detachment faulting mantle serpentinization and serpentinite mud volcanism beneath the porcupine basin southwest of ireland the porcupine basin southwest of ireland provides an opportunity to study the symmetry of rifting at stretching factors approaching continental breakup profiles across the basin image a bright reflection that appears to represent a detachment fault and may in part be a decollement at the top of partially serpentinized mantle although overall the basin appears symmetric the consistent westward structural dip of the detachment implies that at high stretching factors extension was asymmetric farther south the porcupine median high appears in cross section to be a triangular construction overlying tilted fault blocks and onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

highly alkaline high temperature hydrothermal fluids in the early archean ocean abstract based on the petrology of hydrothermally altered archean basaltic greenstones thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high temperature 350 c hydrothermal fluid in an archean subseafloor basalt hosted hydrothermal system the results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high co2 condition and predict a generation of sio2 rich fe poor hydrothermal fluids in the archean subseafloor hydrothermal system the chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic neutral seawater are characterized by inverse ph and chemical polarity to modern hydrothermal systems leading to extensive precipitation of silica and iron oxyhydroxides on under the seafloor such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike the mechanism of silicification and the ph controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the archean ocean such high temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life

non transform offsets along the mid atlantic ridge south of the azores 38 n 34 n ultramafic exposures and hosting of hydrothermal vents ten contiguous non transform offsets ntos along the mid atlantic ridge mar south of the azores between 38 n and 35 40 n have been studied in detail using swath bathymetric acoustic backscatter and deep tow high resolution sidescan sonar tobi data in contrast with discontinuities studied elsewhere at slow spreading ridges these left lateral ntos are consistently broader and larger with complex structural fabrics accommodating the offset they are characterized by a range of elevated and faulted massifs detached from their segment flanks with an irregular acoustic backscatter pattern some of these massifs have been explored and sampled recently during dive cruises revealing that they are composed of upper mantle peridotites and lower crustal rocks and sometimes associated with high temperature hydrothermal venting water column surveys adjacent to these massifs show high ch4 and low tdm total dissolvable manganese concentrations possibly resulting from the process of serpentinization of ultramafic rocks the correlation between the shallow dome like shaped massifs and the high concentrations of ch4 associated with low levels of mn is of particular interest to predict the outcrop of ultramafic rocks within the ntos where no geological data are available the exposure of the ultramafic massifs within the ntos is favored by low magmatic supply and low angle detachment faulting occurring at segment ends the pervasive fracturing and faulting at these discontinuities favor circulation of hydrothermal fluids and occurrence of high temperature vent sites

geochemical constraints on the modes of carbonate precipitation in peridotites from the logatchev hydrothermal vent field and gakkel ridge the ultramafic hosted logatchev hydrothermal field lhf at 15 n on the mid atlantic ridge and the arctic gakkel ridge gr feature carbonate precipitates aragonite calcite and dolomite in voids and fractures within different types of host rocks we present chemical and sr isotopic compositions of these different carbonates to examine the conditions that led to their formation our data reveal that different processes have led to the precipitation of carbonates in the various settings seawater like 87sr 86sr ratios for aragonite in serpentinites 0 70909 to 0 70917 from the lhf are similar to those of aragonite from the gr 0 70912 to 0 70917 and indicate aragonite precipitation from seawater at ambient conditions at both sites aragonite veins in sulfide breccias from lhf also have seawater like sr isotope compositions 0 70909 to 0 70915 however their rare earth element ree patterns show a clear positive europium eu anomaly indicative of a small 1 hydrothermal contribution in contrast to aragonite dolomite from the lhf has precipitated at much higher temperatures 100 c and yet its 87sr 86sr ratios 0 70896 to 0 70907 are only slightly lower than those of aragonite even higher temperatures are calculated for the precipitation of deformed calcite veins in serpentine talc fault schists form north of the lhf these calcites show unradiogenic 87sr 86sr ratios 0 70460 to 0 70499 indicative of precipitation from evolved hydrothermal fluids a simple mixing model based on sr mass balance and enthalpy conservation indicates strongly variable conditions of fluid mixing and heat transfers involved in carbonate formation dolomite precipitated from a mixture of 97 seawater and 3 hydrothermal fluid that should have had a temperature of approximately 14 c assuming that no heat was transferred the much higher apparent precipitation temperatures based on oxygen isotopes 100 c may be indicative of conductive heating probably of seawater prior to mixing the hydrothermal calcite in the fault schist has precipitated from a mixture of 67 hydrothermal fluid and 33 seawater which should have had an isenthalpic mixing temperature of 250 c the significantly lower temperatures calculated from oxygen isotopes are likely due to conductive cooling of hydrothermal fluid discharging along faults rare earth element patterns corroborate the results of the mixing model since the hydrothermal calcite which formed from waters with the greatest hydrothermal contribution has ree patterns that closely resemble those of vent fluids from the lhf our results demonstrate for the first time that 1 precipitation from pure seawater 2 conductive heating of seawater and 3 conductive cooling of hydrothermal fluids in the sub seafloor all can lead to carbonate precipitation within a single ultramafic hosted hydrothermal system

