Ridge Hydrothermal Systems Research Articles

Peptide Synthesis in Early Earth Hydrothermal Systems

We report here results from experiments and thermodynamic calculations that demonstrate a rapid, temperature-enhanced synthesis of oligopeptides from the condensation of aqueous glycine. Experiments were conducted in custom-made hydrothermal reactors, and organic compounds were characterized with ultraviolet-visible procedures. A comparison of peptide yields at 260 degrees C with those obtained at more moderate temperatures (160 degrees C) gives evidence of a significant (13 kJ . mol(-1)) exergonic shift. In contrast to previous hydrothermal studies, we demonstrate that peptide synthesis is favored in hydrothermal fluids and that rates of peptide hydrolysis are controlled by the stability of the parent amino acid, with a critical dependence on reactor surface composition. From our study, we predict that rapid recycling of product peptides from cool into near-supercritical fluids in mid-ocean ridge hydrothermal systems will enhance peptide chain elongation. It is anticipated that the abundant hydrothermal systems on early Earth could have provided a substantial source of biomolecules required for the origin of life.

High‐resolution surveys along the hot spot–affected Gálapagos Spreading Center: 3. Black smoker discoveries and the implications for geological controls on hydrothermal activity

To explore effects of hot spots on mid‐ocean ridge hydrothermal systems, we conducted nested sonar, hydrothermal plume, and near‐bottom photographic surveys along the portion of the Galápagos Spreading Center (GSC) influenced by the Galápagos hot spot, from longitude 95°–89.5°W. We report the first active high‐temperature black smokers to be found on the GSC, at longitudes 94°4.5′W and 91°56.2′–54.3′W; describe two areas of recently inactive smokers, at longitudes 91°23.4′–23.7′W and 91°13.8′W; and document an older inactive site, at longitude 90°33.4′W. All imaged vents issue either from dike‐induced fissures along linear axial volcanic ridges and collapses or from a caldera. Magmatic control of hydrothermal systems also is revealed by spatial clustering of plumes within the topographically elevated middles of volcanic ridge segments with inferred centralized melt supply. In searched areas, smokers are more typical than diffuse flow vents, but total GSC plume incidence is half of that expected from the spreading rate. Why? Dike‐fed fissures provide permeable pathways for efficient hydrothermal extraction of magmatic heat, but cones without calderas do not. Among many point‐source cones surveyed, only the two with calderas had detectable plumes. Possibly, dominance of point‐source over linear‐source melt delivery on the GSC decreases plume incidence. Also, similar maturities of observed vents and their host lava flows indicate that hydrothermally active volcanic segments along the western GSC are contemporaneously in a waning phase of volcanic‐hydrothermal activity. Perhaps ridge/hot spot interaction produces melt pulses that drive near‐synchronous volcanic‐hydrothermal activity on the volcanic segments spanning the hot spot. During active periods, hydrothermally active dike‐fed fissures and calderas may be more abundant than we currently observe.

The Structure and Dynamics of Mid-Ocean Ridge Hydrothermal Systems

Sub-seafloor hydrothermal convection at mid-ocean ridges transfers 25% of the Earth's heat flux and can form massive sulfide ore deposits. Their three-dimensional (3D) structure and transient dynamics are uncertain. Using 3D numerical simulations, we demonstrated that convection cells self-organize into pipelike upflow zones surrounded by narrow zones of focused and relatively warm downflow. This configuration ensures optimal heat transfer and efficient metal leaching for ore-deposit formation. Simulated fluid-residence times are as short as 3 years. The concentric flow geometry results from nonlinearities in fluid properties, and this may influence the behavior of other fluid-flow systems in Earth's crust.

Optical plume velocimetry: a new flow measurement technique for use in seafloor hydrothermal systems

Evidence suggests that fluid flow rates in mid-ocean ridge hydrothermal systems may be strongly influenced by mechanical forces such as ocean tidal loading. However, long time-series measurements of flow have not been collected in these environments. We develop a non-invasive method, called optical plume velocimetry (OPV), suitable for obtaining fluid flow rates through black smoker vents based on image analysis of effluent video. We use video from laboratory flows to evaluate three different methods for estimating the image-velocity field that are based on region-based matching, spectral-analysis of Hovmöller diagrams, and temporal cross-correlation of adjacent pixel values. We find that OPV is most sensitive and least biased when the cross-correlation method is used and conclude that OPV should not be applied to flows that are transitioning between jet-like and plume-like behavior.

