δ37Cl Values Research Articles

Petrogenesis of serpentinites from the Franciscan Complex, western California, USA

Serpentinites from the Franciscan Complex of California, USA, were analyzed for their bulk major and trace element compositions, relict mineral (spinel and pyroxene) compositions, and stable isotope (O, H, Cl) compositions with the goal of determining protolith origin and subsequent serpentinizing fluid sources in order to decipher the tectonic setting of serpentinization. We focused on serpentinite bodies found in the Franciscan Complex (west of Cuesta Ridge; south of San Francisco; Tiburon Peninsula; Healdsburg) (n=12). Three samples from Cuesta Ridge (part of the Coast Range ophiolite) were also analyzed for comparison. Serpentinites from Cuesta Ridge have flat to U-shaped chondrite-normalized REE patterns and spinels with Cr# values >0.60 implying a supra-subduction zone origin. In contrast, Franciscan serpentinites west of Cuesta Ridge and Tiburon Peninsula have positive-sloped REE patterns. This depletion in LREE is typical of abyssal peridotites. Most relict spinels have low Cr# values (<0.3) and relict clinopyroxenes from Tiburon Peninsula have high HREE concentrations, also supporting an abyssal origin. Franciscan serpentinite samples from south of San Francisco and near Healdsburg have U-shaped REE patterns and spinel compositions that lie within the forearc peridotite field with some overlap into the abyssal field and are of more ambiguous origin. All samples are high in fluid-mobile elements with remarkable positive Ce and Y anomalies. We speculate that these anomalies may be due to involvement of highly oxidizing fluids resulting in the preferential scavenging of Ce and Y by ferromanganese oxyhydroxides during serpentinization. All samples (except those south of San Francisco) have δ18O values of +5.4 to +7.9‰, typical values for oceanic serpentinites formed via low-T seawater hydration on the seafloor. δD values of all samples are extremely low (−107 to −90‰), likely the result of post-serpentinization, post-emplacement interaction with meteoric water at low temperature. Samples south of San Francisco lie on the San Andreas fault and have high δ18O values (+7.2 to +9.5‰) likely due to low-T interaction with meteoric water at high fluid–rock ratios. Most of the serpentinites have δ37Cl values between +0.2 and +0.9‰, typical values for serpentinites formed by interaction with seawater. Exceptions are those from the San Andreas fault and one sample from Cuesta Ridge with a high δ37Cl value (+1.7‰) possibly from interaction with a slab-derived fluid.

Origin of Cl− and High δ37Cl Values in Radiocarbon Dated-Fracture Groundwaters at the Tournemire URL (France)

Fracture groundwaters collected from the indurated Toarcian/Domerian clayrock in the IRSN Underground Research Laboratory (URL) at Tournemire (south of France) are rare and limited in volume. The present study aims at resolving an apparent contradiction observed for these groundwaters: how can these groundwaters in transmissive fractures contain indicators of both geologically young and old residence times? Young residence times on the order of a few thousand years are indicated by the presence of 14C, and old residence times appear to be indicated by the extraordinarily positive δ37Cl values, between 5 and 8‰ vs SMOC. These highly enriched δ37Cl values might be attributed to a long diffusive transport (on the order of tens of million years) between the semi-permeable clayrock sequence and its surrounding aquifers, as estimated by modeling in a companion paper [1].Knowledge of the geometry of the fractures and the diffusion parameters in the intact clayrock allowed us to propose a 2D modeling for the transport of chloride and its stable isotopes using the Hytec code. Simulations applied to borehole M3 data show that the measured Cl values in the fracture groundwater are consistent with a diffusive exchange between the unaltered argillite porewater and meteoric waters that infiltrated in the fracture about 26 ka ago and have become isolated since then. This result is in good agreement with previously estimated maximum 14C residence times for these fracture groundwaters. The modeled δ37Cl value at 26 ka would be enriched by only 0.02‰ with respect to the initial imposed value for the argillite, which remains in the analytical uncertainty of ±0.2‰. These simulations provide evidence that the aqueous chloride and high δ37Cl values may reflect values of the unaltered argillite porewater.

Modeling Cl− Concentration and δ37Cl Profiles in Porewater Across a 250m-thick Indurated Argillite at the Tournemire URL (France)

Dissolved chloride in argillite porewater has been studied as a natural analogue for radionuclides potentially released from radioactive waste disposal. The Tournemire URL intersects impervious and compacted argillite. A previously obtained chloride concentration profile of intact rock is symmetric with a maximum concentration of 0.6±0.1 g/L, compared to 19 g/L for the original connate seawater. Dissolved chloride shows high δ37Cl values, ranging between +6 and +8‰ vs. SMOC. The modeled profile considers diffusive exchange between connate seawater and meteoric freshwater. Transport parameters were obtained by radial diffusion experiments. Numerical modeling was performed with the coupled reactive-transport code Hytec. Simulations suggest a diffusive-exchange time of 85±10 Ma for Cl, which correlates with a major erosional period. Simulated δ37Cl values between 1.002 and agree with observed porewater δ37Cl. This study strongly suggests that the dissolved chloride profile in the argillites results from diffusive exchange and indicates that unfractured argillites can provide good confinement.

Assessing Initial Conditions for Chloride Transport Across Low- permeability Argillaceous Rocks, Wellenberg, Switzerland

Information about fluid evolution and solute transport in a low-permeability metamorphic rock sequence has been obtained by comparing chloride concentrations and chlorine isotope ratios of pore water, groundwater, and fluid inclusions. The similarity of δ37Cl values in fluid inclusions and groundwater suggests a closed-system evolution during the metamorphic overprint, and signatures established at this time appear to form the initial conditions for chloride transport after exhumation of the rock sequence.

An online method to determine chlorine stable isotope composition by continuous flow isotope ratio mass spectrometry (CF-IRMS) coupled with a Gasbench II

An online method using continuous flow isotope ratio mass spectrometry (CF-IRMS) interfaced with a Gasbench II was presented to determine chlorine stable isotope composition. Silver chloride (AgCl) was quantitatively derived from chloride by using silver nitrate (AgNO3), and then was reacted with iodomethane (CH3I) to produce methyl chloride (CH3Cl). A GasBench II equipped with a PoraPlot Q column was used to separate CH3Cl from any other gas species. Finally, chlorine stable isotope analysis was carried out on CH3Cl introduced to the IRMS in a helium stream via an active open split. The minimum amount of Cl used in this method is of the order of 1.4 μmol. Inter-laboratory and inter-technique comparisons show that the total uncertainty incorporating both the precision and accuracy of this method is better than 0.007%. Furthermore, ten seawaters sampled from different locations have a narrow δ37Cl value range from −0.008% to 0.010%, with a mean value of (0.000±0.006)%. This supports the assumption that any seawater can be representative of standard mean ocean chloride (SMOC) and used as an international reference material.

Major cation/chlorine ratio and stable chlorine isotopic compositions of sediment interstitial water in the Brazos-Trinity Basin IV from the Gulf of Mexico (IODP 308)

Interstitial water from the Integrated Ocean Drilling Program Expedition 308 (IODP 308), drilling in the Brazos-Trinity Basin IV on the slope of Gulf of Mexico, were analyzed for major anion and cation concentrations and δ37Cl isotopic compositions. In the Brazos-Trinity Basin IV, interstitial water samples from the Hole U1319 show chlorine content mostly centering around 560mM, and δ37Cl values from −0.81‰ to +0.71‰ with an average of −0.18‰, which are close to the seawater values (0‰). Integrated with the Na/Cl and Br/Cl ratios, the low δ37Cl of interstitial water show no relationship with the dissolution of halite or addition of relict brine. The concentrations of K+, Na+, Ca2+ and Mg2+ in the interstitial water show a similar decreasing pattern at shallow depths, reaching minima at ∼13.5m, corresponding to the alkalinity maximum. Relationships between alkalinity and Na+, K+, Ca2+, Mg2+ concentrations suggest that only Ca2+ concentration being controlled by the precipitation of diagenetic carbonates, while Na+, K+ and Mg2+ concentrations can be used as a geochemical indicator to reveal the fluid processes in the sedimentary basin. Abnormity low Na/Cl, K/Cl, and Mg/Cl ratios of interstitial water show a strong link to well-known cycles of sea level change to well imaged deposits in the fill of Brazos-Trinity Basin IV, which suggests that a lateral fluid with low cation/Cl ratios was probably intruded in the sediment strata during low stand sea level period in the basin.

Mineralogical control on the chlorine isotope composition of altered oceanic crust

Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n=50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from <0.01wt.% to 0.09wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. δ37Cl values of whole rock AOC samples range from −1.4 to +1.8‰. High δ37Cl values (≥ ~0.5‰) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero δ37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.

Compound-Specific Chlorine Isotope Analysis: A Comparison of Gas Chromatography/Isotope Ratio Mass Spectrometry and Gas Chromatography/Quadrupole Mass Spectrometry Methods in an Interlaboratory Study

Chlorine isotope analysis of chlorinated hydrocarbons like trichloroethylene (TCE) is of emerging demand because these species are important environmental pollutants. Continuous flow analysis of noncombusted TCE molecules, either by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) or by GC/quadrupole mass spectrometry (GC/qMS), was recently brought forward as innovative analytical solution. Despite early implementations, a benchmark for routine applications has been missing. This study systematically compared the performance of GC/qMS versus GC/IRMS in six laboratories involving eight different instruments (GC/IRMS, Isoprime and Thermo MAT-253; GC/qMS, Agilent 5973N, two Agilent 5975C, two Thermo DSQII, and one Thermo DSQI). Calibrations of (37)Cl/(35)Cl instrument data against the international SMOC scale (Standard Mean Ocean Chloride) deviated between instruments and over time. Therefore, at least two calibration standards are required to obtain true differences between samples. Amount dependency of δ(37)Cl was pronounced for some instruments, but could be eliminated by corrections, or by adjusting amplitudes of standards and samples. Precision decreased in the order GC/IRMS (1σ ≈ 0.1‰), to GC/qMS (1σ ≈ 0.2-0.5‰ for Agilent GC/qMS and 1σ ≈ 0.2-0.9‰ for Thermo GC/qMS). Nonetheless, δ(37)Cl values between laboratories showed good agreement when the same external standards were used. These results lend confidence to the methods and may serve as a benchmark for future applications.

Chlorine isotope variations along the Central American volcanic front and back arc

Volcanic outputs (ash, tephra, and lava samples) of 23 volcanic centers from the volcanic front, secondary front, and back arc in Central America were analyzed for their δ37Cl values with the goal of using chlorine isotopes as a tracer of fluids sourced from the subducting Cocos slab. δ37Cl values range from −2.6 to +3.0‰ with systematic variations along the length of the front. Values from the northernmost (Guatemala and El Salvador) and southernmost (Costa Rica) ends of the front have mantle‐like signatures. In contrast, δ37Cl values are both positive and negative in the center of the front (Nicaragua), implying a sediment and/or serpentinite‐derived component. Geophysical observations are consistent with extensive hydration of the Cocos plate offshore of Nicaragua, in support of a serpentinite‐derived fluid source. Fluids from dehydrating serpentinites may also incorporate Cl from overlying sediments, resulting in a multiple‐source chlorine signature.

The stable isotopic composition (37Cl/35Cl) of dissolved chloride in rainwater

Abstract The stable isotopic composition of dissolved Cl - in rainwater was measured from a coastal and an interior location in eastern Canada. At the interior Bonner Lake, Ontario, site the δ37Cl values of dissolved Cl - in precipitation ranged from −3.5‰ to −1.2‰ (SMOC) with an amount-weighted annual average of −2.3‰. At the coastal site, Bay D’Espoir, Newfoundland, δ37Cl values of dissolved Cl - values ranged from −3.1‰ to 0.0‰ with an amount-weighted annual average of −1.3‰. These negative δ37Cl values provide evidence that atmospheric HCl is 37Cl depleted, presumably from acidification of sea-salt aerosols. Accordingly, dissolved Cl - in the headwaters of two montane rivers in Western Canada had similarly depleted δ37Cl values. These results have implications to the interpretation of the isotopic compositions of dissolved Cl - in surface waters, formation fluids, and groundwaters.

Cl/Br ratios and stable chlorine isotope analysis of magmatic–hydrothermal fluid inclusions from Butte, Montana and Bingham Canyon, Utah

A bulk geochemical study has been carried out on fluid inclusion leachates extracted from quartz veins from porphyry Cu deposits in Butte, Montana, USA and Bingham Canyon, Utah, USA. The leachates mostly represent low-salinity magmatic–hydrothermal fluid inclusions. Their halogen ratios (Br/Cl) of fluid inclusion leachates were determined by ion chromatography, and δ37Cl values of the leachates were measured by continuous-flow isotope ratio mass spectrometry. Br/Cl ratios from early pre-Main stage and later Main stage veins at Butte range from 0.60 to 1.88 × 10−3 M. Ratios are similar in pre-Main stage veins with sericite bearing selvages and Main stage samples ranging from 0.81 to 1.08 × 10−3 and from 0.92 to 1.88 × 10−3 M, respectively, clustering below seawater (1.54 × 10−3 M) and overlapping mantle values (~1–2 × 10−3 M). Two samples associated with early pre-Main stage potassic alteration yield distinctly lower Br/Cl ratios of 0.60 and 0.64 × 10−3 M. Butte δ37Cl values range from −0.8‰ to −2.3‰ with no significant difference between pre-Main stage and Main stage samples. Br/Cl ratios for quartz veins from Bingham Canyon range from 0.18 to 3.68 × 10−3 M. Br/Cl ratios from Bingham range above and below previously reported for porphyry copper deposits. In contrast to Butte, δ37Cl values for Bingham are lower, ranging from −0.9‰ to −4.1‰. In the absence of any processes which can significantly fractionate chlorine isotopes at high temperatures, we suggest that the porphyry system at Bingham, and to a lesser extent at Butte, have inherited negative chlorine isotopic signatures from the subducting slab generated at low temperatures.

Chlorine isotope fractionation associated with volcanic activity at the Kusatsu-Bandaiko hot spring in Japan

Stable chlorine isotope compositions (δ37Cl, per-mil: ‰, vs. a standard sample of sea water) of Kusatsu-bandaiko hot water samples, taken regularly in the years between 1974 and 1995 in the Kusatsu-Shirane volcanic region, Japan, were measured mass-spectrometrically. The results show that the δ37Cl values of the waters taken before 1984 were at around−0.12‰, whereas those after 1984 were at around+0.18‰. The δ37Cl values are thus distinct across 1984, which is consistent with the classification by the Cl to S molar ratio (Cl/S): the higher the Cl/S ratio, the larger the δ37Cl value. The δ37Cl value increased as much as 0.30‰ during 5 years between 1980 and 1984. This isotopic enrichment is likely correlated with increasing Cl/S ratios, suggesting that the heavier isotope (37Cl) may have preferentially increased in the original Cl source of the hot spring across 1984 when volcanic activity likely increased at Mt Kusatsu-Shirane.

Two chloride sources in soils of the McMurdo Dry Valleys, Antarctica

Sulfate, nitrate, and chloride are major water‐soluble anions in soils of the McMurdo Dry Valleys (MDV), Antarctica. We measured the chloride stable isotope composition of 40 soil samples from 16 different soil profiles. The soil profiles come from three different microclimate zones in the MDV: a coastal thaw zone, which fosters a seasonally saturated active layer; a stable upland zone, in which air and soil temperatures are too cold to foster active layers and/or significant snowmelt; and an intermediate, inland mixed zone. Results show that the total water‐soluble chloride content is low in both the coastal and the upland zones, but highly variable in the inland mixed zone (average 770 to 11520 ppm). Individual soil δ37Cl values range from −2.10‰ to +0.56‰, with profile average δ37Cl values broadly decreasing from the coast toward upland valleys. Vertically, the δ37Cl increases with depth in soil profiles developed on basal tills but decreases on sublimation tills. We propose that the spatial [Cl−] and δ37Cl patterns are the result of continuous influx and leaching of two major chloride sources, an airborne sea‐salt chloride (SSC) with a δ37Cl value ≥ ∼0.0‰ and a secondary atmospheric chloride (SAC) with a δ37Cl value ≤ −2.10‰. Our model suggests that sublimation tills have their Cl− input from two boundaries: a till‐atmosphere interface at the top and a buried‐ice‐till interface at the bottom, differing distinctively from basal tills where there is no salt contribution from the bottom. Our model also suggests a higher influx of SSC relative to SAC in the MDV in the past. Overall, characterizing the sources, budgets, and spatial/vertical patterns of chloride constitutes a first step toward unraveling climate, tectonic, and landscape changes archived in MDV soil profiles.

Chlorine isotope chemistry of serpentinites from Elba, Italy, as an indicator of fluid source and subsequent tectonic history

Chlorine concentrations and δ37Cl and δD values are reported for serpentinites in different tectonic positions on Elba, Italy. Serpentinites from high in the nappe sequence contain 0.01–0.1wt% Cl with δ37Cl values from −0.9 to +1.4‰. Samples with positive δ37Cl values occur beneath gabbro and basalt and are interpreted as having been serpentinized by interaction with seawater. Samples with negative δ37Cl values are in direct contact with sedimentary rocks. These samples were juxtaposed against and/or buried by sediments in response to low‐angle normal faulting on the seafloor, and serpentinization probably occurred by subsequent interaction with sedimentary pore fluids. Most samples from a structurally lower nappe and from areas affected by contact metamorphism lost their original Cl during heating. Low δD values for all samples suggest postserpentinization interaction with meteoric water, but neither Cl nor O isotope ratios were affected by this interaction. Our results demonstrate that chlorine can be retained in obducted serpentinites up to low greenschist conditions and that the Cl isotope composition of such serpentinites preserves a record of seafloor tectonic processes.

Influence of air–sea fluxes on chlorine isotopic composition of ocean water: Implications for constancy in δ37Cl—A statistical inference

The behaviors of chlorine isotopes in relation to air–sea flux variables have been investigated through multivariate statistical analyses (MSA). The MSA technique provides an approach to reduce the data set and was applied to a set of 7 air–sea flux variables to supplement and describe the variation in chlorine isotopic compositions (δ37Cl) of ocean water.The variation in δ37Cl values of surface ocean water from 51 stations in 4 major world oceans—the Pacific, Atlantic, Indian and the Southern Ocean has been observed from −0.76 to +0.74‰ (av. 0.039±0.04‰). The observed δ37Cl values show basic homogeneity and indicate that the air–sea fluxes act differently in different oceanic regions and help to maintain the balance between δ37Cl values of the world oceans. The study showed that it is possible to model the behavior of chlorine isotopes to the extent of 38–73% for different geographical regions. The models offered here are purely statistical in nature; however, the relationships uncovered by these models extend our understanding of the constancy in δ37Cl of ocean water in relation to air–sea flux variables.

Polyhalite occurrence in the Werra (Zechstein, upper Permian) peribaltic basin of Poland and Russia: Evaporite facies constraints

Polyhalite is a common constituent of many ancient evaporite sequences, especially Permian and Neogene ones, that is related to the Na−K−Mg−Cl−SO4 type of marine brines in those time intervals. There are four polyhalite deposits in the Zechstein of northern Poland, and more than ten polyhalite-bearing areas in the adjacent part of Russia, and they are commonly accompanied by K−Mg chlorides. Most polyhalite occurrences are related to the upper part of the Lower Werra Anhydrite and in most cases, polyhalite deposits are concentrated at the sulfate platform close to its boundary with platform slope, where they can pass horizontally into polyhalite beds occurring in the Oldest Halite. The bromine content in samples of the Oldest Halite range from 40–120 ppm and the composition of fluid inclusions in halite are characteristic of halite precipitated from seawater concentrated to the early and middle stages of halite precipitation. The δ18O and δ34S values for sulfates are 10.03‰–13.50‰ and 10.03‰–12.14‰, respectively, and the δ37Cl values for halites from −0.1‰ to +0.4‰ support their marine origin. Bromine distribution in the Oldest Halite and the occurrence of anhydrite intercalations indicate fluctuations of the brine density during the Oldest Halite deposition. The formation of polyhalite was preceded by the syndepositional dehydration of the original gypsum deposit and it appears that the anhydrite was then transformed to polyhalite by reaction with marine brines more evolved than those from which precipitated precursor calcium sulfate minerals. These concentrated brines could have been derived from the evaporation of marine brines and/or inflow of K- and Mg-rich brines that were formed in nearby shallow salt pans occurring in sulfate platform areas and thus sulfate platform areas and adjacent slopes of those platforms were predestined for polyhalite formation.

Application of Multi-stable Isotope (13C, 15N, 34S, 37Cl) Assays to Assess Spatial Separation of Fish (Longnose Sucker Catostomus catostomus) in an Area Receiving Complex Effluents

Abstract Incorporation of stable isotope analysis (SIA) into routine environmental effects monitoring programs of receiving waters may enable determination of the spatial extent of biotic exposure and discrimination among sources of complex effluents. To evaluate this hypothesis, longnose sucker (Catostomus catostomus) were collected from four sites along the Athabasca River, Alberta (upstream reference site, two sites downstream of effluents from two pulp and paper mills, and a site downstream of effluent from a municipal sewage treatment plant). Stable isotopes of carbon, nitrogen, sulfur and chlorine were analyzed in bone, gonad, liver and white muscle tissues of the fish. In general, most sites and tissues differed according to δ13C, δ15N and δ34S values. Also, an interaction between site and tissue was observed for δ15N values. A better insight into the usefulness of stable isotopes was obtained through the use of multivariate discriminant function analysis. δ15N and δ34S signatures of gonad and liver tissues of males were most effective at classifying fish according to site (~70% for both tissues). For all tissues except bone, fish from the upstream reference site were most separable from all others, especially females. δ37Cl values for female gonads and male livers were related to sites downstream of the pulp and paper mills. Future research should routinely include SIA of fish tissues, but also of effluents, receiving waters and food web components to better resolve links between specific effluents and fish exposure.

On-Line Technique for the Determination of the δ37Cl of Inorganic and Total Organic Cl in Environmental Samples

Here we describe an on-line method for measuring delta(37)Cl values of chloride bearing salts, waters, and organic materials using multicollector continuous-flow isotope ratio mass spectrometry (CF-IRMS). Pure AgCl quantitatively derived from total Cl in water, inorganic Cl salts, and biological samples was reacted with iodomethane in evacuated 10-mL stopper sealed glass vials to produce methyl chloride gas. A GV Instruments Multicollector CF-IRMS with CH(3)Cl optimized collector geometry was modified to accommodate a headspace single-sample gas injection port prior to a GC column. The GC column was a 2-m Porapak-Q packed column held at 160 degrees C. The resolved sample CH(3)Cl was introduced to the IRMS source in a helium stream via an open split. delta(37)Cl values were calculated by measurement of CH(3)Cl at m/z 52/50 and by comparison to a reference pulse of CH(3)Cl calibrated to standard mean ocean chloride. Sample CH(3)Cl analysis time was approximately 6 min. Injections of 40 microL of pure CH(3)Cl gas yielded a repeatability (+/-SD) of +/-0.06 per thousand for delta(37)Cl (n = 10). Combined GC and IRMS source linearity for CH(3)Cl was <0.2 per thousand/nA (V) peak height. External repeatability, based on processing of seawater and NaCl reference solutions, was better than +/-0.08 per thousand. The smallest sample for delta(37)Cl analysis by this method was approximately 0.2 micromol of Cl. Selected results from a river basin and biological samples study illustrate the potential of on-line chlorine isotope assays in environmental pollution studies.

Water-soluble chlorides in massive seafloor serpentinites: a source of chloride in subduction zones

Dissolved cation and anion species were measured in the water-soluble fraction of massive seafloor serpentinites leached with distilled H2O. Soluble chloride contents ranged from 0.09 to 0.51 wt.% (average=0.26 wt.%), SO42− to 0.09 wt.%, Mg2+ to 0.16 wt.% and Na+ to 0.09 wt.%. The concentration of evolved ions was the same in samples equilibrated with water at 25 and 230 °C, demonstrating that a significant amount of Cl present in serpentinites occurs as water-soluble salts. A nearly equal amount of Cl exists in the leached serpentinite, likely sited in serpentinite itself. The formation of intergranular salts in the serpentinite can be explained by seawater hydrothermal alteration of peridotite. Hydration reactions and H2O-incorporation in the serpentine leave the residual fluid with ever-increasing salinity and ultimately lead to precipitation of water-soluble chlorides. Chlorine isotope ratios in the water-soluble fraction average −1.49‰ (vs. standard mean ocean chlorine, SMOC) from a core at the Iberian Abyssal Plain, and +0.16‰ from a much younger core at Hess Deep. The δ37Cl value of insoluble Cl is ∼0.33‰ higher than the soluble fraction. The nonzero δ37Cl values demonstrate that the Cl in the serpentinites is not an artifact of seawater incorporation during drilling.The following conclusions are presented: (1) Serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle. The Cl flux in serpentinites into the mantle is larger than all other previously identified fluxes. (2) Cl is sited in both a water-soluble and -insoluble component. (3) A high-salinity fluid ‘plume’ is evolved during serpentinite dehydration of a subducting slab. The high-salinity fluid affects metamorphic reactions and melting properties above the slab and can cause intense metasomatism. The unusual bulk composition of the whiteschist from Dora Maira can be explained by channeled passage of a high-salinity fluid. (4) High-salinity fluids reduce the stability field of serpentine, causing its dehydration to occur over a broad interval, which may partly explain the broad locus of intermediate-depth earthquakes.

Extreme Cl/Br and δ37Cl isotope fractionation in fluids of modern submarine hydrothermal systems

Microthermometric studies of sphalerite-hosted fluid inclusions from fracture fillings in felsic igneous rocks of modern polymetallic massive sulfide mineralization from the JADE field (Central Okinawa Trough, Japan) and from mounds on the seafloor in the North Fiji basin reveal evidence for sub-critical two-phase separation of seawater. Chemical analyses of the fluid inclusions indicate intense water–rock interaction between the hydrothermal fluids and their host rocks. The analyses indicate that sub-critical phase separation has a much larger effect on the partitioning of chloride and bromide into liquid and vapor phase than previously reported for vent fluids. This study also shows that, for the first time, chlorine isotope fractionation correlates with Cl and Br fractionation. Using combined Cl/Br ratios and δ37Cl values allows us to discount Br-enriched organic matter or halite precipitation as responsible for the formation of Br-rich fluids. Moreover, the same parameters exclude any significant magmatic component in the mineral-forming fluids as previously proposed for similar modern sulfide deposits and other massive sulfide deposits in the geological record.