EXPERIMENTAL STUDIES OF OXYGEN ISOTOPE FRACTIONATIONS BETWEEN CaCO_3 AND H_2O AT LOW TEMPERATURES

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Abstract:

Stable isotope analyses of CaCO 3 minerals are very important for investigations of paleoclimate and sedimentary environments For this reason, the experimental calibration of oxygen isotope fractionation factors in the system of CaCO 3 H 2O has been an attractive subject in stable isotope geochemistry since its birth However, there are considerably large differences in the oxygen isotope fractionation factors between CaCO 3 and H 2O measured by different investigators, due to CaCO 3 polymorphs in nature ( i e calcite, aragonite and vaterite) As a result, different temperatures are yielded when these different fractionation factors are applied to isotope geothermometry Thus it is of critical importance in low temperature and environmental geochemistry to make the correct and reasonable choice of oxygen isotope fractionation equations in the calcite water and aragonite water systems In this paper the experimental calibration history, approaches and results of oxygen isotope fractionations in the CaCO 3 H 2O system are systematically summarized and reviewed, and the different expressions about the oxygen isotope fractionation between CaCO 3 and H 2O are unified Salt effect and kinetic effect on oxygen isotope fractionation as well as oxygen isotope inheritance in the processes of polymorph transformation are discussed as well The equilibrium equation of oxygen isotope fractionation between calcite and water is recommended on the basis of reprocessing a large number of the known experimental data and comparing them with theoretical calculations However, the theoretical calculation results concerning the aragonite water system has to be confirmed by further experiments

Keywords:

Equilibrium fractionation

Mass-independent fractionation

Topics:

Geology and Paleoclimatology Research

Methane Hydrates and Related Phenomena

Paleontology and Stratigraphy of Fossils

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On the Direction and Magnitude of Oxygen Isotope Fractionation Between Calcite and Aragonite at Thermodynamic Equilibrium

Aquatic Geochemistry (2006)

Gen‐Tao Zhou Yong‐Fei Zheng

Equilibrium fractionation

Carbonate minerals

10.1007/s10498-005-5857-3

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Citations (19)

On the theoretical calculations of oxygen isotope fractionation factors for carbonate-water systems

GEOCHEMICAL JOURNAL (2011)

Yong‐Fei Zheng

Theoretical and experimental aspects of oxygen isotope fractionation in carbonate minerals are critically examined based on a direct comparison of fractionation factors for carbonate-water systems. The results show good agreement between theory and experiment in most cases. In particular, theoretical fractionation factors calculated by the statistico-mechanics method and the increment method are in good agreement with the experimental values for dolomite, siderite, witherite, strontianite and cerussite. These agreements provide corroboration that the two entirely independent approaches of calculation are generally capable of producing thermodynamic equilibrium fractionation factors for the most carbonate-water systems. In particular, the merit of the increment method relative to the statistico-mechanics method is evident for crystalline minerals because it enables the systematic and accurate predictions of oxygen isotope fractionation factors for different structures and compositions of crystalline minerals based only on their crystal chemistry. Thus, the increment method has no limitations to the calculations as commonly encountered by the statistico-mechanics method. Nevertheless, complexity in oxygen isotope fractionations between calcite and the other carbonate minerals can be caused by the oxygen isotope inheritance during polymorphic transformation from aragonite to calcite and by the isotope salt effect on the other carbonates in the presence of aqueous fluids. Therefore, caution must be exercised when interpreting possible disagreements between theory and experiment because of the kinetic effects. In the extreme case, equilibrium oxygen isotope fractionation factors for single carbonate-water systems could be either underestimated or overestimated by one of the theoretical and experimental methods.

Equilibrium fractionation

Carbonate minerals

Mass-independent fractionation

10.2343/geochemj.1.0125

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Citations (46)

Sulfur isotope fractionation between aqueous and carbonate-associated sulfate in abiotic calcite and aragonite

Geochimica et Cosmochimica Acta (2020)

Yigal Barkan Guillaume Paris Samuel M. Webb Jess F. Adkins Itay Halevy

Equilibrium fractionation

10.1016/j.gca.2020.03.022

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