Micro-scale chemical and physical patterns in an interface of hydrothermal dolomitization reveals the governing transport mechanisms in nature: Case of the Layens anticline, Pyrenees, France

Hydrothermal dolomitization is an important diagenetic process that occurs in tectonic environments worldwide and forms conventional reservoirs associated with ore deposits and hydrocarbon accumulation, while forming efficient reservoirs for carbon sequestration. However, the current state of knowledge about the availability and reaction rate of Mg in dolomitizing fluids fails to explain the large volumes of hydrothermal dolomites geobodies observed in extended margins or in fold-and-thrust belts. To better understand this widespread phenomenon, it is essential to recognize the governing and limiting transport mode of the dolomitizing fluid. This contribution investigates the chemical and physical patterns developed between the original calcite and the newly formed dolomite. An extensive analytical study of well-preserved dolomitization interfaces observed at outcrop scale in Callovian-Oxfordian limestones in the Layens anticline (north-western Pyrenees, France) is presented. Through the use of scanning electron microscopy, electron backscattered diffraction, X-ray microtomography, laser ablation inductively coupled plasma mass spectrometry (and mapping), the replacement related variations in elementary content, rock density, crystallographic properties and phase volumes and distribution were constrained. The results indicate a sequence of replacement, beginning with a fluid which starts to infiltrate the host rock by advection in the grain boundary network causing at the same time the replacement of calcite by diffusion-limited dissolution and associated dolomite precipitation. The progressive replacement of calcite grains by dolomite is led by dissolution inside the grain enhanced by replacement related porosity creation, leading to a progressive decrease of local calcite grain size isolated as islands until the replacement is complete. The replacement of calcite by dolomite led to a mass loss without volume change, through generation of ca 11 vol.% porosity. Based on analytical observations of a natural sample, a conceptual model that accounts for the transport mode governing the different steps of hydrothermal dolomitization at crystal-scale is proposed.