Vent fluid chemistry of the Rainbow hydrothermal system (36°N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes

The Rainbow hydrothermal field is located at 36°13.8′N-33°54.15′W at 2300 m depth on the western flank of a non-volcanic ridge between the South AMAR and AMAR segments of the Mid-Atlantic Ridge. The hydrothermal field consists of 10-15 active chimneys that emit high-temperature (∼365 °C) fluid. In July 2008, vent fluids were sampled during cruise KNOX18RR, providing a rich dataset that extends in time information on subseafloor chemical and physical processes controlling vent fluid chemistry at Rainbow. Data suggest that the Mg concentration of the hydrothermal end-member is not zero, but rather 1.5-2 mmol/kg. This surprising result may be caused by a combination of factors including moderately low dissolved silica, low pH, and elevated chloride of the hydrothermal fluid. Combining end-member Mg data with analogous data for dissolved Fe, Si, Al, Ca, and H₂, permits calculation of mineral saturation states for minerals thought appropriate for ultramafic-hosted hydrothermal systems at temperatures and pressures in keeping with constraints imposed by field observations. These data indicate that chlorite solid solution, talc, and magnetite achieve saturation in Rainbow vent fluid at a similar pH_(T,P) (400 °C, 500 bar) of approximately 4.95, while higher pH values are indicated for serpentine, suggesting that serpentine may not coexist with the former assemblage at depth at Rainbow. The high Fe/Mg ratio of the Rainbow vent fluid notwithstanding, the mole fraction of clinochlore and chamosite components of chlorite solid solution at depth are predicted to be 0.78 and 0.22, respectively. In situ pH measurements made at Rainbow vents are in good agreement with pH_(T,P) values estimated from mineral solubility calculations, when the in situ pH data are adjusted for temperature and pressure. Calculations further indicate that pH_(T,P) and dissolved H₂ are extremely sensitive to changes in dissolved silica owing to constraints imposed by chlorite solid solution-fluid equilibria. Indeed, the predicted correlation between dissolved silica and H₂ defines a trend that is in good agreement with vent fluid data from Rainbow and other high-temperature ultramafic-hosted hydrothermal systems. We speculate that the moderate concentrations of dissolved silica in vent fluids from these systems result from hydrothermal alteration of plagioclase and olivine in the form of subsurface gabbroic intrusions, which, in turn are variably replaced by chlorite + magnetite + talc ± tremolite, with important implications for pH lowering, dissolved sulfide concentrations, and metal mobility.     

Geochemistry of hydrothermal fluids from the PACMANUS, Northeast Pual and Vienna Woods hydrothermal fields, Manus Basin, Papua New Guinea

Processes controlling the composition of seafloor hydrothermal fluids in silicic back-arc or near-arc crustal settings remain poorly constrained despite growing evidence for extensive magmatic-hydrothermal activity in such environments. We conducted a survey of vent fluid compositions from two contrasting sites in the Manus back-arc basin, Papua New Guinea, to examine the influence of variations in host rock composition and magmatic inputs (both a function of arc proximity) on hydrothermal fluid chemistry. Fluid samples were collected from felsic-hosted hydrothermal vent fields located on Pual Ridge (PACMANUS and Northeast (NE) Pual) near the active New Britain Arc and a basalt-hosted vent field (Vienna Woods) located farther from the arc on the Manus Spreading Center. Vienna Woods fluids were characterized by relatively uniform endmember temperatures (273-285 °C) and major element compositions, low dissolved CO₂ concentrations (4.4 mmol/kg) and high measured pH (4.2-4.9 at 25 °C). Temperatures and compositions were highly variable at PACMANUS/NE Pual and a large, newly discovered vent area (Fenway) was observed to be vigorously venting boiling (358 °C) fluid. All PACMANUS fluids are characterized by negative δD_H2O values, in contrast to positive values at Vienna Woods, suggesting substantial magmatic water input to circulating fluids at Pual Ridge. Low measured pH (25 °C) values (∼2.6-2.7), high endmember CO₂ (up to 274 mmol/kg) and negative δ³⁴S_H2S values (down to −2.7‰) in some vent fluids are also consistent with degassing of acid-volatile species from evolved magma. Dissolved CO₂ at PACMANUS is more enriched in ¹³C (−4.1‰ to −2.3‰) than Vienna Woods (−5.2‰ to −5.7‰), suggesting a contribution of slab-derived carbon. The mobile elements (e.g. Li, K, Rb, Cs and B) are also greatly enriched in PACMANUS fluids reflecting increased abundances in the crust there relative to the Manus Spreading Center. Variations in alkali and dissolved gas abundances with Cl at PACMANUS and NE Pual suggest that phase separation has affected fluid chemistry despite the low temperatures of many vents. In further contrast to Vienna Woods, substantial modification of PACMANUS/NE Pual fluids has taken place as a result of seawater ingress into the upflow zone. Consistently high measured Mg concentrations as well as trends of increasingly non-conservative SO₄ behavior, decreasing endmember Ca/Cl and Sr/Cl ratios with increased Mg indicate extensive subsurface anhydrite deposition is occurring as a result of subsurface seawater entrainment. Decreased pH and endmember Fe/Mn ratios in higher Mg fluids indicate that the associated mixing/cooling gives rise to sulfide deposition and secondary acidity production. Several low temperature (?80 °C) fluids at PACMANUS/NE Pual also show evidence for anhydrite dissolution and water-rock interaction (fixation of B) subsequent to seawater entrainment. Hence, the evolution of fluid compositions at Pual Ridge reflects the cumulative effects of water/rock interaction, admixing and reaction of fluids exsolved from silicic magma, phase separation/segregation and seawater ingress into upflow zones.     