Reconciling temperatures of metamorphism, fluid fluxes, and heat transport in the upper crust at intermediate to fast spreading mid‐ocean ridges

Current models of the mineralogical and chemical modification of the crust within mid‐ocean ridge hydrothermal systems at intermediate to fast spreading ridges are difficult to reconcile with the temperature distribution predicted by the required fluid fluxes. In particular, the sharp increase in metamorphic grade passing down from the lavas into the sheeted dike complex places important constraints on the flow of fluid through the crust. This distribution of metamorphic temperatures has typically been discussed in terms of a step in permeability in the downwelling limb of a single‐pass convection cell. Simple modeling of this system shows that downwelling fluid cools the crust too efficiently for high‐temperature metamorphism of the upper dikes to occur in the downflow zone. In fact, temperatures throughout most of the sheeted dike complex are too high. Three alternative models are considered. Fluid‐rock reaction during the cooling of each dike from magmatic temperatures is difficult to reconcile with cooling rate estimates for dikes, reaction rates required to grow metamorphic minerals, and the relatively constant composition of many hydrothermal vents. Metamorphism during reheating of the crust after fluid flow slows or stops is difficult to reconcile with the required fluid fluxes during metamorphism, based on the Sr‐isotopic composition of the sheeted dike complex, and cannot produce high enough temperatures in the upper dikes and low temperatures in the lower lavas. In contrast, fluid‐rock reaction in the sheeted dike complex as hot fluids migrate upward after being heated by the underlying magma source appears to be consistent with the data. In this model, downwelling seawater cools the recharge zone, and the fluid only becomes hot enough to significantly alter the crust once it approaches the conductive boundary layer overlying the magma chamber. Metamorphism of the sheeted dike complex then largely occurs in the discharge zone.

Two‐dimensional numerical models of open‐top hydrothermal convection at high Rayleigh and Nusselt numbers: Implications for mid‐ocean ridge hydrothermal circulation

Mid‐ocean ridges host vigorous hydrothermal systems that remove large quantities of heat from the oceanic crust. Inferred Nusselt numbers (Nu), which are the ratios of the total heat flux to the heat flux that would be transported by conduction alone, range from 8 to several hundred. Such vigorous convection is not fully described by most numerical models of hydrothermal circulation. A major difficulty arises at high Nu from the numerical solution of the temperature equation. To avoid classical numerical artifacts such as nonphysical oscillatory behavior and artificial diffusion, we implement the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) technique, which solves the temperature equation using an iterated upwind corrected scheme. We first validate the method by comparing results for models with uniform fluid properties in closed‐ and open‐top systems to existing solutions with Nu ≤ ∼20. We then incorporate realistic fluid properties and run models for Nu up to 50–60. Solutions are characterized by an unstable bottom thermal boundary layer where thermal instabilities arise locally. The pattern of heat extraction is periodic to chaotic. At any Nu > ∼13 the venting temperatures in a given plume are chaotic and oscillate from ∼350° to 450°C. Individual plumes can temporarily stop short of the surface for intervals ranging from tens to hundreds of years at times when other plumes vent with an increased flow rate. The solutions also display significant recirculation, and as a result large areas of downflow are relatively warm with temperatures commonly exceeding 150°C at middepths. Our results have important implications for mid‐ocean ridge hydrothermal systems and suggest the following: (1) The reaction zones of mid‐ocean ridge hydrothermal systems are enlarged by thermal instabilities that migrate laterally toward upflow zones. This will substantially increase the volume of rock involved in chemical reactions compared to steady state configurations. (2) Hydrothermal discharge can stop temporarily as zones of venting are dynamically replaced by zones of seawater recharge. (3) Anhydrite precipitation occurring at temperatures exceeding ∼150°C will likely occur throughout a large portion of recharge zone and will not necessarily clog downflow pathways as efficiently as has been recently inferred.

