salt related structures in the gulf of mexico a field guide for geophysicists in finding economic quantities of oil and gas below a salt sheet the mahogany discovery well in ship shoal south addition offshore louisiana dramatically boosted exploration interest in the gulf of mexico s subsalt play in shallow water interest was initially aroused by the earlier deep water discovery below a salt sheet forming part of mickey mouse a cluster of pancake shaped salt sheets in mississippi canyon block 211 a major technologic obstacle in this play has been the difficulty of imaging the base of salt sheets and the subsalt structures targeted for exploration some recent articles have showcased the spectacular fidelity of 3 d seismic data sets collected on closely spaced survey lines and processed by techniques such as 3 d prestack depth migration and 3 d dip moveout followed by 3 d turning ray kirchhoff migration however at present the cost of this technology limits its use to specific rigorously selected targets

the δ18o of primary and secondary waters in hydrous volcanic glass abstract we present analyses of phyllosilicates volcanic glass and a series of experiments and observations on the 18o of water extracted by rapid thermal pyrolysis at 1450 c using the tcea thermal conversion elemental analyzer the study includes the same hydrous glasses that were previously analyzed for their d values of extracted water seligman et al 2016 we utilize natural phyllosilicates where water is present as oh only natural magmatic and experimentally quenched glasses in which water occurs as h2om and oh and glasses that underwent low temperature secondary hydration by meteoric water almost completely as h2om our study documents that 1 thermal extraction and simultaneous pyrolysis of h2ot into co and h2 produces little or no exchange

reverse time migration of multiples for subsalt imaging some hydrocarbon reservoirs are trapped beneath salt bodies where seismic imaging is greatly challenged due to poor illumination multiple reflections have different propagation wave paths from primary reflections and thus can be used to complement the illuminations where primary reflections from beneath the salt are not acquired consequently migration of multiples can sometimes provide better subsalt images compared to conventional migration which uses primary reflections only in this paper we propose to modify conventional reverse time migration so that multiples can be used as constructive reflection energy for subsalt imaging this new approach replaces the impulsive source wavelet with the recorded data containing both primaries and multiples and uses predicted multiples as the input data instead of primary reflections in the reverse time migration process multiples recorded on the surface are extrapolated backward in time to each depth level and the observed data with both primaries and mult

sulfur in serpentinized oceanic peridotites serpentinization processes and microbial sulfate reduction the mineralogy contents and isotopic compositions of sulfur in oceanic serpentinites reflect variations in temperatures and fluid fluxes serpentinization of 1 ma peridotites at hess deep occurred at high temperatures 200 400 c and low water rock ratios oxidation of ferrous iron to magnetite maintained low o2 and produced a reduced low sulfur assemblage including nife alloy small amounts of sulfate reduction by thermophilic microbes occurred as the system cooled producing low 34s sulfide 1 5 to 23 7 in contrast serpentinization of iberian margin peridotites occurred at low temperatures 20 200 c and high water rock ratios complete serpentinization and consumption of ferrous iron allowed evolution to higher o2 microbial reduction of seawater sulfate resulted in addition of low 34s sulfide 15 to 43 and formation of higher sulfur assemblages that include valleriite and pyrite the high so4 total s ratio of hess deep serpentinites 0 89 results in an increase of total sulfur and high 34s of total sulfur mean 8 in contrast iberian margin serpentinites gained large amounts of 34s poor sulfide mean total s 3800 ppm and the high sulfide total s ratio 0 61 results in a net decrease in 34s of total sulfur mean 5 thus serpentinization is a net sink for seawater sulfur but the amount fixed and its isotopic composition vary significantly serpentinization may result in uptake of 0 4 14 1012 g s yr 1 from the oceans comparable to isotopic exchange in mafic rocks of seafloor hydrothermal systems and approaching global fluxes of riverine sulfate input and sedimentary sulfide output

composition and origin of rhyolite melt intersected by drilling in the krafla geothermal field iceland the iceland deep drilling project well 1 was designed as a 4 to 5 km deep exploration well with the goal of intercepting supercritical hydrothermal fluids in the krafla geothermal field iceland the well unexpectedly drilled into a high silica 76 5 sio2 rhyolite melt at approximately 2 1 km some of the melt vesiculated while extruding into the drill hole but most of the recovered cuttings are quenched sparsely phyric vesicle poor glass the phenocryst assemblage is comprised of titanomagnetite plagioclase augite and pigeonite compositional zoning in plagioclase and exsolution lamellae in augite and pigeonite record changing crystallization conditions as the melt migrated to its present depth of emplacement the in situ temperature of the melt is estimated to be between 850 and 920 c based on two pyroxene geothermometry and modeling of the crystallization sequence volatile content of the glass indicated partial degassing at an in situ pressure that is above hydrostatic 16 mpa and below lithostatic 55 mpa the major element and minor element composition of the melt are consistent with an origin by partial melting of hydrothermally altered basaltic crust at depth similar to rhyolite erupted within the krafla caldera chondrite normalized ree concentrations show strong light ree enrichment and relative flat patterns with negative eu anomaly strontium isotope values 0 70328 are consistent with mantle derived melt but oxygen and hydrogen isotope values are depleted 3 1 and 118 respectively relative to mantle values the hydrogen isotope values overlap those of hydrothermal epidote from rocks altered by the meteoric water recharged krafla geothermal system the rhyolite melt was emplaced into and has reacted with a felsic intrusive suite that has nearly identical composition the felsite is composed of quartz alkali feldspar plagioclase titanomagnetite and augite emplacement of the rhyolite magma has resulted in partial melting of the felsite accompanied locally by partial assimilation the interstitial melt in the felsite has similar normalized sio2 content as the rhyolite melt but is distinguished by higher k2o and lower cao and plots near the minimum melt composition in the granite system augite in the partially melted felsite has re equilibrated to more calcic metamorphic compositions rare quenched glass fragments containing glomeroporphyritic crystals derived from the felsite show textural evidence for resorption of alkali feldspar and quartz the glass in these fragments is enriched in sio2 relative to the rhyolite melt or the interstitial felsite melt consistent with the textural evidence for quartz dissolution the quenching of these melts by drilling fluids at in situ conditions preserves details of the melt wall rock interaction that would not be readily observed in rocks that had completely crystallized however these processes may be recognizable by a combination of textural analysis and in situ analytical techniques that document compositional heterogeneity due to partial melting and local assimilation

the chemistry of geothermal waters in iceland ii mineral equilibria and independent variables controlling water compositions abstract the major element chemistry of icelandic geothermal waters is predictable provided two parameters are known this follows from an attainment of or a close approach to an overall chemical equilibrium in the geothermal systems at temperatures as low as 50 c it is considered that the geothermal system composition temperature and kinetic factors determine which alteration minerals form the system composition is not so much fixed by rock composition as by the rate of leaching of the various constituents from the fresh rock and the composition of inflowing water the water chemistry is determined by the system composition and the external variables acting on the system they include temperature and the mobility of chloride pressure which theoretically should be regarded as an external variable has insignificant effect on water compositions in the range 1 200 bars occurring in the geothermal systems

the geochemistry of trace elements in geothermal fluids iceland abstract trace element geochemistry was studied in geothermal fluids in iceland the major and trace element compositions of hot springs sub boiling and two phase liquid and vapor wells from 10 geothermal areas were used to reconstruct the fluid composition in the aquifers at depth aquifer fluid temperatures ranged from 4 to 300 c ph values between 4 5 and 9 3 and fluids typically contained total dissolved solids

dirty salt velocity inversion the road to a clearer subsalt image for deep water gulf of mexico accurate salt geometry is criticaltosubsaltimaging thisrequiresthedefinitionofboth externalandinternal saltgeometries inrecentyears external salt geometry i e boundaries between allochthonous salt and background sediment has improved a great deal due to advances in acquisition velocity model building and migration algorithms but when it comes to defining internal salt geometry i e intrasalt inclusions or dirty salt no efficient method has yet been developed in common industry practices intrasalt inclusions and thus their velocity anomalies are generally ignored during the model building stages however as external salt geometries reach higher levels of accuracy it becomes more important to consider the once ignored effects of dirty salt we have developed a reflectivity based approach for dirty salt velocity inversion this method takes true amplitude reverse time migration stack volumes as input then estimates the dirty salt velocity based on reflectivity under a 1d assumption results from a 2d synthetic data set and a real 3d wide azimuth data set demonstrated that the reflectivity inversion scheme significantly improves the subsalt image for certain areas in general we believe that this method produces a better salt model than the traditional clean salt velocity approach

advances in velocity model building technology for subsalt imaging seismic imaging of hydrocarbon accumulations below salt is a formidable challenge because complexly shaped salt bodies severely distort wavefronts or scatter seismic energy we have highlighted some recent advances in building a velocity model for subsalt imaging there are three main stages suprasalt velocity determination salt model definition and subsalt velocity update volumetric high resolution tomography that incorporates high velocity contrast boundaries is used to derive a good sediment velocity model before building a salt model to facilitate integration of interpretation and depth processing beam based interactive imaging is used to refine the salt geometry for subsalt velocity update either subsalt tomography or subsalt scan based techniques can be used depending on the quality of subsalt reflections there are concepts and techniques for attaining subsalt images suitable for hydrocarbon exploration beneath complexly shaped salt bodies

