Hydrothermal circulation of seawater through the oceanic crust occurs at such rates that the entire volume of the ocean may be cycled through the oceanic crust in less than 20 Myr (German & Von Damm, 2003;Wheat & Mottl, 2004). This high fluid flux, coupled with extensive fluid-rock interaction, has significant consequences for the thermal structure and rheology of the oceanic lithosphere, geochemical budgets of the ocean, and microbial processes within and at the seafloor (e.g., Lister, 1972;Proskurowski et al., 2006;Wheat & Mottl, 2004). At slow-spreading ridges, hydrated upper mantle rocks (serpentinites) are commonly exposed at the seafloor and constitute highly reactive chemical and thermal systems. Carbon-one of the most important elements on Earth-is stored within the lithosphere in the form of gaseous CO 2 and CH 4 within mineral inclusions or as solid phases, such as carbonate and graphite, and organic compounds. These may persist in ancient mantle domains over extended periods of time.

Petrographic and major element investigations of carbonates from seven 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 (a) fluid composition and flow, (b) temperature, and (c) 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 184°C (based on clumped isotope thermometry) 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 ambient and 196°C, from fluids with highly variable FeO, MnO, and SrO 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 (<9°C) within the uplifted serpentinized peridotites. Major element analyses suggest that aragonite precipitated from seawater, which experienced little interaction with the basement. Combining these results, we propose a model that positions different carbonate occurrences in a conceptual framework considering mafic domains in the peridotites and fluid heterogeneities during progressive exhumation and alteration of the AM.

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.

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.

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