Résumé
Carbon trapping in ophiolitic peridotites contributes to the global carbon cycle between solid Earth and its outer envelopes (through subduction and/or modern alteration). To investigate this process, we performed petro-structural (microtomography, EBSD, EPMA) and geochemical studies (LA-ICP-MS, carbon and oxygen isotopes on bulk and minerals using SHRIMP) of harzburgites cored in the Oman Ophiolite. Studied harzburgites are highly serpentinized (> 90%) and crosscut by 3 generations of carbonates (> 20 Vol%) with compositions from calcite to dolomite (Mg/Ca = 0-0.85). Type 1 carbonates are fine penetrative veinlets and mesh core after olivine. They have low REE (e.g., Yb = 0.08-0.23 X CI-chondrite) and negative Ce anomalies. They have delta (super 13) C (sub PDB) = -15.2 to 1.10 ppm and delta (super 18) O (sub SMOW) = 17.5 to 33.7 ppm, suggesting precipitation temperatures up to 110 degrees C. Type 2 carbonates are pluri-mm veins bounded by cm-thick serpentinized vein selvages, oriented dominantly parallel to mantle foliation. Dynamic recrystallization is observed, indicating polygenetic formation: well crystallized calcite with REE abundances similar to Type 1 carbonates are locally replaced by small dolomite and calcite grains with higher REE (e.g., Yb = 0.35-1.0 X CI-chondrite) and positive Gd anomaly. Type 2 carbonates have delta (super 13) C (sub PDB) = -12.6 to -4.1 ppm and delta (super 18) O (sub SMOW) = 25.0 to 32.7 ppm, suggesting precipitation temperatures from 10 to 60 degrees C. Type 3 carbonates are late pluri-mm to cm veins reactivating Type 2 veins. They consist of small grains of dolomite and calcite with REE abundances similar to recrystallized Type 2 carbonates. Type 3 carbonates have delta (super 13) C (sub PDB) = -8.3 to -5.8 ppm and delta (super 18) O (sub SMOW) = 28.8 to 32.7 ppm, suggesting precipitation temperatures <35 degrees C. delta (super 13) C data indicate an evolution of fluid composition precipitating carbonates from seawater- and sediment-derived fluids to meteoric water. Carbonate formation starts during oceanic lithospheric cooling and occurs as a penetrative process at the expense of olivine (Type 1, at T > 100 degrees C). Formation of carbonate veins (Type 2) indicates localization of fluid flux, while serpentinization remains the dominant alteration process. Low T carbonate veins (Type 3) remain the main flow path through ophiolitic peridotites. Our study suggests that their orientation is controlled by the later stages of oceanic mantle deformation.