benthic foraminifera from the necop study area impacted by the mississippi river plume and seasonal hypoxia abstract benthic foraminifera influenced by the mississippi river plume and seasonal hypoxia were assessed from louisiana inner continental shelf sediment samples surface foraminifera assemblages were representative of in situ populations as established by staining techniques community diversity and richness evenness analyses indicate three regimes high stress sediment dominated intermediate stress hypoxia dominated and low stress low sediment accumulation high oxygen epistominella vitrea and buliminella morgani are useful tracers of rapid sediment accumulation rate and hypoxia a bottom water productivity signal west of the mississippi river plume is indicated by benthic and planktic foraminifera abundance peaks surface benthic foraminifera trends are utilized to interpret changes in historical community structure from hypoxic area sediments deposited since the turn of the century the hypoxia tolerant species buliminella morgani increases markedly upcore while hypoxia intolerant species decrease or disappear diversity and dominance trends temporally correspond to a dramatic increase in u s fertilizer application the results of this study have application to paleoenvironmental research spanning longer geologic timescales the documented relationships between population structure and stressors in river dominated marine systems may provide a useful analog for recognition of these conditions in the fossil record

variations of the isotopic composition of sulfur in enstatite and ordinary chondrites abstract high precision sulfur isotopic analyses 33s 34s and 36s of bulk ordinary and enstatite chondrites demonstrate that systematic variations exist the average 34s values are 0 26 0 07 0 02 0 06 and 0 49 0 16 for enstatite and ordinary and carbonaceous chondrites respectively isotopic variations of different sample specimens of primitive meteorites e g qingzhen and abee have been observed which may be attributed to heterogeneity in the early solar nebula sulfur isotopic fractionations in both bulk samples and mineral separates are mass dependent and no nuclear isotopic anomalies have been detected the sulfur isotopic compositions of both mineral and density separates have been measured the sulfur isotopic compositions of separated chondrules from chainpur and bjurbole are reported significant isotopic difference for the chondrules from the bulk meteorite are noted for both meteorites

superwide angle one way wave propagator and its application in imaging steep salt flanks one way wave propagators have been used extensively in forward and inverse problems the advantages of one way wave propagators are the fast speed of computation and the large savings in computer resources however regular one way wave propagators have inherent angle limitation to within 90 of the preferred direction e g z direction we propose to overcome the limitation by a superwide angle one way method which combines and interpolates two orthogonally propagated one way wavefields in the 2d case a weight function determined by the propagation angle is employed to combine the two one way wavefields in the current implementation we use the wavefield gradient to estimate the propagation angle numerical experiments show that the proposed method has accurate superwide angle 90 propagation and can model turning waves well by applying the superwide angle one way wave propagator on several synthetic model data sets we can demonstrate the validity of the method in imaging steep and overhanging

hydro geochemical and isotopic fluid evolution of the los azufres geothermal field central mexico abstract hydrothermal alteration at los azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system the deepest fluid inclusions proto fluid are liquid rich with low salinity with nacl dominant fluid type and ice melting temperatures t mi near zero 0 c and salinities of 0 8 wt nacl equivalent the homogenization temperature t h 325 5 c the boiling zone shows t h 300 c and apparent salinities between 1 and 4 9 wt nacl equivalent implying a vaporization process and a very important participation of non condensable gases ncgs mostly co 2 positive clathrate melting temperatures fusion with t h 150 c are observed in the upper part of the geothermal reservoir from 0 to 700 m depth these could well be the evidence of a high gas concentration the current water produced at the geothermal wells is nacl rich geothermal brine and is fully equilibrated with the host rock at temperatures between t 300 and 340 c the hot spring waters are acid sulfate indicating that they are derived from meteoric water heated by geothermal steam the ncgs related to the steam dominant zone are composed mostly of co 2 80 98 of all the gases the gases represent between 2 and 9 wt of the total mass of the fluid of the reservoir the authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface boiling is caused by a drop of pressure which favors an increase in the steam phase within the brine ascending towards the surface during this ascent the fluid becomes steam dominant in the shallowest zone and mixes with meteoric water in perched aquifers stable isotope compositions 18 o d of the geothermal brine indicate mixing between meteoric water and a minor magmatic component the enrichment in 18 o is due to the rock water interaction at relatively high temperatures 13 c stable isotope data show a magmatic source with a minor meteoric contribution for co 2 the initial isotopic value 34 s res 2 3 which implies a magmatic source more negative values are observed for shallow pyrite and range from 34 s fes 2 4 to 4 9 indicating boiling the same fractionation tendencies are observed for fluids in the reservoir from results for 18 o

the dynamics of the mississippi river plume impact of topography wind and offshore forcing on the fate of plume waters 1 high resolution numerical simulations of the northern gulf of mexico region using the hybrid coordinate ocean model hycom were employed to investigate the dynamical processes controlling the fate of the mississippi river plume in particular the conditions that favor cross marginal transport the study focuses on the effects of topography wind driven and eddy driven circulation on the offshore removal of plume waters a realistically forced simulation nested in a data assimilative regional gulf of mexico hycom model reveals that the offshore removal is a frequent plume pathway eastward wind driven currents promote large freshwater transport toward the shelf break and the desoto canyon where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters our estimates show that the offshore removal by eddies can be as large as the wind driven shelf transport the proximity of eddies to the shelf break is a sufficient condition for offshore removal and shelf to offshore interaction is facilitated by the steep bottom topography near the delta strong eddy plume interactions were observed when the loop current system impinged against the shelf break causing the formation of coherent narrow low salinity bands that extended toward the gulf interior the offshore pathways depend on the position of the eddies near the shelf edge their life span and the formation of eddy pairs that generate coherent cross shelf flows this study elucidates the dynamics that initiate a unique cross marginal removal mechanism of riverine low salinity nutrient rich waters allowing their export along connectivity pathways induced by a large scale current system

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

stable isotopic studies of mafic sills and proterozoic metasedimentary rocks located beneath the duluth complex minnesota abstract thin 3 20 m picritic to noritic sills are found in the proterozoic metasedimentary footwall rocks of the duluth complex minnesota the sills were emplaced prior to the major intrusions that comprise the duluth complex and underwent contact metamorphism along with the country rocks oxygen hydrogen and sulfur isotopic compositions of the sills indicate a varied history of isotopic exchange between minerals melts and hydrothermal fluids in the high temperature environment below the major plutonic bodies of the midcontinent rift system the pre duluth complex sills exhibit a range in 18o values from 4 9 to 14 8 with values between 6 and 7 generally found in sill interiors high 18o values near sill contacts with high 18o metasedimentary rocks of the pelitic virginia formation or biwabik iron formation coupled with a smooth sigmoidal isotopic profile centered at the contact suggest that oxygen diffusion was an important exchange mechanism the elevated 18o values near the center of the thickest sills are thought to reflect the emplacement of isotopically contaminated basaltic magma dehydration reactions in the pelitic rocks of the contact aureole liberated high 18o fluids that enhanced subsolidus diffusive exchange advective displacement of the diffusion profiles toward the sill interior is less than 40 cm and suggests that layer parallel flow dominated in the dehydration of the contact aureole elevated but uniform 18o values 9 7 to 10 5 in thinner sills suggests that oxygen diffusivity was increased relative to country rocks due to enhanced porosity perhaps related to extensive development of microcracks although d values of the pelitic country rocks record a history of dehydration systematic variations of d 64 to 143 and h2o 0 15 to 5 40 wt content are not found in the sills 18o values of coexisting plagioclase and pyroxene from the sills indicate a close approach to isotopic equilibrium and are consistent with a diffusion dominant exchange process at temperatures near 500 c results of diffusion modelling suggest a duration of isotopic exchange that may have extended from tens of thousands of years to 1 4 ma depending on local controls of porosity and permeability as well as rates of fluid production in the contact aureole localized areas of 18o and d depletion in the sills values as low as 4 9 and 143 respectively denote exchange with meteoric water after interaction with the high 18o metamorphic fluids in the contact aureole although all of the elevated 18o samples in the contact environment may have suffered 18o depletion most exchange with meteoric water appears to be spatially localized and is thought to reflect highly channelized fracture controlled fluid flow sulfur isotopic values of the sills are variable 2 7 to 11 2 and indicative of an evolution involving pre emplacement contamination of basaltic magma and sub solidus exchange with an h2s bearing metamorphic fluid sulfur contents exceed 3 0 wt only within troctolitic to melatroctolitic sills and 34s values of 7 8 to 8 3 are strongly suggestive of pre emplacement contamination by sulfur derived from a proterozoic sedimentary unit high 18o and 34s rocks particularly at sill margins are consistent with either hydrothermal precipitation of fine grained sulfide minerals or isotopic exchange between magmatic sulfides and an h2s bearing metamorphic fluid

chemical transport in geothermal systems in iceland evidence from hydrothermal alteration abstract this study focuses on the chemical changes in basaltic rocks in fossil low and high temperature hydrothermal systems in iceland the method used takes into account the amount of dilution caused by vesicle and vein fillings in the rocks the amount of dilution allows a calculation of the primary concentration of the immobile element zr and by multiplying the composition of the altered rock by the ratio of zr protolith zr altered rock one can compute the mass addition caused by the dilution of the void fillings and also make a direct comparison with the likely protoliths from the same areas the samples were divided into three groups two from tertiary fossil high temperature systems hafnarfjall geitafell and the third group from a low temperature zeolite altered plateau basalt succession the results show that hydrothermally altered rocks are enriched in si al fe mg and mn and that na k and ca are mobile but show either depletion or enrichment the elements that are immobile include zr y nb and probably ti the two high temperature systems show quite similar chemical alteration trends an observation which may apply to icelandic fresh water high temperature systems in general the geochemical data show that the major changes in the altered rocks from icelandic geothermal systems may be attributed to addition of elements during deposition of pore filling alteration minerals a comparison with seawater dominated basalt hosted hydrothermal systems shows much greater mass flux within the seawater systems even though both systems have similar alteration assemblages the secondary mineral assemblages seem to be controlled predominantly by the thermal stability of the alteration phases and secondarily by the composition of the hydrothermal fluids

