two pulses of mineralization and genesis of the zhaxikang sb pb zn ag deposit in southern tibet constraints from fe zn isotopes abstract zhaxikang is one large sb pb zn ag deposit located in the north himalaya of southern tibet to date the genesis of this deposit still remains controversial here we present new pyrite fe and sphalerite zn isotopic data for the first three stages of mineralization fe zn isotopic data for mn fe carbonate that formed during the first two stages of mineralization and zn isotopic data for the slate wall rocks of the jurassic ridang formation to discuss the genesis of the zhaxikang deposit the overall 56 fe and 66 zn values range from 0 80 to 0 43 and from 0 03 to 0 38 respectively the 56 fe values of mn fe carbonates are lighter than those of associated pyrite in six mineral pairs indicating that the iron carbonates are preferentially enriched in light fe isotopes relative to pyrite the sphalerite has lighter 66 zn values than associated mn fe carbonates in three mineral pairs the 56 fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 0 33 to 0 09 through stage 2 0 30 to 0 19 to stage 3 0 16 0 43 in comparison the sphalerite that formed during these stages has 66 zn values that gradually decrease from stage 1 0 16 0 35 through stage 2 0 09 0 23 to stage 3 0 03 to 0 22 these data in conjunction with the observations of hand specimens and thin sections suggest that the deposit was overprinted by a second pulse of mineralization this overprint would account for these fe zn isotopic variations as well as the kinetic rayleigh fractionation that occurred during mineralization the temporally increasing 56 fe and decreasing 66 zn values recorded in the deposit are also coincident with an increase in alteration again supporting the existence of two pulses of mineralization the 56 fe values of the first pulse of ore forming fluid were calculated using theoretical equations yielding values of 0 54 to 0 34 that overlap with those of submarine hydrothermal solutions 1 to 0 however the 56 fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid in addition the second pulse of ore forming fluid has brought some fe and taken away parts of zn which results the lighter 66 zn values of sphalerite and heavier 56 fe values of pyrite from the second pulse of mineralization overall the zhaxikang deposit records two pulses of mineralization and the overprint by the second pulse of mineralization causes the lighter 66 zn values and heavier 56 fe values of modified samples

origin of the neoproterozoic fulu iron formation south china insights from iron isotopes and rare earth element patterns abstract in the neoproterozoic era there was widespread deposition of iron formations in close association with global or near glaciations these snowball earth glaciations likely played a key role in iron formation distribution and deposition however the environmental conditions fe sources and formation mechanisms remain debated here we present the rare earth element geochemistry and fe isotope composition of the synglacial iron formation within the neoproterozoic fulu formation south china the fulu iron formation consists of layers of authigenic minerals mainly hematite and detrital components quartz feldspars fe chlorite and minor biotite positive eu anomalies in one of the fulu localities indicate a hydrothermal influence suggesting that fe was mainly sourced from distal hydrothermal systems the bulk rock fe isotope composition of the fulu iron formation shows a large range with 56fe from 0 23 to 1 78 correlation between bulk rock 56fe values and al fe ratios demonstrates that 56fe variability reflects in part varying proportions of authigenic versus detrital components the fe isotope composition of authigenic hematite is calculated by a linear regression and shows 56fe between 0 83 and 2 21 with an average at 1 54 0 50 2 n 41 using a dispersion reaction model the high 56fe values of hematite constrain local dissolved o2 concentrations of the ocean to less than 0 4 nmol l even in the shallow part of the water column this relationship is consistent with highly reducing conditions in the neoproterozoic oceans favored by isolation from the atmosphere by a sea ice we attribute the extremely positive values to partial iron oxidation in waters that were cold relative to modern surface oceans the dominant occurrence of hematite supports an abiotic precipitation pathway given that biological activity would have introduced organic matter to the sediments and led to partial reduction of fe iii oxides and subsequent formation of magnetite and or siderite as is observed in archean and paleoproterozoic iron formations oxic glacial meltwater and or o2 vertical transfer from the atmosphere to the upper ocean linked to ice dynamics is likely to have mediated the abiotic oxidation we propose that vertical transfer of o2 resulted from the deposition of snow that trapped air bubbles at the top of the glacier coupled to a melting of the bottom of the glacier which in combination delivered a limited but continuous amount of o2 to the ocean

a dna tracer used in column tests for hydrogeology applications tracing techniques are commonly used to investigate groundwater quality and dynamics as well as to measure the hydrogeological parameters of aquifers the last decade has seen a growing interest in environmentally friendly tracers including single stranded dna molecules in this study an electrolytic tracer and a synthetic dna tracer are comparatively evaluated in laboratory scale tests to assess their potential application in field studies aimed at investigating groundwater environments a real time quantitative polymerase chain reaction assay was developed and optimized to detect and quantify the dna tracer while tracer column tests were performed to investigate the dna tracer behavior and to compare it to the electrolytic tracer the results show that the dna tracer has an almost pure convective flow while the kcl tracer experiences dispersive behavior the tracing method proposed can be applied in hydrogeological field studies involving calcareous fractured rock systems with the dna tracer particularly suitable in tracing karst systems which are often characterized by several conduits of flow to test the dna tracer in operation a preliminary test was conducted in the field

shallow marine ironstones formed by microaerophilic iron oxidizing bacteria in terminal paleoproterozoic abstract the microbial origin of precambrian iron formations is debated due to the lack of direct fossil evidence in order to reveal the genesis of ironstones under low oxygen levels integrative studies of sedimentology petrography mineralogy and geochemistry were conducted on the intertidal to shallow subtidal ooidal and stromatolitic ironstones from the terminal paleoproterozoic chuanlinggou formation ca 1 65 1 64 ga of north china using microscopy sem eds icp oes icp ms and mc icp ms techniques mineralogical study shows that the fe rich mineral is predominantly hematite that resulted from dehydration of amorphous fe oxyhydroxide during diagenesis petrographic observation indicates that the iron was oxidized and precipitated from seawater rather than sourced from terrestrial detritus basinward increases of the ironstone abundance eu anomalies from 1 39 to 1 56 and 56fe values of the ironstones from 0 5 to 1 0 suggest that the iron was mainly sourced from seafloor hydrothermal fluids and partially oxidized and precipitated in shallow subtidal to intertidal environments the common existence of fe oxide coated sheaths spiral stalks residual extracellular polymer substances eps and other biogenic fabrics indicates that microaerophilic iron oxidizing bacteria feob may have played an important role in precipitating the chuanlinggou ironstones the extremely low oxygen concentrations implied by the proliferation of microaerophilic feob in the shallow waters the weak positive ce anomalies 0 94 1 12 and low mn concentrations in the ironstones are broadly consistent with the previous result of a cr isotope study thus the establishment of a microaerophilic feob genetic model for the widespread chuanlinggou ironstones in north china provides new insight into the origin of precambrian iron formations and the redox evolution of ocean atmosphere systems during the boring billion

dissolved and particulate fe in a hydrothermal plume at 9 45 n east pacific rise slow fe ii oxidation kinetics in pacific plumes production of fe iii particles in hydrothermal plumes is of fundamental importance to the long term effect of hydrothermal circulation on seawater composition to elucidate the fundamental controls on fe redox kinetics and solution particle partitioning in neutrally buoyant plumes we sampled near field 3 km plume particles at 9 45 n on the east pacific rise in 1996 returning in 1997 to sample both particulate and dissolved phases 0 40 m filter concentrations of dissolved fe varied from 320 to 20 nm in proximal 0 3 km from vent site to distal samples 1 3 km downfield constituting 85 50 of total fe respectively based on vent fluid dilution factors calculated from dissolved mn a mass balance for vent fluid fe at this site indicates that 65 of fe is lost to particulate sulfide settling in the buoyant plume and that particulate fe in distal 1 3 km samples is twice as concentrated as predicted from dilution of particles in proximal plume water these observations are consistent with a calculated fe ii oxidation half time of 3 3 h long enough that fe iii colloid production and aggregation occurs primarily in the neutrally buoyant plume at relatively high dilutions preventing generation of high particulate fe concentrations 11 56 nm observed a general investigation of fe ii oxidation rates in plumes worldwide gives fe ii oxidation half lives as short as 17 min at some atlantic sites and as long as 6 h at some pacific sites the calculations indicate that the distribution of fe particles in plumes depends chiefly on inter basin differences in ambient deep water chemistry primarily ph and dissolved o2 and on local currents driving plume dilution and to a much lesser extent on variations in primary vent fluid composition long term changes in thermohaline circulation or ocean biogeochemistry may therefore alter fe dynamics and minor element fluxes associated with global hydrothermal activity independent of variations in crustal production rates

