Stability of naujakasite in hyperagpaitic melts, and the petrology of naujakasite lujavrite in the Ilímaussaq alkaline complex, South Greenland

• Published in: Mineralogical Magazine
• Main Authors: Andersen, Tom, Sørensen, H.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2005
• Subjects: Agpaitic rocks; Greenland; Ilímaussaq complex; Naujakasite; Nepheline syenite; Oxygen fugacity; Water activity; kola peninsula
• Online Access: https://researchers.mq.edu.au/en/publications/d487f80f-fb45-492a-9d5d-35a576fac45b; https://doi.org/10.1180/0026461056920240; http://www.scopus.com/inward/record.url?scp=20444380324&partnerID=8YFLogxK

Naujakasite (Na 6 (Fe,Mn)Al 4 Si 8 O 26 ) is a characteristic, rock-forming mineral of hyperagpaitic naujakasite lujavrite (arfvedsonite + naujakasite + steenstrupine + albite ± relict nepheline ± microcline ± ussingite) from the Ilímaussaq complex, South Greenland, and is only known from this complex. The qualitative stability relationships of naujakasite-bearing, low-variance liquidus mineral assemblages at near-solidus conditions in lujavritic magma can be modelled within the six-component system Si-Al-Fe-Na-O-H. The modelling suggests that an uncommon combination of elevated water activity and low oxygen fugacity is needed to stabilize arfvedsonite + naujakasite mineral assemblages in highly sodic liquids. These conditions are incompatible with the stable coexistence of aegirine and arfvedsonite. They are met in the arfvedsonite-bearing lujavrites of Ilímaussaq, but apparently not in rocks of similar composition in the Khibina and Lovozero complexes, Kola Peninsula, where aegirine-or aegirine-arfvedsonite-bearing mineral assemblages are characteristic. Liquid lines of descent leading to crystallization of naujakasite have to pass through the albite+nepheline+arfvedsonite liquidus field in the Si-Al-Fe-Na-O-H system, reaching this field at the high-Na side of the albite-nepheline-arfvedsonite plane.