Abstract
For the first time, high-alumina mineral associations including rare minerals of the rhönite-kuratite series were found in post-caldera lava flows of the Kudryaviy and Menshiy Brat volcanoes (Kuril Islands). These lava flows are composed 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 name 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 melt inclusions through water accumulation in the 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.
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