Low-molecular-weight hydrocarbons in natural hydrothermal fluids have been attributed to abiogenic production by Fischer-Tropsch type (FTT) reactions, although clear evidence for such a process has ...been elusive. Here, we present concentration, and stable and radiocarbon isotope, data from hydrocarbons dissolved in hydrogen-rich fluids venting at the ultramafic-hosted Lost City Hydrothermal Field. A distinct "inverse" trend in the stable carbon and hydrogen isotopic composition of C₁ to C₄ hydrocarbons is compatible with FTT genesis. Radiocarbon evidence rules out seawater bicarbonate as the carbon source for FTT reactions, suggesting that a mantle-derived inorganic carbon source is leached from the host rocks. Our findings illustrate that the abiotic synthesis of hydrocarbons in nature may occur in the presence of ultramafic rocks, water, and moderate amounts of heat.
Carbonate‐brucite chimneys are a characteristic of low‐ to moderate‐temperature, ultramafic‐hosted alkaline hydrothermal systems, such as the Lost City hydrothermal field located on the Atlantis ...Massif at 30°N near the Mid‐Atlantic Ridge. These chimneys form as a result of mixing between warm, serpentinization‐derived vent fluids and cold seawater. Previous work has documented the evolution in mineralogy and geochemistry associated with the aging of the chimneys as hydrothermal activity wanes. However, little is known about spatial heterogeneities within and among actively venting chimneys. New mineralogical and geochemical data (87Sr/86Sr and stable C, O, and clumped isotopes) indicate that the brucite and calcite precipitate at elevated temperatures in vent fluid‐dominated domains in the interior of chimneys. Exterior zones dominated by seawater are brucite‐poor and aragonite is the main carbonate mineral. Carbonates record mostly out of equilibrium oxygen and clumped isotope signatures due to rapid precipitation upon vent fluid‐seawater mixing. On the other hand, the carbonates precipitate closer to carbon isotope equilibrium, with dissolved inorganic carbon in seawater as the dominant carbon source and have δ13C values within the range of marine carbonates. Our data suggest that calcite is a primary mineral in the active hydrothermal chimneys and does not exclusively form as a replacement of aragonite during later alteration with seawater. Elevated formation temperatures and lower 87Sr/86Sr relative to aragonite in the same sample suggest that calcite may be the first carbonate mineral to precipitate.
Plain Language Summary
At the Lost City hydrothermal field, warm alkaline fluids are discharging out of uplifted mantle rocks. When vent fluids mix with seawater at the seafloor, carbonate and brucite minerals form spectacular towers up to 60 m high. Systems like Lost City are important because the reaction between water and rocks provides carbon and energy sources for microbial life. However, we still do not fully understand what controls the mineralogy and geochemistry of the Lost City hydrothermal chimneys. In this paper, we suggest that the extent of mixing between the hydrothermal fluids and seawater influences the mineralogy and geochemistry of the chimneys. Calcite, which was previously thought to form only during alteration of aragonite by seawater, can also form during seawater‐hydrothermal fluid mixing. Both calcite and brucite form in the interior of the chimneys where vent fluid is more dominant. Aragonite, on the other hand, forms in the exterior of the structures from seawater‐rich fluids. Lastly, because minerals precipitate rapidly during fluid mixing, the stable isotope geochemistry of the carbonates mostly records the composition and temperature of seawater and not the mixed fluid. Thus, care should be exercised in interpreting mineral geochemical data from similar systems.
Key Points
The mineralogy and geochemistry of Lost City chimneys are controlled by the extent of mixing between hydrothermal fluids and seawater
Brucite and calcite precipitate in vent fluid dominated zones while aragonite forms in the exterior of the structures in seawater‐rich zones
Carbonates precipitate in isotopic disequilibrium and record the O and C stable isotope composition of seawater dissolved inorganic carbon
Although the serpentinite‐hosted Lost City hydrothermal field (LCHF) was discovered more than 20 years ago, it remains unclear whether and how the presence of microbes affects the mineralogy and ...textures of the hydrothermal chimney structures. Most chimneys have flow textures comprised of mineral walls bounding paleo‐channels, which are preserved in inactive vent structures to a varying degree. Brucite lines the internal part of these channels, while aragonite dominates the exterior. Calcite is also present locally, mostly associated with brucite. Based on a combination of microscopic and geochemical analyses, we interpret brucite, calcite, and aragonite as primary minerals that precipitate abiotically from mixing seawater and hydrothermal fluids. We also observed local brucite precipitation on microbial filaments and, in some cases, microbial filaments may affect the growth direction of brucite crystals. Brucite is more fluorescent than carbonate minerals, possibly indicating the presence of organic compounds. Our results point to brucite as an important substrate for microbial life in alkaline hydrothermal systems.
Plain Language Summary
Water‐serpentinite reactions at the Lost City hydrothermal field (LCHF) are considered similar to the interactions between seawater and volcanic rocks of the early Earth. The vent fluids that form as a result of this interaction are rich in organic compounds that serve as food and hydrogen that provides energy for microbial life. Systems such as Lost City are ideal sites for investigating processes relevant to early life on Earth. Dense microbial communities live in the mineral structures that form from mixing between vent fluids and seawater at the LCHF. The effect of these microbial inhabitants on the mineralogy and textures of the hydrothermal chimneys is still unknown. This study aims to better understand the mineralogy of the hydrothermal chimneys at Lost City and the relationships between microbes and minerals. Calcite and brucite form in the interior of the chimneys, while aragonite forms on the exterior. Brucite forms inorganic mineral membranes that bound cavities where microbes may live. Unlike calcite and aragonite, brucite is spatially closely associated with microbial activity.
Key Points
Lost City hydrothermal chimneys preserve flow textures defined by mineral membranes bounding paleo‐channels and form a network of cavities
The backbone of the chimney structure is initially brucite. Carbonates precipitate on brucite as the chimneys continue to form
Brucite is the preferred substrate for the growth of microbial biofilms within the chimneys
We report on the presence of the serpentine-type antigorite in abyssal-serpentinized peridotite. At mid-ocean spreading ridges, antigorite crystallizes under retrograde metamorphic conditions during ...tectonic exhumation of the newly formed oceanic lithosphere. Using optical microscopy and micro-Raman spectroscopy, we identified antigorite in 49 samples drilled at the Hess Deep (East Pacific Rise) and the Atlantis Massif (Mid-Atlantic Ridge, 30°N), and dredged along the Southwest Indian Ridge (62°–65°E). Overall, antigorite is common, but occurs in limited modal amounts. SEM and TEM investigations reveal its frequent crystallization after lizardite and chrysotile via dissolution–recrystallization processes and a local association with olivine or talc. We explain antigorite crystallization by the interaction with seawater-derived hydrothermal fluids moderately enriched in silica (metasomatism). The origin of silica is attributed to alteration of mafic intrusions or pyroxenes. Antigorite can, therefore, be considered a marker of preferential fluid pathways under rock-dominated conditions during exhumation of a portion of the oceanic lithosphere. We also measured the in-situ major and trace-element composition of antigorite and the predating and postdating phases. Most of the elements are immobile during the mineralogical transitions. Other elements (Ni, Ca, Al, and Ti) evolve within the serpentine textures, including antigorite, as a result of chemical exchanges accompanying the development of the sequence of serpentine textures. A further category includes elements that are specifically enriched (Mn, Sn) or depleted (Fluid-Mobile Elements: B, Sr, As, U, Sb, and Cl) in antigorite compared to lizardite and chrysotile. These enrichments and depletions possibly reflect a change of the fluid physicochemical characteristics allowing a change in element mobility during the dissolution–recrystallization accommodating the lizardite/chrysotile-to-antigorite transition. Such depletion in FME is comparable to depletions described in studies of serpentinization and antigorite formation in subduction zone setting, which suggests that the origin of antigorite in some subducted samples could be reevaluated.
A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective ...sink for CO2. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400‐m deep) and BA3A (300‐m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro‐fractures and grain boundaries, causing large‐scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids with δ18O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.
Plain Language Summary
Water‐rock interactions that have biological and economic importance have received increasing interest in recent years. Carbon species, stored in the atmosphere, biosphere, oceans, and lithosphere, are profoundly affected by biotically and abiotically controlled exchange reactions among these reservoirs. Carbon is an essential element for life and contributes to the greenhouse effect in the form of CO2 and CH4. Unraveling water‐rock reactions that transform atmospheric CO2 into thermodynamically stable carbonates helps to better understand long‐term carbon storage in the lithosphere and its long‐term influence on climate and to better evaluate the potential for life within the lithosphere. This study uses samples recovered by drilling mantle sequences of the Wadi Tayin Massif in Oman and presents chemical data to evaluate carbon sources, speciation, and transformations. The Wadi Tayin Massif is located in the Samail Ophiolite and comprises mantle rocks. Mantle rocks react in contact with water to form serpentine and create an environment that promotes the transformation of CO2 into carbonates. Our results demonstrate large‐scale carbonization of the massif and a dominance of dispersed carbonate formation below the surface. Our study provides insights into the carbon cycle and will help to evaluate the potential of mantle rocks to store carbon and sustain life.
Key Points
Two carbonate occurrences are observed: localized dolomite, calcite, and aragonite veins and pervasive dispersed carbonates
Carbonate precipitation in the peridotite occurred in the last 50 kyr at moderate temperatures, post‐dating ocean‐floor serpentinization
The oxygen isotope composition of dispersed carbonates indicates precipitation from highly 18O‐depleted fossil groundwater
How simple abiotic organic compounds evolve toward more complex molecules of potentially prebiotic importance remains a missing key to establish where life possibly emerged. The limited variety of ...abiotic organics, their low concentrations and the possible pathways identified so far in hydrothermal fluids have long hampered a unifying theory of a hydrothermal origin for the emergence of life on Earth. Here we present an alternative road to abiotic organic synthesis and diversification in hydrothermal environments, which involves magmatic degassing and water-consuming mineral reactions occurring in mineral microcavities. This combination gathers key gases (N
, H
, CH
, CH
SH) and various polyaromatic materials associated with nanodiamonds and mineral products of olivine hydration (serpentinization). This endogenous assemblage results from re-speciation and drying of cooling C-O-S-H-N fluids entrapped below 600 °C-2 kbars in rocks forming the present-day oceanic lithosphere. Serpentinization dries out the system toward macromolecular carbon condensation, while olivine pods keep ingredients trapped until they are remobilized for further reactions at shallower levels. Results greatly extend our understanding of the forms of abiotic organic carbon available in hydrothermal environments and open new pathways for organic synthesis encompassing the role of minerals and drying. Such processes are expected in other planetary bodies wherever olivine-rich magmatic systems get cooled down and hydrated.
The serpentinite-hosted Lost City hydrothermal field is a remarkable submarine ecosystem in which geological, chemical, and biological processes are intimately interlinked. Reactions between seawater ...and upper mantle peridotite produce methane- and hydrogen-rich fluids, with temperatures ranging from <40° to 90°C at pH 9 to 11, and carbonate chimneys 30 to 60 meters tall. A low diversity of microorganisms related to methane-cycling Archaea thrive in the warm porous interiors of the edifices. Macrofaunal communities show a degree of species diversity at least as high as that of black smoker vent sites along the Mid-Atlantic Ridge, but they lack the high biomasses of chemosynthetic organisms that are typical of volcanically driven systems.
The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered during IODP Expedition 357 on the Atlantis Massif (AM) was examined to characterize carbon sources and the fate of ...dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in seawater during long‐lived hydrothermal circulation and serpentinization. Carbon isotopes reveal three stages of carbonate formation, starting at least 38,000 yr ago: (a) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50°C–190°C); (b) carbonate vein formation related to high and focused fluid fluxes still at high temperatures (30°C–200°C); and (c) seawater circulation leading to cold carbonate precipitation controlled by late, brittle fractures during uplift, and unroofing of the oceanic core complex. Our study reveals three main DIC sources in the system: (a) DIC from abiotic hydrothermal degradation of dissolved organic matter (OM); (b) DIC from seawater; and (c) DIC from mantle‐derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (∼800 ppm) of total non‐carbonate carbon (NCC) and 13C‐depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favor the transport and incorporation of marine DOC in serpentinites and that carbonates record isotopic signals of OM decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the AM and suggests that serpentinites may be an important sink of DOC from seawater.
Plain Language Summary
Carbon can be found in higher concentrations within the atmosphere, terrestrial biosphere, oceans, and lithosphere. It is an essential element for various abiotic and biotic reactions and is in the form of CO2 and CH4, an important greenhouse gas. The transfer of carbon between different reservoirs is not well constrained. This study uses samples recovered by drilling the lithosphere at the Atlantis Massif (AM) and chemical investigations to evaluate carbon sources, speciation, and transformations. The AM is located in the Atlantic Ocean close to the Mid‐Atlantic‐Ridge and comprises a high amount of mantle rocks exposed at the seafloor. Mantle rocks react in contact with water to form serpentine and create an environment that promotes the transformation of CO2 into carbonates. Our results demonstrate that the oceanic lithosphere is a significant reservoir of organic compounds and inorganic carbonates and that carbonates record the oxidation of organic matter over long periods. Our study can help to evaluate the potential of mantle rocks to store carbon and reduce CO2 emission. However, it may also provide information to evaluate the potential for life within the lithosphere, not only on Earth but also on other planets where serpentinization processes occur.
Key Points
Serpentinization is associated with dispersed carbonate and multiple generations of veins formed at temperatures from 5°C to 200°C
The system has multiple carbon sources: inorganic and organic carbon from seawater, mantle‐derived carbon, and minor in situ production
Abiotic hydrothermal degradation of dissolved organic matter contributes to carbon cycling in peridotite‐hosted hydrothermal systems
Mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system, in which interaction with seawater to produce serpentinite has major consequences for lithospheric ...cooling, global geochemical cycles, and microbial activity. Serpentinite-hosted hydrothermal activity is exemplified by the Lost City Hydrothermal Field (30°N, Mid-Atlantic Ridge) where fluid–rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen, methane, C2+ alkanes, and formate. Such systems have been proposed as possible analogs to the Early Earth environments that gave rise to the first biochemical pathways. Thus, characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance. Here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important, up to 50% of the microbial biomass is synthesized from mantle carbon. Conversely, mantle carbon contributes only ∼10% of the biomass in areas with minimal sulfate reduction. We attribute this difference to greater incorporation of formate or methane by the dominant microbial species, the Lost City Methanosarcinales, in locations where sulfate reducers are able to facilitate this assimilation. The ability of autotrophic communities at Lost City to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments.
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