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

distributions of hydrothermal activity along the mid atlantic ridge interplay of magmatic and tectonic controls abstract hydrothermal activity has been investigated along three different sections of the slow spreading mid atlantic ridge mar 11 20 30 n 36 38 n and the reykjanes ridge 57 45 63 06 n when considered in total the incidence of venting along these three sections of the mar compares well with the predictions of a model in which frequency of venting is linearly related to ridge crest spreading rate at the scale of individual study areas however departure from the model is observed by up to an order of magnitude venting is anomalously rare along the reykjanes ridge but anomalously abundant along the mar 36 38 n whilst such variability may be within the error of the linear spreading rate model we note that the interplay between magmatic and tectonic processes also differs between the three study areas in the case of the reykjanes ridge we propose that the low incidence of venting reported may reflect a limitation of the sampling investigative strategy because the style of venting which predominates may not give rise to conventional black smoker hydrothermal plumes along the oblique and broadly segmented mar 36 38 n we propose that vigorous hydrothermal venting in broad segment end non transform discontinuities may be focussed along deeply penetrating active faults with the requisite heat supply being supported through some combination of along axis magmatic intrusions and thermal release associated with the serpentinisation of crustal peridotites

iron isotope constraints on fe cycling and mass balance in oxygenated earth oceans the fe isotope composition of proterozoic to modern clastic sedimentary rocks and aerosols defines a range ind 56 fe values that is only slightly more variable than the range of fe isotope com positions measured in terrestrial igneous rocks indicating that chemical weathering sedimentary transport and diagenesis play only a minor role in producing fe isotope variations in environ ments where fe redox conditions have been controlled by current levels of atmospheric oxygen in contrast the fe isotope composi tions of hot fluids 300 8c from mid ocean ridge mor spread ing centers define a narrow range that is shifted to lower d 56 fe values by 0 2 0 5 as compared to igneous rocks these new data allow a conceptual model for the fe isotope composition of the oxic oceans that predicts large ranges in fe isotope composition under conditions of changing aerosol and mor fe fluxes such as during periods of major worldwide glaciation

iron isotopes in hot springs along the juan de fuca ridge we present fe isotope data for high temperature hydrothermal fluids sulfides and basalts from the juan de fuca ridge usgs basalt standard bcr 2 and the canyon diablo iron meteorite the fe isotopic compositions of all samples fall within a range of 1 however there is a small but statistically significant isotopic variation between fe dissolved in hydrothermal fluids and fe in canyon diablo specifically the hydrothermal fluids are lighter compared to canyon diablo yielding 56fe 56fe 54fe sample 56fe 54fe canyon diablo 1 1000 values from 0 30 to 0 77 in contrast the bcr 2 basalt and canyon diablo are isotopically indistinguishable measurements of 56fe in juan de fuca ridge basalts and sulfides suggest that fe isotopes are fractionated in hydrothermal systems however the data do not exclude the possibility of small 56fe differences between the rocks from which fluid fe is derived and other igneous fe in which case fractionation in igneous systems is indicated regardless these results are evidence of non biological mass fractionation of fe isotopes in nature further venting of hydrothermal fluids into the overlying water column could provide a source of isotopically light fe to the deep oceans contributing to fe isotopic variations observed in deep sea sediments

styles of positive inversion tectonics in the central apennines and in the adriatic foreland implications for the evolution of the apennine chain italy abstract integration of new field structural and geophysical data with existing information from the apennines chain in italy and its adjacent adriatic foreland indicates that the styles of positive inversion tectonics and the modes of interaction between the extensional and the subsequent compressive structures vary starting from the cretaceous the contractional deformation induced by the mainly north directed convergence of africa adria with respect to the european plate promoted the closure of various arms of the atlantic and the neo tethys oceans which opened in different times and with distinct orientations the mosaic of continental blocks carbonate platforms rift basins and oceanic domains with several geometries and orientations with respect to the axis of the subsequent compression and the resulting heterogeneities within the shallow sedimentary cover and the overall lithosphere strongly influenced both the structural evolution of the apennine orogenic belt and the intra continental deformation within the adriatic foreland field observations reveal that the steeply e and w dipping mesozoic cenozoic normal faults are systematically decapitated by sub horizontal or gently west dipping thrusts propagating with short cut trajectories pre thrusting normal faults were commonly deformed by later thrusts but little evidence seems to support their entire reactivation as high angle reverse faults this suggests that these shallow and steeply dipping discontinuities were not suitable to be reutilized by the superficial thin skinned thrust faults propagating within the sedimentary cover in contrast presumably late paleozoic and mesozoic w dipping normal faults appear moderately reactivated in the adriatic foreland and strong positive inversion tectonics affect the deeper and buried structural levels of the apennine chain within the latter the syn rift sediments in the hangingwall blocks of the fault bounded basins were totally extruded and generated the strong uplift of the thinned adria continental crust finally the contrasting styles of interactions of the pre existing normal faults with later thrusts i e passive truncation or positive reactivation strictly result from the different evolution of the apennine chain and the combined thin and thick skinned modes of deformation of the stretched lithosphere of the adria plate

iron isotope fractionation during oceanic crust alteration abstract the purpose of this work is to study the mobility and budget of fe isotopes in the oceanic crust and in particular during low temperature interaction of seawater with oceanic basalt we carried out this investigation using samples from ocean drilling program odp site 801c drilled during leg 129 and leg 185 in jurassic pacific oceanic crust seaward of the mariana trench the site comprises approximately 450 m of sediment overlying a section of 500 m of basalt which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low temperature 10 30 c ocherous si fe hydrothermal deposits fe was chemically separated from 70 selected samples and 57fe 54fe ratios were measured by mc icp ms isoprobe the isotopic ratios were measured relative to an internal standard solution and are reported relative to the international fe standard irmm 14 based on duplicate measurements of natural samples an external precision of 0 2 2 has been obtained the results indicate that the deep sea sediment section has a restricted range of 57fe which is close to the igneous rock value in contrast large variations are observed in the basaltic section with positive 57fe values up to 2 05 for highly altered basalts and negative values down to 2 49 for the associated alteration products and hydrothermal deposits secondary fe minerals such as fe oxyhydroxides or fe bearing clays celadonite and saponite have highly variable 57fe values that have been interpreted as resulting from the partial oxidation of fe2 leached during basalt alteration and precipitated as fe3 rich minerals in contrast altered basalts at site 801c which are depleted in fe up to 80 display an increase in 57fe values relative to fresh values which suggest a preferential leaching of light iron during alteration the apparent fractionation factor between dissolved fe2 and fe remaining in the mineral is from 0 5 to 1 3 and may be consistent with a kinetic isotope fractionation where light fe is stripped from the minerals alternatively the formation of secondary clays minerals such as celadonite during basalt alteration may incorporate preferentially the heavy fe isotopes resulting in the loss of light fe isotopes in the fluids because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions fe redox state and fe isotope fractionation we evaluated the possible effect of this deep biosphere on fe isotope signatures the fe isotope systematics presented in this study suggest that even though iron behavior during seafloor weathering may be mediated by microbes such as iron oxidizers 57fe variations of more than 4 may also be explained by abiotic processes further laboratory experiments are now required to distinguish between various processes of fe isotope fractionation during seafloor weathering

opposing authigenic controls on the isotopic signature of dissolved iron in hydrothermal plumes iron is a scarce but essential micronutrient in the oceans that limits primary productivity in many regions of the surface ocean the mechanisms and rates of fe supply to the ocean interior are still poorly understood and quantified iron isotope ratios of different fe pools can potentially be used to trace sources and sinks of the global fe biogeochemical cycle if these boundary fluxes have distinct signatures seafloor hydrothermal vents emit metal rich fluids from mid ocean ridges into the deep ocean iron isotope ratios have the potential to be used to trace the input of hydrothermal dissolved iron to the oceans if the local controls on the fractionation of fe isotopes during plume dispersal in the deep ocean are understood in this study we assess the behaviour of fe isotopes in a southern ocean hydrothermal plume using a sampling program of total dissolvable fe tdfe and dissolved fe dfe we demonstrate that 56fe values of dfe 56dfe within the hydrothermal plume change dramatically during early plume dispersal ranging from 2 39 0 05 to 0 13 0 06 2 sd the isotopic composition of tdfe 56tdfe was consistently heavier than dfe values ranging from 0 31 0 03 to 0 78 0 05 consistent with fe oxyhydroxide precipitation as the plume samples age the dfe present in the hydrothermal plume includes stabilised dfe species with potential to be transported to the deep ocean we estimate that stable dfe exported from the plume will have a 56fe of 0 28 0 17 further we show that the proportion of authigenic iron sulfide and iron oxyhydroxide minerals precipitating in the buoyant plume exert opposing controls on the resultant isotope composition of dissolved fe passed into the neutrally buoyant plume we show that such controls yield variable dissolved fe isotope signatures under the authigenic conditions reported from modern vent sites elsewhere and so ought to be considered during iron isotope reconstructions of past hydrothermalism from ocean sediment records