opaque minerals in geothermal well no 7 krafla northern iceland abstract study of the opaque minerals from well no 7 krafla indicates two mineral assemblages 1 hydrothermally altered igneous minerals and 2 secondary minerals that have precipitated from the geothermal fluid at depths down to 2140 m and at temperatures up to more than 340 c chief amongst the chemically precipitated minerals are pyrite pyrrhotite and goethite which is described here for the first time in an icelandic geothermal drill hole the geothermal system at krafla has been periodically disturbed by the influx of volcanic emanations this article attempts to interpret by use of thermochemical calculations the processes affecting the precipitated mineral assemblage

origin of a rhyolite that intruded a geothermal well while drilling at the krafla volcano iceland magma fl owed into an exploratory geothermal well at 2 1 km depth being drilled in the krafl a central volcano in iceland creating a unique opportunity to study rhyolite magma in situ in a basaltic environment the quenched magma is a partly vesicular sparsely phyric glass containing 1 8 of dissolved volatiles based on calculated h 2 o co 2 saturation pressures it degassed at a pressure intermediate between hydrostatic and lithostatic and geothermometry indicates that the crystals in the melt formed at 900 c the glass shows no signs of hydrothermal alteration but its hydrogen and oxygen isotopic ratios are much lower than those of typical mantle derived magmas indicating that this rhyolite originated by anhydrous mantle derived magma assimilating partially melted hydrothermally altered basalts

mass dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles abstract sulfur isotope studies of post archean terrestrial materials have focused on the ratio 34s 32s because additional isotopes 33s and 36s were thought to carry little information beyond the well known mass dependent relationship among multiple isotope ratios we report high precision analyses of 33s and 36s values defined as deviations of 33s and 36s from ideal mass dependent relationships for international reference materials and sedimentary sulfides of phanerozoic age by using a fluorination technique with a dual inlet isotope ratio mass spectrometer measured variations in 33s and 36s are explained as resulting from processes involve branching reactions two or more reservoirs formed or mixing irreversible processes in closed systems rayleigh distillation amplify the isotope effect we outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation the isotope systematics discussed here cannot explain the much larger variation of 33s and 36s observed in archean rock records furthermore phanerozoic samples we have studied show a characteristic 33s and 36s relationship that differs from those measured in archean rocks and laboratory photolysis experiments thus high precision analysis of 33s and 36s can be used to distinguish small non zero 33s and 36s produced by mass dependent processes from those produced by mass independent processes in archean rocks and extraterrestrial materials

geothermal surface alteration of basalts krýsuvik iceland alteration mineralogy water chemistry and the effects of acid supply on the alteration process abstract the geothermal surface alteration of basalts and associated water chemistry at kr suvik sw iceland were studied the geothermal area was characterised with zones of intensive surface alteration steam vents mud pots and hot springs the steam heated geothermal surface waters had ph values between 1 69 and 7 67 and total dissolved solid tds concentrations between 154 and 6660 ppm with cl and so 4 concentration decreasing and increasing with decreasing ph respectively alteration mineral assemblages observed were strongly associated with the surface intensity in areas of most intensive activity the basaltic rocks were altered to amorphous silica anatase and pyrite with a crust of native sulphur at the surface with decreased activity kaolinite became important as well as iron oxyhydroxides and oxides on the flanks of the area montmorillonite was the predominant alteration product based on these observations the surface geothermal activity was divided into three groups 1 high activity areas with active steam vents and mud pots and intensive acid leaching 2 medium activity areas where the ground is hot steam vents and mud pots are uncommon and the surface alteration is less intensive and 3 low activity areas on the margins of the surface geothermal activity the primary factors influencing the steam heated acid sulphate alteration of basalts included the redox state oxidation front supply of acids and ph and the extent of reaction the formation of iron and sulphur containing minerals and the respective elemental mobility depended on the redox conditions with pyrite formation under reduced conditions and goethite and or hematite under oxidised conditions at low ph ca mg k and na were mobile and leached out whereas fe ti and al and to a large degree si were retained in the alteration product at higher ph values 5 the mobility of ca mg k and na was reduced due to the formation of clays

heat flux from magmatic hydrothermal systems related to availability of fluid recharge abstract magmatic hydrothermal systems are of increasing interest as a renewable energy source surface heat flux indicates system resource potential and can be inferred from soil co 2 flux measurements and fumarole gas chemistry here we compile and reanalyze results from previous co 2 flux surveys worldwide to compare heat flux from a variety of magma hydrothermal areas we infer that availability of water to recharge magmatic hydrothermal systems is correlated with heat flux recharge availability is in turn governed by permeability structure lithology rainfall topography and perhaps unsurprisingly proximity to a large supply of water such as the ocean the relationship between recharge and heat flux interpreted by this study is consistent with recent numerical modeling that relates hydrothermal system heat output to rainfall catchment area this result highlights the importance of recharge as a consideration when evaluating hydrothermal systems for electricity generation and the utility of co 2 flux as a resource evaluation tool

sulfur 33 constraints on the origin of secondary pyrite in altered oceanic basement abstract low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur which commonly yields low 34s values suggesting involvement of microbial sulfate reduction in order to test whether secondary sulfide minerals are consistent with a biogenic source we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks namely serpentinized peridotites and altered basalts samples from two peridotite sites iberian margin and hess deep and from a basalt site on the eastern flank of the juan de fuca ridge yield overlapping 34s values ranging from 0 to 44 in contrast sulfides in the basalt site are characterized by relatively low 33s values ranging from 0 06 to 0 04 compared to those from peridotite sites 0 00 to 0 16 the observed 33s signal is significant considering the analytical precision of 0 014 2 we present a batch reaction model that uses observed 34s and 33s relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction the estimated enrichment factors as large as 61 and 53 for peridotite and basalt sites respectively suggest the involvement of microbial sulfate reduction the relatively high 33s values in the peridotite sites are due to sulfate reduction in a closed system environment whereas negative 33s values in the basalt site reflect open system sulfate reduction a larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher h2 production capacity compared to basalt alteration and further supports in situ microbial sulfate reduction coupled with h2 production during serpentinization reactions

origin and history of hydrothermal fluids of the reykjanes and krafla geothermal fields iceland the reykjanes and krafla geothermal fields are both examples of active high temperature systems and show similar assemblages of alteration minerals but the fluid at reykjanes is dominantly sea water whereas that at krafla is meteoric oxygen isotope analyses of surface rock and of drill chip samples from different depths are presented together with results for the krafla fluid which is close to local precipitation 18o 11 9 d 86 8

a time series of vent fluid compositions from 21 n east pacific rise 1979 1981 1985 and the guaymas basin gulf of california 1982 1985 a time series study of four hot spring fields at 21 n on the east pacific rise demonstrates that the solution compositions of mid ocean ridge hydrothermal vents are stable on a decadal time scale the ocean bottom seismometer obs national geographic smoker ngs and south west sw vents show no significant changes in the major element composition of their end member fluids since 1979 two of the vents obs and sw have maintained exit temperatures near 350 c since 1979 the ngs vent has remained close to 270 c since 1981 after cooling from 350 c in 1979 only one vent hanging gardens shows important changes in solution chemistry from 1981 to 1985 in 1985 this vent appears to have cooled 10 c relative to the other vents the depth of the reaction zone as determined by quartz geobarometry has increased by about 1 km at this vent the large fluxes of compositionally stable fluids from these vents cannot be explained by transient conditions and unique circumstances rather the constancy of small interfield variations in solution composition is more consistent with control by rock buffering a 4 year time series study of hydrothermal vents in the guaymas basin also shows no significant changes in the major element composition of the end member fluids interfield differences in solution chemistry have been maintained from 1982 to 1985 this phenomenon is best explained in terms of an equilibrium model

stable isotope geochemistry of the pierina high sulfidation au ag deposit peru influence of hydrodynamics on so42 h2s sulfur isotopic exchange in magmatic steam and steam heated environments abstract the pierina high sulfidation au ag deposit formed 14 5 my ago in rhyolite ash flow tuffs that overlie porphyritic andesite and dacite lavas and are adjacent to a crosscutting and interfingering dacite flow dome complex the distribution of alteration zones indicates that fluid flow in the lavas was largely confined to structures but was dispersed laterally in the tuffs because of a high primary and alteration induced permeability the lithologically controlled hydrodynamics created unusual fluid temperature and ph conditions that led to complete so 4 2 h 2 s isotopic equilibration during the formation of some magmatic steam and steam heated alunite a phenomenon not previously recognized in similar deposits isotopic data for early magmatic hydrothermal and main stage alunite 34 s 8 5 to 31 7 18 o so 4 4 9 to 16 5 18 o oh 2 2 to 14 4 d 97 to 39 sulfides 34 s 3 0 to 4 3 sulfur 34 s 1 0 to 1 1 and clay minerals 18 o 4 3 to 12 5 d 126 to 81 are typical of high sulfidation epithermal deposits the data imply the following genetic elements for pierina alteration mineralization 1 fluid and vapor exsolution from an i type magma 2 wallrock buffering and cooling of slowing rising vapors to generate a reduced h 2 s so 4 6 highly acidic condensate that mixed with meteoric water but retained a magmatic 34 s s signature of 1 3 so 2 disproportionation to hso 4 and h 2 s between 320 and 180 c and 4 progressive neutralization of laterally migrating acid fluids to form a vuggy quartz alunite quartz clay intermediate argillic propylitic alteration zoning magmatic steam alunite has higher 34 s 8 5 to 23 2 and generally lower 18 o so 4 1 0 to 11 5 18 o oh 3 4 to 5 9 and d 93 to 77 values than predicted on the basis of data from similar occurrences these data and supporting fluid inclusion gas chemistry imply that the rate of vapor ascent for this environment was unusually slow which provided sufficient time for the uptake of groundwater and partial to complete so 4 2 h 2 s isotopic exchange the slow steam velocities were likely related to the dispersal of the steam column as it entered the tuffs and possibly to intermediate exsolution rates from magmatic brine the low d values may also partly reflect continuous degassing of the mineralizing magma similarly data for steam heated alunite 34 s 12 3 to 27 2 18 o so 4 11 7 to 13 0 18 o oh 6 6 to 9 4 d 59 to 42 are unusual and indicate a strong magmatic influence relatively high temperatures 140 to 180 c based on 18 o so 4 oh fractionations and partial to complete sulfur isotopic exchange between steam heated sulfate and h 2 s restricted lithologically controlled fluid flow in the host tuffs allowed magmatic condensate to supplant meteoric groundwater at the water table and create the high temperature low ph conditions that permitted unusually rapid so 4 2 h 2 s isotopic equilibration 50 300 days and or long sulfate residence times for this environment late void filling barite 34 s 7 4 to 29 7 18 o so 4 0 4 to 15 1 and later void filling goethite 18 o 11 8 to 0 2 document a transition from magmatic condensate to dominantly meteoric water in steam heated fluids during cooling and collapse of the hydrothermal system these steam heated fluids oxidized the top 300 m of the deposit by leaching sulfides redistributing metals and precipitating barite acanthite gold and goethite hematite gold steam heated oxidation rather than weathering was critical to forming the orebody in that it not only released encapsulated gold but likely enriched the deposit to ore grade au concentrations

