The Lost City hydrothermal field (LCHF) is considered an analogue of Archean alkaline hydrothermal vents where life on Earth may have spawned. Although Lost City was discovered more than 20 years ago, it remains unclear to what extent microbes are involved in the precipitation of the various minerals constituting the hydrothermal chimneys. Most chimneys preserve 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 observe local brucite precipitation on microbial filaments and, in some cases, microbial filaments may affect the growth direction of brucite crystals. The link between brucite and organic compounds is further confirmed by fluorescence microscopy: brucite shows a strong fluorescence not observed in calcite and aragonite. Our results point to brucite as an important link to microbial life in alkaline fluids and is potentially a key aspect for future research on the origin of life.

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 isotope) indicate that 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 form mostly out of oxygen and clumped isotope equilibrium due to rapid precipitation upon vent fluid-seawater mixing. In contrast, the carbonates precipitate close 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.

A large part of hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites, which constitute reactive chemical and thermal systems and potentially represent an effective sink for CO2. Understanding carbonation mechanisms is almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon 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 over 50 kyr) are localized calcite, dolomite, and aragonite veins, which formed between 26 to 43 degrees Celcius and are related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1000 yr. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods of time 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. We also show 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.

Petrographic and major element investigations on carbonates from drill cores recovered during IODP Expedition 357 on the Atlantis Massif (AM) provide information on the genesis of carbonate minerals in the oceanic lithosphere. Textural sequences and mineralogical assemblages reveal three distinct types of carbonate occurrences in ultramafic rocks that are controlled by (i) fluid composition and flow, (ii) temperature of the system, and (iii) the presence of mafic intrusions. The first occurrence of carbonate consists of different generations of calcite that formed syn- to post- serpentinization. These calcites formed at temperatures between 30 and 185°C (based on clumped isotopes) and from a fluid influenced by interaction with mafic intrusions. The second occurrence consists of magnesite, dolomite, calcite and aragonite veins that also formed syn- to post serpentinization. These carbonates formed at temperatures between 4 and 188°C and from fluids with highly variable composition and Mg/Ca ratios, but overall high CO2 and moderate SiO2 concentrations. High FeO (3.3 wt%) and MnO (7.3 wt%) contents indicate high temperatures, high water/rock ratios, and low oxygen fugacity for both carbonate assemblages. The third occurrence consists solely of aragonite veins formed at low-temperatures (5°C) within the uplifted serpentinized peridotites. Chemical data suggest that aragonite precipitated from cold seawater, which underwent little exchange with the basement. Combining these observations, we propose a model that places different carbonate occurrences in a conceptual frame involving mafic intrusions in the peridotites and fluid heterogeneities during progressive exhumation and alteration of the AM.

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 (DOC) and 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: (1) Early dispersed carbonate precipitation, with low water/rock ratios and high temperatures (50 to 190°C); (2) carbonate vein formation related to high and focused fluid fluxes still at higher temperatures (30 to 190°C); and (3) 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: (1) DIC from abiotic hydrothermal degradation of dissolved organic matter; (2) DIC from seawater; and (3) DIC from mantle-derived volatiles. Basement rocks containing dispersed carbonates are characterized by high concentrations (~800 ppm) of total organic carbon (TOC) and 13C-depleted carbonates. We propose that high seawater fluxes in the southern part of the AM likely favour the transport and incorporation of marine dissolved organic carbon in serpentinites and that carbonates record isotopic signals of organic matter decay. Our study indicates that organic carbon accounts for a significant proportion of the total carbon stored in the Atlantis Massif and suggests that serpentinites may be an important sink of DOC from seawater.