iron and carbon isotope evidence for microbial iron respiration throughout the archean abstract banded iron formations bifs are voluminous chemical sediments that are rich in iron oxide carbonate and silica and whose occurrence is unique to the precambrian their preservation in the geological record offers insights to the surface chemical and biological cycling of iron and carbon on early earth however many details regarding the role of microbial activity in bif deposition and diagenesis are unresolved laboratory studies have shown that reaction between carbon and iron through microbial iron respiration 2fe2o3 nh2o ch2o 7h 4fe2 hco3 2n 4 h2o chemical energy can impart fractionation to the isotopic compositions of these elements here we report iron 56fe vs irmm 014 and carbon isotopic 13c vs v pdb compositions of magnetite and of iron rich and iron poor carbonates in bifs from the late archean 2 5 ga hamersley basin australia and the early archean 3 8 ga isua supracrustal belt isb greenland the range of 56fe values measured in the hamersley basin including light values in magnetite and heavy values in iron rich carbonates up to 1 2 are incompatible with their precipitation in equilibrium with seawater rather the data together with previously reported light 13c values in iron rich carbonates record evidence for diagenetic reduction of ferric oxide precursors to magnetite and carbonate through microbial iron respiration i e dissimilatory iron reduction dir iron and carbon isotope data of iron rich metacarbonates from the isb are similar to those of late archean bifs the isotopic signatures of these metacarbonates are supportive of an early diagenetic origin despite metasomatic overprint and preserve evidence of microbial iron respiration within the oldest recognized sedimentary rocks on earth

in situ filtration and preliminary molecular analysis of microbial biomass from the rainbow hydrothermal plume at 36 15 n on the mid atlantic ridge abstract elevated microbial biomass levels play a significant role in altering the chemical composition of hydrothermal plumes in this work a procedure was developed for collecting sufficient quantities of microbial biomass from a hydrothermal plume to allow the extraction of dna suitable for use in molecular analyses of the plume microbial populations during a geophysical survey along 200 km of the mid atlantic ridge mar southwest of the azores triple junction several new sites of hydrothermal activity were identified including the rainbow hydrothermal plume at 36 15 n the strongest such feature yet reported from the mar during the survey in situ filtration systems were used to collect microbial biomass from the water column onto 293 mm diameter 0 2 m pore membrane filters by pumping large volumes 300 500 l of seawater through the filters filter samples were collected from both the neutrally buoyant portion of the rainbow plume at a depth of 2050 m and from above it in the water column at 1500 m microgram quantities of genomic dna were extracted and purified from the biomass on these filters the suitability of the purified dna for use in molecular analyses was demonstrated by polymerase chain reaction pcr amplification of the small subunit ribosomal rrna ssu rrna gene from the two primary prokaryotic domains the bacteria and the archaea as indicated by the binding of domain specific oligonucleotide hybridisation probes to the purified dna the plume microbial populations contained a higher proportion of members of the domain archaea than the populations in the surrounding ambient sea water

transport and reaction of iron and iron stable isotopes in glacial meltwaters on svalbard near kongsfjorden from rivers to estuary to ocean abstract glacial meltwater has been suggested as a significant source of potentially bioavailable iron to the oceans however the supply of dissolved iron dfe in glacial meltwaters is poorly constrained as few sites have been studied and because the chemical processing of fe during transport from glaciers to the adjacent coastal ocean is not well understood in order to better constrain glacial fluxes of dfe to the ocean iron concentrations iron stable isotopes 56fe and other supporting chemical and physical measurements were made along a 4 km long glacial meltwater river on svalbard and in estuarine waters that it flows into dissolved iron concentrations in the bayelva river decreased from a maximum of 734 nm near the glacier to an average value of 116 nm near the mouth of the river measurements in the kongsfjorden estuary suggest that 3 to 10 nm of dfe from the bayelva river is stabilized in glacial waters by the time it mixes into the ocean incubation of bayelva river waters over two weeks in both the light and dark show similar results with the majority of dfe being quickly precipitated and 4 to 7 nm fe stabilized in the dissolved phase evidence suggests that fe is most likely lost from the dissolved phase by aggregation and adsorption of nanoparticulate and colloidal fe to particles dissolved 56fe was between 0 11 and 0 09 for all river samples and did not vary systematically with dfe concentrations we infer that the fe is lost from the dissolved phase by a process that fractionates fe isotopes by less than 0 05 indicating that the fe bonding environment does not change during precipitation this is consistent with doc loss that is much faster than predicted photo oxidation rates suggesting that doc is also lost through adsorption and precipitation dissolved fe concentrations in the bayelva river 15 734 nm and fe concentrations which are stabilized in the dissolved phase 4 7 nm are much lower than some previous estimates of fe in glacial meltwaters with roughly 80 of dfe lost during transit in the bayelva river and roughly 90 of the remaining dfe lost in the estuary this may mean that glaciers are a less significant source of dissolved fe to the global oceans than has been previously hypothesized that cold base glaciers of the type studied here do not contribute significantly to the dissolved fe flux or that the flux of reactive particulate fe to the oceans is more important than the dissolved flux in arctic regions with similar proglacial environments bedrock composition weathering intensity and as precipitation of colloidal and nanoparticulate fe may all play an important role in regulating the glacial meltwater iron flux to the ocean

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

the relationship between near axis hydrothermal cooling and the spreading rate of mid ocean ridges recent seismic observations and new crustal thermal models suggest that near axis 1 km from the spreading axis hydrothermal cooling increases linearly with increasing spreading rate we present here the first corroborating evidence from near axis hydrothermal plume observations comprehensive surveys of plume distributions have now been conducted on multi segment portions of ridge crest with full spreading rates from 20 to 150 km myr these surveys find thatph the fraction of ridge crest length overlain by hydrothermal plumes and full spreading rate us are related byph us where 0 004 myr km we recast this large scale spatial variability into long term temporal variability by postulating that any vent field site is hydrothermally active for an us fraction of geologic time we then use this relationship and the mean of eight studies that have determined instantaneous heat flux ha at vent field scales 75 45 mw km to estimate the time averaged heat flux ha asha us applying these scaled values ofha to the distribution of mid ocean ridge spreading rates yields a global near axis heat flux ha of 8 8 1011 w this value is 10 of the total oceanic hydrothermal heat flux and agrees with a crustal cooling model derived value of ha of 9 1011 w our estimate of ha implies a mean global3he heat ratio of 2 4 8 3 10 13 cm3 j stp and high temperature hydrothermal fluid flow of 1 8 1013 kg yr

hydrogeological and geochemical characterisation of the rock of orvieto the town of orvieto located on the rock of the same name is an example of vulnerable town problems of slope instability connected with the lithological and morphological characteristics of the rock have been thor oughly examined and discussed during previous research studies hydrogeochemical data about groundwater recharging the springs present in the area were never taken into account pollution of the springs is well known but still occurs for unclear reasons the aim of this work is there fore to present the results of a hydrogeological and geo chemical investigation of all the springs along the slopes of orvieto hill and at the foot of the tuffaceous rock to characterize the groundwater flow paths and to suggest a possible source of contamination the research study was carried out during three hydrogeochemical surveys in the years 1998 1999 2003 2004 and 2007 2008

fluxes to sediments underlying the rainbow hydrothermal plume at 36 14 n on the mid atlantic ridge abstract a geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the rainbow hydrothermal field as well as a large biogenic component 80 caco3 these sediments record clear enrichments of the elements fe cu mn v p and as from hydrothermal plume fallout but only minor detrital background material systematic variations in the abundances of hydrothermal elements are observed at increasing distance from the vent site consistent with chemical evolution of the dispersing plume further pronounced ni and cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near field locations radiocarbon dating reveals mean holocene accumulation rates for all four cores of 2 7 to 3 7 cm kyr 1 with surface mixed layers 7 to 10 cm thick from which a history of deposition from the rainbow hydrothermal plume can be deduced deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced e g fe mn cu or scavenged from seawater via the hydrothermal plume e g v p as holocene fluxes into to the cores surface mixed layers are presented which typically are an order of magnitude greater than background authigenic fluxes from the open north atlantic one core collected closest to the vent site indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14c kyr ago the preferred explanation is that this variation reflects the initiation intensification of hydrothermal venting at the rainbow hydrothermal field at this time perhaps linked to some specific tectonic event in this fault controlled hydrothermal setting

