mineral and gas chemistry of hydrothermal fluids on an ultrafast spreading ridge east pacific rise 17 to 19 s naudur cruise 1993 phase separation processes controlled by volcanic and tectonic activity during the naudur cruise in december 1993 23 dives using the french submersible nautile were conducted on the axis of the east pacific rise between 17 and 19 s where the spreading rate is among the fastest in the ocean 14 to 16 cm yr twenty hydrothermal fluids located at the topographic high of each segment in the axial domain were collected between 2573 and 2669 m depth on three segments centered respectively at 17 25 s 18 15 s and 18 26 s the fluids exhibit a very wide range of temperature chemical and gas compositions on the 17 25 s and 18 26 s segments fluids have quite uniform compositions low chlorinities are gas enriched and are low in dissolved metals relative to fluids from the 18 15 s segment which show high chlorinities are less gas enriched and show high metal concentrations chloride and metal depletion associated with gas enrichment is consistent with phase separation whereas ch 4 end members show large variations between sites the 13 ch 4 data are very similar with 13 c values in a narrow range 22 0 to 23 9 versus pee dee belemnite pdb 13 co 2 measured in fluids within the 18 15 s and 18 26 s segments are respectively 7 9 and 5 8 versus pdb similar to 13 c of co 2 trapped in mid oceanic ridge basalts suggesting a magmatic origin the variability in fluid composition is linked to the variability of the accretion process observed on the three segments the uniform venting of low chlorinity fluids in the 17 25 s and 18 26 s segments is connected with volcanic activity which causes boiling with preferential venting of vapor enriched fluids high salinity fluids are emitted on the 18 15 s segment where the ridge is tectonics dominated and subseafloor circulation controlled by faults phase separated effluents induced by volcanic and tectonic activity are delivered to the deep ocean in this area as previously observed on the juan de fuca ridge or in the north fiji basin ridge

mineralogy and chemistry of massive sulfide deposits from the juan de fuca ridge six hydrothermal vent sites and associated benthic communities were located in the axial valley of the southern juan de fuca ridge using transponder navigated bottom photography the hydrothermal deposits form ledges and shallow mounds within a central zone characterized by a linear bathymetric depression and numerous collapse features the flat valley floor adjacent to the central zone consists of ferrobasalt lobate flows and sheet flows sediment cover is minimal vent sites are characterized by concentrations of tube worms clams benthic siphonophores and several unidentified fauna two types of massive sulfide were dredged from one of the vent sites type a samples are angular slabs of dark gray zn rich sulfide with interlayers and a thin partly oxidized crust of fe sulfide these layered sulfide aggregates appear to be fragments of a sulfide wall enclosing an active hydrothermal vent the outer sulfide wall is composed mainly of colloform fe sulfide and fe poor sphalerite deposited under lower temperature conditions whereby sea water and hydrothermal fluid mix above the discharge point with continued sulfide deposition the wall inhibits mixing of sea water and hydrothermal fluid inside the wall the intensifying hydrothermal system deposits a higher temperature assemblage of granular fe rich sphalerite wurtzite pyrite and minor cu fe sulfide the zonation in wurtzite from fe rich cores to fe poor rims may result from a late stage cooling of the hydrothermal fluid and or a change in fluid chemistry the sulfide wall grows outward where a rupture in it permits the escape of high temperature fluid and then deposition of a secondary shell over the breakthrough point as temperature increases earlier formed minerals dissolve and zn fe and pb migrate toward the outer sulfide wall tube worms flourished in the outer wall of type a samples and abandoned tube structures served as conduits for the late stage hydrothermal fluids type b sulfide samples are subrounded spongy textured fragments composed almost entirely of dendritic aggregates of pale fe poor colloform sphalerite and opaline silica this type of sulfide is deposited in open space by moderate to low temperature fluid discharging at a slow but variable rate the fluid becomes increasingly oxidizing resulting in late stage deposition of hematite barite and sulfur type b samples show little evidence of burrowing animals this type of sulfide may be deposited in settings peripheral to sites of focused discharge

hydrothermal activity at the trans atlantic geotraverse hydrothermal field mid atlantic ridge crest at 26 n the first submersible observations of the only known active submarine hydrothermal field on a slow spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity duration and varying intensity of hydrothermal activity at the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge crest near latitude 26 n hydrothermal activity has persisted at this location for at least 1 106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age the hydrothermal activity has been cyclic multistage and episodic prior high temperature hydrothermal venting stages with a periodicity of the order of 1 104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in cu fe and zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the mid atlantic ridge the most recent episode of high temperature venting is inferred to have ceased in the recent past based on metal enrichment cu fe zn in the surficial sediment layer low temperature hydrothermal venting stages with a duration of the order of 1 104 years intervene between the short high temperature stages and produce stratiform deposits of layered and earthy manganese oxide iron oxide hydroxide and silicate bivalve like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs the fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits conductive heat flow anomalies in the sediment column and the record of temperature anomalies and excess 3he in the near bottom water column suggest that the low temperature hydrothermal discharge is intermittent at individual vents on a time scale of years

growth history of a diffusely venting sulfide structure from the juan de fuca ridge a petrological and geochemical study centimeter scale mineralogical and chemical analyses of a diffusely venting 280 c sulfide structure called roane from the mothra hydrothermal field on the juan de fuca ridge reveal a complex growth history these analyses document four well defined zones which from the exterior to the interior of the structure include a barite silica zone fossil worm tube zone a silica sulfide zone silica zone a sulfide silica zone outer sulfide zone and a sulfide gordaite silica zone inner sulfide zone these features are a product of a myriad of processes that include extensive mineral replacement ingress of seawater fracturing and breakout of hot hydrothermal fluids significant conductive cooling and the sustained broad scale outflow of warm fluids from the walls through a porous and permeable matrix roane lacks an open throughgoing chalcopyrite lined central conduit instead it hosts an anastomosing discontinuous network of tortuous channels within the interior of a sponge like matrix of amorphous silica sulfide sulfate and clay minerals megafaunal communities at the summit and sides of this mature diffusely venting chimney provide a constructional framework for new growth isolation of hydrothermal fluid from seawater during progressive fossilization of megafauna and the early formation of barite silica assemblages promote flow of higher temperature fluids within the interior continued isolation of interior higher temperature fluids through increased mineral precipitation and expansion of the structure leads to a zn sulfide pyrite dominated permeable matrix and a network of tortuous channels that form a central porous conduit this conduit is shielded by a weakly silicified metal sulfide inner wall and a strongly silicified outer wall the most outer portions of the walls are highly porous and sites of significant advection and mixing between hydrothermal fluids and seawater the mineralogy and chemistry of roane are strongly affected by ammonia ammonium buffering of the ph during cooling of the hydrothermal fluids the two major results of the buffering are 1 precipitation of zn sulfide at higher temperatures than are typical for bare rock systems leading to the correlation of cu zn cd and se and 2 increased deposition of amorphous silica and clay minerals

trace element distributions in the chalcopyrite wall of a black smoker chimney insights from laser ablation inductively coupled plasma mass spectrometry la icp ms abstract the thin walls of young black smoker chimneys experience steep physico chemical gradients during active venting of hydrothermal fluid and these gradients control trace element precipitation within those walls here we utilise a combination of high sensitivity icpms and uv laser ablation resolution of better than 30 m to demonstrate the existence of non random v ag in te ba au pb and u distributions within the chalcopyrite wall of an immature black smoker chimney the data are the first of their kind to be produced for black smoker chimney walls distributions of in and te are attributed to preferential incorporation into lattices at elevated temperature enrichments of u and v derived from seawater are the product of redox immobilisation on sulphide surfaces the distributions of au ag pb and ba may be related to interactions at the hydrothermal fluid seawater mixing front however comparison of distribution data with reaction transport models of chimney walls suggest a possible ph control on precipitation these data illustrate the power of the la icp ms method and such spatially resolved data have the potential to constrain models of element precipitation both in chimneys and in associated mounds

ferro andesites in the grande ronde basalt their composition and significance in studies of the origin of the columbia river basalt group flows from the grande ronde basalt of the columbia river basalt group include several with the chemical composition of ferro andesites these flows have sio2 55 mgo 4 and also have higher fe mg than the average value for grande ronde basalt they are also distinctly richer in cs rb k ba la ce th and u than the remainder of the grande ronde basalt flows and possess small negative eu anomalies their pb isotopic compositions define a mixing line with a negative slope on a 206pb 204pb versus 207pb 204pb plot their nd isotopic compositions lie between 143nd 144nd 0 51252 and 0 51264 and their sr isotopic compositions lie between 87sr 86sr 0 7055 and 0 7060 these values define the enriched end of the columbia river basalt group spectrum on a sr nd epsilon diagram excluding the saddle mountains basalt these ferro andesite flows also form a compositional end member of the columbia river basalt group and are sufficiently distinctive to warrant special consideration we compare them with

rare earth element geochemistry of massive sulfides sulfates and gossans on the southern explorer ridge massive sulfide sulfate deposits on the southern explorer ridge were analyzed for 14 rare earth elements ree by a modified inductively coupled plasma mass spectrometric technique that included a correction for high ba content bulk samples of finely intermixed sulfides sulfate and amorphous silica contain ree concentrations of 6 ppm ree patterns range from 1 strongly enriched in light ree with positive eu anomalies to 2 relatively flat with positive eu anomalies and slightly negative ce anomalies to 3 slightly enriched in light ree with moderately negative ce anomalies pattern 1 is similar to that of 300 350 c solutions discharging at vents on the east pacific rise and the mid atlantic ridge whereas pattern 3 resembles ree distributions in normal oceanic bottom waters the sulfide sulfate patterns are interpreted to result from variable mixtures of hydrothermal and normal seawater barite in gossans capping the mounds has an ree pattern almost identical to patterns of high temperature vent solutions hydrothermal barite has lower ree contents and a different ree pattern relative to hydrogenous barite formed slowly on the sea floor

