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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.



Benjamin Florian Walter1, Johannes Giebel2,3, Matthew Steele-MacInnis4, Michael Marks5, Jochen Kolb1, Gregor Markl5
1Karlsruhe Institute of Technology, Adenauerring 20b, 76131 Karlsruhe, Germany; 2Technische Universität Berlin, Ernst-Reuter-Platz 1, 10587 Berlin, Germany; 3University of the Free State, 250 Nelson-Mandela-Drive, Bloomfontein 9300, South Africa; 4University of Alberta, 1-26 Earth Sciences Building, Edmonton AB T6G2E3, Canada; 5Eberhard Karls Universität Tübingen, Schnarrenbergstraße 94–96, 72076 Tübingen, Germany
GeoKarlsruhe 2021