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Russian Geology and Geophysics

2011 year, number 11

RHÖNITE IN ALKALI BASALTS: SILICATE MELT INCLUSIONS IN OLIVINE PHENOCRYSTS

V.V. Sharygina, K. Kóthayb, Cs. Szabób, T.Ju. Timinaa, K. Törökb,c, Ye. Vapnikd, and D.V. Kuz'mina,e
aV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
bLithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary
cResearch Group for Environmental Physics and Geophysics of the Hungarian Academy of Sciences, Department of Geophysics, Eötvös University, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary
dDepartment of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P.O. Box 653, 84105 Beer-Sheva, Israel
eGeochemistry Division, Max Planck Institute fur Chemie, Joh.-Joachim-Becher-Weg 27, Mainz, 55128, Germany
Keywords: Rhönite, cinopyroxene, spinel, silicate melt inclusions, olivine phenocryst, alkali basalts
Pages: 1334-1352

Abstract

Silicate melt inclusions containing rhonite Ca2 (Mg,Fe2+)4Fe3+Ti[Al3Si3O20] were studied in olivine phenocrysts from alkali basalts of six different volcanic regions: Udokan Plateau, North Minusa Depression, Tsagan-Khurtei Ridge (Russia), Bakony-Balaton Highland, Nógrád-Gömör Region (Hungary), and Makhtesh Ramon (Israel). Rhonite-bearing silicate melt inclusions are relatively common phenomena in alkali basalts and usually coexist with inclusions lacking rhönite. Inclusions with rhönite generally occur in the core of the olivine phenocrysts. According to heating experiments and CO2 microthermometry, all the rhönite-bearing inclusions in core of the olivine phenocrysts were trapped as silicate melt at T > 1300?C and P > 3-5 kbar. Rhönite crystallized in a narrow temperature range (1180-1260?Ñ) and P < 0.5 kbar. The petrography and thermometry of rhönite-bearing silicate melt inclusions show a general crystallization sequence: Al-spinel → rhönite → clinopyroxene → apatite → ± amphibole, Fe-Ti oxide (ilmenite or Ti-magnetite) → glass.
The majority of rhonites from melt inclusions have Mg/(Mg + Fe2+)>0.5 and belong to Mg-rich species Ca2Mg4Fe3+Ti[Al3Si3O20]. There are no significant differences in chemistry among rhönites from olivine-hosted silicate melt inclusions from phenocryst, from groundmass of alkali basalts, and from alteration products of kaersutitic amphibole mega/xenocrysts and of kaersutite in deep-seated xenoliths in alkali basalts. The rare occurrence of rhönite as essential constituents in rocks may be explained from its microstructural peculiarities. This mineral is an intermediate member of the polysomatic spinel-pyroxene series. Possibly, the structural feature of rhönite does explain why it is an unstable mineral under changing crystallization conditions. In general, the presence and chemistry of rhonite can be used for the rough estimation of temperature, pressure, and oxygen fugacity during the crystallization of alkali basalts.