Magmatic processes and timescales of the 726 CE eruption of the Kameni Volcano (Greece)

The polycyclic Santorini caldera (Greece) has entered a new caldera cycle following the large-magnitude Late Bronze Age (Minoan) eruption¹. In this new caldera cycle, low-magnitude effusive and mildly explosive eruptions have built up the Kameni islands inside the flooded Minoan caldera. However, an eruption deposit up to 34 m thick was recovered during the International Ocean Discovery Program (IODP) Expedition 398 to the South Aegean Volcanic Arc² at various sites inside the Santorini caldera (U1594-U1597) and has been interpreted as produced by an explosive (VEI 5) eruption in 726 CE³. Such explosive eruptions are uncommon in the early stages of caldera cycles when the plumbing system is recharging, and the shallow magma reservoir is recovering from caldera collapse. The indication that Santorini can produce explosive eruptions early in a new caldera cycle elevates the hazard potential of future eruptions for Santorini and neighbouring islands in the eastern Mediterranean.

This study presents a petrological and geochemical investigation of juvenile material from the 726 CE eruption deposit at IODP Site U1595, encompassing the full thickness of the deposit, including light and dark grey pumice, banded pumice, scoria, mafic enclaves, dark cognate lithics and glomerocrysts. We also report the results of diffusion chronometry on the primary crystal phases, including Fe-Mg diffusion in orthopyroxene and clinopyroxene, and Mg diffusion in plagioclase. Crystal chemistry reveals the presence of mafic (Mg# 84-73 and An 86-82), intermediate (Mg# 71-69 and An 67-57), and more silicic (Mg# 67-54 and An 53-37) crystal assemblages derived from compositionally distinct magmatic sources beneath the volcano, as well as a crystal mush zone, with evidence of crystal exchange between these reservoirs. By modelling reverse-, normally- and oscillatory-zoned core-rim profiles using diffusion chronology, we constrain mafic and silicic magma recharge timescales, revealing the complex recharge dynamics of the plumbing system associated with the 726 CE eruption. Plagioclase-, clinopyroxene-, and orthopyroxene-hosted melt inclusions of dacitic composition record water concentrations between 3.1-6.1 wt% H₂O (average of 4.5 wt% H₂O and CO₂ concentrations below detection limits (<50 ppm CO₂), corresponding with shallow and upper-mid crustal storage between 2.6-7.7 km depth. The melt inclusion derived water concentrations are consistent with other explosive eruptions at Santorini⁴. Collectively, these results advance our understanding of post-caldera magma system evolution and the conditions under which magma reservoirs capable of large explosive eruptions can develop early in a new caldera cycle.

¹Druitt et al. (1999) Geological Society of London, Memoirs, https://doi.org/10.1144/GSL.MEM.1999.019.01.1

²Druitt et al. (2024) Proceedings of the International Ocean Discovery Program,

https://doi.org/10.14379/iodp.proc.398.101.2024

³Preine et al. (2024) Nature Geoscience, https://doi.org/10.1038/s41561-024-01392-7

⁴Druitt et al. (2016) Journal of Petrology, https://doi.org/10.1093/petrology/egw015