Kinetic fractionation of carbon and oxygen isotopes during BaCO3 precipitation. (1st July 2020)

• Record Type: Journal Article
• Authors: Sade, Ziv; Yam, Ruth; Shemesh, Aldo; Halevy, Itay
• Is Part Of: Geochimica et cosmochimica acta. Volume 280(2020)
• Page Start: 395
• Page End: 422
• Publication Date: 2020-07-01
• Subjects: Experimental precipitation -- Carbonate mineral -- Aqueous carbonate speciation; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2020.04.025
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Disequilibrium isotopic compositions in carbonate minerals often reflect the integration of several different kinetic isotope effects (KIEs), whose relative contribution to the overall composition depends on the specific mineral formation process and environment. Thus, potential environmental reconstructions from disequilibrium compositions in natural carbonates require ( i ) quantification of KIEs associated with reactions involved in mineral formation, and ( ii ) a theoretical framework linking physical properties of natural environments to the expression and preservation of these KIEs in carbonates. To constrain KIEs associated with carbonate mineral precipitation reactions, we performed a series of rapid witherite (BaCO3 ) precipitation experiments over a range of pH (8.7–13.0), temperature (15–40 °C) and fractional yield of the dissolved inorganic carbon (DIC; a few percent to quantitative precipitation). Our experiments extend the range of pH and temperature explored in previous studies, and include measurements of both carbon and oxygen isotopes. We developed a dynamic model of the DIC system, with which we simulated the experiments. The model results identify isotopic distillation due to formation of aqueous CO2 and mineral precipitation by either CO3 2– or HCO3 – as the main determinants of the carbon and oxygen isotopic evolution of the solid and of the solution. We estimate that the carbon kinetic fractionation factor (KFF) associated with unidirectional precipitation of witherite via CO3 2–, 1000 l n 13 α C O 3 2 - → B a C O 3 k + 1, is 0.5 ± 0.1‰ (15–40 °C), whereas the oxygen KFF, 1000 l n 18 α C O 3 2 - → B a C O 3 k + 1, is –1.0 ± 0.3‰ (15 °C) and –0.5 ± 0.2‰ (25–40 °C). The carbon KFF associated with unidirectional HCO3 – precipitation pathways, 1000 l n 13 α H C O 3 - → B a C O 3 k + 2, + 3, is –0.2 ± 1.0‰ (15–40 °C), whereas the oxygen KFF, 1000 l n 18 α H C O 3 - → B a C O 3 k + 2, + 3, is –6.7 ± 1.1‰ (15 °C), –4.5 ± 1.1‰ (25 °C) and –4.0 ± 1.1‰ (40 °C). Our oxygen KFF of witherite precipitation from CO3 2– agrees well with the available literature estimates at 25 °C. In addition, our carbon and oxygen KFFs are comparable to literature KFFs associated with calcite precipitation, possibly suggesting a similarity of precipitation KFFs among carbonate minerals. Importantly, the relative magnitudes and uncertainties of the precipitation KFFs and the equilibrium fractionation between CO3 2– and HCO3 –, leads to the expectation that in many natural settings, the isotopic composition of carbonate minerals be an insensitive probe of the precipitation pathway.