simultaneous precipitation of magnesite and lizardite from hydrothermal alteration of olivine under high carbonate alkalinity the present study reports original experiments in order to investigate the simultaneous serpentinization and carbonation of olivine with relevance in earth systems e g functioning of hydrothermal fields or in engineered systems e g ex situ and in situ mineral sequestration of co2 for this case specific experimental conditions were examined 200 c saturated vapor pressure 16bar solution solid weight ratio 15 olivine grain size 30 m and high carbonate alkalinity 1m nahco3 under these conditions competitive precipitation of magnesite and serpentine preferentially lizardite type were clearly determined by using conventional analytic tools xrd fesem ftir and tga excluding the fate of the iron initially contained in olivine the alteration reaction for olivine under high carbonate alkalinity can be expressed as follows 2 mg 2 sio 4 2h 2 o h co 3 mg co 3 mg 3 si 2 o 5 oh 4 oh this reaction mechanism implied a dissolution process releasing mg and si ions into solution until supersaturation of solution with respect to magnesite and or serpentine the released iron contained in the olivine has not implied any precipitation of iron oxides or oxy hydroxides in fact the released iron was partially oxidized about 50 via a simple reduction of water 2 fe 2 2h 2 o 2 fe 3 h 2 2 oh in this way the released iron was incorporated in serpentine fe ii and fe iii and in magnesite fe ii this latter was clearly determined by fesem eds chemical analysis on the single magnesite crystals the nucleation and epitaxial growth processes at the olivine fluid interfaces cannot be excluded in our investigated system the experimental kinetic data fitted by using a kinetic pseudo second order model have revealed a retarding process of serpentine formation with respect to magnesite about three times slower in fact the magnesite seems to reach an apparent stabilization after about 20 days of reaction while the serpentine follows a progressive slower evolution we assumed that the magnesite has reached a fast apparent equilibrium with solution because the available carbonate species are not renewed from fluid phase as typically constrained in aqueous carbonation experiments where a given co2 pressure is imposed in the system on the other hand the reactivity of serpentinized olivine chrysotile brucite small amount of residual olivine and high purity chrysotile at the same above investigated conditions and the olivine serpentinization in initial acid ph 0 66 are also reported as complementary information in this study these novel experimental results concerning simultaneous serpentinization and aqueous carbonation of olivine expand the thermodynamic conditions where serpentine and magnesite can simultaneously precipitate this could contribute to a better understanding of fluid rock interactions in natural active hydrothermal fields on earth

extensional deformation with northwest vergence pioneer core complex central idaho the pioneer mountains of idaho contain a detachment fault that separates a lower plate core of precambrian and ordovician metasedimentary rocks and cretaceous and eocene plutonic bodies from a surrounding upper plate of paleozoic sedimentary and tertiary sedimentary and volcanic units the detachment system developed during tertiary extension and it overprinted paleozoic and mesozoic east directed compressional features beneath the detachment both brittle and ductile mylonitic deformation can be observed trends of stretching lineations in the mylonite and striations along the detachment surface both cluster around n65 w composite planar fabrics s and c surfaces in the mylonite and development of the mylonite zone on the northwest side of the core both suggest a top to the west sense of shear both brittle and ductile deformation affect the eocene pluton of the core therefore the deformation must be eocene or younger displacement is estimated at about 17 km the extension direction in the pioneer complex correlates with that of other core complexes in the region north and south of the snake river plain this suggests that the direction of extension in core complexes was controlled by an inherited structural framework which was crustal in scale

