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| Titel |
Kinetic and equilibrium Ba isotope fractionation during carbonate
precipitation |
| VerfasserIn |
Kirsten van Zuilen, Vasileios Mavromatis, Bettina Purgstaller, Andre Baldermann, Thomas F. Nägler, Martin Dietzel |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250151221
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| Publikation (Nr.) |
 EGU/EGU2017-15784.pdf |
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| Zusammenfassung |
| Variations in stable isotope ratios recorded in carbonates are widely used to reconstruct the
physicochemical conditions, e.g., pH, temperature and redox conditions, prevailing at the
time of carbonate mineral formation. Knowledge of isotope fractionation factors during
mineral precipitation under varying environmental conditions is irremissible for the
interpretation of isotope variations in natural sedimentary archives. However, experimentally
derived fractionation factors, of for instance Ca isotopes, are often ambiguous and
incommensurable due to differences in experimental parameters.
Here, Ba isotope fractionation during carbonate mineral formation was investigated
[1]. Time-resolved experiments of witherite (BaCO3) precipitation revealed an
initial kinetic isotope effect with increasing Δ137/134Ba values of the ambient
solution, following Rayleigh fractionation (αwitherite-fluid = 0.99993 ± 0.00004).
After precipitation, the witherite crystals remained in contact with the ambient
solution for about nine days. During this time, chemical steady state was achieved
between solution and witherite; however, the Δ137/134Ba values of the solution
decreased. At isotopic equilibrium, the ambient solution and the witherite crystals
exhibited identical δ137/134Ba values, within the analytical uncertainty of ±0.04
‰Ṫhis observation is interpreted as the result of continuous exchange of Ba2+
ions between witherite and solution after initial preferential uptake of the lighter
Ba isotopes in the precipitating carbonates. Mass balance calculations indicate
that the ion exchange affects several subsurface layers of the crystals. In summary,
Ba isotope exchange between carbonate and ambient solution occurs at chemical
equilibrium, and pristine isotopic signatures in carbonates may thus be reset at low
temperatures.
[1] Mavromatis et al. (2016) Geochim. Cosmochim. Acta 190, 72-84. |
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