Separation of He and CH4 signals on the Mid-Atlantic Ridge at 5°N and 51°N

Abiogenic methane may be produced in submarine hydrothermal systems by degassing of basalts or serpentinization of ultramafic outcrops. The latter process presumably releases little primordial helium and is therefore implicated by high CH₄/³He ratios in vent fluids from the ultramafic-hosted Rainbow field and in methane plumes near ultramafic outcrops. In two segments of the Mid-Atlantic Ridge, at 5.4°N and 51°N, we have observed depth-separated CH₄ and ³He plumes. In both cases, the helium plume was deeper, near the valley floor. It may be that the plumes issue from separate vents, where the helium is discharged near the volcanic axis and the methane is generated by serpentinization on the valley wall. However, at the present time the locations of the vents that produce these plumes are not known. Using a one-pass model, we investigated whether separate venting could arise from heat conduction from a primary, helium-carrying, hydrothermal circulation to a second, shallower fracture loop intersecting ultramafic rock. The model results indicate that the flow rate through the secondary loop would have to be relatively low in order for it to stay warm enough for serpentinization to proceed. In this case, some of the exothermic heat production is lost by conduction, and the temperature increase in the circulating fluid is only a fraction of that expected from a water/rock ratio of 1:1.     

Compositional controls on vent fluids from ultramafic-hosted hydrothermal systems at mid-ocean ridges: An experimental study at 400°C, 500 bars

Olivine (Fo₈₉), orthopyroxene (En₈₅), and clinopyroxene (Di₈₉) were reacted, individually and in combinations, with NaCl-MgCl₂ at 400°C, 500 bars to better assess alteration and mass transfer in ultramafic-hosted hydrothermal systems at mid-ocean ridges. Data indicate that temperature plays a key role in mineral solubility and kinetic processes, which influence the compositional evolution of the fluid. At the temperature and pressure of the experiments, the rate of olivine hydrolysis is sluggish as indicated by the limited extent of mass transfer between the fluid and mineral and absence of hydrous alteration phases. In contrast, reactions involving pyroxenes proceed rapidly, which result in significant increases in dissolved Ca, SiO₂, Fe and H₂, and formation of SiO₂-rich secondary minerals (talc and tremolite) and magnetite. SiO₂ release from pyroxene occurs in non-stoichiometric proportions and is a critical factor governing the stability of secondary minerals, with attendant effects on fluid chemistry.Magnetite and talc-fluid equilibria were used to calculate fluid pH at elevated temperatures and pressures. In general, pH is relatively low in the orthopyroxene- and clinopyroxene-bearing experiments due to constraints imposed by talc-fluid and talc-tremolite-fluid equilibria, respectively. Even in experiments where the olivine/pyroxene ratio is as great as 3, which is typical for abyssal peridotite, the low pH and high Fe concentrations are maintained. This is in sharp contrast to theoretical predictions assuming full equilibrium in the MgO-CaO-FeO-Fe₂O₃-SiO₂-Na₂O-H₂O-HCl system at 400°C, 500 bars.Ultramafic-hosted hydrothermal systems, such as the recently discovered Rainbow system at 36°13.80′N, 33°54.12′W on the Mid-Atlantic Ridge, indicate reaction processes in keeping with results of the present experiments, as suggested by vent fluid chemistry and temperature. In particular, relatively high SiO₂, Ca, H₂, and Fe concentrations characterize the Rainbow vent fluids. Indeed, Fe concentrations are the highest of any vent system yet discovered and require a relatively low pH in the subseafloor reaction zone from which the fluids are derived. This, together with the SiO₂ concentrations of the vent fluids, strongly indicates fluid buffering by silica-rich phases produced during pyroxene dissolution, the likely abundant presence of olivine notwithstanding. Time-series observations at Rainbow are clearly needed to better constrain the temporal evolution of hydrothermal alteration processes of ultramafic rocks in subseafloor reaction zones. In the absence of events permitting fluid continuous access to fresh rock, pyroxene will ultimately be consumed and vent fluids may then reflect changes imposed by bulk compositional constraints characteristic of ultramafic bodies at depth, which would be in better agreement with theoretical phase relations for the fully equilibrated system.     

