Mid-ocean Ridge Hydrothermal Vent Research Articles

Stable potassium (K) isotope characteristics at mid-ocean ridge hydrothermal vents and its implications for the global K cycle

Recent discoveries of significant variations in stable K isotope ratios (41K/39K or δ41K) among various terrestrial samples indicate that K isotopes can be a novel tracer for the global K cycle, but a key observation that seawater δ41K is ∼0.6‰ higher than the bulk silicate Earth remains unexplained. An unconstrained component critical to this puzzle is hydrothermal systems that represent both a major K source and sink in the ocean. Here we report δ41K results on mid-ocean ridge (MOR) hydrothermal fluids from the Gorda Ridge and ∼9°N East Pacific Rise (EPR), including time-series samples that recorded major perturbations in fluid chemistry induced by a local volcanic eruption. Fluid δ41K values range from -0.46‰ to -0.15‰, falling between those of fresh basalts and seawater. δ41K values of “time-zero” fluids collected shortly after the volcanic eruption are shifted towards the seawater value, followed by a return to pre-eruption values within ∼2 years. Fluid δ41K variations are largely influenced by water–rock interactions, but they cannot be solely explained by simple mixing of seawater and K leached from basalts at high temperatures. Instead, these data imply small but significant isotope fractionation that enriches heavy K isotopes in basalts, likely caused by low-temperature alteration during the recharge stage of hydrothermal circulation. Our results preclude MOR hydrothermal systems as the cause for the heavy δ41K value of seawater. Using fluid δ41K data and K isotope fractionation constrained here for hydrothermal systems, a K mass-balance model implies a critical role for a marine sedimentary sink, possibly authigenic clay formation, in the global K cycle. Also, applying the K isotope fractionation constrained here to the published δ41K data from ophiolites shows the possibility for significantly lower seawater δ41K during the Ordovician, which can be explained by enhanced reverse weathering in response to distinct climate and tectonics at that time.

Integrating Multidisciplinary Observations in Vent Environments (IMOVE): Decadal Progress in Deep-Sea Observatories at Hydrothermal Vents

The unique ecosystems and biodiversity associated with mid-ocean ridge (MOR) hydrothermal vent systems contrast sharply with surrounding deep-sea habitats, however both may be increasingly threatened by anthropogenic activity (e.g., mining activities at massive sulphide deposits). Climate change can alter the deep-sea through increased bottom temperatures, loss of oxygen, and modifications to deep water circulation. Despite the potential of these profound impacts, the mechanisms enabling these systems and their ecosystems to persist, function and respond to oceanic, crustal, and anthropogenic forces remain poorly understood. This is due primarily to technological challenges and difficulties in accessing, observing and monitoring the deep-sea. In this context, the development of deep-sea observatories in the 2000s focused on understanding the coupling between sub-surface flow and oceanic and crustal conditions, and how they influence biological processes. Deep-sea observatories provide long-term, multidisciplinary time-series data comprising repeated observations and sampling at temporal resolutions from seconds to decades, through a combination of cabled, wireless, remotely controlled, and autonomous measurement systems. The three existing vent observatories are located on the Juan de Fuca and Mid-Atlantic Ridges (Ocean Observing Initiative, Ocean Networks Canada and the European Multidisciplinary Seafloor and water column Observatory). These observatories promote stewardship by defining effective environmental monitoring including characterizing biological and environmental baseline states, discriminating changes from natural variations versus those from anthropogenic activities, and assessing degradation, resilience and recovery after disturbance. This highlights the potential of observatories as valuable tools for environmental impact assessment (EIA) in the context of climate change and other anthropogenic activities, primarily ocean mining. This paper provides a synthesis on scientific advancements enabled by the three observatories this last decade, and recommendations to support future studies through international collaboration and coordination. The proposed recommendations include: i) establishing common global scientific questions and identification of Essential Ocean Variables (EOVs) specific to MORs, ii) guidance towards the effective use of observatories to support and inform policies that can impact society, iii) strategies for observatory infrastructure development that will help standardize sensors, data formats and capabilities, and iv) future technology needs and common sampling approaches to answer today’s most urgent and timely questions.

