Sylvain Richoz

Despite their geological significance, the thermochemical processes leading to the formation of early carbonate cements remain elusive. To bridge this knowledge gap, we carried out here laboratory-simulated diagenesis on aragonite ooids. They were placed in autoclaves, filled with distilled water, and subjected to varying temperatures and durations. Our results focus on the transformation from aragonite to calcite with temperature-dependent kinetics. At the grain scale, considered as a thermodynamic system on its own, two distinct stages of microstructural evolution were observed. In the first stage, diffusion processes predominate: the ooids experience progressive dissolution while a fringe of calcite cement forms around their periphery. In a second stage, radial diffusion stops and transformations are dominated by surface-controlled processes, characterized by calcite crystal growth within the ooids. The end-result is generally referred to as “neomorphism” in natural analogues. The transition from the diffusion-dominated to the surface-controlled stage is temperature-dependent and does not occur below 150°C. This behaviour can be explained by the progressive growth of the calcite fringe, which isolates the ooid from the surrounding pore water, while internal aragonite dissolution continues within the grain. These experiments show that early calcite cements, similar to those observed in meteoric phreatic environments, can grow without any external CaCO₃ input. They also show that neomorphism in ooids requires the closure of the ooid system, a condition that can occur at any stage of diagenesis. Finally, our experiments support the presence of a diffusive boundary layer, offering a process-based refinement of the traditional ‘thin film’ concept.