magma bright spots mapped beneath krafla iceland using rvsp imaging of reflected waves from microearthquakes abstract the geometry and distribution of magma in the crust remain controversial topics with recent studies questioning the role of large magma chambers in this investigation high resolution 3d reflection images of crustal discontinuities beneath the krafla geothermal field in northern iceland were generated by applying vertical seismic profiling vsp techniques adapted from reflection seismology to microearthquake data exceptionally large amplitude reflections bright spots at a depth of 2 1 km correlate with rhyolitic magma encountered in the iddp 1 borehole although similarly bright reflectors at about 4 km correspond in depth to the top of an inferred magma chamber from previous seismic studies the scattered reflectivity that persists beneath this deeper reflector argues for a distributed magma system rather than a large feeder chamber

modeling the impact of wide azimuth acquisition on subsalt imaging wide azimuth towed streamer wats acquisition improves the subsalt seismic image by suppressing multiples improves the results of 3d surface related multiple elimination srme processing and provides more uniform seismic illumination of subsalt targets a simple model shows that the additional suppression of multiples in the case of wats acquisition is the result of a natural weighting of the traces going into the stack due to the areal nature of the acquisition this simple model also shows that the extent of the additional multiple suppression is strongly dependent on the acquisition effort a sparse acquisition effort will result in little additional multiple suppression the use of 3d srme processing is shown to be more accurate in predicting multiples given input data with multiple azimuths compared to making similar predictions from narrow azimuth data three dimensional srme has the potential to reduce the residual multiples to the same extent as wats acquisition with a higher acquisition eff

organic carbon production and preservation in response to sea level changes in the turonian carlile formation u s western interior basin abstract a primary sea level control over the distribution of total organic and carbonate carbon and organic matter type can be inferred in the early to middle turonian carlile formation fm western interior basin united states the conceptual model relies on chemo and lithostratigraphic correlations of lower to mid turonian strata in the central kwis supported by ammonite biostratigraphy and is based primarily on lithologic gamma ray spectrometric and geochemical facies analysis of the usgs portland no 1 core from central colorado the amoco rebecca bounds no 1 core from western kansas and the hawarden core from northwestern iowa sedimentation in the central marine axial basin of the cretaceous western interior seaway kwis during the turonian mostly reflects deposition by pelagic settling and from nepheloid layers with winnowing by bottom currents relatively high total organic carbon toc carbonate caco3 and rock eval pyrolysis hydrogen index hi values correspond to transgressive or highstand episodes within the overall regressive sequence whereas low values of these parameters characterize regressive intervals the lower fairport shale member of the carlile fm and coeval strata in iowa were deposited during a second order sea level highstand the waning stages of the greenhorn cyclothem an overall shallowing and coarsening upward sequence characterizes the overlying majority of the carlile fm this trend is punctuated by a short term transgressive episode with associated retrograde facies and a disconformity earlier studies document relatively high productivity during the turonian nutrient input to the seaway required to sustain water column productivity is difficult to account for solely by riverine inputs thus a model of transgressive flooding of preconditioned oxygen deficient nutrient rich water from the global ocean into the kwis is invoked this advection of nutrients and low oxygen water also helped to create broadly distributed dysoxic to anoxic conditions in the seaway which would otherwise have been difficult to maintain in a relatively well mixed shallow sea as the seaway regressed river supplied sea surface nepheloid layers provided sufficient nutrient inputs and occasionally established temporary stratification of the water column and thus contributed to maintaining an environment poised to produce and preserve organic matter

hydrothermal flow systems in the midcontinent rift oxygen and hydrogen isotopic studies of the north shore volcanic group and related hypabyssal sills minnesota abstract rift related lavas of the north shore volcanic group nsvg are intruded by plutonic rocks of the duluth complex along the unconformity between the nsvg and the underlying proterozoic metasedimentary rocks animikie group and archean volcano sedimentary and plutonic rocks heat associated with the emplacement of the mafic intrusions generated fluid flow in the overlying plateau lavas 18o values for whole rocks from the nsvg and hypabyssal sills range from 5 5 to 17 7 and 5 3 to 11 5 respectively and most values are higher than those considered normal for basaltic rocks 5 4 to 6 0 in general there is a positive correlation between whole rock 18o and water content which suggests that elevated 18o values are related primarily to secondary mineral growth and isotopic exchange during hydrothermal alteration and metamorphism 18oh2o values computed from amygdule filling minerals such as smectite chlorite and epidote found in low to high temperature metamorphic zones range from 1 to 6 with an average value of 3 smectite in the lower grade zones gives computed dh2o values between 26 and 83 whereas epidote in the higher grade zones gives dh2o values of 15 to 6 fluid isotopic compositions computed from epidote and smectite values are suggestive of the involvement of at least two fluids during the early stages of amygdule filling fluid d and 18o values determined from epidote at the higher metamorphic grades indicate that seawater dominated the deeper portions of the system where greenschist facies assemblages and elevated 18o values were produced in flow interiors as well as margins smectite isotopic compositions suggest that meteoric water was predominant in the shallower portions of the system the increase in 18o values of massive flow interiors with depth is interpreted as a result of rock interaction with a fluid of constant oxygen isotopic composition with increasing temperature the stable isotopic data are supportive of previous suggestions that seawater was involved in the hydrothermal system associated with the midcontinent rift although the origin of the seawater remains problematic it appears that marine incursions may have occurred during the late stages of portage lake volcanism and periodically thereafter

reconstructing the rise of recent coastal anoxia molybdenum in chesapeake bay sediments abstract sporadic direct observations over a 50 yr period inadequately characterize the history of seasonal hypoxia and anoxia in chesapeake bay a large estuary threatened by eutrophication here we undertake a reconstruction of 20th century oxygen depletion in this estuary using mo concentrations in 210pb dated sediments cu concentrations are used to control for anthropogenic influences cores from the central channel display mild mo enrichments above crustal backgrounds up to 5 g g and strong cu enrichments up to 35 g g temporally cu enrichment mostly anthropogenic began earlier and stabilized in the last two thirds of the 20th century in contrast mo enrichment has grown during the last two thirds of the century molybdenum enrichment is mostly hydrogenic except in a section of the channel that receives additional mo from erosion of early miocene shore deposits two geochemical mechanisms promote mo enrichment manganese refluxing concentrates dissolved moo42 at the sediment water interface and sulfide substitution into moo42 produces thiomolybdates which can be fixed by particles the mo enrichment mechanisms operate primarily during periods when bottom waters are anoxic and thiomolybdate formation can occur near the sediment water interface this implies a temporal coupling between water column anoxia and mo fixation even though fixation occurs only within sediments the mo enrichment profiles suggest that chesapeake bay has experienced growing o2 depletion since the first half of the 20th century but especially after 1960

subsalt 3d vsp imaging at deimos field in the deepwater gulf of mexico at deimos field in the deepwater gulf of mexico surface seismic imaging cannot fully image sedimentary structure near the steeply dipping base of salt 3d vertical seismic profile vsp imaging was introduced to the deepwater gom to improve seismic imaging of complex structural and stratigraphic features ray et al 2003 hornby et al 2006 and recently has been used for imaging blind spots in surface seismic coverage hornby et al 2005 hornby et al 2007 with this method 3d surveys are conducted using a surface source vessel and downhole geophones here we investigate the use of 3d vsp imaging to complement the surface results with additional coverage beyond that of the surface seismic adjacent to the complex salt body

trace metal distributions in sulfide scales of the seawater dominated reykjanes geothermal system constraints on sub seafloor hydrothermal mineralizing processes and metal fluxes abstract mineral precipitation in the seawater dominated reykjanes geothermal system on the mid atlantic ridge iceland is caused by abrupt artificially induced pressure and temperature changes as deep high temperature liquids are drawn from reservoir rocks up through the geothermal wells sulfide scales within these wells represent a complete profile of mineral precipitation through a seafloor hydrothermal system from the deep reservoir to the low temperature silica rich surface discharge mineral scales have formed under a range of conditions from high pressures and temperatures at depth 2 km to boiling conditions in the upflow zone and at the surface consistent trace element enrichments similar to those in black smoker chimneys are documented cu zn cd co te v ni mo w sn fe and s are enriched at higher pressures and temperatures in the deepest scales zn and cu bi pb ag as sb ga hg tl u and th are enriched at lower temperatures and pressures nearer to the surface a number of elements e g co se cd zn cu and au are deposited in both high and low pressure scales but are hosted by distinctly different minerals other trace elements such as pb ag and ga are strongly partitioned into low temperature minerals such as galena pb ag and clays ga boiling and destabilization of metal bearing aqueous complexes are the dominant control on the deposition of most metals particularly au other metals e g cu and se may also have been transported in the vapor phase very large enrichments of au ag and pb in the scales e g 948 ppm au 23 200 ppm ag and 18 8 wt pb versus average concentrations in black smoker chimneys likely reflect that some elements are preferentially deposited in boiling systems a mass accumulation of 5 7 t yr of massive sulfide was calculated for one high temperature production well equating to metal fluxes of 1 7 t yr zn 0 3 t yr cu 23 kg yr pb 4 1 kg yr ag and 0 5 kg yr au at least three quarters of the major and trace element load is precipitated within the well before reaching the surface we suggest that a similar proportion of metals may be deposited below the seafloor in submarine hydrothermal systems where significant boiling has occurred mass accumulation estimations over the lifetime of the reykjanes system may indicate significant enrichment of zn pb au and ag relative to both modern and ancient mafic dominated seafloor massive sulfide deposits and highlights the potential for metal enrichment and accumulation in the deep parts of geothermal systems

