosynthetic community on the gulf of mexico continental slope at about 540 m depth were collected by research submersible the study area is characterized by low water temperature mean 7 c high pressure about 5 400 kpa and abundant structure ii gas hydrate bacterial oxidation of hydrate bound methane ch sub 4 is indicated by three isotopic properties of gas hydrate samples relative to the vent gas from which the gas hydrate formed 1 methane bound methane is enriched in sup 13 c by as much as 3 8 pdb peedee belemnite 2 hydrate bound methane is enriched in deuterium d by as much as 37 smow standard mean ocean water and 3 hydrate bound carbon dioxide co sub 2 is depleted in sup 13 c by as much as 22 4 pdb hydrate associated authigenic carbonate rock is also depleted in sup 13 c bacterial oxidation of methane is a driving force in chemosynthetic communities and in the concomitant precipitation of authigenic carbonate rock that modifies sea floor geology bacterial oxidation of hydrate bound methane expands the potential boundaries of life in extreme environments

split beam echo sounder observations of natural methane seep variability in the northern gulf of mexico a method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 khz scientific split beam echo sounder sbes was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern gulf of mexico a total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the sbes beam pattern and the geometry of plume ensonification these profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 khz and showed significant variability between repeat observations at time intervals of 1 h 7 5 months active gas venting was observed during approximately one third of the survey passes at each cluster while gas flux is not estimated directly in this study owing to lack of bubble size distribution data repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours the minimum depths of acoustic plume observations at 18 khz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers suggesting acoustic masking of the gas bubble plumes in these layers minimum plume depth estimates were limited by the sbes field of view in only five instances

domainal fabrics of hematite in schistose shear zone hosted high grade fe ores the product of the interplay between deformation and mineralization abstract schistose high grade hematite orebodies 64 wt fe in the iron quadrangle minas gerais were formed in shear zones by hydrothermal alteration of the paleoproterozoic caue bif during the transamazonian orogenesis the ore is comprised of platy hematite specularite grains that define the foliation and overprint a relict banded martite hematite fabric resembling at first sight a mylonite the ebsd analyses of a m scale schistose orebody from the pau branco mine show that specularite grew as elongated plates with the 00 1 plane parallel to the foliation the population of the measured grain aspect ratio gar is homogenous in different scales and the longest axes of the crystals align with the stretching lineation l x building continuous domains or anastomose around stretched iron oxide aggregates and rootless fold hinges the pole figure of the 00 1 plane shows usually a maximum centered on the pole of the foliation z often elongated on a girdle perpendicular to the lineation l the 10 4 pole figure has the configuration of a symmetric cleft girdle and the corresponding 11 0 and 10 0 pole figures present well developed girdles parallel to the foliation with an elongated maximum centered on x microstructures associated with crystal plastic behavior and dynamic recrystallization are missing and the fabric of the orebody probably results from precipitation of strain controlled oriented hematite plates and anisotropic syntaxial growth of favorably oriented grains with the intervention of hydrothermal fluids during fe enrichment the shear zone provided pathways for the percolation of mineralizing fluids under temperatures that varied from 140 to 350 c or higher under ductile or ductile brittle conditions the orthorhombic fabric and cpo crystallographic preferred orientation of the ore nevertheless contrast with the asymmetry of simple shear as observed in the torsion experiments by siemes et al 2010 siemes et al 2011 probably due to volume loss and possibly a flattening component of deformation in the ore zone

fabric development in brittle to ductile shear zones brittle to ductile shear zones from two separate geological settings are shown to have nucleated on zones of predominantly brittle deformation the shear zones are not simply foliated cataclasites since they contain abundant evidence of dynamic recrystallization of constituent minerals a small quartz diorite lens in the borrego springs shear zone southern california contains centimeter scale cataclasite zones that exhibit a gradual transition into foliated rock alteration of magnesiohornblende to actinolite feldspar to white mica plus quartz and biotite to chlorite produced elongate minerals that define the foliation during the later stages of deformation intracrystalline slip and dynamic recrystallization of quartz and feldspar were important deformation mechanisms