We introduce a database of hydrothermal vent fluid compositions extracted from peer‐reviewed publications. The database includes general fluid parameters (e.g., temperature, salinity, and pH) as well ...as major‐, minor‐, and trace‐element concentrations (including rare earth elements) of dissolved cations, anions, reduced carbon compounds, and gases. In addition, isotopic compositions of elements and molecules are included. Each parameter in the database is given in a uniform unit and enables direct intercomparison of data from the incorporated sources. The database provides detailed information about geographic location, the date of sampling, and a broad set of supplementary information that enables clear identification of each individual sample and the original data source. This type of metadata was used to merge compositional data for discrete vent fluid samples that originate from different publications. Hence, the database provides a more complete set of compositional parameters than the original sources do. To facilitate operability, the database is provided as ®Excel sheet, which enables users to visualize, sort, filter, and evaluate the database in a straightforward way. The sample information metadata enables extraction of available vent fluid data for specific regions, tectonic settings, host rock types, etc. The database will be a useful tool in determining (1) mechanisms that set fluid chemistry, as well as (2) regional and global geochemical fluxes across the seabed. To demonstrate the extent of the database, we examine the global distribution of magnesium, chloride, and sodium concentrations in vent fluid samples that are incorporated in MARHYS Database.
Plain Language Summary
We have compiled data for chemical compositions of hot solutions emitted from the ocean floor. This compilation uses data from journal publications, book chapters, and monographs and includes all chemical parameters that were found in the surveyed sources. The individual format in which the data are presented in the original sources varies considerably, which makes intercomparisons challenging. We have put all data in a common format to simplify the comparison of vent fluids from different sources. The sample information that was acquired along with the chemical data was used to find discrete samples that were analyzed in several publications and to merge the chemical data to unique entries. These merged entries provide a more complete chemical description of the samples than the original sources do. The database is included in an ®Excel sheet to provide interdisciplinary researchers a simple way to access, filter, and evaluate data. MARHYS Database provides an excellent mean to compare existing data with own results and provides a chance to evaluate globally or regionally averaged chemical compositions of hydrothermal solutions. To demonstrate the extent of the database, we present the distribution of magnesium, chloride and sodium concentrations of all samples incorporated in the database.
Key Points
MARHYS Database: A comprehensive dataset of marine hydrothermal vent fluid compositions was compiled
Average values for hydrothermal fluids, end members, and seawater compositions are presented
The database provides a mean to assess sample information and compositions for individual vents and regional/global compilations
This contribution assesses the availability of catabolic energy for microbial life during water-rock reactions in the flanks of mid-ocean ridges, where basaltic and ultramafic rocks interact with ...circulating seawater. In addition to equilibrium thermodynamic computations, results for kinetic reaction paths are presented. In these calculations, it is assumed that dissolution of olivine and basalt glass control the rates of hydrogen forming reactions in ultramafic and basaltic rocks, respectively. The results suggest that all ocean crust basement rocks release enough hydrogen (H2,aq) to support hydrogenotrophic life at low water-to-rock ratios. Olivine dissolution rate control imposes a stronger effect on hydrogen production than phase equilibrium controls, indicating that magnetite formation is not a requirement for production of large amounts of hydrogen in ultramafic rocks. The formation of non-tronite and celadonite are primarily responsible for the formation of the moderate amounts of hydrogen (H2,aq) expected in basaltic ridge flanks. Under conditions of large seawater fluxes required to account for the great global convective heat flow in ridge flanks, however, hydrogen production in basaltic ridge flanks is insufficient for supporting hydrogenotrophic life. It is hence proposed that the role of Fe oxidation in basaltic ridge flanks is greater than previously suggested. A standing stock of 2.4(∗)10(28) cells may be supported by Fe oxidation in basaltic ridge flanks, equivalent of about 10% of the sedimentary deep biosphere. The size of a hydrogenotrophic biomass within the ocean crust is more difficult to estimate because the rates and processes of hydrogen release are insufficiently constrained. In any case, hydrogenotrophy in the ocean crust should be of key importance only in olivine-rich basement rocks and in sedimented ridge flanks with low time-integrated seawater fluxes.
Thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic/ultramafic boundaries. The models were set up to investigate the shifts in fluid–rock ...equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body. Phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 °C, 300 °C, and 400 °C. Phase assemblages typical of rodingite (grossular+diopside±chlorite) are predicted to form at 200 °C and 300 °C, but only in areas where the fluid is essentially unaffected by reactions with gabbro, i.e., near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid. As the fluid becomes more affected by reactions with the gabbro, prehnite or epidote-ss replace garnet and tremolite replaces clinopyroxene. Once the fluid chemistry is completely reset by reactions with gabbro, the predicted assemblage is typical of greenschist facies: albitic plagioclase, actinolite, chlorite, and epidote-ss. These transitions predicted in the model are very similar to what is observed in natural rodingites from different settings. Our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present. Our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids. Mass transfer of Ca is likely by diffusion of the CaOH+ species, which is predicted to show a very steep concentration gradient across a mafic–ultramafic boundary. Rodingites act as Ca traps because of the great stability of diopside and garnet at low SiO2 activities.
Our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 °C and 300 °C, suggesting that very high temperatures on the order of 800 °C are not required in the formation of diopsidite veins.
We investigated the halogen (Cl, F, Br, and I) chemistry of serpentinites that record progressive dehydration during subduction from shallow oceanic environments via increased pressure and ...temperature conditions to complete breakdown of antigorite. The aim is to evaluate the relevance of serpentinites for halogen recycling in subduction zones and for deep mantle recharge of these elements. The halogen compositions of the analyzed samples indicate input from seawater and sedimentary sources during initial serpentinization of either subducting lithospheric mantle during slab bending or forearc mantle by uprising slab fluids. During the first dehydration stage (antigorite
+
brucite
→
olivine
+
H
2O), fluids with high Br/Cl and I/Cl ratios are released resulting in residual serpentinites with lower Br/Cl and I/Cl ratios. Veins associated with this event and with the final antigorite breakdown (antigorite
→
olivine
+
orthopyroxene
+
H
2O) show higher halogen ratios compared to their adjacent wall rocks, and they are similar to those found in arc volcanoes (F/Cl and I/Cl between ca. 0.083–1.5, and ca. 0.00038–0.0013, respectively). All measured deserpentinization samples show a narrow range in δ
37Cl values (between −
0.42‰ and +
0.92‰) overlapping the δ
37Cl values of seafloor serpentinites and confirming that no significant Cl isotope fractionation occurs during subduction dehydration of serpentinites. Our findings document the conservative behavior of halogens during subduction. Mass balance constraints reveal that serpentinites strongly control the halogen chemistry of deep subduction zone fluids and that descent of rock residues after deserpentinization strongly affects the halogen budget of the mantle.
► Halogen concentrations and ratios and Cl isotope data indicate halogen input from sedimentary reservoirs during shallow serpentinization, either at the outer rise during slab bending or in suprasubduction shallow environments. ► During deserpentinization, fluids with high Br/Cl and I/Cl ratios are released which drives the residual serpentinites to lower Br/Cl and Cl/I and during ongoing dehydration. F/Cl ratios increase strongest with increasing degree of dehydration throughout the whole deserpentinization process. The high-pressure slab fluids liberated during the antigorite to olivine reaction have F/Cl and I/Cl ratios that are similar to those of arc volcanoes. ► The overall δ
37Cl values of the deserpentinization fluids are close to 0‰, which are within the range of known arc volcanoes. ► The concentrations and ratios found in the serpentinization–deserpentinization sequence indicate the conservative behavior of halogens during subduction and fluid release. ► Mass balance calculations show that serpentinites seem to strongly control the halogen budget, and likely also the Cl isotope signature, of the deeper subduction zone fluids.
