Rare earth elements are critical constituents for modern technologies, and some of their largest natural resource deposits are related to carbonatite systems. However, the mechanisms leading to rare earth element mineralization and the role of magmatic fluids in carbonatite systems remain poorly understood. Here, we present the first in situ characterization of fluids and their trace-element compositions in natural carbonatite systems by studying secondary quartz-hosted fluid inclusions from Oldoinyo Lengai volcano. By comparing our data to other fluids and melts from various carbonatite systems, we constructed a model for fluid-mediated rare earth element transport and mineralization. We show that carbonatite-related fluids are rich in alkali-carbonate + sulfate + chloride and CO2, but poor in H2O, and they can be significant carriers of rare earth elements (>1600 ppm). We argue that fluid CO2 contents are essential to preclude or slow down the interaction with wall rock during migration and that fluid-mediated rare earth element mineralization occurs when partial pressure of CO2 decreases in the fluid (i.e., during degassing).

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