Sulfur and copper isotopic composition of seafloor massive sulfides and fluid evolution in the 26°S hydrothermal field, Southern Mid-Atlantic Ridge

In-situ sulfur isotopic compositions of morphologically different pyrites from the SMAR (southern Mid-Atlantic Ridge) 26°S hydrothermal field, integrated with δ34S and δ65Cu values in associated chalcopyrite, reflect a recurring flux of magmatic-hydrothermal fluids and subsequent leaching by seawater. The δ34S values of sulfides (−1.27 to +5.25‰) indicate mixing of magmatic sulfur with seawater sulfate-derived sulfur. Compared to other Mid-Ocean Ridge (MOR) hydrothermal vent sites, a significant contribution of magmatic fluids is herein distinctive. Three concentric mineralogical zones around the conduit in an inactive black smoker sample include an inner chalcopyrite-dominated zone, an intermediate zone of idiomorphic and massive pyrite, and an outermost zone of porous and colloform pyrite with/without traces of chalcopyrite. The systematic increase in δ34S from the interior conduit (avg. −0.60‰, n = 8) through the intermediate zone (avg. +2.36‰, n = 14) to the exterior wall (avg. +3.02‰, n = 27), and contributions of seawater sulfate (0 to 10.7%, and to 18.6%, respectively) are most likely related to interaction between hydrothermal fluid and previously precipitated sulfides. A significant contribution of seawater sulfate-derived sulfur occurred likely via permeability or porosity upsurge across the chimney. Chalcopyrite with consistently positive δ65Cu values (+0.17 to +0.48‰), which are typical of mid-ocean ridge (MOR) hydrothermal vent fluids, is suggestive of reaction/mixing with hydrothermal fluids. Sulfur and Cu isotopic compositions of chalcopyrite further imitate the significant contributions of magmatic gases in the formation of metalliferous seafloor massive sulfides.

Barium isotopes in mid-ocean ridge hydrothermal vent fluids: A source of isotopically heavy Ba to the ocean

Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO4) precipitation during mixing between Ba-bearing vent fluids and SO4-bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Bahyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Bahyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO4 concentrations in the past.

Fluid Chemistry of Mid-Ocean Ridge Hydrothermal Vents: A Comparison between Numerical Modeling and Vent Geochemical Data

Seawater-basalt interaction taking place at mid-ocean ridges was studied using numerical modeling to determine the compositional evolution of hydrothermal fluids and associated alteration mineralogy forming within newly emplaced crustal material. Geochemical modeling was carried out in a closed seawater-basalt system at discrete temperature intervals between 2 and 400°C at 500 bars, varying fluid/rock ratios, and secondary mineral assemblages representative of basalt alteration in natural systems. In addition to temperature, the fluid/rock ratio has a fundamental control on the resulting system chemistry. At rock-buffered conditions (low fluid/rock ratios), the mineral-solution equilibrium was characterized by high cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and very low dissolved Mg concentrations due to the precipitation of smectites and chlorite. A complex secondary mineral alteration assemblage dominated by Ca- and Na-bearing minerals including zeolites, calcite, epidote, prehnite, clinozoisite, and albite was predicted to form. The resulting fluid composition was alkaline and reduced relative to ambient seawater, with Eh values ranging between −0.2 and −0.6 V. In contrast, seawater-buffered conditions (high fluid/rock ratios) resulted in lower cation to proton activity ratios foraCa2+/(aH+)2andaNa+/aH+and higher dissolved Mg concentrations comparable to the value of this element in ambient seawater. A more simple mineral assemblage was predicted to form at these conditions with the predominance of Al-Si- and Mg-bearing minerals including kaolinite, quartz, and talc in addition to large amounts of anhydrite. The resulting fluid composition was mildly acidic and oxidized relative to seawater with Eh values ranging between −0.2 and 0 V. These modeling results were compared to a compilation of submarine hydrothermal vent fluid compositions from mid-ocean ridge settings and analogous basalt-dominated environments. The agreement obtained between the simulations and the compiled fluid data indicates that mid-ocean ridge hydrothermal processes can be closely reproduced by mineral-solution equilibria for a broad range of temperatures and fluid/rock ratios.

