Abstract

The haüynophyre emitted from a parasitic vent of the Vulture stratovolcano is a S- and Cl-rich, leucite-melilite-bearing lava flow containing an unusually large amount of sodalite-group minerals (>23 vol.%). Mineralogical and chemical study of phenocrysts has led to the identification of black haüynes, blue lazurites and of Cl-rich white or black noseans. X-ray diffraction (XRD) study confirms the occurrence of nosean having a low symmetry (P23). Raman spectra and XRD data show that S is fully oxidized to SO4 in black haü ynes and in white noseans, while it is partly reduced to form S3 groups in blue lazurites, which also contain H2O molecules. Structural and chemical data strongly question the validity of the Hogarth and Griffin (1976) method widely used to resolve the ratio S6+/S2− in sodalite-group phases from EMPA data. Among euhedral phenocrysts, large lazurites are only faintly zoned. All other phases show variable core-rim chemical zoning and many phenocrysts are partially resorbed and/or colour-zoned. Black haüynes have highly variable S/Cl and slightly lower SiO2/Al2O3 ratios, larger FeTOT contents and more compatible trace elements than lazurites. Thin opaque noseansodalite rims surrounding all crystals are interpreted as a result of rapid crystallization driven by exsolution of a S-scavenging fluid phase. We suggest that the extreme complexity of the mineralogical assemblage reflects variable aSiO2 and aH2O of the silicate melts.

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