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

precious and volatile metals in the perseverance nickel deposit gossan implications for exploration in weathered terrains lateritization of the perseverance nickel sulfide deposit near agnew western australia has produced mineralogical and chemical changes in the sulfides and serpentinized dunite host rocks culminating in a thick goethite quartz magnesite gossan the trace and minor elements au ag pd pt ir as sb tl se co and cr were analyzed in 30 samples to assess their mobility under conditions of extreme chemical weathering gold pd pt se as sb and tl are retained or enriched in samples from the oxidized zone while ag is lost the additional metal must have been derived from the overlying gossan since removed by erosion because the metal abundances in the surrounding barren host rocks are too low to act as a potential source the different sampling techniques employed in the study allow an assessment to be made of geochemical sampling procedures in weathered terrain grab samples generally yield the same trends as do channel samples however the grab samples display greater variability reflecting small scale heterogeneity in the fresh parent rocks the observed absolute enrichments and depletions of the trace elements investigated in this study in the oxide zone at perseverance indicate that their retention or loss is a complex process not simply controlled by the eh and ph fluctuations during lateritization the enrichment of as and sb in all the strongly oxidized weathered material and the retention of tl in the lower parts of the oxide zone indicates that primary dispersion halos of these elements should persist in intensely weathered terrain the use of ir and pd and pt to discriminate barren gossans from those developed above nickel sulfides has been confirmed

geochemical constraints on chemolithoautotrophic metabolism by microorganisms in seafloor hydrothermal systems mixing of hydrothermal fluids and seawater at the ocean floor combined with slow reaction kinetics for oxidation reduction reactions provides a source of metabolic energy for chemolithotrophic microorganisms which are the primary biomass producers for an extensive submarine ecosystem that is essentially independent of photosynthesis thermodynamic models are used to explore geochemical constraints on the amount of metabolic energy potentially available from chemosynthetic reactions involving s c fe and mn compounds during mixing of hydrothermal fluids with seawater for the vent fluid used in the calculations epr 21 n obs the model indicates that mixing environments are favorable for oxidation of h2s ch4 fe2 and mn2 only below 38 c with methanogenesis and reduction of sulfate or s favored at higher temperatures suggesting that environments dominated by mixing provide habitats for mesophilic but not thermophilic aerobes and thermophilic but not mesophilic anaerobes a maximum of 760 cal per kilogram vent fluid is available from sulfide oxidation while between 8 and 35 cal kg vent fluid is available from methanotrophy methanogenesis oxidation of fe or mn or sulfate reduction the total potential for chemosynthetic primary production at deep sea hydrothermal vents globally is estimated to be about 1013 g biomass per year which represents 0 02 of the global primary production by photosynthesis in the oceans thermophilic methanogens and sulfate and s reducers are likely to be the predominant organisms in the walls of vent chimneys and in the diffuse mixing zones beneath warm vents where biological processes may contribute to the high methane concentrations of vent fluids and heavy 34s32s s ratios of vent sulfide minerals the metabolic processes taking place in these systems may be analogs of the first living systems to evolve on the earth

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

polymetallic sulfide exploration on the deep sea floor the feasibility of the mini moses experiment the recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them a magnetometric resistivity method mini moses has been developed for detecting the distribution of sub sea floor electrical resistivity a vertical long wire bipole feeds electric current into the ocean some of the current enters the sea floor and its magnetic field is measured at various distances by a low frequency induction coil magnetometer the method is sensitive to conductive and resistive zones at depth and information about three dimensional resistivity structures can be obtained the small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery powered instrument the induction coil receiver is more sensitive and less noisy than conventional flux gate magnetometers over the selected frequency band this receiver is therefore well adapted to the almost noise free deep sea floor environment b

mineralogy and geochemistry of epigenetic features in metalliferous sediment atlantis ii deep red sea finely laminated and undisturbed except in the area of the southwest basin in this part bedding resedimentation and brecciation are common in addition some sediment cores contain vein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments these veins are the conduits by which new hot brine vents into the deep vein mineralogy is dominated by anhydrite talc smectite pyrite sphalerite and chalcopyrite vertical zonation due to cooling and reaction of the incoming brine as it rises through the metalliferous sediment near the southwest basin west basin transition metalliferous sediment has been locally recrystallized to a hematite magnetite pyroxene assemblage probably due to the intrusion of basalt into the metalliferous sediments modified journal abstract

effect of eolian fe supply change on the phytoplankton productivity and community in central equatorial pacific ocean during the pleistocene a lipid biomarker approach abstract the effect of iron fe fertilization on biological productivity and community structure in the equatorial pacific a region with high nutrient and low chlorophyll concentrations has been debated for many years here we present lipid biomarker and geochemical proxy records in a sediment core from the central equatorial pacific spanning from 1100 to 600 ka to investigate the response of marine phytoplankton communities to eolian fe supply and to assess links to environmental fluctuations during the pleistocene we found a strong link between change in eolian fe input and phytoplankton community structure our data demonstrate that an increase in eolian fe supply across the 800 ka boundary resulted in a shift in the major primary producer from coccolithophores alkenones to diatoms epi brassicasterol and dinoflagellates dinosterol at the study site this strong link between changes in eolian fe supply and the community structure of plankton during 800 600 ka may have been associated with a change in atmospheric and ocean conditions due to southward migration of the intertropical convergence zone itcz the lipid biomarker results show that such a high supply of eolian fe might have led to a change in the dominant phytoplankton group however the opal record which is often used as a primary productivity indicator was not consistent with the biomarker results therefore the results indicate that multi proxy approaches should be used in paleoenvironmental studies because different proxies can reveal different aspects of environmental conditions thus contributing to a better understanding of paleoceanographic conditions

hydrothermal sulfide and oxide deposits on seamounts near 21 n east pacific rise hydrothermal deposits were sampled by submersible from the calderas and summit benches of two seamounts 20 and 30 km west of the east pacific rise at 21 n the deposits document a wide variety of off axis hydrothermal activity including samples from the interiors of seafloor sulfide deposits in contrast to most mid ocean ridge deposits sulfide deposits on green seamount are rich in si fe and cu are poor in zn and contain abundant quartz sulfide chimneys grew by quenching of hydrothermal fluids and deposition of walls of colloform pyrite marcasite zn sulfide and minor galena whereas coarse grained pyrite and chalcopyrite were deposited at higher temperatures 250 c in chimney interiors as chimney walls grew outward into sea water increased temperatures in the inner walls resulted in recrystallization to pyrite and deposition of pyrite and chalcopyrite in pore space cooling in the outer portions of chimneys caused deposition of opal at temperatures of about 80 170 c as chimneys coalesced and opal rich material was heated in the interiors of sulfide edifices opal was recrystallized to chalcedony and quartz faulting has exposed the interiors of sulfide deposits where further recrystallization and deposition of chalcopyrite and quartz occurred at temperatures of 230 320 c clogging of chimneys changes in flow paths and cooling of the deposits led to the deposition of late mixed layer illite smectite iron sulfides fe poor sphalerite opal and barite the sulfide deposits formed over a time span of at least 140 000 to 70 000 yr ago probably coincident with caldera and pit crater formation the ages of the sulfide deposits provide constraints on the last volcanic and tectonic activity and they also suggest that it took a maximum of 260 000 yr for the seamount to reach its present height red seamount is presently hydrothermally active and deposits consisting predominantly of amorphous fe oxyhydroxide are forming on small pillow cones in the caldera at temperatures of 10 15 c hydrothermal nontronite deposits capped by mn oxide crusts cover much of the southern summit bench of red seamount and formed at temperatures of around 30 c iron rich talc and sulfide grains in nontronite provide evidence of former high temperature activity and suggest that a sulfide deposit is present nearby

uptake of elements from seawater by ferromanganese crusts solid phase associations and seawater speciation marine fe mn oxyhydroxide crusts form by precipitation of dissolved components from seawater three hydrogenetic crust samples one phosphatized and two hydrothermal mn oxide samples were subjected to a sequential leaching procedure in order to determine the host phases of 40 elements those host phase associations are discussed with respect to element speciation in seawater the partitioning of elements between the two major phases mn oxide and fe oxyhydroxide can in a first order approximation be explained by a simple sorption model related to the inorganic speciation of the elements in seawater as has been proposed in earlier models free and weakly complexed cations such as alkali and alkaline earth metals mn co ni zn tl i and partly y are sorbed preferentially on the negatively charged surface of the mno2 in hydrogenetic crusts the driving force is a strong coulombic interaction all neutral or negatively charged chloro cd hg tl carbonate cu y pb and u and hydroxide be sc ti fe zr nb in sn sb te hf ta bi th and tl iii complexes and oxyanions v cr as se mo and w bind to the slightly positively charged surface of the amorphous feooh phase while coulombic interaction can explain the sorption of the negatively charged species the binding of neutral species is based on specific chemical interaction organic complexation of elements in deep ocean water seems to be at most of minor importance surface oxidation can explain some strong metal associations e g of co and tl with the mno2 and te with the feooh sorption reactions initially driven by coulombic forces are often followed by the formation of specific bonds between the adsorbate and the atoms of the oxide surface differences in the associations of some metals between the non phosphatized and phosphatized hydrogenetic crusts and between the hydrogenetic and the hydrothermal samples reflect the different physico chemical environments of formation and speciations in oxic seawater vs less oxic fluids especially for the redox sensitive metals such as mo and v these environmental related differences indicate that the methodology of chemical speciation used here in combination with spectroscopic methods may allow for the detection of changes in paleoceanographic conditions recorded during the several tens of millions of years of crust growth

genetic interpretation of lead isotopic data from the columbia river basalt group oregon washington and idaho lead isotopic signatures of basalts from the columbia river basalt group of central oregon washington and western idaho and of the high alumina olivine basalts from southeastern oregon plot mostly in the field of pb isotopic data from volcanic rocks of the cascade range in contrast to the mid ocean ridge basalts morb the pb isotopic data for several of the formations form small tight clusters and the nd and sr isotopic data show discrete variation between these basalt groups pseudoisochrons are not observed a single model will account for the observed isotopic and trace element data from samples from most of the columbia river basalt group this model calls for partial melting of the convecting oceanic type mantle and mixing with or contamination by fluids derived from continental derived sediments it requires that crustal components are not only subducted along the trench but that they are also transported in the low velocity zone of the mantle at least 400 km behind the active volcanic arc into the back arc environment represented by the columbia plateaus province lead isotopes in the saddle mountains basalt the youngest formation suggest that these magmas were formed by partial melting of a 2 6 b y old subcontinental lithosphere that extends as far west as south central washington near ice harbor dam the source material for these flows is characterized by high th u th pb rb sr and nd sm ratios and light ree enrichment an additional component presumably granitic crust is necessary to explain the pb isotopic data of the umatilla and perhaps the pomona members of the saddle mountains basalt this contamination model is supported by published 18o data and petrological interpretation published studies of basalts from the deccan traps the tasmanian dolerites the snake river group and the volcanics of the yellowstone plateau province also suggest that melting of old subcontinental lithosphere beneath the continental crust may be an important process in basalt genesis on a global scale

growth history of hydrothermal black smoker chimneys hollow mineral spires known as black smoker chimneys precipitate around 350 c jets of hydrothermal fluid spouting from the sea floor on the east pacific rise epr at 21 n refs 1 2 i describe and discuss here the formation of mineral zoning across the walls of these structures throughout two major growth phases a sulphate dominated stage and a sulphide replacement stage during the sulphide replacement stage at least four distinct sulphide mineral zonation sequences develop across chimney walls from interior to exterior the apparently successive formation of these sulphide zonation sequences is attributed to evolution of radial thermal and chemical gradients accompanying the gradual thickening and sealing of black smoker chimney walls