Phase separation and convection in heterogeneous porous media: Implications for seafloor hydrothermal systems

We examine experimentally the effect of phase separation of a binary fluid on convection and heat transfer in porous domains with heterogeneous permeability fields. In mid‐ocean ridge hydrothermal systems, a dense brine phase and a coexisting vapor phase form during the phase separation of seawater at supercritical conditions; this process strongly influences the chemistry of vents and the convective transfer of heat. Furthermore, convection and phase separation are likely to occur in oceanic crust that is highly heterogeneous in nature. Because of high temperatures and pressures required for phase separation in saline systems, a binary fluid [H2O‐2‐butoxyethanol], with a lower consolute point of 48.5°C, was used as a proxy for seawater. A saturated pseudo‐two‐dimensional porous domain containing high permeability [k] zones embedded in a lower permeability matrix was heated from below and cooled from above. As with homogeneous systems, chemical stratification develops in heterogeneous domains, with dense fluid accumulating in low‐velocity regions at the bottom of the porous matrix. Furthermore, in both types of porous media, the efficiency of thermal transport is reduced relative to single‐phase systems. Heterogeneities in the permeability field, however, can act to amplify these effects; in systems with vertical high‐k zones, the thickness of the dense‐phase layer increases in the low‐k regions, and thermal transport is suppressed even more. Also, the experiments show that regions of both high and low permeabilities can act as a store for brines and facilitate the formation of much thicker brine layers than are otherwise predicted by models based on homogeneous oceanic crust.

In situ measurement of dissolved chloride in high temperature hydrothermal fluids

The ability to continuously monitor chemical properties of hydrothermal vent effluents for extended periods of time is essential to understanding dynamic processes responsible for the temporally variable nature of mid-ocean ridge hydrothermal systems. Although instruments do exist for some parameters, there has been no sensor capable of measuring the chloride concentration, an indicator of possible phase separation, on a real-time and long-term basis. In this article, we discuss the construction of a novel instrument which measures solution resistance as a proxy for chloride concentration. The sensor consists of four gold electrodes embedded in a cylindrical ZrO 2 ceramic housing. It has been successfully deployed in several high temperature vents at the Main Endeavour Field (MEF) on the Juan de Fuca ridge in the NE Pacific, and calibrated under simulated hydrothermal conditions ranging up to 380 °C and 300 bar. The in situ data clearly demonstrate a tidal influence on the effluent from some high temperature vents possibly relating to a subsurface mixing process involving non-seawater end-members. Non-tidal changes are used to constrain the sequence and type of controls operating on fluids circulating within the subsurface.

Chemical composition of hydrothermal ores from Mid-Okinawa Trough and Suiyo Seamount determined by neutron activation analysis

Neutron activation analysis of 13 hydrothermal ore samples (70 subsamples) collected from the Mid-Okinawa Trough and Suiyo Seamount revealed higher contents of precious metal such as Au and Ag, and those of As, Sb, Ga, and Hg than those from mid-ocean ridge hydrothermal systems. In addition, the Mid-Okinawa Trough samples were richer in Ag and Sb than those from the Suiyo Seamount. The geochemical differences among these hydrothermal ore deposits are regarded as reflecting both differences in the chemical composition of the hosted magma of hydrothermal system and the abundance of sediments that is reacted with hydrothermal fluids.

Geology and metal contents of the Ruttan volcanogenic massive sulfide deposit, northern Manitoba, Canada