glacial interglacial co2 change the iron hypothesis several explanations for the 200 to 280 ppm glacial interglacial change in atmospheric co2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients an hypothesis is presented herein in which arguments are made that new productivity in today s southern ocean 7 4 1013g yr 1 is limited by iron deficiency and hence the phytoplankton are unable to take advantage of the excess surface nitrate phosphate that if used could result in total southern ocean new production of 2 3 1015 g c yr 1 as a consequence of fe limited new productivity holocene interglacial co2 levels preindustrial are as high as they were during the last interglacial 280 ppm in contrast atmospheric dust fe supplies were 50 times higher during the last glacial maximum lgm because of this fe enrichment phytoplankton growth may have been greatly enhanced larger amounts of upwelled nutrients may have been used and the resulting stimulation of new productivity may have contributed to the lgm drawdown of atmospheric co2 to levels of less than 200 ppm background information and arguments in support of this hypothesis are presented

integrated fe and s isotope study of seafloor hydrothermal vents at east pacific rise 9 10 n abstract in this study we report on coupled fe and s isotope systematics of hydrothermal fluids and sulfide deposits from the east pacific rise at 9 10 n to better constrain processes affecting fe isotope fractionation in hydrothermal environments we aim to address three fundamental questions 1 is there significant fe isotope fractionation during sulfide precipitation 2 is there significant variability of fe isotope composition of the hydrothermal fluids reflecting sulfide precipitation in subsurface environments 3 are there any systematics between fe and s isotopes in sulfide minerals the results show that chalcopyrite precipitating in the interior wall of a hydrothermal chimney displays a limited range of 56fe values and 34s values between 0 11 to 0 33 and 2 2 to 2 6 respectively the 56fe values are on average slightly higher by 0 14 relative to coeval vent fluid composition while 34s values suggest significant s isotope fractionation 0 6 0 2 during chalcopyrite precipitation in contrast systematically lower 56fe and 34s values relative to hydrothermal fluids by up to 0 91 and 2 0 respectively are observed in pyrite and marcasite precipitating in the interior of active chimneys these results suggest isotope disequilibrium in both fe and s isotopes due to s isotopic exchange between hydrothermal h2s and seawater so42 followed by rapid formation of pyrite from fes precursors thus preserving the effects of a strong kinetic fe isotope fractionation during fes precipitation in contrast 56fe and 34s values of pyrite from inactive massive sulfides which show evidence of extensive late stage reworking are essentially similar to the hydrothermal fluids multiple stages of remineralization of ancient chimney deposits at the seafloor appear to produce minimal fe isotope fractionation similar affects are indicated during subsurface sulfide precipitation as demonstrated by the lack of systematic differences between 56fe values in both high temperature fe rich black smokers and lower temperature fe depleted vents

discovery of ancient and active hydrothermal systems along the ultra slow spreading southwest indian ridge 10 16 e 1 we report the discovery of active and fossil hydrothermal systems during r v knorr cruise 162 legs vii and ix along a 400 km long segment of the ultra slow southwest indian ridge swir between 10 and 16 e where the effective spreading rate for mantle upwelling is the slowest of any ocean ridge explored to date 8 4 mm yr 1 full rate eight of forty one optical temperature profiles contain hydrothermal plume characteristics that indicate firm evidence for two active vent sites and tentative evidence for as many as three others fossil hydrothermal material was recovered in 6 of the 38 dredge hauls and includes an occurrence of partially oxidized sulfide breccias four deposits ofsepiolite and silica as well as mn oxide and nontronite cemented breccias the massive sulfide deposit likely developed during mixing of upwelling hot fluids with cold seawater within a shallow permeable fault breccia along a deep seated normal fault bounding the southern rift valley wall silica and sepiolite deposits were found mostly on the rift valley walls and likely formed during low to moderate temperature ultramafic hosted hydrothermal activity the abundance of hydrothermal material and the frequency of localized hydrothermal activity is remarkable because the mantle upwelling and magma supply rates and hence the magmatic heat input along this section of the swir are lower than on any other explored segment of the global mid ocean ridge system this observation suggests that high mantle upwelling and magma supply rates are not required to drive mid ocean ridge hydrothermal systems and that a close relationship between magmatic heat input and hydrothermal activity may not be established at the ultra slow end of the ridge spreading spectrum the frequency and distribution of hydrothermal activity in the study area may reflect a largely tectonic control on fluid circulation with hydrothermal vent sites being preferentially associated with long lived faults that provide fluid pathways our results suggest the contribution of magma starved ridge segments to the global ocean lithosphere geochemical budget is potentially much larger than previously thought

iron isotopes constrain biologic and abiologic processes in banded iron formation genesis abstract the voluminous 2 5 ga banded iron formations bifs from the hamersley basin australia and transvaal craton south africa record an extensive period of fe redox cycling the major fe bearing minerals in the hamersley transvaal bifs magnetite and siderite did not form in fe isotope equilibrium but instead reflect distinct formation pathways the near zero average 56fe values for magnetite record a strong inheritance from fe3 oxide hydroxide precursors that formed in the upper water column through complete or near complete oxidation transformation of the fe3 oxide hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of 56fe values 1 addition of marine hydrothermal fe 2 aq 2 complete reduction by bacterial dissimilatory iron reduction dir and 3 interaction with excess fe 2 aq that had low 56fe values and was produced by dir most siderite has slightly negative 56fe values of 0 5 that indicate equilibrium with late archean seawater although some very negative 56fe values may record dir support for an important role of dir in siderite formation in bifs comes from previously published c isotope data on siderite which may be explained as a mixture of c from bacterial and seawater sources several factors likely contributed to the important role that dir played in bif formation including high rates of ferric oxide hydroxide formation in the upper water column delivery of organic carbon produced by photosynthesis and low clastic input we infer that dir driven fe redox cycling was much more important at this time than in modern marine systems the low pyrite contents of magnetite and siderite facies bifs suggests that bacterial sulfate reduction was minor at least in the environments of bif formation and the absence of sulfide was important in preserving magnetite and siderite in the bifs minerals that are poorly preserved in the modern marine record the paucity of negative 56fe values in older early archean and younger early proterozoic bifs suggests that the extensive 2 5 ga hamersley transvaal bifs may record a period of maximum expansion of dir in earth s history

iron isotope composition of some archean and proterozoic iron formations abstract fe isotopes can provide new insight into redox dependent biogeochemical processes precambrian iron formations if are deserving targets for fe isotope studies because they are composed predominantly of authigenic fe phases and record a period of unprecedented iron deposition in earth s history we present fe isotope data for bulk samples from 24 archean and proterozoic if and eight phanerozoic fe oxide rich deposits these data reveal that many archean and early paleoproterozoic iron formations were a sink for isotopically heavy fe in contrast to later proterozoic and phanerozoic fe oxide rich rocks the positive 56 fe values in if are best explained by delivery of particulate ferric oxides formed in the water column to the sediment water interface because if are a net sink for isotopically heavy fe there must be a corresponding pool of isotopically light fe in the sedimentary record earlier work suggested that archean pyritic black shales were an important part of this light sink before 2 35 billion years ago ga it is therefore likely that the persistently and anomalously low 56 fe values in shales are linked with the deposition of isotopically heavy fe in if in the deeper parts of basins if deposition produced a residual isotopically light dissolved fe pool that was captured by pyritic fe in shales local dissimilatory fe reduction in porewater and associated diagenetic reactions resulting in pyrite and carbonate precipitation may have further enhanced fe isotope heterogeneity in marine sediments and an iron shuttle may have transported isotopically light fe from shelf sediments to the basin nevertheless water column processing of hydrothermally delivered fe likely had the strongest influence on the bulk iron isotope composition of archean and paleoproterozoic iron formations and other marine sediments