seafloor bioalteration of sulfide minerals results from in situ incubation studies we present results of incubation studies conducted at low temperatures 4 c in the vicinity of a seafloor hydrothermal vent system we reacted fe s cu and zn bearing minerals including pyrite marcasite chalcopyrite sphalerite elemental sulfur and a portion of a natural chimney sulfide structure for 2 months at the main endeavour segment of the juan de fuca ridge in the pacific ocean our study utilizes fluorescent in situ hybridizations fish scanning and transmission electron microscopy sem tem and light microscopic analysis the surfaces of these minerals are solely colonized by bacteria and not by archaea colonization densities vary over an order of magnitude with the following sequence elemental sulfur chimney sulfide marcasite pyrite sphalerite chalcopyrite and correspond well with the abiotic oxidation kinetics of these materials excepting elemental sulfur which is both the least reactive to oxidizing species and the most heavily colonized colonization densities also correspond with apparent degree of reaction dissolution pitting accumulation of secondary alteration products heavy accumulations of secondary fe oxides on fe bearing minerals most notably on the chimney sulfide form in situ as the result of mineral dissolution and the activity of neutrophilic fe oxidizing bacteria results suggest that mineral oxidizing bacteria play a prominent role in weathering of seafloor sulfide deposits and that microbial utilization of mineral substrates contributes to biomass production in seafloor hydrothermal environments

geology sulfide geochemistry and supercritical venting at the beebe hydrothermal vent field cayman trough the beebe vent field bvf is the world s deepest known hydrothermal system at 4960m below sea level located on the mid cayman spreading centre caribbean the bvf hosts high temperature 401 c black smoker vents that build cu zn and au rich sulphide mounds and chimneys the bvf is highly gold rich with au values up to 93 ppm and an average au ag ratio of 0 15 gold precipitation is directly associated with diffuse flow through beehive chimneys significant mass wasting of sulphide material at the bvf accompanied by changes in metal content results in metaliferous talus and sediment deposits situated on very thin 2 3km thick oceanic crust at an ultraslow spreading centre the hydrothermal system circulates fluids to a depth of 1 8km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies we suggest hydrothermal interaction with chalcophile bearing sulphides in the mantle rocks together with precipitation of au in beehive chimney structures has resulted in the formation of a au rich volcanogenic massive sulphide vms deposit with its spatial distribution of deposit materials and metal contents the bvf represents a modern day analogue for basalt hosted au rich vms systems this article is protected by copyright all rights reserved

drilling shallow water massive sulfides at the palinuro volcanic complex aeolian island arc italy a subseafloor replacement style barite and sulfide occurrence was drilled in shallow waters at the palinuro volcanic complex the northernmost aeolian arc volcano in the tyrrhenian sea italy using a lander type drilling device 11 successful drill holes yielded a total of 13 5 m of core from a sediment filled depression located at a water depth of 630 to 650 m the longest continuous drill core recovered consists of 4 84 m of massive to semimassive barite and sulfides with abundant late native sulfur overprint seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active although black smoker activity does not occur on the seafloor the recovered drill core shows that the subseafloor deposit is zoned with depth the top of the mineralized zone is comprised of a variably silicified vuggy barite sulfide facies that shows notable polymetallic metal enrichment while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time the barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis during the main stage of the mineralization the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite trace famatinite and rare cinnabar lower temperature precipitates formed during the main stage of mineralization include sphalerite galena pyrite opal a and barite which are associated with traces of pb sb as sulfosalts such as bournonite seligmannite or semseyite a distinct mineral assemblage of fine grained anhedral enargite hypogene covellite chalcopyrite and galena is commonly associated with colloform sphalerite galena and pyrite as a late phase of this main stage colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence the deposit is interpreted to have formed from fluids having an intermediate sulfidation state although excursions to high and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition 34s 39 to 3 whereas a more narrow range is observed in the massive pyrite at depth 34s 10 to 0 similar variations were also documented for the late native sulfur overprint overall the negative sulfur isotope ratios at depth the intermediate to very high sulfidation conditions during mineralization and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth biological processes are interpreted to have played a major role during late stages of ore formation the combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite the intermediate to high sulfidation characteristics and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment the barite and sulfide occurrence at palinuro shares many characteristics with porphyry related base metal veins and intermediate sulfidation epithermal deposits suggesting that metallogenic processes associated with arc related magmatic hydrothermal systems are not restricted to the subaerial environment

hydrothermally precipitated mixed layer illite smectite in recent massive sulfide deposits from the sea floor mixed layer illite smectite i s in recent massive sulfide deposits from a seamount near the east pacific rise has been studied by x ray powder diffraction xrd and scanning and transmission electron microscopy sem tem the i s fills pore space between quartz and sulfides and xrd and tem analyses indicate that the i s contains 25 45 smectite with r1 type ordering isisis microprobe and analytical electron microscope analyses indicate a rectorite like composition k being the dominant interlayer cation the occurrence of the i s within a hydrothermally precipitated sulfide deposit the pore filling texture and the internal structure of the i s are all consistent with direct precipitation from hydrothermal solutions direct precipitation of i s from solution has two important implications first some sericite in ancient volcanogenic massive sulfide deposits may have similarly originated as hydrothermal precipitates rather than as alteration products with implications for mobility and transport of al second direct precipitation of r1 ordered i s from fluid implies that during diagenetic evolution of smectite to illite the sequence of smectite random i s ordered i s illite may not be followed

comparative mineralogy and geochemistry of gold bearing sulfide deposits on the mid ocean ridges a comparative study of the mineralogy and geochemistry of sulfide deposits on mid ocean ridges in the northeast pacific and the mid atlantic reveals common characteristics associated with primary gold enrichment average gold contents of 0 8 to 5 ppm au occur in sulfides from southern explorer ridge and axial seamount northeast pacific and from the tag hydrothermal field and snakepit vent field mid atlantic ridge the enrichment of gold in these deposits is consistently related to a phase of late stage low temperature 1 ppm au occur exclusively in pyritic assemblages and commonly with abundant fe poor sphalerite and a suite of complex pb sb as sulfosalts amorphous silica and locally barite or carbonate are important constituents of the gold rich precipitates but do not contain gold themselves high temperature 350 c black smoker assemblages consisting dominantly of pyrite chalcopyrite pyrrhotite isocubanite and abundant anhydrite are uniformly gold poor 0 2 ppm au to the extent that individual sulfides can be mechanically separated chemical analyses by neutron activation indicate that gold is most abundant in sphalerite up to 5 7 ppm au but also occurs in pyrite and marcasite samples of sphalerite with abundant inclusions of fine grained sulfosalts locally contain up to 18 ppm au suggesting that sulfosalts may be repositories for gold no free gold has been observed at 4000 magnification of polished specimens indicating that the gold is present only as submicroscopic inclusions or as a chemical constituent within the sulfides samples from gold rich deposits in the northeast pacific and mid atlantic are compared with similar but relatively gold poor sulfides from the galapagos rift and 13 n on the east pacific rise epr and with barren sulfides from 11 n epr 21 n epr the endeavour ridge and the southern juan de fuca ridge trace element analyses of more than 170 samples show that gold enrichment in almost all of the deposits is associated with high concentrations of ag as sb pb and zn and locally with high cd hg tl and ga in contrast gold is typically depleted in samples with high co se and mo the close association of au with ag as sb and pb may reflect the common behavior of these metals as aqueous sulfur complexes e g au hs 2 at low temperatures similar mineralogical and geochemical associations are observed in sulfide deposits from modern back arc settings and in the ancient geologic record

gradients in the composition of hydrothermal fluids from the endeavour segment vent field phase separation and brine loss hydrothermal fluid samples collected in 1984 1987 and 1988 from a large vent field near 47 57 n on the endeavour segment of the juan de fuca ridge jfr have been analyzed for major and minor elements and gases there are of the order of 100 individual smoker vents on 10 large sulfide structures which are localized along faults and fault intersections across the vent field each sulfide structure has a characteristic fluid composition which varies very little from one vent orifice to the next or from year to year on a given structure however there are large gradients in fluid composition across the vent field with endmember chlorinity increasing from 255 mmol kg in the sw to 505 mmol kg in the ne end member concentrations of major elements are well correlated with chlorinity and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids the gradients in composition across the vent field and measured vent fluid temperatures 400 c are consistent with supercritical phase separation and loss of brine phase below the seafloor the factor of 2 variation in co2 and h2s is larger than expected for loss of a very high chlorinity brine concentrations of iron and manganese are not positively correlated with chlorinity suggesting that temperature and ph are more important in controlling metal solubility elevated ammonia and bromide chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter and high boron concentrations point to a sedimentary source

formation of highly zn enriched sulfide scale at a deep sea artificial hydrothermal vent iheya north knoll okinawa trough artificial hydrothermal vents created by boreholes that discharge hydrothermal fluids and useful for observing secular changes in mineral precipitates and the chemical compositions of hydrothermal fluids are periodically cleaned of scale deposits here we report petrographic and geochemical features of hydrothermal scale with a concentric structure and extreme enrichment in zn recovered as an intact plug from an artificial hydrothermal vent pipe in the okinawa trough the scale consists of sphalerite with accessory galena and chalcopyrite and minor cotunnite pbcl2 barite an unidentified zn sulfate and bi rich minerals it comprises at least five concentric layers alternating between thin reddish brown porous layers composed of relatively fe rich sphalerite accompanied by galena and chalcopyrite and coarse grained dark gray layers dominated by relatively fe poor sphalerite cotunnite occurs only in the innermost reddish brown layer and barite occurs only in the uppermost and innermost layers the scale is composed of 50 wt zn several wt fe and pb and 1 wt cu mn and cd a composition consistent with the solubility of zn and cu in a hydrothermal fluid at 304 311 c and ph 4 7 5 0 we suggest that the concentric layers are related to periodic scale removal operations through which 1 mostly clogged pipe conditions and weak hydrothermal discharge alternate with 2 fully open pipe conditions and vigorous hydrothermal discharge thus affecting sulfur fugacity and cl and so42 activities in the hydrothermal fluid and leading to the formation of concentric layers of precipitated minerals

palaeoceanography bloom without fertilizer iron has been shown to stimulate productivity in certain areas of the modern ocean however it was not the primary driver of carbon burial in the equatorial pacific ocean for the past 10 million years

