Dynamic investigation of high-temperature dolomitization of polycrystalline Carrara marble

Abstract

Dolomitization of limestones is widespread and has great importance regarding fluid/rock interactions as the calcite dolomite transformation theoretically creates up to 13% of porosity while being linked to strategic resources. Yet, the dynamic of dolomitization is still poorly understood and there is a critical need of experimental data to better constrain this mineralogical replacement. In this contribution, we successfully manage to reproduce dolomitization in laboratory in hydrothermal reactors at 200 °C with a Mg and Zn-enriched reactive fluid. 3D time-step observation of the reaction by the means of X-ray microtomography, along with post-mortem chemical and mineralogical analyses with SEM-EBSD and microprobe, respectively, offers an unprecedented view of the reaction mechanisms. Two sets of experiments have been conducted: (1) the Static experiment involved 3 incubations in the same reactive fluid which is progressively depleted in Mg as dolomite content increases in the solid; (2) the Pulsatile experiment is refilled with the initial Mg-rich fluid at each incubation which results in the maintenance of the geochemical disequilibrium between the fluid and the solid. The results show that the Pulsatile sample is more dolomitized after 3 weeks than the Static sample, and that a metastable carbonate is systematically present at the interface between calcite and the dolomite front, propagating along grain boundaries and forming by itself another replacement front. These experiments show the development of microstructures directly comparable to natural dolomitized limestones allowing the identification of interface-coupled dissolution/precipitation mechanisms.