the chemistry of trace elements in surface geothermal waters and steam iceland abstract the geochemistry of trace elements in surface geothermal fluids in iceland was studied the sampled fluids included hot springs mud pots steam vents and soil solutions with temperatures ranging from 4 to 100 c ph between 2 01 and 9 10 and total dissolved solids between 86 and 4375 ppm the surface geothermal waters may be categorized into three groups based on their chemical composition namely nacl waters steam heated acid sulfate waters and mixed waters nacl waters with ph 8 are considered to represent aquifer geothermal fluids that have undergone boiling in the upflow they contained only low concentrations of most metals

ion microprobe analysis of oxygen isotope ratios in quartz from skye granite healed micro cracks fluid flow and hydrothermal exchange quartz grains in hydrothermally altered granites from the isle of skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 m scale ion microprobe analysis of one grain shows a gradient of 13 over 400 m and a greater range in 18o than all quartz previously analyzed on the isle of skye other crystals from the same outcrop are homogeneous digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable the ion probe analysis correlates low 18o values on a micro scale to one set of healed cracks thus quartz exchanges oxygen isotopes primarily by solution and reprecipitation along fractures in contrast to more reactive feldspar that appears to exchange from the grain boundary inward macroscopic models of isotope exchange are not realistic for these rocks the flow of hydrothermal fluids was heterogeneous anisotropic and crack controlled

the dole effect and its variations during the last 130 000 years as measured in the vostok ice core we review the current understanding of the dole effect the observed difference between the delta sup 18 o of atmospheric o sub 2 and that of seawater and its causes extended the record of variations in the dole effect back to 130 kyr before present using data on the delta sup 18 o of o sub 2 obtained from studying the vostok ice core and discuss the significance of temporal variations the dole effect reflects oxygen isotope fractionation during photosynthesis respiration and hydrologic processes evaporation precipitation and evapotranspiration our best prediction of the present day dole effect 20 8 per thousand is considerably lower than the observed value 23 5 per thousand and we discuss possible causes of this discrepancy during the past 130 kyr the dole effect has been 0 05 per thousand lower than the present value on average the standard deviation of the dole effect from the mean has been only per thousand and the dole effect is nearly unchanged between glacial maxima and interglacial periods the small variability in the dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant most periodic variability in the dole effect is in the precession band suggesting that changes in this global biogeochemical termmore reflects variations in low latitude land hydrology and productivity or possibly variability in low latitude oceanic productivity 65 refs 4 figs 3 tabs less

sulfur geochemistry of hydrothermal waters in yellowstone national park wyoming usa ii formation and decomposition of thiosulfate and polythionate in cinder pool abstract cinder pool is an acid sulfate chloride boiling spring in norris geyser basin yellowstone national park the pool is unique in that its surface is partially covered with mm size black hollow sulfur spherules while a layer of molten sulfur resides at the bottom of the pool 18 m depth the sulfur speciation in the pool was determined on four different days over a period of two years samples were taken to evaluate changes with depth and to evaluate the importance of the sulfur spherules on sulfur redox chemistry all analyses were conducted on site using a combination of ion chromatography and colorimetric techniques dissolved sulfide h 2 s thiosulfate s 2 o 3 2 polythionates s x o 6 2 and sulfate were detected the polythionate concentration was highly variable in time and space the highest concentrations were found in surficial samples taken from among the sulfur spherules with depth the polythionate concentrations dropped off the maximum observed polythionate concentration was 8 m thiosulfate was rather uniformly distributed throughout the pool and concentrations ranged from 35 to 45 m total dissolved sulfide concentrations varied with time concentrations ranged from 16 to 48 m sulfate was relatively constant with concentrations ranging from 1150 to 1300 m the sulfur speciation of cinder pool is unique in that the thiosulfate and polythionate concentrations are significantly higher than for any other acid sulfate spring yet sampled in yellowstone national park complementary laboratory experiments show that thiosulfate is the intermediate sulfoxyanion formed from sulfur hydrolysis under conditions similar to those found in cinder pool and that polythionates are formed via the oxidation of thiosulfate by dissolved oxygen this last reaction is catalyzed by pyrite that occurs as a minor constituent in the sulfur spherules floating on the pool s surface polythionate decomposition proceeds via two pathways 1 a reaction with h 2 s yielding thiosulfate and elemental sulfur and 2 by disproportionation to sulfate and thiosulfate this study demonstrates that the presence of a subaqueous molten sulfur pool and sulfur spherules in cinder pool is of importance in controlling the pathways of aqueous sulfur redox reactions some of the insights gained at cinder pool may be relevant to acid crater lakes where sulfur spherules are observed and variations in polythionate concentrations are used to monitor and predict volcanic activity

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

multiple sulfur isotope systematics of icelandic geothermal fluids and the source and reactions of sulfur in volcanic geothermal systems at divergent plate boundaries abstract multiple sulfur isotope systematics of geothermal fluids at krafla northeast iceland were studied in order to determine the source and reactions of sulfur in this system as an example of a geothermal system hosted on a divergent plate boundary fluid temperatures ranged from 192 to 437 c and the fluids have low cl concentration between 10 and 150 ppm with liquid water and vapor being present in the reservoir dissolved sulfide s ii and sulfate svi predominated in the water phase with trace concentrations of thiosulfate s 2 o 3 2 whereas sulfide s ii was the only species observed in the vapor phase the reconstructed sulfur isotope ratios of the reservoir fluids based on samples collected at surface from two phase and vapor only well discharges indicated that 34s and 33s of sulfide in the reservoir fluid ranged from 1 5 to 1 1 and 0 001 to 0 017 respectively whereas 34s and 33s of sulfate were significantly different and ranged from 3 4 to 13 4 and 0 000 to 0 036 respectively depressurization boiling upon fluid ascent coupled with progressive fluid rock interaction and sulfide mineral pyrite formation results in the liquid phase becoming progressively isotopically lighter with respect to both 34s and 33s in contrast h2s in the vapor phase and pyrite become isotopically heavier the observed 33s and 34s systematics for geothermal fluids at krafla suggest that the source of sulfide in the reservoir fluids is the basaltic magma either through degassing or upon dissolution of unaltered basalts at high temperatures insignificant so4 was observed in the fluids but below 230 c significant concentrations of so4 were observed the source inferred to be h2s oxidation the two key factors controlling the multiple sulfur isotope systematics of geothermal fluids are 1 the isotopic composition of the source material and 2 the isotope fractionation associated with aqueous and vapor speciation and how these change as a function of processes occurring in the system including boiling oxidation and fluid rock interaction

environmental significance of foraminiferal assemblages dominated by small sized ammodiscus and trochammina in triassic and jurassic delta influenced deposits abstract the sediment packages analyzed for benthic foraminifera consist of mudstones with interbedded sandstones deposited in shallow delta influenced shelf to deltaic environments the sections are located in spitsbergen the barents sea northern north sea and yorkshire and range in age from late triassic to middle jurassic salient features of the foraminiferal successions are 1 the assemblages consist entirely or dominantly of agglutinated taxa 2 the faunal diversities are extremely low 3 the dominant genera are ammodiscus and trochammina 4 the species are generally of small size compared to usual dimensions within the genera the features listed above suggest that the assemblages were adapted to restricted conditions clearly divergent from those of a normal marine shelf where the main limiting factors were low salinity and reduced amount of dissolved oxygen in unstable storm influenced environments evidence for environmental conditions is obtained from modern analogues although the large evolutionary changes in foraminifera during post jurassic time make it difficult to find such analogues additional information is derived from functional morphology sedimentary features and paleogeography the analyzed sediment packages show close faunal similarities suggesting opening of a marine pathway which connected the paleo arctic ocean with the western european shelf seas in early jurassic a depositional biofacies model of the small sized ammodiscus trochammina assemblages envisages a delta influenced shelf environment where high freshwater influx would have created a density stratified water column with a tendency to develop hypoxic conditions in its deeper parts the depth interval between fair weather and storm wave base the offshore transition zone is indicated as the habitat of the small sized ammodiscus trochammina assemblages in this zone benthic biota would have been stressed by intermittent periods with moderate hypoxia combined with lowered salinity and storm impacts

multiple sulfur isotope constraints on the modern sulfur cycle we present 28 multiple sulfur isotope measurements of seawater sulfate 34sso4 34sso4 and 33sso4 33sso4 from the modern ocean over a range of water depths and sites along the eastern margin of the pacific ocean the average measured 34sso4 34sso4 is 21 24 0 88 2 0 88 2 with a calculated 33sso4 33sso4 of 0 050 0 050 0 014 2 0 014 2 with these values we use a box model to place constraints on the gross fraction of pyrite burial in modern sediments this model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of 34spyrite 34spyrite which is poorly constrained but instead uses new information about the relationship between 34s 34s and 33s 33s in global marine sulfate our calculations indicate that the pyrite burial flux from the modern ocean is between 10 and 45 of the total sulfur lost from the oceans with a more probable range between 20 and 35

