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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]

The formation of large‐volume silicic magmas in arc settings is fundamental for understanding trans‐crustal magmatic systems related to subduction zones. Here, we present an integrated study of the four Quaternary Toba eruptions and pre‐caldera Haranggaol Andesite on Sumatra, Indonesia. This rock suite has significantly enriched Sr‐Nd isotopes (87Sr/86Sr = 0.71220–0.71517, εNd = −8.9 to −10.6) compared with other volcanic rocks on Sunda‐Banda arc, but is similar to the post‐caldera (<74 ka) Sipisupisu basalts near the Toba Caldera. Thermodynamic modeling using Magma Chamber Simulator has revealed that the Toba silicic rocks can be produced by a two‐stage assimilation and fractional crystallization of mantle‐derived basaltic melts with compositions similar to the Sipisupisu basalts in the lower and upper crustal magma reservoirs. Binary modeling of Sr‐Nd isotopes suggests that the rocks near the Toba Caldera can be produced by mixing of 5%–10% of the subducting Nicobar Fan sediments (87Sr/86Sr = 0.73493, εNd = −14 on average) with depleted MORB mantle (DMM). Indeed, decompressional partial melting modeling of bulk mixtures of DMM with 7% subducting sediments using pMELTS indicates that the melts generated can have geochemical compositions similar to the Sipisupisu basalts. We therefore argue that hybridization of the subducting sediments with the mantle wedge could be an alternative scenario responsible for the enriched isotopic characteristics of the rocks near Toba. Prolonged fractionation of mantle‐derived enriched/depleted basaltic melts, accompanied by crustal assimilation and crystal‐melt segregation, could be common processes in generating large‐volume silicic magmas on continental arcs. Plain Language Summary: The Toba volcano in northern Sumatra is a supervolcano that has produced two super‐eruptions, one of which is the largest Quaternary eruption on Earth. The last super‐eruption of the Youngest Toba Tuff at ∼74 ka expelled ∼5,300 km3 dense rock equivalent of magmas, corresponding to a volcanic explosivity index of ∼9.1. This eruption has produced the largest collapsed caldera and caused severe global temperature decrease and environmental impacts. The silicic magmas feeding these super‐eruptions have previously been considered to be produced by melting of the continental crust. We present a new and systematic elemental and Sr‐Nd isotopic data set of the Toba eruptions. Combined with thermodynamic modeling, we show that the Toba silicic magmas were fundamentally the evolutionary products of mantle‐derived basaltic melts. The basaltic melts were isotopically enriched due to addition of subducted terrigenous sediments to the mantle wedge underneath the Toba Caldera. The basaltic melts have undergone a two‐stage evolution in the lower and upper crustal magma reservoirs. Assimilation of the continental crust and crystal‐melt segregation could have occurred accompanied by fractional crystallization in the upper crustal magma reservoir. We therefore propose that large‐volume silicic magmas commonly have a basaltic root, at least in arc settings. Key Points: Toba silicic magmas were products of a two‐stage evolution of mantle‐derived basaltic melts in the lower and upper crustAssimilation, fractionation, and crystal‐melt segregation of andesitic magma in the upper crust feed directly to the Toba silicic eruptionsThe mantle source underneath Toba is enriched due to hybridization of the subducted sediments [ABSTRACT FROM AUTHOR]

Super-eruptions are among the most threatening natural hazards on Earth. The storage and thermal status of magmas within the crust are fundamental to understand the triggering mechanisms of explosive eruptions. This study presents cathodoluminescence (CL) images and Ti contents of quartz crystals from the ∼75 ka Youngest Toba Tuff (YTT), Indonesia. More than half of the quartz crystals did not display compositional zoning, whereas others contained simple zoning with bright rims (100 to 150 μm in width) or fine oscillatory zoning. The crystallization temperatures of unzoned quartz crystals were mostly near or below the eruptive temperatures estimated by Fe-Ti oxides, and similar to those of CL-dark zones in zoned quartz crystals. This indicates that the majority of quartz crystals were crystallized at relatively low and constant temperatures during the repose period prior to the YTT eruption. In comparison, the CL-bright zones of zoned quartz crystals are commonly thin and crystallized within or slightly beyond the range of eruptive temperatures, indicating progressive thermal maturation of the crustal magma reservoir. Combined with greyscale intensity and Ti-in-quartz diffusion modeling, the residence time of quartz crystals within the crustal magma reservoir was estimated to vary from ∼100 to 400 kyrs, which is of similar orders of magnitude with those obtained by zircon U-Pb and allanite U-Th ages. Our results suggest that the large YTT crustal magma reservoir was generally stored at near-solidus temperatures in discrete and thermally heterogeneous magma batches for a prolonged period of hundreds of kyrs. We argue that sustained and stable magma influx, instead of pre-eruptive magma reheating, is essential to maintain the thermally steady-state YTT crustal magma reservoir and leads to rapid aggregation of different magma batches prior to the YTT super-eruption. • The YTT magma reservoir was stored at a near-solidus status for hundreds of kyrs. • Sustained and stable magma influx maintained a thermally steady YTT magma reservoir. • Rapid assembly of thermally heterogeneous magma batches occurred before the eruption. • Quartz could record prolonged evolution histories of large magmatic systems. [ABSTRACT FROM AUTHOR]