The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano's recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490-595 °C, which is 250-450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8-9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000-1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite, ~12% apatite, and ~2 wt% clinopyroxene. SiO 2 solubility only increases from 0.2 to 2.9 wt% at 750-1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism. *

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A new numerical model has been developed that makes it possible to describe the process of formation of a dyke of a combined composition on the basis of the dynamics of a viscous compressible fluid. The numerical thermo-mechanical model shows the processes of magma mingling and taking into account multiphase interaction of melts which are different in composition and properties. The models suggest a mechanism for uplifting of high-density mafic enclaves in a chamber/dyke filled with salic magma by gravitational floating in the enclosing granite magma that has been cooled and lost volatile components. The performed simulation shows that the main parameter controlling the shape and size of the ascending bodies is the difference in densities. The viscosity contrast determines whether interpenetration and hybridization of magmas occur. The limiting ratio of felsic material in the mixture, which is capable of uplifting denser mafic enclaves, is estimated. The duration of melt uplifting in combined dykes is estimated with respect to the viscosity parameters. At a typical rate of 2-3 km per year, it amounts to almost 12 months.