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.