TIMING AND RECURRENCE INTERVALS FOR MAJOR ERUPTIONS FROM AMATITLÁN CALDERA (GUATEMALA)

Defining a precise timeline for past eruptions from explosive volcanoes in continental arcs is imperative to forecast future hazards and mitigate volcanic disasters for these often densely populated regions. However, establishing reliable ages for Quaternary eruptions in the Central American Volcanic Arc (CAVA) has been challenging due to the common lack/alteration of suitable K-rich phases for ⁴⁰Ar/³⁹Ar geochronology but also from their position in time beyond the scope of radiocarbon dating. This especially holds for the active Amatitlán caldera in Guatemala, from which at least six explosive silicic eruptions have produced tephra blanketing neighboring regions that are today inhabited by millions of people. Zircon, a common datable accessory mineral in Amatitlán caldera magmas, was here used to retrieve eruption ages by applying the novel zircon double-dating method (ZDD) that integrates ²³⁸U-²³⁰Th disequilibrium dating and (U-Th)/He thermochronology. Two of its most recent and previously undated eruptions (J-tephra and E-tephra) yielded the first-ever radioisotopic ages of 24 ± 3 ka and 48 ± 6 ka (1σ), respectively. Remarkably, zircon crystallization and ZDD eruption ages for two older and voluminous eruptions (T-tephra and L-tephra) significantly post-date existing plagioclase ⁴⁰Ar/³⁹Ar dates by ca. 30 and 70 kyr, respectively. The ZDD eruption age for T-tephra is 93 ± 4 ka, whereas zircon crystallization ages for L-tephra yield a maximum modelled eruption age of ca. 124 ka. The strong eruption age dichotomy between ZDD and plagioclase ⁴⁰Ar/³⁹Ar dating argues for the presence of inherited or xenocrystic plagioclase in Amatitlán caldera eruptive products. Statistical analysis from the updated eruptive history suggests a significantly shorter eruption recurrence interval of ca. 17 kyr than previously estimated. The new age data, thus, suggest a potentially explosive eruption overdue for the Amatitlán caldera, highlighting the necessity to better understand the current underlying magmatic system and to better constrain its past eruptive history.