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

a model for growth of steep sided vent structures on the endeavour segment of the juan de fuca ridge results of a petrologic and geochemical study petrologic and geochemical studies of vent solids from the main endeavour field mef and the high rise field hrf juan de fuca ridge demonstrate that the steep sided vent structures characteristic of these sites form dominantly by flange growth combined with diffuse flow through sealed portions of structures and incorporation of flanges into structures geochemical calculations suggest that the prevalence of amorphous silica and flanges in endeavour deposits is the result of conductive cooling of vent fluids that have high concentrations of ammonia at endeavour as the temperature of vent fluid decreases ammonia ammonium equilibrium buffers ph and allows more efficient deposition of sulfide minerals and silica from fluids that have a higher ph than conductively cooled ammonia poor fluids present at most other unsedimented mid ocean ridge vent sites deposition of silica stabilizes flanges and allows structures to attain large size it also leads to diffuse flow and further conductive cooling by reducing the permeability and porosity of the structures and of feeder zones thus decreasing entrainment of seawater most inactive vent samples recovered from areas peripheral to the hrf and mef are similar to barite silica rich samples from the explorer ridge and axial seamount and likely formed from precipitation of silica and barite on a biological substrate active white smoker chimneys from the clam bed field located south of the hrf are pyrrhotite rich and likely formed from vent fluids that are depleted in zn and cd and enriched in pb and ba relative to fluids exiting trans atlantic geotraverse tag and cleft segment white smoker chimneys

a dual origin for the hydrothermal component in a metalliferous sediment core from the mid atlantic ridge a highly metalliferous sediment core collected from tag 26 n mid atlantic ridge has been analyzed for uranium series isotopes rare earth elements ree and radiogenic isotopes two types of deposition of hydrothermal material have been identified and characterized the first is sulfide material which has been rapidly deposited and is sometimes clearly turbiditic this material exhibits high u fe and eu sm ratios and low 230th fe and ree fe ratios it is proposed that u is incorporated into the sulfides by diffusion through the oxidized coating with reductive fixation near the sulfide oxide boundary the second hydrothermal component is particulate fallout from the neutrally buoyant plume which is diluted by the background pelagic sedimentation this component exhibits u fe ratios around 2 10 5 an order of magnitude greater than reported plume particulate levels but the 230th fe ree fe and eu sm ratios are similar to plume values nd and pb isotopes have been used to differentiate the hydrothermal and pelagic end members within this sediment core

upwelling and productivity changes inferred from a temperature record in the central equatorial pacific a series of c37 39 alkenones synthesized by prymnesiophyte algae is commonly preserved in marine sediments and can be used for estimating past sea surface temperatures ssts 1 2 here we present an alkenone analysis of sediments taken from the central equatorial pacific ocean core w8402a 14gc 0 57 n 138 57 w which shows that sst varied slightly 2 c but coherently with milankovitch insolation cycles over the past 250 kyr the in phase response of the sst to the precessional component of insolation indicates that this part of the sst time series may be driven by changes in local trade wind strength and the longer 100 kyr eccentricity component indicates a response to basin wide south pacific winds using a simple heat balance model we use our palaeotemperature record to constrain the upwelling rates that have occurred in the past similar sst records from the eastern pacific could constrain the horizontal advection component of the sst record and allow for better estimates of upwelling upwelling and palaeoproductivity then could be compared to determine whether changes in equatorial nutrient levels actually cause equivalent changes in productivity

the influence of hydrothermal fluid composition and advection rates on black smoker chimney mineralogy insights from modeling transport and reaction abstract a detailed study of black smoker chimneys was carried out by drawing together mineralogical observations and measured fluid compositions the modeling technique which establishes a link between vent fluid chemistry and vent deposit mineralogy is used to examine the influence of the physical environment on transport across short length scales centimeters which are subject to steep thermal 350 c and chemical gradients and to predict saturation states of minerals as functions of position within chimney walls owing to short length scales steep gradients and constancy of bounding fluid compositions concentration profiles are controlled by transport the effect of reaction rates on profiles is assumed to be negligible precipitation at points within the chimney wall is thus kinetically controlled local equilibrium does not apply and precipitation must be suppressed when calculating distributions of species and saturation states of minerals model calculations are performed in a series of four steps the first is speciation of measured fluid compositions at in situ temperature and pressure conditions results of calculations are plotted on stability diagrams of log fo2 log f s2 and log fo2 log a fe 2 a cu 2 to examine how closely the predicted saturation states fit with observations of minerals coexisting with the hydrothermal fluid at the vent sites the second step is calculation of heat and mass transport across the chimney wall the third is speciation of the pore fluid compositions calculated to exist in the wall subsequent to transport and the fourth is comparison of calculated mineral saturation states to observations of naturally occurring chimney samples application of the modeling technique to five vent sites which exhibit distinct fluid compositions allows reproduction of observed mineralogy model results demonstrate the sensitivity of geochemical processes to the physical environment porosity tortuosity permeability mineral composition to rates of advection of seawater inward or hydrothermal fluid outward across chimney walls and to endmember fluid composition

geological setting and mineralogical and geochemical investigations on sulfide deposits near 13 n on the east pacific rise geological mineralogical and geochemical investigations of hydrothermal deposits on one of the most studied segments of the east pacific rise near 13 n show that sulfides occur in four main tectonic settings these are 1 in the middle part of the central graben where active zones are located 2 at the top of the central graben where a second line of deposits is observed 3 on the marginal high which has sulfides both on its western tectonic side and at its top where a large deposit is located and 4 on the flank of the southeastern seamount which is the site of the most important deposit of the area the nature of the distribution of the samples through the area studied shows that hydrothermal activity has existed for more than 20 000 years mineral assemblages are dependent on the sample location the sulfide samples exhibit a wide range of mineralogical assemblages varying from high temperature hydrothermal cu fe rich chimneys black smokers which are characteristic structures of young edifices to mature cu fe rich massive sulfides similar to those found in the core of massive sulfide lenses mined on shore with respect to these observations a classification into nine types of sulfide assemblages is proposed hydrothermal activity at the axis is now increasing in intensity increase in temperature decrease in f s2 and f o2 the relative paucity of zn rich massive sulfides on the off axis structures is likely to be a consequence of the depletion of zn in the hydrothermal fluids r mode factor analysis applied to a population of nineteen representative bulk chemical analyses shows fractionation of the reduced sulfur fluid during the discharge into three main groups cu fe co se ca sr zn cd ag as pb and si ba displaying the principal types of sulfide samples collected in the 13 n area microprobe investigations conducted on sulfide minerals define the distribution of trace metal elements such as ag co se cd and as during the process of hydrothermal maturation of the deposits

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

lead isotopes and the sources of the columbia river basalt group a detailed study of 208pb 204pb 207pb 204pb and 206pb 204pb ratios suggests that the number of pb isotopic reservoirs required for the columbia river basalt group crbg as currently defined must be increased from the presently accepted four to at least six the identities of the six reservoirs are two of mid ocean ridge basalt morb slightly contaminated with sediment rl and r2 rl being cascades pb one a probable plume component r3 two of unspecified mantle material but probably of rl type contaminated with local crustal pb r4 and r5 and the sixth a complex enriched reservoir of 2150 ma age r6 rl is an end member for all crbg except the saddle mountains basalts r2 is a second end member for the picture gorge basalt and appears to be unique to it both rl and r2 are related to the juan de fuca subduction zone and the back arc spreading associated with it r3 is the principal source for most of the wanapum basalt as well as for most of the chemically evolved portions of the grande ronde basalt volumetric and tectonic considerations require that one source of the crbg be a plume or at least a nonlocal crustal or uppermost mantle source and r3 is the ideal candidate for that role r4 is the second source for the imnaha basalt and r5 is the source for the 206pb rich varieties of the grande ronde basalt r4 and r5 are identified with contamination by local phanerozoic crust because of their similarity to local crustal pb and because the rocks which contain them also have cu contents correlated with their pb isotopic compositions r6 is the parent for pb in all the saddle mountain basalts this last source appears to have been homogeneous 2150 m y ago but has since split into a number of separate discrete pockets each with its own characteristic pb isotopic signature some of these appear in individual saddle mountain basalt flows while other sources have mixed with r4 to produce individual saddle mountain basalt flows of the ice harbor set r6 is similar in all its isotopic properties to the precambrian basement that is exposed to the immediate north of the columbia river basalt group the overall model for columbia river basalt group genesis is termed plume reinforced back arc magmatism

kinetics of sulfide mineral oxidation in seawater implications for acid generation during in situ mining of seafloor hydrothermal vent deposits abstract growth in global metal demand has fostered a new age of unconventional mining on the seafloor in situ pulverization and extraction of seafloor massive sulfide sms deposits is economically attractive due to minimal overburden and high ore grades however important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining laboratory experiments were performed to evaluate the effects of ph temperature dissolved oxygen and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater these minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow the rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22 c are given in the form r sp k m o 2 a m h b where rsp is the specific rate moles m 2 sec 1 k is the rate constant oxygen and proton concentrations are expressed in molalities m and their reaction orders as a and b respectively the specific rate laws obtained for each sulfide studied are r sp pyrrhotite 10 7 27 m o 2 aq 0 51 0 08 m h 0 08 0 03 r sp chalcopyrite 10 9 38 m o 2 aq 1 16 0 03 m h 0 36 0 09 when used to quantitatively predict maximum acid generation rates these rate laws indicate that acid production from in situ sms mining is insufficient to exceed the buffering capacity of advecting seawater we also calculated the residence times of crushed sulfides in seawater with low po2 0 10 atm ph of 8 23 c and find that depending on grain size mining waste may persist near the seafloor for years the implications are positive in terms of slow acid production but potentially problematic considering the potential ecological effects of an unnatural influx of particulates

systematics of and postulated controls on submarine hydrothermal solution chemistry the chemical compositions of seafloor hydrothermal solutions from the galapagos spreading center 21 n and 13 n on the east pacific rise the southern juan de fuca ridge and the guaymas basin gulf of california are examined for systematic variations several elemental ratios elemental products and empirical geothermometers show much consistency among the various areas in spite of large absolute differences in elemental concentrations the results suggest that most of the major cations in these solutions are solubility controlled with respect to a similar phase assemblage at all of the sites examination of the solution chemistry in conjunction with experimental results and seafloor rock alteration data permits identification of phases which may be important in controlling the solution chemistry as potassium appears to be solubility controlled its use to calculate water rock ratios is inappropriate lithium appears to be a better element to use in these calculations a model to explain the observed solution chemistries as a result of three component mixing is discussed all of the solutions are postulated to be either the result of mixing between a seawater hydrothermal solution which has not undergone a phase separation and is hence gas containing a gas rich vapor phase and a gas poor brine phase or any one of these individual components alone the vapor and brine phases are the result of phase separation which may have occurred at some time in the past essentially all of the reported data fall within the field defined by these three components the solutions apparently have had sufficient time after the mixing event s to reequilibrate with the rock this is true for quartz as well as other chemical components the best test of this model is chloride together with gas and isotropic data as the model predicts that solutions with elevated chlorinities will be depleted in gases and enriched in 18 o and low chlorinity solutions will have elevated gas contents and lower 18 o