The Paleoproterozoic Ruttan Cu–Zn volcanogenic massive-sulfide (VMS) deposit is a large, relatively low grade, bimodal-siliciclastic type deposit in the Rusty Lake volcanic belt of northern Manitoba. The deposit contained over 82.8 million tonnes of massive sulfide, of which 55.7 million tonnes were mined from 1973 to 2002. The deposit consists of a series of moderately to steeply dipping, south-facing lenses that extend along strike at the surface for 1.1 km and to a depth of 1.0 km. These lenses occur within a steeply dipping, bimodal volcanic, volcaniclastic and siliciclastic sequence. In the immediate mine area, transitional calc-alkalic to high-silica (tholeiitic), felsic, and intermediate volcanic/volcaniclastic rocks of the Mine Sequence are host to, and intercalated with, the massive-sulfide lenses. Transitional tholeiitic to calc-alkalic basalt and andesite are present in the footwall sequence, approximately 500 m down-section from the ore horizon. The overlying rocks are predominantly fine-grained volcaniclastics and siliciclastics, but include polyfragmental agglomerate that contains mafic bombs and scoriaceous felsic fragments. Syn-depositional felsic and mafic dikes, sills, and apophyses are ubiquitous throughout the Mine Sequence, including the ore lenses, indicating continued, near-vent magmatism, and volcanism during ore formation. Fabrics in altered hostrocks have consistent, down-plunge stretching lineations to the SSE that suggest the deposit has been elongated by a factor of ~1.2–1.5; otherwise, the deposit is remarkably undeformed. Syn- and post-depositional faults in the mine area have relatively minor displacements up to tens of meters. Proximal (within 200 m) footwall rocks exhibit moderate to strong chloritization, characterized by the upper greenschist to lower amphibolite facies assemblages that include cordierite–almandine–andalusite–sillimanite–biotite ± staurolite ± anthophyllite ± talc, and local silicification. The proximal hanging wall rocks are characterized by sericite ± gahnite alteration, which is restricted to within approximately 75 m of the uppermost lenses. Additional gangue minerals are anhydrite and carbonate minerals (siderite, dolomite, ankerite, and calcite), as well as chlorite, sericite, biotite, talc, and quartz. Carbonate (excluding siderite), potassium feldspar, silicification and epidotization are common distal alteration zones in the footwall to the Mine Sequence several kilometers to the northeast. There are three principal groups of massive sulfide lenses; the East lenses, the West lenses, and the Western Anomaly lenses to the far west. In general, Cu is relatively enriched at the stratigraphic base and in the center of the deposit, whereas Zn is enriched upsection and at the outer margins. Some of the Zn-rich ore exhibits primary mineralogical layering. Parts of the West and Western Anomaly lenses show two layers with Cu-rich bases and Zn-rich tops. The massive sulfide is typically 10–40-m thick; one area along the margin of the main lenses is over 130-m thick and may represent deposition adjacent to a syn-depositional fault. The main sulfide phases are pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena, with tetrahedrite as the most abundant trace phase. Gahnite is ubiquitous in the chlorite-rich assemblages adjacent to the ore lenses. The average base, precious and trace metal contents estimated from Cu and Zn concentrates, and from millhead grades and recoveries. Metals easily transported as chloride and bisulfide complexes in hydrothermal fluids including: Pb, Ag, In, Cu, Cd, Au, and Zn are enriched by 1.5–2.5 orders of magnitude in comparison to the bulk continental crust. Other elements such as Sn, Mo, and As are at near-crustal concentrations, whereas Mn, Ga, and Co are significantly depleted in comparison to the crust. Calculated metal concentrations in the average hydrothermal fluid based on the average metal contents are comparable to, or higher than those measured at sediment covered ridge hydrothermal systems, which precipitate much of their metal budget in the subsurface. Average rare earth element contents for the sulfide are light rare earth element enriched (LaN/YbN=22) and range from 0.45 to 0.02x chondritic values, with a moderate negative Eu anomaly (Eu*=0.51). Metal and trace element contents in the Ruttan exhalite horizon, and in proximal (within 1–2 km) exhalites along strike from the 0.6 million tonne Dar-2 Cu–Zn deposit 12 km south of Ruttan, have positive Eu anomalies, whereas negative Eu anomalies are present at distance. The positive Eu anomalies reflect high temperature paleoseafloor hydrothermal venting and precipitation of Eu2+-enriched clays and possibly carbonates, and indicate proximity to base-metal deposits. Silver and lead are also enriched in the exhalites near the deposits, whereas Mn is enriched at ~1–3 km along strike, but not consistently.

