Applicability and Limitations of the Na–Ca Partitioning between Scapolite and Plagioclase as a Geothermometer

The Na-Ca partitioning between coexisting scapolite and plagioclase constitutes a potential geothermometer. The application is, however, complicated by the potential influence of additional parameters, including the bulk-rock and the fluid composition. The term scapolite refers to a solid solution between the marialite (3NaAlSi₃O₈·NaCl) and meionite (3CaAl₂Si₂O₈·CaCO₃) end-members. Thermodynamic mixing models for scapolite are available for Ca-rich scapolite with CO₃^2- as the only anion (carbonate scapolite) and do not account for Ca-poor scapolite, where Cl^- is an important anion, in addition to CO₃^2-. Even for carbonate scapolite, the bulk rock composition, in particular, the alumina to silica and the Na₂O to CaO ratios, may have an influence on the compositions of coexisting scapolite and plagioclase. We investigate the chemical compositions of scapolite and plagioclase from the southeastern part of the Bohemian massif in Austria. Scapolite is found within calcsilicate gneiss associated with marble layers of the Drosendorf unit. The layers can be traced more or less continuously for at least 75 km in a north-south direction with a slight temperature increase towards the south. We compare the results with predictions from Gibbs energy minimization. The thermodynamic calculations indicate a substantial temperature dependence of the Na-Ca partitioning between coexisting carbonate scapolite and plagioclase. At fixed pressure and temperature and in the presence of calcite, the compositions of scapolite and plagioclase are fixed. Under these specific conditions, the temperature dependence of the Na-Ca partitioning between Ca-rich carbonate scapolite and anorthite-rich plagioclase are a viable geothermometer. In the absence of calcite, the compositions of coexisting scapolite and plagioclase depend on the bulk rock composition and can produce a range of compositions for the same temperature. Still, the Na-Ca partitioning may be used for temperature determination if scapolite-plagioclase pairs of different compositions are available from one sample or one outcrop, so that several scapolite-plagioclase conodes can be determined simultaneously.