origin and fate of hydrothermal fluids at piton des neiges volcano reunion island a geochemical and isotopic o h c sr li cl study of thermal springs abstract as many ocean basaltic volcanoes worldwide piton des neiges reunion island hosts a hydrothermal system that represents a potential geothermal resource despite several prospection campaigns over the past decades this potential has never been confirmed because many aspects of fluid supply and circulation remain unclear to track the origin and fate of hydrothermal fluids we analyzed the geochemistry of thermal springs water and gas by combining conventional tracers major trace elements and o h c sr li isotopes with cl isotopes a geochemical tool under development added to literature data our new results allow us to compare the composition of thermal springs with cold waters rocks gas and fumarolic deposits from la reunion and elsewhere and to untwine the complex history of fluid mixing that constitutes the geothermal system of piton des neiges 18oh2o and dh2o values of springs 8 67 to 4 9 and 56 3 to 23 5 vsmow respectively are positioned close to the local meteoric water line showing that thermal waters are of meteoric origin however their depletion in heavy isotopes relative to cold waters suggests a recharge strongly influenced by cyclones with a possible contribution from high altitude 2000 m rainfalls their absence of 18o enrichment relative to the local meteoric water line indicates that the isotopic exchange between rocks and water is very limited and therefore suggests that their deep temperatures are relatively low and or the water rock ratios are high 13c values in thermal waters 8 0 to 3 2 pdb and gases 6 7 to 5 3 pdb confirm that carbon is essentially of magmatic origin which we interpret as supplied by regional degassing of la reunion hotspot major trace elements and 87sr 86sr 0 704142 to 0 704336 7li 2 and 34 8 lsvec and 37cl 0 35 and 0 40 smoc compositions show that three additional processes contribute to the mineralization of thermal waters 1 interaction with basalt at temperatures 50 c 2 very limited seawater contamination the geothermal system of piton des neiges is thus the result of meteoric water interaction with a trachytic heat source as a solidified intrusion or a magma pocket unrelated to the co2 flux we also emphasize that thermal waters at piton des neiges are exceptionally isolated from seawater compared to other geothermal systems in ocean island volcanoes worldwide which implies a heat source located above the seawater freshwater interface these inferences are of critical importance for the future geothermal exploration on the island as they suggest that the geothermal resource is within reach of relatively shallow drilling depths 1000 m

genesis of the changlingzi zn pb deposit in the southern great xing an range northeastern china constraints from fluid inclusions and h o s pb isotope systematics the changlingzi pb zn deposit is located in the southern great xing an range metallogenic belt of northeast china this deposit experienced two types of mineralization including skarn ore block i

evolution of fluid rock interaction in the reykjanes geothermal system iceland evidence from iceland deep drilling project core rn 17b abstract we describe the lithology and present spatially resolved geochemical analyses of samples from the hydrothermally altered iceland deep drilling project iddp drill core rn 17b the 9 3 m long rn 17b core was collected from the seawater dominated reykjanes geothermal system located on the reykjanes peninsula iceland the nature of fluids and the location of the reykjanes geothermal system make it a useful analog for seafloor hydrothermal processes although there are important differences the recovery of drill core from the reykjanes geothermal system as opposed to drill cuttings has provided the opportunity to investigate evolving geothermal conditions by utilizing in situ geochemical techniques in the context of observed paragenetic and spatial relationships of alteration minerals the rn 17b core was returned from a vertical depth of 2560 m and an in situ temperature of 345 c the primary lithologies are basaltic in composition and include hyaloclastite breccia fine grained volcanic sandstone lithic breccia and crystalline basalt primary igneous phases have been entirely pseudomorphed by calcic plagioclase magnesium hornblende chlorite titanite albitized plagioclase vein epidote and sulfides despite the extensive hydrothermal metasomatism original textures including hyaloclastite glass shards lithic clasts chilled margins and shell fragment molds are superbly preserved multi collector la icp ms strontium isotope ratio 87 sr 86 sr measurements of vein epidote from the core are consistent with seawater as the dominant recharge fluid epidote hosted fluid inclusion homogenization temperature and freezing point depression measurements suggest that the rn 17b core records cooling through the two phase boundary for seawater over time to current in situ measured temperatures electron microprobe analyses of hydrothermal hornblende and hydrothermal plagioclase confirm that while alteration is of amphibolite grade it is in disequilibrium and the extent of alteration is dependent upon protolith type and water rock ratio alteration in the rn 17b core bares many similarities to that of type ii basalts observed in mid atlantic ridge samples

a geochemical model for the formation of hydrothermal carbonates on mars it is often argued that substantially more carbon dioxide and water were degassed from the martian interior than can be found at present in the atmosphere polar caps and regolith calculations have shown that atmospheric escape cannot account for all of the missing volatiles suggestions that carbon dioxide is stored as marine or lacustrine deposits are challenged by earth based and spacecraft remote sensing data moreover recent modelling of the martian atmosphere suggests that rainfall or open bodies of water are in any case unlikely to have persisted for extended periods of time hydrothermal carbonates therefore provide a possible solution to this dilemma using an accessible terrestrial system iceland as a guide to the underlying processes and a host rock composition inferred from the least altered martian meteorite we present a geochemical model for the formation of carbonates in possible martian hydrothermal systems our results suggest that an extensive reservoir of carbonate minerals equivalent to an atmospheric pressure of carbon dioxide of at least one bar could have been sequestered beneath the surface by widespread hydrothermal activity in the martian past

composition of geothermal minerals from saline and dilute fluids krafla and reykjanes iceland abstract in high temperature geothermal areas t 200 c in iceland the alteration minerals formed show a regular zoning with increasing temperature i e smectite chlorite epidote and actinolite to test the effect of varying composition of the hydrothermal circulation fluid drillhole chips from two contrasting areas were analyzed in detail the reykjanes area is fed with sea water while krafla is fed with meteoric water and has a cl concentration of 28 ppm both areas have tholeiitic basalts lavas and hyaloclastites as original reservoir rocks although both systems have the same sequence of alteration minerals the actual composition of the minerals differs between systems reflecting the different chemical composition of the circulating waters in reykjanes with its brine fluid the composition of the original rock appears not to affect the composition of the alteration minerals while the reverse appears to be the case in krafla

relating sulfide mineral zonation and trace element chemistry to subsurface processes in the reykjanes geothermal system iceland abstract the nature and distribution of sulfide minerals and their trace element chemistry in the seawater dominated reykjanes geothermal system was determined through the study of cuttings and core from wells that intersect different regions of the hydrothermal cell from the near surface to depths of 3000 m the observed sulfide mineral zonation and trace element enrichment correlate well with the present day thermal structure of the system isocubanite and pyrrhotite are confined to the deep low permeability regions whereas an assemblage of chalcopyrite and pyrite predominates in the main convective upflow path the presence of marcasite in the uppermost regions of the system reflects weakly acidic conditions ph 2 the presence of chalcopyrite disease in sphalerite may be an indication that the system is experiencing a heating trend following the logic of zone refining in volcanogenic massive sulfide systems sulfide sulfur at all analyzed depths in the reykjanes geothermal system was derived from a mixture of basaltic and reduced seawater sources petrographic evidence suggests that seawater derived hydrothermal fluids have altered primary igneous sulfides in the host rocks a process that has been proposed as a major control of aqueous sulfide production in mid ocean ridge environments calculations show that igneous sulfides in the host basalts likely account for less than 5 of the total available ore metal budget in the system however their contribution to fluid metal budgets is probably significant because of their relatively high solubility the processes documented by this study are likely analogous to those operating in the feeder and deep reaction zones of mid ocean ridge seafloor hydrothermal systems the results show that sulfide mineral zonation and trace element chemistry vary as a function of physicochemical parameters that are relevant to the characterization and exploration of geothermal energy resources

staining algorithm for seismic modeling and migration abstractin seismic migration some structures such as those in subsalt shadow zones are not imaged well the signal in these areas may be even weaker than the artifacts elsewhere we evaluated a method to significantly improve the signal to noise ratio s n in poorly illuminated areas of the model we constructed a phantom wavefield an extension of the wavefield to the complex domain the imaginary wavefield was synchronized with the real wavefield but it contained only the events relevant to a target region of the model which was specified using a staining algorithm the real wavefield interacted with the entire model however all structures except for the target were transparent to the imaginary wavefield which is excited only when the real wavefront arrives at the target structure the real and the imaginary source wavefields were crosscorrelated with the regular receiver wavefield the results were revealed in two images the conventional reverse time migration image and an image of the target

a new model for atmospheric oxygen over phanerozoic time a mathematical model has been constructed that enables calculation of the level of atmospheric o2 over the past 570 my from rates of burial and weathering of organic carbon c and pyrite sulfur s burial rates as a function of time are calculated from an assumed constant worldwide clastic sedimentation rate and the relative abundance and c and s contents of the three rock types marine sandstones and shales coal basin sediments and other non marine clastics red beds arkoses by our model values of o2 versus time using a constant total sedimentation rate agree with those for variable sedimentation derived from present day rock abundances and estimates of erosional losses since deposition this agreement is the result of our reliance on the idea that any increase in total worldwide sediment burial with consequently faster burial of c and s and greater o2 production must be accompanied by a corresponding increase in erosion and increased exposure of c and s on the continents to o2 consumption via weathering it is the redistribution of sediment between the three different rock types and not total sedimentation rate that is important in o2 control to add stability to the system negative feedback against excessive o2 fluctuation was provided in the modeling by the geologically reasonable assignment of higher weathering rates to younger rocks resulting in rapid recycling of c and s we did not use direct o2 negative feedback on either weathering of c and s or burial of c because weathering rates are assumed to be limited by uplift and erosion and the burial rate of c limited by the rate of sediment deposition the latter assumption is the result of modern sediment studies which show that marine organic matter burial occurs mainly in oxygenated shallow water and is limited by the rate of supply of nutrients to the oceans by rivers results of the modeling indicate that atmospheric o2 probably has varied appreciably over phanerozoic time during the late carboniferous and permian periods o2 was higher than previously because of the rise of vascular land plants and the widespread burial of organic matter in vast coal swamps a large decrease in o2 during the late permian was due probably to the drying up of the coal swamps and deposition of a large proportion of total sediment in c and s free continental red beds sensitivity study shows that major parameters affecting results are the mean c concentration in coal basins and the relative sizes of the reservoirs of young rapidly recycled versus old rocks less sensitivity was found for changes over time in total land area undergoing weathering and the use of direct o2 negative feedback on marine carbon burial good agreement for rates of c burial calculated via our model and via independent models which are based on the use of stable carbon isotopes indicates that the dominant factor that has brought about changes in atmospheric o2 level and the isotopic composition of dissolved inorganic carbon in seawater over phanerozoic time is sedimentation and not weathering or higher temperature phenomena such as basalt seawater reaction