temperature variations at diffuse and focused flow hydrothermal vent sites along the northern east pacific rise in the decade following documented volcanic activity on the east pacific rise near 9 50 n we monitored hydrothermal vent fluid temperature variations in conjunction with approximately yearly vent fluid sampling to better understand the processes and physical conditions that govern the evolution of seafloor hydrothermal systems the temperature of both diffuse flow low temperature and focused flow high temperature vent fluids decreased significantly within several years of eruptions in 1991 and 1992 after mid 1994 focused flow vents generally exhibited periods of relatively stable slowly varying temperatures with occasional high and low temperature excursions lasting days to weeks one such positive temperature excursion was associated with a crustal cracking event another with both positive and negative excursions demonstrated a subsurface connection between adjacent focused flow and diffuse flow vents diffuse flow vents exhibit much greater temperature variability than adjacent higher temperature vents on timescales of a week or less temperatures at a given position within a diffuse flow field often varied by 5 10 c synchronous with near bottom currents dominated by tidal and inertial forcing on timescales of a week and longer diffuse flow temperatures varied slowly and incoherently among different vent fields at diffuse flow vent sites the conceptual model of a thermal boundary layer immediately above the seafloor explains many of the temporal and spatial temperature variations observed within a single vent field the thermal boundary layer is a lens of warm water injected from beneath the seafloor that is mixed and distended by lateral near bottom currents the volume of the boundary layer is delineated by the position of mature communities of sessile e g tubeworms and relatively slow moving organisms e g mussels vertical flow rates of hydrothermal fluids exiting the seafloor at diffuse vents are less than lateral flow rates of near bottom currents 5 10 cm s the presence of a subsurface shallow reservoir of warm hydrothermal fluids can explain differing temperature behaviors of adjacent diffuse flow and focused flow vents at 9 50 n different average temperatures and daily temperature ranges are explained by variable amounts of mixing of hydrothermal fluids with ambient seawater through subsurface conduits that have varying lateral permeability

oxidative dissolution of hydrothermal mixed sulphide ore an assessment of current knowledge in relation to seafloor massive sulphide mining highlights seafloor massive sulphides vary in mineralogy and oxidise at different rates galvanic cells play a role in increasing dissolution rates sms deposits that have been inactive for some time may have lost economic value the geochemistry of oxidation products has potential to be an exploration tool potential for anthropogenic release of heavy metals during seafloor mining any risk needs addressing by the isa prior to the formation of mining regulations the weathering process of seafloor massive sulphide sms deposits can be considered analogous to weathering of terrestrial volcanogenic massive sulphides vms deposits however in the context of sms deposits the process occurs in chemically buffered waters of near neutral ph resulting in the formation of insoluble fe oxy hydroxide minerals including goethite and hematite as well as sulphates such as jarosite as a result of this precipitation it is commonly assumed that any sms deposit is unlikely to exhibit a significant loss of metals dissolution and release of heavy metals into the water column however galvanic interactions have never been considered in this seafloor context whilst they have already been shown to have the ability to increase dissolution significantly in terrestrial deposits if heavy metal release is not temporally balanced by precipitation of oxide phases there is the potential that these metal occurrences lose economic value this is specifically significant if there is an industrial focus on exploiting deposits associated with hydrothermal vents that have been inactive for some time not only this but the geochemistry of weathering products gossans that are formed have the potential to be used as tools for exploration furthermore it is unknown what impact galvanic coupling may have with regards to anthropogenic release of heavy metals during seafloor mining of deposits associated with either active or inactive vents disturbance of sediment plume generation and dewatering process this environmental impact needs to be addressed prior to the formulation of regulations for deep sea mining by the international seabed authority the present review examines our current understanding of oxidation and dissolution of a mixed sulphide ore bringing together lines of evidence from a range of literature sources based on this review different seafloor sulphide ore deposits will dissolve by oxidation and release a variety of different metals economic and or toxic all at different rates with galvanic cells playing a role by increasing dissolution rates while precipitation of oxide and oxy hydroxide phases will occur it is unknown in both a natural weathering and anthropogenic mining context whether heavy metal release including economic and toxic metals is temporally balanced by this precipitation and any subsequent adsorption based on our current understanding certain sites will be more predisposed to oxidation as a result of their mineralogy those containing significant pyrrhotite marcasite galena and secondary copper sulphides and or environment higher temperatures and oxygen concentrations lower ph s furthermore certain sites pose more of an environmental risk in terms of toxicity than others with arc related sms deposits associated with higher concentrations of as pb sb cd and hg and ultra mafic hosted sms deposits associated with high concentrations of co and ni the review highlights the need for subsequent studies that investigate the natural weathering process of seafloor sulphide ore deposits including how this process impacts their economic value and consequent geochemical signature of oxides that are produced over time moreover this review underlines the necessity for experiments to elucidate the oxidative dissolution of ore throughout any mining process

supergene alteration of sulphides vii distribution of elements during the gossan forming process abstract the results of many electrochemical weathering experiments involving sulphide ore assemblages are brought together and discussed in terms of the mechanism of reaction of sulphide minerals through to oxide gossan minerals the observed reactions of various sulphide minerals are compared and the behaviour of the resultant cations and anions in solution including their hydrolysis and adsorption properties as they effect the types of gossan materials that are formed is given the interaction between the ph of the reactions and the amount of fe in the sulphides is shown to be the dominant feature which controls the mobility of many elements the experiments demonstrate how a low ph develops near to the surface of oxidising sulphides giving a scale at this point that is depleted in most base metals relative to gossan materials that precipitate from solution gangue minerals are shown to be leached and the elements leached play a role in the formation of gossan materials there is a complex interaction between the fe ii fe iii couple and the cu i cu ii couple that should favour the formation of hematite native cu at low ph and malachite hematite goethite at ph above 6 5

plume dispersed hydrothermal particles a time series record of settling flux from the endeavour ridge using moored sensors abstract settling particles collected with moored sediment traps near an area of active hydrothermal venting in the northeast pacific ocean provide information about the deep sea distribution of hydrothermal material results of an 8 day near field single trap experiment and a 340 day multi instrument 5 trap deployment two kilometers from the endeavour ridge vent field show variability in the quantity and composition of the plume to sediment particle flux over time with depth and with distance from the vent field hydrothermal elemental fluxes vary by a factor of two among the four sampling intervals of the longer experiment a factor of three variation in fe s among these periods reflects temporal changes in phase composition changes in flux with depth for example a 200 fold increase in cu and zn between above and below plume traps are evidence of hydrothermal input comparison of fluxes determined in the near and far field experiments reveals the spatial influence of vent effluent near field particle flux exceeds that measured two kilometers away by a factor of 10 to 20 and is relatively enriched in fe cu and organic carbon but depleted in mn calculations of the residence time of the plume with respect to settling indicate that more than 90 percent of the particulate material is transported farther than two kilometers from the vent field interpretation of elemental mineralogical and grain size analyses suggest that chemical precipitation kinetics other physical and biological particle forming processes changes in vent output and lateral transport of hydrothermal material by abyssal currents are tenable explanations for our observations our findings suggest that these rate interdependent processes be included in realistic models of hydrothermal particle flux

compositions growth mechanisms and temporal relations of hydrothermal sulfide sulfate silica chimneys at the northern cleft segment juan de fuca ridge three active hydrothermal vents forming sulfide mounds and chimneys monolith fountain and pipe organ and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern cleft segment juan de fuca ridge the formation of zoned tubular curich chimneys type i on the monolith sulfide mound is related to focused flow of high temperature to 328 c fluid bulbous chimneys type ii or beehives at the monolith and fountain vents are products of diffuse high temperature to 315 c discharge a broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite rich shells columnar zn sulfide rich chimneys with narrow channelways type iii are constructed where focused and relatively low temperature 261 c fluid vents directly from the basalt substrate the bulk chemistry low cu high pb ag and sio2 contents mineralogy pyrite marcasite wurtzite amorphous silica anglesite colloform and filamentous textures and oxygen isotope characteristics of inactive type iv chimneys indicate a low temperature 250 c origin involving diffuse and sluggish flow patterns and conductive cooling seafloor observations and 210pb data indicate that 1 type iv chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption 2 the high temperature monolith and fountain vents are manifestations of the same heating event shallow emplacement of magma that led to the sheet flow eruption and recent megaplumes and 3 the pipe organ vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption

tectonic controls on magma genesis and evolution in the northwestern united states abstract field chronologic chemical and isotopic data for late cenozoic basaltic rocks from the northwestern united states illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism in the northwestern u s the first major episode of basaltic volcanism was triggered by crustal rifting in a back arc environment east of the westward migrating volcanic arc created by the subduction of the juan de fuca plate beneath the north american plate rifting and volcanism were concentrated by pre existing zones of crustal weakness associated with boundaries between the old archean core of the continent and newly accreted terranes basalts erupted during this time columbia river steens mountain show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust mantle interface due to their encounter with thick crust not yet extended and still containing its low density easily fusible component continued rifting of this crust and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts coupled with a shift in stress orientation roughly 10 5 ma ago allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface in the western part of the region oregon plateau these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere to the east snake river plain however the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying archean crustal section some 2 6 ga ago

lead isotopic data from sulfide minerals from the cascade range oregon and washington abstract lead isotopic studies of mineral deposits associated with tertiary plutons found in the cascade range of oregon and washington demonstrate a rather uniform isotopic composition in various sulfide minerals 206 pb 204 pb 18 84 to 19 05 207 pb 204 pb 15 57 to 15 62 208 pb 204 pb 38 49 to 38 74 show less variation than data from the volcanic rocks of the cascade range and fall within the mixing array defined by the morb regression line and continental sediments an evaluation of the role of crustal assimilation by hydrothermal convection during emplacement was made on five sulfide deposits associated with a single composite batholith the cloudy pass pluton the pb isotopic data and mass balance calculations suggest that only minor amounts of the lead were derived from the overlying precambrian swakane biotite gneiss during emplacement the bulk of the metal that occurs in sulfide deposits in the cascade mineral belt appears to have been derived from subducted continental detritus the variation of the pb isotopic signature of sulfides from specific districts or deposits suggests that there is a correlation with age and structure of the crust 206 pb 204 pb is greater than 18 92 in northern washington and southern oregon where deposits have intruded mesozoic or older crust however the ore deposits between the northern oregon border and central oregon south of eugene have intruded younger crust composed largely of mafic and andesitic volcanic rocks and 206 pb 204 pb lies between 18 84 and 18 92 this region previously called the columbia embayment appears to be underlain by tertiary volcanic rocks lead isotopic data may be used to define the boundaries between discontinuous blocks of mesozoic crust and tertiary volcanic cover

