Oxygen isotope fractionation in double carbonates.
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 (18)O 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.
a CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences , University of Science and Technology of China , Hefei , People's Republic of China.
Yong-Fei
YF
Zheng
b Geochemistry and Stable Isotope Geochemistry Group, Department of Marine Geology , Leibniz Institute for Baltic Sea Research (IOW) , Warnemünde , Germany.
Michael E
ME
Böttcher
D001464:Barium / Q000032:analysis; D002247:Carbon Isotopes / Q000737:chemistry; D002254:Carbonates / Q000032:analysis*; D005591:Chemical Fractionation / Q000379:methods*; D003461:Crystallography; D007854:Lead / Q000032:analysis; D008274:Magnesium / Q000032:analysis; D010100:Oxygen / Q000737:chemistry*; D010103:Oxygen Isotopes / Q000737:chemistry
D002247:Carbon Isotopes; D002254:Carbonates; D010103:Oxygen Isotopes; C005479:magnesium carbonate; D001464:Barium; D007854:Lead; C043262:lead carbonate; C006685:barium carbonate; D008274:Magnesium; D010100:Oxygen
carbon isotopes; crystal structure; double carbonate; isotope exchange; isotope fractionation; oxygen isotopes; temperature dependence