iron isotope and ree y composition of the caue banded iron formation and related iron ores of the quadrilatero ferrifero brazil the minas supergroup banded iron formations bifs of the brazilian quadrilatero ferrifero qf mineral province experienced multiple deformational events synchronous with hypogene mineralization which resulted in the metamorphism of bifs to itabirites and their upgrade to high grade iron ore here we present rare earth element and yttrium ree y compositions together with iron isotope ratios of itabirites and their host iron orebodies from 10 iron deposits to constrain environmental conditions during bif deposition and the effects of hypogene iron enrichment the ree y characteristics of itabirites positive eu anomaly and lree depletion indicate hydrothermal iron contribution to the minas basin iron isotope data and ce anomalies suggest bifs were precipitated by a combination of anoxic biological mediated ferrous iron oxidation and abiotic oxidation in an environment with free oxygen such as an oxygen oasis perhaps related to increase in oxygen concentrations before the great oxidation event goe the similarity of the ree y composition of the itabirites from the different qf deformational domains as well as to other superior type bifs indicates that the metamorphism and synchronous hydrothermal mineralization did not significantly affect the geochemical signature of the original bifs however iron isotope compositions of iron ore vary systematically between deformational domains of the qf likely reflecting the specific mineralization features in each domain

tectonic and volcanic implications of a cratered seamount off nicobar island andaman sea the region of the nicobar earthquake swarm of january 2005 has been explored during a recent cruise using multibeam swath bathymetry seafloor imaging and tv guided sampling to decipher the seafloor morphology nature and tectonic frame work a seamount with well developed crater at the summit was discovered near to the center of the nicobar swarm rock samples collected by tv guided grab from the seamount crater are dacite rhyolite and andesite type with a veneer of ferromanganese oxide coating the aggregates recovered from the slope consisted of manganese globules the geochemistry of the globules suggests 66 97 mn oxide indicative of hydrothermal origin the morphology of the seamount seafloor video footage and geochemistry of the seabed samples suggest that the cratered seamount has erupted in the recent geological past and is dormant at present this is the first documented report of submarine arc volcanism in the andaman sea this finding substantiates the prediction of a submarine volcano east of nicobar island study of the seismicity and the stress pattern in the region suggest that the earthquake swarm in the region occurred due to 2004 sumatra andaman megathrust earthquake the derived stress pattern suggests that the 2004 earthquake greatly reduced the normal stress in the region of west andaman fault from the planes or unclamped them thus unclamping of the region by the 2004 sumatra andaman earthquake led to the initiation of the swarm through predominantly strike slip faulting the newly discovered dormant submarine volcano indicates the volcanic nature of the region we suggest that this submarine volcano is a link between sub aerial volcanoes of barren narcondam islands of the andaman sea and the volcanoes of sumatra

hydrothermal activity along the southwest indian ridge twenty years after the discovery of sea floor hot springs vast stretches of the global mid ocean ridge system remain unexplored for hydrothermal venting the southwest indian ridge is a particularly intriguing region as it is both the slowest spreading of the main ridges1 and the sole modern migration pathway between the diverse vent fauna of the atlantic and pacific oceans2 a recent model postulates that a linear relation exists between vent frequency and spreading rate3 and predicts vent fields to be scarcest along the slowest spreading ridge sections thus impeding migration and enhancing faunal diversity2 here however we report evidence of hydrothermal plumes at six locations within two 200 km long sections of the southwest indian ridge indicating a higher frequency of venting than expected these results suggest that fluxes of heat and chemicals from slow spreading ridges may be greater than previously thought and that faunal migration along the southwest indian ridge may serve as an important corridor for gene flow between pacific and atlantic hydrothermal fields

micro scale tracing of fe and si isotope signatures in banded iron formation using femtosecond laser ablation we have detected micrometre scale differences in fe and si stable isotope ratios between coexisting minerals and between layers of banded iron formation bif using an uv femtosecond laser ablation system connected to a mc icp ms in the magnetite carbonate chert bif from the archean old wanderer formation in the shurugwi greenstone belt zimbabwe magnetite shows neither intra nor inter layer trends giving overall uniform d 56 fe values of 0 9 but exhibits intra crystal zonation bulk iron carbonates are also relatively uniform at near zero values however their individual d 56 fe value is highly composition dependent both siderite and ankerite and mixtures between both are present and d 56 fe end member values are 0 4 for siderite and 0 7 for ankerite the data suggest either an early diagenetic origin of magnetite and iron carbonates by the reaction of organic matter with ferric oxyhydroxides catalysed by fe iii reducing bacteria or more likely an abiotic reaction of organic carbon and fe iii during low grade metamorphism si isotope composition of the old wanderer bif also shows significant variations with d 30 si values that range between 1 0 and 2 6 for bulk layers these isotope compositions suggest rapid precipitation of the silicate phases from hydrothermal rich waters interestingly fe and si isotope compositions of bulk layers are covariant and are interpreted as largely primary signatures moreover the changes of fe and si isotope signatures between bulk layers directly reflect the upwelling dynamics of hydrothermal rich water which govern the rates of fe and si precipitation and therefore also the development of layering during periods of low hydrothermal activity precipitation of only small amounts of ferric oxyhydroxide was followed by complete reduction with organic carbon during diagenesis resulting in carbonate chert layers during periods of intensive hydrothermal activity precipitation rates of ferric oxyhydroxide were high and subsequent diagenesis triggered only partial reduction forming magnetite carbonate chert layers we are confident that our micro analytical technique is able to detect both the solute flux history into the sedimentary bif precursor and the bif s diagenetic history from the comparison between coexisting minerals and their predicted fractionation factors

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

secular variation of iron isotopes in north atlantic deep water a high precision iron isotope time series for a ferromanganese crust demonstrates that the iron isotope composition in north atlantic deep water has changed substantially over the past 6 million years and that iron isotope variations in the crust are closely correlated to those of lead isotopes the close correlation between the two isotope series indicates that the observed iron isotope variations predominantly reflect those of iron input from terrigenous sources and provides no evidence for biologically induced mass fractionation within north atlantic deep water

nitrate reduction sulfate reduction and sedimentary iron isotope evolution during the cenomanian turonian oceanic anoxic event onset of the oae in italy have unusually low bulk d 57 fe values unlike those found in the black shale livello bonarelli deposited during the oceanic anoxic event itself these latter conform to the phanerozoic norm for organic rich sediments pyrite formation in the pre oae black shales has apparently taken place via dissimilatory iron reduction dir within the sediment a suboxic process that causes an approximately 2 fractionation between a lithogenic fe iii oxide source and fe ii aq in contrast bacterial sulfate reduction bsr at least partly in the water column characterized the oae itself and was accompanied by only minor iron isotope fractionation this change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from 10 to 7 mm the gradual albeit irregular increase in fe isotope values during the oae as recorded in the italian section is taken to demonstrate limited isotopic evolution of the dissolved iron pool consequent upon ongoing water column precipitation of pyrite under euxinic conditions given that evidence exists for both nitrate and sulfate reduction during the oae it is evident that redox conditions in the water column were highly variable in both time and space

trace elements and isotope geochemistry c o fe cr of the caue iron formation quadrilatero ferrifero brazil evidence for widespread microbial dissimilatory iron reduction at the archean paleoproterozoic transition abstract the banded iron formation bifs of the caue formation quadrilatero ferrifero brazil are part of the great volume of bifs deposited worldwide around 2 45 ga samples of carbonate ankerite dolomite bif from this unit were collected from a drill core ca 600 m deep in the alegria region geochemical data suggest low clastic contamination y ho 37 7 and seawater like signatures with positive la and y anomalies no true ce anomalies and mean eu eu 1 62 u th shows a sharp increasing downwards pattern up to ca 15 below the 535 m mark values of 13c and 18o are all negative reaching a minimum of 10 0 and 24 6 respectively in the 540 560 m interval values of 56fe are mostly positive between 0 5 and 1 3 the higher values are attained in the same interval values of authigenic 53cr range between 0 01 and 0 26 with an increasing downwards pattern which mirrors the u th variation a possible explanation for both the low 13c and the high 56fe is the activity of microorganisms in the dissimilatory iron reduction dir of the precursor ferric oxides thus we put forward a model for deposition of the caue bifs that starts with the generation of fe ii aq through hydrothermal input to the deeper basin which upon reaching a chemocline in the platform border oxidizes to fe iii and settles as iron oxy hydroxides to the basin floor when reaching the sediment layer fe iii is then reduced by microbial activity to generate magnetite and iron rich carbonates with negative 13c and positive 56fe an important issue is how u vi and cr vi were mobilized to the basin to form the positive u th and 53cr excursions these could reflect oxidative continental weathering but an alternative possibility is that those elements became available through local oxygenation in continental microenvironments which would better explain the lack of ce redox cycling in either case our results together with a growing body of data for bif sequences of similar age around the world e g kuruman south africa and hamersley australia suggest that microbial mediated dir was a common and widespread mechanism for the genesis of bifs at the archean paleoproterozoic transition

