TAG Hydrothermal Field Research Articles

Automated Seafloor Massive Sulfide Detection Through Integrated Image Segmentation and Geophysical Data Analysis: Revisiting the TAG Hydrothermal Field

AbstractAccessible seafloor minerals located near mid‐ocean ridges are noticed to mitigate the projected metal demands of the net‐zero energy transition, promoting growing interest in quantifying the global distributions of seafloor massive sulfides (SMS). Mineral potentials are commonly estimated using geophysical and geological data that lastly rely on additional confirmation studies using sparsely available, locally limited, seafloor imagery, grab samples, and coring data. This raises the challenge of linking in situ confirmation data to geophysical data acquired at disparate spatial scales to obtain quantitative mineral predictions. Although multivariate data sets for marine mineral research are incessantly acquired, robust, integrative data analysis requires cumbersome workflows and experienced interpreters. We introduce an automated two‐step machine learning approach that integrates the mound detection through image segmentation with geophysical data. SMS predictors are subsequently clustered into distinct classes to infer marine mineral potentials that help guide future exploration. The automated workflow employs a U‐Net convolutional neural network to identify mound structures in bathymetry data and distinguishes different mound classes through the classification of mound architectures and magnetic signatures. Finally, controlled source electromagnetic data are utilized together with in situ sampling data to reassess predictions of potential SMS volumes. Our study focuses on the Trans‐Atlantic Geotraverse area, which is among the most explored SMS areas worldwide and includes 15 known SMS sites. The automated workflow classifies 14 of the 15 known mounds as exploration targets of either high or medium priority. This reduces the exploration area to less than 7% of the original survey area from 49 to 3.1 km2.

Detachment-parallel recharge can explain high discharge fluxes at the TAG hydrothermal field

Submarine massive sulfide deposits on slow-spreading ridges are larger and longer-lived than deposits at fast-spreading ridges, likely due to more pronounced tectonic faulting creating stable preferential fluid pathways. The TAG hydrothermal mound at 26∘N on the Mid-Atlantic Ridge (MAR) is a typical example located on the hanging wall of a detachment fault. It has formed through distinct phases of high-temperature fluid discharge lasting 10s to 100s of years throughout at least the last 50,000 yrs and is one of the largest sulfide accumulations on the MAR. Yet, the mechanisms that control the episodic behavior, keep the fluid pathways intact, and sustain the observed high heat fluxes of possibly up to 1700 MW remain poorly understood. Previous concepts involved long-distance channelized high-temperature fluid upflow along the detachment but that circulation mode is thermodynamically unfavorable and incompatible with TAG's high discharge fluxes. Here, based on the joint interpretation of hydrothermal flow observations and 3-D flow modeling, we show that the TAG system can be explained by episodic magmatic intrusions into the footwall of a highly permeable detachment surface. These intrusions drive episodes of hydrothermal activity with vertical discharge and recharge along the detachment. The numerical simulations reveal that the high-temperature circulation system at TAG may be confined to a vertical zone of enhanced permeability that channelizes upflow and a recharge system that is hosted by the detachment surface with a high permeability of 2×10−13 to 10−12m2. This revised flow regime reconciles problematic aspects of previously inferred circulation patterns and allows to identify the prerequisites for generating substantive seafloor mineral systems.

Metal Preservation and Mobilization in Sediments at the TAG Hydrothermal Field, Mid‐Atlantic Ridge

AbstractAt the Trans‐Atlantic Geotraverse hydrothermal field, metalliferous sediments cover extinct hydrothermal mounds and the surrounding seafloor. Here, we report the morphological, mineralogical and geochemical processes that deposit these sediments, remobilize their metals, and affect their preservation. We found that the initial sediment metal tenor is controlled by physical transport of hydrothermal material from its source, followed by diagenetic redistribution and potentially diffuse fluid flow after high‐temperature hydrothermal activity has ceased. We distinguished three different environments: (a) proximal metalliferous sediments on top of extinct mounds are mainly derived from oxidative weathering of primary sulfide structures and are predominantly composed of Fe oxyhydroxides with low contents of Cu, Co, and Zn; metal enrichments in specific layers are likely related to upward flow of low‐temperature hydrothermal fluids; (b) medial distant metalliferous sediments found at the base of the mounds, deposited by mass transport, contain cm‐thick layers of unsorted sulfide sands with high base metal contents (e.g., up to 28% Cu); these buried sulfides continue to undergo dissolution, resulting in metal release into porewaters; (c) distal metalliferous sediments, found in depositional basins a few hundreds of meters from the extinct mounds, include fining‐upwards sequences of thin sulfide sand layers with Fe oxyhydroxides and were deposited by recurrent turbiditic flows. Dissolved metals (e.g., Cu2+ and Mn2+) diffuse upwards under reducing conditions and precipitate within the sediment. Hence, when using hydrothermal sediments to construct reliable geochronological records of hydrothermal activity, distance from source, local seafloor morphology, mass‐transport and depositional, and diagenetic modification should all be considered.

