Letter | Published:

Oxygen Isotope Fractionation between Coexisting Calcite and Dolomite in the Freshwater Upper Carboniferous Freeport Formation

Naturevolume 207pages972973 (1965) | Download Citation



HYDROTHERMAL experiments of Epstein, Graf and Degens1 have suggested that dolomite forms from pre-existing calcite by a replacement mechanism in which the 18O/16O ratio of the original calcite remains unchanged. In an investigation of natural sedimentary dolomite-calcite rocks, Degens and Epstein2 observed no fractionation of the oxygen isotopes between coexisting calcite and dolomite in relatively young deposits and concluded that all dolomite is secondary, that is a replacement product of pre-existing calcite. Older carbonate rocks examined by those authors, however, did contain dolomite which was enriched in oxygen-18 with respect to the associated calcite. This phenomenon was explained by possible isotopic equilibration of the calcite with intrastratal fluids (which are generally enriched in oxygen-16). Dolomite is believed to be more resistant to equilibration. Hence, according to Degens and Epstein, such calcite rocks are partly replaced by diagenetic dolomite without alteration of the isotopic composition of the carbonate. With time, oxygen isotope exchange between formation waters and the calcite, but to a negligible extent between the waters and the dolomite, results in a higher 18O/16O ratio in the dolomite. Thus the difference in δ18Odol and δ18Oct observed in ancient carbonate rocks does not represent original isotopic equilibrium between these two minerals.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1

    Epstein, S., Graf, D. L., and Degens, E. T., in Isotopic and Cosmic Chemistry, 169 (North Holland Publishing Co., Amsterdam, 1963).

  2. 2

    Degens, E. T., and Epstein, S., Geochim. Cosmochim. Acta, 28, 23 (1964).

  3. 3

    Weber, J. N., p. 930 of this issue of Nature.

  4. 4

    Weber, J. N., Science, 145, 1303 (1963).

  5. 5

    Keith, M. L., and Weber, J. N., Geochim. Cosmochim. Acta, 28, 1787 (1964).

Download references

Author information


  1. Department of Geochemistry and Mineralogy, Pennsylvania State University, Pennsylvania

    • JON N. WEBER


  1. Search for JON N. WEBER in:

About this article

Publication history

Issue Date



Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.