hydrothermal precipitates associated with bimodal volcanism in the central bransfield strait antarctica polymetallic sulfide sulfate mineralization enriched in pb ag as sb hg occurs in the bransfield strait a late tertiary quaternary marginal basin close to the antarctic peninsula the mineralization is associated with bimodal volcanism and pelagic and volcaniclastic sediment in rifted continental crust hydrothermal precipitates have been recovered from two shallow 1 050 1 000 m water depth submarine volcanoes hook ridge and three sisters in the central bransfield strait mineralization at hook ridge consists of polymetallic sulfides massive barite and pyrite and marcasite crusts in semilithified pelagic and volcaniclastic sediment native sulfur commonly infills void space and cements the volcaniclastic sediment the polymetallic sulfides are dominated by sphalerite with minor galena enargite tetrahedrite tennantite pyrite chalcopyrite and traces of orpiment cemented by barite and opal a the presence of enargite at hook ridge the abundance of native sulfur and the low fe content of sphalerite indicate a high sulfur activity of the hydrothermal fluids responsible for mineralization the sulfur isotopic composition of hook ridge precipitates documents the complexity of the sulfur sources in this hydrothermal system with variable influence of biological activity and possibly magmatic contributions homogenization temperatures and salinities of fluid inclusions in barite and opal a suggest that boiling may have affected the hydrothermal fluids during their ascent the discovery of massive barite silica precipitates at another shallow marine volcano three sisters volcano attests to the potential for hydrothermal mineralization at other volcanic edifices in the area the characteristics of the mineralization in the bransfield strait with rifting of continental crust the presence of bimodal volcanism including highly evolved felsic volcanic rocks the association with sediments and the pb ag as sb hg enrichment are similar to the setting of massive sulfide deposits in the okinawa trough and distinct from those of sediment dominated hydrothermal systems such as escanaba trough middle valley and guaymas basin the geological setting of the bransfield strait is also broadly similar to that of some of the largest volcanogenic massive sulfide deposits in the ancient record such as the iberian pyrite belt

growth of large sulfide structures on the endeavour segment of the juan de fuca ridge abstract mapping and sampling with dsrv alvin has established that sulfide blocks 0 5 m across dredged from the axial valley of the endeavour segment at 47 57 n are samples of unusually large sulfide structures the steep sided structures up to 30 m in length 20 m in height and 10 15 m across are localized by venting along normal faults at the base of the western axial valley wall and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest high temperature fluids 350 to more than 400 c pass through the massive sulfide structures and enter seawater through small concentric nozzle like features projecting from the top or the sides of the larger vent structures diffuse low temperature flow is pervasive in the vicinity of the active sulfide structures exiting from basalt and sulfide surfaces alike evidence of recent volcanic activity is sparse the two largest samples taken with the dredge would not have been recoverable using the submersible these samples represent massive complex portions of the sulfide structures which were not closely associated with rapid high temperature fluid flow at the time of sampling they contain textural evidence of sealed hydrothermal fluid exit channels mineralogy is dominated by fe sulfides nnd amorphous silica pyrite marcasite wurtzite chalcopyrite and iss are the most common sulfide phases pyrrhotite galena and sphalerite are present in trace amounts barite amorphous silica and chalcedony are the only non sulfide phases anhydrite is not observed in any of the dredge samples although it is common in the chimney like samples recovered by alvin specific mineralogical textural zones within the dredge samples are anaoogous to individual layers in east pacific rise at 21 n and southern juan de fuca ridge samples with two exceptions a coarse grained highly porous fe sulfide rich interior containing sulfidized tubeworm casts and a 2 5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica the porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber the amorphous silica deposited from a seawater hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate these silica rich zones are the densest most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features observations in these samples are consistent with two distinct phases of development phase 1 is analogous to chimney growth and construction at 21 n and ends when flow channels become sealed to rapid flow of through going fluid the flow is evidently redirected within the structure phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater hydrothermal fluid mixtures evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures

tectonic processes in papua new guinea and past productivity in the eastern equatorial pacific ocean phytoplankton growth in the eastern equatorial pacific ocean today accounts for about half of the new production the fraction of primary production fuelled by externally supplied nutrients in the global ocean the recent demonstration that an inadequate supply of iron limits primary production in this region1 supports earlier speculation that in the past fluctuations in the atmospheric deposition of iron bearing dust may have driven large changes in productivity2 but we argue here that only small 2 nm increases in the iron concentration in source waters of the upwelling equatorial undercurrent are needed to fuel intense diatom production across the entire eastern equatorial pacific ocean episodic increases in iron concentrations of this magnitude or larger were probably frequent in the past because a large component of the undercurrent originates in the convergent island arc region of papua new guinea which has experienced intensive volcanic erosional and seismic activity over the past 16 million years cycles of plankton productivity recorded in eastern equatorial pacific sediments may therefore reflect the influence of tectonic processes in the papua new guinea region superimposed on the effects of global climate forcing

deep ocean mineral deposits as a source of critical metals for high and green technology applications comparison with land based resources abstract ferromanganese fe mn crusts are strongly enriched relative to the earth s lithosphere in many rare and critical metals including co te mo bi pt w zr nb y and rare earth elements rees fe mn nodules are strongly enriched in ni cu co mo zr li y and rees compared to fe mn crusts nodules are more enriched in ni cu and li with subequal amounts of mo and crusts are more enriched in the other metals the metal ions and complexes in seawater are sorbed onto the two major host phases feo oh with a positively charged surface and mno 2 with a negatively charged surface metals are also derived from diagenetically modified sediment pore fluids and incorporated into most nodules seafloor massive sulfides sms especially those in arc and back arc settings can also be enriched in rare metals and metalloids such as cd ga ge in as sb and se metal grades for the elements of economic interest in sms cu zn au ag are much greater than those in land based volcanogenic massive sulfides however their tonnage throughout the global ocean is poorly known and grade tonnage comparisons with land based deposits would be premature the clarion clipperton fe mn nodule zone ccz in the ne pacific and the prime fe mn crust zone pcz in the central pacific are the areas of greatest economic interest for nodules and crusts and grades and tonnages for those areas are moderately well known we compare the grades and tonnages of nodules and crusts in those two areas with the global terrestrial reserves and resources nodules in the ccz have more tl 6000 times mn te ni co and y than the entire global terrestrial reserve base for those metals the ccz nodules also contain significant amounts of cu mo w li nb and rare earth oxides reo compared to the global land based reserves fe mn crusts in the pcz have significantly more tl 1700 times te 10 times more co and y than the entire terrestrial reserve base other metals of significance in the pcz crusts relative to the total global land based reserves are bi reo nb and w ccz nodules and pcz crusts are also compared with the two largest existing land based ree mines bayan obo in china and mountain pass in the usa the land based deposits are higher grade but lower tonnage deposits notably both land based deposits have

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

evolution of hydrothermal activity on the juan de fuca ridge observations mineral ages and ra isotope ratios we discuss observations of hydrothermal activity from sites along the juan de fuca ridge and report the ages and radium isotope ratios of samples from eight sites collected on a series of 11 alvin dives in september 1988 this was a pilot program to assess whether uranium and thorium series isotopes could provide age information on the duration of active venting the ages are based on the ingrowth of 228th from 228ra in both active and inactive chimneys and chimney talus composed of barite anhydrite chalcopyrite wurtzite and pyrite samples were obtained from from north to south the fairycastle site northern endeavour ridge the endeavour hydrothermal site and the southern juan de fuca sites named vent 3 vent 1 and plume areas samples from active chimneys 20 yielded 228th 228ra ages in the range 0 2 to 1 2 years we recorded no ages in the range 1 2 to 12 years for inactive chimneys or rubble our tentative conclusions are that the individual chimneys form rapidly and that the presently active chimney structures are 2 years old in the fairycastle site there is at least one episode of hydrothermal activity recorded in the rubble which occurred at least 20 years earlier large sulfide edifices in the endeavour hydrothermal site probably record a minimum of 20 years of discharge for each particular structure chimney structures in the southern juan de fuca ridge may be older they show no evidence of significant recent growth except for an outer barite rich layer the results of this study suggest that hydrothermal chimneys of diverse mineralogy can be dated by measuring the ingrowth of 228th from 228ra decay the technique is most accurate for chimneys that are less than 10 years old the technique in conjunction with detailed petrological analyses could be used to assess the growth rates of individual chimney walls variations in the 228ra 226ra activity ratios among the three areas reflect either differences in fluid residence time depth of generation of hydrothermal fluids or interaction with basaltic rocks previously altered at low temperatures

particle recycling in the tag hydrothermal plume abstract analysis of th isotope distributions in the tag hydrothermal plume yields evidence for pronounced 234 th 230 th isotope fractionation this fractionation can be attributed to a combination of four processes 1 particle ageing 2 differential scavenging of 234 th and 230 th 3 differential focusing of 234 th and 230 th and 4 recycling of old particles we present arguments for the importance of the last of these processes particle recycling particles in the tag hydrothermal plume exhibit grain sizes which fall in the range 1 10 m applying the range of densities reported for amorphous iron oxyhydroxide and iron oxide mineral phases stokes law predicts that these particles should achieve terminal settling velocities ranging between 4 and 8 m d 1 for coarse grained 5 m radius particles and 0 2 0 3 m d 1 for fine grained particles 1 m radius these velocities have been used in a physical model of re entrainment and recycling in neutrally buoyant plumes our results indicate that particle recycling and re entrainment in hydrothermal plumes may be important over length scales of 1 10 km the model predicts that the proportion of particles re entrained is independent of particle size but increases with plume strength for plumes with volumetric fluxes greater than 10 3 m 3 s 1 the proportion of particles re entrained exceeds 50 in agreement with previous calculations based upon 230 th distributions sampling strategies in the past have typically concentrated upon processes operating within 1 km of active vent fields and will need to be revised accordingly in the future

distribution and composition of hydrothermal plume particles from the ashes vent field at axial volcano juan de fuca ridge in 1986 and 1987 buoyant and neutrally buoyant hydrothermal plume particles from the ashes vent field within axial volcano were sampled to study their variations in composition with height above the seafloor individual mineral phases were identified using standard x ray diffraction procedures elemental composition and particle morphologies were determined by x ray fluorescence spectrometry and scanning electron microscopy x ray energy spectrometry techniques the vent particles were primarily composed of sphalerite anhydrite pyrite pyrrhotite chalcopyrite barite hydrous iron oxides and amorphous silica grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume the neutrally buoyant plume consisted of a lower plume which was highly enriched in fe s zn and cu and an upper plume which was highly enriched in fe and mn the upper plume was enriched in fe and mn oxyhydroxide particles and the lower plume was enriched in suspended sulfide particles in addition to the fe and mn oxyhydroxide particles the chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles short term sediment trap experiments indicate that the fallout from the ashes vent field is not as large as some of the other vent fields on the juan de fuca ridge