particle geochemistry in the rainbow hydrothermal plume mid atlantic ridge we report the analysis of 18 large volume 500 1500 l in situ filtered samples of particulate material from the largest hydrothermal plume on the mid atlantic ridge overlying the ultramafic hosted rainbow hydrothermal field at 36 deg 14 n measured particulate iron concentrations reach 614 nm high concentrations of particulate fe oxyhydroxides result from the extremely high fe concentration 24 mm and fe h2s ratio 24 of the vent fluids and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along axis flow two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume these samples are characterized by anomalously compared to neutrally buoyant plume samples high concentrations of mg u and chalcophile elements and low concentrations of mn ca v y and the rare earth elements ree within the neutrally buoyant plume elemental distributions are largely consistent with previously observed behaviors preferential removal of chalcophile elements conservative behavior of oxyanions p v and u and continuous scavenging of y and the ree this consistency is particularly significant in light of the underlying differences in fluid chemistry between rainbow and other studied sites chalcophile elements are preferentially removed from the plume in the order cd zn co cu phosphorus iron and vanadium iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the rainbow plume cave et al 2002 indicates however that while p fe ratios are indeed constant v fe ratios increase progressively from early stage plume particles to sediments ree distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column

processes affecting the isotopic composition of dissolved iron in hydrothermal plumes a case study from the vanuatu back arc abstract hydrothermal discharge is one of multiple fe sources contributing to the marine dissolved fe budget but its relative importance remains controversial and largely unconstrained isotopic characterization of fe sources and mass balance calculations are considered as a potential tool to unravel the complex interplay of various fe fluxes but require a more profound knowledge of the processes that fractionate fe isotopes in hydrothermal plumes we present dissolved fe isotope data from two buoyant plumes and profiles traversing the non buoyant plume above the nifonea vent field in the vanuatu back arc our findings support opposing controls from precipitation of fe sulfides and fe oxyhydroxides that either enrich the heavy or the light isotopes within the residual dissolved fe fraction but also suggest significant influence from size partitioning of oxidized fe following precipitation as well as potential effects from organic complexation at low aggregation of precipitated fe oxyhydroxides or stabilization of significant fe iii aq dissolved 56 fe is only marginally affected by fe ii oxidation which strengthens the role of fe sulfide precipitation during early plume rise in determining the isotope signature of hydrothermal fe exported to the open ocean dissolved 56 fe range from 0 73 0 05 to 0 16 0 05 and suggest that sulfide rich basalt hosted hydrothermal systems and slow oxidation kinetics in the pacific favor export of stabilized dissolved fe with heavier 56 fe values than the true vent fluids 56 fe of 0 5

atmospheric transport of iron and its deposition in the ocean the atmospheric transport of continental weathering products is responsible for much of the mineral material and fe entering the open ocean and is probably the dominant source of nutrient fe in the photic zone in regions where other nutrients are present in high concentrations the flux of fe from the atmosphere may be a limiting factor in primary productivity due to the larger source regions for dust north of the equator 8 times more atmospheric fe is deposited in the northern hemisphere than in the southern hemisphere the mineral aerosol and fe transport and deposition are highly variable due to the episodic nature of dust generation and its transport and deposition processes between 10 and 50 of the total atmospheric fe entering the world ocean appears to dissolve i e it will pass through a 0 4 m pore size filter rapidly when the mineral matter enters the ocean much of the atmospheric fe is present as fe ii apparently produced as a result of photochemical reduction reactions taking place during atmospheric transport this readily soluble fe ii should be available immediately for use as a nutrient by phytoplankton atmospheric transport from the continents is estimated to supply 3 times as much dissolved fe to the oceans as that delivered via rivers

fe isotope variations in the modern and ancient earth and other planetary bodies iron the fourth most abundant element in the earth s crust has four naturally occurring stable isotopes 54fe 5 84 56fe 91 76 57fe 2 12 and 58fe 0 28 and the natural mass dependent isotope variations of fe in the rock record span a range of 4 per mil in 56fe 54fe ratios fig 1 the field of fe isotope geochemistry is relatively new but has received considerable attention because it may allow us to gain a better understanding of how fe is cycled in different environments iron typically occurs as either reduced ferrous fe in oxygen poor environments or as oxidized ferric iron in oxygen rich environments notably only the reduced species is soluble in oxygenated aqueous solutions unless the ph is low in the archean and early proterozoic the earth may have been relatively oxygen poor e g kasting et al 1979 grandstaff 1980 holland 1994 suggesting that there may have been significant quantities of fe 0 9 millimolar dissolved in the oceans as fe ii aq e g ewers 1983 sumner 1997 the extensive iron formations of archean to early proterozoic age may have been deposited from such fe ii rich oceans e g beukes and klein 1992 in the modern oxic oceans however fe contents are exceedingly low 1 nanomolar in the open oceans e g martin and gordon 1988 bruland et al 1991 martin 1992 johnson et al 1997 and it is now recognized that marine productivity is fe limited in parts of the open oceans e g martin and fitzwater 1988 martin et al 1989 1994 the differences in the behavior of fe with redox state and the significant isotope fractionations 1 or more in 56fe 54fe that are associated with redox conditions suggests that fe isotope studies will be extremely useful for tracing

in situ iron isotope ratio determination using uv femtosecond laser ablation with application to hydrothermal ore formation processes abstract the feasibility of in situ stable fe isotope ratio measurements using uv femtosecond laser ablation connected to a multiple collector inductively coupled plasma mass spectrometer mc icp ms has been investigated different types of matrices independently determined by solution mc icp ms after chromatographic separation of fe have been analysed by laser ablation using the isotopically certified iron reference material irmm 014 as the bracketing standard the samples have been pure iron metal jm puratronic fe meteorites north chile glenormiston and toluca the meteorite phases kamacite and taenite in toluca and fe sulphides furthermore fe isotope ratios from hydrothermal hematite siderite and goethite from an old mining area in the schwarzwald germany and of magnetite from the metamorphic biwabik iron formation have been determined the results show that a precision of better than 0 1 2 sigma can be achieved with laser ablation and that all the results obtained agree with those determined by solution icp to better than 0 1 this precision and accuracy is achievable in both raster and spot ablation mode a matrix matched bracketing standard is not required and all these materials can be measured accurately against a metal standard the hydrothermal minerals show significant fe isotope zonations in some samples the range of 56fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district for example isotopic zonations found in secondary fibrous hematites show a continuous change in the 56fe values from 0 5 in the core to 1 8 in the rim primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim while the siderite is thought to record primary fluid histories the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re precipitation of the oxidized fe abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0 4 lighter than the core of the grain if indeed fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals and these in turn record hydrothermal fluid histories then the tools are in place for a detailed reconstruction of the deposit s genesis we expect similar observations from other fe rich deposits formed at intermediate and low temperatures e g banded iron formations laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable fe isotope fractionation

time series studies of vent fluids from the tag and mark sites 1986 1990 mid atlantic ridge a new solution chemistry model and a mechanism for cu zn zonation in massive sulphide orebodies

imaging of crustal heterogeneous structures using a slowness weighted back projection with effects of scattering modes 2 application to the nagamachi rifu fault japan area 1 this is the second paper in a two part series on a newly developed imaging approach for small scale heterogeneities 1 km in the crust with effects of scattering modes in the present paper we estimated a detail three dimensional spatial distribution of small scale heterogeneities around the nagamachi rifu fault northeastern japan in a frequency range of 2 16 hz using the imaging approach presented in the first paper we used seismograms recorded by dense three component seismic arrays that were deployed in this region for 15 explosion sources as one of our important results there are concentrations for only p s but not p p scatterers near the surface trace of the fault p s scatterers in a frequency range of 8 16 hz are localized near the surface trace of the fault implying the possibility of strong heterogeneities with a size of 0 08 km there in the shirasawa caldera region the characteristics of seismic scatterers seem to convert from large p s to large p p relative scattering coefficients with its transition depth range of 5 8 km this feature implies that the materials composed of seismic scatterers may show a systematic variation with depth finally the strength of scattering coefficients is rather weak in the coseismic area of the 15 september 1998 earthquake with a magnitude of 5 2 the largest recent event in this area this result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system