fossil clams from a serpentinite hosted sedimented vent field near the active smoker complex rainbow mar 36 13 n insight into the biogeography of vent fauna hydrothermal circulation at ultramafic hosted sites supports a large variety of high and low temperature hydrothermal vents and associated ecosystems the discovery of abundant fossil vesicomyid and thyasirid shell accumulations at the ridge crest approximately 2 5 km east of the active rainbow vent field on the mid atlantic ridge mar 36 degrees 13 n increased our knowledge regarding the diversity of vent communities at slow spreading ridges bivalve molluscs of the family vesicomyidae were represented by the genus phreagena here we present the first record of this genus in the atlantic ocean this second vesicomyid species known from the mar phreagena sp was found to be associated with a thyasira species that is affiliated with t southwardae at the logatchev vent field on the mar and with t vulcolutre in the gulf of cadiz these two clams have close relationships with seep taxa along the continental margin and were likely associated with sedimented vent fields delta o 18 and delta c 13 analyses of the shells suggested that the burrowing bivalve thyasira could incorporate isotopically light carbon derived from the oxidation of methane in the sediment while the signature of phreagena sp shells denoted a different carbonate source c 14 dating of the shells denoted that the hydrothermal activity in the rainbow area began at least similar to 25 5 kyr bp which is similar to the model of the hydrothermal vent field distribution that was proposed for the logatchev hydrothermal site the results provide new insight regarding the diversity of chemosynthetic fauna on the mar over geologic time ultramafic hosted on axis sedimented vent fields extend the range of habitats for chemosynthetic communities underlying the need to further explore the geology of these types of environments on slow spreading ridges and to determine their role in the ecology of deep sea vent communities

sr isotope and major ion compositional evidence for formation of qarhan salt lake western china abstract qarhan salt lake qsl is the largest potash brine deposit in the continental qaidam basin qb of western china the origins of the qarhan brines whether from migration of qaidam mega paleolakes paleoriver capture or mixing of modern river water and ca cl spring water are controversial details about the origin hydrothermal diagenetic or both of the ca cl inflow waters entering qsl in the qb are unclear here we show from the strontium sr isotope ratios of river waters intercrystalline brines spring waters halite gypsum and corbicula shells from qarhan playa that 87 sr 86 sr increases from bieletan western qsl to huobuxun eastern qsl this trend verifies that river water spring water mixing has been operating at qsl for the past 50 000 years contributions of ca cl inflow waters to the qb have profoundly influenced the brine evolution and mineralogy of salt deposits mixing between low sr 2 concentration low 87 sr 86 sr value of river waters and high sr 2 concentration high 87 sr 86 sr value of ca cl spring inflow waters has produced various intercrystalline brines and salt deposits at qsl ca cl inflow waters and brines are enriched in sr 2 and ca 2 and depleted in mg 2 and so 4 2 these characteristics suggest brine sedimentary rock silicate interactions at burial diagenetic or hydrothermal conditions ca cl inflow waters and brines have a similar sr isotopic range as oilfield brines and celestite deposits in the qb which also suggests a diagenetic hydrothermal origin ca cl brines discharge as springs or seeps along fault zones in the qb they are interpreted to reach the surface of the northern margin of qarhan playa by topographically driven circulation