Calcium isotope systematics at hydrothermal conditions: Mid-ocean ridge vent fluids and experiments in the CaSO4-NaCl-H2O system

Two sets of hydrothermal experiments were performed to explore Ca isotope fractionation and exchange rates at hydrothermal conditions (410–450 °C, 31.0–50.0 MPa). The first set of experiments determined the magnitude of vapor-liquid Ca isotope fractionation and anhydrite solubility in the CaSO4-NaCl-H2O system. The data indicate no statistical difference between the Ca isotopic composition of coexisting vapor and liquid. The second set of experiments utilized an anomalous 43Ca spike to determine the rate of Ca exchange between fluid and anhydrite as a function of total dissolved Ca concentration. Results show that the rate of exchange increases with dissolved Ca concentrations (12–23 mM/kg), but no change in exchange rate is observed when the Ca concentration increases from 23 to 44 mM/kg Ca. 74–142 days are required to achieve 90% anhydrite-fluid Ca isotope exchange at the conditions investigated, while only several hours are necessary for vapor-liquid isotopic equilibrium. The lack of vapor-liquid Ca isotope fractionation in our experiments is consistent with δ44Ca of mid-ocean ridge hydrothermal vent fluids that remain constant, regardless of chlorinity. Moreover, the narrow range of end member fluid δ44Ca, −0.98 to −1.13‰ (SW), is largely indistinguishable from MORB δ44Ca, suggesting that neither phase separation nor fluid-rock interactions at depth significantly fractionate Ca isotopes in modern high-temperature mid-ocean ridge hydrothermal systems.

Anaerobic methane oxidation in metalliferous hydrothermal sediments: influence on carbon flux and decoupling from sulfate reduction

The anaerobic oxidation of methane (AOM) is a globally significant sink that regulates methane flux from sediments into the oceans and atmosphere. Here we examine mesophilic to thermophilic AOM in hydrothermal sediments recovered from the Middle Valley vent field, on the Juan de Fuca Ridge. Using continuous-flow sediment bioreactors and batch incubations, we characterized (i) the degree to which AOM contributes to net dissolved inorganic carbon flux, (ii) AOM and sulfate reduction (SR) rates as a function of temperature and (iii) the distribution and density of known anaerobic methanotrophs (ANMEs). In sediment bioreactors, inorganic carbon stable isotope mass balances results indicated that AOM accounted for between 16% and 86% of the inorganic carbon produced, underscoring the role of AOM in governing inorganic carbon flux from these sediments. At 90°C, AOM occurred in the absence of SR, demonstrating a striking decoupling of AOM from SR. An abundance of Fe(III)-bearing minerals resembling mixed valent Fe oxides, such as green rust, suggests the potential for a coupling of AOM to Fe(III) reduction in these metalliferous sediments. While SR bacteria were only observed in cooler temperature sediments, ANMEs allied to ANME-1 ribotypes, including a putative ANME-1c group, were found across all temperature regimes and represented a substantial proportion of the archaeal community. In concert, these results extend and reshape our understanding of the nature of high temperature methane biogeochemistry, providing insight into the physiology and ecology of thermophilic anaerobic methanotrophy and suggesting that AOM may play a central role in regulating biological dissolved inorganic carbon fluxes to the deep ocean from the organic-poor, metalliferous sediments of the global mid-ocean ridge hydrothermal vent system.

δ44/40Ca variability in shallow water carbonates and the impact of submarine groundwater discharge on Ca-cycling in marine environments

Shallow water carbonates from Florida Bay, the Florida Reef Tract, and a Mexican Caribbean fringing reef at Punta Maroma were studied to determine the range of Ca-isotope variation among a cohort of modern carbonate producers and to look for local-scale Ca-cycling effects. The total range of Ca-isotope fractionation is 0.4‰ at Punta Maroma, yielding an allochem-weighted average δ44/40Ca value of −1.12‰ consistent with bulk sediment from the lagoon with a value of −1.09‰. These values are virtually identical to bulk carbonate sediments from the Florida Reef Tract (−1.11‰) and from one location in Florida Bay (−1.09‰) near a tidal inlet in the Florida Keys. No evidence was found for the ∼0.6‰ fractionation between calcite and aragonite which has been observed in laboratory precipitation experiments. Combining these results with carbonate production modes and δ44/40Ca values for pelagic carbonates taken from the literature, we calculate a weighted average value of −1.12±0.11‰ (2σ) for the global-scale Ca-output flux into carbonate sediments. The δ44/40Ca value of the input Ca-flux from rivers and hydrothermal fluids is –1.01±0.04‰ (2σmean), calculated from literature data that have been corrected for inter-laboratory bias. Assuming that the ocean Ca cycle is in steady state, we calculate a δ44/40Ca value of −1.23±0.23‰ (2σ) for submarine groundwater discharge (SGD) on a global scale. The SGD Ca-flux rivals river flows and mid-ocean ridge hydrothermal vent inputs as a source of Ca to the oceans. It has the potential to differ significantly in its isotopic value from these traditional Ca-inputs in the geological past, and to cause small changes in the δ44/40Ca value of oceans through time.In the innermost water circulation restricted region of northeastern Florida Bay, sediments and waters exhibit a 0.7‰ gradient in δ44/40Ca values decreasing towards the Florida Everglades. This lowering of δ44/40Ca is predominantly caused by local-scale Ca-inputs from SGD, which has a high Ca concentration (450mg/L) and low δ44/40Ca value (−0.96‰). Mixing calculations show that Ca inputs from SGD and surface water runoff from the Florida Everglades contribute between 8% and 60% of the dissolved Ca to the studied waters with salinities between 30 and 14, respectively. Similar degrees of circulation restriction between epeiric seas and oceans in the geological past may have also led to overprinting of sedimentary carbonate δ44/40Ca values in nearshore regions of epeiric seas due to local-scale cycling of seawater through coastal carbonate aquifers. Local Ca-cycling effects may explain some of the scatter in δ44/40Ca values present in the Ca-isotope evolution curve of Phanerozoic oceans.

A new Fe/Mn geothermometer for hydrothermal systems: Implications for high-salinity fluids at 13°N on the East Pacific Rise

Field and experimental investigations demonstrate the chemistry of mid-ocean ridge hydrothermal vent fluids reflects fluid–mineral reaction at higher temperatures than those typically measured at the seafloor. To account for this and, in turn, be able to better constrain sub-seafloor hydrothermal processes, we have developed an empirical geothermometer based on the dissolved Fe/Mn ratio in high-temperature fluids. Using data from basalt alteration experiments, the relationship; T (°C) = 331.24 + 112.41*log[Fe/Mn] has been calibrated between 350 and 450 °C. The apparent Fe–Mn equilibrium demonstrated by the experimental data is in good agreement with natural vent fluids, suggesting broad applicability. When used in conjunction with constraints imposed by quartz solubility, associated sub-seafloor pressures can be estimated for basalt-hosted systems. As an example, this methodology is used to interpret new data from 13°N on the East Pacific Rise, where high-temperature fluids both enriched and depleted in chloride (339–646 mmol/kg), relative to seawater, are actively venting within a close proximity. Accounting for these variable salinities, active phase separation is clearly taking place at 13°N, yet the fluid Fe/Mn ratios and the silica concentrations suggest equilibration at temperatures less than those coinciding with the two-phase region. These data show the chloride-enriched fluid reflects the highest temperature and pressure (∼432 °C, 400 bars) of equilibration, consistent with circulation near the top of the inferred magma chamber. This is in agreement with the elevated CO 2 concentration relative to the chloride-depleted fluids. The noted temperature derived from the Fe/Mn geothermometer is higher than the critical temperature for a fluid of equivalent salinity. This carries the important implication that, despite being chloride-enriched relative to seawater, these fluids evolved as the vapor component of even higher salinity brine.

Isotopic evidence ( 87Sr/ 86Sr, δ 7Li) for alteration of the oceanic crust at deep-rooted mud volcanoes in the Gulf of Cadiz, NE Atlantic Ocean

The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/ 86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ 7Li values, partly close to those of hydrothermal vent fluids (δ 7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/ 86Sr ratios (∼0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/ 86Sr ratios (∼0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location. Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.