Formation, remobilisation and alteration processes at inactive hydrothermal vents: insights from elemental analysis of Cu-(Fe-)S sulfides from TAG, Mid-Atlantic Ridge

Chalcopyrite is the main Cu mineral in mafic-hosted marine hydrothermal systems. Its trace element budget and that of its alteration products may hold valuable information on formation, remobilisation and alteration processes of the hydrothermal system. In this study, we analysed chalcopyrite from five inactive seafloor massive sulfide (SMS) sites from the TAG hydrothermal field on the Mid-Atlantic Ridge by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 24 elements. Twelve of them are discussed in detail. In general, trace element concentrations range between sub-parts per million (ppm) to several hundreds of ppm. The elements Se and Co are incorporated into the lattice at high temperatures of > 300 °C, whereas As, Ge, Ga substitute into the structure at intermediate to low temperatures. Other elements, e.g. Zn, are either accommodated into the mineral lattice or form inclusions, whereas V and Mn, which originate from seawater, get adsorbed onto the mineral surface. Idaite, chalcocite, and covellite exhibit similar trace element patterns to those of the precursor chalcopyrite. However, the secondary copper minerals show enrichment of Ag and Mo. Factors controlling the incorporation are predominantly related to changes in physicochemical conditions with the host rock composition playing only a minor role.

TAG Plume: Revisiting the Hydrothermal Neodymium Contribution to Seawater

We present results on the distribution of eNd and [Nd] from the TAG hydrothermal vent field and adjacent locations collected during the GEOTRACES GA03 cruise in October 2011. Our results show that Nd isotopes directly below and above the plume do not significantly deviate from average NADW (eNd = -12.3±0.2). Within the plume, however, isotope values are shifted slightly towards more radiogenic values up to eNd = -11.4. Interestingly at the same time a significant decrease in [Nd] along with rare earth element (REE) fractionation is observed, indicating enhanced scavenging within the plume despite the change in Nd isotopes. Elemental concentrations of Nd are reduced by 19.6 to 18.5 pmol/kg, coinciding with the maximum increase of mantle derived helium (xs3He) from 0.203 to 0.675 fmol/kg, resulting in an average 1.8 pmol/kg decrease in [Nd] relative to an expected linear increase with depth. The inventory loss of Nd within the plume sums up to 614 nmoles/m2, or 6 %, if a continuous increase of [Nd] with depth is assumed. Compared to BATS and the western adjacent station USGT11-14, the local inventory loss is even higher at 10 %. The tight relationship of xs3He increase and [Nd] decrease allows us to estimate scavenging rates at TAG suggesting 40 mol/yr are removed within the TAG plume. A global estimate using power output along ocean ridges yields an annual Nd removal of 3.44x106 mol/yr, which is about 71% of riverine and dust flux combined or 6 to 8% of the estimated global flux of Nd into the ocean. The change in Nd isotopic composition of up to 0.7 more radiogenic eNd values suggests an exchange process between hydrothermally derived particles and seawater in which during the removal process an estimated 1.1 mol/yr of hydrothermal Nd is contributed to the seawater at the TAG site. This estimate is only 0.1 % of the global Nd signal added to the ocean by boundary exchange processes at ocean margins, limiting the ability of changing the Nd isotopic composition on a global scale in contrast to the more significant estimated sink of elemental Nd in hydrothermal plumes from

Fe-rich cap lithologies in the TAG hydrothermal field: implications for the preservation of extinct seafloor massive sulphide deposits

Fe-rich cap lithologies in the TAG hydrothermal field: implications for the preservation of extinct seafloor massive sulphide deposits

Hydrothermal iron flux variability following rapid sea level changes

AbstractSea level changes associated with Pleistocene glacial cycles have been hypothesized to modulate melt production and hydrothermal activity at ocean ridges, yet little is known about fluctuations in hydrothermal circulation on time scales longer than a few millennia. We present a high‐resolution record of hydrothermal activity over the past 50 ka using elemental flux data from a new sediment core from the Mir zone of the TAG hydrothermal field at 26°N on the Mid‐Atlantic Ridge. Mir sediments reveal sixfold to eightfold increases in hydrothermal iron and copper deposition during the Last Glacial Maximum, followed by a rapid decline during the sea level rise associated with deglaciation. Our results, along with previous observations from Pacific and Atlantic spreading centers, indicate that rapid sea level changes influence hydrothermal output on mid‐ocean ridges. Thus, climate variability may discretize volcanic processing of the solid Earth on millennial time scales and subsequently stimulate variability in biogeochemical interactions with volcanic systems.

The isotopic signature and distribution of particulate iron in the North Atlantic Ocean

Iron (Fe) is a key micronutrient for life in the oceans. Particles play an important role in the marine biogeochemical cycling of Fe as a reservoir of marine Fe that may be directly accessible to phytoplankton, and as sources and sinks for seawater dissolved Fe. Here, we report the stable isotopic composition of Fe (δ56Fe) in suspended (0.8–51μm) particles from the US GEOTRACES GA03 North Atlantic zonal transect, in order to facilitate a better understanding of the marine biogeochemical cycling of Fe. Data are presented both for a total digestion of the particles, and for ‘ligand-leachable’ phases of Fe using a newly-developed pH 8 oxalate–EDTA leach. For total particle digests, the mean δ56Fe across the whole GA03 section was 0.08±0.09‰ (1 S.D.) which is equivalent to the isotope composition of known lithogenic Fe sources to the ocean. In contrast, ligand-leachable Fe was generally lighter than continental material with a mean δ56Fe of −0.30±0.17‰ (1 S.D.). Our data also provide valuable insight into Fe biogeochemical cycling in several key regions. In the deep ocean, but above the depths where near-sediment nephloid layers are present, ligand-leachable Fe is isotopically lighter in the deep Western Basin compared to the deep Central and Eastern Basins suggesting differences in particle surface chemistry between resuspended seafloor sediments, which may predominate in the west, and Saharan dust that predominates in the center and in the east. Within a nephloid layer above reduced continental margin sediments in the Eastern Basin, below the Mauritanian upwelling region, we report the lowest particulate δ56Fe values for both total and ligand-leachable Fe, suggesting a transfer of isotopically light dissolved porewater Fe2+ to the particulate phase. In contrast, δ56Fe values within a nephloid layer near Bermuda are similar to values higher in the water column. Within a hydrothermal plume sampled at the TAG hydrothermal field on the mid-Atlantic Ridge, decreasing Fe concentration and δ56Fe with distance from the vent indicate the water-column precipitation of Fe oxyhydroxides within the plume. Comparison between total and ligand-leachable particulate Fe concentrations throughout the transect also provides useful biogeochemical information. In the chlorophyll maximum, a high proportion of Fe in the labile phase demonstrates a relative lack of lithogenic material, while the high proportion of labile Fe observed between 2000 and 4000m depth to the west of the mid-Atlantic Ridge suggests that hydrothermal oxyhydroxide particles can be transported thousands of kilometers away from vent sites. Throughout the North Atlantic, measurements of total and ligand-leachable Fe concentration and δ56Fe are therefore useful tools for studying the biogeochemical cycling of particulate Fe.

Fluid evolution in an Oceanic Core Complex: A fluid inclusion study from IODP hole U1309 D—Atlantis Massif, 30°N, Mid‐Atlantic Ridge

In the detachment mode of slow seafloor spreading, convex-upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long-lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30°N, Mid-Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase-separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400°C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low-salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.

Deformation and surface uplift associated with serpentinization at mid‐ocean ridges and subduction zones

We employ the classical problem of an inclusion in an elastic half‐space to model effects of sub‐surface serpentinization on crustal deformation, change of stress state, and surface uplift. At the TAG hydrothermal field on the Mid‐Atlantic Ridge, the model suggests that an anomalous salient 3 km in diameter and 100 m high that projects 3.5 km westward from the east valley wall may have resulted from a relatively deep‐seated well‐serpentinized body exhibiting transformational strain. The associated large strains would likely result in sub‐surface fracturing or faulting, but surface uplift may be relatively insensitive to the exact depth and shape of the serpentinized region. Serpentinization of a region beneath the footwall of the TAG detachment fault will tend to promote slip along some overlying normal faults, which may then enhance fluid pathways to the deeper crust to continue the serpentinization process. Our solution for the Miyazaki Plain above the Kyushu‐Palau subduction zone in SW Japan explains the observed uplift of ≈120 m. The small transformational strains associated with serpentinization in this region may promote thrust‐type events in the aseismic slip zone near the upper boundary of the subducting Philippine Sea Plate as well as intraplate earthquakes associated with normal faulting. The rate of serpentinization needed to produce the observed uplift at the Miyazaki Plain is significantly greater than that needed at TAG, though significantly smaller on per unit volume basis. Thermal effects of serpentinization in both regions appear to be small, but uplift data provide an additional constraint on inferring serpentinization from geological and seismological observations.

Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U‐Th)/He ages

Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid‐ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U‐Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid‐Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15°20′N Transform, and Atlantis Massif at 30°N). Closure temperatures for the Pb/U (∼800°C–850°C) and (U‐Th)/He (∼210°C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature‐time history during faulting. Results indicate cooling to ∼200°C in 0.3–0.5 Myr after zircon crystallization, recording time‐averaged cooling rates of ∼1000°C–2000°C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U‐Th)/He zircon ages together with independently determined slip rates allow the distance between the ∼850°C and ∼200°C isotherms along the fault plane to be estimated. Calculated distances are 8.4 ± 4.2 km and 5.0 ± 2.1 km from holes 1275D and 1270D and 8.4 ± 1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50° and a depth of 1.5 km to the 200°C isotherm leads to the prediction that the ∼850°C isotherm lies ∼5–7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high‐temperature fault rocks recovered from many oceanic detachment faults.

Bottom pressure signals at the TAG deep-sea hydrothermal field: Evidence for short-period, flow-induced ground deformation

[1] Bottom pressure measurements acquired from the TAG hydrothermal field on the Mid-Atlantic Ridge (26°N) contain clusters of narrowband spectral peaks centered at periods from 22 to 53.2 minutes. The strongest signal at 53.2 min corresponds to 13 mm of water depth variation. Smaller, but statistically significant, signals were also observed at periods of 22, 26.5, 33.4, and 37.7 min (1–4 mm amplitude). These kinds of signals have not previously been observed in the ocean, and they appear to represent vertical motion of the seafloor in response to hydrothermal flow - similar in many ways to periodic terrestrial geysers. We demonstrate that displacements of 13 mm can be produced by relatively small flow-induced pressures (several kPa) if the source region is less than ∼100 m below the seafloor. We suggest that the periodic nature of the signals results from a non-linear relationship between fluid pore pressure and crustal permeability.

Evolving patterns of the fluids within the TAG hydrothermal field

The mixing of seawater/hydrothermal fluid within the large seafloor hydrothermal sulfide deposits plays a key role in the formation processes of the sulfide deposits. Some issues attract considerable attentions in the study of seafloor hydrothermal system in recent years, such as the relationships among different types of vent fluids, the characteristics of chemical compositions and mineral assemblages of the hydrothermal deposits and their governing factors. Combined with the measured data of hydrothermal fluid in the TAG field, the thermodynamic model of mixing processes of the heated seawater at different temperatures and the hydrothermal fluid is calculated to understand the precipitation mechanism of anhydrite and the genetic relationships between the black and white smoker fluids within the TAG mound. The results indicate that the heating of seawater and the mixing of hydrothermal fluid/seawater are largely responsible for anhydrite precipitation and the temperature of the heated seawater is not higher than 150°C and the temperature of the end-member hydrothermal fluid is not lower than 400°C. Based on the simulated results, the evolving patterns of fluids within the TAG deposit are discussed. The mixed fluid of the end-member hydrothermal fluid and the seawater heated by wall rock undergoes conductive cooling during upflowing within the deposit and forms “White Smoker” eventually. In addition, the end-member hydrothermal fluid without mixed with seawater, but undergoing conductive cooling, vents out of the deposit and forms “Black Smoker”.

Zinc stable isotopes in seafloor hydrothermal vent fluids and chimneys

Many of the heaviest and lightest natural zinc (Zn) isotope ratios have been discovered in hydrothermal ore deposits. However, the processes responsible for fractionating Zn isotopes in hydrothermal systems are poorly understood. In order to better assess the total range of Zn isotopes in hydrothermal systems and to understand the factors which are responsible for this isotopic fractionation, we have measured Zn isotopes in seafloor hydrothermal fluids from numerous vents at 9–10°N and 21°N on the East Pacific Rise (EPR), the TAG hydrothermal field on the Mid-Atlantic Ridge, and in the Guaymas Basin. Fluid δ 66Zn values measured at these sites range from + 0.00‰ to + 1.04‰. Of the many physical and chemical parameters examined, only temperature was found to correlate with fluid δ 66Zn values. Lower temperature fluids (< 250 °C) had both heavier and more variable δ 66Zn values compared to higher temperature fluids from the same hydrothermal fields. We suggest that subsurface cooling of hydrothermal fluids leads to precipitation of isotopically light sphalerite (Zn sulfide), and that this process is a primary cause of Zn isotope variation in hydrothermal fluids. Thermodynamic calculations carried out to determine saturation state of sphalerite in the vent fluids support this hypothesis with isotopically heaviest Zn found in fluids that were calculated to be saturated with respect to sphalerite. We have also measured Zn isotopes in chimney sulfides recovered from a high-temperature (383 °C) and a low-temperature (203 °C) vent at 9–10°N on the EPR and, in both cases, found that the δ 66Zn of chimney minerals was lighter or similar to the fluid δ 66Zn. The first measurements of Zn isotopes in hydrothermal fluids have revealed large variations in hydrothermal fluid δ 66Zn, and suggest that subsurface Zn sulfide precipitation is a primary factor in causing variations in fluid δ 66Zn. By understanding how chemical processes that occur beneath the seafloor affect hydrothermal fluid δ 66Zn, Zn isotopes may be used as a tracer for studying hydrothermal processes.

The role of prokaryotes in subsurface weathering of hydrothermal sediments: A combined geochemical and microbiological investigation

A detailed geochemical and microbiological study of a ∼2 m sediment core from the inactive Alvin mounds within the TAG hydrothermal field was conducted to examine, for the first time, the role of prokaryotes in subsurface weathering of hydrothermal sediments. Results show that there has been substantial post-depositional remobilisation of metal species and diagenetic overprinting of the original high-temperature hydrothermal minerals, and aspects have involved prokaryotic processes. Prokaryotic enumeration demonstrates the presence of a population smaller than the average for deep sea sediments, probably due to the low organic carbon content, but not inhibited by (and hence adapted to) the metal rich environment. There was a small but significant increase in population size associated with the active redox boundary in an upper metal sulphide layer (50–70 cm) around which active metal remobilisation was concentrated (Cu, Au, Cd, Ag, U, Zn and Zn). Hence, subsurface prokaryotes were potentially obtaining energy from metal metabolism in this near surface zone. Close association of numbers of culturable Mn and Fe reducing prokaryotes with subsurface Fe 2+ and Mn 2+ pore water profiles suggested active prokaryotic metal reduction at depth in core CD102/43 (to ∼175 cm). In addition, a prokaryotic mechanism, which is associated with bacterial sulphate reduction, is invoked to explain the U enrichment on pyrite surfaces and Zn and Pb remobilisation in the upper sediment. Although prokaryotic populations are present throughout this metalliferous sediment, thermodynamic calculations indicated that the inferred low pH of pore waters and the suboxic/anoxic conditions limits the potential energy available from Fe(II) oxidation, which may restrict prokaryotic chemolithotrophic biomass. This suggests that intense prokaryotic Fe oxidation and weathering of seafloor massive sulphide deposits may be restricted to the upper portion of the deposit that is influenced by near neutral pH and oxic seawater unless there is significant subsurface fluid flow.

大西洋中央海嶺（TAG熱水サイト）におけるチムニーの成長と化学組成

大西洋中央海嶺（TAG熱水サイト）におけるチムニーの成長と化学組成

Analysis of fluid inclusions in seafloor hydrothermal precipitates: testing and application of an integrated GC/IC technique

The principal research objective was to test an integrated gas (GC) and ion chromatographic (IC) technique for analysis of trapped fluids in seafloor hydrothermal precipitates and to compare the results with independently analyzed vent fluids. Twenty-three samples of chalcopyrite, pyrite, sphalerite, barite and anhydrite from hydrothermal chimneys from four seafloor sites, Axial Seamount (Juan de Fuca Ridge), the Vai Lili vent field (Lau Basin), the TAG hydrothermal field (Mid-Atlantic Ridge), and the 21°N vent field (East Pacific Rise), as well as of pyrite/marcasite and quartz from the TAG mound, were analyzed. A new type of blank for sulphides and sulphates was also developed. Water contents obtained by GC analysis are in agreement with known compositions of seafloor hydrothermal solutions. Also, the volatiles occur in the same order of abundance as in vent fluids CO 2>N 2(±CO±Ar±O 2)≥CH 4>COS>C 2–C 3 hydrocarbons. For all analyzed hydrothermal fluids, the mean Cl − concentrations are similar to those for the respective measured vent fluid compositions. Furthermore, the GC/IC results are directly comparable to salinity data determined by microthermometric methods. NH 4 + can be enriched and is most probably formed by decomposition of organic matter. Volatile concentrations in fluid inclusions and data from vent fluids differ significantly for some of the investigated sites. Furthermore, the GC data indicate systematic variations regarding the volatile contents of trapped fluids from different active seafloor hydrothermal systems. Fluid inclusion volatile data of samples from the ASHES vent field at Axial Seamount define an inclined array consistent with phase separation.

Genesis of ferromanganese crusts from the TAG hydrothermal field

The rare earth element (REE) geochemistry and Nd isotopic composition of marine ferromanganese crusts allow insight into their environment of deposition. We present REE along with major and trace element data from a suite of samples collected near an active hydrothermal site from the Mid-Atlantic Ridge and demonstrate extreme REE variability over short spatial scales. Both positive and negative Ce anomalies are observed, along with varied fractionation of the REEs. Some of the Fe–Mn crusts are formed directly from low-temperature hydrothermal fluids, channelled over 3 km from any high-temperature vent site. A significant hydrothermal component is observed in one Fe–Mn crust sampled. Other samples, collected from the same area, are predominantly hydrogenous and form at a wide range of accumulation rates depending on the hydrothermal plume input. The identification of extreme heterogeneity in source and depositional environment within one hydrothermal field suggests that (1) this variability can be resolved both spatially and temporally within a fossil hydrothermal deposit and (2) the integrated record provided by bulk analysis should be interpreted with caution.

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, rieon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion He-3/He-4 ratios are 2.2-13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (He-3/He-4=7.5-8.2 Ra) and mid-ocean ridge basalt values (He-3/He-4=6-11 Ra) shows that the variation range of He-3/He-4 ratios from sulfide-hosted fluid inclusions is significantly large. Values for Ne-20/Ne-22 are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion Ar-40/Ar-36 ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.

Zinc‐rich Pyrite from the TAG Active Mound, the TAG Hydrotherma Field, Mid‐Atlantic Ridge

Abstract: Pyrite rich in Zn, up to 3.1 wt%, was found in the TAG active mound of the TAG hydrothermal field, the slow‐spreading Mid‐Atlantic Ridge at 26°08'N and 44°49'W. The Zn‐rich pyrite is characterized by an optical homogeneity, a homogeneous distribution of Zn in the back‐scattered electron images, both at a magnification of about 500, a negative correlation between Fe and Zn contents of the pyrite and a rather small unit cell edge (a0 = 5.4117 ± 0.0008Å), strongly indicating that the detected Zn is present in the pyrite in solid solution. Such Zn concentrations are observed exclusively in dendritic pyrite, suggesting that the Znrich pyrite grew from hydrothermal fluids of a high degree of supersaturation due to quenching on the seafloor.