environmental controls on mid ocean ridge hydrothermal fluxes abstract high temperature hydrothermal fluxes at mid ocean ridges are thought to be an important component of oceanic biogeochemical cycles however little consideration has been given to how these fluxes vary as a consequence of changing environmental conditions over earth history here we consider how changes in sea level and ocean chemistry are likely to have impacted on axis high temperature hydrothermal fluxes focusing on phanerozoic conditions changes in sea level lead to changes in hydrostatic pressure near the base of hydrothermal systems where both peak fluid rock reaction temperatures and phase separation occur in general higher global sea level will lead to higher peak temperatures of fluid rock reaction additionally phase separation at higher pressure tends to lead to formation of a more cl rich vapor that constitutes a larger mass fraction of the system these combined factors may serve to significantly modify hydrothermal fluxes even for sea level changes on the scale of 100 m changes in ocean chemistry can also affect axial hydrothermal fluxes in several ways seawater sulfate contents control the amount of anhydrite that forms which has both physical porosity filling and chemical effects the most important aspect of ocean chemistry in controlling the composition of high temperature vent fluids may be ocean salinity if evaporite formation and dissolution has changed ocean salinity substantially over the phanerozoic hydrothermal fluxes could have been greatly modified ocean chemistry also plays a large role in controlling processes operating in hydrothermal plumes and hence the net flux of elements into and out of the ocean associated with hydrothermal systems we conclude that there is a need for substantial further work to quantify the effects of sea level and ocean chemistry on high temperature hydrothermal fluxes including the development of more robust models that integrate field laboratory and theoretical observations

organic synthesis during fluid mixing in hydrothermal systems hydrothermal circulation can lead to fluid mixing on any planet with liquid water and a source of heat aqueous fluids with differing compositions especially different oxidation states are likely to be far from thermodynamic equilibrium when they mix and provide a source of free energy that can drive organic synthesis from co 2 and h 2 and or supply a source of geochemical energy to chemolithoautotrophic organisms results are presented that quantify the potential for organic synthesis during unbuffered fluid mixing in present submarine hydrothermal svstems as well as hypothetical systems that may have existed on the early earth and mars dissolved hydrogen present in submarine hydrothermal fluids owing to the high temperature reduction of h 2 o as seawater reacts with oceanic crustal rocks provides the reduction potential and the thermodynamic drive for organic synthesis from co 2 or bicarbonate as hydrothermal fluids mix with seawater the potential for organic synthesis is a strong function of the h 2 content of the hydrothermal fluid which is in turn a function of the prevailing oxidation state controlled by the composition of the rock that hosts the hydrothermal system hydrothermal fluids with initial oxidation states at or below those set by the fayalite magnetite quartz mineral assemblage show the greatest potential for driving organic synthesis these calculations show that it is thermodynamically possible for 100 of the carbon in the mixed fluid to be reduced to a mixture of carboxylic acids alcohols and ketones in the range 250 50 c as cold seawater mixes with the hydrothermal fluid as the temperature drops larger organic molecules are favored which implies that fluid mixing could drive the geochemical equivalent of a metaholic system this enormous reduction potential probably drives a large portion of the primary productivity around present seafloor hydrothermal vents and would have been present in hydrothermal systems on the early earth or mars the single largest control on the potential for organic synthesis is the composition of the rock that hosts the hydrothermal system

microbial utilization of abiogenic carbon and hydrogen in a serpentinite hosted system abstract mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling global geochemical cycles and microbial activity serpentinite hosted hydrothermal activity is exemplified by the lost city hydrothermal field 30 n mid atlantic ridge where fluid rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen methane c 2 alkanes and formate such systems have been proposed as possible analogs to the early earth environments that gave rise to the first biochemical pathways thus characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important up to 50 of the microbial biomass is synthesized from mantle carbon conversely mantle carbon contributes only 10 of the biomass in areas with minimal sulfate reduction we attribute this difference to greater incorporation of formate or methane by the dominant microbial species the lost city methanosarcinales in locations where sulfate reducers are able to facilitate this assimilation the ability of autotrophic communities at lost city to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

hydrogen generation from mantle source rocks in oman hydrogen gas associated with ca2 oh rich alkaline groundwaters ph 10 12 is at present emanating from ultramafic rocks of the oman ophiolite isotopic and chemical evidence indicates that hydrogen is formed by low temperature redox reactions in a closed groundwater environment this is normally a cryptic process in the hydrosphere but is fortuitously revealed by the unusual hydrogeological conditions in oman where atmospheric oxygen is totally assimilated it is suggested that hydrogen generation in mantle source rocks at depth and in the early archaean surface environment may be more widespread than has hitherto been realised