Biogenic iron oxyhydroxide formation at mid-ocean ridge hydrothermal vents: Juan de Fuca Ridge

Here we examine Fe speciation within Fe-encrusted biofilms formed during 2-month seafloor incubations of sulfide mineral assemblages at the Main Endeavor Segment of the Juan de Fuca Ridge. The biofilms were distributed heterogeneously across the surface of the incubated sulfide and composed primarily of particles with a twisted stalk morphology resembling those produced by some aerobic Fe-oxidizing microorganisms. Our objectives were to determine the form of biofilm-associated Fe, and identify the sulfide minerals associated with microbial growth. We used micro-focused synchrotron-radiation X-ray fluorescence mapping (μXRF), X-ray absorption spectroscopy (μΕXAFS), and X-ray diffraction (μXRD) in conjunction with focused ion beam (FIB) sectioning, and high resolution transmission electron microscopy (HRTEM). The chemical and mineralogical composition of an Fe-encrusted biofilm was queried at different spatial scales, and the spatial relationship between primary sulfide and secondary oxyhydroxide minerals was resolved. The Fe-encrusted biofilms formed preferentially at pyrrhotite-rich (Fe 1− x S, 0 ⩽ x ⩽ 0.2) regions of the incubated chimney sulfide. At the nanometer spatial scale, particles within the biofilm exhibiting lattice fringing and diffraction patterns consistent with 2-line ferrihydrite were identified infrequently. At the micron spatial scale, Fe μEXAFS spectroscopy and μXRD measurements indicate that the dominant form of biofilm Fe is a short-range ordered Fe oxyhydroxide characterized by pervasive edge-sharing Fe–O 6 octahedral linkages. Double corner-sharing Fe–O 6 linkages, which are common to Fe oxyhydroxide mineral structures of 2-line ferrihydrite, 6-line ferrihydrite, and goethite, were not detected in the biogenic iron oxyhydroxide (BIO). The suspended development of the BIO mineral structure is consistent with Fe(III) hydrolysis and polymerization in the presence of high concentrations of Fe-complexing ligands. We hypothesize that microbiologically produced Fe-complexing ligands may play critical roles in both the delivery of Fe(II) to oxidases, and the limited Fe(III) oxyhydroxide crystallinity observed within the biofilm. Our research provides insight into the structure and formation of naturally occurring, microbiologically produced Fe oxyhydroxide minerals in the deep-sea. We describe the initiation of microbial seafloor weathering, and the morphological and mineralogical signals that result from that process. Our observations provide a starting point from which progressively older and more extensively weathered seafloor sulfide minerals may be examined, with the ultimate goal of improved interpretation of ancient microbial processes and associated biological signatures.

Iron isotope, major and trace element characterization of early Archean supracrustal rocks from SW Greenland: Protolith identification and metamorphic overprint

The iron isotope, trace and major element compositions of Eoarchean supracrustal rocks from southern West Greenland (Isua Supracrustal Belt, the islands of Akilia and Innersuartuut) were analyzed in order to identify protoliths and characterize the imprints of metamorphism and metasomatism. Banded iron formations (BIFs) from the Isua Supracrustal Belt (ISB) have trace element characteristics that are consistent with seawater derivation, including high Y/Ho ratios, positive Eu/Eu ∗ anomalies, positive La/La ∗ anomalies, and concave upward REE patterns. These rocks also have heavy Fe isotopic compositions relative to surrounding igneous rocks (∼+0.4‰/amu). The most likely interpretation is that this signature was inherited from partial oxidation in a marine setting of Fe emanating from a source similar to modern mid-ocean ridge hydrothermal vents (∼−0.15‰/amu). Banded quartz-rich rocks from the island of Akilia with high Fe/Ti ratios share many similarities with bona fide BIFs from Isua (heavy Fe isotopic compositions up to +0.4‰/amu, elevated Y/Ho ratios compared to igneous rocks, sometimes positive Eu/Eu ∗ anomalies) suggesting a chemical sedimentary origin. Iron-poor metacarbonates from the southwestern part of the ISB have light Fe isotopic compositions (∼−0.4‰/amu). This is consistent with derivation of these rocks by fluid flow through surrounding ultramafic rocks and deposition as metasomatic carbonates. Iron-rich metacarbonates from the northwest and northeast parts of the ISB have Fe isotopic compositions (from +0.1 to +0.4‰/amu) and trace element patterns (high Y/Ho ratios, positive Eu/Eu ∗ and La/La ∗ anomalies, and concave upward REE) similar to associated BIFs. The most likely interpretation is that these iron-rich metacarbonates were derived from mobilization of Fe in BIFs by metasomatic fluids.