low frequency circulation on the texas louisiana continental shelf for the texas louisiana coast west of 92 5 w long series of data from near freeport texas together with shorter series from other locations show strong response of coastal current to wind stress in agreement with coastal jet concepts we infer from coastal winds scattered current measurements and distributions of sea surface salinity and geopotential that a cyclonic gyre elongated along the shelf is the dominant feature of the prevailing shelf circulation the inshore limb of the gyre is the coastal jet driven by wind with a west or southward downcoast component which prevails along much of the coast except in july august because the coast is concave the shoreward prevailing wind results in a convergence of coastal currents which marks the downcoast extent of the gyre corresponding to the convergence is a seaward flow which forms the southwest limb of the gyre a prevailing countercurrent north or eastward flow along the shelf break includes the outer limb of the gyre the eastern shoreward flowing limb of the gyre corresponds to divergence along the coast centered near 92 5 w the convergence at the western or southern end of the gyre migrates seasonally with the direction of the prevailing wind reaching south of the rio grande mouth in fall and the cameron offing in july the gyre is normally absent in july but reappears in august september when a downcoast wind component develops

influence of hypoxia on biogenic structure in sediments on the louisiana continental shelf abstract as part of a study of the effects of seasonal hypoxia on sediment properties samples were collected during the spring and late summer of 2009 from four sites of similar sediment type and water depth 30 39 m but different recent history of bottom water oxygen concentration on the continental shelf of louisiana sediment profile imaging spi box coring x radiography and computed tomography ct imaging were employed to characterize the biogenic structural differences in surficial sediments among a normoxic control site and three sites subjected to hypoxic events varying in frequency of occurrence results of the ct imagery indicated that macrobenthic biogenic structures were the most numerous at the h7 site that had experienced the least hypoxia in the past 23 years the e4 site that had experienced hypoxia seasonally with a frequency between 50 and 75 of the time had the fewest biogenic structures in spring 2009 but exhibited recovery in terms of their abundance and diameter in summer 2009 e4 also exhibited high rates of bioturbation during the late summer sampling as determined from excess 234 th this suggests that the macrobenthos community at this site was in an active phase of recovery from hypoxia at the a6 site exposed to hypoxia with an annual frequency 75 biogenic structures were numerous but dimensionally small correlating with the average individual size of macrobenthos found there the total volume maxima occupied by biogenic structures in the sediment occurred below the uppermost sediment intervals with the exception of the spring sample from the d5 site that experienced hypoxia between 25 and 50 of the time ct imagery indicating effects of seasonal hypoxia on biogenic structure in the top 10 cm of sediments detected more structures than spi x radiographic imagery or macrobenthos census data the presence of relict burrows probably inflated estimates of biogenic structures in the subcores rendering an integrated result that included creation destruction and preservation of burrows and voids over time

understanding subsalt illumination through ray trace modeling part 1 simple 2 d salt models the complex structure and high velocity contrast of salt in the gulf of mexico create a difficult seismic imaging problem 3 d prestack depth migration presdm of seismic data has allowed imaging of reflectors under the salt sheets and of detached bodies of irregular shape but 3 d presdm cannot fill in shadow zones below salt where little energy is reflected additionally amplitude variations caused by salt structures focusing or dispersing seismic energy are usually poorly handled by 3 d presdm ray trace modeling can clarify subsalt imaging problems and the modeling results should be incorporated into exploration and development plans

sulfur geochemistry of hydrothermal waters in yellowstone national park i the origin of thiosulfate in hot spring waters thiosulfate s sub 2 o sub 3 sup 2 minus polythionate s sub x o sub 6 sup 2 minus dissolved sulfide h sub 2 s and sulfate so sub 4 sup 2 minus concentrations in thirty nine alkaline and acidic springs in yellowstone national park ynp were determined the analyses were conducted on site using ion chromatography for thiosulfate polythionate and sulfate and using colorimetry for dissolved sulfide thiosulfate was detected at concentrations typically less than 2 micro mol l in neutral and alkaline chloride springs with low sulfate concentrations cl sup minus so sub 4 sup 2 minus 25 the thiosulfate concentration levels are about one to two orders of magnitude lower than the concentration of dissolved sulfide in these springs in most acid sulfate and acid sulfate chloride springs cl sup minus so sub 4 sup 2 minus 10 thiosulfate concentrations were also typically lower than 2 micro mol l however in some chloride springs enriched with sulfate cl sup minus so sub 4 sup 2 minus between 10 and 25 thiosulfate was found at concentrations ranging from 9 to 95 micro mol l higher than the concentrations of dissolved sulfide in these waters polythionate was detected only in cinder pool norris geyser basin at concentrations up to 8 micro mol l with an average s chain length from 4 1 to 4 9 sulfur atoms the results indicate that no thiosulfate occurs in the deeper parts of themore hydrothermal system thiosulfate may form however from 1 hydrolysis of native sulfur by hydrothermal solutions in the shallower parts 50 m of the system 2 oxidation of dissolved sulfide upon mixing of a deep hydrothermal water with aerated shallow groundwater and 3 the oxidation of dissolved sulfide by dissolved oxygen upon discharge of the hot spring upon discharge of a sulfide containing hydrothermal water oxidation proceeds rapidly as atmospheric oxygen enters the water the transfer of oxygen is particularly effective if the hydrothermal discharge is turbulent and has a large surface area less

underground fumaroles excess heat effects in vein formation we examine the heat extracted from a fractured hydrothermal system following the sudden dilation of fractures in the host rock this dilation increases the permeability of the system and veins form along irregular fractures where the opening is localized for simplicity we will consider one such vein in this paper fluid in the system is assumed to be initially close to vapor saturation and in thermal equilibrium with the surrounding host rock the rapid opening of the vein leads to approximately isenthalpic vapor separation and causes a pressure drop to propagate through the system as the two phase fluid in the fracture system is constrained to the vapor pressure curve the depressurization is accompanied by a rapid cooling of the fluid fluid flow is also induced from a hydrothermal reservoir toward the dilated discharge vein the temperature difference between the host rock and the two phase fluid in the fracture system drives heat from the rock into the fluid the excess heat 1 1 derived from the host rock in this manner evaporates additional liquid in the two phase flow which develops substantially larger vapor fractions than those due only to the rapid initial adiabatic depressurization of the fluid such additional vapor separation therefore leads to high solute concentrations in the liquid phase and potentially results in faster deposition of minerals in the fracture system and local veins a model is developed here to describe the minimum excess heat transfer i e the worst case scenario into the two phase saturated liquid vapor mixture as it is drawn horizontally from the hydrothermal reservoir toward the discharge vein excess heat effects are shown to be effective within tens to hundreds of meters of actively forming veins and a principal cause of oscillatory banding and lamination fabrics this transient heat extraction process applies to a wide range of shallow crustal deposit styles associated with brittle and brittle ductile deformation these deposit styles range from epithermal bonanza veins sea floor massive sulfide deposits mesothermal veins in shear zones to the higher temperature veins and stockworks of hypothermal and porphyry mineralization 1 fn 1

generalized staining algorithm for seismic modeling and migration abstractthe staining algorithm is introduced to improve the signal to noise ratio s n of poorly illuminated subsurface structures in seismic imaging however the amplitudes of the original and the stained wavefield i e the real and the imaginary wavefields differ by several orders of magnitude and the waveform of the stained wavefield may be greatly distorted we have developed a generalized staining algorithm gsa to achieve amplitude preservation in the stained wavefield the real wavefield and the stained wavefield propagate in the same velocity model a source wavelet is used as the source of the real wavefield however the real wavefield is extracted from the stained area as the source of the stained wavefield the gsa maintains some properties of the original staining algorithm the stained wavefield is synchronized with the real wavefield and it contains only information relevant to the target region by imaging with the stained wavefield we obtain higher s ns in images of target struct

b o th isotope systematics in icelandic tephra abstract we report new boron concentrations and isotope compositions measured by ion microprobe on holocene tephra samples from 6 icelandic volcanoes the b concentrations range over almost a factor of 10 from 1 50 0 10 to 13 10 0 10 ppm in basalt and obsidian respectively the highest 11b 16 9 2 2 and 6 1 1 6 measured in rhyolites from the rift related askja and krafla volcanoes in contrast the lowest 11b values 5 3 and 5 2 observed in samples with the some of the highest 18o 4 92 and 5 18 respectively and 230th 232th 1 067 and 1 030 respectively correspond to a normal mantle signature the variations of boron isotope compositions in the icelandic tephra and their negative correlations with both 18o and 230th 232th strongly support the mixing between mantle derived basalts and crustal rhyolites produced from anatexis of hydrothermally altered basaltic crust the low 18o in combination with high 11b values in rhyolite is best explained by the presence of seawater derived b in a hydrothermal fluid that is mostly of meteoric origin