reduction of co2 during serpentinization of olivine at 300 c and 500 bar co 2 reduction processes occurring during experimental serpentinization of olivine at 300 c and 500 bar confirm that ultramafic rocks can play an important role in the generation of abiogenic hydrocarbon gas data reveal that conversion of fe ii in olivine to fe iii in magnetite during serpentinization leads to production of h 2 and conversion of dissolved co 2 to reduced c species including methane ethane propane and an amorphous carbonaceous phase hydrocarbon gases generated in the process fit a schulz flory distribution consistent with catalysis by mineral reactants or products magnetite is inferred to be the catalyst for methanization during serpentinization because it has been previously shown to accelerate fischer tropsch synthesis of methane in industrial applications involving mixtures of h 2 and co 2 the carbonaceous phase was predominantly aliphatic but had a significant aromatic component although this phase should ultimately be converted to hydrocarbon gases and graphite if full thermodynamic equilibrium were established its formation in these experiments indicates that the pathway for reduction of co 2 during serpentinization processes is complex and involves a series of metastable intermediates

multiple origins of methane at the lost city hydrothermal field abstract the high concentrations of methane in the vent fluids of the lost city hydrothermal field represent the sum of abiotic and biological sources and sinks stable isotopes of carbon are of limited value in discriminating between the various sources of methane because the isotope effects associated with the multiple processes forming and consuming methane are each poorly constrained and the products of these processes are pooled furthermore reservoir effects complicate interpretation the near quantitative reduction of inorganic carbon to methane under highly reducing conditions limits the isotope effects associated with methanogenesis however the carbon isotope compositions of lipids derived from anaerobic methanotrophs suggest that more than one isotopically distinct pool of methane exists at lost city in this analysis we integrate multiple lines of evidence to constrain the relative contribution of various processes at lost city the processes that we consider here include i fischer tropsch type ftt abiotic synthesis of methane and other hydrocarbons ii the sabatier process for the abiotic synthesis of methane alone iii biological methane production by methanosarcinales and iv biological methane consumption by anaerobic and aerobic methanotrophs this analysis suggests that abiotic processes particularly the sabatier reaction are likely to be the dominant source of methane at lost city biological methane is present in the vent fluids but does not compose a high fraction of the total methane pool these observations imply that ultramafic systems could have supplied abundant reduced carbon to the early earth even without biological catalysis

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 1 the results of detailed textural mineral chemical and petrophysical studies shed new light on the poorly constrained fluid rock reaction pathways during retrograde serpentinization at mid ocean ridges uniformly depleted harzburgites and dunites from the mid atlantic ridge at 15 n show variable extents of static serpentinization they reveal a simple sequence of reactions serpentinization of olivine and development of a typical mesh texture with serpentine brucite mesh rims followed by replacement of olivine mesh centers by serpentine and brucite the serpentine mesh rims on relic olivine are devoid of magnetite conversely domains in the rock that are completely serpentinized show abundant magnetite we propose that low fluid flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open system conditions and formation of magnetite by the breakdown of ferroan brucite modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks citation bach w h paulick c j garrido b ildefonse w p meurer and s e humphris 2006 unraveling the sequence of serpentinization reactions petrography mineral chemistry and petrophysics of serpentinites from mar 15 n odp leg 209 site 1274 geophys res lett 33 l13306

