Dear Vratislav, I would propose to read the fundamental papers of Yong-Fei Zheng (see below). I also observed such trends in basalt- and lamprophyre-related carbonates (although formed at much higher temperatures). Possible explanations could be found in fractionations during CO2-H2O degassing and associated changes in the carbonate-fluid isotope fractionation due to XCO2 decrease. Best regards, Attila Zheng, Y.-F. (1990): Carbon-oxygen isotopic covariation in hydrothermal calcite during degassing of CO2. Mineral. Deposita, 25, 246-250. Zheng, Y.-F., Hoefs, J. (1993): Carbon and oxygen isotopic covariations in hydrothermal calcites. Mineralium Deposita, 28, 79-89. Demeny, A., Forizs, I., Molnar, F. (1994): Stable isotope and chemical compositions of carbonate ocelli and veins in Mesozoic lamprophyres of Hungary. European Journal of Mineralogy, 6, 679-690. Demeny, A., Harangi, Sz. (1996): Stable isotope studies on carbonate formations in alkaline basalt and lamprophyre series: evolution of magmatic fluids and magma-sediment interactions. Lithos, 37, 335-349. -- ********************************************************************************* Dr. Attila Demeny Geokemiai Kutatolaboratorium - Laboratory for Geochemical Research Magyar Tudomanyos Akademia - Hungarian Academy for Sciences Budapest, Budaorsi ut 45, H-1112, Hungary Tel: (361-)309-2600/ ext. 1223 (office), 1394 (lab) Tel/fax: (361-)319-3137 e-mail: [log in to unmask] Http://www.core.hu/~demeny *********************************************************************************