EARLY DIAGENESIS OF LACUSTRINE CARBONATES IN VOLCANIC SETTINGS: THE ROLE OF MAGMATIC CO2 (LAKE DZIANI DZAHA, MAYOTTE, INDIAN OCEAN)

Lacustrine carbonates formed in rift settings are increasingly studied not only as archives of Earth chemical and climatic history but also because they represent potential hydrocarbon source rocks and/or reservoirs. The role of magmatic gases in their formation and diagenetic evolution, hence in their reservoir properties, remains unclear. We studied the first meter of carbonate sediments of the Dziani Dzaha volcanic crater lake (Mayotte Island) and developed a reactive-transport model with the CrunchFlow software that allows the quantification of diagenetic reactions by considering depth-dependent burial rate and sediment compaction. The model is constrained by the previoulsy documented solid phase compositions of the lake sediment and by new data consisting of 14C dating of plant macro-remains to characterize the sediment age model, chemical composition of sediment pore waters and chemical and isotopic composition of gases dissolved and bubbling through the lake. These new data reveal a massive magmati...