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Natrocarbonatites; a hidden product of three-phase immiscibility

Tibor Guzmics, Marta Berkesi, Robert J. Bodnar, Andras Fall, Eniko Bali, Ralf Milke, Eniko Vetlenyi and Csaba Szabo
Natrocarbonatites; a hidden product of three-phase immiscibility
Geology (Boulder) (April 2019) 47 (6): 527-530


Earth's only active natrocarbonatite volcanism, occurring at Oldoinyo Lengai (OL), Tanzania, suggests that natrocarbonatite melts are formed through a unique geological process. In the East African Rift, the extinct Kerimasi (KER) volcano is a neighbor of OL and also contains nephelinites hosting melt and fluid inclusions that preserve the igneous processes associated with formation of natrocarbonatite melts. Here, we present evidence for the presence of coexisting nephelinite melt, fluorine-rich carbonate melt, and alkali carbonate fluid. The compositions of these phases differ from the composition of OL natrocarbonatites; therefore, it is unlikely that natrocarbonatites formed directly from one of these phases. Instead, mixing of the outgassing alkali carbonate fluid and the fluorine-rich carbonate melt can yield natrocarbonatite compositions at temperatures close to subsolidus temperatures of nephelinite (<630-650 degrees C). Moreover, the high halogen content (6-16 wt%) in the carbonate melt precludes saturation of calcite (i.e., formation of calciocarbonatite) and maintains the carbonate melt in the liquid state with 28-41 wt% CaO at temperatures > or =600 degrees C. Our study suggests that alkali carbonate fluids and melts could have commonly formed in the geological past, but it is unlikely they precipitated calcite that facilitates fossilization. Instead, alkali carbonates likely precipitated that were not preserved in the fossil nephelinite rocks. Thus, alkali carbonate fluids and melts have been so far overlooked in the geological record because of the lack of previous detailed inclusion studies.

ISSN: 0091-7613
EISSN: 1943-2682
Serial Title: Geology (Boulder)
Serial Volume: 47
Serial Issue: 6
Title: Natrocarbonatites; a hidden product of three-phase immiscibility
Affiliation: Eotvos University Budapest, Lithosphere Fluid Research Laboratory, Budapest, Hungary
Pages: 527-530
Published: 20190415
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 23
Accession Number: 2019-038006
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2019193
Illustration Description: illus.
S02°51'00" - S02°51'00", E35°57'00" - E35°57'00"
Secondary Affiliation: Virginia Polytechnic Institute and State University, USA, United StatesUniversity of Texas at Austin, USA, United StatesUniversity of Iceland, ISL, IcelandFree University of Berlin, DEU, Germany
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201920
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