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

downward continued multichannel seismic refraction analysis of atlantis massif oceanic core complex 30 n mid atlantic ridge 1 detailed seismic refraction results show striking lateral and vertical variability of velocity structure within the atlantis massif oceanic core complex occ contrasting notably with its conjugate ridge flank multichannel seismic mcs data are downward continued using the synthetic on bottom experiment sobe method providing unprecedented detail in tomographic models of the p wave velocity structure to subseafloor depths of up to 1 5 km velocities can vary up to 3 km s over several hundred meters and unusually high velocities 5 km s are found immediately beneath the seafloor in key regions correlation within situand dredged rock samples video and records from submersible dives and a 1 415 km drill core allow us to infer dominant lithologies a high velocity body ies found to shoal near to the seafloor in multiple locations is interpreted as gabbro and is displaced along isochrons within the occ indicating a propagating magmatic source as the origin for this pluton s the western two thirds of the southern ridge is capped in serpentinite that may extend nearly to the base of our ray coverage the distribution of inferred serpentinite indicates that the gabbroic pluton s was emplaced into a dominantly peridotitic host rock presumably the mantle host rock was later altered via seawater penetration along the detachment zone which controlled development of the occ the asymmetric distribution of seismic velocities and morphology of atlantis massif are consistent with a detachment fault with a component of dip to the southeast the lowest velocities observed atop the eastern central dome and conjugate crust are most likely volcanics here an updated model of the magmatic and extensional faulting processes at atlantis massif is deduced from the seismic results contributing more generally to understanding the processes controlling the formation of heterogeneous lithosphere at slow rate spreading centers

habitat characterization of the vema fracture zone and puerto rico trench abstract although many of the regions on and close to the mid ocean ridges have been extensively mapped and sampled the abyssal intraplate regions remain essentially unsampled and unmapped leaving huge gaps in our understanding of their geologic history and present activity prominent bathymetric features in these intraplate regions are fracture zones here we present bathymetric and sampling information from a transatlantic transect along the vema fracture zone ca 11 n covering crustal ages from 109 0 ma on the african plate and 0 62 ma on the south american plate the vema fracture zone is the intraplate trace of the active vema transform plate boundary which offsets the present day mid atlantic ridge by ca 300 km left laterally juxtaposing zero age crust with crust of 20 million years age our results show clear evidence of tectonic activity along most of the fracture zone in most places likely associated with active fluid flow within the active vema transform at crustal ages of ca 10 ma we found clear indications of fluid flow both in the sediments and the overlying water column this region is 120 km from the nearest spreading axis and increases by almost an order of magnitude the maximum off axis distance that active hydrothermal discharge has been found on the oceanic crust sampling of the igneous seafloor was possible at all crustal ages and the accretionary fabric imprinted on the plate during its production was prominent everywhere seafloor sediments show signs of extensive bioturbation in one area high concentrations of spherical mn nodules were also found and sampled at the end of the transect we also mapped and sampled the puerto rico trough a 8000 m deep basin north of the caribbean arc here the seafloor morphology is more complicated and strongly influenced by transpressive tectonics

influence of sea floor spreading on the global hydrothermal vent fauna one remarkable discovery of recent decades is the presence of hundreds of unusual species including fourteen new families at hydrothermal vents these animals unknown from other habitats live in extreme chemical and thermal conditions around vents on spreading centres of the mid ocean ridges and back arc basins chemosynthesis provides an in situ energy source for the thriving vent fauna this habitat has existed through the phanerozoic1 2 and probably since the archaean thus providing sites for long term adaptation we now test the hypothesis that animal distribution among hydrothermal vents is related to tectonic plate history3 4 the predominant migration pathway is most likely to occur along mid ocean ridges rather than by shortest oceanic routes similarity analyses suggest that the distribution patterns of today s vent fauna display the strong imprint of the timing and geometry of ancient plate boundaries study of past ridge geometry provides a method to predict relationships among vent communities yet to be discovered

geologic implications of seawater circulation through peridotite exposed at slow spreading mid ocean ridges peridotite denuded by tectonic extension and exposed at the seafloor adjacent to slow spreading centers hosts hydrothermal circulation of seawater the reaction of seawater with peridotite causes serpentinization which generates a high ph strongly reducing fluid rich in methane and hydrogen and is accompanied by as much as 40 volume expansion complete serpentinization of peridotite requires tectonic activity to open fluid paths sealed by volume expansion diffuse venting of serpentinization fluids causes lithification of calcareous ooze on the seafloor to chalk like limestone this may be the main mechanism of deposition of ophicarbonates common in ophiolites the degree of induration is a function of the fluid flux through the sediment column calcareous ooze infiltrates faults and fractures and can be deformed following lithification focused venting of serpentinization fluids may lead to deposition of large chimneys composed of calcite aragonite and brucite such as those in the recently documented lost city vent field 30 n mid atlantic ridge geophysical implications of serpentinization include 1 creation of magnetic anomalies due to growth of magnetite in serpentinite and 2 lowered seismic velocity integrated studies of geologic and geophysical effects of serpentinization may aid in a more complete understanding of the structure of oceanic lithosphere and the mechanisms that expose mantle peridotite at the seafloor