mass independent isotope effects in archean 2 5 to 3 8 ga sedimentary sulfides determined by ion microprobe analysis we report sulfur isotope anomalies with 33 s the deviation from a mass dependent fractionation line for the three isotope system 34 s 32 s vs 33 s 32 s ranging up to 2 within individual archean sedimentary sulfides from a variety of localities our measurements which are made in situ by multicollector secondary ion mass spectrometry unequivocally corroborate prior bulk measurements of mass independent fractionations mif in sulfur and provide additional evidence for an anoxic atmosphere on the earth before 2 ga this technique also offers new opportunities for exploring ancient sulfur metabolisms preserved in the rock record the presence of mif sulfur in sulfides from a 3 8 ga fe rich quartzite from akilia island west greenland is consistent with a marine sedimentary origin for this rock copyright 2003 elsevier science ltd

degassing regime of hekla volcano 2012 2013 hekla is a frequently active volcano with an infamously short pre eruptive warning period our project contributes to the ongoing work on improving hekla s monitoring and early warning systems in 2012 we began monitoring gas release at hekla the dataset comprises semi permanent near real time measurements with a multigas system quantification of diffuse gas flux and direct samples analysed for composition and isotopes 13c d and 18o in addition we used reaction path modelling to derive information on the origin and reaction pathways of the gas emissions hekla s quiescent gas composition was co2 dominated 0 8 mol fraction and the 13c signature was consistent with published values for icelandic magmas the gas is poor in h2o and s compared to hydrothermal manifestations and syn eruptive emissions from other active volcanic systems in iceland the total co2 flux from hekla central volcano diffuse soil emissions is at least 44 t d 1 thereof 14 t d 1 are sourced from a small area at the volcano s summit there was no detectable gas flux at other craters even though some of them had higher ground temperatures and had erupted more recently our measurements are consistent with a magma reservoir at depth coupled with a shallow dike beneath the summit in the current quiescent state the composition of the exsolved gas is substantially modified along its pathway to the surface through cooling and interaction with wall rock and groundwater the modification involves both significant h2o condensation and scrubbing of s bearing species leading to a co2 dominated gas emitted at the summit we conclude that a compositional shift towards more s and h2o rich gas compositions if measured in the future by the permanent multigas station should be viewed as sign of imminent volcanic unrest on hekla

fluid flow and water rock interaction in the east rift zone of kilauea volcano hawaii the east rift zone of kilauea volcano in hawaii represents a major area of geothermal activity fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes soh drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level calculated 18o and d values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of mauna loa volcano in the interior of the rift zone recharge is dominated by seawater mixed with local meteoric water water rock ratios in the rift area are approximately 2 but strongly 18o enriched fluids in the deeper parts of the soh 2 and soh 4 drill holes on the north side of the rift indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone marine carbonates at the subaerial to submarine transition between 1700 and 1780 m depth in soh 4 have not fully equilibrated with the fluids suggesting that the onset of hydrothermal activity in this area was relatively recent 2000 years this may represent increased volcanic activity along the rift after the end of the ai la au phase of eruptive activity at the kilauea summit approximately 1000 years ago or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift

stable isotopic aspects of fluid flow in the krafla namafjall and theistareykir geothermal systems of northeast iceland abstract water and steam condensates from the krafla namafjall and theistareykir geothermal systems have been studied using hydrogen and oxygen stable isotope ratios to gain more information on thermal fluid flow the effect of rifting on individual fields and the provenance of recharge to these fields at krafla there are minor differences between individual wellfields but recharge is essentially local only one well shows signs of a change in composition over a 6 year production period but a local volcanic eruption in 1984 affected fumarole steam compositions the nearby namafjall field has isotopic compositions suggesting that recharge is ultimately derived from a long distance to the south perhaps by a combination of surface and groundwater flow some local spring and shallow well compositions have been affected by the post 1975 rifting episode the undrilled theistareykir field has an inflow estimated to be of similar isotopic composition to namafjall some fumaroles have an anomalously depleted 18o composition and rayleigh condensation is postulated to explain them

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

geothermal mineral equilibria the dominant reaction determining the chemistry of fluids in a geothermal system of the new zealand type consists of the conversion of primary plagioclase by co2 to calcite and clays with log pco2 15 26 7850 t 273 2 temperature t in c subsequent reactions involving secondary minerals control relative co2 h2s contents the distribution of mineral phases throughout a geothermal system reflects the stepwise conversion of thermodynamically unstable primary phases through a series of intermediate metastable phases to a thermodynamically stable secondary assemblage the relative stabilities of these phases was evaluated on the basis of their solubilities the least soluble aluminiumsilicate representing the thermodynamically most stable phase under a given set of conditions observed assemblages of secondary minerals in geothermal systems represent indicators allowing mineral fluidinteraction conditions to be evaluated on the basis of multi component mineral stability diagrams

evidence for oxygen isotope disequilibrium in selected geothermal and hydrothermal ore deposit systems abstract there is ample evidence from geothermal systems that temperatures estimated from the oxygen isotope fractionation between alteration phases and coexisting aquifer fluids agree closely 25 c with measured bore hole temperatures conversely many hydrothermal systems exhibit varying degrees of apparent fluid rock oxygen isotope disequilibrium measured rock water equilibrium rock water a comparison of data from a variety of hydrothermal systems indicates that an increase in the degree of isotopic equilibration can be related in a general way to an increase in the temperature salinity and intensity of fracturing this relationship is entirely consistent with calculations carried out for a simple closed system model that estimates the fraction of isotope exchange between rock and fluid as a function of temperature rock type salinity of the fluid grain size water rock ratio and the rate of isotope exchange the fact that many silicate fluid reactions and their isotopic counterparts are fast leads us to conclude that observed variations in the degree of exchange may have resulted from local self sealing of the fracture network prior to equilibration the ability for fractures to remain open or to propagate allowing continued fluid flow may be the deciding factor in the attainment of isotopic equilibration

application of radium isotopes to determine crustal residence times of hydrothermal fluids from two sites on the reykjanes peninsula iceland radium isotopes were used to determine the crustal residence times of hydrothermal fluids from two geothermal wells svartsengi and reykjanes from the reykjanes peninsula iceland the availability of rock samples from the subsurface to depths of 2400 m allowed direct comparison of the radium isotopic characteristics of the fluids with those of the rocks within the high temperature and pressure reaction zone the 226 ra activity of the svartsengi fluid wasone fourth of the reykjanes fluid and the 228 ra 226 ra ratio of the svartsengi fluid wastwice that of reykjanes the fluid isotopic character istics were relatively stable for both sites over the 6 years 2000 2006 of the study it was determined using a model that predicts the evolution of the fluid 228 ra 226 ra ratio with time that both sites had fluid residence times from the onset of high temperature water rock reaction of less than 5 years measurement of the short lived 224 ra and 223 ra allowed estimation of the recoil input parameter used in the model the derived timescale is consistent with results from similar studies of fluids from submarine systems and has implications for the use of terrestrial systems in iceland as an exploited energy resource 2007 elsevier ltd all rights reserved

the fluid geochemistry of icelandic high temperature geothermal areas abstract icelandic high temperature geothermal systems are considered to number thirty three thereof three are submarine and seven subglacial all are briefly described but the chemistry of fluids from twenty four of them is considered the fluid in the three submarine areas and those four on land that are closest to the sea are relatively saline but to a differing extent mixed with groundwater the rest contain dilute fluids the fluids of the central highland systems are mostly locally derived but may in some instances be quite old whereas those in the northerly krafla area which is inland and the oxarfjor ur area which is close to the sea appear to be a mixture of local and central highland water but those in the inland hengill geysir namafjall and theistareykir areas appear to have travelled relatively long distances from the central highlands the gas observed is magmatic except in the northerly oxarfjordur area close to the sea where it is apparently derived from organic sediments

depositional environments organic carbon accumulation and solar forcing cyclicity in smackover formation lime mudstones northern gulf coast upper jurassic smackover lime mudstones a major hydrocarbon source rock in the u s gulf coast show three distinct lithofacies the basal laminated lithofacies consists of organic and siliciclastic rich lamina sets that alternate with organic poor clean carbonate lamina sets both lamina sets are composed of lamina couplets interpreted to be varves this lithofacies lacks current induced structures allochems and rip up clasts benthic and planktonic fossils are absent one organic rich lamina set and one organic poor lamina set comprise one cycle 0 5 3 cm thick the middle thin bedded lithofacies is composed of organic and siliciclastic rich carbonate beds that alternate with organic poor clean carbonate beds both bed types are internally laminated one organic rich bed and one organic poor bed comprise one cycle 2 60 cm thick the upper burrowed lithofacies is extensively bioturbated massive shows no evidence of cyclicity and contains little organic carbon sedimentologic characteristics suggest that the basal laminated lithofacies was deposited under anoxic conditions below storm wave base in a basinal environment the middle thin bedded lithofacies is copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received march 5 1996 revised manuscript received august 2 1996 final acceptance december 10 1996 2basin research institute louisiana state university baton rouge louisiana 70803 3lss international 1440 lake front circle suite 180 the woodlands texas 77380 formerly with department of geology and geophysics louisiana state university baton rouge louisiana 70803 4department of geology and geophysics louisiana state university baton rouge louisiana 70803 the study was supported by grants from the center for energy studies and the basin research institute of louisiana state university we thank h d klemme and g t moore for providing unpublished data p meyers and l m pratt reviewed an earlier version of the manuscript and offered valuable comments j wrenn kindly provided characteristics of organic matter we thank j kropog w s leblanc k lyle p o neill and b simpson for technical assistance we appreciate the helpful comments of r l folk b kirkland and c h moore critical comments by k t biddle n b harris b a prather and p d wagner significantly improved the quality of concepts presented

erratum to sulfur isotopic compositions of individual sulfides in martian meteorites alh84001 and nakhla implications for crust regolith exchange on mars earth planet sci lett 184 2000 23 35

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved