the impact of oxic alteration on plume derived transition metals in ridge flank sediments from the east pacific rise we describe transition metal behaviour during early diagenesis in sediments collected along a transect across the eastern flank of the east pacific rise epr at 14 s overlying crust with magnetic ages of 0 36 ma to 4 6 ma the sediments are lithologically simple and consist of biogenic carbonate hydrothermal plume precipitates and minor basaltic debris sedimentation rates range from 1 cm ka in the near axis region 0 36 ma to 0 1 0 2 cm ka across the young ridge flank by 1 9 ma or within 142 km from axis redox indicators porewater mn ii aq and solid phase mo mn ratios indicate fully oxic conditions in near axis sediments changing to mildly suboxic conditions across the ridge flank sequential extraction analysis of sediments from the 0 36 ma site indicates that oxic alteration of fe oxyhydroxides is dominated by transformation of ferrihydrite to goethite implying that alteration occurs via dissolution followed by nucleation and precipitation the extent of alteration amorphous fe total plume derived fe ranges from 0 3 to 0 8 and the time to half conversion is on the order of 100 ka v associated with plume derived ferrihydrite is strongly discriminated against during alteration to goethite and the distribution coefficient dv v fe goethite v fe dissolved is in the range 0 45 0 59 cu zn and ni are delivered to the sediment in association with both fe oxyhydroxides and mn oxides where 70 of cu and zn and 10 of ni are associated with fe oxyhydroxides the fe adsorbed cu and zn are incorporated quantitatively into the lattice structure of goethite during alteration the fe associated ni most likely represents ni co precipitated as ni fe ferrihydrite and incorporated into goethite during alteration co is associated only with mn oxides the oxic alteration of hydrothermal fe oxyhydroxides therefore represents a relatively permanent sink where goethite dissolution is a prerequisite for metal release for a maximum of 70 of plume derived cu and zn 50 of v 10 of ni and 0 of co in oxic sediments the remaining plume derived metals are retained in association with amorphous fe oxyhydroxides and mn oxides although this fraction is highly susceptible to diagenetic remobilization under suboxic conditions

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

upper crustal structure of an active volcano from refraction reflection tomography montserrat lesser antilles to better understand the volcanic phenomena acting on montserrat the sea calipso seis mic experiment seismic experiment with airgun source caribbean andesitic lava island precision seismo geodetic observatory was conducted in 2007 december with the aim of imaging the upper crust and the magmatic system feeding the active soufri ere hills volcano the 3 d survey covered an area of about 50 40 km and involved the deployment of 247 land stations and ocean bottom seismometers obss a subset of the data recorded by four obss and four land stations on a southeast to northwest line has been analysed and traveltimes have been inverted to obtain a 2 d seismic velocity model through the island inverted phases include crustal and sediment p waves and wide angle reflections the resulting velocity model reveals the presence of a high velocity body 3 5 5 5 km s 1 beneath the island with highest velocities beneath the soufri ere and centre hills cor responding primarily to the cores of these volcanic edifices built of a pile of andesite lava domes and subsequent intrusions in the off shore region velocities in the surficial sediment layer vary from 1 5 to 3 0 km s 1 consistent with a mainly calcareous and volcaniclastic composition a wide angle reflector is observed at a depth of 1200 m below the seabed and appears to deepen beneath the island the upper crust beneath this reflector has velocities of 4 0 6 0 km s 1 and is infer red to cor respond to plutonic and hypabyssal rocks and sedimentary material of the old arc the high velocity region beneath the island extends into the crust to a depth of at least 5 km and is believed to be caused by an intrusive complex possibly of intermediate composition a low velocity zone as would be expected in the presence of an active magma chamber was not observed perhaps due to the limited resolution beneath 5 km depth our results so far provide the first wide angle seismic constraints on the upper crustal structure of the island to a depth of 10 km and will help understanding the processes that drive volcanism at montserrat and other island arc volcanoes

textures of the hellyer volcanic hosted massive sulfide deposit tasmania the aging of a sulfide sediment on the sea floor the relatively undeformed polymetallic massive sulfide ore of the hellyer deposit retains many textures developed during the early crystallization history common textures in the least modified ore away from the main feeder zone include pyrite framboids concentric and radial textured spheres dendritic and fibrous forms breccias mineral banding veins and reniform and crustification textures many are typical of early crystallization within a medium at high degrees of supersaturation some result from open space filling and others indicate disturbances of the sulfide mass possibly due to tectonic activity and volume adjustment the banding could be due to current winnowing most of these textures are found in modern midocean ridge and kuroko massive sulfide deposits but at hellyer the framboids are much more common hexagonal outlines in ore sphalerite indicate derivation of much of the sphalerite by inversion of wurtzite the high content of chalcopyrite in the wurtzite derived sphalerite appears in most cases to have developed by exsolution rather than replacement or coprecipitation although there is local evidence of both processes having operated possibly resulting from early asymmetrical crystal growth comparison of hellyer ore and typical kuroko ores reveals other differences than framboid abundance e g metal contents and ratios mineralogy and sulfur isotope compositions the most significant of which are the relatively high salinities of the hellyer ore fluids these together with evidence of a coeval basin imply sulfide deposition in a brine pool as previously proposed the development of a quenched sulfide mud at the bottom of a basin filled with spent ore fluid with little or no involvement of seawater explains the different mineral content e g lack of fe oxides scarcity of barite in the massive sulfide and the pyrite arsenopyrite assemblage the high metal content lack of chimney structures and differences in sulfur isotope compositions the sulfide mud dominated by fe s phases would be an ideal growth medium for framboidal pyrite the weight of experimental evidence concerning the development of framboidal pyrite points to involvement of metastable precursor fe s phases such as mackinawite and greigite in a weakly oxidized environment oxygen may have been added to the reduced acid brine pool as a result of inward diffusion of oxygen from overlying seawater which is shown to have been oxic by the presence of sessile benthic dendroids in the shales overlying the ore deposit however the level of oxidation was insufficient to form marcasite

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

crustal structure and evolution of the mariana intra oceanic island arc a new high resolution velocity model of the mariana arc backarc system obtained from active source seismic profiling demonstrates velocity variations within the arc middle and lower crusts of intermediate to felsic and mafic compositions the characteristics of the oceanic island arc crust are a middle crust with velocity of 6 km s laterally heterogeneous lower crust with velocities of 7 km s and unusually low mantle velocities petrologic modeling suggests that the volume of the lower crust composed of restites and olivine cumulates after the extraction of the middle crust should be significantly larger than is observed suggesting that a part of the lower crust especially the cumulates is seismically a part of the mantle

submarine pyroclastic deposits formed at the soufriere hills volcano montserrat 1995 2003 what happens when pyroclastic flows enter the ocean the soufriere hills volcano montserrat west indies has undergone a series of dome growth and collapse events since the eruption began in 1995 over 90 of the pyroclastic material produced has been deposited into the ocean sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea the coarse components pebbles to boulders are deposited proximally from dense basal slurries to form steep sided near linear ridges that intercalate to form a submarine fan the finer ash grade components are mixed into the overlying water column to form turbidity currents that flow over distances 30 km from the source the total volume of pyroclastic material off the east coast of montserrat exceeds 280 106 m3 with 65 deposited in proximal lobes and 35 deposited as distal turbidites

the role of magma mixing in triggering the current eruption at the soufriere hills volcano montserrat west indies the andesite lava currently erupting at the soufriere hills volcano montserrat contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite the andesite phenocrysts have a range of textures and zonation patterns which suggest that non uniform reheating of the magma occurred shortly before the current eruption reheating resulted in remobilisation of the resident magma and may have induced eruption

remobilization of andesite magma by intrusion of mafic magma at the soufriere hills volcano montserrat west indies the 1995 1999 eruption of the soufriere hills volcano montintroduction serrat has produced a crystal rich andesite containing quenchthe soufriere hills volcano montserrat located in the textured mafic inclusions which show evidence of having been molten lesser antilles island arc in the west indies began to when incorporated into the host magma individual crystals in the erupt on 18 july 1995 after about 350 years of dormancy andesite record diverse histories amphibole phenocrysts vary from young et al 1998b the eruption was preceded by a pristine and unaltered to strongly oxidized and pseudomorphed by period of seismic swarms which began in january 1992 anhydrous reaction products plagioclase phenocrysts are commonly previous earthquake crises have occurred on montserrat reverse zoned often with dusty sieve textures reverse zoned rims are in 1896 1897 1932 1935 and 1966 1967 powell 1938 also common on orthopyroxene phenocrysts pyroxene geothermometry shepherd et al 1971 initial phreatic and phregives an average temperature of 858 20 c for orthopyroxene atomagmatic activity was followed by extrusion of an phenocryst cores whereas reverse zoned rims record temperatures andesitic lava dome in november 1995 pyroclastic flows from about 880 to 1050 c the heterogeneity in mineral rim generated by gravitational collapse of the unstable dome compositions zoning patterns and textures is interpreted as reflecting have been the dominant form of activity throughout the non uniform reheating and remobilization of the resident magma eruption but periods of explosive activity producing body by intrusion of hotter mafic magma convective remobilization pumice flows and falls have also occurred robertson et results in mixing together of phenocrysts that have experienced al 1998 young et al 1998b dome growth ceased different thermal histories depending on proximity to the intruding suddenly in march 1998 and further activity up to may mafic magma the low temperature and high crystallinity are 1999 has mainly involved gravitational collapses of the interpreted as reflecting the presence of a cool highly crystalline dome and periods of ash venting with some small scale magma body beneath the soufriere hills volcano the petrological vulcanian explosive events observations in combination with data on seismicity extrusion rate the soufriere hills volcano is the only active centre and so2 fluxes indicate that the current eruption was triggered by on montserrat the volcano is a composite of at least recent influx of hot mafic magma five andesitic lava domes flanked by pyroclastic deposits rea 1974 identified four other volcanic centres all

three dimensional crustal structure of the mariana island arc from seismic tomography 1 a three dimensional 3 d seismic refraction survey was acquired over the mariana volcanic arc at 14 5 18 5n and 145 147e first arrival traveltimes from this survey and from a separate 2 d survey acquired approximately perpendicular to the arc have been simultaneously inverted for a 3 d p wave velocity model using seismic tomography subject to smoothness constraints the active arc which initiated only 3 4 ma ago has an average crustal thickness of 18 km approximately 40 km to the east the inactive remnant of the rifted eocene arc has an average crustal thickness of 21 km due primarily to a thicker lower crustal layer with velocities of 6 5 7 0 km s 1 crustal production clearly varies both temporally and spatially with some crustal layers including the igneous forearc crust varying in thickness by a factor of up to 2 along strike average p wave velocities within the upper crust of the modern arc are 240 360 m s 1 lower than in the eocene arc but are 280 m s 1 higher within the lower crust middle crust with velocities of 6 0 6 5 km s 1 is best developed beneath the eocene arc these results suggest an evolution of arc structure with increasing age we infer closure of fractures and porosity in the upper crust through hydrothermal circulation and a reduction in the mafic character of the middle to lower crust as a result of intracrustal differentiation although tonalitic rocks may predominate in the transition from upper to middle crust the bulk of the crust is essentially basaltic

recent remobilisation of shallow level intrusions on montserrat revealed by hydrogen isotope composition of amphiboles abstract ion probe measurements of hydrogen isotopes were made on amphiboles representing different stages of the ongoing eruption of the soufriere hills volcano montserrat the majority 80 of the andesitic amphiboles show relative intra and inter crystal d homogeneity with a mean of 38 12 consistent with known primary magmatic values the remainder 20 of amphiboles show marked d heterogeneity with a mean of 6 30 the heterogeneous amphiboles have 100 m rims with d values similar to the homogeneous crystals but core values which are significantly heavier than primary magmatic values early in the eruption 50 of amphiboles are isotopically heterogeneous but post spring 1996 all amphiboles are homogeneous we interpret these ion probe data in terms of development of a shallow andesitic magma chamber by repeated emplacement of andesitic magma in the shallow crust over at least the last 100 yr we suggest such shallow intrusions emplaced during the volcano seismic crises preceding the ongoing eruption solidified some regions interacted with hydrothermal fluids and isotopic exchange took place generating the heavy hydrogen isotope signatures in the heterogeneous amphiboles on onset of the ongoing eruption such old igneous material was remobilised with intimate mixing between the new magma batch and the old intrusions indicated by the close proximity of homogeneous and heterogeneous crystals the isotopically primary magmatic rims on the heterogeneous crystals indicate hydrogen isotope exchange over a period of a few weeks consistent with timescales of magma ascent at the soufriere hills volcano once the old igneous material was flushed out by spring 1996 the eruption proceeded by extruding material dominated by the new batch of magma this resulted in a marked increase in extrusion rate

hydrothermal scavenging at the mid atlantic ridge radionuclide distributions we have carried out a study of u series radionuclide distributions in the neutrally buoyant plume overlying the tag hydrothermal vent field at 26 n mid atlantic ridge limited uptake of u accompanies the formation of fe oxyhydroxide particulate material within the hydrothermal plume the particulate u concentrations are too small for measurable dissolved u anomalies to be produced within the water column at plume height however such uptake followed by deposition in the underlying sediments could account for 10 of the u enrichment that has been reported previously for metalliferous ridge crest sediments particulate 234th 230th and 228th activities in the tag hydrothermal plume represent 65 of total th activities typical of the deep ocean low dissolved th activities coincide with the high particulate th activities indicating that hydrothermal scavenging strongly affects th isotope distributions in the water column close to hydrothermal vents much of the scavenging occurs during the initial formation of fe rich hydrothermal precipitates but some further scavenging continues as the material is dispersed away from its source distributions of particulate 210pb and 231pa activities in the neutrally buoyant plume are also extremely high 7 12 dpm 102 l and 30 160 dpm 106 l respectively like th 210pb and231pa show evidence for scavenging within the hydrothermal plume total th activities at plume height are 50 higher than background this focusing of th into the plume is consistent with the entrainment and re emplacement at plume height of particulate material recycled from beneath the neutrally buoyant plume

deposition of cohesive sediment from turbulent plumes gravity currents and turbidity currents models for the deposition of cohesive sediment from turbulent plumes or buoyant jets gravity currents and turbidity currents are provided in this paper the cohesive sediment is made up of small particles that aggregate together to form larger flocs which are in turn broken up by turbulent shear the equilibrium mean floc size and thus the equilibrium mean fall speed is a function of the turbulent dissipation rate and the sediment concentration the flows are modeled by using integral and box models with dissipation related to bulk flow properties for plumes it is shown that there is a well defined equilibrium fall speed at the virtual origin and that the fall speed changes relatively slowly in the momentum dominated part of the flow within one jet length or so of the source if the flocs are assumed to adjust instantaneously to their equilibrium size an integral model for a turbulent plume carrying cohesive sediment can be described in terms of two parameters the angle between the plume an

seismological evidence for variable growth of crust along the izu intraoceanic arc 1 the processes that create continental crust in an intraoceanic arc setting are a matter of debate to address this issue we conducted an active source wide angle seismic study to examine along arc structural variations of the izu intraoceanic arc the data used were acquired over a 550 km long profile along the volcanic front from sagami bay to tori shima the obtained structural model showed the existence of felsic to intermediate composition middle crust with a p wave velocity vp of 6 0 6 5 km s 1 in its upper part and 6 5 6 8 km s 1 in its lower part the thickness of the middle crust varied markedly from 3 to 13 km the underlying lower crust also consisted of two layers vp of 6 8 7 2 km s 1 in the upper part and vp of 7 2 7 6 km s 1 in the lower part the upper of these layers was interpreted to consist of plutonic gabbro and the lower layer was interpreted to be mafic to ultramafic cumulates average crustal velocities calculated from our model showed remarkable lateral variation which correlated well with arc volcanism low average crustal seismic velocities 6 7 km s 1 due to thick middle crust were obtained beneath basaltic volcanoes e g o shima miyake jima hachijo jima aoga shima while higher average velocities 7 1 km s 1 were obtained beneath rhyolitic volcanoes e g nii jima kurose south hachijo caldera myoji knoll and south sumisu caldera we concluded from these observations that continental crust grows predominantly beneath the basaltic volcanoes of the izu arc and that rhyolitic volcanism may be indicative of a more juvenile stage of crustal evolution or remelting of preexisting continental crust or both

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

submarine deposition of volcaniclastic material from the 1995 2005 eruptions of soufriere hills volcano montserrat abstract soufriere hills volcano montserrat has been erupting since 1995 during the current eruption a large part of the material produced by the volcano has been transported into the sea modifying the morphology of the submarine flanks of the volcano we present a unique set of swath bathymetric data collected offshore from montserrat in 1999 2002 and 2005 from 1999 to 2002 pyroclastic flows associated with numerous dome collapses entered the sea to produce 100 mm 3 deposit from 2002 to 2005 the 290 mm 3 submarine deposit is mainly from the 12 13 july 2003 collapse these data allow us to estimate that by may 2005 at least 482 mm 3 of material had been deposited on the sea floor since 1995 we compare on land characteristics and volumes of dome collapse events with the submarine deposits and propose a new analysis of their emplacement on the submarine flanks of the volcano the deposition mechanism shows a slope dependence with the maximum thickness of deposit before the break in the slope probably because of the type of the dense granular flow involved we conclude that from 1995 to 2005 more than 75 of the erupted volume entered the sea

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 of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

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

constrained circulation at endeavour ridge facilitates colonization by vent larvae understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid ocean ridge system remains a major challenge in oceanic research1 2 among the factors considered important in the recruitment of deep sea larvae are metabolic lifespan the connectivity of the seafloor topography and the characteristics of the currents3 here we use current velocity measurements from endeavour ridge to examine the role of topographically constrained circulation on larval transport along ridge we show that the dominant tidal and wind generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest and that hydrothermal plumes rising from vent fields in the valley drive a steady near bottom inflow within the valley extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid ocean ridges shields larvae from cross axis dispersal into the inhospitable deep ocean this effect augmented by plume driven circulation within rift valleys having active hydrothermal venting helps retain larvae near their source larvae are then exported preferentially down ridge during regional flow events that intermittently over ride the currents within the valley

late quaternary sedimentation in the lesser antilles island arc investigation of patterns and processes of late quaternary sedimentation in the lesser antilles island arc indicates that the major components of deep water sediment are volcaniclastic silt and clay redeposited shallow water carbonate mainly aragonite and high mg calcite pelagic carbonate mainly low mg calcite and windblown dust from africa although sediment derived from the adjacent south american continent is included in accreted trench deposits marine deposition of this sediment within the arc is insignificant except at the south end of the forearc distribution of sedimentary components within the lesser antilles arc is controlled primarily by arc volcanism a miocene shift in the locus of volcanism which caused the arc to bifurcate created two sedimentologic regimes in the southern part of the arc which is characterized by volcanically active islands with high relief deep water sediment is dominantly volcaniclastic in contrast in the northern part of the arc where a broad arc platform separates high relief active volcanoes from low relief extinct volcanoes surrounded by wide carbonate shelves deep water sediment is composed mainly of redeposited shallow water carbonate pelagic carbonate and eolian transported saharan dust superposed on the facies patterns determined by arc volcanism is a subtle but distinct record of quaternary climate change glacial interglacial periods are recorded in deep water sediment by variations in sedimentation rate composition and texture volcaniclastic sedimentation responded to lowstands of sea level by increased sedimentation rates and thicker and or more frequent turbidites reflecting intensified currents in interisland passages and increased erosion of exposed insular shelves deep water carbonate sedimentation records decreased carbonate productivity on exposed shelves and in the open ocean during glacial lowstands as well as variations in carbonate preservation associated with changes in ocean circulation deposition of saharan dust increased during glacial periods because of intensified wind strengths

the structure mass and interactions of the hydrothermal plumes at 26 n on the mid atlantic ridge abstract water column surveys by combined nephelometer ctd nctd tows contributed to the 1985 discovery of the first black smokers on the mid atlantic ridge subsequent regional water column mapping has helped define the extent mass and interactions of the suspended particulate matter phase spm of the hydrothermal plumes emanating from the known and other nearby sources the results of 29 nctd cast tows covering 25 30 km 2 of ridge segment indicate the presence of as many as two additional sources based on spm concentration gradients and plume top doming over source areas plume doming documented here for the first time from field observations conforms strikingly with laboratory experiments and can serve as a marker for source field location a comparison of the plumes spm with potential temperature and salinity distributions indicates close correlation in water column anomaly patterns for each confirming modification of the regional potential temperature and salinity structure by hydrothermal plumes which is expressed by wide separation and sloping of isotherms and isohalines

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

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

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

pyroclastic stratigraphy of the soufriere hills volcano montserrat implications for the present eruption the stratigraphy of the soufriere hills volcano is dominated by dense andesite block and ash flow deposits forty one radiocarbon ages permit a reconstruction of the eruptive history over the past 31 000 years a single stratigraphic member yielding radiocarbon ages of 22 000 to 20 000 years b p is composed of block and ash flow deposits with semi vesiculated clasts concentrated in the upper third of each deposit the present eruption is also producing block and ash flow deposits with both dense and semi vesiculated clasts it marks a return to a more explosive driven type of activity as occurred 22 000 to 20 000 years ago deposits of this member have a broad sheet like form and volumes in excess of any produced so far by the present eruption

implications of magma transfer between multiple reservoirs on eruption cycling volcanic eruptions are episodic despite being supplied by melt at a nearly constant rate we used histories of magma efflux and surface deformation to geodetically image magma transfer within the deep crustal plumbing of the soufriere hills volcano on montserrat west indies for three cycles of effusion followed by discrete pauses supply of the system from the deep crust and mantle was continuous during periods of reinitiated high surface efflux magma rose quickly and synchronously from a deflating mid crustal reservoir at about 12 kilometers augmented from depth during repose the lower reservoir refilled from the deep supply with only minor discharge transiting the upper chamber to surface these observations are consistent with a model involving the continuous supply of magma from the deep crust and mantle into a voluminous and compliant mid crustal reservoir episodically valved below a shallow reservoir at about 6 kilometers

particle dynamics in the rising plume at piccard hydrothermal field mid cayman rise processes active in rising hydrothermal plumes such as precipitation particle aggregation and biological growth affect particle size distributions and can exert important influences on the biogeochemical impact of submarine venting of iron to the oceans and their sediments however observations to date of particle size distribution within these systems are both limited and conflicting in a novel buoyant hydrothermal plume study at the recently discovered high temperature 398 c piccard hydrothermal field mid cayman rise we report optical measurements of particle size distributions psds we describe the plume psd in terms of a simple power law model commonly used in studies of upper and coastal ocean particle dynamics observed psd slopes derived from spectral beam attenuation and laser diffraction measurements are among the highest found to date anywhere in the ocean and ranged from 2 9 to 8 5 beam attenuation at 650 nm ranged from near zero to a rarely observed maximum of 192 m 1 at 3 5 m above the vent we did not find large 100 m particles that would settle rapidly to the sediments instead beam attenuation was well correlated to total iron suggesting the first order importance of particle dilution rather than precipitation or dissolution in the rising plume at piccard our observations at piccard caution against the assumption of rapid deposition of hydrothermal particulate metal fluxes and illustrate the need for more particle size and composition measurements across a broader range of sites globally

chemistry of hydrothermal solutions from the southern juan de fuca ridge fluids from three vent fields on the southern juan de fuca ridge were sampled in september 1984 using the dsrv alvin the fluids are uniquely high in both chloride which ranges up to twice the seawater value and in metal content simple vapor liquid phase separation could not have produced both the high chlorinity and gas concentrations observed in these fluids the cause of the elevated chlorinity can not be uniquely identified but may be the result of either or a combination of two processes 1 subsurface formation of a degassed brine and subsequent mixing of a small amount of this brine with a hydrothermal seawater which has not undergone a phase separation and 2 dissolution of a chloride rich phase combined with a possible small loss of gas during sampling procedures although measured temperatures were all less than 300 sup 0 c quartz geothermometry suggests that the fluids have equilibrated at greater than 340 sup 0 c quartz geobarometry is also in agreement with geophysical estimates of depth to the local magma chamber copyright american geophysical union 1987

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

sedimentation from buoyant jets an integral model is developed to describe sedimentation from a turbulent buoyant jet injected at an angle into a stationary uniform ambient fluid entrainment is modeled using the standard entrainment assumption and sediment is assumed to fall from the jet where the outward component of the fall velocity normal to the jet boundary exceeds the inward entrainment velocity when appropriately scaled by source momentum and buoyancy fluxes turbulent buoyant jets may be described in terms of a single parameter the angle 0 between the flow and the horizontal at the virtual origin which is close to the actual source for large initial densimetric froude numbers including sedimentation in the model introduces a further parameter ws which is the ratio of the fall speed of the sedimenting particles to a typical entrainment velocity and so ws is a nondimensional fall speed an important result is that this ratio is independent of the source speed so that the proportion of the sediment load deposited nea

chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume the aging of the hydrothermal plume over the endeavour segment of the juan de fuca ridge was estimated by measuring the222rn3he ratio in the plume as it dispersed despite uncertainties in the source function of hydrothermal input it wa determined that the relative sequence of removal from the plume ish2 c 222rn ch4 mn where c is a measure of particle concentration and the mean life of222rn is 5 5 days h2 is removed from the plume within hours of input while mn is not removed within the two week timescale of the radon helium clock entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone this is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient like profile in the deep ocean thus we found that significant fractions of the si and226ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height

dispersal patterns for hydrothermal plumes in the south pacific using manganese as a tracer abstract manganese profiles over the spreading axis of the east pacific rise epr between 22 15 s and 19 25 s disclose a series of plumes which are formed by buoyant emanations of hot hydrothermal vent fluids the plume ceilings or anomaly maxima can occur several hundred meters above the sea floor these plumes are dispersed laterally and are detectable as mid depth manganese anomalies 2000 km from the spreading axis along this section the plumes are dispersed mainly westward but at 30 s the plumes are advected eastward the asymmetrical dispersal pattern inferred from these data is coincident with the asymmetrical distribution of metalliferous sediments about the epr which suggests that plumes generated by high temperature venting play a dominant role in the formation of these sediments

surface sediment geochemistry and hydrothermal activity indicators in the dragon horn area on the southwest indian ridge abstract the fluids found in sulfide producing hydrothermal vents are rich in ore forming elements these elements usually precipitate as solid particles that are dispersed by the plumes and deposited as sediments around the hydrothermal field to assess how the geochemical features of such sediments may be used in seafloor sulfide exploration this study analyzed the surface sediment geochemistry in the dragon horn area on the southwest indian ridge the results indicate that the sediments are mainly composed of pelagic material basalt and ultramafic debris mn fe oxyhydroxides and mn oxides hydrothermal components the debris content in this area is lower than that in sediments found at fast and medium spreading mid ocean ridges with relatively high amounts of ultramafic components indicating that ultramafic rock outcrops are present in the study area precipitated hydrothermal elements in the sediments exhibit zonation in the spatial distributions of zn cu fe and mn the sediments proximal to the hydrothermal field display high cu zn and fe contents and low mn contents hydrothermal cu and zn phases occurred within a limited range of the hydrothermal field and zn was found to precipitate earlier than cu during dispersal from the plume hydrothermal fe distributed similarly with cu spatially but with a larger dispersion distance hydrothermal mn was able to migrate beyond the ridge valley to precipitate in the ridge flank sediments two hydrothermal cu and zn anomalies were identified on the south ridge flanks which suggest the presence of undiscovered hydrothermal field the sediments in the study area are additionally characterized by relatively low mn contents and high fe mn molar ratios the relatively reducing environment of the plume may facilitate the migration of hydrothermal mn over large distances which probably accounts for the lower mn contents in the sediments the cu cu fe molar ratio model was proposed to identify mafic and ultramafic related hydrothermal field the low cu fe molar ratios and ree characteristics of the sediments suggest that ultramafic rocks are likely not the only source of the ore forming element for hydrothermal mineralization low temperature diffuse flow on the ridge flanks of the study area probably results from serpentinization of ultramafic rocks

thallium isotope variations in seawater and hydrogenetic diagenetic and hydrothermal ferromanganese deposits results are presented for the first in depth investigation of tl isotope variations in marine materials the tl isotopic measurements were conducted by multiple collector inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts diagenetic fe mn nodules hydrothermal manganese deposits and seawater samples the natural variability of tl isotope compositions in these samples exceeds the analytical reproducibility 0 05 by more than a factor of 40 hydrogenetic fe mn crusts have 205tl of 10 to 14 whereas seawater is characterized by values as low as 8 205tl represents the deviation of the 205tl 203tl ratio of a sample from the nist srm 997 tl isotope standard in parts per 104 this 2 difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of tl onto ferromanganese particles an equilibrium fractionation factor of 1 0021 is calculated for this process ferromanganese nodules and hydrothermal manganese deposits have variable tl isotope compositions that range between the values obtained for seawater and hydrogenetic fe mn crusts the variability in 205tl in diagenetic nodules appears to be caused by the adsorption of tl from pore fluids which act as a closed system reservoir with a tl isotope composition that is inferred to be similar to seawater nodules with 205tl values similar to seawater are found if the scavenging of tl is nearly quantitative hydrothermal manganese deposits display a positive correlation between 205tl and mn fe this trend is thought to be due to the derivation of tl from distinct hydrothermal sources deposits with low mn fe ratios and low 205tl are produced by the adsorption of tl from fluids that are sampled close to hydrothermal sources such fluids have low mn fe ratios and relatively high temperatures such that only minor isotope fractionation occurs during adsorption hydrothermal manganese deposits with high mn fe and high 205tl are generated by scavenging of tl from colder more distal hydrothermal fluids under such conditions adsorption is associated with significant isotope fractionation and this produces deposits with higher 205tl values coupled with high mn fe

history and geochemistry of a metalliferous sediment core from the mid atlantic ridge at 26 n fourteen thousand years of hydrothermal deposition are recorded in a metalliferous sediment core recovered from the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge at 26 n an estimated 26 to 96 of the sediment at this core site is hydrothermally derived as determined from the caco3 al and fe data layers of essentially pure vent precipitates contain 3 caco3 and 0 5 al with high concentrations of fe 43 cu 4 1 zn 1 2 mn 1 1 v 480 ppm pb 175 ppm cd 32 ppm and hg 3 8 ppm sediment accumulation rates vary from 1 to 30 g cm2 1000 y throughout the core a function of the intensity of hydrothermal inputs distinct hydrothermal events are recorded at 6000 and 8500 y b p in layers containing 90 vent derived material vertical metal profiles and interelement relationships in the core result from variable deposition of oxides and sulfides oxidation and dissolution of sulfide phases and scavenging of metals from seawater

a model for the formation of hydrothermal manganese crusts from the pitcairn island hotspot the distribution of elements in the hydrothermal mn crusts from the pitcairn island hotspot can be explained by considering processes occurring within the hotspot volcano these include the role of boiling of the ascending hydrothermal fluid which leads to sulphide deposition and seawater intrusion into the upper part of the volcano this in turn results in the development of an oxycline above which feooh can precipitate the high mn fe ratios and low contents of ni and zn in the massive crusts are thought to reflect the rapid incorporation of the ore elements into sulphide minerals such that only a small proportion of these elements are available for incorporation in the crusts the small positive eu anomaly in the crusts on a nasc normalized basis reflects the lower temperatures in the hydrothermal fluids at the pitcairn island hotspot 25dgc compared to those at mid ocean ridges c 350 c the oxyanions cro42 vo43 aso43 and po43 the rare earth elements ree and th are scavenged by feooh particles within the upper part of the seamount and can therefore be incorporated into the crusts associated with the fe rich phase molybdenum and uranium are scavenged in only negligible amounts by the feooh molybdenum is therefore most probably scavenged from seawater by manganese oxides during the actual formation of the mn crusts the alkali metals li rb and cs remain in solution in the hydrothermal fluids and are selectively incorporated into the buserite structure of the mn crusts in the absence of strong competition from the divalent transition metal ions the compositional characteristics of the hydrothermal mn crusts can therefore be explained in terms of a simple model of element scavenging and uptake occurring mainly within the seamount a laser ablation icpms profile in one of the hydrothermal crusts ds 63 4 revealed varying ree concentrations and patterns in the different layers of the crust these data indicate that the hydrothermal component was variable during the formation of the crust it was calculated to be almost 100 in the upper 18 0 mm of this crust apart from the samples at 6 0 8 0 mm and about 80 in the lower layers

near field variability of the tag non buoyant plume 26 n mid atlantic ridge we report the initial results of a nine hour monitoring of the non buoyant plume above the tag hydrothermal vent field at 26 n on the mid atlantic ridge a ctd conductivity temperature and pressure and nephelometer particle sensor were continuously raised and lowered through the non buoyant plume for nine hours at a site situated 100 m south of the hydrothermal mound this method permits a detailed record of the variability of the non buoyant plume close to the vent field to be made our results show that the particulate maximum of the non buoyant plume appears 150 m lower in the water column at the start and end of the record compared with profiles recorded towards the middle of the record a corresponding structure also appears in the temperature and salinity records with the result that the plume particle maximum resides on the 27 8035 isopycnal throughout modelling of the rise height of an axisymmetric plume can only account for 95 m of the variability in the final level of neutral buoyancy plume bending due to currents is a likely explanation for the recorded temporal variability

the geochemistry of atlantic hydrothermal particles particles were collected from the dilute portion of neutrally buoyant hydrothermal plumes from four mid atlantic ridge sites mark 23 n tag 26 n broken spur 29 n lucky strike 37 n comparison of data from proximal portions of the tag atlantic german et al 1991 this study and north cleft pacific feely et al 1994 plumes show that oxyanion e g v scavenging is more efficient at tag possibly due to a higher proportion of fe removed as sulfides at north cleft and or the more vigorous mixing in the high energy tag buoyant plume chalcophile elements e g cu show two stage removal they are precipitated as sulfides during initial mixing of vent fluids with seawater and are sedimented from the buoyant plume in the dilute plume they are scavenged from seawater by fe oxyhydroxides the ree show continued scavenging in the neutrally buoyant plume and lower levels in 1993 samples compared to 1988 samples german et al 1990 suggesting that the amount of scavenging is related to particle recycling

natural and anthropogenic radionuclide distributions in the northwest atlantic ocean we have used in situ pumps which filter large volumes of sea water through a 1 m cartridge prefilter and two mno2 coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans two sites in the northwest atlantic show subsurface maxima of the fallout radionuclides137cs 239 240pu and241am although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions comparison of predicted and measured water column inventories suggests that at least 35 50 of the pu and241am are supplied to the deep water by advection the depth distributions of the naturally occurring radionuclides232th 228th and230th reflect their sources to the oceans 232th shows high dissolved concentrations in surface waters presumably as a result of atmospheric or riverine supply activities of232th decrease with depth to values 0 01 dpm 1000 l 228th shows high activities in near surface and near bottom water due to the distribution of its parent 228ra dissolved230th produced throughout the water column from234u decay increases with depth to 3000 m values in the deep water 3000 m are nearly constant 0 6 0 7 dpm 1000 l and the distribution of this tracer and perhaps other long lived particle reactive tracers as well may be affected by the advection inferred from pu and241am data the ratio of particulate to dissolved activity for both230th and228th is 0 15 0 20 this similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium moreover in mid depths228th tends to have a higher particulate dissolved ratio than230th suggesting uptake and release of230th and228th by different processes this could occur if228th produced in surface water were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking 230th produced throughout the water column may more closely approach a sorption equilibrium at all depths 230th 241am and239 240pu are partitioned onto particles in the sequence th am pu with 15 of the230th on particles compared with 7 for am and 1 for pu distribution coefficients kd are 1 3 1 6 107 for th 5 6 106 for am and 7 10 105 for pu the lower reactivity for pu is consistent with analyses of pu oxidation states which show 85 oxidized v vi pu however thekd value for pu may be an upper limit because pu like228th may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase

sub surface dynamics and eruptive styles of maars in the colli albani volcanic district central italy protezione civile italy 2004 2006 agreement with istituto nazionale di geofisica e vulcanologia ingv project v3 1

sedimentation from gravity currents generated by turbulent plumes sedimentation from radially spreading gravity currents generated at the top of ascending sediment laden plumes is described by a model which assumes that sediment is dispersed homogeneously by turbulence in the gravity current resulting in an exponential decrease in the concentration of sediment with time as particles settle out of the lower boundary of the current for radial spreading this model predicts a gaussian distribution of sediment accumulation away from the source with an exponential constant b which depends on flow rate q and particle settling velocity v b nv q in the experiments described sedimentation occurs from gravity currents generated by ascent of buoyant particle laden plumes of fresh water in a tank of salty water the sediment accumulation shows close agreement with the theoretical model and the gaussian decay constant b can be determined from a maximum in the accumulated mass of sediment per unit distance and from the slope of the line in s s0 br2 where r is the radial distance s is the sediment mass flux per unit area and s0 is the value of s at r 0 data from the dispersal of volcanic ejecta from a large c 24 km high plinian eruption column in the azores also show good agreement with the theory confirming that it is general and independent of scale and the nature of the fluid the experimental data also show a change in sedimentation behaviour at distances from the source corresponding to the corner of the plume where it diverts into a lateral gravity current and there is an abrupt decrease in vertical velocity sedimentation of coarse grain sizes between the source and the corner occurs from the inclined plume margins and does not behave as predicted by the theoretical model

brine pool deposition for the zn pb cu massive sulphide deposits of the bathurst mining camp new brunswick canada ii ocean anoxia during mineralisation abstract current models of massive sulphide ore genesis in the bathurst mining camp new brunswick involve settling of sulphide particles from a stagnating low salinity hydrothermal plume spreading laterally in an anoxic ocean layer with minimal sulphate content there is fragmentary evidence of ocean anoxia in the form of local fine lamination in the shales that host some of the deposits but the total organic carbon s fe u th ni co v ni and v cr relationships indicate deposition under oxic or dysoxic conditions vanadium and mn values range from oxic to anoxic and sulphate reducing to non sulphate reducing but mn may be anomalously low due to derivation by erosion of acidic volcanic rocks the somewhat equivocal physical and chemical data combined with the likely disturbing effects of penecontemporaneous volcanism considerably weaken the case for an anoxic bottom layer in a static ocean the presence of barite with ambient seawater 34 s values in brunswick no 12 ore and the abundance of sulphate in modern euxinic basin waters make a sulphate free layer unlikely even if anoxic sulphate bearing low salinity fluids mixing with seawater would lead to growth of barite bearing chimneys and baritic rubble mounds which are not observed a model involving brine pool deposition better explains the major features of the bathurst ores

hydrothermal scavenging on the juan de fuca ridge 23othxs 10be and rees in ridge flank sediments abstract we have investigated the geochemistry of a hydrothermally enriched sediment core recovered from the western flank of the n cleft segment juan de fuca ridge 8km west of the megaplume area previously identified near 45 n the core contains varying biogenic lithogenic and hydrothermal components as reflected in caco3 al and fe contents respectively horizons of pronounced hydrothermal input in core top sediments and at depth exhibit increased concentrations of fe cu zn and pb and shifts in pb isotopic compositions toward nonradiogenic morb hydrothermal values ree concentrations co vary with hydrothermal fe down core and shale normalised ree distribution patterns exhibit both negative ce anomalies and positive eu anomalies indicative of input from plume particle fall out unsupported 230thxs activities down core are consistent with continuous slow sediment accumulation rates of 0 54 cm ky for 200 ky since the deposition of the deeper fe rich horizon 10be 0 and 9be isotope concentrations also co vary with hydrothermal fe down core and exhibit 10 be o 9 be ratios which approach that of pacific ocean deep water indicative of a seawater scavenging source 10 be 0 230 oth xs 0 ratios throughout most of core gc88 6 are greater than mean pacific ocean values indicating that hydrothermal scavenging can lead to significant net removal of dissolved 10be into ridge flank sediments

late pleistocene tephrochronology of marine sediments adjacent to montserrat lesser antilles volcanic arc the recent history of the soufriere hills volcano montserrat lesser antilles volcanic arc is deduced using data obtained from a submarine core collected in 2002 the core contains concentrations of ash and several tephra layers which are identified by the abundance of glass shards dense and poorly vesiculated particles and scoria the tephra layers have been dated using micropalaeontology and stable isotope stratigraphy tephra layers in a marine sediment core off the coast of montserrat record the volcanic history of south soufriere hills soufriere hills volcano back to 250 ka eight layers are composed of dense juvenile ash related to dome eruptions five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions a marker sequence of basalt tephra layers is dated at 124 147 ka and is correlated with construction of the south soufriere hills basaltic stratocone pelagic sediments between the main tephra layers have low abundances of volcanogenic components c 10 4 years of dormancy or low activity

lithospheric structure of an active backarc basin the taupo volcanic zone new zealand seismic data from both explosive and earthquake sources have been used to model the crustal and upper mantle velocity structure beneath the taupo volcanic zone tvz an active backarc basin in central north island new zealand volcanic sediments with p wave velocities of 2 0 3 5 km s 1 reach a maximum thickness of 3 km beneath the central tvz underlying these sediments to 16 km depth is material with velocities of 5 0 6 5 km s 1 interpreted as quartzo feldspathic crust east and west of the tvz crust with similar velocities is found to depths of 30 and 25 km respectively beneath the tvz material with p wave velocities of 6 9 7 3 km s 1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust the base of the crust at 30 km depth under the tvz is marked by a strong seismic reflector interpreted as the moho modelling of arrivals from deep 40 km earthquakes near the top of the underlying subducting pacific plate reveals a region with low mantle velocities of 7 4 7 8 km s 1 beneath the crust of the tvz this region of low mantle velocities is best explained by the presence of partially hydrated upper mantle resulting from dehydration of hydrous minerals e g serpentinite carried down by the underlying subducting plate within the lower crust beneath the tvz a region of high 0 34 poisson s ratio is observed indicating the presence of at least 1 per cent partial melt this melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust where it provides a source for the voluminous rhyolitic magmas of the tvz

manganese geochemistry near high temperature vents in the mid atlantic ridge rift valley in july 1985 the first high temperature hydrothermal vents ever discovered on the mar were found at 26 n tag a transponder navigated survey of a 4 8 km area was begun when on board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the epr these plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures using relay transponder navigation with a rosette ctd system it was possible to sample bottom waters with temperature anomalies of several decidegrees samples collected near vent orifices possessed silica anomalies of up to 17 m and dissolved manganese anomalies as large as 360 nmol kg these manganese and silica anomalies are linearly related and predict amn si ratio of 20 mmol mol for these mar vent fluids a value essentially identical to that measured in 350 c fluid from the epr at 21 n the distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site a rough estimate of the convective heat loss from this section of the mar based on z profiles is 1 2 108 cal s which is similar to the heat loss estimated for the vent field on the epr at 21 n

real time observation of dispersed hydrothermal plumes using nephelometry examples from the mid atlantic ridge abstract as part of the 1984 1985 noaa vents program on the mid atlantic ridge nephelometry was used to provide real time detection and tracking of dispersed hydrothermal plumes at all nine 1984 study sites hydrothermal activity was detected by in situ real time nephelometer measurements and later confirmed by dissolved mn and particulate fe measurements these same techniques were employed in a site specific survey of the trans atlantic geotraverse tag area in 1985 where large water column anomalies in turbidity and in dissolved mn helped lead to the discovery of high temperature black smokers the optical response of the nephelometer was to hydrothermally derived particulate matter thus strong correlations existed between the nephelometer readings and total suspended matter r 0 98 n 34 and particulate fe r 0 88 n 32 in addition digital nephelometer data correlated well with dissolved mn r 0 88 n 78 throughout a large concentration range 0 2 31 0 nmol kg these data provide good evidence for the utility of in situ nephelometer measurements for locating and surveying plumes from hydrothermal vents it also appears possible within limits to predict concentrations of in situ total suspended matter of particulate fe and of dissolved mn

petrologic evidence for pre eruptive pressure temperature conditions and recent reheating of andesitic magma erupting at the soufriere hills volcano montserrat w i the recent eruption of the soufriere hills volcano in montserrat july 1995 to present september 1997 has produced an andesitic dome sio 2 59 61 wt the eruption has been caused by invasion of mafic magma into a preexisting andesitic magma storage region p 130 mpa 5 km depth the composition of the andesite has remained essentially constant throughout the eruption but heating by the mafic magma increased the andesite temperature from 830 c to 880 c prior to being heated the stable mineral assemblage in the andesite was plagioclase amphibole orthopyroxene titanomagnetite ilmenite quartz the rise in temperature from 830 c to 880 c fo 2 1 log unit above nno has caused quartz to become unstable and has also caused changes in silicate and fe ti oxide mineral compositions the andesitic magma is likely saturated with an h 2 o rich vapor phase in the upper part of the magma storage region melt h 2 o content is 4 7 wt

theory applied to the mid atlantic ridge hydrothermal plumes the finite difference approach a system of differential equations describing the entrainment of ambient seawater during the rise of a buoyant hydrothermal plume has been solved by an explicit finite difference method chosen for its ease of application an equation for tracer concentration has been added so that concentrations in the neutrally buoyant plume can be calculated for those elements whose background profiles vary over the depth range of the buoyant plume the height of plume rise can be related directly to the thermal and hydrothermal vent flux for the tag vent field 26 n mar five plume layers have been identified the calculated heat flux is 500 940 mw and the fluid flux is 0 210 0 392 m3 s 1 the fluid flux to the neutrally buoyant plume is 1460 2740 m3 s 1 calculated plume concentrations of mn compared with measurements made at tag in 1988 indicate that there is either 50 removal of manganese in the buoyant plume or that low temperature vent fluids contribute about 50 of the total thermal output of the tag vent field