Abstract
Ultramafic rocks are one of the most important reservoirs of carbon and have a significant impact on the global carbon cycle. Carbonated serpentinites, ophicarbonates, are exposed in ophiolites, which can be remnants of oceanic lithospheres. Multiple generations of fluids for carbonation are recorded in ophicarbonates in Oman ophiolite, and Chenaillet and Lago Nero ophiolite, Western Alps. Oman ophiolite records oceanic lithosphere in fast spreading ridges, whereas Chenaillet and Lago Nero ophiolite in slow spreading ridges. Carbonate veins in serpentinites of Oman ophiolite have been proposed as formed in the ocean (Type 1), during obduction (Type 2), and after obduction (Type 3) based on trace elements and oxygen and carbon isotope ratios of carbonates in a previous study. The ophicarbonate of Chenaillet ophiolite in the Western Alps has been classified into four groups in a previous study based on texture and oxygen isotope data. Carbonate veins (C1) and carbonate cores (C2) in the breccia of serpentinite were formed on the ocean floor along the detachment fault through abyssal hydrothermal circulation. Carbonate matrix (C3) filling between breccia of serpentinites. Finally carbonate veins (C4) cross the matrix and breccia. Ophicarbonate of Lago Nero ophiolite suffered from metamorphism during subduction and consists of a deformed carbonate matrix (DMx) filling the breccia of serpentinite and branching carbonate veins (BCV) in the serpentinite. Carbonated fluids were classified based on microthermometry of fluid inclusions in each carbonate structure. (i) fluid inclusions in the Type 1 vein of the Oman, the C3 matrix of the Chenaillet, and the DMx of the Lago Nero show a wide range of salinities from 0–11 wt. % NaCl eq. This suggests mixing of brine and vapor separated by seawater boiling (Kelly & Delaney, 1987). Type 1 vein can be precipitated from fluids heated by a geothermal gradient or magma, while the C3 matrix and DMx can be precipitated from fluids that was boiled by rapid depressurization due to fracturing. (ii) Most of the fluid inclusions in the large calcite of the Type 2 veins of the Oman show low salinities close to fresh water of 0–1 wt.% NaCl eq. This suggests that they are formed by meteoric water. Some of the large calcite contact with microcrystal of dolomite, which fills the intervening spaces and have fluid inclusions > 4 wt.% NaCl eq. Half of the fluid inclusions in the microcrystaline dolomite in Type 2 carbonate veins show >3.5 wt. % NaCl eq. Fluids with salinity higher than seawater can be subducted slab-derived fluids. (iii) Fluid inclusions in the C1 vein, theC2 core of the Chenaillet, and the BCV of the Lago Nero show salinities of 4–6 wt.% NaCl eq. Thesecarbonates can be formed by fluids with increased salinity due to serpentinization (Debure et al., 2019;Vesi et al., 2023)