Physical controls on the salinity of mid-ocean ridge hydrothermal vent fluids

Variations in the salinity of black smoker effluents (0.1–∼ 8 wt.% NaCl) relative to seawater (3.2 wt.% NaCl) are attributed to phase separation and segregation of the resulting brines and vapors. However, models of phase separation predict brines with substantially higher salinities than observed at vents and such brines are commonly observed in fluid inclusions from fossil hydrothermal systems. It has been postulated that the range of observed salinities is controlled by the density of upwelling fluids. Here we present models of hydrothermal circulation that predict the observed maximum salinity when an upper layer of high-permeability is included, and also reproduce black smoker temperatures when the upflow zone is surrounded by a low-permeability shell. Pressure gradients across the permeability boundary act as a density filter impeding the passage of high-salinity fluids, while the shell provides sufficient insulation to tap hot fluids to the surface. Our models fit the observations when the permeabilities of the upper layer differ from the permeability of the lower layer by factors of ∼ 10 and ∼ 100 in upflow and downflow regions, respectively, and when the permeability of the shell is one tenth that of the lower layer. The permeability structure we propose is consistent with observations in oceanic crust and inferences from ophiolites. While a previous study argues that black smoker temperatures are a consequence of the thermodynamic properties of seawater, our work suggests that very specific permeability configurations are required to match both the temperature and maximum salinity.

Isotopic effects in fracture-dominated reactive fluid–rock systems

A mathematical model is presented that describes the effects of pore fluid aqueous diffusion and reaction rate on the isotopic exchange between fluids and rocks in reactive geo-hydrological systems where flow is primarily through fractures. The model describes a simple system with parallel equidistant fractures, and chemical transport in the matrix slabs between fractures by aqueous diffusion through a stagnant pore fluid. The solid matrix exchanges isotopes with pore fluid by solution–precipitation at a rate characterized by a time constant, R (yr −1), which is an adjustable parameter. The effects of reaction on the isotopes of a particular element in the fracture fluid are shown to depend on the ratio of the diffusive reaction length for that element ( L) to the fracture spacing ( b). The reaction length depends on the solid–fluid exchange rate within the matrix, the partitioning of the element between the matrix pore fluid and the matrix solid phase, the porosity and density of the matrix, and the aqueous diffusivity. For L/ b < 0.3, fluid–rock isotopic exchange is effectively reduced by a factor of 2 L/ b relative to a standard porous flow (single porosity) model. For L/ b > 1, the parallel fracture model is no different from a porous flow model. If isotopic data are available for two or more elements with different L values, it may be possible to use the model with appropriate isotopic measurements to estimate the spacing of the primary fluid-carrying fractures in natural fluid–rock systems. Examples are given using Sr and O isotopic data from mid-ocean ridge (MOR) hydrothermal vent fluids and Sr isotopes in groundwater aquifers hosted by fractured basalt. The available data for MOR systems are consistent with average fracture spacing of 1–4 m. The groundwater data suggest larger effective fracture spacing, in the range 50–500 m. In general, for fractured rock systems, the effects of fracture–matrix diffusive exchange must be considered when comparing isotopic exchange effects for different elements, as well as for estimating water age using radioactive and cosmogenic isotopes.

Predicted scattering of sound by diffuse hydrothermal vent plumes at mid-ocean ridges

Amplitude and phase fluctuations of monochromatic acoustic signals traveling through diffuse mid-ocean ridge hydrothermal vent plumes are modeled using existing theory in an attempt to find suitable frequencies and path lengths for plume monitoring. Weak-scattering solutions are evaluated numerically, with model parameters adjusted to match observed plume characteristics. Constraints required for weak-scattering solutions to be valid can be met for transmission ranges of 500–2000 m and frequencies of 20–80 kHz. Therefore, because fluid structure and scattering strength are more closely linked for weak scattering than for stronger scattering, inversion for fluid statistical properties may be possible, enabling diffuse vent monitoring. Such monitoring would be subject to geometric assumptions such as transmission entirely within a statistically homogeneous plume. Performance-limiting phase fluctuations have also been computed for a 13–17 kHz geodetic survey system.

