High-Alumina Daughter Phases in Olivine-Hosted Melt Inclusions from Kudryavy and Menshiy Brat Volcanoesv (Medvezhia Caldera, Iturup Island)

For the first time, high-alumina mineral assemblages including rare minerals of the rhönite–kuratite series were found in postcaldera lava flows of the Kudryavy and Menshiy Brat volcanoes (Kuril Islands). These lava flows consist of olivine-bearing basalts and andesite-basalts of the calc-alkaline series. Partially crystallized olivine-hosted melt inclusions in these rocks contain high-alumina daughter phases. Residual glass with up to 25 wt % Al2O3, Al-pyroxenes, and spinel with minerals of the rhönite subgroup were identified. The rhönite–kuratite series minerals are characterized by strong variations in Al, Fe2+, Fe3+, Mg, Si, as well as very low Ti, which makes it possible to call it a low-Ti Fe2+ analogue of rhönite with the formula (Ca,Mg,Na)4(Mg,Fe,Fe, Al,Ti,Mn)12O4[Si8.2Al3.8O36]—(Ca,Na,Mg)4(Fe,Mg,Al,Fe,Ti)12O4[Si6.78Al5.22O36]. It is assumed that high-alumina daughter phases form in olivine-hosted inclusions via water accumulation in trapped melts during crystallization of the host olivine on the walls of melt inclusions. Water-rich melts can inhibit early crystallization of plagioclase, which promotes enrichment of the residual melt in Al2O3.