New Age for the Archaeos Tuff: Using 40Ar/39Ar and U-Pb to Redefine Ancestral Santorini’s Tephrochronostratigraphy and Implications for Rapid Magma Accumulation

IODP Expedition 398 to the Christiana-Santorini-Kolumbo Volcanic Field unearthed the largest shallow submarine explosive eruption from ancestral Santorini: The Archaeos Tuff (AT). This ~90 km³ eruption erupted at water depths of 200–1000 m, forming a megaturbidite up to 150 m thick in the surrounding marine basins¹. Shipboard biostratigraphic constraints initially suggested an age of 520 ± 10 ka, though nannofossils are subject to post-eruptive upward remobilization in active marine basins¹. Here, we present preliminary ⁴⁰Ar/³⁹Ar and U-Pb age determinations for AT from 5 samples at sites U1591 and U1593 that redefine AT as a stratigraphic marker approximately 250 kyrs older than shipboard estimates. Plagioclase and sufficiently degassed glass were separated from recovered pumice and lapilli by crushing, heavy liquid and/or magnetic separation, and subsequently leached and handpicked. Following irradiation, the samples were analyzed on the ARGUS-VI noble gas multi-collector mass spectrometer in the ⁴⁰Ar/³⁹Ar Geochronology lab at Oregon State University. In parallel, zircons were separated from the residual sample powders (<150 µm) as well as one AT onshore equivalent and analyzed by secondary ionization mass spectrometry (SIMS) at Curtin University. Preliminary age determinations from Sites U1591 and U1593 in the Christiana and Anhydros Basins, respectively, of glass and plagioclase indicate a new stacked eruptive age that is tentatively constrained at 764 ± 3.5 . Tentative U-Pb crystallization ages from zircons collected from the same samples, after filtering for high-radiogenic (>90% ²⁰⁶Pb) spots, range from a minimum crystallization age of 775 ± 18 Ma, over a a (geologically meaningless) central age of 810 ± 19 Ma (MSWD = 3.1; n = 49), to a minor xenocrystic population with ages between 2.1 and 8.6 Ma (n = 5). Compared to other highly explosive arc systems, the equivalence between ⁴⁰Ar/³⁹Ar ages and those of a significant zircon population with near-eruption crystallization ages suggests rapid magma accumulation prior to the AT event. This highlights the importance of integrated, high-resolution radiometric age constraints in tephrochronostratigraphy and multi-chronometer assessments of recharge timescales in arc settings.

¹Druitt, T., Kutterolf, S., Ronge, T.A. et al. Giant offshore pumice deposit records a shallow submarine explosive eruption of ancestral Santorini. Commun Earth Environ 5, 24 (2024). https://doi.org/10.1038/s43247-023-01171-z