Theoretical Determination of Isotope Fractionation Factors in Sedimentary Carbonates

D. Fraser Steele Department of Earth Sciences, Liverpool University, Liverpool L69 3BX, UK

The carbonate minerals are widely used in stable isotope geochemistry for palaeoenvironmental investigations. Fundamental to these investigations is the use of the oxygen isotope ratios to determine the temperatures at which sedimentary carbonate minerals were deposited.

The carbonate - water oxygen isotope fractionation factor has been determined empirically at earth surface and elevated temperatures for calcite and aragonite using direct precipitation (O'Neil et al., 1969; Tarutani et al., 1969) methods. However, empirical determination of oxygen isotope fractionation factors for other carbonates such as dolomite and magnesite at typical sedimentary temperatures has never been done. This is because of the great difficulty in synthesising these minerals at ambient and near-ambient temperatures. It is therefore necessary to extrapolate from high temperature hydrothermal exchange results down to sedimentary temperatures.

Using statistical thermodynamics methods, theoretical oxygen isotope fractionation factors for calcite have been calculated previously (Bottinga, 1968; O'Neil et al., 1969; Golyshev et al., 1981; Kieffer, 1982). These compare favourably with empirical determinations. There is, however, a dearth of such data for other carbonates, the work of Golyshev et al. (1981) being the exception. It is the purpose of this project to employ the mathematical model developed by Kieffer (1982) to calculate theoretical fractionation factors from published vibrational spectra and elastic data for dolomite and magnesite. Calculated carbonate-water oxygen fractionation factors (1000 lna) at 298K are shown in Table 1.

Table 1. Comparison of theoretical fractionation factors at 298K with previously published results.

Mineral This work Previous results Ref.

Calcite 28.1 28.0 1

Dolomite 33.4 31.4 - 34.5 2

Magnesite 35.0 31.1 - 36.2 3

1: Friedman & O'Neil, 1977;

2: Land, 1983;

3: Aharon, 1988

Comparison with the methods used previously (Bottinga, 1968; O'Neil et al., 1969; Golyshev et al., 1981) is possible, and the importance of accurate spectral data for the carbonates under investigation will be discussed. Further use of the Kieffer model includes the extension of this work to include other carbonates for which elastic data is not available.
A theoretical investigation into the effect of magnesium substitution in calcite on oxygen isotope fractionation, and comparison with the currently accepted fractionation of magnesian calcites (Tarutini et al., 1969) will also be presented.


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Bottinga, Y., J. Phys. Chem. 72, 800-808 (1968).

Friedman, I. & O'Neil J.R., Data of Geochem. (U.S. Govt Print. Off., 1977).

Golyshev, S.I., Padalko, N.L. & Pechenkin S.A., Geochem. International 18, 85-99 (1981).

Kieffer, S.W., Rev. Geophys. Space Phys. 20, 827-849 (1982).

Land, L. S. SEPM Special Publication 28, 87-110 (1983).

O'Neil, J.R., Clayton, R.N. & Mayeda, T.K., J. Chem. Phys. 51, 5547-5558 (1969).

Tarutani, T., Clayton, R.N. & Mayeda , T.K., Geochim. Cosmochim. Acta 33, 987-996 (1969).