Oxygen isotope fractionation in double carbonates

Abstract

Oxygen isotope fractionations in double carbonates of different crystal structures were calculated by the increment method. Synthesis experiments were performed at 60 °C and 100 °C to determine oxygen and carbon isotope fractionations involving PbMg[CO₃]₂. The calculations suggest that the double carbonates of calcite structure are systematically enriched in ¹⁸O relative to those of aragonite and mixture structures. Internally consistent oxygen isotope fractionation factors are obtained for these minerals with respect to quartz, calcite and water at a temperature range of 0–1200 °C. The calculated fractionation factors for double carbonate–water systems are generally consistent with the data available from laboratory experiments. The experimentally determined fractionation factors for PbMg[CO₃]₂, BaMg[CO₃]₂ and CaMg[CO₃]₂ against H₂O not only fall between fractionation factors involving pure carbonate end-members but are also close to the calculated fractionation factors. In contrast, experimentally determined carbon isotope fractionation factors between PbMg[CO₃]₂ and CO₂ are much closer to theoretical predictions for the cerussite–CO₂ system than for the magnesite–CO₂ system, similar to the fractionation behavior for BaMg[CO₃]₂. Therefore, the combined theoretical and experimental results provide insights into the effects of crystal structure and exchange kinetics on oxygen isotope partitioning in double carbonates.

Keywords:

• carbon isotopes
• crystal structure
• double carbonate
• isotope exchange
• isotope fractionation
• oxygen isotopes
• temperature dependence

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Erratum

Acknowledgements

This contribution is dedicated to Jochen Hoefs on the occasion of his 75th birthday. He is thanked for hosting YFZ to study stable isotope geochemistry in the University of Göttingen and for providing MEB to use the mass spectrometer at the Geochemical Institute. We are grateful to Dr Gerhard Strauch and Taylor & Francis for the invaluable support in realizing this special issue. Thanks are due to Dr Zhengrong Wang and the two anonymous reviewers for their comments on this manuscript, which helped in the improvement of the presentation.

Funding

This study was supported by funds from the National Natural Science Foundation of China [41621022] and by Leibniz Institute for Baltic Sea Research.