Evidence of polygenetic carbon trapping in the Oman Ophiolite: Petro-structural, geochemical, and carbon and oxygen isotope study of the Wadi Dima harzburgite-hosted carbonates (Wadi Tayin massif, Sultanate of Oman)
Résumé
The Wadi Dima area (Oman Ophiolite) exposes partially altered to highly serpentinized harzburgites that are cross-cut by intense (>20 Vol%) carbonate veining. We identified a sequence of 3 types of carbonate veins with compositions ranging from calcite to dolomite (Mg/Ca = 0-0.85). Type 1 carbonates occur as a fine diffuse vein network, locally replacing olivine cores, penetrative into the serpentinized harzburgites. They have depleted trace elements abundances (e.g., Yb < 0.2 × C1-chondrite) relative to other Wadi Dima carbonates, exhibit negative Ce and positive Y, U anomalies and a broad range in δ13CV-PDB (-5 to -15‰) and δ18OSMOW (18 to 31‰). These compositions are consistent with precipitation after seawater-derived fluids and/or fluids in equilibrium with mantle rocks and serpentines during cooling of oceanic lithosphere (110 to 15°C). Type 2 carbonates are localized in veins, which acted as main flow paths for fluids interacting with peridotites in the exhumed Oman mantle lithosphere (50°C-10°C). The orientation of these veins is controlled by the crystallographic anisotropy of Oman mantle peridotites. Type 2 carbonates record two stages. The first involved the formation of large calcite crystals of composition similar to Type 1 carbonates (trace element depleted; δ13CV-PDB B = -4 to -9‰ and δ18OSMOW = 26 to 30‰), which during the second stage were recrystallized to form dolomite and calcite microcrystals (trace element enriched; δ13CV-PDB = -7 to -13‰ and δ18OSMOW = 29 to 32‰), after fluids sampling different sources including contributions of sediment-derived components. They were most likely formed at shallow depths and record the transition from oceanic to continental settings during late Cretaceous ophiolite obduction. Type 3 veins reactivate Type 2 veins. They comprise dominantly calcite and dolomite microcrystals (Light REE enriched patterns) with isotopic compositions (δ13CV-PDB B ~ -7 to -8‰; δ18OSMOW~ 28 to 32‰) consistent with precipitation at low temperatures (T°<30°C) from surface/meteoric fluids. Type 3 veining is probably triggered by ophiolite uplift during the Oligocene to early Miocene. Our study presents new insights into the role of the initial mantle anisotropy in the orientation of the vein network and of principal flow paths during serpentinization and carbonatization of mantle peridotites. It also highlights the highly variable carbon isotope composition of carbonates and suggest different origins for these heterogeneities: the carbon isotope composition of the early Type 1 carbonates dispersed in the poorly connected peridotites is locally modified by serpentinization reactions whilst the carbon isotope compositions of Type 2 and 3 carbonates record mixing of fluids from different sources in high flow veins.