Constraints on the timing of Quaternary volcanism and duration of magma residence at Ciomadul volcano, east–central Europe, from combined U–Th/He and U–Th zircon geochronology

• Combined use of U–Th and (U–Th)/He zircon geochronology for eruption dating
• Eruption ages of the youngest volcano of eastern–central Europe
• Coeval large explosive event with the Campanian Ignimbrite eruption
• Prolonged zircon crystallization in a crustal magma chamber over ca. 300–350 ka
• Long lasting (> 100 ka) cold crystal mush storage beneath Ciomadul

High-spatial resolution zircon geochronology was applied to constrain the timescales of volcanic eruptions of the youngest, mostly explosive volcanic phase of Ciomadul volcano (Carpathian–Pannonian region, Romania). Combined U–Th and (U–Th)/He zircon dating demonstrates that intermittent volcanic eruptions occurred in a time range of 56–32 ka. The reliability of the eruption dates is supported by concordant ages obtained from different dating techniques, such as zircon geochronology, radiocarbon analysis, and infrared stimulated luminescence dating for the same deposits. The new geochronological data suggest that volcanism at Ciomadul is much younger (< ca. 200 ka) than previously thought (up to 600 ka). A dominantly explosive volcanic phase occurred after an apparent lull in volcanism that lasted for several 10's of ka, after a period of lava dome extrusion that defines the onset of the known volcanism at Ciomadul. At least four major eruptive episodes can be distinguished within the 56–32 ka period. Among them, relatively large (sub-plinian to plinian) explosive eruptions produced distal tephra covering extended areas mostly southeast from the volcano. The 38.9 ka tephra overlaps the age of the Campanian Ignimbrite eruption and has an overlapping dispersion axis towards the Black Sea region. The wide range of U–Th model ages of the studied zircons indicates prolonged existence of a low-temperature (< 800 °C) silicic crystal mush beneath Ciomadul. The main zircon crystallization period was between ca. 100 and 200 ka, coeval with the older, mostly extrusive lava dome building stage of volcanism. Even the youngest U–Th model ages obtained for the outermost 4 μm rim of individual zircon crystals predate the eruption by several 10's of ka. The zircon age distributions suggest re-heating above zircon saturation temperatures via injection of hot mafic magmas prior to eruption. Intermittent intrusions of fresh magma could play a significant role in keeping the intrusive silicic magmatic reservoir in a partially melted for prolonged period. The previous history of Ciomadul suggests that melt-bearing crystal mush resided beneath the volcano, and was rapidly remobilized after a protracted (several 10's of ka) lull in volcanism to trigger several eruptions in a comparatively short time window. This classifies Ciomadul as a volcano with ‘Potentially Active Magma Storage’ (PAMS) which we propose to be common among the seemingly inactive volcanoes in volcanic arc regions. The potential for reactivation of these systems should be included into volcanic hazard assessments.