Mixing and eruption of mafic magmas of the Sipisupisu volcano, the Toba volcanic system, Indonesia.

Texture and particularly compositional variations of zoned minerals are important to understand trans-crustal magma plumbing systems of volcanoes. The Sipisupisu volcano in Sunda arc, northern Sumatra, occurs as a stratocone on the northwestern tip of the Toba Caldera and belongs to the large Toba volcanic system, which has produced two super-eruptions in the Quaternary. Compositionally zoned olivine, plagioclase and clinopyroxene grains, together with orthopyroxene reaction rims around olivine from basalts and basaltic andesites of the Sipisupisu volcano indicate open-system processes characterized by multiple episodes of magma recharge and mixing. Based on compositional variations of a variety of zoned olivine and clinopyroxene crystals, melts ascended from the large basaltic magma reservoir at the Moho have resided in at least three staging magma reservoirs within the crust. One magma reservoir is Si-saturated and located < 13 km deep in the upper crust, underlain by two Si-undersaturated ones at a depth from ~ 13 to 25 km. Fe–Mg diffusion modeling shows that olivine residence time from mixing of compositionally more evolved magmas to eruption varies from ~ 7 days to 9 months, whereas that from recharge of more primitive magmas to eruption is ~ 6 months to 5 years. This timescale is, on average, longer than olivine residence time of the majority of mafic arc volcanoes worldwide and may favor prolonged crystal fractionation. The multiple staging magma reservoirs and prolonged crystal residence time of the Sipisupisu volcano could potentially be linked to the large basaltic magma reservoir at depth. Finally, mixing of Si-saturated and Si-undersaturated magmas probably triggered Sipisupisu eruption. [ABSTRACT FROM AUTHOR]