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

isotope and trace element geochemistry of augustine volcano alaska implications for magmatic evolution k e johnson r s harmon j m richardson s moorbath and d f strong department of geology 1112 turlington hall university of florida gainesville fl 32611 usa us army research office p o box 12211 research triangle park nc 27709 usa geoscience branch ontario geological survey willet green miller centre sudbury ont pse 6b5 canada department of earth sciences university of oxford parks road oxford oxi 3pr uk centre for earth and ocean research university of victoria victoria b c v8w 3p6 canada

active hydrothermal vents and sulfide deposits on the southern juan de fuca ridge massive sulfide deposits rich in zinc and silver were recovered from the juan de fuca ridge 500 km west of oregon in september 1981 the samples recovered are composed largely of zinc sulfide with lesser amounts of iron lead and copper sulfide most of the deposits occur at a series of hydrothermal vents within a relatively continuous depression in the center of a smooth 1 km wide valley along the ridge axis the depression appears to be formed by collapse of a lava lake possibly modified by extensional faulting the axial valley floor outside the depression is underlain by fresh glassy ferrobasalt sheet and lobate flows two types of sulfide mineral deposits were dredged from one of the hydrothermal vents 1 angular slabs of dark gray zinc sulfide and 2 subrounded fragments of porous light gray zinc sulfide the samples contain fresh sphalerite zoned wurtzite pyrite and minor amounts of marcasite galena and chalcopyrite cubanite the spreading rate of the juan de fuca ridge and the composition of the sulfide samples are generally similar to the east pacific rise lat 21 n site however the texture and geologic setting of the sulfide deposits are significantly different

discrete and diffuse heat transfer atashes vent field axial volcano juan de fuca ridge abstract heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ashes vent field located in the caldera of axial volcano on the central juan de fuca ridge the discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor the investigation centered on the high temperature portion of the field a 100 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge 326 c from seven individual vents and diffuse flow from intervening areas a total of 4 4 2 10 6 w was estimated for the discrete heat flux based on measurements of effluent temperature flow rate and orifice diameter at all the known individual vents by d s v alvin in september 1987 a total of 15 75 10 6 w was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of alvin s instrument platform the study indicates that 1 the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component and that 2 the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high temperature portion of the ashes vent field

links between iron input and opal deposition in the pleistocene equatorial pacific ocean iron is often a limiting nutrient in ocean regions that have a constant supply of other macro nutrients and changes in iron supply over time have been linked to fluctuations in primary productivity marine sediments from the equatorial pacific ocean show that over the past million years iron input was linked to the export and burial of biogenic silica

rift topography linked to magmatism at the intermediate spreading juan de fuca ridge new seismic observations of crustal structure along the juan de fuca ridge indicate that the axial rift topography reflects magma induced deformation rather than alternating phases of magmatism and tectonic extension as previously proposed contrary to predictions of the episodic models crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2 1 2 6 km the shallow rift valley or axial graben associated with each juan de fuca segment is 50 200 m deep and 1 8 km wide and is well correlated with a magma body in the subsurface analysis of graben dimensions height and width shows that the axial graben narrows and graben height diminishes where the magma body disappears rather than deepening and broadening as expected for rift topography due to tectonic extension we propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor in this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion rather than episodic magma delivery from the mantle

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

a miocene subcontinental plume in the pacific northwest geochemical evidence most of the recent discussions on the number and kinds of components which can be distinguished in the columbia river basalt group crbg magmas have used arguments developed from isotopic evidence in this paper we consider relative contents of excluded trace elements interpreted by means of spidergrams and abundance ratios and compare data from the crbg and related lavas with those of selected oceanic basalts although many recent models for crbg petrogeneses do not include a plume component spidergrams for the american bar flows of the imnaha basalt of the crbg have broad humps at ba th and nb ta this feature suggests that an enriched mantle plume component is present in the american bar magmas an inference which is weakly confirmed by 87sr 86sr data this plume may have been identical to that presently beneath yellowstone the inferred hot spot track lies almost directly east west and requires a vector of motion of the pacific northwest part of the north american plate significantly different from that inferred by minster and jordan 1978 31 our proposed vector for the north american plate agrees however with that suggested by the anahim belt a volcanic lineament in southern british columbia that has been proposed as a hot spot track this proposed vector requires alterations in current models for pacific and north american plate movement during the cenozoic

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

sulfide oxidation and distribution of metals near abandoned copper mines in coastal environments prince william sound alaska usa abstract the oxidation of sulfide rich rocks mostly leftover debris from cu mining in the early 20th century is contributing to metal contamination of local coastal environments in prince william sound alaska analyses of sulfide water sediment precipitate and biological samples from the beatson ellamar and threeman mine sites show that acidic surface waters generated from sulfide weathering are pathways for redistribution of environmentally important elements into and beyond the intertidal zone at each site volcanogenic massive sulfide deposits composed of pyrrhotite and or pyrite chalcopyrite sphalerite with subordinate galena arsenopyrite and cobaltite represent potent sources of cu zn pb as co cd and hg the resistance to oxidation among the major sulfides increases in the order pyrrhotite sphalerite chalcopyrite pyrite thus pyrrhotite rich rocks are typically more oxidized than those dominated by pyrite the pervasive alteration of pyrrhotite begins with rim replacement by marcasite followed by replacement of the core by sulfur fe sulfate and fe al sulfate the oxi dation of chalcopyrite and pyrite involves an encroachment by colloform fe oxyhydroxides at grain margins and along crosscutting cracks that gradually consumes the entire grain the complete oxidation of sulfide rich samples results in a porous aggregate of goethite lepidocrocite and amorphous fe oxyhydroxide enclosing hydrothermal and sedimentary silicates an inverse correlation between ph and metal concentrations is evident in water data from all three sites among all waters sampled pore waters from ellamar beach gravels have the lowest ph 3 and highest concentrations of base metals to 25 000 g l which result from oxidation of abundant sulfide rich debris in the sediment high levels of dissolved hg to 4100 ng l in the pore waters probably result from oxidation of sphalerite rich rocks the low ph and high concentrations of dissolved fe al and so4 are conducive to precipitation of interstitial jarosite in the intertidal gravels although pore waters from the intertidal zone at the threeman mine site have circumneutral ph values small amounts of dissolved fe2 in the pore waters are oxidized during mixing with seawater resulting in precipitation of fe oxyhydroxide flocs along the beach seawater interface at the beatson site surface waters funneled through the underground mine workings and discharged across the waste dumps have near neutral ph 6 7 7 3 and a relatively small base metal load however these streams probably play a role in the physical transport of metalliferous particulates into intertidal and offshore areas during storm events somewhat more acidic fluids to ph 5 3 occur in stagnant seeps and small streams emerging from the beatson waste dumps amorphous fe precipitates in stagnant waters at beatson have high cu 5 2 wt and zn 2 3 wt concentrations that probably reflect adsorption onto the extremely high surface area of colloidal particles conversely crystalline precipitates composed of ferrihydrite and schwertmannite that formed in the active flow of small streams have lower metal contents which are attributed to their smaller surface area and therefore fewer reactive sorption sites seeps containing precipitates with high metal contents may contribute contaminants to the marine environment during storm induced periods of high runoff preliminary chemical data for mussels mytilus edulis collected from beatson ellamar and threeman indicate that bioaccumulation of base metals is occurring in the marine environment at all three sites

petrochemistry of the yellowknife volcanic suite at yellowknife n w t the petrochemistry of the yellowknife volcanic belt at yellowknife kam duck and banting formations shows the suite to be more chemically complex than previously thought the kam formation 65 basalt 27 basaltic andesite 17 andesite and 2 dactite has a strong tholeiitic trend involving pyroxene fractionation which shows iron enrichment to feo feo mgo 0 75 after altered samples are removed from consideration it can be shown that the kam is chemically analogous to modern ocean floor or ocean rise tholeiites the andesitic members of the kam were partially melted from a hornblendic region presumably former oceanic crust possibly with a slight sialic component the duck formation 11 basalt 21 basaltic andesite 59 andesite and 9 dacite shows considerable evidence of alkali mobility with varied carbonate content it is thought to have been largely derived as partial melts from former oceanic crust via hornblende fractionation the banting formation 14 basalt 7 basaltic andesi

copper isotope systematics of the lucky strike rainbow and logatchev sea floor hydrothermal fields on the mid atlantic ridge in this paper we present the copper isotope signatures of black smoker sulfides massive sulfides and their alteration products and provide new insights into mineralization processes and applications of copper isotope geochemistry to sea floor hydrothermal systems on the mid atlantic ridge the hydrothermal systems studied include the lucky strike field at 37 17 n on a basaltic substrate and the rainbow and logatchev fields situated on ultramafic rock copper isotope variation in the hydrothermal precipitates was examined in conjunction with s isotopes and se and co concentrations the comparison between 65cu and se contents shows that subsurface precipitation of cu rich sulfides does not control significantly the 65cu values of the hydrothermal chimneys it appears that the major cause of copper isotope fractionation in hydrothermal systems up to 3 is the sea floor oxidation of primary copper sulfides enrichment in the heavy copper isotope can be explained by processes occurring at the sea floor such as hydrothermal reworking of previously altered sulfides by high temperature fluid massive sulfides characterized by negative 65cu values have undergone extensive recrystallization in these mineralogical assemblages isotopically heavy copper typical of altered sulfides has been leached and redeposited in the external zones or incorporated in hydrothermal fluids copper isotopes are therefore a promising tool for the study of supergene processes and the recycling of previously oxidized sulfides as well as a means of characterizing the degree of hydrothermal reworking of large sulfide deposits further studies of copper isotope fractionation under controlled laboratory experimental conditions are required to identify copper isotope fractionation during sulfide alteration in particular the possible biological mediation of copper isotope fractionation during sulfide oxidation may be an important direction for further studies

gold and native copper in supergene sulphides from the mid atlantic ridge in 1985 black smokers and massive sulphide deposits were discovered in the tag hydrothermal field in the rift valley of the mid atlantic ridge near latitude 26 08 n ref 1 sulphide samples from chimneys and mounds were recovered by dredging at depths between 3 620 and 3 670 m mineralogical and chemical analyses of the samples reveal 1 primary unaltered zn fe cu sulphides containing 0 8 4 0 p p m au 0 02 0 12 oz per ton and 2 secondary cu rich sulphides with native copper formed by supergene alteration of primary cu fe sulphides containing up to 16 4 p p m au 0 48 oz per ton as free native gold this is the first reported occurrence of secondary enrichment of gold and copper in recent submarine sulphides the high gold grades and native copper associated with secondary cu sulphides resemble occurrences in some supergene gossans overlying ancient massive sulphide deposits on land2

post jotnian basic intrusions in the fennoscandian shield and the break up of baltica from laurentia a palaeomagnetic and ams study abstract a palaeomagnetic and anisotropy of magnetic susceptibility ams study has been performed on dolerite sills of the central scandinavian dolerite group csdg in the fennoscandian shield the dolerites occur in four previously known complexes in central sweden and finland and from the results of this palaeomagnetic study another complex has been identified in northern sweden these complexes cover an area of at least 100 000 km2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites the measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca nw or se directed magma flows the dolerites of the csdg are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings in a palaeomagnetic reconstruction of baltica versus laurentia at ca 1 27 ga the two continents were joined with ne greenland attached to nw baltica ams data from a few dolerites and a basalt in ne greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in sweden this may suggest a common magma source located at the reconstructed contact between baltica and laurentia both the dolerites in greenland and those in sweden are of tholeitic composition indicating an intraplate origin which supports the interpretation of joined continents at that time the tensional regime that is reflected by the huge sill complexes is in our interpretation related to the break up of baltica from laurentia at ca 1 27 ga ago

