What control hydrothermal dolomitization? Experimental replacement with time-step monitoring by X-ray microtomography

Hydrothermal dolomitization of limestones, i.e. fluid-mediated stoichiometric substitution Ca_solid ↔ Mg_fluid replacing CaCO3 with dolomite CaMg(CO3)₂, plays a key role in the structural integrity and permeability of the rock that can have dramatic consequences for earthquake hazards, reservoir quality, civil engineering. This particular reaction creates km-scale geobodies usually related to ore deposits or hydrocarbons, and being very efficient bodies for carbon sequestration. As for numerous hydrothermal reactions, various chemo-physical models built from chemical analysis and experiments in analogous replacement compete to explain this mineralogical transformation. Yet, relevant comparison to natural systems remains limited, and the way to explain the creation of large dolomite geobodies remains unexplained. Only recently dolomitization has been successfully recreated in laboratory under a reasonable timescale [1, 2], a few hours to a week according to the fluid reactivity.

This project proposes to use non-destructive imagery methods (xCT) coupled with hydrothermal reactors to reproduce dolomitization in-situ. We choose to study two natural samples representing two end-members: (1) Carrara marble which contains homogeneous polymineralic calcite grains; (2) Layens limestones (French Pyrenees) which is a marble already dolomitized naturally. These two samples were incubated in hydrothermal Teflon reactors in a Mg-enriched aqueous solution [3] at 200°C for different time steps. Microtomography have been acquired at various stages of the reaction, allowing us to track the propagation of the dolomitization front within the samples. This approach allows us to mimic natural dolomitization over time and provides a detailed study of the morphology of the reaction front between calcite and dolomite. Quantifying and describing the microstructures related to replacements (pores, fractures, grains orientation and size) help unravelling how dolomitization can propagate in nature.

 

 

References

[1] L. Jonas, T. Müller, R. Dohmen, L. Baumgartner, B. Pultlitz, Geology (2015)

[2] J. Weber, M. Cheshire, M. Bleuel, D. Mildner, Y-J. Chang, A. Ievlev, K. Littrell, J. Ilavsky, A. Stack, L. Anovitz, Geochimica et Cosmochimica Acta 303 (2021)

[3] V. Vandeginste, O. Snell, M. Hall, E. Steer, A. Vandeginste, Nature communications (2019)