Rhönite in alkali basalts: silicate melt inclusions in olivine phenocrysts

Silicate melt inclusions containing rhönite 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). Rhönite-bearing silicate melt inclusions are relatively common phenomena in alkali basalts and usually coexist with inclusions containing no 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 °C) 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.Majority of rhönites from melt inclusions have Mg/(Mg + Fe2+) > 0.5 and belong to Mg-rich species Ca2Mg4Fe3+Ti[Al3Si3O20]. There are no essential differences in chemistry among rhönites from olivine-hosted silicate melt inclusions from phenocryst, from groundmass of alkali basalts, 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 constituent 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 in under changing crystallization conditions. In general, the presence and chemistry of rhönite may be used for rough estimation of temperature, pressure and oxygen fugacity during crystallization of alkali basalts.