An experimental analogue for convection and phase separation in hydrothermal systems

Experiments were conducted to explore the behavior of convection and heat transfer during phase separation in porous media. Phase separation is considered to be an important process in many mid‐ocean ridge hydrothermal systems in which a dense brine separates from a lighter, less saline vapor phase at supercritical temperatures. As this process occurs at high temperatures and pressures in seawater systems, a binary fluid (H2O‐2‐butoxyethanol) with a lower consolute point of 48.5°C, was used as a proxy for the hydrothermal fluid. In the experiments, a cell containing glass beads saturated with the binary fluid was heated from below and cooled from above. It was demonstrated that chemically differentiated regions can exist as part of a steady state convective regime, with the denser fluid phase separating and accumulating in a stagnant bottom layer. Furthermore, in upwelling regions phase separating fluids can become entrained. Phase separation was also found to lower the efficiency of convective thermal transfer, indicated by the deviation from the calculated single‐phase Rayleigh‐Nusselt curve. It is proposed that the reduction of convective heat transfer at supercritical conditions may be a crucial factor in controlling heat transfer through the oceanic crust.

Dynamics and storage of brine in mid‐ocean ridge hydrothermal systems

Mid‐ocean ridge hydrothermal systems are known to vent fluids with salinities substantially different from seawater as a result of phase separation and segregation of the resulting vapor and brine phases. Time series of vent temperature and salinity (chlorinity) show that some black‐smoker vent fields such as the Main Endeavour Field on the Juan de Fuca Ridge have vented fluids with salinities well below seawater for over a decade, which raises important questions concerning the fate of brines in these systems. One widely accepted model is that high‐density brines formed by supercritical phase separation sink to the base of hydrothermal systems, leading to the development of a two‐layer system in which a recirculating brine layer underlies a single‐pass seawater cell. We first present theoretical arguments to constrain the dynamics of such a deep brine layer in a system still undergoing phase separation, and we conclude that if brines are stored in a basal layer, they are unlikely to convect because they will be stably stratified. One consequence of this result is that the brine layer beneath black smoker systems has to be thin (<10 m) to match the high heat fluxes. However, estimates of the rate at which brines are accumulating in the crust below the main field on the Endeavour segment of the Juan de Fuca Ridge suggest that the brine layer is likely at least 100 m thick. To resolve this apparent paradox, we propose an alternative model. We argue that interfacial tensions between fluid and solid phases will likely favor the segregation of vapor into the main fractures and brine into the smaller fissures and backwaters. This allows the vapor to flow efficiently through the system and transport large heat fluxes while most of the porosity in the lower part of the system fills with brines. It is generally believed that the pressure gradients in mid‐ocean ridge hydrothermal systems are close to cold hydrostatic. At the high temperatures and pressures characteristic of the deeper parts of these systems, brines with salinities as high as 20 wt % NaCl have densities around 800–900 kg m−3 and will be buoyant in a cold‐hydrostatic system. Rather than sinking to the base of the system, it is possible that brines produced by supercritical phase separation rise slowly until they reach a level of neutral buoyancy as they cool or enter high‐permeability regions in which the pressure gradients decrease.

Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust

The mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems. We have investigated the role of zones of focussed fluid flow in the plutonic complex of the Oman ophiolite in the near-axis cooling of the oceanic crust. Lineaments identified on aerial photographs, that occur at 1 km spacing, show evidence for extensive hydrothermal fluid flow through regions 10 to 50 m wide. Fluid flow is initiated in these regions at 800°C and continues at least into the lower greenschist facies. Strontium-isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the Moho. Computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are 1x108 kgm2. Modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones. Heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the Sr-isotopic composition of samples from the focussed fluid flow zones. The combination of independent estimates of the fluid and heat fluxes, such as these, can provide more rigorous constraints on the thermal history than either approach used in isolation. Our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust. Significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid-ocean ridges. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass. Thus, if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought.

