A common origin of carbonatite magmas

The more than 500 fossil Ca-carbonatite occurrences on Earth areat odds with the only active East African Rift carbonatite volcano,Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas.The volcano9s recent major explosive eruptions yielded a mixof nephelinitic and carbonatite melts, supporting the hypothesis thatcarbonatites and spatially associated peralkaline silicate lavas arerelated through liquid immiscibility. Nevertheless, previous eruptiontemperatures of Na-carbonatites were 490–595 °C, which is 250–450°C lower than for any suitable conjugate silicate liquid. This studydemonstrates experimentally that moderately alkaline Ca-carbonatitemelts evolve to Na-carbonatites through crystal fractionation.The thermal barrier of the synthetic Na-Ca-carbonate system, heldto preclude an evolution from Ca-carbonatites to Na-carbonatites,vanishes in the natural system, where continuous fractionation ofcalcite + apatite leads to Na-carbonatites, as observed at OldoinyoLengai. Furthermore, saturating the Na-carbonatite with mineralspresent in possible conjugate nephelinites yields a parent carbonatitewith total alkali contents of 8–9 wt%, i.e., concentrations thatare realistic for immiscible separation from nephelinitic liquids at1000–1050 °C. Modeling the liquid line of descent along the calcitesurface requires a total fractionation of ∼48% calcite, ∼12% apatite,and ∼2 wt% clinopyroxene. SiO 2 solubility only increases from 0.2to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallizationof silicates. The experimental results suggest a moderately alkalineparent to the Oldoinyo Lengai carbonatites and therefore a commonorigin for carbonatites related to alkaline magmatism.