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

comparison of dissolution under oxic acid drainage conditions for eight sedimentary and hydrothermal pyrite samples the abiotic oxidative dissolution behaviors of eight natural pyrite samples five sedimentary and three hydrothermal from various geological environments were compared under oxic conditions at ph 3 and 6 in a highly controlled batch reactor dissolution system the three sedimentary pyrite samples associated with coal had greater specific surface areas and also exhibited greater apparent dissolution rates and extent than the other two sedimentary and three hydrothermal samples under both ph conditions however after normalizing for surface area the dissolution rate constants for the different pyrite samples were similar the greatest difference was between the two non coal sedimentary pyrite samples pyrite morphology and the presence of trace metals could contribute to the differences in dissolution behavior as reflected in the normalized dissolution rates the sulfur iron ratio observed in the aqueous solution at ph 3 increased with time but was always less than 2 0 predicted from the stoichiometry of dissolution for all the pyrite samples during the 24 h experimental duration this can be explained by the disproportionation dissociation of thiosulfate an initial product of pyrite dissolution to elemental sulfur and sulfate which does not occur in a 1 1 ratio the results of this work indicate the importance of extracting and using the specific pyrite s relevant to particular mining areas in order to understand pyrite dissolution rates and the influence of environmental conditions on those rates

a diatom record of co 2 decline since the late miocene abstract extratropical sea surface temperature records from alkenones record a dramatic cooling of up to 17 c over the last 14 ma but the relationship between this cooling and greenhouse gas forcing has been elusive due to sparse and contrasting reconstructions of atmospheric co2 for the time period alkenone carbon isotopic fractionation during photosynthesis has previously been used to estimate changes in pco2 over this interval but is complicated by significant changes in cell size of the alkenone producing coccolithophorids over this time period in this study we reconstruct carbon isotopic fractionation during photosynthesis e p using organic compounds trapped within the frustules of pennate diatoms in sediments from the eastern equatorial pacific ocean at ocean drilling program site 846 over the last 13 ma physical separation of pennate diatoms prior to measuring carbon isotopic fractionation enables us to obtain a record with constant cell geometry eliminating this factor of uncertainty in our pco2 reconstruction in the past 11 ma e p declines from 15 5 to 10 3 using the classic diffusive model and taking into account variations in opal content alkenone concentration and coccolith sr ca as indicators of past productivity and growth rate and sea surface temperature records from the site we estimate a decline in pco2 from 454 41 to 250 15 ppmv between 11 and 6 ma models accounting for changing the significance of active carbon uptake for photosynthesis which likely produce more accurate co2 estimates suggest a significant larger pco2 decline of up to twice that shown by the classic diffusive model in average from 794 233 ppmv at 11 ma to 288 25 ppmv at 6 ma considering growth rates varying between 0 5 and 1 7 day 1 large uncertainties in the pco2 estimated between 8 and 11 ma using the active uptake model are related to the growth rate used for calculations together these results suggest co2 forcing for this period of steep decline in temperatures

hydrothermal exploration of mid ocean ridges where might the largest sulfide deposits be forming highlights black smokers on slow spreading ridges can host larger seafloor massive sulfide sms deposits than on fast ridges the largest slow ridge sms deposits are typically associated with long lived tectonic fractures sustained fluid flow ultramafic tectonic hosted sites exhibit high average cu 10wt and au 3ppm contents in their surface sms samples hydrothermal plume surveys are consistent with equal abundances of magmatic vs tectonic vent control along ultraslow ridges the first black smokers on any ultraslow ridges exhibit high cu au sms contents at both magmatic and tectonic vent sites abstract here we review the relationship between the distribution of modern day seafloor hydrothermal activity along the global mid ocean ridge crest and the nature of the mineral deposits being formed at those sites since the first discovery of seafloor venting a sustained body of exploration has now prospected for one form of hydrothermal activity in particular high temperature black smoker venting along 30 of the global mid ocean ridge crest while that still leaves most of that 60 000 km continuous network to be explored some important trends have already emerged first it is now known that submarine venting can occur along all mid ocean ridges regardless of spreading rate and in all ocean basins further to a first approximation the abundance of currently active venting as deduced from water column plume signals can be scaled linearly with seafloor spreading rate a simple proxy for magmatic heat flux what can also be recognized however is that there is an excess of high temperature venting along slow and ultra slow spreading ridges when compared to what was originally predicted from seafloor spreading magmatic heat budget models an examination of hydrothermal systems tracked to source on the slow spreading mid atlantic ridge reveals that no more than half of the sites responsible for the black smoker plume signals observed in the overlying water column are associated with magmatic systems comparable to those known from fast spreading ridges the other half of all currently known active high temperature submarine systems on the mid atlantic ridge are hosted under tectonic control these systems appear both to be longer lived than and to give rise to much larger sulfide deposits than their magmatic counterparts presumably as a result of sustained fluid flow a majority of these tectonic hosted systems also involve water rock interaction with ultramafic sources importantly from a mineral resource perspective this subset of tectonic hosted vent sites also represents the only actively forming seafloor massive sulfide deposits on mid ocean ridges that exhibit high concentrations of cu and au in their surface samples 10 wt average cu content and 3 ppm average au along ultraslow spreading ridges first detailed examinations of hydrothermally active sites suggest that sulfide deposit formation at those sites may depart even further from the spreading rate model than slow spreading ridges do hydrothermal plume distributions along ultraslow ridges follow the same 50 50 distribution of black smoker plume signals between magmatic and tectonic settings as the slow spreading mar however the first three black smoker sites tracked to source on any ultra slow ridges have all revealed high temperature vent sites that host large polymetallic sulfide deposits in both magmatic as well as tectonic settings further deposits in both types of setting have now been revealed to exhibit moderate to high concentrations of cu and au respectively an important implication is that ultra slow ridges may represent the strongest mineral resource potential for the global ridge crest despite being host to the lowest magmatic heat budget

sources of fe to the equatorial pacific ocean from the holocene to miocene abstract biological productivity in the modern equatorial pacific ocean a region with high nutrients and low chlorophyll is currently limited by the micronutrient fe in order to test whether fe was limiting in the past and to identify potential pathways of fe delivery that could drive fe fertilization i e dust delivery from eolian inputs vs fe supplied by the equatorial undercurrent we chemically isolated the terrigenous material from sediment along a cross equatorial transect in the central equatorial pacific at 140 w and at ocean drilling program site 850 in the eastern equatorial pacific we quantified the contribution from each potential fe bearing terrigenous source using a suite of chemical and isotopic discrimination strategies as well as multivariate statistical techniques we find that the distribution of the terrigenous sources i e asian loess south american ash papua new guinea and ocean island basalt varies through time latitude and climate regardless of which method is used to determine accumulation rate there also is no relationship between flux of any particular fe source and climate moreover there is no connection between a particular fe source or pathway eolian vs undercurrent to total productivity during the last glacial maximum pleistocene glacial episodes and the miocene biogenic bloom this would suggest an alternative process such as an interoceanic reorganization of nutrient inventories may be responsible for past changes in total export in the open ocean rather than simply fe supply from dust and or equatorial undercurrent processes additionally perhaps a change in fe source or flux is related to a change in a particular component of the total productivity e g the production of organic matter calcium carbonate or biogenic opal

hydrothermal activity in the lau back arc basin sulfides and water chemistry the submersible nautile completed 22 dives during the nautilau cruise r v nadir april 17 may 10 1989 for a detailed investigation of the southern lau basin near tonga the objective of the scientific team from france germany and tonga was to understand the process of sea floor ore formation associated with hydrothermal circulation along the valu fa back arc ridge behind the tonga kermadec trench the four diving areas between lat 21 25 s and 22 40 s in water 2000 m deep were selected on the basis of results from cruises of the r v jeancharcot and r v sonne the nadir cruise provided proof of hydrothermal activity in all four areas over more than 100 km as indicated by the widespread occurrence of hydrothermal deposits and by heat flow conductivity and temperature measurements near the sea bottom the most spectacular findings were high temperature white and black smokers and associated fauna and ore deposits hydrothermal water chemistry and sulfide composition data presented here indicate that this hydrothermal field is very different from the hydrothermal fields in oceanic ridges this difference is seen in the water chemistry of the hydrothermal fluid ph 2 and high metal content and the chemical composition of sulfides enrichment in ba as and pb

mineralogy and geochemistry of hydrothermal precipitates from kairei hydrothermal field central indian ridge the kairei hydrothermal field was the first confirmed active submarine hydrothermal system on the central indian ridge it has been suggested to be related to mafic as well as ultramafic host rocks based on vent fluid composition and the presence of ultramafic rocks in its vicinity in this study detailed geochemical and mineralogical analyses have been carried out on the hydrothermal precipitates from the kairei vent field in order to investigate the possible presence of indications for an ultramafic substrate at this vent site the studied samples included fragments of sulfide chimneys massive sulfides and talc bearing and silicified breccias three mineralization stages were identified 1 a high temperature stage consisting largely of chalcopyrite isocubanite and pyrite 2 a medium to low temperature stage characterized by the mineral assemblages of sphalerite and pyrite and 3 a weathering stage characterized by secondary cu sulfides bornite digenite covellite and idaite fe oxihydroxides opal a and cu chloride paratacamite and atacamite the sulfide geochemistry is characterized by high concentrations of cu and zn cu zn up to 29 3 wt n 17 and au mean 5 28 ppm n 17 which is comparable to results from seafloor massive sulfides collected from ultramafic hosted sites in the atlantic ocean but differs from those of typical mafic hosted deposits the high concentrations of cu and au at the kairei hydrothermal field could be an indication for the involvement of ultramafic rocks in the subseafloor ultramafic hosted au rich sulfide deposits may not be restricted to the atlantic ocean and may be common along all slow and intermediate spreading ridges

geology and venting characteristics of the mothra hydrothermal field endeavour segment juan de fuca ridge the 500 m long mothra hydrothermal field is the largest venting site in areal extent on the endeavour segment of the juan de fuca ridge within this field there are at least five actively venting sulfide complexes spaced 40 to 200 m apart along a trend of 020 the clusters are composed of multiple steep sided pinnacles that rise up to 20 m above the seafloor most of the sulfide structures are awash in diffusely venting fluids 30 200 c that support rich and diverse macrofaunal and microbial communities isolated black smoker chimneys vent 302 c fluids the linear arrays of sulfide complexes delineate a zone of active faulting near the western wall of the axial valley this fault network serves as the conduit for the rising fluids

glacial to interglacial fluctuations in productivity in the equatorial pacific as indicated by marine barite an empirical correlation between marine barite baso4 accumulation rate in core top sediment samples from two equatorial pacific transects at 140 w and 110 w and the estimated primary productivity of the overlying water column were used to evaluate glacial to interglacial changes in productivity fluctuations in barite accumulation rates down core indicate that during glacial periods of the past 450 000 years the productivity in the central and eastern equatorial pacific was about two times that during intervening interglacial periods this result is consistent with other evidence that productivity was high in the eastern and central equatorial pacific during the last glacial