geologic implications of seafloor character and carbonate lithification imaged on the domal core of atlantis massif abstract we document the seafloor character on atlantis massif an ocean core complex located at 30 n on the mid atlantic ridge with an emphasis on the distribution of carbonate features seafloor imagery near bottom backscatter and bathymetry were analyzed on the central dome and the western shoulder of the exposed footwall to the detachment and on the eastern block a hanging wall to the fault we merged argo ii still images to produce photo mosaics and evaluated these together with video imagery acoustic reflectivity and basic rock composition the seafloor was classified as unconsolidated sediment lithified carbonate crust consolidated carbonate cap exposed basement or rubble and the spatial distribution of each type was assessed unconsolidated sediment exposed basement and rubble were documented in all three regions studied lithified carbonate crust was also present on the western shoulder and eastern central dome consolidated carbonate cap was found on the eastern block the formation of the carbonate rock is interpreted to reflect precipitation and or sediment cementation via fluids derived from serpentinization both processes occur at the nearby lost city hydrothermal field the newly documented locations of seafloor carbonate lithification therefore mark pathways of past possibly recent fluid flux from subsurface water rock reaction zones and represent an additional constituent of the carbon cycling hosted by oceanic lithosphere

reactions between basalt and co2 rich seawater at 250 and 350 c 500bars implications for the co2 sequestration into the modern oceanic crust and the composition of hydrothermal vent fluid in the co2 rich early ocean abstract this study aims to understand how basaltic rocks absorb co 2 in high temperature alteration zones in the subseafloor and to reconstruct hydrothermal alteration processes such as carbonatization of archean greenstones to this end we conducted two laboratory experiments simulating hydrothermal reactions between basalt synthesized under quartz fayalite magnetite oxygen fugacity and co 2 rich nacl fluid ph 6 5 at 25 c at high temperature and pressure as the water rock reactions progressed at 250 c and 350 c 500 bars total carbonic acid concentration co 2 reduced from its initial 400 mmol kg to near 0 and 100 mmol kg respectively meanwhile calcite was formed in the basalt as an alteration mineral this indicates that calcite destabilizes as temperature increases in the h 2 o co 2 basalt system and that crustal basalts can absorb almost all co 2 in the fluid as calcite at least at temperatures and initial co 2 concentrations below 250 c and 400 mmol kg respectively although the second aim was realized in the experiments minerals such as sericite dolomite ankerite and siderite present in archean greenstones were not identified in the alteration products possibly because k mg and fe were lacking in the initial solutions steady state concentrations of sio 2 mg and k in the fluids during water rock reactions were similar to those of high temperature fluids 250 c in modern basalt hosted hydrothermal systems however the final experimental ph in situ was 6 6 and 7 2 at 250 c and 350 c respectively higher than that in modern hydrothermal fluids approximately 5 and higher than the neutral ph 5 5 5 6 at 250 350 c 500 bars the results suggest that the presence of abundant co 2 in the initial fluid induced carbonatization of basalt consequently ph was buffered by precipitation and dissolution of calcite because ph in situ was elevated the dissolved fe and mn concentrations in the fluid were two to three orders of magnitude lower than those of modern hydrothermal fluids in modern oceans high temperature hydrothermal vent fluids are the second largest iron source after riverine input however because alkaline metal poor hydrothermal fluids are generated in co 2 rich systems co 2 rich seafloor hydrothermal systems may have behaved as iron sinks in early oceans