Oxygen isotope study of mid-ocean ridge hydrothermal fluids: Implication for the oxygen-18 budget of the oceans

New data are reported for the 18O composition of hot hydrothermal fluids from various sites with different tectonic settings. The measured δ 18O, in the range 0.53–2.3‰ confirm earlier data suggesting that mid-ocean ridge hydrothermal vents are a major source of 18O to the ocean. In light of this new data, we attempt to re-evaluate the marine 18O budget from direct quantification of the main sources and sinks of oceanic oxygen-18. Within the uncertainty of the flux estimates, 18O inputs and outputs are found to balance each other, showing that oceanic 18O is presently at a steady-state. Using these present fluxes, we examine the past evolution of the 18O composition of the ocean and speculate that the initial 18O oceanic content may have evolved rapidly towards its present value. This suggests, as already pointed out in previous studies, that ocean water may have had a rather constant 18O composition over most of the Earth's history and that δ 18O measurements in ancient cherts and carbonates may primarily record the evolution of the ocean temperature.

Halide systematics of submarine hydrothermal vents

CHLORIDE is the dominant anion in submarine hydrothermal fluids and plays a fundamental part in controlling their chemistry1. The endmember Cl concentrations in virtually all submarine hot springs deviate from ambient sea water (∼541 mmol kg–1) and range from 188 mmol kg–1 at the ASHES vents on Axial Volcano2 to 1,090 mmol kg–1 at the Southern Juan de Fuca Ridge3. A variety of mechanisms have been proposed to explain these Cl enrichments and depletions including: rock hydration and dehydration4, phase separation2–3,5–7, and the formation and dissolution of a retrograde soluble phase8. Support for the latter hypothesis came from hydrothermal fluid/basalt experiments which showed large changes in dissolved Cl at specific reaction conditions9 and also from the presence of Cl-rich minerals phases in ultramafic rocks10. We have carried out the first systematic study of Br and I in mid-ocean ridge hydrothermal vent fluids to constrain the models of halide diversity better by ascertaining their behaviour relative to Cl. The cohesiveness of the Br and Cl data and the similarity of Br/Cl to the seawater ratio suggests a single, non-fractionating process controls the salinity of sediment-starved mid-ocean ridge hydrothermal systems. The thermal decomposition of organic matter provides an additional source of Br and I in sediment-hosted systems.

A time series of vent fluid compositions from 21°N, East Pacific Rise (1979, 1981, 1985), and the Guaymas Basin, Gulf of California (1982, 1985)

A time series study of four hot spring fields at 21°N on the East Pacific Rise demonstrates that the solution compositions of mid‐ocean ridge hydrothermal vents are stable on a decadal time scale. The Ocean Bottom Seismometer (OBS), National Geographic Smoker (NGS), and South West (SW) vents show no significant changes in the major element composition of their end‐member fluids since 1979. Two of the vents, OBS and SW, have maintained exit temperatures near 350°C since 1979. The NGS vent has remained close to 270°C since 1981 after cooling from 350°C in 1979. Only one vent, Hanging Gardens, shows important changes in solution chemistry from 1981 to 1985. In 1985 this vent appears to have cooled ∼10°C relative to the other vents. The depth of the reaction zone, as determined by quartz geobarometry, has increased by about 1 km at this vent. The large fluxes of compositionally stable fluids from these vents cannot be explained by transient conditions and unique circumstances. Rather, the constancy of small interfield variations in solution composition is more consistent with control by rock buffering. A 4‐year time series study of hydrothermal vents in the Guaymas Basin also shows no significant changes in the major element composition of the end‐member fluids. Interfield differences in solution chemistry have been maintained from 1982 to 1985. This phenomenon is best explained in terms of an equilibrium model.

Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites

The authors report Pb isotope ratios of sulfides deposited at seven recently active mid-ocean ridge (MOR) hydrothermal vents. Sulfides from three sediment-starved sites on the Juan de Fuca Ridge contain Pb with isotope ratios identical to their local basaltic sources. Lead in two deposits from the sediment-covered Escanaba Trough, Gorda Ridge, is derived from the sediments and does not appear to contain any basaltic component. There is a range of isotope ratios in a Guaymas Basin deposit, consistent with a mixture of sediment and MOR basalt Pb. Lead in a Galapagos deposit differs slightly from known Galapagos basalt Pb isotope values. The faithful record of Pb isotope signatures of local sources in MOR sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite-hosted deposits and continental crustal averages in sediment-hosted deposits. The preservation of primary ophiolitic or continental crustal Pb isotope signatures in ancient MOR sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes.