Hydrothermal End-member Research Articles

Chromium stable isotope distributions in the southwest Pacific Ocean and constraints on hydrothermal input from the Kermadec Arc

Special attention has been given to chromium (Cr) as a paleoproxy tracing redox cycling throughout Earth’s history, due to differences in the solubility of its primary redox species at Earth’s surface (Cr(III) and Cr(VI)) and isotope fractionation associated with their interconversion. In turn, chromium’s paleoproxy potential has motivated studies of the modern ocean to better understand which processes drive its cycling and to constrain their impact on the Cr isotope composition (δ53Cr) of seawater. Here, we present total dissolved seawater Cr concentrations and δ53Cr along the GEOTRACES GP13 section. This section is a zonal transect extending from Australia in the subtropical southwest Pacific Ocean. Surface signals of local biological Cr cycling are minimal, in agreement with distributions of dissolved major nutrients as well as biologically-controlled trace metals in this low productivity, oligotrophic environment. Depth profiles have Cr concentration minima in surface waters and maxima at depth, and are largely shaped by the advection of nutrient- and Cr-rich subsurface waters rather than vertically-driven processes. Samples close to the sediment–water interface indicate important benthic Cr fluxes across the section.The GP13 transect crosses the hydrothermally-active Kermadec Arc. Hydrothermal fluids (consisting of <15% background seawater) were collected from three venting sites at the Brothers Volcano (along the Kermadec Arc). These fluids yielded near-crustal δ53Cr values (−0.17 to +0.08‰) and elevated [Cr] (7.5–23 nmol kg−1, hydrothermal endmember [Cr] ≈ 8–27 nmol kg−1), indicating that the Kermadec Arc may be an isotopically light Cr source. Dissolved [Fe] enrichments have been reported previously in deep waters (∼1600–3000 m) along the GP13 transect, east of the Kermadec Arc. These same waters show elevated [Cr] compared to Circumpolar Deep Water ([Cr] = 3.88 ± 0.11, δ53Cr = 0.89 ± 0.08, n = 32), with an average [Cr] accumulation of 0.71 ± 0.11 nmol kg−1 (1 SD), and an estimated δ53Cr of +0.46 ± 0.30‰ (2 SD, n = 9) for the accumulated Cr. Comparing high-temperature vent and neutrally buoyant plume data, hydrothermal-sourced Cr is likely negligable compared to Cr contributions from other processes (benthic fluxes, release from particles), and the advection of more Cr-rich Pacific Deep Water. It is unlikely that hydrothermal vents would be a major contributor within the regional or global biogeochemical Cr cycle, even if hydrothermal fluxes change by orders of magnitude, and therefore δ53Cr trends in the paleorecord may be attributable, at least in part, to major changes in other controls on Cr (e.g. widespread anoxia).

Mantellic degassing of helium in an extensional active tectonic setting at the front of a magmatic arc (central Mexico)

Abstract The physicochemical and isotopic characteristics of groundwater and dissolved gas of central Mexico provide valuable information about the geologic and tectonic context of the area. Low–high-enthalpy manifestations (up to 98 °C in springs and more than 100 °C in geothermal wells) are distributed within the San Juan del Río, Querétaro, and Celaya hydrologic basins, located at the boundary between the current Mexican magmatic arc and an extensional continental area with intraplate volcanism called Mesa Central Province. Groundwaters in the study area represent a mixture between the cold water end-member with a Ca2+-Mg2+-HCO3-composition and a hydrothermal end-member enriched in Na+, K+, SO42−, and Cl-. Cold and hot groundwaters δ2H and δ18O plot along the same evaporation lines and do not exhibit a magmatic input. Dissolved and free gas do not show a typical volcanic composition signature. He and Ne isotope composition provide evidence of an important contribution of non-atmospheric noble gases. Although helium composition mainly has a crustal origin (21–83%), the mantellic contribution (1–39%) is higher than expected for an area lacking recent volcanism. A volatilerich magma aging at depth was discarded as the source of this mantellic helium signature but points out a recent mantellic contribution. Thus, we propose that mantellic helium comes from the sublithospheric mantle into the shallow crust through the highly permeable tectonic boundaries between the geologic provinces, namely the N−S Taxco−San Miguel de Allende and Chapala-Tula fault systems. Mantellic helium flow rates through these fault systems were estimated to have values ranging from 0.1 m/yr to 2.9 m/yr. This He flux range implies that aside from subduction, mantle volatile degassing enhanced by crustal fault systems is the main degassing process in the region studied.

Sr isotopic ratios of hydrothermal fluids from the Okinawa Trough and the implications of variation in fluid–sediment interactions

Sr isotope ratios of hydrothermal fluids were observed at five sediment-associated sites in the Okinawa Trough to investigate the diversity of subseafloor fluid–rock–sediment interactions. The estimated 87Sr/86Sr ratios of the hydrothermal endmember fluids at the five sites were all higher than those at the sediment-starved sites. The endmember Sr isotopic ratios of hydrothermal fluids were diverse within the Okinawa Trough, ranging from 0.7077 at the Iheya North Knoll site to 0.712 at the Yonaguni Knoll IV site. To our knowledge, 0.712 is the highest value reported to date for seafloor hydrothermal fluids. This variation is likely attributable to the relative contributions of multiple subseafloor Sr reservoirs, which are 87Sr-poor volcanic rock and 87Sr-rich hemipelagic sediments containing clay minerals of terrestrial origin. These data support a model based on the carbon isotope ratio of CH4, which indicates whether volcanic rocks or terrestrial sediments are distributed in the high-temperature reaction zone of the hydrothermal system.

High-Ca vent fluids discharged from the Lutao arc volcanic hydrothermal system are associated with albitization and recycling of marine carbonate

The chemical and isotopic characteristics of calcium (Ca) in subduction zones are closely related to the budget of Ca and carbon cycles. Here we investigate the ultra-high Ca concentrations that characterize the hydrothermal fluids discharged from two types of vents, named the Zhudanqu brine vent (ZDQ) and the Huwaichi vapor spring (HWC), in the Lutao hydrothermal system at the north Luzon arc. The Ca concentrations of up to 159 mM and Ca/Cl ratio of up to 0.26 in the ZDQ vent fluids are possibly the highest ever reported for Ca enrichment in global seawater-circulated hydrothermal/geothermal systems. The differences in chemical compositions between the ZDQ and the HWC vent fluids are primary controlled by subcritical phase separation. The brine phase constitutes the ZDQ vent fluids, while the HWC vent fluids represent mixtures of the vapor phase and seawater. Both the vapor and the brine phases exhibit similar δ44/40Ca values (0.72 ± 0.05‰), suggesting no significant Ca isotope fractionation has occurred during phase separation.The hydrothermal endmember before phase separation (the “Lutao endmember”) presents depletions of 213 ± 15 mM of Na, 24.4 ± 0.4 mM of SO42−, and 10.2 mM of K, and enrichment of 130.2 ± 5.5 mM of Ca with respect to the percolated seawater. The total gained Ca is 154.6 ± 5.9 mM with a δ44/40Ca value of 0.67‰ – 0.77‰ (0.72 ± 0.05‰), considering anhydrite precipitation during hydrothermal circulation. The Holocene raised coral reef is unlikely to contribute substantial Ca into the Lutao system. Much of the gained Ca (111.6 ± 7.5 mM) is produced by high-degree albitization of the Lutao host rock, which is promoted by the low water/rock ratio (~ 2), slightly alkaline conditions, and relatively lower temperature of the Lutao system with respect to most mid-ocean ridge hydrothermal systems. Ca derived from this process inherits the Ca isotopes of plagioclase in the Lutao host rocks (δ44/40Ca = 0.82 ± 0.06‰). According to mass and isotopic balances, the recycled marine carbonate is proposed to contribute 43 ± 13.4 mM Ca with a δ44/40Ca value of 0.46−0.63+0.35‰ into the Lutao system. Such isotopically lighter Ca is derived from either pore fluids expulsed from underlying Philippine Sea sediments, or more probably, carbonate-bearing subduction fluids from the subducting South China Sea sediments and slab. The carbonate solubility in the subduction fluids could maintain at 600 mM near the reaction zone. The carbonate-rich fluids were subsequently migrated into the Lutao reaction zone and released an additional 43 ± 13.4 mM Ca via dolomitization. A small amount (~ 9%) addition of carbonate-rich fluids would not significantly change the budgets of Na, Mg, and Cl but could generate substantial Ca enrichment and Ca isotopic variations.

Response of a hydrothermal system to escalating phreatic unrest: the case of Turrialba and Iraz\xfa in Costa Rica (2007\u20132012)

This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica, manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012) coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140 years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year (~ 2400 m3/year by San Cayetano-Río Sucio). Despite Irazú being the currently less active volcano, it is a highly efficient rock remover, which, on the long term can have effects on the stability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse, landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/s above pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that the drying of the summit hydrothermal system of Turrialba could tap deeper than previously thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

Geochemical and mineralogical composition of ferromanganese precipitates from the southern Mariana arc: Evaluation, formation, and implications

The Mariana intraoceanic volcanic arc system in the western Pacific Ocean hosts abundant ferromanganese (Fe-Mn) precipitates. A suite (n = 22) of Fe-Mn precipitates were collected from the southern portion of the arc and their mineralogies and chemical compositions were determined. These results were used to decipher their genetic assemblage, assess their potential as a source of trace metals, and place them into context with respect to Fe-Mn precipitates sampled at higher latitudes in the Mariana arc and from other locations, globally. Minerals identified include vernadite, birnessite, 10 Å manganate, manganite, hematite, goethite, maghemite, calcite, rhodochrosite, quartz, phillipsite, various feldspar, pyroxene, and clay minerals. Element discrimination diagrams indicate that the samples are predominantly of hydrothermal and hydrogenetic-hydrothermal (i.e., mixed) origin, with most reflecting some influence of both. Rare earth element and Y (REY) profiles are distinguished by negative Ce and Y anomalies and positive Eu anomalies. Together, the samples form a continuum from the hydrogenetic to hydrothermal endmembers. Samples with the largest hydrogenetic component are friable with branching oxide/oxyhydroxide growth structures, contain mostly vernadite, and have the greatest concentrations of most metals (including the REY elements). Hydrothermal input produces denser, cemented deposits that contain more 10 Å and 7 Å manganate minerals and lower minor-metal contents. Calculated growth rates range from 6 mm to >190 m Ma−1. Average metal contents of Fe-Mn precipitates from the southern Mariana arc are low relative to hydrogenetic Fe-Mn crusts and hydrothermal Fe-Mn deposits from the northern Mariana arc and elsewhere, globally, and are therefore unlikely to be viable exploration targets.

Changing Brine Inputs Into Hydrothermal Fluids: Southern Cleft Segment, Juan de Fuca Ridge

AbstractIn 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge. Coupled with previous sampling efforts, this collection represents a 32‐year record of discharge from a single chimney structure, the longest record to date. Remarkably, the fluid has remained brine‐dominated for more than three decades. This brine formed during phase separation and segregation prior to initial observations in 1984. Although the chloride concentration of the discharging fluid has decreased with time, the fluid temperature has remained nearly constant for at least 3.3 years and probably for 15 or even 22 years. Compositions of the discharging fluids are consistent with inputs from a deep‐sourced brine, which was last equilibrated at &gt;400°C at a depth consistent with the base of the sheeted dikes and the brittle‐ductile transition. This brine mixed (diffusion or dispersion) with a likely non‐phase‐separated, hydrothermal fluid prior to discharge. A survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol/kg shows, with the exception of Fe, a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions, spreading rates, and water and magma depths. Calculated deep‐sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas. A better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals.

Imprint of Kairei and Pelagia deep-sea hydrothermal systems (Indian Ocean) on marine dissolved organic matter

Only few studies exist that investigate the dynamics of deep-sea dissolved organic matter (DOM) derived from hydrothermal vents. In this study, we provide first insight into the molecular composition of DOM associated with Indian Ocean hydrothermal systems covering the full range from hot focussed endmember fluids over diffuse fluids to open ocean hydrothermal plumes and deep seawater. We combined geochemical analyses with molecular characterization of DOM using ultra-high resolution mass spectrometry (FT-ICR-MS). We studied two vent systems with fluids venting >330 °C and up to 97% of hydrothermal endmember: the Kairei vents (Central Indian Ridge) with brine phase separation, and the newly discovered Pelagia vents (South-East Indian Ridge). The hot fluids in both systems were highly enriched in dissolved Fe, Si, K, Li, Mn and Zn compared to seawater. The molecular composition of DOM from hot fluids differed substantially from that of diffuse fluids and plumes, in which the composition was highly dominated by the seawater DOM signature. Low O/C ratio average in hot fluids (<0.38) indicated potential input of more reduced (O-poor) DOM compounds from the vents into the surrounding seawater, independently of the vent location and of whether the fluid has undergone phase separation. To test the importance of pure thermal degradation, we compared our samples to fluids subjected to abiotic thermal degradation under laboratory conditions and we observed that our natural samples largely differed (>94% Bray Curtis dissimilarity) from the experimental ones, suggesting additional degradation processes of organic compounds at the Indian Ocean hydrothermal systems.

A fundamental role of carbonate–sulfate melts in the formation of iron oxide–apatite deposits

Genetic models for iron oxide–apatite deposits are controversial and span a spectrum from orthomagmatic to hydrothermal endmembers. This lack of consensus is rooted in uncertainties as to the nature and origin of ore-forming fluids in these systems. Here, we present a fluid-inclusion study of mineralizing fluids at two iron oxide–apatite deposits (Buena Vista, Nevada and Iron Springs, Utah). We found that the inclusions in both systems comprise both aqueous brine and ubiquitous iron-rich carbonate–sulfate melts. These melts were found throughout the paragenesis of both deposits and show a tremendous capacity to transport ferric iron. Hence, we argue that orthomagmatic fluids played a role in mineralization at both Buena Vista and Iron Springs, and that the main ore-forming fluid was an iron-rich carbonate–sulfate melt formed by the assimilation and anatexis of evaporite-bearing carbonate rocks. The geological conditions that give rise to carbonate–sulfate melts are also a common feature of other classic iron oxide–apatite systems worldwide. Hence, we argue that the process of assimilation, anatexis and immiscibility of carbonate–sulfate melts is fundamental to iron oxide–apatite formation and provides a common link between iron oxide–apatite systems in different geological settings. Iron-rich carbonate–sulfate melts are fundamental to the formation of iron oxide–apatite ore deposits, according to a detailed fluid-inclusion study that characterized the mineralizing fluids for two mineralizing systems in the United States.

Subcritical Phase Separation and Occurrence of Deep-Seated Brines at the NW Caldera Vent Field, Brothers Volcano: Evidence from Fluid Inclusions in Hydrothermal Precipitates

The northwestern caldera wall of Brothers volcano in the southern Kermadec arc features several clusters of hydrothermal venting in a large area that extends from near the caldera floor (~1700 mbsl) almost up to the crater rim (~1300 mbsl). Abundant black smoker-type hydrothermal chimneys and exposed stockwork mineralization in this area provide an excellent archive of hydrothermal processes that form seafloor massive sulfide deposits. Using sulfate precipitates from chimneys and stockwork recently recovered by remotely operated vehicles, we conducted fluid inclusion microthermometry and Sr isotope studies to determine the role of phase separation and mixing between vent fluid and seawater. The variability in the vast majority of fluid inclusion salinities (i.e., 0.1–5.25 wt.% NaCl eq.) and entrapment temperatures of up to 346°C are indicative of phase-separated hydrothermal fluids. Large salinity variations in samples with entrapment temperatures mostly below the boiling temperature for the sample’s depth show that the majority of fluids ascending below the NW Caldera are phase separating in the subsurface and cooling, prior to discharge. In several samples, entrapment temperatures of over 343°C suggest that phase-separating fluids have at least sporadically exited the seafloor at the NW Caldera site. Isobaric-isenthalpic mixing trends between coexisting phase-separated vapors and brines with seawater are consistent with phase-separated fluids at near-seafloor pressures of ~170 bar and suggest that the vast majority of the ascending fluids continue to phase separate to within tens to hundreds of meters below seafloor prior to mixing with seawater. A small subset of the most saline fluid inclusions (up to 18.6 wt.% NaCl eq.) is unlikely formed by near-seafloor phase separation and is considered to be produced either by supercritical phase separation or by the contribution of a magmatic brine from near the magmatic-hydrothermal interface. 87Sr/86Sr values of sulfate samples range from 0.7049 (i.e., near hydrothermal end-member) to 0.7090 (i.e., near seawater) and show that the crystals grew from vapor- and brine-derived fluids in a hydrothermally dominated mixing regime. Our work provides new insights into mineral growth conditions, mixing regimes, and in particular, the extent and character of subseafloor phase separation during the formation of hydrothermal vents and their underlying stockwork in seawater-dominated, arc-related hydrothermal systems.

Gas geochemistry of hot springs at the Tengchong field, Southwest China: Controlled by the spatial distribution of magmatic chamber

The Tengchong volcanic field, located at the collision zone between the Indian and Eurasian plates, is well known for its large-scale Holocene volcanic eruptions and extensive hydrothermal activities. Bubbling gases from eight geothermal springs at the Tengchong field were collected and analyzed for their chemical and isotopic compositions (3He/4He, 20Ne, N2, Ar, δ13CCH4 and δ13CCO2). The results show that the gas samples could be categorized into a group I dominated by N2 and a group II with CO2 as major component. The air-corrected 3He/4He ratios ranged from 0.17 Ra to 4.76 Ra and the calculated mantle contributions dramatically varied from 1.9% to 77.9% for group II. By contrast, an invariant air-corrected 3He/4He ratio of about 0.30 Ra and a relatively constant mantle contribution of 4.0% to 5.0% was obtained for group I. The sampling sites just on top of the magma chamber received significant mantle-derived helium and magmatic gas components (at the proximal site of the magma chamber: >95% magmatic gas contributions). The calculated N2/Ar ratio of the hydrothermal endmember for group II (sample sites close to the magma chamber) was 61.7–66.2, suggesting that the reaction zone temperature is between 223 °C and 252 °C. The influence of the magma chamber decreases with increasing distance of the sampling sites from the chamber apex. The sampling sites (group I) at the marginal area discharge gases with relatively low 3He/4He ratios and magmatic contributions (19–24%). The temperature of the reaction zone for this group was estimated to be 74 °C based on the N2/Ar ratio of 42. This study provided important constraints to relate hydrothermal gas geochemistry directly with the spatial distribution of the underlying magma chamber. Plain language summaryGeochemical characteristics of gases discharged from hydrothermal systems are usually affected by deep magmatic processes, shallow hot fluids circulation, and water-rock interactions in the reaction zone. In this study, we analyzed the gas abundances and isotopic compositions of hydrothermal gas discharged from the Yunnan-Tibet geothermal belt. We used a gas mixing model to quantify the contributions of mantle-derived/magmatic endmember and hydrothermal endmember. The 3He/4He ratios, magmatic contribution, and temperature of the reaction zone decrease significantly with increasing distance from the magma chamber underneath the Tengchong field, highlighting the intimate relationships between gas transport, hydrothermal circulation and the extent of the magma chamber.

Spatially and temporally variable sulfur cycling in shallow-sea hydrothermal vents, Milos, Greece

Shallow-sea hydrothermal systems are ideal for studying the relative contributions to sedimentary sulfur archives from ambient sulfur-utilizing microbes and from fluxes of hydrothermally derived sulfur. Here we present data from a vent field in Palaeochori Bay, Milos, Greece using a suite of biogeochemical analytical tools that captured both spatial and temporal variability in biotic and abiotic sulfur cycling. Samples were collected along a transect from a seagrass meadow to an area of active venting. The abundance and isotopic composition of sulfide captured in situ, together with geochemistry from sedimentary porewaters and the overlying water column and solid phase sulfide minerals, record evidence of ephemeral activity of microbial sulfate reduction as well as sulfide oxidation. The sulfur and oxygen isotope composition of porewater sulfates indicate active sulfate reduction within the transition zone between the vents and seagrass, rapid recycling of biologically produced sulfide within non-vent sediments, and reoxidation of abiotic sulfide within the vent field. A phylogenetic survey of sediments also indicates the pervasive presence of a suite of putative sulfur-metabolizing bacteria, including sulfate reducers and sulfide oxidizers, many of which can utilize intermediate valence sulfur compounds. The isotopic composition of pyrite in these sediments consistently records a microbially influenced signature (δ34Spy of −4.4 to −10.8‰) relative to the hydrothermal endmember (δ34S ~ + 2.5‰), independent of distance from the vent source. The narrow range of pyrite δ34S across sediments with a highly variable hydrothermal influence suggests that physical mixing (e.g., by storm events) homogenizes the distribution of biogenic and hydrothermal Fe-sulfides throughout the region, overprinting the spatially and temporally variable interplay between biological and hydrothermal sulfur cycling in these environments.

Geochemistry and isotope composition (Sr, Pb, δ66Zn) of Vulcano fumaroles (Aeolian Islands, Italy)

We present and discuss temperatures, major and trace element gas geochemistry, radiogenic isotopes (Pb, Sr) and the first Zn isotope data of fumarole condensates and altered rocks from the Vulcano fumarolic field. The fumaroles of the La Fossa cone, sampled on 5th May 2015, have temperatures ranging between 233 and 427 °C. They plot compositionally on the mixing trend between the magmatic and hydrothermal end-members defined by previous studies, but are strongly displaced towards the hydrothermal component. Correlations of radiogenic (Sr, Pb) and stable isotopes of Zn with δ13CCO2 and several trace elements of the fumarolic acid condensates support mixing between the above mentioned distinct (magmatic and hydrothermal) fluids. The magmatic end-member has a less radiogenic Sr (87Sr/86Sr ~0.7045) and heavier Zn isotope composition (δ66Zn ~0.3‰). The hydrothermal fluid end-member has a more radiogenic 87Sr/86Sr signature (>0.7055), which could be due to leaching of radiogenic Sr from the crystalline basement rocks or reflect seawater Sr. It is also characterized by lighter Zn isotope composition (δ66Zn < −0.3), most likely reflecting equilibrium fractionation of Zn isotopes during precipitation of sphalerite (ZnS) from the hydrothermal fluid. Nonetheless, some scatter in the correlation trends suggests either the involvement of additional (subordinate) source(s) or temporal variations in isotopic and chemical compositions of the two above end-members.Pb isotope compositions of the fumaroles sampled in this study show a shift towards less radiogenic values compared to pre-2001 fumaroles. This could indicate either a change in the hydrothermal circulation pattern (leading to leaching of rocks with different isotopic compositions) or the involvement of a new, isotopically distinct, magmatic fluid.The alteration zones around the fumarole vents are also characterized by systematic correlations of Pb and Zn isotopes with major and trace elements. We interpret these as the result of the addition onto the substratum volcanic rock of Pb and Zn from the fumaroles. Zn isotope signatures of the alteration zones are significantly heavier than those of the corresponding fumaroles (δ66Zn = 0.3–1.6) probably due to equilibrium fractionation occurring during sphalerite precipitation at the vent discharge.

Salinity of the Archaean oceans from analysis of fluid inclusions in quartz

Fluids trapped in inclusions in well-characterized Archaean hydrothermal quartz crystals were analyzed by the extended argon–argon method, which permits the simultaneous measurement of chlorine and potassium concentrations. Argon and nitrogen isotopic compositions of the trapped fluids were also determined by static mass spectrometry. Fluids were extracted by stepwise crushing of quartz samples from North Pole (NW Australia) and Barberton (South Africa) 3.5–3.0-Ga-old greenstone belts. The data indicate that fluids are a mixture of a low salinity end-member, regarded as the Archaean oceanic water, and several hydrothermal end-members rich in Cl, K, N, and radiogenic parentless 40Ar. The low Cl–K end-member suggests that the salinity of the Archaean oceans was comparable to the modern one, and that the potassium content of the Archaean oceans was lower than at present by about 40%. A constant salinity of the oceans through time has important implications for the stabilization of the continental crust and for the habitability of the ancient Earth.

Gas discharges from the Kueishantao hydrothermal vents, offshore northeast Taiwan: Implications for drastic variations of magmatic/hydrothermal activities

The chemical compositions of gas discharges from the Kueishantao (KST) hydrothermal field changed dramatically from 2000 to 2014. In this study, we established a gas mixing model for the KST gases. The N2, Ar, and CO2 contents were mixed from a magmatic endmember with CO2 of about 990 mmol/mol, a hydrothermal and an atmospheric endmember enriched in N2 and Ar. More than 71% KST gas components were mantle-derived/magmatic. The calculated endmember N2/Ar ratio and Ar contents of the hydrothermal endmember (percolated fluid) are about 140 and 5.28–5.52 mmol/mol, respectively. This relatively elevated N2/Ar ratio was probably caused by the thermogenic addition of N2. The log(CH4/CO2) values of the KST gas samples correlate well with the mixing temperature that estimated from the mixing ratio between the percolated fluid and the magmatic endmember. It is indicated that the KST CH4 and CO2 may have attained chemical equilibrium. The temporal variations of the KST gas compositions are determined by the mixing ratio, which is dependent on the magmatic activity underneath the KST field. With the decreasing of magmatic activity since 2005, the proportion of the hydrothermal endmember increased, along with the increasing of N2, Ar, and CH4 contents. This study proposed an effective model to quantitatively assess the sources of gas components discharged from submarine hydrothermal vents. In addition, it is suggested that the mixing between a magmatic and a hydrothermal endmember may play an important role in the concentrations of CO2 and CH4 in hydrothermal gas discharges.

Fluid chemistry of the low temperature hyperalkaline hydrothermal system of Prony Bay (New Caledonia)

Abstract. The terrestrial hyperalkaline springs of Prony Bay (southern lagoon, New Caledonia) have been known since the nineteenth century, but a recent high-resolution bathymetric survey of the seafloor has revealed the existence of numerous submarine structures similar to the well-known Aiguille de Prony, which are also the location of high-pH fluid discharge into the lagoon. During the HYDROPRONY cruise (28 October to 13 November 2011), samples of waters, gases and concretions were collected by scuba divers at underwater vents. Four of these sampling sites are located in Prony Bay at depths up to 50 m. One (Bain des Japonais spring) is also in Prony Bay but uncovered at low tide and another (Rivière des Kaoris spring) is on land slightly above the seawater level at high tide. We report the chemical composition (Na, K, Ca, Mg, Cl, SO4, dissolved inorganic carbon, SiO2(aq)) of 45 water samples collected at six sites of high-pH water discharge, as well as the composition of gases. Temperatures reach 37 °C at the Bain des Japonais and 32 °C at the spring of the Kaoris. Gas bubbling was observed only at these two springs. The emitted gases contain between 12 and 30% of hydrogen in volume of dry gas, 6 to 14% of methane, and 56 to 72% of nitrogen, with trace amounts of carbon dioxide, ethane and propane. pH values and salinities of all the 45 collected water samples range from the seawater values (8.2 and 35 g L−1) to hyperalkaline freshwaters of the Ca-OH type (pH 11 and salinities as low as 0.3 g L−1) showing that the collected samples are always a mixture of a hyperalkaline fluid of meteoric origin and ambient seawater. Cl-normalized concentrations of dissolved major elements first show that the Bain des Japonais is distinct from the other sites. Water collected at this site are three component mixtures involving the high-pH fluid, the lagoon seawater and the river water from the nearby Rivière du Carénage. The chemical compositions of the hyperalkaline endmembers (at pH 11) are not significantly different from one site to the other although the sites are several kilometres away from each other and are located on different ultramafic substrata. The very low salinity of the hyperalkaline endmembers shows that seawater does not percolate through the ultramafic formation. Mixing of the hyperalkaline hydrothermal endmember with local seawater produces large ranges and very sharp gradients of pH, salinity and dissolved element concentrations. There is a major change in the composition of the water samples at a pH around 10, which delimitates the marine environment from the hyperalkaline environment. The redox potential evolves toward negative values at high pH indicative of the reducing conditions due to bubbling of the H2-rich gas. The calculation of the mineral saturation states carried out for the Na-K-Ca-Mg-Cl-SO4-DIC-SiO2-H2O system shows that this change is due to the onset of brucite formation. While the saturation state of the Ca carbonates over the whole pH range is typical of that found in a normal marine environment, Mg- and Mg-Ca carbonates (magnesite, hydromagnesite, huntite, dolomite) exhibit very large supersaturations with maximum values at a pH of around 10, very well marked for the Bain des Japonais, emphasizing the role of water mixing in mineral formation. The discharge of high-pH waters of meteoric origin into the lagoon marine environment makes the hydrothermal system of Prony Bay unique compared to other low temperature serpentinizing environments such as Oman (fully continental) or Lost City (fully marine).

Subseafloor phase equilibria in high-temperature hydrothermal fluids of the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17′N)

As part of an integrated study conducted at the Lucky Strike Seamount (Mid-Atlantic Ridge, 37°17′N) in 2008, gas-tight sampling devices were used to collect high-temperature (∼300°C) hydrothermal fluids issuing from sulfide structures distributed throughout the vent field located in the summit depression. Compared with previous observations from 1993 to 1997, the most substantial changes in vent fluid compositions are dramatically increased CO2 concentrations (∼5×, up to 133mmol/L) and the observation of vent fluids enriched in dissolved chloride relative to seawater. Combined with an increase in δ13CCO2 values by ∼4‰ in 2008, the elevated CO2 indicates replenishment of the magmatic heat source and may be indicative of a recent magmatic event. The additional supporting fluid chemistry is, however, similar to that of the previous sampling intervals, necessitating a reassessment of the subseafloor controls on vent fluid chemistry at Lucky Strike in the context of recently obtained geophysical data that provides the depth/extent of a steady-state magma chamber. Two-phase behavior is indicated by the chloride variability in the vent fluids; and comparison with experimental data for the associated chloride-dependent partitioning of minor/trace elements suggests the possibility of a similar source fluid for all the vent structures, while limiting the likelihood of shallow phase separation and subseafloor mixing for the hydrothermal end-members. A recently calibrated Fe/Mn geothermometer indicates minimum subseafloor equilibration temperatures of 350–385°C. However, constraints imposed by dissolved Si/Cl in conjunction with geophysical observations are consistent with peak reaction conditions at temperatures of 430–475°C and pressures near the top of the axial magma chamber (∼410–480bars), where magmatic CO2 becomes entrained in the circulating fluids. The distance between the magma chamber and the seafloor at Lucky Strike is substantially greater than at most faster spreading ridges; and we propose the resulting increased residence time in the up-flow zone leads to the re-equilibration of temperature sensitive transition metals at conditions less extreme than those associated with peak reaction. Agreement between experimental data, thermodynamic model calculations, and dissolved concentrations of Fe, Cu, Zn, H2, and H2S in the Lucky Strike fluids reinforce the hypothesis of pH-redox equilibria for transition metals at relatively oxidizing conditions and temperatures predicted by the empirical Fe/Mn geothermometer. In-situ pH measurements of the high-temperature fluids exiting the seafloor are also consistent with the model calculations.

Geochemically induced shifts in catabolic energy yields explain past ecological changes of diffuse vents in the East Pacific Rise 9°50'N area

The East Pacific Rise (EPR) at 9°50'N hosts a hydrothermal vent field (Bio9) where the change in fluid chemistry is believed to have caused the demise of a tubeworm colony. We test this hypothesis and expand on it by providing a thermodynamic perspective in calculating free energies for a range of catabolic reactions from published compositional data. The energy calculations show that there was excess H2S in the fluids and that oxygen was the limiting reactant from 1991 to 1997. Energy levels are generally high, although they declined in that time span. In 1997, sulfide availability decreased substantially and H2S was the limiting reactant. Energy availability dropped by a factor of 10 to 20 from what it had been between 1991 and 1995. The perishing of the tubeworm colonies began in 1995 and coincided with the timing of energy decrease for sulfide oxidizers. In the same time interval, energy availability for iron oxidizers increased by a factor of 6 to 8, and, in 1997, there was 25 times more energy per transferred electron in iron oxidation than in sulfide oxidation. This change coincides with a massive spread of red staining (putative colonization by Fe-oxidizing bacteria) between 1995 and 1997.For a different cluster of vents from the EPR 9°50'N area (Tube Worm Pillar), thermodynamic modeling is used to examine changes in subseafloor catabolic metabolism between 1992 and 2000. These reactions are deduced from deviations in diffuse fluid compositions from conservative behavior of redox-sensitive species. We show that hydrogen is significantly reduced relative to values expected from conservative mixing. While H2 concentrations of the hydrothermal endmember fluids were constant between 1992 and 1995, the affinities for hydrogenotrophic reactions in the diffuse fluids decreased by a factor of 15 and then remained constant between 1995 and 2000. Previously, these fluids have been shown to support subseafloor methanogenesis. Our calculation results corroborate these findings and indicate that the 1992-1995 period was one of active growth of hydrogenotrophic communities, while the system was more or less at steady state between 1995 and 2000.

The roles of magmatic and hydrothermal processes in PGE mineralization, Ferguson Lake deposit, Nunavut, Canada

The Ferguson Lake Ni–Cu–Co–platinum-group element (PGE) deposit in Nunavut, Canada, occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites. Massive to semi-massive sulfide occurs toward the structural hanging wall of the metagabbro, and a low-sulfide, high-PGE style of mineralization (sulfide veins and disseminations) locally occurs ~30–50 m below the main massive sulfide. Water–rock interaction in the Ferguson Lake Ni–Cu–Co–PGE deposit is manifested mostly as widespread, post-metamorphic, epidote–chlorite–calcite veins, and replacement assemblages that contain variable amounts of sulfides and platinum-group minerals (PGM). PGM occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high-PGE zones, at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide, in undeformed sulfide veins and adjacent chlorite and/or epidote halos, in hornblende adjacent to hydrothermal veins, and in plagioclase–chlorite aggregates replacing garnet cemented by sulfide. The PGM are mostly represented by the kotulskite (PdTe)–sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi2), merenskyite (PdTe2), mertieite II (Pd8[Sb,As]3), and sperrylite (PtAs2) and occur in variety of textural settings. Those that occur in massive and interstitial sulfides, interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures <600°C, are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulfide/silicate veins and their epidote–chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite). The chemistry and textural setting of the various PGM supports a genetic model that links the magmatic and hydrothermal end-members of the sulfide–PGM mineralization. The association of PGM with magmatic sulfides in the massive sulfide and high-PGE zones has been interpreted to indicate that PGE mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization. However, the occurrence of PGM in undeformed sulfide-bearing veins and in their chlorite–epidote halos and differences in PGM chemistry indicate that hydrothermal fluids were responsible for post-metamorphic redistribution and dispersion of PGE.

N2, Ar, and He as a tool for discriminating sources of volcanic fluids with application to Vulcano, Italy

A detailed analysis of published data on the N2, Ar, and He content and Ar and He isotopic composition of fumarolic fluids from Vulcano crater (south Italy) supports a model with two endmembers comprising magmatic and hydrothermal fluids with correspondingly low and high H2O content. The magmatic component with the highest 3He/4He and highest absolute concentrations of N2, Ar, and He also has the lowest N2/Ar and N2/He ratios (∼300 and ∼500, respectively). In contrast, the hydrothermal endmember, with the lower 3He/4He and lower absolute N2, Ar, and He abundances, has high N2/Ar (∼1,000) and high N2/He (>3,000) ratios. The hydrothermal component is also characterized by the highest 40Ar/36Ar ratios (>1,000) and is proposed to be the main carrier of metamorphic gases from the arc crust.