Serpentinization and heat generation: constraints from Lost City and Rainbow hydrothermal systems

The discovery of ultramafic hosted hydrothermal systems at Rainbow (36°N MAR) and Lost City, a vent site approximately 15 km west of the MAR at 30°N, provides unique perspectives on chemical and heat-generating processes associated with serpentinization at a range of chemical and physical conditions. Heat balance calculations together with constraints imposed by geochemical modeling indicate that significant changes in temperature are not likely to occur at either vent system as a result of the exothermic nature of olivine hydrolysis. At Rainbow, the relatively high temperatures in subseafloor reaction zones (in excess of 400°C), which must be linked to magmatic processes, inhibit olivine hydrolysis, effectively precluding mineralization-induced heating effects. Geochemical modeling of the Lost City vent fluids indicates temperatures in excess of those measured (40-75°C). The relatively high subseafloor temperatures (∼ 200 ± 50°C) requires conductive cooling of the fluids on ascent to the seafloor—a scenario in keeping with the mineralization of chimney structures actually observed. Although the intermediate temperatures predicted for subseafloor reaction zones at Lost City could be expected to enhance olivine to serpentine conversion, dissolved Cl, K/Cl and Na/Cl ratios of the Lost City vent fluids are virtually unchanged from seawater values and indicate little hydration of olivine, which is a necessary condition for exothermic heat generation by serpentinization. Apparently the fluid/rock mass ratio is too high or fluid residence times too low for this to occur to any significant extent. Thus, in spite of the off-axis location of the Lost City vents and apparent lack of a localized heat source, mineralization reactions likely play an insignificant role in accounting for hydrothermal circulation. It is more likely that tectonic processes associated with the slow spreading MAR, permit access of seawater to relatively deep and still hot lithospheric units and/or near axis magmatic heat sources, before venting. Additional chemical and physical (temperature, flow rate) data for Lost City and similar hydrothermal systems are needed to test key elements of the proposed model.     

Kinetics of H2-O2-H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions

Hydrothermal experiments were conducted to evaluate the kinetics of H_2(aq) oxidation in the homogeneous H₂-O₂-H₂O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 °C and 242-497 bar). The kinetics of the water-forming reaction that controls the fundamental equilibrium between dissolved H_2(aq) and O_2(aq), are expected to impose significant constraints on the redox gradients that develop when mixing occurs between oxygenated seawater and high-temperature anoxic vent fluid at near-seafloor conditions. Experimental data indicate that, indeed, the kinetics of H_2(aq)-O_2(aq) equilibrium become slower with decreasing temperature, allowing excess H_2(aq) to remain in solution. Sluggish reaction rates of H_2(aq) oxidation suggest that active microbial populations in near-seafloor and subsurface environments could potentially utilize both H_2(aq) and O_2(aq), even at temperatures lower than 40 °C due to H_2(aq) persistence in the seawater/vent fluid mixtures. For these H₂-O₂ disequilibrium conditions, redox gradients along the seawater/hydrothermal fluid mixing interface are not sharp and microbially-mediated H_2(aq) oxidation coupled with a lack of other electron acceptors (e.g. nitrate) could provide an important energy source available at low-temperature diffuse flow vent sites.More importantly, when H_2(aq)-O_2(aq) disequilibrium conditions apply, formation of metastable hydrogen peroxide is observed. The yield of H₂O_2(aq) synthesis appears to be enhanced under conditions of elevated H_2(aq)/O_2(aq) molar ratios that correspond to abundant H_2(aq) concentrations. Formation of metastable H₂O₂ is expected to affect the distribution of dissolved organic carbon (DOC) owing to the existence of an additional strong oxidizing agent. Oxidation of magnetite and/or Fe⁺⁺ by hydrogen peroxide could also induce formation of metastable hydroxyl radicals (•OH) through Fenton-type reactions, further broadening the implications of hydrogen peroxide in hydrothermal environments.     