iron isotope fractionation in a buoyant hydrothermal plume 5 s mid atlantic ridge fe isotopes are a potential tool for tracing the biogeochemical redox cycle of fe in the ocean specifically it is hypothesized that fe isotopes could enable estimation of the contributions from multiple fe sources to the dissolved fe budget an issue that has received much attention in recent years the first priority however is to understand any fe isotope fractionation processes that may occur as fe enters the ocean resulting in modification of original source compositions in this study we have investigated the fe inputs from a basalt hosted deep sea hydrothermal system and the fractionation processes that occur as the hot chemically reduced and acidic vent fluids mix with cold oxygen rich seawater the samples collected were both end member vent fluids taken from hydrothermal chimneys and rising buoyant plume samples collected directly above the same vents at 5 s mid atlantic ridge our analyzes of these samples reveal that for the particulate fe species within the buoyant plume 25 of the fe is precipitated as fe sulfides the isotope fractionation caused by the formation of these fe sulfides is fe ii fes 0 60 0 12 the source isotope composition for the buoyant plume samples collected above the red lion vents is calculated to be 0 29 0 05 this is identical to the value measured in end member vent fluids collected from the underlying tannenbaum chimney the resulting isotope compositions of the fe sulfide and fe oxyhydroxide species in this buoyant plume are 0 89 0 11 and 0 19 0 09 respectively from mass balance calculations we have been able to calculate the isotope composition of the dissolved fe fraction and hypothesize that the isotope composition of any stabilised dissolved fe species exported to the surrounding ocean may be heavier than the original vent fluid such species would be expected to travel some distance from areas of hydrothermal venting and hence contribute to not only the dissolved fe budget of the deep ocean but also it s dissolved fe isotope signature

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

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

black smokers massive sulphides and vent biota at the mid atlantic ridge the discovery of black smokers massive sulphides and vent biota in the rift valley of the mid atlantic ridge demonstrates that this assemblage of hydrothermal phenomena is not limited to intermediate to fast spreading oceanic ridges hydrothermal exchange processes may thus be important at the ridges which extend through the atlantic ocean and western indian ocean comprising more than half of the 55 000 km global length of seafloor spreading centres

zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys many of the heaviest and lightest natural zinc zn isotope ratios have been discovered in hydrothermal ore deposits however the processes responsible for fractionating zn isotopes in hydrothermal systems are poorly understood in order to better assess the total range of zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation we have measured zn isotopes in seafloor hydrothermal fluids from numerous vents at 9 10 n and 21 n on the east pacific rise epr the tag hydrothermal field on the mid atlantic ridge and in the guaymas basin fluid 66zn values measured at these sites range from 0 00 to 1 04 of the many physical and chemical parameters examined only temperature was found to correlate with fluid 66zn values lower temperature fluids 250 c had both heavier and more variable 66zn values compared to higher temperature fluids from the same hydrothermal fields we suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite zn sulfide and that this process is a primary cause of zn isotope variation in hydrothermal fluids thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest zn found in fluids that were calculated to be saturated with respect to sphalerite we have also measured zn isotopes in chimney sulfides recovered from a high temperature 383 c and a low temperature 203 c vent at 9 10 n on the epr and in both cases found that the 66zn of chimney minerals was lighter or similar to the fluid 66zn the first measurements of zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid 66zn and suggest that subsurface zn sulfide precipitation is a primary factor in causing variations in fluid 66zn by understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid 66zn zn isotopes may be used as a tracer for studying hydrothermal processes

iron sources and dissolved particulate interactions in the seawater of the western equatorial pacific iron isotope perspectives this work presents iron isotope data in the western equatorial pacific marine aerosols and top core margin sediments display a slightly heavy fe isotopic composition 56 fe of 0 33 0 11 2sd and 0 14 0 07 respectively samples reflecting the influence of papua new guinea runoff sepik river and rabaul volcano water are characterized by crustal values in seawater fe is mainly supplied in the particulate form and is found with a 56 fe between a0 49 and 0 34 0 07 the particulate fe seems to be brought mainly by runoff and transported across continental shelves and slopes aerosols are suspected to enrich the surface vitiaz strait waters while hydrothermal activity likely enriched new ireland waters dissolved fe isotopic ratios are found between a0 03 and 0 53 0 07 they are almost systematically heavier than the corresponding particulate fe and the difference between the signature of both phases is similar for most samples with 56 fe dfe pfe 0 27 0 25 2sd this is interpreted as an equilibrium isotopic fractionation revealing exchange fluxes between both phases the dissolved phase being heavier than the particles suggests that the exchanges result in a net nonreductive release of dissolved fe this process seems to be locally significantly more intense than fe reductive dissolution documented along reducing margins it may therefore constitute a very significant iron source to the ocean thereby influencing the actual estimation of the iron residence time and sinks the underlying processes could also apply to other elements

the marine environments encompassing the neoproterozoic glaciations evidence from c sr and fe isotope ratios in the hecla hoek supergroup in svalbard abstract the neoproterozoic era records several important events in earth history the associations between bif deposition snowball earth events and the redox state of seawater are the key to explain the re appearance of banded iron formations bif during the neoproterozoic unraveling ancient iron cycles provides important information about the linkage between the precipitation of bif and the redox condition in the ocean but current iron isotopic data are limited to sediments deposited only during the sturtian glaciations and ediacaran we conducted a detailed geological survey of the tonian to ediacaran sedimentary succession in nordaustlandet island svalbard our chemostratigraphies of 13 c carb from pre sturtian to post marinoan sedimentary successions in the polarisbreen group are consistent with those in coeval sediments we first analyzed iron isotope ratios 56 54 fe of individual pyrite grains with diverse shapes in carbonates black shales quartz arenites and diamictites in order to constrain the redox condition in seawater we found large variations in iron isotope ratios from ca 2 to ca 4 positive 56 54 fe values over 1 0 are only preserved in euhedral and aggregated pyrite grains in black shales of the backlundtoppen formation and the macdonaldryggen member of the elbobreen formation a partial oxidation of ferrous iron in seawater is necessary to explain such high 56 54 fe values which means that deep seawater was ferruginous ferrous iron rich before and after the sturtian glaciation this stands in opposition to the traditional idea that a glacial cover on an ocean is necessary for the accumulation of ferrous iron in neoproterozoic seawater this study presents the first evidence from iron isotopes that supports a deep ferruginous ocean around the sturtian glaciation if the deep ocean was intrinsically iron rich it is likely that neoproterozoic bifs formed by thermohaline circulation during the glaciation and by massive oxidation due to the expansion of oxic oceans after the glaciation

hydrography and high temperature heat flux of the rainbow hydrothermal site 36 14 n mid atlantic ridge on the basis of an extensive set of conductivity temperature depth lowered acoustic doppler current profiler ladcp and nephelometry profiles and tow yos the hydrography flow field and particle plume of the rainbow hydrothermal site on the mid atlantic ridge are analyzed in the rift valley the water column is less dense and stratified than both eastern and western off ridge water with t s characteristics consistent with inflow across a sill from the east the buoyant hydrothermal plumes rise into a strong boundary current flowing along the slope of a topographic high which partially blocks the rift valley below 1950 m the bulk of the neutrally buoyant plume is advected across a sill which forms both the narrowest and the shallowest part of the valley and acts as a hydraulic control point for the flow below 2000 m large amplitude internal waves consistent with tidal forcing are observed near the sill but ladcp measurements suggest that the tidal signal is not strong enough to lead to flow reversal at plume depth above 2000 m the mean current across the topography generates lee waves which radiate energy upward and downstream density averaged light scattering profiles show the hydrothermal particle plume to be gaussian in depth even in the near field where many of the individual profiles are characterized by multiple peaks and the horizontal variability is highest the temperature anomalies associated with the mean near source particle plume are of order 5 10 3 c that is the plume is cold fresh as expected from the background hydrography of the deep atlantic using the flow field light scattering and hydrographic anomaly observations the heat flux associated with the hydrothermal particle plume at rainbow is estimated to lie between 1 and 5 gw

dna and fluorescein tracer tests to study the recharge groundwater flowpath and hydraulic contact of aquifers in the umbria marche limestone ridge central apennines italy the purpose of this paper is to highlight the hydrogeological contact among aquifers in karst and fissured systems and study the different flowpaths present in said aquifers given the complex hydrogeological and tectonics settings detailed surveys in the umbria marche limestone ridges and tracer field tests were used to achieve this goal dna and fluorescent tracers were injected into a sinkhole and thereafter recovered at different points allowing both for the identification of contact among aquifers as well as the main and secondary directions of groundwater the tracer tests have shown that the majority of the groundwater flow is directed towards the axis of the tectonic structures emerging in more depressed areas in addition they have confirmed the influence of karstification on the recharge which allows for a more detailed characterisation of water circulation in the unsaturated zone of the maiolica and the calcare massiccio limestone aquifers thus offering confirmation of the hydrogeological survey results and providing additional information that was previously unknown furthermore the dna tracer provided good results in the field especially in the karst systems and fissured rocks thus confirming its appropriacy for flowpath investigations and vulnerability analyses of springs in particular dna is suitable for very large amounts of groundwater owing to its very low detection limit further investigations and studies are however required to assess its validity also for the evaluation of hydrogeological parameters

