A global review of carbonatite-hosted fluid inclusions and the role of fluid release on carbonatite magma ascent

Carbonatites crystallize from mantle-derived carbonate- and volatile-rich melts that exsolve large amounts of fluids during their ascent through and emplacement into the crust. A global review of available fluid inclusion data for carbonatitic systems from variable emplacement depths identified four types of fluid inclusions: (type-I) vapour-poor H₂O-NaCl fluids with <50 wt.% salinity; (type-II) vapour-rich H₂O-NaCl-CO₂ fluids with <5 wt.% salinity; (type-III) multi-component fluids with high salinity without CO₂; and (type-IV) multi-component fluids with high salinity and high CO₂. This global data set indicates initial release of type-I saline brines that may either separate into immiscible type-II and -III fluids (eruption?) or may continuously evolve into type-IV fluids (sealing?). Moreover, fluid inclusions in early magmatic apatite crystallization suggest initial fluid release (type-I) at depths of 12-16 km (brittle-ductile transitions zone), which may be related to a sudden pressure drop initiated by crustal fracturing during rapid, forceful and discontinuous magma ascent.

Our model for the ascent of carbonatitic magmas is adopted from a jackhammer-like process, which explains the apparent absence of shallow carbonatite magma chambers, reflects the observed intrusion geometries, identifies fenitization as a process induced by fluids released during magma ascent and final emplacement, and demonstrates the formation of fluid induced brecciation related to magma ascent. The proposed model of a self-sustaining system is also in agreement with a turbulent ascent and high ascent rates, which allows for the transport of mantle xenoliths through the crust as observed in several cases.