Reactions between olivine and CO2-rich seawater at 300 °C: Implications for H2 generation and CO2 sequestration on the early Earth

To understand the influence of fluid CO₂ on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth, we monitored the reaction between San Carlos olivine and a CO₂-rich NaCl fluid at 300 °C and 500 bars. During the experiments, the total carbonic acid concentration (ΣCO₂) in the fluid decreased from approximately 65 to 9 mmol/kg. Carbonate minerals, magnesite, and subordinate amount of dolomite were formed via the water-rock interaction. The H₂ concentration in the fluid reached approximately 39 mmol/kg within 2736 h, which is relatively lower than the concentration generated by the reaction between olivine and a CO₂-free NaCl solution at the same temperature. As seen in previous hydrothermal experiments using komatiite, ferrous iron incorporation into Mg-bearing carbonate minerals likely limited iron oxidation in the fluids and the resulting H₂ generation during the olivine alteration. Considering carbonate mineralogy over the temperature range of natural hydrothermal fields, H₂ generation is likely suppressed at temperatures below approximately 300 °C due to the formation of the Mg-bearing carbonates. Nevertheless, H₂ concentration in fluid at 300 °C could be still high due to the temperature dependency of magnetite stability in ultramafic systems. Moreover, the Mg-bearing carbonates may play a key role in the ocean-atmosphere system on the early Earth. Recent studies suggest that the subduction of carbonated ultramafic rocks may transport surface CO₂ species into the deep mantle. This process may have reduced the huge initial amount of CO₂ on the surface of the early Earth. Our approximate calculations demonstrate that the subduction of the Mg-bearing carbonates formed in komatiite likely played a crucial role as one of the CO₂ carriers from the surface to the deep mantle, even in hot subduction zones.     

Composition and origin of hydrothermal petroleum and associated lipids in the sulfide deposits of the Rainbow field (Mid-Atlantic Ridge at 36°N)

The lipid components in hydrothermal sulfide deposits from the Rainbow vent field (Mid-Atlantic Ridge at 36°N) were studied by gas chromatography/mass spectrometry. The Rainbow vent field is one of two known active hydrothermal systems related to abyssal circulation, where high-temperature fluids are formed during serpentinization of ultrabasic crustal rocks. The major amount of the extractable organic matter from the sulfides consists of normal and branched alkanes, UCM, PAHs, terpenoids, and fatty acids. The branched alkanes are comprised of unique gem-diethylalkane series, possibly from sulfide oxidizing bacteria, and biphytanes from archaea. The characteristic lipid and biomarker compounds found in the hydrothermal samples support a predominantly biological origin of the bitumens from the thermal transformation of the biomass of microorganisms (bacteria and archea) and minor macrofauna of this vent field. A search for molecular evidence for abiogenic thermocatalytic synthesis of organic compounds was negative. However, methane in the hydrothermal fluids and possibly a minor amount of the alkanes in the sulfides may be of an abiogenic origin in the Rainbow vent field.     

Chemical and isotopic constraints on water/rock interactions at the Lost City hydrothermal field, 30°N Mid-Atlantic Ridge

Low temperature vent fluids (<91 °C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30°N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H₂,CH₄) while attaining elevated pH values, indicative of the serpentization processes that govern water/rock interactions deep in the oceanic crust. Here, we present a series of theoretical models to evaluate the extent of hydrothermal alteration and assess the effect of cooling on the systematics of pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate that the mineral assemblage composed of diopside, brucite and chrysotile likely dictates fluid pH at moderate temperature serpentinization processes (<300 °C), by imposing constraints on the aCa⁺⁺/a²H⁺ ratios and the activity of dissolved SiO₂. Based on Sr abundances and the ⁸⁷Sr/⁸⁶Sr isotope ratios of vent fluids reported from Lost City, estimated water/rock mass ratios (w/r = 2-4) are consistent with published models involving dissolved CO₂ and alkane concentrations. Combining the reported δ¹⁸O values of vent fluids (0.7‰) with such w/r mass ratios, allows us to bracket subseafloor reaction temperatures in the vicinity of 250 °C. These estimates are in agreement with previous theoretical studies supporting extensive conductive heat loss within the upflow zones. Experimental studies on peridotite-seawater alteration suggest that fluid pH increases during cooling which then rapidly enhances boron removal from solution and incorporation into secondary phases, providing an explanation for the highly depleted dissolved boron concentrations measured in the low temperature but alkaline Lost City vent fluids. Finally, to account for the depleted ¹¹B composition (δ¹¹B ∼25-30‰) of vent fluids relative to seawater, isotopic fractionation between tetrahedrally coordinated aqueous boron species with BO₃-bearing mineral sites (e.g. in calcite, brucite) is proposed.     

Fluid elemental and stable isotope composition of the Nibelungen hydrothermal field (8°18′S,Mid-Atlantic Ridge): Constraints on fluid–rock interaction in heterogeneous lithosphere

Depending on the geological setting, the interaction of submarine hydrothermal fluids with the host rock leads to distinct energy and mass transfers between the lithosphere and the hydrosphere. The Nibelungen hydrothermal field is located at 8°18′S, about 9 km off-axis of the Mid-Atlantic Ridge (MAR). At 3000 m water depth, 372 °C hot, acidic fluids emanate directly from the bottom, without visible sulfide chimney formation. Hydrothermal fluids obtained in 2009 are characterized by low H₂S concentrations (1.1 mM), a depletion of B (192 μM) relative to seawater, lower Si (13.7 mM) and Li (391 μM) concentrations relative to basaltic-hosted hydrothermal systems and a large positive Eu anomaly, and display a distinct stable isotope signature of hydrogen (?²H_H2O = 7.6–8.7‰) and of oxygen (?¹⁸O_H2O = 2.2–2.4‰).The heavy hydrogen isotopic signature of the Nibelungen fluids is a specific feature of ultramafic-hosted hydrothermal systems and is mainly controlled by the formation of OH-bearing alteration minerals like serpentine, brucite, and tremolite during pervasive serpentinization. New isotopic data obtained for the ultramafic-hosted Logatchev I field at 14°45′N, MAR (?²H_H2O = 3.8–4.2‰) display a similar trend, being clearly distinguished from other, mafic-hosted hydrothermal systems at the MAR.The fluid geochemistry at Nibelungen kept stable since the first sampling campaign in 2006 and is evident for a hybrid alteration of mafic and ultramafic rocks in the subseafloor. Whereas the ultramafic-fingerprint parameters Si, Li, B, Eu anomaly and ?²H_H2O distinguish the Nibelungen field from other hydrothermal systems venting in basaltic settings at similar physico-chemical conditions and are related to the interaction with mantle rocks, the relatively high concentrations of trace alkali elements, Pb, and Tl can only be attributed to the alteration of melt-derived gabbroic rocks. The elemental and isotopic composition of the fluid suggest a multi-step alteration sequence: (1) low- to medium-temperature alteration of gabbroic rocks, (2) pervasive serpentinization at moderate to high temperatures, and (3) limited high-temperature interaction with basaltic rocks during final ascent of the fluid. The integrated water/rock ratio for the Nibelungen hydrothermal system is about 0.5.The fluid compositional fingerprint at Nibelungen is similar to the ultramafic-hosted Logatchev I fluids with respect to key parameters. Some compositional differences can be ascribed to different alteration temperatures and other fluid pathways involving a variety of source rocks, higher water/rock ratios, and sulfide precipitation in the sub-seafloor at Logatchev I.     

Particle geochemistry in the Rainbow hydrothermal plume, Mid-Atlantic Ridge

We report the analysis of 18 large volume (500-1500 L) in situ filtered samples of particulate material from the largest hydrothermal plume on the Mid-Atlantic Ridge, overlying the ultramafic-hosted Rainbow hydrothermal field at 36° 14′N. Measured particulate iron concentrations reach 614 nM. High concentrations of particulate Fe oxyhydroxides result from the extremely high Fe concentration (∼24 mM) and Fe/H₂S ratio (∼24) of the vent fluids, and persist to at least 10 km away from the vent site due to the advection of plume material with the ambient along-axis flow. Two of the nine pairs of pump deployments appear to have intercepted the buoyant or otherwise very young portion of the hydrothermal plume. These samples are characterized by anomalously (compared to neutrally buoyant plume samples) high concentrations of Mg, U, and chalcophile elements, and low concentrations of Mn, Ca, V, Y, and the rare earth elements (REE). Within the neutrally buoyant plume, elemental distributions are largely consistent with previously observed behaviors: preferential removal of chalcophile elements, conservative behavior of oxyanions (P, V, and U), and continuous scavenging of Y and the REE. This consistency is particularly significant in light of the underlying differences in fluid chemistry between Rainbow and other studied sites. Chalcophile elements are preferentially removed from the plume in the order Cd>Zn>Co>Cu. Phosphorus/iron and vanadium/iron ratios for the neutrally buoyant plume are consistent with global trends with respect to the concentration of dissolved phosphate in ambient seawater. Comparison of buoyant and neutrally buoyant plume ratios with data from hydrothermal sediments underlying the Rainbow plume (Cave et al., 2002) indicates, however, that while P/Fe ratios are indeed constant V/Fe ratios increase progressively from early stage plume particles to sediments. REE distributions in the buoyant and neutrally buoyant plume appear most consistent with a continuous scavenging process during dispersion through the water column.     

