Prolonged near-solidus and steady-state magma storage for the Youngest Toba Tuff: Evidence from TitaniQ thermometry and diffusion chronometry.

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]