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

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

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

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

大西洋洋中脊TAG热液区硫化物铅和硫同位素研究

位于大西洋洋中脊26.08°N的TAG热液区是目前已知的赋存在无沉积物覆盖的洋中脊区的一个最大的海底热液硫化物矿床.新测得来自ODP-158航次钻孔的9件热液硫化物的铅、硫同位素组成;2件铁锰氧化物和1件底盘玄武岩的铅同位素组成.结果表明,矿石硫化物的铅同位素组成206Pb/204Pb为18.2343～18.3181,207Pb/204Pb为15.4717～15.5061,208Pb/204Pb为37.7372～37.8417;它们位于该区底盘玄武岩(206Pb/204Pb=18.1454,207Pb/204Pb=15.4572,208Pb/204Pb=37.6534)和近洋底铁锰氧化物(206Pb/204Pb,207Pb/204Pb,208Pb/204Pb分别为18.6907～18.9264,15.5615～15.6279,38.1164～38.3687)的铅同位素组成之间.三者呈线性相关关系,说明硫化物中铅来源于地幔(玄武岩)与海水(铁锰氧化物)的两端元混合.硫化物的硫同位素组成δ34S为6.2‰～9.5‰,它明显高于地幔玄武岩的硫同位素组成(δ34S=±0‰),也高于东太平洋海隆EPR21°N(δ34S=0.9‰～4.0‰)和大西洋洋中脊MAR23°N(δ34S=1.2‰～2.8‰)等热液活动区硫化物的硫同位素组成,这一特征反映了TAG热液体系中硫来源于地幔玄武岩硫与海水硫酸盐无机还原作用产生的硫的两端元混合.因此,铅硫同位素研究为现代大洋底热液硫化物矿床形成过程中矿质来源及流体混合作用提供了十分有益的信息.     

Sulfur in peridotites and gabbros at Lost City (30°N, MAR): Implications for hydrothermal alteration and microbial activity during serpentinization

Variations in sulfur mineralogy and chemistry of serpentinized peridotites and gabbros beneath the Lost City Hydrothermal Field at the southern face of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) were examined to better understand serpentinization and alteration processes and to study fluid fluxes, redox conditions, and the influence of microbial activity in this active, peridotite-hosted hydrothermal system. The serpentinized peridotites are characterized by low total sulfur contents and high bulk δ³⁴S values close to seawater composition. Low concentrations of ³⁴S-enriched sulfide phases and the predominance of sulfate with seawater-like δ³⁴S values indicate oxidation, loss of sulfide minerals and incorporation of seawater sulfate into the serpentinites. The predominance of pyrite in both serpentinites and gabbros indicates relatively high fO₂ conditions during progressive serpentinization and alteration, which likely result from high fluid fluxes during hydrothermal circulation and evolution of the Lost City system from temperatures of ∼250 to 150 °C. Sulfate and sulfide minerals in samples from near the base of hydrothermal carbonate towers at Lost City show δ³⁴S values that reflect the influence of microbial activity. Our study highlights the variations in sulfur chemistry of serpentinized peridotites in different marine environments and the influence of long-lived, moderate temperature peridotite-hosted hydrothermal system and high seawater fluxes on the global sulfur cycle.     

Carbon geochemistry of serpentinites in the Lost City Hydrothermal System (30°N, MAR)

The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ¹³C_TC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ¹³C_TOC: −28.9‰ to −21.5‰) and variations in δ¹³C_TC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C₁₅ to C₃₀. Longer-chain hydrocarbons (up to C₄₀) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C₁₆ to n-C₂₀ alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.     

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

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

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

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

Geochemistry of a sediment push-core from the Lucky Strike hydrothermal field,Mid-Atlantic Ridge

Hydrothermal sediment mineralogy and geochemistry can provide insights into seafloor mineralization processes and changes through time. We report a geochemical investigation of a short (22 cm) near-vent hydrothermal metalliferous sediment core from the Lucky Strike site (LS), on the Mid-Atlantic Ridge (MAR). The sediment was collected from the base of an active white smoker vent and comprises pure hydrothermal precipitates, mainly chalcopyrite, sphalerite, pyrite and barite, with negligible detrital and biogenic inputs. Geochemically, the core is enriched in elements derived from high-temperature hydrothermalism (Fe, Cu, Zn and Ba) and depleted in elements derived from low-temperature hydrothermalism (Mn), and metasomatism (Mg). The U/Fe content ratio is elevated, particularly in the deeper parts of the core, consistent with uptake from seawater associated with sulphide alteration. Rare earth elements (REE) concentrations are low and chondrite-normalized patterns are characteristic of high-temperature vent fluids with an enrichment in light REE and a pronounced positive Eu anomaly. A stronger positive Eu anomaly associated with higher La_n/Sm_n at the core top is controlled by barite precipitation. The hydrothermal influence on the REE decreases downcore with some evidence for a stronger seawater influence at depth. Nd isotopes also exhibit an increased detrital/seawater influence downcore. Pb isotope ratios are uniform and plot on the Northern Hemisphere Reference Line in a small domain defined by LS basalts and exhibit no detrital or seawater influence. Lucky Strike sediments are derived from high-temperature mineralization and are overprinted by a weak seawater–sediment interaction when compared with other Atlantic hydrothermal sites such as TAG. The larger seawater input and/or a larger detrital contribution in deeper layers can be explained by variable hydrothermal activity during sediment formation, suggesting different pulses in activity of the LS hydrothermal system.     

Iron partitioning and hydrogen generation during serpentinization of abyssal peridotites from 15°N on the Mid-Atlantic Ridge

Aqueous dihydrogen (H₂,aq) is produced in copious amounts when seawater interacts with peridotite and H₂O oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly understood in this process is the partitioning of iron and its oxidation state in serpentine, although both impose an important control on dihydrogen production. We present results of detailed petrographic, mineral chemical, magnetic and Mößbauer analyses of partially to fully serpentinized peridotites from the Ocean Drilling Program (ODP) Leg 209, Mid-Atlantic Ridge (MAR) 15°N area. These results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite-seawater reaction path models. In samples from Hole 1274A, mesh-rims reveal a distinct in-to-out zoning from brucite at the interface with primary olivine, followed by a zone of serpentine + brucite ± magnetite and finally serpentine + magnetite in the outermost mesh-rim. The compositions of coexisting serpentine (Mg# 95) and brucite (Mg# 80) vary little throughout the core. About 30-50% of the iron in serpentine/brucite mesh-rims is trivalent, irrespective of subbasement depth and protolith (harzburgite versus dunite). Model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water-to-rock ratio during serpentinization. At temperatures above 330 °C the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen depends on the availability of an external silica source. At these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen, hence conditions are not reducing enough to form awaruite. At T < 330 °C, hydrogen generation is facilitated by the formation of brucite, as dissolution of olivine to form serpentine, magnetite and brucite requires no addition of silica. The model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from <150 to 250 °C and bulk water-to-rock ratios between 0.1 and 5. These conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization. The development of the common brucite rims around olivine is either due to an arrested reaction olivine → brucite → serpentine + brucite, or reflects metastable olivine-brucite equilibria developing in the strong gradient in silica activity between orthopyroxene (talc-serpentine) and olivine (serpentine-brucite).     

Tholeiitic and alkali basalts from the Mid-Atlantic Ridge at 43 ° N

Tholeiitic basalts dredged from the Mid-Atlantic Ridge (MAR) axis at 43 ° N are enriched in incompatible trace elements compared to the ‘ normal’ incompatible element depleted tholeiites found from 49 ° N to 59 ° N and south of 33 ° N on the MAR. The most primitive 43 ° N glasses have MgO/FeO*= 1.2 and coexist with olivine (Fo_90–91) and chrome-rich spinel. The tholeiitic basalts from the MAR 43 ° N are distinct from the strongly incompatible trace element depleted tholeiities found elsewhere in the Atlantic, and have trace element features typical of island tholeiities and MAR axis tholeiites from 45 ° N. Petrographic, major, and compatible trace element trends of the axial valley tholeiites at 43 ° N are consistent with shallow-level fractionation; in particular, evolution from primitive liquids with forsteritic olivine plus chrome spinel as liquidus phases to fractionated liquids with plagioclase plus clinopyroxene as major crystallizing phases. However, each dredge haul has distinctive incompatible trace element abundances. These trace element characteristics require a hetrogeneous mantle or complex processes such as open system fractional crystallization and magma mixing. Alkali basalts (～5% normative nepheline) were dredged from a prominent fracture zone at 43 ° N. Typical of alkali basalts they are strongly enriched (compared to tholeiites) in incompatible elements. Their highly fractionated rare-earth element (REE) abundances require residual garnet during partial melting. The 43 ° N tholeiites and alkali basalts could be derived from a garnet peridotite source with REE contents equal to 2 × chondrites by ～5% and 1% melting, respectively. Alternatively, they could be derived from a moderately light REE enriched source by ～25% and 9.5% melting, respectively.     

Elevated concentrations of formate, acetate and dissolved organic carbon found at the Lost City hydrothermal field

Fluids from the ultramafic-hosted Lost City hydrothermal field were analyzed for total dissolved organic carbon and dissolved organic acids. Formate (36-158 μmol/kg) and acetate (1-35 μmol/kg) concentrations are higher than in other fluids from unsedimented hydrothermal vents, and are a higher ratio of the total dissolved organic carbon than has been found in most marine geothermal systems. Isotopic evidence is consistent with an abiotic formation mechanism for formate, perhaps during serpentinization processes in the sub-surface. Further support comes from previous studies where the abiological formation of low molecular weight organic acids has been shown to be thermodynamically favorable during hydrothermal alteration of olivine, and laboratory studies in which the reduction of carbon dioxide to formate has been confirmed. As the second most prevalent carbon species after methane, formate may be an important substrate to microbial communities in an environment where dissolved inorganic carbon is limited. Acetate is found in locations where sulfate reduction is believed to be important and is likely to be a microbial by-product, formed either directly by autotrophic metabolic activity or indirectly during the fermentative degradation of larger organic molecules. Given the common occurrence of exposed ultramafic rocks and active serpentinization within the worlds ocean basins, the abiotic formation of formate may be an important process supporting life in these high pH environments and may have critical implications to understanding the organic precursors from which life evolved.     

PGE fractionation in seafloor hydrothermal systems: examples from mafic- and ultramafic-hosted hydrothermal fields at the slow-spreading Mid-Atlantic Ridge

The distribution of platinum group elements (PGEs) in massive sulfides and hematite–magnetite±pyrite assemblages from the recently discovered basalt-hosted Turtle Pits hydrothermal field and in massive sulfides from the ultramafic-hosted Logatchev vent field both on the Mid-Atlantic Ridge was studied and compared to that from selected ancient volcanic-hosted massive sulfide (VHMS) deposits. Cu-rich samples from black smoker chimneys of both vent fields are enriched in Pd and Rh (Pd up to 227 ppb and Rh up to 149 ppb) when compared to hematite–magnetite-rich samples from Turtle Pits (Pd up to 10 ppb, Rh up to 1.9 ppb). A significant positive correlation was established between Cu and Rh in sulfide samples from Turtle Pits. PGE chondrite-normalized patterns (with a positive Rh anomaly and Pd and Au enrichment), Pd/Pt and Pd/Au ratios close to global MORB, and high values of Pd/Ir and Pt/Ir ratios indicate mafic source rock and seawater involvement in the hydrothermal system at Turtle Pits. Similarly shaped PGE chondrite-normalized patterns and high values of Pd/Pt and Pd/Ir ratios in Cu-rich sulfides at Logatchev likely reflect a similar mechanism of PGE enrichment but with involvement of ultramafic source rocks.     

Diagenetic alterations of copper sulfides in modern ore-bearing sediments of the Logatchev-1 hydrothermal field (Mid-Atlantic Ridge 14°45′ N)

Sulfide ore samples recovered by corers from and beyond Orebody 1 in the Logatchev-1 hydrothermal field (Mid-Atlantic Ridge, 14°45′ N) are studied by optical, electron microscopic, X-ray microspectral, and X-ray diffraction methods. The major and ore minerals are identified. Sulfides of the Cu-S system, the major ore-forming minerals in sediments, are investigated in detail. Specific features of their composition, structure, secondary alterations, and distribution in sediments of the Logatchev-1 field are considered. It has been established that sulfide concretions in modern sediments primarily consist of nonstoichiometric minerals of the chalcocite-digenite series, i.e., djurleite (Cu_1.96S) and roxbyite (Cu_1.75–1.86S). It is assumed that copper sulfides primarily precipitated from hydrothermal solutions as high-temperature hexagonal chalcocite that was replaced after the hydrothermal activity by djurleite, roxbyite, and other nonstoichiometric minerals of the Cu-S system. Based on the comparison of their paragenetic associations with those of copper sulfides in hydrothermal chimneys, the paper discusses constraints of the diagenetic transformation of sulfides in ore-bearing sediments and the halmyrolysis of modern hydrothermal edifices located in contact with seawater. Roxbyite recently discovered in oceanic sediments plays a specific role in these processes.     

Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites

Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30–100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo_91–92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En_89.1–87.6Fs_8.2-8.0Wo_2.7–4.4; Al₂O₃=1.82–2.64 wt%; Cr₂O₃=0.63–0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe²⁺) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C.Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10–15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.