Helium and carbon gas geochemistry of pore fluids from the sediment-rich hydrothermal system in Escanaba Trough

Ocean Drilling Program (ODP) Leg 169, which was conducted in 1996 provided an opportunity to study the gas geochemistry in the deeper part of the sediment-rich hydrothermal system in Escanaba Trough. Gas void samples obtained from the core liner were analyzed and their results were compared with analytical data of vent fluid samples collected by a submersible dive program in 1988. The gas geochemistry of the pore fluids consisted mostly of a hydrothermal component and was basically the same as that of the vent fluids. The He isotope ratios (R/R_A=5.6–6.6) indicated a significant mantle He contribution and the C isotopic compositions of the hydrocarbons [δ¹³C(CH₄)=−43‰, δ¹³C(C₂H₆)=−20‰] were characterized as a thermogenic origin caused by hydrothermal activity. On the other hand, the pore fluids in sedimentary layers away from the hydrothermal fields showed profiles which reflected lateral migration of the hydrothermal hydrocarbons and abundant biogenic CH₄. Helium and C isotope systematics were shown to represent a hydrothermal component and useful as indicators for their distribution beneath the seafloor. Similarities in He and hydrocarbon signatures to that of the Escanaba Trough hydrothermal system were found in some terrestrial natural gases, which suggested that seafloor hydrothermal activity in sediment-rich environments would be one of the possible petroleum hydrocarbon generation scenarios in unconventional geological settings.     

Numerical model to determine the composition of H2O-NaCl-CaCl2 fluid inclusions based on microthermometric and microanalytical data

Natural fluids approximated by the H₂O-NaCl-CaCl₂ system are common in a wide range of geologic environments, including sedimentary basins associated with hydrocarbon occurrences and MVT deposits, submarine hydrothermal systems, and other metamorphic, magmatic and hydrothermal environments. We present a comprehensive numerical model and Microsoft® Excel©-based computer program to determine the compositions of fluid inclusions in the H₂O-NaCl-CaCl₂ system based on microthermometric and microanalytical data. The model consists of six polynomial correlation equations that describe liquid salinity as a function of NaCl/CaCl₂ ratio and melting temperature on each of the ice, hydrohalite, halite, antarcticite, CaCl₂·4H₂O and CaCl₂·2H₂O vapor-saturated liquidus surfaces. The cotectic and peritectic boundaries are determined from the intersections of the liquidus surfaces. The model is implicitly internally consistent and topologically correct.The model expands upon the compositional range of applicability and the data types that can be used for compositional determination. It reproduces experimental data for all compositions that lie within the H₂O-NaCl-CaCl₂·4H₂O compositional triangle in the H₂O-NaCl-CaCl₂ system and yields accurate reproductions of the H₂O-NaCl and H₂O-CaCl₂ binaries. Furthermore, in comparison to previously published models, the one presented here eliminates systematic errors, wavy isotherms and cotectic and peritectic curves with local “bumps.”     

Fluxes to sediments underlying the Rainbow hydrothermal plume at 36°14′N on the Mid-Atlantic Ridge

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO₃) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of “hydrothermal” elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr⁻¹, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than “background” authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 ¹⁴C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.     

Rhondonite solubility and thermodynamic properties of aqueous MnCl2 in the system MnOSiO2HClH2O

The solubility of rhodonite, represented by the reaction MnSiO₃ (rhodonite) + 2HCl⁰ = MnCl₂⁰ + SiO₂ (quartz) + H₂O, was investigated experimentally in the temperature range 400°–700°C at 1 and 2 kbar by rapid-quench hydrothermal techniques and the Ag-AgCl buffer methods. Variations in the molalities of associated hydrogen chloride (m_HCl⁰) as a function of the molalities of total Mn indicate that Mn in the fluid in equilibrium with the assemblage rhodonite + quartz is predominantly associated as MnCl₂⁰. The Mn:Cl in the fluid ?2, indicating that Mn⁺² is the dominant oxidation state.The solubility data were used to calculate the equilibrium constant of the above reaction as a function of temperature, pressure, and the difference in Gibbs free energy of formation between MnCl₂⁰ and HCl⁰. The equilibrium constants of solubility for Mn minerals for which thermochemical data are available were also calculated. Calculated mineral solubilities were used in conjunction with the data of Frantz et al. (1981) to calculate the composition of supercritical fluids in equilibrium with Mn-bearing phases and assemblages. At 400°C and 1000 bars, supercritical fluids in equilibrium with olivines of compositions similar to those present in MORB tend to be enriched in Mn, despite the low mole fraction of tephroite in the olivine. Supercritical fluids in equilibrium with the assemblage quartz-hematite-rhodonite at 500° and 400°C and 1000 bars show high concentrations of Mn relative to Fe. Manganese concentrations in the fluids increase with decrease in the mole fraction of H, whereas Fe concentrations decrease. The data indicate that H fugacity plays a significant role in the separation of Mn from Fe in chloride-bearing hydrothermal fluids at supercritical temperatures.     