Marine dissolved organic matter (DOM) is a large (660Pg) pool of reduced carbon that is subject to thermal alteration in hydrothermal systems and sedimentary basins. In natural high-temperature ...hydrothermal systems, DOM is almost completely removed, but the mechanism and temperature dependence of this removal have not been studied to date. We investigated molecular-level changes to DOM that was solid-phase extracted (SPE-DOM) from the deep ocean of the North Pacific Ocean. This complex molecular mixture was experimentally exposed to temperatures between 100 and 380°C over the course of two weeks in artificial seawater, and was then characterised on a molecular level via ultrahigh-resolution Fourier-transform ion cyclotron mass spectrometry (FT-ICR-MS). Almost 93% of SPE-DOM was removed by the treatment at 380°C, and this removal was accompanied by a consistent pattern of SPE-DOM alteration across the temperatures studied. Higher molecular weight and more oxygen rich compounds were preferentially removed, suggesting that decarboxylation and dehydration of carboxylic acid and alcohol groups are the most rapid degradation mechanisms. Nitrogen containing compounds followed the same overall trends as those containing just C, H and O up to 300°C. Above this temperature, the most highly altered samples contained very little of the original character of marine DOM, instead being mainly composed of very low intensity N- and S- containing molecules with a high H/C ratio (>1.5). Our results suggest that abiotic hydrothermal alteration of SPE-DOM may already occur at temperatures above 68°C. Our experiments were conducted without a sedimentary or mineral phase, and demonstrate that profound molecular alteration and almost complete removal of marine SPE-DOM requires nothing more than heating in a seawater matrix.
Serpentinite mud volcanism in the Mariana forearc provides a window into the shallow portions of an active subduction zone. Fluid–rock interactions and related mass transfers into the mantle wedge ...can be assessed by studying the trace element compositions of slab-derived fluids and serpentinized mantle wedge materials brought to the seafloor by the serpentinite mud volcanoes. We investigated variably serpentinized ultramafic clasts from the Yinazao, Fantangisña, and Asùt Tesoru mud volcanoes recovered on International Ocean Discovery Program Expedition 366 to examine the transfer of fluid-mobile elements (FMEs) from the slab to the wedge. These mud volcanoes sample the slab–wedge interface at depths of ~13–18 km and estimated temperatures of 80–250 °C. Our samples represent the serpentinized forearc and exhibit a multi-phase serpentinization history, as apparent from microfabrics, mineralogy, and in situ major and trace elemental analyses of distinct generations of serpentine. Initial hydration of the forearc mantle occurred under reducing conditions by Si-rich fluids. Early serpentine is characterized by generally high concentrations of Li, Sr, Rb, Cs, and Ba. Subsequent fluid–rock interactions were driven by Si-rich and FME-poor fluids and at later stages by Si- and FME-poor fluids in the mud volcano conduits, the latter of which resulted in the abundant formation of Fe-rich brucite. Iowaite and hematite indicate that less reducing conditions prevailed during the alteration of clasts after their emplacement at the seafloor. Concentrations of B are generally high but our dataset does not allow distinguishing slab- from seawater-derived B.
Serpentine from the shallow-sourced Yinazao exhibits high Rb/Cs ratios of ≤37, highest concentrations of Li, but lowest Rb, Sr, Ba, and Cs contents. The serpentinizing fluids were derived from expulsion of sedimentary pore waters and by the breakdown of opal in the subducted sediments. Serpentine at the intermediate-sourced Fantangisña has Rb/Cs ratios of <10, similar Li, Sr, and Ba concentrations as Yinazao, but higher Rb and Cs contents. These patterns likely reflect dewatering and FME-release from clays in the subducted sediments. Fluids at the deeply sourced Asùt Tesoru as well originate from clay-breakdown, but increased concentrations of Rb, Sr, Cs, and Ba are further indicative of beginning dehydration of altered oceanic crust.
Including data from the South Chamorro serpentinite mud volcano (18 km slab depth; Kahl et al., 2015, Lithos), we provide a detailed record of slab dehydration reactions at shallow forearc depths and the related mobilization of FMEs as well as their transport into the mantle wedge. Our study demonstrates that slab-derived fluids undergo extensive alteration during the interaction with mantle wedge peridotite. Pore waters from the serpentinite mud volcanoes hence provide incomplete insight into the processes at depth; fluid signatures at the slab–wedge interface as well as their across-forearc changes are best recorded in early hydration products such as serpentine.
Display omitted
•Fluid-mobile element contents in mantle wedge serpentine are a function of P/T conditions in the slab.•Across-forearc changes in concentrations and ratios imply varying element sources.•Rb/Cs ratios fingerprint fluid and element sources at shallow depths.•We present a continuous record of dehydration reactions in the subducting plate.
At deep-sea hydrothermal vents, primary production is carried out by chemolithoautotrophic microorganisms, with the oxidation of reduced sulfur compounds being a major driver for microbial carbon ...fixation. Dense and highly diverse assemblies of sulfur-oxidizing bacteria (SOB) are observed, yet the principles of niche differentiation between the different SOB across geochemical gradients remain poorly understood. In this study niche differentiation of the key SOB was addressed by extensive sampling of active sulfidic vents at six different hydrothermal venting sites in the Manus Basin, off Papua New Guinea. We subjected 33 diffuse fluid and water column samples and 23 samples from surfaces of chimneys, rocks and fauna to a combined analysis of 16S rRNA gene sequences, metagenomes and real-time in situ measured geochemical parameters. We found Sulfurovum Epsilonproteobacteria mainly attached to surfaces exposed to diffuse venting, while the SUP05-clade dominated the bacterioplankton in highly diluted mixtures of vent fluids and seawater. We propose that the high diversity within Sulfurimonas- and Sulfurovum-related Epsilonproteobacteria observed in this study derives from the high variation of environmental parameters such as oxygen and sulfide concentrations across small spatial and temporal scales.
Species within the genus Alcanivorax are well known hydrocarbon-degraders that propagate quickly in oil spills and natural oil seepage. They are also inhabitants of the deep-sea and have been found ...in several hydrothermal plumes. However, an in-depth analysis of deep-sea Alcanivorax is currently lacking. In this study, we used multiple culture-independent techniques to analyze the microbial community composition of hydrothermal plumes in the Northern Tonga arc and Northeastern Lau Basin focusing on the autecology of Alcanivorax. The hydrothermal vents feeding the plumes are hosted in an arc volcano (Niua), a rear-arc caldera (Niuatahi) and the Northeast Lau Spreading Centre (Maka). Fluorescence in situ hybridization revealed that Alcanivorax dominated the community at two sites (1210-1565 mbsl), reaching up to 48% relative abundance (3.5 × 10
cells/ml). Through 16S rRNA gene and metagenome analyses, we identified that this pattern was driven by two Alcanivorax species in the plumes of Niuatahi and Maka. Despite no indication for hydrocarbon presence in the plumes of these areas, a high expression of genes involved in hydrocarbon-degradation was observed. We hypothesize that the high abundance and gene expression of Alcanivorax is likely due to yet undiscovered hydrocarbon seepage from the seafloor, potentially resulting from recent volcanic activity in the area. Chain-length and complexity of hydrocarbons, and water depth could be driving niche partitioning in Alcanivorax.
Hydrothermal systems located at intra-oceanic volcanic arcs and in back-arc basins reveal a complex sulfur cycling. Here we present multiple sulfur isotope data for dissolved sulfide and sulfate in ...hydrothermal fluids from vent sites at the southern Kermadec and the northern Tonga island arcs and the NE Lau Basin.
δ34S values for H2S range from −7.4 to +4.6‰ for hydrothermal fluids from arc volcanoes and from +0.2 to +4.5‰ for those located in the NE Lau Basin. Ranges for Δ33S values are −0.010 to +0.033‰ and −0.016 to +0.011‰, respectively. SO42− in fluids from hydrothermal vents at Macauley caldera, the Brothers volcanic cones, and Niua North show ranges in δ34S and Δ33S from +16.7 to +24.7‰ and +0.013 to +0.049‰. The multiple sulfur isotope data suggest a contribution to the hydrothermal sulfur budget from the disproportionation of magmatic SO2 for most hydrothermal vent sites from the Kermadec arc, whereas H2S at the NE Lau Basin vent sites is thought to result mainly from two-component mixing between host rock-derived sulfur and seawater sulfate with an additional local contribution from SO2 disproportionation. Data from Brothers NW Caldera suggest isotope exchange between H2S and SO42−, whereas lowering of fluid acidity by dissolution of mineral phases occurs at the Brothers Lower Cone. δ34S values between −8.5 and +3.1‰ and Δ33S values between +0.009 and +0.036‰ for elemental sulfur indicate changing redox conditions and/or a decrease of magmatic SO2 in the hydrothermal fluids at Macauley, Haungaroa, Brothers Lower Cone, Niua North and Niuatahi.
We conclude that acid sulfate venting in particular shows the highest variability in sulfur isotopic composition of hydrothermal sulfur, irrespective of whether located at an intra-oceanic arc or in a back-arc basin. Sulfur isotope heterogeneity is due to variable contributions from SO2 disproportionation and host rock compositions.
The footwalls of oceanic detachment faults commonly expose shear zone rocks that appear to have compositions intermediate between those of mantle peridotite and magmatic rocks. These compositions ...either reflect metasomatic mass transfers or they relate to the impregnation of lithospheric mantle with basaltic or more evolved melts. We studied chlorite‐amphibole‐rich shear zone rocks from a detachment fault zone in the 15°20′N Fracture Zone area, Mid‐Atlantic Ridge, to examine their origin and role in strain localization. Geochemical compositions of these rocks imply that they formed by mixing between peridotite and gabbro. Textural observations indicate a strong contrast between the deformation intensity of these hybrid peridotite‐gabbro rocks and the host serpentinized peridotite. Geothermometry data give formation temperatures of >500 °C for synkinematic amphibole, zircon, rutile, and titanite. Chlorite appears intergrown with these phases and likely grew at similar temperatures. These results are compliant to thermodynamic computations that predict comparable mechanically weak mineralogies when hydrating hybrid rocks at 500 to 600 °C, whereas secondary assemblages after pure peridotite or gabbro are considerably stronger. Consequently, metamorphic weakening takes place to a much greater extent in rocks with a hybrid ultramafic–mafic composition than in purely ultramafic or gabbroic lithologies. Deformation may enhance fluid flow, which will in turn increase the extent of hydration and mechanical weakening. A positive feedback loop between hydration and strain localization may hence develop and facilitate the concentration of extensional tectonics into long‐lived, high‐displacement faults. We suggest that hybrid lithologies may play a key role in detachment faulting at slow spreading ridges worldwide.
Plain Language Summary
At slow spreading mid‐ocean ridges, the formation of oceanic crust is commonly accommodated by geologic faulting, whereby tectonic plates are displaced relative to one another. Some of these faults, termed detachments, are active over millions of years and displacement can sum up to several tens of kilometers. It is not well understood why detachment faults form and remain active over such long periods of time. We present evidence from drill core from the Mid‐Atlantic Ridge that fault evolution and longevity are favored by a process that is related to the presence of a particular mixture of rocks: where magma has intruded and crystallized in the prevalent mantle rocks, chemical reactions with seawater that commonly penetrates deep into the seafloor cause the formation of mechanically weak minerals. Extensive tectonic strain is subsequently localized into these weak spots where it leads to deformation and faulting. This faulting enables further inflow of seawater that results in further weakening—a positive feedback loop is initiated. Comparison to other detachment faults implies that such rock mixtures are present at many slow spreading ridges, and we suggest that the proposed process significantly contributes to the formation of new seafloor around the globe.
Key Points
Impregnation of mantle peridotite by mafic or more evolved melts is common in oceanic detachment faults
Seawater interaction at >500 °C turns impregnated peridotite into weak secondary assemblages in which strain localizes
Strain localization initiates deformation and faulting and facilitates the concentration of extensional tectonics into long‐lived detachments
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