Geochemical controls on hydrothermal fluids from the Kairei and Edmond Vent Fields, 23°–25°S, Central Indian Ridge

Prior to 2000, no mid‐ocean ridge hydrothermal systems were identified and sampled in the Indian Ocean. The Kairei field, Central Indian Ridge, was discovered and sampled in 2000 (Gamo et al., 2001; Hashimoto et al., 2001). This paper reports the chemical composition of hydrothermal fluids collected in 2001 from both the Kairei and newly discovered Edmond hydrothermal systems. Data are used to infer subsurface processes and conditions at the two sites, as well as to place them within the global context of known hydrothermal sites on the mid‐ocean ridge system. While both sites are located on the intermediate spreading Central Indian Ridge, their hydrologic systems have affinities with those observed on slow spreading ridges. Although the compositions of fluids from the four individual vents sampled at Edmond and the three individual vents sampled at Kairei vary respectively, our interpretation is that a single source fluid at each site is subsequently modified by processes including phase separation, subsurface mixing with seawater, and deposition and/or remobilization of metal‐sulfide deposits to generate the observed range of compositions. Both fields are located ≥6 km from the neovolcanic zone, on steps on the east wall of the axial valley, with the Edmond field ∼160 km north of Kairei and almost 1000 m deeper (3300 versus 2450 m). The Edmond fluids are extremely hot, with maximum measured temperatures of 382°C compared to 365°C for Kairei. All of the fluids sampled have chlorinities greater than local ambient seawater due to phase separation of seawater at supercritical conditions. Visual observations suggest both sites have been active for a substantial period of time, again similar to observations of slow spreading ridges. Our chemical data suggest that the substrate underlying Kairei is highly altered, while that at Edmond is not. Significant albitization appears to be occurring below the Edmond field, but not at Kairei. The Edmond fluids are by far the hottest brines yet observed venting from a mid‐ocean ridge hydrothermal site, resulting in unusually high concentrations of several transition metals.

The dynamics of mid-ocean ridge hydrothermal systems: Splitting plumes and fluctuating vent temperatures

We present new, accurate numerical simulations of 2D models resembling hydrothermal systems active in the high-permeability axial plane of mid-ocean ridges and show that fluid flow patterns are much more irregular and convection much more unstable than reported in previous simulation studies. First, we observe the splitting of hot, rising plumes. This phenomenon is caused by the viscous instability at the interface between hot, low-viscosity fluid and cold, high-viscosity fluid. This process, known as Taylor–Saffman fingering could potentially explain the sudden extinguishing of black smokers. Second, our simulations show that for relatively moderate permeabilities, convection is unsteady resulting in transiently varying vent temperatures. The amplitude of these fluctuations typically is 40 °C with a period of decades or less, depending on the permeability. Although externally imposed events such as dike injections are possible mechanisms, they are not required to explain temperature variations observed in natural systems. Our results also offer a simple explanation of how seismic events cause fluctuating temperatures: Earthquake-induced permeability-increase shifts the hydrothermal system to the unsteady regime with accompanying fluctuating vent temperatures. We demonstrate that realistic modelling of these high-Rayleigh number convection systems does not only require the use of real fluid properties, but also the use of higher order numerical methods capable of handling high-resolution meshes. Less accurate numerical solutions smear out sharp advection fronts and thereby artificially stabilize the system.

ESTUDO DE INCLUSÕES FLUIDAS EM MINERALIZAÇÃO SULFETADA VULCANOGÊNICA HIDROTERMAL E DE OURO OROGÊNICO NA SERRA DO IPITINGA, AMAZÔNIA, BRASIL