Hydrocarbons and oxidized organic compounds in hydrothermal fluids from Rainbow and Lost City ultramafic-hosted vents

The first building blocks of life could be produced in ultramafic-hosted hydrothermal systems considering the large amounts of hydrogen and methane generated by serpentinisation and Fischer–Tropsch-Type synthesis, respectively, in those systems. The purpose of this study was to detect and characterise organic molecules in hydrothermal fluids from ultramafic-hosted hydrothermal systems in the Mid-Atlantic Ridge (MAR) region. During the EXOMAR cruise 2005, fluids from the Rainbow (36°14′N) and the Lost City (30°N) hydrothermal fields were collected and treated by Stir Bar Sorptive Extraction (SBSE) and Solid Phase Extraction (SPE). The extracts were analysed by Thermal Desorption–Gas Chromatography–Mass Spectrometry (TD–GC–MS) and GC–MS, respectively. Compared to nearby deep seawater, hydrothermal fluids were clearly enriched in organic compounds, with a more diverse spectrum of molecules. We observed a very similar range of organic compounds in fluids from both sites, with a dominance of aliphatic hydrocarbons (C9–C14), aromatic compounds (C6–C16) and carboxylic acids (C8–C18). The occurrence of these compounds is supported by other field studies on serpentinites and sulfide deposits. Literature on thermodynamic data and experimental work has suggested the possible abiogenic origin of hydrocarbons and organic acids. In addition, it has been shown elsewhere that catalytic reactions producing hydrocarbons likely occur at both Lost City and Rainbow hydrothermal fields as suggested by the evolution of δ¹³C with increasing C number for methane, ethane, propane and butane. In order to investigate the origin of the organic molecules in the fluids, compound-specific carbon isotope ratio measurements were performed on n-alkanes and carboxylic acids, for which the δ¹³C values were in the range of ? 46 to ? 20‰ (vs. V-PDB). These preliminary data did not allow conclusive support or rejection of an abiogenic origin of the compounds. Indeed, predicting δ¹³C signatures in hydrothermal systems is likely to be complicated, due to differences in source δ¹³C signatures (i.e., of the C building blocks), and a variety of, mostly unknown, fractionation steps which may occur along the synthesis pathways. In addition, even though a fraction of the compounds detected in the fluids is likely abiotically produced, a dominance of biogenic sources and/or processes might hide their characteristic signature.     

Mineralogy, geochemistry, and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge

Petrological, geochemical, and Nd isotopic analyses have been carried out on rock samples from the Rainbow vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted hydrothermal system located on the Mid-Atlantic Ridge characterized by vigorous high-temperature venting (∼365°C) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe, and rare earth element (REE) contents (Douville et al., Chemical Geology 184:37–48, 2002). Serpentinization has occurred under a low-temperature (<270°C) retrograde regime, later overprinted by a higher temperature sulfide mineralization event. Retrograde serpentinization reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal fluids. The following units were identified within the deposit: (1) nonmineralized serpentinite, (2) mineralized serpentinite—stockwork, (3) steatite, (4) semimassive sulfides, and (5) massive sulfides, which include Cu-rich massive sulfides (up to 28wt% Cu) and Zn-rich massive sulfide chimneys (up to 5wt% Zn). Sulfide mineralization has produced significant changes in the sulfide-bearing rocks including enrichment in transition metals (Cu, Zn, Fe, and Co) and light REE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich volcanic-hosted massive sulfide deposits and different ¹⁴³Nd/¹⁴⁴Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing, and high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely under subcritical conditions (boiling). An explanation for the high chlorinity is not straightforward unless mixing with high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered. This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat and metals to the system. Co/Ni ratios proved to be good tools used to discriminate between rock units, degree of sulfide mineralization, and positioning within the hydrothermal system. Deeper units have Co/Ni <1 and subsurface and surface units have Co/Ni >1.     

