Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates – Relation between textural control on effective element fluxes and resulting carbonate phase

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Jonas, L, Müller, T, Dohmen, R, Immenhauser, A, Putlitz, B
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2017
• Subjects: Arctica islandica
• Online Access: https://eprints.whiterose.ac.uk/88546/; https://eprints.whiterose.ac.uk/88546/8/Jonas%20et%20al%202016%20GCA.pdf; https://doi.org/10.1016/j.gca.2016.09.034

Dolomitization, i.e., the secondary replacement of calcite or aragonite (CaCO3) by dolomite (CaMg[CO3]2), is one of the most volumetrically important carbonate diagenetic processes. It occurs under near surface and shallow burial conditions and can significantly modify rock properties through changes in porosity and permeability. Dolomitization fronts are directly coupled to fluid pathways, which may be related to the initial porosity/permeability of the precursor limestone, an existing fault network or secondary porosity/permeability created through the replacement reaction. In this study, the textural control on the replacement of biogenic and abiogenic aragonite by Mg-carbonates, that are typical precursor phases in the dolomitization process, was experimentally studied under hydrothermal conditions. Aragonite samples with different textural and microstructural properties exhibiting a compact (inorganic aragonite single crystal), an intermediate (bivalve shell of Arctica islandica) and open porous structure (skeleton of coral Porites sp.) were reacted with a solution of 0.9 M MgCl2 and 0.015 M SrCl2 at 200 °C. The replacement of aragonite by a Ca-bearing magnesite and a Mg-Ca carbonate of non-stoichiometric dolomitic composition takes place via a dissolution-precipitation process and leads to the formation of a porous reaction front that progressively replaces the aragonite precursor. The reaction leads to the development of porosity within the reaction front and distinctive microstructures such as gaps and cavities at the reaction interface. The newly formed reaction rim consists of chemically distinct phases separated by sharp boundaries. It was found that the number of phases and their chemical variation decreases with increasing initial porosity and reactive surface area. This observation is explained by variations in effective element fluxes that result in differential chemical gradients in the fluid within the pore space of the reaction rim. Observed reaction rates are highest for the replacement of the ...