The Ipitinga Group (2.27 Ga) at the Serra do Ipitinga consists of a meta-volcano-sedimentaryunit of mafic and ultramafic metavo1canics, cordierite-antophyllite-and quartzo-chlorite-bearing rocks under1ying chemical (oxide and silicatetype BIFs) and clastic metasedimentary rocks. Part of the mafic and ultramafic lavas was hydrothermally altered, which gave rise to quartz-chlorite-bearing rocks and hydrothermal syndepositional exhalative vo1canogenic sulfide mineralization, formed by pyrrhotite, pyrite, chalcopyrite and sphalerite with Au and Ag and traces of galena , molibdenite and silver telluride. Ali the meta-volcano-sedimentary unit was deformed by ductile-brittle shearing and transformed in present-day NW-SE trending shear belts with a subvertical - NE dip. During this defomational shearing episode, hydrothermal fluids yielded gold-sheared Iodes. Twelve types of fluid inclusions identified in the Ipitinga Group distinguish two fluid systems related to three peculiar hydrothermal events. The quartz of the quartz-chlorite-bearing roeks (hydrothermally - altered basalts), the main host of the volcanogenic sulfide mineralization, eontains eleven types of fluid inclusions. Eight of these oceur only in these rocks, which can be considered to have resulted from fluid-rock reactions related to the first hydrothermal event (1). This event involved an aqueous earbonic fluid consisting mainly of CH 4 and Hp, with traces of N 2 and H 2 S. CO 2 -bearing inclusions are subordinate and contain traces of CH 4 , H 2 S and N 2. The predominance of CH 4 over CO 2 in the aqueous earbonic fluid system is coherent to proposed models for the generation of exhalative volcanogenic sulfide mineralization in mid - oceanic ridge hydrothermal systems. The presence of CH 4 in the first hydrothermal event in the Ipitinga Group is evidence of an ocean-floor metarnorphic-hydrotherrnal process, in an environment similar to present-day ocean spreading eenters. Pressure and temperature values for the sulfide mineralization were considered to be in the range of 0.7 - 2.3 kbar and 250 - 450°C, respectively. Aqueous saline fluid related to the second hidrothermal event (II) oceurs only in the cordierite-anthophyllite rocks. Quartz veins are the products of a third hydrothermal event related to duetile shearing (III) and contain inclusions of an aqueous saline fluid which can also be found in quartz of the quartz-chlorite-bearing roeks. The fluid inelusion study also pointed out the relative chronology for the hydrothermal-metarnorphic events in the Ipitinga Group.

The Escanaba Trough, Gorda Ridge hydrothermal system: Temporal stability and subseafloor complexity

The composition and temperature of vent fluids sampled from the active hydrothermal system in Escanaba Trough, Gorda Ridge in 2000 and 2002 remain unchanged from the only time this field was previously sampled, in 1988. ODP Leg 169 drilled nine bore holes at this site in 1996, some within meters of the vents, yet this disturbance has not impacted the measured compositions or temperatures of the fluids exiting at the seafloor. The fluids have maximum measured temperatures of 218°C and contain ∼20% more chloride than local ambient seawater. Our interpretation is that the fluid compositions are generated by supercritical phase separation of seawater, with much of the water-rock reaction occurring within the ∼400m thick sedimentary section that overlies the basalt at this site. The ODP drilling results provide information on the mineralogy and composition of materials below the seafloor, as well as direct constraints not typically available on the physical conditions occurring below the seafloor hydrothermal system. Calculations utilizing geochemical modeling software suggest the fluids are close to saturation with a suite of minerals found subsurface, suggesting equilibrium between the fluids and substrate. These results provide an explanation for why the fluids have remained chemically stable for 14 yrs. The pore water data from drilling suggest that the hydrology and chemistry of the hydrothermal system are much more complex within the sediment cover than would be expected from the surface manifestations of the hydrothermal system. While the pore waters have chloride contents both greater and less than the local seawater, only fluids with higher chloride contents vent at the seafloor. Our calculations suggest that at the current conditions the “brines” (fluids with chlorinity greater than seawater) are actually less dense than the “vapors” (fluids with chlorinity less than seawater). These density relationships may provide an explanation for why the “brines” are now venting preferentially to the “vapors,” a situation opposite to what is usually observed or inferred.

Modeling the effects of tidal loading on mid‐ocean ridge hydrothermal systems

Tidal signals are observed in numerous time‐series measurements obtained from mid‐ocean ridge hydrothermal systems. In some instances these tidal signals are clearly the result of ocean currents, but in other instances it appears that the signals may originate in the subseafloor formation. In order to explore the effect of ocean tidal loading on mid‐ocean ridge hydrothermal systems, we apply a one‐dimensional analytical model of tidal loading on a poroelastic half‐space and develop a two‐dimensional numerical model of tidal loading on a poroelastic convection cell. The one‐dimensional models show that for a reasonable range of fluid, elastic, and hydrological properties, the loading efficiency may vary from near zero to near unity and the diffusive penetration depth for tidal pressure signals may vary from tens of meters to kilometers. The two‐dimensional models demonstrate that tides may generate significant vertical and horizontal pressure gradients in mid‐ocean ridge hydrothermal systems as a result of spatial variations in fluid temperatures and the elastic and hydrological properties of the crust. These continuum models predict that outflow temperature perturbations will be very small (<10−4°C), but in real systems where the continuum hypothesis does not always apply, the perturbations may be on the order of ∼0.1°C. The models predict relatively large perturbations to fluid velocity at the seafloor. For high‐temperature vents the outflow perturbations normalized to the mean flow velocity increase as the permeability decreases. Flow reversals at the seafloor are predicted in some regions of net low‐temperature outflow and net inflow during the tidal cycle. In the subseafloor, tidally induced flow perturbations are likely to significantly enhance mixing and fluid exchange below the seafloor in regions of slow flow and in regions where there are strong gradients in temperature or in the mechanical and hydrological properties of the crust. Tidally enhanced mixing and fluid exchange may significantly influence the extent and character of microbial production in the subseafloor.

Hydrothermal Fe fluxes during the Precambrian: Effect of low oceanic sulfate concentrations and low hydrostatic pressure on the composition of black smokers

Modern mid-ocean ridge hydrothermal systems typically release vent fluids to the ocean with dissolved H 2S in excess of Fe. These fluids are the consequence of high-temperature interactions between sulfate-rich seawater and mid-ocean ridge basalt at conditions near the critical point for seawater. The precipitation of FeS and FeS 2 during venting titrates most of the Fe from the fluid, significantly reducing the net flux of Fe to the open ocean. Here we suggest that hydrothermal fluids emanating from Precambrian seafloor systems older than ∼1.8 Ga had Fe / H 2S ratios >> 1, and with fH 2 higher than today, because seawater lacked its primary oxidant, dissolved sulfate ion. This predominance of Fe over H 2S would have promoted the establishment of an iron-rich deep ocean and the deposition of banded iron formations (BIF). Accordingly, the end of BIF deposition at ∼1.8 Ga was the result of the buildup of sulfate in seawater from oxidative weathering, and its return at 750 Ma the result of reductions in seawater sulfate concentrations during Snowball Earth episodes, enhanced by elevated Fe concentrations during depressurization of hydrothermal systems by large eustatic sea-level falls. Moreover, Precambrian chemosynthetic vent communities may have been based on H 2 synthesis rather than on H 2S oxidation, as they largely are today.

The potential role of ridge-flank hydrothermal systems on oceanic germanium and silicon balances

Abstract Sediment pore-water samples and warm spring fluids from three ridge flank hydrothermal systems (40–75°C) were analyzed for Ge, Si, and Mg to quantify the contribution of these systems to oceanic Ge and Si balances. These warm hydrothermal fluids have elevated concentrations of Ge and Si relative to bottom seawater, indicating that these systems are net sources of Ge and Si to the oceans. End-member Ge:Si molar ratios range from 12 to 34 μmol/mol, which is similar to, but elevated relative to, the average ratio from high temperature (350°C) midocean ridge hydrothermal systems. The source for this Ge and Si is suggested to be a combination of seawater reaction with basalt and diffusive exchange with overlying basal pore waters, which are in pseudo equilibrium with amorphous silica. Because the Ge:Si molar ratio in both basalt and sediment is much less than those measured in the dissolved phase, a net preferential removal of Si relative to Ge must occur within basaltic basement. As suggested for high temperature hydrothermal systems, we suggest that this removal mechanism is likely a consequence of quartz precipitation and the formation of secondary clays (i.e., smectite). We estimate that ridge flank hydrothermal systems likely provide ∼10% of the Ge and Si fluxes to the oceans.