Quartz solubility in the two-phase and critical region of the NaCl-KCl-H2O system: Implications for submarine hydrothermal vent systems at 9°50′N East Pacific Rise

Experiments were performed to investigate quartz solubility in Cl-bearing aqueous solutions at temperature (365-430 °C) and pressure conditions (219-381 bars) near and within the two-phase region of the NaCl-KCl-H₂O system. Dissolved SiO₂ concentrations increased with pressure along a given isotherm, although the magnitude of this decreased with increasing proximity to the two-phase boundary. Upon intersection of the two-phase boundary, however, significant concentrations of dissolved SiO₂ characterized vapor-rich fluids at both subcritical and supercritical conditions. For these fluids, dissolved silica concentrations ranged from 2.81 to 14.6 mmolal, increasing with dissolved chloride concentration. The experimental data permit regression of a density-based relationship, taking account of non-ideal activity-concentration effects, which can be used to better constrain temperatures and pressures from dissolved SiO₂ and chloride in high temperature vent fluids at mid-ocean ridges. Accordingly, pressure and temperature conditions in subseafloor hydrothermal reaction zones at 9°50′N East Pacific Rise (EPR) were estimated applying data from this experimental study to interval (1991-2002) and new field data (2004). Results indicate reaction zone at conditions ranging from 420 to 430 °C at 600 to 1500 m below seafloor. Thus, conditions predicted for 9°50′N East Pacific Rise (EPR) vent fluids suggest that supercritical phase separation might be more common than previously thought.     

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 CO₂ 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.     

显生宙全球海水化学成分演化及其对蒸发岩沉积的约束

通过搜集显生宙以来不同地质时期内海相碳酸盐岩鲕粒及胶结物矿物成分、钾盐矿床矿物种类及组合特征、蒸发岩盆地中石盐流体包裹体成分,并利用这些资料与人工海水模拟实验得到的石盐中Br分配特征的对比,得出海水成分在5.5亿年以来的显生宙期间,经历了五个阶段:其中晚元古代至寒武纪早期、二叠纪早期至中生代早期、新生代早期至现今,这些时期的原始海水组成特征系数m(SO_4~(2-))+m(HCO_3~-)/2m(Ca~(2+)),为Na-Mg-K-SO_4-Cl型海水,此期间沉积的钾盐矿床的钾镁盐矿物主要为钾盐镁矾、无水钾镁矾、杂卤石、硫酸镁石等含MgSO_4矿物,海相鲕粒和碳酸盐胶结物矿物成分为文石;而寒武纪早期至石炭纪、中生代早期至新生代早期,原始海水组成特征系数m(Ca~(2+))m(SO_4~(2-))+m(HCO_3~-)/2,为Na-Mg-KCa-Cl型海水,此期间沉积的钾镁盐矿物主要为光卤石和钾石盐,甚至含有溢晶石,海相鲕粒和碳酸盐胶结物矿物成分为方解石。根据石盐流体包裹体成分计算得出:显生宙期间,海水K+含量大部分时间变化幅度较小,为9.3～11.5mmol/kg H_2O(除了石炭纪和晚元古代),平均为10.55mmol/kg H_2O。Mg~(2+)含量在早寒武世≥67mmol/kg H_2O、晚志留世至中泥盆世31～41mmol/kg H_2O、晚古生代≥48mmol/kg H22O、晚白垩世34mmol/kg H_2O和现代55.1mmol/kg H_2O。Ca~+含量在晚元古代至古生代早期≤11mmol/kg H_2O、古生代早期至石炭纪22～35mmol/kg H_2O、石炭纪至中生代早期≤17mmol/kg H_2O、中生代早期至新生代早期19～39mmol/kg H_2O及新生代早期至今7～21mmol/kg H_2O。SO_4~(2-)含量在晚元古代至古生代早期≥23mmol/kg H_2O、古生代早期至石炭纪5～17mmol/kg H_2O、石炭纪至中生代早期13～22mmol/kg H_2O、中生代早期至新生代早期5～19mmol/kg H_2O及新生代早期至今12～29.2mmol/kg H_2O。海水Ca~(2+)与SO_4~(2-)含量的相对变化是控制海相钾盐矿床钾镁盐矿物类型的基本因素。同时,利用以上数据计算得到的显生宙各时期海水[m(Mg~(2+))+m(SO_4~(2-))]/[m(K~+)+m(Ca~(2+))]的变化与各时期海相蒸发岩系石盐层底部的Br含量变化具有同步性,进一步验证了显生宙期间海水成分是不断变化的,是约束海相蒸发岩钾盐矿物类型的主要因素。海水成分变化的控制因素为洋中脊热液和陆地水,其中洋中脊热液起主要作用,而控制这些因素变化的根本原因为板块构造运动。