Figure 3
Diagrams of (a) δ26Mg versus SiO2 and (b) δ26Mg versus 1/MgO, for calcitic and dolomitic dykes and ore bodies of the Bayan Obo deposit.
Also shown are the δ26Mg ranges of the mantle and the upper continental crust (UCC)31,33. Sedimentary carbonates have δ26Mg = −3.09 ± 2.66‰59. The calcitic dykes have highly varied δ26Mg. Samples with high SiO2 and MgO have δ26Mg close to mantle values, indicating addition of SiO2 and mantle MgO. Most samples plot about a mixing line between the mantle (average δ26Mg = −0.25 ± 0.07‰)33 and the calcitic carbonatite sample (No. 07.104-2.5) that has the lowest Mg content and δ26Mg value. Four of the calcitic carbonatite samples have δ26Mg higher than the mantle value, however, which cannot be explained by simple mixing between these two end-members. Material from the upper continental crust may also have been involved.