fluid mixing and the deep biosphere of a fossil lost city type hydrothermal system at the iberia margin subseafloor mixing of reduced hydrothermal fluids with seawater is believed to provide the energy and substrates needed to support deep chemolithoautotrophic life in the hydrated oceanic mantle i e serpentinite however geosphere biosphere interactions in serpentinite hosted subseafloor mixing zones remain poorly constrained here we examine fossil microbial communities and fluid mixing processes in the subseafloor of a cretaceous lost city type hydrothermal system at the magma poor passive iberia margin ocean drilling program leg 149 hole 897d brucite calcite mineral assemblages precipitated from mixed fluids ca 65 m below the cretaceous paleo seafloor at temperatures of 31 7 4 3 c within steep chemical gradients between weathered carbonate rich serpentinite breccia and serpentinite mixing of oxidized seawater and strongly reducing hydrothermal fluid at moderate temperatures created conditions capable of supporting microbial activity dense microbial colonies are fossilized in brucite calcite veins that are strongly enriched in organic carbon up to 0 5 wt of the total carbon but depleted in 13c 13ctoc 19 4 we detected a combination of bacterial diether lipid biomarkers archaeol and archaeal tetraethers analogous to those found in carbonate chimneys at the active lost city hydrothermal field the exposure of mantle rocks to seawater during the breakup of pangaea fueled chemolithoautotrophic microbial communities at the iberia margin possibly before the onset of seafloor spreading lost city type serpentinization systems have been discovered at midocean ridges in forearc settings of subduction zones and at continental margins it appears that wherever they occur they can support microbial life even in deep subseafloor environments

molecular and isotopic analysis of anaerobic methane oxidizing communities in marine sediments convergent lines of molecular carbon isotopic and phylogenetic evidence have previously indicated hinrichs k u hayes j m sylva s p brewer p g delong e f 1999 methane consuming archaebacteria in marine sediments nature 398 802 805 that archaea are involved in the anaerobic oxidation of methane in sediments from the eel river basin offshore northern california now further studies of those same sediments and of sediments from a methane seep in the santa barbara basin have confirmed and extended those results mass spectrometric and chromatographic analyses of an authentic standard of sn 2 hydroxyarchaeol hydroxylated at c 3 in the sn 2 phytanyl moiety have confirmed our previous tentative identification of this compound but shown that the previously examined product was the mono tms rather than di tms derivative further analyses of 13 c depleted lipids appreciably more abundant in samples from the santa barbara basin have shown that the archaeal lipids are accompanied by two sets of products that are only slightly less depleted in 13 c these are additional glycerol ethers and fatty acids the alkyl substituents in the ethers mostly monoethers with some diethers are non isoprenoidal the carbon number distributions and isotopic compositions of the alkyl substituents and of the fatty acids are similar suggesting strongly that they are produced by the same organisms their structures n alkyl and methyl branched n alkyl require a bacterial rather than archaeal source the non isoprenoidal glycerol ethers are novel constituents in marine sediments but have been previously reported in thermophilic sulfate and nitrate reducing organisms which lie near the base of the rrna based phylogenetic tree based on previous observations that the anaerobic oxidation of methane involves a net transfer of electrons from methane to sulfate it appears likely that the non archaeal 13 c depleted lipids are products of one or more previously unknown sulfate reducing bacteria which grow syntrophically with the methane utilizing archaea their products account for 50 of the fatty acids in the sample from the santa barbara basin at all methane seep sites examined the preservation of aquatic products is apparently enhanced because the methane oxidizing consortium utilizes much of the sulfate that would otherwise be available for remineralization of materials from the water column

a serpentinite hosted ecosystem the lost city hydrothermal field the serpentinite hosted lost city hydrothermal field is a remarkable submarine ecosystem in which geological chemical and biological processes are intimately interlinked reactions between seawater and upper mantle peridotite produce methane and hydrogen rich fluids with temperatures ranging from 40 to 90 c at ph 9 to 11 and carbonate chimneys 30 to 60 meters tall a low diversity of microorganisms related to methane cycling archaea thrive in the warm porous interiors of the edifices macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the mid atlantic ridge but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems