Modelling of early diagenesis of lacustrine carbonates associated with Mg-silicates
Résumé
Diagenetic processes occuring in time and space are critical in the evolution of sedimentary rocks. They need to be assessed to improve our abilities for palaeoenvironmental interpretations. A numerical model was developed with the reactive transport code CrunchFlow to assess the chemical and physical processes occurring during the early diagenesis of lacustrine carbonates formed in rift settings, using as a case study the carbonate sediments associated with Mg-silicates of the alkaline volcanic crater lake, Dziani Dzaha. The model relies on the solid phase compositions of the first meter of sediments, the porosity, the pore water chemistry and an age model for the sediment based on radiocarbon measurements. Chemical and isotope analyses reveal the inflow of magmatic CO 2 and intense microbial methanogenesis activity in the lake. The alkaline pH of the lake induce oversaturation of porewaters relative to aragonite, hydromagnesite and saponite. Carbonates form close to equilibrium and dominate the mineralogy of the shallow sediment while kinetic effects inhibit the formation of saponite that precipitate only at depth. Magmatic CO 2 inflow and microbial degradation of organic matter cause a decrease of pH that destabilized hydromagnesite. The model brings new insights on the palaeoenviroments and on the early diagenetic processes leading to the lacustrine carbonates formed in rift settings. It quantifies the mechanisms involved in the early diagenetic processes (e.g. input of mantellic CO 2) without which minerals reactivity, pH and porosity would not be described over the sediment depth. This study represents a first step towards the forward modeling of the evolution of the solid and fluid phases of carbonate sediments from their deposition at sediment surface to their current settings in the sedimentary column.