TY - JOUR

T1 - Unravelling calcite-to-aragonite evolution from a subsurface fluid - Formation pathway, interfacial reactions and nucleation effects

AU - Eichinger, Stefanie

AU - Boch, Ronny

AU - Baldermann, Andre

AU - Goetschl, Katja

AU - Wenighofer, Robert

AU - Hoffmann, Rene

AU - Stamm, Franziska

AU - Hippler, Dorothee

AU - Grengg, Cyrill

AU - Immenhauser, Adrian

AU - Dietzel, Martin

PY - 2023/10/9

Y1 - 2023/10/9

N2 - 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.

AB - 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.

U2 - 10.1016/j.chemgeo.2023.121768

DO - 10.1016/j.chemgeo.2023.121768

M3 - Article

VL - 2023

JO - Chemical geology

JF - Chemical geology

SN - 0009-2541

IS - ??? Stand: 16. Oktober 2023

M1 - 121768

ER -