Unravelling calcite-to-aragonite evolution from a subsurface fluid - Formation pathway, interfacial reactions and nucleation effects
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Authors
Organisational units
External Organisational units
- Technische Universität Graz
- Ruhr-Universität Bochum
- Geoconsult ZT GmbH
- Fraunhofer Research Institution for Energy Infrastructures and Geothermal Systems IEG
Abstract
Calcite-aragonite alternations are documented in sedimentary deposits worldwide, but their formation is still poorly understood and individual CaCO3 precipitation pathways are rarely confirmed experimentally. Therefore, (sub)recent CaCO3 sinter formation in a historic subsurface adit at Erzberg (Austria) was used as a natural laboratory to monitor and assess the calcite-to-aragonite evolution pathway and intergrowth mechanism in terms of solid-liquid-atmosphere dynamics and their relevance for solid-liquid interface reactions and nucleation effects. Our results indicate an initial homogeneous nucleation of low-Mg calcite (LMC: ~3 ± 1 mol% MgCO3), induced by CO2 degassing from the percolating geogenic fluid, which is originated from seepage of local meteoric water and incongruent dissolution of Mg-Ca-Fe-bearing minerals from the host rock. Progressive LMC growth leads to an increase in the aqueous molar ratio of Mg/Ca, causing a Mg/Ca zonation pattern with transitions to high-Mg calcite (HMC: up to 7 mol% MgCO3). At a critical Mg concentration, the available Mg calcite crystal surfaces are acting as a nucleation site for heterogenous aragonite formation. In this way, fast growing acicular aragonite crystals are initiated, which impede further calcite growth. The calcite-to-aragonite transition is thus controlled by the reaction kinetics and mechanisms of Mg-calcite formation and the chemical evolution of the precipitating solution at the nano- to micro-spatial scale, creating Mg-enriched HMC surface sites for aragonite to be nucleated and preferentially grown. In the present case, the dynamics of the formation of calcite-aragonite sequences are triggered by distinct local environmental changes, in particular seasonal variations in seepage fluid flow behavior and progress in CO2 degassing. These considerations are relevant for a better understanding of proxy signal development and preservation in calcareous sedimentary sequences forming under highly dynamic environmental conditions.
Details
Original language | English |
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Article number | 121768 |
Number of pages | 32 |
Journal | Chemical geology |
Volume | 2023 |
Issue number | ??? Stand: 16. Oktober 2023 |
DOIs | |
Publication status | E-pub ahead of print - 9 Oct 2023 |