field and laboratory studies of metal uptake and release by hydrothermal precipitates data for suspended particles from hydrothermal plumes on the mid atlantic ridge mar support uptake of dissolved v and as during formation of fe oxides and release of cu and cd during alteration of sulfides to complement these field observations we carried out laboratory time series experiments to determine the stability of v as cu and cd in precipitates from hydrothermal plumes on the mar and juan de fuca ridge for v and as the metal fe ratio in plume particles remained uniform after sampling in contrast to v and as particulate cu and cd values decreased relative to fe over time in laboratory studies using both plume particles and mound derived sulfides the combined field and laboratory results show that scavenging is an important removal mechanism for seawater v and as whereas oxidative dissolution of hydrothermal precipitates in seawater adds cu and cd to the ocean

iron isotope fractionation during leaching of granite and basalt by hydrochloric and oxalic acids abstract transport of iron fe within hydrothermal and soil environments involves the transferral into aqueous solutions by leaching of complex polyminerallic rocks understanding the isotope fractionation mechanisms during this process is key for any application of the fe isotope system to biogeochemical studies here we reacted biotite granite and tholeiite basalt with 0 5 m hydrochloric acid and 5 mm oxalic acid solutions at ambient temperature solution aliquots were recovered over a seven day period and analysed for major and trace element concentrations and fe isotopic compositions in all experiments fe initially released into solution was isotopically lighter with 56fesolution rock as low as 1 80 in the granite hydrochloric acid system the oxalic acid experiments showed similar patterns but smaller fractionation in all experiments the 56fesolution rock reduced over time which would be in line with the formation of a leached layer as proposed before brantley s l liermann l j guynn r l anbar a icopini g a and barling j 2004 fe isotopic fractionation during mineral dissolution with and without bacteria geochim cosmochim acta 68 15 3189 3204 granite and basalts reacting with hydrochloric acid reached apparent steady state values of 0 60 0 15 and 0 40 0 20 respectively whilst experimental values with oxalic acid were 1 0 0 15 and 0 50 0 15 during the granite experiments alteration of biotite to chlorite followed by dissolution of chlorite were likely the dominant processes whilst in the basalt experiments dissolution of pigeonite was likely the principal source of fe variations in ph during the hydrochloric acid experiments were minimal remaining below 0 5 at all times in oxalic acid solutions the ph increased to over 4 leading likely to precipitation of secondary minerals and adsorption co precipitation of fe onto mineral surfaces these processes could contribute to the greater fractionation observed in the final stages of the oxalic acid experiments our results highlight the importance of mineralogy and fluid composition on the fe isotope systematics during weathering the fractionation processes identified for granites and basalts are in line with those inferred from field observations in soils sediments groundwater and hydrothermal deposits and from laboratory studies of single mineral leaching

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

topographic control of a dispersing hydrothermal plume deep sea hydrothermal vents represent a major source of heat and chemicals to the oceans and support endemic chemosynthetic biological communities to fully understand the impact of hydrothermal activity upon the oceans however requires investigation of both the physical and the biogeochemical processes which are active in hydrothermal plumes and which serve to determine the net hydrothermal flux to the oceans we have recently conducted a detailed multidisciplinary study of the lateral dispersion of the hydrothermal plume emitted from the rainbow vent site near 36 15 n mid atlantic ridge combining velocity measurements from a lowered adcp optical back scatter measurements from a deep tow ctd and methane measurements from bottle samples we are able for the first time in the atlantic to trace a neutrally buoyant plume for a distance of over 50 km the path of the plume is seen to be heavily controlled by the local topography with a general northeast movement of water both particle and methane concentrations decrease downstream over the length of the observed plume the dataset provides an excellent opportunity to study the mixing and biogeochemical processes active in a hydrothermal plume and estimate fluxes of biogeochemical constituents

redox changes in a seafloor hydrothermal system recorded in hematite chalcopyrite chimneys abstract hydrothermal chimneys composed of hematite and chalcopyrite grow at the irina ii vent site logatchev hydrothermal field mid atlantic ridge hematite and chalcopyrite form layers that repeatedly alternate within the chimney wall the repeating alternation of hematite and chalcopyrite implies for repeating changes in the physico chemical parameters of the vent fluid the modeled stability diagrams of hematite and chalcopyrite at the physico chemical conditions of the irina ii hydrothermal site showed that the precipitation of these two fe minerals is primarily governed by the activity of h2s and ph fe isotope studies suggest that the successive deposition of chalcopyrite 56 54fe 0 20 and hematite 56 54fe 0 60 layers in the irina ii chimneys is a result of redox changes in the hydrothermal fluid the replacement of hematite by either magnetite or chalcopyrite at some places also suggests for a redox inversion in the hydrothermal system from oxic to reduced chondrite normalized rare earth element distribution patterns of the hematite layers show a strong negative eu anomaly which is interpreted to be a result of crystal chemical control on the rare earth element fractionation during hematite precipitation sr isotope systematics indicate significant and variable incorporation of seawater from 40 to 80 in the hydrothermal system nd pb isotope studies show that nd and pb in chalcopyrite and hematite layers are derived mainly from the underlying mafic and ultramafic rocks the chimneys are young 5 13 1 28 kyrs b p and their wall thickened inward the duration of redox cycles oxic and reducing within the scale of a single chimney is about several hundred years the hematite layers are enriched in w in sn and u indicating that primary hematite hydrothermal deposits at the seafloor may be a potential source for these strategic trace elements high concentrations of these elements in the studied hematite layers can be explained by their easy ion substitution for fe3 in the hematite crystal lattice whereas the most plausible source of the elevated w in and sn contents are the hydrothermal fluids the seawater is the possible major source of u

distribution of rees and yttrium among major geochemical phases of marine fe mn oxides comparative study between hydrogenous and hydrothermal deposits abstract mineralogy and geochemistry of the ferromanganese oxides collected from two seamounts in the andaman sea have been investigated with special reference to the solid phase partitioning behavior of rare earth elements rees bulk elemental compositions indicate that the oxides from a volcanic cratered seamount csm are hydrothermal while those from another seamount sm2 are hydrogenous in origin the distributions of rees and yttrium in four geochemical phases viz easily exchangeable mn oxide fe oxyhydroxide and residual of csm oxides were distinctly different from that of sm2 oxides in csm oxides the rees and yttrium are mainly associated with fe oxyhydroxide phase while in sm2 oxides both mn oxide and fe oxyhydroxide phases contain the major fraction of these elements the exchangeable and residual phases of both hydrothermal and hydrogenous oxides have extremely low ree content the shale normalized negative ce anomalies and relative enrichment of heavy ree characteristic of hydrothermal oxides are retained in fe oxyhydroxide phase in hydrogenous oxides the positive ce anomalies occurred in both fe and mn oxide phases while negative y anomalies were found only in fe oxyhydroxide phase the phase distribution of rees and yttrium in hydrothermal oxides is described in terms of co precipitation and adsorption of their halide complexes and available free ions in hydrothermal solutions instead adsorption of major carbonate species from seawater primarily regulates the partitioning of rees among the co existing phases of hydrogenous oxides

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

the effect of plume processes on the fe isotope composition of hydrothermally derived fe in the deep ocean as inferred from the rainbow vent site mid atlantic ridge 36 14 n abstract the rainbow hydrothermal vent site which is the largest known point source for dissolved fe delivered to the deep north atlantic ocean has remained invariant in its fe isotope composition over at least the past 16 000 years based on analysis of metalliferous sediments beneath the plume because of the conservative behavior of fe in the rainbow plume 56 fe values of particles in the neutrally buoyant plume 0 18 0 05 and underlying sediments 0 19 0 05 are indistinguishable from the 56 fe values of the high temperature fluid sources 0 23 0 04 particles from the near vent buoyant stage of the plume however have higher 56 fe values 0 15 to 1 20 relative to the original vent fluid consistent with fractionation during oxidation of fe ii aq to fe iii aq isotope compositions become invariant in the plume once all fe ii aq is fully oxidized preserving the original composition of the vent fluid the constant fe isotope compositions of the vent fluids over time implies that changes in seawater fe isotope composition of the north atlantic ocean as they are recorded in fe mn crusts requires changes in the relative fluxes of fe to the ocean

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